Ultrafast magnetism as seen by x-rays

Abstract

Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for magnetic recording industry. Exchange interaction is the strongest force in magnetism, being responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically perturbed on an ultrashort timescales pertinent to the characteristic time of the exchange interaction? Here we demonstrate that femtosecond measurements of X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism. In particular, we show that upon femtosecond optical excitation the ultrafast spin reversal of Gd(FeCo) -a material with antiferromagnetic coupling of spins -occurs via a transient ferromagnetic state. The latter one emerges due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction.

Magnetic order dynamics in optically excited multiferroicTbMnO3

“…In other materials, such as CuO or La0.5Sr1.5MnO4, it was observed that pumping with 800 nm light leads to an ultrafast suppression of magnetic order in less than 1 ps and that the magnetic phase transformation is delayed by approximately ¼ of the period of the low lying spin gap excitation [10,12]. Such ultrafast changes in magnetization (less than 1 ps) have also been observed in metallic ferromagnets via all optical pump/probe schemes and optical pump/XMCD (X-ray magnetic circular dichroism) probe experiments [14][15][16].…”

Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys

“…For elemental ferromagnets the emerging dynamics can be probed using conventional magneto-optical methods [1,2]. For magnets composed of several distinct elements, such as ferrimagnetic or ferromagnetic alloys, the individual spin dynamics of the different elements can be probed employing ultrafast excitation in combination with the femtosecond-resolved x-ray magnetic circular dichroism (XMCD) technique [3,4]. An astonishing example of such element-specific ultrafast magnetization dynamics was first measured on ferrimagnetic GdFeCo alloys [5].…”

Thermally induced magnetization switching in Gd/Fe multilayers

“…However, it has been discovered that reduction of the magnetic field pulse durations below about 2-3 ps may result in a stochastic magnetization switching 4 . One of the most intriguing alternatives to magnetic field-induced magnetization switching is making use of a subpicosecond laser pulse [5][6][7][8] . Since the first observation of subpicosecond demagnetization of a Ni film subjected to a 60-fs laser pulse, it has been shown that such a pulse is able to cause ultrafast changes in the magnetic state 9 .…”