In order to interpret recent experimental observations of superconducting vortices interacting with tilted columnar defects in high-temperature superconducting materials, we have extended to the case of anisotropic materials our Fourier space approach for the calculation of the electron optical phase shift experienced by the high-energy electrons in a transmission electron microscope. The case of a London vortex having its core not perpendicular to the specimen surfaces is considered. The same configuration is also analyzed in the framework of a simplified pancake model and the influence of the number of stacks on the phase shift and images is investigated. The results obtained by the two models are compared between them and with the experimental results. The agreement between theory and experiment confirms that anisotropy plays a major role in affecting the electron microscopy images.
Two types of Fresnel contrasts of superconducting vortices in a Lorentz micrograph, corresponding to pinned and unpinned vortices, were obtained by a newly developed 1 MV field-emission transmission electron microscope on a Bi 2 Sr 2 CaCu 2 O 8þ (Bi-2212) thin specimen containing tilted linear columnar defects introduced by heavy ion irradiation. The main features of the Fresnel contrasts could be consistently interpreted by assuming that the vortices are pinned along the tilted columnar defects and by using a layered or an anisotropic model to calculate the phase shift of the electron wave. The confirmed validity of both models strongly indicates that superconducting vortices in high-critical temperature (high-T c ) layered materials have an anisotropic structure.
We studied what effect exchange bias had on magnetic noise under high-temperature or low-aspect-ratio conditions. We found that a steep increase in magnetic noise started at around 150 ˚C with a low exchange bias (Jk ~ 0.4 erg/cm 2), and attributed this to the decrease in exchange bias. We measured the dependence of magnetic noise with a high exchange bias (Jk ~ 1.0 erg/cm 2) on temperature and found that magnetic noise was reduced under high-temperature conditions (~ 250 ˚C). We also found that magnetic noise could be diminished at room temperature even for a TMR head with a low aspect ratio (~ 0.5). This indicates that the pinned layer instability increased by reducing the head size was improved with a high exchange bias.
We describe a thermal magnetic noise simulation of tunneling magnetoresistive (TMR) heads. We calculate the phase and amplitude distributions of the local noise resistance derived from the magnetization fluctuation of the free layer in the several gigahertz range and consider their relations with the magnetic noise spectrum. The results show that the phase distribution of the noise resistance strongly influences the second or higher resonant peaks in the magnetic noise spectrum of TMR heads. As an example, we show that the second resonant peak appears in the spectrum when the asymmetric magnetic field is applied to the free layer.Index Terms-Magnetic noise, tunneling magnetoresistive (TMR) head.
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