On the tricritical point in MnSi under high pressures

The magnetic susceptibility of a MnSi single crystal is measured in the region of the ferromagnetic phase transition under pressures up to 0.8 GPa in compressed helium. It is found that the tricritical point on the phase-transition curve corresponds to a much lower pressure and a considerably higher temperature (P _tr ≈ 0.355 GPa and T _tr ≈ 25.2 K) than was reported earlier (P _tr ≈ 1.2 GPa and T _tr ≈ 12 K). New results impose certain limitations on theoretical analysis of tricritical phenomena in MnSi.     

Friedel oscillation in the spin density at the surface of an itinerant ferromagnet

Using the spin-dependent generalization of the Hohenberg-Kohn-Sham density functional formalism, the separate up- and down-spin densities have been calculated self-consistently for a jellium model of a ferromagnetic surface. The magnetization dependence of the work function, surface potential, surface moment and surface energy is calculated and compared with ferromagnetic gadolinium.     

Magnetic blue phase in the chiral itinerant magnet MnSi

Chiral nematic liquid crystals sometimes form blue phases characterized by spirals twisting in different directions. By combining model calculations with neutron-scattering experiments, we show that the magnetic analogue of blue phases does form in the chiral itinerant magnet MnSi in a large part of the phase diagram. The properties of this blue phase explain a number of previously reported puzzling features of MnSi such as partial magnetic order and a two-component specific-heat and thermal-expansion anomaly at the magnetic transition.     

Phase transition of ZnS at high pressures and temperatures

The high-pressure behaviour of zinc sulphide, ZnS, has been investigated, using an in situ X-ray powder diffraction technique in a diamond anvil cell, at pressures and temperatures up to 35 GPa and 1000 K, respectively. The pressure-induced phase transition from a zincblende (B3) to a rocksalt (B1) structure was observed. This transition occurred at 13.4 GPa and at room temperature, and a negative dependence on temperature for this transition was confirmed. The transition boundary was determined to be P (GPa) = 14.4 ? 0.0033 × T (K).     

Phase equilibria of multicomponent mineral systems at high pressures and high temperatures

Abstract

The substance of the Earth's upper mantle was essentially differentiated in the course of deep-seated magmatic processes. It is for the most part formed by peridotitic as well as pyroxenitic and eclogitic The most deep-seated ones pertain to the garnet-peridotitic facies. Liquidus phase relations between the minerals of primary garnet lherzolite (compositional estimations are given in Refs. 1–3) account for the regularities of the formation, evolution, crystallization of multicomponent silicate magmatic melts and petrogenesis of garnet-peridotitic mantle rocks.     

Quantum phase transitions in the itinerant ferromagnet ZrZn2

We report a study of the ferromagnetism of ZrZn2, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures T as a function of pressure p. We find that the ordered ferromagnetic moment disappears discontinuously at p(c)=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetization isotherms up to 12 T in the p-T plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn2, thus strongly suggesting that QPT in itinerant ferromagnets are always first order.     

Experimental study of the magnetic phase transition in the MnSi itinerant helimagnet

Magnetic susceptibility, heat capacity, thermal expansion, and resistivity of a high-quality single crystal of MnSi were carefully studied at ambient pressure. The calculated change in magnetic entropy in the temperature range 0–30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the behavior of the thermal expansion coefficient in the range 30–150 K, which correlates to a large enhancement of the linear electronic term in the heat capacity. A surprising similarity between variation of the heat capacity, the thermal expansion coefficient, and the temperature derivative of resistivity through the phase transition in MnSi is observed. Specific forms of the heat capacity, thermal expansion coefficient, and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as a combination of sharp first-order features and broad peaks or shallow valleys of yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state in MnSi slightly above the transition temperature. Present experimental findings bring the current views on the phase diagram of MnSi into question. The text was submitted by the authors in English.     

Phase transitions in uranium dioxide at high pressures and temperatures

The melting curve is plotted for uranium dioxide with fluorite structure in a pressure range from ?2.5 to +100 GPa. This curve has a peak at the point 3348 K, 6 GPa, and has a negative derivative at high pressures. The pressure corresponding to a polymorphic transition of uranium dioxide (37 GPa) at a temperature of 1015 K is determined. The slope of the equilibrium curve of the polymorphic transition in UO₂ in the temperature range 300–1000 K is ? 56 K/GPa.     

Electronic structure and spin fluctuations in the helical ferromagnet MnSi

The influence of spin fluctuations on the magnetic properties of the ferromagnetic helimagnet MnSi has been studied in the Hubbard model taking into account the antisymmetric relativistic Dzyaloshinskii–Moriya interaction for band electrons. The obtained equations of the magnetic state indicate the correlation between the fine structure of the density of electronic states and the magnetization and coefficient of mode–mode coupling. It has been shown that the position of the Fermi energy in the immediate proximity on the point of the local minimum of the density of electronic states leads to large zero spin fluctuations at low magnetization of the helimagnet. When approaching from down the Néel point (approximately, at 0.9T_N), the zero fluctuation disappear, and the temperature rise of thermal spin fluctuation is accompanied by the change in the sign of the coefficient of mode–mode coupling. A magnetic field perpendicular to the helicoids plane brings about the formation and subsequent “collapse” of the helimagnetic cone. However, the condition of the change in the sign of the coefficient of mode–mode coupling divides the MnSi phase diagram into two parts, one of which corresponds to the ferromagnetic state induced by the field, and the other corresponding to the paramagnetic state. In this case, the h–T diagram has a specific region, inside which the paramagnetic and the ferromagnetic state are instable. The boundaries of the region agree with the experimental data on the boundaries of the anomalous phase (a phase). It has been found that the results of calculations of the temperature dependence of the magnetic susceptibility agree with the experimental data.     

Phase transitions of lanthanide monophosphides with NaCl-type structure at high pressures

Lanthanide monophosphides LnP (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Tm and Yb) with a NaCl-type structure have systematically been prepared at high temperatures. Using synchrotron radiation, X-ray diffractions of LnP have been studied up to 61 GPa at room temperature. The NaCl---CsCl transition for CeP is found at around 25 GPa. First-order phase transitions of LnP (Ln = La, Pr and Nd) with the crystallographic change occur at around 24, 26 and 30 GPa, respectively. The structure of the high pressure phases of these phosphides is a body center tetragonal structure (Ln: 0, 0, 0; P: 1/2, 1/2, 1/2; space group P4/mmm), which can be seen as the distorted CsCl-type structure. The Pr---P distance in the high pressure form of PrP is 2.789 Å. This almost agrees with the sum of covalent radii of Pr and P. The Pr---P bond has the covalent character at very high pressures. Similar results are also obtained for LaP and NdP. The pressure-induced phase transitions of SmP, GdP, TbP, TmP and YbP occur at around 35, 40, 38, 53 and 51 GPa, respectively. The structure of the high pressure phase is unknown. The phase transitions of LnP with many f-electrons are not due to the mechanism of the ordinary NaCl---CsCl transition. The transition pressures of LnP increase with decreasing the lattice constants in the NaCl-type structure, which decrease with increasing atomic number of the lanthanide atoms.     

Spin fluctuation of itinerant electrons in MnSi probed by μ+

Conclusion To our knowlge, this is the first experimental work to show the divergent behavior of 1/T₁ at T_C in weak itinerant ferromagnetic systems, and we conclude that there is general agreement between the present results and the SCR theory.Work supported by Japan Society for the Promotion of Science, the Toray Science Foundation, the Grant-in-Aid of the Japanese Ministry of Education, Culture and Science, and the Atomic Energy Control Board and National Research Council of Canada.     

Magnetic dynamics of two-dimensional itinerant ferromagnet Fe_3GeTe_2

Among the layered two-dimensional ferromagnetic materials(2 D FMs),due to a relatively high T_C,the van der Waals(vdW) Fe_3 GeTe_2(FGT) crystal is of great importance for investigating its distinct magnetic properties.Here,we have carried out static and dynamic magnetization measurements of the FGT crystal with a Curie temperature TC ≈ 204 K.The M-H hysteresis loops with in-plane and out-of-plane orientations show that FGT has a strong perpendicular magnetic anisotropy with the easy axis along its c-axis.Moreover,we have calculated the uniaxial magnetic anisotropy constant(K_1)from the SQUID measurements.The dynamic magnetic properties of FGT have been probed by utilizing the high sensitivity electron-spin-resonance(ESR) spectrometer at cryogenic temperatures.Based on an approximation of single magnetic domain mode,the K_1 and the effective damping constant(α_(eff)) have also been determined from the out-of-plane angular dependence of ferromagnetic resonance(FMR) spectra obtained at the temperature range of 185 K to T_C.We have found large magnetic damping with the effective damping constant α_(eff) ～ 0.58 along with a broad linewidth(ΔH_(pp) 1000 Oe at 9.48 GHz,H ‖ c-axis).Our results provide useful dynamics information for the development of FGT-based spintronic devices.     

Electronic structure of half-metallic ferromagnet Co<Subscript>2</Subscript>MnSi at high-pressure

In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co₂MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (σ-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L2₁ structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around ~70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at Γ-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.     

Iron at high negative pressures

With the object of verifying the presence of a region of anomalous iron compressibility at negative pressures, as predicted by the ab initio calculations, the reflection of compression pulses from the surfaces of iron single crystals was detected. No evidence of the expected formation of rarefaction shock waves was observed in the range of attained tensile stresses up to 7.6 GPa. The breaking stresses achieved 25-50% of the theoretical iron ultimate strength for a load duration of ～10^?8 s. The dependence of breaking strength on the extension rate did not reveal any singularities in the region of assumed anomaly in iron compressibility.     

Phase transitions in cerium at high pressures (up to 15 GPa) and high temperatures

Phase transitions in cerium have been studied by the electrical resistance method in the 15-GPa pressure range at high temperatures. At pressures above 10 GPa, cerium represents a mixture of stable and metastable phases, the composition of this mixture being dependent on the trajectory in the P-T plane that leads to a given point. Transformations in both stable and metastable components of the mixture proceeding rather independently display a complicated picture of phase transitions. It was assumed that only the α (fcc) and α′ (α-U) phases are stable at pressures above the well-known γ-α transition, the other phases being metastable. The proposed bow-shaped equilibrium phase diagram includes an extremely wide hysteresis region, where stable and metastable phases can coexist. The fcc α phase alone survives upon heating above 50°C at 15 GPa.     

Influence of high magnetic fields on the low temperature heat capacity of the itinerant electron ferromagnet Sc3In

The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at T_c becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above T_c the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields ( ≈ 5 T). Our results show that the spin fluctuations in Sc₃In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, T_s, of this itinerant electron ferromagnet is about the same as T_c.     

Phase transformations of the amorphous Zn-Sb alloy under high pressures

Abstract

Phase transformations occurring in initially amorphous Zn₄₁ Sb₅₉ semiconductor at pressures to 10 GPa and temperatures to 350C were studied using the measurement of electrical resistance, in situ energy dispersive X-ray diffraction and neutron diffraction on quenched high-pressure phases at ambient pressure. The studied T- P region involves the regions of reversible and irreversible crystallisation and phase transitions between the equilibrium crystalline low-pressure and high-pressure phases.     

Phase relations in the Fe-P system at high pressures and temperatures from ab initio computations

ABSTRACT

Based on the first-principles calculations within the density functional theory and crystal structure prediction algorithms iron phosphide phases stable under pressure of the Earth’s core and temperatures up to 4000?K were determined. A new low-temperature modification FeP-P2₁/c stable above ～75?GPa was predicted. Fe₂P with the allabogdanite structure has been established to be stable in the low-temperature region at ambient conditions. At 750?K it transforms into the barringerite structure. The transition from Fe₃P with schreibersite structure to Fe₃P-Cmcm was observed at 27?GPa, and the phase transition boundary is nearly isobaric. Fe₂P and FeP are thermodynamically stable at the Earth’s inner core pressures and 0?K according to the obtained results, whereas Fe₃P stabilizes with respect to decomposition to Fe?+?Fe₂P at high temperatures above ～3200?K.     

Phase diagram of RDX (hexahydro-1,3,5-trinitro-s-triazine) at high pressures and temperatures

Abstract

The phase diagram of RDX-h₆ (hexahydro-1,3,5-trinito-s-triazine) and RDX-d₆ has been studied by Raman spectroscopy to more than 13 GPa at 295 K and 7.5 GPa between 150 and 450 K. Two stable high pressure phases have been found. γ-RDX or RDX-III forms from α-RDX above 3.8 GPa below 380 K. β-RDX forms when α- or γ-RDX are heated, can be retained metastably at low temperatues, and may be related to a very unstable form occasionally recovered at ambient pressure. Deuterium isotopic substitution and shear increase the temperature where β-RDX begins to form on heating.     

Thermopower of calcium at high pressures

Calcium at megabar pressures undergoes numerous structural transitions and has a complex phase diagram. At the same time, according to the recent theoretical investigations, an anomalous behavior of many physical properties, including a transition to the state of a narrow-gap semiconductor, can be expected even in the region of stability of the normal-pressure phase of calcium with the fcc structure at moderate pressures P ～ 5–15 GPa. Data on the thermopower of calcium in the pressure range up to 9 GPa have been reported. The thermopower in this pressure range is positive, has a smooth maximum at 5–6 GPa, and decreases quite rapidly at higher pressures. The absolute values of the thermopower (5–12 μV/K) indicate that calcium in this pressure range is a metal. The difference between the thermopowers in the direct and inverse passages in the range of 5–7.5 GPa is fairly noticeable (～10%). The possible reasons for such an anomalous behavior, as well as new calculations of the band structure of calcium, have been discussed.