The penetration of magnetic flux into a thin superconducting film of Nb3Sn with critical temperature 17.8K and critical current density 6MA/cm^2 was visualized using magneto-optical imaging. Below 8 K an avalanche-like flux penetration in form of big and branching dendritic structures was observed in response to increasing perpendicular applied field. When a growing dendritic branch meets a linear defect in the film, several scenarios were observed: the branch can turn and propagate along the defect, continue propagation right through it, or "tunnel" along a flux-filled defect and continue growth from its other end. The avalanches manifest themselves in numerous small and random jumps found in the magnetization curve.Comment: 3 pages, 4 figures, submitted to Cryogenics. Revision: M(H) data adde

Flux distributions in thin superconducting NbN films placed in a perpendicular magnetic field have been studied using magneto-optical imaging. Below 5.5 K the flux penetrates in the form of abrupt avalanches resulting in dendritic structures. Magnetization curves in this regime exhibit extremely noisy behavior. Stability is restored both above a threshold temperature T* and applied field H*, where H* is smaller for increasing field than during descent. The dendrite size and morphology are strongly T dependent, and fractal analysis of the first dendrites entering into a virgin film shows that dendrites formed at higher T have larger fractal dimension.Comment: 3 pages, 5 figure

Materials in which charge and spin degrees of freedom interact strongly offer applications known as spintronics. Following a remarkable success of metallic spintronics based on the giant-magnetoresistive effect, tremendous efforts have been invested into the less developed semiconductor spintronics, in particular, with the aim to produce three-terminal spintronic devices, e.g. spin transistors. One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers from a ferromagnetic semiconductor into a nonmagnetic semiconductor, preferably one of those currently used for industrial applications such as Si -a workhorse of modern electronics. Ferromagnetic semiconductor EuO is long believed to be the best candidate for integration of magnetic semiconductor with Si. Although EuO proved to offer optimal conditions for effective spin injection into silicon and in spite of considerable efforts, the direct epitaxial stabilization of stoichiometric EuO thin films on Si without any buffer layer has not been demonstrated to date. Here we report a new technique for control of EuO/Si interface on submonolayer level which may have general implications for the growth of functional oxides on Si. Using this technique we solve a long-standing problem of direct epitaxial growth on silicon of thin EuO films which exhibit structural and magnetic properties of EuO bulk material. This result opens up new possibilities in developing all-semiconductor spintronic devices.Modern information technology is based on the fundamental dichotomy: it utilizes charge of electrons to process information in semiconductors and their spin to store information in magnetic materials. Strong correlation of spin and charge degrees of freedom in the same material makes it possible to manipulate magnetically stored information with electric fields and/or modify fast logic gates by changing the magnetisation of their components. In metal multilayers, such effects are manifest in giant magnetoresistance, where the orientations of the macroscopic magnetisation in adjacent layers determine the electrical resistance of the structure [1,2]. Metallic spintronic devices, such as hard disk read heads and magnetic random access memory are among the most successful technologies of the past decades. However, metals cannot enhance signals -the prerequisite for transistor technology readily offered by semiconductors.The development of semiconductor spintronics requires the ability to inject, modulate and detect spin-polarized carriers in a single device, preferably made of technologically important materials currently used in integrated circuits such as Si or GaAs [3,4]. Thus far, the spin of the carriers has played a minor role in semiconductor devices mainly because Si and GaAs are nonmagnetic. On the other hand, the enhanced spin-related phenomena realized in diluted magnetic semiconductors (DMS) (especially 2 GaMnAs films [5]) open the way for applications in spintronics [6]. The interplay between electrical and magneti...

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We report on magnetic properties and X-band electron spin resonance (ESR) study (80–430 K) of diluted magnetic semiconductors Ge1−xMnxTe (x=0.07–0.44) revealing ferromagnetism with the Curie temperature achieving 90 K. Effective magnetic moment peff per Mn2+ ion estimated from the Curie constant in the range 0.15≤x≤0.26 increases from 2.27 to 2.95 μB. The temperature evolution of ESR spectra was found to have a complicated character. We observed the presence of two distinct absorption lines of Dysonian type, associated with two resonance centers in these diluted magnetic semiconductors. Possible origin of these two lines has been analyzed and clustering effect was proposed to be most likely responsible for anomalous behavior. Anomalies in temperature dependencies of the linewidths and g-factors were observed at temperatures lower than 160 K. We connect these anomalies with magnetic fluctuations in the vicinity of transition to ferromagnetic phases. The anomalous broadening of the linewidth in paramagnetic region occurs with decreasing of the temperature similarly to ESR behavior of related A M2nB6 diluted magnetic semiconductors. This broadening is satisfactorily described in the framework of modified Huber’s theory. At the same time, the g-factor is essentially independent on both temperature and alloy composition. Average values of effective g-factor for two resolved spectral components at room temperature were found to be g1=2.02–2.07 and g2≈1.99.