Linear Magnetic Birefringence Research Articles

Effect of antiferromagnetic order on a propagating single-cycle THz pulse

Employing polarization sensitive terahertz (THz) transmission spectroscopy, we explored how the waveform of initially single-cycle linearly polarized THz pulses changes upon propagation through a thick antiferromagnetic crystal of CoF2. The changes upon propagation through CoF2 are found to depend strongly on both the incoming polarization and temperature. In particular, the ellipticity and polarization rotation acquired by initially linearly polarized light are quantified and explained in terms of magnetic linear birefringence and dichroism. Although the magneto-optical effects are often considered to be relatively weak, our experiments reveal that the polarization of the THz pulse substantially changes along the pulse duration. The pulse shape is further complicated by features assigned to the formation of magnon-polaritons. The findings clearly show the importance of accounting for propagation effects in antiferromagnetic spintronics and magnonics.

Laser-induced THz magnetism of antiferromagnetic CoF2

Excitation, detection, and control of coherent THz magnetic excitation in antiferromagnets are challenging problems that can be addressed using ever shorter laser pulses. We study experimentally excitation of magnetic dynamics at THz frequencies in an antiferromagnetic insulator CoF2 by sub-10 fs laser pulses. Time-resolved pump-probe polarimetric measurements at different temperatures and probe polarizations reveal laser-induced transient circular birefringence oscillating at the frequency of 7.45 THz and present below the Néel temperature. The THz oscillations of circular birefringence are ascribed to oscillations of the magnetic moments of Co2+ ions induced by the laser-driven coherent E g phonon mode via the THz analogue of the transverse piezomagnetic effect. It is also shown that the same pulse launches coherent oscillations of the magnetic linear birefringence at the frequency of 3.4 THz corresponding to the two-magnon mode. Analysis of the probe polarization dependence of the transient magnetic linear birefringence at the frequency of the two-magnon mode enables identifying its symmetry.

Magneto-optical study of metamagnetic transitions in the antiferromagnetic phase of \u03b1-RuCl3

α-RuCl3 is a promising candidate material to realize the so far elusive quantum spin liquid ground state. However, at low temperatures, the coexistence of different exchange interactions couple the effective pseudospins into an antiferromagnetically zigzag (ZZ) ordered state. The low-field evolution of spin structure is still a matter of debate and the magnetic anisotropy within the honeycomb planes is an open and challenging question. Here, we investigate the evolution of the ZZ order parameter by second-order magneto-optical effects, the magnetic linear dichroism and magnetic linear birefringence. Our results clarify the presence and nature of metamagnetic transitions in the ZZ phase of α-RuCl3. The experimental observations show the presence of initial magnetic domain repopulation followed by a spin-flop transition for small in-plane applied magnetic fields (≈1.6 T) along specific crystallographic directions. In addition, using a magneto-optical approach, we detected the recently reported emergence of a field-induced intermediate phase before suppressing the ZZ order. The results disclose the details of various angle-dependent in-plane metamagnetic transitions quantifying the bond-anisotropic interactions present in α-RuCl3.

Tailored Magnetic Linear Birefringence in Wedge-Shaped Co Nanocluster Assemblies

The purpose of this paper is to present an experimental method to induce strong magnetic linear birefringence in two-dimensional assemblies of Co nanoclusters grown on glass plates. Additionally, we have also correlated the magnitude and characteristics of that nonlinear magneto-optical effect with the thickness and profile of those disordered nanostructures. For those aims, we have grown Co nanocluster assemblies on amorphous substrates, by means of pulsed laser ablation in off-axis geometry. This approach enabled us to obtain magnetic media with an intended and pronounced thickness profile, i.e., wedge-shaped assembly, to investigate the orientation and behavior of surface magnetization regarding both the thickness gradient direction and in-plane magnetic field. That study was accomplished by measuring the magneto-optical effects in reflection and transmission configurations, unveiling an out-of-plane magnetization whose magnitude depends closely on the thickness gradient direction. That component, arising from a graded magnetic anisotropy along the wedged nanostructure, adds a reversal mechanism to the surface magnetization, thus being responsible for the magnetic linear birefringence in our ultrathin Co assemblies.

A novel pulsed magnet for magnetic linear birefringence measurements

In this paper, we describe a novel pulsed magnet, called foil coil, which can deliver a field transverse to the light propagation of more than 10 T over about 0.8 m operating without cryogenic equipment. It has been designed for linear magnetic birefringence measurements. We report on testing the coil and also show some physics data taken in vacuum during its commissioning in the framework of the Biréfringence Magnétique du Vide (BMV) apparatus, with special attention to noise induced by the pulse itself. Finally, we compare the preliminary results obtained here with data from the previous BMV coil.

Spin Dynamics Slowdown near the Antiferromagnetic Critical Point in Atomically Thin FePS3.

Two-dimensional (2D) magnetic materials have attracted much recent interest with unique properties emerging at the few-layer limit. Beyond the reported impacts on the static magnetic properties, the effects of reducing the dimensionality on the magnetization dynamics are also of fundamental interest and importance for 2D device development. In this report, we investigate the spin dynamics in atomically thin antiferromagnetic FePS3 of varying layer numbers using ultrafast pump-probe spectroscopy. Following the absorption of an optical pump pulse, the time evolution of the antiferromagnetic order parameter is probed by magnetic linear birefringence. We observe a strong divergence in the demagnetization time near the Néel temperature. The divergence can be characterized by a power-law dependence on the reduced temperature, with an exponent decreasing with sample thickness. We compare our results to expectations from critical slowing down and a two-temperature model involving spins and phonons and discuss the possible relevance of spin-substrate phonon interactions.

Monte Carlo study of the BMV vacuum linear magnetic birefringence experiment

QED vacuum can be polarized and magnetized by an external electromagnetic field, therefore acting as a birefringent medium. This effect has not yet been measured. In this paper, after having recalled the main facts concerning Vacuum Magnetic Birefringence polarimetry detection method and the related noise sources, we detail our Monte Carlo simulation of a pulsed magnetic field data run. Our Monte Carlo results are optimized to match BMV experiment 2014 data. We show that our Monte Carlo approach can reproduce experimental results giving an important insight to the systematic effects limiting experiment sensitivity.

Magnetic linear birefringence of light in Tb3Ga5O12

The spectra of the magnetic linear birefringence (MLB) in the terbium-gallium garnet TbGG (Tb3Ga5O12) have been studied within the visible spectral range for temperature T = 90 K. Analysis of the temperature and spectral dependences of MLB has made it possible to determine the “effective” frequencies of the allowed f→d electric dipolar transitions in rare-earth (RE) ions of Tb3+ ions responsible for the formation of quadratic magneto-optical effect in this garnet. The results of the performed magneto-optical experiments in TbGG have confirmed the Akulov's relations (for the quadratic magnetic effects) arising from the phenomenological consideration of MLB for the values of Δn measured at the orientation of the external field in the plane (110) along the main crystallographic directions of the garnet crystal. From analysis of the experimental data on MLB was obtained that for the TbGG MLB changes quadratically with the field within the temperature range 90–300 K. At the same time with a decrease in temperature, its value grows inversely proportional to the square of temperature. A certain difference was also found in the “effective" frequencies of the allowed transitions in Tb3+ ions of the garnet, which is determined from the optical absorption spectra of TbGG on the one hand, and from the analysis of the spectral dependence of MLD on the other one. It has been assumed that this difference can be due to significant distortions of the TbGG crystal lattice caused by this significant magnetostriction at low temperatures, leading to the "mixing" of the 9D term states with the "optically resolved" states of the 7D term.

Measurement of Birefringence Using the Magneto-Optical Modulator in the Magnetomechanical Resonance Mode

In order to detect magnetic linear birefringence of new materials, we have developed an installation using the magneto-optical modulator in the mode of magnetomechanical resonance. We have theoretically substantiated and experimentally applied radiation with phase-frequency dependencies of the Stokes vector on magnetomechanical resonance frequency for measuring magnetic linear birefringence. Our results demonstrate that the radiation in which the variable components of the Stokes vector have different initial phases after passing through the sample with linear birefringence leads to additional phase delays of the signal at the output of the photopolarimeter. As an example, the results of measurements of the magnetic linear birefringence in the film of yttrium iron garnet have been introduced. The minimum value of the linear birefringence which was measured in this experiment 0.018 deg. The advantage of the installation is the independence of the measurements results from the changes of the intensity in the optical system.

Morphology and Magnetic Structure of the Ferritin Core during Iron Loading and Release by Magnetooptical and NMR Methods.

Ferritins are proteins, which serve as a storage and transportation capsule for iron inside living organisms. Continuously charging the proteins with iron and releasing it from the ferritin is necessary to assure proper management of these important ions within the organism. On the other hand, synthetic ferritins have great potential for biomedical and technological applications. In this work, the behavior of ferritin during the processes of iron loading and release was examined using multiplicity of the experimental technique. The quality of the protein's shell was monitored using circular dichroism, whereas the average size and its distribution were estimated from dynamic light scattering and transmission electron microscopy images, respectively. Because of the magnetic behavior of the iron mineral, a number of magnetooptical methods were used to gain information on the iron core of the ferritin. Faraday rotation and magnetic linear birefringence studies provide evidence that the iron loading and the iron-release processes are not symmetrical. The spatial organization of the mineral within the protein's core changes depending on whether the iron was incorporated into or removed from the ferritin's shell. Magnetic optical rotatory dispersion spectra exclude the contribution of the Fe(II)-composed mineral, whereas joined magnetooptical and nuclear magnetic resonance results indicate that no mineral with high magnetization appear at any stage of the loading/release process. These findings suggest that the iron core of loaded/released ferritin consists of single-phase, that is, ferrihydrite. The presented results demonstrate the usefulness of emerging magnetooptical methods in biomedical research and applications.

Low temperature Voigt effect in the terbium gallium garnet crystal.

Magnetic linear birefringence and dichroism are investigated for the paramagnetic terbium gallium garnet (TGG) single crystal in the temperature range 8-100 K. The reciprocal nature is confirmed for the linear birefringence. Furthermore a theoretical model is validated that describes the intermixing of linear and circular birefringence. The ellipticity and rotation of the polarization ellipse are investigated in the light of these measurements. These otherwise minuscule magnetically induced effects are amplified at cryogenic temperatures and are determined by a phase-sensitive technique based on the Fourier decomposition of detected signal intensities. The correspondent measurements also allow us to determine the Curie-Weiss constant corroborating the presence of a magnetically frustrated spin system. Additionally we show how the Voigt geometry enables determining the direction of a magnetic field.

Terahertz-Driven Nonlinear Spin Response of Antiferromagnetic Nickel Oxide.

Terahertz magnetic fields with amplitudes of up to 0.4 Tesla drive magnon resonances in nickel oxide while the induced dynamics is recorded by femtosecond magneto-optical probing. We observe distinct spin-mediated optical nonlinearities, including oscillations at the second harmonic of the 1THz magnon mode. The latter originate from coherent dynamics of the longitudinal component of the antiferromagnetic order parameter, which are probed by magneto-optical effects of second order in the spin deflection. These observations allow us to dynamically disentangle electronic from lattice-related contributions to magnetic linear birefringence and dichroism-information so far only accessible by ultrafast THz spin control. The nonlinearities discussed here foreshadow physics that will become essential in future subcycle spin switching.

Circular and linear magnetic birefringences in xenon at λ = 1064 nm.

The circular and linear magnetic birefringences corresponding to the Faraday and the Cotton-Mouton effects, respectively, have been measured in xenon at λ = 1064 nm. The experimental setup is based on time dependent magnetic fields and a high finesse Fabry-Pérot cavity. Our value of the Faraday effect is the first measurement at this wavelength. It is compared to theoretical predictions. Our uncertainty of a few percent yields an agreement at better than 1σ with the computational estimate when relativistic effects are taken into account. Concerning the Cotton-Mouton effect, our measurement, the second ever published at λ = 1064 nm, agrees at better than 1σ with theoretical predictions. We also compare our error budget with that established for other experimental published values.

Anisotropy of the Linear Magnetic Birefridence of Europium Iron Garnet

The results of investigation of magnetic linear birefringence (MLB) and magnetic linear dichroism (MLD) of Eu3Fe5O12 (EuIG) iron garnet on 7F07F6 optical transitions upon variation of the directions of the magnetization vector I relative to the electric vector E linearly polarized light that propagates through single crystal iron garnets are presented. The measurements were made on Eu3Fe5O12 single-crystal samples in the form of plates 100 μm thick cut in the (110) and (100) plane at the temperatures T = 82 and in a magnetic field H= 25 kOe. The absorption spectra of the linearly polarized light were studied. It is shown that MLB and dichroism in the region of the 7F07F6 absorption band reach values 10-3. First experimentally discovered nonreciprocity of MLB spectra and dichroism with the change of the relative orientation of the magnetization vector I and the light wave vector. This effect may use as a base for the design of the different transducers, for example, magnetooptical optical channels commutator.

Specific Futures of Optical Anisotropy in Terbium Iron and Terbium Gallium Garnets

We reported magnetooptical properties of Tb3+ in single crystals of Tb3Fe5O12 and Tb3Ga5O12 for ion occupying sites of D2 symmetry in the garnets structure. It is shown that in the employed Voigt geometry the magnetic linear birefringence and the dichroism reach values 10-4, and have a strong dependence on the wavelength and a strong anisotropy. The absorption spectra were obtained at temperatures of 30K, 100K using magnetic field up to 25 kOe applied parallel and perpendiculare to the electric vector E linearly polarized light on the 7F67F0 and 7F67F1 optical transitions region. The aim of this research was revealing of a role of contributions of exchange interaction and a crystal field in splitting of energy levels of the basic condition 7F6 ion Tb3+ multiplet in Tb ferrite-garnet by studying of character of spectra magnetic linear dichroism (MLD) paramagnetic and ferrimagnetic crystals placed in an external magnetic field. More over, the assumption about nonreciprocity of magnetic linear birefringence (MLB) spectra and dichroism with the change of the relative orientation of the magnetization vector I and the light wave vector has been experimentally confirmed. This effect may use as a base for the design of the different transducers, for example, magnetooptical optical channels commutator.

Systematic study of magnetic linear dichroism and birefringence in (Ga,Mn)As

Magnetic linear dichroism and birefringence in (Ga,Mn)As epitaxial layers is investigated by measuring the polarization plane rotation of reflected linearly polarized light when magnetization lies in the plane of the sample. We report on the spectral dependence of the rotation and ellipticity angles in a broad energy range of $0.12--2.7\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ for a series of optimized samples covering a wide range on Mn dopings and Curie temperatures and find a clear blueshift of the dominant peak at energy exceeding the host material band gap. These results are discussed within the framework of the $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ and mean-field kinetic-exchange model of the (Ga,Mn)As band structure. We infer that disorder-induced nondirect transitions significantly influence magneto-optical properties of (Ga,Mn)As.

Specific Features of ${\hbox{Tb}}{^{3+}}$ Magnetooptics in Terbium Iron and Terbium Gallium Garnets (${\hbox{Tb}}_{3}{\hbox{Fe}}_{5}{{\hbox{O}}_{12}}$ and ${\hbox{Tb}}_{3}{\rm Ga}_{5}{{\hbox{O}}_{12}}$)

We reported magnetooptical properties of Tb3+ in single crystals of Tb3Fe5O12 and Tb3Ga5O12 for ion occupying sites of D2 symmetry in the garnets structure. It is shown that in the employed Voigt geometry the magnetic linear birefringence and the dichroism reach values 10-4, and have a strong dependence on the wavelength and a strong anisotropy. The absorption spectra were obtained at temperatures of 30 K, 82 K, 100 K using magnetic field up to 25 kOe applied parallel and perpendicular to the electric vector E linearly polarized light on the 7F6-7F0 and 7F6-7F1 optical transitions region. The aim of this research was revealing of a role of contributions of exchange interaction and a crystal field in splitting of energy levels of the basic condition 7F6 ion Tb3+ multiplet in Tb ferrite-garnet by studying of character of spectra of magnetic linear dichroism (MLD) paramagnetic and ferrimagnetic crystals placed in an external magnetic field. Moreover, the assumption about nonreciprocity of magnetic linear birefringence (MLB) spectra and dichroism with the change of the relative orientation of the magnetization vector I and the light wave vector has been experimentally confirmed. This effect may be used as a base for the design of the different transducers, for example, magnetooptical optical channels commutator.

Vacuum magnetic linear birefringence using pulsed fields: status of the BMV experiment

We present the current status of the BMV experiment. Our apparatus is based on an up-to-date resonant optical cavity coupled to a transverse magnetic field. We detail our data acquisition and analysis procedure which takes into account the symmetry properties of the raw data with respect to the orientation of the magnetic field and the sign of the cavity birefringence. The measurement result of the vacuum magnetic linear birefringence k CM presented in this paper was obtained with about 200 magnetic pulses and a maximum field of 6.5 T, giving a noise floor of about 8 × 10-21 T-2 at 3σ confidence level.

Features of optical anisotropy of europium and terbium iron garnets

The results of investigation of magnetic linear birefringence (MLB) and magnetic linear dichroism of Tb3Fe5O12 (TbIG) iron garnet (IG) on 7F6–7F0 and 7F6–7F1 optical transitions and Eu3Fe5O12 (EuIG) iron garnet on 7F0–7F6 at the variation of the directions of the magnetization vector I relative to the electric vector E linearly polarized light that propagates through single crystal iron garnets are presented. The measurements were made on Tb3Fe5О12 and Eu3Fe5O12 single-crystal samples in the form of plates 100μm thick cut in the (110) and (100) plane a temperature of T=82K and in a magnetic field H=22kOe. The absorption spectra of the linearly polarized light were studied. It is shown that MLB and dichroism in the region of the 7F6–7F0 and 7F0–7F6 absorption bands reach values 10−3. The nonreciprocity of MLB spectra and dichroism with the change of the relative orientation of the magnetization vector I and the light wave vector is first experimentally discovered. This effect may be used as a base for the design of the different transducers such as a magnetooptical optical channels commutator.

Coherent control of terahertz radiation from antiferromagnetic magnons in NiO excited by optical laser pulses

We investigated terahertz (THz) radiation from twinned NiO(110) single crystals excited by optical laser pulses with various polarization states. We found that the polarity of THz radiation from optically excited coherent antiferromagnetic (AFM) magnons can be controlled by switching the helicity of circular polarization and by rotating the direction of linear polarization of the laser pulses. The dependence of THz radiation on the polarization of the laser pulses suggests that AFM magnons are excited by the inverse Faraday effect in a twinned crystal with linear magnetic birefringence.