摘要:

We have measured at low temperatures the ac differential susceptibility and the electrical resistivity for a series of Gd(Cu1−xGax) pseudo binary alloys for x between 0.10 and 0.22. The Gd(CuGa) system is particularly interesting in this concentration region since high temperature susceptibility measurements have shown a zero paramagnetic &Vthgr; for x?0.15. The intermetallic compound GdCu is antiferromagnetic TN?140 K while GdGa is ferromagnetic Tc?210 K. It was hoped by randomly mixing the Cu and Ga in the x region where Tcand TN→0, a spin glass‐like state could be formed. The susceptibility exhibits a broad maximum around 38 K for all alloys which is relatively concentration independent. At very low temperatures (1–10 K) a shoulder develops in the susceptibility which does vary with x. The resistivity shows an x dependent minimum at high temperatures 80–120 K, followed by a lower temperature maximum at 45–80 K. These behaviours indicate that a simple spin glass freezing is absent and that a complicated form of long range magnetic order is present.

ISSN:0021-8979    

DOI:10.1063/1.326947

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We report first detailed magnetoresistance (MR) studies of amorphous magnetically dilute FexNi1−xP14B6alloys with x=3,5,7,11 and 13, in applied fields, B, to 22 T and for temperatures above 0.1 K. The resistance minimum, observed in all these alloys, occurs at a maximum temperature of 100 K for x=5 alloy, a consentration at which a transition from spin glass to mictomagnetic behavior has been reported.1The (MR) in all these alloys, which can be as large as −1%, has a strong T‐dependence over a wide temperature range both below and above the spin freezing temperature Tf(or Tcfor x≳11). Above Tf(Tc) the B dependence of (MR) is −ve but with a curvature indicative of cluster effects. Also even at 0.2 K no saturation effects are observed in the −ve (MR) up to 22 T. The overall rise in resistance R(T)/Rmindecreases with increasing Fe conc. for x≳5. It is striking that for all these alloys the resistance is found to increase with decreasing T even at 0.1 K with no indications of any spin freezing effects. Thus the low T resistivity behavior of these amorphous alloys is markedly different from those usually observed in crystalline spin glasses. Supported by NSF grants DMR 77−08599 and DMR 76−80994 resp.1David Onn et al., J. Appl. Phys. 49(3), 1730 (1978).

ISSN:0021-8979    

DOI:10.1063/1.326949

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

In recent years detailed predictions were made concerning the temperature dependence of the critical resistivity. It is shown here that experimental results obtained for a ferromagnetic metal (Fe), a ferromagnetic semi‐conductor (p‐type CdCr2Se4), an antiferromagnetic semiconductor (MnTe) and an antiferromagnetic metal (Dy) confirm the theoretical predictionsquantitatively. The quantitative comparison of the theory and the experimental results yielded the specific heat parameters, &agr;=&agr;′=−0.21, A/A′=1.48 for Fe, &agr;=&agr;′=−0.01, A/A′=1.37 for MnTe and &agr;=&agr;′=−0.04, A/A′=1.44 for Dy. These results are in reasonable agreement with results from specific heat measurements and with results obtained in renormalization‐group calculations.

ISSN:0021-8979    

DOI:10.1063/1.326950

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A T=0 real‐space renormalization‐group method is presented and applied to the quantum 1D transverse field Ising model in the presence of a longitudinal field. This is an indirect way to study the equation of state of the equivalent 2D classical Ising model at finite temperature. When the longitudinal field is purely imaginary we obtain a new critical behavior which is related to the Yang‐Lee edge singularity of the corresponding classical model. We extract the edge exponent &sgr; in agreement with previous series expansion estimates.

ISSN:0021-8979    

DOI:10.1063/1.326767

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Quasi‐1‐D magnetic systems with on the one hand an Ising‐Heisenberg type spin anisotropy and on the other hand an alternating (dimerized) character have many interesting features in common and a few interesting differences in their phase behavior and general magnetic properties. This report reviews results rather scattered in the literature in addition to presenting new results. These rather complex quantum models present a theoretical challenge. It is also hoped that this work will be helpful to magnetochemists interested in identifying the underlying magnetic character of their systems, and to experimentalists in general.

ISSN:0021-8979    

DOI:10.1063/1.326951

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We have used the Monte Carlo renormalization group method to study the order‐disorder phase transition of the three and four dimensional, three‐state Potts model. In both cases, the transition appears to be first order. However, no evidence for a discontinuity fixed point associated with these transitions was found. Instead, the existence of a nearby second order fixed point just beyond the first order transition could be inferred from the RG flow in the ordered as well as in the disordered state. These results imply that the thermodynamic quantities are analytically continuable into the metastable regions, and that apart from discontinuities caused by the intersection of the two free‐energy branches, no additional singularities occur at the transition.

ISSN:0021-8979    

DOI:10.1063/1.326900

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The cubic model, described by the Hamiltonian ⋅=−J&Sgr;<ij≳ &sgr;i&sgr;j&dgr;&agr;i&dgr;&agr;jwhere &sgr;i=±1 and &agr;i=x, y, or z, is subjected to a Monte Carlo simulation. For an 8×8×8 fcc lattice, it is found that the system exhibits a first order transition at kBTo/J=3.45±0.01 in fairly close agreement with results obtained previously by the Kikuchi approximation. Discontinuities at the phase transition and stability limits for the metastable states on either side of the phase transition are also investigated.

ISSN:0021-8979    

DOI:10.1063/1.326901

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

An improved computational technique of the high‐temperature series expansion for magnetic systems with single‐ion anisotropy of arbitrary strength is presented. The method facilitates the calculation of higher‐order terms in the expansion. While applications have been made to obtain susceptibility and specific heat series for spin‐one ferromagnets with general single‐ion anisotroy, only the general method will be shown.

ISSN:0021-8979    

DOI:10.1063/1.326902

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

It is shown that the Migdal transformations should not be considered to be true lattice transformations since the same recursion relation may be derived for any lattice. A profitable point of view is to consider the various recursion transformations as approximate integrals of their common differential limit. It is then possible to construct extensions of the Migdal transformations by examining the renormalization group property and partition function invariance of a potential‐moving scheme order by order in (b‐1) where b is the length rescaling factor. Explicit conditions are given in a quasi‐continuum approximation to 0((b−1)2).

ISSN:0021-8979    

DOI:10.1063/1.326903

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We study the consequences of recently proposed recursions [1] for the anisotropic spin‐1/2 Heisenberg model, based on a generalization of the Migdal approximation [2]. In the isotropic case [3], they give the correct lower critical dimension, 2, and yield &ngr;=1/&egr; in (2+&egr;) dimensions as &egr;→0. In three dimensions, &ngr;=1.40 and &fgr;=1.56. The recursions give a finite entropy and linear specific heat as T→0. They give a reasonable description of Ising‐like anisotropy, but a strange and qualitatively incorrect picture of the XY side: first, there is no XY fixed point (fp) at T=0, and in three dimensions a flow from the finite‐T XY fp leads to the T=0 Heisenberg fp. Second, there is a jump (as opposed to flow) behaviour in the first iteration of any point in a large region of the anisotropy‐temperature plane. Third, the finite‐T fp persists in fewer than 2 dimensions, and a new finite‐T XY fp also appears; both are spurious. We also study the sensitivity of the results on b, and find that unlike the Migdal approximation for classical systems, here even the lower critical dimension in the isotropic case depends on b.

ISSN:0021-8979    

DOI:10.1063/1.326905

出版商:AIP    

年代:1979

数据来源: AIP