M.L. Fdez-Gubieda

FeNi-based magnetoimpedance multilayers: Tailoring of the softness by magnetic spacers

The microstructure and magnetic properties of sputtered permalloy films and FeNi(170 nm)/X/FeNi(170 nm) (X = Co, Fe, Gd, Gd-Co) sandwiches were studied. Laminating of the thick FeNi film with various spacers was done in order to control the magnetic softness of FeNi-based multilayers. In contrast to the Co and Fe spacers, Gd and Gd-Co magnetic spacers improved the softness of the FeNi/X/FeNi sandwiches. The magnetoimpedance responses were measured for [FeNi/Ti(6 nm)]2/FeNi and [FeNi/Gd(2 nm)]2/FeNi multilayers in a frequency range of 1–500 MHz: for all frequencies under consideration the highest magnetoimpedance variation was observed for [FeNi/Gd(2 nm)]2/FeNi multilayers.

Anisotropy effects in magnetic hyperthermia: A comparison between spherical and cubic exchange-coupled FeO/Fe3O4 nanoparticles

Spherical and cubic exchange-coupled FeO/Fe3O4 nanoparticles, with different FeO:Fe3O4 ratios, have been prepared by a thermal decomposition method to probe anisotropy effects on their heating efficiency. X-ray diffraction and transmission electron microscopy reveal that the nanoparticles are composed of FeO and Fe3O4 phases, with an average size of ∼20 nm. Magnetometry and transverse susceptibility measurements show that the effective anisotropy field is 1.5 times larger for the cubes than for the spheres, while the saturation magnetization is 1.5 times larger for the spheres than for the cubes. Hyperthermia experiments evidence higher values of the specific absorption rate (SAR) for the cubes as compared to the spheres (200 vs. 135 W/g at 600 Oe and 310 kHz). These observations point to an important fact that the saturation magnetization is not a sole factor in determining the SAR and the heating efficiency of the magnetic nanoparticles can be improved by tuning their effective anisotropy.

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.

Influence of the short-range order on the magnetic properties of metallic glasses

The short-range order of metallic glasses has been investigated by x-ray absorption spectroscopy in order to explore the relation between the local structure and the magnetic properties. The structural information has been further contrasted with the results obtained from the use of other local probes such as Mossbauer spectroscopy. The existence of chemical preferences leading the metalloids, B and Si, to locate near one particular metallic species, Fe and Co respectively, is in the origin of the strong increase of the short-range order in the Co rich side, characterized by the appearance of a BCC-like structure around Fe for . Such an effect causes a decrease of the root mean square deviation of distance between Fe and its near-neighbour metallic atoms in the Co rich side , which is strongly correlated with the decrease of standard deviation of the distribution of hyperfine fields. The nearest-neighbour distances Fe-M and Co-M (M = Fe, Co) decrease for , and the Co-M one shows a minimum at y = 0.06. This behaviour has been used to explain some features of the dependence on the composition of the Curie temperature and the spin wave stiffness constant as well as the isomer shift of the alloys.

Enhanced magnetic anisotropy and heating efficiency in multi-functional manganese ferrite/graphene oxide nanostructures.

A promising nanocomposite material composed of MnFe2O4 (MFO) nanoparticles of ∼17 nm diameter deposited onto graphene oxide (GO) nanosheets was successfully synthesized using a modified co-precipitation method. X-ray diffraction, transmission electron microscopy, and selected area electron diffraction confirmed the quality of the synthesized samples. Fourier transform infrared measurements and analysis evidenced that the MFO nanoparticles were attached to the GO surface. Magnetic measurements and analysis using the modified Langevin model evidenced the superparamagnetic characteristic of both the bare MFO nanoparticles and the MFO-GO nanocomposite at room temperature, and an appreciable increase of the effective anisotropy for the MFO-GO sample. Magnetic hyperthermia experiments performed by both calorimetric and ac magnetometry methods indicated that relative to the bare MFO nanoparticles, the heating efficiency of the MFO-GO nanocomposite was similar at low ac fields (0-300 Oe) but became progressively larger with increasing ac fields (>300 Oe). This has been related to the higher effective anisotropy of the MFO-GO nanocomposite. In comparison with the bare MFO nanoparticles, a smaller reduction in the heating efficiency was observed in the MFO-GO composites when embedded in agar or when their concentration was increased, indicating that the GO helped minimize the physical rotation and aggregation of the MFO nanoparticles. These findings can be of practical importance in exploiting this type of nanocomposite for advanced hyperthermia. Magnetoimpedance-based biodetection studies also indicated that the MFO-GO nanocomposite could be used as a promising magnetic biomarker in biosensing applications.

Magnetic phase diagram of superantiferromagnetic TbCu2 nanoparticles

The structural state and static and dynamic magnetic properties of TbCu2 nanoparticles are reported to be produced by mechanical milling under inert atmosphere. The randomly dispersed nanoparticles as detected by TEM retain the bulk symmetry with an orthorhombic Imma lattice and Tb and Cu in the 4e and 8h positions, respectively. Rietveld refinements confirm that the milling produces a controlled reduction of particle sizes reaching ≃6 nm and an increase of the microstrain up to ≃0.6%. The electrical resistivity indicates a metallic behavior and the presence of a magnetic contribution to the electronic scattering which decreases with milling times. The dc-susceptibility shows a reduction of the Néel transition (from 49 K to 43 K) and a progressive increase of a peak (from 9 K to 15 K) in the zero-field-cooled magnetization with size reduction. The exchange anisotropy is very weak (a bias field of ≃30 Oe) and is due to the presence of a disordered (thin) shell coupled to the antiferromagnetic core. The dynamic susceptibility evidences a critical slowing down in the spin-disordered state for the lowest temperature peak associated with a spin glass-like freezing with a tendency of zv and β exponents to increase when the size becomes 6 nm (zv ≃ 6.6 and β ≃ 0.85). A Rietveld analysis of the neutron diffraction patterns 1.8 ≤ T ≤ 60 K, including the magnetic structure determination, reveals that there is a reduction of the expected moment (≃80%), which must be connected to the presence of the disordered particle shell. The core magnetic structure retains the bulk antiferromagnetic arrangement. The overall interpretation is based on a superantiferromagnetic behavior which at low temperatures coexists with a canting of surface moments and a mismatch of the antiferromagnetic sublattices of the nanoparticles. We propose a novel magnetic phase diagram where changes are provoked by a combination of the decrease of size and the increase of microstrain.

Correlation among the structural and magnetic properties of CoCu granular alloys

We have analyzed the magnetic and structural properties of CoCu melt-spun granular alloys annealed at increasing temperatures by means of their room-temperature hysteresis loops and high resolution x-ray diffraction (XRD) experiments. The magnetic analysis has been performed taking into account two contributions to the total magnetization: one from superparamagnetic nanometer Co clusters responsible also of the giant magnetoresistance (GMR) and another ferromagnetic; originated by a Co-rich phase observed by XRD. We have also detected a percentage of Co that remains diluted into the Cu matrix at high annealing temperatures (Tann⩾500 °C) due to the mixcibility of Co and Cu at those temperatures, and this is in the origin of the drop of the GMR.

EXAFS study of short-range order in (FexCo1−x)75Si15B10 metallic glasses☆

Short-range order studies by means of extended X-ray absorption fine structure (EXAFS) and Mossbauer spectroscopy on (FexCo1−x)75Si15B10 amorphous alloys have been performed over the full range of composition. EXAFS studies provide strong evidence for differing chemical and topological environments around Fe and Co in these metallic glasses. While a featureless structure around Co atoms is obtained over the entire compositional range, a strong short-range order appears around Fe atoms in the low Fe concentration range. A non-homogeneous distribution of metalloids with preferential coordination of Fe with B and Co with Si with decreasing Fe concentration is corroborated by Mossbauer spectroscopy.

Influence of the Si Substrate on the Transport and Magnetotransport Properties of Nanostructured Fe-Ag Thin Films

A systematic study of electrical and magnetic transport properties of FexAg100-x (50<x<55)thin films deposited by both sputtering and pulsed laser ablation onto two p-type Si substrates with different resistivities is reported in this work. Resistance, magnetoresistance and Hall effect were measured in the temperature range 75-350 K. Magnetization was measured in the temperature range 5-300 K, showing no appreciable differences for samples deposited onto one substrate or the other. A near room temperature transition in the resistance has been observed only for samples deposited onto the lower resistance substrate. Before this transition, the results obtained agree well with the conduction being almost completely confined to the metallic film, but after the transition, apparently most of the carriers change to the substrate. This is corroborated by the positive magnetoresistance and the change in the sign of the carriers indicated by the Hall measurements after the transition. The experimental results agree well with a three layer model that includes an interface whose resistance increases exponentially with decreasing temperature.

Structure and magnetic properties in CoCu granular alloys

Abstract Granular alloys, composed of magnetic clusters embedded in non-magnetic metallic matrices, can develop giant magnetoresistance effect after suitable preparation and thermal treatments. The structural effect of annealing on the structure of Co10Cu90 samples has been directly probed by in situ time resolved X-ray diffraction (TR-XRD) during thermal treatment. TR-XRD definitively proves the occurrence of an anomalous behaviour in the thermally activated segregation process that is related to the evolution of magnetotransport properties in these materials.