Gen Feng

Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes: Influence of annealing temperature

Low frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. The authors investigated the noise for different degrees of crystallization and CoFeB∕MgO interface quality depending on the annealing temperature. The authors report an extremely low 1∕f noise, compared to magnetic junctions with Al2O3 barriers. The origin of the low frequency noise is discussed and it is attributed to localized charge traps with the MgO barriers.

Annealing of CoFeB/MgO based single and double barrier magnetic tunnel junctions: Tunnel magnetoresistance, bias dependence, and output voltage

Co40Fe40B20/MgO single and double barrier magnetic tunnel junctions (MTJs) were grown using target-facing-target sputtering for MgO barriers and conventional dc magnetron sputtering for Co40Fe40B20 ferromagnetic electrodes. Large tunnel magnetoresistance (TMR) ratios, 230% for single barrier MTJs and 120% for the double barrier MTJs, were obtained after postdeposition annealing in a field of 800 mT. The lower TMR ratio for double barrier MTJs can be attributed to the amorphous nature of the middle Co40Fe40B20 free layer, which could not be crystallized during postannealing. A highly asymmetric bias voltage dependence of the TMR can be observed for both single and double barrier MTJs in the as-deposited states and after field annealing at low temperature. The asymmetry decreases with increasing annealing temperature and the bias dependence becomes almost symmetric after annealing at 350 °C. Maximum output voltages of 0.65 and 0.85 V were obtained for both single and double barrier MTJs, respectively, after...

Influence of annealing on the bias voltage dependence of tunneling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes

Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance (TMR) values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB∕MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V1∕2=1.88V when the junctions are processed at 200°C. The largest output voltage, 0.62V, is obtained after annealing at 300°C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence.

Magnetic dead layers in sputtered Co40Fe40B20 films

The magnetic moment of Co40Fe40B20 electrodes has been investigated as a function of thickness for films prepared by magnetron sputtering on Si∕SiO2∕MgO and Si∕SiO2 substrates. On MgO, the metal film becomes discontinuous and superparamagnetic with no stable ferromagnetic moment below 1.0nm, whereas on Si∕SiO2, there appears to be a 0.7nm dead region for all film thicknesses. The dead layer is attributed to an interdiffusion region at the interface with the substrate where there are weakly coupled noncollinear spins and a 0.2nm dead layer associated with the Ta cap. The discontinuous ferromagnetic films maybe useful for the wide-range linear field sensors.

Reduced low frequency noise in electron beam evaporated MgO magnetic tunnel junctions

We compare low frequency noise in magnetic tunnel junctions with MgO barriers prepared by electron-beam evaporation with those prepared by radiofrequency sputtering, both showing a high tunneling magnetoresistance. The normalized noise parameter in the parallel state of junctions with evaporated barriers is at least one order of magnitude lower than that in junctions with sputtered barriers, and exhibits a weaker bias dependence. The lowest normalized noise is in the 10−11 μm2 range. A lower density of oxygen vacancies acting as charge trap states in the evaporated MgO is responsible for the lower noise.

High inverted tunneling magnetoresistance in MgO-based magnetic tunnel junctions

Inverted tunneling magnetoresistance, where resistance decreases as the free layer in a magnetic tunnel junction flips its direction of magnetization after saturation, has been observed at zero bias in magnetic tunnel junctions with a thin CoFeB layer in the pinned synthetic antiferromagnetic CoFe∕Ru∕CoFeB stack. Magnetoresistance values as high as −55% at room temperature are measured in MgO-based tunnel junctions when the thickness of the pinned CoFeB layer is 1.5nm. The inverted magnetoresistance is associated with imbalance of the synthetic antiferromagnetic pinned layer. Asymmetric bias dependence with a magnetoresistance sign change is observed for a 0.5nm pinned CoFeB layer.Inverted tunneling magnetoresistance, where resistance decreases as the free layer in a magnetic tunnel junction flips its direction of magnetization after saturation, has been observed at zero bias in magnetic tunnel junctions with a thin CoFeB layer in the pinned synthetic antiferromagnetic CoFe∕Ru∕CoFeB stack. Magnetoresistance values as high as −55% at room temperature are measured in MgO-based tunnel junctions when the thickness of the pinned CoFeB layer is 1.5nm. The inverted magnetoresistance is associated with imbalance of the synthetic antiferromagnetic pinned layer. Asymmetric bias dependence with a magnetoresistance sign change is observed for a 0.5nm pinned CoFeB layer.

Magnetic Properties of Exchange-Biased Multilayer With Perpendicular Magnetic Anisotropy

Perpendicular exchange-biased [Co/Pt]₃/Co(t_Co)/IrMn andIrMn/Co(t_Co)/Pt/[Co/Pt]₃ multilayers have been fabricated, changing the thickness (t_Co) of the cobalt that is next to the IrMn layer. The crystal structure, interface roughness and magnetic properties were characterized by X-ray diffraction (XRD), X-ray reflectivity (XRR), atomic force microscopy (AFM) and extraordinary Hall Effect (EHE). The multilayers are flat, and the roughness is about 0.2 nm across the 2-¿m scanning range. The magnetization reversal is dependent on t_Co, and the exchange bias and coercivity vary with t_Co. The exchange bias reaches a maximum value (about 13.9 mT) for top-pinned multilayers at t_Co = 0.45 nm, while it reaches a maximum (about 3.5 mT) for bottom-pinned multilayers at t_Co = 0.6 nm. For both stacks the coercivity is maximum (about 30 mT) at t_Co = 0.8 nm, and stable for t_Co > 1.0 nm. These results are well understood in terms of the Co spin orientation at the Co/IrMn interface.

Magnetic noise in MgO-based magnetic tunnel junction rings

Magnetization switching is investigated in ring-shaped MgO-based magnetic tunnel junctions with 168% tunneling magnetoresistance. Besides the forward and reverse onion states, two vortex states and several metastable states are observed for the ferromagnetic free layer. Electrical noise is used to characterize the low frequency magnetization dynamics; a stationary 1/f noise spectrum is observed within each magnetic state but they are separated by noise peaks which show a 1/f2 spectrum that is associated with slow random telegraph fluctuations. In the 1/f region, the normalized magnetic noise parameter, αmag, is shown to be consistent with the fluctuation-dissipation theorem.

MgO-based double barrier magnetic tunnel junctions with thin free layers

The free layer thickness (tfree) in double barrier magnetic tunnel junctions (DMTJs) based on crystalline MgO barriers and CoFeB ferromagnetic layers has been varied from 0.5 to 3.0 nm in order to investigate its effect on the magnetic and electrical properties. One obvious feature of DMTJs with tfree≤1 nm is the absence of sharp free layer switching in the TMR curves, which can be explained by the superparamagnetic nature of discontinuous CoFeB layer, which breaks into nanodots when it is very thin. Normal free layer switch is observed when tfree=2.0 and 3.0 nm. Another difference is a rapid increase in junction resistance and tunnel magnetoresistance at low temperature for DMTJs with thin tfree, which is attributed to the Coulomb blockade effect. We also observed a small conductance peak in the dI/dV curve at low bias only in the parallel configuration and at temperatures below 100 K. This is related to the Kondo scattering process on the nanodots, which constitutes the discontinuous free layer. We foun...

Magnetic Properties of Exchange-Biased [Co/Pt](n) Multilayer With Perpendicular Magnetic Anisotropy

Perpendicular exchange-biased [Co/Pt]₃/Co(t_Co)/IrMn andIrMn/Co(t_Co)/Pt/[Co/Pt]₃ multilayers have been fabricated, changing the thickness (t_Co) of the cobalt that is next to the IrMn layer. The crystal structure, interface roughness and magnetic properties were characterized by X-ray diffraction (XRD), X-ray reflectivity (XRR), atomic force microscopy (AFM) and extraordinary Hall Effect (EHE). The multilayers are flat, and the roughness is about 0.2 nm across the 2-¿m scanning range. The magnetization reversal is dependent on t_Co, and the exchange bias and coercivity vary with t_Co. The exchange bias reaches a maximum value (about 13.9 mT) for top-pinned multilayers at t_Co = 0.45 nm, while it reaches a maximum (about 3.5 mT) for bottom-pinned multilayers at t_Co = 0.6 nm. For both stacks the coercivity is maximum (about 30 mT) at t_Co = 0.8 nm, and stable for t_Co > 1.0 nm. These results are well understood in terms of the Co spin orientation at the Co/IrMn interface.