Controlling the polarity of the transient ferromagneticlike state in ferrimagnets

Abstract

It was recently observed that the two antiferromagnetically coupled sublattices of a rare earth-transition metal ferrimagnet can temporarily align ferromagnetically during femtosecond laser heating, but always with the transition metal aligning in the rare earth direction. This behavior has been attributed to the slower magnetization dynamics of the rare earth sublattice. The aim of this work was to assess how the difference in the speed of the transition metal and rare earth dynamics affects the formation of the transient ferromagnetic-like state and consequently controls its formation. Our investigation was performed using extensive atomistic spin simulations and analytic micromagnetic theory of ferrimagnets, with analysis of a large area of parameter space such as initial temperature, Gd concentration and laser fluence. Surprisingly, we found that at high temperatures, close to the Curie point, the rare earth dynamics become faster than those of the transition metal. Subsequently we show that the transient state can be formed with the opposite polarity, where the rare earth aligns in the transition metal direction. Our findings shed light on the complex behavior of this class of ferrimagnetic materials and highlight an important feature which must be considered, or even exploited, if these materials are to be used in ultrafast magnetic devices.

Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys

“…Initial theoretical comparisons of the element-specific demagnetization in GdFeCo were done recently by Atxitia el al. [10] who obtained a good agreement with experimental observations. However, in this work the exchange integrals as well as the magnetic atomic moments were taken from phenomenological considerations contrary to the present work where all the parameters are obtained from first-principles calculations.…”

“…The element-selective technique allowed us moreover to observe the element-specific dynamics of the so-called "all-optical switching" (AOS) [8] in GdFeCo alloys, finding that it unexpectedly proceeds through a transient ferromagneticlike state where the FeCo sublattice magnetization points in the same direction as that of the Gd sublattice before complete reversal [5,9]. Recent theoretical works supported the distinct demagnetization times observed experimentally [10][11][12] and their crucial role on the transient ferromagneticlike state. AOS has been also demonstrated for other rare-earth transition-metal ferrimagnetic alloys as TbFe [13], TbCo [14], TbFeCo [15], DyCo [16], HoFeCo [16], synthetic ferrimagnets [16][17][18], and very recently in the hard-magnetic ferromagnet FePt [19].…”

“…The case of GdFeCo ferrimagnetic alloys is paradigmatic since this was the first material where the so-called ultrafast all-optical switching (AOS) of the magnetization has been observed [8]. The element-dependent magnetization dynamics in GdFeCo alloys has meanwhile been thoroughly studied [9,10,12,[20][21][22][23]. From a fundamental viewpoint, however, it is also important to understand the element-specific magnetization dynamics in multielement ferromagnetic alloys.…”

“…[27] for single-element ferromagnets) has been suggested by Koopmans et al [28] on the basis of the ratio between the magnetic moment and the Curie temperature μ/T C . Since for ferromagnetic alloys each [10] have theoretically estimated the demagnetization times in GdFeCo alloys proposing that the demagnetization times scale with the ratio of the magnetic moment to the exchange energy of each element and a similar relation is expected for ferromagnetic alloys. The demagnetization times of Fe and Ni in Py were also theoretically investigated by Schellekens and Koopmans in Ref.…”

Ultrafast relaxation rates and reversal time in disordered ferrimagnets

“…The temperature where Γ T T = Γ RR is called the coupling temperature T co . 15 For relatively low temperatures, T /T C < T co,1 and for temperatures T /T C > T co,2 , we have Γ T R , Γ RT ≪ Γ T T , Γ RR . Therefore, the longitudinal relaxation time of each sublattice in these temperature regions can be written as τ || ν ≃ 1/Γ νν .…”