I. Orue

Magnetic behavior of Fe‐Nb and Fe‐Zr alloys nanocrystallized by means of flash annealing

Recently nanocrystalline alloys obtained from Fe‐rich amorphous metallic glasses have shown excellent soft magnetic properties for technical applications. They include 4d elements like Nb or Zr for obtaining nanometer‐size crystals after heat treatments of about 1 h at temperatures around 500 °C. Flash annealing, using an electrical current flowing along the sample, is a useful method for performing heat treatments in short periods of time without need of controlled atmosphere. Grain sizes in the range 8–20 nm were determined from x‐ray diffraction in crystallized alloys prepared by flash annealing technique. These are grain sizes even smaller than the ones obtained in furnace treated samples. Magnetic softening of the alloys is observed after the heating. Magnetization work, permeability, coercitive force, etc., are very similar after applying both types of treatments.

Magnetic influence on the martensitic transformation entropy in Ni-Mn-In metamagnetic alloy

We study the martensitic transformation (MT) of metamagnetic shape memory alloy Ni50Mn34.5In15.5 in the magnetic fields up to 12 T. The observed dependence of the MT temperature, Tm, on the field is highly nonlinear. As far as magnetization change, ΔM, remains field-independent, a depart from linearity of Tm(H) function is attributed to a decrease of the transformation entropy, ΔS. This decrease correlates with the parameter (TC-Tm), controlled by magnetic field, where TC is the Curie temperature of austenite, and with the dependence of ΔS on the width of the MT temperature interval deduced from a ferroelastic model of MT.

Magnetic and transport properties of Fe-Zr-B-(Cu) amorphous alloys

FeZrB metallic glasses present magnetic properties that are enhanced compared to the pure FeZr ones. In particular, a large increase of the Curie temperature has been found. Magnetic and Mossbauer measurements show a decrease of the spin-glass character and a parallel homogenization of the hyperfine-field distribution as the boron concentration increases. Resistivity versus temperature measurements show a change in behaviour with B content: in the samples with small amounts of boron, a minimum in the resistivity versus temperature curves appears near the Curie temperature, while samples with high boron content show a low-temperature minimum, characteristic of most metallic glasses. The analysis of the results suggests that the evolution of the magnetic behaviour is related to changes in the density of states at the Fermi level, rather than to changes in the Fe - Fe distances. This is in agreement with published data on the specific heat of FeZr and FeB glasses. The influence of boron is shown to greatly enhance the weak itinerant ferromagnetism of FeZr glasses, leading to stronger ferromagnetic behaviour. The characteristic features of the resistivity are analysed in terms of localization effects on the conduction electrons, which extend to higher temperatures in the low-boron-content alloys.

Magnetic domains in Ni–Mn–Ga martensitic thin films

A series of martensitic Ni52Mn24Ga24 thin films deposited on alumina ceramic substrates has been prepared by using RF (radio-frequency) magnetron sputtering. The film thickness, d, varies from 0.1 to 5.0 µm. Magnetic domain patterns have been imaged by the MFM (magnetic force microscopy) technique. A maze domain structure is found for all studied films. MFM shows a large out-of-plane magnetization component and a rather uniform domain width for each film thickness. The domain width, δ, depends on the film thickness as in the whole studied range of film thickness. This dependence is the expected one for magnetic anisotropy and magnetostatic contributions in a perpendicular magnetic domain configuration. The proportionality coefficient is also consistent with the values of saturation magnetization and magnetic anisotropy determined in the samples.

Interfacial magnetic coupling between Fe nanoparticles in Fe–Ag granular alloys

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2–4 nm) nanoparticles and fcc Ag (10–12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.

Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mössbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe86Zr7Cu1B6. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mössbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (≈330 K) at 360 K. We have also performed Mössbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, α-Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.

MFM Domain Imaging of Textured Ni-Mn-Ga/MgO(100) Thin Films

Magnetic, micromagnetic, and texture properties of the martensitic thin films prepared by magnetron sputtering of Ni49.5Mn28Ga22.5 target on MgO(100) wafer are studied as a function of film thickness. The films exhibit 220-fiber texture. Moreover, in contrast to thick films, a considerable in-plane texture component is giving rise to the pronounced in-plane magnetic anisotropy found in the thin film. The texture features together with a large magnetocrystalline anisotropy lead to the particular domain structure and different magnetic anisotropy components of films depending on the film thickness and substrate nature.

Magnetoimpedance in narrow NiFe/Au/NiFe multilayer film systems

Magnetoimpedance (MI) in RF sputtered magnetic/metallic multilayers of composition NiFe/Au/NiFe has been investigated with the purpose of clarifying the effect of in-plane size. The film width was varied between 20 /spl mu/m and 200 /spl mu/m, the length was 2 or 5 mm and the layer thickness was 0.5 /spl mu/m. The films of 50 /spl mu/m wide demonstrated the highest MI ratio of more than 140% for frequencies of 200-400 MHz. We believe this is one of the best obtained results regarding the film dimensions and sensitivity. However, when the width is further decreased, the MI ratio drops considerably due to flux leakage through the inner conductor. This effect was analyzed within a 2-D model of a three-layer symmetrical film system, having magnetically open-loop structure, giving a reasonable agreement with the experimental results in 50 and 20 /spl mu/m wide films.

Magnetic and Mossbauer study of amorphous and nanocrystalline Fe/sub 86/Zr/sub 7/Cu/sub 1/B/sub 6/ alloys

Amorphous alloys of composition Fe/sub 86/Zr/sub 7/Cu/sub 1/B/sub 6/ are prepared and annealed in the temperature range from 570 to 950 K, obtaining several degrees of nanocrystallization. Magnetic and Mossbauer measurements show the different phases that appear in the samples after the heating, and their percentage, depending on the annealing temperature. The Curie temperature of the amorphous phase is shown to remain almost unchanged during the crystallization. Mossbauer spectroscopy reveals subtle changes in the structure. >

Influence of Magnetic Field on Magnetostructural Transition in Ni46.4Mn32.8Sn20.8 Heusler Alloy

A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.