arXiv subjects

E. E. Fullerton

Publications and source records attributed to E. E. Fullerton.

At least 19 recordsLinked to original sources

arXiv2024

Imaging ultrafast electronic domain fluctuations with X-ray speckle visibility

Speckle patterns manifesting from the interaction of coherent X-rays with matter offer a glimpse into the dynamics of nanoscale domains that underpin many emergent phenomena in quantum materials. While the dynamics of the average structure can be followed with time-resolved X-ray diffraction, the ultrafast evolution of local structures in nonequilibrium conditions have thus far eluded detection due to experimental limitations, such as insufficient X-ray coherent flux. Here we demonstrate a nonequilibrium speckle visibility experiment using a split-and-delay setup at an X-ray free-electron laser. Photoinduced electronic domain fluctuations of the magnetic model material Fe$_{3}$O$_{4}$ reveal changes of the trimeron network configuration due to charge dynamics that exhibit liquid-like fluctuations, analogous to a supercooled liquid phase. This suggests that ultrafast dynamics of electronic heterogeneities under optical stimuli are fundamentally different from thermally-driven ones.

cond-mat.str-el↗

arXiv2023

Ultrafast Emergence of Ferromagnetism in Antiferromagnetic FeRh in High Magnetic Fields

Ultrafast heating of FeRh by a femtosecond laser pulse launches a magneto-structural phase transition from an antiferromagnetic to a ferromagnetic state. Aiming to reveal the ultrafast kinetics of this transition, we studied magnetization dynamics with the help of the magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400 K) and magnetic fields (up to 25 T). Three different types of ultrafast magnetization dynamics were observed and, using a numerically calculated H-T phase diagram, the differences were explained by different initial states of FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that ultrafast heating of FeRh in the canted antiferromagnetic phase launches practically the fastest possible emergence of magnetization in this material. The magnetization emerges on a time scale of 2 ps, which corresponds to the earlier reported time-scale of the structural changes during the phase transition.

cond-mat.mtrl-sci↗

arXiv2021

Micro-structuration effects on local magneto-transport in [Co/Pd]IrMn thin films

We measured the local magneto-transport (MT) signal with an out-of-plane magnetic field, including magneto-resistance (MR) and Extraordinary Hall effect (EHE), in exchange-biased [Co/Pd]IrMn thin multilayers that are micro-structured with a 100 micron window. We found that when measured locally around the window, the MT signal deviate from the expected behavior. We studied possible causes, including film micro-structuration, electrical contact geometry as well as magnetic field tilt from the normal direction. These MT measurements were carried using the Van-der-Pauw method, with a set a four microscopic contacts directly surrounding the window, and a set of four contacts positioned several millimeters away from the window. We found that tilting the magnetic field direction with respect to the normal does not significantly affect the MT signal, whereas the positioning and geometry of the contacts seem to highly affect the MT signal. When the contacts are directly surrounding the window, the shape of the EHE signal is drastically deformed, suggesting that the electron path is disturbed by the presence of the window and the proximity of the electric contacts. If, on the other hand, the contacts are sufficiently far apart, the MT signal is not significantly affected by the presence of the window. Furthermore, the deformed EHE signal measured on the inner contacts can be modeled as a mix of the EHE and MR signals measured on the outer contacts.

cond-mat.mtrl-sci↗

arXiv2021

Non-equilibrium self-assembly of spin-wave solitons in FePt nanoparticles

Magnetic nanoparticles such as FePt in the L10-phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magneto-crystalline anisotropy. This in turn reduces the magnetic exchange length to just a few nanometers enabling magnetic structures to be induced within the nanoparticles. Here we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved X-ray diffraction and micromagnetic modeling that spin-wave solitons of sub-10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin-precession frequency of 0.1 THz positions this system as a platform to develop miniature devices capable of filling the THz gap.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast kinetics of the antiferromagnetic-ferromagnetic phase transition in FeRh

Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is a test specimen for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation can induce ferromagnetism in antiferromagnetic FeRh, but the mechanism and dynamics of this transition are topics of intense debates. Employing double-pump THz emission spectroscopy has enabled us to dramatically increase the temporal detection window of THz emission probes of transient states without sacrificing any loss of resolution or sensitivity. It allows us to study the kinetics of emergent ferromagnetism from the femtosecond up to the nanosecond timescales in FeRh/Pt bilayers. Our results strongly suggest a latency period between the initial pump-excitation and the emission of THz radiation by ferromagnetic nuclei.

cond-mat.mtrl-sci↗

arXiv2019

THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing

Femtosecond laser excitation of a Co/Pt bilayer results in the efficient emission of picosecond THz pulses. Two known mechanisms for generating THz emission are spin-polarized currents through a Co/Pt interface, resulting in helicity-independent electric currents in the Pt layer due to the inverse spin-Hall effect and helicity-dependent electric currents at the Co/Pt interface due to the inverse spin-orbit torque effect. Here we explore how roughness, crystal structure and intermixing at the Co/Pt interface affect the efficiency of the THz emission. In particular, we varied the roughness of the interface, in the range of 0.1-0.4 nm, by tuning the deposition pressure conditions during the fabrication of the Co/Pt bilayers. To control the intermixing at the Co/Pt interface a 1-2 nm thick CoxPt1-x alloy spacer layer was introduced with various compositions of Co and Pt. Finally, the crystal structure of Co was varied from face centered cubic to hexagonal close packed. Our study shows that the roughness of the interface is of crucial importance for the efficiency of helicity-dependent THz emission induced by femtosecond laser pulses. However, it is puzzling that intermixing while strongly enhancing the helicity-independent THz emission had no effect on the helicity-dependent THz emission which is suppressed and similar to the smooth interfaces.

cond-mat.mtrl-sci↗

arXiv2018

Single-shot multi-level all-optical magnetization switching mediated by spin-polarized hot electron transport

All-optical ultrafast magnetization switching in magnetic material thin film without the assistance of an applied external magnetic field is being explored for future ultrafast and energy-efficient magnetic storage and memories. It has been shown that femto-second light pulses induce magnetization reversal in a large variety of magnetic materials. However, so far, only GdFeCo-based ferrimagnetic thin films exhibit magnetization switching via a single optical pulse. Here we demonstrate the single-pulse switching of Co/Pt multilayers within a magnetic spin-valve structure ([Co/Pt] / Cu / GdFeCo) and further show that the four possible magnetic configurations of the spin valve can be accessed using a sequence of single femto-second light pulses. Our experimental study reveals that the magnetization final state of the ferromagnetic [Co/Pt] layer is determined by spin-polarized hot electrons generated by the light pulse interactions with the GdFeCo layer. This work provides a new approach to deterministically switch ferromagnetic layers and a pathway to engineering materials for opto-magnetic multi-bit recording.

cond-mat.mtrl-sci↗

arXiv2017

Room-temperature observation and current control of skyrmions in Pt/Co/Os/Pt thin films

We report the observation of room-temperature magnetic skyrmions in Pt/Co/Os/Pt thin-film heterostructures and their response to electric currents. The magnetic properties are extremely sensitive to inserting thin Os layers between the Co-Pt interface resulting in reduced saturation magnetization, magnetic anisotropy and Curie temperature. The observed skyrmions exist in a narrow temperature, applied-field and layer-thickness range near the spin-reorientation transition from perpendicular to in-plane magnetic anisotropy. The skyrmions have an average diameter of 2.3μm and transport measurements demonstrate these features can be displaced with current densities as low as J = 2x10^4 A/cm^2 and display a skyrmion Hall effect.

cond-mat.mes-hall↗

arXiv2017

Beyond a phenomenological description of magnetostriction

We use ultrafast x-ray and electron diffraction to disentangle spin-lattice coupling of granular FePt in the time domain. The reduced dimensionality of single-crystalline FePt nanoparticles leads to strong coupling of magnetic order and a highly anisotropic three-dimensional lattice motion characterized by a- and b-axis expansion and c-axis contraction. The resulting increase of the FePt lattice tetragonality, the key quantity determining the energy barrier between opposite FePt magnetization orientations, persists for tens of picoseconds. These results suggest a novel approach to laser-assisted magnetic switching in future data storage applications.

cond-mat.mtrl-sci↗

arXiv2017

Helicity-dependent all-optical domain wall motion in ferromagnetic thin films

Domain wall displacement in Co/Pt thin films induced by not only fs- but also ps-laser pulses is demonstrated using time-resolved magneto-optical Faraday imaging. We evidence multi-pulse helicity-dependent laser-induced domain wall motion in all-optical switchable Co/Pt multilayers with a laser energy below the switching threshold. Domain wall displacement of about 2 nm per 2- ps pulse is achieved. By investigating separately the effect of linear and circular polarization, we reveal that laser-induced domain wall motion results from a complex interplay between pinning, temperature gradient and helicity effect. Then, we explore the microscopic origin of the helicity effect acting on the domain wall. These experimental results enhance the understanding of the mechanism of all-optical switching in ultra-thin ferromagnetic films.

cond-mat.mtrl-sci↗

arXiv2017

Resonant properties of dipole skyrmions in amorphous Fe/Gd multilayers

The dynamic response of dipole skyrmions in Fe/Gd multilayer films is investigated by ferromagnetic resonance measurements and compared to micromagnetic simulations. We detail thickness and temperature dependent studies of the observed modes as well as the effects of magnetic field history on the resonant spectra. Correlation between the modes and the magnetic phase maps constructed from real-space imaging and scattering patterns allows us to conclude the resonant modes arise from local topological features such as dipole skyrmions but does not depend on the collective response of a closed packed lattice of these chiral textures. Using, micromagnetic modeling, we are able to quantitatively reproduce our experimental observations which suggests the existence of localized spin-wave modes that are dependent on the helicity of the dipole skyrmion. We identify four localized spin wave excitations for the skyrmions that are excited under either in-plane or out-of-plane r.f. fields. Lastly we show that dipole skyrmions and non-chiral bubble domains exhibit qualitatively different localized spin wave modes.

cond-mat.mtrl-sci↗

arXiv2017

Photoinduced Enhancement of the Charge Density Wave Amplitude

Symmetry breaking and the emergence of order is one of the most fascinating phenomena in condensed matter physics. It leads to a plethora of intriguing ground states found in antiferromagnets, Mott insulators, superconductors, and density-wave systems. Exploiting states of matter far from equilibrium can provide even more striking routes to symmetry-lowered, ordered states. Here, we demonstrate for the case of elemental chromium that moderate ultrafast photo-excitation can transiently enhance the charge-density-wave (CDW) amplitude by up to 30% above its equilibrium value, while strong excitations lead to an oscillating, large-amplitude CDW state that persists above the equilibrium transition temperature. Both effects result from dynamic electron-phonon interactions, providing an efficient mechanism to selectively transform a broad excitation of the electronic order into a well defined, long-lived coherent lattice vibration. This mechanism may be exploited to transiently enhance order parameters in other systems with coupled degrees of freedom.

cond-mat.str-el↗

arXiv2016

Tailoring magnetic energies to form dipole skyrmions and skyrmion lattices

The interesting physics and potential memory technologies resulting from topologically protected spin textures such as skyrmions, has prompted efforts to discover new material systems that can host these kind of magnetic structures. Here we use the highly tunable magnetic properties of amorphous Fe/Gd multilayer films to explore the magnetic properties that lead to dipole-stabilized skyrmions and skyrmion lattices that form from the competition of dipolar field and exchange energy. Using both real space imaging and reciprocal space scattering techniques we determined the range of material properties and magnetic fields where skyrmions form. Micromagnetic modeling closely matches our observation of small skyrmion features (~50 to 70nm) and suggests these class of skyrmions have a rich domain structure that is Bloch like in the center of the film and more Néel like towards each surface. Our results provide a pathway to engineer the formation and controllability of dipole skyrmion phases in a thin film geometry at different temperatures and magnetic fields.

cond-mat.mtrl-sci↗

arXiv2016

Synthesizing Skyrmion Molecules in Fe-Gd Thin Films

We show that properly engineered amorphous Fe-Gd alloy thin films with perpendicular magnetic anisotropy exhibit room-temperature skyrmion molecules, or a pair of like-polarity, opposite-helicity skyrmions. Magnetic mirror symmetry planes present in the stripe phase, instead of chiral exchange, determine the internal skyrmion structure and the net achirality of the skyrmion phase. Our study shows that stripe domain engineering in amorphous alloy thin films may enable the creation of skyrmion phases with technologically desirable properties.

cond-mat.mtrl-sci↗

arXiv2016

Accumulative magnetic switching of ultra-high-density recording media by circularly polarized light

Manipulation of the magnetization by external energies other than magnetic field, such as spin-polarized current1-4, electric voltage5,6 and circularly polarized light7-11 gives a paradigm shift in magnetic nanodevices. Magnetization control of ferromagnetic materials only by circularly polarized light has received increasing attention both as a fundamental probe of the interactions of light and magnetism but also for future high-density magnetic recording technologies. Here we show that for granular FePt films, designed for ultrahigh-density recording, the optical magnetic switching by circularly polarized light is an accumulative effect from multiple optical pulses. We further show that deterministic switching of high anisotropy materials by the combination of circularly polarized light and modest external magnetic fields, thus revealing a pathway towards technological implementation.

cond-mat.mes-hall↗

arXiv2014

Condensation of collective charge ordering in Chromium

Here we report on the dynamics of the structural order parameter in a chromium film using synchrotron radiation in response to photo-induced ultra-fast excitations. Following transient optical excitations the effective lattice temperature of the film rises close to the Néel temperature and the charge density wave (CDW) amplitude is reduced. Although we expect the electronic charge ordering to vanish shortly after the excitation we observe that the CDW is never completely disrupted, which is revealed by its unmodified period at elevated temperatures. We attribute the persistence of the CDW to the long-lived periodic lattice displacement in chromium. The long-term evolution shows that the CDW revives to its initial strength within 1 ns, which appears to behave in accordance with the temperature dependence in equilibrium. This study highlights the fundamental role of the lattice distortion in charge ordered systems and its impact on the re-condensation dynamics of the charge ordered state in strongly correlated materials.

cond-mat.str-el↗

arXiv2014

Large exchange-dominated domain wall velocities in antiferromagnetically coupled nanowires

Magnetic nanowires supporting field- and current-driven domain wall motion are envisioned for new methods of information storage and processing. A major obstacle for their practical use is the domainwall velocity, which is traditionally limited due to the Walker breakdown occurring when the forcing field or current reaches a critical threshold value. We show through numerical and analytical modeling that the Walker breakdown limit can be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires. These coupled nanowires allow for giant domain-wall velocities driven by field and/or current via spin transfer torque as compared to conventional nanowires.

cond-mat.mes-hall↗

arXiv2013

Spin-transfer assisted thermally activated switching distributions in perpendicularly magnetized spin valve nanopillars

We present switching field distributions of spin-transfer assisted magnetization reversal in perpendicularly magnetized Co/Ni multilayer spin-valve nanopillars at room temperature. Switching field measurements of the Co/Ni free layer of spin-valve nanopillars with a 50 nm x 300 nm ellipse cross section were conducted as a function of current. The validity of a model that assumes a spin-current dependent effective barrier for thermally activated reversal is tested by measuring switching field distributions under applied direct currents. We show that the switching field distributions deviate significantly from the double exponential shape predicted by the effective barrier model, beginning at applied currents as low as half of the zero field critical current. Barrier heights extracted from switching field distributions for currents below this threshold are a monotonic function of the current. However, the thermally-induced switching model breaks down for currents exceeding the critical threshold.

cond-mat.mes-hall↗