Cavity Magnon Polaritons and Inverse Spin Hall Effect in Easy-Axis Antiferromagnets

Abstract

Contemporary spintronics, utilizing the electronic spin for information processing and microelectronics, is mostly based on ferromagnetic device architectures. In view of long term perspectives to enable enhanced data processing speeds and downscaling for on-chip information processing, spintronics with antiferromagnets is a promising avenue [1]. Antiferromagnets exhibit the key advantage over ferromagnets that their resonance frequency is enhanced by the exchange coupling of the sublattices, and thus generally in the terahertz regime. In compensated antiferromagnets, the absence of a net moment however strongly impedes simple access to their ultrafast dynamics, especially in thin films, and the development of ultra-fast antiferromagnet-based devices [2].Experimental access to the spin dynamics can be facilitated by spin-to charge conversion mechanisms such as the spin pumping effect [3,4] or by probing the light-matter interaction of the spin system via a coupling to cavity resonator photons [5]. The spin pumping effect generates alternating (AC) and continous (DC) spin currents in an adjacent conductor which can be electrically detected, for instance, by the inverse spin hall effect (ISHE) [6]. On the other hand, light-matter interaction is also at the heart of cavity spintronics which describes the generation and manipulation of cavity-magnon polaritons (CMP) for a general understanding of these interactions and applications for information processing and spintronics. The CMP is the associated quasiparticle to the hybridization of cavity resonator photons and collective spin excitations, i.e. magnon modes and the hallmark for a coherent information transfer in the strong coupling regime [5]. Such coherent information transfer is key for the implementation of microwave-to-optics transducers or quantum-based information processing schemes such as quantum memories [7]. Probing the spin dynamics electrically by spin-to-charge mechanisms or via the light-matter interaction of a strongly coupled photon-magnon system allows to study the physics of spin dynamics from different perspectives as it is at the crossroads of spintronics, magnonics and photonics. In this work, we access the spin dynamics of a collinear (easy axis) antiferromagnetic system by using a bulk chromium oxide (Cr2O3) sample as a model system. We both investigate the hybridization between the spin precession in the low frequency left-handed mode with cavity resonator photons and its temperature dependence via recording the corresponding ISHE voltages.We measure a CMP in Cr2O3 at 150 K and we observe several, small couplings to resonator modes below the spin flop field at 6 T. For instance, for a resonator mode ~39.6 GHz, we find a coupling strength of approximately 50 MHz. For the spin pumping experiment, we find a non-monotonous temperature dependence of the ISHE voltage signal with a peak at 30 K of the low frequency left-handed mode. The opposite signs of the ISHE voltages between the left-handed mode of Cr2O3 and the right-handed mode of ferrimagnetic YIG confirm the mode polarity. We compare our findings to the work of J.Li et al., who studied the temperature dependence of the high frequency right-handed mode of Cr2O3 [8]. We find qualitatively the same temperature dependence towards higher signals with the signal vanishing above 100 K as the net spin current polarization decreases to higher temperatures [8].Hence, we study the spin dynamics of our model system Cr2O3 for GHz frequencies based on electrical detection and light matter interaction. Notably, establishing antiferromagnets in cavity spintronics could bridge frequency regimes from the GHZ to the optical range. Further, the demonstration of a coupling and, eventually, coherent information exchange is a promising way towards quantum information processing schemes with antiferromagnets [9]. **