Speaker
Description
The temperature plays an essential role in many novel and exciting phenomena such as domain wall motion under thermal gradients, temperature-assisted magnetization switching, ultra-fast magnetization dynamics, etc. In standard micromagnetics temperature is included as the white noise represented by random fields, however, this approach is valid for temperatures far from the Curie temperature only. At high temperatures the micromagnetics based on the Landau-Lifshitz-Bloch (LLB) equation which includes longitudinal magnetization dynamics has been developed in recent years [1,2]. Typically, the influence of temperature on the magnetization dynamics is studied. However, there is an inverse effect when the magnetization dynamics produces temperature change, for example in the magnetocaloric effect or during the heating under an ac applied field. A promising application in this respect is the magnetic hyperthermia treatment for cancer. In the modeling of the temperature rise during the ac-field application, the released heat is usually evaluated in the global sense from the hysteresis cycle area, which corresponds to the whole thermodynamic ensemble. However, this does not allow to describe, for example, the timescale of the temperature rise or the local individual heating around one nanoparticle, aspects that may play a key role in the development of accurate and safe biomedical protocols. As a different example, in the domain wall dynamics one can expect large local magnetization changes (especially if the domain wall has high velocity as is the case of antiferromagnets). These can lead to a local dynamical temperature rise.
Recently, we have developed a self-consistent micromagnetic dynamical approach for both magnetization and temperature [3]. The approach consists in the simultaneous solution of the (quantum) LLB micromagnetic equation coupled to the equation for the temperature dynamics. The latter equation is derived from the self-consistent quantum mechanical treatment of the spin-phonon Hamiltonian and the density matrix approach. A simpler derivation considers simple thermodynamical reciprocity relations.
In this talk we will discuss the main features of the novel micromagnetic approach. Not too close to the Curie temperature and in the absence of precessional effects, the global temperature rise coincides with that of the quasi-static hysteresis area evaluation. The temperature rise exists only for non-reversible processes, i.e. when the local magnetization torque is zero, consequently the magnetisation precession always produces some temperature change. Another source of the temperature change is the presence of the longitudinal relaxation term, active either in short timescale or close to the Curie temperature. Finally, we will show some examples and possible multiscale approaches of these phenomena based on novel classical spin-lattice models [4]. These new models might constitute a step forward towards more realistic modeling of many interesting phenomena where the magnetic and temperature dynamics are relevant, like heat-assisted magnetic recording, ultrafast magnetism, magnetic refrigeration, magnetic hyperthermia or spincaloritronics.
[1] N. Kazantseva, et al Phys. Rev. B 77, 184428 (2008).
[2] J. Mendil et al, Sci. Rep. 4, 3980 (2014).
[3] P. Nieves, D. Serantes, O. Chubykalo-Fesenko, Phys. Rev. B 94 (2016) 014409.
[4] J. Tranchida et al., Journal of Computational Physics, 372 (2018) 406–425.
