A. A. Timopheev

Direct and converse magnetoelectric effects in Metglas/LiNbO3/Metglas trilayers

Electromechanical and magnetoelectric properties of Metglas/LiNbO3/Metglas trilayers have been studied in the frequency range from 20u2009Hz to 0.4u2009MHz. A trilayer of Metglas/PMN-PT/Metglas prepared in the same way was used as a reference. Though PMN-PT has much larger charge piezocoefficients than LiNbO3 (LNO), the direct magnetoelectric voltage coefficient is found to be comparable in both trilayers due to the much lower dielectric permittivity of LNO. The magnitude of the direct magnetoelectric effect in the LNO trilayers is about 0.4u2009V/cmu2009Oe in the quasistatic regime and about 90u2009V/cmu2009Oe at the electromechanical resonance. Calculations show that the magnetoelectric properties can be significantly improved (up to 500u2009V/cmu2009Oe) via controlling the cut angle of LNO, choosing the appropriate thickness ratio of the ferroelectric/ferromagnetic layers and a better bonding between Metglas and LNO. Advantages of using LiNbO3-type ferroelectrics in magnetoelectric composites are discussed.

Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures

Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.

Anomalous Hall effect in two-phase semiconductor structures: The role of ferromagnetic inclusions

The Hall effect in InMnAs layers with MnAs inclusions of 20-50 nm in size is studied both theoretically and experimentally. We find that the anomalous Hall effect can be explained by the Lorentz force caused by the magnetic field of ferromagnetic inclusions and by an inhomogeneous distribution of the current density in the layer. The hysteretic dependence of the average magnetization of ferromagnetic inclusions on an external magnetic field results in a hysteretic dependence of RH(Hext). Thus we show the possibility of a hysteretic RH(Hext) dependence (i.e. observation of the anomalous Hall effect) in thin conductive layers with ferromagnetic inclusions in the absence of carriers spin polarization.

NiFe/CoFe/Cu/CoFe/MnIr spin valves studied by ferromagnetic resonance

Ion-beam deposited (Glass/Ta/NiFe/CoFe/Cu/CoFe/MnIr/Ta) spin valves (SVs) with a Cu-spacer thickness (tCu) varying from 14 to 28u2009A have been studied by ferromagnetic resonance (FMR) and magnetoresistance (MR) measurements. With respect to the interlayer coupling strength between the free and fixed ferromagnetic layers, the samples have been divided in those with a weak coupling (for tCu > 16u2009A) and a strong coupling regimes (for tCu ≤ 16u2009A). The FMR behavior in these two regimes is quite different. For the weakly coupled series, there are two well-defined FMR peaks stemming from the free and fixed layers. Their in-plane angular dependences exhibit 180° and 360° symmetries, respectively. For the strongly coupled SVs, the resonance modes are hybridized and possess features of both layers simultaneously. The main coupling mechanism between the two layers, as concluded from the FMR and MR measurements, is the Neel “orange-peel” magnetostatic interaction, accompanied by a direct exchange due to pinholes in the...

Dynamic Measurements of Magnetoelectricity in Metglas-Piezocrystal Laminates

This chapter describes an experimental technique, developed experimental setup and respective experimental study of the dynamic properties of direct magnetolelectric (ME) effect measured in metglas-piezocrystal laminates. We have prepared a variety of different magnetoelectric laminates by bonding magnetostrictive metglas foils onto single-crystalline substrates of LiNbO3 (LNO), GaPO4 (GPO) and PMN-PT. The measurements have been performed as a function of the crystal cut, magnitude and orientation of the magnetic bias field and the frequency of the modulation field. Despite much weaker PE coefficients of LNO and GPO, direct ME effects have been found to have comparative magnitudes in the samples based on them and on PMN-PT. Greatly enhanced ME coefficients in certain resonance modes are explored and their relations to the material properties of the crystals and the geometry of the composites are investigated. We demonstrate that control of the PE crystal’s orientation can be successfully used in order to obtain almost any desired quasi-static and resonant anisotropic ME properties for some given application. Such unique features as chemical stability, linear piezoelectricity, thermal robustness open up a real perspective to use lead-free LNO and α-GPO based ME tri-layers, e.g., as vector magnetic field sensors working in a wide range of temperatures.

Splitting of standing spin-wave modes in circular submicron ferromagnetic dot under axial symmetry violation

The spin wave dynamics in patterned magnetic nanostructures is under intensive study during the last two decades. On the one hand, this interest is generated by new physics that can be explored in such structures. On the other hand, with the development of nanolithography, patterned nanoelements and their arrays can be used in many practical applications (magnetic recording systems both as media and read-write heads, magnetic random access memory, and spin-torque oscillators just to name a few). In the present work the evolution of spin wave spectra of an array of non-interacting Permalloy submicron circular dots for the case of magnetic field deviation from the normal to the array plane have been studied by ferromagnetic resonance technique. It is shown that such symmetry violation leads to a splitting of spin-wave modes, and that the number of the split peaks depends on the mode number. A quantitative description of the observed spectra is given using a perturbation theory for small angles of field inclination from the symmetry direction. The obtained results give possibility to predict transformation of spin wave spectra depending on direction of the external magnetic field that can be important for spintronic and nanomagnetic applications.

Engineering the Magnetoelectric Response in Piezocrystal-Based Magnetoelectrics: Basic Theory, Choice of Materials, Model Calculations

This chapter presents a theoretical basis of the anisotropic magnetoelectric (ME) effect in tri-layers of metglas and piezoelectric (PE) single crystals. The properties of various common PE and magnetostrictive substances are discussed, and arguments for the choice of the most appropriate materials are made. A linear description of the ME effects in terms of electric, magnetic and elastic material fields and material constants is presented. An averaging quasi-static method is used to illustrate the relation between the material constants, their anisotropy and the transversal direct ME voltage and charge coefficients. Subsequently, the aforementioned model is employed in the calculation of the maximum expected direct ME voltage coefficient for a series of tri-layered Metglas/Piezocrystal/Metglas composites as a function of the PE crystal orientation. The ME effects are shown to be strongly dependent on the crystal orientation, which supports the possibility of inducing large ME voltage coefficients in composites comprising lead-free PE single crystals such as LiNbO3, LiTaO3, α-GaPO4, α-quartz, langatate and langasite through the optimization of the crystal orientation.

Resonant and non-resonant microwave absorption as a probe of the magnetic dynamics and switching in spin valves

We use the resonant and non-resonant microwave absorption to probe the dynamic and static magnetic parameters of weakly coupled spin valves. The sample series include spin valve structures with varying thickness of the non-magnetic metallic spacer and reference samples comprised only a free or fixed magnetic layer. Beside the common resonance absorption peaks, the observed microwave spectra present step-like features with hysteretic behavior. The latter effect is a direct manifestation of the interlayer coupling between the ferromagnetic layers and provides two static magnetic parameters, the switching field and coercivity of the fixed layer. The analysis of the microwave absorption spectra under in-plane rotation of the applied magnetic field at different spacer thicknesses permits a deeper insight in the magnetic interactions in this system as compared to the conventional magnetometry. We combine the standard Smit-Beljers formalism for the angular dependence of the resonance fields with a Landau-Lifshit...