Scanning Hall probe microscopy has been used to make a microscopic study of flux structures and dynamics in yttrium barium copper oxide thin-film disks containing a regular 10-m-period square array of 2.5-m-sized holes ͑antidots͒. Images obtained after field cooling the sample to 77 K in very low fields reveal that the holes can trap two flux quanta at this temperature. Scans obtained after zero-field cooling ͑ZFC͒ to 77 K and a subsequent applied field cycle clearly display preferential flux channeling along chains of antidots in the direction of maximum induction gradient. Remarkably, upon reversal of field sweep direction, we observe flux "streaming" out of the holes towards the sample edges with almost uniform density flux "stripes" bridging the holes in the exit direction. We estimate that the antidots can preferentially trap about 15 flux quanta in these ZFC experiments. Classical electrodynamics simulations of our samples appear to be in good qualitative agreement with our results, indicating that many of the observed phenomena may be geometrical effects that depend primarily on the shape and topology of the sample, and potential applications are discussed.
Scanning Hall probe microscopy is a noninvasive magnetic imaging technique with potential for having a major impact in the data storage industry if high-resolution Hall effect sensors can be developed with sufficiently low-noise figures at room temperature. To meet this requirement, we have developed a series of second-generation quantum-well Hall probes whereby the careful design of an AlGaAs∕InGaAs∕GaAs pseudomorphic heterostructure, chip layout, metal interconnects, and passivation layers has allowed a dramatic reduction of low-frequency noise sources. In addition, the Johnson noise-limited minimum detectable fields of these sensors are more than an order of magnitude lower than those used in early microscopes. The key figures-of-merit of the sensors are presented and their performance illustrated in an imaging study of a yttrium–iron–garnet thin film at room temperature.
We have imaged the micromagnetic structure of 50 nm Ga0.91Mn0.09As films after different low-temperature anneals. Samples annealed in vacuum for 10 min display a very random domain structure with small (∼3–5 μm) domains. In contrast a sample which was further annealed in air for 50 h exhibited the highest Curie temperature and very large (∼100 μm) domains. Even within large domains we resolve magnetic disorder which has not been removed by the annealing procedure. Micron-sized regions near domain walls remain ferromagnetic well above TC in all the films, possibly indicating the presence of regions with above average Mn density or very small MnAs precipitates, which act as pinning centers and strongly influence the coercive fields of the films.
Scanning Hall probe microscopy has been used to study the magnetic domain structure of GaMnAs thin films after various low temperature anneals to increase the Curie temperature (TC). Samples with in-plane magnetization, which received short low temperature anneals in vacuum directly after growth, exhibit very small (∼2–5 μm) rather random domains. In stark contrast similar samples, which additionally received very long low temperature anneals in air, display large (10–100 μm) domains, which still contain clearly resolvable magnetic disorder. Preliminary scans of air-annealed samples with out-of-plane magnetization also reveal a very irregular, rather fine (1–3 μm) domain structure. In all samples micron-sized regions at domain walls frequently remain ferromagnetic well above the average TC, indicating either the presence of ferromagnetic precipitates (e.g., MnAs) or material there with higher than average Mn concentration.
We have used scanning Hall probe microscopy to image domain structures and magnetization reversal in optimally annealed Ga0.91Mn0.09As films grown on (311)B GaAs substrates. Unmagnetized films exhibit a disordered mazelike domain structure consistent with a composite state of regions with magnetization along [0,1,0] and [0,0,1] out-of-plane easy axes. The characteristic stripe width of ∼3μm exhibits almost no temperature dependence in the range of 5–90K, consistent with recent theoretical predictions, while the peak domain fields drop almost linearly with increasing temperature. With an applied field perpendicular to the zero-field-cooled film magnetization proceeds by the motion of rather ordered stripe-shaped domains which form preferentially along one of the [0,1,0]∕[0,0,1] easy axes. Surprisingly, stripelike domains are not clearly observed during reversal from the magnetized state, which appears to involve the propagation of magnetic “bubbles.” Weak image contrast in the magnetized state points to the existence of residual magnetic disorder in the films on an ∼2–3μm length scale. Abrupt breaks within single images indicate the occurrence of large Barkhausen events when domain walls suddenly jump over ∼1μm distances. This implies the existence of strong pinning sites on this length scale and this, as well as the residual magnetic disorder, may be related to microscopic Mn-rich regions formed during sample growth.
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