Experimental observation of transition from type I to type II ultrafast demagnetization dynamics in chemically disordered Fe60Al40 thin film, driven by laser fluence

Abstract

Understanding of the laser-induced ultrafast demagnetization dynamics is one of the most challenging and hot topics in magnetism research due to its potential applications in magnetic storage devices and the field of spintronics. Recently, a laser-induced switching of ferromagnetism, driven by a disorder–order transition on FeAl thin films, has been experimentally demonstrated. The switching of ferromagnetic ordering by ultrafast laser pulses in FeAl thin films may open new possible applications of this material such as magnetic data storage and manipulation. Since the speed of the magnetic switching of magnetic states in thin films is one of the critical parameters for these applications, here we used time resolved magneto-optical Kerr measurements to investigate the demagnetization dynamics of Fe[Formula: see text]Al[Formula: see text] thin films at room temperature. We have for the first time observed a clear transition from one-step dynamics (type I) to two-step (type II) dynamics in the same material by increasing pump laser fluence. This experimental observation may give a strong confirmation that the ultrafast demagnetization process can be treated as a thermal process and is driven by the difference between temperatures of the electron and spin systems.

Laser‐Induced Positional and Chemical Lattice Reordering Generating Ferromagnetism

“…The magnetic flux is oriented parallel to the substrate due to the high shape anisotropy of the prepared lamella. The well-defined magnetic phase contrast observed in Figure 4c,g is consistent with the ferromagnetic behavior measured in previous studies, [7,8] thereby evidencing the predominant presence of chemical disorder within the irradiated area in both cases. The reference electron holography measurement of the unirradiated area reveals no detectable magnetic flux lines as demonstrated in Section S.V, Supporting Information.…”

“…In chemical order, atoms of a given species are assigned to specific sites of the ordered lattice, whereby chemical disorder denotes the occupation of the lattice points by a random sequence of atomic species. [ 6–8 ] This description results in a hierarchy through which chemical order can only exist if there is positional order. Subtle changes in both positional and chemical order have been demonstrated to yield significant modifications in the material's optical properties, for example, reflectance, as well as in the magnetic behavior, such as the exchange coupling, and can result in the onset of ferromagnetism.…”

“…Its chemical order consists of pure Fe planes separated by Al‐rich planes, resulting in a Fe–Fe nearest‐neighbor coordination of 2.7, corresponding to a paramagnetic state. [ 7,8,16,17 ] In contrast, the second alloy, Fe 60 V 40 , exhibits positional and chemical disorder in its initial state (Figure 1, right). [ 10,18–21 ] By modifying the chemical order in both alloys through energy deposition via penetrating ions [ 8,16,22–24 ] or laser irradiation, [ 7,9,25 ] a formation into a positionally ordered but chemically disordered state, referred to as A2 phase, can be induced (Figure 1, irradiated areas).…”

“…[ 7,8,16,17 ] In contrast, the second alloy, Fe 60 V 40 , exhibits positional and chemical disorder in its initial state (Figure 1, right). [ 10,18–21 ] By modifying the chemical order in both alloys through energy deposition via penetrating ions [ 8,16,22–24 ] or laser irradiation, [ 7,9,25 ] a formation into a positionally ordered but chemically disordered state, referred to as A2 phase, can be induced (Figure 1, irradiated areas). [ 8,9,16 ] This phase transition is accompanied by an increased nearest‐neighbor coordination and the onset of localized ferromagnetism in the positionally ordered and chemically disordered regions as indicated by the white arrows in Figure 1.…”