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

Abstract

Abstract Unraveling the origin of ultrafast demagnetization in multisublattice ferromagnetic materials requires femtosecond x-ray techniques to trace the magnetic moment dynamics on individual elements, but this could not yet be achieved in the hard x-ray regime. We demonstrate here the first ultrafast demagnetization dynamics in the ferromagnetic heavy 5d-transition metal Pt using circularly-polarized hard x-rays at an x-ray free electron laser (XFEL). The decay time of laser-induced demagnetization of L1 0 -FePt is determined to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Pt</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.61</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">ps</mml:mi> </mml:math> using time-resolved x-ray magnetic circular dichroism at the Pt L 3 edge, whereas magneto-optical Kerr measurements indicate the decay time for the total magnetization as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">total</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>0.1</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>ps</mml:mi> </mml:math> . A transient magnetic state with a photo-modulated ratio of the 3d and 5d magnetic moments is demonstrated for pump–probe delays larger than 1 ps. We explain this distinct photo-modulated transient magnetic state by the induced-moment behavior of the Pt atom and the x-ray probing depth. Our findings pave the way for the future use of XFELs to disentangle atomic spin dynamics contributions.