Spin Wave Resonances in GaMnAs

Ferromagnetic resonance (FMR) measurements were carried out on a series of GaMnAs thin films with different thicknesses grown by low temperature molecular beam epitaxy. Clear spin wave resonances were observed in addition to FMR when the applied magnetic field was perpendicular to the film plane. The spin wave spectra show a nearly linear dependence of the resonance mode positions on the mode number, suggesting that the magnetization profile of the GaMnAs films is not uniform in the growth direction. A first-order analysis of these effects is presented along with the experimental data.     

Ferromagnetic Behavior of Fe+ Implanted Si(100) Semiconductor

Fe ions have been implanted into Si (100) single crystals using ion implantation technique. The Fe ions have been accelerated to 45 keV with a dose of 5×10¹⁷ ion/cm² at room temperature. The ions have been sent to the substrate??s surface at normal incidence. The temperature dependence of magnetization measurement was explored at the temperature range of 10?C300 K. The implanted Si substrate was studied with Ferromagnetic Resonance (FMR) technique and Vibrating Sample Magnetometer (VSM). The FMR spectra were recorded by applying external magnetic field in different experimental geometries. FMR spectra were analyzed and the magnetic properties, which are the g-factor, effective magnetization and uniaxial anisotropy parameter, were estimated by simulation of the experimental data. The sample showed two-fold magnetic anisotropic symmetry. By fitting the Si-2p region obtained through XPS measurements it is observed that Fe and Fe compounds are present in the material.     

Magnetic Properties of Single Crystalline Co2MnAl Heusler Alloy Thin Films

Ferromagnetic resonance measurements were carried out for single crystalline Co₂MnAl thin films as a function of the angle between applied external magnetic field and normal of the film surface. In- and out-of-plane angular dependence of the resonance field and only out-of-plane angular dependence of the linewidth of FMR spectra were analyzed using the Landau?CLifshitz?CGilbert equation. The easy and hard axes were determined by analyzing the in-plane angular dependence of resonant field. The films annealed at various temperatures showed four-fold magnetic anisotropy symmetry and the damping factor was estimated from the analysis of the angular dependency of FMR spectra.     

Ferromagnetic and spin wave resonances in thin layer of expanded austenite phase

Four samples of austenite coatings deposited by reactive magnetron sputtering on silicon substrate at four different temperatures and pressures were investigated by ferromagnetic resonance (FMR) method at room temperature. The expanded austenite phase S (γ _N ) layers with thickness in the 160–273 nm range and concentration of magnetic atoms: 72 % Fe, 18 % Cr and 10 % Ni, were obtained. The coatings with nanometric size grains were strongly textured and grown mostly in [100] direction, perpendicular to the sample surface. Intense FMR spectra were recorded at various angles between the static magnetic field direction and the sample surface. A strong magnetic anisotropy of the main uniform FMR mode was observed and the effective magnetization 4πM _eff determined. Spin wave resonance (SWR) modes were observed in all investigated samples in out-of-plane geometry of the magnetic field. The resonance fields of SWR modes in our samples varied linearly with the spin wave mode number. The value of the effective magnon stiffness constant was determined assuming a parabolic shape of the magnetization variation across the sample thickness.     

Thickness and Temperature Dependence of Exchange Bias in Co/CoO Bilayers

The effects of the ferromagnetic (FM) Co layer thickness on magnetic anisotropies and exchange bias (EB) properties of Co/CoO bilayers have been investigated by using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. The temperature and thickness dependence of EB were studied in temperature range of 4?C320?K. FMR and VSM measurements show that Co/CoO bilayers exhibit negative exchange bias just below the blocking temperature (T _B). Room-temperature in-plane FMR measurements reveal that the Co layer was epitaxially grown on MgO substrate with four-fold magneto-crystalline symmetry. The data also show that the easy axis of magnetization is in the film plane and parallel to the ??110?? crystallographic directions of MgO substrate in all samples.     

Change of RF susceptibility and parametric oscillations of the magnetization of stripe films in magnetic fields normal to the surface

Multidomain ferromagnetic resonance (FMR) for Permalloy magnetic fills (thickness 2700 Å) 83 percent Ni with rotatable anisotropy was experimentally observed with an external magnetic field applied normal to the sample surface. The lower frequency branch of multidomain FMR was excited when the RF magnetic fieldhwas oriented along the film plane; forhoriented parallel to the fieldH_{perp}the higher frequency branch was excited. The dependence of the resonant fieldH_{perp}on the frequency of fieldhfor multidomain and single-domain FMR was determined. Increasing the RF fieldhoriented alongH_{perp}gave rise to oscillations of magnetization with the frequency(1/2)f, wherefis the pumping frequency. The dependence of the amplitude of these oscillations on the strength of the RF fieldhand on the magnetizing fieldH_{perp}was investigated.     

Spin-Wave Excitations in Evaporated Co/Pt Multilayers

The magnetic properties of evaporated Co/Pt multilayers have been studied by magnetic measurements and ferromagnetic resonance (FMR). The spin-wave resonances were observed in some multilayers in FMR experiments, which implied that spin waves were sustained by the whole and propagated through Pt layers. The relation of the resonance field H _res with the mode number n obeys the so-called n ² law and the interlayer coupling strength J _I has been determined. The temperature dependence of the spontaneous magnetization can be well described by Bloch’s law, in all multilayers. The increase of the spin-wave parameter B with decreasing cobalt thickness has been discussed. A spin-wave theory has been used to explain the temperature dependence of the magnetization and the approximate values for the bulk exchange interaction J _b and surface exchange interaction J _S for various Co/Pt multilayers have been obtained.     

Ferromagnetic Resonance Investigation of Hexaferrite Nanoparticles Prepared by Sol-Gel Auto-Combustion Method

In this study, the magnetic and electromagnetic wave-absorbing properties of barium and strontium ferrite nanopowders prepared by a sol-gel technique were investigated. To study the structural characteristics of hexaferrites, X-ray diffraction analysis was used. Investigation of the morphologies of nanoparticles was carried out by field emission scanning electron microscopy. A vibrating sample magnetometer was used in order to examine the magnetic properties of synthesized hexaferrites at room temperature. Ferromagnetic resonance (FMR) was used to investigate ferromagnetic resonance of the powders. Experimental results indicated that the materials had hexagonal structures with desirable magnetic properties. A low-field absorption signal was observed with the same phase as the FMR absorption in barium hexaferrites.     

Ferromagnetic Resonance Study of Fe/Cu Multilayer Thin Film

Fe/Cu/Fe multilayer thin film was grown on Si (100) substrate by using magnetron sputtering technique at room temperature. Dynamic and static magnetisations of the film have been investigated using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques in the temperature range of 10–300 K. From the room-temperature in-plane FMR measurements, a growth-induced uniaxial magnetic anisotropy was observed. Out-of-plane FMR measurements exhibited a large magnetic anisotropy due to a large saturation magnetisation of Fe. A computer code was written to simulate the experimental FMR data and to obtain the magnetic parameters of the Fe/Cu/Fe multilayer thin film. g value, effective magnetisation, uniaxial anisotropy field and perpendicular anisotropy constant from the fitting of the angular dependence of the resonance field at both the in-plane and out-of plane geometries were determined. The exchange bias effect was observed from the low-temperature VSM measurements. The saturation magnetisation and coercive field decrease with increasing temperature due to the increase of the thermal fluctuations.     

Ferromagnetic Resonance Studies of Exchange Biased CoO/Fe Bilayer Grown on MgO Substrate

Dynamic and static magnetizations of an exchange biased bilayer system which is constructed as a proximity of a CoO layer on an Fe-layer grown on the (100) oriented MgO substrate by ion beam sputtering technique have been investigated by ferromagnetic resonance (FMR) and vibrating sample magnetometry (VSM) techniques. The room-temperature FMR measurements reveal that the Fe layer is epitaxially grown on MgO substrate with four-fold magnetocrystalline anisotropy and the hard magnetization axis of the sample is the [100] crystallographic directions of MgO substrate. We have determined the g-value, effective magnetization, magnetocrystalline anisotropy constants and contributions to FMR linewidth due to the intrinsic Gilbert damping and inhomogeneity of magnetization by using Landau–Lifshitz–Gilbert (LLG) equation. We observed an unusual FMR line shape attributed to impedance switching of resonance cavity and complex component of conductivity of sample system. The low-temperature FMR measurement shows asymmetric hysteretic behavior of resonance field related to magnetic coupling of ferromagnetic and antiferromagnetic layers. From both FMR and VSM measurements between 10–300 K, the magnetocrystalline anisotropy is observed to dominate above blocking temperature, while unidirectional anisotropy is observed to dominate below blocking temperature over internal magnetic anisotropy. FMR spectra have a comparatively small linewidth between 40–100 Oe, which indicates to a high crystallinity of the Fe film. Gilbert constant was calculated as 0.007 from the linewidth fitting of FMR spectra. This small value is a suitable for reducing the critical switching current used in magnetic tunneling junction. Detailed exchange bias studies were carried out for hard and easy axis of the sample in the temperature range of 10–300 K. From both low-temperature FMR and VSM measurements, the blocking temperature of the system was determined as ～60 K.     

Magnetic characterization of nanocrystalline iron samples with different size distributions

Nanocrystalline iron was obtained by fusing magnetite and promoters. The oxidized form was reduced with hydrogen and passivated (sample P0). The average nanocrystallite size in sample P0 was d(P0) =16 nm and the width of size distribution was σ(P0) = 18 nm. Samples of nanocrystalline iron with narrower diameter ranges and larger and smaller average crystallite sizes were also synthesized. They were: sample P1 (d(P1) = 28 nm, σ(P1) = 5 nm), sample P2 (d(P2) = 22 nm, σ(P1) = 5 nm), sample P3 (d(P3) = 12 nm, σ(P1) = 9 nm). These four samples were studied at room temperature by dc magnetization measurements and ferromagnetic resonance at microwave frequency. Correlations between samples sizes distributions (average size and width of the sizes) and magnetic parameters (effective magnetization, anisotropy field, anisotropy constant, FMR linewidth) were investigated. It was found that the anisotropy field and effective magnetization determined from FMR spectra scale linearly with nanoparticle sizes, while the effective magnetic anisotropy constant determined from the hysteresis loops decreases with nanoparticle size increase.     

NiO/FeCo磁性多层膜的高频磁性

制备了一系列具有铁磁/反铁磁交换偏置作用的[NiO/Fe65Co35]10多层膜,使用振动样品磁强计(VSM)测量了样品的静磁参数,利用微带线法测量了样品4GHz-10GHz的磁谱,首次制备并测得了自然共振频率(fr)在6GHz以上,最高到fr=9.6GHz的薄膜样品。结果表明交换偏置场(Hex)、各向异性场(Hua)、以及矫顽力(Hc)随铁磁层厚度(tFM)增大而减小;基于Landau-Lifishitz(L-L)方程对静磁参量和磁谱进行了比较,发现样品自然共振频率较L-L方程计算值偏大30%以上。     

Magnetic resonance study of γ-Fe2O3 nanoparticles dressed in oxygen based free radicals

Two composites consisting of γ-Fe₂O₃ (maghemite) nanoparticles covered by two different oxygen-based free radicals derived from a 4-(methylamino)phenol sulphate and 8-hydroxy-1,3,6-trisulfonic trisodium salt acid were prepared and investigated by the magnetic resonance method in the 4–300 K range. Both composites displayed broad and very intense ferromagnetic resonance (FMR) lines originating from γ-Fe₂O₃ agglomerated nanoparticles. The FMR spectrum was fitted satisfactorily at each temperature by two Landau-Lifshitz functions reflecting the existence of magnetic anisotropy in the investigated system. The temperature dependence of the obtained FMR parameters (resonance field, linewidth, integrated intensity) was studied and the results were interpreted in terms of magnetic interactions between free radicals and nanoparticle agglomerates. A comparison with previously studied similar systems containing maghemite nanoparticles was made and conclusions about the role of free radicals were drawn.     

Direct Current Effects on High-Frequency Properties of Patterned Permalloy Thin Films

We have investigated experimentally direct current (dc) effects on high-frequency properties of two different permalloy (Py) submicrometer patterns of 0.24 $mu$m and 0.55 $mu$m width, 10 $mu$m length, and 100 nm thickness. The natural ferromagnetic resonance (FMR) frequencies for the two samples are about 8.5 and 11.5 GHz. A 50 mA dc produces a FMR frequency reduction of about 1 GHz in both samples. We extracted susceptibility spectra for the samples from the measurement data. We studied inductance variations of Py embedded transmission lines for different dc levels. With 50 mA dc, the operational frequencies of the inductances decreased by 9% and 12.5%. We also tested effects of magnetic fields generated by external magnets on the submicrometer patterns for comparison. To obtain the same magnetization rotation angle, the external magnetic field needs to be about five times larger than the Ampere field created by the direct current. This behavior is unique and may be associated with the increased thermal energy from the Joule heating effects.      

Defect mediated magnetic interaction and high Tc ferromagnetism in Co doped ZnO nanoparticles

Structural, optical and magnetic studies have been carried out for the Co-doped ZnO nanoparticles (NPs). ZnO NPs are doped with 3% and 5% Co using ball milling and ferromagnetism (FM) is studied at room temperature and above. A high Curie temperature (Tc) has been observed from the Co doped ZnO NPs. X-ray diffraction and high resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption and photoluminescence studies on the doped samples show change in band structure and oxygen vacancy defects, respectively. Micro-Raman studies of doped samples shows defect related additional strong bands at 547 and 574 cm(-1) confirming the presence of oxygen vacancy defects in ZnO lattice. The field dependence of magnetization (M-H curve) measured at room temperature exhibits the clear M-H loop with saturation magnetization and coercive field of the order of 4-6 emu/g and 260 G, respectively. Temperature dependence of magnetization measurement shows sharp ferromagnetic to paramagnetic transition with a high Tc = 791 K for 3% Co doped ZnO NPs. Ferromagnetic ordering is interpreted in terms of overlapping of polarons mediated through oxygen vacancy defects based on the bound magnetic polaron (BMP) model. We show that the observed FM data fits well with the BMP model involving localised carriers and magnetic cations.     

Ferromagnetism and ferromagnetic resonance in Mn and As co-implanted Si and GaAs

Ferromagnetism persisting above 375 K and anisotropic ferromagnetic resonance (FMR) spectra have been detected for the first time in Si co-implanted with Mn and As and annealed under appropriate conditions. For comparison, semi-insulating GaAs samples have been implanted with the same ions and subsequently annealed. They also exhibit ferromagnetism with a Curie temperature well in excess of 375 K. High-resolution transmission electron microscopy (HR TEM) performed on the samples with the best magnetic characteristics has shown the presence of nanoclusters due to the segregation of the implanted species in both Si and GaAs. The angular dependence of the FMR spectra reveals the existence of magnetic entities with the hard magnetization axes aligned along the four equivalent 〈1 1 1〉 crystal axes. The spectra are very similar in Si and GaAs, indicating that hexagonal MnAs clusters might be formed in Si.     

Magnetic properties of gamma-Fe2O3/poly(ether-ester) nanocomposites

The magnetic properties of gamma-Fe2O3 nanoparticles embedded in a thermoplastic elastomer poly(ether-ester) copolymer by the in situ polycondensation reaction process have been investigated by means of magnetization and ferromagnetic resonance (FMR) measurements at low filler concentrations of 0.1 and 0.3 wt% with the magnetic additive introduced in the polymer matrix in powder and solution form. The magnetic behavior of the magnetopolymeric nanocomposites indicates significant interparticle interaction effects that depend mainly on the dispersion state of the magnetic nanoparticles as well as their concentration, consistent with the variation of the particle microstructure characterized by magnetic aggregates in the nanometer and micron scale for the solution and powder dispersions, respectively. The magnetization and FMR results at different filler concentrations and dispersions show a close correspondence to the relaxation processes of the copolymer, implying the coupling of polymeric and magnetic properties.     

The magnetic anisotropy of thin epitaxial CrO<Subscript>2</Subscript> films studied by ferromagnetic resonance

The magnetic anisotropy of thin epitaxial films of chromium dioxide (CrO₂) has been studied as a function of the film thickness by the ferromagnetic resonance (FMR) technique. CrO₂ films with various thicknesses in the range from 27 to 535 nm have been grown on (100)-oriented TiO₂ substrates by chemical vapor deposition using CrO₃ as a solid precursor. In a series of CrO₂ films grown on the substrates cleaned by etching in a hydrofluoric acid solution, the FMR signal exhibits anisotropy and is strongly dependent on the film thickness. The magnetic properties of CrO₂ films are determined by a competition between the magnetocrystalline and magnetoelastic anisotropy energies, the latter being related to elastic tensile stresses caused by the lattice mismatch between the film and the substrate. In the films of minimum thickness (27 nm), this strain-induced anisotropy is predominant and the easy magnetization axis switches from the [ 001] crystallographic direction (characteristic of the bulk magnet) to the [ 010] direction.     

Determination of magnetic anisotropy constants for bubble garnet epitaxial films using field orientation dependence in ferromagnetic resonances

Cubic magnetocrystalline anisotropy constant K₁ as well as uniaxial anisotropy constant K_u and gyromagnetic ratio γ are precisely determined for (111) magnetic garnet epitaxial films from measurements on FMR field orientation dependence. Strict FMR conditions are derived from total free energy expressions, where the differences between magnetization direction and applied field direction are taken into consideration. By applying magnetic field in (110) plane, FMR is measured to obtain the three best fitting parameters of K₁, K_u and γ. Present analysis is compared with Cronemeyer et al's analysis. Influence of sample misalignment on measurement accuracy is also presented. Similarly, for (110) garnet films with orthorhombic magnetic anisotropy, measurements are carried out for two crystallographic planes of (001) and (110), and the four best fitting parameters of K₁, K_u, δ and γ are determined.