Extraordinary Hall Effect Measurements Research Articles

Blocking phenomena in granular magnetic alloys through magnetization, Hall effect, and magnetoresistance experiments

Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution.

Room temperature magnetic (Ga,Mn)N: a new material for spin electronic devices

A new dilute magnetic semiconductor (Ga,Mn)N grown by metal organic chemical vapor deposition (MOCVD) is reported. Vibrating sample magnetometer (VSM) and extraordinary Hall effect (EHE) measurements verified a ferromagnetic component at room temperature. The direction of the easy axis and the Curie temperature varies with the growth conditions, the latter ranging from 38°C to 75°C. Secondary ion mass spectroscopy (SIMS) confirms diffusion of Mn into the GaN to a depth of 380 Å.

Control of the magnetic domain wall propagation in Pt/Co/Pt ultra thin films using direct mechanical AFM lithography

We create mesoscopic point and line defects by scanning probe lithography to control the magnetization reversal process in Pt/Co/Pt ultra thin film devices. The domain wall propagation near the defects is studied by the Kerr microscopy and extraordinary Hall effect measurements. The observed domain wall pinning is used to block and channel the domain expansion and to create artificial domain patterns.

Propagation of a magnetic domain wall in the presence of AFM fabricated defects

We can control the magnetization reversal process in Pt/Co/Pt ultra thin film devices by using mesoscopic defects fabricated by atomic force microscope (AFM) lithography. Holes and grooves locally cutting the Co layer are written by direct mechanical indentation of the metal samples by the AFM tip. The smallest lateral feature size of these artificial structures (down to 20 nm for a hole) is comparable to the intrinsic Barkhausen length (/spl sim/25 nm) of the films. The influence of the AFM fabricated structures on the magnetization reversal process in micron sized devices was studied by Kerr microscopy and extraordinary Hall effect measurements. Single point defects act as mesoscopic domain wall pinning centers, while lines effectively block the domain wall propagation. Study of the influence of well characterized defects should help understand better the magnetization reversal processes in the films.

Magnetoresistance anomaly in DyFeCo thin films

Microstructured rare-earth–transition-metal DyFeCo films have been investigated using magnetoresistance and extraordinary Hall-effect measurements. The Hall loops reveal variation of coercive fields depending on the linewidth and the composition of the films. The magnetoresistance curves, with changes up to as high as 1.3%, show positive/negative magnetoresistance peaks centered on the coercive fields depending on the linewidth of the films only. The variation of the coercivity can be attributed to the magnetic moment canting between the Dy and FeCo subcomponents and the existence of the diverged magnetization on the edges, and the anomalous magnetoresistance peaks observed are discussed with the existing theories.

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

Epitaxial ferromagnetic thin films grown directly on III–V compound semiconductors can lead to the integration of magnetism and high-speed III–V electronics/photonics, offering possibilities of hybrid ferromagnetic-semiconductor devices. Despite stringent material requirements for this purpose, we have successfully grown some of Mn-based metallic compounds sharing common atoms with the underlying III–V using molecular beam epitaxy (MBE). First, it is shown that ferromagnetic MnGa thin films with perpendicular magnetization were obtained on (001) GaAs substrates. The c axis, which is the easy axis for magnetization, of the tetragonal unit cell is found to be desirably aligned perpendicular to the film plane. Second, in order to derive more functionality and to increase the freedom in material design, we have also created a new class of magnetic superlattices (SLs) of metallic compounds, MnGa/NiGa, on GaAs substrates. The MnGa/NiGa SLs show even stronger perpendicular magnetization and evenly-spaced multiple steps in the hysteresis loops in extraordinary Hall effect measurements. Third, ferromagnetic hexagonal MnAs, an As-based compound which is more compatible with existing III–V MBE technology, is shown to be successfully grown on GaAs. It was found that the MBE-grown MnAs/GaAs heterostructures have a unique epitaxial relationship and a mono-atomic template plays a critical role in determining the epitaxial orientation and magnetic properties.

Microstructural and Magnetotransport (EHE) Properties of Epitaxial τ-MnAl on (100)GaAs Substrates by Pulsed Laser Deposition

AbstractWe report on the crystal quality and magnetotransport properties (EHE) of epitaxial (001) τ-MnA1/GaAs(100) grown by the laser ablation deposition technique. Films (10–30 nm thick) were grown by two methods: (1) ablating a τ-MnAl target, prepared in-house; (2) alternate deposition of ultra thin layers of Mn and Al (˜6 periods) followed by annealing at different temperatures. For both deposition approaches ultrathin coherent epitaxial τ-MnAl films have been grown at temperatures in the range 250–420°C that is below the ε-phase transformation temperature. Reflection high energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HRTEM) were used to characterize the crystalline quality of the deposited films. The Extraordinary Hall Effect measurements (EHE) indicate that the deposited films are perpendicularly magnetized showing coercivity up to 6 kOe at room temperature. The epitaxial growth of thin film ferromagnetic materials on lattice matching semiconductor substrates offers the possibility of integrating magnetic and semiconducting devices.

Interface Quality and Magnetic Properties of τ MnAl/Co Superlattices On GaAs

ABSTRACTτ MnAl / Co superlattices are grown epitaxially on GaAs (100) substrates by molecular beam epitaxy. High angle XRD spectra are analysed and indicate that good structural quality is achieved. The behaviour of the superlattice upon annealing is described and compared with inplane and out of plane magnetization data. Extraordinary hall effect measurements show low field switching comparable to MnAl thin films. Arguments on how structural properties are affecting the magnetic properties are discussed.

Magnetotransport properties of MBE-grown magnetic superlattices of Mn-based intermetallics on GaAs heterostructures

We have studied magnetic and magnetotransport properties of two kinds of new epitaxial magnetic superlattices, MnGa/NiGa and MnAl/NiAl, consisting of ferromagnetic tetragonal MnGa (MnAl) and non-magnetic CsCl-type NiGa (NiAl), grown by molecular beam epitaxy (MBE) on (001) GaAs substrates. Strong perpendicular magnetization is evidenced in both types of superlattices, with the values of remanent magnetization higher than those of MBE-grown MnGa and MnAl thin films reported previously. At room temperature, the hysteresis loops in the extraordinary Hall effect (EHE) measurements on the MnGa/NiGa superlattices exhibit evenly-spaced multiple steps, while the MnAl/NiAl superlattices show rectangular single-stepped EHE hysteresis loops with nearly perfect squareness. The capability of growing these kinds of new epitaxial magnetic superlattices gives a new degree of materials design of the magnetic thin films on GaAs substrates, offering the possibility of integrating magnetic devices with III–V semiconductor electronics and photonics.

Epitaxial MnGa/NiGa magnetic multilayers on GaAs

We have grown a new class of epitaxial metallic multilayers consisting of ultrathin ferromagnetic tetragonal MnGa and nonmagnetic (CsCl-type) NiGa on (001) GaAs substrates by molecular beam epitaxy. Reflection high energy electron diffraction and cross-sectional transmission electron microscopy analyses show that MnGa/NiGa multilayers with atomically abrupt interfaces are formed with the expected epitaxial orientations, and, in particular, that the c-axis of the tetragonal structure of the MnGa film is aligned perpendicular to the substrate. Perpendicular magnetization of the MnGa/NiGa multilayers was evidenced by both vibrating sample magnetometer and extraordinary Hall effect measurements at room temperature, with higher values (Mr=267–302 emu/cm3) of remanent magnetization than those of previously reported MnGa and MnAl thin films. The capability of growing this new class of materials will allow a new degree of artificial materials design on semiconductor substrates.

Epitaxial growth of ferromagnetic ultrathin MnGa films with perpendicular magnetization on GaAs

We have successfully grown ferromagnetic MnGa ultrathin films on GaAs substrates by molecular beam epitaxy. Reflection high energy electron diffraction and cross-sectional transmission electron microscopy show that monocrystalline MnGa films are grown with the c axis of the tetragonal unit cell normal to the (001) GaAs substrates. Both magnetization measurements by vibrating sample magnetometer and extraordinary Hall effect (EHE) measurements indicate perpendicular magnetization, with the remnant magnetization of 225 emu/cm3 and EHE resistivity in the range of 0.5–4 μΩ cm at room temperature. The material possesses properties ideal for certain nonvolatile magnetic memory coupled with underlying III-V circuitry.

Epitaxial Ferromagnetic MnGa and (MnNi) Ga Thin Films with Perpendicular Magnetization on GaAs

ABSTRACTWe have successfully grown thermodynamically stable ferromagnetic MnxGa1-x (x=0.55∼0.60) thin films with thicknesses ranging from 3 nm to 60 nm on GaAs substrates by molecular beam epitaxy. The c-axis of the tetragonal structure of the MnGa film is shown to be aligned perpendicular to the substrate. Both Magnetization Measurements and extraordinary Hall effect measurements indicate perpendicular magnetization of the MnGa films, exhibiting squarelike hysteresis characteristics. Furthermore, we have investigated the effect of Ni additions as a substitution for mn in (Mn60-yNiy) Ga40 alloy thin films with y=0 – 30 at% Ni. With increasing Ni, the perpendicular component of the magnetization becomes smaller up to y=18 where the magnetization is in-plane. At y=30, the magnetization is again perpendicular.

The influence of thermal annealing on the resistivity and extraordinary Hall effect of amorphous Co-Gd-Mo films

Resistivity and extraordinary Hall effect measurements were made on low bias sputtered Co-Gd-Mo films of 70 nm thickness. Investigations were performed with as-deposited films as well as after isothermal vacuum annealing at temperatures up to 360°C, keeping the films always in an amorphous state. During annealing two processes can be observed which influence the resistivity and Hall effect. At lower annealing temperatures up to 250°C ordering effects are dominant while above 250°C oxidation takes over.

Extraordinary Hall-Effect Measurements on Ni, Some Ni Alloys, and Ferrites

Extraordinary Hall-effect measurements over a wide range of temperature are reported for 99.9 Ni, 99.4 Ni, 95 Ni, several 80% Ni alloys, ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, and ${(\mathrm{NiO})}_{0.75}$ ${(\mathrm{FeO})}_{0.25}$ (${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$). The resistivity $\ensuremath{\rho}$ of all samples over the same temperature range is also reported. At high temperatures, the extraordinary Hall coefficient ${R}_{1}$ for 99.9 Ni is proportional to ${\ensuremath{\rho}}^{2}$ as predicted by theory. However, the dependence upon $\ensuremath{\rho}$ is smaller in the 99.9 Ni sample at lower temperatures and in the samples with higher impurity concentration at all temperatures. The 80% Ni samples exhibit positive ${R}_{1}$, while ${R}_{1}$ in the high-concentration Ni samples is negative. ${R}_{1}$ for both ferrites is negative below about 400\ifmmode^\circ\else\textdegree\fi{}C and positive above this temperature.The theoretical work on metals generally indicates that ${R}_{1}$ is proportional to ${\ensuremath{\rho}}^{2}$ and the constant of proportionality $r$ is weakly temperature dependent and of the order of unity. In the high-concentration Ni samples $r$ is negative, of the order of unity, and exhibits a significant temperature dependence at low temperatures. In the 80% Ni samples, $r$ is positive and of the order of ${10}^{\ensuremath{-}2}$. At room temperature, $r$ is of the order of ${10}^{\ensuremath{-}3}$ in ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ and of the order of ${10}^{\ensuremath{-}6}$ in ${(\mathrm{NiO})}_{0.75}$ ${(\mathrm{FeO})}_{0.25}$ (${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$). In both ferrites, $r$ is strongly temperature dependent. The change of sign of $r$ with temperature observed in both ferrites has not been previously observed in metals or alloys.