Thermal or nonthermal? That is the question for ultrafast spin switching in GdFeCo

• Published in: Journal of physics. Condensed matter Vol. 25; no. 36; p. 366002
• Main Authors: Zhang, G P, George, Thomas F
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 11.09.2013; Institute of Physics
• Subjects: Cobalt - chemistry; Condensed matter; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Criteria; Critical-point effects, specific heats, short-range order; Exact sciences and technology; Femtosecond; Ferromagnetism; Gadolinium - chemistry; Helicity; Iron; Iron - chemistry; Lasers; Magnetic Phenomena; Magnetic properties and materials; Numerical simulation studies; Order disorder; Physics; Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.); Temperature; Time Factors; Femtosecond; Radiation effects; Magnetic ordering; Two-level systems; Thermalization; Ferromagnetic materials; Iron alloys; Fermi Dirac distribution; Switching; Helicity; Gadolinium alloys; Laser radiation; Ferromagnetism; Magnetism; Cobalt alloys; Transition element alloys; Ultrafast process
• DOI: 10.1088/0953-8984/25/36/366002
• ISSN: 0953-8984

GdFeCo is among the most interesting magnets for producing laser-induced femtosecond magnetism, where light can switch its spin moment from one direction to another. This paper aims to set a criterion for the thermal/nonthermal mechanism: we propose to use the Fermi-Dirac distribution function as a reliable criterion. A precise value for the thermalization time is needed, and through a two-level model, we show that since there is no direct connection between the laser helicity and the definition of thermal/nonthermal processes, the helicity is a poor criterion for differentiating a thermal from a nonthermal process. In addition, we propose a four-site model system (Gd2Fe2) for investigating the transient ferromagnetic ordering between Gd and Fe ions. We find that states of two different kinds can allow such an ordering. One state is a pure ferromagnetic state with ferromagnetic ordering among all the ions, and the other is the short-ranged ferromagnetic ordering of a pair of Gd and Fe ions.