Antiferromagnetic-paramagnetic Phase Transition Research Articles

ESR study of dynamics of the phase transition in Wurster's blue perchlorate

Wurster's blue perchlorate (WBP) crystal was prepared by an electrochemical oxidation, and its ESR spectra were measured over the range of 100–350 K. Each spectrum shows a single line with an exchange-narrowed Lorentz shape. The ESR spectra have a maximum in the intensity at 193 K, and a minimum in the linewidth near the temperature, which corresponds to the well-known paramagnetic-antiferromagnetic phase transition. The linewidths show remarkable anisotropy, in particular, in the low-temperature region. The angular dependence of linewidth can be accounted for by a one-dimensional spin array along the a-axis of WBP crystal. The linewidth exhibits the most drastic decrease, when the field is applied along the a-axis. This is because, at the transition point, the cation radicals would more fluctuate along the a-axis than along other axes, since the rearrangement of the Wurster's blue radical cation occurs at the temperature along the a-axis.

Heat capacity of two-dimensional mixed conductor NaCrS 2

The heat capacity of NaCrS 2 has been measured by adiabatic calorimetry between 9 and 300 K. The antiferromagnetic-paramagnetic phase transition is observed as a second-order type of anomaly at 17.20 K. The enthalpy and entropy of transition are estimated as 90 J mol −1 and 6 J K −1 mol −1, respectively. The results are analyzed and discussed in comparison with those of AgCrS 2 and CuCrS 2.

The quasi-one-dimensional antiferromagnet CsMnCl3.2H2O. I. A random-phase approximation model

A self-consistent random-phase approximation model has been used to simulate the magnetic properties of quasi-one-dimensional and three-dimensional antiferromagnets at low temperatures, and in various applied fields. To avoid problems associated with crystal-field terms, the authors have replaced the quadratic D(Sz2-1/2S(S+1)) term, which is responsible for easy-axis alignment, with anisotropic magnetic exchange. The resultant calculations have been used to discuss sublattice magnetisation, specific heats and nuclear-magnon coupling, in various applied fields, particularly near the antiferromagnetic-paramagnetic phase transition. Their principal conclusion is that magnetic moments in quasi-one-dimensional antiferromagnets are likely to be characterised by a large zero-point motion. This feature has important implications for nuclear orientation experiments in the milli-kelvin regime.

Heat capacities, magnetic phase transition at 42.02 K and order-disorder phase transition at 673 K of AgCrS 2

Heat capacities of AgCrS 2 were measured by adiabatic calorimetry between 7 and 327 K and by a.c. calorimetry from room temperature up to 720 K. The antiferromagnetic-paramagnetic phase transition was detected as a diffused first-order type of anomaly at 42.02 K. The enthalpy and entropy of transition were determined as 360 J · mol −1 and 9.3 J · K −1 · mol −1, respectively. Phase transition due to the order-disorder of Ag + ions was observed as a typical second-order type of anomaly at 673 K with ΔH t = 1.9 kJ · mol −1 and ΔS t = 3.0J · K −1 · mol −1.

A study of the temperature and pressure dependence of the electrical resistivity of a dilute Cr-Ge system

The electrical resistivity of a series of dilute Cr-Ge alloys containing up to 1.5 atm % Ge, was measured as a function of temperature and pressure. The measurements clearly demonstrate the existence of resistivity anomalies at the incommensurate-commensurate spin density wave transition temperature ( T IC) in contrast with recently reported results. The complete magnetic phase diagram, determined for the first time from electrical resistivity measurements, contains a triple point in contrast with previous neutron diffraction results but in agreement with thermal expansion measurements. It was found that the incommensurate spin density wave state is absent in alloys with more than 1 atm % Ge. The Néel temperatures and incommensurate-commensurate transition temperatures are affected differently by pressure. Pressure decreases T N in all the alloys while it increases T IC for those alloys in which the incommensurate-commensurate transition occurs. The decrease of T N with pressure is much larger for the commensurate-paramagnetic than for the incommensurate-paramagnetic transition. The electrical resistivity of the alloys at room temperature behaves anomalously with applied pressure. This anomalous behaviour is attributed to an antiferromagnetic-paramagnetic phase transition that is induced in the alloys by applied pressure.

Brillouin scattering and ultrasonic propagation in the nearly cubic antiferromagnet KNiF3

Spin waves, elastic waves and their magnetostrictive coupling in KNiF3are studied as functions of temperature and of applied magnetic field by Brillouin scattering and ultrasonic measurements. The light scattering data provide the magnetic and elastic coefficients; in addition, they enable magnetostrictive coefficients to be evaluated, in the framework of the usual magnetoelastic description and, more fruitfully, of a Landau model describing the antiferromagnetic-paramagnetic phase transition. Various features concerning the size of the magnetic domains and their repartition in the crystals are derived from the authors' experiments. The existing differences between ultrasonic and Brillouin scattering results in the magnetically ordered phase are discussed.

Temperature dependence of NQR frequencies in CuBr 2 and MgBr 2·2Et 2O

The temperature dependence of NQR frequencies is studied in CuBr2 and MgBr2·Et2O. In CuBr2 the resonance disappears as the temperature is decreased between 77 and 105 K. Since this may be related to a paramagnetic-antiferromagnetic phase transition, the corresponding Neel temperature is much lower than the value 193 K reported in the literature. In MgBr2·Et2O a phase transition associated with a change in the conformation of the ether molecule is clearly seen in the NQR frequency vs temperature diagram.

Magnetic and electrical properties related to structural transitions in Na xVS 2 and Na xVSe 2

The magnetic and electrical properties of polycrystalline samples of Na x VS 2 and Na x VSe 2 have been measured. For x = 1, compounds with semiconducting properties and with localized magnetic moments of the V 3+ (d 2) ions exist. These forms reveal a clear paramagnetic-an tiferro magnetic phase transition at about 50 K with a simultaneous first-order cooperative Jahn-Teller lattice distortion. The magnetic structure of NaVSe 2, an essentially MnBr 2 type of magnetic ordering, has been determined by neutron diffraction. The forms with smaller Na contents are metallic; they show several anomalies in their magnetic and electrical behaviour as a function of temperature; these are considered to be due to charge density waves.

Magnetization studies in transition-metal niobates. II. FeNb2O6

The magnetization and susceptibility of flux-grown Fe${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$ single crystals have been measured from 80 K down to 1.4 K. An antiferromagnetic ordering was observed at ${T}_{N}=5.5\ifmmode\pm\else\textpm\fi{}0.5$ K. The antiferromagnetic-paramagnetic phase transition induced by an external field along the orthorhombic $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ axis seems to be of second order in the whole range from the ${T}_{N}$ down to 1.4 K. The experimental results are interpreted in the mean-field approximation using a four-sublattice model for the ${\mathrm{Fe}}^{2+}$ spins. A nearly accidental doublet ground state is assumed for the ${\mathrm{Fe}}^{2+}$ ions. The $g$ tensors are found to be $g=7.7\ifmmode\pm\else\textpm\fi{}0.4$ in the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}\ensuremath{-}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ plane at angles of \ifmmode\pm\else\textpm\fi{} (${17}^{0}$ \ifmmode\pm\else\textpm\fi{} ${3}^{0}$) to the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ axis. These are also the two easy directions of the four-sublattice ${\mathrm{Fe}}^{2+}$ spin system. The ${\mathrm{Fe}}^{2+}$-${\mathrm{Fe}}^{2+}$ interaction is essentially of the exchange type with a small dipolar contribution. The intrasublattice ferromagnetic interaction is found to be larger than the intersublattice antiferromagnetic coupling.

Magnetization studies in transition-metal niobates: I. NiNb2O6

The principal magnetic susceptibilities of Ni${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$ were measured from ambient down to 1.4 K. At ${T}_{N}=6.0\ifmmode\pm\else\textpm\fi{}0.3$ K an antiferromagnetic ordering was observed. An antiferromagnetic-paramagnetic phase transition was induced by external magnetic fields parallel to the crystallographic $\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ axis in the temperature range below ${T}_{N}$. The experimental results are interpreted in terms of anisotropic bilinear interactions between ${\mathrm{Ni}}^{2+}$ ions within the manifold of the ground term $^{3}A_{2}$. A two-sublattice uniaxial antiferromagnet model is used to describe the ${\mathrm{Ni}}^{2+}$ spins in the mean-field approximation. The bilinear ${\mathrm{Ni}}^{2+}$ -${\mathrm{Ni}}^{2+}$ interactions are essentially of the exchange type with a small dipolar contribution. Along all three orthorhombic directions the ferromagnetic intrasublattice interaction is found to be stronger than the intersublattice antiferromagnetic coupling. The ${\mathrm{Ni}}^{2+}$ $g$ tensor, taken to be isotropic, is $g=2.4\ifmmode\pm\else\textpm\fi{}0.1$ and the uniaxial anisotropy term $\ensuremath{-}K{S}_{z}^{2}$ in the effective spin Hamiltonian with $S=1$ yields $K=7.9\ifmmode\pm\else\textpm\fi{}0.7$ K/spin.

Magnetic and structural phase transitions in sodium intercalates NaxVS2 and NaxVSe2

Non-stoichiometric compounds Na x VS 2 (Se 2 ) were synthesized by reaction from the elements. They crystallize in two different structures: a metallic form with Pauli-paramagnetic behaviour and a semiconducting form with paramagnetic behaviour at room temperature. The semiconducting form shows a sharp paramagnetic-antiferromagnetic phase transition at ∼50 K with a simultaneous first order co-operative Jahn-Teller lattice distortion.

Magnetic effects on the ortho-parahydrogen conversion over α-Cr 2O 3, CoO, and MnO

Catalyzed ortho-parahydrogen conversion rates were measured on two antiferromagnetic oxides, α-Cr 2 O 3 and CoO over a range of temperature and both with and without an extrinsic magnetic field. A few measurements were made on MnO. The conversion mechanism was, in all cases, nondissociative. Results for the three catalysts, while qualitatively similar, differed in detail. A correlation between the zero field conversion rate and the extrinsic field rate suggests a relation to the paramagnetic-antiferromagnetic phase transition in catalysts of high surface site density. A positive extrinsic field effect was observed only at temperatures above the Neel point ( T N ). Negative effects were found, in some cases, both above and below T N . The latter results were characterized by a maximum rate decrease just below T N and by a rate saturation effect that was a function of applied field strength. The several effects observed suggest that, on high site density surfaces, two or more mechanisms are operative in an extrinsic field. These, and other related results, show that the current state of conversion theory over magnetic surfaces is still incomplete.

The role of harmonics in the first order antiferromagnetic to paramagnetic transition in chromium

The results of Rice (see abstr. A13376 of 1971) and Malaspinas and Rice (see abstr. A1642 of 1972) on the effects of imperfect Fermi surface nesting in chromium are discussed to find the conditions under which imperfect Fermi surface nesting might give a first order antiferromagnetic-paramagnetic phase transition. These conditions are found to be not conducive to a stable antiferromagnetic state. A new mechanism for such a first order transition is proposed based on the existence of harmonics of the spin density wave. In this theory the transition is predicted to be always second order when TN approaches zero. Since Kimball and Falicov (see abstr. A31753 of 1968) found the same result in their model it is likely that the authors' mechanism for a first order transition is the same as theirs. The presence of impurities is shown to reduce the tendency for a first order transition for alloys of chromium.

Magnetic Phase Transition of Organic Free Radical p-Cl-BDPA

The heat capacity of polycrystalline organic free radical, p -Cl-BDPA, has been measured between 1.5 and 20 K. The broad maximum near 5 K in the magnetic heat capacity corresponds the short range magnetic ordering of the unpaired electron. At 3.25 K a sharp peak was observed, which indicates paramagnetic-antiferromagnetic phase transition. The magnetic entropy of this transition yields 101.5% of the expected Rln2. Below T N the magnetic heat capacity was in good agreement with the three-dimensional spin wave theory of antiferromagnet.

HEAT CAPACITY ANOMALY IN THE CRYSTALLINE ORGANIC FREE RADICAL, p-Cl-BDPA

Abstract The heat capacity has been measured on polycrystalline sample of 1,3-bisdiphenylene-2-p-chlorophenyl-allyl (abbreviated as p-Cl-BDPA). The sharp heat capacity anomaly was observed at 3.25 K, which may imply a paramagnetic-antiferromagnetic phase transition. This phenomenon accords well with the other results obtained from the susceptibility, ESR, and NMR measurements.

Antiferromagnetic Linear Chains in the Crystalline Free Radical BDPA

The static magnetic susceptibility from 1.6 to 300°K and the heat capacity in fields of 0 and 5 kG from 0.9 to 10.2°K have been measured on polycrystalline samples of the complex of 1,3-bisdiphenylene-2-phenylallyl with benzene, commonly abbreviated BDPA. The data are shown to correspond to antiferro-magnetic Heisenberg linear chains characterized by a nearest-neighbor exchange integral of −4.6°K. Weak interchain coupling results in a paramagnetic—antiferromagnetic phase transition at 1.695°K in zero field, as evidenced by the sharp heat capacity anomaly at that temperature. Resolution of the heat capacity into a lattice T3 contribution and a magnetic contribution yields a Debye temperature of 52.5°K and a magnetic entropy per mole of 101% of the expected Rln2.

Magnetic susceptibility anomaly in nearly equiatomic MnNi alloys

AbstractThe crystallographic and magnetic properties of nearly equiatomic nickel–manganese alloys have been investigated by X‐ray and neutron diffraction techniques as well as by classical susceptibility measurements. It has been found that the antiferromagnetic–paramagnetic phase transition takes place in most of the cases as a result of the θ → β allotropic transformation. It has been observed that the temperature dependence of the susceptibility in alloys containing somewhat more than 50% nickel is strongly influenced by an external magnetic field. Samples cooled down slowly in a magnetic field of several kOe have a room temperature susceptibility three to four times higher than before the heat treatment. The increased susceptibility shows an unidirectional anisotropy which is explained by the non‐equal distribution of the excess nickel atoms between the two manganese sublattices.

THE EFFECTS OF PRESSURE ON THE PARAMAGNETIC–ANTIFERROMAGNETIC TRANSITION OF DYSPROSIUM

The shift of temperature with pressure of the paramagnetic–antiferromagnetic phase transition of dysprosium at 177° K has been found to be −(0.55±0.02) × 10−3 deg K/(kg/cm2). Pressure was applied to the substance by compressing the metal between two pistons in a cylinder. The transition point was determined by measuring the magnetic moment of the sample as a function of temperature at various pressures up to 8400 kg/cm2. The results appear to be consistent with current theories of the magnetic properties of dysprosium.