MnBi Films Research Articles

Control of magnetic properties of MnBi and MnBiCu thin films by Kr+ ion irradiation

Mn52Bi48 (15 nm) and Mn54Bi24Cu21 (15 nm) thin films were prepared by the magnetron sputtering and vacuum annealing at 350 °C, and the variations of their structures and magnetic properties with 30 keV Kr+ ion irradiation were studied. The MnBi and MnBiCu films exhibited saturation magnetizations Ms of 180 emu/cc and 210 emu/cc, the coercivities Hc of 10 kOe and 3.4 kOe, respectively. The Ms and Hc of the MnBi abruptly vanished by the irradiation of ion dose at 3 × 1014 ions/cm2, while those of the MnBiCu film gradually decreased with increasing the ion dose and became zero at 5 × 1013 ions/cm2. The different trend on the ion irradiation between MnBi and MnBiCu films is understood by the surface structure of the film, i.e., the MnBi has convex islands on its surface, which protect the underneath NiAs-type MnBi from the irradiation, while the MnBiCu has rather flat surface, and its crystal structure was uniformly modified by the irradiation. From the surface flatness and the uniformity of the MnBiCu film, as well as the low annealing temperature of 350 °C, it was concluded that the MnBiCu film is one of the attractive materials for high-density ion irradiation bit patterned media.

Anomalous Hall effect and electron transport in ferromagnetic MnBi films

The electron transport properties of highly c-axis oriented MnBi thin films of various thicknesses have been investigated. Samples are metallic but the low temperature resistivity shows an unusual T3 dependence. Transverse Hall effect measurements show that both the ordinary and anomalous Hall coefficients decrease with decreasing temperature below 300 K, but the ordinary Hall coefficient (R0) undergoes a sign reversal around 105 K, where the magnetic anisotropy also changes sign. Analysis of the Hall data for various samples shows that the anomalous Hall coefficient (Rs) exhibits a strong ρ2 dependence, where ρ is the longitudinal resistivity.

Spin correlations and electron transport in MnBi:Au films

The structural, magnetic, and electron transport properties of Mn55−xAuxBi45 (x = 0, 4.5) thin films prepared by magnetron sputtering have been investigated. The magnetization of the MnBi films decreases and the coercivity increases due to Au doping. The temperature dependence of resistivity between 2 to 300 K shows that the films are metallic but the 4.5% Au-doped film shows a Kondo behavior with resistance minimum at 10.2 K. The magnetoresistance is anisotropic and the positive transverse magnetoresistance is significantly enhanced (16.3% at 70 kOe) by Au doping. We interpret these data in terms of a model in which Au atoms preferentially substitute for Mn atoms on the Mn lattice, and some Mn atoms are displaced to interstitial sites in the NiAs structure. These interstitial Mn atoms are coupled antiferromagnetically to the Mn atoms on the original Mn lattice leading to the large decrease in magnetization, Kondo effect, and the positive magnetoresistance.

Transport spin polarization of high Curie temperature MnBi films

We report on the study of the structural, magnetic and transport properties of highly textured MnBi films with the Curie temperature of 628K. In addition to detailed measurements of resistivity and magnetization, we measure transport spin polarization of MnBi by Andreev reflection spectroscopy and perform fully relativistic band structure calculations of MnBi. A spin polarization from 51\pm1 to 63\pm1% is observed, consistent with the calculations and with an observation of a large magnetoresistance in MnBi contacts. The band structure calculations indicate that, in spite of almost identical densities of states at the Fermi energy, the large disparity in the Fermi velocities leads to high transport spin polarization of MnBi. The correlation between the values of magnetization and spin polarization is discussed.

Electrodeposition and characterization of manganese–bismuth system from chloride based acidic bath

In this work, Mn–Bi system has been successfully electrodeposited on Cu/Si substrates from aqueous ammonium chloride containing electrolyte. A thermodynamic study of the electrolysis bath highlights the possible formation of manganese complexes ( MnCl + , MnCl 2 , MnC l 3 − ) and bismuth complexes ( BiCl 2+ , BiC l 2 + , BiCl 3 , BiC l 4 − , BiC l 5 2 − , BiC l 6 3 − ) . The mechanism process involving Mn and Bi electrodeposition is investigated by cyclic voltammetry. Mn–Bi films can be grown under potentiostatic control. The physico-chemical, morphological and structural characterizations of the deposits were carried out by scanning electron microscopy (SEM) and X-rays analysis (XRD). The results reveal a granular surface quality and a heterogeneous chemical composition of the films. The energy dispersive spectroscopy (EDS) analysis reveals the presence of manganese and bismuth peaks whose relative intensities vary according to the imposed potential and the position on the sample. The X-rays diffraction analysis showed three different crystalline phases: α-manganese (body centred cubic system), γ-manganese (body centred tetragonal system) and bismuth (rhombohedral system).

Growth of Mn-Bi films on Si(111): Targeting epitaxial MnBi

MnBi is of high interest for application as active layer in magneto-optic data storage media for several decades. Here the molecular-beam-epitaxy growth of Mn-Bi films on Si(111) surfaces is investigated under ultrahigh vacuum conditions by means of reflection high-energy electron diffraction, low-energy electron diffraction with spot-profile analysis, Auger-electron spectroscopy, magneto-optic Kerr spectroscopy, and transmission electron microscopy to reveal the microscopic film structure, film composition, and magnetic properties. The film parameters and growth protocols are varied but none of the chosen conditions lead to the formation of MnBi from the two elements. The affinity of Mn to the substrate material Si must be regarded as major reason for the fact that no MnBi forms.

Micromagnetism and high temperature coercivity of MnBi/Al multilayers

The micromagnetic properties of multilayered MnBi/Al films have been investigated and compared to pure MnBi thin films. Pure MnBi films reveal an anomalous increase of the high temperature coercivity, which has been explained on the basis of a hybrid domain wall pinning model. The multilayer-type preparation of MnBi/Al thin films results in significantly reduced MnBi particle size of approximately 40 nm. The smaller particle size leads to a change of the dominant magnetization reversal process from one driven by domain wall movement toward coherent rotation. This was investigated via magnetic force microscopy imaging and micromagnetic calculations. The absence of domain walls during magnetization reversal results in a clear suppression of the increase of the high temperature coercivity observed in pure MnBi films.

Temperature dependence of structure and magnetic properties in MnBi-based alloy films

MnBi alloy films, which have large magnetocrystalline anisotropy and high remanence, are synthesized with small dopants of rare-earth elements (RE=Dy, Sm, or Tb) by vacuum evaporation method. In contrast with most ferromagnetic materials, the low temperature phase (LTP) of MnBi film shows a large coercivity, Hc, over an extended temperature range. The hexagonal crystal structure of MnBi LTP thus has good thermal stability. The experiments found that doping further extends the temperature range through which the LTP is stable. Moreover, MnBi-doped films still exhibit square hysteresis loops at high temperatures. We attribute the unusual temperature dependence of the coercivity to an increased perpendicular anisotropy field, Ha, with temperature. For this reason, MnBi-based alloy films have great potential as permanent magnets at high temperatures.

High-temperature coercivity and Kerr spectroscopy on MnBi/Al multilayers

The magneto-optic properties of multilayered MnBi/Al films have been investigated and compared to pure MnBi films. The Al interlayers act as diffusion barrier during the MnBi formation process leading to a reduced MnBi particle size of approximately 50 nm. The smaller particle size changes the dominant magnetization reversal process from domain wall movement towards coherent rotation. This leads to a suppression of the anomalous increase of the high-temperature coercivity observed in pure MnBi. The comparison of the experimental polar Kerr spectra with theory elucidates the role of oxygen in MnBi films.

MnBi Thin Films Prepared by Molecular-Beam Epitaxy Technique

The Epitaxial growth of MnBi films has been studied by means of molecular beam epitaxy (MBE) technique. Single crystals of MgO (001) and Al2O3 (00.1) were employed as substrates. Without any buffer layers, MnBi thin films grown on these substrates were polycrystalline. We then prepared MnBi films using either a Au or a Cu layer as a buffer on the substrates. Using Au buffer layers (10.2)-oriented MnBi films were obtained, yet the magnetic property of these films are poor. On the other hand, using Cu (111) buffer layers MnBi films with a sufficient flatness and better magnetic property were obtained, although some trace of (10.2)-orientated grains was observed.

Epitaxial Growth of MnBi Films and Characterization

After reviewing the state of the art of the preparation and the properties of MnBi thin films, we report on the first successful epitaxial growth of MnBi single crystalline films with very high magnetic energy products and coercivity fields up to 20 kOe. The best films have been grown by molecular beam epitaxy on buffered BaF2 substrates. The epitaxy on GaAs substrates will also be discussed.

Structure, magnetic, and magneto-optical properties of MnBi films grown on quartz and (001)GaAs substrates

MnBi films are prepared on quartz and on (001) orientated GaAs substrates by molecular beam epitaxy in ultrahigh vacuum environment. Both kinds of substrates are used simultaneously. The influence of the substrate material is investigated with respect to the structural, the magnetic, and magneto-optical properties of the MnBi films. By evaporating a 100 nm thick SiOx buffer layer the homogeneity of the composition is improved and a grain size of about 100 nm is achieved without adding other elements. In contrast to previous investigations, our measured magneto-optical Kerr rotation spectra show no Kerr rotation peak near 3.35 eV. These results confirm theoretical predictions whereupon this peak is attributed to oxygen which occupies interstitial sites in the regular MnBi lattice.

Decoupling the structural and magnetic phase transformations in magneto-optic MnBi thin films by the partial substitution of Cr for Mn

The first-order nature of the magnetic phase transformation at 360 °C and the presence of a Bi-rich eutectic at 265 °C have inhibited the application of MnBi thin films for high density magneto-optical data storage. It is suggested that partial substitution of Cr for Mn should both lower the Curie temperature, Tc, and decouple the lattice and magnetic transitions so as to allow reversible Curie point writing. It is found experimentally that 10% substitution of Cr for Mn reduces the apparent Tc to ∼250 °C while retaining a Kerr rotation angle greater than 1° at 633 nm, as measured through the silica glass substrate. Observations of increasing Hc with temperature in both MnBi and (Mn,Cr)Bi thin films suggest that the low-temperature phase is ferrimagnetic.

Method to measure spectra of both the magneto-optical Kerr and the Faraday effect

Details of an integrated apparatus to measure spectra of both the magneto-optical Kerr and the Faraday effect by the use of a rotating analyzer are given. The absolute values of both the Kerr rotation angle theta(k) and the ellipticity epsilon(k) can be determined by means of an achromatic quarter-wavelength retarder. The system is simple and reliable, and fully controlled by a computer. The measured values of the Kerr and Faraday effects for one MnBi(Al) film sample and for one GaP crystal sample are given. Since the magnitude of the Faraday rotation signal is proportional to the sample thickness, one must be careful in determining the fundamental energy gap by use of the Faraday spectrum only. Employment of the Fourier-transform technique will enable the system to measure both the Kerr and the Faraday effect in a wider data range from one-hundredth to a few hundred degrees. (C) 1997 Society of Photo-Optical Instrumentation Engineers.

Suppression of the increase of high-temperature coercivity in MnBi thin films by Al interlayers

By tailoring the microcrystalline structure of MnBi films, using Al interlayers, a reduction of the high-temperature coercivity by a factor of 3 is achieved. The separation of Bi/Mn bilayers by Al interlayers acts as a diffusion barrier perpendicular to the surface. After annealing, the MnBi layers contain single-domain particles surrounded by an Al matrix exhibiting no significant increase of the coercive field with increasing temperature.

Magnetic behavior of MnBi0.47Al0.15 alloy films

MnBiAl films were prepared by vacuum interdiffusion alloying. We report on the magneto-optical properties and low temperature magnetization of this material. By eliminating the Faraday contribution from the glass substrates, a remanant Kerr rotation of 3.22° at 632.8 nm is measured at room temperature. At a wavelength of 376 nm, the remanent Kerr rotation is 2.25°. Superconducting quantum interference device magnetometer examination showed that there is no spin transition at low temperature for the MnBiAl films. By decreasing the temperature from 280 to 5 K, the perpendicular remanant magnetization of the MnBi0.47Al0.15 films remains along the c axis and increases about 12% in magnitude, while MnBi changes its easy axis from the c axis to the base plane at around 84 K. This suggests that both the lattice shrinkage and chemical interactions among Mn, Bi, and Al atoms play important roles in the magnetic properties of MnBi0.47Al0.15 alloy films.

Magneto-optic study of the high-temperature coercivity of MnBiAl and MnBiPt films

We present evidence that domain-wall pinning in MnBi films is responsible for the unusual increase of the high-temperature coercivity. The MnBi crystallite size has been reduced by depositing Bi/Mn/Al multilayers. The average diameter of the MnBi particles is smaller than the critical diameter of single-domain particles. Therefore, domain-wall pinning does not enhance the high-temperature coercivity as observed for pure MnBi films.

Dependence of magnetization reversal on the crystallite size in MnBi thin films: Experiment, theory, and computer simulation

A micromagnetic model is proposed to explain the magnetization reversal of thermally evaporated MnBi thin films. The model assumes that the films consist of grains on a square lattice with perpendicular magnetic anisotropy. In order to describe the magnetic reversal of a particular grain, dipole coupling, wall energy, Zeeman energy, and an energy barrier for the reversal of a single grain are taken into account. Disorder is included by random fluctuations of the lateral grain size in the Zeeman-energy term. The simulation is carried out by using a Monte Carlo method. The experimentally observed decrease in coercivity with increasing film thickness is accurately described by the model and can be explained by an increase in lateral grain size. The asymmetry of the slope of the hysteresis curve of thicker MnBi films can be understood in terms of disorder. In the presence of fluctuations in grain size, large grains reverse more easily than small ones leading to an asymmetry of the slope of the hysteresis curve. \textcopyright{} 1996 The American Physical Society.

Influence of Al capping layers on growth, topography, and magnetic properties on MnBi thin films

Bi/Mn bilayers have been deposited on fused-quartz substrates at room temperature and annealed at elevated temperatures of 300–380 °C. To investigate the influence of capping layers on MnBi crystallite size and magnetic properties, some of the Bi/Mn bilayers were protected by Al and SiOx layers. The coercive fields of the resulting MnBi films without a protective layer reach values of up to 1.25 T. In case of depositing an Al capping layers prior to annealing, the coercive fields are decreasing strongly showing coercive fields in the range of 0.6 T. In contrast, using SiOx as a capping layer, the Kerr hysteresis loops show a nonlinearity near the coercive field indicating an inhomogeneous film. The change in the coercive field is explained by the influence of capping layers on the MnBi crystallite size during annealing.

Optical and magneto-optical properties of MnBi film.

The spectral features of Kerr rotation and ellipticity of MnBi film have been found to greatly change with temperature from 85 to 475 K, and the Kerr rotation at 700 nm increases over 50%. And the complex refractive index at room temperature and the dielectric and conductivity tensors at 85, 300, and 475 K have been determined with the reflectance of an opaque film, the transmittance of a semitransparent film and the Kerr effect. There are two distinct peaks in the \ensuremath{\omega}${\mathrm{\ensuremath{\sigma}}}_{\mathit{xy}}^{\mathrm{\ensuremath{''}}}$ spectrum, corresponding well to that of Kerr rotation. By comparing those results with the reported calculations, it becomes clear that the Kerr rotations originate principally from the interband transitions of Mn 3d\ensuremath{\downarrow} electrons (\ensuremath{\Elzxh}\ensuremath{\omega}2.5 eV) and of Mn 3d\ensuremath{\uparrow} electrons (\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\gtrsim}2.5 eV). And the changes of the exchange splitting and the spin-orbit interaction strength are responsible for the strong temperature dependence of Kerr effect. Specific Faraday spectra at different temperatures have also been calculated with the above optical and dielectric constants and are much different from the reported data. \textcopyright{} 1996 The American Physical Society.