Magnetically dilute metallic glasses. II. 4\u3ci\u3ef\u3c/i\u3e moments

Magnetic-susceptibility and high-field magnetization measurements are presented for amorphous Zr40Cu600-xMx (M denoting Gd and Tb), with x ranging from zero to ten. Effective moments of magnetic solutes were determined by fitting susceptibility data to the Curie-Weiss expression. The moments of Gd and Tb are very close to those expected for trivalent ions and the paramagnetic Weiss temperatures are positive. Saturation did not occur in any of the samples, even at 80 kOe and 1.3 K; however, the alloys containing Gd approached normalized magnetization values of unity. High-field hysteresis loops were used to obtain the temperature dependence of the coercive fields Hc and to determine ordering temperatures, defined as those temperatures for which Hc vanishes. The zero-field susceptibility for both the Gd and Tb alloys shows sharp peaks at low temperatures. The magnetically ordered state of the Gd alloys is characterized as spin-glass-like; that is, the spins are frozen in random directions and there are both ferromagnetic and antiferromagnetic exchange interactions, with the former dominant. The Tb alloys also have low-temperature magnetic states characterized as spin-glass-like in this sense. However, in the Tb alloys, the presence of local random anisotropy affects the high-field magnetization considerably. Attempts to fit the high-field magnetization of the Tb alloys to the local-random-anisotropy theory of Harris et al. are described

Ab-initio density functional studies of stepped TaC surfaces

We report on density functional total energy calculations of the step formation and interaction energies for vicinal TaC(001) surfaces. Our calculations show that double and triple-height steps are favored over single-height steps for a given vicinal orientation, which is in agreement with recent experimental observations. We provide a description of steps in terms of atomic displacements and charge localization and predict an experimentally observable rumpled structure of the step-edges, where the Ta atoms undergo larger displacements compared to the C atoms.Comment: 4 pages, 4 figure

Magnetically dilute metallic glasses. I. 3\u3ci\u3ed\u3c/i\u3e moments

Magnetic-susceptibility and high-field magnetization measurements are presented for amorphous Zr40Cu50M10 (M denoting Fe, Mn) and Nb50Ni50-xFex, with x ranging from zero to about ten. Curie-Weiss fits to the susceptibility data were made to determine effective moments peff and paramagnetic Weiss temperatures, ⊖. The small values of peff suggest the presence of moment instabilities depending on local environment, and the negative ⊖ values suggest antiferromagnetic local-moment-conduction-electron interactions. The Fe alloys develop hysteresis at low temperatures indicating the presence of a magnetically ordered state. These alloys can be characterized as cluster glasses because of the apparent importance of magnetic clusters and the development of an ordered state of spin-glass character. However, the Mn alloys show no evidence of magnetic order down to 1.3 K

Electronic properties and superconductivity of Zr-Pd glasses

Electrical-resistivity measurements between 1.4 and 300 K and room-temperature magnetic-susceptibility measurements were performed on the metallic glasses ZrxPd1-x, for x=0.59, 0.63, 0.67, and 0.70. The samples, prepared by a modified hammer-anvil technique, exhibit negative temperature coefficients of the resistivity at temperatures from about 10 to 300 K. These data can be understood in terms of the extended Ziman theory if it is assumed that Zr and Pd contribute about 2 and 1 conduction electrons, respectively. Assuming that the magnetic susceptibility is a qualitative measure of the electronic density of states at the Fermi level, N(EF), it is shown that N(EF) does not seem to decrease with composition as the eutectic composition is approached. Thus, for this system, there is no obvious correlation between the thermodynamic stability of the liquid phase and the electronic density of states. The superconducting transition temperature (Tc) scales linearly with x and the valence-electron-per-atom ratio (z). However, as in the case for other intertransition-metal glasses composed of distant neighbors in the Periodic Table, Tc (z) falls below the Collver-Hammond Tc (z) curve for amorphous metals and alloys of neighboring metals of the 4d and 5d transition series

Effect of random anisotropy on magnetic properties of amorphous systems

We consider the random anisotropy model for amorphous magnetism by making a local-mean-field approximation (LMFA) on arrays of spin-one particles. Hysteresis loops and the temperature (T) dependence of several thermodynamic quantities are presented for various values of the ratio of the strength of the exchange (J) to the strength of the uniaxial anisotropy (D). Using the LMFA limits us to systems with a small number (N) of spins, of which we explicitly consider N=64, 216, and 1000. We assume periodic boundary conditions on a system with N1/3 spins along an edge, nearest-neighbor coupling of constant strength, and six nearest neighbors (as for a simple cubic lattice). For J\u3e0 the free energy of spin-glass-like states is higher than that of corresponding states with remanent magnetization. The dependence of the coercive field (Bc) on J and D is discussed and the apparent discrepancy of Chi and Alben vis à vis Callen, Liu, and Cullen concerning the behavior of Bc for large D is clarified. A calculation of the temperature dependence of Bc is presented which is reminiscent of experimental results. This random anisotropy is found to give rise to a second peak in the specific heat for suitable values of D/J. The magnetic susceptibility (χT) is calculated for both positive and negative J and shows positive and negative paramagnetic Curie-Weiss temperatures, respectively. The slopes of the χT-1 (T) curves for T well above the critical temperature (Tc) have values that are roughly equal to 3/2, the value appropriate to D=0 and S=1. The local order parameter q is used to identify Tc, which correlates well with the critical temperature identified from other thermodynamic quantities. The presence of the random anisotropy is found to reduce Tc by up to about 25%. The results of several temperature-dependent calculations are summarized in a phase diagram and regions of paramagnetic, random ferromagnetic, and random antiferromagnetic (or spin-glass-like) behavior are identified

Electronic and magnetic properties of amorphous and crystalline Zr\u3csub\u3e40\u3c/sub\u3eCu\u3csub\u3e60-x\u3c/sub\u3eFe\u3csub\u3ex\u3c/sub\u3e alloys

Electronic states and localized magnetic moments and their interactions were studied in amorphous and crystalline Zr40Cu60-xFex alloys for 0≤x≤12. Electrical resistivity, magnetic susceptibility, and high-field magnetization measurements were performed. In the dilute crystalline alloys Curie-Weiss behavior is seen in the susceptibility and is associated with localized moments (μeff≈3μB) on the iron atoms. At higher iron concentrations ferromagnetism is observed. Curie-Weiss behavior also is seen in the susceptibility of the dilute (0eff≈0.7μB). There is some evidence that the local environment of the Fe atoms is important and may depend sensitively on the quench rate used in making the samples or, perhaps, on room-temperature ageing effects in the samples. The dilute amorphous alloys exhibit a negative dρ/d T from 1.4 to 300 K. This is not to be associated with Kondo spin-flip scattering but it is consistent with several other mechanisms including localized-spin-fluctuation scattering, s-d scattering in a nonmagnetic model, scattering from tunneling states in the amorphous alloy, or quasi-liquidmetal-pseudopotential scattering. A recent theory due to Nagel and Tauc on the nearly-free-electron approach to metallic glass alloys is shown to be consistent with this last idea and also is used to account for other features exhibited by the amorphous Zr-Cu system. In the concentrated (x\u3e6) amorphous alloys, resistance maxima and magnetic hysteresis are seen at low temperatures. For x=12 a random ferromagnetic state develops with T0=30 K, which is some five times smaller than T0 for the corresponding crystalline alloy. The saturation moment in the amorphous alloy is also considerably smaller than in the crystalline case. This behavior is similar to other systems in which the crystalline-to-amorphous transition greatly weakens the magnetism