Magneto-optical Susceptibility Research Articles

Terahertz-driven magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet thin film near a compensation point

Magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet film $({\mathrm{Y}}_{3\ensuremath{-}z}{\mathrm{Bi}}_{z})[{\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Ga}}_{x}][{\mathrm{Fe}}_{3\ensuremath{-}y}{\mathrm{Ga}}_{y}]{\mathrm{O}}_{12}$ has been studied using a terahertz pump--optical probe spectroscopy. Two magnetic modes are observed whose frequencies intersect at a magnetization compensation point. The experimental dependence of the excited modes on terahertz pulse polarization, an external magnetic field and the temperature are analyzed. The theoretical description based on symmetry and the Lagrangian formalisms is proposed. Magnetic modes crossing is explained as interplay between exchange and anisotropy energies equally contributing near the compensation point. Simulations based on the Landau-Lifshitz-Gilbert equations show that a difference in magneto-optical susceptibilities of tetrahedral and octahedral iron sublattices can substantially enhance the sensitivity to the magnetic modes.

Longitudinal Magneto-Optical Kerr Effect of Nanoporous CoFeB and W/CoFeB/W Thin Films

Nanoporous Co40Fe40B20 (CoFeB) and sandwich tungsten (W)/CoFeB/W thin films were fabricated via an anodic aluminum oxide (AAO) template-assisted magneto sputtering process. Their thickness-dependent magneto-optical Kerr effect (MOKE) hysteresis loops were investigated for enhanced Kerr rotation. Control of the Kerr null points of the polarized reflected light can be realized via the thicknesses of the CoFeB layers and W layers. Simulation of the thickness-dependent phase difference change by the finite element method reveals the existence of the two Kerr null points for W/CoFeB/W thin films, matching the experimental result very well. However, there are two additional Kerr null points for pure CoFeB thin films according to the simulation by comparing with the experimental result (only one). Theoretical analysis indicates that the different Kerr null points between the experimental result and the simulation are mainly due to the enhanced inner magnetization in the ferromagnetic CoFeB layer with the increased thickness, which is usually omitted in the simulation. Clearly, the introduction of non-ferromagnetic W layers can experimentally regulate the Kerr null points of ferromagnetic thin films. Moreover, construction of W/CoFeB/W sandwich thin films can greatly increase the highest magneto-optical susceptibility and the saturated Kerr rotation angle when compared with CoFeB thin films of the same thickness.

Linear and Nonlinear Magneto-optical Phenomena in Epitaxial Films of Europium Chalcogenides EuX (X = O, Se, Te)

The studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu2+ ions, is revealed.

Линейные и нелинейные магнитооптические явления в эпитаксиальных плeнках халькогенидов европия EuX (X=O, Se, Te)

AbstractThe studies of magneto-optical effects in epitaxial films of magnetic semiconductors—europium chalcogenides EuX (X = O, Se, Te) are presented: linear and quadratic magneto-optical Kerr effects, photo-induced Faraday effect, magnetically-induced optical second and third harmonics generation, the inverse Faraday effect, and the phenomenon of optical orientation. A large quadratic magneto-optical Kerr effect was found in EuO and (Eu, Gd)O films. Photo-induced spin polarons with a huge value of the magnetic moment were studied in europium chalcogenides EuTe and EuSe using the optical pump-probe technique. In the same materials, a new type of nonlinear magneto-optical susceptibility, which is responsible for the optical second harmonic generation and associated with the magnetic ordering of the spins of Eu^2+ ions, is revealed.

Optically probed symmetry breaking in the chiral magnetCu2OSeO3

We report on the linear optical properties of the chiral magnet Cu2OSeO3, specifically associated with the absence of inversion symmetry, the chiral crystallographic structure, and magnetic order. Through spectroscopic ellipsometry, we observe local crystal-field excitations below the charge-transfer gap. These crystal-field excitations are optically allowed due to the lack of inversion symmetry at the Cu sites. Optical polarization rotation measurements were used to study the structural chirality and magnetic order. The temperature dependence of the natural optical rotation, originating in the chiral crystal structure, provides evidence for a finite magneto-electric effect in the helimagnetic phase. We find a large magneto-optical susceptibility on the order of V(540nm)~10^4 rad/(T*m) in the helimagnetic phase and a maximum Faraday rotation of ~165deg/mm in the ferrimagnetic phase. The large value of V can be explained by considering spin cluster formation and the relative ease of domain reorientation in this metamagnetic material. The magneto-optical activity allows us to map the magnetic phase diagram, including the skyrmion lattice phase. In addition to this, we probe and discuss the nature of the various magnetic phase transitions in Cu2OSeO3.

The linear optical and magneto-optical susceptibilities and geometric effects of nine La@C82 (C82) crystal isomers

Using the extended Su-Schrieffer-Heeger model in conjunction with the local field method and Ewald method, we have calculated the linear susceptibility χ for nine different La@C82 (C82) crystal isomers. Our numerical calculation results indicate that the spectral shapes of the susceptibilities tensor of the La@C82 (C82) crystal turn out to be determined mainly by the geometrical distributions of the pentagons in the fullerene and endohedral structures.

Magneto-optical properties of the iron garnet Tb3Fe5O12 near the magnetic compensation temperature

The magneto-optical susceptibility and magnetic hysteresis loops of the Faraday effect, which accompany the technical magnetization of the iron garnet Tb3Fe5O12, have been investigated experimentally in the temperature region near the magnetic compensation point of this ferrimagnet T c = 249 K. It has been found that, during the technical magnetization as the temperature approaches the magnetic compensation point T c , the velocity of domain walls increases, whereas the magneto-optical susceptibility has local maxima to the right and to the left from T c . Mentioned features of magneto-optical properties of the iron garnet Tb3Fe5O12 are associated with the resonance natural frequency of the domain wall oscillations in the crystal with the frequency of the alternating magnetic field. It has been shown that the used theoretical model of the magnetic resonance of domain walls makes it possible to consistently describe (at the qualitative level) the revealed regularities of the variation of the Faraday effect in the iron garnet Tb3Fe5O12 near T c .

Micromagnetic investigation of all-optical switching

This Letter investigates all-optical magnetization switching from micromagnetic perspective. The influence of circularly polarized light on a magnetic sample was considered to be both directly through the inverse Faraday effect and indirectly aided by thermal induced effects by the laser beam. Dependence of all-optical switching on pulse duration, laser intensity and the magneto-optical susceptibility strength is studied. An important aspect of this investigation is the analysis of how the cooling process influences the successive switches, especially through limiting the successive writing time. • The effects of laser exposure time and laser intensity on the stability of all-optical switching are analyzed. • The correlation between times involved into all-optical magnetization reversal is established. • The influence of cooling process on successive switches is analyzed.

Specific features of Eu 3+ and Tb 3+ magnetooptics in gadolinium-gallium garnet (Gd 3Ga 5O 12)

We reported magnetooptical properties of Eu 3+(4f (6)) and Tb 3+(4f (8)) in single crystals of Gd 3Ga 5O 12 (GGG), Y 3Ga 5O 12 (YGG), and Eu 3+(4f (6)) in Eu 3Ga 5O 12 (EuGG) for both ions occupying sites of D 2 symmetry in the garnet structure. Absorption, luminescence, and magnetic circular polarization of luminescence (MCPL) spectra of Tb 3+ in GGG and YGG and absorption and magnetic circular dichroism (MCD) of Eu 3+ in EuGG were studied. The data were obtained at 85 K and room temperature (RT). Magnetic susceptibility of Eu 3+ in EuGG was also measured between 85 K and RT. The magnetooptical and magnetic susceptibility data were modeled using the wavefunctions of the crystal-field split energy (Stark) levels of Eu 3+ and Tb 3+ occupying D 2 sites in the same garnets. The results reported gave a precise determination of these Stark level assignments and confirmed the symmetry labels (irreducible representations) of the closely-spaced Stark levels (quasi-doublets) found in the 5D 1 (Eu 3+) and 5D 4 (Tb 3+) multiplets. Ultraviolet (UV) excitation (<300 nm) of the 6P J and 6I J states of Gd 3+ in the doped GGG crystals led to emission from 5D 4 (Tb 3+) and 5D 1 and 5D 0 (Eu 3+) through radiationless energy transfer to the 4f ( n−1) 5d band of Tb 3+ and to UV quintet states of Eu 3+. The temperature-dependent emission line shapes and line shifts of the magnetooptical transitions excited by UV radiation suggested a novel way to explore energy transfer mechanisms in this rare-earth doped garnet system.

Magneto-optical investigations of the domain structure and magnetization of the ferrite-garnet Tb0.2Y2.8Fe5O12 in the temperature range of the spontaneous spin-reorientation phase transition

The transformation of a domain structure and technical magnetization of the Tb0.2Y2.8Fe5O12 single crystal in the temperature range of the spontaneous orientational phase transition have been investigated by the magneto-optical method. It has been shown that the phase transition is extended in a certain temperature range in which domains of the low-temperature and high-temperature magnetic phases coexist. It has been found that the evolution of the domain configuration in the temperature range of spin reorientation substantially depends on the presence of mechanical stresses in the crystal. Anomalies in the temperature dependences of the coercivity and magneto-optical susceptibility of the crystal due to the transformation of its domain structure during the phase transition have been revealed. The experimental results have been interpreted within the existing theory of orientational phase transitions in cubic crystals.

Optical Excitation of a Forbidden Magnetic Resonance Mode in a Doped Lutetium-Iron-Garnet Film via the Inverse Faraday Effect

The effective magnetic field induced by a femtosecond pulse of circularly polarized light, via the inverse Faraday effect, is shown to excite a magnetic-dipole forbidden exchange spin resonance in a lutetium iron garnet. An external magnetic field cannot excite this mode, as the iron sublattices have the same gyromagnetic ratio and no net torque can be applied between them. However, since the sublattices have different magneto-optical susceptibilities, the inverse Faraday effect induces different effective fields on different iron sites, allowing excitation.

Paradigm of the time-resolved magneto-optical Kerr effect for femtosecond magnetism

The magneto-optical Kerr effect (MOKE) is a powerful tool for studying changes in the magnetization of ferromagnetic materials. It works by measuring changes in the polarization of reflected light. However, because the conventional theoretical basis for interpreting a MOKE signal assumes measurement with continuous-wave light1,2, its use for understanding high-speed magnetization dynamics of a material probed with femtosecond optical pulses3,4 has been controversial5,6,7,8,9,10. Here we establish a new paradigm for interpreting time-resolved MOKE measurements, through a first-principles investigation of ferromagnetic nickel. We show that the time-resolved optical and magnetic responses energetically follow their respective optical and magneto-optical susceptibilities. As a result, the one-to-one correspondence between them sensitively depends on the incident photon energy. In nickel, for photon energies below 2 eV the magnetic response is faithfully reflected in the optical response, but above 2 eV they decouple. By constructing a phase-sensitive polarization versus magnetization plot, we find that for short pulses the magnetic signals are delayed by 10 fs with respect to the optical signals. For longer pulses, the delay shortens and the behaviour approaches the continuous-wave response. This finally resolves the long-standing dispute over the interpretation in the time-resolved MOKE measurements and lays a solid foundation for understanding femtomagnetism3,4.

Use of the Faraday optical transformer for ultrafast magnetization reversal of nanomagnets

We propose a new strategy for ultrafast magnetization reversal of nanomagnets. Due to the Inverse Faraday Effect, circularly polarized optical pulses induce a pulsed magnetic flux in materials with large magneto-optical susceptibility. Alternatively, intense optical pulses can induce a pulsed magnetic flux by means of ultrafast demagnetization of a metallic thin film or multilayer with a perpendicular magnetic anisotropy. The time varying magnetic flux induces a transient electro-motive force and electric current in a conducting loop on the surface of the illuminated material, and hence a transient magnetic field. The magnetic field pulses due to the transient current appear to be too short for use in the magnetic field or spin-current induced precessional switching of magnetization. However, our calculations suggest that the magnetic field could lead to ultrafast switching of a nanomagnet overlaid on the surface of the conductor and demagnetized by the same optical pulse. In the case of magnetic pulses due to the Inverse Faraday Effect, the switching direction could be controlled by the helicity of the optical pulse.

Imaging of domain wall motion in small magnetic particles through near-field microscopy

We report magneto-optical scanning near-field optical microscopy images for 4×4×0.08μm3 and 16×16×0.08μm3 low magnetic anisotropy Co70.4Fe4.6Si15B10 particles. Measuring magneto-optical differential susceptibility we acquired images of the domain wall movement driven by an applied magnetic field with a spatial resolution better than λ∕4. For the 4×4×0.08μm3 sized particle, a sequence of 27 magneto-optical differential susceptibility images reveals the evolution of the magnetic domain structure between positive and negative saturation fields passing through the four-domain flux-closure magnetization structure. On the 16×16×0.08μm3 particle, we studied the role of the oscillating driving field on the susceptibility distribution. Comparing the different magneto-optical differential susceptibility sequences, the role of the shape anisotropy on the field induced domain wall movement is evidenced. Micromagnetic simulations were used to provide a better understanding of the domain wall movement.

Generation of femtosecond current pulses using the inverse magneto-optical Faraday effect

A new method of generating ultrashort current pulses is proposed that is based on the optical pumping of a mesoscopic structure comprising a metal ring with a core made of a material possessing giant magneto-optical susceptibility. The main dynamic characteristics of the proposed device are calculated.

Magnetic characterization of microscopic particles by MO-SNOM.

This paper reports on the development of a magneto-optical scanning near-field optical microscope and the experimental near-field study of the domain structure for a model magnetic particle of 16 x 16 micro m(2) of a Co(70.4)Fe(4.6)Si(15)B(10) amorphous thin film, deposited on a silicon substrate. We present the topographic, optical and magneto-optical differential susceptibility (MODS) images of the particle. Imaging by using the local MODS reveals the domain structure. These images are also used for positioning the tip in order to acquire local hysteresis loops, with submicrometre spatial resolution.

Nonlinear magneto-optical response of s- and d-wave superconductors.

The nonlinear magneto-optical response of $s$- and $d$-wave superconductors is discussed. We carry out the symmetry analysis of the nonlinear magneto-optical susceptibility in the superconducting state. Due to the surface sensitivity of the nonlinear optical response for systems with bulk inversion symmetry, we perform a group theoretical classification of the superconducting order parameter close to a surface. For the first time, the mixing of singlet and triplet pairing states induced by spin-orbit coupling is systematically taken into account. We show that the interference of singlet and triplet pairing states leads to an observable contribution of the nonlinear magneto-optical Kerr effect. This effect is not only sensitive to the anisotropy of the gap function but also to the symmetry itself. In view of the current discussion of the order parameter symmetry of High-T$_c$ superconductors, results for a tetragonal system with bulk singlet pairing for various pairing symmetries are discussed.

Enhancement of the magneto-opticl Kerr rotation in nonlinear optical response

Using an electronic theory we calculate the frequency-dependent nonlinear magneto-optical Kerr rotation and show that the second harmonic polarization ellipsis is more strongly influenced by magnetism than in the linear case. We find for the magnetization dependence of the nonlinear and linear magneto-optical susceptibility tensor elements a linear dependence on magnetization. The influence of the symmetry of matrix elements for s or d transitions on the second harmonic intensity is discussed. Furthermore, the ab initio nonlinear Kerr spectra of Fe layers of different thicknesses are presented.

The magneto-optical birefringence and initial susceptibility of very dilute magnetic fluids

Abstract In this paper the behaviour of birefringence and the initial susceptibility in the low concentration range ( ϵ =1.34×10 -5 to 0.01) have been examined for magnetite ferrofluids. Our measurements show that the initial degree of clustering does not change appreciably by diluting the sample. In low fields the birefringence results from the shape anisotropy of single particles. In the case where particle interactions are pronounced the birefringence results from the interactions within the pre-existing clusters in the sample.

A theoretical investigation on the magneto-optical effect of CeF 3 and NdF 3

In this paper the calculations of magneto-optical Verdet constants and susceptibilities of CeF 3 and NdF 3 compounds in the temperature range 40–300 K are presented. It is found that the magneto-optical effects of these compounds are mainly caused by the intra-ionic electric dipole transitions between the 4f and 5d energy levels split by the crystal and magnetic fields. The mixed-valent behavior of the Ce ions in CeF 3 has an important effect on the Faraday rotation and susceptibility. The theoretical results are in agreement with experiments.