Magnetic Tunnel Junction Sensors Research Articles

Low aspect ratio micron size tunnel magnetoresistance sensors with permanent magnet biasing integrated in the top lead

Optimized single magnetic bead detection requires magnetic sensors with an improved field detection limit and a sensor area matching the bead cross section (micron size spatial resolution). This work shows that square micron sized tunnel magnetoresistance sensors can be fabricated with integrated longitudinal permanent magnet (PM) biasing that show a linearized field response and low noise. PMs (Co66Cr16Pt18) were integrated in the top lead of an AlOx based magnetic tunnel junction sensor for transfer curve linearization. Sensors patterned with dimensions down to 3 × 3 μm2 present field detection limits of 6 nT/Hz0.5 and 3 nT/Hz0.5 at 10 kHz and 500 kHz, respectively, corresponding to a field sensitivity of 1.8% per mT (linear field detection range ΔB = −8 mT + 8 mT) and αH = 3.9 ± 0.3 × 10−10 μm2 (RxA = 1.9 kΩ μm2).

Magnetic Tunnel Junction Sensors With Conetic Alloy

Al2O3 magnetic tunneling junction (MTJ) sensors were fabricated with Conetic alloy Ni77Fe14Cu5Mo4 deposited as the free layer and pinned layer for its soft magnetic properties. It was observed that the Al2O3 MTJ sensors with Conetic exhibited relatively small easy-axis coercivity. Tunneling magnetoresistance (TMR) and noise measurements were carried out to characterize the sensors. TMR of 9.5% and Hooge parameter of 3.825 × 10-7 μm2 were achieved without any hard-axis field. Hard-axis bias field was applied to eliminate the hysteresis and improve the linear field response of the MTJ sensor. The hysteresis was removed by applying an external magnetic field along the hard axis at 8 Oe and the sensor sensitivity was 0.4 %/Oe within a linear region at room temperature. The relationship between the Hooge parameter and hard-axis field was also investigated and the result demonstrated that the 1/f noise can be suppressed by an optimized hard-axis bias field. This work shows that it is feasible to use Conetic alloy as the soft magnetic layers in MTJ sensors for its small coercivity, and a hard-axis bias field can be used to linearize the sensor response and suppress the 1/f noise.

Review of Noise Sources in Magnetic Tunnel Junction Sensors

Noise problem limits the sensitivity of magnetic tunnel junction (MTJ) sensors for ultra-low magnetic field applications. Noise analysis not only helps in finding ways to eliminate noise disturbances but also essential for understanding the electronic and magnetic properties of MTJs. These approaches provide insight for optimizing the design of MTJ sensors before fabrication. This paper reviews the noise sources in MTJ sensors reported in recent years. Both the origins and mathematical derivations of the noise sources are presented, illustrating how different factors affecting the performance of MTJ sensors. A brief outlook of challenges in the future is also given.

On-chip measurement of the Brownian relaxation frequency of magnetic beads using magnetic tunneling junctions

We demonstrate the detection of the Brownian relaxation frequency of 250 nm diameter magnetic beads using a lab-on-chip platform based on current lines for exciting the beads with alternating magnetic fields and highly sensitive magnetic tunnel junction (MTJ) sensors with a superparamagnetic free layer. The first harmonic out-of-phase component of the MTJ response gives the imaginary part of the magnetic bead susceptibility, which peaks at the Brownian relaxation frequency. This work paves the way to on-chip implementation of Brownian magnetorelaxometry in innovative “lab-on-a-bead” assays for biomolecular recognition.

Magnetic scanometric DNA microarray detection of methyl tertiary butyl ether degrading bacteria for environmental monitoring

A magnetoresistive biosensing platform based on a single magnetic tunnel junction (MTJ) scanning probe and DNA microarrays labeled with magnetic particles has been developed to provide an inexpensive, sensitive and reliable detection of DNA. The biosensing platform was demonstrated on a DNA microarray assay for quantifying bacteria capable of degrading methyl tertiary butyl ether (MTBE), where concentrations as low as 10 pM were detectable. Synthetic probe bacterial DNA was immobilized on a microarray glass slide surface, hybridized with the 48 base pair long biotinylated target DNA and subsequently incubated with streptavidin-coated 2.8 μm diameter magnetic particles. The biosensing platform then makes use of a micron-sized MTJ sensor that was raster scanned across a 3 mm by 5 mm glass slide area to capture the stray magnetic field from the tagged DNA and extract two dimensional magnetic field images of the microarray. The magnetic field output is then averaged over each 100 μm diameter DNA array spot to extract the magnetic spot intensity, analogous to the fluorescence spot intensity used in conventional optical scanners. The magnetic scanning result is compared with results from a commercial laser scanner and particle coverage optical counting to demonstrate the dynamic range and linear sensitivity of the biosensing platform as a potentially inexpensive, sensitive and portable alternative for DNA microarray detection for field applications.

Impedance spectroscopy of micron sized magnetic tunnel junctions with MgO tunnel barrier

We have studied the magnetoimpedance of micron sized magnetic tunnel junction sensors with 1.7 nm MgO tunnel barrier. We performed ac impedance spectroscopy in the frequency range between 100 Hz–40 MHz as a function of applied magnetic field in the sensing direction. We model our devices with a simple RLC circuit. Fitting the model to our data results in frequency independent R, L, and C, and our low frequency results are in agreement with dc measurements. Despite excellent agreement with published result on interface capacitance for MgO barrier magnetic tunnel junctions similar to ours we do not observe any magnetocapacitance in our devices.

Magnetic tunnel junctions with large tunneling magnetoresistance and small saturation fields

There is a continuing need for greater sensitivity in magnetic tunnel junction (MTJ) sensors. We have found a new approach to achieving large tunneling magnetoresistance (TMR) with a very soft free layer. The high TMR is achieved by conventional means of annealing a bottom pinned MTJ that has Ta and Ru capping layers. The soft free layer is achieved by etching almost to the MgO tunnel barrier and depositing a thick soft magnetic film. The results are far superior to annealing the MTJ with the thick soft layer already deposited.

Scanning Magnetoresistance Microscopy for Imaging Magnetically Labeled DNA Microarrays

We demonstrate the use of a magnetic tunnel junction (MTJ) sensor as a probe for non-contact scanning microscopy of magnetically labeled DNA microarrays. Induced magnetic stray fields from 2.8 mum diameter magnetic particles are detected using the MTJ sensor, while two dimensional scanning generates the magnetic map of the DNA microarray with a spatial resolution of 1 mum over a large scan area exceeding 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Current particle-sensor spacing of ~20 mum results in a detection resolution of ~30 magnetic particles within each 100 mum diameter DNA spot. Our results highlight the use of scanning magnetoresistive microscopy as a convenient and powerful technique for the accurate detection and identification of biomolecules tagged with magnetic particles.

Wide range and tunable linear magnetic tunnel junction sensor using two exchange pinned electrodes

A magnetic tunnel junction sensor with both the detection and the reference layers pinned by IrMn is fabricated. Using the differences in the blocking temperatures of the IrMn films with different thicknesses, crossed anisotropies can be induced between the detection and the reference electrodes. The pinning of the sensing electrode ensures a linear and reversible output. It also allows tuning both the sensitivity and the linear range of the sensor. The sensitivity varies linearly with the ferromagnetic thickness of the detection electrode. It is demonstrated that an increased thickness leads to a rise of sensitivity and a reduction of the operating range.

A Portable and Autonomous Magnetic Detection Platform for Biosensing

This paper presents a prototype of a platform for biomolecular recognition detection. The system is based on a magnetoresistive biochip that performs biorecognition assays by detecting magnetically tagged targets. All the electronic circuitry for addressing, driving and reading out signals from spin-valve or magnetic tunnel junctions sensors is implemented using off-the-shelf components. Taking advantage of digital signal processing techniques, the acquired signals are processed in real time and transmitted to a digital analyzer that enables the user to control and follow the experiment through a graphical user interface. The developed platform is portable and capable of operating autonomously for nearly eight hours. Experimental results show that the noise level of the described platform is one order of magnitude lower than the one presented by the previously used measurement set-up. Experimental results also show that this device is able to detect magnetic nanoparticles with a diameter of 250 nm at a concentration of about 40 fM. Finally, the biomolecular recognition detection capabilities of the platform are demonstrated by performing a hybridization assay using complementary and non-complementary probes and a magnetically tagged 20mer single stranded DNA target.

Quantitative detection of DNA labeled with magnetic nanoparticles using arrays of MgO-based magnetic tunnel junction sensors

We have demonstrated the detection of 2.5μM target DNA labeled with 16nm Fe3O4 nanoparticles (NPs) using arrays of magnetic tunnel junction sensors with (001)-oriented MgO barrier layers. A MTJ sensor bridge was designed to detect the presence of magnetic NPs bonded with target DNA. A raw signal of 72μV was obtained using complementary target DNA, as compared with a nonspecific bonding signal of 25μV from noncomplementary control DNA. Our results indicate that the current system’s detection limit for analyte DNA is better than 100nM.

Field detection in single and double barrier MgO magnetic tunnel junction sensors

Single and double barrier MgO based magnetic tunnel junction sensors were processed with area of 200μm2 and small aspect ratios (&amp;lt;2). These sensors present resistance area products ranging from 10to70kΩμm2 and tunneling magnetoresistance (TMR) values of up to 160%. Sensor linearization was mostly obtained by thinning the CoFeB free layer to 15.5A, close to the free layer transition from ferromagnetic to superparamagnetic behavior. Three different thicknesses were studied for the free layer CoFeB: 1.55, 1.8, and 3nm. The CoFeB thickness decrease required for the linearization implies a loss in TMR. Field detection range of ∼80pT∕Hz0.5 was obtained for both double and single barrier sensors (at 500KHz). Out of the 1∕f regime, the detection range decreases to ∼2pT∕Hz0.5 for the single barrier sensor. The field detection dependence on bias voltage was also studied at 500KHz and outside the 1∕f dominated regime.

Dependence of noise in magnetic tunnel junction sensors on annealing field and temperature

The minimum detectable field of magnetoresistive sensors is limited by their intrinsic noise. Magnetization fluctuations are one of the crucial noise sources and are related to the magnetization alignment at the antiferromagnetic-ferromagnetic interface. In this study, we investigated the low frequency noise of magnetic tunnel junctions (MTJs) annealed in the temperature range from 265to305°C and magnetic fields up to 7T, either in helium or hydrogen environments. Our results indicate that the magnetic fluctuators in these MTJs changed their frequency based on annealing field and temperature. The noise of the MTJs at low frequency can be reduced by annealing in high magnetic field (7T) and further improved by annealing in a hydrogen environment.

Detection of 130nm magnetic particles by a portable electronic platform using spin valve and magnetic tunnel junction sensors

An integrated biosensor with magnetic tunnel juntions (MTJs) and spin valve (SV) sensor was used for 130nm particle detection. A platform drives an external magnet generating an in-plane dc+ac magnetic field on the sensor at frequencies up to 375Hz, provides a current to bias the sensor, and performs the signal acquisition and treatment. The signal-to-noise ratio of the SV and MTJ was characterized. Bead detection (130nm in diameter) was performed using both sensors leading to a detection limit of 3×108particles∕ml for SV and 3×1010particles∕ml for MTJ.

Detection of DNA labeled with magnetic nanoparticles using MgO-based magnetic tunnel junction sensors

We have demonstrated the detection of 2.5μM target DNA labeled with 16nm Fe3O4 nanoparticles (NPs) and 50nm commercial MACS™ NPs using arrays of magnetic tunnel junction sensors with (001)-oriented MgO barrier layers. Signal-to-noise ratios of 25 and 12 were obtained with Fe3O4 and MACS™ NPs, respectively. These data show conclusively that MgO-based MTJ sensor arrays are very promising candidates for future applications involving the accurate detection and identification of biomolecules tagged with magnetic nanoparticles.

Selectable resistance-area product by dilute highly charged ion irradiation

Considerable effort worldwide has been invested in producing low resistance-area (RA) product magnetic tunnel junction sensors for future hard drive read heads. Here the authors present a method of producing tunnel barriers with a selectable RA value spanning orders of magnitude. A single process recipe is used with only the dose of highly charged ions (HCIs) varied. The HCIs reduce the tunnel barrier integrity, providing enhanced conduction that reduces the overall RA product. The final RA product is selected by appropriate choice of the HCI density; e.g., 100HCIs∕μm2 typically results in the RA product being reduced by a factor of 100.

Improvement of pulse diagnostic apparatus with array sensor of magnetic tunneling junctions

To obtain the spatial feature of arterial pulse, a spatial pulse diagnostic apparatus was designed using a two-dimensional magnetoresistive sensor array. The magnetic-field distribution for the magnet array was simulated using the finite-element method. The field distribution of parallel magnet arrays was observed to be more sensitive and uniform than that of the perpendicular ones. Moreover, the spatial displacements of the magnet array coincided with the output signal of the magnetic tunneling junction sensor array.

2차원 배열 자기저항소자를 이용한 공간 맥진파형의 전산모사 분석

To get the spatial feature of arterial pulse, we designed spatial pulse diagnostic apparatus (SPDA) using a 2-dimensional magnetoresistive sensor array. The magnetic field distribution fur magnet may was simulated using finite element method. We recognized that the field distribution of parallel magnet mays was more sensitive and uniformed than that of perpendicular one. Also the spatial displacements of magnet array were agreed with the output signal of magnetic tunnel junction (MTJ) sensor array.

In situ detection of single micron-sized magnetic beads using magnetic tunnel junction sensors

We have demonstrated the use of highly sensitive magnetic tunnel junction (MTJ) sensors for the detection of individual micron-sized magnetic labels. By integrating the MTJ sensor into a microfluidic channel, we were able to detect the presence of moving superparamagnetic beads (Dynabeads® M-280) in real time by direct measurement of the magnetic dipole fields associated with single beads. The dipolar fields of a single bead were sufficient to obtain a signal of 80μV with signal to noise ratio of 24dB in an applied field of 15Oe. Our data show conclusively that MTJ sensors are very promising candidates for future applications involving the accurate detection and identification of biomolecules with magnetic labels.

Study of Longitudinal Stabilization Using In-Stack Biasing

The in-stack stabilization of unshielded and shielded magnetic tunnel junction (MTJ) sensors have been studied experimentally by quasi-static and recording tests as well as theoretically by micromagnetic modeling. Results showed the viability of in-stack stabilization over a range of design and material conditions. Performance tradeoff studies and design optimization results led to the fabrication of MTJ read heads with good magnetic stability and high readback sensitivity.