Cross coupling between electric and magnetic orders in a multiferroic metal-organic framework

Giant Exchange Bias in Antiferromagnetic Mixed-Valence MOFs

Regulating magnetism of a metal-organic framework (MOF) is highly desirable in magnetoelectric devices. Single-crystalline MOFs of [(CH3)2NH2]2[FeIIIFe2-xIICoxII(HCOO)9] (x = 0, 0.2, 0.3, 0.6) with an unprecedented (412·63)1(49·66)2 topology were synthesized. These MOFs exhibit distinct antiferromagnetism, and the Néel temperature (TN) changes from 32.8 to 29.0 K with increasing x. After field cooling from above TN to 2 K, these MOFs demonstrate giant exchange bias (EB) featured by a significant vertical and simultaneous horizontal shift in an almost linear curve of magnetization (M) versus the field (H). As the temperature rises, the M-H curve becomes hysteretic, with significantly increased coercivity and a diminished EB field. In combination with the Monte Carlo simulations, this EB effect is attributed to a large enough (negligible) anisotropy of FeII (FeIII) ions and antiferromagnetic exchange coupling between FeII and FeIII ions. This work provides a clue to designing a linear magnetic field sensor by using MOFs.

Anomalous Magnetodielectric Effect in the Metal-Organic Framework [CH3NH3][Co(HCOO)3]

We have investigated the anisotropic magnetodielectric effect in the perovskite metal-organic framework [CH3NH3][Co(HCOO)3] single crystals. At 2 K, the spin reorientation along the [101] direction induces a notable dielectric peak, while the ferromagnetic ground state along the [010] direction gives rise to a pronounced positive magnetodielectric effect, which is attributed to the strong magnetic anisotropy of the [CH3NH3][Co(HCOO)3] compounds. At the critical temperature of magnetic ordering, the maximum magnetodielectric effect is observed when both magnetic and electric fields are applied along the [010] or [101] directions, with Δε/ε values of -0.31% along the [101] direction and -0.23% along the [010] direction under a 9 T field. This finding suggests that strong spin fluctuations at the magnetic ordering temperature enhance the magnetodielectric effect. Even in the paramagnetic state up to 150 K, a weak magnetodielectric effect is still observed, which may be attributed to the magnetostrictive effect. Our study provides insights into the mechanisms behind the magnetodielectric effect in metal-organic frameworks, emphasizing the role of magnetic anisotropy and spin-lattice coupling.

Resonant Quantum Magnetodielectric Effect in Multiferroic Metal-Organic Framework [CH3NH3]Co(HCOO)3

The observation of both resonant quantum tunneling of magnetization (RQTM) and resonant quantum magnetodielectric (RQMD) effect in the perovskite multiferroic metal-organic framework [CH3NH3]Co(HCOO)3.is reported. An intrinsic magnetic phase separation emerges at low temperatures due to the hydrogen-bond-modified long-range super-exchange interaction, leading to the coexistence of canted antiferromagnetic order and single-ion (Co2+) magnets. Subsequently, a stair-shaped magnetic hysteresis loop along the [101] direction characterizing the RQTM appears below the magnetic blocking temperature. More interestingly, the magnetic field dependence of dielectric permittivity exhibits pronounced peaks at the critical fields corresponding to the RQTM, a phenomenon termed the RQMD effect which enables electrical detection of the RQTM. The magnetostriction shows a similar behavior to the magnetodielectric effect at 2 and 5 K, which suggests that the magnetodielectric effect in this multiferroic metal-organic framework is related to the spin-lattice coupling.

Quasi-one-dimensional alternating spin-1/2 antiferromagnetism in perovskite metal formate framework [(NH2)2CH]Cu(HCOO)3

The formamidinium copper formate [(NH2)2CH]Cu(HCOO)3(FMD-Cu) with a perovskite-like structure based on a nonporous metal-organic framework (MOF), is presented for its synthesis and magnetic properties. The magnetic properties and their couplings to the structure are derived from detailed magnetic susceptibility and heat capacity measurements. We also discuss the spin exchange couplings based on density functional theory (DFT) calculations. As a result, FMD-Cu exhibits the unusual quasi-one-dimensional antiferromagnetic (AFM) characteristics with the Néel temperatureTN= 12.0 K and an intrachain coupling constantJ/kB≈ 76.3 K. We also estimate the effective interchain couplingJ*/kB≈ 4.24 K, suggesting that FMD-Cu is close to an ideal candidate for one-dimensional magnet. Furthermore, the heat capacity shows a transition to an antiferromagnetic ordering state appears aroundTN. Besides, the nonzero parameterγ= 0.089 J mol-1K-1obtained from the linear relationship,γT, to the low temperature-dependent zero-field heat capacity data, can be associated with the magnetic excitations in insulating quasi-one-dimensional AFM Heisenberg spin-1/2 chains. The experimental estimate and DFT calculations are entirely consistent with a model of FMD-Cu in which AFM exchange interactions originating from Jahn-Teller distortion of the Cu2+(3d9) ions, leaving a sublattice of coupled ferromagnetic (FM) chains. Hence, FMD-Cu is proposed as a canonical model of a quasi-one-dimensional Heisenberg spin-1/2 antiferromagnetic material.

Ferroics in Hybrid Organic-Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

Hybrid organic-inorganic perovskites (HOIPs) afford highly versatile structure design and lattice dimensionalities; thus, they are actively researched as material platforms for the tailoring of ferroic behaviors. Unlike single-phase organic or inorganic materials, the interlayer coupling between organic and inorganic components in HOIPs allows the modification of strain and symmetry by chirality transfer or lattice distortion, thereby enabling the coexistence of ferroic orders. This review focuses on the principles for engineering one or multiple ferroic orders in HOIPs, and the conditions for achieving multiferroicity and magnetoelectric properties. The prospects of multilevel ferroic modulation, chiral spin textures, and spin orbitronics in HOIPs are also presented.

Pressure effect on the magnetism and crystal structure of magnetoelectric metal-organic framework [CH3NH3][Co(HCOO)3]

[CH3NH3][Co(HCOO)3] is the first perovskite-like metal-organic framework exhibiting spin-driven magnetoelectric effects. However, the high-pressure tuning effects on the magnetic properties and crystal structure of [CH3NH3][Co(HCOO)3] have not been studied. In this work, alongside ac magnetic susceptibility measurements, we investigate the magnetic transition temperature evolution under high pressure. Upon increasing the pressure from atmospheric pressure to 0.5 GPa, TN (15.2 K) remains almost unchanged. Continuing to compress the sample results in TN gradually decreasing to 14.8 K at 1.5 GPa. This may be due to pressure induced changes in the bond distance and bond angle of the O-C-O superexchange pathway. In addition, by using high pressure powder X-ray diffraction and Raman spectroscopy, we conducted in-depth research on the pressure dependence of the lattice parameters and Raman modes of [CH3NH3][Co(HCOO)3]. The increase in pressure gives rise to a phase transition from the orthorhombic Pnma to a monoclinic phase at approximately 6.13 GPa. Our study indicates that high pressure can profoundly alter the crystal structure and magnetic properties of perovskite type MOF materials, which could inspire new endeavors in exploring novel phenomena in compressed metal-organic frameworks.

Electrical detection and modulation of magnetism in a Dy-based ferroelectric single-molecule magnet

Electrical control of magnetism in single-molecule magnets with peculiar quantum magnetic behaviours has promise for applications in molecular electronics and quantum computing. Nevertheless, such kind of magnetoelectric effects have not been achieved in such materials. Herein, we report the successful realization of significant magnetoelectric effects by introducing ferroelectricity into a dysprosium-based single-molecule magnet through spatial cooperation between flexible organic ligands and halide ions. The stair-shaped magnetization hysteresis loop, alternating current susceptibility, and magnetic relaxation can be directly modulated by applying a moderate electric field. Conversely, the electric polarization can be modulated by applying a small magnetic field. In addition, a resonant magnetodielectric effect is clearly observed, which enables detection of quantum tunnelling of magnetization by a simple electrical measurement. The integration of ferroelectricity into single-molecule magnets not only broadens the family of single-molecule magnets but also makes electrical detection and modulation of the quantum tunnelling of magnetization a reality.

Raman study of pressure-induced phase transitions in imidazolium manganese- hypophosphite hybrid perovskite

By using Raman spectroscopy, we demonstrate that [IM]Mn(H₂POO)₃ is a highly compressible material that undergoes three pressure-induced phase transitions. Using a diamond anvil cell we performed high-pressure experiments up to 7.1 GPa, using paraffin oil as the compression medium. The first phase transition, which occurs near 2.9 GPa, leads to very pronounced changes in the Raman spectra. This behavior indicates that this transition is associated with very large reconstruction of the inorganic framework and collapse of the perovskite cages. The second phase transition, which occurs near 4.9 GPa, is associated with subtle structural changes. The last transition takes place near 5.9 GPa and it leads to further significant distortion of the anionic framework. In contrast to the anionic framework, the phase transitions have weak impact on the imidazolium cation. Pressure dependence of Raman modes proves that compressibility of the high-pressure phases is significantly lower compared to the ambient pressure phase. It also indicates that the contraction of the MnO₆ octahedra prevails over that of the imidazolium cations and hypophosphite linkers. However, compressibility of MnO₆ strongly decreases in the highest pressure phase. Pressure-induced phase transitions are reversible.

Atypical Magnetic Behavior in the Incommensurate (CH3NH3)[Ni(HCOO)3] Hybrid Perovskite

A plethora of temperature-induced phase transitions have been observed in (CH₃NH₃)[M(HCOO)₃] compounds, where M is Co(II) or Ni(II). Among them, the nickel compound exhibits a combination of magnetic and nuclear incommensurability below Néel temperature. Despite the fact that the zero-field behavior has been previously addressed, here we study in depth the macroscopic magnetic behavior of this compound to unveil the origin of the atypical magnetic response found in it and in its parent family of formate perovskites. In particular, they show a puzzling magnetization reversal in the curves measured starting from low temperatures, after cooling under zero field. The first atypical phenomenon is the impossibility of reaching zero magnetization, even by nullifying the applied external field and even compensating it for the influence of the Earth's magnetic field. Relatively large magnetic fields are needed to switch the magnetization from negative to positive values or vice versa, which is compatible with a soft ferromagnetic system. The atypical path found in its first magnetization curve and hysteresis loop at low temperatures is the most noticeable feature. The magnetization curve switches from more than 1200 Oe from the first magnetization loop to the subsequent magnetization loops. A feature that cannot be explained using a model based on unbalanced pair of domains. As a result, we decipher this behavior in light of the incommensurate structure of this material. We propose, in particular, that the applied magnetic field induces a magnetic phase transition from a magnetically incommensurate structure to a magnetically modulated collinear structure.

Pressure Control of the Structure and Multiferroicity in a Hydrogen-Bonded Metal-Organic Framework

Multiferroic materials with the cross-coupling of magnetic and ferroelectric orders provide a new platform for physics study and designing novel electronic devices. However, the weak coupling strength of ferroelectricity and magnetism is the main obstacle for potential applications. The recent research focuses on enhancing the coupling effect via synthesizing novel materials in a chemical route or tuning the multiferroicity in the physical way. Among them, pressure is an effective method to modify multiferroic materials, especially when the chemical doping has reached its tuning limit. In this work, we systemically studied the multiferroic properties in a hydrogen-bonded metal-organic framework (MOF) [(CH₃)₂NH₂]Ni(HCOO)₃ under high pressure. X-ray diffraction and Raman scattering reveal that a structural phase transition occurs in a pressure region of 6-9 GPa, and the crystal structure is greatly modified by pressure. With the ac magnetic susceptibility, pyroelectric current, and dielectric constant measurements, we obtain the multiferroic property evolution under high pressure and create a temperature-pressure phase diagram. Our study demonstrates that the pressure can modify the magnetic superexchange interaction and hydrogen bonding simultaneously in these perovskite-like MOFs. The multiferroic phase region has been expanded to higher temperature due to the pressure-enhanced spin-phonon coupling effect.

Spin-Electric Coupling with Anisotropy-Induced Vanishment and Enhancement in Molecular Ferroelectrics

Manipulating quantum properties by electric fields using spin-electric coupling (SEC) effects promises spatial addressability. While several studies about inorganic materials showing the SEC functionality have been reported, the vastly tunable crystal structures of molecular ferroelectrics provide a range of rationally designable materials yet to be exploited. In this work, Mn²⁺-doped molecular ferroelectrics are chosen to experimentally demonstrate the feasibility of achieving the quantum coherent SEC effect in molecular ferroelectrics for the first time. The electric field pulse applied between Hahn-echo pulses in electron paramagnetic resonance (EPR) experiments causes controllable phase shifts via manipulating of the zero-field splitting (ZFS) of the Mn(II) ions. Detailed investigations of the aMn crystal showed unexpected SEC vanishment and enhancement at different crystal orientations, which were elucidated by studying the spin Hamiltonian and magnetic anisotropy. With the enhanced SEC efficiency being achieved (0.68 Hz m/V), this work discovers an emerging material library of molecular ferroelectrics to implement coherent quantum control with selective and tunable SEC effects toward highly scalable quantum gates.

Principles of melting in hybrid organic-inorganic perovskite and polymorphic ABX3 structures

Four novel dicyanamide-containing hybrid organic-inorganic ABX3 structures are reported, and the thermal behaviour of a series of nine perovskite and non-perovskite [AB(N(CN)2)3] (A = (C3H7)4N, (C4H9)4N, (C5H11)4N; B = Co, Fe, Mn) is analyzed. Structure-property relationships are investigated by varying both A-site organic and B-site transition metal cations. In particular, increasing the size of the A-site cation from (C3H7)4N → (C4H9)4N → (C5H11)4N was observed to result in a decrease in T m through an increase in ΔS f. Consistent trends in T m with metal replacement are observed with each A-site cation, with Co < Fe < Mn. The majority of the melts formed were found to recrystallise partially upon cooling, though glasses could be formed through a small degree of organic linker decomposition. Total scattering methods are used to provide a greater understanding of the melting mechanism.

Spin-Dependent Polaron Dynamics in Organic Ferromagnets

The spin-dependent polaron dynamics in organic ferromagnets under driven electric fields are investigated by using the extended Su-Schrieffer-Heeger (SSH) model coupled with a nonadiabatic dynamics method. It is found that the spin-down polaron with the same spin orientation as the radicals drifts faster than the spin-up one under the same driven electric field. In an applicable range of driven electric fields, the velocity of the spin-down polaron is about 3.4 times that of the spin-up one. The dynamical property of the polaron with each spin (up or down) is asymmetric upon the reversal of the driven electric fields. The diverse dynamical properties of polarons with specific spins can be attributed to the spin nondegenerate polaron energy levels, the dipole moment generated by the asymmetrical polaron charge distributions and the strong electron-lattice coupling in organic ferromagnets. Our findings are expected to be useful for improving organic ferromagnet based spintronic devices.

Structural, magnetic and photoluminescent properties of new hybrid hypophosphites: discovery of the first noncentrosymmetric and two cobalt-based members

Hybrid organic-inorganic perovskites comprising hypophosphite ligands are emerging functional materials exhibiting magnetic, photoluminescence, negative thermal expansion and negative linear compressibility behaviour. This work reports five novel hypophosphite perovskites, [A]M(H2POO)3 (A=...

The cation-dependent structural, magnetic and optical properties of a family of hypophosphite hybrid perovskites

Hypophosphite hybrid perovskites have recently received widespread attention due to their diverse structural and magnetic properties, negative thermal expansion and photoluminescence behaviour. Herein, we report two new three-dimensional hybrid perovskites containing unusually large organic cations, pyrrolidinium and 2-hydroxyethylammonium. We report the crystal structures of these new manganese-hypophosphite frameworks and their magnetic and optical properties. We also report the magnetic and optical properties of two previously discovered analogues, dimethylammonium and imidazolium manganese hypophosphites. Both new compounds crystallize in a monoclinic structure, space group P2₁/n, with ordered organic cations at room temperature. Magnetic studies show that all studied compounds are examples of canted antiferromagnets but the weak ferromagnetic contribution and the ordering temperature are significantly modulated by the type of organic cation located in the cavity of the framework. We discuss the origin of this behaviour. Upon ultraviolet excitation, all compounds exhibit broadband photoluminescence associated with the ⁴T_1g(G) → ⁶A_1g(S) transition of octahedrally coordinated Mn²⁺ ions. The position of the PL band depends on the type of organic cation, being the most blue-shifted for the imidazolium analogue (646 nm) and the most red-shifted for the pyrrolidinium counterpart (689 nm). The most interesting property of the studied hypophosphites is, however, the strong temperature dependence of the photoluminescence intensity, suggesting the possible application of these compounds in non-contact optical thermometry.

Inorganic-Organic Hybrid Molecular Materials: From Multiferroic to Magnetoelectric

Inorganic-organic hybrid molecular multiferroic and magnetoelectric materials, similar to multiferroic oxide compounds, have recently attracted increasing attention because they exhibit diverse architectures, a flexible framework, fascinating physics, and potential magnetoelectric functionalities in novel multifunctional devices such as energy transformation devices, sensors, and information storage systems. Herein, the classification of multiferroicity and magnetoelectricity is briefly outlined and then the recent advances in the multiferroicity and magnetoelectricity of inorganic-organic hybrid molecular materials, particularly magnetoelectricity and the relevant magnetoelectric mechanisms and their categories are summarized. In addition, a personal perspective and an outlook are provided.

Elastic Properties and Energy Dissipation Related to the Disorder-Order Ferroelectric Transition in a Multiferroic Metal-Organic Framework [(CH3)2NH2][Fe(HCOO)3] with a Perovskite-Like Structure

The elastic properties and the coupling of ferroelasticity with ferromagnetism and ferroelectricy are crucial for the development of multiferroic metal-organic frameworks (MOFs) with strong magnetoelectric coupling. Elastic properties and energy dissipation related to the disorder-order ferroelectric transition in [(CH₃)₂NH₂][Fe(HCOO)₃] were studied by differential scanning calorimetry (DSC), low temperature X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). DSC result indicated the transition near 164 K. XRD showed the first-order structural transition from rhombohedral R3-c to monoclinic Cc at ~145 K, accompanied by the disorder-order transition of proton ordering in the N-H…O hydrogen bonds in [(CH₃)₂NH₂]⁺ as well as the distortion of the framework. For single crystals, the storage modulus was ~1.1 GPa and the loss modulus was ~0.02 GPa at 298 K. DMA of single crystals showed quick drop of storage modulus and peaks of loss modulus and loss factor near the ferroelectric transition temperature ~164 K. DMA of pellets showed the minimum of the normalized storage modulus and the peaks of loss factor at ~164 K with weak frequency dependences. The normalized loss modulus reached the maximum near 145 K, with higher peak temperature at higher frequency. The elastic anomalies and energy dissipation near the ferroelectric transition temperature are caused by the coupling of the movements of dimethylammonium cations and twin walls.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex

We investigate giant magnetoelectric coupling at a Mn³⁺ spin crossover in [Mn^IIIL]BPh₄ (L = (3,5-diBr-sal)₂323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη² coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Magnetodielectric Response in a Layered Mixed-Valence Ferrimagnetic Molecular Compound

The magnetodielectric effect is closely related to multiferroic or magnetoelectric coupling; thus, it can be used to predict magnetoelectric coupling, especially in compounds with special magnetic properties. The magnetodielectric response can often be used to predict many interesting and meaningful physical coupling mechanisms. Therefore, fabricating magnetodielectric materials is an effective step toward the development of magnetoelectric materials. Herein, we synthesize the mixed-valence layered ferrimagnetic molecular compound (C₆N₂H₁₄)Fe^III₂Fe^IIF₈(HCOO)₂ (1) and demonstrate that it exhibits both slow magnetic relaxation behavior and long-range magnetic order. This long-range order occurs because of the coexistence and competition between two typical magnetic interactions, namely, an Fe^III-F-Fe^II superexchange and a long-distance superexchange Fe^II-O-C-O-Fe^III-F-Fe^III path in the interlayer and interchain spin frustration. Notably, this compound also demonstrates two abnormal dielectric relaxation processes: the first process is dominated by dynamic guest cations, while the other process is related to the increasing magnetic correlation. Over a wide temperature range below 170 K, the magnetodielectric effect reveals that the magnetic correlation maybe promotes electron dynamics and leads to magnetodielectric coupling. These findings pave a novel path for designing magnetodielectric molecular materials.

Red/green-light emission in continuous dielectric phase transition materials: [Me3NVinyl]2[MnX4] (X = Cl, Br)

The luminescence of dielectric phase transition materials is one important property for technological applications, such as low-energy electron excitation. The combination of dielectric phase transitions and luminescence within organic–inorganic hybrids would lead to a new type of luminescent dielectric phase transition multifunctional material. Here, we report two novel A₂BX₄ organic–inorganic hybrid complexes [Me₃NVinyl]₂[MnCl₄] 1 and [Me₃NVinyl]₂[MnBr₄] 2, ([Me₃NVinyl] = trimethylvinyl ammonium cation). The complexes 1 and 2 were found to undergo continuous reversible phase transitions as well as switch dielectric phase transitions. Strikingly, intensive red luminescence and green luminescence were obtained under UV excitation respectively to reveal potential application of the two complexes in multi-functional materials along with dielectric switches and so on.

The luminescence of dielectric phase transition materials is one important property for technological applications, such as low-energy electron excitation.

Cadmium and manganese hypophosphite perovskites templated by formamidinium cations: dielectric, optical and magnetic properties

The first cadmium hypophosphite perovskite exhibiting reddish-orange emission, glass-like behaviour and order–disorder phase transition.

Vibrational Properties and DFT Calculations of Perovskite-Type Methylhydrazinium Manganese Hypophosphite

Recently discovered hybrid perovskites based on hypophosphite ligands are a promising class of compounds exhibiting unusual structural properties and providing opportunities for construction of novel functional materials. Here, we report for the first time the detailed studies of phonon properties of manganese hypophosphite templated with methylhydrazinium cations ([CH3NH2NH2][Mn(H2PO2)3]). Its room temperature vibrational spectra were recorded for both polycrystalline sample and a single crystal. The proposed assignment based on Density Functional Theory (DFT) calculations of the observed vibrational modes is also presented. It is worth noting this is first report on polarized Raman measurements in this class of hybrid perovskites.

Constructing Asymmetrical Ni-Centered {NiN2O4} Octahedra in Layered Metal-Organic Structures for Near-Room-Temperature Single-Phase Magnetoelectricity

Layered metal-organic structures (LMOSs) as magnetoelectric (ME) multiferroics have been of great importance for realizing new functional devices in nanoelectronics. Until now, however, achieving such room-temperature and single-phase ME multiferroics in LMOSs have proven challenging due to low transition temperature, poor spontaneous polarization, and weak ME coupling effect. Here, we demonstrate the construction of a LMOS in which four Ni-centered {NiN₂O₄} octahedra form in layer with asymmetric distortions using the coordination bonds between diphenylalanine molecules and transition metal Ni(II). Near room-temperature (283 K) ferroelectricity and ferromagnetism are observed to be both spontaneous and hysteretic. Particularly, the multiferroic LMOS exhibits strong magnetic-field-dependent ME polarization with low-magnetic-field control. The change in ME polarization with increasing applied magnetic field μ₀H from 0 to 2 T decreases linearly from 0.041 to 0.011 μC/cm² at the strongest ME coupling temperature of 251 K. The magnetic domains can be manipulated directly by applied electric field at 283 K. The asymmetrical distortion of Ni-centered octahedron in layer spurs electric polarization and ME effect and reduces spin frustration in the octahedral geometry due to spin-charge-orbital coupling. Our results represent an important step toward the production of room-temperature single-phase organic ME multiferroics.

Developing the Pressure-Temperature-Magnetic Field Phase Diagram of Multiferroic [(CH3)2NH2]Mn(HCOO)3

We combined Raman scattering and magnetic susceptibility to explore the properties of [(CH₃)₂NH₂]Mn(HCOO)₃ under compression. Analysis of the formate bending mode reveals a broad two-phase region surrounding the 4.2 GPa critical pressure that becomes increasingly sluggish below the order-disorder transition due to the extensive hydrogen-bonding network. Although the paraelectric and ferroelectric phases have different space groups at ambient-pressure conditions, they both drive toward P1 symmetry under compression. This is a direct consequence of how the order-disorder transition changes under pressure. We bring these findings together with prior magnetization work to create a pressure-temperature-magnetic field phase diagram, unveiling entanglement, competition, and a progression of symmetry-breaking effects that underlie functionality in this molecule-based multiferroic. That the high-pressure P1 phase is a subgroup of the ferroelectric Cc suggests the possibility of enhanced electric polarization as well as opportunity for strain control.

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+ doped [NH4][Zn(HCOO)3] hybrid formate framework

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (β = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal-oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase.

Phonon properties and mechanism of order-disorder phase transition in formamidinium manganese hypophosphite single crystal

The detailed temperature-dependent IR and Raman spectra were used to study and understand the mechanism of structural phase transition occurring at 175 K in manganese hypophosphite templated with formamidinium (FA⁺) ions, [FA]Mn(H₂POO)₃, which adopts a perovskite-like architecture. The structural transformation between the C2/c and the P2₁/c monoclinic phases has a complicated nature and is mainly driven by re-orientational motions of the FA⁺ cations but it is also accompanied by a significant distortion of the MnO₆ octahedral units as well as steric-forced changes of the PH₂ groups determining the off-center shifts of FA⁺ cations in the cages. The re-orientational motions of formamidinium cations at 175 K are followed by slight changes of their geometry and re-arrangement of hydrogen bonds (HBs). The strong temperature-dependences of bands corresponding to vibrations involving hydrogen bonding reveal the highly-dynamic character of this phase transition and strong nature of created HBs. The most pronounced changes are observed for the modes corresponding to the formamidinium cation, proving that the phase transition has an order-disorder character.

Metal-Organic Framework Magnets

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

Investigation of structural, electronic and magnetic properties of breathing metal-organic framework MIL-47(Mn): a first principles approach

The structural, electronic and magnetic properties of the MIL-47(Mn) metal-organic framework are investigated using first principles calculations. We find that the large-pore structure is the ground state of this material. We show that upon transition from the large-pore to the narrow-pore structure, the magnetic ground-state configuration changes from antiferromagnetic to ferromagnetic, consistent with the computed values of the intra-chain coupling constant. Furthermore, the antiferromagnetic and ferromagnetic configuration phases have intrinsically different electronic behavior: the former is semiconducting, the latter is a metal or half-metal. The change of electronic properties during breathing posits MIL-47(Mn) as a good candidate for sensing and other applications. Our calculated electronic band structure for MIL-47(Mn) presents a combination of flat dispersionless and strongly dispersive regions in the valence and conduction bands, indicative of quasi-1D electronic behavior. The spin coupling constants are obtained by mapping the total energies onto a spin Hamiltonian. The inter-chain coupling is found to be at least one order of magnitude smaller than the intra-chain coupling for both large and narrow pores. Interestingly, the intra-chain coupling changes sign and becomes five times stronger going from the large pore to the narrow pore structure. As such MIL-47(Mn) could provide unique opportunities for tunable low-dimensional magnetism in transition metal oxide systems.

Static disorder in a perovskite mixed-valence metal–organic framework

Effects of A-site and M-site substitutions on the structural properties of perovskite dimethylammonium iron formate.

Novel hypophosphite hybrid perovskites of [CH3NH2NH2][Mn(H2POO)3] and [CH3NH2NH2][Mn(H2POO)2.83(HCOO)0.17] exhibiting antiferromagnetic order and red photoluminescence

Hybrid perovskites based on hypophosphite ligands constitute an emerging family of compounds exhibiting unusual structures and offering a platform for construction of novel functional materials. We report the synthesis, crystal structure, and magnetic and optical properties of novel undoped and HCOO⁻-doped manganese hypophosphite frameworks templated by methylhydrazinium cations. The undoped compound crystallizes in a three-dimensional perovskite-like orthorhombic structure, space group Pnma, with ordered organic cations located in windows between the perovskite cages expanding along the a-direction. Both conventional anti-phase tilting, unconventional in-phase tilting and columnar shifts in the a-direction are present. Doping with HCOO⁻ ions has a insignificant influence on the crystal structure but leads to a decrease of the unit cell volume. Magnetic studies indicate that these compounds order antiferromagnetically at T_N = 6.5 K. Optical studies indicate that they exhibit red photoluminescence under 266 nm excitation with the activation energy for thermal quenching of 98 and 65 meV for the undoped and doped sample, respectively. For the undoped sample, the emission lifetime reaches 5.05 ms at 77 K but it decreases to 62.26 μs at 300 K. The low value of the activation energy and huge temperature dependence of photoluminescence intensity suggest a high potential of these hypophosphites for non-contact temperature sensing.

The first perovskite-type hypophosphite-linked dense metal–organic framework exhibiting red emission and antiferromagnetic order at 6.5 K.

Ferroelectricity and antiferromagnetism in organic–inorganic hybrid (1,4-bis(imidazol-1-ylmethyl)benzene)CuCl4·H2O

A new [CuCl₄]²⁻ based organic–inorganic hybrid, (bix)CuCl₄·H₂O (bix = 1,4-bis(imidazol-1-ylmethyl)benzene), is synthesized via simple solution method, which shows the coexistence of ferroelectric and antiferromagnetic ordering in (bix)CuCl₄·H₂O.

First-principles study of the structural, electronic, magnetic, and ferroelectric properties of a charge-ordered iron(ii)-iron(iii) formate framework

Density functional theory calculations have been performed for the structural, electronic, magnetic, and ferroelectric properties of a mixed-valence Fe(ii)-Fe(iii) formate framework [NH₂(CH₃)₂][FeⁱⁱⁱFeⁱⁱ(HCOO)₆]. Recent experiments report a spontaneous electric polarization, and our calculations are in agreement with the reported experimental value. Furthermore, we shed light onto the microscopic mechanism leading to the observed value, as well as on how to possibly enhance the polarization. The interplay between charge ordering, dipolar ordering of DMA⁺ cations, and the induced structural distortions suggest new interesting directions to explore in these complex multifunctional hybrid perovskites.

Magnetic-Field Tuning of Hydrogen Bond Order-Disorder Transition in Metal-Organic Frameworks

The ordering of polar hydrogen bonds may break space inversion symmetry and induce ferroelectricity or antiferroelectricity. This process is usually immune to external magnetic fields so that magnetic control of hydrogen bonds is very challenging. Here we demonstrate that the ordering of hydrogen bonds in the metal-organic frameworks [(CH_{3})_{2}NH_{2}]M(HCOO)_{3} (M=Fe, Co) can be manipulated by applying magnetic fields. After cooling in a high magnetic field, the order-disorder transition of hydrogen bonds shifts to a lower or higher temperature, depending on antiferroelectricity or ferroelectricity induced by hydrogen bond ordering. Besides, the order-disorder transition leads to a giant thermal expansion, exceeding ∼3.5×10^{4} and ∼2×10^{4}  ppm for M=Fe and Co, respectively, which is much higher than that of inorganic ferroelectrics. The influence of magnetic field on hydrogen bond ordering is discussed in terms of the magnetoelastic coupling.

Multiferroicity and magnetoelectric coupling in the paramagnetic state of the metal-organic framework [(CH3)2NH2]Ni(HCOO)3

The metal-organic framework [(CH₃)₂NH₂]Ni(HCOO)₃ (DMA-Ni) has an ABX₃ perovskite-like structure. At T _C ~ 181 K, DMA-Ni displays a first-order ferroelectric transition, which is triggered by the disorder-order transition of hydrogen bonds. In addition, this compound exhibits a spin-canted antiferromagnetic order below T _N ~ 37.6 K through the long-distance superexchange interaction, and a spin-reorientation transition appears near 15 K. The coexistence of magnetic and ferroelectric orders at low temperature testifies the multiferroic properties of DMA-Ni. Besides, the magnetoelectric (ME) coupling exists in the paramagnetic state, where the ferroelectric polarization can be modified by applying high magnetic fields. This behavior is attributed to the local magnetostriction effect.

Stability and flexibility of heterometallic formate perovskites with the dimethylammonium cation: pressure-induced phase transitions

We report the high-pressure Raman studies and DFT calculations of DMANaCr and DMAKCr perovskite formates.

A breakthrough in the intrinsic multiferroic temperature region in Prussian blue analogues

Thin films of [(Fe^II_xCr^II_1−x)]_1.5[Cr^III(CN)₆]·yH₂O (x ≈ 0.30–0.35, y ≈ 1.77) (1) on FTO substrates (namely film 1) were synthesized with an electrochemical method. Investigation of the ferroelectricity of film 1 at different temperatures reveals that it exhibits ferroelectric behaviour in the temperature range from 10 K to 310 K. Study of the X-ray absorption (XAS) of the crushed film 1 and simulation of the structure of film 1 and crushed film 1 by using the Materials Studio software indicate that the vacancy defects and interactions between the film and FTO substrate make a key contribution to the ferroelectricity of film 1. Owing to the magnetic phase transition point being up to 210 K, film 1 is a multiferroic material and its magneto/electric coexistence temperature can be as high as 210 K.

Prussian blue analogue film exhibits ferroelectric from 10 to 310 K and works up to 210 K as a molecular-based multiferroic material.

Raman and single-crystal X-ray diffraction evidence of pressure-induced phase transitions in a perovskite-like framework of [(C3H7)4N] [Mn(N(CN)2)3]

The [TPrA][Mn(dca)₃] perovskite shows highly anisotropic compression and the presence of three pressure-induced phase transitions near 0.4, 3 and 5 GPa into lower symmetry phases.

Incommensurate structures of the [CH3NH3][Co(COOH)3] compound

The present article is devoted to the characterization of the structural phase transitions of the [CH3NH3][Co(COOH)3] (1) perovskite-like metal-organic compound through variable-temperature single-crystal neutron diffraction. At room temperature, compound 1 crystallizes in the orthorhombic space group Pnma (phase I). A decrease in temperature gives rise to a first phase transition from the space group Pnma to an incommensurate phase (phase II) at approximately 128 K. At about 96 K, this incommensurate phase evolves into a second phase with a sharp change in the modulation vector (phase III). At lower temperatures (ca 78 K), the crystal structure again becomes commensurate and can be described in the monoclinic space group P21/n (phase IV). Although phases I and IV have been reported previously [Boča et al. (2004). Acta Cryst. C60, m631-m633; Gómez-Aguirre et al. (2016). J. Am. Chem. Soc. 138, 1122-1125; Mazzuca et al. (2018). Chem. Eur. J. 24, 388-399], phases III and IV corresponding to the Pnma(00γ)0s0 space group have not yet been described. These phase transitions involve not only the occurrence of small distortions in the three-dimensional anionic [Co(HCOO)3]- framework, but also the reorganization of the [CH3NH3]+ counter-ions in the cavities of the structure, which gives rise to an alteration of the hydrogen-bonded network, modifying the electrical properties of compound 1.

The structural, phonon and optical properties of [CH3NH3]M0.5CrxAl0.5−x(HCOO)3 (M = Na, K; x = 0, 0.025, 0.5) metal–organic framework perovskites for luminescence thermometry

We report the structural, phonon and optical properties of perovskite-type heterometallic formates templated by methylammonium cations.

Theoretical investigation of multiferroic metal-organic framework magnet [CH3NH3][Co(HCOO)3]: Green's function method

For the first presented magnetic ordering-induced multiferroics with a metal-organic framework (MOF) of formula [CH₃NH₃][Co(HCOO)₃], we theoretically investigate its multiple ferroics. It is found that Dzyaloshinskii-Moriya interaction is a main cause that leads to non-zero magnetization, and electric polarization, and the induced electric polarization can be regulated by magnetic fields. As an assistant mechanism, magnon-magnon interaction and quantum fluctuation play an important role on ferroelectrics and magnetism. Our methods are based on the double-time Green's function and Holstein-Primakoff transformation. Theoretical results can be compared with experiments, though there are some discrepancies.

Spin-reorientation-induced magnetodielectric coupling effects in two layered perovskite magnets

Spin-reorientation-induced magnetodielectric coupling effects were discovered in two layered perovskite magnets, [C6H5CH2CH2NH3]2[MCl4] (M = Mn2+ and Cu2+), via isothermal magnetodielectric measurements on single-crystal samples. Specifically, peak-like dielectric anomalies and spin-flop transitions appeared simultaneously at around ±34 kOe for the canted antiferromagnet (M = Mn2+) at below 44.3 K, while a low-field (1 kOe) controlled magnetodielectric effect was observed in the "soft" ferromagnet (M = Cu2+) at below 9.5 K. These isothermal magnetodielectric effects are highly reproducible and synchronous with the field-induced magnetization at different temperatures, well confirming the essential role of spin reorientation on inducing magnetodielectric coupling effects. These findings strongly imply that the layered perovskite magnets are new promising organic-inorganic hybrid systems to host magnetodielectric coupling effects.

Pressure dependence of spin canting in ammonium metal formate antiferromagnets

High-pressure single-crystal X-ray diffraction at ambient temperature and high-pressure SQUID measurements down to 2 K were performed up to ∼2.5 GPa on ammonium metal formates, [NH₄][M(HCOO)₃] where M = Mn²⁺, Fe²⁺, and Ni²⁺, in order to correlate structural variations to magnetic behaviour. Similar structural distortions and phase transitions were observed for all compounds, although the transition pressures varied with the size of the metal cation. The antiferromagnetic ordering in [NH₄][M(HCOO)₃] compounds was maintained as a function of pressure, and the magnetic ordering transition temperature changed within a few kelvins depending on the structural distortion and the metal cation involved. These compounds, in particular [NH₄][Fe(HCOO)₃], showed greatest sensitivity to the degree of spin canting upon compression, clearly visible from the twenty-fold increase in the low-temperature magnetisation for [NH₄][Fe(HCOO)₃] at 1.4 GPa, and the change from purely antiferromagnetic to weakly ferromagnetic ordering in [NH₄][Mn(HCOO)₃] at 1 GPa. The variation in the exchange couplings and spin canting was checked with density-functional calculations that reproduce well the increase in canted moment within [NH₄][Fe(HCOO)₃] upon compression, and suggest that the Dzyaloshinskii-Moriya (DM) interaction is evolving as a function of pressure. The pressure dependence of spin canting is found to be highly dependent on the metal cation, as magnetisation magnitudes did not change significantly for when M = Ni²⁺ or Mn²⁺. These results demonstrate that the overall magnetic behaviour of each phase upon compression was not only dependent on the structural distortions but also on the electronic configuration of the metal cation.

Structure-Property Relations in Multiferroic [(CH3)2NH2] M(HCOO)3 ( M = Mn, Co, Ni)

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH₃)₂NH₂] M(HCOO)₃ ( M = Mn²⁺, Co²⁺, Ni²⁺) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH₃)₂NH₂]Mn(HCOO)₃ takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across T_C reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH₃)₂NH₂]Co(HCOO)₃ reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co²⁺ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

Switching of the Chiral Magnetic Domains in the Hybrid Molecular/Inorganic Multiferroic (ND4)2[FeCl5(D2O)]

(ND₄)₂[FeCl₅(D₂O)] represents a promising example of the hybrid molecular/inorganic approach to create materials with strong magneto-electric coupling. Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of this material. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, deducing the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K.