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Femtosecond laser excitations of FeRh/Pt bilayers launch an ultrafast pulse of electric photocurrents in the Pt-layer and subsequently result in the emission of electromagnetic radiation in the THz spectral range. Analysis of the THz emission as a function of the polarization of the femtosecond laser pulse, external magnetic field, sample temperature, and sample orientation shows that the photocurrent can emerge due to vertical spin pumping and photo-induced inverse spin-orbit torque at the FeRh/Pt interface. The vertical spin pumping from FeRh into Pt does not depend on the polarization of light and originates from ultrafast laser-induced demagnetization of the ferromagnetic phase of FeRh. The photo-induced inverse spin-orbit torque at the FeRh/Pt interface can be described in terms of a helicity-dependent effect of circularly polarized light on the magnetization of the ferromagnetic FeRh and the subsequent generation of a photocurrent. © 2020 Author(s).

Magnetic imaging based on nitrogen-vacancy (NV) centers in diamond has emerged as a powerful tool for probing magnetic phenomena in fields ranging from biology to physics. A key strength of NV sensing is its local-probe nature, enabling high-resolution spatial images of magnetic stray fields emanating from a sample. However, this local character can also form a drawback for analysing the global properties of a system, such as a phase transition temperature. Here, we address this challenge by using statistical analyses of magnetic-field maps to characterize the first-order temperature-driven metamagnetic phase transition from the antiferromagnetic to the ferromagnetic state in FeRh. After imaging the phase transition and identifying the regimes of nucleation, growth, and coalescence of ferromagnetic domains, we statistically characterize the spatial magnetic-field maps to extract the transition temperature and thermal hysteresis width. By analysing the spatial correlations of the maps and their dependence on an external magnetic field, we investigate the magnetocrystalline anisotropy and detect a reorientation of domain walls across the phase transition. The employed statistical approach can be extended to the study of other magnetic phenomena with NV magnetometry or other sensing techniques., Comment: 11 pages, 4 figures

The spin-phonon interaction in spin density wave (SDW) systems often determines the free energy landscape that drives the evolution of the system. When a passing energy flux, such as photoexcitation, drives a crystalline system far from equilibrium, the resulting lattice displacement generates transient vibrational states. Manipulating intermediate vibrational states in the vicinity of the critical point, where the SDW order parameter changes dramatically, would then allow dynamical control over functional properties. Here we combine double photoexcitation with an X-ray free-electron laser (XFEL) probe to control and detect the lifetime and magnitude of the intermediate vibrational state near the critical point of the SDW in chromium. We apply Landau theory to identify the mechanism of control as a repeated partial quench and sub picosecond recovery of the SDW. Our results showcase the capabilities to influence and monitor quantum states by combining multiple optical photoexcitations with an XFEL probe. They open new avenues for manipulating and researching the behaviour of photoexcited states in charge and spin order systems near the critical point., Precise control of vibrational states coupled to electronic degrees of freedom could enable control over charge or magnetic order in a material. Here, the authors use a double-pulse photoexcitation combined with an X-ray probe to control vibrational states near the critical point of spin density wave in Cr films.

The Dzyaloshinskii-Moriya interaction (DMI) in magnetic systems stabilizes spin textures with preferred chirality, applicable to next-generation memory and computing architectures. In perpendicularly magnetized heavy-metal/ferromagnet films, the interfacial DMI originating from structural inversion asymmetry and strong spin-orbit coupling favors chiral N\'eel-type domain walls (DWs) whose energetics and mobility remain at issue. Here, we characterize a new effect in which domains expand unidirectionally in response to a combination of out-of-plane and in-plane magnetic fields, with the growth direction controlled by the in-plane field strength. These growth directionalities and symmetries with applied fields cannot be understood from static treatments alone. We theoretically demonstrate that perpendicular field torques stabilize steady-state magnetization profiles highly asymmetric in elastic energy, resulting in a dynamic symmetry breaking consistent with the experimental findings. This phenomenon sheds light on the mechanisms governing the dynamics of N\'eel-type DWs and expands the utility of field-driven DW motion to probe and control chiral DWs., Comment: 20 pages, 4 figures, 10 supplementary figures

Magnetocaloric effect (MCE) in an antiferromagnetic holmium (Ho) film is studied near the spin reorientation temperatures. A series of magnetization isotherms from 60 to 150 K around the N\'eel temperature, ${T}_{N}\ensuremath{\approx}130\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ were recorded for both in-plane and out-of-plane magnetic field orientations. A change in entropy, $\mathrm{\ensuremath{\Delta}}{S}_{M}$ of $\text{--}5\phantom{\rule{0.28em}{0ex}}\mathrm{J}/\mathrm{kg}\mathrm{K}$ was found near ${T}_{N}$ for an in-plane field orientation. A large change in $\mathrm{\ensuremath{\Delta}}{S}_{M}$ of \ensuremath{-}11.8 J/kg K due to a fan-helix spin transition at $T=90\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ is observed for an in-plane field orientation. Spin transition at both ${T}_{N}$ and at the fan-helix transition exhibit larger MCE in the in-plane field orientations in comparison with the perpendicular orientation. The value of the refrigerant capacity extracted from the temperature dependence of $\mathrm{\ensuremath{\Delta}}{S}_{M}(T)$ is found to be larger for the ``in- plane'' orientation by a factor of two.

We report on the photoinduced dynamics of the magneto-optical Faraday effect and the transmittivity of ferromagnetic semiconducting phase of EuO. Excitation with 8-fs laser pulses launches significantly different dynamics of the Faraday effect compared to magnetic refraction. It is argued that the effects are dynamic probes of the magnetization and the exchange interaction in the material which have distinctly different dynamics at the sub-100 fs time scale.

In bilayers of ferromagnets and heavy metals, which form the so-called spintronic emitters, the phenomena of ultrafast demagnetization and the inverse spin Hall effect (ISHE) conspire to yield remarkably efficient emission of electric pulses in the THz band. Light-induced demagnetization of the ferromagnet launches a pulse of spin current into the heavy metal, wherein it bifurcates into a radiative charge transient due to the ISHE. The influence of temperature on this combined effect should depend on both the magnetic phase diagram and the microscopic origin of spin Hall conductivity, but its exact dependence remains to be clarified. Here, we experimentally study the temperature dependence of an archetypal spintronic emitter, the Co/Pt bilayer, using electro-optic sampling of the emitted THz pulses in the time domain. The emission amplitude is attenuated with decreasing temperature, consistent with an inverse spin Hall effect in platinum of predominantly intrinsic origin.

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., Comment: 17 pages, 7 figures, 1 table

Time-resolved magneto-optical imaging reveals that the dynamics of the helicity-dependent all-optical switching (HD-AOS) of Co/Pt ferromagnetic multilayers occurs on the time scales from nanoseconds to seconds. We find HD-AOS proceeds by two stages. First, for an optimized laser fluence, the ultrashort laser pulse demagnetizes the film to 25% of the initial magnetization. Subsequent laser pulses aids nucleation of small reversed domains. The observed nucleation is stochastic and independent of the helicity of laser light. At the second stage circularly polarized light breaks the degeneracy between the magnetic domains promoting a preferred direction of domain wall motion. One circular polarization results in a collapse of the reversed magnetic domains. The other polarization causes the growth of reversed magnetic domain from the nucleation sites, via deterministic displacement of the domain wall resulting in magnetization reversal. This mechanism is supported by further imaging studies of deterministic laser-induced displacement of the domain walls when excited by circularly polarized optical pulses.

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., 8 pages, 4 figures

The generation of electrical field signals in the terahertz frequency (THz) range has gained increasing attention in recent years. The use of antiferromagnets (AFM) has been proposed as a possible alternative to generate high frequency signals using spin transfer torque (STT) induced damping compensation. In this work, we simulated a potential mechanism for laser-induced THz signals in the AFM phase of FeRh/Pt bilayer films using micromagnetic model. The FeRh film is modeled as two Fe-sublattices coupled via intra-lattice exchange field, and subjected to a sub-picosecond thermal pulse. A partial canting between the magnetizations of two Fe-sublattices, is observed within the first picosecond after the excitation. This short lived state relaxes abruptly into the initial AFM phase, injecting a spin current into the Pt layer via spin pumping, which will eventually be converted into charge current oscillating at THz frequency.

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., Comment: 16 pages, 3 figures + supplementary information

We estimated the magnetic damping constant α of Co2FeAl (CFA) Heusler alloy films of different thicknesses with an MgO capping layer by means of time-resolved magneto-optical Kerr effect and ferromagnetic resonance measurements. CFA films with thicknesses of 1.2 nm and below exhibited perpendicular magnetic anisotropy arising from the presence of the interface with MgO. While α increased gradually with decreasing CFA film thickness down to 1.2 nm, it was increased substantially when the thickness was reduced further to 1.0 nm. Based on the microstructure analyses and first-principles calculations, we attributed the origin of the large α in the ultrathin CFA film primarily to the Al deficiency in the CFA layer, which caused an increase in the density of states and thereby in the scatterings of their spins.

Combining femtosecond transmission measurements with picosecond time-resolved photo-emission electron microscopy, both using x-ray magnetic circular dichroism, new insights into the all-optical magnetization switching mechanism in GdFe based rare-earth transition metal ferrimagnetic alloys is provided, with emphasis on the role played by the magnetization compensation temperature TM of the alloy.

The goal of this work is to define conditions for the most efficient ultrafast optical control of magnetism. Results show that tuning the composition of the GdFeCo alloys towards the magnetization compensation point as well as reducing the sizes of structures allowing one to reduce the energy of the laser pulse required for ultrafast demagnetization or magnetization reversal.

The ability of a femtosecond laser pulse to manipulate and reverse the magnetization in a ferrimagnetic Gd24Fe66.5Co9.5 thin film was studied experimentally as a function of temperature. For a fixed energy of the laser pulse, the dynamics of magnetization showed different behavior depending on whether the sample temperature was below or above the magnetization compensation point (T M ). The conditions for full ultrafast demagnetization and magnetization reversal were easily achieved below T M , while the same laser excitation caused just 50% demagnetization above T M . This interesting change in magnetization dynamics is qualitatively explained in terms of effective changes in the magnitudes of magnetizations of atomic sublattices.

We present an experimental study of all-optical helicity-dependent switching (AO-HDS) of ferromagnetic Pt/Co/Pt heterostructures with perpendicular magnetic anisotropy. The sample is patterned into a Hall cross and the AO-HDS is measured via the anomalous Hall effect. This all-electrical probing of the magnetization during AO-HDS enables a statistical quantification of the switching ratio for different laser parameters, such as the threshold power to achieve AO-HDS and the exposure time needed to reach complete switching at a given laser power. We find that the AO-HDS is a cumulative process, a certain number of optical pulses is needed to obtain a full and reproducible helicity-dependent switching. The deterministic switching of the ferromagnetic Pt/Co/Pt Hall cross provides a full “opto-spintronic device,” where the remanent magnetization can be all-optically and reproducibly written and erased without the need of an external magnetic field.

Laser-induced ultrafast demagnetization in ferrimagnetic Gd${}_{x}$Fe${}_{100\ensuremath{-}x\ensuremath{-}y}$Co${}_{y}$ thin films was studied experimentally as a function of Gd concentration ($x=18$, 22, 24, 30%, and $y\ensuremath{\approx}9$--10%), pump fluence, and sample temperature. The results showed that the conditions for full demagnetization at the ultrafast time scale in Gd${}_{x}$Fe${}_{100\ensuremath{-}x\ensuremath{-}y}$Co${}_{y}$ thin metal films are easily achieved below the magnetization compensation point (${T}_{M}$) and, furthermore, when the ratio between Gd and Fe concentrations is not too large. Consequently, the ultrafast demagnetization strongly depends on the initial temperature of these alloys compared to their ${T}_{M}$. These results provide further insight into the unconventional ultrafast dynamics of multisublattice metallic magnets.

Employing magnetization-sensitive microscopy techniques, we address the light-induced magnetization dynamics of patterned samples of rare-earth transition metal alloys. We experimentally find that smaller structures require a lower energy density to undergo all-optical switching of the magnetization. With the aid of simulations we explain this reduction in terms of enhanced light absorption by interference of the light within the structure. This results in a decrease of about 60% in the energy densities required for all-optical switching compared with those in continuous thin films of the same alloy. Moreover, we envisage that an energy lower than 10 fJ should be sufficient to switch a 20 × 20 nm 2 structure.

We investigate the spin dynamics driven by terahertz magnetic fields in epitaxial thin films of cobalt in its three crystalline phases. The terahertz magnetic field generates a torque on the magnetization which causes it to precess for about 1 ps, with a sub-picosecond temporal lag from the driving force. Then, the magnetization undergoes natural damped THz oscillations at a frequency characteristic of the crystalline phase. We describe the experimental observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the results from the relativistic theory of magnetic inertia, we find that the angular momentum relaxation time $\eta$ is the only material parameter needed to describe all the experimental evidence. Our experiments suggest a proportionality between $\eta$ and the strength of the magneto-crystalline anisotropy., Comment: 6 pages, 4 figures, Supplemental Material