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Proceedings Paper

The nature of non-equilibrium ultrafast demagnetization in ferromagnetic nickel
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Paper Abstract

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-ofequilibrium 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

Paper Details

Date Published: 20 December 2019
PDF: 12 pages
Proc. SPIE 11209, Eleventh International Conference on Information Optics and Photonics (CIOP 2019), 1120938 (20 December 2019); doi: 10.1117/12.2548814
Show Author Affiliations
Zhensheng Tao, Fudan Univ. (China)
Univ. of Colorado (United States)
Phoebe Tengdin, Univ. of Colorado (United States)
Wenjing You, Univ. of Colorado (United States)
Cong Chen, Univ. of Colorado (United States)
Xun Shi, Univ. of Colorado (United States)
Dmitriy Zusin, Univ. of Colorado (United States)
Yingchao Zhang, Univ. of Colorado (United States)
Christian Gentry, Univ. of Colorado (United States)
Adam Blonsky, Univ. of Colorado (United States)
Mark Keller, National Institute of Standards and Technology (United States)
Peter M. Oppeneer, Uppsala Univ. (Sweden)
Henry C. Kapteyn, Univ. of Colorado (United States)
Margaret M. Murnane, Univ. of Colorado (United States)

Published in SPIE Proceedings Vol. 11209:
Eleventh International Conference on Information Optics and Photonics (CIOP 2019)
Hannan Wang, Editor(s)

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