Elektronische spezifische Wärme und Antiferromagnetismus in Chromlegierungen

The specific heats of 20 alloys of Cr with Mo, W, V, Ru and Os within the range of the body-centered cubic phase have been measured between 1.5 and 4.2°K and separated in their electronic and lattice parts. From the temperature dependence of the electrical resistance between 70 and 340°K the Néel temperatures of Cr-alloys with 0–16 at.-% Mo could be determined. A comparison of the electronic specific heats and the Néel temperatures of the investigated Cr-alloys reveals a considerable reduction of the electronic density of states within the range of concentrations with high Néel temperatures.     

Extended random phase approximation for layered copper oxides antiferromagnets

The role of interlayer coupling constant on the Néel temperature of layered copper oxides and other magnetic properties have been studied. The theoretical framework is based on anisotropic Heisenberg model and the two sublattice approach. The higher-order Green functions are decoupled using the second random phase approximation. The Green’s function related to the localized spin correlation functions has been exploited. A self consistent expression for the sublattice magnetization and a corrected form for the Néel temperature are obtained. The free-spin-wave results are obtained under a specific approximation. The theoretical results are compared with that of the random phase approximation and existing experimental results.     

Caracterisation et Etude par effet Mössbauer d'une nouvelle variete haute pression de FeOOH

A new high pressure form of FeOOH, isostructural with InOOH, has been prepared by hydrothermal synthesis under very high pressure conditions. Characterization of the new phase was performed by D.T.A., T.G.A., infrared and Mössbauer spectroscopy. The compound behaves as an antiferromagnet with a Néel temperature of approximately 570° K. The hyperfine magnetic field at 0° K is about 525 kOe. Thermal decomposition occurs below the Néel temperature. Comparison with other FeOOH phases is made.     

Ultrasonic determination of the phase diagram in CsNiCl3

The phase diagram of CsNiCl₃ has been determined ultrasonically. There is a quadratic increase in the Néel temperature with increasing magnetic field. A possible spin-flop transition has been detected at approximately 22 kG.     

Possible phase transition of anisotropic frustrated Heisenberg model at finite temperature

The frustrated spin-1/2 J_1a–J_1b–J₂ antiferromagnet with anisotropy on the two-dimensional square lattice was investigated, where the parameters J_1aand J_1b represent the nearest neighbor exchanges and along the x and y directions, respectively. J₂ represents the next-nearest neighbor exchange. The anisotropy includes the spatial and exchange anisotropies. Using the double-time Green’s function method, the effects of the interplay of exchanges and anisotropy on the possible phase transition of the Néel state and stripe state were discussed. Our results indicated that, in the case of anisotropic parameter 0≤η<1, the Néel and stripe states can exist and have the same critical temperature as long as J₂ = J_1b/2. Under such parameters, a first-order phase transformation between the Néel and stripe states can occur below the critical point. For J₂ ≠J_1b/2, our results indicate that the Néel and stripe states can also exist, while their critical temperatures differ. When J₂>J_1b/2, a first-order phase transformation between the two states may also occur. However, for J₂<J_1b/2, the Néel state is always more stable than the stripe state.     

Theoretical study of magnetic susceptibility of orbitally unquenched compound KCoF3

The effect of residual orbital magnetic moment of Co²⁺ in KCoF₃ on the magnetic susceptibility has been studied. For the calculation for both the ordered state and the paramagnetic state we have applied the correlated effective field approximation developed by Lines. An excellent value of the Néel temperature is obtained and, except near the Néel temperature, the calculated susceptibility agrees well with the experimental results over the whole temperature range.     

The temperature dependence of the angular variation of,and the critical point exponent for the EPR linewidth in the two-dimensional canted antiferromagnetic (C2H5NH3)2MnBr4: Evidence for a structural phase transition

The angular variation of the EPR linewidths in single crystals of (C₂H₅NH₃)₂MnBr₄ has been measured as a function of temperature. The angular dependence is well characterized by δH(θ) = a + b(3 cos²θ ? 1) + c(3 cos²θ ? 1)². The temperature dependence of the expansion coefficients is reported, and the effect of critical point fluctuations near the Néel temperature as well as a linear temperature dependence at high temperature are observed. A sharp decrease in linewidths at 160°K is attributed to a structural phase transition. The Néel temperature is determined to be 46°K (± 1°) from linewidth measurements of a powder sample. The linewidths diverge exponentially near the Néel temperature with a critical point exponent of 1.5.     

Effect of the crystalline electric field on the Néel temperatures of RCu2 compounds

Values for the Néel temperature of the RCu₂ compounds (R=Tb–Tm) have been calculated using a molecular-field model including crystalline-electric-field effect as presented by Noakes and Shenoy. The calculated results show that the unusual behavior, at the Néel temperatures, of these compounds can be explained on the basis of this model.     

Exchange bias in a bilayer ferromagnet-antiferromagnet system with close phase transition temperatures

The temperature dependence of the exchange bias in a bilayer ferromagnet-antiferromagnet system with an uncompensated surface has been calculated within the phenomenological Landau theory of phase transitions for the case where the Néel temperature of the antiferromagnet is close to and higher than the Curie temperature of the ferromagnet.     

A study of the conduction mechanism of single crystal Li0.5Fe2.5O4 through the transition points

The differential thermal analysis, electrical resistivity and thermopower are studied as a function of temperature in the range 300–1100 K. The measurements show two transition points at 905 and 1030 K. The first point is linked with the magnetic transition at the Néel temperature (T_N), and the second with the order-disorder phase. The conduction mechanism through the Néel point is discussed using the spin polaron model.     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

Magnetic properties of undoped YBa 2 Cu 3 O 6 + x system

In the present paper, we study the magnetic properties of bilayer cuprate antiferromagnets. In order to evaluate the expressions for spin-wave dispersion, sublattice magnetization, Néel temperature and the magnetic contribution to the specific heat, the double time Green's function technique has been employed in the random phase approximation (RPA). The spin wave dispersion curve for a bilayer antiferromagnetic system is found to consist of one acoustic and one optic branch. The “optical magnon gap” has been attributed solely to the intra-bilayer exchange coupling (J _⊥ ) as its magnitude does not change significantly with the inter-bilayer exchange coupling (J_z). However J_z is essential to obtain the acoustic mode contribution to the magnetization. The numerical calculations show that the Néel temperature (T _N ) of the bilayer antiferromagnetic system increases with the J_z and a small change in J_z gives rise to a large change in the Néel temperature of the system. The magnetic specific heat of the system follows a T² behaviour but in the presence of J_z it varies faster than T². Received 13 July 2000 and Received in final form 14 May 2001     

Role of rotational symmetry in magnetism of multiband models

The role of the complete Coulomb interaction matrix in the determination of the magnetic susceptibility of the two-band Hubbard model has been investigated in the dynamical mean-field theory. The off-diagonal component of the U-matrix has been found to reduce the Néel temperature, which agrees with the available experimental data. The magnitude of the effect upon a change in the dimension of the U-matrix is discussed. The results have been compared with the thermodynamic characteristics of an isolated Hubbard atom.     

Effect of pressure on the Néel temperature and the lattice parameter of MnTe

The Néel temperature of MnTe has been measured at high pressures up to 70 kbar and found to increase with pressure. The rate of increase changes discontinuously near 40 kbar. The lattice parameters were measured by X-ray diffraction techniques, and also change anomalously near 40 kbar. From these results, it appears that some kind of magnetic phase transition occurs at this point. A magnetic phase diagram is proposed.     

First-principles investigation of the electronic structure and magnetism of eskolaite

The electronic structure and magnetism of eskolaite are studied by using first-principles calculations where the on-site Coulomb interaction and the exchange interaction are taken into account and the LSDA+U method is used. The calculated energies of magnetic configurations are very well fitted by the Heisenberg Hamiltonian with interactions in five neighbour shells; interaction with two nearest neighbours is found to be dominant. The Néel temperature is calculated in the spin-3/2 pair-cluster approximation. It is found that the measurements are in good agreement with the calculations of lattice parameters, density of states, band gap, local magnetic moment, and the Néel temperature for the values of U and J that are close to those obtained within the constrained occupation method. The band gap is of the Mott--Hubbard type.     

Ground state phase diagram of a spin-2 antiferromagnet on the square lattice

We use the vertex state model approach to construct optimum ground states for a large class of quantum spin-2 antiferromagnets on the square lattice. Optimum ground states are exact ground states of the model which minimize all local interaction operators. The ground state contains two continuous parameters and exhibits a second order phase transition from a disordered phase with exponentially decaying correlation functions to a Néel ordered phase. The behaviour is very similar to that of the corresponding ground state of a quantum spin-3/2 model on the hexagonal lattice, which has been investigated in an earlier paper. Received 8 April 1999     

Exotic entanglement scaling of Heisenberg antiferromagnet on honeycomb lattice

The scaling behaviors of entanglement entropy (EE) against dimension cut-off of density matrix renormalization group (DMRG) in an anisotropic Heisenberg model on honeycomb lattice are investigated. In the gapped dimer phase, the entanglement spectrum (ES) exhibits large gaps and the EE shows an unexpected linear scaling before convergence. In contrast in the gapless Néel phase, the ES decays in a much smoother way, and the EE scales logarithmically. Our calculations show that the linear scaling in the dimer phase originates from one dominant Schmidt number plus n (nearly) degenerate Schmidt numbers that are much smaller than the dominant one. The non-trivial entanglement-scaling properties of the dimer and Néel phases could potentially be used for their detections.     

High-Order Coupled Cluster Method (CCM) Calculations for Quantum Magnets with Valence-Bond Ground States

In this article, we prove that exact representations of dimer and plaquette valence-bond ket ground states for quantum Heisenberg antiferromagnets may be formed via the usual coupled cluster method (CCM) from independent-spin product (e.g. Néel) model states. We show that we are able to provide good results for both the ground-state energy and the sublattice magnetization for dimer and plaquette valence-bond phases within the CCM. As a first example, we investigate the spin-half J ₁–J ₂ model for the linear chain, and we show that we are able to reproduce exactly the dimerized ground (ket) state at J ₂/J ₁=0.5. The dimerized phase is stable over a range of values for J ₂/J ₁ around 0.5, and results for the ground-state energies are in good agreement with the results of exact diagonalizations of finite-length chains in this regime. We present evidence of symmetry breaking by considering the ket- and bra-state correlation coefficients as a function of J ₂/J ₁. A radical change is also observed in the behavior of the CCM sublattice magnetization as we enter the dimerized phase. We then consider the Shastry-Sutherland model and demonstrate that the CCM can span the correct ground states in both the Néel and the dimerized phases. Once again, very good results for the ground-state energies are obtained. We find CCM critical points of the bra-state equations that are in agreement with the known phase transition point for this model. The results for the sublattice magnetization remain near to the “true” value of zero over much of the dimerized regime, although they diverge exactly at the critical point. Finally, we consider a spin-half system with nearest-neighbor bonds for an underlying lattice corresponding to the magnetic material CaV₄O₉ (CAVO). We show that we are able to provide excellent results for the ground-state energy in each of the plaquette-ordered, Néel-ordered, and dimerized regimes of this model. The exact plaquette and dimer ground states are reproduced by the CCM ket state in their relevant limits. Furthermore, we estimate the range over which the Néel order is stable, and we find the CCM result is in reasonable agreement with the results obtained by other methods. Our new approach has the dual advantages that it is simple to implement and that existing CCM codes for independent-spin product model states may be used from the outset. Furthermore, it also greatly extends the range of applicability to which the CCM may be applied. We believe that the CCM now provides an excellent choice of method for the study of systems with valence-bond quantum ground states.     

Effects of Co on the Néel temperature and thermal expansion in dilute CrFe alloys

The Néel temperature and thermal expansion characteristics of dilute CrFe alloys containing Co have been investigated. It has been found that the Néel temperature of Cr_99-xFe₁Co_x alloys (0.5?x?1) increases with increasing Co content and that these ternary alloys exhibit a large spontaneous volume magnetostriction, indicating Invar characteristics around room temperature. These behaviors are extremely different from those of CrFe and CrCo binary alloys.     

Crystallographic distortion in CrAs0.50Sb0.50

X-ray diffraction has shown that CrAs_0.50Sb_0.50 undergoes at the Néel temperature a crystallographic distortion (NiAs → MnP) accompanied by a discontinuous volume change. Neutron diffraction revealed in this compound a helimagnetic structure very similar to the ‘double spiral’ in CrAs discovered recently. The pressure dependence of the Néel temperature is estimated in the framework of the Bean Rodbell theory and compared with the CrAs value.