Planar Hall Effect Sensor Research Articles

On-chip measurements of Brownian relaxation of magnetic beads with diameters from 10 nm to 250 nm

We demonstrate the use of planar Hall effect magnetoresistive sensors for AC susceptibility measurements of magnetic beads with frequencies ranging from DC to 1 MHz. This wide frequency range allows for measuring Brownian relaxation of magnetic beads with diameters ranging from 10 nm to 250 nm. Brownian relaxation is measured for six different magnetic bead types and their hydrodynamic diameters are determined. The hydrodynamic diameters are found to be within 40% of the nominal bead diameters. We discuss the applicability of the different bead types for volume-based biosensing with respect to sedimentation, magnetic trapping, and signal per bead. Among the investigated beads, we conclude that the beads with a nominal diameter of 80 nm are best suited for future on-chip volume-based biosensing experiments using planar Hall effect sensors.

Signal dependence on magnetic nanoparticles position over a planar Hall effect biosensor

Graphical abstractBy experiments and micromagnetic simulations, the sensor signal dependence on the spatial locations of magnetic nanoparticles used as labels in biodetection applications was evidenced.Display Omitted Highlights? Disk-shaped Permalloy planar Hall effect sensors have been obtained. ? High field sensitivity has been obtained in a very simple structure. ? Experiments for detection of maghemite nanoparticles have shown good sensitivity. ? Micromagnetic simulations have been performed to explain the experimental data. ? The sensor signal depends on the spatial locations of magnetic nanoparticles. Experimental and micromagnetic studies have been carried out on the planar Hall effect (PHE) sensor signal dependence on the spatial locations of magnetic nanoparticles used as labels in biodetection applications. Disk-shaped structures, made from Permalloy, 1mm diameter and 20nm thick, were deposited on to oxidised Si substrate. To have a better control of the sensor sensitivity and linearity, a DC biasing field has been applied parallel to the driving current. The magnetic nanoparticles of maghemite were placed on different positions over the sensor surface using small water droplets and well defined magnetic field gradients. In our experiments the magnetic field was applied perpendicular to the sensors surface in order to avoid the saturation of the PHE signal. The results of our experiments are explained by means of micromagnetic simulations where magnetostatic interactions between magnetic nanobeads and sensor are clearly highlighted. With such simple and low cost sensors, magnetic moments lower than 7i?10-6emu can been detected which means a total mass of maghemite nanobeads lower than 2i?10-7g.

High-sensitivity planar Hall sensor based on simple gaint magneto resistance NiFe/Cu/NiFe structure for biochip application

The planar Hall effect (PHE) sensor based on a simple giant magneto resistance (GMR) trilayer structure NiFe/Cu/NiFe has been designed and fabricated successfully using conventional clean room fabrication methods. The PHE sensor is integrated by 24 sensor patterns with dimensions of 50 × 50 μm. Influence of individual layer thickness to sensitivity of sensor has been investigated. Sensitivity and planar Hall voltage increases with the decrease of Cu-layer thickness. The results are discussed in terms of the reinforcement of the antiferromagnetic interaction between NiFe layers and shunting current through the layer Cu. The optimum configuration has been found in the structure with the Cu-layer of 1 nm. In this case a single planar Hall effect sensor exhibits a high sensitivity of about 8 μV Oe−1 and a maximal of the signal change as large as ▵V ∼ 55 μV. These values are comparable to those of the typical PHE sensor based on complex GMR spin-valve structure. With a high sensitivity and simple structure, this sensor is very promising for practical detection of magnetic beads and identifying multiple biological agents in the environment.

Planar Hall Effect Sensors With Subnanotesla Resolution

We report the fabrication of elliptical planar Hall effect sensors made of Permalloy with response determined by shape-induced uniaxial anisotropy. By using ac excitation and by optimizing the sensor thickness and the amplitude of the excitation current, we have obtained a magnetic field resolution which is better than 600 pT/ √Hz at 1 Hz and close to 1 nT/ √Hz at 0.1 Hz. We discuss possible routes for further improvement of the obtained resolution.

Magnetic Sensor-Based Detection of Picoliter Volumes of Magnetic Nanoparticle Droplets in a Microfluidic Chip

We have designed, fabricated and tested an integrated microfluidic chip with a Planar Hall Effect (PHE) sensor. The sensor was constructed by sequentially sputtering Ta/NiFe/Cu/NiFe/IrMn/Ta onto glass. The microfluidic channel was fabricated with poly(dimethylsiloxane) (PDMS) using soft lithography. Magnetic nanoparticles suspended in hexadecane were used as ferrofluid, of which the saturation magnetisation was 3.4 emu/cc. Droplets of ferrofluid were generated in a T-junction of a microfluidic channel after hydrophilic modification of the PDMS. The size and interval of the droplets were regulated by pressure on the ferrofluid channel inlet. The PHE sensor detected the flowing droplets of ferrofluid, as expected from simulation results. The shape of the signal was dependent on both the distance of the magnetic droplet from the sensor and the droplet length. The sensor was able to detect a magnetic moment of <TEX>$2{\times}10^{-10}$</TEX> emu at a distance of 10 <TEX>${\mu}m$</TEX>. This study provides an enhanced understanding of the magnetic parameters of ferrofluid in a microfluidic channel using a PHE sensor and will be used for a sample inlet module inside of integrated magnetic lab-on-a-chip systems for the analysis of biomolecules.

Microbeads Detection Using Spin-Valve Planar Hall Effect Sensors

In this paper we present a micromagnetic approach to describe the detection of magnetic nanobeads using planar Hall effect sensors. The magnetic beads polarized by a dc magnetic field generate a field, which can affect the magnetization state of spin-valve sensor, leading in principle, to a detectable signal. For magnetic nanobeads we assumed a superparamagnetic behaviour. Three detection geometries are discussed and some specific behaviours were highlighted by micromagnetic simulations. We found that when the polarising field is applied parallel with the sensor surface a very weak signal can be obtained. This is because at working fields, for which the magnetic nanobeads are magnetised, the sensor saturates. We identified other setups that can overcome this shortcoming and deliver a net signal.

Measurements of Brownian relaxation of magnetic nanobeads using planar Hall effect bridge sensors

We compare measurements of the Brownian relaxation response of magnetic nanobeads in suspension using planar Hall effect sensors of cross geometry and a newly proposed bridge geometry. We find that the bridge sensor yields six times as large signals as the cross sensor, which results in a more accurate determination of the hydrodynamic size of the magnetic nanobeads. Finally, the bridge sensor has successfully been used to measure the change in dynamic magnetic response when rolling circle amplified DNA molecules are bound to the magnetic nanobeads. The change is validated by measurements performed in a commercial AC susceptometer. The presented bridge sensor is, thus, a promising component in future lab-on-a-chip biosensors for detection of clinically relevant analytes, including bacterial genomic DNA and proteins.

Optimisation of Spin-Valve Planar Hall Effect Sensors for Low Field Measurements

In this paper are presented results of galvanomagnetic measurements and micromagnetic simulations performed on low-field magnetic sensors based on the planar Hall effect (PHE). Disc-shaped structures of the type Co/Cu/Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> , 4 mm diameter, deposited by magnetron sputtering onto Si substrate, were used to build magnetic sensors. At this stage of study, no uniaxial anisotropy axes were defined during the samples deposition. Two types of applications have been considered: (i) magnetic field measurements and (ii) rotation sensing. In order to obtain a coherent rotation of the magnetization inside the PHE sensor under the action of an applied magnetic field, H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">appl</sub> , DC or AC magnetic biasing fields were used. By these means the magnetic sensitivity and the hysteresis width of the PHE signal can be tuned. Sensitivities between 0.07 and 0.17 μV·A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ·m have been obtained for a driving current of 10 mA. Micromagnetic simulations were used to explain some field angular behavior of these sensors.

Planar Hall effect sensors with shape-induced effective single domain behavior

We show that shape anisotropy induces effective single domain behavior in elliptical structures of thin permalloy films with long axis ranging between several microns to several millimeters, provided that the ratio of the film long and short axes is large enough. We also show that the thin film elliptical structures exhibit a wide range of effective anisotropy fields, from less than 10 Oe up to more than 100 Oe. We discuss the advantage of shape anisotropy in the fabrication of planar Hall effect sensors with high field resolution.

Planar Hall effect sensor with magnetostatic compensation layer

Demagnetization effects in cross-shaped planar Hall effect sensors cause inhomogeneous film magnetization and a hysteretic sensor response. Furthermore, when using sensors for detection of magnetic beads, the magnetostatic field from the sensor edges attracts and holds magnetic beads near the sensor edges causing inhomogeneous and non-specific binding of the beads. We show theoretically that adding a compensation magnetic stack beneath the sensor stack and exchange-biasing it antiparallel to the sensor stack, the magnetostatic field is minimized. We show experimentally that the compensation stack removes nonlinear effects from the sensor response, it strongly reduces hysteresis, and it increases the homogeneity of the bead distribution. Finally, it reduces the non-specific binding due to magnetostatic fields allowing us to completely remove beads from the compensated sensor using a water flow 60 times smaller than a flow that failed to remove beads from an uncompensated sensor.

Planar Hall Effect Ring Sensors for High Field-Sensitivity

Planar Hall effect sensor has been explored using multi-layer cross-shaped and bridge geometry. We present planar Hall effect in a ring-shaped geometry experimentally that shows progress of sensor sensitivity as well as output signals. Sensitivity improves about 170 times compare to cross-shaped geometry and about 1.4 times to bridge geometry in conventional measurement system. These values become 2.5 times larger at 20o measurement system. The presented ring geometry may take great potential in Planar Hall effect sensor applications.

Size-dependent effects in exchange-biased planar Hall effect sensor crosses

Exchange-biased planar Hall effect magnetic field sensor crosses with arm width w have been studied as function of w. For large values of w, the magnetic behavior is hysteresis-free and follows the single domain Stoner–Wohlfarth model. When w is decreased, hysteresis is observed in the sensor response. For intermediate values of w, the magnetization reversal takes place in two steps, and for small values of w, the magnetization reversal takes place in a single step. Based on electrical measurements, magnetic force microscopy, and micromagnetic simulations, the observations are explained by an increasing magnetic shape anisotropy of the arms of the cross. We propose a simple analytical model that captures the essential physics of the observations and parameterizes the effects of the cross-shape on the central part of the cross.

Bead magnetorelaxometry with an on-chip magnetoresistive sensor

Magnetorelaxometry measurements on suspensions of magnetic beads are demonstrated using a planar Hall effect sensor chip embedded in a microfluidic system. The alternating magnetic field used for magnetizing the beads is provided by the sensor bias current and the complex magnetic susceptibility spectra are recorded as the 2nd harmonic of the sensor response. The complex magnetic susceptibility signal appears when a magnetic bead suspension is injected, it scales with the bead concentration, and it follows the Cole-Cole expression for Brownian relaxation. The complex magnetic susceptibility signal resembles that from conventional magnetorelaxometry done on the same samples apart from an offset in Brownian relaxation frequency. The time dependence of the signal can be rationalized as originating from sedimented beads.

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.

High field-sensitivity planar Hall sensor based on NiFe/Cu/IrMn trilayer structure

A trilayer structure, which has weak exchange coupling and high active current, has been optimized emphasizing for high field-sensitivity planar Hall effect (PHE) sensor. To illustrate the high field sensitivity of the PHE sensor, three different structures are fabricated: a bilayer thin film Ta(3)/NiFe(10)/IrMn(10)/Ta(3) (nm), a spin-valve thin film Ta(3)/NiFe(10)/Cu(1.2)/NiFe(2)/IrMn(10)/Ta(3) (nm), and a trilayer thin film Ta(3)/NiFe(10)/Cu(0.12)/IrMn(10)/Ta(3) (nm). The characterized results reveal that the field sensitivity of PHE sensor based on trilayer thin film is about one order larger than that of bilayer and is about twice larger than that of spin-valve thin film. Moreover, in trilayer structure, the thinner spacer layer gives the better performance. When the nominal thickness of spacer Cu layer is the smallest, the PHE sensor exhibits the best performance, i.e., in this experiment, it is about 0.12 nm.

Spin-valve planar Hall sensor for single bead detection

The planar Hall effect (PHE) sensor with a junction size of 3 μm × 3 μm for a single micro-bead detection has been fabricated successfully using a typical spin-valve thin film Ta(5)/NiFe(16)/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) nm. The PHE sensor exhibits a sensitivity of about 7.2 μV Oe −1 in the magnetic field range of ±7 Oe approximately. We have performed an experiment to illustrated the possibility of single micro-bead detection by using a PHE sensor. A single micro-bead of 2.8 μm diameter size is secluded from 0.1% dilute solution of the Dynabeads ® M-280 dropped on the sensor surface and is located on the sensor junction by using a micro magnetic needle. The comparison of the PHE voltage profiles in the field range from 0 to 20 Oe in the absence and presence of a single micro-bead identifies a single Dynabeads ® M-280, the maximal signal change as large as Δ V ∼ 1.1 μV can be obtained at the field ∼6.6 Oe. The results are well described in terms of the reversal of a basic single domain structure.

Optimization of the Multilayer Structures for a High Field-Sensitivity Biochip Sensor Based on the Planar Hall Effect

We have investigated the planar Hall effect (PHE) in three multilayer structures such as a bilayer, a spin-valve and a weak exchange bias coupling bilayer structure introduced a very thin Cu spacer layer between the antiferromagnetic and ferromagnetic layers. These thin films are Ta(3)/NiFe(10)/IrMn(10)/Ta(3) (nm), Ta(3)/NiFe(10)/Cu(1.2)/NiFe(2)/IrMn(10)/Ta(3) (nm), and Ta(3)/NiFe(10)/Cu(0.2)/IrMn(10)/Ta(3) (nm), respectively. The active layers in all three structures were kept constant. The field-sensitivity of the fabricated PHE sensors obtained for the respected structures are about 1.6 muV middot Oe-1, 5 muV middot Oe-1, and 12 muV middot Oe-1, respectively. The results suggest that the sensor based on a weak exchange bias coupling structure has the highest field-sensitivity compared with the others. The proposed weak exchange bias coupling structure emphasizes for the development of the PHE sensor materials.

Optimization of planar Hall effect sensor for magnetic bead detection using spin-valve NiFe/Cu/NiFe/IrMn structures

Present paper deals with the planar Hall effect (PHE) of Ta(5 nm)/NiFe(tf)/Cu(1.2 nm)/NiFe(tp)/IrMn(15 nm)/Ta(5 nm) spin-valve structures. Experimental investigations are performed for 50 × 50 μm2 junctions with various thicknesses of free and pinned layer tf = 4, 8, 10, 15, 20 nm and tp = 2, 3, 6, 8, 9, 12 nm. The results show that the thicker free layers, the higher PHE signal is obtained. In addition, the thicker pinned layers, the lower PHE signal. The highest PHE sensitivity S of 15.6 mΩ/Oe is obtained in the spin-valve configuration with tf = 20 nm and tp = 2 nm. This optimum structure is rather promising for micro magnetic bead detections.

Planar Hall effect in biosensor with a tilted angle of the cross-junction

A typical planar Hall effect (PHE) sensor junction consists of two Hall bars that the bars appear normal to each other and the junction can have the required four terminals for current and voltage measurements. We are now introducing a tilted angle of the cross-junction and studying the role of the PHE therein. The results show that although there is a tilted angle of the cross-junction, the PHE voltage is remained constant. The result is interpreted by assuming the sensor material with the behavior of a basic single domain structure under the external magnetic field reversals. The calculations of the model are found to be in good concurrence with the experimental results.

Sensitivity Dependence of the Planar Hall Effect Sensor on the Free Layer of the Spin-Valve Structure

Planar Hall effect (PHE) sensors with the junction size of 50 mum times 50 mum were fabricated successfully by using spin-valve thin films Ta(5)/NiFe(x) /Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) with x = 4, 8, 10, 12, 16. The magnetic field sensitivity of the PHE sensors increases with increasing thickness of ferromagnetic (FM) free layer. The sensitivity of about 95.5 m Omega/(kA/m) can be obtained when the thickness of the FM-free layer increases up to 16 nm. The enhancement of sensitivity is explained by the shunt current from other layers. The PHE profiles are well described in terms of the Stoner-Wohlfarth energy model. The detection of magnetic micro-beads label Dynabeadsreg M-280 is demonstrated and the results revealed that the sensor is feasible for high-resolution biosensor applications.