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.

We report on muon spin rotation (µSR) measurements of the internal magnetic field distribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single crystals, from which we have simultaneously determined the hole doping dependences of the in-plane Ginzburg-Landau (GL) length scales in the underdoped regime. We find that Tc has a sublinear dependence on 1/λ 2 ab , where λ ab is the in-plane magnetic penetration depth in the extrapolated limits T → 0 and H → 0. The power coefficient of the sublinear dependence is close to that determined in severely underdoped YBCO thin films, indicating that the same relationship between Tc and the superfluid density is maintained throughout the underdoped regime. The GL coherence length ξ ab (vortex core size) is found to increase with decreasing hole doping concentration, and exhibit a field dependence that is explained by proximity-induced superconductivity on the CuO chains. Both λ ab and ξ ab are enhanced near 1/8 hole doping, supporting the belief by some that stripe correlations are a universal property of high-Tc cuprates.

Muon spin-rotation experiments ͑supported by magnetization measurements͒ have been carried out in the canonical 4f mixed-valence narrow-gap semiconductor SmS from 10 to 900 K in magnetic fields up to 3.5 T. A bound state of an electron around a positive muon is found to form up to about 800 K. This state is a magnetic polaron: the electron wave function is confined within R Ϸ 0.5 nm ͑the first two coordination spheres͒ due to its exchange interaction with Sm magnetic moments. As such, it may serve as a model system for the hypothetical bound state suggested to account for a transition from divalent Sm 2+ to trivalent Sm 3+ , which is invoked to explain the transformation of SmS from a paramagnetic insulator into a magnetic metal at high pressure.The problem of spin and charge fluctuations close to a magnetic instability in mixed-valence ͑MV͒ systems has attracted considerable attention ͑see, e.g., Ref. 1 and references therein͒. In strongly correlated 4f electron systems, the class of MV materials known as narrow-gap semiconductors or Kondo insulators ͑SmB 6 , YbB 12 , TmSe, etc.͒ supports hybridization between localized 4f states and itinerant 5d-6s states causing instabilities in charge and magnetic configurations. These materials have been studied for almost four decades 2 but the mechanism of the valence fluctuation remains mysterious. Among them, the canonical MV system SmS makes an ideal material in which to study charge fluctuations in the vicinity of a quantum critical point because of the relative ease with which the Sm ion changes its ion charge state. Although a consensus has been reached that the remarkable properties of SmS may be understood in terms of a Sm 2+ ͑4f 6 ; J =0͒ → Sm 3+ ͑4f 5 ; J =5/ 2͒ + e͑d , s͒ transition, 3 the mechanism of the electron capture/release remains a subject of current interest.Unlike classical magnetic semiconductors ͑Eu chalcogenides, magnetic spinels, etc.͒ which experience metalinsulator transitions ͑MIT͒ close to the magnetic ordering temperature 4,5 at ambient pressure SmS remains a paramagnetic semiconductor within a NaCl crystal structure down to low temperature. 2 As the pressure is increased, SmS exhibits two successive phase transitions. First it undergoes an isostructural transition at the remarkably low pressure of p BG Ϸ 0.65 GPa ͑at room temperature͒, 6 involving a valence change from a Sm 2+ to a homogeneous mixed-valent ͑2.6-2.8͒ state. 2 This first-order phase transition is characterized by a huge volume collapse of up to 15% accompanied by a color change from black to golden. 7 In the black phase, the Fermi level E F falls into a gap between a 4f 6 level and an unoccupied 5d band, thus making Sm 2+ S 2− a nonmagnetic narrow-gap semiconductor. 8 In the golden phase, the 4f 6 level may be pushed into the conduction band, 8,9 resulting in a mix of divalent 4f 6 and trivalent 4f 5 configurations. In this phase the high-temperature resistivity is metallic but at low temperature the ground state is a strongly correlated semiconductor as long as the pressure i...

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...