Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

Abstract

Diffraction is used to determine the spatial and temporal evolution of the magnetic disordering. We observe a magnetic melt front that grows from the substrate interface into the film, at a speed that suggests electronically driven propagation.Light control and ultrafast phase front propagation at hetero-interfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices. 3/25! ! !In transition metal oxides, rearrangements in electronic and magnetic properties can be triggered by the application of magnetic [5] and electric fields [6], or pressure [7,8].Switching has also been demonstrated in these materials using femtosecond optical excitation, at near-visible [9,10,11,12,13,14,15], mid-infrared [16,17,18,19,20] edge. The bandwidth of the X-ray pulses was reduced to below 1 eV by a grating monochromator. Diffracted X-rays were detected as function of the time delay relative to the mid-infrared excitation pulses. An avalanche photodiode enabled pulse-to-pulse normalization of the diffracted to the incident light intensity. where the latter is limited by the jitter between the FEL and the optical laser. We compare these dynamics to the time needed for the film to become metallic, as measured by the transient reflectivity in the 1−5 THz range induced by the same mid-infrared excitation (green dots in Fig. 1(d)). These two similar timescales, which reflect only average changes over the whole film, suggest an intimate connection between the insulator-to-metal transition and the melting of magnetic order.In Figure 2, we plot the transient θ-2θ scans for the (1/4 1/4 1/4) diffraction peak, sensitive to the out-of-plane antiferromagnetic ordering. In equilibrium, i.e. at negative time delay, a narrow diffraction peak and Laue oscillations are observed, attesting to the presence of magnetic order across the entire 30-nm film height, with sharp magnetic boundaries.Figure 2 further shows that a significant peak broadening and a suppression of the Laue oscillations accompany the strong photo-induced reduction in peak intensity. The broadening of the diffraction peak implies that the excitation melts the magnetic order only over a fraction of the film along the sample growth direction. Secondly, the suppression of the Laue oscillations indicates that the boundary between the ordered and disordered regions of the film is not sharp.We also find that throughout these dynamics the in-plane correlation length, as measured by transverse rocking curves (θ scans), remains unchanged (see Supplementary 6/25! ! ! Information). Hence, the dynamics discussed here are one dimensional, evolving along the sample growth direction.!The spatial distribution of the magnetic order at a time delay τ was analyzed quantitatively with the following expression for kinematic diffractionHere, the magnetic profile is represented by the space-and time-dependent structure factor F(z,τ), where ! = 4! sin ! ! is the magnitude of the scattering wave vector (with θ th...

Multiple Supersonic Phase Fronts Launched at a Complex-Oxide Heterointerface

“…4 that the IMT and the emergence of itinerant charges may be driving rearrangements also in the magnetic and structural properties in the nickelate film. This analysis substantiates our earlier proposal of a phase transition driven by charge redistribution [8]. Furthermore, we can safely exclude a propagating acoustic (strain) wave as a driving force for the observed dynamics.…”

“…Ref. 8), and the structural phase front propagating at about 4·10 3 m/s (black data points, extracted from off-resonant x-ray diffraction presented in Fig. 1), equal to the NdNiO 3 speed of sound (grey solid line).…”

Controlling competing orders via nonequilibrium acoustic phonons: Emergence of anisotropic effective electronic temperature

“…Experimentally, a nonequilibrium distribution of acoustic phonons, which we denote by n B (ω q ), can be generated by ultrafast strain of interfaces [6,7,[28][29][30]. A periodic driving of such nonequilibrium phonons is necessary to establish a steadystate non-thermal phononic distribution, otherwise the phonons would relax back to equilibrium.…”

“…Although acoustic phonons do not couple directly to light, they can be excited by rapidly applying lattice strain via an interface or via piezoelectric forces [7,[28][29][30]. The small energies required to excite acoustic phonons, as compared to optical phonons, generate less heating.…”

Element Selective Probe of the Ultra-Fast Magnetic Response to an Element Selective Excitation in Fe-Ni Compounds Using a Two-Color FEL Source

“…The combination of an element selective excitation with an element selective probe offers new paths for unraveling the fundamental mechanisms that drive magnetization loss by separately addressing the weight of possible contributions in this complex problem. Important classes of magnetic materials, in this respect, are ferrites [8,9] and transition metal (TM) rare-earth (RE) compounds [10][11][12][13]. Beyond their applicative interest, the (O-mediated) TM-TM coupling in magnetic oxides and the (5d-mediated) 3d-4f coupling in TM-RE compounds make these materials ideal candidates for resonant-pump resonant-probe magnetization dynamics studies.…”