F.P. Missell

Magnetic viscosity and texture in sintered NdFeB and NdDyFeB magnets

Abstract We have measured the magnetic viscosity parameter Sv for a number of well-characterized NdFeB and NdDyFeB sintered magnets, as a function of magnetic field H and for temperatures T above room temperature. The results are compared for magnets showing different textures or degrees of orientation (i.e. isotropic vs. anisotropic) in an attempt to observe an angular dependence of the effective barrier energy with applied magnetic field. For the NdDyFeB magnets, Sv shows no dependence upon texture, while, for the NdFeB magnets, the results are consistent with the measured texture.

Stress dependence of the magnetic properties of amorphous Co-Fe-Si-B alloys

Abstract For the amorphous alloys Co 75− x Fe x Si 15 B 10 ( x = 0, 2, 4, 4.6) and Co 75 Si 5 B 20 , we have studied the stress depe ndence of the coercive field, the initial susceptibility, the effective anisotropy, and the reduced remanence. The experimental results are in good agreement with a model due to Vazquez, Fernengel and Kronmuller. From our measurements, we obtain the magnetostriction, as well as information about quenched-in stresses and anisotropy mechanisms in these materials.

Magnetic and Magnetotransport Properties of Co Thin Films on Si

A systematic study of magnetic and magnetotransport properties of thin Co films on Si is reported in this work. The Co films of thicknesses 30, 160 and 440 A were prepared by magnetron sputtering from a Co target onto a Si(100) substrate held at room temperature. Resistance, magnetoresis-tance and Hall effect were measured in the temperature range 5-350 K. Magnetization was measured in the same temperature range using a SQUID magnetometer. Complete surface hysteresis loops were measured from 4 to 300 K by means of the magneto-optical Kerr effect, in order to follow the behavior of the coercivity and magnetic easy axes of the samples. The transport and magnetotransport properties display a peculiar effect as functions of temperature. The films behave as pure metallic Co below 250 K. However, the resistance drops with heating from 250 to 280 K, regaining its typical metallic behavior at temperatures higher than 280 K. The thinner the film, the larger is the resistance drop in the temperature interval 250-280 K, reaching a factor of 4.5 for the 30 A thick film. The Hall effect contains both ordinary and extraordinary contributions. From ordinary Hall effect measurements, one finds that the conduction is electronic at low temperatures and turns to hole-like above the transition point. Magnetoresistance changes from negative at low T to positive at high temperatures. These facts indicate that the observed effect is related to the metallic layer, which undergoes a conducting channel switching when the temperature is increased.

Activation energy spectrum of induced anisotropy in Co70.4Fe4.6Si15B10

Abstract Isothermal annealing curves of the magnetic anisotropy, induced for H =0 in non-magnetostrictive Co 70.4 Fe 4.6 Si 15 B 10 , have been obtained for as-quenched and pre-annealed samples. Utilizing a micromagnetic theory of induced anisotropy based upon a distribution of two-level systems, a spectrum of activation energies has been obtained for reversible processes. The activation energy spectrum also accounts for the increase in the coercive field upon annealing.

Induced anisotropy in nanocrystalline FeCuNbSiB

The kinetics of induced anisotropy K/sub ind/ was studied in nanocrystalline Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 13.5/B/sub 9/, as well as in the amorphous precursor and in amorphous Fe/sub 78/B/sub 13/Si/sub 9/. The nanocrystalline alloy was produced from the precursor by annealing at 813 K for 1 h and possessed an average FeSi grain size of 13 nm, as determined from X-ray diffraction. Annealing in a 0.2 T field at 723-773 K, above T/sub c/ of the amorphous phase, resulted in low values of K/sub ind/. The data were compared to the micromagnetic theory of Kronmuller to determine activation energy spectra. K/sub ind/ for the nanocrystalline alloy is well described by this theory, however, with an activation energy spectrum that is much narrower than for the amorphous alloys. The limiting value of the anisotropy is K/sub /spl infin///spl ap/13 J/m/sup 3/ consistent with that expected for the anisotropy in Fe-20at%.Si with the DO/sub 3/ structure. >

Stress and annealing dependence of magnetic properties of amorphous Co-Fe-Si-B Alloys

We re-examine a model relating the stress (σ) dependence of the macroscopic magnetic properties (initial susceptibility χ i , coercive field H c , and effective anisotropy K eff ) to the domain structure and quenched-in stress field of an amorphous alloy. The model parameters (saturation magnetostriction λ s , volume average of quenched-in stresses \bar{\sigma}_{c} , and strain-independent anisotropy K o ) are shown to self-consistently determine measured values of K eff The evolution of these parameters for strain-relieving anneals of amorphous Co 75 Si 15 B 10 and Co 71 Fe 4 Si 15 B 10 is examined.

Antiferromagnetism in K2[FeCl5(H2O)] and its dependence upon hydrostatic pressure

Abstract We have measured the isothermal magnetization M of antiferromagnetic K 2 [FeCl 5 (H 2 O)] as a function of applied magnetic field H , for fields up to 80 kOe and for several hydrostatic pressures up to P ∼ 10 kbar. We obtained the P =1 atm exchange parameters of this material for an orthorhombic model. The spin-flop boundary of K 2 [FeCl 5 (H 2 O)] was studied for several pressures. For the spin-flop fields H SF , a relation of the form H SF (T) = H SF (0) (1 + BT 3 2 + CT 2 + DT 5 2 ) adequately describes the experimental points up to relatively high temperatures, for all pressures. The isotherms H SF vs. P show an abrupt change of slope at P ≲ 1 kbar, possibly due to a pressure-induced exchange of the intermediate and hard magnetic axes.

Magnetic permeability aftereffect in amorphous CoMn alloys

Abstract Amorphous ribbons with compositions Co 77− x Mn x Si 14 B 9 ( x = 0−15) and Co 75− x Mn x FeMoSi 14 B 9 ( x = 1−7) were prepared by melt-spinning. The room-temperature values of the specific magnetization σ s , the saturation magnetostriction λ s and permeability aftereffect Δμ/μ were determined for comparison with the theory of Allia and Vinai. The crystallization temperature T x was determined for all compositions. The influence of the alloy stability and casting conditions on the permeability aftereffect is discussed.

Magnetic properties of rapidly quenched iron-based alloys

Abstract We have measured the total magnetic losses of Fe-6.4wt%Si, amorphous Fe 78 B 13 Si 9 and amorphous and nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 , in the range 1–10 3 Hz. The total loss was found to be lowest in nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 . We have applied Bertottis formalism for this alloy in both amorphous and nanocrystalline forms. The experimental loss data were well reproduced with V 0 = 0.18–0.20 A / m for amorphous ribbons and with V 0 = 0.12 A / m for nanocrystalline ribbons.

Induced anisotropy and magnetostriction in Co70.4Fe4.6Si15B10

Abstract We have carried out isothermal annealings of as-cast and relaxed samples of the amorphous alloy Co70.4Fe4.6Si15B10 in the presence of an in-plane, transverse magnetic field (H = 5 kOe) at temperatures 463–598 K. The induced anisotropy Kind as well as the change in the saturation magnetorestriction on annealing Δλs was measured as a function of annealing time. For these two quantities, similar two-peaked spectra of activation energies were obtained, corresponding to reversible processes. We conclude that Kind and Δλs are proportional in this alloy.