The low field magnetisation of Y4Co3

The onset of magnetic order at 6 K and a superconducting transition at 2 K was previously observed in Y₄Co₃. In this paper we investigate the magnetisation in very low fields, in the vicinity of the magnetic and superconducting transition (T_c). Below T_c the M-H curves are characterised by broad hysteresis loops. This type of the behaviour indicates that the sample consists of different regions. Some of these regions are superconducting below T_c and others are magnetically ordered below 6 K.     

Pressure-induced magnetic transition in Fe2P

The weak field ac susceptibility and the resistivity of Fe₂P single crystals were measured as functions of temperature from 4.2–300 K and as functions of hydrostatic pressures up to 20 kbar, using a newly designed clamp-type pressure cell. The Curie temperature, and the first-order transition temperature, decreased rapidly with increasing pressure, and ferromagnetism vanished at about 13 kbar at 0 K. A second-order transition temperature, as well as the first-order transition, appeared in the region below 170 K and above 5 kbar (triple point) and a new pressure-induced magnetic phase was found. The phase is proposed to be antiferromagnetic for reasons discussed in the paper.     

Pressure dependence of the superconducting transition temperature of a (Mo0.6Ru0.4)86B14 metallic glass

We have studied the pressure dependence of the superconducting transition temperature of amorphous (Mo_0.6Ru_0.4)₈₆B₁₄ for hydrostatic pressures up to P ～ 9 kbar. The transition temperature T_c decreases with pressure at a rate dT_c/dP=-(9±1) mK kbar^-1. We estimate the Grüneisen parameter and the volume dependence of the electron-phonon coupling constant.     

Selected studies of magnetism at high pressure

Summary Most previous studies of magnetism in various compounds under extreme conditions have been conducted over a wide pressure range at room temperature or over a wide range of cryogenic temperatures at pressures below 20 GPa (200 kbar). We present some of the most recent studies of magnetism over an extended range of temperatures and pressures far beyond 20 GPa,i.e. in regions of pressure-temperature (P-T) space where magnetism has been largely unexplored. Recent techniques have permitted investigations of magnetism in selected 3d transition metal compounds in regions ofP-T where physical properties may be drastically modified; related effects have often been seen in selected doping studies at ambient pressures. We present⁵⁷Fe and¹²⁹I M?ssbauer isotope studies covering the range 300–4 K to sub-megabar pressures in compounds such as Sr₂FeO₄, LaFeO₃ and FeI₂, representative of a broad class of 3d transition metal compounds. At ambient pressure the electronic structure of the transition metal atom in these antiferromagnetic insulators extends from 3d ⁴ to 3d ⁶ and has a distinct influence on the pressure evolution of their magnetic properties. M?ssbauer studies of these compounds are considered in conjunction with available structural and electrical transport data at pressure. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

(Fe0.1Co0.55Ni0.35)78Si8B14金属玻璃的晶化过程及压力的影响(Ⅰ)——晶化时的相析出过程

用X射线衍射法研究了(Fe_0.1Co_0.55Ni_0.35)₇₈Si₈B₁₄金属玻璃在常压下及20kbar高压下晶化过程中的析出相及析出过程。结果表明在上述压力下晶化过程都分成两个阶段,分别对应于初级晶化和共晶晶化。在常压下,初级晶化时析出fcc-Co晶体,而共晶晶化对应着Ni₃₁Si₁₂和(FeCoNi)₃(SiB)相的析出。随着回火温度的增高或时间的延长,(FeCoNi)₃(SiB)相逐渐转变为(FeCoNi)₂₃B₆相。20kbar高压下的晶化析出过程与常压下不同的是:提高了晶化温度,在共晶晶化阶段出现了Co₂B相。此外,压力还阻止(FeCoNi)₂₃B₆相的形成。从热力学和动力学的角度讨论了压力对金属玻璃晶化过程的影响。 关键词：     

High field magnetization of Y2Co7 and YCo3 hydrides

The magnetization curves for the Y₂Co₇H_x and the YCo₃H_x systems have been measured at 4.2 K in the pulsed high magnetic fields up to 280 kOe. The metamagnetic transition is observed in the β and the γ hydrides except for Y₂Co₇H₃ and YCo₃H₁, which is interpreted in terms of the itinerant electron metamagnetism.     

Superconductivity in polymeric sulfur nitride (SN)x at high pressure

Investigations of the pressure dependence of the superconducting transition temperature T_c up to 17 kbar, and of the normal conductivity up to 50 kbar are reported. It is observed that below 8 kbar, the value of T_c increases linearly with the pressure. In addition, there is a significant drop of T_c at about 9 kbar which may be due to a phase transition.     

Superconducting transition temperature of a paramagnetic material close to magnetic ordering

An explicit expression for the superconducting transition temperatureT _c in a paramagnetic material is derived, when the transition occurs just before a possible magnetic ordering. As first noted by Uspenskii, such a transition may arise from electronic mechanism itself, without the necessary role played by the usual phonon-exchange mechanism. The result is discussed in terms of some recent experimental observations on the binary alloy Y₉Co₇.     

Non-equilibrium effects in the magnetic behavior of Co3O4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co₃O₄ nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co₃O₄ nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.     

Raman study of rutile (TiO2) at high pressures

The Raman spectra and polymorphism of rutile have been investigated under hydrostatic pressures up to 90 kbar at room temperature. A transition previously observed in rutile at 30 kbar in a Drickamer-type cell under nonhydrostatic conditions was observed to begin at approximately 70 kbar in a 4:1 mixture of methanol and ethanol. The small amount (10–20%) of the high-pressure phase synthesized from rutile, however, did not increase even though the sample was left under pressure for a period of 1 month at ambient temperature. On the basis of factor group analysis, in situ powder x-ray diffraction data, and comparison of the Raman spectrum of the high-pressure modification with that of TiO₂-II (α-PbO₂-type structure synthesized from anatase powder at 40 kbar and 400°C), it is evident that a high pressures rutile transforms irreversibly to TiO₂-II.     

Pressure and disorder effects on the half-metallic character and magnetic properties of the full-Heusler alloy Co2FeSi

We investigate the pressure and site disorder effects on the half-metallicity and magnetic properties of the full-Heusler alloy Co₂FeSi using first-principles density functional theory within the GGA and GGA+U schemes. The calculated lattice constant, bulk modulus and total magnetic moments are in excellent agreement with recent experiments. The volume compression leads to a slight increase of the minority band gap, i.e., the half-metallic properties of Co₂FeSi can maintain under pressure. The disorder calculations reveal that Fe–Co type disorder significantly destroys the half-metallic character and reduces the spin polarization of Co₂FeSi while disorder between Fe and Si can maintain half-metallic properties. Our results also show that the Fe–Co type disorder leads to degradation of the magnetism while the Fe–Si type disorder affects hardly the magnetism as observed in Co₂FeSi.     

Calorimetric investigation under high pressure of the heavy fermion superconductor CeCu2Si2

The heavy fermion compound CeCu₂Si₂ is commonly regarded as a Kondo lattice system. Though it has been shown that the heavy mass quasiparticles participate in its superconductivity below ～ 0.7 K, a detailed understanding of the interdependence of the superconducting and the Kondo lattice parameters is still to be developed. The application of pressure is one useful approach to study this problem. In this paper we present results of specific heat measurements between 0.3 K and 2 K under pressures up to 5.9 kbar. While in our sample T_c hardly changes, the normal state specific heat, which is exclusively of electronic origin in the present temperature range, is rapidly decreased in a monotonous way, qualitatively corresponding to the expected rise of the Kondo temperature with pressure. In contrast to this behaviour, a strong nonlinear change of the jump Δc(T_c) passing through a maximum near 3 kbar is observed. We suggest that this reflects changes of the Kondo lattice coherence structure in the quasiparticle density of states near E_F.     

The magnetic properties of FexCo1-x and FexTi1-x alloys

High pressures and high magnetic field behaviour give indications on the nature of magnetism encountered in transition metal alloys. We present experiments performed under pressure up to 7 kbar, in fields up to 150 kOe, between 1.4 K and 300 K. The results are used to discuss the occurence of ferromagnetism in Fe_xCo_10xSi and Fe_xCo_1-xTi systems. These systems exhibit two critical concentrations for the onset of ferromagnetism. Their magnetic properties are very sensitive to magnetic fields and high pressures and this allows us to characterize them as weak itinerant ferromagnets. A more localized behaviour is evidenced for the iron rich alloys and the appearance to ferromagnetism is different for cobalt rich side and for iron rich one.     

Pressure dependence of the magnetic transition fields of the quasi two-dimensional antiferromagnet (C2H5NH3)2 CuCℓ4

Using magnetic susceptibility measurements, we have studied the pressure dependence of the magnetic transition fields of powder samples of antiferromagnetic (C₂H₅NH₃)₂CuC?₄, for hydrostatic pressures up to P ～ 7 kbar. The spin-flop field H₁ decreases linearly with pressure $\text{(}\text{dH}{}_1\text{dP}\text{= ?(20±4)}\text{G}\text{kbar}\text{)}$, while the field corresponding to the paramagnetic transition H₂ shows an increase, ～ 70% for P～7 kbar, that is roughly quadratic with P.     

Hydrostatic pressure study of the structural phase transitions and superconductivity in single crystals of (Ba1−xKx)Fe2As2 (x=0 and 0.45) and CaFe2As2

We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe₂As₂, BaFe₂As₂, and (Ba_0.55K_0.45)Fe₂As₂, by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe₂As₂ and BaFe₂As₂ (lightly doped with Sn) display structural phase transitions near 170 and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba_0.55K_0.45)Fe₂As₂ is superconducting for T<30 K. The effect of pressure on BaFe₂As₂ is to shift the onset of the crystallographic transformation down in temperature at the rate of ~−1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba_0.55K_0.45)Fe₂As₂ is to shift the onset of superconductivity to lower temperatures at the rate of ~−0.21 K/kbar. The effect of pressure on CaFe₂As₂ is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P<5 kbar. However, higher pressures bring about another phase transformation characterized by reduced-resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (H_c2) data in (Ba_0.55K_0.45)Fe₂As₂ and CaFe₂As₂ are discussed.     

Hydrostatic pressure effect on Tc of Ba0.9K0.1Pb0.75Bi0.25O3

The superconducting transition temperature of Ba_0.9K_0.1Pb_0.75Bi_0.25O₃ has been found to be suppressed smoothly by the application of hydrostatic pressure at a rate of —(2.9 ± 0.2) × 10^?5 kbar^?1 up to 15 kbar. The implications of these results are discussed.     

Pressure dependence of the superconducting transition temperature of Th4H15

Measurements of the superconducting transition temperature of Th₄H₁₅ under hydrostatic pressure up to 28 kbar are reported. The initial linear increase is found to be $\text{+42}\text{mK}\text{kbar}$.     

Pressure induced superconductivity in BaMo6S8

The resistance and a.c. magnetic susceptibility of BaMo₆S₈ have been measured under pressure. A resistance-anomaly, indicative of the reported structural transformation at ～ 145 K, was observed. Non-bulk superconductivity with a pressure dependent signal size was induced by a quasi-hydrostatic pressure ? 15 kbar but not a hydrostatic pressure up to 18 kbar down to 1.2 K.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Anomalous Tc-behavior of LaSn3 under pressure

The superconducting transition temperature T_c of LaSn₃ has been measured up to ～ 22 kbar and was observed to increase through a maximum under hydrostatic compression. The anomalous T_c-behavior is attributed to a pressure induced Fermi surface topology change.