Probing ultrafast spin dynamics with high-harmonic magnetic circular dichroism spectroscopy

Abstract

Magnetic circular dichroism in the extreme ultraviolet (XUV) spectral range is a powerful technique for element-specific probing of magnetization in multicomponent magnetic alloys and multilayers. We combine a high-harmonic generation source with a λ/4 phase shifter to obtain circularly polarized XUV femtosecond pulses for ultrafast magnetization studies. We report on simultaneously measured resonant magnetic circular dichroism (MCD) of Co and Ni at their respective M 2,3 edges and of Pt at its O edge, originating from interface magnetism. We present a time-resolved MCD absorption measurement of a thin magnetic Pt/Co/Pt film, showing simultaneous demagnetization of Co and Pt on a femtosecond time scale.

Ultrafast demagnetization of Pt magnetic moment in L10-FePt probed by magnetic circular dichroism at a hard x-ray free electron laser

“…Since FePt is considered as one of the most promising materials for high-density magnetic recording, its demagnetization behavior under laser-excitation has drawn attention recently [3,45,57]. The elementdependent demagnetization of CoPt, NiPd, and FePt alloys have been studied recently with trMOKE in the EUV regime (energies of 50− 70 eV) [19][20][21]. Hofherr et al [21] observed for FePt similar demagnetization times on both Fe and Pt, in contrast to our measurements that show a disproportionate demagnetization.…”

“…The EUV range has also been used to investigate recently the spin dynamics of composite ferromagnetic films as e.g. CoPt, NiPd, and FePt [19][20][21]. These studies indicated a small or almost no difference in the demagnetization behavior of the ferromagnetic element (Co, Fe, Ni) and the nonmagnetic element (Pd, Pt), but the signals at the Pt O edge or Pd N edge were not sufficiently large to determine a precise demagnetization mechanism.…”

Element Specificity of Transient Extreme Ultraviolet Magnetic Dichroism

“…Femtosecond pulses in the same spectral range were generated via high harmonic generation in neon, resulting in discrete harmonic emission peaks separated by 2 ω = 2×1.55 eV. Here, photons are circularly polarized by a 4-mirror phase shifter [22,28] and energetically dispersed after transmission through the magnetized sample. In both measurements we record the transmitted intensity (I ± ) of circularly polarized photons through the magnetic sample for its two magnetization directions (±).…”

“…This can be clearly seen for the Co peak at 60 eV where Pt O 23 -edges contributes significantly and for the Pt O 23 -edge where transitions from Co states contribute significantly. This raises the question of whether the dynamics as traced by the intensity of the peak at 60 eV and 54 eV can corresponds to the dynamics of the Co and Pt local moment respectively, as is generally assumed to be the case [15,[29][30][31][32][33][34].…”

Multi-color imaging of magnetic Co/Pt heterostructures

“…Furthermore, it is noteworthy that the Pt N 6 resonance is below the Al L-edge, which allows the use of Al metallic filters against visible and infrared radiation for time resolved, optical pump-XUV probe studies using free electron laser or high harmonic radiation 17,23 . Because the induced magnetization of Pt is known to be confined to a few atomic layers at the Co/Pt boundary, 19,24 envisioned time resolved experiments will hence not only track the lateral spatial magnetization profiles after ultrafast laser excitation, but will also give a detailed view on the physics of interface magnetism 21 …”