Giant Anisotropic Magnetocaloric Effect in Double-perovskite Gd2CoMnO6 Single Crystals

Giant and highly anisotropic magnetocaloric effects in single crystals of disordered-perovskite RCr0.5Fe0.5O3 (R = Gd, Er)

Magnetic anisotropy is crucial in examining suitable materials for magnetic functionalities because it affects their magnetic characteristics. In this study, disordered-perovskite RCr_0.5Fe_0.5O₃ (R = Gd, Er) single crystals were synthesized and the influence of magnetic anisotropy and additional ordering of rare-earth moments on cryogenic magnetocaloric properties was investigated. Both GdCr_0.5Fe_0.5O₃ (GCFO) and ErCr_0.5Fe_0.5O₃ (ECFO) crystallize in an orthorhombic Pbnm structure with randomly distributed Cr³⁺ and Fe³⁺ ions. In GCFO, the long-range order of Gd³⁺ moments emerges at a temperature of T_Gd (the ordering temperature of Gd³⁺ moments) = 12 K. The relatively isotropic nature of large Gd³⁺ moment originating from zero orbital angular momentum exhibits giant and virtually isotropic magnetocaloric effect (MCE), with a maximum magnetic entropy change of [Formula: see text] ≈ 50.0 J/kg·K. In ECFO, the highly anisotropic magnetizations result in a large rotating MCE characterized by a rotating magnetic entropy change [Formula: see text] = 20.8 J/kg·K. These results indicate that a detailed understanding of magnetically anisotropic characteristics is the key for exploring improved functional properties in disordered perovskite oxides.

Magnetocaloric effect and Griffiths phase analysis in a nanocrystalline Ho2NiMnO6 and Ho2CoMnO6 double perovskite

Rare-earth double perovskite oxides have intriguing magnetocaloric properties at cryogenic temperatures. In this study, Ho₂NiMnO₆ and Ho₂CoMnO₆ were synthesized using the sol-gel method, which crystallized in a monoclinic structure in the P2₁/n space group. The magnetic phase transition was observed at 81.2 K for Ho₂NiMnO₆ and 73.5 K for Ho₂CoMnO₆. The presence of a paramagnetic matrix and short-range ferromagnetic clusters causes magnetic disorder in these double perovskites, resulting in Griffiths phase formation. The Arrott plot confirms that compounds undergo second-order phase transition. At an applied magnetic field of 5 T, the maximum magnetic entropy change (-ΔS) for the studied compounds is 1.7 and 2.2 J kg^-1 K^-1, respectively. The transition metals Ni and Co in a double perovskite cause lattice distortion in the structural parameters and oxidation states of manganese (Mn³⁺/Mn⁴⁺), which changes the magnetic and magnetocaloric properties. The quantitative approach provides a systematic study of magnetocaloric properties of the rare earth double perovskite compounds with ferromagnetic 3d transition elements.

Rotating magnetocaloric effect in highly anisotropic TbIII and DyIII single molecular magnets

The magnetocaloric effect (MCE) was investigated in highly anisotropic single crystals of two single molecule magnets (SMMs): [Ln^III(Zn^IIL)₂]CF₃SO₃, where Ln = Tb, Dy and L = tripodal hexadentate Schiff base ligand. The structure of these paramagnetic compounds consists of identically oriented linear trinuclear clusters in a trigonal system with an easy direction c∥Zn-Ln-Zn array and a hard plane ab⊥Zn-Ln-Zn array. The magnitude of MCE measured for c∥H was significantly greater than MCE for ab∥H at a wide temperature range regardless of the studied SMM. Therefore, the rotating magnetocaloric effect (RMCE) was evaluated. The maxima of the magnetic entropy change for RMCE were obtained at 2.0 K and moderate fields: 3.9 J K^-1 kg^-1 at µ₀H = 1.3 T for Ln = Tb and 3.3 J K^-1 kg^-1 at µ₀H = 1.1 T for Ln = Dy. The relative efficiency of RMCE compared to the MCE measured in c∥H was as high as 99% at low magnetic fields.

Behavior of magnetoelectric hysteresis and role of rare earth ions in multiferroicity in double perovskite Yb2CoMnO6

Double-perovskite multiferroics have been investigated because alternating orders of magnetic ions act as distinct magnetic origins for ferroelectricity. In Yb₂CoMnO₆, the frustrated antiferromagnetic order emerging at T_N = 52 K induces ferroelectric polarization perpendicular to the c axis through cooperative O^2- shifts via the symmetric exchange striction. In our detailed measurements of the magnetoelectric properties of single-crystalline Yb₂CoMnO₆, we observe full ferromagnetic-like hysteresis loops that are strongly coupled to the dielectric constant and ferroelectric polarization at various temperatures below T_N. Unlike Lu₂CoMnO₆ with non-magnetic Lu³⁺ ions, we suggest the emergence of additional ferroelectric polarization along the c axis below the ordering temperature of magnetic Yb³⁺ ions, T_Yb ≈ 20 K, based on the spin structure established from recent neutron diffraction experiments. While the proposed description for additional ferroelectricity, ascribed to the symmetric exchange striction between Yb³⁺ and Co²⁺/Mn⁴⁺ magnetic moments, is clearly given, anomalies of dielectric constants along the c axis are solely observed. Our interesting findings on magnetoelectric hysteresis and the possible development of additional ferroelectricity reveal notable characteristics of double perovskites and provide essential guidance for the further examination of magnetoelectric functional properties.

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.

Tunable magnetization steps in mixed valent ferromagnet Eu2CoMnO6

Magnetic properties can be manipulated to enhance certain functionalities by tuning different material processing parameters. Here, we present the controllable magnetization steps of hysteresis loops in double-perovskite single crystals of Eu2CoMnO6. Ferromagnetic order emerges below TC ≈ 122 K along the crystallographic c axis. The difficulty in altering Co2+ and Mn4+ ions naturally induces additional antiferromagnetic clusters in this system. Annealing the crystals in different gas environments modifies the mixed magnetic state, and results in the retardation (after O2-annealing) and bifurcation (after Ar-annealing) of the magnetization steps of isothermal magnetization. This remarkable variation offers an efficient approach for improving the magnetic properties of double-perovskite oxides.

Structural, magnetic, and magnetocaloric properties of the multiferroic host double perovskite compound Pr2FeCrO6

We have investigated the crystal structure and the nature of the magnetic ground state of the polycrystalline compound Pr2FeCrO6 (PFCO) through X-ray diffraction (XRD), magnetization, and magnetocaloric effect studies. Analysis of the XRD pattern reveals that the PFCO compound exhibits a B-site disordered orthorhombic crystal structure. The random distribution of Fe3+ and Cr3+ magnetic sublattices at the B-sites of the crystallographic unit cell helps to generate several fascinating magnetic properties. The compound exhibits three distinct anomalies in both the temperature dependence of the magnetization and the magnetic entropy change (-ΔS) curves, namely, (i) a G-type canted antiferromagnetic (AFM) ordering of the transition metal ions (TN1), (ii) a progressive spin reorientation (SR) transition (TSR), and (iii) an AFM ordering of Pr3+ sublattices at very low temperature (TN2). Surprisingly, a novel "diamagnetism-like" behavior appears in the low-temperature region for low applied field values. Moreover, we have also constructed the thermal evolution of the magnetic crystal structures in different transition regions with the help of irreducible representations of the crystal symmetry. Overall, our study of B-site disordered PFCO may help to encourage basic fundamental and applied research on disordered rare-earth and transition metal-based perovskite systems due to their interesting magnetic properties over a broad temperature range.

Viable Materials with a Giant Magnetocaloric Effect

This review of the current state of magnetocalorics is focused on materials exhibiting a giant magnetocaloric response near room temperature. To be economically viable for industrial applications and mass production, materials should have desired useful properties at a reasonable cost and should be safe for humans and the environment during manufacturing, handling, operational use, and after disposal. The discovery of novel materials is followed by a gradual improvement of properties by compositional adjustment and thermal or mechanical treatment. Consequently, with time, good materials become inferior to the best. There are several known classes of inexpensive materials with a giant magnetocaloric effect, and the search continues.

Rotating magnetocaloric effect and unusual magnetic features in metallic strongly anisotropic geometrically frustrated TmB4

We have investigated the rotating magnetocaloric effect (R-MCE) of TmB₄ - an anisotropic magnetic system with geometrical frustration of Shastry-Sutherland type. The R-MCE was obtained from detailed temperature dependencies of heat capacity in various magnetic fields of a single crystalline sample for crystal axes orientations c || B and c ⊥ B. The received results exhibit rather complex distributions of positive and negative entropy ΔS(T, B) and temperature ΔT(T, B) differences below and above T_N when the direction of the magnetic field changes between directions c || B and c ⊥ B. The calculated results were confirmed by direct R-MCE measurements which, moreover, show an interesting angular dependence of R-MCE in the ordered phase, which seems to be related with the change of the effective magnetic field along the c axis during sample rotation. Thus, our study presents a new type of magnetic refrigerant with a rather large R-MCE for low temperature magnetic refrigeration, and points to further interesting magnetic features in the ordered phase of this frustrated system.