E. R. Nowak

Progress toward a thousandfold reduction in 1∕f noise in magnetic sensors using an ac microelectromechanical system flux concentrator (invited)

The potential advantage of some magnetic sensors having a large response is greatly decreased because of the 1∕f noise. We are developing a device, the microelectromechanical system (MEMS) flux concentrator, that will mitigate the effect of this 1∕f noise. It does this by placing flux concentrators on MEMS structures that oscillate at kilohertz frequencies. By shifting the operating frequency, the 1∕f noise will be reduced by one to three orders of magnitude depending upon the sensor and the desired operating frequency. We have succeeded in fabricating the necessary MEMS structures and observing the desired kilohertz normal-mode resonant frequencies. Only microwatts are required to drive the motion. We have used spin valves for our magnetic sensors. The measured field enhancement provided by the flux concentrators agrees to within 4% with the value estimated from finite element calculations. No difference was detected in noise measurements on spin valves with and without the flux concentrators. This resul...

Low frequency magnetoresistive noise in spin-valve structures

We report on resistance noise in spin-valve structures that is due to reconfigurations in domain structure of the magnetic layers. 1/f noise from the free layer and pinned layer is evident and its magnitude is in good agreement with predictions from the fluctuation dissipation relation using the imaginary (dissipative) component of the measured resistance susceptibility. In addition, we find that the imaginary component is dependent on applied magnetic field, being larger for layers that exhibit pronounced magnetic hysteresis. A magnetoresistive 1/f noise parameter is proposed, and benchmark values for a variety of spin-valve devices are reported.

In-situ characterization of rapid crystallization of amorphous CoFeB electrodes in CoFeB/MgO/CoFeB junctions during thermal annealing

We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson–Mehl–Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template.

1∕f noise in magnetic tunnel junctions with MgO tunnel barriers

Electrical noise measurements are reported for magnetic tunnel junctions having magnesium oxide tunnel barriers. These junctions have resistance-area products (RAPs) of order 10–100MΩμm2 and exhibit zero-bias tunneling magnetoresistance ratios (TMRs) as high as 120% at room temperature. The TMR is bias dependent and decreases to half its maximum value for biases near 300 mV. The dominant low-frequency electrical noise is due to resistance fluctuations having a 1∕f-like power spectral dependence and a nonmagnetic origin. The normalized 1∕f noise parameter, α, is found to be of order 10−7 to 10−6 which compares favorably to magnetic tunnel junctions consisting of an aluminum oxide barrier with comparable RAPs but lower TMR. At high biases, α is found to decrease which we attribute to defect-assisted tunneling mechanisms.

Validation of the microelectromechanical system flux concentrator concept for minimizing the effect of 1/f noise

With the microelectromechanical system (MEMS) flux concentrator, we have been able to increase the operating frequency of small magnetic sensors above the region where 1/f noise dominates. The device accomplished this by modulating the field via the oscillatory motion of flux concentrators on MEMS flaps. Electrostatic comb drives were used to drive the MEMS flaps. We have demonstrated an increase in the signal to noise ratio at 1 Hz, that the power signal correctly depends on V4 where V is the amplitude of the voltage energizing the comb drives, and that the signal increases dramatically with vacuum packaging.

Evolution of barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions during annealing

The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1/f spectrum and it is quantified by a Hooge-like parameter α given in units of μm2. Unannealed devices have the highest noise levels and their α parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance (TMR) is observed for short annealing times (on the order of minutes) at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430 °C exhibit the same minimum noise levels, α≈2×10−10 μm2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attr...

Spin-polarized transport in hybrid (Zn,Cr)Te/Al2O3/CO magnetic tunnel junctions

Tunnel magnetoresistance (TMR) of 21% is observed at low temperature in hybrid magnetic tunnel junctions (MTJs) composed of a magnetic semiconductor (Zn,Cr)Te and Co electrodes separated by an alumina barrier. The TMR is observed up to 250K, which is a considerable improvement over previous work on MTJs with semiconductor electrodes. The temperature and bias dependence of the TMR are consistent with a transport model involving spin-polarized tunneling and spin-independent hopping through impurity states.

Magnetic noise evolution in CoFeB/MgO/CoFeB tunnel junctions during annealing

We report on the evolution of equilibrium magnetoresistive (MR) 1/f noise due to the exchange-biased magnetic layer in MgO-based magnetic tunnel junctions as a function of annealing time at 380 and 430 °C. The resistance susceptibility and MR noise are observed to increase rapidly with annealing time at a fixed temperature. The magnetic losses responsible for MR noise are not significantly affected by the structural crystallization at the CoFeB/MgO interface during short annealing times. After prolonged annealing, the decrease in magnetic losses is attributed to reduced disorder in the magnetic layers that result in thermally driven fluctuations in local micromagnetic structure.

Large magnetoresistance of thick polymer devices having La0.67Sr0.33MnO3 electrodes

We report magnetoresistance (MR) measurements for structures with micrometer-thick regioregular, polythiophene (rr-P3HT) polymer layers between two ferromagnetic contacts. Hole spin transport through the polymer layer leads to a relative MR value in 300 mT fields of 0.3% at 300 K and increasing to 18% at 25 K. The inferred intrinsic spin lifetime and diffusion length are about 7 ms and 0.4 μm, respectively. The spin transport coherence length is enhanced by the electric field, leading to an enhancement in MR with increasing applied voltage.

Advances in magnetometry through miniaturization

Recent innovations may lead to magnetic sensors that are smaller, more sensitive, and/or cost less than current magnetometers. Examples of this are the chip scale atomic magnetometer, magnetic tunnel junctions with MgO barriers, and a device for minimizing the effect of 1∕f noise, the microelectromechanical system (MEMS) flux concentrator. In the chip scale atomic magnetometer, researchers have been able to fabricate the light source, optics, heater, optical cell, and photodiode detector in a stack that passes through a silicon wafer. Theoretical and subsequent experimental work has led to the observation of magnetoresistance values of 400% at room temperature in magnetic tunnel junctions with MgO barriers. This large magnetoresistance occurs because electrons in the majority band can tunnel more easily through the MgO barrier than electrons in the minority band. The MEMS flux concentrator has the potential to increase the sensitivity of magnetic sensors at low frequencies by more than an order of magnitu...