Effect of GaAs Intermediary Layer Thickness on the Properties of (Ga,Mn)As Tri-Layer Structures

We have investigated the magnetic properties, current–voltage characteristics, and tunnel magnetoresistance of (Ga,Mn)As/GaAs/(Ga,Mn)As tri-layers having different thickness of the intermediary GaAs layer (1–7 nm). When the thickness of GaAs layer is less than 6 nm, the two (Ga,Mn)As layers couples ferromagnetically. For the tri-layers with thicker GaAs, the GaAs acts as an effective barrier for the carrier transport and decouples the two (Ga,Mn)As layers, which results in tunnel magnetoresistance ratio as high as 100% at 5 K.     

Influence of Codoping on the Magnetoresistance of Paramagnetic (Ga,Mn)As

We studied the correlation between magnetoresistance (MR) effects and the type of conducti- vity in paramagnetic Ga_1–x Mn _x As with x < 0.5%. Series of samples of (Ga,Mn)As with different codoping levels of Te have been prepared by metal-organic vapor-phase epitaxy (MOVPE). With increasing Mn-content from doping levels to x = 0.5% in paramagnetic Ga_1–x Mn _x As, the MR at 1.6 K changes continuously from positive to strongly negative. This manifests the complex interplay between p–d exchange induced valence band splitting and the metal–insulator transition. Codoping with Te leads eventually to a dominant conduction band transport. It is characterized by a small negative contribution at low magnetic fields to the parabolic MR similar to that found in highly n-doped diamagnetic semiconductors. It shows that the Mn-induced conduction band splitting is much smaller than the valence band splitting, i.e., |N₀| |N₀|.     

Electric Field Effect on the Magnetic Properties of III–V Ferromagnetic Semiconductor (In,Mn)As and ((Al),Ga,Mn)As

We have investigated the magnetotransport properties of field-effect transistors (FET) having a III–V ferromagnetic semiconductor channel layer. One can control not only the ferromagnetic transition temperature T _C but also the magnetization and the coercive force of (In,Mn)As channel layers isothermally and reversibly by gate electric fields. A small change of the magnetization upon application of gate electric fields is also observed in FETs with a (Ga,Mn)As channel. Results on a (Al,Ga,Mn)As channel FET are also presented.     

Temperature Peculiarities of Magnetic Anisotropy in (Ga,Mn)As: The Role of the Hole Concentration

It is demonstrated by SQUID measurements that (Ga,Mn)As films can exhibit perpendicular easy axis at low temperatures, even under compressive strain, provided that the hole concentration is sufficiently small. In such films, the easy axis assume a standard in-plane orientation when the temperature is increased towards the Curie temperature. The findings are shown to corroborate the predictions of the mean-field Zener model for strained (III,Mn)V ferromagnetic semiconductors.     

Electrical Electron Spin Injection with a p+-(Ga,Mn)As/n+-GaAs Tunnel Junction

We demonstrate electrical electron spin injection in a p⁺-(Ga,Mn)As/n⁺-GaAs tunnel junction with an n-GaAs/(In,Ga)As/p-GaAs light emitting diode (LED). By applying a reverse bias to the p⁺-(Ga,Mn)As/n⁺-GaAs junction (forward bias to the LED), we observed clear hysteresis in electroluminescence (EL) polarization. The magnitude of the EL polarization, which does not depend critically on the spacer layer thickness up to 800 nm, is found to be about five times greater than that of the hole spin injection.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

Photoinduced Magnetization Rotation and Precessional Motion of Magnetization in Ferromagnetic (Ga,Mn)As

Dynamics of photoinduced magnetization have been studied in ferromagnetic semiconductor (Ga,Mn)As thin films by time-resolved polar Kerr rotation. Ultrafast (150 fs) magnetization rotation has been realized using both chirality of excitation light and p–d exchange interaction. We also report on the occurrence of photoinduced magnetization precession in longer time domain. This is initiated by a change in hole concentration and resultant change in magnetic anisotropy. The overall behaviors of these photoinduced magnetization dynamics are discussed.     

Manifestation of Local Magnetic Domain Reversal by Spin-Polarized Carrier Injection in (Ga,Mn)As Thin Films

Recently, we have reported that the spin-polarized holes generated by the irradiation with circularly polarized light can change the magnetization orientation of III–V ferromagnetic semiconductor (Ga,Mn)As via p–d exchange interaction. In this paper, we report that a small portion of change does not return to the initial state after the light is turned off. This residual component, named as the memorization effect, exhibits ferromagnetic characteristics. This fact strongly suggests that small magnetic domains having the perpendicular magnetic axis are rotated by photogenerated carrier spins.     

Hole Spin Polarization in Metallic Ferromagnetic GaMnAs Multilayers and Superlattices: Lateral and Bloch Miniband Transport

It is shown that spin-polarized currents occur in metallic and ferromagnetic Ga_1–x Mn _x As/GaAs multilayered structures, as a result of the magnetic interaction between holes and the Mn ions. The magnetic layers act as potential barriers for holes with spins aligned parallel to the layer magnetization, and as potential wells for the inverse spin polarization. In the case of currents in-plane, holes with spin parallel and antiparallel to this magnetization move in different regions. By choosing properly the magnetic and the nonmagnetic layers widths, a spin-polarized transport with a difference of an order of magnitude on the mobilities for each spin polarization is predicted to occur. Spin-polarized minibands are also shown to occur in a superlattice based on the same structure. We calculated the dependence of the spin polarization with the superlattice parameters, and we discuss how this polarization affects the Bloch miniband transport in such ferromagnetic superlattice.     

Channel Thickness Dependence of the Magnetic Properties in (Ga,Mn)As FET Structures

We have fabricated a series of field-effect transistor structures with a thin (Ga,Mn)As channel with thickness t of 3.5, 4.0, 4.5, and 5.0 nm, and investigated the effect of electric-field E on their magnetic properties. The Curie temperature T _C showed a clear dependence on the magnitude of E, and its controllable range became larger with decreasing t and reached 15 K for the device with t=3.5 nm, which corresponded to 32% of T _C of the layer.     

Spin Carrier Exchange Interactions in (Ga,Mn)N and (Zn,Co)O Wide Band Gap Diluted Magnetic Semiconductor Epilayers

(Ga,Mn)N and (Zn,Co)O wide band gap diluted magnetic semiconductor epilayers have been investigated by magneto-optical spectroscopy. In both cases, absorption bands observed below the energy gap allow us to study the nature of the valence and spin state of the incorporated magnetic element. Exchange interactions between magnetic ions and carriers can be observed by analyzing the magnetic circular dichroism in transmission or the exciton Zeeman splitting in reflection for (Zn,Co)O. A first estimation of the exchange integrals can be given for both materials.     

Magneto-Optical Spectroscopy on (Ga,Mn)As Based Layers—Correlation Between the p–d Exchange Integral and Doping

It is well established that the ferromagnetism of (Ga,Mn)As is mediated by holes. An important parameter in this context is the p–d exchange integral N ₀ as the RKKY interaction parameter J _RKKY is proportional to (N ₀)². In magnetic semiconductors, magnitude and sign of N ₀ are strongly dependent on the local electronic configuration of the Mn-ion and on the free carrier density. To investigate the correlation between the p–d exchange interaction and doping in Ga_1–x Mn _x As, paramagnetic samples with x 0.1% have been intentionally n-doped with Te. High-field Hall measurements at 1.6 K confirm that with increasing degree of Te co-doping, the samples change from p- to n-type. The magnetic field induced splitting of the excitonic and electronic states has been studied by various magneto-optical spectroscopic techniques at 1.8 K and external magnetic fields up to 7.5 T. The p–d exchange integral changes from N ₀ > 0 (ferromagnetic coupling) to N ₀ < 0 (antiferromagnetic coupling) with increasing degree of Te co-doping. Thus, N ₀ can be tuned and can even change its sign in dependence on the doping conditions of the Mn-containing layers.     

Regular Arrays of (Zn,Mn)S Quantum Wires with Well-Defined Diameters in the Nanometer Range

Zn_1–x Mn _x S, with x varying between 0.01 and 0.30, were formed inside the ordered pore systems of different mesoporous SiO₂ matrices. Because of the highly ordered structure of the hosts, regular arrays of Zn_1–x Mn _x S quantum wires with lateral dimensions of 3 and 5.5 nm, respectively, separated by 2-nm SiO₂ barriers were obtained. The wires were characterized using photoluminescence (PL) and PL excitation (PLE) spectroscopy at liquid Helium temperatures. The PL of the wires is dominated by the ⁴T₁ ⁶A₁ internal transition of the Mn²⁺(3d⁵) ions. The corresponding PLE spectra show higher internal Mn transitions as well as the band to band transition. The energies of the internal Mn transitions are typical for Mn²⁺ on a cation site of (II,Mn)VI semiconductors. Because of the comparable bandgaps of the SiO₂ and the Zn_1–x Mn_xS as well as the small exciton Bohr radius in (Zn,Mn)S quantum confinement effects in the wires are less than about 150 meV.     

Magnetic Behaviors of Ferromagnetic Semiconductor Zn1?xCrxTe Grown by MBE

The magnetic and structural properties of MBE-grown films of Zn_1–xCr_xTe were investigated. The magnetization versus magnetic field (M–H) measurement of Zn_1–xCr_xTe (x = 0.01–0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T_C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M–T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.     

Rotation of Ferromagnetically Coupled Mn Spins in (Ga,Mn)As by Hole Spins

We present experimental data obtained from ferromagnetic p-(Ga,Mn)As layers that indicate the collective rotation of ferromagnetically coupled Mn spins by optically generated spin-polarized holes through the p–d exchange interaction. The rotation occurs reversibly between the in-plane and perpendicular directions, causing a large change in perpendicular magnetization without the application of a magnetic field. Pump-and-probe experiments suggest that the rotation of Mn spins take place in the picosecond time domain.     

Ferromagnetism in (Zn,Cr)Se Layers Grown by Molecular Beam Epitaxy

Magnetization measurements of MBE grown epilayers of (Zn,Cr)Se with relatively large Cr content of 0.014 and 0.021 are presented. We evidence the presence of a strong ferromagnetic coupling between Cr ions, but also suggest a significant clustering due to a pronounced superparamagnetic behavior found in the layers. We estimate the intraparticle Curie temperature to be about 100 K, which combined with other arguments appears to indicate that some magnetic properties of Cr-rich layers might be dominated by the presence of small grains of A^II–Cr₂–B₄ ^VI spinels.     

Controlling the Charge and the Spin Polarization Distributions in (In,Ga,Mn)As-Based Diluted Magnetic Semiconductor Multilayered Structures

A self-consistent Luttinger–Kohn (LK) calculation is used to obtain the electronic structure of Mn-doped semiconductor multilayers. In the particular, case of a (In,Ga)As/GaAs heterostructure, the ternary alloy has a smaller gap than GaAs, and introduces strain due to the differences on the lattice parameters. The aim of this work is to understand how the charge and the spin polarization densities can be controlled by the parameters of the structure in such way that they appear concentrated either on the magnetic or in the non-magnetic region. We show that by changing the In concentration the valence band mismatch competes with the energy splitting due to the magnetic interaction, and we can manipulate the charge and the spin polarization densities in the structure.     

Electrical Electron Spin Injection with a p+-(Ga,Mn)As/n+-GaAs Tunnel Junction

We demonstrate electrical electron spin injection in a p⁺-(Ga,Mn)As/n⁺-GaAs tunnel junction with an n-GaAs/(In,Ga)As/p-GaAs light emitting diode (LED). By applying a reverse bias to the p⁺-(Ga,Mn)As/n⁺-GaAs junction (forward bias to the LED), we observed clear hysteresis in electroluminescence (EL) polarization. The magnitude of the EL polarization, which does not depend critically on the spacer layer thickness up to 800 nm, is found to be about five times greater than that of the hole spin injection.