Interface-driven unusual anomalous Hall effect in $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers

The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create novel chiral spin structures (e.g., skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological Hall effect (THE). In this work, however, we present an unusual anomalous Hall effect (UAHE) in $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers and demonstrate that the features extremely similar to THE can be generated without involving any chiral spin structures. Low-temperature magnetic force microscopy has been used to explore the magnetic field-dependent behavior of spin structures, and the UAHE as a function of magnetic field does not peak near the maximal density of magnetic bubbles. Our results unambiguously evidence that the UAHE in $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers shows no correlation with chiral spin structures but is driven by the modified interfacial properties, indicating a wealth of underlying and interesting physics.

Figure 1

(a–c) The Hall resistivities after subtracting ordinary Hall contributions $\left({\rho }_{XY}-{\rho }_O\right)$ of the $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers under perpendicularly applied magnetic fields in the temperature range from 5 to 300 K; (d–f) MR of the $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers under perpendicularly applied magnetic fields in the temperature range from 5 to 300 K.

Figure 2

(a–c) ${\rho }_A$ vs ${\rho }_{{}_{XX}}^2$ for $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers. Solid red lines refer to a linear fit of ${\rho }_A=\alpha {\rho }_{XX0}+\mathrm{b}{\rho }_{XX}^2$. (d) Temperature dependence of skew scattering coefficients $\alpha$ for $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers.

Figure 3

(a–c) The additional Hall resistivities after subtracting ordinary and anomalous Hall contributions $\mathrm{\Delta }{\rho }_{XY}$ of the $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers as functions of perpendicularly applied magnetic fields at different temperatures from 5 to 300 K. $\mathrm{\Delta }{\rho }_{{}_{XY}}^{\max }$ denotes the peak (300 K) or valley (5 K) values with sweeping magnetic field from positive to negative. (d) Temperature dependence of $\mathrm{\Delta }{\rho }_{{}_{XY}}^{\max }$ for the $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers. Lines are provided to guide the eye. Magnetic force microscopy images for the $\mathrm{M}{\mathrm{n}}_{1.55}\mathrm{Ga}/\mathrm{Al}$ (e) and $\mathrm{M}{\mathrm{n}}_{1.55}\mathrm{Ga}/\mathrm{Pt}$ bilayers (f) at 300 K after applying a positive perpendicular magnetic field of 5 T and then decreasing the magnetic field down to zero. The scale size is $7\mu \text{m}\times 5\mu \text{m}$.

Figure 4

M-H (a) and $\mathrm{\Delta }{\rho }_{XY}\text{−}H$ (b) curves for $\mathrm{M}{\mathrm{n}}_{1.55}\mathrm{Ga}/\mathrm{Pt}$ bilayers at 3.5 K. (c–h) Magnetic force microscopy images after applying a positive perpendicular magnetic field of 5 T and then decreasing the magnetic field down to +2 T, 0 T, −0.5 T, −1.0 T, and −2.0 T, respectively. The scale size is $7\mu \mathrm{m}\times 7\mu \mathrm{m}$.

Figure 5

(a) Mn $2p$ XAS spectra of the $\mathrm{M}{\mathrm{n}}_{1.25}\mathrm{Ga}/\mathrm{Pt}$ and $\mathrm{M}{\mathrm{n}}_{1.25}\mathrm{Ga}/\mathrm{Al}$ bilayers. (b) Temperature dependence of $\mathrm{\Delta }{\sigma }_{{}_{XY}}^{\max }$ for the $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers. (c) Temperature dependence of $d^2\mathrm{\Delta }{\sigma }_{XY}^{max}/d{T}^2$ and $d^2\alpha /d{T}^2$ for the $\mathrm{M}{\mathrm{n}}_{1.55}\mathrm{Ga}/\mathrm{Pt}$ bilayers. (d) Angle dependence of $\left[{\sigma }_{XX}\left(\theta \right)-{\sigma }_{XX}\left(0\right)\right]$ at 5 K for $\mathrm{M}{\mathrm{n}}_{1.25}\mathrm{Ga}/\mathrm{Al}$ and $\mathrm{M}{\mathrm{n}}_{1.25}\mathrm{Ga}/\mathrm{Pt}$ bilayers. The current is applied along the X direction, and $\theta =0^{\circ }$ denotes the magnetic field (9 T) is applied along the Z direction. The signals in $\mathrm{M}{\mathrm{n}}_{1.25}\mathrm{Ga}/\mathrm{Pt}$ bilayers are multiplied by a factor of 10 shown as “$\times 10$.”