A study of the combined effect of CoFeB free layer thickness, shape anisotropy, and annealing temperature on the transfer curves of MgO based magnetic tunnel junctions is presented. The response of the free layer changes gradually as its thickness decreases and shows hysteresis-free response for CoFeB⩽15.5Å. For junctions with CoFeB=15.5Å, the onset of hysteresis-free response depends on junction area and annealing temperature. Linearized magnetic tunnel junctions with dimensions down to 1.5×3μm2 and sensitivity as high as 7.7%∕Oe were achieved.

Low-frequency noise was studied in MgO magnetic tunnel junctions (MTJs). The junctions were analyzed under saturating magnetic fields to minimize the noise of magnetic origin. Low-frequency noise of magnetically saturated MTJs is dominated by two different types of electrical noise: 1 noise and random telegraph noise (RTN). 1 noise is always present and represents the ultimate limitation for low-frequency applications. The RT noise component has a higher contribution for lower resistance area (RA) product samples and its magnitude increases with bias voltage. The 1 noise of different MTJs can be compared using the Hooge parameter ( ). This paper shows a systematic decrease in with increasing bias voltage for MTJs with varying RA. All the MTJs studied, for both memory and sensor applications with single and double barrier, exhibit higher variations while saturated in the anti-parallel state (AP) and no significative dependence on bias polarity was observed. Variations above one order of magnitude were observed for MTJs with RA 10 k m 2 in the AP. For the same bias voltage, the lowest 's were always obtained in the parallel state (P). The lowest( 1 E 10 m 2 ) was obtained for the lower RA sample ( 500 m 2 ) saturated in P, with an applied bias voltage of 600 mV.Index Terms-1/ noise, double barrier, low-frequency noise, magnetic tunnel junction, MgO, random telegraph noise.

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15  V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.

Single and double barrier MgO based magnetic tunnel junction sensors were processed with area of 200μm2 and small aspect ratios (<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.

We report on a magnetic field sensor based on CoFeB/MgO/CoFeB magnetic tunnel junctions. By taking advantage of the perpendicular magnetic anisotropy of the CoFeB/MgO interface, the magnetization of the sensing layer is tilted out-of-plane which results in a linear response to in-plane magnetic fields. The application of a bias voltage across the MgO tunnel barrier of the field sensor affects the magnetic anisotropy and thereby its sensing properties. An increase of the maximum sensitivity and simultaneous decrease of the magnetic field operating range by a factor of two is measured. Based on these results, we propose a voltage-tunable sensor design that allows for active control of the sensitivity and the operating filed range with the strength and polarity of the applied bias voltage.Comment: 4 pages, 4 figures, lette

Two sets of low resistance MgO junctions were patterned into junctions with areas from 1 to 24μm2. By properly choosing the operating conditions the background noise can be placed at the level of equivalent fields of ∼10−11 and ∼10−12T∕Hz0.5 calculated for 30 Oe linear range junctions of types 1 (150Ωμm2, tunnel magnetoresistance (TMR)=150%) and 2 (30Ωμm2, TMR=100%), respectively. Such room temperature sensitivities can only be achieved at frequencies where the 1∕f noise contribution is negligible. From the 1∕f noise Hooge constant (αH=2.66×10−9μm2 at R×A∼150Ωμm2 and αH=1.24×10−9μm2 at R×A∼30Ωμm2) the 1∕f noise corner in the optimum biasing conditions is predicted to be located between 10 and 70 MHz, depending on junction area and resistance.

We report on a highly efficient spin diode effect in an exchange-biased spin-valve giant magnetoresistance (GMR) strips. In such multilayer structures, symmetry of the current distribution along the vertical direction is broken and, as a result, a non-compensated Oersted field acting on the magnetic free layer appears. This field, in turn, is a driving force of magnetization precessions. Due to the GMR effect, resistance of the strip oscillates following the magnetization dynamics. This leads to rectification of the applied radio frequency current and induces a direct current voltage VDC . We present a theoretical description of this phenomenon and calculate the spin diode signal, VDC , as a function of frequency, external magnetic field, and angle at which the external field is applied. A satisfactory quantitative agreement between theoretical predictions and experimental data has been achieved. Finally, we show that the spin diode signal in GMR devices is significantly stronger than in the anisotropic magnetoresistance permalloy-based devices.

Spin diode effect in a giant magnetoresistive strip is measured in a broad frequency range, including resonance and off-resonance frequencies. The off-resonance dc signal is relatively strong and also significantly dependent on the exchange coupling between magnetic films through the spacer layer. The measured dc signal is described theoretically by taking into account magnetic dynamics induced by Oersted field created by an ac current flowing through the system.