Ferromagnetic Semiconductor Films Research Articles

Depolarization effects on spin-orbit torque magnetization switching in (Ga,Mn)(As,P) ferromagnetic semiconductor film

Depolarization effects on spin-orbit torque magnetization switching in (Ga,Mn)(As,P) ferromagnetic semiconductor film

Anomalous Hall effect and ordinary Hall effect in variable range hopping regime of inhomogeneous ferromagnetic semiconductor

Anomalous Hall effect (AHE) and ordinary Hall effect (OHE) in variable range hopping regime of ferromagnetic semiconductor are important for understanding the localized impurity states at different energy levels within the band gap, and provides insight into the specific percolation networks of the thermally excited spin-polarized carriers. Here we report the experiments of both OHE and AHE in Mott and Efros variable range hopping regimes of inhomogeneous In0.27Co0.73O1-v ferromagnetic semiconductor films with the tunable localized electron density and localization length. We found an unusual scaling relation between the AHE coefficient RAH and the longitudinal resistivity ρxx, RAH∝ρxxβ with β>2 , which contrasts the scaling relation of OHE coefficient ROH∝ρxxα with α<1. More strikingly, the sign of AHE coefficient reverses with decreasing temperature for specific electron density. We argue that these surprising results are related to the specific impurity energy band structure of the oxygen vacancies within the band gap of inhomogeneous In0.27Co0.73O1-v ferromagnetic semiconductor films.

Carrier localization in quaternary Ga1−xMnxAs1−yPy ferromagnetic semiconductor films

Motivated by the fact that holes in the ${\mathrm{Ga}}_{1--x}{\mathrm{Mn}}_{x}\mathrm{As}$ family of ferromagnetic semiconductors play a key role in determining their ferromagnetic properties, we have measured hole concentrations in a series of three ${\mathrm{Ga}}_{1--x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1--y}{\mathrm{P}}_{y}$ alloys grown by molecular beam epitaxy with varying amounts of phosphorus. This was carried out by Hall effect measurements, after eliminating the effect of the anomalous Hall term, and thus isolating the ordinary Hall term that directly provides the concentration of freely moving holes. Comparing these Hall effect results with total hole concentrations obtained from the number of acceptors and compensating donors as given by structural and magnetization measurements, we find that the number of itinerant holes (i.e., holes that contribute to the Hall effect) is significantly less than the total hole concentration. This indicates that a considerable fraction of the holes arising from Mn acceptors are localized. We find, furthermore, that the degree of such localization increases with the concentration of phosphorus in the ${\mathrm{Ga}}_{1--x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1--y}{\mathrm{P}}_{y}$ alloy. Our results indicate that the Curie temperature (and, by extension, other magnetic properties) described by the Zener model in ferromagnetic semiconductors of the ${\mathrm{Ga}}_{1--x}{\mathrm{Mn}}_{x}\mathrm{As}$ family are determined by the itinerant holes rather than by the total hole concentration. Finally, our results also indicate that ferromagnetism in these alloys vanishes when the total hole concentration falls below a certain Mott-like threshold, suggesting that the holes (both localized and itinerant) reside in the acceptor impurity band created by the presence of Mn acceptors.

Programmable bias field observed in graded ferromagnetic semiconductor films with broken symmetry

We report on the observation of a field-induced magnetic bias effect in a $\mathrm{G}{\mathrm{a}}_{0.94}\mathrm{M}{\mathrm{n}}_{0.06}\mathrm{A}{\mathrm{s}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ thin film with digitally graded phosphorus content. Although phosphorus concentration in the sample is changed in steps from $y\ensuremath{\approx}0.03$ to $y\ensuremath{\approx}0.28$, the magnetometry and magnetotransport data display a coherent magnetic response typical for single-layer in-plane magnetized films with cubic and weak [110] uniaxial anisotropy. Unexpectedly, low-temperature planar Hall resistance loops exhibit remarkable asymmetry tunable by application of strong in-plane initial magnetic field ${H}_{\mathrm{Ini}}$. We discuss this unusual memory effect, resembling the magnetization asymmetry in the exchange biased magnetic bilayers, in terms of a unidirectional bias field ${H}_{b}$ induced by ${H}_{\mathrm{Ini}}$. We show that such bias field defines the delayed or accelerated nucleation of domains with chiral domain walls performing the magnetization reversal.

Inhomogeneous magnetization of a thin film of a ferromagnetic semiconductor in an electric field

A theoretical model describing the spontaneous magnetization of a ferromagnetic semiconductor (InMn)As film in the presence of an external electric field directed across the film is considered. It is assumed that the ions of a manganese impurity with spin 5/2 are acceptors, have a uniform spatial distribution inside the semiconductor, and do not change their position under the action of an external field. The motion of holes with spin ½ changes their spatial distribution under the action of the field. The exchange interaction between manganese ions and holes allows the formation of magnetization that is non-uniform across the film thickness.In particular, the existence of a piecewise continuous solution describing the presence of a phase transition boundary for magnetization inside a ferromagnetic semiconductor film is shown.

The Spatial Distribution of Magnetization in a Ferromagnetic Semiconductor Thin Film

A model for the description of the distribution of magnetization across the thickness of a ferromagnetic semiconductor film is considered. Applying a constant electric field perpendicular to the film surface makes it possible to change the Curie temperature. The obtained formulas determine the dependence that this distribution has on the values of the physical parameters of the film.

Magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor

We report a systemeatic investigation of magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor films by magneto-transport. Hall measurements showed a transition of the easy magnetization direction from in-plane to out-of plane with incorporation of the P into the GaMnAs films. Quantitative information on magnetic anisotropy of the films is obtained by fitting the angular dependence of Hall resistance data to magnetic free energy using the coherent rotation model. Values of magnetic anisotropy parameters show that in-plane anisotropy decreases and out-of-plane anisotropy increases with increasing P content in these films. The out-of-plane magnetic anisotropy in GaMnAsP layers is further enhanced by low temperature annealing. By optimizing the growth and annealing conditions, we were able to obtain a Curie temperature of 125 K in such quaternary films, with strong out-of-plane anisotropy. This study showed that the magnetic anisotropy of the GaMnAsP films can be controlled by adjusting the concentration of the P, and by appropriate post-growth annealing.

Aligned ZnO:Co nanorod arrays: Electrophoretic deposition fabrication and magnetic manipulation

ZnO:Co thin films with various morphologies were synthesized via two-step electrophoretic deposition at controlled conditions. Their morphologies, structures, and magnetic properties were systemically investigated. Dense solid, granulated, and nanorod-array morphological films can be obtained by carefully tuning the deposition conditions. The structural results indicate that Co doped into the ZnO lattice occupy the Zn2+ ion sites in a high-spin Co2+ state. Magnetic observations show that all the films have room-temperature ferromagnetisms but their magnetisms are different because of their different morphologies and structures. The magnetism of the nanorod-array film is considerably higher than those of the granulated and solid films, which indicate that the magnetic properties of ZnO:Co films can be manipulated by tuning their morphologies and structures. The mechanism of electrophoretic deposition and magnetism differences of the films are discussed in detail. These ferromagnetic semiconductor films with controllable morphologies and structures are expected to present extensive applications in the future.

Cyclotron resonance in ferromagnetic InMnAs and InMnSb

We present experimental and theoretical studies of the magneto-optical properties of $p$-type In${}_{1\ensuremath{-}x}$Mn${}_{x}$As and In${}_{1\ensuremath{-}x}$Mn${}_{x}$Sb ferromagnetic semiconductor films in ultrahigh magnetic fields oriented along [001]. Samples were fabricated by molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). To model the results, we used an 8-band Pidgeon-Brown model generalized to include the wave vector dependence of the elec-tronic states along ${k}_{z}$ as well as the s-d and p-d exchange interactions with the localized Mn $d$ electrons. The Curie temperature is taken as an input parameter and the average Mn spin is treated in mean-field theory. We compared Landau level and band structure calculations with observed cyclotron resonance (CR) measurements. While differences between the CR measurements are seen for MBE and MOVPE samples, our calculations indicate that they arise from differences in the carrier densities. In addition, the difference in the carrier densities suggests significantly larger average spin for the MOVPE structures; this fact could be responsible for higher Curie temperatures in this material system.

Oscillation of coercivity between positive and negative in MnxGe1−x:H ferromagnetic semiconductor films

Amorphous MnxGe1−x:H ferromagnetic semiconductor films prepared in mixed Ar with 20% H2 by magnetron co-sputtering show global ferromagnetism with positive coercivity at low temperatures. With increasing temperature, the coercivity of MnxGe1−x:H films first changes from positive to negative, and then back to positive again, which was not found in the corresponding MnxGe1−x and other ferromagnetic semiconductors before. For Mn0.4Ge0.6:H film, the inverted Hall loop is also observed at 30 K, which is consistent with the negative coercivity. The negative coercivity is explained by the antiferromagnetic exchange coupling between the H-rich ferromagnetic regions separated by the H-poor non-ferromagnetic spacers. Hydrogenation is a useful method to tune the magnetic properties of MnxGe1−x films for the application in spintronics.

Study of influence of domain structure on observed magnetoresistance anomalies in GaMnAs

Magneto-transport properties of a Ga0.93Mn0.07As ferromagnetic semiconductor film with strong epitaxial strain (Ga0.7In0.3As buffer) have been studied. The observed magnetoresistance showed peculiar peaks at the magnetic fields corresponding to magnetization switching probed by Hall voltage. Computer simulations showed that these anomalies could originate from the formation of complex, island-like magnetic domains, and their propagation in the sample.

Effects of Sn doping on the morphology and properties of Fe-doped In2O3 epitaxial films

(Sn, Fe)-codoped In2O3 epitaxial films were deposited on (111)-oriented Y-stabilized ZrO2 substrates by pulsed laser deposition with constant Fe concentration and different Sn concentrations. The influence of Sn concentration on the crystal structure and properties of Fe-doped In2O3 ferromagnetic semiconductor films has been investigated systematically. Experimental results indicate that Sn doping can effectively reduce the surface roughness and suppresses breakup of the films into separated islands. At the same time, the optical band gap increases and the electrical properties improve correspondingly. However, although the carrier density increases dramatically with the Sn doping, no obvious change of the ferromagnetism is observed. This is explained by a modified bounded magnetic polaron model.

Magnetic Anisotropy of GaMnAs Film and Its Application in Multi-valued Memory Devices

The magnetic anisotropy of GaMnAs ferromagnetic semiconductor film was investigated by magneto-transport measurements. Values of cubic and uniaxial anisotropies were obtained by analyzing the angular dependences of Hall effects based on magnetic free energy. The presence of strong cubic anisotropy gave four magnetic easy axes in the film's plane, resulting in two-step switching behavior in field scans of the planar Hall effect. Asymmetric loops containing four resistance plateaus were observed in minor scans of the planar Hall effect owing to the formation of stable multi-domain structures during magnetization reversal. Four-valued memory function based on the four observed Hall resistance states was demonstrated by using appropriate sequences of magnetic field pulses.

Three Dimensional Local Structure Analysis of ZnSnAs2:Mn by X-ray Fluorescence Holography

X-ray fluorescence holographic study on a room-temperature ferromagnetic semiconductor film of ZnSnAs2:Mn was performed using a strong X-ray beam of third generation synchrotron radiation of SPring-8. The real space reconstructions of the environments around Mn atoms were successfully visualized from the observed holograms despite the very small amount of Mn atoms. The reconstructions revealed that the Mn atoms occupy the cation (Zn or Sn) site.

Three Dimensional Local Structure Analysis of ZnSnAs2:Mn by X-ray Fluorescence Holography

X-ray fluorescence holographic study on a room-temperature ferromagnetic semiconductor film of ZnSnAs2:Mn was performed using a strong X-ray beam of third generation synchrotron radiation of SPring-8. The real space reconstructions of the environments around Mn atoms were successfully visualized from the observed holograms despite the very small amount of Mn atoms. The reconstructions revealed that the Mn atoms occupy the cation (Zn or Sn) site.

Controllable spin-polarized electrical transport in wide-band-gap oxide ferromagnetic semiconductors

A family of wide-band-gap ternary oxide ferromagnetic semiconductor films with high transition metal concentration was prepared. The resistivity of these films can be changed up to four orders of magnitude by varying the composition or the concentration of the oxygen vacancies. Moreover, all these films show common features in electrical transport, i.e., Mott variable range hopping (VRH) in the lower resistivity range, Efros VRH in the middle resistivity range, and “hard gap” resistance in the higher resistivity range. The above phenomena are well understood by considering the relative magnitude of three characterization lengths, i.e., Coulomb screening length, localization length of the carriers, and optimal hopping distance. Furthermore, spin polarization ratio of these magnetic semiconductors was obtained by fitting the experimental results of electrical transport. Therefore, the wide gap oxide ferromagnetic semiconductors with controllable spin-polarized electrical transport are expected to have application in spintronics devices as a spin injection source.

Magneto-transport Properties of GaMnAs:Si FerromagneticSemiconductors

The magnetic properties of a series of GaMnAs:Si ferromagnetic semiconductor films, in which the Mn concentration ranges from 7% to 10%, were investigated by using magneto-transport measurements. The temperature dependence of the resistivity revealed a systematic increase in the Curie temperature (T c) with increasing Mn concentration in the series. Since the T0 of the undoped GaMnAs ferromagnetic semiconductor decreases with increasing Mn concentration above 6%, the observation of a systematic increase of T c with increasing Mn concentration in our GaMnAs:Si series indicates the effectiveness of our counter doping for the incorporation of a a large amount of 7% Mn in the system. The field scan of the planar Hall effect (PHE) showed a typical two-step switching behavior at low temperatures, indicating the presence of a strong cubic anisotropy. The switching fields, however, systematically decreased with increasing Mn concentration in the series. The angular dependences of the switching fields were fitted by using the magnetic free energy and Cowburn's model to obtained the domain pinning energy, which showed systematically smaller values as the Mn concentration of the sample was increased. The temperature dependences of the pinning energies indicated a change in the uniaxial anisotropy from the [110] to the [110] direction with increasing Mn concentration in the series.

Four Stable Magnetization States Formed in the Single Layer of GaMnAs Ferromagnetic Film

AbstractA GaMnAs ferromagnetic semiconductor film under compressive strain has strong biaxial in-plane anisotropy, which generates four stable magnetization directions at a zero magnetic field. This feature results in a double switching behavior in the main loop of planar Hall resistance (PHR) spectrum. The minor scan of PHR measurement exhibited staggered asymmetric loops due to the formation of the stable muti-domain structures. We demonstrated the observed four stable PHR states can be served as quaternary logic states for spin memory device.

3DAP analysis of (Ga,Mn)As diluted magnetic semiconductor thin film

The distribution of Mn in a Ga 0.963Mn 0.037As ferromagnetic semiconductor film has been characterized by the three-dimensional atom probe (3DAP) technique. Atom probe specimens were directly prepared from the (Ga,Mn)As film grown epitaxially on a p-type GaAs substrate by the lift-out technique using a scanning electron microscope/focused ion beam system. The atom probe elemental map revealed that the Mn atoms in the Ga 0.963Mn 0.037As are uniformly dissolved without forming any nanometer-sized clusters.

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.