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Equilibrium susceptibility x eq of frozen iron nitride magnetic fluids is estimated as the convergent value of the relaxation curves for various initial states. In the lower temperature range in which the field cooled susceptibility x FC shows a plateau, x eq of the dense sample is nearly the same as x FC , while x eq of the diluted sample increases with decreasing temperature. These indicate that blocked moments are observed for the isolated particles owing to a finite measurement time and that the magnetic moments of the interacting particles freeze cooperatively as seen in a spin glass. [S0031-9007 (97)04918-1] PACS numbers: 75.50.Lk, 75.50.MmGlassy behavior due to random anisotropy and dipolar interactions has been intensively investigated in ferromagnetic fine particle systems [1][2][3]. However, there is little agreement as to the existence of the spin glasslike phase. One reason is that most properties for the cooperative freezing of spin glass are similar to those for the blocking of isolated particles in appearance. For example, a plateau for the temperature dependence of the field cooled susceptibility has been observed for both systems. Recently, typical spin glass dynamics has been observed in a concentrated system of g-Fe 2 O 3 fine particles by Jonsson et al., where the relaxation depends on the time spent at constant temperatures before applying the magnetic field [4]. However, the existence of this dynamics does not by itself imply that there is a thermodynamic spin glass phase. To distinguish a thermodynamic phase from a nonequilibrium state, the equilibrium properties, in addition to critical phenomena, should be examined. In the spin glass phase, it is known that the equilibrium susceptibility x eq is almost independent of the temperature, and that the nonlinear susceptibility x 2 diverges at the transition. On the other hand, x eq of the blocked magnetic moments is predicted not to be constant and to be superparamagnetic. In this Letter, x eq and x 2 are estimated and the results for isolated particles and interacting particles are compared with the superparamagnetic model and spin glass.The samples are iron nitride e-Fe 3 N magnetic fluids with kerosene as the carrier liquid [5]. Band structure calculation [6] has shown that e-Fe 3 N is a ferromagnetic substance with magnetic moment per Fe of about 1.9m B and that the easy axis is along the c axis of the hexagonal structure. Electron microscopy shows that the particles have isotropic shapes and uniform size, as shown in Fig. 1. There is a single domain within each particle for this size. Therefore, we can take the magnetization of each particle as a rigid dipole moment m [the mean value is ͗m͑T ͒͘]. The small angle x-ray scattering profiles show no particle agglomeration. Measurements were made on samples cooled in a zero field to 150 K below the freezing point for kerosene. These samples, therefore, are randomly oriented single-domain particles embedded in solidified kerosene. Sample d1 is an as-prepared magnetic fluid and the other samples ...

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Fermi level tuning has been successfully demonstrated in Co-based full-Heusler alloy Co(2)FeAl(0.5)Si(0.5) (CFAS). The half-metallic band gap of CFAS was proved by the behavior of differential conductance of CFAS/(MgAl(2))O(x)/CoFe magnetic tunneling junctions with an unexplored crystalline (MgAl(2))O(x) barrier. CFAS exhibits the highest effective spin polarization (P_{eff}) at 300 K and the weakest temperature dependence of P_{eff} among all known half metals. Further study shows that P_{eff} of CFAS decays with increasing temperature (T) following T;{3/2} law perfectly, which indicates that the depolarization of CFAS is determined by spin wave excitation only.

The magnetic properties and the crystal structure of MgV2O4 and Mg(V0.85Al0.15)2O4 have been studied. Both compounds are the normal cubic spinels with highly frustrated magnetic lattice. Around T2=65 K, MgV2O4 has magnetic orders accompanied with the cubic-tetragonal transition. Below T2, the susceptibility shows complex behavior. In Mg(V0.85Al0.15)2O4, the spin-glasslike state appears. The V51-Knight shift of MgV2O4 has an anomalous temperature dependence, which is not simply related by that of the susceptibility.

We fabricated fully epitaxial Fe/MgAl2O4/Fe(001) magnetic tunnel junctions using plasma oxidation of an Mg/Al bilayer. The MgAl2O4 showed a (001)-oriented spinel-type structure, and there were few misfit dislocations at the interfaces between the MgAl2O4 and the two Fe layers due to a small lattice mismatch (∼1%). Tunnel magnetoresistance (TMR) ratios up to 117% (165%) were obtained at room temperature (15 K). The bias voltage for one-half of the zero-bias TMR ratio (Vhalf) was relatively large, ranging from 1.0 to 1.3 V at room temperature, which is attributed to the small misfit dislocation density.

Effects of temperature changes on aging phenomena are studied for the spin-glass-like phase of a ferromagnetic fine particle system. The results obtained during the short experimental periods are, in appearance, asymmetric about temperature changes. However, the extension of the periods shows that the effects of temperature changes are symmetric. These results can be explained by the droplet model well. [S0031-9007(99)

A current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve using epitaxial layers of Co2FeAl0.5Si0.5 (CFAS) Heusler alloy as ferromagnetic electrodes is reported. A multilayer stack of Cr/Ag/CFAS/Ag/CFAS/Co75Fe25/Ir22Mn78/Ru was deposited on a MgO (001) single crystal substrate. Epitaxial growth of the Cr, Ag, and CFAS layers in the (001) orientation up to the top CFAS layer was confirmed. Large MR ratios of 6.9% at room temperature and 14% at 6 K were observed for the CPP-GMR device. High spin polarization of epitaxial CFAS is the possible reason for high MR ratios.