Ferromagnetic Resonance Signal Research Articles

Study of spin transfer torque in serially connected pillars by means of ferromagnetic resonance

We investigate the influence of the spin polarized current on magnetic damping in the ferromagnetic layers of pillar structures by means of ferromagnetic resonance (FMR). The pillars are prepared and electrically contacted by high-resolution electron beam lithography and electron beam evaporation in a multi-step process. FMR measurements are carried out on a pillar array, in which all pillars are connected in series, so that the current density reaches values high enough to observe the effect of the spin transfer torque on the FMR signal. The temperature rise due to the high current density is avoided by cooling. An influence of the spin transfer torque effect on the intrinsic damping in the ferromagnet is observed. The fabrication of the serial connection can be easily applied to arbitrary structures.

Characterization of YIG nanopowders by mechanochemical synthesis

Yittrium iron garnet, Y 3Fe 5O 12 (YIG), is a material used widely in electronic devices for the microwave region as well as the magnetic bubble domain-type memories. Yittrium iron garnet (Y 3Fe 5O 12) was produced by mechanochemical synthesis from Y 2O 3 and Fe 2O 3 with a particle size of around 150 nm. Dense ceramics with theoretical density of around 95% was obtained from mechanochemically activated powders after sintering at 1425 °C for 10 h without calcination step. Ferro-magnetic resonance (FMR) spectrum and microwave absorption spectrum of the YIG nanoparticles were studied at different frequencies and magnetic fields. The signal exhibits a resonance character at each frequency. It was observed that the location of the FMR signal peak at the field axes monotonically shifts to higher field with increasing frequency. The dielectric properties of YIG pellets produced from nanopowder was studied over a wide range of frequency and as a function of temperature. It was observed that the dielectric constant, dielectric loss and electrical conductivity gradually increase with temperature.

Microwave power absorption analysis in the devitrification process of Co-based amorphous ribbons

Melt-spun Co 66 Fe 4 B 12 Si 13 Nb 4 Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5–20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon’s plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon’s plane was normal to the AC magnetic field. In both cases, the ribbon’s axis was parallel to the DC magnetic field. For both orientations, two absorptions were observed: the first corresponds to a low field microwave absorption (LFA) centered in zero dc magnetic field, and a higher field absorption corresponding to the ferromagnetic resonance (FMR). In the geometry 1, a single FMR spectrum was observed for all the samples, with a shift in resonant field as annealing increased. For geometry 2, evidence of the superposition of two FMR signals was observed. FMR spectra are therefore due to a combination of two different magnetic phases corresponding to the amorphous matrix and nanocrystallites. Deconvolution calculations were carried out on FMR spectra to separate the contributions. Their behavior as a function of annealing time was in good agreement with the magnetic softening, also obtained with LFA results. The differences in microwave absorption, for both geometries, can be explained by differences in the electromagnetic wave propagation volume.

Colloidal Fe-doped ZnO nanocrystals: Facile low temperature synthesis, characterization and properties

Colloidal Fe-doped ZnO nanocrystals, Zn 1− x Fe x O ( x = 0.00, 0.05, 0.07 and 0.1) have been prepared by thermal decomposition of metal precursors at 200 °C with hexadecylamine (HDA) as solvent and surfactant. The nanocrystals so prepared can be easily dispersed in non-polar solvents like chloroform and toluene. The nanocrystals have been structurally characterized using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). Magnetization measurements on a representative sample, Zn 0.95Fe 0.05O using a vibrating sample magnetometer (VSM) reveal that the nanocrystals exhibit a weak ferromagnetic behavior at 300 K. This observation is further confirmed by the electron paramagnetic resonance spectrum of Zn 0.95Fe 0.05O nanocrystals, which shows a distinct ferromagnetic resonance signal at room temperature.

Potential of EPR imaging to detect traces of primitive life in sedimentary rocks

In the prospect of the search for traces of primitive life on Earth and Mars, we investigated the possibility to detect and visualize the spatial distribution of minute amounts of organic matter in ancient rocks, in a non-destructive way, by Electron Paramagnetic Resonance Imaging (EPRI). We studied a series of non- or moderately metamorphosed siliceous rocks (cherts) of different ages ranging from 45 Ma to 3490 Ma and embedding fossile organic matter. In the case of the oldest cherts containing only mature insoluble organic matter (IOM), with IOM • radicals characterized by a single Electron Paramagnetic Resonance (EPR) line, we could obtain three-dimensional images with sub-millimetric resolution of the organic matter distribution inside samples containing as low as 10 14–10 15 radicals per gram. In the case of younger cherts, containing less mature organic matter, and thus several types of organic radicals, we showed that selective imaging of each type of radical was possible provided that the EPR spectra did not overlap. Selective imaging of either the organic radicals or of the oxygen vacancy (E' centres) of the mineral matrix, which are ubiquitous in siliceous rocks, was possible, selecting either one or the other paramagnetic species with high power in-phase, 1st harmonic detection or with 90°-out-of-phase, 2nd harmonic detection of the EPR. The influence of ferromagnetic inclusions in the mineral matrix on the EPRI of the organic matter was also addressed. Image artifacts due to the ferromagnetic resonance signal of these inclusions could be easily removed by background substraction from the EPR spectra before image reconstruction. We thus showed that selective imaging by EPR of minute amounts of fossile organic matter in ancient rocks could be possible despite the magnetic complexity of such materials.

Room temperature ferromagnetism in Fe-doped BaSnO 3

Polycrystalline bulk samples of BaSn 1− x Fe x O 3, with x = 0.03, 0.04 and 0.05 were prepared by solid-state reaction method. These Fe-doped BaSnO 3 systems exhibit ferromagnetism at room temperature with an average magnetic moment of 0.047, 0.038 and 0.025 μ B/ F e and Curie temperature of 510, 462 and 446 K, respectively. The observed magnetic properties are not attributed to any of the known impurity phases or clusters. The presence of ferromagnetic interaction at room temperature is supported by the ferromagnetic resonance (FMR) signals observed in the electron spin resonance spectra. The ferromagnetic order between Fe-ions is expected to be mediated by an electron trapped at the oxygen anion vacancy, called the F-centre exchange (FCE) interaction.

Characteristics of MnxSi1-xTe Compound Studied by Electron Magnetic Resonance and Other Experiments

The magnetic and other physical characteristics of <TEX>$Mn_xSi_{1-x}Te$</TEX> have been investigated by electron magnetic resonance (EMR), X-ray diffraction (XRD) and other experiments. <TEX>$Mn_xSi_{1-x}Te$</TEX> is found to have corundum structure for manganese contents up to 10% and also to be ferromagnetic for temperatures below 80 K. While ferromagnetic resonance signal coexists with the usual paramagnetic resonance signal, invariance of the g-factor inferred from the electron paramagnetic resonance signals throughout all temperature ranges clearly confirms that the manganese ions are in the electronic 3d5 state. The temperature dependence of EMR line-width is the same as other diluted magnetic semiconductors. From the EMR signals relaxation times <TEX>$T_2$</TEX> and <TEX>$T_1$</TEX> of <TEX>$Mn_xSi_{1-x}Te$</TEX> compounds are estimated to be about <TEX>$4.4{\times}10^{-10}s$</TEX> and <TEX>$9.3{\times}10^{-8}s$</TEX> respectively and are found to vary slightly with temperature or composition change. Exchange narrowing of the EMR line-width becomes dominant for the sample in which the substitution ratio, x = 30%. For one sample, in which x = 0.5%, spin glass-like behavior is indicated by EMR signals for temperatures lower than 60 K. This behavior may authentic for samples within a certain range of x.

EMR study of magnetic inhomogeneities in crystalline bulk and nanometer-sized La 0.7Ca 0.3MnO 3

Electron magnetic resonance (EMR) was studied on crystalline bulk and nanometer-sized samples of the same La 0.7Ca 0.3MnO 3 manganite compound aiming to compare their magnetic homogeneities. The results obtained show that single-crystalline bulk sample is less homogeneous than the nanocrystalline one. Except for higher homogeneity, the nanometer-sized sample also demonstrates a weaker magnetic anisotropy. As a result, well-pronounced coexistence of different magnetic phases (coexistence of ferromagnetic and paramagnetic resonance signals) is observed in the bulk sample, while it is absent in the nanocrystalline one. It is suggested that such strong difference is directly connected to the difference in structural state of the samples.

Characterisation and intrinsic magnetic resonance properties of nickel nanoparticles

Nickel nanoparticles have been extensively characterised by atomic force microscopy (AFM), scanning electron microscopy (SEM) and confocal micro-Raman spectroscopy. AFM underestimates the particle size compared to SEM measurements. It is shown that Raman spectroscopy can detect the nanometre-thick NiO layer on the particles having frequency shifts of the modes indicative of phonon confinement. The magnetic properties of the particles are studied by ferromagnetic resonance (FMR) of magnetic field aligned particles. The alignment is achieved by suspending the particles in the liquid crystal MBBA and freezing the liquid in a 0.4 T DC magnetic field. The in-field solidification locks the direction of maximum magnetisation of the particles parallel to the direction of the applied DC magnetic field. This removes the effects of dynamical particle fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the magnetic nanoparticles. The intensity of the FMR signal decreased with lowering temperature for the particles frozen in the liquid in a 0.4 T DC magnetic field. The effect is suggested to be due to a reduction of the microwave skin depth with lowering temperature.

Unusual magnetic properties of Mn-doped ThO2 nanoparticles

We report the synthesis of Th1–xMnxO2 (x = 0, 0.001, 0.002, 0.004, and 0.01 wt%) nanoparticles by the urea combustion method using thorium nitrate gel followed by heat treatment at a higher temperature (T). The obtained Th1–xMnxO2 nanocrystals were characterized by x-ray diffraction (XRD), direct-current magnetization (M) measurements and electron paramagnetic resonance (EPR). XRD analysis revealed that Th1–xMnxO2 crystallizes in the cubic structure (Fm3m). M measurements showed ferromagnetic ordering at room temperature for Th0.99Mn0.01O2 samples annealed at 775 K. An intense and broad ferromagnetic resonance (FMR) having linewidth of ∼1200 G, was observed at relatively lower fields in the EPR spectra of Th0.99Mn0.01O2 samples annealed at 775 K, indicating the presence of a ferromagnetic phase at room temperature. EPR measurements were used to estimate the number of spins involved in the ferromagnetic ordering. Out of the total Mn present in Th0.99Mn0.01O2 samples, about 25% of the Mn2+ ions were found to be responsible for the ferromagnetic ordering. In addition to the FMR signal, a weak hyperfine sextet was observed at g = 2.0048 (55Mn, I = 5/2), which corresponds to the −1/2 ↔ +1/2 transition of Mn2+ ions, suggesting its presence at thorium sites (uncoupled spins). X-ray photoelectron spectra indicated that the manganese ions exist mainly as Mn2+, Mn3+, and Mn4+. The room-temperature ferromagnetism may be attributed to the coupling between these Mn2+ ions at thorium sites in ThO2 rather than due to the formation of any metastable secondary phases.

Field-induced spin–flop transitions of interacting nanosizedα-Fe2O3 particles dispersed in a silica glass matrix

Nanoparticles of 10 mol% Fe2O3 doped in silica glass (Fe10) samples prepared by a sol–gel methodfollowed by calcination at various temperatures in the range700–1000 °C are studied by x-ray, transmission electron microscopy and magnetic methods, includingelectron paramagnetic resonance (EPR). X-ray studies reveal the presence of bothγ-Fe2O3 and α-Fe2O3 nanocrystals in varying proportion, the latter being more abundant in samples subject tocalcination at higher temperature. Nanocrystals have mean sizes in the range 10–40 nm andare larger in the samples calcined at higher temperatures. The relatively narrow EPR linehaving its origin in superparamagnetism is observed at room temperature and is transformedto an asymmetrically broad ferromagnetic resonance signal at 77 K. Mössbauer spectra of the700 °C calcined sample at room temperature show two doublet structures due toγ-Fe2O3 and α-Fe2O3 nanoparticlessignifying their superparamagnetic character, whereas those of samples calcined at higher temperatures(≤1000 °C) displaytwo sextets indicating that the nanoparticles are magnetically ordered. At higher calcination temperatures(≥900 °C) the hyperfine lines become asymmetrically broadened and the average hyperfine fielddiminishes in a way typical for interacting magnetic nanoparticles. Zero-field-cooledmagnetization and hysteresis studies of Fe10 samples calcined at higher temperatures(≥800 °C) in the temperature range 5–300 K reveal unusual ferromagnetic behaviour with substantialmagnetization and large coercivities at low temperature (5 K). The presence of a highirreversibility field and shifted hysteresis loop in the Fe10 sample calcined at1000 °C at 5 K has been verified from field-cooled magnetization versus the magnetic field curve. Onlyα-Fe2O3 nanoparticles generatedin the Fe2O3:SiO2 sample calcinedat 1000 °C exhibit a Morintransition like bulk α-Fe2O3 crystals but at a muchlower temperature ∼100 K. Spin–flop like transitions have been observed for the first time at temperatures abovethe Morin temperature, possibly induced by an external dc magnetic field of appropriatemagnitude in conjunction with inter-nanoparticle exchange.

Magnetic inhomogeneities in crystalline bulk and nanometer sized La0.7Ca0.3MnO3: ESR probing

AbstractElectron spin resonance was studied on three (two bulk and one nanometer sized) samples of the same La0.7Ca0.3MnO3 manganite compound aiming to compare their magnetic homogeneities. The results obtained show that both ceramic and single crystalline bulk samples are less homogeneous than nano‐crystalline one. Except for higher homogeneity, the nano sample demonstrates also a weaker magnetic anisotropy. As a result, well pronounced coexistence of different magnetic phases (coexistence of ferromagnetic and paramagnetic resonance signals) is observed in bulk samples, while it is absent in nano. It is suggested that such strong difference is directly connected to the difference in structural state of the samples. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

FMR Study of Carbon Coated Cobalt Nanoparticles Dispersed in a Paraffin Matrix

Agglomerated cobalt magnetic nanoparticles coated with carbon, dispersed in a paraffin matrix, were prepared and investigated by FMR (ferromagnetic resonance) at room temperature. Four samples with different C/Co content, ranging from 0.175 to 1.011, dispersed at low concentration in paraffin were investigated. Very intense and broad FMR spectra with different intensities, line widths and positions of the resonance fields were recorded for the samples. A strong dependence of the FMR signal intensity and resonance on the concentration of magnetic nanoparticles was observed. Various magnetic interactions affecting the observed FMR spectra have been analyzed. It was found that with increasing concentrations of magnetic nanoparticles the magnetic dipole interaction between the agglomerates plays a more important role.

Unusual Room Temperature Ferromagnetism in Bulk Sintered GaP Doped with Copper

Robust room temperature ferromagnetism is obtained in single phase Gallium Phosphide doped with Cu2+ prepared by simple solid state reaction route. The saturation magnetization at 300 K is 1.5times10-2 emu/g and the coercivity was found to be 125 Oe. A strong ferromagnetic resonance signal confirms the long range magnetic order which persists to temperatures as high as 739 K. X-ray absorption spectroscopy (XAS) indicate that Cu is in a +2 state. Ab initio calculations also show that the ferromagnetic ordering is energetically favorable in Cu doped GaP. When the spin-orbit coupling is included we get an enhanced total magnetic moment of 0.31 muB with a local moment on Cu 0.082 and on P 0.204 muB. per atom

Ferromagnetic resonance in an ensemble of nanoparticles with randomly distributed anisotropy axes

Spectra of absorbed power as probed by ferromagnetic resonance (FMR) are numerically calculated within a macro-spin model for single domain nanoparticles using Landau–Lifshitz–Gilbert dynamics. Randomly distributed anisotropy axis and a distribution of anisotropy energies result in a significant broadening of the FMR signal as compared to an ensemble of particles all having the same anisotropy. Additionally, a temperature dependence of the shift of the resonance frequency is obtained which is in a qualitative agreement with experimental data on single domain nanoparticles.

Electron spin resonance study of bulk and nanosized La 0.875Sr 0.125MnO 3

The structural, transport and electron spin resonance properties of bulk and nanosized La 0.875Sr 0.125MnO 3 prepared by a sol–gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T C . In addition to a sharp peak in the vicinity of T C , another sharp peak close to T OO is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.

Electrically detected ferromagnetic resonance

We study the magnetoresistance properties of thin ferromagnetic CrO2 and Fe3O4 films under microwave irradiation. Both the sheet resistance ρ and the Hall voltage VHall characteristically change when a ferromagnetic resonance (FMR) occurs in the film. The electrically detected ferromagnetic resonance (EDFMR) signals closely match the conventional FMR, measured simultaneously, in both resonance fields and line shapes. The sign and the magnitude of the resonant changes Δρ∕ρ and ΔVHall∕VHall can be consistently described in terms of a Joule heating effect. Bolometric EDFMR thus is a powerful tool for the investigation of magnetic anisotropy and magnetoresistive phenomena in ferromagnetic micro- or nanostructures.

Temperature-dependent magnetic resonance force microscopy studies of a thin Permalloy film

We used magnetic resonance force microscopy (MRFM) to study a 50nm thick continuous Permalloy film. We mechanically measured the ferromagnetic resonance signal in the temperature range between 10 and 70K in the presence of a static magnetic field applied normal to the surface of the film. The measurements show a decrease of the ferromagnetic resonance field with increasing temperature. We attribute this behavior to the temperature-dependent changes of the saturation magnetization. Our experiments demonstrate the potential of MRFM to perform quantitative ferromagnetic resonance measurements as a function of temperature.

Observation of a Griffiths-like phase in bilayered manganites

The authors report the observation of a Griffiths-like phase in La2−2xSr1+2xMn2O7 (x=0.30, 0.33, and 0.40) and (La0.8Eu0.2)2−2xSr1+2xMn2O7 (x=0.33) single crystals by means of electron spin resonance, magnetic susceptibility, and magnetoresistance measurements. In the temperature region TC&amp;lt;T&amp;lt;350K, the resonance signal consists of a ferromagnetic resonance line and a paramagnetic resonance line, which suggests that the system is not in a pure paramagnetic phase. The ferromagnetic resonance signal disappears and the magnetic susceptibility starts to deviate from the Curie-Weiss law at the same temperature T*≈350K, independent of doping level and anisotropy. These results indicate that a Griffiths-like phase exists at TC&amp;lt;T&amp;lt;T* in bilayered manganites, which is due to intrinsic inhomogeneity caused by quenched disorder.

Magnetization and structural studies of Mn doped ZnO nanoparticles: Prepared by reverse micelle method

Single phase Mn (2.5 at%) doped ZnO nanocrystalline samples were synthesized by reverse micelle method as characterized by Rietveld refinement analysis of X-ray diffraction data, high resolution transmission electron microscopy and selected area electron diffraction analyses. The X-ray photoelectron spectroscopy and electron paramagnetic resonance (EPR) studies indicated that manganese exist as Mn 2+ in ZnO lattice. DC magnetization measurements as a function of field and temperature, of 2.5 at% Mn doped ZnO nanoparticles annealed at 675 K, showed room temperature ferromagnetism (RTF). This observation is further confirmed by the EPR spectrum of the sample, which shows a distinct ferromagnetic resonance signal at room temperature. These results indicate that the observed RTF in Mn-doped ZnO may be attributed to the substitutional incorporation of Mn at Zn sites.