Magnetization-polarization cross-control near room temperature in hexaferrite single crystals

Prediction of Room Temperature Electric Field Reversal of Magnetization in the Family of A_{4}B_{3}O_{9} Layered Oxides

The promise of a strong magnetoelectric coupling in a multiferroic material is not only of fundamental interest, but also forms the basis of next generation memory devices where the direction of magnetization can be reversed by an external electric field. Using group-theory led first-principles calculations, we have identified a hitherto unknown polar phase of the A_{4}B_{3}O_{9} layered oxides, where the polar mode couples to the magnetic modes through a rare Γ-point magnetoelectric-multiferroic coupling scheme such that the net magnetization can be directly reversed by an electric field switching of the polar mode. Furthermore, in agreement with previous experimental observations, we predict room temperature magnetism in A_{4}B_{3}O_{9} oxides that indicates the promising practical applications of these compounds in the next generation memory devices.

Controlling the magnetic structure in W-type hexaferrites

W-type hexaferrites with varied Co/Zn ratios were synthesized and the magnetic order was investigated using neutron powder diffraction. In SrCo₂Fe₁₆O₂₇ and SrCoZnFe₁₆O₂₇ a planar (Cm'cm') magnetic ordering was found, rather than the uniaxial ordering (P6₃/mm'c') found in SrZn₂Fe₁₆O₂₇ which is common in most W-type hexaferrites. In all three studied samples, non-collinear terms were present in the magnetic ordering. One of the non-collinear terms is common to the planar ordering in SrCoZnFe₁₆O₂₇ and uniaxial ordering in SrZn₂Fe₁₆O₂₇, which could be a sign of an imminent transition in the magnetic structure. The thermomagnetic measurements revealed magnetic transitions at 520 and 360 K for SrCo₂Fe₁₆O₂₇ and SrCoZnFe₁₆O₂₇, and Curie temperatures of 780 and 680 K, respectively, while SrZn₂Fe₁₆O₂₇ showed no transition but a Curie temperature at 590 K. This leads to the conclusion that the magnetic transition can be adjusted by fine-tuning the Co/Zn stoichiometry in the sample.

Room-Temperature Type-II Multiferroic Phase Induced by Pressure in Cupric Oxide

According to previous theoretical work, the binary oxide CuO can become a room-temperature multiferroic via tuning of the superexchange interactions by application of pressure. Thus far, however, there has been no experimental evidence for the predicted room-temperature multiferroicity. Here, we show by neutron diffraction that the multiferroic phase in CuO reaches 295 K with the application of 18.5 GPa pressure. We also develop a spin Hamiltonian based on density functional theory and employing superexchange theory for the magnetic interactions, which can reproduce the experimental results. The present Letter provides a stimulus to develop room-temperature multiferroic materials by alternative methods based on existing low temperature compounds, such as epitaxial strain, for tunable multifunctional devices and memory applications.

Magnetic field reversal of electric polarization and pressure-temperature-magnetic field magnetoelectric phase diagram of the hexaferrite Ba0.4Sr1.6Mg2Fe12O22

Pressure, as an independent thermodynamic parameter, is an effective tool to obtain novel material system and exotic physical phenomena not accessible at ambient conditions, because it profoundly modifies the charge, orbital and spin state by reducing the interatomic distance in crystal structure. However, the studies of magnetoelectricity and multiferroicity are rarely extended to high pressure dimension due to properties measured inside the high pressure vessel being a challenge. Here we reported the temperature-magnetic field-pressure magnetoelectric (ME) phase diagram of Y type hexaferrite Ba_0.4Sr_1.6Mg₂Fe₁₂O₂₂derived from static pyroelectric current measurement and dynamic magnetodielectric in diamond anvil cell and piston cylinder cell. We found that a new spin-driven ferroelectric phase emerged atP= 0.7 GPa and sequentially ME effect disappeared aroundP= 4.3 GPa. The external pressure may enhance easy plane anisotropy to destabilize the longitudinal conical magnetic structure with the suppression of ME coefficient. These results offer essential clues for the correlation between ME effect and magnetic structure evolution under high pressure.

A unique and discrete Ce(iii) macrocyclic complex exhibits ferroelectric, dielectric, and slow relaxation of magnetization properties

A [Ce(L1)(NO3)3] (1) was found to exhibit ferroelectric and magnetic bistability simultaneously. The ferroelectric to paraelectric transition was observed at 303 K and a small external electric field was required to switch the spontaneous polarization in 1.

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.

Nanoscale Magnetization Reversal by Magnetoelectric Coupling Effect in Ga0.6Fe1.4O3 Multiferroic Thin Films

The control of magnetism by electric means in single-phase multiferroic materials is highly desirable for the realization of next-generation magnetoelectric (ME) multifunctional devices. Nevertheless, most of these materials reveal either low working temperature or antiferromagnetic nature, which severely limits the practical applications. Herein, we selected room-temperature multiferroic Ga_0.6Fe_1.4O₃ (GFO) with ferrimagnetism to study electric-field-induced nanoscale magnetic domain reversal. The GFO thin film fabricated on the (111)-orientated Nb-doped SrTiO₃ single-crystal substrate was obtained through the pulsed laser deposition method. The test results indicate that the thin film not only exhibits ferroelectricity but also ferrimagnetism at room temperature. More importantly, reversible and nonvolatile nanoscale magnetic domains reversal under pure electrical fields is further demonstrated by taking advantage of its ME coupling effect with dependent origins based on iron ions. When providing an appropriate applied voltage, clear magnetic domain structures with large size can be easily manipulated. Meanwhile, the change ratio of the electrically induced magnetizations in the defined areas can reach up to 72%. These considerable merits of the GFO thin film may provide a huge potential in the ME multifunctional devices, such as the multi-value, low-energy-consuming, and nonvolatile memory and beyond.

Flux Growth and Magnetic Properties of Helimagnetic Hexagonal Ferrite Ba(Fe1-x Sc x )12O19 Single Crystals

Fabricating large, high-crystalline-quality single-crystal samples of hexagonal ferrite Ba(Fe_1-x Sc _x )₁₂O₁₉ is the first important step to elucidating its helimagnetic structure and developing it for further applications. In this study, single crystals of Ba(Fe_1-x Sc _x )₁₂O₁₉ of various Sc concentrations x were successfully grown by the spontaneous crystallization method using Na₂O-Fe₂O₃ flux. We determined the optimal starting composition of reagents for Ba(Fe_1-x Sc _x )₁₂O₁₉ growth as a function of x. In situ monitoring of the crystal nucleus generation accelerated the success of crystal growth. The obtained crystals comprised black and lamellate structures with a size of 13 mm × 8 mm × 2 mm and a surface of {001} orientation. X-ray diffraction and elemental analysis revealed that the obtained crystals were composed of single-phase Ba(Fe_1-x Sc _x )₁₂O₁₉ of high crystalline quality. The lattice constants a and c increased linearly with increasing x, thereby following Vegard's law. The temperature dependence of magnetization and the magnetization curves at 77 K of the x = 0.128 crystal exhibited behavior characteristics of helimagnetism. Neutron diffraction measurements of the x = 0.128 crystal exhibited magnetic satellite reflection peaks below 211 K, providing evidence that Ba(Fe_1-x Sc _x )₁₂O₁₉ behaves as a helimagnetic material.

Single-Crystal Growth and Room-Temperature Magnetocaloric Effect of X-Type Hexaferrite Sr2Co2Fe28O46

X-type hexaferrites have been receiving considerable attention due to their promising applications in many magnetic-electronic fields. However, the growth of single-crystal X-type hexaferrite is still a challenge. Herein we reported, for the first time, the preparation of single crystal X-type hexaferrite Sr₂Co₂Fe₂₈O₄₆ (Sr₂Co₂X) with high-quality and large size using floating-zone method with laser as the heating source. The crystals show rhombohedral symmetry with space group of R-3m (No. 166, a = 5.8935(1) Å and c = 83.7438(17) Å). Co²⁺ and Fe³⁺ oxidation states were confirmed by the X-ray absorption near-edge spectroscopy. The prepared Sr₂Co₂X exhibits a spin reorientation transition from easy-cone to easy-axis at T₂ of 343 K and a ferrimagnetism-paramagnetism transition at Curie temperature (T_C) of ∼743 K. The spin reorientation transition was accompanied by magnetocaloric effect (MCE). Both conventional and inverse MCEs were observed near T₂ with a magnetic field applied along the c-axis. The maximum value of the magnetic entropy change along the c-axis was evaluated to be 1.1 J/kg·K for a magnetic field change of 5 T.