Conservation of angular momentum and the inverse Faraday effect

Abstract

Precession of magnetization via the inverse Faraday effect is investigated with a view of determining the fundamental limit on the precession speed. Such a limit could have important consequences for ultrafast magnetic switching. The angular momentum required for precession is shown to be supplied by the light. This indicates that there is no fundamental obstruction to magnetization reversal on the time scale of a laser pulse provided that a material with a sufficiently strong magneto-optical response can be found. DOI: 10.1103/PhysRevB.79.212412 PACS number͑s͒: 75.60.Jk, 75.40.Gb, 78.20.Ls, 85.70.Li The ability to control magnetization on a subpicosecond time scale is growing in importance as the speed of electronic devices increases. The current generation of technology employs magnetic fields to induce magnetization dynamics. However, due to the difficulty of creating ultrashort magnetic pulses and the recent discovery that magnetic switching by strong magnetic fields can be unpredictable, 1 alternative techniques of controlling magnetization are under intense investigation. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Optical methods 2-13 are particularly promising due to the availability of ultrashort laser pulses. However, the fundamental mechanisms of optically induced demagnetization and magnetic switching are not fully understood. In particular, the question of which reservoir supplies the angular momentum needed for demagnetization remains controversial. This reservoir plays a decisive role in determining the maximum demagnetization speed so resolving this issue is important for technological applications.Most experiments on optical demagnetization and magneto-optical switching employ thermal methods. An optical pulse is absorbed, the electrons are driven far from equilibrium, and the sample is almost completely demagnetized within a few hundred femtoseconds. [3][4][5][6][7][8][9][10] If this is performed in the presence of a magnetic field, the magnetization can be reversed. [2][3][4][5] Some estimates show that thermal demagnetization occurs too rapidly for phonon processes to be relevant and it has been suggested that angular momentum is transferred between the spin and orbital components of the electrons. 9 On the other hand, it is possible that the nonequilibrium electrons experience a spin-phonon interaction that is much stronger than usual. In this case, the phonons could provide the angular momentum. 8,10 Transfer of angular momentum by the absorption of photons has also been considered. 9,13,18 Theoretical arguments based on the number of photons absorbed 9 and experiments using circularly polarized light with nickel 18 indicate that the photon angular momentum is irrelevant, although experiments with GdFeCo yielded the opposite conclusion. 13 Thermomagnetic switching is associated with an increase in temperature, so devices employing these methods will suffer a significant cooling time before new information can be written to them. The inverse Faraday effect ͑IFE͒ offers the p...

Theoretical investigation of the inverse Faraday effect via a stimulated Raman scattering process

“…One of the open questions is the evolution of the magnetic momentum of a medium excited by a laser pulse. 18,21,24 It cannot be answered without the knowledge of the laserinduced transitions, which lead to the change of the magnetic state of a system in the inverse Faraday effect experiments. In order to get a detailed insight into such transitions, we study the stimulated Raman scattering process, which has been suggested to be responsible for this effect.…”

“…[6][7][8][9][10][11][12][13][14] It was shown that such laser pulses act as an effective magnetic field in oxidic materials, which are weak ferromagnets [6][7][8][9][10] and even compensated antiferromagnets 12 and paramagnets. 13 However, the mechanisms of laser-induced magnetization dynamics are still poorly understood in spite of much experimental [6][7][8][9][10][11][12][13][14][15][16][17][18] and theoretical [19][20][21][22][23][24][25][26][27] effort. One of the open questions is the evolution of the magnetic momentum of a medium excited by a laser pulse.…”

Manipulation of ferromagnets via the spin-selective optical Stark effect

“…8,11,12 Several theoretical mechanisms have been proposed, such as the optical Barnett effect or the inverse Einstein-de Haas effect, 13 light-induced circular currents in the collisionless limit, 14,15 the impulsive stimulated Ramanlike process, 8 and photonic angular momentum transfer via deflection of the scattered photons. 16 A dissipative IFE under THz irradiation has been computed for dirty metals with extrinsic spin-orbit interaction. 17,18 Also experimentally the situation is not clear.…”

Ab InitioTheory of Coherent Laser-Induced Magnetization in Metals

“…The IFE is an optomagnetic counterpart of the magneto-optical Faraday effect, that is, the circularly polarized laser light imparts a magnetization in the material which exerts a torque on the pre-existing magnetization and assists the magnetization switching. However, although various models for the IFE have been proposed [11][12][13][14][15][16][17] there does as yet not exist any knowledge as to how the induced magnetization, or optomagnetic field arises, and even less is known about the materials dependence of the IFE. As materials specific theory is lacking it is neither known for which materials large effects are predicted nor how the IFE could be optimized to trigger reversal with minimal laser power.…”