Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature (T(C)) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1-)(x)Mn(x)Te and is used to predict materials with T(C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.

(Accepted by PRB Rap. com.)We have calculated the spin-polarization effects of a current in a two dimensional electron gas which is contacted by two ferromagnetic metals. In the purely diffusive regime, the current may indeed be spin-polarized. However, for a typical device geometry the degree of spin-polarization of the current is limited to less than 0.1%, only. The change in device resistance for parallel and antiparallel magnetization of the contacts is up to quadratically smaller, and will thus be difficult to detect.Spin-polarized electron injection into semiconductors has been a field of growing interest during the last years [1][2][3][4]. The injection and detection of a spin-polarized current in a semiconducting material could combine magnetic storage of information with electronic readout in a single semiconductor device, yielding many obvious advantages. However, up to now experiments for spininjection from ferromagnetic metals into semiconductors have only shown effects of less than 1% [5,6], which sometimes are difficult to separate from stray-field-induced Hall-or magnetoresistance-effects [2]. In contrast, spininjection from magnetic semiconductors has already been demonstrated successfully [7,8] using an optical detection method.Typically, the experiments on spin-injection from a ferromagnetic contact are performed using a device with a simple injector-detector geometry, where a ferromagnetic metal contact is used to inject spin polarized carriers into a two dimensional electron gas (2DEG) [5]. A spin-polarization of the current is expected from the different conductivities resulting from the different densities of states) for spin-up and spin-down electrons in the ferromagnet. For the full device, this should result in a conductance which depends on the relative magnetization of the two contacts [1].A simple linear-response model for transport across a ferromagnetic/normal metal interface, which nonetheless incorporates the detailed behaviour of the electrochemical potentials for both spin directions was first introduced by van Son et al. [9]. Based on a more detailed (Boltzmann) approach, the model was developed further by Valet and Fert for all metal multilayers and GMR [10]. Furthermore, it was applied by Jedema et al. to superconductor-ferromagnet junctions [11]. For the interface between a ferromagnetic and a normal metal, van Son et al. obtain a splitting of the electrochemical potentials for spinup and spindown electrons in the region of the interface. The model was applied only to a single contact and its boundary resistance [9]. We now apply a similar model to a system in which the material properties differ considerably.Our theory is based on the assumption that spinscattering occurs on a much slower timescale than other electron scattering events [12]. Under this assumption, two electrochemical potentials µ ↑ and µ ↓ , which need not be equal, can be defined for both spin directions at any point in the device [9]. If the current flow is one dimensional in the x-direction, the electroc...

Recent works aiming at understanding magnetotransport phenomena in ferromagnetic III-V and II-VI semiconductors are described. Theory of the anomalous Hall effect in p-type magnetic semiconductors is discussed, and the relative role of side-jump and skew-scattering mechanisms assessed for (Ga,Mn)As and (Zn,Mn)Te. It is emphasized that magnetotransport studies of ferromagnetic semiconductors in high magnetic fields make it possible to separate the contributions of the ordinary and anomalous Hall effects, to evaluate the role of the spins in carrier scattering and localization as well as to determine the participation ratio of the ferromagnetic phase near the metal-insulator transition. A sizable negative magnetoresistance in the regime of strong magnetic fields is assigned to the weak localization effect.

The magnetic state of a single magnetic ion (Mn2+) embedded in an individual quantum dot is optically probed using microspectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn2+ ion (S=5/2) is analyzed in detail. The exciton-Mn2+ exchange interaction shifts the energy of the exciton depending on the Mn2+ spin component and six emission lines are observed at zero magnetic field. Magneto-optic measurements reveal that the emission intensities in both circular polarizations are controlled by the Mn2+ spin distribution imposed by the exchange interaction with the exciton, the magnetic field, and an effective manganese temperature which depends on both the lattice temperature and the density of photocreated carriers. Under magnetic field, the electron-Mn interaction induces a mixing of the bright and dark exciton states.

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped Zn 1Ϫx Mn x Te epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance, and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida ͑RKKY͒ model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.

A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulationdoped p-type Cd0.96Mn0.04Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible cross-over between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid.Soon after the discovery of carrier-controlled ferromagnetism in Mn-doped III-V [1] and II-VI [2] semiconductor compounds, it has become clear that these systems offer unprecedented opportunities to exploit the powerful methods developed for tuning carrier densities in semiconductor quantum structures, in order to control the magnetic characteristics in these systems [2,3,4,5,6]. Such a control opens new prospects for information storage and processing, as well as it makes it possible to examine the behavior of strongly correlated systems as a function of externally controllable parameters. In the case of III-V magnetic semiconductors, Koshihara et al.[3] detected an enhancement of ferromagnetism by illumination of an (In,Mn)As/GaSb heterostructure, an effect assigned to the presence of an interfacial electric field that drives the photo-holes to the magnetically active (In,Mn)As layer. More recently, Ohno et al.[6] demonstrated that a gate voltage of ±125 V changes the Curie temperature T C by about 1 K in a field-effect transistor structure containing an (In,Mn)As quantum well (QW).In the case of II-VI diluted magnetic semiconductors (DMS), Mn does not introduce any carriers. Hence, holemediated ferromagnetic interactions can be induced by modulation-doping of heterostructures [7]. Due to the valence band structure, T C is typically lower in II-VI than in III-V DMS. At the same time, however, it may be expected [2,5] that, owing to the small background hole density, the strength of the carrier mediated ferromagnetic interactions can be tuned over a wider range in II-VI than in III-V DMS.In this paper, we present photoluminescence (PL) studies of modulation-doped p-type (Cd,Mn)Te QW. The (Cd,Zn,Mg)Te barriers are doped either p-or n-type, so that p-i-p or p-i-n structures are formed. The QW in these systems are ferromagnetic below about 3 K. We show that, depending of the sample layout, the ferromagnetism is either destroyed or enhanced during illumination by photons with energy greater than the band gap of the barrier material. In both cases, the switching process is isothermal and reversible. Moreover, we demonstrate that the reverse biasing of the p-i-n diode by a voltage smaller than 1 V turns the ferromagnet into a paramagnetic material. Importantly, this strong effect of light and electric field can be readily explained by considering the distributi...

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A novel magnetoresistance effect, due to the injection of a spin-polarized electron current from a dilute magnetic into a nonmagnetic semiconductor, is presented. The effect results from the suppression of a spin channel in the nonmagnetic semiconductor and can theoretically yield a positive magnetoresistance of 100%, when the spin flip length in the nonmagnetic semiconductor is sufficiently large. Experimentally, our devices exhibit up to 25% magnetoresistance.