[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] The hyperfine field at metal lattice sites in a nanostructured HfO₂ thin film was studied by the Perturbed Angular Correlation (PAC) technique using ¹⁸¹Ta as probe nuclei. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The resulting film thickness was about 25 nm. The ¹⁸¹Ta isotopes are formed in Hf sites of the sample as disintegration product of radioactive ¹⁸¹Hf, which were produced by neutron activation of the very thin film by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements of the electric field gradient at the ¹⁸¹Ta probe sites were performed with the thin film in the temperature range 295 - 1273 K in vacuum. Results showed that the film has a single crystalline structure and two different quadrupole frequencies could be observed corresponding to ¹⁸¹Ta at monoclinic structure and with oxygen vacancy in its neighborhood.

[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] In this article we report the damage and annealing behavior as well as lattice site location of Fe atoms in GaN. The Fe ions were homogeneously implanted in GaN films with an energy of 150 keV at room temperature. A two-step annealing (650 degree C 15 min and then 1000 degree C 2 min) was performed to remove the implantation-induced damage and to drive the dopants into the lattice site. The structure of GaN films before and after the implantation as well as at each stage of the annealing was characterized by Rutherford backscattering/channeling combined with particle induced x-ray emission and high resolution x-ray diffraction. The Fe⁺ implanted GaN films exhibits an expanded lattice. After the two-step annealing, the lattice distortion does not fully recover. Angular scans along both [0001] and [10 bar 11] directions show that the Fe atoms occupy the lattice site of Ga atoms in the case of low dose implantation after annealing. However, for the high dose implanted GaN, about 75% of the implanted Fe atoms substitutes Ga atoms, showing that the damage is not annealed out and the dopants are not completely activated. [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] The γ-γ Perturbed Angular Correlation technique was used to study the hyperfine interaction of ¹⁸¹Ta at the Hf site(s) in UFe₄Al₈ at room temperature and 12 K. The data at room temperature are well described by two electric field gradients, while at low temperature two combined hyperfine interactions have to be considered, one with the magnetic hyperfine field collinear with the c-axis and another with the magnetic hyperfine field in the basal plane. The results are compared with previous Moessbauer and neutron diffraction experiments and the lattice site of Hf is discussed.

[en] Conversion electron-gamma PAC measurements of the 49-37 keV cascade in ⁸⁰Br through the intermediate 2^- state with T_1/2=7.4 ns were performed with a system of two magnetic lens spectrometers and two BaF₂ scintillation detectors. The parent ^80mBr activity with a halflife of 4.4 hrs was implanted into Ni, Zn and graphite at the ISOLDE separator at CERN. The observed interaction frequency in the nickel matrix is in good agreement with the known value of the hyperfine field for Br in Ni and the magnetic moment of the 2^- state. From the measured quadrupole interaction in Zn and graphite the electric field gradients at Br were obtained.

[en] In this work the perturbed angular correlation technique is used to locally monitor the epitaxy and thermal stability of the hafnium layer in a Zr/Hf/Zr multilayer. The studies reveal that the system is highly homogeneous and presents high epitaxy. Measurements at high temperatures show that no HfO_x formation occurs up to 562 K but at this temperature the Hf layer is no longer oxygen-defect free.

[en] The results reported in this work show the relevance of the lithium deficiency for the Li lattice site occupation of dopants in congruent lithium niobate crystals co-doped with 0.1 mol% Cr₂O₃ and different concentrations of HfO₂, up to 3 mol%. The same behavior is observed in a crystal doped with 1 mol% of HfO₂ and co-doped with 4 mol% MgO where after annealing most of the Hf/Ta probes occupy the Nb lattice site.

[en] The hyperfine field at ¹⁸¹Ta lattice sites in a nanostructured HfO₂ thin film was studied by the perturbed angular correlation (PAC) technique. The thin oxide film was deposited by pulsed laser ablation on a silicon substrate kept at 673 K. The thickness was about 25 nm. The radioactive ¹⁸¹Hf ions were produced by neutron activation of the very thin film in the Portuguese research reactor by the reaction ¹⁸⁰Hf(n,γ)¹⁸¹Hf. PAC measurements were carried out at room temperature after annealing at different temperatures up to 1,473 K in air. The PAC technique allows determining the electric field gradient at the ¹⁸¹Ta probe sites. The ¹⁸¹Ta isotopes appear in the sample as disintegration product of ¹⁸¹Hf.