Mott–Hubbard transition in a 2D He fluid monolayer

We discuss recent observations of the heat capacity and magnetization of a fluid ³He monolayer adsorbed on graphite plated with a bilayer of HD. Approaching the density at which the monolayer solidifies into a commensurate solid, we observe an apparent divergence of the effective mass. However, the inferred values of F₀^a tend to a constant. We interpret this in terms of a Mott–Hubbard transition between a 2D Fermi liquid and a magnetically disordered solid occurring via the Brinkman–Rice–Anderson–Vollhardt scenario.     

Anisotropic magnetic properties and superzone gap formation in CeGe single crystal

Single crystals of CeGe and its non-magnetic analog LaGe have been grown by the Czochralski method. The CeGe compound crystallizes in the orthorhombic FeB-type crystal structure with the space group Pnma (#62). The anisotropic magnetic properties have been investigated for well oriented single crystals by measuring the magnetic susceptibility, electrical resistivity and heat capacity. It has been found that CeGe orders antiferromagnetically at 10.5?K. Both transport and magnetic studies have revealed large anisotropy, reflecting the orthorhombic crystal structure. The magnetization data at 1.8?K reveal metamagnetic transitions along the [010] direction at 4.8 and 6.4?T and along the [100] direction at a critical field of 10.7?T, while the magnetization along the [001] direction increases linearly without any anomaly up to a field of 16?T. From the magnetic susceptibility and the magnetization measurements it has been found that the [010] direction is the easy axis of magnetization. The electrical resistivity along the three crystallographic directions exhibits an upturn at T(N), indicating superzone gap formation below T(N) in this compound. We have performed crystal electric field analysis on the magnetic susceptibility and the heat capacity data and found that the ground state is a doublet, and the energies of splitting from the ground state to the first and second excited doublet states were estimated to be 39 and 111?K, respectively.     

Magnetic reversal of the ferroelectric polarization in a multiferroic spinel oxide

Ferroelectric transition has been detected in a ferrimagnetic spinel oxide of CoCr2O4 upon the transition to the conical spin order below 25 K. The direction [110] of the spontaneous polarization is normal to both the magnetization easy axis [001] and to the propagation axis [110] of the transverse spiral component, in accord with the prediction based on the spin-current model. The reversal of the spontaneous magnetization by a small magnetic field (approximately 0.1 T) induces the reversal of the spontaneous polarization, indicating the clamping of the ferromagnetic and ferroelectric domain walls.     

Observation of a liquid-gas-type transition in the pyrochlore spin ice compound Dy2Ti2O7 in a magnetic field

Low temperature magnetization measurements on the pyrochlore spin ice compound Dy2Ti2O7 reveal that the ice-rule breaking spin flip, appearing at H approximately 0.9 T applied parallel to the [111] direction, turns into a novel first-order transition for T<0.36 K which is most probably of a liquid-gas type. T-linear variation of the critical field observed down to 0.03 K suggests the unusual situation that the entropy release across the transition remains finite [approximately 0.5 (J/K) x mol-Dy] as T-->0, in accordance with a breaking of the macroscopic degeneracy in the intermediate "kagomé ice" state.     

Magnetic behavior of Eu(3)Ni(4)Ga(4): antiferromagnetic order and large magnetoresistance

The results of the magnetic susceptibility, isothermal magnetization, heat capacity, electrical resistivity and magnetoresistance measurements on polycrystalline Eu(3)Ni(4)Ga(4) are presented. Eu(3)Ni(4)Ga(4) forms in Na(3)Pt(4)Ge(4)-type cubic crystal structure (space group [Formula: see text]). The temperature dependence of the magnetic susceptibility of Eu(3)Ni(4)Ga(4) confirms the divalent state (Eu(2+)) of Eu ions with an effective magnetic moment μ(eff)?=?7.98?μ(B). At low fields, e.g.?at 0.01?T, a magnetic phase transition to an antiferromagnetically ordered state occurs at T(N)?=?10.9?K, which is further confirmed by the temperature dependence of the heat capacity and electrical resistivity. The field dependence of isothermal magnetization at 2?K reveals the presence of two field induced metamagnetic transitions at H(c1) and H(c2)?=?0.55 and 1.2?T, respectively and a polarized phase above H(PO)?=?1.7?T. The reduced jump in the heat capacity at the transition temperature, ΔC|(T(N))?=?13.48?J/mol-Eu?K would indicate an amplitude modulated (AM) antiferromagnetic structure. An interesting feature is that a large negative magnetoresistance, MR?=?[ρ(H)?-?ρ(0)]/ρ(0), is observed in the vicinity of magnetic transition even up to 2T(N). Similar large magnetoresistance has been observed in the paramagnetic state in some Gd and Eu based alloys and has been attributed to the magneto-polaronic effect.     

Three-dimensional Kasteleyn transition: spin ice in a [100] field

We examine the statistical mechanics of spin-ice materials with a [100] magnetic field. We show that the approach to saturated magnetization is, in the low-temperature limit, an example of a 3D Kasteleyn transition, which is topological in the sense that magnetization is changed only by excitations that span the entire system. We study the transition analytically and using a Monte Carlo cluster algorithm, and compare our results with recent data from experiments on Dy2Ti2O7.     

CO adsorbed on MgO(100): a high resolution LEED study

The monolayer structure of CO molecules adsorbed on MgO(100) single crystal surfaces cleaved in situ has been analysed by LEED within the 30–56 K temperature range. At T ⩽ 40 K CO forms a 2 × 4 commensurate 2D solid phase. A sharp uniaxial transition occurs above this temperature, along the [10] surface direction which locks the monolayer into a new commensurate 2 × 3 phase stable over a temperature range of 8 K. Above 50 K, this second commensurate phase expands itself uniaxially in a sharp transition toward a solid with disorder increasing with temperature. This succession of transitions is an interesting illustration of the incomplete “devil's staircase”.     

Low temperature magnetism in the rare-earth perovskite GdScO_3

The magnetic phase diagram of rare-earth perovskite compound, GdScO_3, has been investigated by magnetization and heat capacity. The system undergoes an antiferromagnetic phase transition at T_N= 2.6K, with an easy axis of magnetization along the a axis. The magnetization measurements show that it exists a spin-flop transition around 0.3 T for the applied field along the a axis. The critical magnetic field for the antiferromagnetic-to-paramagnetic transition is near 3.2 T when temperature approaches zero. By scaling susceptibilities, we presume this point(B = 3.2 T, T = 0 K) might be a fieldinduced quantum critical point and the magnetic critical fluctuations can even be felt above TN.     

Antiferromagnetic ordering in EuPtGe3

The magnetic properties of single crystalline EuPtGe(3), crystallizing in the non-centrosymmetric BaNiSn(3)-type crystal structure, have been studied by means of magnetization, electrical resistivity, heat capacity and (151)Eu M?ssbauer spectroscopy. The susceptibility and heat capacity data indicate a magnetic transition at T(N) = 11 K. The M?ssbauer data confirm this conclusion, but evidence a slight first-order character of the transition. Analysing the magnetization data using a mean field model with two antiferromagnetically coupled sublattices allows us to explain some aspects of the magnetic behaviour, and to derive the first- and second-neighbour exchange integrals in EuPtGe(3).     

Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5

We report specific heat measurements of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical field H(c2), with magnetic fields in the [110], [100], and [001] directions, and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for fields above 10 T for H parallel [110] and H parallel [100]. In the same range of magnetic fields, we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. We obtain similar results for H parallel [001], with the FFLO state occupying a smaller part of the phase diagram.     

Evolution of magnetic properties in the normal spinel solid solution Mg(1-x)Cu(x)Cr2O4

We examine the evolution of magnetic properties in the normal spinel oxides Mg(1-x)Cu(x)Cr2O4 using magnetization and heat capacity measurements. The end-member compounds of the solid solution series have been studied in some detail because of their very interesting magnetic behavior. MgCr2O4 is a highly frustrated system that undergoes a first-order structural transition at its antiferromagnetic ordering temperature. CuCr2O4 is tetragonal at room temperature as a result of Jahn-Teller active tetrahedral Cu2+ and undergoes a magnetic transition at 135 K. Substitution of magnetic cations for diamagnetic Mg2+ on the tetrahedral A site in the compositional series Mg(1-x)Cu(x)Cr2O4 dramatically affects magnetic behavior. In the composition range 0 ≤ x ≤ ≈0.3, the compounds are antiferromagnetic. A sharp peak observed at 12.5 K in the heat capacity of MgCr2O4 corresponding to a magnetically driven first-order structural transition is suppressed even for small x. Uncompensated magnetism--with open magnetization loops--develops for samples in the x range ≈0.43 ≤ x ≤ 1. Multiple magnetic ordering temperatures and large coercive fields emerge in the intermediate composition range 0.43 ≤ x ≤ 0.47. The Néel temperature increases with increasing x across the series while the value of the Curie-Weiss Θ(CW) decreases. A magnetic temperature-composition phase diagram of the solid solution series is presented.     

Metamagnetic transition in Na 0.85 CoO2 single crystals

We report the magnetization, specific heat, and transport measurements of a high quality Na(0.85)CoO2 single crystal in applied magnetic fields up to 14 T. At high temperatures, the system is in a paramagnetic phase. It undergoes a magnetic phase transition below approximately 20 K. For the field H||c, the measurement data of magnetization, specific heat, and magnetoresistance reveal a metamagnetic transition from an antiferromagnetic state to a quasiferromagnetic state at about 8 T at low temperatures. However, no transition is observed in the magnetization measurements up to 14 T for H perpendicular c. The low temperature magnetic phase diagram of Na(0.85)CoO2 is determined.     

Thermodynamic and magnetic properties of HoRh2Si2: A comparison between experiments and calculations in a crystal field model

The origin of two peaks observed in the magnetization and the heat capacity curves of HoRh₂Si₂ is investigated. The thermodynamic and the magnetic properties of this compound are calculated using a Hamiltonian including crystal field and isotropic exchange interaction terms which are comparable in magnitude. Two components (parallel and perpendicular to the c-axis) of the magnetization are evaluated by the molecular field approximation. The two peaks in the magnetization and the heat capacity can be reproduced by choosing appropriate values for crystal field parameters and molecular field constant. The lower transition temperature corresponds to the temperature at which the perpendicular component of the magnetization disappears, and the higher one, to that at which the parallel component of the magnetization disappears.     

Exchange bias in bulk layered hydroxylammonium fluorocobaltate (NH?OH)?CoF?

The magnetic properties of layered hydroxylammonium fluorocobaltate (NH(3)OH)(2)CoF(4) were investigated by measuring its dc magnetic susceptibility in zero-field-cooled (ZFC) and field-cooled (FC) regimes, its frequency dependent ac susceptibility, its isothermal magnetization curves after ZFC and FC regimes, and its heat capacity. Effects of pressure and magnetic field on magnetic phase transitions were studied by susceptibility and heat capacity measurements, respectively. The system undergoes a magnetic phase transition from a paramagnetic state to a canted antiferromagnetic state exhibiting a weak ferromagnetic behavior at T(C) = 46.5 K and an antiferromagnetic transition at T(N) = 2.9 K. The most spectacular manifestation of the complex magnetic behavior in this system is a shift of the isothermal magnetization hysteresis loop in a temperature range below 20 K after the FC regime-an exchange bias phenomenon. We investigated the exchange bias as a function of the magnetic field during cooling and as a function of temperature. The observed exchange bias was attributed to the large exchange anisotropy which exists due to the quasi-2D structure of the layered (NH(3)OH)(2)CoF(4) material.     

Magnetic, structural, and thermal properties of CoV?O?

In this paper, we investigate the electric, magnetic, structural, and thermal properties of spinel CoV(2)O(4). The temperature dependence of magnetization shows that, in addition to the paramagnetic-to-ferrimagnetic transition at T(C) = 142 K, two magnetic anomalies exist at 100 K, T(1) = 59 K. Consistent with the anomalies, the thermal conductivity presents two valleys at 100 K and T(1). At the temperature T(1), the heat capacity shows one peak, which cannot be attributed to the structural transition as revealed by the x-ray diffraction patterns for CoV(2)O(4). Below the transition temperature T(1), the ac susceptibility displays the characteristics of a glass. The series of phenomena at T(1) and the orbital state on V(3+) sites are discussed.     

Long-term relaxation of magnetoresistance in a granular ferromagnet

Long-term relaxations (of the logarithmic type) are revealed in the tunnel magnetoresistance of Fe/SiO₂ nanocomposites, which are due to variation of the magnetization of the nanocomposites. Good qualitative agreement between experimental results and the recently developed concepts of the behavior of magnetization of granular ferromagnets [7] proves that the revealed relaxations are associated with the spin-glass nature of the magnetic state of such systems. It is further demonstrated that it is, in principle, impossible to observe such relaxations using the anomalous Hall effect (proportional to magnetization) because of physical reasons, i.e., mesoscopic fluctuations of the Hall voltage as a result of the magnetic field effect and variation of magnetization.     

Low-Temperature Phase Transitions in the Trigonal Modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9

The trigonal (P-3 m1) modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4-300 K. In Cs 3 Bi 2 Br 9 a second-order phase transition was found at T C = 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs 3 Sb 2 I 9 has a sequence of phase transitions at T C = 85 K, T i = 78 K and T L = 72.1 K. The monoclinic structure below 85 K is isomorphic with the low-temperature structure of Cs 3 Bi 2 Br 9 . According to calorimetric data the lock-in transition at 72.1 K is discontinuous.     

Heat capacity study of the first monolayer of adsorbed methane on grafoil

The heat capacity of CH₄ adsorbed on Grafoil below monolayer completion has been determined experimentally for the first time. The specific heat at constant density shows two characteristic features, very broad, weak anomalies at 47.6 K (around one third of a monolayer) and (for occupation numbers below two thirds of a monolayer) very narrow, strong peaks at 56.35 K. The anomalies at 47.6 K are interpreted as being due to a registry — out-of-registry transition of the two dimensional (2D) solid CH₄ adsorbate into an incommensurate expanded structure. The anomalies at 56.35 K define the 2D triple point of adsorbed CH₄. For higher occupation numbers the liquid-solid phase boundary shifts to higher temperatures. The present data make partial re-definition of the 2D-CH₄ phase diagram necessary.     

Magnetic Anisotropy and Hidden Martensitic Transition in V3Si

Magnetization, electrical resistivity and heat capacity have been measured on a single crystal V₃Si in the range of (2-25) K and in magnetic field up to 14 T. A different behavior of magnetization for two orientations of the crystal has been found. In one orientation the magnetization displays a clear ferromagnetic character and below T _c coexistence of ferro-magnetism and superconductivity with a peak-effect in the vicinity of upper critical field H _c2. The specific heat measurements show sharp lambda anomaly corresponding to a transition to superconductive state and an additional anomaly around 15 K when applied field suppresses the superconductivity below this temperature.     

Heat capacity of Ar monolayer adsorbed on basal plane graphite

The specific heat of Ar monolayer film adsorbed on basal planes of graphite has been measured for film coverage ranging from 0.31 to 0.97 monolayers for 4.2 < T < 68 K. It is found that a broad specific heat peak centered around 50 K exists over most of the submonolayer coverages until reaching monolayer completion. A comparison with recent neutron scattering and LEED studies relates the anomaly to a possible higher order phase transition between 2D solid and 2D fluid.