Spin Filtering in Epitaxial Spinel Films with Nanoscale Phase Separation

Robust room temperature perpendicular magnetic anisotropy and anomalous Hall effect of sputtered NiCo2O4film

Inverse spinel ferrimagnetic NiCo2O4(NCO) exhibits volatile physical properties due to the complex ion/valence occupation, which complicates the study its intrinsic properties. In this work, robust room temperature perpendicular magnetic anisotropy (PMA) is distinctly observed in high-quality RF-sputtered NCO film down to 3 uc (2.4 nm), confirmed by the room temperature anomalous Hall effect. The NCO films show a good metallic conductivity with a dimensional driven metal-insulator transition. The scaling relation between anomalous Hall conductivity (σxy) and the longitudinal conductivity (σxx) reveals the dirty metal behavior in conjunction with the contribution of intrinsic Berry phase or disorder-enhanced electron correlation contribute to the anomalous Hall effect for thick films while the dirty scaling law dominates for the thin films. This work introduces an oxide candidate with robust room temperature PMA as well as massive production ability for the functional spintronic applications.

Underlying Mechanisms and Tunability of the Anomalous Hall Effect in NiCo2 O4 Films with Robust Perpendicular Magnetic Anisotropy

Their high tunability of electronic and magnetic properties makes transition-metal oxides (TMOs) highly intriguing for fundamental studies and promising for a wide range of applications. TMOs with strong ferrimagnetism provide new platforms for tailoring the anomalous Hall effect (AHE) beyond conventional concepts based on ferromagnets, and particularly TMOs with perpendicular magnetic anisotropy (PMA) are of prime importance for today's spintronics. This study reports on transport phenomena and magnetic characteristics of the ferrimagnetic TMO NiCo2 O4 (NCO) exhibiting PMA. The entire electrical and magnetic properties of NCO films are strongly correlated with their conductivities governed by the cation valence states. The AHE exhibits an unusual sign reversal resulting from a competition between intrinsic and extrinsic mechanisms depending on the conductivity, which can be tuned by the synthesis conditions independent of the film thickness. Importantly, skew-scattering is identified as an AHE contribution for the first time in the low-conductivity regime. Application wise, the robust PMA without thickness limitation constitutes a major advantage compared to conventional PMA materials utilized in today's spintronics. The great potential for applications is exemplified by two proposed novel device designs consisting only of NCO films that open a new route for future spintronics, such as ferrimagnetic high-density memories.

Magnetotransport Anomaly in Room-Temperature Ferrimagnetic NiCo2 O4 Thin Films

The inverse spinel ferrimagnetic NiCo₂ O₄ presents a unique model system for studying the competing effects of crystalline fields, magnetic exchange, and various types of chemical and lattice disorder on the electronic and magnetic states. Here, magnetotransport anomalies in high-quality epitaxial NiCo₂ O₄ thin films resulting from the complex energy landscape are reported. A strong out-of-plane magnetic anisotropy, linear magnetoresistance, and robust anomalous Hall effect above 300 K are observed in 5-30 unit cell NiCo₂ O₄ films. The anomalous Hall resistance exhibits a nonmonotonic temperature dependence that peaks around room temperature, and reverses its sign at low temperature in films thinner than 20 unit cells. The scaling relation between the anomalous Hall conductivity and longitudinal conductivity reveals the intricate interplay between the spin-dependent impurity scattering, band intrinsic Berry phase effect, and electron correlation. This study provides important insights into the functional design of NiCo₂ O₄ for developing spinel-based spintronic applications.

Thermal Driven Giant Spin Dynamics at Three-Dimensional Heteroepitaxial Interface in Ni0.5Zn0.5Fe2O4/BaTiO3-Pillar Nanocomposites

Traditional magnetostrictive/piezoelectric laminated composites rely on the two-dimensional interface that transfers stress/strain to achieve the large magnetoelectric (ME) coupling, nevertheless, they suffer from the theoretical limitation of the strain effect and of the substrate clamping effect in real ME applications. In this work, 3D NZFO/BTO-pillar nanocomposite films were grown on SrTiO₃ by template-assisted pulsed laser deposition, where BaTiO₃ (BTO) nanopillars appeared in an array with distinct phase transitions as the cores were covered by NiZn ferrite (NZFO) layer. The perfect 3D heteroepitaxial interface between BTO and NZFO phases can be identified without any edge dislocations, which allows effective strain transfer at the 3D interface. The 3D structure nanocomposites enable the strong two magnon scattering (TMS) effect that enhances ME coupling at the interface and reduces the clamping effect by strain relaxation. Thereby, a large FMR field shift of 1866 Oe in NZFO/BTO-pillar nanocomposite was obtained at the TMS critical angle near the BTO nanopillars phase transition of 255 K.

Anisotropic sensor and memory device with a ferromagnetic tunnel barrier as the only magnetic element

Multiple spin functionalities are probed on Pt/La₂Co_0.8Mn_1.2O₆/Nb:SrTiO₃, a device composed by a ferromagnetic insulating barrier sandwiched between non-magnetic electrodes. Uniquely, La₂Co_0.8Mn_1.2O₆ thin films present strong perpendicular magnetic anisotropy of magnetocrystalline origin, property of major interest for spintronics. The junction has an estimated spin-filtering efficiency of 99.7% and tunneling anisotropic magnetoresistance (TAMR) values up to 30% at low temperatures. This remarkable angular dependence of the magnetoresistance is associated with the magnetic anisotropy whose origin lies in the large spin-orbit interaction of Co²⁺ which is additionally tuned by the strain of the crystal lattice. Furthermore, we found that the junction can operate as an electrically readable magnetic memory device. The findings of this work demonstrate that a single ferromagnetic insulating barrier with strong magnetocrystalline anisotropy is sufficient for realizing sensor and memory functionalities in a tunneling device based on TAMR.

Fabrication of Epitaxial Fe3O4 Film on a Si(111) Substrate

The application of magnetic oxides in spintronics has recently attracted much attention. The epitaxial growth of magnetic oxide on Si could be the first step of new functional spintronics devices with semiconductors. However, epitaxial spinel ferrite films are generally grown on oxide substrates, not on semiconductors. To combine oxide spintronics and semiconductor technology, we fabricated Fe₃O₄ films through epitaxial growth on a Si(111) substrate by inserting a γ-Al₂O₃ buffer layer. Both of γ-Al₂O₃ and Fe₃O₄ layer grew epitaxially on Si and the films exhibited the magnetic and electronic properties as same as bulk. Furthermore, we also found the buffer layer dependence of crystal structure of Fe₃O₄ by X-ray diffraction and high-resolution transmission electron microscope. The Fe₃O₄ films on an amorphous-Al₂O₃ buffer layer grown at room temperature grew uniaxially in the (111) orientation and had a textured structure in the plane. When Fe₃O₄ was deposited on Si(111) directly, the poly-crystal Fe₃O₄ films were obtained due to SiO_x on Si substrate. The epitaxial Fe₃O₄ layer on Si substrates enable us the integration of highly functional spintoronic devices with Si technology.