Yingjian Chen

Influence of Ba content on grain size and dynamics of crystallization in barium ferrite thin films

The effects of Ba content on the dynamics of the crystallization process, which ultimately determines the grain size, were studied in barium ferrite thin films. Rapid thermal annealing was used to crystallize the amorphous as-deposited barium ferrite films. The annealing time and temperature dependent crystalline volumes were determined from the magnetization of the films, and were analyzed by means of the Johnson–Mehl–Avrami relation. Microstructural studies showed that the films with higher Ba content had higher nucleation rates and lower growth rates; consequently, they had finer grain size and smaller grain aspect ratio. Magnetic measurements showed that the higher Ba content films also had higher coercivities and smaller magnetic switching volumes.

Temperature dependent magnetic properties of barium-ferrite thin-film recording media

Temperature dependencies of magnetic properties were measured in barium-ferrite thin films with grains having in-plane aligned as well as randomly oriented easy axes. The temperature dependence of H/sub c/ was shown to be similar to that of the effective anisotropy field, which is determined by the crystalline and shape anisotropies. The measured H/sub c/ values in the easy-axis aligned and randomly oriented films are significantly smaller than those predicted by the Stoner-Wohlfarth (SW) coherent rotation model. Such discrepancies are believed to be caused by incoherent rotation. Deviations from the SW model were also found within a wide temperature range in the angular dependence of coercivity and remanent coercivity, the field dependence of torque and rotational hysteresis. These led to the conclusion that temperature has only weak affects on the rotation mode of the magnetization. The discrepancies between the thermal activation volume and the grain size which were found previously at room temperature, and were believed to be yet another signature of incoherent rotation, were shown to also exist at a low temperature of 173 K.

CrPt3 thin film media for perpendicular or magneto-optical recording

The magnetic properties of CrPt3 L12 ferrimagnetic thin films have been studied. Films were produced by sputtering multilayers of Cr and Pt onto silicon nitride coated silicon substrates. The as-deposited films are nonmagnetic. An anneal at ∼800u200a°C results in ferrimagnetic behavior with a perpendicular easy-axis. X-ray diffraction and transmission electron microscopy (TEM) measurements show that (111) CrPt3 is the only crystalline phase present after annealing. Rocking curves with a full width at half maximum as low as 1.8° indicate good crystallographic orientation. Magnetic properties of the films vary with composition, annealing temperature and time, layer thickness, and sputtering conditions. The films exhibit large coercivities, Hc, that can be tuned in the range 1500–8000 Oe. Saturation magnetization, Ms, is typically 150–200 emu/cc. Squarenesses, S, as high as 0.99 have been found. A uniaxial magnetic anisotropy constant, Ku, of up to 8×106erg/cc was achieved. TEM micrographs show a 35 nm average g...

Thermal activation and switching in c-axis aligned barium ferrite thin film media

The thermal activation volume and the magnetic switching volume were measured in c-axis uniaxially in-plane oriented barium hexaferrite thin film magnetic recording media. Excellent agreement among the experimental data obtained independently from the magnetic viscosity measurements and the measurements of the time dependence of coercivity implies that the thermally driven process is the cause of both phenomena. The presence of incoherent switching during magnetization reversal is indicated by the in-plane angular dependence of coercivity and is believed to cause the much smaller activation volume and switching volume than the grain size as determined from a TEM micrograph.

Effects of Cr2O3 or Y2O3 doping in barium ferrite thin‐film medium for high‐density recording

The effects of Cr2O3 or Y2O3 doping on the surface morphology and magnetic properties in barium ferrite thin films have been studied. Thin films were deposited by co‐sputtering a barium ferrite target with a Cr2O3 or Y2O3 target. The deposited amorphous films were homogenized at a temperature of 600u2009°C for over 10 h followed by annealing at 800u2009°C in a tube furnace. It was observed that, whereas stoichiometric barium ferrite shows 5000 A×500 A acicular grainlike features, films doped with Cr2O3 or Y2O3 had equiaxed topographic features with an average size of 500 A. Magnetic studies showed that the coercivity of the doped films was over 3500 Oe and was larger than that of stoichiometric films. A weak magnetostatic interaction was found in all the films. Studies of the magnetic after effect indicated excellent thermal stability in all the films.

Microstructural origin of soft magnetic properties of sendust films prepared by N2 reactive sputtering

Sendust films were deposited using dc magnetron sputtering at room temperature in a mixture of Ar and N2 gas. The soft magnetic properties of the sendust films were, in general, improved with the addition of N2. The optimum soft magnetic properties were achieved when 6% N2 gas was used. Transmission electron microscopy studies showed that the sendust film without N2 contains average grain sizes of 200 nm; whereas the films with increasing N2 content contain increasing percentages of much smaller grains of 20 nm. The smaller grains have a random crystalline orientation with respect to each other. The cross-sectional morphological structures were studied using a scanning electron microscope. In the film without N2, power-law cone structures were observed. In the N2 containing films, needlelike morphological structures were formed. X-ray photoelectron spectroscopy studies on these films indicate that N2 included in the films reacts preferentially with Al to form AlN. Excess N2 may then react with Si, possibl...

Incoherent Rotation In Barium Ferrite Thin Film Recording Media

The angular dependence of the remanent magnetic properties of uniaxially in-plane oriented barium ferrite films were found to deviate significantly from the Stoner-Wohlfarth (SW) coherent rotation model. The in-plane torque and the rotational hysteresis were studied in both aligned and randomly oriented films. They were found to be different from that calculated from the SW coherent rotation model, and suggested incoherent rotation of magnetization during the switching process. Incoherent rotation was found to be dependent on grain size and intergranular interactions.

Observation of magnetic domain structure and switching in barium ferrite thin films

Magnetic domain structure and switching mechanism were investigated in individual barium ferrite grains with sizes ranging from 0.5-7 /spl mu/m. Magnetic force microscopy (MFM) was used to image grains in sputtered Pt-doped barium ferrite thin films. The switching thresholds of the different grains were measured by applying successively larger magnetic fields to the sample and observing which grains switched. Application of a field to as-annealed samples swept out multidomain structures at a field of H/sub ini//spl ap/1000 Oe, which was relatively independent of grain size. It was found that, after the grains had been saturated, a different field H/sub rev/ had to be applied to saturate the grain in the opposite direction in large size grains (about 4 /spl mu/m) H/sub ini//spl ap/H/sub rev/; whereas for grains with sizes of 1-3 /spl mu/m, H/sub rev/ varies from approximately equal to H/sub ini/ to many times larger. Domain wall motion was found to dominate the reversal process in grains larger than 4 /spl mu/m, as suggested by discontinuous magnetization jumps and an inverse-cosine angular dependence of H/sub rev/. Reversal in grains with sizes between 0.5 and 1 /spl mu/m is believed to be dominated by incoherent rotation, as indicated by the angular dependence of H/sub rev/.

Effects of grain isolation on magnetic properties of c-axis uniaxially in-plane oriented barium hexaferrite thin films

Magnetic properties of c-axis uniaxially in-plane aligned Ba-ferrite thin films were studied. Ba-ferrite thin films on [110] a-plane single-crystal sapphire substrates were deposited by low-power RF diode sputtering. Different degrees of grain isolation were achieved via postdeposition annealing at various temperatures in a rapid thermal annealing furnace. In this study, a magnetic evaluation of intergranular interaction was carried out using the well-established remanent magnetization /spl Delta/M curve. The results presented indicate a positive magnetostatic interaction, the strength of which is correlated with grain separation when measured along the magnetic easy axis direction. A predominant negative magnetostatic interaction was observed when measured along the magnetic hard axis direction, the magnitude of which is relatively independent of further grain separation.

Barium ferrite thin films without a dead layer

A barium ferrite layer with low-Ba content was used as an underlayer for a high-Ba content barium ferrite thin film. During the post-deposition annealing, the underlayer which has a lower nucleation rate than the top layer prevented grains from nucleating in the interdiffused region at the interface with the substrate. As a result, the magnetic properties were dramatically improved as indicated by much higher coercivity squareness S* and narrower switching field distribution.