Determination of sub-ps lattice dynamics in FeRh thin films

Abstract

Understanding the ultrashort time scale structural dynamics of the FeRh metamagnetic phase transition is a key element in developing a complete explanation of the mechanism driving the evolution from an antiferromagnetic to ferromagnetic state. Using an X-ray free electron laser we determine, with sub-ps time resolution, the time evolution of the (–101) lattice diffraction peak following excitation using a 35 fs laser pulse. The dynamics at higher laser fluence indicates the existence of a transient lattice state distinct from the high temperature ferromagnetic phase. By extracting the lattice temperature and comparing it with values obtained in a quasi-static diffraction measurement, we estimate the electron–phonon coupling in FeRh thin films as a function of laser excitation fluence. A model is presented which demonstrates that the transient state is paramagnetic and can be reached by a subset of the phonon bands. A complete description of the FeRh structural dynamics requires consideration of coupling strength variation across the phonon frequencies.

Finite size effects on the metamagnetic phase transition in a thick B2 FeRh nanocluster film

“…A substantial distribution in nanograin size, porosity, and potential dispersion around the equiatomic composition could contribute to the observed broad thermal hysteresis. Interestingly, despite the nanogranular microstructure of the presently characterised thick film, the magnetic field dependence of the transition temperature closely resembles that observed in continuous FeRh films 23 and wires, 13 i.e. nearly linear with a slope of −9.1 K/T as seen in Fig.…”

“…16 A sharp transition has been observed in a 50 nm thick FeRh (001) film grown over MgO (001), closing the loop in ∼40 K with similar shapes of the two branches, 18 while Loving et al have reported a loop over ∼300 K with a coolingdown branch significantly wider than the heating-up branch. 22 Nevertheless, a linear correlation between the transition temperature and the applied external field 13,23 is consistently observed, indicating that despite differences between the material microstructure and system dimensions, the intrinsic properties of FeRh B2 alloys dominate this first order metamagnetic transition. Despite a lot of studies on thin films with low defect density, 19,20,24,25 the study of single B2 FeRh nanomagnets is rather rare.…”

“…Near the point of equiatomic composition, FeRh bulk alloys in the CsCl-type (B2) chemically ordered phase present a metamagnetic transition from the antiferromagnetic (AF) state at low temperature, with μ Fe = 3.3μ B and no observable magnetic moment for Rh, to the ferromagnetic (F) state, with μ Fe = 3.2μ B and μ Rh = 0.9μ B , above a critical temperature T t of 370 K. This transition is accompanied by a 0.5% lattice parameter expansion. [1][2][3][4] The competition between both magnetic orders of FeRh occurs relatively close to room temperature (RT) and holds great potential for applications in magnetocaloric refrigeration due to its large negative magnetocaloric effect, [5][6][7][8][9][10] in ultrafast spintronics by heating FeRh with a laser to generate a spin current due to the uncompensated magnetic moment, [11][12][13] as well as in antiferromagnetic spintronics 14 and recording data. 15 Although the magnetic properties of FeRh nanostructures have been extensively studied for more than 25 years, the mechanism behind the AF to F transition is still widely debated and not well understood due to the strong correlation between structural, electronic and magnetic order in such nanoalloys.…”

Stabilising transient ferromagnetic states in nanopatterned FeRh with shape-induced anisotropy

“…The electrical measurements show that changing the applied magnetic field from OOP to IP, results in a change in the T T of 6 K (figure 6(a)), which is not sufficient to explain the strong divergence in behaviour seen when comparing axial (figure 3) and tangential (figure 4). Based on the finite-element modelling simulations and previous studies [33], a laser pulse with a fluence of 4.0 mJ cm −2 , is expected to induce a temperature rise of > 150 K. As such, the excited sample will transition to the fully FM state, irrespective of field direction. However, the demagnetisation field strength as a function of angle offers a more plausible argument.…”

“…Despite the large body of work on FeRh thin films, the effect of lateral confinement on the transition dynamics remains under-reported. The dynamics of the FeRh phase transition have been explored with probing of the electronic [15], magnetic [31,32], and lattice [33] transformations across a range of timescales from sub-ps [15] up to ns [34,35]. It is known that the AF → FM transition mechanism differs from the reverse transition as a consequence of the different domain sizes (AF domain sizes are <100 nm, while FM domain sizes ≈1 µm) [36][37][38].…”