Positive, linear in field, and isotropic magnetoresistance in fields up to 60 T is found in geometrically constrained ferromagnets, such as thin films of iron, nickel, and cobalt and their granular mixtures with nonmagnetic materials. The resistivity measured as a function of temperature shows a minimum at temperatures reaching a remarkably high 92 K, followed by logarithmic dependence at low temperatures. We propose to explain both phenomena by a modified version of the quantum electron-electron interaction theory. The agreement is only qualitative while the observed magnitude of the magnetoresistance slope is much larger than the calculated one.

We report on the temperature-and field-driven metal-insulator transition in disordered Ge:Mn magnetic semiconductors accompanied by magnetic ordering, magnetoresistance reaching thousands of percents, and suppression of the extraordinary Hall effect by a magnetic field. Magnetoresistance isotherms are shown to obey a universal scaling law with a single scaling parameter depending on temperature and fabrication. We argue that the strong magnetic disorder leads to localization of charge carriers and is the origin of the unusual properties of Ge:Mn alloys.Interest in magnetic semiconductors was triggered by the development of spintronics in metallic magnetic materials and prospects to empower the semiconducting electronics by the spin-dependent degree of freedom. Compatibility with the existing silicon technology recently attracted much attention to the group-IV semiconductors doped with magnetic impurities. The novel materials appeared remarkably interesting and revealed a number of unusual properties not yet well understood. Huge positive magnetoresistance ͑MR͒ of hundreds to thousands of percent, 1-4 reversal of magnetoresistance from positive to negative after annealing, 4 and large Hall effect with nonmonotonic field dependence 5-7 are among them. Interpretation of the data is difficult owing to the complicated structure of these alloys. As a result, almost no general relations were derived from the experiments, and only special models were proposed to explain the data in each case.In this Rapid Communication we report on the high-field magnetic and magnetotransport properties of several Ge:Mn samples produced by ion implantation. The material was found to pass from metallic-like to insulator-like state, either at low temperatures in zero field or under an applied field at elevated temperatures. We show that all the magnetoresistance data can be scaled on a universal curve with a single parameter H s , which tends to zero at the paramagnet-toferromagnet transition. The same scaling procedure is applicable to the published data 4 obtained in samples produced by the molecular-beam epitaxy, which indicates the generality of the scaling approach. We argue that establishment of an inhomogeneous magnetization landscape leads to localization of the charge carriers and, respectively, to a huge positive magnetoresistance and a total suppression of the extraordinary Hall effect.Two Ge:Mn samples discussed here were fabricated by implanting Mn + ions into commercial single-crystalline Ge͑100͒ wafers with resistivity of 40-57 ⍀ cm. Mn + ions were implanted with an energy of 100 keV at fluences of 1 ϫ 10 16 and 2 ϫ 10 16 that produce average volume concentrations of Mn of about 2 and 4 at. % in the projected depth range of about 120 nm. During the implantation, the samples were held at 300°C to avoid amorphization. Structure of the samples is strongly nonuniform, depending on the concentration, and contains diluted Mn, amorphous semiconducting Mn-rich nanoclusters, and ferromagnetic metallic Mn 5 Ge 3 clusters. Detailed str...

Extraordinary Hall effect (EHE) is a spin-dependent phenomenon that generates voltageproportional to magnetization across a current carrying magnetic film. Magnitude of the effect can be artificially increased by stimulating properly selected spin-orbit scattering events. Already achieved sensitivity of the EHE-based sample devices exceeds 1000 Ω/T, which surpasses the sensitivity of semiconducting Hall sensors. Linear field response, thermal stability, high frequency operation, sub-micron dimensions and, above all, simplicity, robustness and low cost manufacture are good reasons to consider a wide scale technological application of the phenomenon for magnetic sensors and memory devices.2 Anisotropic magnetoresistance [1,2], planar Hall effect [3,4], spin-dependent tunneling [5,6] and the extraordinary or anomalous Hall effect (EHE) [7,8] are spin-dependent electronic transport phenomena known for many years. However, it is the discovery of the giant magnetoresistance (GMR) [9,10] that gave birth to the term spintronics and triggered a world-wide outburst of the spin-related research. Extraordinary Hall Effect (EHE) in magnetic materials was discovered more than a century ago [7], extensively studied both theoretically and experimentally [8], and left out of the mainstream research for the last thirty years. The possibility to use the effect for technical applications, such as magnetic sensors and nonvolatile magnetic random access memories (MRAM), has been mentioned more than three decades ago [11], but no significant progress was reported until recently. A probable reason for this is that although EHE in bulk magnetic materials can be significantly higher than the ordinary Hall effect in normal metals, its magnitude remained far beyond the sensitivity of semiconductors and magnetic sensors based on the anisotropic magnetoresistance [12]. The renewed interest in EHE has only recently arisen when some recipes to enhance the effect were found [13][14][15].The Hall effect in magnetic materials is commonly described [8,16] by the phenomenological equationwhere ρ H is the Hall resistivity, B, H and M are components of the magnetic induction, applied field and magnetization normal to the film plane, and D is the demagnetization factor. R 0 is the ordinary Hall coefficient related to the Lorentz force acting on moving charge carriers. R EHE , the extraordinary Hall coefficient, is associated with a break of the right-left symmetry at spin-orbit scattering in magnetic materials.. Demagnetization factor D is equal to 1 when field is applied perpendicular to a homogeneous magnetic film. In this case Eq.1 is simplified toVoltage measured between Hall contacts located perpendicular to the direction of an electric current is given by:where I is current and t thickness of the film. Fig.1 presents a typical field dependence of the Hall resistance (in a 4 nm thick Ni film at room temperature with field applied normal to the film plane.Magnetization normal to the film increases with field till saturation at about ±0.2 T. The EH...

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Metal-clad MgB2 tapes with Cu, Ni, Fe, and stainless steel sheaths, fabricated by the powder-in-tube method, have been studied using x-ray diffraction and magnetoresistance measurements. Tapes subjected to different mechanical and thermal processings have been used to probe the ab-plane texturing. Only moderate rolling-induced texturing has been observed experimentally, with a maximal texture factor, ΔF00l, of about 0.22. ΔF00l is found to be dependent on both the sheath material and tape processing prehistory. Electrical resistivity measurements in high magnetic fields (parallel and perpendicular to the tape plane) show that even poor texturing, with ΔF00l=0.065, may result in a significant anisotropy of magnetoresistance. The anisotropy of the upper critical field, Bc2, has been derived from the experimental texturing and magnetoresistance data, with the anisotropy factor of the order 5 at 4.2K. It is shown that a maximal magnetic field shift of the resistively probed superconducting transition associated with the tape core texturing may reach 4.5T at 4.2K.