Nicoleta Lupu

Surface magnetization processes in soft magnetic nanowires

The surface magnetization processes taking place in simple permalloy (Py) and FeGa nanowires, Py/Cu, CoFeB/Cu, CoNiP/Cu, FeGa/Py, and FeGa/CoFeB multilayered nanowires have been studied by magneto-optical Kerr effect (MOKE) magnetometry. The results indicate a strong correlation between the direction of the anisotropy axis relative to the direction of the applied field and the plane of incidence of the laser spot, as well as the effect of dipolar interactions between the nanowires or between the ferromagnetic layers on the magnetization reversal. The larger laser spots are inducing more noise in the MOKE hysteresis loops because of the dimensional imperfections along the nanowires.

Development of Fe–Nb–Cr–B Glassy Alloys With Low Curie Temperature and Enhanced Soft Magnetic Properties

Fe_67.7Nb_0.3Cr₁₂B₂₀ glassy melt-spun ribbons and glass-coated microwires with T_C around 310-320 K (~35-45°C) (the lowest ever reported for metallic glasses) have been prepared by rapid solidification. Excepting the thick melt-spun ribbons of 40 μm, all samples have a glassy structure in the as-quenched state, consisting of very small Fe-Cr and boride (Fe₃B) clusters of sizes ranging from several nanometer up to several tens of nanometer, embedded within the residual amorphous matrix. The ferromagnetic behavior of the glassy materials deteriorates drastically with an increase in the temperature. The melt-spun ribbons become paramagnetic above 34-35°C, whereas the transition temperature for the glass-coated microwires is around 45°C, because of the very specific magnetic domain structure and magnetic anisotropy distribution. The observed stable behavior of low <i>T</i>_C Fe_67.7Nb_0.3Cr₁₂B₂₀ glassy alloys up to very high frequencies (10 GHz) is another important feature for sensing and biomedical applications. The temperature sensor based on low <i>T</i>_C Fe_67.7Nb_0.3Cr₁₂B₂₀ glassy melt-spun ribbons has a very good sensitivity in a narrow temperature range (about 1°C).

Electrochemical deposition of FeGa/NiFe magnetic multilayered films and nanowire arrays

Results concerning the preparation and characterization of electrochemically deposited (Fe1−xGax∕Fe1−yNiy)n (x=0.1–0.3at.%; y=0.4–0.8at.%) multilayered films and nanowires arrays are reported for the first time. The layers have been deposited successively by changing the electrodeposition potential. The combination of Fe1−xGax magnetostrictive material and Fe1−yNiy soft magnetic material shows good magnetic softness (Hc does not exceed 60Gs) and gives novel magnetostrictive behavior caused by the formation of twisted spin structures.

Magnetic Characterization of Submicron Wires and Nanowires Using Digital Integration Techniques

A new procedure for performing magnetic measurements on a single ultrathin magnetic wire is presented. The proposed solution is based on the use of digital integration techniques. Such a new measuring method was required by the recent development of novel magnetic materials such as rapidly solidified amorphous nanowires and submicron wires, for which a fast and reliable method of measuring the axial hysteresis loop was lacking. The new procedure implies two methods of measuring the hysteresis loop: one which requires a large number of measurements in order to determine the profile of the hysteresis loop, and another one which removes all the noise from rectangular loops associated with magnetically bistable samples. The first method can be employed for any ultrathin wire shaped sample, while the second one is suitable only for bistable samples with rectangular hysteresis loops, although it is ultrafast.

Design and preparation of new Fe-based bulk amorphous alloys toroids

We investigate dc and ac magnetic properties of Fe/sub 56/Co/sub 7/Ni/sub 7/Zr/sub 6/M/sub 1.5/Nb/sub 2.5/B/sub 20/ (M = Zr, Ti, Ta, Mo) amorphous alloys toroids prepared by mold casting. Electrical resistivities of 180-230 /spl times/ 10/sup -8/ /spl Omega//spl middot/m, coercive fields up to 10 A/m, dc-magnetic permeabilities of about 1 /spl times/ 10/sup 5/ and ac-magnetic permeabilities up to 3 /spl times/ 10/sup 4/, saturation magnetizations of 0.9-1.2 T, and Curie temperatures between 550 and 600 K are measured in the as-cast state. We discuss the influence of the preparation conditions, composition, frequency (5 Hz-10 MHz), and treatments on the electrical and magnetic characteristics and present the advantages of using bulk amorphous alloys with soft magnetic properties in new physical and engineering applications.

Magnetotransport Phenomena in [NiFe/Cu] Magnetic Multilayered Nanowires

[Py(20-50 nm)/Cu(5-20 nm)] x n multilayered nanowires were electrodeposited by switching between the deposition potentials of the two constituents [respectively, -1.4 V and -0.3 V for permalloy (NiFe) and Cu deposition]. The magnetotransport properties of single multilayered nanowires have been measured. The MR curve is symmetrically anhysteretic, independent of the thickness of the nonmagnetic Cu layer. The MR ratio can reach values of 2%-2.5%, depending on the thickness of the nonmagnetic Cu layer. The presence of nonmagnetic Cu layers produces a decrease of the axial magnetic anisotropy in the multilayered nanowires, and consequently the magnetic permeability increases at the surface of the nanowires leading to the enhancement of the dc magnetic field effect over the MI response, which can go up to 100%.

Low TC Fe-Cr-Nb-B glassy submicron powders for hyperthermia applications

Fe79.7−xNb0.3CrxB20 (x = 11.5–13.5 at. %) submicron powders with glassy structure and sizes between a few tens of nm and 1 μm have been prepared by high-energy ball milling from glassy melt-spun ribbons (MSRs) precursors. The glassy structure existing in the MSRs, and consisting of Fe-Cr clusters embedded within the (Fe,Nb)3B residual amorphous matrix, is preserved by milling the annealed MSRs in oleic acid and n-heptane. The glassy structure of the submicron powders confers the specific magnetic properties and allows tuning the Curie temperatures in the range of 15–50 °C. The precipitation of α-Fe in excess shifts the Curie temperature to 780 °C. The use of surfactants (oleic acid or heptane) avoids partially the agglomeration of powders, and their individual size goes down to 25–40 nm. The heating efficiency of the submicron powders in a.c. fields (H = 350 mT and f = 153 kHz) evidences a working regime in the range of 33–45 °C, depending on Cr content, indicating the suitability of using the Fe-Cr-Nb-B ...

Magnetic Properties of Electrodeposited FeGa/CoFeB Multilayered Films and Nanowire Arrays

Results concerning the preparation and magnetic characterization of electrochemically deposited [Fe₈₀ Ga₂₀ /Co₇₅ Fe₁₀ B₁₅ ]_n multilayered films and nanowire arrays are reported for the first time. The deposition bath consisted in a single mixed solution of FeSO₄middot7H₂O, Ga₂(SO₄)₃middotH₂O, CoSO₄middot7H₂O, C₂H ₁₀BN, and H₃BO₃. The layers have been deposited successively by changing the electrodeposition potential. The composition and thickness of the different layers were tailored through optimizing deposition parameters such as buffer additives and applied voltage. Two hundred sequences multilayered nanowires were electrodeposited into commercially available nanoporous alumina membranes with pore diameters of 100 nm and lengths of 50 mum. Multilayered films 4-5 mu m thick have been obtained by electrodepositing 50 consecutive sequences of FeGa and CoFeB. The multilayered structures consisting FeGa magnetostrictive alloy and CoFeB amorphous magnetic material are magnetically soft (H_c does not exceed 1.6 kA/m).

The magnetic and structural properties of the high-coercivity Nd50Fe40Al10 amorphous alloys

Abstract Melt-spun amorphous ribbons with thicknesses between 25 and 150 μm and amorphous rods with diameters between 0.5 and 2 mm, prepared by copper mould suction casting, having nominal composition Nd50Fe40Al10, were measured. The magnetic properties were measured as a function of temperature from 5 to 800 K. The dependence of the coercive field on the thickness of the samples and temperature is ascribed to two magnetic phases: Fe–Nd ferromagnetic clusters and a Nd-based matrix. The differences observed between zero-field-cooled (MZFC) and field-cooled (MFC) magnetisations as well as the presence of a cusp on the MZFC curve, and its displacement as a function of the magnitude of the external field are consistent with two types of magnetic order: spin-glass-like short-range order and long-range ferromagnetic order.

Magnetic behavior of Nd/sub 50/Fe/sub 40/Al/sub 10/ bulk amorphous alloys

The temperature and thickness dependence of the hysteresis loop, magnetization, coercive field, and AC-susceptibility of melt-spun amorphous ribbons and amorphous cast rods of the composition Nd/sub 50/Fe/sub 40/Al/sub 10/ were measured between 5 and 300 K. The large values of the coercivity are ascribed to the existence of very small magnetic clusters dispersed in the amorphous matrix. The Mossbauer spectra can be fitted to two Fe sites with magnetic moments ranging from 1.47 to 2.00 /spl mu//sub B/.