Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses

Abstract

The demand for ever-increasing density of information storage and speed of manipulation has triggered an intense search for ways to control the magnetization of a medium by means other than magnetic fields. Recent experiments on laser-induced demagnetization and spin reorientation use ultrafast lasers as a means to manipulate magnetization, accessing timescales of a picosecond or less. However, in all these cases the observed magnetic excitation is the result of optical absorption followed by a rapid temperature increase. This thermal origin of spin excitation considerably limits potential applications because the repetition frequency is limited by the cooling time. Here we demonstrate that circularly polarized femtosecond laser pulses can be used to non-thermally excite and coherently control the spin dynamics in magnets by way of the inverse Faraday effect. Such a photomagnetic interaction is instantaneous and is limited in time by the pulse width (approximately 200 fs in our experiment). Our finding thus reveals an alternative mechanism of ultrafast coherent spin control, and offers prospects for applications of ultrafast lasers in magnetic devices.

Photon‐Gated Spin Transistor

“…[13,14] To achieve optical manipulation of magnetic ordering, much work has been done, providing the possibility to manipulate spins in an optical way, such as the optical excitation of magnetic precession, [15] the all-optical magnetization reversal, [16] and the photon-induced phase transition and spin transition, [17][18][19] confirming the optical control of the spins. So far, gate operation of spin-FET features has been realized by electric field [20][21][22] , magnetic field [11] and interface resonance [23,24] , which can be considered as an electrical-field effect in nature.…”

Exploring nanomagnetism with soft X-ray microscopy

“…to understand the time scale on which magnetization can evolve upon excitation e.g. with ultrafast optical pulses [8,9].…”

Conservation of angular momentum and the inverse Faraday effect

“…As with the thermomagnetic methods, the source of angular momentum in the IFE has not been identified despite much theoretical and experimental work. [11][12][13][19][20][21] I show below that the angular momentum is provided by the photons. No other angular momentum reservoir is needed.…”

“…The inverse Faraday effect ͑IFE͒ offers the possibility of nonthermally controlling the magnetization and avoiding this problem. 11,12 The inverse Faraday effect is the generation of an effective magnetic field using nonresonant circularly polarized light. [19][20][21] It is the "inverse" of the well-known Faraday effect-the rotation of linearly polarized light propagating through a medium in a magnetic field 22,23 -because it is derived from the same free energy.…”

“…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.…”