Nonlinear Optical Studies of Neel Order and Magnetic Domain Structure in Antiferromagnetic RuO2*

A challenge in antiferromagnetic (AF) spintronics is the lack of suitable table-top techniques to determine spin orientations and magnetic domain structure. RuO2, a collinear AF, has been recently theoretically predicted to exhibit unexpected properties such as anomalous Hall effect [1] and a linear magneto-optical effect [2] that arise from the fact that opposite magnetic sublattices in RuO2 are connected by a combination of time reversal and rotation operations.  We use a modified Onsager’s relation suitable for AF materials to show that the same symmetry property results in a symmetric second order optical susceptibility tensor. We use optical second harmonic generation (SHG) to study Neel order and spin domains in thin RuO2 films. Our results show that spins are oriented along the c-axis, resulting in magnetic space group P4₂’/mnm’. However, small canting of the spins is seen in thin films. Interestingly, strain is seen to play an important role in the AF domain size. Whereas AF domain sizes in RuO2 grown on TiO2 substrates are as small as few tens of nm, domain sizes in films grown on r-Al2O3 are much larger than a micron. We will discuss results from linear magneto-optical imaging of spin domains in RuO2. [1] Sci. Adv. 6, eaaz8809 (2020) [2] Phys. Rev. B 104, 024401(2021).