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We investigate transport through 6–10 nm thin epitaxial GaAs(001) barriers sandwiched between polycrystalline iron films. Apart from a pronounced tunneling magnetoresistance effect at low magnetic fields, we observe a distinct negative magnetoresistance (MR) at low and a positive MR at higher temperatures. We show that the negative MR contribution is only observed for the ferromagnetic iron contacts but is absent if iron is replaced by copper or gold electrodes. Possible explanations of the negative MR involve suppression of spin-flip scattering or Zeeman splitting of the tunneling barrier, but neither of these explanations is fully consistent with the data. [ABSTRACT FROM AUTHOR]

The conductivity and Hall effect of p-type doped Zn[sub (1-x)]Mn[sub x]Te layers are described and analyzed in terms of a two-fluid model where part of the carriers are weakly localized as bound magnetic polarons. At low temperature, the Hall effect is dominated by the spin-dependent component (extraordinary Hall effect), in good agreement with a crude estimate taking into account the properties of the valence band. The long range ferromagnetic interaction appears to be induced by the weakly localized holes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using SQUID magnetometry have found that in (Ga, Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions. It can be rotated from the [110] direction to the [110] direction by low temperature annealing and we show that this change is hole density-related. We demonstrate that the magnitude of uniaxial anisotropy as well its dependence on hole-concentration and temperature can be explained in terms of the p-d Zener model of ferromagnetism assuming a small trigonal-like distortion. [ABSTRACT FROM AUTHOR]

Demonstrates electric-field control of ferromagnetism in a thin-film semiconducting alloy, using an insulating-gate field-effect transistor structure. Ability to vary isothermally and reversibly the transition temperature of hole-induced ferromagnetism which is demonstrated; Methods of the study.

The focus of studies on ferromagnetic semiconductors is moving from material issues to device functionalities based on phenomena often associated with the anisotropy properties of these materials. This is driving a need for a method to locally control the anisotropy in order to allow the elaboration of devices. Here the authors present a method which provides patterning induced anisotropy that not only can be applied locally but also dominates over the intrinsic material anisotropy at all temperatures. [ABSTRACT FROM AUTHOR]

We report on the fabrication of pseudomorphic wurtzite Ga1-xMnxN grown on GaN with Mn concentrations up to 10% using molecular beam epitaxy. According to Rutherford backscattering, the Mn ions are mainly at the Ga-substitutional positions, and they are homogeneously distributed according to depth-resolved Auger-electron spectroscopy and secondary-ion mass-spectroscopy measurements. A random Mn distribution is indicated by transmission electron microscopy, and no Mn-rich clusters are present for optimized growth conditions. A linear increase of the c-lattice parameter with increasing Mn concentration is found using x-ray diffraction. The ferromagnetic behavior is confirmed by superconducting quantum-interference measurements showing saturation magnetizations of up to 150 emu/cm3. [ABSTRACT FROM AUTHOR]

We report on a monotonic reduction of Curie temperature in dilute ferromagnetic semiconductor (Ga,Mn)As upon a well controlled chemical-etching/oxidizing thinning from 15 nm down to complete removal of the ferromagnetic response. The effect already starts at the very beginning of the thinning process and is accompanied by the spin reorientation transition of the in-plane uniaxial anisotropy. We postulate that a negative gradient along the growth direction of self-compensating defects (Mn interstitial) and the presence of surface donor traps gives quantitative account on these effects within the p-d mean field Zener model with adequate modifications to take a nonuniform distribution of holes and Mn cations into account. [ABSTRACT FROM AUTHOR]

Direct determination of the hole-induced ferromagnetic contribution to the nearest-neighbor (NN) Mn–Mn exchange interaction in Zn1-xMnxTe is reported. The difference in the NN exchange constants for insulating and strongly p-type specimens was found by comparing the spectra of inelastic magnetic neutron scattering from isolated NN spin pairs in the samples. Our results are in good agreement with the predictions of a Zener/RKKY model, and thus indicate that the ferromagnetic exchange is mediated by weakly localized holes. [Copyright &y& Elsevier]

The article reviews our recent studies on quantum Hall ferromagnetism (QHF) in diluted magnetic semiconductors. A giant and nonlinear Zeeman splitting in these materials offers a unique opportunity to examine QHF since crossing of Landau levels (LL) can be achieved in perpendicular magnetic fields. We carried out magnetoresistance studies on modulation-doped, gated heterostructures of Cd1−xMnxTe/Cd1−yMgyTe:I. We put into evidence the formation of Ising quantum Hall ferromagnet with Curie temperature TC as high as 2 K. QHF in our device is manifested by anomalous magnetoresistance maxima, their hysteretic behavior, and time-dependent resistance, similar to earlier observations in III–V heterostructures. However, in our system these phenomena are much stronger, especially when either 2− or 1−, and 0+ LL are brought into coincidence. The magnitude of the QHF spikes depends dramatically on the history of the sample, shows hysteresis when either magnetic field or gate voltage are swept, stretched-exponential time evolution characteristic of glassy systems, and strong Barkhausen noise. Our study suggests that these metastabilities stem from the slow dynamics of ferromagnetic domains. [Copyright &y& Elsevier]

This work presents an experimental demonstration of time-reversal asymmetry of electron states propagating along the boundary separating areas with opposite magnetic fields. For this purpose we have fabricated a hybrid ferromagnet– semiconductor device in the form of a Hall cross with two ferromagnets deposited on top. The magnets generated two narrow magnetic barriers of opposite polarity in the active Hall area. We have observed that if the signs of the barriers are reversed, the bend resistance changes its sign. Using the Landauer–Bu¨ttiker theory, we have demonstrated that this is a direct consequence of asymmetric transmission of the “snake” and the “cycloidal” trajectories formed at the boundary separating the regions with opposite magnetic field directions. [Copyright &y& Elsevier]

Experimental results obtained on Zn1−xMnxTe layers and Cd1−xMnxTe quantum wells are surprisingly well understood in the framework of a mean field model of the carrier-induced ferromagnetic transition, provided the real structure of the valence band is properly taken into account. Here we put the emphasis on effects leading to deviations from the standard model: RKKY oscillations in Zn1−xMnxTe when the Mn concentration is not high enough with respect to the carrier density, electronic disorder and localization at low carrier density in Zn1−xMnxTe epilayers and Cd1−xMnxTe quantum wells. Importantly, the modulation of magnetic properties through modulation of the carrier density, either optically or using the electric potential in a pin diode, is demonstrated. [Copyright &y& Elsevier]

Electric-field control of carrier-induced ferromagnetism is demonstrated for field-effect transistor structure of magnetic semiconductor (In,Mn)As. By varying the gate electric field one can control the ferromagnetic transition temperature isothermally and reversibly. [Copyright &y& Elsevier]

By using highly sensitive millikelvin superconducting quantum interference device magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga1-xMnxN. The interpretation of the results in the frame of tight-binding theory and of Monte Carlo simulations allows us to assign the spin interaction to ferromagnetic superexchange and to point out the limited accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators. [ABSTRACT FROM AUTHOR]