Epitaxial MnAs Films Research Articles

In Depth Spatially Inhomogeneous Phase Transition in Epitaxial MnAs Film on GaAs(001).

Most studies on MnAs material in its bulk form have been focused on its temperature-dependent structural phase transition accompanied by a magnetic one. Magnetostructural phase transition parameters in thin MnAs films grown on substrates present however some differences from the bulk behavior, and local studies become mandatory for a deeper understanding of the mechanisms involved within the transition. Up to now, only surface techniques have been carried out, while the transition is a three-dimensional phenomenon. We therefore developed an original nanometer scale methodology using electron holography to investigate the phase transition in an epitaxial MnAs thin film on GaAs(001) from the cross-section view. Using quantitative magnetic maps recorded at the nanometer scale as a function of the temperature, our work provides a direct in situ observation of the inhomogeneous spatial distribution of the transition in the layer depth and brings new insights on the fundamental transition mechanisms.

Observation of magnetic domain structures in epitaxial MnAs film on GaAs(001) with temperature hysteresis

The saturation magnetization of MnAs films epitaxially grown on GaAs(001) substrates exhibited temperature hysteresis in the temperature range of 10–45 °C. We investigated the cause of the temperature hysteresis using temperature- and field-controlled magnetic force microscopy (MFM). The MFM results showed that inside the ferromagnetic α-MnAs stripes of the film at 30 °C, 180° domains were formed during cooling but a single domain state was developed during heating. Despite the cooling procedure, a single domain state was found inside the α-MnAs stripes when a magnetic field of 800 Oe was applied. From these results, the spontaneous magnetization of the α-MnAs phase was ascribed to the temperature hysteresis.

Magnetocrystalline anisotropy and uniaxiality of MnAs/GaAs(100) films

We present an investigation of the magnetic behavior of epitaxial MnAs films grown on GaAs(100). We address the dependence of the magnetic moment, ferromagnetic transition temperature (Tc), and mag ...

Magnetoresistance of a Spin Metal–Oxide–Semiconductor Field-Effect Transistor with Ferromagnetic MnAs Source and Drain Contacts

Transport characteristics were investigated in a spin metal–oxide–semiconductor field-effect transistor (spin MOSFET) with ferromagnetic MnAs source and drain (S/D) contacts. A bottom-gate type spin MOSFET was fabricated by photolithography using an epitaxial MnAs film grown on a silicon-on-insulator (SOI) substrate. In-plane magnetoresistance showed a square like hysteretic behavior, when measurements were performed with constant source–drain and source–gate biases. From the comparison with the magnetization-related resistance change resulting from the MnAs contacts, a highly possible origin of the feature obtained for the spin MOSFET is the spin-valve effect originating from the spin-dependent transport in the Si channel.

Direct observation on the temperature-dependent change of magnetic domains in epitaxial MnAs film on GaAs (0 0 1)

We present a systematic change of the magnetic domain structure with temperature in epitaxial ferromagnetic MnAs film on GaAs (0 0 1), observed in a wide temperature range of 15–45 °C by magnetic force microscopy. Interestingly, it is found that, as temperature increases, the domain structure within the ferromagnetic α-MnAs stripes shows a mixture of head-on and simple domains at 15 °C and then, takes a complete transition to simple ones above 15 °C. This change could be understood by change in the demagnetizing factor of the cross-section of the ferromagnetic stripes with temperature.

First order phase transition in MnAs nanodisks

AbstractThe first order phase transition between ferromagnetic α‐MnAs and paramagnetic β‐MnAs in nanoscale disks etched out of an epitaxial MnAs film on GaAs(001) is studied by means of X‐ray diffraction and magnetic force microscopy. The temperature hysteresis loop is observed to be expanded with decreasing disk size. The hysteresis is absent in the original continuous epitaxial film from which the disks are fabricated. The hysteresis is explained as a competition between the energy gain due to the phase transition and the energy loss due to the elastic strain. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Special Issue: Frontiers in Molecular‐Beam Epitaxy toward Novel Devices

AbstractThis special issue of physica status solidi (b) entitled “Frontiers in Molecular‐Beam Epitaxy toward Novel Devices” with 27 contributions from leading semiconductor scientists has been prepared to honor Klaus H. Ploog, Paul Drude Institute for Solid State Electronics Berlin, on the occasion of his 65th birthday.The cover picture shows an azimuthal reflection high‐energy electron diffraction (RHEED) scan of a 6.5 monolayer thick MnAs film on GaAs [1] overlaid on a GaAs wafer. The diffraction pattern is a cut through the reciprocal lattice of the epitaxial MnAs film, reflecting the hexagonal structure of MnAs. The growth of MnAs by molecular‐beam epitaxy and its structural and magnetic properties have been extensively studied under Klaus Ploog's directorship of the Paul Drude Institute. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magnetostructural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thickness-dependent magnetic domain change in epitaxial MnAs films on GaAs(001)

The authors report the change of the magnetic domain structure, dependent on the film thickness of MnAs films epitaxially grown on GaAs(001), investigated by magnetic force microscopy. Interestingly, as the film thickness decreases, the domain structure within the ferromagnetic α-MnAs stripes changes from a head-on domain structure to a simple 180° one around a thickness of 250nm. This result is understood by the change in the demagnetizing factor of the ferromagnetic stripes with the film thickness.

Surface reconstructions of epitaxial MnAs films grown on GaAs(111)B

This study makes reference to the surface structure of MnAs(0001) epilayers grown by molecular beam epitaxy through a multitechnical method that combines scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), grazing incidence x-ray diffraction (GIXD), and ab initio calculations. Two surface reconstructions were studied, i.e., the $(2\ifmmode\times\else\texttimes\fi{}2)$ and the $(3\ifmmode\times\else\texttimes\fi{}1)$. Photoemission measurements provided quantitative information about (i) the surface coverage of both surfaces and (ii) the atomic environment of surface atoms. An atomic structural model has been proposed for the observed reconstructions. On the basis of STM observations and ab initio calculations the $(2\ifmmode\times\else\texttimes\fi{}2)$ phase is constituted by As trimers adsorbed on an underlying Mn layer of bulk MnAs, while x-ray diffraction analysis, coupled with STM, showed that the $(3\ifmmode\times\else\texttimes\fi{}1)$ reconstructed surface is constituted of long and narrow As chains aligned with the ⟨110⟩ GaAs substrate equivalent directions.

Effect of Magnetic Field on the Magnetic Domain Structure of MnAs Film on GaAs(001)

We have observed the magnetic domain evolution under an external magnetic field in an epitaxial MnAs film on GaAs(001) by magnetic force microscopy. Owing to the strain involved, the ferromagnetic alpha-MnAs and the paramagnetic beta-MnAs phases coexist as self-aligned stripes at room temperature. It was found that a complex magnetic domain structure appeared in the ferromagnetic alpha-phase region at the demagnetized state (H=0 Oe). As the magnetic field increased, the magnetic domain structure was gradually changed, and reached a completely saturated state at H=600 Oe. Especially, at H=300 Oe, the observed magnetic domain wall distribution is well matched with the topographical shape

Nucleation at the phase transition near 40°C in MnAs nanodisks

The phase transition near 40°C of both as-grown thin epitaxial MnAs films prepared by molecular beam epitaxy on GaAs(001) and nanometer-scale disks fabricated from the same films is studied. The disks are found to exhibit a pronounced hysteresis in the temperature curve of the phase composition. In contrast, supercooling and overheating take place far less in the samples of continuous layers. These phenomena are explained in terms of the necessary formation of nuclei of the other phase in each of the disks independent from each other. The influence of the elastic strains in the disks is reduced considerably.

Real-time direct observation of temperature-dependent domain reversal behavior in epitaxial MnAs film on GaAs(001)

We have investigated temperature-dependent domain reversal behavior in the MnAs film epitaxially grown on GaAs(001) using a magneto-optical microscope capable of real-time direct observation of domain evolution. Interestingly enough, the domain reversal in the temperature range of 20°C∼35°C shows the domain wall-motion process with the sawtooth type and, then, it changes to the nucleation-dominant process above 37.5°C. This change could be understood by the decrease of the dipolar interaction energy and the disconnection of the ferromagnetic α-MnAs stripes, induced by the decrease of the α-MnAs volume ratio with increasing temperature.

Temperature dependence of the growth morphology in molecular beam epitaxy grown MnAs

The necessity for employing a low substrate temperature for MnAs growth in molecular beam epitaxy has been clarified through an investigation for the effect of the growth temperature on the microscopic structural morphology. The atomic registry of MnAs films on GaAs substrate was examined by transmission electron microscopy. The MnAs films revealed a single-crystalline nature at ∼300 °C, but the morphology changed to polycrystalline with an increase in the substrate temperature. The variation in the degree of the magnetic anisotropy of the films was consistent with the change of the film morphology along with the substrate temperature. The uniaxial in-plane magnetic anisotropy of the epitaxial MnAs film was correlated with the surface reconstruction on the GaAs substrate.

Apparent split between magnetic and structural phase transitions in epitaxial MnAs films

We investigate the magnetic field dependence of the magnetic and structural phase transition of an epitaxial MnAs layer grown on GaAs (1 0 0) by using SQUID magnetometry and X-ray diffraction. The phase transition temperatures obtained from magnetic and structural results revealed an apparent splitting for magnetic field less than ∼1 kOe, while for higher magnetic fields a behavior similar to that reported for bulk MnAs is observed.

Apparent split between magnetic and structural phase transitions in epitaxial MnAs films

We investigate the magnetic field dependence of the magnetic and structural phase transition of an epitaxial MnAs layer grown on GaAs (1 0 0) by using SQUID magnetometry and X-ray diffraction. The phase transition temperatures obtained from magnetic and structural results revealed an apparent splitting for magnetic field less than ∼1 kOe, while for higher magnetic fields a behavior similar to that reported for bulk MnAs is observed.

Effect of strain on the local phase transition temperature of MnAs/GaAs(001)

We present measurements of the influence of local strain on the phase transition behavior of epitaxial MnAs films on GaAs(001). As shown previously, stripes of ferromagnetic α-MnAs and paramagnetic β-MnAs coexist around room temperature. Temperature-dependent atomic force and magnetic force microscopy reveals that the characteristic temperature T*, at which the as-grown films transform to the paramagnetic β-phase, is locally shifted up towards the value of unstrained bulk MnAs. The film areas exhibiting a higher T* were identified as regions in which the strain in the MnAs film was allowed to relax.

Low-energy electron microscopy/x-ray magnetic circular dichroism photoemission electron microscopy study of epitaxial MnAs on GaAs

Epitaxial MnAs films on GaAs(001) substrates are studied at room temperature and in the completely ferromagnetic state below room temperature with low-energy electron microscopy, x-ray magnetic circular dichroism photoemission electron microscopy, and low-energy electron diffraction. The combination of these techniques shows a clear relation between the two-phase structure of the layers and their magnetic domain structure.

Growth of nanosized MnAs/Si(111) magnetoelectronic heterostructures and their magnetooptical study

Thin (6–12 nm) epitaxial MnAs films were MBE-grown on Si(111) substrates under different technological conditions. The films feature essentially different surface morphology. This manifests itself in the formation, on the silicon surface, of hexagonal-shaped crystallites, whose dimensions vary depending on the growth conditions. The volume and surface magnetic properties of the films were studied using the magnetooptical Kerr effect and optical second harmonic generation. The Kerr effect was found to scale linearly with the effective thickness of the magnetic layer. The thickness of the magnetically disordered transition layer formed near the interface with the substrate was estimated. The surface and volume hysteresis properties of the films were found to be different. A contribution to the second-harmonic intensity was observed which is an odd function of magnetization. This effect originates from the interference of the magnetic and nonmagnetic contributions to the nonlinear polarization.

Epitaxial growth of MnAs on single-crystalline Mn–Zn ferrite substrates

We have grown MnAs thin films on (1 1 1), (1 1 0), and (1 0 0)Mn–Zn ferrite substrates by molecular beam epitaxy (MBE) and succeeded in growing epitaxial single-crystalline MnAs films with the growth plane of (0 0 0 1) on the (1 1 1)Mn–Zn ferrite substrates. Atomic force microscopy (AFM) measurements have revealed that the surface of the MnAs epitaxial film is atomically flat. The reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) measurements have shown that the MnAs epitaxial films have in-plane alignment of [1 1 2 ̄ 0] MnAs|| [1 1 ̄ 0] Mn–Zn ferrite. On the other hand, polycrystalline MnAs has grown on (1 1 0) and (1 0 0)Mn–Zn ferrite substrates even under the same growth conditions.