X-ray detected ferromagnetic resonance in thin films

We discuss the physical content of X-ray Detected Magnetic Resonance (XDMR), i.e. a novel spectroscopy which uses XMCD to probe the resonant precession of the local magnetization in a strong microwave pump field. We focus on the simplest case of a steady-state precession of elemental moments in the non-linear regime of angular foldover. Like XMCD, XDMR is element and edge selective and could become a unique tool to investigate how precessional dynamics can locally affect the spin and orbital magnetization of p- or d-projected DOS. This should be possible only in the limit where there is no overdamping due to ultrafast orbit-lattice relaxation.     

Elastically driven ferromagnetic resonance in nickel thin films

Surface acoustic waves (SAWs) in the GHz frequency range are exploited for the all-elastic excitation and detection of ferromagnetic resonance (FMR) in a ferromagnetic-ferroelectric (Ni/LiNbO(3)) hybrid device. We measure the SAW magnetotransmission at room temperature as a function of frequency, external magnetic field magnitude, and orientation. Our data are well described by a modified Landau-Lifshitz-Gilbert approach, in which a virtual, strain-induced tickle field drives the magnetization precession. This causes a distinct magnetic field orientation dependence of elastically driven FMR that we observe in both model and experiment.     

Voltage control of millimeter-wave ferromagnetic resonance in multiferroic heterostructures thin films

Multiferroic heterostructures thin films, SrBi₂Ta₂O₉/BaFe₁₂O₁₉ (SBT/BaM), were grown on Pt/TiO₂/SiO₂/Si substrates by using magnetron sputtering and Sol-gel ways. X-ray diffractometer (XRD) analysis showed that only SBT and BaM phases appeared in the multiferroic heterostructures. Magnetic hysteresis loops revealed that the saturated magnetization was 3.7 kG and the M-H characteristics of SBT/BaM were not influenced by the presence of the SBT layer. Ferromagnetic resonance (FMR) measurement showed the lowest FMR linewidth of 205 Oe at 50 GHz. Additionally, when direct-current electric field was applied to SBT layer, as a result of which mechanical deformation of the ferromagnetic layer occurred that leads to a frequency shift in ferromagnetic resonance and the magnetoelectric coupling effect (α) is 1.8 MHz*cm/kV. Our findings indicate that these SBT/BaM thin films have a significant potential for the usage in millimeter wave tunable devices.     

New types of inhomogeneous nuclear spin-spin resonance in thin antiferromagnetic films

Several new resonant size effects in the bulk nuclear-spin dynamics of a thin magnetic film are investigated theoretically. The physical mechanism responsible for their appearance in a finite magnet is the indirect exchange of nuclear spins through a field of virtual “Lamb” phonons. Fiz. Tverd. Tela (St. Petersburg) 39, 1817–1823 (October 1997)     

Nernst-Ettingshausen effect in thin ferromagnetic films

A new thermomagnetic effect in a ferromagnetic film is discussed. When a temperature gradient is established along the x axis in a ferromagnetic sample, a transverse electric field arises in the same plane as the spontaneous-magnetization vector (in the case H = 0, where H is the external magnetic field). A phenomenological expression is given for the transverse electric field as a function of the square of the magnetization. Theory is given for the Nernst-Ettingshausen effect due to the spin-orbit interaction in a ferromagnetic film in the effectivemass approximation, and the Nernst-Ettingshausen constant is derived. The calculation is carried out by the density-matrix method derived by Kohn and Luttinger.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 28–32, November, 1969.     

Dipolar interactions in ferromagnetic thin films

We introduce a discrete model describing the motion of a zigzag domain wall in a disordered ferromagnet with in-plane magnetization, driven by an external magnetic field. The main ingredients are dipolar interactions and anisotropy. We investigate the dynamic hysteresis by analyzing the effects of external field frequency on the coercive field by Monte Carlo simulations. Our results are in good agreement with experiments on Fe/GaAs films reported in literature, and we conclude that dynamic hysteresis in this case can be explained by a single propagating domain wall model without invoking domain nucleation.     

Magnetic anisotropy in thin ferromagnetic films

The magnetic properties of thin ferromagnetic films are studied taking into account the magnetic anisotropy term in the Hamiltonian. In the second approximation equations are obtained for the magnetization of the monatomic layers parallel to the surface of the thin film. From these equations one obtains the Curie temperature, which depends on the thickness of the thin film and the ratio a between the anisotropy constant and the exchange energy between two neighbours. A value can be chosen for such that the thin film becomes ferromagnetic only for a thickness greater than a definite value and in this manner the theoretical results can be fitted to the experimental data. The situation in cobalt thin films is dealt with in particular.

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The author extends his thanks to the research workers of CIFA 1 as well as to Dr. L. Valenta for information on the same subject.     

Magnons in surface-rearranged ferromagnetic thin films

By a Green function approach, spin waves in a surface-rearranged ferromagnetic thin film are derived both analytically and numerically. Heisenberg exchange, bulk and surface anisotropy between nearest neighbors and external magnetic field are taken into account for an sc film with {001} surfaces. Because of the anisotropies, the dynamical matrix defined from the Green function equations is not Hermitian, so we generalize the Bogoliubov canonical transformation to derive the spin wave spectrum. The spin waves propagating inside the film result from the superposition of two sine or hyperbolic sine waves. The amplitude and polarization of spin waves are shown to be quite sensitive to the details of the surface rearrangements, whereas spin wave energies are not so sensitive to such rerrangements, except when soft modes occur in the unrearranged configuration. In that case, we show that when the surface rearrangement is taken into account, soft modes disappear in the spin wave spectrum.     

高斯型非均匀应力对铁磁薄膜磁化性质的影响

采用Monte-Carlo方法研究了高斯型非均匀应力对的铁磁薄膜磁化性质的影响.结果表明: 与易轴平行的拉应力和与易轴垂直的压应力能够增大系统的矫顽场, 而与易轴平行的压应力和与易轴垂直的拉应力则会减小系统的矫顽场.在矫顽场增大(减小)的同时, 系统还伴随着剩磁及其矩形度的增大(减小).更有意义的是, 在与易轴平行的压应力或与易轴垂直的拉应力作用下, 在应力的集中区域会出现“易轴旋转”的现象. 这种产生“易轴旋转”的应力集中区域的范围强烈地依赖于应力的强度和分布宽度. 关键词： 铁磁薄膜 非均匀应力 Monte-Carlo方法 磁滞回线     

Enhanced gilbert damping in thin ferromagnetic films

The precession of the magnetization of a ferromagnet is shown to transfer spins into adjacent normal metal layers. This "pumping" of spins slows down the precession corresponding to an enhanced Gilbert damping constant in the Landau-Lifshitz equation. The damping is expressed in terms of the scattering matrix of the ferromagnetic layer, which is accessible to model and first-principles calculations. Our estimates for permalloy thin films explain the trends observed in recent experiments.     

Vanishing magnetic interactions in ferromagnetic thin films

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.     

Surface spin waves in ferromagnetic thin films

Surface spin-wave modes are calculated for (100) surface of simple cubic structure. An isotropic part as well as an anisotropic part are introduced in the spin Hamiltonian. The anisotropic factor is allowed to vary on the first two surface layers. Dispersion curves are derived by a Green function formalism.The authors are indebted to Dr A. P. Legrand for his help in numerical calculations and to Dr J. Korringa for his continuous encouragement and suggestions.     

High-frequency susceptibility and ferromagnetic resonance in multidomain thin films of the yttrium-iron garnet type

The field dependence of the high-frequency susceptibility and the ferromagnetic resonance were experimentally studied in a thin (d≈0.1 µm) (111)-oriented single-crystal film of substituted yttrium-iron garnet with the factor q?1. It was shown that the anomaly in the high-frequency susceptibility observed in a magnetic field H parallel to the normal to the film surface in the magnetization saturation region (H≈H_s) has a dual nature; more specifically, this anomaly is associated with an abrupt collapse of the stripe domain structure and a ferromagnetic resonance in the experimental configuration H ∥ [111] and h ⊥ H. In this case, the film transition from the inhomogeneous multidomain state to the homogeneous (single-domain) state at the point H≈H_s has no indications of a second-order phase transition. The experimental frequency-field dependence of ferromagnetic resonance (FMR) in the sample under study, having a characteristic minimum at the point ω₀=5 MHz and H_FMR=H_s, agrees qualitatively and quantitatively with calculations. The influence of the cubic magnetic anisotropy and the film thickness on the FMR spectrum and the orientation of the spontaneous magnetization in domains with respect to the film plane in the zero field H was theoretically studied.     

Direct detection of nonlinear ferromagnetic resonance in thin films by the magneto-optical Kerr effect

The longitudinal magneto-optical Kerr effect is used to obtain a calibrated measure of the dynamic magnetization response over the ferromagnetic resonance (FMR) profile for in-plane magnetized Permalloy films excited with high power in-plane transverse microwave fields at 1.25 to 3.75 GHz and in-plane precession angles up to about 20 degrees. The data provide a profound demonstration of the Suhl threshold effect for parametric spin wave generation for angles above about 14 degrees, the magnetization precession lock-up just above threshold, and the complicated response over the full FMR profile at very high powers.     

Anisotropic magnetoresistance in thin inhomogeneous gold films

We investigate the magnetoresistance (MR) of thininhomogeneous gold films applying the magnetic field perpendicular as well as parallel to the film plane. The MR-data show a strong anisotropy which can be well explained within the theory of weak electron localization (WEL) in 2d for both field orientations. The important results is that fitting the MR-data for both orientations by the corresponding theoretical expressions we obtain nearly identical values for the phase coherence lengthsL and its temperature dependence. This confirms (i) the correctness of the fitting process and (ii) the validity of the theory. From this we conclude that WEL in 2d can also be used to describe the MR ofinhomogeneous films.     

Magnetization of thin films of ferromagnetic superconductors

The magnetization of thin films of ferromagnetic superconductors is theoretically studied. The symmetric and antisymmetric magnetic orders appear in the films depending on the thickness of the films. The magnetization for the symmetric order monotonically increases with decreasing temperature. However, the magnetization curves for the antisymmetric order has a sharp peak near the magnetic phase transition temperature in a weak external magnetic field.     

On the theory of ferromagnetic thin films

As is known, the second approximation in the calculation of the partition function by the traces method of ferromagnetic thin films gives wrong results for the coordination number equal to eight. In order to obtain correct results even for this case, the third order approximation of the partition function is developed and thus the magnetic properties of body-centred cubic iron thin films are studied. The dependence of the Curie temperature on the thickness, for different values of the ratio between the anisotropy constant and the exchange energy between two neighbours, is discussed. A value can be chosen for this ratio such that the thin film becomes ferromagnetic only for a thickness greater than a definite value.

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