Variable magnetic field and temperature magnetic force microscopy

Magnetic force microscopy (MFM) studies of epitaxial MnAs films on GaAs(001) have been performed as a function of the applied magnetic field and the sample temperature. For this purpose, we combined a stable variable-temperature sample stage with a compact magnet assembly to fit a commercial magnetic force microscope. In order to keep the thermal drift that affects MFM measurements low, we employed a permanent magnet that can be rotated in a yoke assembly guiding the magnetic flux to the sample. PACS 68.37.Rt; 68.35.Rh; 75.70.-i; 75.70.Kw     

Direct observation of magnetization reversal of epitaxial MnAs films

The magnetization reversal of MnAs epitaxial films on GaAs (0 0 1) substrates was investigated using a Kerr microscope. The direct observation of the change in domain structure under magnetic fields revealed characteristic magnetization reversal process of MnAs films with a ladder-type domain structure. The nucleus of the magnetization reversal region appears and propagates to neighboring α-MnAs lines, and then the ladder-type structures cover all over the surface. Finally the domain wall displacement occurs to expand the domain. The change in magnetic domain reflects the characteristic ridge/groove structures of MnAs films.     

Magnetic pole pinning at rectangular defects on MnAs/GaAs(0 0 1)

Strong magnetic poles at characteristic rectangular defects have been observed using a magnetic force microscope on a MnAs(  1 0 0) thin film with the thickness of 30 nm. The MnAs thin film was epitaxially grown on a GaAs(0 0 1) substrate. The magnetic poles were in one-arranging direction, being independent of the magnetization direction of the film. The poles were pinned at the edges of the rectangular defects until just below the Curie temperature, and formed a stable magnetic-field loop on the MnAs surface. The stability of the magnetic pole pinning shows the distinctive feature of the magnetic domain structure on the surface with a strong anisotropy, which was built in the heterostructure of MnAs and GaAs.     

Epitaxial ferromagnetic thin films and heterostructures of Mn-based metallic and semiconducting compounds on GaAs

We present two approaches to integrate magnetic materials with III–V semiconductors. One is epitaxial ferromagnetic metallic films and heterostructures on GaAs (0 0 1) substrates. Although crystal structure, lattice constant, chemical bonding and other properties are dissimilar, ferromagnetic hexagonal MnAs thin films and MnAs/NiAs ferromagnet/nonmagnet heterostructures (HSs) are grown on GaAs by molecular beam epitaxy (MBE). Multi-stepped magnetic hysteresis are controllably realized in MnAs/NiAs HSs, making this material promising for the application to multi-level nonvolatile recording on semiconductors. The other approach is to prepare a new class of GaAs based magnetic semiconductor, GaMnAs, by low-temperature molecular beam epitaxy (LT-MBE) on GaAs (0 0 1). New III–V based superlattices consisting of ferromagnetic semiconductor GaMnAs and nonmagnetic semiconductor AlAs are also successfully grown. Structural and magnetic properties of these new heterostructures are presented.     

Ferromagnetism of MnAs studied by heteroepitaxial films on GaAs(001)

Thin epitaxial films of MnAs--promising candidates for the spin injection into semiconductors--are well known to undergo simultaneously a first-order structural and magnetic phase transition at 10-40 degrees C. The evolution of stress and magnetization of MnAs/GaAs(001), both measured quantitatively with our cantilever beam magnetometer at the coexistence region of alpha-MnAs and beta-MnAs, reveal an orthorhombically distorted unit cell of the ferromagnetic phase, which provides important clues on the origin of ferromagnetism in MnAs.     

Direct observation of room temperature magnetism in (In,Mn)As thin films by magnetic force microscopy

Variable-temperature magnetic force microscopy (MFM) has been performed over the temperature range of 298-348 K on ferromagnetic (In,Mn)As thin films deposited by metal-organic vapor phase epitaxy (MOVPE). Ferromagnetic domains were observed with submicron resolution in both single and two phase (In,Mn)As films, persisting up to 328 K. Isolated cylindrical domains ranging from 100 to 350 nm in diameter with densities of 2-5 × 10⁸ cm⁻² were observed in phase pure films. Longer range magnetic order, in the form of ribbon-like domains up to 1 μm in length, are present in the regions between the cylindrical domains. Two phase (In,Mn)As films produced a well-resolved complex domain structure consisting of 180° parallel and antiparallel domains. Excellent agreement between the temperature dependence of the relative magnetization obtained by MFM and superconducting quantum interference device measurements was observed.     

Unforeseen properties of MnAs epilayers grown on GaAs semiconductor

This paper reviews some recent works performed on MnAs/GaAs thin films and other related structures grown by molecular beam epitaxy. The impact of epitaxy on the magneto-structural properties of MnAs and possible applications of MnAs epilayers are discussed. A brief account of recent results obtained on the magneto-transport in MnAs/GaAs/MnAs magnetic tunnel junctions is also given, highlighting several appealing and promising properties of this system for spintronics applications.     

Magnetization reversal of elliptic Co/Si/Co nanodisks in the field of a magnetic-force microscope probe

The processes of local magnetization reversal of elliptic Co/Si/Co nanodisks under the action of a nonuniform magnetic field of a magnetic-force microscope (MFM) probe have been investigated. The specific features of the distribution of the phase MFM contrast from particles with ferromagnetic and antiferromagnetic configurations of the magnetic moments in neighboring Co layers have been discussed. It has been shown experimentally that, under the action of the probe field, there occur orientational transitions of two types: transitions from the ferromagnetic configuration to the antiferromagnetic configuration due to the reorientation of the magnetization of the upper layer and transitions in the antiferromagnetic configuration with a change in the orientation of the magnetic moment in both ferromagnetic layers. The presented results of micromagnetic simulation of the processes of transformation of the magnetization in such particles under the action of the MFM probe field explain the main regularities of the magnetization reversal processes.     

Magnetic behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.     

Thickness dependent magnetic and structural properties of Co films grown on GaAs (100)

The structural, magnetic and transport properties measurements carried out on Co thin films deposited by electron beam evaporation on GaAs substrate as a function of layer thickness ranging from 50 Å to 1000 Å are presented here. Structural measurements show the film to be amorphous in nature at lower thickness which becomes crystalline at higher thickness. Magnetic measurements show an increase in saturation magnetization (M_S) with film thickness. M_S values are found to vary from 521 emu/cm³ to 1180 emu/cm³ for thicknesses ranging from 50 Å to 1000 Å. The coercivity and saturation field value shows a systematic decrease up to 600 Å thickness and increase thereafter. Various microstructural parameters were also calculated using GIXRR technique. A clear grain growth is observed in AFM technique with film thickness and its influence on transport properties was also seen. Different surface morphology and magnetic domain structures were obtained on different thin film samples by AFM and MFM techniques, respectively. XPS measurements reveal formation of CoAs phase at the interface between Co and GaAs. All these results are discussed and interpreted in detail in this communication.     

Magnetic and electrical transport properties of delta-doped amorphous Ge:Mn magnetic semiconductors

We report on the growth and characterization of delta-doped amorphous Ge:Mn diluted magnetic semiconductor thin films on GaAs (0 0 1) substrates. The fabricated samples exhibit different magnetic behaviors, depending on the Mn doping concentration. The Curie temperature was found to be dependent on both the Mn doping concentration and spacing between the doping layers. A sharp drop in magnetization and rise in resistivity are observed at low temperature in samples with high Mn doping concentrations, which is also accompanied by a negative thermal remanent magnetization (TRM) in the higher temperature range. The temperature at which the magnetization starts to drop and the negative TRM appears show a correlation with the Mn doping concentration. The experimental results are discussed based on the formation of ferromagnetic regions at high temperature and antiferromagnetic coupling between these regions at low temperature.     

Structural and magnetic properties of ferromagnetic In1−xMnxAs1−yPy layers

The structural and magnetic properties of epitaxial In_1−xMn_xAs_1−yP_y quaternary layers with Mn content ranging from 0.01 to 0.04 and phosphorous content ranging from 0.11 to 0.21 were studied. X-ray diffraction indicated that the films were two phase consisting of an InMnAsP solid solution and hexagonal MnAs nanoprecipitates. Addition of phosphorus promoted precipitate formation. Films were ferromagnetic showing hysteretic behavior in the field dependence of magnetization at 5 and 298 K. From field-cooled magnetization measurements ferromagnetic transitions were observed at 280 and 325 K. The zero field-cooled magnetization versus temperature measurements showed irreversibility for T<300 K that was attributed to the presence of MnAs nanoprecipitates. The calculated coercivity using the Neel model was 1380 G compared to the experimental value of 380 G at 5 K. The difference was attributed to a strong inter-cluster exchange that stabilizes the ferromagnetic state.     

Magnetic properties of MnAs nanocrystals embedded in GaAs

A phenomenological model for explaining the magnetic properties of MnAs nanocrystals embedded in GaAs is proposed. It is shown that experimental data of DC magnetization as a function of temperature, obtained according to zero-field-cooled and field-cooled protocols, can be understood assuming a transition of the system from a low temperature state in which very slow dynamics is observed (frozen state) to a high-temperature state in which dynamics is fast (quasi superparamagnetic state).     

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.     

Local observation of reverse-domain superconductivity in a superconductor-ferromagnet hybrid

Nanoscale magnetic and superconducting properties of the superconductor-ferromagnet Nb/PbFe12O19 hybrid were studied as a function of applied magnetic fields. Low-temperature scanning laser microscopy (LTSLM) together with transport measurements were carried out in order to reveal local variations of superconductivity induced by the magnetic field template produced by the ferromagnetic substrate. Room temperature magnetic force microscopy (MFM) was performed and magnetization curves were taken at room and low temperature to investigate the magnetic properties of the hybrid. Comparative analysis of the LTSLM and the MFM images has convincingly demonstrated the presence of the reverse-domain superconductivity.     

Fe/MnAs bilayers: Magnetic anisotropy and the role of the interface

We have studied different aspects of the magnetic behavior of Fe(5 nm)/MnAs(100 nm) bilayers epitaxially grown on GaAs(1 0 0). Ferromagnetic resonance (FMR) measurements were performed in order to characterize the magnetic anisotropies of the films and the interlayer coupling between them. The chemical composition of the interface was investigated by X-ray photoemission spectroscopy (XPS).     

Evolution of ferromagnetism with sputtering gas in Mn:ZnO films

The effect of introducing nitrogen and oxygen in the sputtering working gas on the magnetic properties of Mn:ZnO thin films has been investigated. A set of films has been characterized by X-ray diffraction, X-ray absorption near edge structure (XANES) and optical absorption spectroscopy to correlate its magnetic properties with Mn electronic characteristics. Mn²⁺ substituting Zn²⁺ in the wurtzite structure has been obtained for the films presenting considerably high saturation magnetization values. The change in the magnetic behaviour seems to be associated with the electronic carrier density in the films.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

Electrically driven spin dynamics of paramagnetic impurities

The spin dynamics of dilute paramagnetic impurities embedded in a semiconductor GaAs channel of a conventional lateral spin valve has been investigated. It is observed that the electron spin of paramagnetic Mn atoms can be polarized electrically when driven by a spin valve in the antiparallel configuration. The transient current through the MnAs/GaAs/MnAs spin valve bears the signature of the underlying spin dynamics driven by the exchange interaction between the conduction band electrons in GaAs and the localized Mn electron spins. The time constant for this interaction is observed to be dependent on temperature and is estimated to be 80 ns at 15 K.     

Specific features of observing magnetization inhomogeneities on the surface of permalloy thin films by means of highly sensitive magnetic-force-microscopy probes

The results of studying regions of inhomogeneous magnetization on the surface of permalloy thin films with the use of fabricated highly sensitive probes of magnetic force microscopy (MFM) are presented. The technological features of manufacturing MFM probes with a high sensitivity to magnetic-field gradient are analyzed. Regions of ordering of the vertical component of the magnetic field are revealed, and domain walls are visualized in the thin films under study. Nanoscale measurements of the domain-wall thicknesses are performed.