Crystal chirality magneto-optical effects in collinear antiferromagnets (original) (raw)
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Physical Review Materials, 2021
Polar and chiral Ni 3 TeO 6 was found to exhibit a colossal magnetoelectric (ME) effect associated with collinear antiferromagnetic order. We have investigated the evolution of its magnetic state with the substitution of the Ni 2+ spins by isotropic Mn 2+ spins, namely (Ni, Mn) 3 TeO 6 (NMTO). The ground state of NMTO maintains an Ising nature, but a new phase with XY-type magnetic anisotropy is discovered at an intermediate temperature range. Neutron powder diffraction experiments reveal that (1) Mn ions tend to occupy a specific transition-metal site, consistent with the first-principle calculation, and (2) the intermediate phase is an incommensurate (IC) helical magnetic state propagating along the c axis with q IC = (0, 0, 1.5 ± δ). We also found that the balance between these two magnetic states can be readily manipulated with magnetic fields, which results in significant ME effects. This selective site occupancy of NMTO allows for a unique competition between collinear and helical magnetic structures with minimal chemical disorder. Thus, NMTO serves as a model system to study site-specific chemical control of noncollinear magnetism and ME coupling.
Chiral magnetic order at surfaces driven by inversion asymmetry
Nature, 2007
Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed singlehandedness of biomolecules 1 ) and in particle physics (in the chargeparity violation of electroweak interactions 2 ). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field 3,4 . This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur 5 . Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure 6,7 , causing a variety of excitations or manipulations of the magnetization 8,9 and giving rise to microwave emission, magnetization switching, or magnetic motors.
Ab Initio Cycloidal and Chiral Magnetoelectric Responses in Cr2O3
Ab initio cycloidal and chiral magnetoelectric responses in Cr2O3, 2016
We present a thorough density functional theory study of the magneto-electric (ME) effect in Cr2O3. The spin-lattice ME tensor α was determined in the low-field and spin flop (SF) phases, using the method of dynamical magnetic charges, and found to be the sum of three distinct components. Two of them, a large relativistic "cycloidal" term and a small longitudinal term, are independent on the spin orientation. The third, only active in the SF phases is also of relativistic origin and arises from magnetic-field-induced chirality, leading to a non-toroidal ME response.
Order and disorder in the magnetization of the chiral crystal CrNb3S6
Physical Review B, 2019
Competing magnetic anisotropies in chiral crystals with Dzyaloshinskii Moriya exchange interactions can give rise to non-trivial chiral topological magnetisation configurations with new and interesting properties. One such configuration is the magnetic soliton, where the moment continuously rotates about an axis. This magnetic system can be considered to be one dimensional and, because of this, it supports a macroscale coherent magnetisation, giving rise to a tunable chiral soliton lattice (CSL) that is of potential use in a number of applications in nanomagnetism and spintronics. In this work we characterise the transitions between the forced-ferromagnetic (F-FM) phase and the CSL one in CrNb 3 S 6 using differential phase contrast imaging in a scanning transmission electron microscope, conventional Fresnel imaging, ferro-magnetic resonance spectroscopy, and mean-field modelling. We find that the formation and movement of dislocations mediate the formation of CSL and F-FM regions and that these strongly influence the highly hysteretic static and dynamic properties of the system. Sample size and morphology can be used to tailor the properties of the system and, with the application of magnetic field, to locate and stabalise normally unstable dislocations and modify their dimensions and magnetic configurations in ways beyond that predicted to occur in uniform films.
Effect of anisotropy on magneto-optical properties of uniaxial crystals: Application to CrO2
Physical Review B, 1996
The effect of the magnetization direction m on the dielectric tensor of uniaxial crystals is described by a simple dependence of the gyration vector g() on m. It is shown that the vectors g() and m as well as the orbital magnetic moment ͗L ͘ and m are generally aligned noncollinearly in contrast to an isotropic case. Formulas describing the polar Kerr effect are derived for crystals with their principal axis cЌm and for polycrystals having c randomly oriented in the sample plane. Using these analytical results and performing ab initio calculations, we correctly reproduce anisotropy in optical spectra of CrO 2 and the main features in magneto-optical spectra of polycrystalline films of CrO 2. The maximal optical anisotropy and orientation dependence of g() of 100% are found in the energy interval បр 2.1 eV coinciding with the direct half-metallic ferromagnetic gap of CrO 2. The noncollinearity effects in this interval are also very large. The obtained results correlate well with strong orientation dependence of ͗L ͘ found in our calculations. ͓S0163-1829͑96͒07525-X͔
Electromagnetooptical Effects in Ferri-And Antiferromagnets
Journal of the Magnetics Society of Japan, 1987
Optical methods open new horizons in studying magnetoelectric interaction in magnetically ordered crystals. In the present paper we shall report on some new optical effects observed in two model crystals: ~n antiferromagnetic Cr 2 0 and in ferrimagnetic Y~Fe~Ot2. In Cr70~ we observed a nonreciprocal rotation or ~lane-po larized light induced by ac eIeetric field applied along optical axis. The temperature variation of this rotation was found to differ substantially from that of magnetoelectric effect in a low-frequency range. New antiferromagnetic mechanism of nonreciprocal rotation was suggested to exist in an optical spectral range. In Cr ° a second order optical magnetoelectric effect was observed for the first tiffie 3 which was related to an antiferromagnetic domain switching. In Y,Fe 5 0 17 electromagnetooptical effects were observed mainly in the regions of domain waIl movement and rotation of magnetization. Much better resolution was obtained in comparison to a low-frequency study of magnetoelectric interaction. A forbidden, linear in electric field, effect was observed in Y,Fe~012 and we relate it to crystal imperfections, which destroy the symmetry center.
Driving spin chirality by electron dynamics in laser-excited antiferromagnets
Communications Physics
Despite recent successes in the area of ultrafast manipulation of magnetic order, optical generation and manipulation of complex spin textures is hindered by an insufficient theoretical understanding of underlying processes. In particular an important aspect of subtle connection between the electronic and magnetic degrees of freedom is not properly accounted for in existing theories. Here, we uncover a distinct physical mechanism for imprinting spin chirality into collinear magnets with short laser pulses. By simultaneously treating the laser-ignited evolution of electronic structure and magnetic order, we show that their intertwined dynamics can result in an emergence of quasi-stable chiral states. We find that laser-driven chirality does not require any auxiliary external fields or intrinsic spin–orbit interaction to exist, and it can survive on the time scale of nanoseconds even in the presence of thermal fluctuations, which makes the uncovered mechanism relevant for understandin...
Paramagnetic magnetostriction in the chiral magnet CrNb3S6 at room temperature
Physical Review B, 2022
We report that the magnetostriction (MS) effects occur in a paramagnetic state of a chiral magnet CrNb 3 S 6. Through a series of experimental tests at room temperature, structural changes were observed at the level of a unit cell. The structural parameters are dependent of the strength and direction of magnetic field (H) even at temperature excessively higher than the magnetic ordering temperature T c of 127 K. The present paramagnetic MS prominently appeared under H the ab plane (easy plane) as opposed to under H the c axis. Features observed in the paramagnetic MS effect significantly differ from those of the spontaneous MS in the vicinity of T c [Phys. Rev. B 102, 014446 (2020)]. In this material, the orbital angular momentum L of Cr originates from the hybridization between Cr and Nb, and L is strongly coupled with the crystal structure [Phys. Rev. B 99, 174439 (2019)]. The present study clarified that the symmetry of the CrS 6 octahedron is sensitive to H even at room temperature. The paramagnetic spin-orbit coupling should induce the distortion of CrS 6 octahedron, resulting in the changes in Cr-Nb(4 f) distance via the change in the hybridization between Cr-a 1g and Nb-4d z 2 orbitals.