Spin resonances in amorphous Y-Fe alloys

We present spin resonance measurements on amorphous Y_1?xFe_x alloys using both bulk (a-YFe₂) and thin film (x = 0.70, 0.77, 0.83) samples. At 300 K, in every sample, we have found one strong line (S₁) whose frequency and angular dependances imply that it is a uniform mode ferromagnetic resonance in a very thin (≈20–40 nm) surface layer. Previous static measurements indicate that these alloys have no long range order at 300 K. The effective magnetization of the surface layer is a non-monotonic function of the Fe content. Rutherford back scattering experiments on bulk samples are used to demonstrate that there is no significant variation in composition, with depth. We show that S₁ cannot be attributed to segregation, surface contamination or crystallinity. We propose that the surface layer is another amorphous phase of the alloy. A second resonance (S₃) has been ascribed to some of the superparamagnetic spin clusters known to be present in these systems. As reported earlier, we observe other resonances whose origin is not fully understood.     

Broad-Band FMR Linewidth of Co_2MnSi Thin Films with Low Damping Factor:The Role of Two-Magnon Scattering

The low Gilbert damping factor,which is usually measured by ferromagnetic resonance,is crucial in spintronic applications.Two-magnon scattering occurs when the orthogonality of the ferromagnetic resonance mode and other degenerate spin wave modes was broken by magnetic anisotropy,voids,second phase,surface defects,etc.,which is important in analysis of ferromagnetic resonance linewidth.Direct fitting to linewidth with Gilbert damping is advisable only when the measured linewidth is a linear function of measuring frequency in a broad band measurement.We observe the nonlinear ferromagnetic resonance linewidth of Co_2MnSi thin films with respect to measuring frequency in broad band measurement.Experimental data could be well fitted with the model including two-magnon scattering with no fixed parameters.The fitting results show that two-magnon scattering results in the nonlinear linewidth behavior,and the Gilbert damping factor is much smaller than reported ones,indicating that our Co_2 MnSi films are more suitable for the applications of spin transfer torque.     

Spin glass behavior and non-ergodicity in amorphous iron-boron alloys

We report FMR measurements on amorphous Fe_xB_1-x thin films with x=0.47, 0.49 and 0.53. At low T we observe anomalies characteristic of the FM-SG transition. In addition, in the 0.47 and 0.49 alloys the resonance field, below ～80K, depends upon the rate of cooling. We suggest that this behavior is symptomatic of non-ergodicity in the spin glass state.     

Surface demagnetizing field as a source of uniaxial surface anisotropy in GdCoMo thin amorphous films

Surface magnetic anisotropy energy was studied for (Gd_0.26Co_0.74)_0.96Mo_0.04 and (Gd_0.29Co_0.71)_0.96Mo_0.04 thin amorphous films by means of microwave spectroscopy at the X-band within the temperature range 4–295 K. Excitations of surface spin waves were observed in the spin wave resonance spectra. The experiment was performed in a rotating external magnetic field. The angular dependence of the resonance field for the uniform mode (spin wave vector k=0) and the surface mode made it possible to determine the surface uniaxial anisotropy constant K_s and its temperature dependence. An inhomogeneity of the saturation magnetization M_s within a close-to-surface layer of thickness d can generate the surface anisotropy energy with anisotropy constant K_s given by the formula: K_s=4πM^b_s (M^b_s−M^surf_s)d, where the indexes b and surf correspond to the bulk and surface values, respectively. The temperature dependence of K_s calculated by means of the formula agrees qualitatively with temperature dependence of K_s found in the experiment.     

Surface modes in magnetic thin amorphous films of GdCoMo alloys

Surface modes in spin wave resonance in thin amorphous films of (Gd_1-xCo_x)_1-yMo_y alloys were studied. The samples were obtained with rf sputtering technique and a bias voltage was applied. Technological conditions were carefully pre-determined for which surface modes were excited in the resonance experiment. One surface mode was present for samples just after deposition and two modes could be observed in some cases for the same samples kept at room temperature for two or three months. The surface anisotropy constant K_s was determined from the surface inhomogeneity (SI) model with symmetrical or non-symmetrical boundary conditions for one or two surface modes, respectively. The fitted K_s values agree with theoretically predicted ones and they are also compatible with numbers found by experimentalists for monocrystalline, polycrystalline or amorphous films.For all samples we also determined the critical angles θ_c's between the external magnetic field and the normal to the film plane for which the position of the surface mode coincides with the position of the first volume mode. The corresponding critical angles ?_c's for the magnetization differ from π/4 which suggests the presence of surface inhomogeneities of the magnetization distribution.     

Pulsed magnetic resonance as a probe of longitudinal spin waves and magnetic second sound

Under appropriate conditions, longitudinal spin waves and spin wave second sound, a magnetic temperature wave, can be observed in quantum liquids and solid using pulsed magnetic resonance. A “hole-burning” experiment yields directly the spectrum ω(k) of the propagating magnetic wave.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

Antiferroelectric spin wave resonance

Antiferroelectric spin wave resonance (AFESWR) and electric spin moment, mathematically predicted by Dirac, were identified for the first time. It has also been found, in addition to well-known SSH-model, that topological quasiparticles, called σ-quasiparticles, can be formed in CC σ-bonds of organic polymers. Topological σ-polaron lattice is proposed to be responsible for the observed AFESWR both in starting polyvinylidene fluoride (PVDF) films and in carbynoid films, which are the products of PVDF-dehydrohalogenation. Dynamical systems of σ-polarons and π-solitons, identified in the samples studied, are substantionally nonlinear and can be registered in stochastic regime by usual IR absorption measurements.     

Spin-Wave Resonance in Chemically Deposited Fe-Ni Films: Measuring the Spin-Wave Stiffness and Surface Anisotropy Constant

Single-layer Fe _x Ni_{1 - x} thin magnetic films have been investigated by the spin-wave resonance technique in the entire concentration range. The surface anisotropy and exchange stiffness constants for the films with a Ni content from 30 to 80 at % have been measured from the experimental standing spin wave spectra. The surface exchange spin wave penetration depth δ _C = 20–30 nm has been determined from the dependences of the surface anisotropy and exchange coupling constants on the Fe₂₀Ni₈₀ film thickness in the range of 250–400 nm.     

Der Einfluß der Dipolwechselwirkung auf die Magnetisierung dünner ferromagnetischer Schichten

The magnetization of thin films is calculated for low temperatures, taking into account the exchange interaction, an external magnetic field, and the dipole interaction. The calculations are performed within a quantized phenomenological spin wave theory. For thin enough films, within the temperature range considered, only the lowest spin wave band contributes to the decrease of the magnetization. The influence of the dipole interaction is as follows: The magnetization decreases less rapidly with growing temperature than predicted by calculations within the Heisenberg model; the decrease depends considerably on the angle between the magnetization and the film plane; even atT=0K there is a small increase of the magnetization with growing external field.     

Tunneling states in ferromagnetic glasses

We study the interaction between structural defects, represented by two level systems, and spin waves in a ferromagnetic glass. The damping and energy shift of the spin wave due to this interaction have a “resonant” and a “relaxation” contribution in analogy with the case of phonons. Ferromagnetic resonance in amorphous ferromagnets can provide useful information on structural relaxation in these materials.     

Optically induced electron spin resonance in doped amorphous silicon

We report the first observations of optically induced electron spin resonance signals in doped and undoped amorphous silicon. We also report the observation of equilibrium surface or interface spin densities ～ 10¹³cm^?2 and volume spin densities ～ 6 × 10¹⁵ cm^?3. The small number of spins observed in equilibrium compared to the large optically induced spin density shows that most electrons are spin paired in equilibrium. We conclude that this implies a very small mean effective correlation energy, U ～ kT.     

Magnetic surface reconstruction in thin film: Possibilities of production and exploitation

Various theoretical techniques are considered for the production of magnetic surface reconstruction (MSR) in ferromagnetic thin films. The problem is discussed within the framework of the surface field model assuming the existence of a unidirectional anisotropy on the surface described by an effective field K_s It is shown that knowledge of the conditions for the occurrence of MSR together with complementary information accessible from critical spin wave resonance permits the complete determination of the properties of the field K_s.     

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ～ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (T _C1), to the crystallization temperature (T _ac), and to the Curie temperature in the crystalline phase (T _C2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH～1?12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ～ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ～ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.     

Defects in thin film silicon at the transition from amorphous to microcrystalline structure

Defects in thin film silicon with different structure all the way from amorphous to microcrystalline were investigated by electron spin resonance with emphasis on amorphous material prepared close to the transition to crystalline growth. Electron beam irradiation and stepwise annealing is used for reversible variation of the defect density over three orders of magnitude. The electron irradiation enhances mainly the native paramagnetic defects. Additional resonances are found as satellites to the central line, which anneal rapidly at temperatures below 100 °C. These features are most pronounced for the amorphous material prepared close to the transition to crystalline growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ESR in quasi 2-d ferromagnets: The coexistence of paramagnetic excitations and magnetostatic spin waves and new interference effects

The quasi 2-dimensional layer structures (C_nH_2n+1NH₃)₂CuCl₄ with n = 1,2 have been investigated by electron spin resonance. New phenomena arise for samples with well defined natural surfaces: (i) well resolved magnetostatic volume and surface spin wave modes at 4.2 K comparable in quality with spectra of YIG, (ii) a magnetostatic spin wave spectrum which is coincident with the paramagnetic resonance over a wide temperature range, (iii) interference effects between these two types of resonances.     

Magnetic properties of amorphous Co75W25

Saturation magnetization M(T), spin wave stiffness D and Curie temperature T_c of amorphous Co₇₅W₂₅ were determined by magnetic measurements. Tungsten reduces these quantities more than metalloids or 3d-transition metals do. T_c is below room temperature. The ratio D/T_c, however, is equal to that observed on a fcc single crystal of Co₉₂Fe₈ and on amorphous Co-Ti alloys.     

Microscopic Theory of the Two-Dimensional Quantum Antiferromagnet in a Paramagnetic Phase

We have developed a consistent theory of the Heisenberg quantum antiferromagnet in the disordered phase with a short range antiferromagnetic order on the basis of the path integral for spin coherent states. In the framework of our approach we have obtained the response function for the spin fluctuations for all values of the frequency ω and the wave vector k and have calculated the free energy of the system. We have also reproduced the known results for the spin correlation length in the lowest order in 1/N. We have presented the Lagrangian of the theory in a form which is explicitly invariant under rotations and found natural variables in terms of which one can construct a natural perturbation theory. The short wave spin fluctuations are similar to those in the spin wave theory and they are on the order of the smallness parameter 1/2s where s is the spin magnitude. The long-wave spin fluctuations are governed by the nonlinear sigma model and are on the order of the smallness parameter 1/N, where N is the number of field components. We also have shown that the short wave spin fluctuations must be evaluated accurately and the continuum limit in time of the path integral must be performed after the summation over the frequencies ω.     

Anisotropic low temperature thermal resistance in polycrystalline magnetic thin films

The scattering of spin waves and the thermal resistance due to this scattering are calculated for polycrystalline magnetic thin films with a small overall uniaxial anisotropy. The scattering is caused by the random orientation of crystallites, and also by the magnetization ripple, which is likewise produced by this random orientation. The wavelength spectra of the ripple and of the spin waves at low energy are essentially influenced by the magnetostatic interaction, which is taken into account within Harte's linearized thin film approximation, and for comparison also within Hoffmann's approximation. Due to the anisotropy of the spectra, the contribution of the spin waves to the thermal resistance of permalloy films should be highly anisotropic at temperatures of 2 K and below.     

High-Field and Multifrequency ESR in the Two-Dimensional Triangular Lattice Antiferromagnet NiGa2S4

We report the experimental results of electron spin resonance in high magnetic fields up to about 53 T on the quasi-two-dimensional triangular lattice antiferromagnet NiGa₂S₄. The temperature dependence of the resonance field at 584.8 GHz shows a steep change below about 30 K, indicating a development of the short-range correlation. The frequency dependence of the resonance field at the lowest temperature for H∥c is explained by one of the helical resonance modes. These experimental results suggest that the short-range order is well developed at low temperatures, and the resonance mode is described by a conventional spin wave theory.