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Exchange interactions determine the correlations between microscopic spins in magnetic materials. Probing the dynamics of these spin correlations on ultrashort length and time scales is, however rather challenging, since it requires simultaneously high spatial and high temporal resolution. Recent experimental demonstrations of laser-driven two-magnon modes - zone-edge excitations in antiferromagnets governed by exchange coupling - posed questions about the microscopic nature of the observed spin dynamics, the mechanism underlying its excitation, and their macroscopic manifestation enabling detection. Here, on the basis of a simple microscopic model, we derive the selection rules for cubic systems that describe the polarization of pump and probe pulses required to excite and detect dynamics of nearest-neighbor spin correlations, and can be employed to isolate such dynamics from other magnetic excitations and magneto-optical effects. We show that laser-driven spin correlations contribute to optical anisotropy of the antiferromagnet even in the absence of spin-orbit coupling. In addition, we highlight the role of subleading anisotropy in the spin system and demonstrate that the dynamics of the antiferromagnetic order parameter occurs only in next-to-leading order, determined by the smallness of the magnetic anisotropy as compared to the isotropic exchange interactions in the system. We expect that our results will stimulate and support further studies of magnetic correlations on the shortest length and time scale., 17 pages, 5 figures

In this work, we tackle the problem of the spatially selective optical excitation of spin dynamics in structures with multiple magnetic layers. The 120 fs circularly polarized laser pulses were used to launch magnetization precession in an all-dielectric magneto-photonic crystals (MPC) formed by magnetic layers sandwiched between and inside two magnetic Bragg mirrors. Optical pump-probe experiments reveal magnetization precession triggered via ultrafast inverse Faraday effect with an amplitude strongly dependent on the pump central wavelength: maxima of the amplitude are achieved for the wavelength tuned at the cavity resonance and at the edge of the photonic bandgap. The optical impact on the spins caused by the inverse Faraday effect and spectrum of this effect are found to correlate mostly to the direct Faraday effect. We show that even though the pump laser pulses propagate along the whole structure tuning their wavelength allows localization of a larger spin precession either in the cavity layer or in the Bragg mirror layers selectively. The results pave the way to the ultrafast optical control of magnetization dynamics at a sub-wavelength scale that is vital for modern magneto-photonics and magnonics.

Coherent THz optical lattice and hybridized phonon–magnon modes are triggered by femtosecond laser pulses in the antiferromagnetic van der Waals semiconductor FePS3. The laser-driven lattice and spin dynamics are investigated in a bulk crystal as well as in a 380 nm-thick exfoliated flake as a function of the excitation photon energy, sample temperature and applied magnetic field. The pump-probe magneto-optical measurements reveal that the amplitude of a coherent phonon mode oscillating at 3.2 THz decreases as the sample is heated up to the Néel temperature. This signal eventually vanishes as the phase transition to the paramagnetic phase occurs, thus revealing its connection to the long-range magnetic order. In the presence of an external magnetic field, the optically triggered 3.2 THz phonon hybridizes with a magnon mode, which is utilized to excite the hybridized phonon–magnon mode optically. These findings open a pathway toward the optical control of coherent THz photo–magnonic dynamics in a van der Waals antiferromagnet, which can be scaled down to the 2D limit. published

Contains fulltext : 286742.pdf (Publisher’s version ) (Open Access)

Magnetic tunnel junction (MTJ) is a leading contender for next generation high-density nonvolatile memory technology. Fast and efficient switching of MTJs between different resistance states is a challenging problem, which can be tackled by using an unconventional stimulus-a femtosecond laser pulse. Herein, we report an experimental study of the laser-induced magnetization dynamics in a Co

Picosecond strain pulses are a versatile tool for investigation of mechanical properties of meso- and nano-scale objects with high temporal and spatial resolutions. Generation of such pulses is traditionally realized via ultrafast laser excitation of a light-to-strain transducer involving thermoelastic, deformation potential, or inverse piezoelectric effects. These approaches unavoidably lead to heat dissipation and a temperature rise, which can modify delicate specimens, like biological tissues, and ultimately destroy the transducer itself limiting the amplitude of generated picosecond strain. Here we propose a non-thermal mechanism for generating picosecond strain pulses via ultrafast photo-induced first-order phase transitions (PIPTs). We perform experiments on vanadium dioxide VO2 films, which exhibit a first-order PIPT accompanied by a lattice change. We demonstrate that during femtosecond optical excitation of VO2 the PIPT alone contributes to ultrafast expansion of this material as large as 0.45%, which is not accompanied by heat dissipation, and, for excitation density of 8 mJ cm−2, exceeds the contribution from thermoelastic effect by a factor of five. Ultrafast driving of vanadium dioxide can induce a large structural phase transition, which can be used to generate picosecond strain pulses. Here the authors show that the photo-induced phase transition can contribute 0.45% strain without causing undesirable heating.

The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (Fe$_{0.81}$Ga$_{0.19}$) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization $M_S$ and magnetic anisotropy parameters $K$ recover within a nanosecond, and the ratio between $K$ and $M_S$ satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of $M_S$ and $K$ to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale., 8 pages, 7 figures

We present a concept of a tunable optical excitation of spin waves and filtering their spectra in a ferromagnetic film with 180$^{\circ}$ N\'eel domain wall. We show by means of micromagnetic simulation that the fluence of the femtosecond laser pulse and its position with respect to the domain wall affect the frequencies of the excited spin waves, and the presence of the domain wall plays crucial role in control of the spin waves' spectrum. The predicted effects are understood by analyzing the changes of the spin waves' dispersion under the impact of the laser pulse., Comment: 7 pages, 5 figures

We use a picosecond ultrasonic technique to evaluate the photoelastic parameters at the wavelength of 1028 nm in epitaxial vanadium dioxide $({\mathrm{VO}}_{2})$ nanolayers grown on c-cut sapphire substrates. In the experiments, we monitor the picosecond evolution of the reflectivity of ${\mathrm{VO}}_{2}$ in insulating and metallic phases under the impact of a picosecond longitudinal strain pulse injected into the nanolayer from the side of the substrate. We show that in a 145-nm-thick granular nanolayer, the temporal features of the reflectivity are clearly dependent on the phase state of ${\mathrm{VO}}_{2}$, showing the change of the photoelastic parameters upon the insulator-metal transition. Analytical consideration and numerical simulations of the optical response to the picosecond strain pulse show that the temporal evolution of the reflectivity strongly depends on the complex photoelastic parameter. The analysis enables us to obtain the values of the photoelastic parameters in the studied nanolayer in both insulating and metallic phases. We find that for a 145-nm film of ${\mathrm{VO}}_{2}$ in an insulating state the imaginary part of the photoelastic constant is negligible. This means that in the insulating phase the strain does not affect the optical absorption of ${\mathrm{VO}}_{2}$. In the metallic phase, the photoelastic parameter of ${\mathrm{VO}}_{2}$ is found to be similar to that typical for metals with positive real and negative imaginary parts. We further show that the optical response to the strain pulse in the layer consisting of disconnected ${\mathrm{VO}}_{2}$ nanohillocks with an average height of 70 nm is governed by their morphology and is different from what is predicted in the plane ${\mathrm{VO}}_{2}$ films.

Using a magneto-optical pump-probe technique with micrometer spatial resolution, we show that magnetization precession can be launched in individual magnetic domains imprinted in a ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ layer by elastic coupling to ferroelectric domains in a ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_{3}$ substrate. The dependence of the precession parameters on the strength and orientation of the external magnetic field reveals that laser-induced ultrafast partial quenching of the magnetoelastic coupling parameter of ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ by approximately 27% along with 10% ultrafast demagnetization triggers the magnetization precession. The relation between the laser-induced reduction of the magnetoelastic coupling and the demagnetization is approximated by an $n(n+1)/2$ law with $n\ensuremath{\approx}2$. This correspondence confirms the thermal origin of the laser-induced anisotropy change. Based on analysis and modeling of the excited precession, we find signatures of laser-induced precessional switching, which occurs when the magnetic field is applied along the hard magnetization axis and its value is close to the effective magnetoelastic anisotropy field. The precession-excitation process in an individual magnetoelastic domain is found to be unaffected by neighboring domains. This makes laser-induced changes of magnetoelastic anisotropy a promising tool for driving magnetization dynamics and switching in composite multiferroics with spatial selectivity.

The ability to switch ferroics (ferro-, ferri-, antiferromagnets, ferroelectrics, multiferroics) between two stable bit states is one of the keystones of modern data storage technology. Due to many new ideas, originating from fundamental research during the last 50 years, this technology has developed in a breath-taking fashion. Finding a conceptually new way to control ferroic state of a medium with the lowest possible production of heat and at the fastest possible timescale is a new challenge in fundamental condensed matter research. Controlling ferroic state of media by light is a promising approach to this problem. Photomagnetism and photoferroelectricity have long been intriguing and the development of femtosecond laser sources made this approach even more appealing. Laser pulse is the shortest stimulus in contemporary experimental physics of condensed matter. While commercial lasers are able to produce pulses with duration of the order of tens of femtosecond, advanced laser sources can generate intense pulses of light even at the sub-femtosecond timescale. Seeking understanding a response of magnetically-ordered media to ultrashort excitation led to foundation of new research field of ultrafast magnetism, discoveries of all-optical magnetic switching in various metallic and dielectric materials. Despite obvious analogies between magnetically-ordered and ferroelectric materials, the issue of the ultrafast switching of the order parameter in the latter class of ferroics has been given very little attention. This raises an obvious question about the possibility of optical switching of the spontaneous polarization in ferroelectrics and the prospects of information recording in ferroelectrics by means of light. Here we briefly review the main findings of earlier studies of optical control of spontaneous magnetization and polarization, highlight recent developments of ultrafast magnetism and magnetic recording with femtosecond laser pulses, and discuss a new field of ultrafast ferroelectricity. Analyzing the literature, we derive the most promising strategies for optical recording in ferroic media and speculate about applicability of the strategy proven to be efficient in magnetically-ordered media, to ferroelectrics and multiferroics. (C) 2020 The Author(s). Published by Elsevier B.V.

In this paper, multiple magnetic transitions of Sm0.5Pr0.5FeO3 single crystal were systematically studied. By substituting Pr3+ in SmFeO3, spin reorientation temperature (TSR) was effectively decreased from 450 K–480 K (SmFeO3) to 180 K–220 K (Sm0.5Pr0.5FeO3). When the applied magnetic field is 500 Oe, spin switching was observed at 119 K (Tssw) for Sm0.5Pr0.5FeO3 and 180 K for SmFeO3, in which a spontaneous spin-flip transition of Sm/Pr- and Fe-sublattices at the same time, accompanied by an exchange of their FM vector directions. The compensation temperature (Tcomp) was raised from 4 K to 90 K which corresponds to zero magnetization. The possible mechanism for the modulation of magnetic transitions in SmFeO3 single crystal by Pr3+ substitution is discussed in detail. The dynamics of spin reorientation for Sm0.5Pr0.5FeO3 is also verified by Terahertz time-domain spectroscopy (THz-TDS), with the resonant excitation of quasi-antiferromagnetic (AFM) mode.

Sm0.4Er0.6FeO3 single crystals were successfully grown by optical floating zone method; high quality samples with various orientations were manufactured. Based on these samples, Magnetic property of Sm0.4Er0.6FeO3 single crystals were investigated systemically by means of the temperature dependence of magnetization. It indicated that compositional variations not only alter the spin reorientation temperature, but also the compensation temperature of the orthoferrites. Unlike single rare earth orthoferrites, the reversal transition temperature point of Sm0.4Er0.6FeO3 increases as magnetic field increases, which is positive for designing novel spin switching or magnetic sensor device.

In this work, Sm 0.3 Tb 0.7 FeO 3 single crystal was successfully grown by optical floating zone method. Sm 0.3 Tb 0.7 FeO 3 samples with a-, b-, and c-orientation were manufactured by means of Laue photograph. Magnetic properties of Sm 0.3 Tb 0.7 FeO 3 single crystals are studied over a wide temperature range from 2 to 400 K. Spin reorientation transition from Γ 2 to Γ 4 are observed by means of the temperature dependence of magnetization It indicated the reorientation transition temperature of Sm 1−x Tb x FeO 3 single crystals is lowered with the contents of Tb contents rising based on this work and our previous works, thus the spin reorientation transition temperature can be adjusted through changing the compound in orthoferrites materials, which means that we can get orthoferrites single crystals with high magnetism property in various temperature through material design.

Picosecond strain pulses are a versatile tool for investigation of mechanical properties of meso- and nano-scale objects with high temporal and spatial resolutions. Generation of such pulses is traditionally realized via ultrafast laser excitation of a light-to-strain transducer involving thermoelastic, deformation potential, or inverse piezoelectric effects. These approaches unavoidably lead to heat dissipation and a temperature rise, which can modify delicate specimens, like biological tissues, and ultimately destroy the transducer itself limiting the amplitude of generated picosecond strain. Here we propose a non-thermal mechanism for generating picosecond strain pulses via ultrafast photo-induced first-order phase transitions (PIPTs). We perform experiments on vanadium dioxide VO2 films, which exhibit a first-order PIPT accompanied by a lattice change. We demonstrate that during femtosecond optical excitation of VO2 the PIPT alone contributes to ultrafast expansion of this material as large as 0.45%, which is not accompanied by heat dissipation, and, for excitation density of 8 mJ cm−2, exceeds the contribution from thermoelastic effect by a factor of five., Ultrafast driving of vanadium dioxide can induce a large structural phase transition, which can be used to generate picosecond strain pulses. Here the authors show that the photo-induced phase transition can contribute 0.45% strain without causing undesirable heating.

Strain engineering is a powerful technology which exploits stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique modulates in space the carrier density and mobility, the optical absorption and, in strongly correlated systems, the phase, e.g. insulator/metal or ferromagnetic/paramagnetic. However, while successfully accessing nanometer length scale, strain engineering is yet to be brought down to ultrafast time scales allowing strain-assisted control of state of matter at THz frequencies. In our work we demonstrate a control of an optically-driven insulator-to-metal phase transition by a picosecond strain pulse, which paves a way to ultrafast strain engineering in nanostructures with phase transitions. This is realized by simultaneous excitation of VO$_2$ nanohillocks by a 170-fs laser and picosecond strain pulses finely timed with each other. By monitoring the transient optical reflectivity of the VO$_2$, we show that strain pulses, depending on the sign of the strain at the moment of optical excitation, increase or decrease the fraction of VO$_2$ which undergoes an ultrafast phase transition. Transient strain of moderate amplitude $\sim0.1$% applied during ultrafast photo-induced non-thermal transition changes the fraction of VO$_2$ in the laser-induced phase by $\sim1$%. By contrast, if applied after the photo-excitation when the phase transformations of the material are governed by thermal processes, transient strain of the same amplitude produces no measurable effect on the phase state., Comment: Main text: 22 pages, 4 figures. Supplimental Material: 8 pages, 9 figures. Submitted

We study experimentally the influence of the laser-induced temperature gradient on the parameters of propagating magnetostatic surface waves in thin film of the ferromagnetic metallic alloy Galfenol Fe0.81Ga0.19. The material has a pronounced magnetocrystalline anisotropy and exhibits the long-distance propagation of magnetostatic surface waves excited with femtosecond laser pulses. The excitation pulse heats up the sample locally, what leads to the spatial-temporal change of magnetization and anisotropy parameters of the film, and thus excites the magnetostatic surface waves. We show experimentally that the spectrum of the excited waves narrows as they propagate in such a gradient medium. By changing the orientation of external magnetic field with respect to anisotropy axes of the sample, we control whether the low- or high-frequency part of the spin waves spectrum is suppressed.

We suggest a model of synthetic ferrimagnetic semiconductor structure based on GaAs-AlGaAs quantum well doped by two Mn delta-layers. The coupling between the delta-layers is mediated by extra holes, and can be switched between ferro- and antiferromagnetic one by gating the structure. A proper choice of Mn concentrations in the delta-layers and of local degree of disorder enables fabrication of a ferrimagnetic structure supporting ultrafast switching of magnetization by short pulses of electric bias without an external magnetic field. The switching mechanism in the structure relies on kinetic spin exchange between the two delta-layers which is mediated by exchange scattering of electric-pulse heated holes by magnetic ions within the layers. Owing to specific interplay between characteristics of the exchange scattering, spin decay times, and the heat withdraw in the suggested synthetic ferrimagnetic semiconductor, the necessary parameters of electric-bias pulse are within the technologically accessible range, and do not contradict typical thermal kinetics of semiconductor structures., 16 pages, 1 figure, submitted to JMMM

We report on experimental picosecond acoustic studies of an ultrafast photoinduced insulator-to-metal and structural transition in VO2 nanostructures epitaxially grown on Al2O3 substrates with different orientations. Applying a pump-probe technique with combined excitation of a sample with picosecond strain and femtosecond laser pulses we demonstrate that dynamical strain of moderate amplitude of 0.1% has a pronounced impact on ultrafast photoinduced phase transition in VO2 nanohillocks. This enables novel path for controlling such transitions at picosecond and nanometer scales. Our experiments also allowed characterizing elastic and photo-elastic properties of the photo-induced metallic phase in VO2 and to relate them to the properties of the equilibrium phase. Furthermore, we demonstrate the generation of picosecond strain pulses upon laser-induced excitation of thin epitaxial VO2.

Excitation of four coherent phonon modes of different symmetries has been realized in copper metaborate CuB$_2$O$_4$ via impulsive stimulated Raman scattering (ISRS). Phonons were detected by monitoring changes in the linear optical birefringence using the balanced-detection (BD) technique. We compare the results of BD-ISRS experiment to the polarized spontaneous Raman scattering spectra. We show that the agreement between the two sets of data obtained by these allied techniques in a wide phonon frequencies range of 4-14 THz can be achieved by rigorously taking into account the symmetry of the phonon modes, and the corresponding excitation and detection selection rules. It is also important to account for the difference between incoherent and coherent phonons in terms of their contributions to the Raman scattering process. This comparative analysis highlights the importance of the ratio between the frequency of a particular mode, and the pump and probe spectral widths. We demonstrate analytically that the pump and probe pulse durations of 90 and 50 fs, respectively, used in our experiments, limit the highest frequency of the excited and detected coherent phonon modes to 12 THz, and define their relative amplitudes.

We demonstrate spin pumping, i.e. the generation of a pure spin current by precessing magnetization, without application of microwave radiation commonly used in spin pumping experiments. We use femtosecond laser pulses to simultaneously launch the magnetization precession in each of two ferromagnetic layers of a Galfenol-based spin valve and monitor the temporal evolution of the magnetizations. The spin currents generated by the precession cause a dynamic coupling of the two layers. This coupling has dissipative character and is especially efficient when the precession frequencies in the two layers are in resonance, where coupled modes with strongly different decay rates are formed., 5 pages, 3 figures

Contains fulltext : 181970.pdf (Publisher’s version ) (Open Access) Light propagation effects can strongly influence the excitation and the detection of laser-induced magnetization dynamics. We investigated experimentally and analytically the effects of crystallographic linear birefringence on the excitation and detection of ultrafast magnetization dynamics in the rare-earth orthoferrites (Sm 0.5 Pr 0.5 )FeO 3 and (Sm 0.55 Tb 0.45 )FeO 3 , which possess weak and strong linear birefringence, respectively. Our finding is that the effect of linear birefringence on the result of a magneto-optical pump-probe experiment strongly depends on the mechanism of excitation. When magnetization dynamics, probed by means of the Faraday effect, is excited via a rapid, heat-induced phase transition, the measured rotation of the probe pulse polarization is strongly suppressed due to the birefringence. This contrasts with the situation for magnetization dynamics induced by the ultrafast inverse Faraday effect, where the corresponding probe polarization rotation values were larger in the orthoferrite with strong linear birefringence. We show that this striking difference results from an interplay between the polarization transformations experienced by pump and probe pulses in the birefringent medium. 10 p.

Physical review / B 96(18), 184420 (2017). doi:10.1103/PhysRevB.96.184420, We report a high-resolution resonant inelastic extreme ultraviolet (EUV) scattering study of the quantum Heisenberg antiferromagnet KCoF$_3$. By tuning the EUV photon energy to the cobalt M$_{23}$ edge, a complete set of low-energy 3d spin-orbital excitations is revealed. These low-lying electronic excitations are modeled using an extended multiplet-based mean-field calculation to identify the roles of lattice and magnetic degrees of freedom in modifying the resonant inelastic x-ray scattering (RIXS) spectral line shape. We have demonstrated that the temperature dependence of RIXS features upon the antiferromagnetic ordering transition enables us to probe the energetics of short-range spin correlations in this material., Published by APS, Woodbury, NY

Using time-resolved magneto-optical pump-probe technique we demonstrate excitation of magnetization precession in a single crystalline bulk magnetite Fe3O4 below and in the vicinity of the Verwey and spin-reorientation (SR) phase transitions. Pronounced temperature dependence of the precession amplitude is observed suggesting that the excitation occurs via laser-driven spin-reorientation transition. Similarity observed between the characteristic features of the laser-induced ultrafast SR and Verwey transitions suggests that they both rely on the same microscopic processes.

Scientific reports 6, 38796 (2016). doi:10.1038/srep38796, In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M$_{2,3}$-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup., Published by Nature Publishing Group, London

Spin-lattice coupling is one of the most prominent interactions mediating response of spin ensemble to ultrafast optical excitation. Here we exploit optically generated coherent and incoherent phonons to drive coherent spin dynamics, i.e. precession, in thin films of magnetostrictive metal Galfenol. We demonstrate unambiguously that coherent phonons, also seen as dynamical strain generated due to picosecond lattice temperature raise, give raise to magnetic anisotropy changes of the optically excited magnetic film; and this contribution may be comparable to or even dominate over the contribution from the temperature increase itself, considered as incoherent phonons.

Ultrafast optical excitation of a metal ferromagnetic film results in a modification of the magnetocrystalline anisotropy and induces the magnetization precession. We consider two main contributions to these processes: an effect of noncoherent phonons, which modifies the temperature dependent parameters of the magnetocrystalline anisotropy and coherent phonons in the form of a strain contributing via inverse magnetostriction. Contrary to earlier experiments with high-symmetry ferromagnetic structures, where these mechanisms could not be separated, we study the magnetization response to femtosecond optical pulses in the low-symmetry magnetostrictive galfenol film so that it is possible to separate the coherent and noncoherent phonon contributions. By choosing certain experimental geometry and external magnetic fields, we can distinguish the contribution from a specific mechanism. Theoretical analysis and numerical calculations are used to support the experimental observations and proposed model.

Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of magnetic state of thin films and nanostructures via inverse magnetostriction on a picosecond time scale. Here we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga) either by injecting picosecond strain pulse into it from a substrate or by generating dynamical strain of complex temporal profile in the film directly. In the former case we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case optically-generated strain emerged abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession anymore. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on mixed, compressive and shear, character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study., Comment: 12 pages, 5 figures

Bulk holmium iron garnet Ho3Fe5O12 is a cubic ferrimagnet with Curie temperature TC = 567 K and magnetization compensation point in the range 130–140 K. The magneto-optical data are presented for a holmium iron garnet Ho3Fe5O12 film, ∼10 μm thick, epitaxially grown on a (111)-type gadolinium-gallium garnet Gd3Ga5O12 substrate. A specific feature of this structure is that the parameters of the bulk material, from which the film was grown, closely match the substrate ones. The temperature and field dependences of Faraday rotation as well as the temperature dependence of the domain structure in zero field were investigated. The compensation point of the structure was found to be Tcomp = 127 K. It was shown that the temperature dependence of the characteristic size of domain structure diverges at this point. Based on the obtained results we established that the magnetic anisotropy of the material is determined by both uniaxial and cubic contributions, each characterized by different temperature dependence. A ...

We explore a thermal mechanism of changing the anisotropy by femtosecond laser pulses in dielectric ferrimagnetic garnets by taking a low symmetry (YBiPrLu)3(FeGa)5O12 film grown on the (210)-oriented Gd3Ga5O12 substrate as a model media. We demonstrate by means of spectral magneto-optical pump-probe technique and phenomenological analysis, that the magnetization precession in such a film is triggered by laser-induced changes of the growth-induced magnetic anisotropy along with the well-known ultrafast inverse Faraday effect. The change of magnetic anisotropy is mediated by the lattice heating induced by laser pulses of arbitrary polarization on a picosecond time scale. We show that the orientation of the external magnetic field with respect to the magnetization easy plane noticeably affects the precession excited via the anisotropy change. Importantly, the relative contributions from the ultrafast inverse Faraday effect and the change of different growth-induced anisotropy parameters can be controlled by varying the applied magnetic field strength and direction. As a result, the amplitude and the initial phase of the excited magnetization precession can be gradually tuned., 16 pages, 10 figures, 2 table

In the last decade, a new area of research, referred to as femtomagnetism, has developed within the field of magnetism, which studies the excitation and control of magnetic medium dynamics on time scales comparable to or even much shorter than those of spin-lattice, spin-orbit, and exchange interactions. Among the many femtomagnetic processes studied to date, the opto-magnetic interaction of femtosecond laser pulses with media is of particular interest. This interaction is based on nondissipative Raman-type mechanisms and enables coherent spin dynamics to be efficiently and selectively excited and its parameters to be controlled. This review considers the key features of ultrafast opto-magnetic phenomena and how they relate to magneto-optical effects. A number of experimentally observed examples of ultrafast spin dynamics excited via opto-magnetic inverse Faraday and Cotton–Mouton effects are considered, and their microscopical nature is discussed. An experimental example is given demonstrating that combining ultrafast opto-magnetic phenomena with other laser-induced processes allows magnetization to be controlled on a picosecond time scale.

The optical spectra of single crystals of hexagonal rare-earth manganites RMnO3 (R=Sc, Y, Er) are studied in the range from 0.7 to 5.4 eV. It is found that the spectra substantially differ from the spectra of orthorhombic manganites in both the positions of spectral features and their polarization anisotropy. It is shown that the optical absorption edge is determined by an abnormally strong (k≃1) and narrow electric dipole transition with the center at approximately 1.6 eV with light polarization in the basal plane of the crystal. This transition can be treated with confidence as charge transfer from oxygen to manganese. The experimental results are in many instances substantially different from the first-principles calculations of the electronic structure of YMnO3 published recently and, hence, may serve as a reliable basis for the further improvement of computational methods.

Optical control of spins in the highly symmetric Heisenberg antiferromagnet KNiF3 is demonstrated. It is shown that Impulsive Stimulated Raman Scattering of light by magnons, combined with time-resolved magneto-optical detection, allows the generation and detection of antiferromagnetic resonances with unprecedented high sensitivity.

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.

We present a femtosecond spectroscopic magneto-optical investigation of the coherent and incoherent spin dynamics in the antiferromagnetic dielectric ${\text{KNiF}}_{3}$. The pathways of the photoinduced energy flow to spins were controlled by tuning the pump photon energy. In particular, we demonstrate that laser pulses, with photon energy tuned to a nearly-zero-absorption region, excite the spin system without any signatures of heating of electrons or phonons. In this regime the ultrafast excitation of coherent spin waves is followed by a gradual increase of the spin temperature solely due to decoherence of the laser-generated magnons, as revealed by our simultaneous measurement of both the transversal and the longitudinal component of the spin dynamics.

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.

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.

The question of how, and how fast, magnetization can be reversed is a topic of great practical interest for the manipulation and storage of magnetic information. It is generally accepted that magnetization reversal should be driven by a stimulus represented by time-non-invariant vectors such as a magnetic field, spin-polarized electric current, or cross-product of two oscillating electric fields. However, until now it has been generally assumed that heating alone, not represented as a vector at all, cannot result in a deterministic reversal of magnetization, although it may assist this process. Here we show numerically and demonstrate experimentally a novel mechanism of deterministic magnetization reversal in a ferrimagnet driven by an ultrafast heating of the medium resulting from the absorption of a sub-picosecond laser pulse without the presence of a magnetic field.

Time-resolved magneto-optical imaging of laser-excited rare-earth orthoferrite $(\mathrm{SmPr}){\mathrm{FeO}}_{3}$ demonstrates that a single 60 fs circularly polarized laser pulse is capable of creating a magnetic domain on a picosecond time scale with a magnetization direction determined by the helicity of light. Depending on the light intensity and sample temperature, pulses of the same helicity can create domains with opposite magnetizations. We argue that this phenomenon relies on a twofold effect of light which (i) instantaneously excites coherent low-amplitude spin precession and (ii) triggers a spin reorientation phase transition. The former dynamically breaks the equivalence between two otherwise degenerate states with opposite magnetizations in the high-temperature phase and thus controls the route of the phase transition.

We present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth---transition metal (RE--TM) alloys Gd${}_{x}$Fe${}_{90\ensuremath{-}x}$Co${}_{10}$ ($20%lxl28%$). Using single-shot time-resolved magneto-optical microscopy and multiscale simulations, we identified and described the unconventional path followed by the magnetization during the reversal process. This reversal does not involve precessional motion of magnetization but is governed by the longitudinal relaxation and thus has a linear character. We demonstrate that this all-optically driven linear reversal can be modeled as a result of a two-fold impact of the laser pulse on the medium. First, due to absorption of the light and ultrafast laser-induced heating, the medium is brought to a highly nonequilibrium state. Simultaneously, due to the ultrafast inverse Faraday effect the circularly polarized laser pulse acts as an effective magnetic field of the amplitude up to $\ensuremath{\sim}$20 T. We show that the polarization-dependent reversal triggered by the circularly polarized light is feasible only in a narrow range (below 10%) of laser fluences. The duration of the laser pulse required for the reversal can be varied from $\ensuremath{\sim}$40 fs up to at least $\ensuremath{\sim}$1700 fs. We also investigate experimentally the role of the ferrimagnetic properties of GdFeCo in the all-optical reversal. In particular, the optimal conditions for the all-optical reversal are achieved just below the ferrimagnetic compensation temperature, where the magnetic information can be all-optically written by a laser pulse of minimal fluence and read out within just 30 ps. We argue that this is the fastest write-read event demonstrated for magnetic recording so far.

We present and analyze high-resolution $\ensuremath{\alpha}$-, $\ensuremath{\sigma}$-, and $\ensuremath{\pi}$-polarized absorption spectra related to d-d electronic transitions in tetragonal metaborate CuB${}_{2}$O${}_{4}$ where copper Cu${}^{2+}$ ions occupy two crystallographically distinct 4$b$ and 8$d$ positions. The spectra are characterized by exceptionally rich fine structure in the spectral range of 1.4--2.4 eV. Six zero-phonon (ZP) lines originating from the electronic transitions within the Cu${}^{2+}$ ions in both positions are distinguished and identified. Symmetry analysis explains polarization properties of the ZP lines in the 8$d$ positions but only partially explains them in the 4$b$ positions. Reliable assignment of all six ZP lines to specific transitions allowed us to calculate genuine cubic $\mathit{Dq}$ and tetragonal $\mathit{Ds}$ and $\mathit{Dt}$ crystal-field parameters for both positions. We show that the ($3{r}^{2}\ensuremath{-}{z}^{2}$) state, the energy of which is the measure of the Jahn-Teller splitting, is the highest 3$d$ state for both types of Cu${}^{2+}$ ion positions. Using the obtained crystal-field parameters as the reference values, we estimated $\mathit{Dq}$, $\mathit{Ds}$, and $\mathit{Dt}$ for several other cuprates with different Cu-O bond lengths. In particular, the 3$d$ level splitting in La${}_{2}$CuO${}_{4}$, Nd${}_{2}$CuO${}_{4}$, CuGeO${}_{3}$, Sr${}_{2}$CuO${}_{2}$Cl${}_{2}$, and Cu${}_{3}$B${}_{7}$O${}_{13}$Cl was analyzed. Our estimates suggest that the Jahn-Teller splitting in some of these cuprates is larger than it was assumed previously.

We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO3 (R=Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the range of 0.6-5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe(3+) ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices., Comment: 8 pages, 3 figures, 22 references. Copyright (2012) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics and may be found at http://link.aip.org/link/?jap/111/056105

Complex optical dielectric function in the tetragonal rare-earth cuprates ${R}_{2}{\text{CuO}}_{4}$ ($R=\text{La}$, Pr, Nd, and Sm) and in the tetragonal bismuth cuprate ${\text{Bi}}_{2}{\text{CuO}}_{4}$ is studied in the spectral range of 0.6--5.4 eV using a method of optical ellipsometry. The dielectric spectra are studied for the two main polarizations and analyzed in terms of a cluster model for ${\text{CuO}}_{4}^{6\ensuremath{-}}$ complexes taking into account intracenter $p\text{\ensuremath{-}}d$ and intercenter $d\text{\ensuremath{-}}d$ charge-transfer (CT) transitions. The band gap in the rare-earth cuprates is defined by an electric-dipole-allowed CT transitions centered at 1.54--1.59 eV in Pr, Nd, and Sm cuprates, and 2.1 eV in La cuprate. Optical response of ${\text{Bi}}_{2}{\text{CuO}}_{4}$ strongly differs from the rare-earth cuprates which we relate with strong covalency of Bi-O bonding and strong ionicity of $\text{Cu}(3d)\text{-O}(2p)$ bonding. These features are manifested in suppression of low-energy intense intracenter $p\text{\ensuremath{-}}d$ and intercenter $d\text{\ensuremath{-}}d$ CT transitions, and by appearance of strong intense absorption bands near 5 eV. Regardless the strong distinctions of optical response, on one hand, of La, Pr, Nd, and Sm cuprates, and on the other hand, of the Bi cuprate, the dielectric gap in these compounds shows comparable values defined by a superposition of intracenter $p\text{\ensuremath{-}}d$ CT transitions and two-center $d\text{\ensuremath{-}}d$ CT transitions. Thus these cuprates should be classified as compounds intermediate between CT and Mott-Hubbard insulators.

We demonstrate experimentally that excitation of a Co-substituted ferrimagnetic yttrium iron garnet thin film with linearly polarized 100 fs laser pulses triggers large-angle magnetization precession with an amplitude, phase, and frequency determined by the characteristics of the laser pulse. The precession results from a light-induced anisotropy field with a characteristic lifetime of 20 ps, the direction of which is determined by the polarization of the light. Its strength for a pump intensity of $25\text{ }\text{mJ}/{\text{cm}}^{2}$ is 250 G which is comparable to the intrinsic anisotropy of the sample. By choosing the proper laser-pulse parameters, we were able to excite a precession with an amplitude as large as $20\ifmmode^\circ\else\textdegree\fi{}$ and a precession frequency modified by up to 50%.

Coherent magnons and phonons are excited by subpicosecond laser pulses in the weak ferromagnet ${\text{FeBO}}_{3}$. Impulsive stimulated Raman scattering (ISRS) is proven to be the microscopic mechanism of the excitation. It is shown that coherent magnons can be excited by both linearly and circularly polarized laser pulses where the efficiency of the process depends on the mutual orientation of the magnetic and crystallographic axes and the light propagation direction. The strong ellipticity of the ferromagnetic magnon mode is demonstrated, both experimentally and theoretically, to be essential for the excitation and observation of such coherent magnons. Because of this ellipticity, the amplitude of the coherent magnons excited by linearly polarized light may exceed by 2 orders of magnitude the amplitude of those excited by circularly polarized light. The primary difference between the excitation of coherent magnons by linearly polarized pulses via ISRS and via the earlier reported process of photoinduced magnetic anisotropy is discussed. Furthermore, the ISRS process is found to be responsible for the excitation of two optical phonon branches (8.4 and 12.1 THz) observed in our experiments. A coherent excitation, with a temperature-independent frequency of 0.7 THz, has also been observed in the magnetically ordered phase but could not be assigned to any optical phonon modes known in ${\text{FeBO}}_{3}$. The well-pronounced dependence of the amplitude of this mode on temperature suggests that this mode of nonmagnetic origin becomes Raman active only in the magnetically ordered phase and, therefore, can be excited and observed only below the N\'eel temperature.

Polarization-dependent excitation of coherent spin precession by 150 fs linearly polarized laser pulses is observed in the easy-plane antiferromagnet FeBO3. We show that the mechanism of excitation is impulsive stimulated Raman scattering. This process is shown to be determined not only by the magneto-optical constants of the material, but also by the properties of the spin precession itself. Though carrying no angular momentum, the linearly polarized laser pulses act on the spins as effective fields that can be considered as an ultrafast inverse Cotton-Mouton effect.

Spectral dependences of the relative permittivity ɛ = ɛ1 − iɛ2 of a uniaxial weak ferromagnet FeBO3 are measured for two principal polarizations in the energy range 0.6–5.6 eV. The positions have been determined for the charge-transfer transitions that make the main contribution to absorption above the bandgap and determine the refractive-index dispersion below the bandgap. The isotropic magnetic contribution to the refractive index has been detected by studying the temperature dependence of the refractive index in the range 100–700 K; its value (≈2 × 10−2) is found to be record high for magnetic dielectrics. The energy shift of the effective oscillator, which characterizes the shift of the positions of the charge-transfer transitions due to magnetic ordering, is determined from these data within the framework of a single-oscillator model. The value of the exchange striction in FeBO3 is determined from optical measurements.

We show that the optical properties of ${\mathrm{LiCu}}^{1+}{\mathrm{Cu}}^{2+}{\mathrm{O}}_{2}$ in the spectral range of $0.6\text{--}5.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ radically differ from those of all other known ${\mathrm{Cu}}^{1+}$, ${\mathrm{Cu}}^{2+}$, and mixed-valent oxide cuprates. An extremely strong, sharp, and highly anisotropic optical feature with ${\ensuremath{\epsilon}}_{2}^{xx}=26$ is observed at $3.27\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ which we assign to an excitonlike transition in the ${\mathrm{O}}^{2\ensuremath{-}}\text{\ensuremath{-}}{\mathrm{Cu}}^{1+}\text{\ensuremath{-}}{\mathrm{O}}^{2\ensuremath{-}}$ dumbbells. Our findings thoroughly disagree with reported ab initio calculations and can be explained by an exciton-type model that includes strong electron-hole correlations and a crystal-field splitting of the ${\mathrm{Cu}}^{1+}$ states. The excitonic effects in ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$ appear strongly enhanced due to the shortening of the dumbbell lattice spacing which is the shortest one among known cuprates. Our experimental data along with the model reveal a previously unknown regularity in the electronic structure of cuprates.