THz spintronics of antiferromagnetic FeRh/Pt

Abstract

Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is an example of a material for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation of antiferromagnetic FeRh induces ferromagnetism and the dynamics of the emergent order on the characteristic time-scale of the exchange interaction has long been a topic of intense debates. The first-order magnetic phase transition in FeRh has attracted considerable attention in material science, magnetocalorics, magnetic recording and spintronics.Aiming to reveal femto-, pico and nanosecond kinetics of the emergent ferromagnetism in FeRh, we employed the concepts of spintronic THz emitter and THz emission spectroscopy. In particular, analyzing THz emission from a FeRh/Pt bilayer excited by a single laser pulse, we reveal the main spin-dependent sources of the laser-induced THz emission [1]. We argue that stroboscopic measurements of first-order phase transitions are hampered by thermodynamically allowed co-existence of two competing phases. In particular, the stroboscopic measurements of THz emission from FeRh may result in the absence of temperature hysteresis and domination of the measured signal not by laser-induced magnetization dynamics in the antiferromagnetic phase, but by the dynamics triggeed in co-existing ferromagnetic nuclei.To minimize the influence of the latter we employed double-pump THz emission spectroscopy. We performed a systematic analysis of the emitted radiation from FeRh/Pt bilayer and revealed the experimental conditions for ultrafast magnetometry with the best signal-to-noise ratio. We show that although femtosecond laser pulse launches ultrafast spin dynamics in the antiferromagnet, nuclei generated by femtosecond laser pulse during the first 20 ps of their lifetime do not emit THz, insusceptible to the external magnetic field and, strictly speaking, have not established ferromagnetic spin order yet [2]. **