Ultrafast Generation of Ferromagnetic Order via a Laser-Induced Phase Transformation in FeRh Thin Films

Abstract

It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations.

Identifying growth mechanisms for laser-induced magnetization in FeRh

“…2͑a͒ the Kerr rotation during the first 700 ps after pump excitation of a 100 nm FeRh film is presented for increasing pump powers. We note that the fast nucleation of ϳ20% of the final magnetic order during the first ps, reported before, 8 is not separately recognizable in this long time scan. A clear threshold behavior is elucidated in the representation of Fig.…”

“…As to the observation of two successive peaks in the data at the highest fluency, it has been argued before 7,8 that such a behavior could be expected because within a complete heating cycle the film passes the transition temperature twice, and at temperatures above T P the magnetization is a monotonously decreasing function. Data of Fig.…”

“…8 Vibrating sample magnetometery ͑VSM͒ displayed a phase transition at 350 K, and a hysteresis of ϳ10 K as usually observed for such films. TRMOKE was performed with 800 nm pump laser pulses ͑ϳ80 fs, 8 MHz rep. rate͒ and time delayed second harmonic probe laser pulses of 400 nm.…”

“…1͑b͒. Laser heating such a film, and thus driving it through its AF to F phase transition, 8 showed a first onset of ferromagnetism within the first ps, which implied the phase transition to be of electronic origin. On a longer time scale a continued growth of F order is witnessed, possibly due to a domain expansion or alignment.…”

“…6 Very recently, two groups independently showed the possibility of generating ferromagnetic order at a sub-ps time scale in FeRh thin films. 7,8 FeRh is a particularly interesting system that displays an antiferromagntic ͑AF͒ phase below, and a ferromagnetic ͑F͒ phase above a phase transition temperature ͑T P ͒ around 370 K, depicted in Fig. 1͑b͒.…”