[en] Exchange enhancement through thermal anneal in bottom-pinned Mn₇₆Ir₂₄ spin valves is investigated. Samples were fabricated by ion beam deposition (IBD), post-annealed in vacuum (10^-6Torr) at 270 degreeC for 10 min, then cooled in a 3 kOe applied field. For a bilayer structure, glass/Ta 40 Aa/NiFe 30 Aa/MnIr 60 Aa/CoFe 25 Aa/Ta 40 Aa, the exchange field (H_ex) reaches 1148 Oe (J_ex=0.4erg/cm²) after anneal. X-ray diffraction (XRD) analysis shows strong enhancement of <111> texture upon anneal, while grain size obtained from XRD and transmission electron microscopy for as-deposited and annealed states shows no major change. With increasing MnIr thickness, the exchange field decreases, and blocking temperature (T_b) increases, reaching 295 degreeC for t_MnIr=180Aa. Spin valves built with the same exchange bilayer (Ta 20 Aa/NiFe 30 Aa/MnIr 60 Aa/CoFe 25 Aa/Cu 22 Aa/CoFe 20 Aa/NiFe 40 Aa/Ta 40 Aa) show H_ex=855Oe (J_ex=0.3erg/cm²) and magnetoresistance (MR)=7.1%. The incorporation of nano-oxide layers in spin valves increases the MR signal to 11%. No signal degradation is found in these specular structures for anneals up to 310 degreeC. [copyright] 2001 American Institute of Physics

[en] The thermal stability of magnetic tunnel junctions with ultrathin (<8 Aa) Al₂O₃ barriers was studied and compared with 15 Aa barriers. The tunnel magnetoresistance (TMR) decay cannot be explained only by Mn diffusion into the pinned CoFe layer since this diffusion starts above 300 degreeC independently of the barrier thickness, while the TMR degradation already occurs at 250 - 270 degreeC for the thinner barriers. The thermal stability is probably controlled by changes at the CoFe/Al₂O₃ interfaces and/or barrier structure. Structural analysis of 15 Aa barriers after annealing at 435 degreeC, shows the existence of an interface region (8 - 12 Aa thick) where CoFe and Al₂O₃ are found. This interfacial region can be explained by the increased roughness in the bottom electrode after annealing, as measured by atomic-force microscopy (from 1.5 to 4 Aa). Ultrathin barriers show a similar trend. The use of low-resistance junctions using thin barriers requires good control of the roughness of the low-resistance bottom electrodes. This is done by preannealing and low-angle ion-beam smoothing 500-Aa-thick Cu or Al films, which will then keep a roughness <2 Aa during processing temperatures up to 400 degreeC. Low-resistance junctions (R x A∼40 - 60 Ωμm²) with 7 Aa barriers grown on 600 Aa Al buffers after the surface treatment show 25% TMR after annealing at 270 degreeC. [copyright] 2001 American Institute of Physics

[en] Tunnel junctions with AlN (AlNxOy) barriers and CoFe and FeTaN electrodes were studied. The AlN barrier was formed by nitridizing a 10 Aa thick Al layer using radio frequency N₂ plasma. The nitrogen and oxygen content in the barriers was determined by Rutherford backscattering spectroscopy (RBS) on specially prepared barriers deposited on DLC coated substrates. RBS results indicate less than 10% O₂ incorporation in the barrier. Top-pinned junctions formed by nitridizing a 10 Aa thick Al layer (CoFe electrodes) show resistance x area products from 73Ωμm² to 8.5kΩμm² for increasing nitridation x from 30 to 200 s, with corresponding tunneling magnetoresistance (TMR) values from 13% to 33%. Bottom-pinned junctions with FeTaN electrodes were also fabricated. Maximum TMR signal is 17% after anneal at 225 degreeC for 30 min. In both cases (CoFe or FeTaN electrodes), TMR degrades for anneals above 250 degreeC. [copyright] 2001 American Institute of Physics

[en] Spin tunnel junctions fabricated with one interposed Fe - FeOx layer between the Al₂O₃ barrier and the top CoFe pinned electrode show large tunneling magnetoresistance (TMR) (40%) for anneals up to 380 degreeC. The annealing temperature T_TMR^*, where maximum TMR occurs, increases with the inserted Fe - FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment (which usually starts to decay below 300 degreeC in the normal junctions) increases with annealing temperature up to 380 degreeC and remains at a maximum until 450 degreeC. The large TMR at high temperature is related with the diffusion of extra Fe (from the Fe - FeOx layer) into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe₃O₄, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoelectron spectra for the Fe2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R x A values and barrier parameters versus annealing temperature for samples with 7 and 25 Aa Fe - FeOx also reflects the above structural changes in the inserted layer. [copyright] 2001 American Institute of Physics

[en] Annealing is a major step in the fabrication of magnetic tunnel junctions (MTJs). It sets the exchange bias between the pinned and antiferromagnetic layers, and helps to increase the tunnel magnetoresistance (TMR) in both amorphous and crystalline junctions. Recent research on MTJs has focused on MgO-based structures due to their high TMR. However, the strict process control and mandatory annealing step can limit the scope of the application of these structures as sensors. In this paper, we present AlOx-based MTJs that are produced by ion beam sputtering and remote plasma oxidation and show optimum transport properties with no annealing. The microfabricated devices show TMR values of up to 35% and using NiFe/CoFeB free layers provides tunable linear ranges, leading to coercivity-free linear responses with sensitivities of up to 5.5%/mT. The top-pinned synthetic antiferromagnetic reference shows a stability of about 30 mT in the microfabricated devices. Sensors with linear ranges of up to 60 mT are demonstrated. This paves the way for the integration of MTJ sensors in heat-sensitive applications such as flexible substrates, or for the design of low-footprint on-chip multiaxial sensing devices. (paper)

[en] Magnetic tunnel junction (MTJ) micropillars were fabricated with integrated thermometers and a heater line (HL) for thermovoltage measurements. This novel thermometer configuration enabled a direct measurement of Δ T across the MTJ micropillar. The MTJ devices were patterned from a CoFeB/MgO/CoFeB stack, with a 1.2 nm to 1.6 nm MgO wedge across the wafer, resulting in resistance area products in the range of 0.7 kΩ · µ m²  <   R   ×   A   <  55 kΩ · µ m². This allowed the measurement of thermoelectric properties as a function of the tunnel barrier thickness. The thermometers showed a homogeneous heating behavior for all devices across the wafer. Combining the in-stack temperature measurements and finite element simulations the thermal profile across the MTJ structure and the thermopower were estimated with a noticeable improvement of the measurement accuracy. The studied MTJ structures showed tunneling magnetoresistance (TMR) ratios up to 125%, and tunneling magnetothermopower (TMTP) up to 35%. (paper)

[en] Magnetoresistive sensors using spin valves and magnetic tunnel junctions are reviewed, considering applications as readers in hard disk drives, as well as applications where the ultimate field detection limits are required (from nT down to pT). The sensor noise level in quasi-DC or high-frequency applications is described, leading to sensor design considerations concerning biomedical and read head applications. Magnetic tunnel junction based sensors using MgO barriers appear as the best candidates for ultra-low field (pT) detection, either in the high-frequency regime, or for quasi-DC applications

[en] Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the metal-insulator multilayer [Co₈₀Fe₂₀(1.6 nm)/Al₂O₃(3 nm)]₉ which exhibits dominant dipolar coupling between the ferromagnetic layers. Our PNR measurements at the coercive field reveal a novel and unexpected magnetization state of the sample, exhibiting an oscillating magnetization depth profile from CoFe layer to CoFe layer with a period of five bilayers along the multilayer stack. With the help of micromagnetic simulations we demonstrate that competition between long- and short-ranged dipolar interactions apparently gives rise to this unprecedented phenomenon

[en] This work combines an electrophysiological system with a magnetoresistive chip to measure the magnetic field created by the synaptic/action potential currents. The chip, with 15 spin valve sensors, was designed to be integrated in a recording chamber for submerged mice brain slices used for synaptic potential measurements. Under stimulation (rectangular pulses of 0.1 ms every 10 s) through a concentric electrode placed near the CA3/CA1 border of the hippocampus, the spin valve sensor readout signals with 20 μV amplitude and a pulse length of 20 to 30 ms were recorded only in the pyramidal cell bodies region and can be interpreted as being derived from action potentials/currents.

[en] This work reports for the first time results on MgO tunnel junctions prepared by ion beam. The MgO barrier was deposited from a ceramic MgO target using an assisted beam, following the deposition and assisted beam phase diagram which relate the beam profile with the current and energy. The deposition rate for MgO is calculated with and without assisted beam, and compared with the experimental values. The MgO film growth on Ta/CoFeB/MgO simple stacks was optimized aiming at a (002) preferred orientation for the MgO growth, measured by x-ray diffraction. The optimum assist beam energy was tuned for each deposition beam condition (+800,+1000,+1200 V), using assist beams of 40 mA (∼130 μA/cm²) with 0 to +600 V. Without assist beam, no texture is observed for the MgO, while the (002) orientation appears for assisted deposition. The optimum range of assist voltages is large, being limited by the onset of etching at high voltages, reducing the deposition rate. Magnetic tunnel junctions were deposited with the structure Ta 50 A/Ru 200 A/Ta 50 A/Mn₇₈Ir₂₂ 150 A/Co₉₀Fe₁₀ 30 A/Ru 8 A/Co₅₆Fe₂₄B₂₀ 40 A/MgO t/Co₅₆Fe₂₄B₂₀ 30 A/Ru 30 A/Ta 50 A, with the MgO barrier deposited with the conditions optimized by x rays. The effect of the assist beam energy on the junction properties (magnetoresistance and magnetization) are discussed. Tunnel magnetoresistance values up to 110%, with RA products of 100-400 Ω μm², for 11 A thick MgO barriers are obtained using assisted deposition with a +100 V assist beam, which is a major improvement of the ∼30% of TMR, if no beam is used