Transport spin polarization in RuO$_2$ films

Abstract

Altermagnets host spin-split electronic bands without net magnetization, enabling spin-polarized transport in the absence of conventional ferromagnetism. RuO$_2$ has been proposed as a candidate altermagnet, yet experimental reports remain conflicting, particularly between bulk-sensitive probes and thin-film measurements. Here we investigate the electronic transport properties of epitaxial RuO$_2$ thin films using anomalous Hall effect measurements and point-contact Andreev reflection spectroscopy. We observe transport spin polarization and a strongly orientation-dependent anomalous Hall response, while magnetometry reveals no detectable net magnetization. The anomalous Hall effect appears only in ultrathin (110)-oriented films, consistent with symmetry-driven Néel-vector physics, and the measured transport spin polarization is systematically higher for (110)-oriented films than for (001)-oriented films, consistent with the crystallographic anisotropy of the spin-split bands. These results are consistent with altermagnetic behavior in RuO$_2$, with the experimentally accessible signatures confined to near-surface regions. They also establish superconducting transport spectroscopy as a metrology for identifying and characterizing altermagnet candidates.

Figures (3)

• Figure 1: Structural and magnetic properties of 60 nm RuO$_2$ thin films with a 3 nm Au cap. (a,b) X-ray diffraction of films deposited on four different substrates, (a) (110) oriented RuO$_2$ and (b) (001) oriented RuO$_2$ with their respective substrate peaks labeled ($\ast$). (c) SQUID magnetometry of the film deposited on Al$_2$O$_3$ and a bare substrate comparison with magnetization calculated from the measured areas and the nominal thickness of the film plus substrate. Magnetometry data acquired at 5 K for a magnetic field applied in-plane on a quartz paddle.
• Figure 2: Electrical characterization of single layer, 1.8 nm thick RuO$_2$ films. (a-d) Temperature dependence of resistivity ($\rho_{xx}$) in zero applied magnetic field. (e-h) Magnetic field dependence of $\rho_{xx}$ at 1.8 K. (i-l) Hall resistivity ($\rho_{xy}$) at 1.8 K. $\rho_{xx}$ and $\rho_{xy}$ are determined through measurements in the van der Pauw orientation on continuous films with an applied current of 100 $\mu$A.
• Figure 3: Point contact Andreev reflection measurements on 60 nm RuO$_2$ samples. (a) Exemplar conductance for the (001)/TiO$_2$ sample with best fit to the BKT model (see text). (b) The polarization, P, as a function of barrier strength, Z. The dashed line represents the linear fit of points from (110)-oriented RuO$_2$ films and the solid line represents the linear fit of points from (001)-oriented RuO$_2$ films.