[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] A simple tight-binding theoretical model is proposed for spin-dependent, current-in-plane transport in highly coherent spin valve structures under specularity conditions. Using quantum-mechanically coherent and spatially quantized Fermi states in the considered multilayered system, a system of partial Boltzmann kinetic equations is built for relevant subbands to yield the expressions for conductance in parallel or antiparallel spin valve states and thus for the magneto-conductance. It is shown that specularity favors the magnetoresistance for this structure reaching a high value (very close to 100%). This result is practically independent of the model parameters, in particular it does not even need that the lifetimes of majority and minority carriers be different (as necessary for the quasiclassical regimes). The main MR effect in the considered limit is due to the transformation of coherent quantum states, induced by the relative rotation of magnetization in the FM layers. Numerical calculations based on the specific Boltzmann equation with an account of spin-dependent specular reflection at the interfaces is also performed for a typical choice of material parameters

[en] Magnetic tunnel junctions (MTJs) are under investigation since they offer great potential for applications in magnetic memories. An interesting effect in TJs concerns non-volatile resistive switching of non-magnetic origin. We report magnetic (magnetoresistance) and structural (resistive; R) switching in MgO-based MTJs (barrier thicknesses t = 0.75, 1.35 nm). As-grown MTJs display R-switching only in the thinnest series, while thicker barrier samples need an electroforming step for R-switching to occur. Forming changes the electrical resistance temperature dependence, from tunnel- to metallic-like, revealing the formation of conductive bridges across the barrier which, leading to local high electrical fields and temperatures, are essential for resistive switching.

[en] In this article, a method to measure the electrical resistivity/conductivity of metallic thin films during layer growth on specific underlayers is described. The in situ monitoring of an underlayer electrical resistance, its change upon the incoming of new material atoms/molecules, and the growth of a new layer are presented. The method is easy to implement and allows obtaining in situ experimental curves of electrical resistivity dependence upon film thickness with a subatomic resolution, providing insight in film growth microstructure characteristics, specular/diffuse electron scattering surfaces, and optimum film thicknesses.

[en] Amorphous Nd–Co films with perpendicular magnetic anisotropy have been nanostructured in a lateral magnetic multilayer geometry in order to analyse and modify in a controlled way the configuration of its characteristic stripe domains as well as their rotation processes. Magnetic force microscopy measurements reveal that, actually, the artificial thickness modulation results in size effects on the magnetic stripes, so that the stripe domains configuration can be tuned with the lateral multilayer periodicity, and, due to the consequent reduction of the rotatable magnetic anisotropy, it is possible to modify the stripe rotation processes for in-plane magnetization reversal. (paper)

[en] Current induced resistance switching (CIS) was recently observed in thin tunnel junctions (TJs) with ferromagnetic (FM) electrodes and was attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. Here the CIS effect is studied in TJs with two thin (20 A) non-magnetic (NM) Ta electrodes inserted above and below the insulating barrier. We observe resistance (R) switching for positive applied electrical current (flowing from the bottom to the top lead), characterized by a continuous decrease in resistance and associated with current-driven displacement of metallic ions from the bottom electrode into the barrier (thin barrier state). For negative currents, displaced ions return to their initial positions in the electrode and the electrical resistance gradually increases (thick barrier state). We measured the temperature (T) dependence of the electrical resistance of both thin- and thick-barrier states (R_b and R_B, respectively). Experiments showed a weaker R(T) variation when the tunnel junction is in the R_b state, associated with a smaller tunnel contribution. By applying large enough electrical currents we induced large irreversible R decreases in the studied TJs, associated with barrier degradation. We then monitored the evolution of the R(T) dependence for different stages of barrier degradation. In particular, we observed a smooth transition from tunnel- to metallic-dominated transport. The initial degradation stages are related to irreversible decreases in barrier thickness (without the formation of pinholes). Only for later stages of barrier degradation do we have the appearance of metallic paths between the two electrodes that, however, do not lead to metallic-dominated transport for small enough pinhole radius

[en] A multi-functional Gd₅Si_1.3Ge_2.7 thin film deposited by pulsed laser ablation in the form of an ensemble of nanoparticles was studied for 18 thermal cycles via electron transport measurements together with structural and magnetic characterization. A general negative thermal dependency of the resistivity (ρ) is observed, which contrasts with the metallic-like behavior observed in bulk Gd₅Si_xGe_4−x compounds. This general trend is interrupted by a two-step, positive-slope transition in ρ(T) throughout the [150, 250] K interval, corresponding to two consecutive magnetic transitions: a fully coupled magnetostructural followed by a magnetic order on heating. An avalanche-like behavior is unveiled by the ∂ρ/∂T(T) curves and is explained based on the severe strains induced cyclically by the magnetostructural transition, leading to a cycling evolution of the transition onset temperature ($\mathrm{\partial <!--\; ???\; -->}{T}_{\mathrm{h}}^{\mathrm{\prime <!--\; ???\; -->}\mathrm{\prime <!--\; ???\; -->}}$/∂n ∼ 1.6 K/cycle, n being the number of cycles). Such behavior is equivalent to the action of a pressure of 0.56 kBar being formed and building up at every thermal cycle due to the large volume induced change across the magnetostructural transition. Moreover the thermal hysteresis, detected in both ρ and magnetization versus temperature curves, evolves significantly along the cycles, decreasing as n increases. This picture corroborates the thermal activation energy enhancement—estimated via an exponential fitting of the ∂ρ/∂T(T) in the avalanche regime. This work demonstrates the importance of using a short-range order technique, to probe both magnetic and magnetostructural transitions and their evolution with thermal cycles. (paper)

[en] Although Cd and Ca ions have the same valence and cation size, their incorporation into vacancy-doped La manganites induce different properties. While the incorporation of Ca leads to high Tc up to 250 K and induces a metallic-like behaviour, the incorporation of Cd severely reduces Tc and promotes insulator-like behaviour. In this work, the Cd hyperfine fields have been measured with the Perturbed Angular Correlations (PAC) technique after implantation and annealing of ^111mCd in La-Cd-MnO₃ samples. The PAC results are compared with measurements of the resistivity and magnetization performed on the same samples. The mixed La and Mn site Cd occupancy is suggested as a possibility to explain the properties of the La-Cd-MnO₃ system

[en] We report the observation of spin-dependent quantum well (QW) resonant tunneling in textured CoFeB free layers of single MgO magnetic tunnel junctions (MTJs). The inelastic electron tunneling spectroscopy spectra clearly show the presence of resonant oscillations in the parallel configuration, which are related with the appearance of majority-spin Δ₁ QW states in the CoFeB free layer. To gain a quantitative understanding, we calculated QW state positions in the voltage-thickness plane using the so-called phase accumulation model (PAM) and compared the PAM solutions with the experimental resonant voltages observed for a set of MTJs with different CoFeB free layer thicknesses (t_fl = 1.55, 1.65, 1.95, and 3.0 nm). An overall good agreement between experiment and theory was obtained. An enhancement of the tunnel magnetoresistance with bias is observed in a bias voltage region corresponding to the resonant oscillations

[en] Magnetic fields generated by current lines are the standard way of switching between resistance (R) states in magnetic random access memories. A less common but technologically more interesting alternative to achieve R-switching is to use an electrical current crossing the tunnel barrier. Such current induced magnetization switching (CIMS) or current induced switching (CIS) effects were recently observed in thin magnetic tunnel junctions, and attributed to spin transfer (CIMS) or electromigration of atoms into the insulator (CIS). In this work, electromigration-driven resistance changes (resistive switching) are superimposed with thermally induced pinned layer reversal (magnetic switching), producing a reproducible, three-state memory device. The tunnel junctions under study show a tunnel magnetoresistance of 14% with a RA product of 50 Ω μm². The reversible electromigration-driven resistance changes amount to 7-8% of the full resistance change and more than 10⁴ R-switching events can be current induced without significant damage to the tunnel junction. Typical critical current densities are of the order of 2 x 10⁶ A cm^-2