The magnetocaloric effect (MCE) is described by the change in temperature of a material by magnetic field variation and is a crucial subject in magnetism; it is motivated by the desire to enhance energy-efficient magnetic refrigeration for clean technology. Despite the recent discovery of the giant cryogenic MCE in double perovskites, the role of magnetic anisotropy has not yet been clearly discussed, because of the averaging effect of polycrystalline samples. Here, we investigated the anisotropic MCE in the single-crystal double perovskite Gd2CoMnO6. In addition to the ferromagnetic order of the Co2+ and Mn4+ moments, the large Gd3+ moments align below T
 Gd = 21 K, exhibiting an isotropic nature. Because of the intricate temperature development of magnetically hysteretic behaviour and metamagnetism, the change in magnetic entropy along the c-axis appears to be relatively small. On the contrary, the smaller but almost reversible magnetization perpendicular to the c-axis leads to a large MCE with a maximum entropy change of 25.4 J/kg·K. The anisotropic MCE generates a giant rotational MCE, estimated as 16.6 J/kg·K. Our results demonstrate the importance of magnetic anisotropy for understanding the MCE and reveal essential clues for exploring suitable magnetic refrigerant compounds aiming at magnetic functional applications.

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Giant Anisotropic Magnetocaloric Effect in Double-perovskite Gd 2 CoMnO 6 Single Crystals.

We investigated the anisotropy of the magnetic and magnetocaloric properties of single-crystalline double-perovskite $\mathrm{T}{\mathrm{b}}_{2}\mathrm{CoMn}{\mathrm{O}}_{6}$, which crystallizes in a monoclinic $P{2}_{1}/n$ structure. Due to dissimilar magnetic anisotropy, the ferromagnetic order of the $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ moments emerges along the $c$ axis at ${T}_{\mathrm{C}}=100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the larger $\mathrm{T}{\mathrm{b}}^{3+}$ moments align perpendicular to the $c$ axis, below ${T}_{\mathrm{Tb}}=15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The intricate temperature development of the metamagnetism along the $c$ axis results in a large negative change in the magnetic entropy at low temperature. On the other hand, the larger but almost reversible magnetization, perpendicular to the $c$ axis, results in a small and positive entropy change. This highly anisotropic magnetocaloric effect (MCE) leads to a giant rotational MCE, estimated to be $20.8\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{kg}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Our findings, based on the magnetic anisotropy in $\mathrm{T}{\mathrm{b}}_{2}\mathrm{CoMn}{\mathrm{O}}_{6}$, enrich fundamental and applied research on magnetic materials, considering the distinct magnetic characteristics of double perovskites.

Anisotropic magnetic properties and giant rotating magnetocaloric effect in double-perovskite T b 2 CoMn O 6

Tuning the composition of perovskite oxides results in double perovskites with cation ordering. One of the most important families of double perovskites is A2B′B′′O6. With great structural variability, A2B′B′′O6 are promising candidates for a broad range of applications such as magnetic, optical, electrical, and catalytic applications. Rare earth elements, as the ensemble of fifteen lanthanide elements as well as yttrium and scandium, represent an important part of the periodic table due to their unique 4f electrons. The incorporation of rare earth elements into double perovskites may open great opportunities to novel properties and applications. A significant number of rare earth A2B′B′′O6 double perovskites have been reported. However, there is no systematic overview of these materials to date. In this review, we summarize the compositions, syntheses, and applications of rare earth A2B′B′′O6 double perovskites. We hope that it will encourage more researchers to enter this emerging area.

Rare earth double perovskites: a fertile soil in the field of perovskite oxides

Studies of structural, dielectric relaxor and electrical characteristics of lead-free double Perovskite:Gd2NiMnO6

Structural, magnetic and magnetocaloric properties in R2CoMnO6 (R = Dy, Ho, and Er)

We have successfully synthesized the series of the double-perovskite R2CoMnO6 (R  =  rare earth: La to Lu) single crystals and have investigated their magnetic properties. The ferromagnetic order of Co(2+)/Mn(4+) spins emerges mainly along the c axis. Upon decreasing the size of rare earth ion, the magnetic transition temperature decreases linearly from 204 K for La2CoMnO6 to 48 K for Lu2CoMnO6, along with the enhancement of monoclinic distortion. The temperature and magnetic-field dependences of magnetization reveal the various magnetic characteristics such as the metamagnetic transition in R  =  Eu, the isotropic nature of rare earth moment in R  =  Gd, and the reversal of magnetic anisotropy in R  =  Tb and Dy. Our results offer comprehensive information for understanding the roles of mixed-valent magnetic ions and rare earth magnetic moments on the magnetic properties.

http://iopscience.iop.org/article/10.1088/0953-8984/27/42/426002/pdf

Investigation of the magnetic properties in double perovskite R2CoMnO6 single crystals (R = rare earth: La to Lu).

Magnetic and dielectric properties of the double perovskite Gd2NiMnO6 were investigated. Ferromagnetic order of alternating Ni2+ and Mn4+ spins arises below K and an additional order of Gd3+ spins occurs at K. The formation of short-range ferromagnetic clusters accompanied by a Griffiths phase-like feature below K is also suggested. Under the low enough dielectric loss at low temperature regime excluding the influence of the extrinsic effect, a highly nonlinear variation of the dielectric constant was achieved in application of the magnetic fields. Our finding offers an efficient approach to accomplish intrinsically coupled functionality utilizing both magnetic and dielectric quantities.

https://iopscience.iop.org/article/10.1088/0022-3727/48/44/445001/pdf

S. H. Oh

Papers

Investigation of the magnetic properties in double perovskite R2CoMnO6 single crystals (R = rare earth: La to Lu).

Nonlinear magnetodielectric effect in double-perovskite Gd2NiMnO6