Movement of magnetic domain walls induced by single femtosecond laser pulses

Abstract

International audienceWe present a microscopic investigation of how the magnetic domain structure in ultrathin films changes after direct excitation by single ultrashort laser pulses. Using photoelectron emission microscopy in combination with x-ray magnetic circular dichroism in the resonant absorption of soft x rays, we find that individual laser pulses of ≈60 fs duration and a central wavelength of 800 nm lead to clear changes in the domain structure of a Co layer of three atomic monolayers thickness in an epitaxial Co/Cu/Ni trilayer on a Cu(001) single-crystal substrate. A relatively small enhancement of the sample base temperature by 40 K is sufficient to lower the threshold of laser fluence for domain wall motion by about a factor of two. Pump-probe measurements with a laser fluence just below this threshold indicate that the laser-induced demagnetization of the sample is far from complete in these experiments. Although the domain wall motion appears similar to thermal domain wall fluctuations, quantitatively it cannot be explained by pure thermal activation of domain wall motion by the transient rise of sample temperature after the laser pulse, but it is likely to be triggered by a laser-induced depinning of domain walls

Steering of magnetic domain walls by single ultrashort laser pulses

“…The motion of domain walls after excitation by laser pulses observed here is similar to the stochastic domain-wall motion observed in Co/Cu/Ni trilayers [11], except that now the domain-wall motion occurs always into one direction, guided by a gradient in the laser pulse and/or the external magnetic field. The laser-induced domain-wall motion had been explained by a thermal motion of the domain wall after depinning by the laser pulse, for example by thermal excitation during the transient rise of sample temperature or by transient spin currents acting on the domain wall via spin torque [11]. We assume that in the present experiment the laser pulse also depins the domain wall, which then moves in the predetermined direction to the next pinning site deep enough to pin the wall.…”

“…Laser-induced domain-wall motion is then observed after individual laser pulses, similar to the result presented in Ref. [11], while the direction of domain-wall motion is defined by the direction of the external magnetic field. corresponds to the maximum of the negative gradient of the laser pulse intensity, represented in Fig.…”

“…In addition to the depinning and stochastic thermal domain-wall motion observed previously [11], here a force pushing the domain wall in a defined direction, away from the hotter to colder areas, has been identified experimentally. We first discuss spin-polarized charge currents as possible source for this directed domain-wall motion, which due to spin transfer torque could exert a force on a magnetic domain wall [32].…”

“…This allows to use ultrashort laser pulses for writing or processing magnetic information. As demonstrated by some of us previously, ultrashort laser pulses are also able to move magnetic domain walls, even if their duration is much shorter than the time the domain wall needs to travel [11].…”

“…A stochastic back-and-forth motion of domain walls over several hundreds of nanometers could be triggered by ultrashort laser pulses [11,12], which has been attributed to laserpulse-induced depinning of domain walls and successive thermal domain-wall motion [11].…”