Ultrafast magnetism

This research field has roots in the applictions for magnetic data processing and storage. The coupling of spin excitations to electrons and phonons is a formidable challenge to experiment and theoretical modeling. We focus on understanding the coupling of phonons and spin excitations in both directions: Coherent and incoherent phonons (strain and heat) can be used to control magnetization dynamics (spin waves) via inverse magnetostriction and changes of the anisotropy fields. Directly exciting spins or heating them optically via the electronic absorption can induce considerable spin disorder, which drives local stress on the lattice. This is particularly rewarding in cases where the stress driven by spin-entropy leads to negative thermal expansion. Our specialty is to combine ultrafast x-ray difffraction (UXRD) and transient magneto-optical Kerr effect (MOKE) experiments in order so selectively probe the spin and lattcie systems.

The investigated thin film as well as nanostructured heterostructures are composed of materials ranging from Ni, FePt and rare earths (Ho, Dy, Gd, TbFe2) via magnetic perovskite oxides such as SrRuO3 and LaSrMnO3 to iron garnets (Bi:YIG).

Interactions in solids
Photo: Alexander von Reppert

Related publications

Schematic of the laser pump--x-ray probe experiment on a thin SRO film and measured transient SRO strain below and above the magnetic phase transition.
Image: Maximilian Mattern

Mattern M., Pudell J.-E., Laskin G., von Reppert A., and Bargheer M.

Analysis of the temperature- and fluence-dependent magnetic stress in laser-excited SrRuO3

Structural Dynamics 8, 024302 (2021).

We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Grüneisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime.

Schematic of the laser pump--x-ray probe experiment on a thin SRO film and measured transient SRO strain below and above the magnetic phase transition.
Image: Maximilian Mattern

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Sketch of multilayer sample where laser pulses excite and probe the triggered lattice and magnetization dynamics
Image: Marwan Deb/Steffen Zeuschner

Deb M., Popova E., Zeuschner S. P., Hehn M., Keller N., Mangin S., Malinowski G., and Bargheer M.

Generation of spin waves via spin-phonon interaction in a buried dielectric thin film

Physical Review B 103, 024411 (2021).

In this paper, we investigate the magnetic, optical, and lattice responses of a Pt/Cu/Bi1Y2Fe5O12/Gd3Ga5O12 heterostructure to femtosecond laser excitation of the opaque Pt/Cu metallic bilayer. The electronic excitation generates coherent and incoherent phonons, which trigger high-frequency standing spin waves (SWs) in the dielectric Bi1Y2Fe5O12 layer via a phonon-induced change of magnetic anisotropy. We find that the incoherent phonons (heat) can induce a fast (<1ps) and slow (>1000ps) decrease of the magnetic order by different spin-phonon interaction scenarios. These results open perspectives for generating high-frequency SWs in buried magnetic garnets.

Sketch of multilayer sample where laser pulses excite and probe the triggered lattice and magnetization dynamics
Image: Marwan Deb/Steffen Zeuschner

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Sketch of the time-resolved x-ray diffraction experiment on nanogranular FePt samples
Image: Alexander von Reppert

Reppert A. v., Willig L., Pudell J.-E., Zeuschner S. P., Sellge G., Ganss F., Hellwig O., Arregi J. A., Uhlíř V., Crut A., and Bargheer M.

Spin stress contribution to the lattice dynamics of FePt

Science Advances 6, eaba1142 (2020)

Invar-behavior occurring in many magnetic materials has long been of interest to materials science. Here, we show not only invar behavior of a continuous film of FePt but also even negative thermal expansion of FePt nanograins upon equilibrium heating. Yet, both samples exhibit pronounced transient expansion upon laser heating in femtosecond x-ray diffraction experiments. We show that the granular microstructure is essential to support the contractive out-of-plane stresses originating from in-plane expansion via the Poisson effect that add to the uniaxial contractive stress driven by spin disorder. We prove the spin contribution by saturating the magnetic excitations with a first laser pulse and then detecting the purely expansive response to a second pulse. The contractive spin stress is reestablished on the same 100-ps time scale that we observe for the recovery of the ferromagnetic order. Finite-element modeling of the mechanical response of FePt nanosystems confirms the morphology dependence of the dynamics.

Sketch of the time-resolved x-ray diffraction experiment on nanogranular FePt samples
Image: Alexander von Reppert

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UXRD data recorded at various temperatures which evidence the generation of unconventional picosecond strain pulses
Image: Alexander von Reppert

Reppert A. v., Matter M., Pudell J.-E., Zeuschner S. P., Dumesnil K., and Bargheer M.

Unconventional picosecond strain pulses resulting from the saturation of magnetic stress within a photoexcited rare earth layer

Structural Dynamics 7, 024303 (2020).

Optical excitation of spin-ordered rare earth metals triggers a complex response of the crystal lattice since expansive stresses from electron and phonon excitations compete with a contractive stress induced by spin disorder. Using ultrafast x-ray diffraction experiments, we study the layer specific strain response of a dysprosium film within a metallic heterostructure upon femtosecond laser-excitation. The elastic and diffusive transport of energy to an adjacent, non-excited detection layer clearly separates the contributions of strain pulses and thermal excitations in the time domain. We find that energy transfer processes to magnetic excitations significantly modify the observed conventional bipolar strain wave into a unipolar pulse. By modeling the spin system as a saturable energy reservoir that generates substantial contractive stress on ultrafast timescales, we can reproduce the observed strain response and estimate the time- and space-dependent magnetic stress. The saturation of the magnetic stress contribution yields a non-monotonous total stress within the nanolayer, which leads to unconventional picosecond strain pulses.

UXRD data recorded at various temperatures which evidence the generation of unconventional picosecond strain pulses
Image: Alexander von Reppert

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Static and time-resolved MOKE data on continuous and granular FePt thin film samples
Image: Lisa Willig

Willig L., von Reppert A., Deb M., Ganss F., Hellwig O., and Bargheer M.

Finite-size effects in ultrafast remagnetization dynamics of FePt

Physical Review B 100, 224408 (2019).

We investigate the ultrafast magnetization dynamics of FePt in the L10 phase after an optical heating pulse, as used in heat-assisted magnetic recording. We compare continuous and nano-granular thin films and emphasize the impact of the finite size on the remagnetization dynamics. The remagnetization speeds up significantly with increasing external magnetic field only for the continuous film, where domain-wall motion governs the dynamics. The ultrafast remagnetization dynamics in the continuous film are only dominated by heat transport in the regime of high magnetic fields, whereas the timescale required for cooling is prevalent in the granular film for all magnetic field strengths. These findings highlight the necessary conditions for studying the intrinsic heat transport properties in magnetic materials.

Static and time-resolved MOKE data on continuous and granular FePt thin film samples
Image: Lisa Willig

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Schematic representation of standing spin waves in Bi:YIG and their respective damping coefficients as determined from MOKE experiments
Image: Marwan Deb

Deb M., Popova E., Hehn M., Keller N., Petit-Watelot S., Bargheer M., Mangin S., Malinowski G.

Damping of Standing Spin Waves in Bismuth-Substituted Yttrium Iron Garnet as Seen via the Time-Resolved Magneto-Optical Kerr Effect

Physical Review Applied 12, 044006 (2019).

We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.

Schematic representation of standing spin waves in Bi:YIG and their respective damping coefficients as determined from MOKE experiments
Image: Marwan Deb

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MOKE data on standing spin waves in iron garnet at various external magnetic fields
Image: Marwan Deb

Deb M., Popova E., Hehn M., Keller N., Petit-Watelot S., Bargheer M., Mangin S., and Malinowski G.

Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets

Physical Review Letters 123, 027202 (2019).

We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (∼33  mT) by a spin dynamics with a frequency up to 15 GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics.

MOKE data on standing spin waves in iron garnet at various external magnetic fields
Image: Marwan Deb

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UXRD data with extraction of Bragg peak shifts of individual layers in a giant magnetostriction thin film sample
Image: Steffen Zeuschner

Zeuschner, S. P., Parpiiev T., Pezeril T., Hillion A., Dumesnil K., Anane A., Pudell J.-E., Willig L., Rössle M., Herzog M., Reppert A. v., and Bargheer M.

Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction

Structural Dynamics 6, 024302 (2019).

We combine ultrafast X-ray diffraction (UXRD) and time-resolved Magneto-Optical Kerr Effect (MOKE) measurements to monitor the strain pulses in laser-excited TbFe2/Nb heterostructures. Spatial separation of the Nb detection layer from the laser excitation region allows for a background-free characterization of the laser-generated strain pulses. We clearly observe symmetric bipolar strain pulses if the excited TbFe2 surface terminates the sample and a decomposition of the strain wavepacket into an asymmetric bipolar and a unipolar pulse, if a SiO2 glass capping layer covers the excited TbFe2 layer. The inverse magnetostriction of the temporally separated unipolar strain pulses in this sample leads to a MOKE signal that linearly depends on the strain pulse amplitude measured through UXRD. Linear chain model simulations accurately predict the timing and shape of UXRD and MOKE signals that are caused by the strain reflections from multiple interfaces in the heterostructure.

UXRD data with extraction of Bragg peak shifts of individual layers in a giant magnetostriction thin film sample
Image: Steffen Zeuschner

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Spatiotemporal lattice dynamics in Holmium and corresponding transient Bragg peak shifts in its paramagnetic and antiferromagnetic phases
Image: Jan-Etienne Pudell

Pudell J.-E., Reppert A. v., Schick D., Zamponi F., Rössle M., Herzog M., Zabel H., and Bargheer M.

Ultrafast negative thermal expansion driven by spin disorder

Physical Review B 99, 094304 (2019).

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 in 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 Néel 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
increasing on a 12-ps timescale. These two timescales were recently measured for the spin disordering in Ho
[Rettig et al., Phys. Rev. Lett. 116, 257202 (2016)]. As a consequence, we observe an unconventional bipolar
strain pulse with an inverted sign traveling through the heterostructure.

Spatiotemporal lattice dynamics in Holmium and corresponding transient Bragg peak shifts in its paramagnetic and antiferromagnetic phases
Image: Jan-Etienne Pudell

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The different transient strain dynamics in continuous and granular FePt thin films (top) are connected to the their different morphological constraints (bottom).
Image: Alexander von Reppert

Reppert A. v., Willig L., Pudell J.-E., Rössle M., Leitenberger W., Herzog M., Ganss F., Hellwig O., and Bargheer M.

Ultrafast laser generated strain in granular and continuous FePt thin films

Applied Physics Letters 113, 123101 (2019).

We employ ultrafast X-ray diffraction to compare the lattice dynamics of laser-excited continuous and granular FePt films on MgO (100) substrates. Contrary to recent results on free-standing granular films, we observe in both cases a pronounced and long-lasting out-of-plane expansion. We attribute this discrepancy to the in-plane expansion, which is suppressed by symmetry in continuous films. Granular films on substrates are less constrained and already show a reduced out-of-plane contraction. Via the Poisson effect, out-of-plane contractions drive in-plane expansion and vice versa. Consistently, the granular film exhibits a short-lived out-of-plane contraction driven by ultrafast demagnetization which is followed by a reduced and delayed expansion. From the acoustic reflections of the observed strain waves at the film-substrate interface, we extract a 13% reduction of the elastic constants in thin 10 nm FePt films compared to bulk-like samples.

The different transient strain dynamics in continuous and granular FePt thin films (top) are connected to the their different morphological constraints (bottom).
Image: Alexander von Reppert

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