M. A. Arranz

Interdiffusion and magnetic properties of Gd1-xCox/Co multilayers

The interdiffusion process in Gd1-xCox/Co multilayered systems has been investigated, in several series of samples made by sputtering, with different values of the Co content in the alloy (x). Grazing-incidence x-ray profiles and the electrical resistivity, together with magnetic measurements at room temperature, allow us to monitor and quantify the interdiffusion between layers as well as their magnetic properties. It is shown that this interdiffusion occurs on increasing the Co content of the alloy layer up to around 60% Co. In the x = 0.60 series the interdiffusion was found to be negligible and therefore a very good structure was obtained, while the ferrimagnetic structure between layers is preserved. The improved multilayered structure obtained in this system could lead to the development of new technological applications.

On the limits of uniaxial magnetic anisotropy tuning by a ripple surface pattern

Ion beam patterning of a nanoscale ripple surface has emerged as a versatile method of imprinting uniaxial magnetic anisotropy (UMA) on a desired in-plane direction in magnetic films. In the case of ripple patterned thick films, dipolar interactions around the top and/or bottom interfaces are generally assumed to drive this effect following Schlomanns calculations for demagnetizing fields of an ideally sinusoidal surface [E. Schlomann, J. Appl. Phys. 41, 1617 (1970)]. We have explored the validity of his predictions and the limits of ion beam sputtering to induce UMA in a ferromagnetic system where other relevant sources of magnetic anisotropy are neglected: ripple films not displaying any evidence of volume uniaxial anisotropy and where magnetocrystalline contributions average out in a fine grain polycrystal structure. To this purpose, the surface of 100 nm cobalt films grown on flat substrates has been irradiated at fixed ion energy, fixed ion fluency but different ion densities to make the ripple pattern at the top surface with wavelength Λ and selected, large amplitudes (ω) up to 20 nm so that stray dipolar fields are enhanced, while the residual film thickness t = 35–50 nm is sufficiently large to preserve the continuous morphology in most cases. The film-substrate interface has been studied with X-ray photoemission spectroscopy depth profiles and is found that there is a graded silicon-rich cobalt silicide, presumably formed during the film growth. This graded interface is of uncertain small thickness but the range of compositions clearly makes it a magnetically dead layer. On the other hand, the ripple surface rules both the magnetic coercivity and the uniaxial anisotropy as these are found to correlate with the pattern dimensions. Remarkably, the saturation fields in the hard axis of uniaxial continuous films are measured up to values as high as 0.80 kG and obey a linear dependence on the parameter ω2/Λ/t in quantitative agreement with Schlomanns prediction for a surface anisotropy entirely ruled by dipolar interaction. The limits of UMA tuning by a ripple pattern are discussed in terms of the surface local angle with respect to the mean surface and of the onset of ripple detachment.

Electrical resistivity and interdiffusion in Gd1−xCox/Co multilayers

The interdiffusion process between Gd and Co has been investigated in several series of multilayered films consisting of Co and different GdCo alloys (Gd1−xCox)/Co (x=0, 0.37, and 0.60). Grazing incidence x-ray profiles provided a preliminary insight into the structure of the samples. Electrical resistivity measurements were performed in a temperature range between 8 and 300 K. For the three series, the room temperature electrical resistivity shows a clear fall once a critical Co thickness is surpassed, which we interpret as due to the growth of pure polycrystalline Co between the alloy layers. A simple model allows us to quantify the amount of Co diffused into the alloy layer. The results will be discussed considering the possible decrease of interdiffusion effects when Co is deposited onto the more stable GdCo alloys. This leads to a well defined multilayered structure, something that is essential for practical applications and for the theoretical study of its underlying physics.

Surface magnetization and the role of pattern defects in various types of ripple patterned films

We present a detailed study of the magnetic properties of cobalt films with wide-area nanoscale ripple patterns, either on their surface only, or on both the film surface and substrate interface. Angular dependence vectorial-resolved magnetometry measurements and magnetic force microscopy with in situ magnetic field have been used to determine the magnetization reversal processes to correlate them to the different patterned nanostructures. All the samples show well-defined uniaxial magnetic anisotropy with the anisotropy axis lying along the ripple direction. Atomic force microscopy of the different types of pattern reveals various pattern defects: height corrugation and breaks of continuity along the ripple direction, and overlapping ripples and Y-shaped defects (pattern dislocation) across the pattern. In spite of the existence of such customary defects of erosive-regime patterns, the type of low-amplitude, surface-patterned films remarkably behave as a macrospin over almost the whole in-plane angular range (340°), with negligible spread of anisotropy axis or energy. In turn, it is found that high-amplitude surface-patterned films develop an angular distribution of anisotropy axes, probably related to the large distribution of amplitudes in a pattern of short ripples, and a significant distribution of anisotropy fields ΔH k/H k up to 15%. On the other hand, films grow on pre-patterned silicon with a significantly longer mean ripple length, and develop a larger anisotropy energy with H k up to 110 mT, probably because of the double interface effect. The switching fields close to the magnetization easy axis of all types of ripple pattern are not well reproduced by the macrospin approximation, but the observed pattern defects seem to be not responsible for the domain wall pinning that occurs with the field applied along the ripple direction.

The Kondo contribution to the electrical resistivity in UCu5−xNix and the non-Fermi liquid behaviour of UCu4Ni

We report on electrical resistivity measurements performed on polycrystalline samples of UCu5?xNix ?(x = 0.25, 1). In order to extract the Kondo contribution to the resistivity, the experiments were carried out over a wide temperature range (0.4?800 K). From the analysis of our results, we conclude that the Kondo temperature takes values of TK ~ 240?K for x = 1 and TK ~ 245?K for x = 0.25, and that for both Ni concentrations the dominant part of the remarkably high residual resistivity (?(0) ~ 400????cm) corresponds to the Kondo contribution. These results are discussed in comparison with previous analysis of specific heat and magnetic susceptibility data that produced similar values of TK. We interpret our results in terms of disorder-driven non-Fermi liquid behaviour for UCu4Ni, as indicated by the anomalous temperature dependences of the electrical, thermal and magnetic properties previously observed in this compound.

Study of the different magnetoresistance sources in Ag/Co multilayers

We report results on magnetoresistance and magnetic properties of sputtered Ag/Co multilayers and their relation to structural properties. We found two components of the magnetoresistance: isotropic and anisotropic. The first one is found to be related to cobalt particles at the interfaces between magnetic and nonmagnetic layers and also to cobalt particles diluted into the silver layers. The other contribution is related to ferromagnetic multidomain Co layers. The results on magnetoresistance and magnetization at low fields, and conductivity measurements, give clear proof of a transition from granular to continuous structure of the magnetic layer. For example, in a Ag/Co multilayer series with silver thickness of 20 A, such a transition occurs for a cobalt thickness around 5 A.

Nanoscale ripple formation in Co/Si(100) thin films with Ar+ beam etching

We have investigated the formation of nanoscale ripples on etched Co/Si(100) films with Ar+ beam in grazing incidence. Topography and dimensions of those nanoscale patterns were characterized by means of atomic force microscopy. Polycrystalline cobalt thin films were deposited by d.c. magnetron sputtering onto Si(100) wafers and, later transferred in situ to a process chamber for the production of ripples. Their average width, Wd, and separation between them, i.e. their periodicity Λ, were found to monotonously increase first with the etching time and, finally, reach saturation values for long irradation times (around 30 min). The same Ar+ beam etching applied on thicker Co films resulted in much wider and higher ripples, providing a more defined nanostructure for ulterior uniaxial magnetic anisotropy measurements. These changes in the ripple dimensions on increasing the Co film thickness are discussed in terms of the surface roughness in the as-deposited film.

Spin-glass-like static and dynamic properties of mechanically alloyed Fe–Re–Cr

We report the observation of spin-glass-like features in a Fe61Re30Cr9 sample obtained after high energy milling for 460 h. X-ray diffraction showed a broad nanocrystalline peak. The zero-field-cooled and field-cooled magnetization curves exhibited a clear irreversibility, the former showing a relatively sharp maximum for low fields at Tmax≈60 K. In addition, the temperature dependence of the magnetic AC susceptibility (χac) also showed a peak at approximately the same temperature. The frequency sensitivity of such maximum (p=ΔTmax/TmaxΔ log w≈0.02) is small compared to that of typical superparamagnets. However, we discuss the insufficiency of this set of phenomena to assert the existence of a true spin-glass transition and propose the blocking of interacting ferromagnetic nanoparticles as the origin of the observed behavior. The effect of annealing, rendered into nanocrystallite growth, on the χac maximum is studied in an attempt to test this explanation.

Angular tuning of the magnetic birefringence in rippled cobalt films

We report the measurement of magnetically induced birefringence in rippled Co films. For this purpose, the magneto-optical properties of ion beam eroded ferromagnetic films were studied using Kerr magnetometry and magnetic birefringence in the transmitted light intensity. Upon sufficient ion sculpting, these ripple surface nanostructures developed a defined uniaxial anisotropy in the in-plane magnetization, finely tuning the magnetic birefringence effect. We have studied its dependence on the relative orientation between the ripple direction and the magnetic field, and found this effect to be dramatically correlated with the capability to neatly distinguish the mechanisms for the in-plane magnetization reversal, i.e., rotation and nucleation. This double refraction corresponds univocally to the two magnetization axes, parallel and perpendicular to the ripples direction. We have also observed that tuned birefringence in stack assemblies of rippled Co films, which enables us to technically manipulate the number and direction of refraction axes.

Magnetic Kondo scattering in the electrical resistivity of sputtered Ti-Co films

We have investigated the influence of the granular microstructure in the electrical resistivity of RF-sputtered Ti100−xCox thin films. Their x-rays characterization has been correlated with dc resistivity measurements from 7 to 300 K. A minimum in the resistivity, ρ(T), centered around Tm∼50 K, was recorded for Ti93Co7 samples grown at 30 W. Contrarily, samples grown at 110 W and the same x value displayed no evidences of that low-temperature anomaly. This phenomenon correlated with the change of the majority Ti granular phase: hcp-Ti for the sample grown at 30 W and bcc-Ti for the 110 W sample. That minimum arises from the great difference in Co solubility into both Ti granular phases (0.8 at. % in hcp-Ti and 14.5 at. % in bcc-Ti). Hence, the very small quantity of Co diluted into the hcp-Ti grains allows us to consider Co particles as uncorrelated magnetic impurities which scatter the conduction electrons. This magnetic Kondo scattering would be the mechanism responsible for the observed minimum in the ...