Spontaneous in-plane anomalous Hall response observed in a ferromagnetic oxide

Recent observation of anomalous Hall effect (AHE) induced by magnetic field or spin magnetization lying in the Hall deflection plane has sparked interest in diverse mechanisms for inducing the Hall vector component perpendicular to the applied magnetic field. Such off-diagonal coupling, which is strictly constrained by symmetry of the system, provides new degrees of freedom for engineering Hall responses. However, spontaneous response as extensively studied for out-of-plane AHE remains unexplored. Here we elucidate in-plane AHE in a typical ferromagnetic oxide SrRuO$_3$. The (111)-orientated ultrathin films with in-plane easy axes of spin magnetization exhibit spontaneous AHE at zero field, which is intrinsically coupled to the in-plane spin magnetization and controllable via its direction. Systematic measurements by varying azimuthal and polar field angles further reveal complex Hall responses shaped by higher-order terms allowed by trigonal distortion of the films. Our findings highlight versatile and controllable in-plane Hall responses with out-of-plane orbital ferromagnetism.