Miina Leiviskä
Spin Hall magnetoresistance (SMR) is a magnetoresistive effect present in bilayers comprising of a magnet (here insulating) and a heavy metal [1]. It relies on the concerted action of spin Hall effect, inverse spin Hall effect, and dissipation of spin accumulation at the interface. The dissipation of the spin accumulation occurs via various channels, including spin-transfer torque and magnon-mediated spin current transfer. Altermagnets are a new class of collinear compensated magnets with a unique anisotropic spin ordering with characteristic spin- degenerate nodes and alternating even-parity spin polarization that breaks the time-reversal symmetry [2]. In this work, we present a set of SMR measurements in heterostructures of Pt and an insulating and ferroelectric altermagnet candidates Ba2CoGe2O7 [3] and Sr2CoSi2O7. We will first detail the fabrication of devices for SMR experiments on bulk crystals and then discuss characteristic SMR features of the systems under study. We show that in both systems the SMR ratio is unexpectedly large and shows anisotropy depending on in which crystal direction the current is applied. We systematically rule out device-to-device variations, magnetic domains, and magnetocrystalline anisotropy as the main origin of this current-direction anisotropy of the SMR ratio, and also address the possible effects of ferroelectric polarization on the SMR. Finally, we will suggest mechanisms through which the crystal symmetries that characterize the altermagnetic phase of Ba2CoGe2O7 and Sr2CoSi2O7 could give rise to the observed anisotropy.
References
[1] H. Nakayama, M. Althammer, Y.-T. Chen, et al. Phys. Rev. Lett. 110, 206601 (2013).
[2] L. Šmejkal, J. Sinova, and T. Jungwirth. Phys. Rev. X 12, 031042 (2022).
[3] M. Leiviskä, R. Firouzmandi, K. Ahn et al. Phys. Rev. Mat. 9, 084403 (2025)