Four-fold Anisotropic Magnetoresistance in Antiferromagnetic Epitaxial Thin Films of MnPt$_{x}$Pd$_{1-x}$

TL;DR

This work investigates thickness-dependent anisotropic magnetoresistance in epitaxial L1$_0$ MnPt$_{x}$Pd$_{1-x}$ thin films on MgO. Thick films exhibit a dominant two-fold, non-crystalline AMR driven by domain reconfiguration and spin canting, while thin films develop a four-fold crystalline AMR tied to uncompensated moments stabilized by interfacial Mn–O hybridization that modulates the density of states at the Fermi level as the Neel vector rotates. The authors support these findings with structural characterization, magnetization and exchange-bias measurements, and ab-initio calculations showing interface-enhanced DOS and DOS-controlled AMR, providing a mechanism for Neel-vector–driven four-fold AMR in ultrathin antiferromagnetic films. The results offer a route to control antiferromagnetic order and AMR in spintronic devices through film thickness, interfacial engineering, and disorder.

Abstract

Antiferromagnets are emerging as promising alternatives to ferromagnets in spintronics applications. A key feature of antiferromagnets is their anisotropic magnetoresistance (AMR), which has the potential to serve as a sensitive marker for the antiferromagnetic order parameter. However, the underlying origins of this behavior remains poorly understood, particularly, in thin film geometries. In this study, we report the observation of AMR in epitaxial thin films of the collinear L1$_{0}$ antiferromagnet MnPt$_{x}$Pd$_{1-x}$. In the thicker films, AMR is dominated by a non-crystalline two-fold component, which emerges from domain reconfiguration and spin canting under applied magnetic field. As the film thickness is reduced, however, a crystalline four-fold component emerges, accompanied by the appearance of uncompensated magnetic moment, which strongly modifies the magnetotransport properties in the thinner films. We demonstrate that interfacial interactions lead to a large density of states (DOS) at the Fermi level. This enhanced DOS, combined with disorder in the thinner films, stabilizes the uncompensated moment and results in a four-fold modulation of the DOS as the Neel vector rotates, explaining the observed AMR behavior.

Figures (5)

• Figure 1: (a) Crystal structure of MnPt$_{x}$Pd$_{1-x}$ (b) Epitaxial relationship of MnPt$_{x}$Pd$_{1-x}$ with MgO substrate. (c) Out-of-plane $\theta$ - $2\theta$ scan of a 40 nm thick MnPt$_{x}$Pd$_{1-x}$ thin film. Substrate peaks are marked by asterisks. (d) Azimuthal $\phi$-scan of the asymmetric planes of MgO(111) and MnPt$_{x}$Pd$_{1-x}$(101) establishing the epitaxial relationship MnPt$_{x}$Pd$_{1-x}$[110]$||$MgO[100]. (e) AFM image of a 10 nm thick sample with a measured RMS roughness of 1.2 nm over a 10$\mu m$$\times$10$\mu m$ field of view.
• Figure 2: Magnetization as a function of magnetic field recorded at 100 K for (a) 10 and (b) 40 nm thick MnPt$_{x}$Pd$_{1-x}$ films. Diamagnetic contribution of the MgO substrate has been subtracted. Inset in (a) shows magnetization per unit area for 10 and 40 nm thick films before subtraction of the diamagnetic contribution from the substrate. Magnetization hysteresis loop in MnPt$_{x}$Pd$_{1-x}$(10 nm)/Fe(7 nm) bilayer sample measured (c) at 30 K under different cooling magnetic field and (e) at different temperatures under a cooling field of 1 T. Corresponding variation of the exchange ($H_{eb}$) and coercive ($H_{c}$) fields are shown in (d) and (f), respectively.
• Figure 3: (a) Schematic of the orientation of different Hall bar devices used in this study. The relevant angles are also shown. The inset shows an optical micrograph image of a Hall bar device (b) AMR of 10 and 40 nm thick D1 devices oriented along MnPt$_{x}$Pd$_{1-x}$[110] direction. (c-d) Comparison of AMR measured in D1(blue), D2(red), D3(magenta), and D4(green) devices for (c) 40 and (d) 10 nm thick films. Evolution of the two-fold symmetric component, $C_{2}$, with magnetic field in (e) 40nm (f) 10 nm thick films. (g) Magnetic field dependence of the four-fold symmetric component, $C_{4}$, in 10 nm thick films. (h) Evolution of the uniaxial component, $C_{u}$, in 40 and 10 nm thick films. All the AMR curves are recorded at 30 K under a 9 T magnetic field.
• Figure 4: (a) Schematic of the two-domain model, as described in the text. The two-domain model predicts a two-fold AMR with a $\pi/2$ phase shift as observed for the D1/D3 devices in the experiment. (b) Evolution of $\Delta\rho_{j}=\rho_{j}(H)-\rho_{j}(H=0)$ with magnetic field for different $\phi$ values with $\theta=0^{\circ}$. The fits to eqn. 3, described in the main text, are shown with black dashed lines. (c) Canting of magnetic moments considered in the model described in the text. The spin canting model accurately predicts two-fold AMR for D2/D4 devices without any phase shift, in contrast to what is observed in the D1/D3 devices. (d) Evolution of $\Delta\rho_{j}^{(\phi)}=\rho_{j}^{(\phi)}-\rho_{j}^{(\phi=90^{\circ})}$ with magnetic field for different $\phi$ values with $\theta=45^{\circ}$. The fits to eqn. 5, described in the main text, are shown with black dashed lines.
• Figure 5: (a) $\Delta \rho_{j}$ as a function of magnetic field applied along out-of-plane(001) direction recorded at 30 K for D1 devices of 40 nm and 10 nm thick MnPt$_{x}$Pd$_{1-x}$ films. Inset shows a schematic that defines the angles described in eqn. 6, described in the main text. (b) In-plane MR for a 10 nm thick film in a D1 device for different magnetic field directions. In (a) and (b), fits to $Hln(H)$ and $H^{2}$ dependence are shown in black and red dashed lines, respectively. Charge density plots around the Mn atoms in MnPt$_{x}$Pd$_{1-x}$ (c) in the bulk (d) at the MnPt$_{x}$Pd$_{1-x}$/MgO hetero-interface. Significant charge redistribution due to orbital overlap between O $p_{z}$ and Mn $d_{z^{2}-r^{2}}$ can be observed in (d). (e) Layer-resolved density of states of the MnPt$_{x}$Pd$_{1-x}$ atomic layers. L1 and L2 correspond to the MnPt$_{x}$Pd$_{1-x}$ layer at the interface and next to the interface, respectively. Partial DOS(PDOS) of Mn atoms is shown in red while the sum of PDOS of Pt and Pd atoms is shown in blue. DOS at E$_{F}$ is progressively reduced for MnPt$_{x}$Pd$_{1-x}$ layers further away from the interface. (f) Change in DOS (CDOS) of MnPt$_{x}$Pd$_{1-x}$ as a function of the direction of the Neel vector rotated within the (001) plane. CDOS is defined as the DOS (along a Neel vector) minus the average DOS. For the thin film calculation, Mn termination is considered (see text)