More Like this

1. 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. 

   more » « less
2. 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. 

   more » « less
3. Microstructural Underpinnings of Giant Intrinsic Exchange Bias in Epitaxial NiCo ₂ O ₄ Thin Films

   https://doi.org/10.1002/aelm.202400149  

   Yang, Detian; Subedi, Arjun; Liu, Chao; Ambaye, Haile; Lauter, Valeria; Dowben, Peter_A; Liu, Yaohua; Xu, Xiaoshan (November 2024, Advanced Electronic Materials)

   Abstract Understanding intrinsic exchange bias in nominally single‐component ferromagnetic or ferrimagnetic materials is crucial for simplifying related device architectures. However, the mechanisms behind this phenomenon and its tunability remain elusive, which hinders the efforts to achieve unidirectional magnetization for widespread applications. Inspired by the high tunability of ferrimagnetic inverse spinel NiCo₂O₄, the origin of intrinsic exchange bias in NiCo₂O₄(111) films deposited on Al₂O₃(0001) substrates are investigated. The comprehensive characterizations, including electron diffraction, X‐ray reflectometry and spectroscopy, and polarized neutron reflectometry, reveal that intrinsic exchange bias in NiCo₂O₄(111)/Al₂O₃(0001) arises from a reconstructed antiferromagnetic rock‐salt Ni_xCo_1‐xO layer at the interface between the film and the substrate due to a significant structural mismatch. Remarkably, by engineering the interfacial structure under optimal growth conditions, it can achieve exchange bias larger than coercivity, leading to unidirectional magnetization. Such giant intrinsic exchange bias can be utilized for realistic device applications. This work establishes a new material platform based on NiCo₂O₄, an emergent spintronics material, to study tunable interfacial magnetic and spintronic properties. 

   more » « less
4. Intrinsic exchange bias from interfacial reconstruction in an epitaxial Ni _x Co _y Fe _{3− x −y} O ₄ (111)/ $\alpha$ -Al ₂ O ₃ (0001) thin film family

   https://doi.org/10.1088/1361-648X/ad789d  

   Yang, Detian; Liu, Yaohua; Xu, Xiaoshan (September 2024, Journal of Physics: Condensed Matter)

   Abstract Intrinsic exchange bias is known as the unidirectional exchange anisotropy that emerges in a nominally single-component ferro-(ferri-)magnetic system. In this work, with magnetic and structural characterizations, we demonstrate that intrinsic exchange bias is a general phenomenon in (Ni, Co, Fe)-based spinel oxide films deposited on $\alpha$-Al₂O₃(0001) substrates, due to the emergence of a rock-salt interfacial layer consisting of antiferromagnetic CoO from interfacial reconstruction. We show that in Ni_xCo_yFe_3−x−yO₄(111)/ $\alpha$-Al₂O₃(0001) films, intrinsic exchange bias and interfacial reconstruction have consistent dependences on Co concentrationy, while the Ni and Fe concentration appears to be less important. This work establishes a family of intrinsic exchange bias materials with great tunability by stoichiometry and highlights the strategy of interface engineering in controlling material functionalities. 

   more » « less
5. Epitaxial NiCo ₂ O ₄ film as an emergent spintronic material: Magnetism and transport properties

   https://doi.org/10.1063/5.0095326  

   Xu, Xiaoshan; Mellinger, Corbyn; Cheng, Zhi Gang; Chen, Xuegang; Hong, Xia (July 2022, Journal of Applied Physics)

   The ferrimagnetic inverse spinel NiCo 2 O 4 has attracted extensive research interest for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo 2 O 4 present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this Perspective, we discuss the recent advances in epitaxial NiCo 2 O 4 thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo 2 O 4 films is explained by considering the strong spin–lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo 2 O 4 thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band-intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiCo 2 O 4 thin films for nanoscale spintronic applications. 

   more » « less