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

Gradient Strain-Induced Room-Temperature Ferroelectricity in Magnetic Double-Perovskite Superlattices

Single-phase multiferroics suffer from a fundamental contradiction between polarity and magnetism in d⁰ electronic configuration, motivating studies of unconventional ferroelectricity in magnetic oxides. However, low critical temperature and polarization still need to be overcome. Here, it is reported that the switchable polarization behavior at room temperature in [(La₂ NiMnO₆ )/(La₂ CoMnO₆ )]_n double-perovskite magnetic superlattice films is achieved by engineering a microstructure with gradient strains, and the ferromagnetic Curie temperature did not show a rapid decrease. The synergy of gradient strains and superlattice components plays a decisive role in inducing ferroelectricity via the tilting or rotation of various oxygen octahedra. Such distortion responses to gradient strains are accompanied by slight magnetic fluctuations, maximizing the preservation of the initial magnetic exchange interactions, which alleviates the contradiction of multiferroic coexistence to a certain extent. This work confirms the room-temperature ferroelectricity in double-perovskite superlattices and provides a preferred strategy for confronting the difficulty of multiferroic coexistence in single-phase materials.

Tunable Magnetic Properties in Sr2FeReO6 Double-Perovskite

Double-perovskite oxides have attracted recent attention due to their attractive functionalities and application potential. In this paper, we demonstrate the effect of dual controls, i.e., the deposition pressure of oxygen (P_O2) and lattice mismatch (ε), on tuning magnetic properties in epitaxial double-perovskite Sr₂FeReO₆ films. In a nearly lattice matched Sr₂FeReO₆/SrTiO₃ film, the ferrimagnetic-to-paramagnetic phase transition occurs when P_O2 is reduced to 30 mTorr, probably due to the formation of Re⁴⁺ ions that replace the stoichiometric Re⁵⁺ to cause disorders of B-site ions. On the other hand, a large compressive strain or tensile strain shifts this critical P_O2 to below 1 mTorr or above 40 mTorr, respectively. The observations can be attributed to the modulation of B-site ordering by epitaxial strain through affecting elemental valence. Our results provide a feasible way to expand the functional tunability of magnetic double-perovskite oxides that hold great promise for spintronic devices.

Synthesis, dielectric, magnetic, and photoluminescence properties of two new hybrid rare-earth double perovskites

Two new organic–inorganic hybrid double perovskites (R3HQ)₄CsSm(NO₃)₈ (1) (R3HQ = (R)-(-)-3-quinuclidinol) and (R3HQ)₄CsEu(NO₃)₈ (2) were synthesized and characterized. Compounds 1 and 2 exhibit obvious phase transitions at 379 and 375 K, respectively, confirmed by differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction. The rapid switching between high- and low-dielectric states makes it a typical dielectric material with a switchable dielectric constant for thermal stimulus response. Furthermore, 1 and 2 show attractive photoluminescence and paramagnetic behavior, and the fluorescence quantum yield of 2 reached 14.6%. These results show that compounds 1 and 2 can be used as excellent candidates for multifunctional intelligent materials, which also provides a new way for development of multifunctional materials.

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.

Rapid Thermal Annealing of Double Perovskite Thin Films Formed by Polymer Assisted Deposition

The annealing process is an important step common to epitaxial films prepared by chemical solution deposition methods. It is so because the final microstructure of the films can be severely affected by the precise features of the thermal processing. In this work we analyze the structural and magnetic properties of double perovskite La₂CoMnO₆ and La₂NiMnO₆ epitaxial thin films prepared by polymer-assisted deposition (PAD) and crystallized by rapid thermal annealing (RTA). It is found that samples prepared by RTA have similar values of saturation magnetization and Curie temperature to their counterparts prepared by using conventional thermal annealing (CTA) processes, thus indicating low influence of the heating rates on the B-B’ site cationic ordering of the A₂BB’O₆ double perovskite structure. However, a deeper analysis of the magnetic behavior suggested some differences in the actual microstructure of the films.

High-Pressure Synthesis of a B-site Co2+/Mn4+ Disordered Quadruple Perovskite LaMn3Co2Mn2O12

A new oxide, LaMn₃Co₂Mn₂O₁₂, was synthesized under high-pressure (7 GPa) and high-temperature (1423 K) conditions. The compound crystallizes in an AA'₃B₄O₁₂-type quadruple perovskite structure with space group Im3̅. The Rietveld structural analysis combined with soft X-ray absorption spectroscopy reveals the charge combination to be LaMn³⁺₃Co²⁺₂Mn⁴⁺₂O₁₂, where the La³⁺ and Mn³⁺ are 1:3 ordered respectively at the A and A' sites, whereas the Co²⁺ and Mn⁴⁺ are disorderly distributed at the B site. This is in sharp contrast to R₂Co²⁺Mn⁴⁺O₆ (R = La and rare earth) double perovskites, in which the Co²⁺ and Mn⁴⁺ charge states are always orderly distributed with a rocksalt-type fashion, giving rise to a long-range magnetic ordering. As a result, LaMn₃Co₂Mn₂O₁₂ displays spin glassy magnetic properties due to the random Co²⁺ and Mn⁴⁺ distribution, as demonstrated by dc and ac magnetic susceptibility as well as specific heat measurements. Possible factors that affect the B-site degree of order in perovskite structures are discussed.

Aqueous Chemical Solution Deposition of Functional Double Perovskite Epitaxial Thin Films

Double perovskite structure (A₂ BB'O₆ ) oxides exhibit a breadth of multifunctional properties with a huge potential range of applications in fields as diverse as spintronics, magneto-optic devices, or catalysis, and most of these applications require the use of thin films and heterostructures. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films combining high performance with high throughput and low cost. In addition, the physical properties of these materials are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Thus, promoting spontaneous cationic ordering has become a relevant issue. In this work, our recent achievements by using polymer-assisted deposition (PAD) of environmentally friendly, water-based solutions for the growth of epitaxial ferromagnetic insulating double perovskite La₂ CoMnO₆ and La₂ NiMnO₆ thin films on SrTiO₃ and LaAlO₃ single-crystal substrates are presented. It is shown that the particular crystallization and growth process conditions of PAD (very slow rate, close to thermodynamic equilibrium conditions) promote high crystallinity and quality of the films, as well as favors spontaneous B-site cationic ordering.

Strong magnetoelectric coupling in mixed ferrimagnetic-multiferroic phases of a double perovskite

Exploring new magnetic materials is essential for finding advantageous functional properties such as magnetoresistance, magnetocaloric effect, spintronic functionality, and multiferroicity. Versatile classes of double perovskite compounds have been recently investigated because of intriguing physical properties arising from the proper combination of several magnetic ions. In this study, it is observed that the dominant ferrimagnetic phase is coexisted with a minor multiferroic phase in single-crystalline double-perovskite Er₂CoMnO₆. The majority portion of the ferrimagnetic order is activated by the long-range order of Er³⁺ moments below T_Er = 10 K in addition to the ferromagnetic order of Co²⁺ and Mn⁴⁺ moments arising at T_C = 67 K, characterized by compensated magnetization at T_Comp = 3.15 K. The inverted magnetic hysteresis loop observed below T_Comp can be described by an extended Stoner-Wohlfarth model. The additional multiferroic phase is identified by the ferroelectric polarization of ~0.9 μC/m² at 2 K. The coexisting ferrimagnetic and multiferroic phases appear to be strongly correlated in that metamagnetic and ferroelectric transitions occur simultaneously. The results based on intricate magnetic correlations and phases in Er₂CoMnO₆ enrich fundamental and applied research on magnetic materials through the scope of distinct magnetic characteristics in double perovskites.

Giant Anisotropic Magnetocaloric Effect in Double-perovskite Gd2CoMnO6 Single Crystals

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.

Structural distortions, orbital ordering and physical properties of double perovskite R2CoMnO6 calculated by first-principles

The structural distortions, orbital ordering, magnetic and electronic properties of double perovskite R₂CoMnO₆ (R  =  rare-earth element) have been systematically calculated by first-principles. Structural distortions, including Co-O and Mn-O bond length splitting, the antiferroelectric motions of R ions, the tilting of octahedral (the resulted Co-O-Mn bond angle) are obviously affected by the rare-earth ions' radius. The bond length splitting behavior of Co-O and Mn-O are rather different because of the Jahn-Teller active ion Co²⁺ and the Jahn-Teller nonactive ion Mn⁴⁺. Taking Gd₂CoMnO₆ as an example, the t _2g orbitals of Co ions are predicted to be orbital ordered. That is, the spin down channel of d _xz orbital for one Co ion and d _yz orbital for another Co ion are basically vacant. Finally, the physical properties, including the magnetic Curie temperature and electronic band gap of R₂CoMnO₆ are almost linear dependent on the average value of cos² θ (θ is the Co-O-Mn exchange-angle).