Pulse-Width Dependence in Current-Driven Magnetization Reversal Using GaMnAs-Based Double-Barrier Magnetic Tunnel Junction

We have investigated the current pulse width dependence on current-driven magnetization reversal in double-barrier structures using GaMnAs-based magnetic tunneling junctions (MTJ) in order to clarify the origin of low threshold current density for current-driven magnetization reversal. Comparing with the case of single-barrier MTJ, the pulse-width dependence reveals that threshold current density is reduced by double-barrier MTJ. We confirmed that the threshold current density in the order of 10⁴ A/cm² is estimated considering the effect of current pulse width.     

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.     

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.     

The Effects of Mn Substitution on Magnetization Reversal Properties in Gd0.7Ca0.3MnO3

Low temperature magnetization in polycrystalline Gd_0.7Ca_0.3Mn_1?x M _x O₃ (M=Cr, Ga, Ru; x=0, 0.2) has been investigated. The samples were prepared via the conventional solid state reaction method. For all the samples, the paramagnetic to ferrimagnetic (PM-FiM) transition temperature (say T _max) can be well defined from the temperature dependent ac susceptibility data. The negative magnetization suppresses due to 20 % Cr or Ga doping at the Mn site. On the other hand, below the compensation temperature (T _comp), the nature of the Ru doping sample is almost similar to that of undoped Gd_0.7Ca_0.3MnO₃. The nonmagnetic Ga doping drastically reduces magnetization and T _max shifts to a lower temperature. However, Ru and Cr doping increase the value of T _max. The network of canted Mn³⁺/Mn⁴⁺ moments changes with the substitution of Cr, Ga, and Ru at the Mn site giving rise to the variation of the internal magnetic moment. Thus, the antiparallel coupling of the Gd moments with Mn/M changes and affects the low temperature magnetization reversal properties.     

Anomalous Conductivity in Random Magnetic (Ga,Cr) As

The electrical conductivity of molecular beam epitaxy grown zinc blende Ga_1–x Cr _x As, x = 0.1, exhibits anomalous behavior below room-temperature. The room-temperature resistivity is small, = 0.1 cm, and comparable to (Ga,Mn)As. Near room temperature, the conductivity is activated, following = ₀ exp(–E _A/kT ), with an activation energy of E _A = 63 meV. In this activated region, Hall measurements also show activated behavior in the hole concentration where p 10²⁰ cm^–3, indicating that Cr can also act as an acceptor similar to Mn. For decreasing temperature, the resistivity increases rapidly because of hopping or tunneling conduction, and becomes strongly insulating at low temperatures. From T = 20 to 200 K the conductivity follows exp[–(T ₁/T)^1/2] over eight orders of magnitude range of conductivity, possible evidence of tunneling between metallic-like polarons.     

Photoinduced Magnetism in Semiconductor-Based Structures

This paper reviews the most recent progress in the studies of photocarrier-induced magnetism, phototunable magnetization reversal, as well as photoenhanced magnetization in the magnetic alloy semiconductor heterostructure (In,Mn)As/(Ga,Al)Sb and the GaAs–Fe granular structure. Emphases are given to point out that the interplay between the nonmagnetic constituents and magnetic constitutes can be mediated by carrier spins.     

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.     

Is High TC Possible in (Ga,Mn)N?: Monte Carlo Simulation vs. Mean Field Approximation

The magnetic properties of diluted magnetic semiconductors (DMSs) are calculated from first-principles by mapping the ab initio results on a classical Heisenberg model. By using the Korringa–Kohn–Rostoker coherent potential approximation method within the local density approximation, the electronic structure of (Ga,Mn)N and (Ga,Mn)As is calculated. Effective exchange coupling constants J_ijs are calculated by using the formula of Liechtenstein et al. (A.~I. Liechtenstein, M. I. Katsnelson, V. P. Antropov, and V. A. Gubanov, 1987, J.~Magn. Magn. Mater. Vol. 67, p. 65). It is found that the range of the exchange interaction in (Ga,Mn)N is very short due to the exponential decay of the impurity wave function in the gap. On the other hand, in (Ga,Mn)As, the interaction is weaker but long ranged because the extended valence hole states mediate the ferromagnetic interaction. Monte Carlo simulations show that the T_C values of (Ga,Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates T_C. Even in (Ga,Mn)As the percolation effect is still important.     

Growth and Spectroscopy of CdSe: Mn Quantum Dots

In this paper we report the growth and optical properties of ZnSe/CdSe:Mn magnetic quantum dots by Atomic Layer Epitaxy. For the uncapped samples, dot densities of the order of 10⁹ cm^–2 were measured by Atomic Force Microscopy. The ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV, and a spectrally broad emission at 2.15 eV from the internal Mn²⁺ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL line width was measured as a function of Mn concentration. For large Mn contents the temporal change in magnetization causes a broadening of the single dot PL line of up to 4 meV FWHM. However, for low concentrations the single dot PL line widths were resolution limited at <0.2 meV.     

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.     

Improving Néel Domain Walls Dynamics and Skyrmion Stability Using He Ion Irradiation

Ion irradiation with light ions is an appealing way to finely tune the magnetic properties of thin magnetic films and in particular the perpendicular magnetic anisotropy (PMA). In this work, the effect of He⁺ irradiation on the magnetization reversal and on the domain wall dynamics  of Pt/Co/AlOx trilayers is illustrated. Fluences up to 1.5 × 10¹⁵ ions cm⁻² strongly decrease the PMA, without affecting neither the spontaneous magnetization nor the strength of the interfacial Dzyaloshinskii–Moriya interaction (DMI). This confirms experimentally the robustness of the DMI interaction against interfacial chemical intermixing, already predicted by theory. In parallel with the decrease of the PMA, a strong decrease of the domain wall depinning field is observed after irradiation. This allows the domain walls to reach large maximum velocities with a lower magnetic field compared to that needed for the pristine films. Decoupling PMA from DMI can, therefore, be beneficial for the design of low energy devices based on domain wall dynamics. When the samples are irradiated with larger He⁺ fluences, the magnetization gets close to the out-of-plane/in-plane reorientation transition, where ≈100nm size magnetic skyrmions are stabilized. It is observed that as the He⁺ fluence increases, the skyrmion size decreases while these magnetic textures become more stable against the application of an external magnetic field, as predicted by theoretical models developed for ultrathin films with labyrinthine domains.     

Anisotropic Quantum Transport through a Single Spin Channel in the Magnetic Semiconductor EuTiO3

Magnetic semiconductors are a vital component in the understanding of quantum transport phenomena. To explore such delicate, yet fundamentally important, effects, it is crucial to maintain a high carrier mobility in the presence of magnetic moments. In practice, however, magnetization often diminishes the carrier mobility. Here, it is shown that EuTiO₃ is a rare example of a magnetic semiconductor that can be desirably grown using the molecular beam epitaxy to possess a high carrier mobility exceeding 3000 cm² V⁻¹ s⁻¹ at 2 K, while intrinsically hosting a large magnetization value, 7 μ_B per formula unit. This is demonstrated by measuring the Shubnikov–de Haas (SdH) oscillations in the ferromagnetic state of EuTiO₃ films with various carrier densities. Using first-principles calculations, it is shown that the observed SdH oscillations originate genuinely from Ti 3d-t_2g states which are fully spin-polarized due to their energetical proximity to the in-gap Eu 4f bands. Such an exchange coupling is further shown to have a profound effect on the effective mass and fermiology of the Ti 3d-t_2g electrons, manifested by a directional anisotropy in the SdH oscillations. These findings suggest that EuTiO₃ film is an ideal magnetic semiconductor, offering a fertile field to explore quantum phenomena suitable for spintronic applications.     

Impact of Li<Superscript>3+</Superscript> ion irradiation on magnetic properties of Mn doped ZnO: correlation with defects and structural property

Present work aims to study the impact of 50 MeV Li³⁺ ion irradiation on magnetic properties and their correlation with the substitutional incorporation of Mn at Zn site (Mn_Zn) and defects in Mn:ZnO (Mn doped ZnO). 2 at% Mn:ZnO has been synthesized by solid-state reaction method and irradiated with fluences 10¹², 10¹³ and 10¹⁴ ions/cm². The concomitant changes have been probed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetization measurements. XRD patterns indicate single-phase nature of the samples and Mn_Zn nicely varied with fluence of irradiation. Temperature dependent dc and ac magnetization (M-T) measurement reveals a minor presence of spin glass nature or superparamagnetism (SPM) though all of the samples are strongly intrinsically ferromagnetic and field dependent magnetization (M-H) measurement actually confirms this observation. It is noteworthy to observe that the ordering of spins and the magnetic parameters viz. saturation magnetization (M_S), remnant magnetization (M_R) and coercivity (H_C) can be tuned by fluences of irradiation. Finally, proportion of defect [particularly zinc vacancy (V_Zn)] and Mn_Zn seems to dictate the trend of variation of magnetization (M_S and M_R). Temperature dependent ac magnetization measurement and magnetic relaxation measurement actually detect a weak spin glass nature of the samples and eliminate the possibility of SPM.     

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.     

Alloying (In,Mn)As and (Ga,Mn)As: Ferromagnetic (In,Ga,Mn)As Lattice-Matched to InP

An effect of alloying two ferromagnetic semiconductors (In,Mn)As and (Ga,Mn)As on the ferromagnetic properties of resultant (In,Ga,Mn)As alloys is reported. For conditions close to lattice-matching to InP substrates, y = 0.53 in (In _y Ga_1–y )_1–x Mn _x As, ferromagnetism up to Curie temperatures T _C = 100–110 K could be achieved for a Mn composition x = 0.13. Trends in the Curie temperature in (In,Ga,Mn)As are compared with (Ga,Mn)As and (In,Mn)As as a function of Mn content. Hole concentrations determined from magnetotransport, taking into account the anomalous Hall contribution to Hall resistance, gives p/Mn = 0.03 ratio to Mn composition in metallic case for x = 0.13. We mention the possible role of chemical ordering (short range) of Mn impurity atoms on hole concentration and, consequently, for the ferromagnetic properties.     

Schottky barrier height enhancement on n-In0.53Ga0.47As by (NH4)2S x surface treatment

(NH₄)₂S _x Surface treatment was found to increase the barrier height (_Bn) for Au/In_0.53Ga_0.47As Schottky junctions from 0.26 eV to 0.58 eV at 300 K as determined from Richardson plots. The ideality factorn thus decreased from 2.7 to 1.6 and the reverse saturation current densityJ ₀ from 9.4 Acm^–2 to 3.4×10^–5A cm^–2. The values of the effective Richardson constant were also evaluated. The chemical state of In_0.53Ga_0.47As surfaces before and after (NH₄)₂S _x modification, examined by X-ray photoelectron spectroscopy (XPS), indicated bond formation of S with In, Ga and As.     

Magneto-Transport Properties of p-Type Zn1?xMnxTe:N Grown by MBE

The magneto-transport properties of nitrogen-doped p-Zn_1–x Mn _x Te films grown by MBE were studied, with an emphasis on the dependence on the Mn composition and the hole concentration. The Mn composition x and hole concentration p were varied in the range of x = 0.026–0.068 and p = 8.2 × 10¹⁸–2.6 × 10¹⁹ cm^–3. In the measurement of magnetoresistance (MR), a large negative component was observed at low temperatures. In the dependence on the Mn composition, it was found the magnitude of the negative component of MR became larger and survived up to higher temperatures in the sample with a higher Mn composition. The observed negative MR is attributed to the localization of holes due to the magnetic polaron formation. The origin of the dependence on the Mn composition is discussed.     

Properties of Quaternary Alloy Magnetic Semiconductor (InGaMn)As Grown on InP

We have studied properties of quaternary alloy magnetic semiconductor (InGaMn)As grown on InP substrates by low-temperature molecular-beam epitaxy (LT-MBE). A large MCD peak whose intensity is larger than 500 mdeg for (InGaMn)As was observed. This peak intensity was about three times larger than that of typical (GaMn)As films. Relatively high Curie temperature of 83 K of [(In_0.53Ga_0.47)_0.88Mn_0.12]As was observed by Hall measurements. The carrier concentration of [(In_0.53Ga_0.47)_0.88Mn_0.12]As was estimated to be more than 1.0 × 10²¹ cm^–3 by using the Curie–Weiss fitting of the Hall coefficient R _H, indicating that more than 40% of Mn atoms are activated. This means that (InGaMn)As has a higher activation ratio of Mn as acceptors than (GaMn)As.     

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.