摘要:

Single crystal magnetic susceptibility results from 2.5 K to 270 K and specific heat results from 3 K to 16 K are reported for TTF⋅PtS4C4(CF3)4, (TTF=tetrathiafulvalene). The combined results are analyzed using a simple model which ignores differences between the two types ofS=1/2 spin carriers and involves a system of ferromagnetic chains treated ’’exactly’’, with interchain antiferromagnetic interaction evaluated in a mean field approximation. Above an apparent ordering transition at 8 K, the susceptibility is well described by the model irrespective of whether the ferromagnetic exchange is Heisenberg, Ising or intermediate to these. The magnetic contribution to the specific heat is obtained using earlier results for the isostructural Au compound. Comparison with specific heat calculations for the Heisenberg, Ising and intermediate cases successfully narrows the ambiguity to an intermediate anisotropic exchange close to the Heisenberg limit.

ISSN:0021-8979    

DOI:10.1063/1.325030

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A spin‐lattice dimerization occurring in a system of one‐dimensional (1D) magnetic spins coupled to the lattice (Spin‐Peierls instability) is studied for classical and quantum Heisenberg and for one‐ and two‐band Hubbard models. Using the exact solutions it is shown for the classical case that second or first order lattice phase transitions (PT) are possible. For the isotropic quantums=1/2 Heisenberg and Hubbard models using the variational principle it is shown that PT can only be of second order while the spin anisotropy and the presence of a second electronic band yield the first order PT. Some applications to PT in VO2are also made.

ISSN:0021-8979    

DOI:10.1063/1.325031

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We present an investigation of the effect of an applied external magnetic field on the low temperature magnetic properties of the linear Heisenberg antiferromagnet (CH3)4NMnCl3(TMMC). The proton and deuteron spin‐lattice relaxation rateT−11has been measured in the temperature interval 1.3–4.2 K and for applied field ranging from 1 kG to 88 kG. Large effects are observed both for the field parallel and perpendicular to the chain axis c. In particular, for H⊥c, a peak inT−11occurs at the 3dordering temperatureTN. In this way the investigation of the field dependence ofTNreported by Dupas and Renard up to 10 kG is extended to 88 kG. In this paper we limit the discussion to the effect of H onTN, for H⊥c. The results are explained by considering the effect of both the dipolar anisotropy and the magnetic field on the spin correlations. It is shown that for H⊥c the system should be described by anXYmodel forT<20 K and it crosses over to an Ising model forkT∼g&mgr;B[S(S+1)H]1/2. The corresponding changes in the correlation length are responsible for the strong field dependence ofTN. At low fields we obtain theoretically the correct H2dependence ofTN. For high fields one suggests that the correlation length and henceTNare affected by the crystal defects.

ISSN:0021-8979    

DOI:10.1063/1.325032

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The crystal structure of the title compound (TMSO≡C4H8SO) consists of linear chains of Cu ions with three bridging ligands (2 Cl, 1 O) between each pair of Cu ions. The susceptibility of the salt has been measured between 2.0 and 120 K on a PAR vibrating sample magnetometer and from 80 to 300 on a Faraday balance. The high temperature data shows positive deviation from Curie behavior, with a Curie‐Weiss constant of 18.7 K. However, considerable interactions between chains exist and the &khgr;TvsTplot shows a maximum atT=7.5 K. The data has been analyzed with several one‐dimensional models. At present, the best fit is obtained with an Heisenberg linear chain (J=24.6 cm−1) with a mean field correction for interchain interactions (ZJ′=−1.3 cm−1).

ISSN:0021-8979    

DOI:10.1063/1.325033

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A detailed and quantitative study of the low‐temperature spin‐wave dynamics has been made for the classical Heisenberg‐coupled chain using computer simulation. Results for the spin‐wave frequencies are presented and compared with existing predictions.

ISSN:0021-8979    

DOI:10.1063/1.324980

出版商:AIP    

年代:1978

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.324981

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The inhomogeneous generalization of the Migdal‐Kadanoff approximate recursion relations is used to study the properties of Ising magnets with spatially random (quenched) couplings. Simple calculations in two dimensions produce qualitatively reasonable phase diagrams and critical exponents for both a dilution problem (nearest‐neighbor couplings 0 andJ) and a spin‐glass system (nearest‐neighbor coupling +Jand −J. Other related work is briefly reviewed.

ISSN:0021-8979    

DOI:10.1063/1.324982

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We report on the experimental investigation of two types of crossover in the ordered state near the spin‐flop (SF) multicritical point in the orthorhombic antiferromagnet, GdAlO3. (a) The magnetization discontinuity, &Dgr;M(t), at the SF transition increases with power &bgr;˜=0.88±0.02 close to the SF multicritical point. Close to a critcal edge point of the SF transition shelf, however, &bgr;˜≈0.32. The bicritical‐critical crossover extends over several orders of magnitude. (b) The SF transition shelf was mapped out for different directions of H⊥. The resulting tetracritical point–bicritical line crossover appears to be rather complicated and the experimental results are not yet well understood.

ISSN:0021-8979    

DOI:10.1063/1.324983

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The bicritical region of the phase diagram of manganous chloride tetrahydrate has been investigated by the determination of the heat capacity as a function of temperature at constant applied magnetic field. The experiments were repeated at field increments of 10 to 50 G in the region of the bicritical point. Plots of the heat capacity maxima in theH‐Tplane clearly exhibit an umbilicus at the bicritical point. These data have been fit to the expression given by Fisher and Nelson following the procedure for locating the appropriate scaling axes given by Fisher. The ratio of the amplitudes of the slope of the phase boundaries in the region of the bicritical point has been determined to be 1.000±0.006, which is not consistent with the theoretical prediction of 2.51 for a uniaxial antiferromagnet. These results and also the variation of the behavior of the phase boundaries in the bicritical region as a function of the angle between the applied magnetic field and the spin axis is discussed.

ISSN:0021-8979    

DOI:10.1063/1.324984

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

We study the critical dynamics of stochastic models appropriate for describing bicritical and tetracritical points in anisotropic antiferromagnetic systems. The dynamic exponents and the transient exponents are calculated by renormalized field theory up to two‐loop order. In the presence of reversible mode‐coupling terms, two‐loop contributions establish bicritical dynamic scaling in the restricted sense and invalidate recent predictions based on mode‐coupling arguments. In the case of ann‐component relaxational model total dynamic scaling is found toO(&egr;2) both at the bicritical (n≲3) and the tetracritical (4≲n<11) points.

ISSN:0021-8979    

DOI:10.1063/1.324985

出版商:AIP    

年代:1978

数据来源: AIP