J. Vergara

Controlled anisotropic GMR in laser ablated Cu-Co thin films

Abstract Controlled microstructured Cu-Co thin films have been fabricated by pulsed-laser ablation during which the target was rotated between 10 and 32 rpm. The samples exhibit giant magnetoresistance after the films are annealed. The films obtained at low rotation velocities require a higher magnetic field ( ≈ 0.15 T) to achieve the maximum resistance in the perpendicular MR as compared to 0.05 T for the parallel MR. On the other hand, the films obtained at higher rotation velocities require quantitatively similar magnetic fields in both orientations, about 0.05 T. This effect is discussed in terms of shape and crystalline anisotropy of magnetic nanoparticles.

Effect of the decomposition process in the magnetic properties of disordered FePd alloys

The influence of the decomposition process from disordered to ordered phases on the magnetic properties of several FePd-based alloys (Fe 75 Pd 25 and Fe 21 Pd 79 ) has been investigated by X-ray diffraction and VSM magnetometry. The starting material mainly consist of the non-equilibrium disordered FePd phases (BCC for Fe 75 Pd 25 and FCC for Fe 21 Pd 79 ), which upon thermal treatments evolve to their respective equilibrium phase distributions; namely, the coexistence of α-Fe and γ 1 -FePd ordered phases in Fe 75 Pd 25 , and the precipitation of the γ 2 -FePd 3 ordered phase in the Fe 21 Pd 79 sample. In the Fe-rich sample, the precipitation of the ordered γ 1 -FePd equilibrium phase leads to a magnetic hardening with respect to the initial disordered state. The Fe 21 Pd 79 presents an order-disorder transition that determines the evolution of the magnetic behavior of the sample. By annealing, γ 2 -FePd 3 and disordered γ-FePd regions coexist. The increase in the volume fraction of the ordered γ 2 -FePd 3 phase with the thermal treatments, plays the dominant role in the evolution of the magnetic properties of the sample.

Magnetic anisotropy in isotropic and nanopatterned strongly exchange-coupled nanolayers.

In this study, the fabrication of magnetic multilayers with a controlled value of the in-plane uniaxial magnetic anisotropy field in the range of 12 to 72 kA/m was achieved. This fabrication was accomplished by the deposition of bilayers consisting of an obliquely deposited (54°) 8-nm-thick anisotropic Co layer and a second isotropic Co layer that was deposited at a normal incidence over the first layer. By changing the thickness value of this second Co layer (X) by modifying the deposition time, the value of the anisotropy field of the sample could be controlled. For each sample, the thickness of each bilayer did not exceed the value of the exchange correlation length calculated for these Co bilayers. To increase the volume of the magnetic films without further modification of their magnetic properties, a Ta spacer layer was deposited between successive Co bilayers at 54° to prevent direct exchange coupling between consecutive Co bilayers. This step was accomplished through the deposition of multilayered films consisting of several (Co8 nm-54°/CoX nm-0°/Ta6 nm-54°) trilayers.

A universal scaling property of the giant magnetoresistance in nanocrystallized Cu-Co melt-spun ribbons

Abstract We present the field and temperature dependencies of the magnetoresistance in melt spun Cu86Co14 and Cu92Co8 ribbons nanocrystallized by annealing them at higher temperatures. It is found that the giant magnetoresistance, GMR, values measured at 5K, 77K, and room temperature scale as a function of B (T+ θ) in the regime where the nanocrystals are superparamagnetic. Furthermore, by taking into account the evolution of the magnetic properties of the nanocrystals with annealing, all the GMR data are found to be collapsible to a universal function, with the Langevin variable mB (T+ θ) , where m is a scaling factor dependent on the magnetic property of the annealed state.

Magnetically Anisotropic Ni2MnGa Thin Films: Coating Glass and Si Micro-Cantilevers Substrates

Ni2MnGa thin films, with thickness between 30 and 60 nm, were pulsed-laser deposited at room temperature on Si micro-cantilevers and glass substrates. Two different deposition processes were performed: normal deposition and off¬-normal. After annealing in an inert atmosphere, in-plane isotropic magnetic hysteresis loops were measured for the normal deposited films. In contrast, in-plane anisotropic hysteresis loops were obtained from the off-normal deposited ones. An in-plane easy direction for the magnetisation, perpendicular to the incidence plane of the plasma during deposition, was measured with an anisotropy field of ≈100 Oe and an easy coercive field of ≈24 Oe. The mechanical behaviour of the magnetically anisotropic coated micro-cantilevers and their response to a decreasing temperature permitted observing the martensitic transformation of the Ni2MnGa thin films.

Generating and measuring the anisotropic elastic behaviour of Co thin films with oriented surface nano-strings on micro-cantilevers

In this research, the elastic behaviour of two Co thin films simultaneously deposited in an off-normal angle method was studied. Towards this end, two Si micro-cantilevers were simultaneously coated using pulsed laser deposition at an oblique angle, creating a Co nano-string surface morphology with a predetermined orientation. The selected position of each micro-cantilever during the coating process created longitudinal or transverse nano-strings. The anisotropic elastic behaviour of these Co films was determined by measuring the changes that took place in the resonant frequency of each micro-cantilever after this process of creating differently oriented plasma coatings had been completed. This differential procedure allowed us to determine the difference between the Youngs modulus of the different films based on the different direction of the nano-strings. This difference was determined to be, at least, the 20% of the Youngs modulus of the bulk Co.PACS: 62.25.-g; 81.16.Rf; 68.60.Bs; 81.15.Fg; 68.37.Ef; 85.85.+j

Influence of the target morphology on anisotropic GMR of CuCo laser ablated thin films

Abstract A correlation between the structure of the target surface and the magneto-transport properties of pulsed laser ablated CuCo thin films is presented in this work. During the deposition process, if the target rotation velocity is kept at 7 rpm, craters and cones, with an average roughness of 9 μm, appear on the target surface after 15 min deposition time. A post-deposition annealing induces the appearance of giant magneto-resistance in the films, and coercivities up to 1700 Oe. However, as the target rotation speed is increased up to 32 rpm, more craters and cones are present on the surface but its average rugosity decreases to 8 μm. The maximum coercivity for the samples deposited in these conditions is 1100 Oe. A possible mechanism to explain this behavior is discussed.

Laser Ablated Pure Non-Crystalline Co Thin Films for Inductors for Ultra-High Frequencies

Abstract : Non-crystalline Co thin films have been prepared by pulsed laser ablation deposition. From the M-H hysteresis loops measurements, a soft magnetic behavior is observed. Neel type magnetic domain walls are observed in the as-deposited films. The spontaneous magnetization, M(sub s) (T = 300 K), is approx. 860 emu/cu cm. After annealing at 500 deg C, M(sub s) (T = 300 K) is approx. 1460 emu/cu cm. The extrapolated to zero K resistance decreases almost two orders of magnitude from the as deposited samples to the crystallized heated at 500 deg C ones. A trilayer Co/Cu/Co has shown a real part magnetic susceptibility of 120 at 100 MHz. In the 100 MHz to 1 GHz frequency range, a perpendicular bias magnetic field increased this value up to 270, remaining almost constant for all range.

Structurally Oriented Nano-Sheets in Co Thin Films: Changing Their Anisotropic Physical Properties by Thermally-Induced Relaxation

We show how nanocrystalline Co films formed by separated oblique nano-sheets display anisotropy in their resistivity, magnetization process, surface nano-morphology and optical transmission. After performing a heat treatment at 270 °C, these anisotropies decrease. This loss has been monitored measuring the resistivity as a function of temperature. The resistivity measured parallel to the direction of the nano-sheets has been constant up to 270 °C, but it decreases when measured perpendicular to the nano-sheets. This suggests the existence of a structural relaxation, which produces the change of the Co nano-sheets during annealing. The changes in the nano-morphology and the local chemical composition of the films at the nanoscale after heating above 270 °C have been analysed by scanning transmission electron microscopy (STEM). Thus, an approach and coalescence of the nano-sheets have been directly visualized. The spectrum of activation energies of this structural relaxation has indicated that the coalescence of the nano-sheets has taken place between 1.2 and 1.7 eV. In addition, an increase in the size of the nano-crystals has occurred in the samples annealed at 400 °C. This study may be relevant for the application in devices working, for example, in the GHz range and to achieve the retention of the anisotropy of these films at higher temperatures.