Magnetotransport Anomaly in Room‐Temperature Ferrimagnetic NiCo₂O₄ Thin Films

Abstract

While previous studies of NCO have mainly centered on nanostructured and mesoporous materials, [6][7][8]15,16] it is of high research interest to examine the system in the epitaxial thin film form, especially for developing spintronic applications such as spinel-based magnetic tunnel junctions (MTJs). [17,18] To date, reports on high-quality NCO thin films have been scarce and mostly focused on the disorder-driven metal-insulator transition, [3,5,11,14,19,20] while the intrinsic magnetotransport properties in the metallic phase remain uncharted.In this work, we report the magnetotransport anomalies observed in 5-30 unit cell (uc) thick high-quality epitaxial NCO thin films. Below the magnetic ordering temperature, NCO exhibits strong out-of-plane magnetic anisotropy, linear magnetoresistance (MR), and robust anomalous Hall effect (AHE) that persist above room temperature. The longitudinal resistance exhibits a weak temperature dependence that cannot be directly correlated with the magnetic state of the system. The anomalous Hall conductivity (σ xy ) shows an unusual nonmonotonic temperature dependence, which changes sign in the thinner films. The relation between σ xy and the longitudinal conductivity (σ xx ) at the intermediate temperature range points to the dirty metal behavior in conjunction with the contribution from band intrinsic Berry phase effect, while the low-temperature characteristic can be attributed to disorderenhanced electron correlation. Our work provides critical information on how to manipulate the competing energy scales in this system to achieve tailored functionalities for spintronic applications.We deposited high-quality 5-30 uc epitaxial NCO films on (001)-oriented MgAl 2 O 4 (MAO) substrates. The bulk cubic lattice constant of NCO is 8.128 Å, subjecting to a 0.55% compressive strain when grown on MAO (a = 8.083 Å). X-ray diffraction (XRD) measurements show (001) growth with no impurity phases (Figure 1b). The calculated c-axis lattice constant is 8.196 Å, comparable with the reported values for strained films. [11,19] Figure 1c,d shows the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of a 17 uc NCO film, which reveal a homogeneous inverse spinel structure with high crystallinity. Atomic force microscopy (AFM) measurements show smooth surface morphology in these samples (Figure 1e), with typical root mean square roughness of 2-3 Å.The inverse spinel ferrimagnetic NiCo 2 O 4 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 NiCo 2 O 4 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 NiCo 2 O 4 films. The anomalous Hall resistance exhibits a nonmonotonic temperature dependence th...

Perpendicular magnetic anisotropy in conducting NiCo2O4 films from spin-lattice coupling

“…Inverse spinel NiCo2O4 (NCO) has recently been demonstrated to be conducting due to the mixed valence [24,25], and ferrimagnetic [24,[26][27][28] from the antialignment of Ni and Co moments above room temperature. In this work, we demonstrate that NCO has a remarkable magnetic anisotropy and spin-lattice coupling which can be employed to generate PMA up to 0.23 MJ/m 3 with a -0.3% epitaxial strain [Fig.…”

Spin reorientation in tetragonally distorted spinel oxide NiCo$_2$O$_4$ epitaxial films

“…In particular, a large tunnel magnetoresistance effect has recently been reported in magnetic tunnel junctions fabricated using NCO electrodes, supporting the half-metallic nature [4,7,8]. In addition, NCO(001) films grown on MgAl 2 O 4 (MAO) substrates exhibited perpendicular magnetic anisotropy (PMA) [9][10][11][12] of K eff u ≈ 0.3 MJ/m 3 at room temperature. The fact that both the half-metallicity and PMA are realized at once indicates that the NCO films possess good potential for application in spintronic devices.…”

Optically-induced magnetization switching in NiCo2O4 thin films using ultrafast lasers

“…We previously reported that NCO thin films realize the ultrafast demagnetization of approximately 0.4 ps at 300 K, thus indicating that NCO films have the spin polarization as large as approximately 0.7 25 . Owing to these suitable features, NCO thin films have been garnering attention as spintronic materials with potential applications in next-generation high-density non-volatile memory devices that are stable at variable temperatures [26][27][28][29] .…”