Matthew J. Carter

Ferromagnetic resonance and damping properties of CoFeB thin films as free layers in MgO-based magnetic tunnel junctions

We have investigated the magnetization dynamics of sputtered Co40Fe40B20 thin films in a wide range of thicknesses used as free layers in MgO-based magnetic tunnel junctions, with the technique of broadband ferromagnetic resonance (FMR). We have observed a large interface-induced magnetic perpendicular anisotropy in the thin film limit. The out-of-plane angular dependence of the FMR measurement revealed the contributions of two different damping mechanisms in thick and thin film limits. In thinner films (<2 nm), two-magnon scattering and inhomogeneous broadening are significant for the FMR linewidth, while the Gilbert damping dominates the linewidth in thicker films (� 4n m). Lastly, we have observed an inverse scaling of Gilbert damping constant with film thickness, and an intrinsic damping constant of 0.004 in the CoFeB alloy film is determined. V C 2011 American Institute of Physics. [doi:10.1063/1.3615961]

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.

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.

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.

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...

DC and AC Characterization of MgO Magnetic Tunnel Junction Sensors

We have fabricated multiple MgO magnetic tunnel junctions (MTJ) with 1.7 nm oxide, which are connected in series, and layed out in a serpentine geometry. We performed DC tunnel magnetoresistance measurements and AC impedance spectroscopy with crossed DC magnetic fields in the easy and hard axis directions. A simple RLC circuit model is used to fit our data and characterize the dependence on capacitance (C) and inductance (L) of magnetization orientation of the MTJ sensors. We have found in our samples that C and L are higher in antiparallel than in parallel configuration. We discuss possible reasons for the existence of this field dependence and show the evolution of magnetic field vs. capacitance curve from memory mode into sensing mode at high frequencies.

Magnetic characterization of magnetic tunnel junction devices using circle transfer curves

We describe new characterization methods that allow an accurate determination of all of the magnetic parameters that govern the behavior of magnetoresistive devices. These characterization methods are explained and used to measure the magnetic properties of MgO-based magnetic tunnel junction (MTJ) devices with magnetoresistance values of over 150%. We will show that the analysis of so-called “circle transfer curves,” which are measurements of the device magnetoresistance in a rotating, constant-magnitude applied field, can accurately determine the magnitude and direction of the free layer anisotropy as well as the pinned layer orientation and exchange bias strength. We also show how a measurement of the MTJ’s remnant resistance curve, obtained by saturating the MTJ at different field angles and then removing the applied field, can provide additional information on the free layer anisotropy characteristics. We will also compare our results with values extracted from traditional Stoner-Wohlfarth asteroid cu...