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.     

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.     

Effect of Exchange Bias on Magnetic Anisotropies in Fe/CoO Bilayers

We report on the structural and magnetic properties of exchange-biased Fe/CoO bilayers grown on MgO (001) substrates by using rf-sputtering. For varying Fe thicknesses (4 nm, 10 nm, and 20 nm) the ferromagnetic resonance (FMR) spectra of bilayers have been studied as a function of temperature at X-band frequency. The resonance lines of FMR spectra have a relatively small linewidth indicating a high crystallinity of the Fe films. The room-temperature FMR data also show that the easy axis of Fe is in the film plane and parallel to the [110] crystallographic direction of MgO substrate. In addition, M versus H loops were recorded at selected temperatures by using VSM magnetometry. The VSM measurements indicate that the Fe thickness and temperature dependence of exchange-bias properties are in good agreement with the previous results on similar systems. However, the blocking temperature of the exchange-biased system is strongly reduced compared to the bulk values. This reduction in the blocking temperature is explained by both the thickness and superstoichiometric structure of antiferromagnetic CoO layer.     

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.     

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.     

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.     

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.     

Annealing Effect on Structural and Magnetic Properties of Cu2MnAl Heusler Alloy Films

Cu₂MnAl Heusler alloy films were grown on MgO (001) substrates by using the ion beam sputtering technique. The films were post-annealed at varying temperatures in order to investigate the influence of annealing on crystal structure and magnetic properties. The structural properties of Cu₂MnAl films have been investigated by using x-ray diffraction (XRD) and magnetic properties have been investigated by both vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques. The experimental data indicates that the crystal structure of the films strongly depends on the annealing temperature. When the films were annealed at 200?°C, the saturation magnetization (M _s =250?emu/cm³) achieved its maximum and the coercive field (H _c ??7?Oe) reached its minimum with B2 ordered structure. In addition, FMR results have revealed that the Cu₂MnAl film annealed 200?°C has the highest effective magnetization. The combination of structural and magnetic characterization indicates that the optimum growth temperature is 200?°C for the Cu₂MnAl Heusler alloy films on MgO substrates.     

Effect of Annealing Temperature on the Physical Properties of Zn-ferrite Nanoparticles

Thermally switched cationic exchange in magnetically ordered ZnFe₂O₄ (ZFO) nanoparticles synthesized via the coprecipitation route has been studied. The ZFO sample synthesized at 343 K and further annealed at 573 K is devoid of any X-ray diffraction (XRD) peak. However, the samples annealed at temperatures 673–973 K show clear XRD peaks indexed to a cubic spinel ferrite. The average crystallite sizes of the ZFO were calculated using Scherrer’s formula and found in the range of 10–16 nm. The lattice parameter of ZFO nanoparticles increased from 8.212 to 8.524 nm as the annealing temperature was raised from 673 to 973 K. Despite the fact that ZFO is antiferromagnetic in bulk, the ZFO nanoparticles annealed at temperatures ≥673 K show a magnetic hysteresis loop. The value of spontaneous magnetization has been decreased from 22 to 16 emu/gm with an increase in the annealing temperature. The electron spin resonance spectra show a single resonance peak with linewidth ranging from 147 to 243 Oe.     

Magnetic Studies of Vapor-Deposited Co/Si(100) and Co/Glass Thin Films

Magnetic force microscopy and alternating gradient field magnetometry techniques were used to investigate the static magnetic properties of vapor-deposited cobalt films with different thicknesses ranging from 50 to 195 nm. Brillouin light scattering (BLS) and ferromagnetic resonance (FMR) techniques were performed to study the dynamic properties of these films. Despite thicknesses well above the theoretical critical thickness that allows the presence of stripe domains, only the thicker films exhibit a magnetic stripe domain structure. The magnetocrystalline anisotropy factors, deduced and computed from BLS and FMR measurements, were found to decrease with thickness. Values of these computed effective anistotropy factors, of up to 7×10⁶ erg?cm^?3, have been found. All these results will be discussed and correlated.     

Magnetic Properties of Fe 3 O 4 Nanoparticles Synthesized by Coprecipitation Method

In this paper, we elucidate several specific magnetic properties of Fe ₃ O ₄nanoparticles synthesized by coprecipitation method. The characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) showed the particles to be of spinel structure and spherical shapes whose diameter could be controlled in the range from 14 to 22 nm simply by adjusting the precursor salts concentration and coprecipitation temperature. Magnetic properties of the Fe ₃ O ₄ nanoparticles measured by using vibration sample magnetometer (VSM) indicated the saturation magnetization and blocking temperature to increase with the particles size. Fe ₃ O ₄ nanoparticles with crystal size smaller than 22 nm exhibits superparamagnetic behavior at room temperatures. Characteristic magnetic parameters of the particles including saturation magnetization, effective anisotropy constant, and magnetocrystalline anisotropy constant have been determined. The observed decrease of saturation magnetization was explained on the base of core-shell model. A simple analysis indicated that the shell thickness decreases with an increase in particle size.     

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.     

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.     

Tuning the permeability spectra with a half-free ferromagnetic underlayer in (NiFe/IrMn)n exchange-biased multilayers

The influence of a half-free ferromagnetic underlayer on the static and microwave magnetic properties in [NiFe/IrMn]_n exchange bias multilayer thin film system has been systematically investigated. By changing the thickness of the half-free ferromagnetic underlayer, the linewidth of the ferromagnetic resonance (FMR) in this system can be tuned from 0.9 to 2.4 GHz. Theoretical fitting of the FMR frequencies based on the Landau-Lifshitz-Gilbert equation is also carried out to quantitatively identify the effective anisotropy fields and Gilbert damping parameters. The results provide an effective and flexible way to tailor the microwave permeability spectra and broaden the frequency linewidth toward the low frequency range in [ferromagnetic/antiferromagnetic]_n exchange bias multilayer films system. This approach has potential application for tunable wideband high frequency noise filters.     

Epitaxial growth and annealing control of FMR properties of thick homogeneous Ga substituted yttrium iron garnet films

Homogeneous, 30 μm to 70 μm thick Ga substituted yttrium iron garnet films have been grown on Y or Al substituted gadolinium gallium garnet substrates having lattice parameters matched to that of the films. Resonance field and FMR linewidth measurements as a function of frequency and annealing experiments revealed that the magnetization and cubic anisotropy of the films are identical to data from flux grown bulk single crystals, the FMR losses of the films are only slightly higher. For films grown with supercooling ΔT < 50 °C a negative, growth induced, uniaxial anisotropy was found which could be removed by annealing in air at 1100 °C. A compensation of the temperature drift of the FMR frequency can be adjusted in the Ga substituted films by changing the frozen-in Ga-Fe cation distribution by annealing and quenching from different temperatures > 800 °C.     

Evolution With Temperature of the Magnetic Anisotropy of Ba Ferrite Particles

The magnetic anisotropy of pure and Co/Ti-doped Ba ferrite particles is analyzed through the evaluation of the dependence on temperature of the constants of magnetocrystalline and shape anisotropy, which both are present in the platelet-like Ba ferrite particles with hexagonal structure. In undoped Ba ferrite, the magnetocrystalline anisotropy constant is predominant on the conflicting shape anisotropy constant at all temperatures, which indicates that the magnetic anisotropy is uniaxial, with preferred direction for the magnetization along the c axis of the hexagonal particles. In doped particles, where the magnetocrystalline anisotropy is weakened by the ionic substitutions, while at high temperatures the magnetic anisotropy is substantially uniaxial with c as axis of easy magnetization, when the temperature decreases, the shape anisotropy constant becomes larger than the magnetocrystalline anisotropy constant, and consequently, the magnetic anisotropy is not uniaxial, but it presents multiple preferred directions for the magnetization.     

Low Temperature and High Magnetic Field Dependence and Magnetic Properties of Nanocrystalline Cobalt Ferrite

The DC magnetic hysteresis loop measurements were carried out for temperatures varying from 5 to 300 K over a field range of ±10 T on nanocrystalline (～35 nm) cobalt ferrite samples (crystallized to \(Fd\bar {3} m\) space group with cubic symmetry) to validate the law of approach at low temperature for the nanocrystalline cobalt ferrite. A magnetocrystalline anisotropy constant and saturation magnetization have been obtained by analyzing the magnetization curve in saturation using the “law of approach (LA) to saturation.” The magnetocrystalline anisotropy constant is found to be almost constant in the temperature range of 5 to 150 K due to the freezing of spin at low temperature. Also, spin freezing leads to a decrease of coercivity with the increase in the temperature.     

Enhanced electric field tunable magnetic properties of lead-free Na0.5Bi0.5TiO3–MnFe2O4 multiferroic composites

Strong magnetoelectric (ME) interaction was exhibited at both dc and microwave frequencies in a lead-free multiferroic particulate composites of Na_0.5Bi_0.5TiO₃ (NBT) and MnFe₂O₄ (MFO) multiferroic, which were prepared by sol–gel route. The room temperature permeability measurements were carried out in the frequency range of 1 MHz–1 GHz. A systematic study of structural, magnetic and ME properties were undertaken. The room temperature ferromagnetic resonance (FMR) was studied. Strong ME coupling is demonstrated in 70NBT–30MFO composite by an electrostatically tunable FMR field shift up to 428 Oe (at E = 4 kV/cm), which increases to a large value of 640 Oe at E = 8 kV/cm. Furthermore, these lead-free multiferroic composites exhibiting electrostatically induced magnetic resonance field at microwave frequencies provide great opportunities for electric field tunable microwave devices.     

Influence of different conducting substrates on magnetic properties of electrodeposited Ni–Fe thin films

Electrodeposition of Ni–Fe soft magnetic alloy on copper and stainless steel substrates was performed in chloride bath. The deposition parameters such as current density, pH, temperature and deposition time have been investigated. From the investigation the optimized deposition parameters were current density 3.5 mA/cm², pH 3, temperature 30 °C and deposition time 15 min. The Ni–Fe magnetic alloys deposited on copper and stainless steel substrates under optimized deposition parameters are subjected to various characterizations. The structural and surface morphology of the Ni–Fe films were detected by using X-ray diffractogram (XRD) and scanning electron microscope (SEM) respectively. The constituents in the films were determined by energy dispersive X-ray spectroscopy (EDAX) technique. The magnetic properties such as the coercivity (H_c) and saturation magnetization of the films were studied with the help of vibrating sample magnetometer (VSM). From the magnetic studies it is concluded that the grain size are create a considerable impact on magnetic behavior of the films on both the substrates. The films prepared on stainless steel substrate of 0.1 M concentration at optimized deposition parameters exhibits higher coercivity (5010 Oe) which seems to be ideal for magnetic sensor applications.     

Solvothermal preparation of iron nanosheets

Iron nanosheet has been prepared in the presence of N-Methylaniline by solvothermal method. The structural property of the sample has been studied by X-ray diffraction and transmission electron microscopy. The magnetic properties have been extensively investigated by hysteresis loops and temperature-dependent magnetization curves. XRD result exemplifies that the as-prepared product has been identified as iron with body centered cubic structure. TEM observation showed that the as-prepared product composed of sheet-like nanostructure with size was around 25 nm. FTIR result suggests that the N-Methylaniline molecules are on the surface of the Fe Nano sheet. Magnetic measurements showed that the prepared iron nanosheet was ferromagnetic behavior at 300 K. Compared with bulk iron, Fe nanosheet exhibits significant increase in coercive force due to the presence of shape anisotropy. Moreover, the temperature dependent magnetization curves show no blocking temperature up to 300 K, which indicates the prepared sheet, is ferromagnetic character at room temperature.