In-plane current induced nonlinear magnetoelectric effects in single crystal films of barium hexaferrite.

This report is on the observation and analysis of nonlinear magnetoelectric effects (NLME) for in-plane currents perpendicularly to the hexagonal axis in single crystals and liquid phase epitaxy grown thin films of barium hexaferrite. Measurements involved tuning of ferromagnetic resonance (FMR) at 56-58 GHz in the multidomain and single domain states in the ferrite by applying a current. Data on the shift in the resonance frequency with input electric power was utilized to estimate the variations in the magnetic parameter that showed a linear dependence on the input electric power. The NLME tensor coefficients were determined form the estimated changes in the magnetization and uniaxial anisotropy field. The estimated NLME coefficients for in-plane currents are shown to be much higher than for currents flowing along the hexagonal axis. Although the frequency shift of FMR was higher for the single domain resonance, the multi-domain configuration is preferable for device applications since it eliminates the need for a large bias magnetic field. Thus, multidomain resonance with current in the basal plane is favorable for use in electrically tunable miniature, ferrite microwave signal processing devices requiring low operating power.

Nonlinear magnetoelectric effects in Al-substituted strontium hexaferrite.

This report is on the observation and theory of electric field E induced non-linear magnetoelectric (NLME) effects in single crystal platelets of ferrimagnetic M-type strontium aluminum hexagonal ferrite. Using microwave measurement techniques, it was found that a DC electric field along the hexagonal c-axis results in significant changes in the saturation magnetization and uniaxial magneto-crystalline anisotropy field and these changes are proportional to the square of the applied static electric field. The NLME effects were present with or without an external bias magnetic field. The E-induced variation in magnetic order parameters is attributed to weakening of magnetic exchange and spin-orbit interactions since conduction electrons in the ferrite are effectively excluded from both interactions while being in transit from one Fe ion to another. We present a phenomenological theory which considers magneto-bielectric effects characterized by a quadratic term in electric field E in the free energy density. The coefficients for the NLME coupling terms have been calculated from experimental data and they do show variations with the Al substitution level and the largest rates of change of the saturation magnetization and anisotropy constant change with the applied power were observed for x = 0.4. It was also clear from the study that strength of the NLME effect does not depend on the amount Al substitution, but critically depends on the electrical conductivity of the sample with the highest NLME coefficients estimated for the sample with the highest conductivity. Results of this work are of importance for a new family of electric field tunable, miniature, high frequency ferrite devices.

Effect of laser pulse propagation on ultrafast magnetization dynamics in a birefringent medium.

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 (Sm0.5Pr0.5)FeO3 and (Sm0.55Tb0.45)FeO3, 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.

Visualization of oscillatory stresses in transparent media using a ß-Ga2O3 adaptive detector.

The phase-modulated optical signal produced by light propagation through stressed transparent media is detected using the non-steady-state photoelectromotive force technique. The mechanical system, including the glass plate and piezoelectric transducer, demonstrates resonant behavior in the vicinity of 100 kHz. The measured distribution of the optical phase is a bell-shaped surface for these frequencies. The simulation of the piezo-optic response shows qualitative coincidence with the experimental data and provides maps of the stress field. The characterization of the ß-Ga2O3 adaptive detector is performed for the light wavelength λ=532 nm.

Anomalous multi-order Raman scattering in LaMnO3: a signature of quantum lattice effects in a Jahn-Teller crystal.

The multi-order Raman scattering is studied up to fourth order for a detwinned LaMnO3 crystal. Based on a comprehensive data analysis of the polarization-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm(-1). We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn-Teller Mn(3+) ion due to the lattice anharmonicity and that the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.

Magnetic and micro-Raman studies of hexagonal-DyMnO3.

Dc-susceptibility measurements and Raman active phonon frequencies of hexagonal DyMnO(3) retrace the Mn(3+) ions antiferromagnetic transition at T(N) ~ 70 K and their spin reorientation at T(SR) ~ 48 K. The temperature evolution of Raman active mode frequencies and their over-hardening are associated with Dy(3+) and Mn(3+) ion displacements below T(N) and with a spin-phonon coupling that involves apical oxygen.

Coherent control of the route of an ultrafast magnetic phase transition via low-amplitude spin precession.

Time-resolved magneto-optical imaging of laser-excited rare-earth orthoferrite (SmPr)FeO3 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.

Metamagnetic transitions in electron-doped single crystals of manganites Ca(1-x)(Ln)(x)MnO3, (Ln = La, Ce; x ≤ 0.12).

The magnetization curves of Ca(1-x)(Ln)(x)MnO(3) single crystals, where Ln denotes La or Ce, x ≤ 0.12, have been measured in pulsed magnetic fields up to 350 kOe. The metamagnetic transitions for compositions with x = 0.10 and 0.12 have been observed in the temperature range 77-240 K. The hysteresis around the transition for the sweep-up and sweep-down branches of the magnetization curve is wide for x(Ce) = 0.10 and 0.12, and relatively narrow for x(La) = 0.12. The maximum magnetization value reaches ~50% from its theoretical value for x(Ce) = 0.10 and 0.12, and ~24% for x(La) = 0.12 in a magnetic field H = 350 kOe. The metamagnetic transition has been attributed to the melting of orbital/charge ordering in the dielectric antiferromagnetic C-type phase, which is accompanied by the growth of the volume of the conductive phase with antiferromagnetic G-type ordering and ferromagnetic contribution.

Relaxations as key to the magnetocapacitive effects in the perovskite manganites.

We present a detailed dielectric study of the relaxation effects that occur in several perovskite rare-earth manganites, including the multiferroics TbMnO(3) and DyMnO(3). We demonstrate that the strong magnetocapacitive effects, observed for electrical fields E parallel c, are nearly completely governed by magnetic-state induced changes of the relaxation parameters. The multiferroic materials, which undergo a transition into a spiral magnetic state, show qualitatively different relaxation behavior than those compounds transferring into an A-type antiferromagnetic state. We ascribe the relaxations in both cases to the off-center motion of the manganese ions, which in the multiferroic systems also leads to the ferroelectric ordering.

Magnetic and magnetoelectric excitations in TbMnO3.

Magnetic and magnetoelectric excitations in the multiferroic TbMnO3 have been investigated at terahertz frequencies. Using different experimental geometries we can clearly separate the electroactive excitations (electromagnons) from the magnetoactive modes, i.e., antiferromagnetic resonances. Two antiferromagnetic resonances were found to coincide with electromagnons. This indicates that both excitations belong to the same mode and the electromagnons can be excited by a magnetic ac field as well. In spite of the 90 degrees rotation of the magnetic cycloid in external magnetic fields, the electromagnons are observable for electric ac fields parallel to the a axis only.

Observation of a griffiths phase in paramagnetic La1-xSrxMnO3.

We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.

Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses.

The demand for ever-increasing density of information storage and speed of manipulation has triggered an intense search for ways to control the magnetization of a medium by means other than magnetic fields. Recent experiments on laser-induced demagnetization and spin reorientation use ultrafast lasers as a means to manipulate magnetization, accessing timescales of a picosecond or less. However, in all these cases the observed magnetic excitation is the result of optical absorption followed by a rapid temperature increase. This thermal origin of spin excitation considerably limits potential applications because the repetition frequency is limited by the cooling time. Here we demonstrate that circularly polarized femtosecond laser pulses can be used to non-thermally excite and coherently control the spin dynamics in magnets by way of the inverse Faraday effect. Such a photomagnetic interaction is instantaneous and is limited in time by the pulse width (approximately 200 fs in our experiment). Our finding thus reveals an alternative mechanism of ultrafast coherent spin control, and offers prospects for applications of ultrafast lasers in magnetic devices.

Spin-controlled Mott-Hubbard bands in LaMnO3 probed by optical ellipsometry.

Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of LaMnO3 in the range 0.5-5.6 eV at temperatures 50