Optical characterization of thermal-stress induced by spin waves in thin-film ferrimagnetic structures

It is shown that intense spin-dipole waves (SDWs) excited in thin yttrium iron garnet (YIG) films induce an in-plane thermal stress (σ) of 1-2 MPa in a YIG/GGG structure (where GGG is gadolinium gallium garnet). In YIG/GGG with normal magnetization, σ shifts its ferromagnetic resonance frequency by ≈1 MHz, which is comparable to the linewidth of the absorption curve of YIG/GGG resonators. The effect was characterized by an optical technique that detects σ in the GGG substrate. It was also demonstrated that this effect can be used for the optical-microwave spectroscopy of spin waves in thin ferromagnetic films, by using thermal mapping of SDWs in the substrate. We have shown that this opens up the possibility of determining the contribution of the two-particle magneto-elastic interaction to the microwave heating of the sample.     

Multiferroics: Different ways to combine magnetism and ferroelectricity

Multiferroics — materials which are simultaneously (ferro)magnetic and ferroelectric, and often also ferroelastic — attract now considerable attention, both because of the interesting physics involved and as they promise important practical applications. In this paper, I give a survey of microscopic factors determining the coexistence of these properties, and discuss different possible routes to combine them in one material. In particular, the role of the occupation of d-states in transition metal perovskites is discussed, possible role of spiral magnetic structures is stressed, and the novel mechanism of ferroelectricity in magnetic systems due to combination of site-centred and bond-centred charge ordering is presented. Microscopic nature of multiferroic behaviour in several particular materials, including magnetite Fe₃O₄, is discussed.     

Pattern formation and direct measurement of the spatial resolution in a photorefractive liquid crystal light valve

In a photorefractive liquid crystal light valve, acting as a Kerr-like nonlinear optical medium, we show the appearance of optical patterns induced by a single mirror feedback. The spatial wavelength of the patterns scales with the distance between the mirror and the valve and the contrast of the patterns decreases for decreasing this distance. We use these properties to setup a new optical scheme for the measurement of the spatial resolution of the nonlinear device.     

Observation of indirect parallel pumping of magneto-elastic modes in layered YIG/GGG structures

In this work we report new experimental results on nonlinear excitation of magnetoelastic (ME) modes in layered YIG/GGG waveguide structures at GHz frequencies, obtained by a guided-wave light scattering technique. It is shown that the fundamental spin-dipole wave (SDW) mode induces a secondary microwave field at double frequency that can efficiently excite shear elastic modes of the structure. The distinctive feature of a mechanism for ME coupling proposed is that it is free from the selection rules and provides continuous excitation of elastic modes within a wide frequency range by means of the standard microstripe line excitation system.     

Magnetic properties and specific heat of Dy1−xLaxNi2 compounds

Magnetic and specific heat measurements have been carried out on polycrystalline series of single-phase Dy_1−xLa_xNi₂ (0?x?1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with x?0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie-Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T.     

Third-order optical nonlinearity of (Ba0.7Sr0.3)TiO3 ferroelectric thin films fabricated by soft solution processing

(Ba_0.7Sr_0.3)TiO₃ (BST) ferroelectric thin films with perovskite crystal structure were fabricated by soft solution processing on a quartz substrate. The third-order nonlinear optical properties were investigated by using Z-scan technique. Positive nonlinear refractive index and nonlinear absorption coefficient were determined to be 4×10⁻⁷ esu and 1.2×10⁻⁶ m/w, respectively. The real part and imaginary part of third-order optical nonlinear susceptibility were calculated and the values were 6.43×10⁻⁸ and 5.14×10⁻⁸ esu, respectively. All of these results show ferroelectric BST thin film is promising for applications in nonlinear optical devices.     

Generalized wave coupling theory of linear electro-optic effect in absorbent medium

Starting from Maxwell’s equations and considering the linear electro-optic effect as a perturbation, we present a generalized wave coupling theory of linear electro-optic effect in absorbent medium. We give the rigorous solution of the resultant equations for a light wave propagating along any direction with an external dc electric field along an arbitrary direction. As an application, we use the theory to discuss the influence of absorption on the light wave in a KTP crystal. The results demonstrate that the absorption coefficients influence not only the amplitude but also the phase of the light wave.     

Magnetodielectric coupling in Ca3Co2O6 triangular Ising lattice

Geometric frustration in magnetic systems is a key ingredient for magnetodielectric coupling. Ca₃Co₂O₆ system is a model of triangular Ising lattice and presenting exotic magnetic properties. With a partially disordered antiferromagnetic ground state and two steps in magnetisation to a ferrimagnetic state and to a ferromagnetic state, this oxide is a good system for studying the effect of frustration in magnetodielectric coupling. Dielectric constant measurements of single crystals of Ca₃Co₂O₆ have been done showing the sensitivity of this technique to frustration.     

Time resolved magnetic ordering and photo-induced gyrotropy at room temperature in CdMgTe-CdMnTe semiconductor micro-cavities

Using a picosecond pump and probe time-resolved technique we evidence a single pump pulse photo-induced magnetic ordering in a Mn-doped semiconductor magneto-photonic microcavity operating in the strong coupling regime at room temperature. This nanosecond duration magnetization is attributed to a magnetic ordering of the Mn-impurities mediated through photo-generated holes and enhanced through the confinement. It is distinct from the preceding short lived photo-induced spin orientation of carriers also evidenced by our technique for circularly polarized pump beams. The photo-generated magnetic flux density amounts to a 1 kG for beam fluences of few tens μJ cm⁻² and effective Mn concentrations of 5 nm⁻³; large photo-induced magneto-optic Kerr rotations are also evidenced.     

Magnetostriction measurements on the multidirectional magnetization performance of SiFe steel

By means of a 3-phase excited rotational single sheet tester in combination with three sets of strain gauges, multi-directional magnetostriction (MS) was studied for highly grain oriented (HGO) SiFe. Emphasis was put on a new type of material with hyper-stress coating based on titanium nitride. Experiments were performed for elliptical, Lozenge-type and circular magnetization. In all states that are relevant for practice, the new type showed lowest levels of MS, especially for the rolling direction and the normal direction. By means of a domain model, which is based on the results of MS, the superior behavior is explained by a suppression of oblique domains.     

Giant uniaxial stress-permeability effect on electrical parameters of heterotypic MnZn ferrite devices and electromagnetic effect

The effects of uniaxial stress on permeability and electrical parameters of heterotypic manganite zinc (MnZn) ferrite devices have been investigated. Giant stress-permeability, stress-capacitance and stress-impedance that are independent of skin effects have been simultaneously observed to exist in a wide range of frequency at room temperature. All the uniaxial stress effects enhance with increasing the permeability of the ferrite. The stress-inductance is same as the stress-impedance and reverse to the stress-capacitance in phase. The stress effects under uniaxial pulling force are analogical with those under uniaxial pressing force. A composite of electrostrain/stress-permeability has been fabricated. Its electromagnetic effects have been observed to be homologous with the stress effects and can also exist in wide range of frequency but display some maximums. Analysis shows that both stress and electromagnetic effects originate from the variation of the magnetic domain structure in the ferrites caused by applied mechanical stress.     

Measurement of the stress sensitivity of magnetostriction in electrical steels under distorted waveform conditions

Electrical steel laminations used in the construction of transformer cores are subject to stresses introduced during their construction and analysis of the effect of this on the magnetostriction of the lamination has been investigated previously. It has been shown that higher harmonics of magnetostriction are of greater importance than the fundamental when considering transformer noise. Whereas previous studies have concentrated on the magnetostriction harmonics generated by sinusoidal magnetization, this investigation seeks to understand the relationship between harmonics present in the magnetization waveform and those in the magnetostriction waveform. A measurement system has been designed based on a similar principle to one previously described. In this case, a single Labview Virtual Instrument (VI) is used for the control of the applied stress, controlled magnetization and measurement of magnetostriction together with other magnetic parameters such as specific total loss, specific apparent power, permeability, coercivity and remanence. An adaptive digital feedback algorithm is utilized for control of arbitrary waveform which may be constructed from discrete harmonics or read from an input waveform. As well as measuring peak magnetostriction the software utilizes an FFT to calculate the harmonics of magnetostriction at each stress point. The effect of harmonics introduced into the magnetization waveform on the magnetostriction harmonics will be shown at various applied stresses. A harmonic, Harm_B in the flux density waveform is shown to have the effect of producing a dominant harmonic in the magnetostriction given by (Harm_B+1)/2.     

Enhanced photo-response of thin-film structures with nanocrystals

Thin-film multilayers with dielectric and semiconductor nanolayers of 200-10 nm thicknesses have been deposited by thermal evaporation onto irradiation-resistive substrates using pure crystals as evaporated targets. Some multilayers were γ-irradiated in air at room temperature with dose of 83 kGy. X-ray diffraction and microscopy studies reveal that the multilayers consist of nanometer-sized crystals with cubic structure and defined size. Film structures were oriented along the (1 1 1) plane. Absorption spectra of non-irradiated LiF nanocrystals of 100 nm size and those of initial crystals give evidence of metal colloids presence. Photoluminescence spectra of γ-irradiated nanostructures with various LiF content show the enhancement of F₃⁺-colour centres excitation in the region of metal colloids absorption and the increase is observed between emission intensities of F₃⁺ and F₂ centers with respect to initial crystals γ-coloured in identical conditions. Emission intensities of both centers under excitation in the M band correlate with LiF content. These effects, which are related to high-quality nanocrystals, but at the same time depend strongly on the defect content, especially as far as their 1-2 ps nonlinearities are concerned, could depend on nanocrystal purity and metal excess collection in their boundaries regions.     

Preparation and multi-properties of insulated single-phase BiFeO3 ceramics

Six types of BiFeO₃ ceramic samples, with subtle differences in synthesis conditions, were prepared. The comparison of their phases, electrical resistivity, and porosity revealed that the use of Bi₂O₃ and Fe₂O₃ powders of <1 μm size and a rapid liquid-phase sintering process of 855 °C for 5 min at 100 °C/s is beneficial to synthesize poreless single-phase BiFeO₃ samples with high electrical resistivity of ∼5×10¹² Ω cm. Deoxygenated Bi_xFe_yO_{1.5x+1.5y−δ} (x≠y, δ≥0) impurities were identified and found to be the main cause of low electrical resistivity and high porosity in the multi-phase samples. Large saturation polarization of 16.6 μC/cm² and low leakage current density of 30 mA/m², both at a high electric field of 145 kV/cm, were measured in the optimized single-phase samples at room temperature besides a large piezoelectric d₃₃ coefficient of 27 pC/N and an obvious canted antiferromagnetic behavior.     

Femtosecond Z-scan measurement of third-order optical nonlinearities in anatase TiO2 thin films

Anatase phase TiO₂ films have been grown on fused silica substrate by pulsed laser deposition technique at substrate temperature of 750 °C under the oxygen pressure of 5 Pa. From the transmission spectra, the optical band gap and linear refractive index of the TiO₂ films were determined. The third-order optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. The real and imaginary parts of third-order nonlinear susceptibility χ⁽³⁾ were determined to be −7.1 × 10⁻¹¹esu and −4.42 × 10⁻¹²esu, respectively. The figure of merit, T, defined by T=βλ/n₂, was calculated to be 0.8, which meets the requirement of all-optical switching devices. The results show that the anatase TiO₂ films have great potential applications for nonlinear optical devices.     

Multiferroic properties of Bi0.8La0.2FeO3/CoFe2O4 multilayer thin films

Bi_0.8La_0.2FeO₃/CoFe₂O₄ (BLFO/CFO) multilayer thin films (totally 20 layers BLFO and 19 layers CFO) were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. X-ray diffraction and transmission electron microscope measurements show that the films are polycrystalline and consisted of multilayered structure. Ferroelectric hysteresis loops with remnant polarization and saturated polarization of 4.2 and 13.3 μC/cm², respectively, were observed. On the other hand, the films show well-shaped magnetization hysteresis loops with saturated and remnant magnetization of 34.7 and 11.4 emu/cm³, respectively, which are significantly larger than pure BLFO thin films deposited under the same conditions. These results indicate that constructing epitaxial superlattice might be a promising way to fabricate multiferroics with improved properties.     

Controllable microstructures and multiferroic properties of Pb(Zr0.53Ti0.47)O3-CoFe2O4 composite films

1-3 and 2-2 types Pb(Zr_0.53Ti_0.47)O₃-CoFe₂O₄ (PZT-CFO) composite films with controllable microstructures, consisted by CFO nanopillar embedded in PZT matrix and PZT-CFO gratings respectively, have been fabricated on Pt/Ti/SiO₂/Si substrates by combining lithography technology and pulsed laser deposition. X-ray diffraction confirms that the films are well crystallized under optimized postannealing conditions. Scanning electron microscope reveals that the periodic microstructures can be well controlled. Especially, intrinsic room temperature ferroelectric and ferrimagnetic behaviors are observed simultaneously. The structure-properties relationship is discussed. Our results may provide an alternative method to design and prepare multiferroic composite films with controllable microstructures.     

Time-resolved photo-induced nonlinear magneto-optical Kerr effect for the study of spin dynamics at a GaAs(001) surface

Received: 20 September 1998     

Low-frequency magnetoelectric effects in ferrite-piezoelectric nanostructures

A theoretical model is presented for low-frequency magnetoelectric (ME) coupling in ferrite-piezoelectric nanobilayers and nanopillars of ferrite and piezoelectrics. We take into account the lattice mismatch and clamping effects of the substrate in determining the ME voltage coefficient. The model could be used to estimate the ME couplings from known material parameters (piezoelectric modules, magnetostriction, stiffness and geometrics) or data on ME coupling could be used to obtain composite parameters. It is shown that increasing the substrate thickness results in a decrease in the ME interaction in nanobilayers due to the clamping caused by the substrate. For nanopillars of NFO in PZT matrix, the substrate pinning effects are negligible as long as the length of the pillar is much greater than its radius.