Magnetic properties and hydrostatic pressure effect on the curie temperature of (Co,Mn)2B amorphous alloys

The temperature dependence of magnetization and magnetic susceptibility and hydrostatic pressure effect on the Surie temperature (dT_c/dP) are measured for (Co_1-xMn_x)₂B (0?x?0.4) amorphous alloys and the results are compared with those of crystalline compounds with the same composition. The Curie temperature decreases linearly with an increasing Mn content but magnetization shows a maximum around x=0.15. The reciprocal magnetic susceptibility of all the prepared alloys obeys the Curie-Weiss law above T_c. The magnitude of the negative value of dT_c/dP decreases linearly with increasing x from about 1.1 K/kbar (x=0) to zero (x=0.4), the composition dependence of which is opposite to that of the crystalline compound. The composition dependence of the average magnetic moment per transition metal atom and the Curie temperature and dT_c/dP are analysed on the basis of the local environment and the pair order interaction mode, respectively.     

Effect of pressure on the Curie temperature of CoCr2O4

The pressure effect on the Curie temperature T_c of CoCr₂O₄ was measured under pressures up to 11.5 kbar. The pressure derivative of T_c is found to be +0.25 K/kbar. On the basis of this result, the relationship between the magnetic transition temperature and volume is discussed.     

Note on the magnetic properties of amorphous rare-earth nickel alloys

The magnetic properties of several amorphous rare-earth nickel alloys close to 30 at% Ni have been studied in the temperature range 4.2–300 K. The Curie temperatures range from 2 K in Pr₆₉Ni₃₁. Alloys in which the rare-earth component has an orbital moment show large intrinsic coercive forces below T_c. These coercive forces vary considerably with temperature, the temperature dependence of the alloys Dy₆₉Ni₃₁ and Tb₆₉Ni₃₁ is satisfactorily described by means of an exponential law of the form H_c(_T) = H_c(0) exp(-αT).     

Pressure effect on magnetization for (CoTm)90Zr10 (Tm = Cr,Mo) amorphous alloys

Temperature dependences of the forced volume magnetostriction dω/dH and the saturation magnetization σ_s for (CoTm)₉₀Zr₁₀ (Tm = Cr, Mo) amorphous alloys have recently been measured by the 3-terminal capacitance method and a vibrating sample magnetometer and magnetic balance at temperatures from 77 K to the Curie temperature T_c or the crystallization temperature. The pressure coefficient of σ_s0 at 0 K, d ln σ_s0/dp, is estimated from (dω/dH)₀ extrapolated to 0 K using the thermodynamical relation. The values of d ln σ_s0/dp increase in negative value with increasing Tm concentration. The relation between d ln σ_s0/dp and the pressure coefficient of T_c, d ln T_c/dp, estimated indirectly from dω/dH is discussed.     

Pressure effect on the anomalous electrical conductivity of magnetite

The temperature dependence of the electrical conductivity of magnetite was measured under hydrostatic pressure up to 18.4 kbar. It is found that the temperature of the conductivity maximum in the high temperature phase is more rapidly reduced by pressure (dT_m/dP = -4.1 K/kbar) than the Verwey temperature (dT_v/dP = -0.27 K/kbar). The discontinuous change of the conductivity at T_v appears to increase with applied pressure as a result of a lowering of T_m.     

Phase transitions and the magnetocaloric effect in Mn rich Ni–Mn–Ga Heusler alloys

In Mn rich polycrystalline Heusler alloys, Ni₅₀Mn_25+−xGa_25−x, prepared by Arc melting, it is found that the structural/first-order magnetic transition temperature T_m increases as the Mn content increases. The Curie temperature T_c is higher than that of Ni rich alloys (Ni_50+xMn_25−xGa₂₅ ) of the same series, and is less affected by composition x. Magnetic entropy change of |ΔS_M| also increases as Mn content increases, while behaviour of the field dependence of ΔS_M is similar to that of single crystal Ni_52.6Mn_23.1Ga_24.3.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Pressure dependence of the Curie temperature of some Ni3Al compounds

Pressure experiments on four ferromagnetic Ni₃Al compounds with Curie temperatures between 30 K and 72 K show a decrease of T_c with increasing pressure. Values for the parameter α(= -T_c?T_c/?p) are between 17 and 26.     

Pressure-induced phase transformation in In2Bi

We have made measurements of the pressure dependence of the superconducting transition temperature, T_c, for In₂Bi and related alloys. For In₂Bi- phase alloys, a large discontinuity in T_c is seen at 15–20 kbar, which we associate with a phase transformation first seen by Bridgman [1]. Our measurements suggest that this transformation is produced by the decomposition of In₂Bi into In₅Bi₃ and an In-rich phase. In the low pressure phase, T_c shows a minimum at 9–15 kbar whereas it depends linearly on pressure in the high pressure phase with ?T_c/?P equal to -4.9 × 10^-5 K bar^-1.     

Pressure dependence of temperature variation of AC susceptibility for Fe50Ni37Mn13 and comparison to Fe68.1Ni31.9

Fe₅₀Ni₃₇Mn₁₃, which is a ferromagnetic alloy with FCC crystal structure, has been reported to show the Invar effect below the Curie temperature; however, this alloy shows a typical anti-Invar effect above the Curie temperature. In this paper, we discuss the pressure dependence of the temperature variation of the alternative current (AC) susceptibility at a frequency of 1 kHz for Fe₅₀Ni₃₇Mn₁₃ at various pressures up to 7.5 GPa above 77 K; we then compare the results with those for Fe_68.1Ni_31.9, which were obtained in a previous study. Fe₅₀Ni₃₇Mn₁₃ was in a ferromagnetic state throughout the entire pressure range measured. T_C decreases in inverse proportion to the increasing pressure; dT_C/dp and dln T_C/dp for Fe₅₀Ni₃₇Mn₁₃ are −26 K GPa⁻¹ and −0.07 GPa⁻¹, respectively. Further, the temperature variation of the shape of the χ′-T curve for different pressures indicates continuous combining of magnetic interactions occurs at high pressures. These results are similar to those obtained for Fe_68.1Ni_31.9.     

Pressure and field dependence of the magnetic transition in GdBa2Cu3O7-x

The influence of hydrostatic pressure and of magnetic field strenght is presented for the low temperature antiferromagnetic ordering temperature (T_N=2.3 K) of GdBa₂Cu₃O_7-x. Data are presented for both superconducting and normal samples, the superconducting sample having a sharp 95 K transition and the oxygen-depleted normal sample being a semiconductor. For both systems the Néel temperatures, extrapolated to zero measuring field, are identical: T_N = (2.33±0.03) K. The effect of pressure is to raise the transition temperature slightly for both samples, dT_N/dP=+0.03 K/kbar for the superconducting sample and +0.04 K/kbar for the normal sample. The temperature dependence of the heat capacity made in several fixed external magnetic fields and the isothermal magnetization for T<T_N provide a measure of the antiferromagnetic-paramagnetic phase boundary, which shows T_N approaching T=0 K at about 2.5 T.     

Pressure dependence of the weak itinerant ferromagnetism of Y(Co1-xAlx)2

Abstract

Y(Co_1-xAl_x)₂ is a weak itinerant ferromagnet for 0.12 < x < 0.20 with a maximum Curie point of about 25 K near x = 0.15. The pressure dependence of the Curie point for samples with 0.14 < x < 0.18 was measured in the temperature and pressure range 5–25 K and 0–8 kbar using a liquid filled lock cell. It was found that T_c was linear in pressure and extrapolated to OK at P_c = 9 ± 1 kbar for all samples. The volume expansion from YCo₂ to x = 0.15 is equivalent to a chemical pressure of -40 kbar so ferromagnetism cannot occur at the lattice constant of YCo₂.     

Electrical resistance of Fe-Zr and Fe-Ni-Zr amorphous alloys under hydrostatic pressure

The effect of pressure on the electrical resistance of Fe₉₀Zr₁₀ and (Fe_0.96Ni_0.04)₉₀Zr₁₀ amorphous alloys has been investigated. The minimum point in the electrical resistance versus temperature curve shifts toward lower temperature range with increasing hydrostatic pressure, corresponding to the shift of the Curie temperature. The decrement of the electrical resistance below the Curie temperature T_c is about twice that above T_c, suggesting a large compressibility in the ferromagnetic state of the amorphous Invar alloys.     

LaAg under hydrostatic pressure: Superconductivity and phase transformation

The electrical resistivity of LaAg has been measured from 1–300 K at hydrostatic pressures to 12.4 kbar and the superconducting transition temperature T_c determined inductively to 23 kbar. For P ? 6.2 kbar a lattice transformation is observed at a temperature T_M which increases rapidly with pressure. T_c shows an oscillatory pressure dependence, increasing initially from T_c (0) = 1.062 K. There is no obvious correlation between the pressure dependence of T_M and T_c.     

Uniaxial piezoresistance measurements on magnetite

The dependence of the Verwey temperature T_v on the uniaxial pressure P was determined. It was found that along the [111] axis dT_v/dP ≈ 10K/kbar while along the [110] and the [001] axes, dT_v/dP ≈ 1 K/kbar. The results emphasize the important role played by the rhombohedral distortion in the Verwey transition.     

Pressure effect on magnetic properties of RCo12B6 (R=Y,Ce) compounds

The effect of pressure on magnetic properties of YCo₁₂B₆ and CeCo₁₂B₆ was studied in temperature range 5–300 K at pressures up to 9 kbar. The Curie temperature T_C and spontaneous magnetization M_S decrease with pressure for both compounds. The decrease can be attributed mostly to the volume dependence of both, the Co magnetic moment and the exchange interactions. The hybridization of the p–d states as a consequence of small distances between the Co and B atoms can be one reason of the relatively low pressure effects (ΔT_C/Δp=?0.39±0.02 K/kbar, d ln M_S/dp=?0.0013±0.0002 kbar^?1) in YCo₁₂B₆. Higher volume sensitivity of magnetic properties of CeCo₁₂B₆ in comparison with YCo₁₂B₆ can be attributed to the pressure induced changes of the Ce f- and Co d-states.     

The pressure influence on the incommensurate-commensurate ferroelectric phase transition in [4-NH2C5H4NH][SbCl4]

The dielectric properties of the [4-NH₂C₅H₄NH] SbCl₄ (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure-temperature phase diagram was given. The paraelectric-ferroelectric phase transition (II→III) temperature (T_c) increases linearly with increasing pressure with a slope dT_c/dp=21×10⁻² K/MPa. The pressure dependence of Curie-Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant (C₊) was pressure dependent whereas the C₋ constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.     

Dependence on interatomic distance of exchange coupling in rare-earth Al2 compounds

The effect of pressure on the Curie temperature of the heavy rare-earth dialuminides has been measured up to 3.5 kbar. The derivatives dT_c/d_p are 0.00(4), 0.09, 0.21, 0.39, 0.60 and 0.71 K/kbar respectively for TmAl₂ ErAl₂, HoAl₂, DyAl₂, TbAl₂ and GdAl₂ thus indicating a clear positive correlation between dT_c/dp and the de Gennes factor G. The results are discussed in terms of the RKKY model. However, the phenomenological parameter D/d, where D is the nearest-neighbour R-R distance and d the diameter of the 4f orbital, is used to reflect the non-δ-function character of the sf-coupling parameter Г. The experimental values of Г²D² and T_p/G are found to decrease linearly with increasing D/d. This is in keeping with the RKKY proportionality of $\text{T}{}_{\mathrm{c}}\text{?T}{}_{\mathrm{p}}$ to Γ²E^?1_FG. For GdAl₂, TbAl₂ and DyAl₂ the quantity $\text{?}\text{(}\text{d}\text{T}{}_{\mathrm{c}}\text{d}\text{p}\text{)}\text{k}{}_{\mathrm{l}}\text{(}\text{D}\text{d}\text{)}\text{G}$ coincides within the error limits with the slope of the T_p/G vs D/d plot.     

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.