Planar Hall Effect Measurements Research Articles

Detection of in-depth helical spin structures by planar Hall effect

We developed a method to determine the magnetic helicity and to study reversal mechanisms in exchange biased nanostructures using Planar Hall Effect (PHE). As a test case, we use an in-depth helical spin configuration that occurs during magnetization reversal in exchange coupled Ni/FeF2 heterostructures. We show the way to induce and determine the sign of the helicity from PHE measurements on a lithographically patterned cross. The helicity sign can be controlled by the angle between the externally applied magnetic field and a well-defined unidirectional anisotropy axis. Furthermore, the PHE signal reveals complex reversal features due to small deviations of the local unidirectional anisotropy axes from the crystallographic easy axis. The simulations using an incomplete domain wall model are in excellent agreement with the experimental data. These studies show that helical spin formations in nanomagnetic systems can be studied using laboratory-based magnetotransport.

Thickness dependence of uniaxial anisotropy fields in GaMnAs films

Our investigation of thin GaMnAs films with different thicknesses revealed that the magnetic properties of this material strongly depend on film thickness. For this study, a single GaMnAs film was selectively etched, and its properties were then investigated by planar Hall effect measurements. A particularly important conclusion from the results is the emergence of a uniaxial anisotropy field along the [100] crystalline direction, which increases rapidly with increasing film thickness. We argue that such thickness dependence of the [100] uniaxial anisotropy results from the crystal structure of the film, rather than from the effects of the interface between the GaMnAs and the substrate.

Magnetization reversal process in Fe/Si (001) single-crystalline film investigated by planar Hall effect**Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921801 and 2012CB933102), the National Natural Science Foundation of China (Grant Nos. 11374350, 11034004, 11274361, 11274033, 11474015, and 61227902), and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20131102130005).

A planar Hall effect (PHE) is introduced to investigate the magnetization reversal process in single-crystalline iron film grown on a Si (001) substrate. Owing to the domain structure of iron film and the characteristics of PHE, the magnetization switches sharply in an angular range of the external field for two steps of 90° domain wall displacement and one step of 180° domain wall displacement near the easy axis, respectively. However, the magnetization reversal process near the hard axis is completed by only one step of 90° domain wall displacement and then rotates coherently. The magnetization reversal process mechanism near the hard axis seems to be a combination of coherent rotation and domain wall displacement. Furthermore, the domain wall pinning energy and uniaxial magnetic anisotropy energy can also be derived from the PHE measurement.

Effect of thermal annealing on the magnetic anisotropy of GaMnAs ferromagnetic semiconductor

We have systematically investigated the influence of annealing on the magnetic anisotropy properties of GaMnAs film using an epilayer with a Mn concentration of 6.2%. The GaMnAs epilayer was grown by molecular beam epitaxy and the planar Hall effect measurement was used to monitor the magnetic anisotropy of the film. We found significant annealing-induced changes in the magnetic anisotropy properties of the GaMnAs film that depended on the annealing conditions. For example, the cubic anisotropy that gave a four-fold symmetry of magnetic easy axes decreased while the uniaxial anisotropy that gave a two-fold symmetry of magnetic easy axes increases in the samples annealed temperature below 300 °C. In particular, the uniaxial anisotropy along the [010] direction in as-grown GaMnAs film changed to the [100] direction by rotating by 90° after the sample was annealed at 300 °C for 3 h. This investigation thus indicates that the magnitude and the direction of the magnetic anisotropy in the GaMnAs film can be effectively controlled by choosing an appropriate annealing time and temperature.

Buffer layer dependence of magnetic anisotropy in Fe films grown GaAs substrate

The effect of buffer layer on the magnetic anisotropy of Fe film has been investigated by using the planar Hall effect measurement. The Fe films were grown on four different buffer layers, such as GaAs, ZnSe, ZnTe, and Ge, by molecular beam epitaxy. The field and angle dependent planar Hall effect measurements revealed the presence of two types of magnetic anisotropies, i.e., cubic crystalline anisotropy along the 〈100〉 directions and uniaxial anisotropy along the 〈110〉 crystallographic directions, in all four Fe films investigated in the study. However, the relative strength of the two types of anisotropies varies significantly depending on the type of buffer layer. Specially, in the Fe film grown on the Ge buffer layer, the uniaxial anisotropy almost disappeared and the entire magnetic anisotropy was dominated by cubic anisotropy with a four-fold symmetry. This suggests that the growth of Fe film on the Ge surface is different from its growth on other surfaces such as GaAs, ZnSe, and ZnTe buffers, where the asymmetric reconstruction process of anions (i.e., As, Se, or Te) is present at the surface.

Magnetization reversal behavior of GaMnAs film grown on Si substrate investigated by planar Hall measurements

Magnetization reversal in a GaMnAs film grown on a Si substrate was investigated using planar Hall effect measurements. The angular dependence of the planar Hall resistance (PHR) shows that cubic crystalline anisotropy along the directions dominates the film’s magnetic anisotropy. However, the magnetization reversal behavior varies significantly with the applied field strength: as the magnetic field strength decreases, the PHR amplitude decreases, and hysteresis appears in data obtained with clockwise and counterclockwise rotation of the field directions. Furthermore, asymmetry in the PHR amplitude between the two opposite field directions appears during the angle scan and increases with decreasing field strength. We show that these features arise from a broad distribution of magnetic domains having different pinning fields.

Effect of light illumination on the [100] uniaxial magnetic anisotropy of GaMnAs film

The effect of light illumination on the magnetic anisotropy of GaMnAs has been investigated by using the planar Hall effect measurement. In as-grown GaMnAs, typical types of magnetic anisotropies, cubic crystalline anisotropy along the 〈100〉 directions and uniaxial anisotropy along the [110] direction, were clearly observed in the planar Hall resistance (PHR) measurements. The behavior of the PHR, however, changed after light illumination by showing significant asymmetry between the [100] and the [010] directions, which was explained by the appearance (or enhancement) of a new type of uniaxial anisotropy along the [100] direction in the film. The experiment reveals that light illumination can significantly alter the magnetic anisotropies of the GaMnAs film by changing the magnitude of existing anisotropies as well as inducing anisotropy along the [100] direction.

Epitaxial Co metal thin film grown by pulsed laser deposition using oxide target

We report here the growth of epitaxial Co-metal thin film on c-plane sapphire by pulsed laser deposition (PLD) using Co:ZnO target utilizing the composition inhomogeneity of the corresponding plasma. Two distinct plasma composition regions have been observed using heavily alloyed Co0.6Zn0.4O target. The central and intense region of the plasma grows Co:ZnO film; the extreme tail grows only Co metal with no trace of either ZnO or Co-oxide. In between the two extremes, mixed phases (Co+Co-oxides+Co:ZnO) were observed. The Co metal thin film grown in this way shows room temperature ferromagnetism with large in-plane magnetization ~1288emucm−3 and a coercivity of ~230Oe with applied field parallel to the film–substrate interface. Carrier density of the film is ~1022 cm−3. The film is epitaxial single phase Co metal which is confirmed by both X-ray diffraction and transmission electron microscopy characterizations. Planar Hall Effect (PHE) and Magneto Optic Kerr Effect (MOKE) measurements confirm that the film possesses similar attributes of Co metal. The result shows that the epitaxial Co metal thin film can be grown from its oxides in the PLD.

Change in planar hall effect ratio of Ni–Co films produced by electrodeposition

Abstract Ni–Co films were produced by the electrodeposition technique and their magnetotransport properties were studied. The anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) ratios were found using the van der Pauw setup at room temperature. It was observed that the PHE ratios were larger than the obtained AMR ratios. While the maximum changes in longitudinal and transversal magnetoresistance ratios were 6.8% and 11.0%, respectively, the change in PHE values was up to 500%. In the PHE measurements, the magnetoresistance orientation depends on the electrical resistance values which occur in branches of the films.

Anomalous transport in half-metallic ferromagnetic CrO2

We have investigated the transport properties of CrO${}_{2}$ thin films deposited on TiO${}_{2}$ and sapphire substrates and find subtle differences with respect to earlier reports. The films are good metals down to low temperatures, with residual resistivities of the order of 6 $\ensuremath{\mu}\ensuremath{\Omega}$ cm for films deposited on TiO${}_{2}$ and two times higher for films on sapphire substrates. Magnetoresistance (MR) measurements in high fields show an as yet unobserved nonmonotonic behavior, which is particularly pronounced around the sign change that takes place from negative to positive at a temperature around 100 K. Moreover, both the ordinary and anomalous Hall coefficients show considerable changes around 100--150 K, suggesting a change in carrier density together with the onset of the influence of spin defects in this temperature window. At lower temperatures, the MR is a linear function of the applied field, which can be explained as intergrain tunneling MR. This interpretation is also suggested by the angular MR. Planar Hall effect measurements reveal that the CrO${}_{2}$ thin films are not in a single magnetic domain state even for films deposited on an isostructural TiO${}_{2}$ substrate.

Investigation of the magnetic anisotropy in ferromagnetic GaMnAs films by using the planar hall effect

The magnetic anisotropy properties of GaMnAs ferromagnetic films have been investigated by using planar Hall effect measurements. The field scan of the planar Hall resistance (PHR) showed a two-step switching behavior indicating a strong cubic anisotropy along the 〈100〉 directions. The difference in the behaviors of the PHR for two applied field directions, \(\left[ {1\bar 10} \right]\) and [110], was understood via the well-known uniaxial anisotropy along the [110] direction. In addition to such known effects, we also found the presence of an asymmetry for the [010] and the [100] directions. This new asymmetry phenomenon was explained by introducing an additional uniaxial anisotropy field Hu2 along the [100] and the \(\left[ {\bar 100} \right]\) directions, which coincide with the two directions of cubic anisotropy. The values of the anisotropy fields, cubic (Hc), first uniaxial (Hu1), and second uniaxial (Hu2), were obtained by analyzing the angle dependence of the PHR. Although the value of Hu2 is small, its effect is clearly observed at high temperatures above 25 K, where the transition of the magnetization occurred before the field direction had been reversed during the magnetization reversal process.

Planar Hall effect in a single GaMnAs film grown on Si substrate

We carried out systematic planar Hall effect (PHE) measurements on a layer of GaMnAs grown on a Si substrate. The field scans of the planar Hall resistance (PHR) data obtained at 4K show a transition behavior, which is similar to the magnetization reorientation process often observed in GaMnAs film grown on a GaAs substrate that has both anisotropies along the <100> and the <110> directions. The dependence of the PHR on the applied field direction revealed the presence of an asymmetry between the <100> and the <110> directions in the magnetic energy of the film. The directions of the magnetic easy axes determined by the PHR value at zero field were about 9° away from the <110> directions. The PHR further shows the absence of the difference in the anisotropy between the <110> directions, indicating no preference of magnetic anisotropy for either the [110] or the [1¯10]direction in the GaMnAs grown on a Si substrate.

In-Plane Magnetic Anisotropy and Temperature Dependence of Switching Field in (Ga, Mn) as Ferromagnetic Semiconductors

We explore the magnetic anisotropy of GaMnAs ferromagnetic semiconductor by Planar Hall Effect (PHE) measurements. Using low magnitude of applied magnetic field (i.e., when the magnitude H is smaller than both cubic Hc and uniaxial Hu anisotropy field), we have observed various shapes of applied magnetic field direction dependence of Planar Hall Resistance (PHR). In particular, in two regions of temperature. At T < Tc/2, the "square-shape" signal and at T > Tc/2 the "zigzag-shape" signal of PHR. They reflect different magnetic anisotropy and provide information about magnetization reversal process in GaMnAs ferromagnetic semiconductor. The theoretical model calculation of PHR based on the free energy density reproduces well the experimental data. We report also the temperature dependence of anisotropy constants and magnetization orientations. The transition of easy axis from biaxial to uniaxiale axes has been observed and confirmed by SQUID measurements.

Magnetic and Transport Properties of As-grown and Annealed GaMnAs Thick Layers

We studied post-growth annealing of GaMnAs thin films with large film thickness t up to 100 nm. Our results show that for films of up to t = 100 nm comparable values of Curie temperature TC and saturation magnetization Ms can be obtained as for t = 25 nm. Hence, the out-diffusion based annealing process is not thickness limited. Furthermore, anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) measurements of as-grown and annealed samples are carried out to characterize the magnetic anisotropies of the different films.

Asymmetry in the angular dependence of the switching field of GaMnAs film

Planar Hall effect measurements were carried out on two GaMnAs ferromagnetic films with different Mn concentrations (6.2% and 8.3% Mn). The switching fields of magnetization taken from field scans of the planar Hall effect showed significantly different angular dependences in the two samples. While the angular dependence of the switching field for the sample with 8.3% Mn had a symmetric rectangular shape in the polar plot, that of the other sample with 6.2% Mn exhibited a clearly asymmetric behavior, with large steps at the 〈110〉 crystallographic directions. This switching field behavior was analyzed by considering pinning fields for crossing the 〈110〉 directions. The fitting of step features appearing at the 〈110〉 directions revealed the presence of a new uniaxial anisotropy field Hu2 along the [100] direction, in addition to the commonly observed cubic Hc anisotropy field (along the 〈100〉 directions) and uniaxial anisotropy Hu1 fields (along either the [110] or the [11¯0] direction) in the GaMnAs film.

Micromagnetic analysis and development of high sensitivity spin-valve magnetic sensors

In this paper we present a micromagnetic analysis and development of low-field magnetic sensors based on the planar Hall effect (PHE) in Permalloy (Ni80Fe20) thin films and spin-valve structures. We used these sensors for field measurements, biomolecules detection in lab-on-a-chip applications and angle positioning devices. Both field and angular PHE measurements were performed. Some distortions and hysteretic effects were observed at low magnetic fields for the angular dependence of the PHE voltage. Micromagnetic simulations were performed to explain the field and the angular dependence of the PHE voltage. We used dc biasing fields to improve the response of the PHE sensor and we compare these results with our micromagnetic simulations.

The effects of geometry on magnetic response of elliptical PHE sensors

We present planar Hall effect measurements of elliptical permalloy sensors having different aspect ratios and thicknesses along with extensive numerical simulations and analytical analysis. We identify an upper limit for the sensor minor axis on the order of 1 μm above which hysteresis effects intensify. We also find that the increased ratio between the ellipse axes and thickness enhances the magnetic response of the sensor.

Magnetization reorientation inGaxMn1−xAsfilms: Planar Hall effect measurements

The process of magnetization reorientation in a ferromagnetic semiconductor GaMnAs films was investigated using planar Hall effect measurements. In addition to the well-known two-step switching behavior that occurs during this process, we have observed two additional distinct features in field scan data of the planar Hall resistance (PHR). First, the region of the external field required to begin and complete the reorientation of magnetization from one easy axis to another strongly depends on the direction of the applied field. And second, the maximum amplitude of PHR is significantly reduced during magnetization reversal when the applied field is oriented near one of the easy axes of the GaMnAs film. We provide an explanation of these phenomena using the magnetic field dependence of the free-energy density and assuming the coexistence of multiple domains with three different directions of magnetization in the sample.

The effect of carrier density on magnetic anisotropy of the ferromagnetic semiconductor (Ga, Mn)As

Planar Hall effect (PHE) measurements are used to investigate magnetic anisotropy in two (Ga, Mn)As samples which differ by the hole concentration, but are otherwise identical. The difference in the hole density is controlled via modulation doping by Be. Angular dependence of PHE measured at 13 K reveals that the uniaxial easy axis in the sample with a lower hole concentration lies along the [110] direction, and along [ 1 ̄ 10 ] in the sample with higher doping. This difference in the orientation of uniaxial easy axes in the two samples demonstrates that the magnetization of GaMnAs can be manipulated just by varying the carrier density.

Electrical and micromagnetic characterization of rotation sensors made from permalloy multilayered thin films

Planar Hall effect (PHE) measurements were performed on permalloy (Py)-based thin films and multilayered structures like FeMn/Py/Cu/Py. FeMn is used for pinning the magnetization of the adjacent Py layer by exchange biasing effect. Here, we present some results of our measurements made on square- and ring-shaped thin-film structures used as rotation sensors. At low magnetic fields, i.e., less than 200 Oe, we observed hysteretic effects and distortions from the expected sinusoidal shape of the angular dependence of the PHE voltage. This is due to hysteretic behaviour of the magnetic material and some coupling effects in the multilayered structure. In order to have a better understanding of this behaviour, micromagnetic simulations were performed to obtain the angular dependence of the sample magnetization for different intensities of the rotating magnetic field. To get better results, the film plane was divided into a number of single domains which interact between them and with the applied magnetic field.