A Theory of Anisotropic Magnetoresistance in Altermagnets and Its Applications

Abstract

Altermagnets, a newly discovered class of magnets, integrate the advantages of both ferromagnets and antiferromagnets, such as enabling anomalous transport without stray fields and supporting ultrafast spin dynamics, offering exciting opportunities for spintronics. A key challenge in altermagnetic spintronics is the efficient reading and writing of information by switching the Neel vector orientations to represent binary 0 and 1. Here, we develop a microscopic theory of the magnetoresistance effect in altermagnets and propose that magnetoresistance anisotropy can serve as an effective mechanism for the electrical readout of the Neel vector. Our theory describes a two-step charge-spin-charge conversion process governed by the interplay between spin splitting and spin Hall effects: a longitudinal electric field induces transverse drift spin currents, which induce significant spin accumulation at the boundaries, generating a diffusive spin current that is converted back into a longitudinal charge current. By switching the Neel vector, a substantial change in magnetoresistance, akin to giant magnetoresistance in ferromagnets, is realized, enabling an electrically readable altermagnetic memory. Our microscopic theory provides deeper insights into the fundamental physics of the magnetoresistance effect in altermagnets and offers valuable guidance for designing next-generation ultradense and ultrafast spintronic devices based on altermagnetism.

Figures (3)

• Figure 1: (a,b) Schematics of transverse spin currents arising from the SHE and SSE, which originate from isotropic and anisotropic spin-splitting bands, respectively. $\boldsymbol{E}$ denotes the electric field applied along the $x$-axis; $\boldsymbol{J}_c$ is the longitudinal charge current, decomposed into spin-up ($\boldsymbol{J}_\uparrow$) and spin-down ($\boldsymbol{J}_\downarrow$) components. $\boldsymbol{J}_{\text{SH}}$ and $\boldsymbol{J}_{\text{SS}}$ are the transverse spin currents along the $z$-axis, generated by the SHE and SSE, respectively. The spin polarization direction of the SSE is pinned to the Néel vector, and is schematically set to be antiparallel to that of the SHE, i.e., along the $y$-axis with $n_y =\sin\alpha =-1$. (c,d) The interplay between SHE and SSE leads to low and high MR states when the Néel vector is parallel ($n_y =\sin\alpha=+1$) or antiparallel ($n_y =\sin\alpha= -1$) to the spin polarization of the SHE-induced spin current, respectively. The MR effect originates from a two-step charge-spin-charge conversion process. In the first step, longitudinal charge currents ($\boldsymbol{J}_c$) are converted into transverse drift spin currents ($\boldsymbol{J}^{\text{dri}}_{\text{S}}$) via the combined action of SHE and SSE, generating significant spin accumulation $\mu^{y}_{s}(z)$ along the $z$-direction. In the second step, the diffusive spin currents ($\boldsymbol{J}^{\text{dif}}_{\text{S}}$) reflected from the sample boundaries are converted back into charge currents ($\delta\boldsymbol{J}_{L}$) through the respective inverse processes. Parallel (antiparallel) alignment yields a larger (smaller) drift spin current, $\boldsymbol{J}^{\text{dri}}_{\text{S}} = \boldsymbol{J}_{\text{SH}} + \boldsymbol{J}_{\text{SS}}$ ($\boldsymbol{J}_{\text{SH}} - \boldsymbol{J}_{\text{SS}}$), resulting in a larger (smaller) reflected spin current that is converted back into charge current with higher (lower) efficiency, $\theta_{\text{SH}} + \theta_{\text{SS}}$ ($\theta_{\text{SH}} - \theta_{\text{SS}}$), thereby leading to low (high) MR.
• Figure 2: (a,b) The temperature ($T$) modulation of $\Delta\rho _{0}$ and $\Delta\rho _{1}$ for several values of $n_s\mathcal{J}$. (c,d) Longitudinal resistivity as a function of the direction of the Néel vector ($n_y=\sin\alpha$) for different values of $\theta^0_{\text{SS}}$ and $T$, with $\Delta\rho_0/\Delta\rho_1\simeq 1.5$. Other parameters: $\theta _{\mathrm{SH}}=0.05$, $\text{S}=3/2$, $\ell _{0}=3.0$ nm, and $d_{N}=5$ nm.
• Figure 3: (a,b) The temperature ($T$) modulation of $\Delta\rho _{0}$ and $\Delta\rho _{1}$ for several values of $n_s\mathcal{J}$. (c,d) Longitudinal resistivity as a function of the direction of the Néel vector ($n_y=\sin\alpha$) for different values of $\theta^0_{\text{SS}}$ and $T$. Here, $\text{S}=5/2$, $T_N=300$ K, and other parameters are same as Fig. \ref{['MMR']}.