Magnetic excitations and femtomagnetism of FeRh: A first-principles study

Abstract

The paper is partly motivated by recent pump-probe experiments with ultrashort laser pulses on antiferromagnetic FeRh that have shown the generation of magnetization within a subpicosecond time scale. On the other hand, the physical mechanism of the thermal antiferromagnetic-ferromagnetic (AFM-FM) phase transition in FeRh, known for many decades, remains a topic of controversial discussions. The selection of the magnetic degrees of freedom as well as the treatment of the magnetic excited states differ strongly in recent models by different authors. We report a density functional theory (DFT) investigation of FeRh. For the study of excited states, DFT calculations with constraints imposed on the directions and values of the atomic moments are employed. We show that the formation of the Rh moment as a consequence of the AFM-FM phase transition cannot be described within the Stoner picture. Instead, an implicit spin splitting of the Rh states takes place in the AFM phase, resulting in the intra-atomic spin polarization of the Rh atoms. This property is a consequence of the strong hybridization between Rh and Fe states. The Fe-Rh hybridization is an important factor in the physics of FeRh. We demonstrate that the ferromagnetic Fe-Rh exchange interaction is robust with respect to the crystal volume variation, whereas the antiferromagnetic Fe-Fe exchange interaction is strongly volume dependent. These different volume dependencies of the competing exchange interactions lead to their strong compensation at certain crystal volume. We perform Monte Carlo simulations and show that the calculated thermodynamics depends on the way the magnetic degrees of freedom are selected. We argue that the excited states resulting from the variation of the value of the Rh moment treated as degree of freedom are important for both the equilibrium thermodynamics of FeRh and the femtomagnetic phenomena in this system. We also study the spin mixing caused by spin-orbit coupling. The obtained value of the Elliott-Yafet spin-mixing parameter is comparable with earlier calculations for the ferromagnetic 3d metals. We draw the conclusion that the Elliott-Yafet mechanism of the angular-momentum transfer between electrons and lattice plays an important role in the femtomagnetic properties of FeRh.

Exchange interactions and local-moment fluctuation corrections in ferromagnets at finite temperatures based on noncollinear density-functional calculations

“…This point of view has been pushed for example in order to understand the antiferromagnetic-to-ferromagnetic transition of FeRh. 57,58 In this section we develop a method of treating such systems, emphasizing the choise of the energetically relevant magnetic degrees of freedom at temperatures up to the critical temperature, as well as the practical implementation of these degrees of freedom to models that can be solved via Monte Carlo simulations. The additional challenge, compared to the theory developed in the previous sections, stems from two facts.…”

Testing spin-flip scattering as a possible mechanism of ultrafast demagnetization in ordered magnetic alloys

“…However, even for a single crystalline material detailed investigations of the dynamic band structure on short time scales are a formidable task and beyond the scope of this paper. Nevertheless the value for the spin-flip rate is always in the range of calculated values [22] for the spin-mixing parameter b 2 , with a sf = p b 2 and p ranging from 1.8 to 4. Furthermore b 2 is calculated to be smaller on the Fe site than on the Rh site, which is also qualitatively reflected in the element selective measurements (Fig 4b).…”

“…Concentrating on theoretically more easily accessible ordered crystalline alloys, can help understanding the origin of the different demagnetization dynamics of the alloy components, by comparing experiments to theoretical calculations. In this letter, we concentrate on an ordered multi-sublattice system, the ordered alloy, FeRh, a single-crystalline material with a well-known electronic band structure [22]. This ordered alloy undergoes a first order phase transition from an antiferromagnetically ordered phase to a ferromagnetically ordered phase at temperatures slightly above room temperature.…”

“…Furthermore the observed element dependent shift in t 0 can be qualitatively described within this theory. The strong hybridization between the Fe and Rh states results in a strong coupling of the Fe and Rh moments [22] and may therefore explain the relatively small differences between the demagnetization traces, in contrast to the larger differences observed in the much weaker coupled 3d-transition metals rare earth alloys [17]. However, the observed dependence of the spin-flip rate a sf with a clear visible minimum as a function of quenching (or fluence) cannot be understood within a rigid band approach and remains an unsolved problem awaiting further theoretical input.…”

Néel Temperature of Antiferromagnets for Phase Transitions Driven by Spin-wave Interactions

“…20 Already in the AFM phase, before the FM state appears, they found that the local magnetic moment in Rh atoms must be remarkably different from zero. More remarkable, as the temperature increases, they show that within the AFM states, Fe local magnetic moments can adopt a non parallel situation in a canted configuration.…”