An effective magnetic field from optically driven phonons (original) (raw)
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Physical Review B, 2017
Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the order parameter are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite (Fe 3 O 4) light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions.
Optical coupling to spin waves in the cycloidal multiferroic BiFeO3
Physical Review B, 2008
The magnon and optical phonon spectrum of an incommensurate multiferroic such as BiFeO3 is considered in the framework of a phenomenological Landau theory. The resulting spin wave spectrum is quite distinct from commensurate substances due to soft mode anisotropy and magnon zone folding. The former allows electrical control of spin wave propagation via reorientation of the spontaneous ferroelectric moment. The latter gives rise to multiple magneto-dielectric resonances due to the coupling of optical phonons at zero wavevector to magnons at integer multiples of the cycloid wavevector. These results show that the optical response of a multiferroic reveals much more about its magnetic excitations than previously anticipated on the basis of simpler models.
Ferroics: magnetic-compass lattice and optical phonon dispersions of dipolar crystals
We report a simple safe and attractive pedagogic demonstration with magnetic compasses that facilitates an intuitive understanding of the concept that ferromagnetism and ferroelectricity do not result from dipole-dipole interactions alone. Phonon dispersion relations were calculated for a 3-dimensional simple-cubic dipole crystal and a 2-dimensional square-lattice dipole crystal. The latter calculation corresponds to the compass demonstration that confirm the antiferro ground state structure. A sum rule for the three optical phonon frequencies is discussed. A mathematical nonanalyticity in the longitudinal optical phonon are also illustrated.
Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3
Journal of Applied Physics, 2014
Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO 3 over a temperature range of 20-300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a N eel transition at T N $ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below T N . Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near T N . An additional phonon anomaly was identified at T * $ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman frequencies between T N and T * suggest competing WFM and AFM interactions. A close match between the phonon frequency of B 3g (3)-octahedral rotation mode with the square of sublattice magnetization between T N and T * is indicative of the presence of spin-phonon coupling in multiferroic YCrO 3 . V C 2014 AIP Publishing LLC. [http://dx.
Physical Review B
Helical magnets are extremely promising, as they have fascinating magnetic, electric, and phononic properties. Here, we report a spectrum of simultaneously occurring and highly entangled intriguing phenomena induced by helical spin ordering in a noncentrosymmetric and spin-frustrated system Fe 3 (PO 4)O 3. These phenomena include magnetodielectric effect in the form of a frequency-independent pronounced dielectric peak, clear magnetostriction effect manifested as a dramatic downturn in the thermal variation of lattice parameters, and strong spin-phonon coupling (which displays a unique anomalous hardening and softening of various phonon modes) at temperatures as high as T N = 163 K. The observed dielectric peak is seemingly associated with a structural distortion via the strong magnetostriction effect.
In this paper, we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite YbFeO 3 at temperatures below and above the spin-reorientation (SR) transition T SR = 7.6 K, in magnetic fields applied along the a, b, and c axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of ∼60 meV, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of ∼1 meV within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at T SR drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinonlike fluctuations. At T < T SR , the Yb spin chains have a well defined field-induced ferromagnetic (FM) ground state, and the spectrum consists of a sharp single-magnon mode, a two-magnon bound state, and a two-magnon continuum, whereas at T > T SR only a gapped broad spinonlike continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem J Yb-Yb , mainly neglected in previous studies, creates unusual quantum spin dynamics on the low-energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be "hidden" by a dominating interaction.
Tuning inelastic light scattering via symmetry control in the two-dimensional magnet CrI3
Nature Nanotechnology
The coupling between spin and charge degrees of freedom in a crystal imparts strong optical signatures on scattered electromagnetic waves. This has led to magneto-optical effects with a host of applications, from the sensitive detection of local magnetic order to optical modulation and data storage technologies. Here, we demonstrate a new magnetooptical effect, namely, the tuning of inelastically scattered light through symmetry control in atomically thin chromium triiodide (CrI3). In monolayers, we found an extraordinarily large magneto-optical Raman effect from an A1g phonon mode due to the emergence of ferromagnetic order. The linearly polarized, inelastically scattered light rotates by ~40⁰, more than two orders of magnitude larger than the rotation from MOKE under the same experimental conditions. In CrI3 bilayers, we show that the same A1g phonon mode becomes Davydov-split into two modes of opposite parity, exhibiting divergent selection rules that depend on inversion symmetry and the underlying magnetic order. By switching between the antiferromagnetic states and the fully spin-polarized states with applied magnetic and electric fields, we demonstrate the magnetoelectrical control over their selection rules. Our work underscores the unique opportunities provided by 2D magnets for controlling the combined time-reversal and inversion symmetries to manipulate Raman optical selection rules and for exploring emergent magneto-optical effects and spin-phonon coupled physics. Main text: Raman scattering measures light inelastically scattered from collective quasiparticle excitations. Since it is highly sensitive to material parameters such as crystal symmetry and local electronic states, Raman spectroscopy has provided a powerful probe of a broad range of condensed matter phenomena, such as charge density waves 1 , superconductivity 2 , ferroelectricity 3 , and topological physics 4. In particular, Raman scattering from spin-phonon excitations has yielded incisive information on magnetic materials. For instance, in recently developed 2D van der Waals magnets, Raman scattering has been used to reveal magnetic order and phase transitions 5-7 down to a single layer 8-10. Chromium triiodide (CrI3), a van der Waals magnet, was shown to be a layered antiferromagnet in its few-layer form: spins within each layer are ferromagnetically (FM) coupled with strong outof-plane anisotropy, while the interlayer exchange is antiferromagnetic (AFM) 11. For bilayers, the system undergoes a spin-flip transition upon the application of a moderate magnetic field 11 , switching from a layered AFM state to a fully spin-polarized state. In addition, magneto-optical effects manifest strongly and in distinctly novel ways in CrI3. Examples include the very large magnetic-optical Kerr effect (MOKE) 11,12 and spontaneous helical light emission 13 from ferromagnetic monolayers, and electric-field induced Kerr rotation 14-16 and giant second-order nonreciprocal optical effects 17 in antiferromagnetic bilayers.
Applied Physics Letters, 2013
Ultrafast magnetization dynamics of a rare-earth Bi-doped garnet were studied using an optical pump-probe technique via the inverse Faraday effect. We observed a wide range of frequency modes of the magnetization precession, covering two orders of magnitude. The excitation efficiency of low-frequency precessions in the GHz range, together with a significant beating effect, strongly depended on the amplitude of the external magnetic field. On the contrary, high-frequency precession was independent of the external magnetic field. The obtained results may be exploited in the development of wide class of microwave and magneto-optical devices. V C 2013 AIP Publishing LLC. [http://dx.