Exchange relaxation as a mechanism of the ultrafast reorientation of spins in a two-sublattice ferrimagnet

Abstract

In the exchange approximation, an exact solution is obtained for the sublattice magnetizations evolution in a two-sublattice ferrimagnet. Nonlinear regimes of spin dynamics are found that include both the longitudinal and precessional evolution of the sublattice magnetizations, with the account taken of the exchange relaxation. In particular, those regimes describe the spin switching observed in the GdFeCo alloy under the influence of a femtosecond laser pulse.PACS numbers: 75.10. Hk, Magnetic materials have various applications in modern electronics and informatics, but probably the most important research direction is still the creation of information storage and processing systems. The challenge of designing magnetic devices with ever increasing information density and recording speed requires solving certain fundamental problems of the magnetism dynamics. The possibility to manipulate the magnetization by means of femtosecond laser pulses opens wide opportunities in this direction. This field has been incepted by the work [1], where a fast (within a time shorter than a picosecond) reduction of nickel magnetization after the exposure to a 100 femtosecond laser pulse has been observed, as well as the subsequent relaxation of the magnetization with a characteristic time of the order of picoseconds. The authors explained the initial drop in the magnetization either by an extremely rapid heating of the sample above the Curie point, see review [2], or by spin-dependent super-diffusive electron transfer in the laser-excited metal [3]. Further work in this area followed for various materials, and unexpected and rather unusual effects were discovered. In the ferrimagnetic rare earth and transition metal alloy GdFeCo, a femtosecond pulse lead, in the first stage, to a similar spin reduction (i.e., the reduction of the magnetization of sublattices) as for nickel, but the subsequent evolution turned out to be fundamentally different. Instead of a simple relaxation to the initial value, within about the same time (a few picoseconds), both sublattice magnetizations changed their signs, i.e., a switching of the net magnetic moment took place [4], and during this picosecond-scale evolution there occurred an a priori energetically unfavorable state with parallel sublattice moments. Such a magnetization switching effect is of a threshold type, and is observed only for sufficiently strong pulses. It has been detected in films as well as in microparticles [5] and nanoparticles [6], both for ferromagnets with and without a compensation point [5]. There is also a way of "selective" switching: due to the magnetic dichroism, the absorbed energy of a circularly polarized pulse depends on the direction of the magnetic moment of the particles, and a pulse of certain polarization would only switch the moments of the particles which are in a matching state [7]. All that makes possible to create a purely optically-controlled magnetic memory with a picosecond recording speed.Although an analytical explanation of this effect is highly desi...

Relativistic theory of spin relaxation mechanisms in the Landau-Lifshitz-Gilbert equation of spin dynamics

“…A nonlocal damping mechanism due to spatial dispersion of the exchange field was proposed by Bar'yakhtar on the basis of phenomenological considerations such as symmetry arguments and Onsager's relations [10]. This leads to a modified expression for the damping term in the Landau-Lifshitz-Bar'yakhtar equation which contains the derivative of the exchange field ∇ 2 B xc [10,20]. The existence of such nonlocal damping term can be related to the continuity equation connecting the spin density and spin current; it is important for obtaining the correct asymptotic behavior of spin wave damping at large wavevectors k [20] known for magnetic dielectrics, see [59].…”

“…This leads to a modified expression for the damping term in the Landau-Lifshitz-Bar'yakhtar equation which contains the derivative of the exchange field ∇ 2 B xc [10,20]. The existence of such nonlocal damping term can be related to the continuity equation connecting the spin density and spin current; it is important for obtaining the correct asymptotic behavior of spin wave damping at large wavevectors k [20] known for magnetic dielectrics, see [59]. Such nonlocal damping is important, too, for describing spin current flow in magnetic metallic heterostructures [78].…”

“…Brataas et al proposed a scattering theory formulation [15] of the Gilbert damping which is equivalent to a Kubo linear-response formulation. A different form of the relaxation term caused by spatial dispersion of the exchange interaction-this in contrast to the isotropic medium assumption made in the LL equation-was proposed by Bar'yakhtar and co-workers [10,20,21].…”

Experimental and Theoretical Study of Stripe Magnetic Domain Structure Drift in Iron Garnet Crystals

“…[2] it was suggested that translational motion of domain structure (DS) is caused by various inhomogeneities that lead to di erent conditions on the neighboring DW. Analytical theory of drift was proposed in [3].…”

Relaxation of Strongly Suppressed Modulus of Magnetization Following Ultrafast Demagnetization

“…The LLBar equations are well suited for description of non-uniform states, like magnetic solitons [14,15] and Bloch points [16], and give the explanation of the reversal effects [17,18]. These equations provide an explanation of recent experiments on magnetization recovery in laserpumped Ni-Ru-Fe heterostructures [19], where the importance of the nonlocal character of the magnetization recovery is established [13].…”