X-ray diffraction study of a sequence of phase transitions in Cs2HgCl4 crystals

The temperature behavior of lattice parameters and diffraction patterns of the reciprocal lattice in Cs₂HgCl₄ crystals is studied by x-ray diffraction in the temperature range from 4.2 to 300 K. A sequence of phase transitions is observed and attributed to the evolution of incommensurate and commensurate modulations along the crystallographic a and c axes of a unit cell in the initial Pnma structure.     

The magnetic phases of NdCu2

The magnetic phase diagram of a NdCu₂ single crystal is investigated by means of specific-heat measurements and neutron diffraction as a function of temperature and external magnetic field applied along the crystallographic b-direction. In the low temperature region we observe three commensurate phases AF1, F1, and F2. Their magnetic structures all consist of ferromagnetic (bc)-planes with different stacking sequences along the a-direction comprising 5, 3 and 8 chemical unit cells, respectively. Above 2.8 T the system is in a ferromagnetically aligned state (F3). Furthermore, there is an incommensurate phase AF3 between the low temperature commensurate phases and the paramagnetic state and, in a very narrow temperature region of only 0.2 K, an intermediate phase AF2 between AF1 and AF3. The Nd-moments are oriented along the b-direction in all phases.     

Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies κ = J′a ² c/(g _L μ _B )². For increasing values of κ, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120° structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF₃, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are seperated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.     

The invar effect and phase transitions in Cs2ZnI4 crystals

An in situ x-ray diffraction study of Cs₂ZnI₄ crystals performed in the 4.2–300 K temperature range is reported. The lattice parameter measurements have revealed three anomalies corresponding to phase transitions. The thermal expansion coefficient along the c axis was found to vanish in the region of incommensurate and commensurate modulated phases, 120–96 K (the invar effect). A possible crystallographic model relating modulated atomic displacements to the invar effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 137–142 (January 1999)     

Modification of the magnetic structure of UAs — Appearance of an incommensurate phase below TN in UAs0.97Se0.03, studied by neutron diffraction from a single crystal

The magnetic structure of UAs_0.97Se_0.03 has been studied by neutron diffraction from a single crystal in zero applied magnetic field. It is found to be antiferromagnetic, of type-IA (++--) below T_o = 75.6 K and of type-I (+-+-) above T_o. The type-I persists till T_IC = 113.5 K, while above it and up to T_N = 122 K an incommensurate phase appears, thereby modifying the magnetic structure in pure UAs. The k-value of the wavevector $\text{K}$ (along cubic axes) is changing from 0.642 at T_N to 0.652 at T_IC. The transitions at T_o and T_IC are first-order while the transition at T_N is second-order. The ordered magnetic moment is 2.15 μ_B at T = 4.2 K, it varies smoothly to 1.95 μ_B at T = 75.4 K and drops drastically to 1.47 μ_B at T = 76 K.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Physical properties and phase diagram of the magnetic compound Cr<Subscript>0.26</Subscript>NbS<Subscript>1.74</Subscript> at high pressures

We report the results of a study of magnetic, electrical, and thermodynamic properties of a single crystal of the magnetic compound Cr_0.26NbS_1.74 at ambient and high pressures. Results of the measurements of magnetization as a function of temperature reveal the existence of a ferromagnetic phase transition in Cr_0.26NbS_1.74. The effective number of Bohr magnetons per Cr atom in the paramagnetic phase of Cr_0.26NbS_1.74 is µ_eff ≈ 4.6µB, which matches the literature data for Cr1/3NbS2. Similarly, the effective number of Bohr magnetons per Cr atom in the saturation fields is rather close in both substances and corresponds to the number of magnetons in the Cr⁺³ ion. In contrast to the stoichiometric compound, Cr_0.26NbS_1.74 does not show a metamagnetic transition, that indicates the lack of a magnetic soliton. A high-pressure phase diagram of the compound reveals the quantum phase transition at T = 0 and P ≈ 4.2 GPa and the triple point situated at T ≈ 20 K and P ≈ 4.2 GPa.     

Mössbauer study of spin structure transformation from an incommensurate to a commensurate state

We present crystallographic and magnetic properties of NiCr_1.98 ⁵⁷Fe_0.02O₄ by using X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy. The lattice constants a₀ were determined to be 8.318 Å. The ferrimagnetic Neel temperature (T _N) for NiCr_1.98 ⁵⁷Fe_0.02O₄ is determined to be 90 K. The Mössbauer absorption spectra for all chromites at 4.2 K show two well developed sextets superposed with small difference of hyperfine fields (H _hf) caused by Cr^3?+? ions in two different magnetic sites. The values of the isomer shifts show that the charge states of Fe are Fe^3?+? for all temperature range. Ni-chromites Mössbauer spectra below T _N present aline broadening due to a Jahn–Teller distortion and show that spin structure behavior of Cr ions change from an incommensurate to a commensurate state.     

Effects of a magnetic field on a modulated phase

The effects of a magnetic field on a modulated phase are studied. A modulated phase is found to have two critical fields H₁ and H₂. For a large enough magnetic field, H₁ and H₂ can be approximated by a linear law. As a result, the minimum magnetic field needed to destroy a modulated phase is a constant. The minimum magnetic field also greatly depends on the order of a commensurate phase. A very high order commensurate phase and an incommensurate phase cannot survive a magnetic field. The behaviour of a magnetoelastic chain in a magnetic field can be described by a harmless devil's staircase. The inverse temperature is found to play a role similar to that of a special magnetic field. The deeper physics underlying these new phenomena is the breaking of the left-right symmetry of a phase diagram. As a result a controllable path to a modulated phase is found.     

Structure and orientational ordering of nitrogen molecules physisorbed on graphite

The structure and orientational ordering of nitrogen molecules physisorbed on graphite have been studied by low-energy diffraction (LEED). A two-sublattice in-plane herringbone structure with glide lines along two perpendicular directions is inferred from LEED patterns at T < 30 K from the monolayer where the molecular centers have the commensurate $\text{(}\text{3}\text{×}\text{3}\text{)}$ 30° structure. The orientational order-disorder transition of this commensurate phase was examined by superlattice spot intensity and angular profile measurements for 20 < T < 38 K. A rapid drop in superlattice intensity is observed near 27 K. The persistence of some intensity to 38 K. is suggestive of residual short-range orientational ordering and perhaps finite size or heterogeneity effects. For increasing coverage at T = 15 K, there is first a transition to a previously unobserved uniaxial incommensurate phase and then a transition to an apparently triangular incommensurate phase. The orientational superlattice spots are clearly present in the uniaxial phase, but are much weaker in the triangular incommensurate phase. At 31 < T < 35 K, an apparently triangular incommensurate phase with no detectable orientational superlattice spots is observed. The lattice constant versus equilibrium vapor pressure curve has been determined in the latter case assuming a continuous transition. The lattice constants of the incommensurate phases are used to place limits on the extent of possible phase-coexistence regions between the commensurate, uniaxial incommensurate, and triangular incommensurate phases. The LEED patterns from the bilayer at T = 15 K indicate a double-period superlattice structure of the triangular incommensurate phase which does not have the glide line symmetries of the commensurate monolayer. Some effects of heterogeneity on these phase transitions are discussed. A phase diagram for 10 < T < 40 K is proposed.     

Magnetic structure of Er<Subscript>5</Subscript>Ge<Subscript>3</Subscript> at 4.2 K

A symmetry analysis of the possible magnetic structures of Er₅Ge₃ in the ground state is performed using the results of measurements of elastic magnetic neutron scattering at 4.2 K. It is shown that the minimum discrepancy factor R _m ≈9.5% corresponds to a modulated collinear magnetic structure in which the magnetic moments of erbium atoms are oriented along the a ₃ axis of the unit cell of the crystal structure and induce an antiferromagnetic longitudinal spin wave (AFLSW). The magnetic structure is characterized by the wave vector k=2π(0, 0, μ /a ₃) (where μ≈0.293) and the modulation period λ≈3.413a ₃. The magnetic ordering temperature T _N ≈38 K is determined from the temperature dependence of the intensity of magnetic reflections. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 9, 2003, pp. 1653–1659. Original Russian Text Copyright ? 2003 by Vokhmyanin, Dorofeev.     

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB₆ single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ～ 7 and ～4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB₆ at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.     

Time relaxation of dielectric constant in the commensurate phase of TlGaSe2

The results of measurements of the time dependences of the dielectric constant of TlGaSe₂ in the commensurate ferroelectric phase are presented. From the result of the observation of the decay of ε at different stabilized temperatures below the commensurate phase transition temperature after cooling from the incommensurate phase, the presence of two different characteristic relaxation time constants with the same temperature behaviour has been revealed. This peculiarity is considered as a result of a coexistence of two polar sublattices in the temperature range below 110 K. According to these results, the previously reported dielectric anomaly at about 103 K is considered as a final lock-in phase transition accompanied by the forming of the antiferroelectric state in TlGaSe₂.     

Temperature hysteresis of magnetic phase transitions in Tb1 − x Ce x Mn2O5 (x = 0, 0.20, 0.25)

The temperature evolution of the magnetic structure of multiferroics Tb_{1 ? x} Ce _x Mn₂O₅ (x = 0, 0.20, 0.25) has been investigated using the neutron scattering methods. It has been found that, despite the qualitative similarity of the magnetic states and the series of phase transitions for pure TbMn₂O₅ (TMO) and doped crystals Tb_{1 ? x} Ce _x Mn₂O₅ (TCMO, x = 0.20 and 0.25), there are significant differences in their properties. In contrast to TMO, where there are three magnetic phases, TCMO can include two magnetic phases that coexist in a wide temperature range and exhibit a rather wide temperature hysteresis. One of these phases with wave vector k ₁ = (0.5, 0, k _z1), k _z1 = 0.25, is commensurate and arises at temperatures below T _N ～ 39 K (for x = 0.2) and T _N ～ 38 K (x = 0.25). The second phase is incommensurate with wave vector k ₂ = (1/2, 0, k _z2), k _z2 = 0.256(2), and appears upon cooling at T = 21 K (x = 0.2) and T = 19 K (x = 0.25). Upon further cooling to 16 K, the component k _z2 increases to 0.292(2) and then remains constant. The component k _z1 increases to the value of 0.280(2) upon cooling in the range from 15 to 10 K and then remains constant down to 1.5 K. With an increase in the temperature, the components k _z1 and k _z2 undergo reverse changes to their initial values, but these changes occur at temperatures 7 K higher than those observed with a decrease in the temperature. For TMO, two phases also coexist, but the temperature hysteresis in this case is considerably smaller than for TCMO. This is explained by different densities of domain walls and different sizes of domains in pure and doped crystals.     

μSR investigation of the magnetic ordering in EuAs3

We have investigated the magnetic ordering and the incommensurate-commensurate phase transition in EuAs₃ by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below T_L=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from T_L≈10.3 K up to T_N≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature T_N≈11 K and also at the lock-in transition T_L≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.     

Anomalous states of the structure of (NH4)2SO4 crystals in the temperature range 4.2–300 K

Crystals of the (NH₄)₂SO₄ ammonium sulfate are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameters exhibits an anomalous behavior, namely, a global hysteresis, and an anomalous increase in the lattice parameter a and the unit cell volume at temperatures below the ferroelectric phase transition point (T _c=223 K). The series of superstructure reflections observed corresponds to an incommensurate composite structure. Analysis of the temperature behavior of the mismatch parameters for the matrix (host) and superstructure (guest) lattices demonstrates that the (NH₄)₂SO₄ compound undergoes a number of phase transitions, including a transition to a three-dimensionally incommensurate composite phase and transitions to commensurate (along one of the crystallographic directions) composite phases.     

Electron paramagnetic resonance in deuterated nickel fluosilicate

Electron paramagnetic resonance measurements in single crystals of NiSiF₆. 6D₂O were made at K, Ku and Ka bands at 4.2 K and between 77 K and 300 K. The measured g values were in the range 2.23–2.26, while the zero-field splitting parameter D varied from ?(0.185 ± 0.005) cm^?1 at 4.2 K to ?(0.53 ± 0.01) cm^?1 at 298 K. The parameters of the trimolecular hexagonal unit cell were determined to be approximately a = 9.28 Å, c = 9.58 Å from powder X-ray diffraction measurements at room temperature.     

Colossal magnetostriction effect in HoMn2O5

We have investigated the detailed field and temperature dependence of the dielectric constant, electric polarization, magnetization and magnetostriction in orthorhombic HoMn₂O₅ single crystals. HoMn₂O₅ displays incommensurate antiferromagnetic ordering below 39 K, becoming commensurate on further cooling. The commensurate-incommensurate transition takes place at low temperatures. The inherent magnetic frustration in this material is lifted by a small lattice distortion, primarily involving shifts of the Mn³⁺ cations and giving rise to a canted antiferroelectric phase. Colossal magnetostriction effect was observed and a novel phase transition diagram was build.     

Magnetoabsorption and magnetization of Zn1−xMnxTe mixed crystals

Excitonic magnetoabsorption (on samples of thickness less than 1 μm) of Zn_1?xMn_xTe (x < 0.1) was measured for magnetic fields up to B = 5T at T = 2.2 K. A strong (about 80 meV) exchange splitting of the exciton was observed. In the absence of magnetic field the variation of energy gap with composition of the crystals, as well as the coefficients dEg/dT were studied at 2.2 < T < 275 K. The magnetization measurements were performed at temperatures of T = 2.2 K and 4.2 K for B < 15T on the same samples that were optically tested. As a result, the s-d exchange constants were evaluated: N_oα = 0.19 eV, N_oβ = ?1.1 eV.