Ryan Stearrett

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.

BaGa2Pn2 (Pn = P, As): new semiconducting phosphides and arsenides with layered structures.

Reported are the synthesis, the structural characterization, and the electronic band structures of two new Zintl phases: BaGa2P2 and BaGa2As2. Both compounds are isoelectronic and isotypic and crystallize in a monoclinic system with a new structure type (Pearson symbol mP20). The structures have been established by single-crystal X-ray diffraction, space group P2(1)/c (Z = 4), with lattice parameters as follows: a = 7.3363(13)/7.495(5) Å; b = 9.6648(17)/9.901(6) Å; c = 7.4261(13)/7.643(5) Å; beta = 115.373(2) degrees/115.381(8) degrees for BaGa2P2/BaGa2As2, respectively. The atomic arrangements in both cases are devoid of disorder and are best rationalized as polyanionic layers, (infinity)(2)[Ga2Pn2]2- (Pn = P, As), with Ba2+ cations separating them. The layers, in turn, can be viewed as the result of condensation of Ga2Pn6 units, which are isosteric with the ethane molecule in its staggered conformation. Structural parallels with other known Zintl phases are presented. The electronic structures, computed using the tight-binding linear muffin-tin orbital methods (TB-LMTO), are discussed as well.

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...

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.

Magnetic tunneling junction based magnetic field sensors: Role of shape anisotropy versus free layer thickness

Al2O3- and MgO-based magnetic tunnel junction (MTJ) sensors were designed and fabricated using microfabrication techniques. This study revealed that in the case of Al2O3-based sensors, the shape anisotropy in the free NiFe electrode resulted in a linear and hysteresis-free tunneling magnetoresistance (TMR) curve. These sensors exhibited TMR values between 27% and 30% and sensitivity up to 0.4%/Oe over a magnetic field range of − 40 to 40 Oe. In the case of CoFeB/MgO/CoFeB MTJ sensors, shape anisotropy alone was not sufficient to achieve a linear and hysteresis-free MR response. A superparamagnetic free layer was used to achieve the desired sensor response. MgO-based sensors had about 90% TMR and 1.1%/Oe sensitivity over the same field range as Al2O3-based MTJs.

Influence of growth and annealing conditions on low-frequency magnetic 1/f noise in MgO magnetic tunnel junctions

Magnetic 1/f noise is compared in magnetic tunnel junctions with electron-beam evaporated and sputtered MgO tunnel barriers in the annealing temperature range 350 - 425 °C. The variation of the magnetic noise parameter (αmag) of the reference layer with annealing temperature mainly reflects the variation of the pinning effect of the exchange-bias layer. A reduction in αmag with bias is associated with the bias dependence of the tunneling magnetoresistance. The related magnetic losses are parameterized by a phase lag e, which is nearly independent of bias especially below 100 mV. The similar changes in magnetic noise with annealing temperature and barrier thickness for two types of MgO magnetic tunnel junctions indicate that the barrier layer quality does not affect the magnetic losses in the reference layer.

SHOT NOISE SUPPRESSION IN INDIVIDUAL AND SERIES ARRAYS OF MAGNETIC TUNNEL JUNCTIONS

Charge-current shot noise is investigated in single magnetic tunnel junctions and devices having multiple junctions that are connected in series. The ratio of the measured shot noise in single junctions to the expected Poisson value, namely the Fano factor, F, is observed to vary from 1 to well below 0.5. Deviations from F = 1 are attributed to localized states (defects) located in the tunnel barrier or at the interfaces with the magnetic electrodes. For series arrays of junctions, the Fano factor scales inversely with the number (1 ≤ N ≤ 30) of junctions in series, even for junctions exhibiting sub-Poissonian (F < 1) shot noise. The 1/N scaling is consistent with the incoherent tunneling of electrons across junctions and indicates that each junction behaves as an individual noise source. The advantages of incorporating series arrays of magnetic tunnel junctions into devices for magnetic field sensing are discussed.