The nature of non-equilibrium ultrafast demagnetization in ferromagnetic nickel

• It has long been known that ferromagnets undergo a phase transition from ferromagnetic to paramagnetic at the Curie temperature, associated with critical phenomena such as a divergence in the heat capacity. A ferromagnet can also be transiently demagnetized by heating it with an ultrafast laser pulse. However, to date the connection between out-of-equilibrium and equilibrium phase transitions was not known, nor how fast the out-of-equilibrium phase transitions can proceed. In this work, by combining time- and angle-resolved photoemission (Tr-ARPES) with time-resolved transverse magneto-optical Kerr (Tr-TMOKE) spectroscopies, we show that the same critical behavior also governs the ultrafast magnetic phase transition in nickel. This is evidenced by several observations. First, we observe a divergence of the transient heat capacity of the electron spin system preceding material demagnetization. Second, when the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: the spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, while demagnetization and the collapse of the exchange splitting occur on much longer timescales. Third, we find that the transient electron temperature alone dictates the magnetic response. By comparing results obtained from different methods, we show that the critical behaviors are essential for fully explaining the fluence-dependent magnetization dynamics measured using magneto-optical spectroscopy.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

NATURVETENSKAP  -- Fysik -- Atom- och molekylfysik och optik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Atom and Molecular Physics and Optics (hsv//eng)

Nyckelord

Ultrafast demagnetization

High-harmonic generation

Time-resolved ARPES

Time-resolved TMOKE

Publikations- och innehållstyp

ref (ämneskategori)

kon (ämneskategori)