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1. Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo ₂ O ₄ films

   https://doi.org/10.1063/5.0097869  

   Chen, Xuegang; Wu, Qiuchen; Zhang, Le; Hao, Yifei; Han, Myung-Geun; Zhu, Yimei; Hong, Xia (June 2022, Applied Physics Letters)

   The inverse spinel ferrimagnetic NiCo₂O₄possesses high magnetic Curie temperature T_C, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo₂O₄thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo₂O₄films on MgAl₂O₄substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while T_Cfor 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity [Formula: see text] and its scaling relation with the longitudinal conductivity ([Formula: see text]) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo₂O₄. 

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2. Magnetotransport Anomaly in Room‐Temperature Ferrimagnetic NiCo ₂ O ₄ Thin Films

   https://doi.org/10.1002/adma.201805260  

   Chen, Xuegang; Zhang, Xiaozhe; Han, Myung‐Geun; Zhang, Le; Zhu, Yimei; Xu, Xiaoshan; Hong, Xia (November 2018, Advanced Materials)

   Abstract The inverse spinel ferrimagnetic NiCo₂O₄presents 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 NiCo₂O₄thin 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 NiCo₂O₄films. 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 NiCo₂O₄for developing spinel‐based spintronic applications. 

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3. Bulk-like dielectric and magnetic properties of sub 100 nm thick single crystal Cr2O3 films on an epitaxial oxide electrode

   https://doi.org/10.1038/s41598-020-71619-1  

   Vu, N. M.; Luo, X.; Novakov, S.; Jin, W.; Nordlander, J.; Meisenheimer, P. B.; Trassin, M.; Zhao, L.; Heron, J. T. (December 2020, Scientific Reports)

   null (Ed.)

   Abstract The manipulation of antiferromagnetic order in magnetoelectric Cr 2 O 3 using electric field has been of great interest due to its potential in low-power electronics. The substantial leakage and low dielectric breakdown observed in twinned Cr 2 O 3 thin films, however, hinders its development in energy efficient spintronics. To compensate, large film thicknesses (250 nm or greater) have been employed at the expense of device scalability. Recently, epitaxial V 2 O 3 thin film electrodes have been used to eliminate twin boundaries and significantly reduce the leakage of 300 nm thick single crystal films. Here we report the electrical endurance and magnetic properties of thin (less than 100 nm) single crystal Cr 2 O 3 films on epitaxial V 2 O 3 buffered Al 2 O 3 (0001) single crystal substrates. The growth of Cr 2 O 3 on isostructural V 2 O 3 thin film electrodes helps eliminate the existence of twin domains in Cr 2 O 3 films, therefore significantly reducing leakage current and increasing dielectric breakdown. 60 nm thick Cr 2 O 3 films show bulk-like resistivity (~ 10 12 Ω cm) with a breakdown voltage in the range of 150–300 MV/m. Exchange bias measurements of 30 nm thick Cr 2 O 3 display a blocking temperature of ~ 285 K while room temperature optical second harmonic generation measurements possess the symmetry consistent with bulk magnetic order. 

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4. Strained single crystal high entropy oxide manganite thin films

   https://doi.org/10.1063/5.0206767  

   Zhao, Zhibo; Waqar, Moaz; Jaiswal, Arun_Kumar; Raghavan, Aaditya_Rangan; Fuchs, Dirk; Lin, Jing; Brezesinski, Torsten; Bhattacharya, Subramshu_S; Hahn, Horst; Pan, Xiaoqing; et al (July 2024, Applied Physics Letters)

   The ability to accommodate multiple principal cations within a single crystallographic structure makes high entropy oxides (HEOs) ideal systems for exploring new composition–property relationships. In this work, the high-entropy design strategy is extended to strained single-crystal HEO-manganite (HEO-Mn) thin films. Phase-pure orthorhombic films of (Gd0.2La0.2Nd0.2Sm0.2Sr0.2)MnO3 were deposited on three different single-crystal substrates: SrTiO3 (STO) (100), NdGaO3 (110), and LaAlO3 (LAO) (100), each inducing different degrees of epitaxial strain. Fully coherent growth of the thin films is observed in all cases, despite the high degree of lattice mismatch between HEO-Mn and LAO. Magnetometry measurements reveal distinct differences in the magnetic properties between epitaxially strained HEO-Mn thin films and their bulk crystalline HEO counterparts. In particular, the bulk polycrystalline HEO-Mn shows two magnetic transitions as opposed to a single one observed in epitaxial thin films. Moreover, the HEO-Mn film deposited on LAO exhibits a significant reduction in the Curie temperature, which is attributed to the strong variation of the in-plane lattice parameter along the thickness of the film and the resulting changes in the Mn–O–Mn bond geometry. Thus, this preliminary study demonstrates the potential of combining high entropy design with strain engineering to tailor the structure and functionality of perovskite manganites. 

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5. Recent progress on topological semimetal IrO ₂ : electronic structures, synthesis, and transport properties

   https://doi.org/10.1088/1361-648X/ad3603  

   Zhang, T. X.; Coughlin, A. L.; Lu, Chi-Ken; Heremans, J. J.; Zhang, S. X. (April 2024, Journal of Physics: Condensed Matter)

   Abstract 5dtransition metal oxides, such as iridates, have attracted significant interest in condensed matter physics throughout the past decade owing to their fascinating physical properties that arise from intrinsically strong spin-orbit coupling (SOC) and its interplay with other interactions of comparable energy scales. Among the rich family of iridates, iridium dioxide (IrO₂), a simple binary compound long known as a promising catalyst for water splitting, has recently been demonstrated to possess novel topological states and exotic transport properties. The strong SOC and the nonsymmorphic symmetry that IrO₂possesses introduce symmetry-protected Dirac nodal lines (DNLs) within its band structure as well as a large spin Hall effect in the transport. Here, we review recent advances pertaining to the study of this unique SOC oxide, with an emphasis on the understanding of the topological electronic structures, syntheses of high crystalline quality nanostructures, and experimental measurements of its fundamental transport properties. In particular, the theoretical origin of the presence of the fourfold degenerate DNLs in band structure and its implications in the angle-resolved photoemission spectroscopy measurement and in the spin Hall effect are discussed. We further introduce a variety of synthesis techniques to achieve IrO₂nanostructures, such as epitaxial thin films and single crystalline nanowires, with the goal of understanding the roles that each key parameter plays in the growth process. Finally, we review the electrical, spin, and thermal transport studies. The transport properties under variable temperatures and magnetic fields reveal themselves to be uniquely sensitive and modifiable by strain, dimensionality (bulk, thin film, nanowire), quantum confinement, film texture, and disorder. The sensitivity, stemming from the competing energy scales of SOC, disorder, and other interactions, enables the creation of a variety of intriguing quantum states of matter. 

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