M. Munakata

B-concentration dependence on anisotropy field of CoFeB thin film for gigahertz frequency use

B was added to the high moment CoFe thin films (Co/sub 35/Fe/sub 65/, Co/sub 25/Fe/sub 75/, and Co/sub 15/Fe/sub 85/) using synchronous triple-RF magnetron sputtering to obtain the films with large anisotropy fields greater than 500 Oe. These films required inductor core materials operating at the frequencies of several gigahertz. Magnetic properties and the crystalline structure of the CoFeB films were investigated. It was found that the B addition increased the anisotropy field to 400-700 Oe and that the increase in anisotropy field was associated with changes in the crystalline structure, crystallite sizes, and crystallite orientation. The results indicated that CoFeB films have potential for use in magnetic cores and inductors operating at gigahertz frequencies.

The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner–Wohlfarth nanoparticles

The influence of interparticle interaction on the processes of magnetization reversal is considered for an ensemble of oriented Stoner–Wohlfarth nanoparticles. This is done through a solution of a kinetic equation describing the relaxation of the total magnetization to its equilibrium value in an effective mean field which includes a term proportional to the instantaneous value of the magnetization. It is shown that the interparticle interaction influences the temperature dependence of a coercive field. Under certain conditions, the presence of the interparticle interaction can lead to the formation of the so-called superferromagnetic state with the correlated directions of the magnetic moments of the particles. If the system is unable to come to the equilibrium during the time interval necessary to perform measurements, some measured quantities become dependent on the measurement time. It is shown that the blocking temperature Tb and the temperature dependence of coercive field at T

High-resistive (CoFeB)-(SiO/sub 2/) soft magnetic amorphous film for micro-cores in a few MHz range

Highly resistive (CoFeB)-(SiO/sub 2/) amorphous films were deposited on glass substrates and polyimide sheets by synchronous dual-rf magnetron sputtering. SiO/sub 2/ volume fraction dependence of magnetic properties and resistivity was investigated. The as-deposited films for SiO/sub 2/ volume fraction /spl upsi/=20 vol.% had low coercivities of H/sub c/=0.2-0.3 Oe, together with high resistivities of 1500-2200 /spl mu/ /spl Omega/ cm. The film exhibited an anisotropy field 40 Oe, a relative permeability of 200 and a saturation magnetization of 7.3 kG. The core loss of the films was 1-1.5 J/m/sup 3/ at 1 MHz for maximum flux density B/sub m/=0.1 T. The loss was found to be as low as those of Co-based ultra-thin ribbons. The films are useful as thin film cores in the micromagnetic elements which work in the MHz range.

Coercivity anomaly in the superferromagnetic state of an ensemble of nanoparticles with oriented anisotropy

An anomalous angular dependence of the coercive field of physically nonpercolated granular CoFeB–SiO2 films with oriented anisotropy of the granules, where the presence of the superferromagnetic phase has been established earlier, is observed experimentally at temperatures above the blocking temperature. The angular dependence of the coercive field is similar to that typically observed in the case of magnetization reversal through an inhomogeneous magnetic state of single granules in the ensemble. As the temperature becomes lower than the blocking temperature, the angular dependence changes to that typical of the magnetization reversal through a coherent rotation of granules’ magnetic moments. It is shown theoretically that the anomaly is determined by a singularity of the angular dependence of the critical field causing the absolute loss of the superferromagnetic phase stability in the ensemble of weakly interacting ferromagnetic nanoparticles.

Transport anisotropy in hetero-amorphous (CoFeB)–SiO2 thin films

We here present resisitivity and magnetoresistance measurements of two granular (CoFeB)–SiO2 amorphous thin films, with a different amount of metallic content. The films were deposited onto substrates sitting on a rotating cylinder. This induces anisotropy in the film plane, which is higher for the sample with less metallic content. This film exhibits typical isotropic giant magnetoresistance (GMR), while the film with higher-metallic content has less anisotropic resistivity, and shows a mixture of GMR and anisotropic magnetoresistance (AMR). The AMR appears at fields below 500 Oe. We believe that this has not been observed before in amorphous samples.

Intergranular interactions in nanogranular (CoFeB)x–(SiO2)1−x films with temperature and angular variations in coercivity

The temperature (4.5–550K) and angular (in the film plane) dependences of the coercivity field for physically nonpercolated nanogranular (CoFeB)x–(SiO2)1−x films with an oriented intraplane granule anisotropy have some unusual properties compared to those owing to blocking of thermally activated reorientation of the magnetic moments of the granules. At temperatures above the blocking temperature Tb≈350–370K, the dependence of the coercivity on the measurement duration is weak compared to that below Tb. Its angular (in the film plane) dependence differs qualitatively from that expected with blocking of superparamagnetic particles. This behavior is explained by the appearance of superferromagnetic ordering of the granule magnetic moments at T>Tb owing to interactions among them. As the temperature is reduced below 100K, the coercive field Hc for magnetization in the easy direction increases significantly more than expected for a blocked superparamagnetic ensemble. Here Hc≠0 for magnetization along the hard ...

Low-temperature onset of the spin glass correlations in the ensemble of oriented Stoner–Wohlfarth nanoparticles

We reveal the low-temperature anomaly in the temperature and angular dependencies of the coercivity in granular (CoFeB)x–(SiO2)1−x films with oriented in-plane anisotropy. Namely, at T<100 K the in-plane angular dependence of coercive field acquires two maxima corresponding to easy and hard (in a film plane) directions. This signifies the emergence of coercivity for hard direction in a film plane. The experimental results are explained in terms of a random field model, which describes the onset of spin glass-like correlations in the ensemble of oriented weakly interacting Stoner–Wohlfarth particles.

Ferromagnetic resonance, magnetic susceptibility, and transformation of domain structure in CoFeB film with growth induced anisotropy

Field dependence of magnetic susceptibility in nanocrystalline CoFeB film was studied in a wide frequency range from 500 kHz to 15 GHz. Anomalies of the susceptibility were detected exciting CoFeB ...

Effect of Interaction in the Magnetization Reversal Relaxation of Superparamagnetic Granular CoFeB -SiO2 Films

In order to study the magnetic interactions influence on the relaxation processes in superparamagnetic media, the magnetostatic measurements were carried out for the granular (CoFeB)x-(SiO2)1-x films with x values near percolation threshold. The observed temperature dependences of the coercivity for the samples magnetized along in-plane easy axis have shown two parts linear on with different inclination angles, which cross each other at the blocking temperature (Tb). The first part of the curve, which corresponds to the temperature range below Tb, depends on the measuring time and is related to the thermal activation nature of magnetization reversal process in the blocked superparamagnetic state. Second part of the curve, which corresponds to the temperature range above Tb, is practically independent on the measuring time and is related to existence of correlated state of superparamagnetic grains moments, i.e. to superferromagnetic state. The method of the magnetization reversal curves simulation based on the numerical solving of kinetic equation has been applied to explain the experimental results.

Preparation and Magnetic Properties of Sm-Co Films with High Substrate Temperatures

Thin magnetic films with an approximate composition of Sm2Co7.8 were prepared on quartz substrates at high temperatures from 150° to 750°C by rf sputtering method. Film structures and magnetic properties have been studied. With the increase of the substrate temperature, the coercive force increases and then decreases through a well-reproducible maximum value of 11 kOe at 400°±50°C. It is shown by X-ray analyses that fine crystallites of SmCo5, Sm2Co7 and Sm2Co17 phases coexist in films with high coercive forces.