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Investigation of ion beam deposited spin valve interface structure by 59Co nuclear magnetic resonance
Abstract
59Co nuclear magnetic resonance was used to study the bulk and interface microstructure in the unpinned Co layer in a series of highly (111)-textured ion beam deposited spin valves. The intent of the study is to determine the degree of Co interface intermixing and any possible variation of the Co layer microstructure as a result of weakening (111) texture of the NiFe/Co/Cu/Co/NiFe/FeMn film stack. We found that the interfaces are diffused over ∼2 monolayers on both sides of the Co layer, involving an equivalent of about 3 Å of Co per interface and vary only slightly with deteriorating texture. The main structural change is the drop of perfect fcc stacking in the Co layer. © 1999 American Institute of Physics.
... The most perfect texture is characteristic of the sample no. 1 (FWHM = 3.35°). The range of the change in the FWHM of the samples studied is typical of the spin valves prepared by dc magnetron sputtering [69,70]. The more perfect texture (FWHM = 2.5°) in the spin valves was obtained, for example, by the authors of [70] with the use of an rf magnetron sputtering in combination with the dc sputtering. ...
The microstructure and the magnetic and magnetotransport properties have been investigated depending on the types of the magnetic and nonmagnetic materials, composition, nanostructure, and regimes of thermomagnetic treatments for two types of metallic nanostructures, namely, magnetic multilayers and spin valves, which exhibit the effect of a giant magnetoresistance (GMR). Magnetosensitive materials that are promising for the applications in microelectronics have been synthesized: CoFe/Cu multilayers with record values of the GMR effect (81%); NiFeCo/Cu multilayers with a sensitivity of 0.5%/Oe; and microobjects prepared on the basis of spin valves, which possess the anhysteretic field dependence of the magnetoresistance and a GMR effect equal to 9%. Special attention is given to the methods of decreasing hysteresis. A comparison of the obtained results with the results for similar types of materials published earlier has been carried out.
... Большие возможности для определения состо яния слоев и интерфейсов предоставляет месс бауэровская спектроскопия [4][5][6][7], но примене ние этого метода ограничено в силу того, что для исследований требуется слой Fe (желательно обо гащенное изотопом 57 Fe). Значительно более ши рокий спектр многослойных структур может быть исследован с помощью метода ядерного магнитно го резонанса (ЯМР) [8]. В частности, метод ЯМР на 59 Co применялся для исследования магнитных свойств и структурных особенностей сверхрешеток [Co/Cu] n [9,10]. ...
... The measurements showed that the most perfect texture is characteristic of sample 1, in which the angle of orientation spread of the texture is γ = 3.35°. On the whole, the range of the variation of γ in the samples studied is typical of spin valves prepared by dc magne tron sputtering [15,16]. A more perfect texture (γ = 2.5°) was obtained in spin valves in [16], where high frequency magnetron sputtering in combination with dc magnetron sputtering was employed. ...
Dc magnetron sputtering was used to prepare spin valves based on an Mn75Ir25 antiferromagnet with a composite free layer Ni80Fe20/Co90Fe10 and different thicknesses of the magnetic and nonmagnetic layers. Investigations of the microstructure and texture have been performed using X-ray diffraction and transmission electron microscopy. Field dependences of the magnetoresistance were measured for samples with a different degree of perfection of the axial texture 〈111〉: from sharp (with an orientation spread angle of 3.3°) to strongly smeared (with an orientation spread angle of 19.3°). It has been found that the width of the low-field loop of the magnetoresistance hysteresis increases monotonically with a decrease in the texture strength from sample to sample. The results are interpreted with allowance for the dependence of the hysteresis-loop width on the mutual orientation of the external magnetic field and easy axes of individual crystallites of the textured film.
The method of nuclear magnetic resonance has been used to investigate the structural features of the interfaces in [Co/Cu] n sublattices prepared using the method of magnetron sputtering. Changes in the structural characteristics of the interfaces and magnetoresistive properties of the sublattices have been analyzed depending on the number n of Co/Cu bilayers. Correlations have been found between the magnitude of the magnetoresistance and structural characteristics of the interfaces.
Магнетронным напылением на постоянном токе были приготовлены спиновые клапаны на основе антиферромагнетика Mn75Ir25 с композитным свободным слоем Ni80Fe20/Co90Fe10 и различными толщинами магнитных и немагнитных слоев. Методами рентгеновской дифракции и просвечивающей электронной микроскопии проведены исследования микроструктуры и текстуры. Полевые зависимости магнитосопротивления измерены для образцов с различной степенью совершенства аксиальной текстуры 〈111〉, от ярко выраженной – с углом рассеяния 3.35°, до почти отсутствующей – с углом рассеяния 19.3°. Обнаружено, что ширина низкополевой петли гистерезиса магнитосопротивления монотонно возрастает с увеличением среднего угла рассеяния текстуры от образца к образцу. Интерпретация полученных результатов дана с учетом зависимости ширины петли гистерезиса от взаимной ориентации внешнего магнитного поля и осей легкого намагничивания отдельных кристаллитов текстурированной пленки.
An internal-field 59Co NMR study of cobalt-containing Fischer-Tropsch synthesis catalysts supported on different alumina modifications was reported. The Co/δ-Al2O3 sample was shown to contain single-domain fcc packing and stacking faults, whereas Co/γ-Al2O3 gave signals from the fcc domain walls, hcp and stacking faults, thus indicating differences in the particle size of the studied samples. T
2 relaxation times were measured; their distribution in a spectrum is non-uniform, which allows signals to be distinguished by their relaxation times. Quantitative measurements of the relative atoms content in different packings revealed that the catalysts have mostly a defect structure. A brief historical background was presented to characterize the internal-field 59Co NMR technique, the related problems, and different approaches to acquired data interpretation.
A 59Co NMR study has been carried out in a series of Co/Cu multilayers with tCu = 20 Å and 2 Å < tCo < 30 Å. The Co layer is found to have a strained fcc structure with hcp stacking faults above 15 Å Co. The lateral size of columnar grains is estimated to be 45 Å. A transformation from continuous to granular Co layer is evidenced below 10 Å Co. Below 3 Å all deposited Co exists in form of a diffused CoCu alloy and Co clusters disappear. These structural changes, together with the decrease of a number of domain walls inferred from the rapid drop of NMR enhancement factor are to be related with the previously observed disappearance of magnetoresistive hysteresis.
A coherent, computer controlled, frequency swept spin echo spectrometer designed for fast automatic spectrum acquisition in modern ferromagnetic materials is described.
Co alloys with Cr, Fe, Ru and Cu in the low concentration range have been studied by 59Co NMR. Regularly spaced satellite lines are observed that develop with increasing impurity concentration. The satellite intensities as a function of the impurity concentration have been compared with the binomial distribution law of particular Co environments. Deviations from the exact binomial law in some cases are briefly discussed against the properties of their respective phase diagrams. From this comparison it is concluded that satellite frequencies are primarily determined by the number of impurity atoms in the first neighbour shell: each additional impurity atom in the shell introduces a constant (element-dependent) modification to the hyperfine field value on a Co site. This assignment of satellite frequency shifts to given configurations in the first coordination shell provides references for structural investigations in related binary metallic systems such as metallic multilayers. Preliminary results for CoPt alloys are also presented which do not follow the systematics of the previous alloys but rather suggest an extended oscillating perturbation around Pt impurities.
Layered … /Cu/Co/Cu … of various origins have been investigated by NMR. The paper reports on the influence of different preparation techniques (UHV evaporation, dc magnetron or rf diode sputtering), of different buffer layers and of Co and Cu thickness on the bulk and interface atomic structure of the Co layers.
Owing to the sensitivity of the hyperfine field to the topological and chemical environment of the probe nuclei, NMR spectra can be considered as detailed histograms of the short range order ruling the structure of the material under investigation. Complementary to diffraction techniques this gives a local insight into the structure in the direct space. We present here recent developments of the method which have been used for structural investigations of cobalt layers imbedded in Co/X multilayers and particularly of buried interfaces. Indeed the use of spectrum modelling allows a detailed, quantitative, description of the way intermixing takes place at the interfaces. Co/Cu multilayers, a case of weakly miscible elements, have been specially investigated. Such a case is the most likely to yield sharp interfaces. Actually, the interface structure is shown to depend drastically on the preparation conditions and techniques, resulting in very different interfaces from abrupt to largely mixed. Solid solution forming elements (Co/Ru or Co/Cr) have also been studied for which interdiffusion at the interfaces is more expected. Indeed thick diffusion profiles, across several atomic planes, are observed which are much less dependent on the preparation technique than in the former case. The case of compound forming elements (Co/Fe) shows the largest interface admixture with a structure that seems also reminiscent of the bulk alloy phase diagram.
Correlations in the scattering from the ordering of impurities due to interdiffusion between metals at interfaces are taken into account. The constructive interference between scattering amplitudes produces enhanced spin-dependent scattering from the interface between magnetic and nonmagnetic metals.
We indicate one microstructural feature which can control the GMR
ratio in a series of highly (111)-textured, ion-beam deposited
“top” spin valves. The textural dispersion, as measured by
x-ray diffraction rocking curves, is shown quantitatively to influence
the resistivity, and indirectly, the GMR ratio, producing a
factor-of-three variation from 2% to 6%. Concurrent variations in the
degree of antiparallel alignment are ruled out. To study more closely
the dependence of resistivity on textural dispersion, the trend has been
reproduced on single Cu films of comparable thickness. Fits to
resistivity vs. thickness, AFM, and TEM characterization of the Cu films
indicate that the resistivity variations come from spin-independent
scattering in the bulk rather than at the surfaces of the films. The GMR
variation is attributed to variations in the grain-boundary scattering;
however, differing contributions of the grain boundary reflectivity and
of the grain size could not be separated