Ultrafast negative thermal expansion driven by spin disorder

Abstract

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...

Heat Transport without Heating?—An Ultrafast X‐Ray Perspective into a Metal Heterostructure

“…[37] The spatiotemporal strain η(z, t) (Figure 3) is used to calculate the X-ray diffraction pattern by dynamical X-ray diffraction theory. [26,30,38] γ S (mJ cm −3 K −2 ) 0.74 [33] 0.10 [33] 1.06 [33] 0.38 [51] -C ph (J cm −3 K −1 ) 2.85 [52] 3.44 3.94 2.33 [51] 1.80 [53] κ 0 e (W m −1 K −1 ) 66 [54] 396 [33] 81 [33,55] 52 κ ph (W m −1 K −1 ) 5 [54] 5 9.6 [55] 5 1 [53] g (PW m −3 K −1 ) 400 [56] 63 [56] 360 [56] 100 - [53] v s (nm −1 ps) 4.2 [57,58] 5.2 [56,90] 6.3 [61] 4.2 5.7 [53] Γ e 2.4 [62] (0.9) 0.9 [62] (1.1) 2.0 [62] 1.3 [62] -Γ ph 3.0 [62] (2.5) 1.7 [62] (2.1) 1.65 [62] 1.5 [62] 0.3 [53] www.afm-journal.de www.advancedsciencenews.com…”

“…Femtosecond pumpprobe experiments using X-rays can yield important additional insights, particularly when probing specific subsystems containing the heat energy. [22][23][24][25][26][27] Recently, ultrafast X-ray diffraction (UXRD) experiments revealed the unconventional heat transport in a bilayer of 5 nm Au and 10 nm Ni. It does not matter…”

Measurement of transient strain induced by two-photon excitation

“…The stress σ on the lattice resulting from the energy density deposited by the pump pulse ρ Q = F a /d is given by the macroscopic Grüneisen parameter via σ = ρ Q [3,46,47,56]. In the one-dimensional (1D) geometry of a homogeneously excited thin film with a cubic symmetry aligned to the sample surface, the strain response perpendicular to the surface is given by η = σ /C 11 , where C 11 is the cubic elastic constant.…”

“…Lattice dynamics is a unifying theme common to all laserexcited condensed matter systems, since their phonon subsystem provides a large bath for energy, entropy, and angular momentum transfer [1][2][3][4]. The ultrafast increase of the energy density in materials drives coherent atomic motion, i.e., sizable picosecond strain pulses [5][6][7], that add to the incoherent thermal expansion.…”

“…Consequently, a rectangular-shaped bipolar strain wave is launched upon NIR excitation and a semiexponential-shaped bipolar strain wave is launched upon NUV excitation [8]. The average strain in the Bi:YIG layer is shaped accordingly [52], which we monitor directly with UXRD [3,8,53]. The different intensity profiles also determine the deposition of heat.…”