摘要:

Transverse magnetoresistance has been measured in AuFe polycrystals betweem 2.5 K and 300 K in magnetic fields up to 18 KG. For a sample with 2.9 at. &percent; Fe (T0=16 K) the magnetoresistance at low temperature first increases with the field, peaks at around 1 KG and then decreases. The peak height decreases rapidly with increasing temperature. At temperatures above 6 K there is no measurable positive magnetoresistance even at 50 gauss. The negative component above 2 KG varies roughly as (&Dgr;&rgr;/&rgr;) = −a(T) Bnwith n=1.9±0.1. The amplitude a(T) saturates at low temperatures (2.5 K<T<8 K) and then falls off gradually with temperature. At around 125 K the negative component becomes undetectable. While no explanation could be found for the behaviour of the positive magnetoresistance, the negative one could be qualitatively explained in terms of the spin‐disorder scattering and the freezing of the spins below T0. The alloys with higher Fe content (6.6, 10, 13, 18 at. &percent; Fe) show that the magnetoresistance between 77 K and 300 K is negative but the field and the temperature dependence of (&Dgr;&rgr;/&rgr;) changes significantly as one passes through the spin glass, mictomagnetic and inhomogeneous ferromagnetic regimes.

ISSN:0021-8979    

DOI:10.1063/1.327244

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Employing both matrix diagonalization and equation‐of‐motion techniques we have calculated the density of spin wave modes and the dynamic structure factor for the spin glass PdMn (10 at&percent;). We used a functional form for the exchange interaction suggested by high field magnetization measurements with an overall multiplicative factor adjusted to give agreement with the low temperature specific heat. Our results for the dynamic structure factor for momentum transfer Q=0.8 A˚−1are compared with recent inelastic neutron scattering data.

ISSN:0021-8979    

DOI:10.1063/1.327245

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The insulating Sr1−xEuxS system exhibits spin‐glass properties for x≳0.05 due to competing ferro and antiferromagnetic interactions. A scaling law x/Tf=f(ln &ngr;) represents the frequency (&ngr;) dependence of Tf, the temperature of the maximum of &khgr;a.c.(or spin‐glass freezing temperature). This frequency dependence of Tfisatleastan order of magnitude bigger than in a canonical spin‐glass such as AuFe. The dipolar interaction is responsible for the anisotropy inside magnetic clusters. For x<0.05 the a.c. susceptibility, &khgr;a.c., doesn’t exhibit a single maximum, but after an initial maximum or plateau around Tpit increases to a second rounded maximum, at much lower temperatures. Below Tp, which is concentration independent, a remanent magnetization appears proportional to x2. Furthermore, at this temperature deviations from a Curie law of &khgr;a.c.increase more rapidly than x. Moreover, the frequency dependence of Tppermits a determination of a blocking energy. This energy is in agreement with the anisotropy barrier due to the dipolar interaction for a ferromagnetic pair of first neighbors. All these results support the idea of short range order for individual pairs, and small magnetic clusters which are blocked below Tpon random anisotropy directions determined by the intra‐cluster dipolar interaction. A similar change in the magnetic properties of other spin‐glasses is expected.

ISSN:0021-8979    

DOI:10.1063/1.327246

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The magnetic properties of Nax V2 O5−&bgr; vanadium bronzes have been studied on single crystals with x=0.20, 0.30 and 0.33. Curie behaviour has been found for all samples. For x=0.33, a magnetic transition takes place at T1?18 K. The initial reversible susceptibility, thermoremanent and isothermal remanent magnetizations have been measured at T<18 K for different crystallographic orientations. The data suggest an antiferromagnetic behaviour with the direction of antiferromagnetism perpendicular to the b&drarr;‐monoclinic axis. The remanent magnetizations are compared to a spin glass behaviour due to a possible position disorder of the V4+. It is also proposed that there could be a weak ferromagnetism along the b&drarr;‐axis.

ISSN:0021-8979    

DOI:10.1063/1.327247

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We examine the thermodynamic properties of a set of Ising spins interacting via various random potentials Jijin a modified Bethe‐Peierls‐Weiss (BPW) approximation and obtain the spin glass order parameter, the internal energy and the specific heat. When the effective number of neighbors z→∞, we show that the magnetic properties of the system are identical in the (i) BPW‐method, (ii) the selfconsistent mean‐random‐field (MRF) method, and (iii) the Sherrington‐Kirkpatrick derivation. The entropy at T=0 is still negative and our method suggests how including correlations would result in a more physically correct solution as T→0. We also obtain the phase diagram as a function of z in the MRF‐method from z=2 to z=20 and find that for z≳8 the phase diagram differs little from the z=∞ case. Similarly, we obtain the thermodynamic properties of the random spins with an RKKY potential in the BPW method. We find that for low concentrations the method gives a discontinuous slope in the magnetic susceptibility &khgr; and the specific heat CMat the spin‐glass transition temperature Tg, however the maximum in CMand &khgr; occur well below Tg.

ISSN:0021-8979    

DOI:10.1063/1.327195

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A recently proposed CPA theory (Tahir‐Khelietal), for treating randomly diluted antiferromagnets with uniaxial anisotropy is revised. It is shown that such a procedure, which ignores dynamical effects occasioned by fluctuations in the local anisotropy field, leads to unphysical results beyond certain critical concentrations at which the theory breaks down. A study of the weakness, the relevance and validity of this theory is presented. The theory is expected to be relevant only when applied to systems with relatively small anisotropy. In fact, owing to its simplicity, within its range of validity, we apply a such CPA to analyse the behaviour of the zero‐field antiferromagnetic resonance in the mixed system of manganese and zinc fluorides as function of zinc concentration. Our results are compared with those from several recent experiments and a very good agreement is found. Moreover, the general features and complexity of an alternative CPA formalism which takes into account dynamical effects of fluctuations in both exchange and anisotropy fields, are also briefly discussed.

ISSN:0021-8979    

DOI:10.1063/1.327196

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A coherent potential approximation (C.P.A.) theory has been developed in order to examine the bond dilution problem in an antiferromagnet which contains alternating strong and weak magnetic exchange interactions. Specifically, we have calculated the concentration dependence of the transition temperature at which the system displays long‐range order. This is done by examining the Green function for the system in the paramagnetic phase. The existence of the strong exchange interaction creates pairs of highly correlated spins in the paramagnetic state. The transition temperature is identified by introducing the weak coupling between these spin pairs and locating the temperature at which the excitation of a pair from the singlet ground state to the lowest excited states becomes soft. When only interpair bonds are broken, a linear concentration vs transition temperature behavior is predicted. This prediction is compared with recent experimental results for the diluted paired antiferromagnet, RbMn1−xMgxCl3.

ISSN:0021-8979    

DOI:10.1063/1.327197

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Magnetic susceptibility and electron paramagnetic resonance experiments have been conducted on the RbMn1−xMgxCl3system at temperatures ranging from 4.2 to 300 K and in magnetic fields up to 100 kOe. In the pure manganese compound the neighboring magnetic ions are alternately coupled by strong and weak exchange interactions, leading to a paired antiferromagnetic structure. The effect of diluting the manganese with up to 50&percent; magnesium, a nonmagnetic species, has been examined. It is found that most of the results are consistent with what one finds in an ordinary diluted antiferromagnet. The notable exception exists in the concentration dependence of the ordering temperature. In the low dilution regime (x<.15) a nonlinear concentration dependence on the Neel temperature is found. This is attributed to the formation of chains of weakly interacting spins. These act to circumvent the magnetically stiff pairs produced at each of the strong exchange interactions, and thereby partially offset the effect of the broken magnetic bonds produced during the dilution process.

ISSN:0021-8979    

DOI:10.1063/1.327198

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We consider a bond model of a disordered Heisenberg ferromagnet in which spin values are equal to 1/2 but two distinct exchange integrals are distributed randomly among bonds. The coherent potential approximation in the form of Oguchi, Ono and Ishikawa is extended so as to include the pair effects of the bond clustering by adapting the treatment of the alloy problem by Schwartz and Ehrenreich. The self‐consistency condition which determines the coherent exchange integral is derived. The density of states of magnons is calculated from the configurationally averaged Green function. The applications for the one‐dimensional case and the two‐dimensional case are discussed.

ISSN:0021-8979    

DOI:10.1063/1.327199

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Low field d.c. magnetization measurements on weakly ferromagnetic and spin glass‐like Ni‐Cu alloys are reported. (AIP)

ISSN:0021-8979    

DOI:10.1063/1.327200

出版商:AIP    

年代:1979

数据来源: AIP