Displacive-type ferroelectricity from magnetic correlations within spin-chain

Temperature dependent structural properties of Mn1.90M0.10O3(M = Cr and Fe)

The polycrystalline samples of Mn1.90Cr0.10O3(MCO) and Mn1.90Fe0.10O3(MFO) have been investigated for their temperature dependent magnetic and structural properties. The Cr and Fe substitutions have significant effect on the magnetic and structural properties of Mn2O3. Like pristine Mn2O3, the Cr and Fe substituted samples MCO and MFO also exhibit two antiferromagnetic transitions; one at ∼77 K, ∼80 K, respectively and another at ∼40 K. Our room temperature synchrotron x-ray powder diffraction (SXRD) results confirm that both the MCO and MFO samples crystallize in cubic symmetry. The temperature dependent SXRD results demonstrate the cubic to orthorhombic structural transition for the studied samples. The pristine Mn2O3shows cubic to orthorhombic transition around 310 K, whereas this structural transition shifted towards lower temperature side with these substitutions i.e. around 240 K for MCO and 260 K for MFO. Interestingly, the centrosymmetricPcabto non-centrosymmetricPca21change in symmetry is also resolved at the ferroelectric ordering temperature for MCO.

Structural correlation to ferroelectric order, non-Griffiths like phase and magnetocaloric effect in YbCrO4

We report a ferroelectric order around ∼99 K (T_FE), which is considerably above the long range ferrimagnetic order at 25 K (T_N). The value of saturation electric polarization is considerable as ∼570μC m^-2for a poling field of 5 kV cm^-1. The ferroelectric order is associated with a significant magnetoelectric coupling below ∼90 K. A weak signature ofT_FEis observed in the dielectric constant, which is associated with a linear magnetodielectric response at 18 K(<TN). A transition to a polar structure has been correlated with the occurrence of ferroelectric order. A non-Griffiths like phase is observed around ∼48 K, which is also linked with the structural distortion. A reasonable value of the magnetic entropy change of 5.07 J Kg^-1 K^-1is noted atT_Nfor a change in field of 50 kOe, which involves a strong magnetoelastic coupling. The results indicate considerable structural instabilities, which is linked with the observed multifunctional properties of YbCrO₄.

Magnetic and transport properties of the mixed 3d-5d-4fdouble perovskite Sm2CoIrO6

Iridium-based double perovskites having mixed 3d-5d-4fmagnetic sub-lattices are expected to exhibit exotic magnetic phenomenon. In this paper, we report a study of structural, magnetic and transport properties of the mixed 3d-5d-4fdouble perovskite Sm₂CoIrO₆(SMCO), which crystallizes in monoclinic structure with space groupP2₁/nand the crystal symmetry remains same throughout the measured temperature down to 15 K. High resolution synchrotron x-ray diffraction reveals an isostructural phase transition around 104 K. Magnetization measurements on polycrystalline samples indicate that SMCO orders ferrimagnetically atT_FiM= 104 K; while, a second transition is observed below 10 K due to the rare-earth (Sm³⁺) ordering. The ferrimagnetic transition is well-understood by Néel's two-sublattice model, which is primarily ascribed to antiferromagnetic coupling between Co²⁺and Ir⁴⁺sub-lattices. Electronic transport measurement shows the insulting behaviour of SMCO, which follows Mott variable-range hopping conduction mechanism. However, dielectric measurements as a function of temperature rules out the presence of magneto dielectric coupling in this compound.

Green sonochemical synthesis of BaDy2NiO5/Dy2O3 and BaDy2NiO5/NiO nanocomposites in the presence of core almond as a capping agent and their application as photocatalysts for the removal of organic dyes in water

The present work reports the sonochemical synthesis of DBNO NC (dysprosium nickelate nanocomposite) using metal nitrates and core almond as a capping agent. In addition, the effects of the power of ultrasound irradiation were investigated. The BaDy2NiO5/Dy2O3 and BaDy2NiO5/NiO nanocomposites were synthesized with sonication powers of 50 and 30 W, respectively. The agglomerated nanoparticles were obtained using different sonication powers, including 15, 30, and 50 W. The results showed that upon increasing the sonication power, the particle size decreased. After characterization, the optical, electrical, magnetic, and photocatalytic properties of the NC were studied. The nanocomposites showed an antiferromagnetic behavior. In this study, the photocatalytic degradations of two dyes, AR14 and AB92, were investigated in the presence of DBNO NC. Furthermore, the effects of the amount of photocatalyst, the concentration of the dye solution, the type of organic dye, and light irradiation on the photocatalytic activity of the nanocomposite were studied. The results showed that with an increasing amount of catalyst and decreasing concentration of dye, the photocatalytic activity of the nanocomposite was increased. This activity for the degradation of AR14 is higher than that of AB92. Both AR14 and AB92 dyes show higher photocatalytic degradation under UV irradiation than under Vis irradiation.

The Critical Role of Stereochemically Active Lone Pair in Introducing High Temperature Ferroelectricity

In this paper, a comparative structural, dielectric, and magnetic study of two langasite compounds Ba₃TeCo₃P₂O₁₄ (absence of lone pair) and Pb₃TeCo₃P₂O₁₄ (Pb²⁺ 6s² lone pair) have been carried out to precisely explore the development of room temperature spontaneous polarization in the presence of a stereochemically active lone pair. In the case of Pb₃TeCo₃P₂O₁₄, mixing of both Pb 6s with Pb 6p and O 2p helps the lone pair to be stereochemically active. This stereochemically active lone pair brings a large structural distortion within the unit cell and creates a polar geometry, while the Ba₃TeCo₃P₂O₁₄ compound remains in a nonpolar structure due to the absence of any such effect. Consequently, polarization measurement under varying electric fields confirms room temperature ferroelectricity for Pb₃TeCo₃P₂O₁₄, which was not the case for Ba₃TeCo₃P₂O₁₄. A detailed study was carried out to understand the microscopic mechanism of ferroelectricity, which revealed the exciting underlying activity of a polar TeO₆ octahedral unit as well as Pb-hexagon.

Observation of magnetoelastic and magnetoelectric coupling in Sc doped BaFe12O19 due to spin-glass-like phase

The present work reports magnetic, magnetoelastic and magnetoelectric (ME) response of scandium (Sc) doped barium hexaferrite, BaFe₁₀Sc₂O₁₉. DC magnetization shows that partial substitution of non-magnetic Sc for Fe in barium hexaferrite results in a reduction of Curie temperature (T _C) from 730 K known for the parent compound BaFe₁₂O₁₉ to 430 K. Magnetization measurements show that, in BaFe₁₀Sc₂O₁₉, in addition to the magnetic transition at 250 K corresponding to longitudinal conical magnetic structure, another magnetic anomaly occurs in the vicinity of 50 K (T _max). Ac susceptibility and magnetic relaxation show that the magnetic transition at T _max is associated with spin glass like dynamics. Field dependence of this glassy transition temperature follows the Almeida-Thouless (A-T) line expected for spin glass-like behaviour. Unit cell volume obtained from the neutron diffraction (ND) measurements shows deviation from the Debye-Gruneisen behaviour below 50 K, revealing the magnetoelastic coupling. Existence of magnetoelastic coupling is also confirmed by Raman spectra as Raman modes show anomalous changes around 50 K and also indicates presence of lattice modulation. Further, the magnetic structure obtained from ND data shows that incommensurate longitudinal conical ferrimagnetic structure persists from 210 K to 3 K. The integrated intensity of (0 0 2) peak and magnetic moments undergoes a subtle change below 50 K that seems to favour coexistence of long range magnetic ordering and spin glass-like dynamics. Significant magneto-dielectric effect was observed around 50 K. Temperature dependent studies of dielectric constant and pyroelectric current indicate the presence of ferroelectricity even in zero magnetic field. Further, existence of ME coupling below 50 K is confirmed by temperature dependence of pyroelectric current under magnetic fields up to 70 kOe. In short, this work identifies a new magnetic anomaly around 50 K, which is spin-glass-like inducing magnetoelastic and ME anomalies, even in the absence of external magnetic fields.

Unveiling spin-glass transition and antiferromagnetic order by μSR studies in spin-chain Sm2BaNiO5

We report the zero-field and longitudinal field muon spin relaxation studies in a spin-chain compound Sm₂BaNiO₅. Two magnetic transitions, that have not been previously detected by the heat capacity and magnetization measurements, are confirmed at 46 and 9 K. The antiferromagnetic order is suggested at 46 K. Analysis of the muon spin polarization unveils the spin-glass transition at 9 K. Time-field scaling relation of the muon spin polarization verifies the spin-spin autocorrelation function following the cut-off power law, which is approximated by the Ogielski form, as employed numerically for characterizing the spin-glasses.

Pyrocurrent anomalies and intrinsic magnetodielectric behavior near room temperature in Li2Ni2Mo3O12, a compound with distorted honeycomb and spin-chains

Keeping current interests to identify materials with intrinsic magnetodielectric behaviour near room temperature and with novel pyroelectric current anomalies, we report temperature and magnetic-field dependent behavior of complex dielectric permittivity and pyroelectric current for an oxide, Li₂Ni₂Mo₃O₁₂, containing magnetic ions with (distorted) honey-comb and chain arrangement and ordering magnetically below 8 K. The dielectric data reveal the existence of relaxor ferroelectricity behaviour in the range 160-240 K and there are corresponding Raman mode anomalies as well in this temperature range. Pyrocurrent behavior is also consistent with this interpretation, with the pyrocurrent peak-temperature interestingly correlating with the poling temperature. ⁷Li NMR offer an evidence for crystallographic disorder intrinsic to this compound and we therefore conclude that such a disorder is apparently responsible for the randomness of local electric field leading to relaxor ferroelectric property. Another observation of emphasis is that there is a notable decrease in the dielectric constant with the application of magnetic field to the tune of about -2.4% at 300 K, with the magnitude varying marginally with temperature. Small loss factor values validate the intrinsic behaviour of the magnetodielectric effect at room temperature.

Size reduction-induced chain breaking in Haldane-chain compounds SrNi2-x Mg x V2O8 (x = 0 and 0.1)

We report size reduction-induced chain breaking in the spin-1 Haldane-chain SrNi_2-x Mg _x V₂O₈ (x  =  0 and 0.1) by magnetization and electron spin resonance measurements. For x  =  0.0, the magnetic susceptibility of all samples can be well described by a temperature-independent term, a Curie-Weiss term and a Haldane-gap term. This implies that a reduced sample grain size breaks the long chain and creates a considerable number of S  =  1/2 edge spins, resulting in the enhancement of magnetization and the decrease of Haldane gap in the samples. These edge spins as well as the other paramagnetic ions at grain boundary and surface might be weakly coupled with each other. For the Mg-doped sample with x  =  0.1, there are more S  =  1/2 spins creased in relative to x  =  0.0 because of a combined effect of lattice defects, Mg-doping and reduced size. In addition, the antiferromagnetic resonance of x  =  0.1 is also presented.

Enhancement of magnetic ordering temperature and magnetodielectric coupling by hole doping in a multiferroic DyFe0.5Cr0.5O3

We report the results of our investigation of magnetic, thermodynamic and dielectric properties of Ca substituted half-doped orthochromite, Dy_0.6Ca_0.4Fe_0.5Cr_0.5O₃. Magnetic susceptibility and heat capacity data bring out that this compound undergoes two antiferromagnetic transitions, one at ~132 and the other at ~22 K. These values are higher than those of DyFe_0.5Cr_0.5O₃. This finding highlights that non-magnetic hole doping in form of Ca⁺² in the place of magnetic Dy⁺³ tends to enhance magnetic transition temperatures in this half-doped orthochromite. We attribute it to possible change in the valence state of Cr/Fe-ion ions due to hole doping. Dielectric anomalies are also seen near the magnetic ordering temperatures indicating magnetodielectric coupling, which is confirmed by magnetic field dependent dielectric studies. The most notable observation is that magnetodielectric coupling strength gets significantly enhanced as compared to DyFe_0.5Cr_0.5O₃. The results reveal that it is possible to tune magnetodielectric coupling by hole doping in this system.

Effect of rare-earth (Er and Gd) substitution on the magnetic and multiferroic properties of DyFe0.5Cr0.5O3

We report the results of our investigations on the influence of partial substitution of Er and Gd for Dy on the magnetic and magnetoelectric properties of DyFe0.5Cr0.5O3, which is known to be a multiferroic system. Magnetic susceptibility and heat capacity data, apart from confirming the occurrence of magnetic transitions at ~121 and 13 K in DyFe0.5Cr0.5O3, bring out that the lower transition temperature only is suppressed by rare-earth substitution. Multiferroic behavior is found to persist in Dy0.4Ln0.6Fe0.5Cr0.5O3 (Ln  =  Er and Gd). There is an evidence for magnetoelectric coupling in all these materials with qualitative differences in its behavior as the temperature is changed across these two transitions. Remnant electric polarization is observed for all the compounds. The most notable observation is that electric polarization is seen to get enhanced as a result of rare-earth substitution with respect to that in DyFe0.5Cr0.5O3. Interestingly, a similar trend is seen in the magnetocaloric effect, consistent with the existence of magnetoelectric coupling. The results thus provide evidence for the tuning of magnetoelectric coupling by rare-earth substitution in this family of oxides.

Magnetoelectric coupling and exchange bias effects in multiferroic NdCrO3

We report ferroelectricity around  ∼88 K that appears well below T N (∼25 K), unlike other members of RCrO3 series. A synchrotron diffraction study suggests that the occurrence of ferroelectricity in NdCrO3 is coupled to the structural transformation from centrosymmetric Pnma to a non-centrosymmetric Pna21 space group. A strong magnetoelectric coupling is observed in the electric polarization [P(T)]. This coupling is significantly influenced by the magnetic field cooling effect, suggesting an exchange bias effect in P(T). This exchange bias effect is also revealed by the systematic shift of the magnetic hysteresis loops below T(N). The rare occurrence of an exchange bias effect in both the magnetic and electric polarizations associated with a strong magnetoelectric coupling is of fundamental interest, as well as being attractive for technological applications close to liquid nitrogen temperature.