FMR Studies Research Articles

Effect of pentavalent vanadium ion doping on magnetic properties of nickel ferrite determined via 57Fe IFNMR and FMR studies

We have synthesized a series of vanadium-doped nickel ferrite samples to investigate the impact of this substitution on the material's magnetic properties. The samples' phase purity has been verified by XRD analysis. Micrographs taken with a SEM instrument show the accelerated development of the grains. All of the samples are ferrimagnetic, as shown by Vibrating Sample Magnetometry (VSM), but the saturation magnetization (MS) decreases up to (x = 0.20), after which it increases. The samples are also studied by FMR and using the enhanced simulation conditions, the magnetocrystalline anisotropy constant (K1) of nickel ferrite is calculated, and it is found to be in excellent agreement with the previous reports. The effect of anisotropy on the strength of the signal is investigated using 57Fe-IFNMR (echo amplitude). Intriguingly, we have found that the magnetocrystalline anisotropy measured by FMR is inversely correlated with the echo amplitude measured by IFNMR.

Ferromagnetic Resonance in Magnetic Oxide Nanoparticules: A Short Review of Theory and Experiment

This review article aims to provide a comprehensive overview of recent FMR studies on magnetic oxide nanoparticles and their potential applications. The use of the FMR technique is a powerful tool to study the magnetic properties of magnetic nanoparticles and can provide valuable information on their behavior. For this, we will start by discussing the purpose of these magnetic nanoparticles and their application in various fields, including biomedical applications, energy storage, and environmental remediation. We will then discuss the methods used to prepare magnetic nanoparticles and the theory behind FMR including the superparamagnetic effect. Additionally, we will present the most recent studies on FMR for magnetic oxide nanoparticles by highlighting the effect of temperature and doping on the magnetic properties of these nanoparticles.

Ferromagnetic resonance in Mn5Ge3 epitaxial films with weak stripe domain structure

Extensive X-band and Q-band FMR experiments have been performed in the Mn5Ge3 epitaxial films with thicknesses varying between 4.5 and 68 nm. FMR signals were recorded in the temperature range between 15 and 295 K, at different orientations of magnetic field with respect to the film plane. In addition to the acoustic FMR mode with well defined resonance field, originating from inside the magnetic domains, a broad absorption line has been observed at low fields and attributed to the unresolved spectrum of FMR modes having the origin in flux closure caps. The FMR results have been discussed in the context of the domain structure computed with the use of OOMMF micromagnetic calculations and giving good agreement with the experimental hysteresis curves. From the Q-band experiments, where the FMR signal is observed in the magnetically saturated sample, the uniaxial anisotropy constant in films with different thicknesses has been determined as a function of temperature. This FMR study provides the evidence that the strong uniaxial anisotropy observed in epitaxial thin films of Mn5Ge3 leads to the formation of a stripe domain structure above 25 nm, in agreement with the published reports on magnetization studies in these films. It also eliminates a possible confusion that may arise from previously published FMR studies on films grown with the same method, which led their authors to conclude that the shape anisotropy can force the magnetization to the in-plane orientation in this thickness range and even above it.

Optimization of perpendicular anisotropy of Ta-inserted double CoFeB/MgO interface MTJ’s for STT-MRAM

Ta inserts in double magnetic tunnel junctions have been shown to induce perpendicular magnetic anisotropy. We fabricated the central layers of a CoFeB/MgO based double magnetic tunnel junction with a Ta insertion layer between the free layers of the magnetic tunnel junctions. The thickness of the Ta insert and CoFeB layer were varied from 0.5 to 1.1nm and 0.9 to 1.7nm respectively, to find which minimum thickness of Ta will induce perpendicular anisotropy in the MTJ. FMR studies were performed to measure the interfacial anisotropy Ku,i. We demonstrated that the most effective stack minimized the CoFeB thickness while maximizing the Ta thickness. This was balanced with the need to keep both CoFeB layers from decoupling or becoming magnetically dead.

Spin-torque-driven ferromagnetic resonance in point contacts

We demonstrate the technique of spin-torque-driven ferromagnetic resonance (ST-FMR) in point contacts, which enables FMR studies in sample volumes as small as a few cubic nanometers. In our experiments, we use point contacts ∼10 nm in size to inject both dc and microwave currents into F/N/F/AFM exchange-biased spin valves where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer and one of the Fs is pinned by an adjacent antiferromagnetic (AFM) layer. High current densities produce the spin-transfer torque on magnetic moments in a small contact region and drive it to resonance at appropriate frequency of the applied microwaves. The resulting magnetodynamics are detected electrically via a small rectified dc voltage, which appears across the contact at resonance. The width of the resonance varies linearly with the applied dc bias as expected for spin transfer in spin valves. Potentially, the point-contact technique extends the applicability of ST-FMR to higher/lower frequencies, smaller sample volumes, and a broader range of materials.

Changes in magnetic properties of ultrathin cobalt films as induced by Mo, V, Au overlayers

Magneto-optical (based on polar and longitudinal Kerr effects) and FMR studies were carried out on ultrathin cobalt films deposited on Au(1 1 1) and covered with either a V or Mo cover layer, and finally by an Au protective layer. Co thickness at which the magnetization reorientation transition from an easy axis into an easy plane occurs is influenced by the type of cover layer. Even 0.1 nm thick V-coverage is efficient enough to induce full reorientation. A strong decrease of domain structure size (down to the submicrometer scale) was observed while approaching reorientation phase transition (RPT). The influence of a Mo-overlayer on Co in-plane magnetic anisotropy is shown to be much weaker in comparison to the influence of Mo-buffer.

FMR studies of single permalloy layers sandwiched by Au

Results of FMR studies performed at X-band on two series of thin permalloy (Py) layers sandwiched by Au are reported and analyzed. These two series are characterized by an Au layer thickness of 12 and 6 nm, respectively. The FMR results are compared to the ones obtained for a series of (Py/Al 2O 3) control films. A strong interfacial increase of the intrinsic damping is revealed. The spin–orbit perpendicular anisotropy field is also dominated by the metallic interface contribution. Remarkable phenomena evidenced in the angular variation of the FMR spectrum may be well understood in terms of the theoretical description we have developed of the observable signal in a bimetallic (ferro/normal) film.

Magnetic anisotropy FMR studies of Co films grown between Mo or Au buffer and Mo or Au overlayers

The magnetic in-plane anisotropy as a dependence on Co film thickness in X/Co/X (where X=Mo or Au) sandwich structures were investigated by the ferromagnetic resonance method in the Co thickness range where magnetization is mainly in-plane oriented. The in-plane anisotropy was observed in all Au/Co/Au, Au/Co/Mo, Mo/Co/Au and Mo/Co/Mo samples. Large (up to 1.5 kOe) in-plane anisotropy field was found for Mo/Co/Au sample which is related with anisotropic strain distribution during growth of Co(0 0 0 1) on Mo(1 1 0) buffer layer. On the other hand, Mo as the overlayer has much smaller contribution to the in-plane anisotropy, which might be connected with pseudomorphic growth of Mo on Co with C3 symmetry.

Magnetization and FMR studies of [Fe/Cr]n structures with ultrathin iron layers

AbstractMagnetic multilayer structures [Fe(t Fe)/Cr(t Cr)]n with ultrathin (t Fe < 5 Å) iron layers prepared by a molecular beam epitaxy were studied using SQUID‐magnetometry and ferromagnetic resonance technique. For the samples with t Fe ≳ 3 Å, the obtained magnetization curves and resonance spectra are typical for regular magnetic superlattices. On the contrary, the samples with t Fe ≲ 3 Å demonstrate superparamagnetic‐like behavior. These samples show a strong temperature dependence of their static and resonance magnetic properties. To explain the obtained results, we propose a theoretical model considering a cluster structure of thin iron layers. Magnetization curves and resonance spectra are calculated using an effective field approximation taking into account an interlayer interaction and cluster size distribution. The calculated dependencies show a qualitative agreement with the experimental data. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Arrays of Ni Nanowires in Alumina Membranes: Magnetic Properties and Spatial Ordering

AbstractFor Abstract see ChemInform Abstract in Full Text.

Arrays of Ni nanowires in alumina membranes: magnetic properties and spatial ordering

Magnetic characteristics of arrays of Ni nanowires embedded in porous alumina are reviewed as a function of their spatial ordering. The different steps for the controlled production of highly-ordered nanowires is firstly described. Nanopores are formed into an hexagonal symmetry arrangement by self-organized process during anodization of pure Al. Parameters of the anodization allow us to control their diameter, hexagonal lattice parameter and size of crystalline domains. Subsequently, Ni nanowires are grown inside the pores by electrodeposition. Control of the pores filling and of geometrical ordering characteristics has been performed by SEM, HRSEM, RBS and AFM techniques. The magnetic characterisation of the arrays has been achieved by SQUID and VSM magnetometers, while information on the magnetic state of individual nanowires is obtained by MFM. Experimental studies are presented, particularly coercivity and remanence, for arrays with different degree of ordering (crystalline domains up to around 1 μm), and for ratio diameter to lattice parameter (diameter ranging between 20 and 180 nm, and distance between 35 and 500 nm). FMR studies have allows us to obtain complementary information of the anisotropy and magnetic characteristics. A modelling of multipolar interacting nanowires is introduced to account for the influence of short and long range ordering degree of the arrays.

Microwave FMR studies on ultrathin Fe/GaAs layer structures

The experimental results of microwave FMR studies on ultrathin Fe/GaAs layer structures with various thicknesses of Fe films and GaAs substrates using flip-chip arrangement are reported. First, the major technical aspects of the MBE-grown Fe/GaAs layer structures are summarized. The measured results on the FMR absorption levels versus the thicknesses of the Fe films and the GaAs substrates are then presented. The optimum sample preparation conditions and the thicknesses of Fe film and GaAs substrate for enhancement of the FMR absorption levels in the Fe/GaAs-based bandstop filters are also suggested.

An Investigation of the Value of Spin-Echo-Based fMRI Using a Stroop Color–Word Matching Task and EPI at 3 T

This study examines the value of spin-echo-based fMRI for cognitive studies at the main magnetic field strength of 3 T using a spin-echo EPI (SE-EPI) sequence and a Stroop color–word matching task. SE-EPI has the potential advantage over conventional gradient-echo EPI (GE-EPI) that signal losses caused by dephasing through the slice are not present, and hence although image distortion will be the same as for an equivalent GE-EPI sequence, signal voids will be eliminated. The functional contrast in SE-EPI will be lower than for GE-EPI, as static dephasing effects do not contribute. As an auxiliary experiment interleaved diffusion-weighted and non-diffusion-weighted SE-EPI was performed in the visual cortex to further elucidate the mechanims of functional contrast. In the Stroop experiment activation was detected in all areas previously found using GE-EPI. Additional frontopolar and ventral frontomedian activations were also found, which could not be detected using GE-EPI. The experiments from visual cortex indicated that at 3 T the BOLD signal change has contributions from the extravascular space and larger blood vessels in roughly equal amounts. In comparison with GE-EPI the absence of static dephasing effects would seem to result in a superior intrinsic spatial resolution. In conclusion the sensitivity of SE-EPI at 3 T is sufficient to make it the method of choice for fMR studies that require a high degree of spatial localization or where the requirement is to detect activation in regions affected by strong susceptibility gradients.

FMR studies of ultrathin permalloy layers sandwiched by Al 2O 3

We report and analyze the results of detailed FMR studies performed on films corresponding to permalloy ultrathin layers sandwiched by Al 2O 3. It is shown that calculations based on a theoretical description of the inhomogeneous linewidth, including the effect of exchange narrowing, enable to reproduce rather well the magnitude and angular variation of the observed width.

Sources of error in comparing functional magnetic resonance imaging and invasive electrophysiological recordings.

Several authors have recently reported studies in which they aim to validate functional magnetic resonance (fMR) imaging against the accepted gold standard of invasive electrophysiological monitoring. The authors have conducted a similar study, and in this paper they identify and quantify two characteristics of these data that can make such a comparison problematic. Eight patients in whom surgery for epilepsy was performed and five healthy volunteers underwent fMR imaging to localize the part of the sensorimotor cortex responsible for hand movement. In the patient group subdural electrode mats were subsequently implanted to identify eloquent regions of the brain and the epileptogenic zone. The fMR imaging data were processed to correct for motion during the study and then registered with a postimplantation computerized tomography (CT) scan on which the electrodes were visible. The motion during imaging in the two groups studied, and the deformation of the brain between the preoperative images and postoperative scans were measured. The patients who underwent epilepsy surgery moved significantly more during fMR imaging experiments than healthy volunteers performing the same motor task. This motion had a particularly increased out-of-plane component and was significantly more correlated with the stimulus than in the volunteers. This motion was especially increased when the patients were performing a task on the side affected by the lesion. The additional motion is hard to correct and substantially degrades the quality of the resulting fMR images, making it a much less reliable technique for use in these patients than in others. Also, the authors found that after electrode implantation, the brain surface can shift more than 10 mm relative to the skull compared with its preoperative location, substantially degrading the accuracy of the comparison of electrophysiological measurements made in the deformed brain and fMR studies obtained preoperatively. These two findings indicate that studies of this sort are currently of limited use for validating fMR imaging and should be interpreted with care. Additional image analysis research is necessary to solve the problems caused by patients' motion and brain deformation.

Structural and magnetic properties of iron and cobalt implanted silicone polymers

40 keV Fe+ and Co+ ions were implanted in both viscous and solid silicone substrates at doses of (0.1–1.8)×1017 ions/cm2 and an ion current density of 4 μA/cm2. The microstructure and magnetic properties of the Fe- and Co-implanted silicone layers were then investigated as a function of the initial relaxation state (viscosity) of the polymer target. TEM investigations showed that granular Fe and Co films consisting of fine isolated particles are synthesized in the solid substrates at doses higher than 0.3×1017 ions/cm2. At the same dose multiparticle iron agglomerates and closely packed large Co particles with a mean size of 200 nm are formed in substrates implanted in the viscous state. FMR studies revealed essential differences in the magnetic properties of the metal films synthesized at different viscosity of the silicone substrate. The temperature dependence of the FMR spectra and the dose dependence of the effective magnetization of the synthesized films were studied.

FMR studies on ultrathin metallic films grown on Al 2O 3 surfaces

We investigated the ferromagnetic resonance of cobalt and iron films consisting of small islands deposited on a sapphire single crystal (Al 2O 3(0 0 0 1)) under UHV conditions. STM measurements performed on similar deposits on a thin alumina film (Al 2O 3/NiAl(1 1 0)) revealed a three-dimensional growth mode. On the basis of these structural data we investigated the magnetic anistotropy via angular-dependent ferromagnetic resonance measurements. Cobalt islands exhibit a uniaxial out-of-plane anisotropy which alignes the magnetization parallel to the substrate. This in-plane magnetization is due to the demagnetizing field which is calculated as the dipole sum atom by atom. Furthermore, the influence of adsorbates like O 2 or CO on the magnetism of thin films was investigated revealing a partial reduction in intensity. The adsorbates only influence the outer atoms and a magnetic core still remains.

FMR studies of magnetic ordering structure in Fe/Cr superlattices

The FMR acoustic mode in parallel magnetic field orientation in 10—38 GHz frequency range at room temperature was studied for a series of multilayer structures [Fe(11 Å)/Cr(d)] 12 with different thickness d of Cr spacer (5 Å < d < 36 Å). Expressions for acoustic and optic modes in parallel and perpendicular magnetic field orientation were obtained. The comparison of the experimental results with these formulas allowed to determine the magnetization dependence on the magnetic field and to evaluate bilinear and biquadratic exchange coupling for these samples.

FMR studies of cobalt thin films deposited by MOCVD

Ferromagnetic resonance has been used to investigate the magnetic anisotropy in thin films of cobalt and Co/ZnO multilayers deposited by MOCVD. In general, the samples exhibited an overall in-plane magnetic anisotropy, the strength of which decreased with decreasing cobalt layer thickness. A buffer layer of ZnO was found to decrease the strength of the anisotropy with respect to a film of the same thickness deposited onto glass. In some relatively thin single films, two resonance modes were observed. This was also the case in multilayers deposited onto a ZnO buffer layer. The origins of these modes are discussed.

New phenomena in FMR and EPR studies of amorphous oxide films and their crystallization

(BiDy) 3Fe 5O 12 films were made in the amorphous state by rf magnetron sputtering from a sintered Bi 2Dy 1Fe 5O 12 ceramic target onto quartz substrates and studied by X-ray diffraction. The films were annealed at temperatures ranging from 500 to 700°C for 1 h in air and also at 650°C for different periods of time from 5 to 60 min. X-ray diffraction was used to determined the phase components of the samples. In the X-ray patterns, the amorphous films had two diffuse broad maxima in X-ray diffraction patterns centred at the double diffraction angles 2 θ=31° and 52°, respectively. The films crystallized at 650°C and all the crystallized samples have a single phase of garnet. The FMR linewidth increased with the decreases in annealing temperature and annealing time. No FMR and EPR of Fe 3+ were observed in the amorphous films even though they contained large amounts of Fe 3+ ions, while the EPR of Fe 3+ was observed in slide glass with the same gain.