Showing papers on "Antiferromagnetism published in 1980"

Low-temperature properties of the random Heisenberg antiferromagnetic chain

Abstract: The one-dimensional quantum spin-\textonehalf{} Heisenberg antiferromagnetic model with randomly distributed interaction strengths is solved approximately for several different distributions. Ground-state energy and low-temperature properties are evaluated. Universal qualitative features are found in the specific heat and the magnetic susceptibility, which display a power-law dependence on temperature. Such features hold for nonsingular distributions as well as for distributions with power-law divergence at the origin. The approximate method of solution is based on successive eliminations of spins coupled by the maximum coupling constant.

3d transition-metal intercalates of the niobium and tantalum dichalcogenides. I. Magnetic properties

Abstract: The Hall coefficient and resistivity of the first-row transition-metal intercalates, M1/3TaS2 (M = V, Cr, Mn, Fe, Co, Ni), Mnl/4TaS2, M1/3NbS2 (M = V, Cr) and Fel/4NbSe2 have been measured as a function of temperature. These intercalation complexes show either ferromagnetic or antiferromagnetic orderings and there are large anomalies in both transport properties at the respective magnetic-ordering temperatures. The Hall coefficient in the ferromagnetic intercalates varies linearly with the magnetic susceptibility for temperatures above the Curie temperature, Tc, and as the square of the resistivity for temperatures below Tc. We have observed that the Hall coefficient of Fe1/3TaS2 varies non-linearly with the applied field near the Curie temperature. We have found a similar variation of the spin-disorder resistivities of the complexes, M1/3TaS2 and M1/3NbS2 (M = Mn, Fe, Co, Ni) with the intercalate ion.

Nuclear Antiferromagnetic Resonance in Solid He 3

Abstract: Detailed measurements of the low-field antiferromagnetic resonance spectrum of spin-ordered bcc 3He exhibit large shifts from the Larmor frequency, with a zero-field resonant frequency near zero temperature of Ω0/2π≃825 kHz. Analysis of the spectrum leads to stringent constraints on possible sublattice structures. The temperature dependence of Ω0 shows low-temperature behavior expected from spin-wave theory, and indicates a first-order transition at 1.03 mK.

Inelastic-light-scattering study of magnon softening in ErFe O 3

Abstract: The observation of Raman scattering due to one-magnon excitation is reported for the first time on canted antiferromagnetic ErFe${\mathrm{O}}_{3}$ which has a zero-angular-momentum ground state. The magnon softening phenomenon, associated with the spin-reorientation phase transitions of this crystal, is studied through the thermal behavior of one-magnon scattering. The temperature dependence of two antiferromagnetic magnon modes is analyzed on the basis of a simple free-energy form for this spin system. Some peculiar features such as the failure of the softmode frequency to vanish at the spin-reorientation temperatures are discussed briefly. It was found that the weak ferromagnetic moment in this canted antiferromagnet must be taken into account for a satisfactory explanation of the polarization properties of the one-magnon spectra. However, the theoretical detail requires further clarification. The prominent intensity enhancement of the soft magnon line near the transition temperature is interpreted to be a result of population increase due to the mode softening associated with spin reorientation.

Manganese hexacyanomanganate: Magnetic interactions via cyanide in a mixed valence Prussian blue type compound

Abstract: A Prussian blue type compound of the stoichiometric composition Mn(CN)3⋅xH2O (x=0.57) is formed as the final product of hydrolysis of Na3[Mn(CN)6] in perchloric acid. IR and visible spectra as well as structural and magnetic data show that it is a mixed valence compound consisting of sixfold N‐coordinated Mn(II) and sixfold C‐coordinated Mn(IV) in a cubic face‐centered lattice. The compound exhibits ferrimagnetism with a Curie temperature of 48.7 K. A molecular field treatment assuming two collinear spin sublattices provides an adequate description of the observed magnetic behavior. The calculation of exchange integrals, which was based on a Heisenberg model, reveals that the magnetic ordering is almost exclusively induced by an antiferromagnetic interaction between Mn(II) and Mn(IV) ions. This interaction is described in terms of a super exchange involving cyanide as bridging ligand.

Exact results and critical properties of the Ising model with competing interactions

Abstract: The random Ising model with competing interactions is investigated on the basis of the gauge-invariant formulation of the problem. Exact results for the internal energy, specific heat and gauge-invariant correlation function are derived. The critical exponent alpha is shown to be negative at the phase boundary of the paramagnetic and ferromagnetic phases if the latter exists at fairly low concentration of antiferromagnetic bonds.

Phase boundary of Ising antiferromagnets near H = H c and T = 0 : Results from hard-core lattice gas calculations

Abstract: Ising antiferromagnets in a near-critical magnetic field at low temperatures are equivalent to hard-core lattice gases. Using this connection and the existing series-expansion results for hard-core lattice gases, we determine the slope of the phase boundary at $T=0$ for the square (sq), plane-triangular (pt), simple cubic (sc), and body-centered cubic (bcc) antiferromagnets. The slope is negative for the sq and pt lattices and nearly zero for the sc case. For the bcc lattice a positive slope is obtained, indicating that the phase boundary bulges above the zero-temperature critical field. We also test M\"uller-Hartmann and Zittarz's postulate for the critical curve of the sq Ising antiferromagnet. A renormalization-group treatment of the hard-square lattice gas yields a critical activity ${z}^{*}=3.7959\ifmmode\pm\else\textpm\fi{}0.0001$, which is in agreement with series-expansion and finite-lattice estimates but at variance with the postulated ${z}^{*}=4$. The same calculation gives $\ensuremath{ u}=0.999\ifmmode\pm\else\textpm\fi{}0.001$ for the correlation-length exponent, thus supporting the conjecture that the transition of the hard-square lattice gas belongs to the Ising universality class.

Amorphization of Layered Ferro- and Antiferromagnetic Systems

Abstract: The amorphyzation of a crystalline layered ferro- and antiferromagnet with randomly distributed interlayer exchange parameters is investigated by the use of newly developed effective field theory, Two typical amorphizations of interlayer exchange parameters are studied. The phase diagrams are obtained. The analytical behaviours of magnetization near the spin glass phase boundary at T =0 are studied in details for each case. We find that the order of transition from the ferromagnetic phase to the spina glass phase depends on the distribution function of interlayer exchange parameters.

NMR Investigations on the Spin Fluctuations in Itinerant Antiferromagnets. I. V3Se4 and V5Se8

Abstract: The properties of the spin density fluctuations in the itinerant antiferromagnetic compounds V 3 S 4 and V 5 S 8 have been investigated in both the paramagnetic and the antiferromagnetic states by using the pulsed NMR technique. Although it has been proposed that V 3 S 4 is one of weak itinerant antiferromagnets because of the low ordering temperature, small staggered moment M Q and the temperature dependence of M Q , the temperature dependence of the nuclear spin-lattice relaxation rate 1/ T 1 are not in accord with the prediction based on the self-consistent renormalization theory of spin fluctuations for weak itinerant antiferromagnets. In V 5 S 8 , the observed temperature independent process of 1/ T 1 in the paramagnetic state and the temperature dependences of the bulk susceptibility and M Q are not explained from either weakly magnetic limit or localized moment limit, suggesting this material is in the intermediate magnetic regime.

Phase transitions of some fully frustrated models

Abstract: Fully frustrated antiferromagnets with triangular plaquettes are investigated. For the two-dimensional triangular lattice, the Ising antiferromagnet is in the class of the XY model with transition temperature T=0. For d>or=3 the authors investigate the face-centred cubic lattice. For even d there is a proper phase transition with long-range order. For odd d the fluctuation spectrum has a reduced dimensionality (d-1). This is reflected in an infinite ground state degeneracy and a reduced dimensionality of the ground state order (d-n). For d=3 a phase transition occurs for the Ising model but not for n>or=3. The relationship of the fully frustrated phase to the disordered spin glass is also discussed. It is suggested that the two phases might be distinct near d=6 but coalesce at some lower dimensionality.

Spin and phonon excitations in actinide systems

Abstract: Recent Chalk River neutron scattering experiments on antiferromagnetic UN, on ferromagnetic UTe and US, and on non-magnetic UPd 3 , are reviewed. The spin excitations may be sharp or diffuse and the phonons exhibit anomalies reminiscent of mixed-valent compounds. A very detailed study of the dispersion of the sharp collective excitations seen in dhcp UPd 3 has shown that the uranium is in a stable f 2 , J = 4, ground state subject to a conventional crystal field and exchange. Thus, UPd 3 is the canonical 5f system, analogous to dhcp Pr in the 4f series. Although UN is cubic, large anisotropies are found in its critical scattering, and large anisotropy gaps are found in the spin wave spectra of UN and UTe.

Magnetic susceptibility and the phase transition of NpO2

Abstract: The magnetic susceptibility and the induced magnetic moment of NpO2 single crystals were measured between 3 and 300 K and in fields up to 1.5 T. The resistivity was measured above 150 K on pressed powder pellets. A band gap of 0.4 eV is found. Three theoretical models are discussed to explain the nature of the phase transition at T0 = 25.4 K and the absence of Np magnetic moment below T0. The crystal field model shows that the Γ(2)8 quartet of 5f3, split by quadrupolar interaction- induced distortion which sets in below T0, may have a quenched moment at the theoretically expected value x = −0.74 of the crystal field parameter. Other models based on the Np3+(5f4) configuration explain the transition either as an antiferromagnetic ordering of s = 1 2 holes located in that half of the oxygen cubes which contain no Np in the fluorite structure, or as an order-disorder transformation of O- and O2− ions. The calculated susceptibility is in fair agreement with the measurement. Further experiments are proposed to test the models.

Magnetic correlations in Eu x Sr1−x S and the ferromagnet-spin glass transition

Abstract: Neutron (Bragg and small angle) scattering and susceptibility measurements are used to study magnetic ordering in Eu x Sr1−xS with ferromagnetic nearest neighbor exchangeJ1 and antiferromagnetic next-nearest neighbor exchangeJ2. We present data for 0.50≦x≦0.70 which cannot be analyzed within the merely geometrical treatments of percolation theory. Breakdown of ferromagnetism occurs atx c =0.51, far above the percolation thresholdx p =0.136, and a spin-glass phase is observed in the intermediate concentration regime. Close tox c , the ferromagnetic state is also displaced by the spinglass phase at lower temperatures. Both properties are a general characteristic of diluted systems with competing interactions. An effective decoupling of finite magnetic clusters from the ferromagnetic net arises from frustration, which enhances the ground-state entropy. Anomalous properties below the Curie temperatureT c as well as atT c support this microscopic picture.

Neutron Diffraction Experiment on a Randomly Mixed Antiferromagnet with Competing Spin Anisotropies

Abstract: Neutron diffraction experiment has been done on a randomly mixed antiferromagnet with competing spin anisotropies, Fe (1- x ) Co x Cl 2 . Front the experiment the spin structures in the three ordered phases which appear in the concentration versus transition temperature phase diagram, namely, the Fe-rich and Co-rich antiferromagnetic phases and the new phase are determined. The spin structure in the Fe-rich (Co-rich) phase is essentially identical with that of pure FeCl 2 (CoCl 2 ). In the new phase, although overall spin structure is the same as those of the pure materials, both of the two spin components are observed which are associated with competing anisotropies. The new phase is an OAF phase in the sense that an inclination of the sublattice magnetizations both from the easy axes of the pure substances means the simultaneous ordering of the spin components. The temperature dependences, of the spin components are discussed in favour of the result of the renormalization group theory.

Magnetic properties and superexchange in single chloride-bridged copper(II) chain compounds

Abstract: Magnetic susceptibility data have been measured in the temperature range 1.75 to 60 K for the following structurally characterized, single chloride-bridged copper(II) chain compounds: dichlorobis(dimethyl sulphoxide)copper(II), dichlorobis(imidazole)copper(II), aquo(caffeine)dichlorocopper(II), and dichloro[2-(2′-methylaminoethyl)pyridine]copper(II). The first two compounds exhibit antiferromagnetic intrachain interactions with exchange coupling constants of –6.1 and –2.1 cm–1, respectively, while the second two compounds exhibit ferromagnetic interactions with J values of 0.48 and 1.58 cm–1 respectively. Except for dichlorobis(imidazole)copper(II), which undergoes long-range magnetic ordering below 7.7 K, the exchange coupling constants become more negative as the angle at the chloride bridge increases from 113.6° in dichloro[2-(2′-methylaminoethyl)pyridine]copper(II) to 144.6° in dichlorobis(dimethyl sulphoxide)copper(II).