Semiconductor electronics has so far been based on the transport of charge carriers while storage of information has mainly relied upon the collective interactions of spins. A new discipline known as spintronics proposes to exploit the strong mutual influence of magnetic and electrical properties in magnetic semiconductors, which promise new types of devices and computer technologies. The mechanism for such phenomena involves the concept of magnetic polarons-microscopic clouds of magnetization composed of charge carriers and neighboring magnetic ions-which determine most of the electrical, magnetic, and optical properties of the material. In spite of the importance of this quasiparticle, its observation remains a formidable challenge. Here we report that, using the positive muon as both a donor center and a local magnetic probe, we have been able to generate and detect the magnetic polaron and determine its size and magnetic moment in the magnetic semiconductor EuS.

Muon spin rotation spectroscopy reveals localized electron states in the geometrically frustrated metallic pyrochlore Cd2Re2O7 at temperatures from 2 to 300 K in transverse magnetic fields up to 7 T. Two distinctive types of localized states, with characteristic radii of about 0.5 and 0.15 nm, are detected at high and low temperature, respectively. These states may be spin polarons, formed due to strong exchange interaction between itinerant electrons and the magnetic 5d electrons of Re ions, which may determine the peculiar electronic and magnetic properties of Cd2Re2O7.

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We have determined locations for the donor and acceptor levels of muonium in six semiconductor materials (Si, Ge, GaAs, GaP, ZnSe, and 6H-SiC) as a test of defect-level pinning for hydrogen. Within theoretical band alignments, our results indicate a common energy for the equilibrium charge-transition level Mu(+/-) to within experimental uncertainties. However, this is nearly 0.5 eV higher than the energy at which the equivalent level for hydrogen was predicted to be pinned. Corrections for zero-point energy account for only about 10% of this difference. We also report experimental results for the (negative-U) difference between donor and acceptor levels for Mu to be compared with calculated values for H impurities in the same materials.

Muon spin rotation/relaxation spectroscopy has been employed to study electron localization around a donor center -the positive muon -in the 3d magnetic spinel semiconductor CdCr2Se4 at temperatures from 2 to 300 K in magnetic fields up to 7 T. A bound state of an electron around a positive muon -a magnetic polaron -is detected far above the ferromagnetic transition up to 300 K. Electron localization into a magnetic polaron occurs due to its strong exchange interaction with the magnetic 3d electrons of local Cr 3+ ions, which confines its wave function within R ≈ 0.3 nm, allowing significant overlap with both the nearest and next nearest shells of Cr ions.PACS numbers: 75.50. Pp, 72.25.Dc, 72.20.Jv, 76.75.+i The semiconductors currently in use as working media in electronics and information technology (Si, Ge, GaAs etc.) are nonmagnetic; therefore the spin of the carriers has so far played a minor role in semiconductor devices. One way to enhance spin-related phenomena for semiconductor spintronics applications [1,2] is to incorporate magnetic ions (typically Mn) into nonmagnetic semiconductors to realize dilute magnetic semiconductors (DMS) [3,4]. The interplay between electric and magnetic properties in ferromagnetic (FM) Mn-doped III-V DMS has recently been demonstrated [5][6][7]. Combined with nonmagnetic semiconductors, these DMS may also serve as polarized spin injectors in spintronics devices [8,9]. The FM in these p-type materials results from a long-range coupling between the Mn atoms mediated by holes generated by Mn substitution at the trivalent cation site [10].Unfortunately, the ferromagnetism in III-V Mn-doped DMS is limited by low concentration of magnetic ions. Molecular beam epitaxy results in a non-equilibrium enhancement of the otherwise low solubility of transition metals in III-V hosts, but still allows incorporation of no more than about 7-8% of Mn atoms; above this critical concentration Mn tends to cluster and phase separate [11]. Even at lower concentrations, spatial homogeneity may be affected by adding Mn [10] and nanoscale-range magnetic inhomogeneities can occur [12].By contrast, intrinsic magnetic semiconductors (MS) such as the 4f Eu chalcogenides or 3d Cr spinels exhibit spontaneous homogeneous ferromagnetic order without any doping. When doped, these MS show semiconducting behavior (both n-and p-type), which indicates strong mutual influence between electrical and magnetic properties [13,14]. Successful demonstration of the epitaxial growth of EuO and CdCr 2 Se 4 on technologically important semiconductors Si, GaN, GaAs and GaP [15][16][17] makes them very attractive working media for spintronics applications. These materials offer several important advantages over DMS, such as higher magnetization, spatial homogeneity and wider ranges of conductivity tunable by doping. The longer spin lifetimes and spin-scattering lengths of electrons in Si, , as well as much higher electron mobilities compared with those of holes, makes the ability to support n-type conductivity in MS especi...