Studies of electron diffusion in photo-excited Ni using time-resolved X-ray diffraction

Persson, Anders; Jarnac, Amélie; Wang, Xiaocui; Enquist, Henrik; Jurgilaitis, A.; Larsson, Jörgen

doi:10.1063/1.4967470

Cited by 4 publications

(3 citation statements)

References 29 publications

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“…In this paragraph we highlight the potential of UXRD for deriving the spatial form of the stress driving the observed strain. [38,39] In Fig. 3a) we analyze the UXRD data recorded for the Nb layer in the PM state of Ho, zooming into the pronounced compression signal of the Nb layer.…”

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Ultrafast negative thermal expansion driven by spin disorder

et al. 2019

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We measure the transient strain profile in a nanoscale multilayer system composed of Yttrium, Holmium and Niobium after laser excitation using ultrafast X-ray diffraction. The strain propagation through each layer is determined by transient changes of the material-specific Bragg angles. We experimentally derive the exponentially decreasing stress profile driving the strain wave and show that it closely matches the optical penetration depth. Below the Neel temperature of Ho, the optical excitation triggers negative thermal expansion, which is induced by a quasi-instantaneous contractive stress, and a second contractive stress contribution rising on a 12 ps timescale. These two timescales have recently been measured for the spin-disordering in Ho [Rettig et al, PRL 116, 257202 (2016)]. As a consequence we observe an unconventional bipolar strain pulse with an inverted sign travelling through the heterostructure.In most of the research on ultrafast magnetism the lattice was only considered as an angular momentum sink.[1-3] Ultrafast effects on the lattice induced by demagnetization have been discussed surprisingly rarely.[4-7] Time-resolved magneto-optical Kerr measurements and optical picosecond ultrasonics are the workhorse for many researchers. [1,2,[8][9][10][11][12] Specialized techniques allow for assigning timescales to specific electronic processes and orbitals or bands. [3,[13][14][15][16]] This is particularly relevant in the magnetic rare earths, where the exchange interaction between the localized 4f spin and orbital magnetic moments is mediated by the itinerant 5d6s conduction electrons via the RKKY interaction. [17,18] Ultrafast electron diffraction (UED) or ultrafast x-ray diffraction (UXRD) experiments that directly observe the transient lattice strain induced by ultrafast demagnetization have been discussed only sporadically. [4,5,[19][20][21] Several ultrafast diffraction studies on the transition metals Ni and Fe [22][23][24] discuss the strain waves excited by electron and phonon stresses σ e and σ ph , and theory predicts relevant electron-phonon (e-ph) coupling constants [25] even with mode-specificity.[26] Very recently granular FePt films were studied by UED. The rapid out-of-plane lattice contraction could be convincingly ascribed to changes of the free energy of the spin system.[5] The macroscopic Grüneisen coefficients (Gc) Γ e,ph describe the efficiency for generating stress σ e,ph = Γ e,ph ρ Q e,ph by a heat energy density ρ Q e,ph .[27] If Γ e = Γ ph , ultrafast diffraction allows inferring the time-dependent σ(t) from the observed transient strain ε(t).[22, 23, 28] Hooke's law relates ε linearly to σ and hence to the energy densities ρ Q e,ph deposited in each subsystem. This concept was extended to stress resulting from spin-excitations in Ni and Fe [29,30] but the experimental verification remained ambiguous. [22][23][24] Thermodynamic analysis affirms that the Gcs generally measure how entropy S depends on strain ε.[31] The phenomenon of negative thermal expansion (NTE) generally oc...

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Ultrafast negative thermal expansion driven by spin disorder

et al. 2019

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“…In particular, due to the relatively large heat capacity of the lattice, an accurate description of electron-lattice equilibration is important. Nonetheless, literature values for the electron-phonon coupling parameter G ep of nickel vary by more than an order of magnitude [1,3,26,30,[34][35][36][37][38][39][40][41]. So far, experimental studies of ultrafast lattice heating in nickel have mostly employed optical techniques [36,37,42], which are sensitive to both the electronic and the lattice responses.…”

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confidence: 99%

“…The most direct technique to study the lattice is diffraction, but there are only few studies that measured the lattice heating directly with time-resolved diffraction [43,44]. In addition, often the electron-phonon coupling was de-duced from observables without considering the energy cost of demagnetization [3,34,36,37,41]. The large spread in literature values for G ep can manifest itself in an imprecise description of the electron-lattice equilibration and makes different models less comparable.…”

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confidence: 99%