B. D. Schrag

Effect of film roughness in MgO-based magnetic tunnel junctions

We have systematically investigated the dependence of tunnel magnetoresistance in MgO-based magnetic tunnel junctions as a function of Ar pressure during sputtering. The MgO surface roughness, and therefore device magnetoresistance, depends strongly on Ar gas pressure. Magnetoresistance of up to 236% was achieved at room temperature after thermal annealing at 425°C and with optimal sputtering conditions. The long mean free path of target atoms at low background pressures increases their kinetic energy at the substrate surface, resulting in smooth surface morphology and correspondingly improved device performance.

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.

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.

Thermal stability of magnetic tunneling junctions with MgO barriers for high temperature spintronics

Thermal stability of MgO-based magnetic tunnel junctions has been investigated from room temperature up to 500°C, in both the memory and sensor configurations. Junctions showed magnetoresistances of over 200% at room temperature and over 100% at 300°C. Below 375°C, the resistance of the parallel state remains constant, while the antiparallel state resistance linearly decreases with temperature. Above that, a rapid increase in the resistance of both states was observed, along with an irreversible loss of magnetoresistance. Junctions in the sensor configuration exhibited a constant sensitivity of 1.0%/Oe at temperatures up to 300°C before getting degraded.

Submicron electrical current density imaging of embedded microstructures

We have developed a scanning magnetic microscopy technique for noninvasively imaging submicron magnetic fields from embedded microscopic electrical circuits. We are able to extract from the field data a complete profile of current densities using a mathematical algorithm. As an example, we provide current density images of micron-scale passivated conductors undergoing electromigration.

Low frequency noise in highly sensitive magnetic tunnel junctions with (001) MgO tunnel barrier

Low frequency voltage noise was measured in highly sensitive magnetic tunnel junctions with MgO tunnel barrier. The voltage noise is observed to scale linearly with the magnetic field sensitivity. Fluctuations in noise, possibly due to local domain nucleation or annihilation inside the free layer, are also observed. Results indicate that an external hard-axis bias field can significantly suppress the magnetization fluctuations of the free layer and lower the magnetic field noise.

Field sensing characteristics of magnetic tunnel junctions with (001) MgO tunnel barrier

We map the magnetic field sensitivity and low-frequency 1∕f voltage noise of high magnetoresistance MgO-based magnetic tunnel junctions in an orthogonal magnetic field arrangement. Large sensitivity values of over 1%/Oe are obtained only when a sufficiently large hard-axis bias field is applied. The low-frequency voltage noise is observed to scale with the field sensitivity. The magnetic field noise map reveals that the signal-to-noise ratios of these devices get gradually better at higher hard-axis bias fields.

Thermal stability, sensitivity, and noise characteristics of MgO-based magnetic tunnel junctions (invited)

Thermal stability, sensitivity, and noise of micron-scale magnetic tunnel junctions based on MgO tunnel barriers have been studied for both the memory and sensing configurations. Junctions show solid high-temperature performance with substantial magnetoresistance observed even at 500°C. At temperatures above 375°C, the junctions begin to experience irreversible degradation due to interlayer diffusion. The thermal stability of these devices depends strongly on the exchange bias of the device and hence on the properties of the antiferromagnetic layer. Sensitivities as high as 3.3%∕Oe have been obtained at room temperature for junctions configured as low-field sensors. Sensitivity values are constant up to temperatures of 300°C, above which performance decays due to a loss of exchange bias and overall magnetoresistance. Noise spectra are 1∕f at frequencies up to 51kHz, and sensors have a resultant field noise better than 1nT∕Hz0.5 at 100kHz. A comparison is made with devices fabricated with alumina tunnel ba...

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.

Current density mapping and pinhole imaging in magnetic tunnel junctions via scanning magnetic microscopy

We have applied a magnetoresistive microscopy technique to the imaging of current densities and pinhole formation in magnetic tunnel junction devices. In this work, we demonstrate how the magnetic field distribution at the surface of the device can be used to understand the flow of current within the junction itself. By imaging the current-induced fields before and after pinhole formation in several different junctions, we find that many junctions exhibit an unexpectedly complicated current distribution after high-voltage-induced breakdown. Further, we have seen that pinhole locations can be correlated with current inhomogeneities observed before junction breakdown. Finally, we present the results of finite-element simulations which are in good agreement with experimental results.