Anisotropy and dynamic properties of Co2MnGe Heusler thin films

Co₂MnGe films of 30 and 50 nm in thickness were grown by RF-sputtering. Their magnetic anisotropies, dynamic properties and the different excited spin wave modes have been studied using conventional ferromagnetic resonance (FMR) and Microstrip line FMR (MS-FMR). From the in-plane and the out-of-plane resonance field values, the effective magnetization (4πM_eff) and the g-factor are deduced. These values are then used to fit the in-plane angular-dependence of the uniform precession mode and the field-dependence of the resonance frequency of the uniform mode and the first perpendicular standing spin wave to determine the in-plane uniaxial, the four-fold anisotropy fields, the exchange stiffness constant and the magnetization at saturation. The samples exhibit a clear predominant four-fold magnetic anisotropy besides a smaller uniaxial anisotropy. This uniaxial anisotropy is most probably induced by the growth conditions.     

Spin dynamics in oriented ferromagnetic nanowires Ge<Subscript>0.99</Subscript>Co<Subscript>0.01</Subscript>

The magnetic properties of one-dimensional oriented nanowires Ge_0.99Co_0.01 grown in pores of anodized aluminum oxide membranes are investigate using ferromagnetic resonance spectroscopy. The electron spin resonance signals of the magnetically ordered cobalt subsystem and the charge-carrier subsystem are identified. It is revealed that the anisotropy field at 4 K is equal to 400 Oe and aligned parallel to the nanowire axis. The transverse relaxation time of spin waves at 4 K is estimated to be ～10^?10 s. It is shown that the magnetic properties of nanowires are predominantly determined by the ferromagnetism of Co and GeCo alloy clusters.     

Thermodynamic magnetization of a strongly correlated two-dimensional electron system

We measure thermodynamic magnetization of a low-disordered, strongly correlated two-dimensional electron system in silicon. Pauli spin susceptibility is observed to grow critically at low electron densities—behavior that is characteristic of the existence of a phase transition. A new, parameter-free method is used to directly determine the spectrum characteristics (Landé g-factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the inter-level interactions between quasiparticles are avoided. It turns out that, unlike in the Stoner scenario, the critical growth of the spin susceptibility originates from the dramatic enhancement of the effective mass, while the enhancement of the g-factor is weak and practically independent of the electron density.     

The ground state and EPR spectrum in monoclinic KY(WO4)2:Nd single crystal

The electron paramagnetic resonance (EPR) of Nd³⁺ ion in KY(WO₄)₂ single crystal was investigated at T=4.2 K using an X-band spectrometer. The observed resonance absorption represents the complex superposition of three spectra corresponding to neodymium isotopes with different nuclear momenta. The EPR spectrum is characterized by a strong g-factor anisotropy. The temperature dependences of the g-factor were caused by strong spin-orbit and orbit-lattice coupling. The resonance lines become broader as temperature increases due to the short spin-lattice relaxation time.     

Magnetic resonance of spin clusters and triplet excitations in a spin-Peierls magnet with impurities

The magnetic resonance spectrum of spin clusters formed in spin-Peierls magnets in the vicinity of impurity ions is investigated. The observed temperature dependences of the effective g-factor and the linewidth of the electron paramagnetic resonance (EPR) in crystals of Cu_1?x Ni_xGeO₃ are described in the model of the exchange narrowing of the two-component spectrum with one component ascribed to spin clusters and exhibiting an anomalous value of the g-factor and the other related to triplet excitations. An estimation of the size of the suppressed dimerization region around the impurity ion is obtained (this region includes about 30 copper ions). The dependence of the effective g-factor and the EPR linewidth on the impurity concentration at low temperatures indicates the interaction of clusters.     

具有单轴各向异性的磁性超晶格中的自旋波

应用转移矩阵的方法研究具有单轴各向异性的磁性超晶格中的自旋波谱,得到自旋波所满足的色散方程,分析在一定的参量条件下,单轴各向异性强度对自旋波谱的影响。 关键词：     

Dependence of electron spin g-factor on magnetic field in quantum wells

The dependence of electron spin g-factor on magnetic field has been investigated in GaAs/AlGaAs quantum wells. We have estimated the electron g-factor from spin precession frequency in time-resolved photoluminescence measurements under a magnetic field in different configurations; the magnetic field perpendicular (g_⊥) and parallel (g_∥) to the quantum confinement direction. When the angle between the magnetic field and the confinement direction is 45°, we have found that g-factor varies depending on the direction of magnetic field and the circular polarization type of excitation light (σ⁺ or σ^?). These dependences of g-factor exhibit main features of Overhauser effect that nuclear spins react back on electron spin precession. The value of g_⊥ and g_∥ corrected for the nuclear effects agree well with the results of four-band k·p perturbation calculations.     

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically T _g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T _g/T). The glass cross-over temperature T_g related to correlations among solitons is equal to the average energy barrier and scales like T _g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap. Received 12 December 2001     

Spin waves on chains of YIG particles: dispersion relations,Faraday rotation,and power transmission

We calculate the dispersion relations for spin waves on a periodic chain of spherical or cylindrical Yttrium Iron Garnet (YIG) particles. We use the quasistatic approximation, appropriate when kd ? 1, where k is the wave number and d the interparticle spacing. In this regime, because of the magnetic dipole-dipole interaction between the localized magnetic excitations on neighboring particles, dispersive spin waves can propagate along the chain. The waves are analogous to plasmonic waves generated by electric dipole-dipole interactions between plasmons on neighboring metallic particles. The spin waves can be longitudinal (L), transverse (T), or elliptically polarized. We find that a linearly polarized spin wave undergoes a Faraday rotation as it propagates along the chain. The amount of Faraday rotation can be tuned by varying the off-diagonal component of the permeability tensor. We also discuss the possibility of wireless power transmission along the chain using these coupled spin waves.     

Measurement of the conduction electron Zeeman splitting in platinum

The cyclotron orbit g-factor in platinum has been investigated using the de Haas-van Alphen effect. Several new features of the spin splitting zero (SSZ) contours of the Γ-centered electron sheet have been established. Six closed spin splitting zero contours of the fundamental of this sheet have been mapped out together with several branches of spin splitting zero contours of the second harmonic. The results give an almost complete determination of the anisotropy of the cyclotron orbit g-factor of the Γ-centered electron sheet in platinum.     

Longitudinal response of the spin system of a metal to modulated EPR saturation at arbitrary modulation frequency and detuning of the saturating field

The theory of the longitudinal (with respect to an external magnetic field) response of a combined spin system of localized paramagnetic centers (s subsystem) and free charge carriers (e subsystem) of a solid semiconductor to modulated saturation of EPR is developed. In contrast to relevant studies made earlier, the general case is considered of an arbitrary modulation frequency and arbitrary detuning of the saturating microwave field with respect to the central EPR frequency. A theoretical approach is used in which normal modes are considered in analyzing coupled oscillations of the spin magnetizations of the s and e subsystems. It is shown that, in the case of relaxation coupling between the subsystems, the longitudinal response recorded at the modulation frequency can be represented as the sum of the responses of the normal modes, each of which is described by a universal resonance lineshape that is different, in general, from the Lorentzian lineshape characteristic of EPR signals. In the extreme cases of weak and strong coupling, simple analytical formulas are derived. The results presented form a theoretical basis for applying the method of modulated longitudinal response for measuring very short longitudinal spin relaxation times in semiconductors with paramagnetic impurities. As an example, experimental data are presented for activated carbon containing stable free radicals.     

Anisotropy of the electron g factor in semiconductors with cubic symmetry

The spin-orbit corrections to electronic states in bulk cubic semiconductors without the center of inversion in an ultraquantum magnetic field are investigated. It is shown that the spin-orbit interaction results in a shift of the Landau levels and the appearance of additional terms in the relationship for the electron g factor. The corrections to the g factor lead to a deviation of the macroscopic magnetization from the direction of the magnetic field, the dependence of the spin resonance frequency on the magnetic field orientation with respect to the principal crystallographic axes, and anisotropy of the spin relaxation through the D’yakonov-Perel’ mechanism.     

Giant Zeeman splitting of light holes in GaAs/AlGaAs quantum wells

We have developed a theory of the longitudinal g-factor of light holes in semiconductor quantum wells. It is shown that the absolute value of the light-hole g-factor can strongly exceed its value in the bulk and, moreover, the dependence of the Zeeman splitting on magnetic field becomes non-linear in relatively low fields. These effects are determined by the proximity of the ground light-hole subband, lh1, to the first excited heavy-hole subband, hh2, in GaAs/AlGaAs-type structures. The particular calculations are performed in the framework of Luttinger Hamiltonian taking into account both the magnetic field-induced mixing of lh1 and hh2 states and the mixing of these states at heterointerfaces, the latter caused by chemical bonds anisotropy. A theory of magneto-induced reflection and transmission of light through the quantum wells for the light-hole-to-electron absorption edge is also presented.     

Optical orientation of particles with a random g-factor in semiconductor nanostructures

In real quasi-two-dimensional semiconductor nanostructures (quantum wells, quantum dots), the transverse g-factor of holes is a stochastic quantity. This fact should be taken into account in analyzing the optical orientation and Hanle effect of holes. The Hall effect for an ensemble of particles with a “random” g-factor has been treated theoretically. In the case where the spin relaxation time of a hole with a characteristic g-factor is shorter than the hole lifetime, there can occur a narrowing of the depolarization contour and an increase in its amplitude. In the opposite case of long spin relaxation times (trions in quantum dots), a formula has been derived, which generalizes the previously obtained result to the case of an arbitrary tilt angle of the magnetic field with respect to the plane of the layer (Hanle effect in the tilted form).     

Brillouin scattering from ultra thin iron films

The spin-wave excitations of ultrathin iron films (1 to 5 nm) on sapphire substrates have been studied by inelastic light scattering using Brillouin spectroscopy. The room temperature magnetization J, magnetic anisotropy field B_an, and the g-factor have been determined by fitting the measured ω-B results for surface spin waves to the related Damon-Eshbach theory. For thicknesses below 4 nm the film magnetization J_f decreases linearly with film thickness and is found substantially smaller than J_f values determined by static magnetization measurements. Equivalent reductions in J_f, however, were also obtained in light scattering studies of ultrathin Fe films on GaAs substrates reported previously in the literature [2].     

Parametric amplification of ultrasound in ferromagnetic piezoelectric composites at the vicinity of the orientational phase transition

Direct parametrical amplification of ultrasound was theoretically considered for the ferromagnetic/piezoelectric thin-layered structure composites, for which within the long-wavelength approach some, material specific, yet homogeneous bulk properties could be assumed. Having chosen appropriate homogeneous components, it is possible to construct a desirable composite with such properties that are absent in the starting-up “building blocks” (components).Sound frequencies are assumed to be such that only the dispersion due to magnetic subsystem is thought relevant; the piezoelectric components are set to have a nonlinear dielectric response. Presence of orientational phase transition for the ferromagnetic subsystem allows to vary frequency of a ferromagnetic resonance, and consequently to alter the dispersion at the chosen frequency of sound.While solving coupled equations describing dynamics of mechanical, magnetic and piezoelectric subsystems, an effective sound amplification for realistic values of physical parameters is shown possible.