Magnetic properties of ball-milled Fe–Mn alloys

BCC, FCC and HCP phases in Fe–13.7 wt% Mn alloys were studied by Mössbauer spectroscopy and X-ray diffraction, after ball milling. The relative amounts of the HCP and FCC phases increase with milling times up to 9 h and decline afterwards. Preliminary AC susceptibility measurements show that the blocking temperatures change for different milling times.     

Critical behavior of electrical resistivity in amorphous Fe-Zr alloys

Electrical resistivity (ρ) of the amorphous (a-)Fe_100−c Zr _c (c=8.5, 9.5 and 10) alloys has been measured in the temperature range 77 to 300 K, which embraces the second-order magnetic phase transition at the Curie temperature point T _c. Analysis of the resistivity data particularly in the critical region reveals that these systems have a much wider range of critical region compared to other crystalline ferromagnetic materials. The value of T _c and specific heat critical exponent, α has the same values as those determined from our earlier magnetic measurements. The value of α for all the present investigated alloys are in close agreement with the values predicted for three-dimensional (3D) Heisenberg ferromagnet systems, which gives contradiction to the earlier results on similar alloys. It is observed from the analysis that the presence of quenched disorder does not have any influence on critical behavior.     

Magnetic induction heating of FeCr nanocrystalline alloys

In this work the thermal effects of magnetic induction heating in (FeCr)_73.5Si_13.5Cu₁B₉Nb₃ amorphous and nanocrystalline wires were analyzed. A single piece of wire was immersed in a glass capillary filled with water and subjected to an ac magnetic field (frequency, 320 kHz). The initial temperature rise enabled the determination of the effective Specific Absorption Rate (SAR). Maximum SAR values are achieved for those samples displaying high magnetic susceptibility, where the eddy current losses dominate the induction heating behavior. Moreover, the amorphous sample with Curie temperature around room temperature displays characteristic features of self-regulated hyperthermia.     

Magnetic viscosity in Nd60Fe30Al10 amorphous alloys

Ferromagnetic amorphous Nd₆₀Fe₃₀Al₁₀ alloys melt spun at wheel speeds between 5 and 20 m/s exhibit hard magnetic properties, which are found to be very sensitive to the cooling conditions. The magnetization reversion mechanisms leading to the rather high coercive forces found are investigated by thermally activated magnetic relaxation experiments; the mean fluctuation field and the activation volume are measured—in specimens cooled at different rates—at the critical field for extensive magnetization reversion. These preliminary results show a qualitative agreement with the predictions of a ferromagnetic cluster model.     

Magnetic properties of 2D nano-islands I: Ising spin model

An Ising spin effective field theory (EFT) is developed as a framework for a detailed analysis of the magnetic properties of two-dimensional (2D) nano-islands on a nonmagnetic substrate with an out of plane magnetization. The Hamiltonian with nearest neighbor exchange interactions and single-atom magnetic anisotropy defines the ground state. The calculation yields the single site spin correlations, the magnetizations, and the isothermal susceptibilities for the core and periphery domains, and the island core phase diagrams. The choice of a spin S=1 for the atoms permits the analysis of the effects of spin fluctuations via the single site spin correlations. In particular we investigate the effects due to the different anisotropies and reduced dimensionalities for the core and periphery domains. The present model calculations are developed for different 2D nano-islands lattices. Detailed theoretical results are presented for the square and hexagonal lattices, with numerical applications for the 2D Co nano-islands on Pt. The derived transition temperature for the hexagonal lattice nano-islands is in good agreement with the experimental data for Co nano-islands on Pt. Though both the core and the periphery domains have the same order-disorder transition temperature, the magnetization of each domain attains this transition differently. The temperature behavior of the spin correlations is also fundamentally different for the periphery and core sites, which entails distinctly different isothermal susceptibilities, and yields statistically averaged nano-islands susceptibilities that do not correspond to a second order phase transition. The experimental susceptibility results for 2D Co nano-islands on Pt can be interpreted within our EFT Ising model without reference to a transition from a blocking state of the particle to a superparamagnetic behavior. The results for the different lattices are formally comparable, and demonstrate the robustness and general character of the model.     

Magnetic, resistive and magnetoresistive properties of melt spun CoCu alloys

_Co10Cu90${\mathrm{Co}}_{10}{\mathrm{Cu}}_{90}$ alloys were prepared by melt spinning in a twin roller device at 5 m/s and 23 m/s tangential wheel velocities. The low temperature (5–300 K) magnetic and electron transport properties were investigated in as cast alloys and in samples annealed for 1 h at 923 K. Magnetoresistance (MR) measurements were performed between 35 K and 295 K, with applied field up to 0.85 T. For each condition, M–H loops and M(T) curves were measured and analyzed to estimate the mean Co particle size and the blocking temperature distribution, respectively. In the as cast condition, particle sizes between 2.3 nm and 5.7 nm and mean blocking temperatures between 31 K and 247 K are found; these alloys, cooled at these relatively low rates, show appreciable room temperature MR values (0.5–1.5% for 0.85 T). A minimum resistance is found at about 30 K in alloys with compositions up to 15 at% Co.     

Magnetic and Rheological Characterization of Fe3O4 Ferrofluid: Particle Size Effects

Magnetite particles of different diameters were synthesized by chemical co-precipitation technique and the same are dispersed in dodecane to prepare a magnetic fluid. The results of X-ray diffraction, magnetization measurements, ac susceptibility and viscosity measurements are analyzed and discussed in the text.     

Magnetic properties and structure of electrodeposited Zn-Co alloys granular thin films

We present several results concerning the preparation by means of electrolysis and characterization of Zn-Co alloys thin films. Films of Zn, Co and Zn-Co with various compositions (8-16 at% Co) were prepared in sulfate baths, using potentiostatic control, envisaging applications in the domain of corrosion resistant magnetic sensors. The effects of applied voltage on the magnetic properties, microstructure and phase content of the electrodeposited Zn-Co films were investigated. The applied voltage significantly influenced the film composition and their magnetic properties. These films, when deposited at an applied voltage of 4.5 V exhibited multiphase behavior due to the inclusion of new phases (cobalt hydroxide), whereas at 3.0 V, only Zn-Co alloys were deposited. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).     

Magnetic properties of nanocrystalline FeNiCo x ZrB alloys (x=0, 10, 20)

Magnetic properties of amorphous and nanocrystalline Fe₃₁Ni₅₀Zr₇B₁₂, Fe₃₁Ni₄₀Co₁₀Zr₇B₁₂ and Fe₃₁Ni₃₀Co₂₀Zr₇B₁₂ alloys were studied by an unconventional “rf-Mössbauer” technique. Introduction of Co atoms into FeNiZrB alloy leads to a large increase of anisotropy field that suppresses the rf collapse effect. The rf induced crystallization effect observed in Co-containing alloys was attributed to the rf sidebands effect which induced in the alloys mechanical deformations via the magnetostriction. This effect is particularly strong in amorphous alloys and in nanocrystalline alloys containing significant fraction of amorphous matrix and is absent in Co-free alloy.     

Magnetic properties of Fe-based glass-coated microwires

Axial hysteresis loops of glass-coated amorphous Fe₇₀B₁₅Si₁₀C₅ microwire have been measured as a function of both the diameter of metallic nucleus (from 3.7–14.9 μm) and the thickness of the coating (4.0–9.6 μm). They exhibit low-field rectangular hysteresis loops with a single and large Barkhausen jump even for samples as short as 5 mm long. Coercivity remarkably increases (roughly from 1 to 10 Oe) and remanence decreases (from 1 to 0.45 T), respectively, as the ratio of metallic nucleus radius to the total radius of the wire decreases from 0.63 to 0.16. The strength of the internal stresses induced during the fabrication depends on this ratio, and the easy axes of the corresponding magnetoelastic anisotropies determine the actual value of coercivity and remanence for these microwires.     

Magnetic properties of Cr-Fe-Mn alloys

The magnetic behaviour of a Cr_80−xFe₂₀Mn_x alloy system with x=2, 7, 10, 13 and 22 has been investigated in the temperature range 2-400 K through measurements of magnetization, electrical resistivity, magnetoresistivity, specific heat and thermal expansion. The temperature vs. Mn concentration magnetic phase diagram of the system is rich in magnetic behaviour with ferromagnetic (FM), antiferromagnetic (AFM) and paramagnetic phase regions and a spin-glass (SG) region at the lowest temperatures. Phase transition temperatures amongst these different magnetic phases could be identified from well-defined anomalies of magnetic origin that are displayed by graphs of the above-mentioned physical properties as a function of temperature. The time relaxation of the thermoremanent, isothermal remanent and field-cooled magnetizations below and above the SG freezing temperature show unusual aspects. These relaxations do not follow the usual superposition principle that is expected for typical SG materials. Negative giant magetoresistance (GMR) is observed in the alloys at 4 K. The GMR initially increases sharply on increasing the Mn content in the alloy system, followed by a tendency towards a saturation negative value for concentrations of more than about 10 at% Mn. Low-temperature plots of C_p/T vs. T², where C_p is the specific heat, present anomalous behaviour for alloys with x=2, 10 and 22. For x=2 the plot shows an upturn at the lowest temperatures that changes over to a prominent downturn for x=10 and 22. This behaviour is attributed to Fe concentration fluctuations in the alloys, confirming the theoretical model of Matthews.     

Magnetic properties and metastable states of Co-Ir films

The phase composition, crystal structure, and magnetic properties of Co_{1 ? x} Ir _x alloy films with x = 0.35–0.8 and fabricated by chemical deposition are studied. Saturation magnetization vs. composition and temperature, coercivity, and magnetization reversal of the films are investigated experimentally. It is shown that in the region of equiatomic composition, Co-Ir films have a two-phase hexagonal close-packed structure that alternates with disordered regions, producing halos in electron diffraction patterns.     

Magnetic properties of amorphous alloys

The magnetic properties of existing amorphous iron based materials, i.e., Fe--metalloid, Fe--early transition metals and Fe--rare earth alloys, are briefly discussed for some representative alloys. The spin orientation of amorphous Fe--metalloid alloys has been determined by the angular dependence of hyperfine interactions. It is shown that in iron--early transition metals ferromagnetic order is not long-ranged, but determined by magnetic clusters. The magnetic hyperfine field distributions of Fe-rich iron--early transition metals consist of a high and a low field tail. The magnetic structure has been investigated for two representative Fe--RE (RE = Er, Ce) amorphous alloys. For the first time, the magnetic coupling phenomenon in amorphous/crystalline multilayers has been discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic properties of CuRh alloys

By means of rapid quenching techniques single phased samples of Cu_xRh_1?x(0?x?1) were obtained. For these alloys the Knight shift of ⁶³Cu and ¹⁰³Rh has been determined employing pulsed NMR at low temperatures, furthermore the magnetic susceptibility was measured for temperatures between 4.2 and 300 K. While the Knight shift of ¹⁰³Rh is dominated by s-electron contributions in spite of a high density of d-states at the Fermi level, for the susceptibility, however, the d-electron contributions prevail. In addition the susceptibility shows a pronounced maximum at about 10 at.% Cu. Using the extrapolated Knight shift of copper (x→0) we estimate a net copper hyperfine field of — 15 T in close agreement with the corresponding values for CuPd and CuPt.     

Magnetic properties of Zr doped Y9Co7

Detailed measurements of magnetization and ac susceptibility at low temperatures of 1% Zr-substituted Y₉Co₇ are presented. All results are indicative of itinerant weak ferromagnetism withT _c ∼ 9.5 K. The zero-field magnetizations followT ² orT ^4/3 behaviour as in the Ni-substituted system. The estimated critical exponents areβ=0.38±0.03,γ=1.16±0.05. It is argued that the main effect of the non-magnetic Zr-substitution in Y₉Co₇ is to stabilize the ferromagnetic ordering by suppressing the ‘hopping’ of Co atoms along thec-axis sites of the hexagonal structure.     

Ni-WC composite coatings by carburizing electrodeposited amorphous and nanocrystalline Ni-W alloys

In situ formation of tungsten carbide in the matrix of FCC nickel has been achieved by carburizing of the electrodeposited Ni-W alloy coatings. The size of the carbide particles ranges between 100 and 500 nm. The carbide phase is also present in the form of very small precipitates inside the nickel grains. The size of such precipitates is between 10 and 40 nm. The carburizing environment was created by introducing a flowing mixture of vaporized 95.5% alcohol (0.25 ml/min, liquid) and argon (0.5 L/min, gas) into the carburizing furnace. Supersaturated nature of electrodeposited amorphous and nanocrystalline alloys, in addition to high diffusivity, have been attributed for the formation of carbide phase in the deposits at a temperature range of 700-850 °C. The carbide-metal interface is clean and the composite coatings are compact. Hardness values up to about 1100 KHN are achieved. Hardness increases with tungsten content and carburizing temperature.     

Magnetic properties of Ni/Au core/shell studied by Monte Carlo simulations

The magnetic properties of ferromagnetic Ni/Au core/shell have been studied using Monte Carlo simulations within the Ising model framework. The considered Hamiltonian includes the exchange interactions between Ni–Ni, Au–Au and Ni–Au and the external magnetic field. The thermal total magnetizations and total magnetic susceptibilities of core/shell Ni/Au are computed. The critical temperature is deduced. The exchange interaction between Ni and Au atoms is obtained. In addition, the total magnetizations versus the external magnetic field and crystal filed for different temperature are also established.