Magnetic interactions in BiFe₀.₅Mn₀.₅O₃ films and BiFeO₃/BiMnO₃ superlattices

Ferroelectricity and Ferromagnetism Achieved via Adjusting Dimensionality in BiFeO3/BiMnO3 Superlattices

Integrating characteristics of materials through constructing artificial superlattices (SLs) has raised extensive attention in multifunctional materials. Here, we report the synthesis of BiFeO₃/BiMnO₃ SLs with considerable ferroelectric polarizations and tunable magnetic moments. The polarization of BiFeO₃/BiMnO₃ SLs presents a decent value of 12 μC/cm², even as the dimensionality of BiFeO₃ layers per period is reduced to about five-unit cells when keeping the BiMnO₃ layers same. Moreover, it is found that the tunable magnetic moments of SLs are linked intimately to the dimensionality of BiFeO₃ layers. Our simulations demonstrate that the superexchange interaction of Fe-O-Mn tends to be antiferromagnetic (AFM) with a lower magnetic domain formation energy rather than ferromagnetic (FM). Therefore, as the dimensionality of BiFeO₃ per period is reduced, the AFM superexchange interaction between BiFeO₃ and BiMnO₃ in the SLs becomes weak, promoting a robust magnetization. This interlayer modulation effect in SLs presents an alluring way to accurately control the multiple order parameters in a multiferroic oxide system.

Tuning ferroelectricity and ferromagnetism in BiFeO3/BiMnO3 superlattices

Multiferroic materials with multifunctional characteristics play a critical role in the field of microelectronics. In a perovskite oxide, ferroelectric polarization and ferromagnetism usually cannot coexist in a single-phase material at the same time. In this work, we design a superlattice structure composed of alternating BiFeO₃ and BiMnO₃ layers and illustrate how tuning the supercell size of epitaxial BiFeO₃/BiMnO₃ superlattices facilitates ferroelectric polarization while maintaining relatively strong ferromagnetism. A comprehensive investigation reveals that the enhanced ferroelectric polarization of BiMnO₃ layers originates from the induction effect induced by a strong polarization field generated by the adjacent ferroelectric BiFeO₃ layers. For the magnetic behavior, we consider the existence of interfacial antiferromagnetic superexchange interaction of Fe-O-Mn between BiFeO₃ and BiMnO₃ layers in our superlattices. This modulation effect of artificial superlattices provides a platform to accurately control the multiple order parameters in a multiferroic oxide system.

A rock-salt-type Li-based oxide, Li3Ni2RuO6, exhibiting a chaotic ferrimagnetism with cluster spin-glass dynamics and thermally frozen charge carriers

The area of research to discover new Li containing materials and to understand their physical properties has been of constant interest due to applications potential for rechargeable batteries. Here, we present the results of magnetic investigations on a Li compound, Li₃Ni₂RuO₆, which was believed to be a ferrimagnet below 80 K. While our neutron diffraction (ND) and isothermal magnetization (M) data support ferrimagnetism, more detailed magnetic studies establish that this ferrimagnetic phase exhibits some features similar to spin-glasses. In addition, we find another broad magnetic anomaly around 40–55 K in magnetic susceptibility (χ), attributable to cluster spin-glass phenomenon. Gradual dominance of cluster spin-glass dynamics with a decrease of temperature (T) and the apparent spread in freezing temperature suggest that the ferrimagnetism of this compound is a chaotic one. The absence of a unique freezing temperature for a crystalline material is interesting. In addition, pyroelectric current (I_pyro) data reveals a feature in the range 40–50 K, attributable to thermally stimulated depolarization current. We hope this finding motivates future work to explore whether there is any intriguing correlation of such a feature with cluster spin-glass dynamics. We attribute these magnetic and electric dipole anomalies to the crystallographic disorder, intrinsic to this compound.

Spin-glass behavior and pyroelectric anomalies in a new lithium-based oxide, Li3FeRuO5

The results of dc and ac magnetization, heat capacity, (57)Fe Mössbauer spectroscopy, dielectric, pyroelectric current and isothermal magneto-capacitance measurements of a recently reported lithium-rich layered oxide, Li3FeRuO5, related to LiCoO2-type (rhombohedral, space group R3[combining macron]m), are presented. The results reveal that the compound undergoes spin-glass freezing at 15 K. There is a peak around 34 K in pyroelectric data, which cannot be attributed to ferroelectricity, but to the phenomenon of thermally stimulated depolarization current. As revealed by magnetocapacitance data above and below the magnetic ordering temperature, magnetic and electric dipoles appear to be coupled, thereby offering evidence for magnetodielectric coupling.

Influence of Fe substitution on the Jahn-Teller distortion and orbital anisotropy in orthorhombic Y(Mn1-xFex)O3 epitaxial films

Multiferroic YMn1-xFexO3(020) (x = 0.125, 0.25, 0.50) epitaxial thin films with an orthorhombic structure (space group Pbnm) were prepared on a YAlO3(010) substrate by pulsed-laser deposition. Upon Fe substitution, the b-axis was clearly shortened, whereas the a- and c-axes were slightly lengthened based on XRD analysis. To understand the influence of orbital polarization and the Jahn-Teller effect of Mn(3+) on Fe substitution and also the local octahedral-site distortion of Fe(3+) in an environment of Jahn-Teller-active Mn(3+) ions in YMn1-xFexO3 films, we measured the polarization-dependent X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges, and also simulated the experimental spectra using configuration-interaction multiplet calculations. Although Δeg for the Mn(3+) ion decreased from 0.9 eV in pure YMnO3 to 0.6 eV in the half-Fe-substituted sample, a single eg electron was still strongly constrained to the d3y(2)-r(2) orbital for all the Fe concentrations tested. The largest Δeg, 0.5 eV, for the Fe(3+) ion was derived for a sample with 12.5% Fe substitution, and gradually decreased to 0.15 eV for the half-Fe-substituted sample. The local octahedral-site distortion of the Fe(3+) ion inside the YMnO3 lattice was similar to that of the Mn(3+) ion, whereas the Jahn-Teller distortion and GdFeO3-type distortion of the Mn(3+) ion were decreased by the spherical high-spin Fe(3+) ions. The combination of the experimental and theoretical data provides both profound insight into the variation of the Jahn-Teller distortion and orbital anisotropy and instructive information about the magnetic structures in these orthorhombic YMn1-xFexO3 thin films.

Enhanced exchange bias and improved ferromagnetic properties in Permalloy-BiFe0.95Co0.05O3 core-shell nanostructures

Hybrid core-shell nanostructures consisting of permalloy (Ni80Fe20) and multiferroic(BiFeO3, BFO/BiFe0.95Co0.05O3, BFC) materials were synthesized by a two-step method, based on wet chemical impregnation and subsequent electrodeposition within porous alumina membranes. Structural and magnetic characterizations have been done to investigate doping effect on magnetic properties and exchange bias. The magnetometry analysis revealed significant enhancements of the exchange bias and coercivity in NiFe-BFC core-shell nanostructures as compared with NiFe-BFO core-shell nanostructures. The enhancements can be attributed to the effective reduction of ferromagnet domain sizes between adjacent layers of core-shell structure. It indicates that it is possible to improve properties of multiferroic composites by site-engineering method. Our approach opens a pathway to obtain optimized nanostructured multiferroic composites exhibiting tunable magnetic properties.