Effect of pressure on the Curie temperature of annealed Fe-Ni-Cr-Mo-Si-B amorphous alloys

Effect of high pressure on the Curie temperature of annealed Fe₃₀Ni3₆Cr₁₂Mo₂Si₅B₁₅ amorphous alloys has been investigated. It has been shown that the annealed samples e hibit a smaller pressure dependence ofT _c than as-quenched one.Dedicated to Dr. S. Krupika on the occasion of his 65th birthday.We would like to thank Dr. P. Duhaj (Institute of Physics, Slovak Academy of Sciences, Bratislava) for the preparation of amorphous samples and Mrs. H. Kuchárska for technical assistance.     

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.     

Effect of Fe addition on the crystallization behaviour and Curie temperature of CoCrSiB-based amorphous alloys

The crystallization behaviour and Curie temperature of melt-spun Co_{71? x} Fe _x Cr₇Si₈B₁₄ (x?=?0, 2, 3.2, 4, 6, 8 and 12?at.%) amorphous alloys have been studied. Differential scanning calorimetry (DSC) showed two stages of crystallization. The first stage of crystallization (T _X1) in the alloy with 6?at.%?Fe was the highest and it had the highest activation energy. X-ray diffraction studies revealed that the primary crystalline phase is hcp-(CoCr)₂Si for Fe-free alloy, whereas (CoFeCr)₂Si and (CoFeCr)₃Si phases were formed with the addition of Fe. hcp-Co was also formed along with these phases. The secondary crystalline phases were fcc-Co and various boron-rich phases. The Curie temperature of the alloys also changed with the addition of Fe to the system. Like the primary crystallization temperature, the Curie temperature of the alloys did not vary systematically with the Fe content. The addition of Fe to the Co-based system changes the nearest-neighbour interaction. This changes the exchange interaction between the transition metal elements. Due to the asymmetry in the Bethe–Slater curve, a systematic variation with Fe addition was not observed in the Curie temperature measurement.     

About the local Curie temperature distribution of Invar alloys

Strongly disordered ferromagnetic alloys which exhibit magnetic inhomogeneities are described in the frame of a generalized Ginzburg-Landau theory by averaging the Ginzburg-Landau energy of a subsystem with a definite Curie temperature over the Curie temperature of all subsystems of the inhomogeneous magnetic system. It is shown that this leads to the rough scaling of a smooth phase transition. Furthermore the distribution function of the Curie temperature is derived from the experimental data of the specific heat, the volume thermal expansion coefficient and the compressibility.     

Effect of hydrostatic pressure on the Curie temperature of the Heusler alloys Ni2MnZ(Z = Al,Ga, In,Sn and Sb)

The pressure derivative of the Curie temperature dT_c/dp of the Heusler alloys Ni₂MnZ(Z = Al, Ga, In, Sn and Sb) has been obtained from the results of temperature dependence of initial permeability under pressure up to about 6 kbar. For all alloys the Curie temperatures increase linearly with increasing pressure at the rate of dT_c/d_p: +0.7 K/kbar for Ni₂MnAl, +1.0 K/kbar for Ni₂MnGa, +0.9 K/kbar for Ni₂MnIn, +1.4 K/kbar for Ni₂MnSn and +4.1 K/kbar for Ni₂MnSb. On the basis of these results, the interatomic dependence of the exchange interaction for Heusler alloys is discussed. The magnetic susceptibilities of those alloys are also reported.     

Formation of PdFe alloys on silica supported catalysts

Silica supported PdFe systems were studied with Pd∶Fe ratios from 5∶1 to 1∶10. Formation of α-iron and bimetallic particles was found during the 670 K hydrogen activation. The 5∶1 PdFe system of extreme catalytic activity consisted of disordered fcc PdFe bimetallic and Fe+2 components. No evidence for occurence of other bimetallic structures was found on catalysts even after 1120 K reduction.     

Effects of hydrogenation on the thermopowers of PdNi and PdCo alloys

The first thermopower data for Ni and Co impurities in β-PdH are reported. A contrast in the behavior for these impurities in Pd with their behavior in β-PdH is shown to provide a valuable measure of both the complexity and the extent of the ferromagnetic to dilute-magnetic-alloy conversion which occurs in these alloys as the H concentration increases.     

The Curie temperature of dilute ferromagnetic alloys near the percolation threshold

Upper limit for the Curie temperature T_c(x) of the dilute Heisenberg ferromagnet near percolation threshold is derived. Qualitative arguments are put forward that this limit gives a correct dependence of T_c on the concentration of magnetic atoms x.     

Microstructures and behavior near the Curie temperature of Gd and GdC alloys

Measurements have been made, in the neighbourhood of the Curie temperature, of heat capacities and electrical resistivity coefficients of Gd with controlled C impurities added. In the present communication, the microstructures of the GdC specimens are studied, both to explore the role of microstructure in critical behavior near magnetic transitions and to find a basis for anomalous behavior of the 1% C specimen. Relatively small changes in critical behavior among the alloys is accompanied by remarkably great differences in microstructure.     

Nuclear orientation study of low temperature magnetization of54Mn impurity in ferromagnetic PtFe alloys

Low-temperature nuclear orientation of⁵⁴Mn in ferromagnetic Pt₉₅Fe₅ and Pt₉₀Fe₁₀ hosts has been investigated in 12–50 mK temperature range at external magnetic fields up to 1.2 T. Temperature dependence of the effective magnetic field B_eff on Mn has been observed and interpreted as an effect of misalignment of Mn magnetic moments with respect to the macroscopic magnetization direction at host magnetic saturation.     

Effect of the pressure on Al-Si alloys

Abstract

The aim of this communication is to study the effect of the high hydrostatic pressure on the phase diagram of an AS12UN commercial alloy by DTA. The high pressure range is 0-360 MPa. Microphotographies of the recovered samples are compared with samples produced by dynamic high pressure in a die-casting set in a 4000 T press on a industrial plant.     

时效工艺对PtCo合金磁性能的影响

研究了时效温度对PtCo合金磁性能的影响.磁性能测量结果显示,随着时效温度的增加合金的剩磁逐渐减小,时效温度为690 ℃时,合金的矫顽力达到最大值.第一步时效处理后,合金的有序度S随时效温度的提高而增加,长程有序度的增加说明合金有序相的晶粒尺寸也在增加,这是合金矫顽力变化的根本原因.第二步时效处理后,PtCo合金的微观组织结构不发生改变,有序相的各向异性场增大,使合金的矫顽力增大. 关键词： 时效温度 PtCo合金 磁性能 有序度     

Chemical and structural disorder effects on the Curie temperature

The disorder effects on the Curie temperature of ferromagnetic and ferroelectric systems are studied by factorizing the spin–spin (or dipole–dipole) interaction into a chemical (on-site) and a structural (off-site) part. Assuming the statistical independence of the two contributions, the Curie temperature T_c is calculated in the limit of small disorder and in the mean-field approximation. The chemical disorder always enhances T_c . In the absence of spin waves (Ising-like systems), the structural disorder enhances T_c in turn. The only negative contribution to T_c is found in Heisenberg-like systems, and is ascribed to the interplay between structural disorder and spin waves. A comparison is made with other mean-field theories that adopt a different representation of the disorder. The application of the results obtained to real systems is considered, with special reference to recent experimental data on ferroelectric perovskites. An approximate expression, consistent with the mean-field approach, is suggested to estimate the relative weight of the chemical and structural disorder effects, even when an exact factorization is impossible, as is the case of the exchange interactions.     

Oxidation of iron,cobalt and nickel at the Curie temperature

The large differences in the logarithmic oxidation kinetics reported as a change in activation energy with the oxidation rate decreasing from ferromagnetic to paramagnetic states are discussed for iron, cobalt and nickel. The observed anomalies at the Curie point are explained by a mechanism which takes into account both surface-state charge at the metal/oxide interface and space charge in the growing oxide.     

Effect of Al concentration and hydrostatic pressure on the néel temperature of CrAl alloys

As determined from ? vs T measurements, the T_N vs Al content curve for CrAl alloys exhibits three distinct regions. The first region extends from 0 to about 2 at.% Al. Here the Néel temperature decreases linearly in two stages, with an abrupt change in slope occurring at about 1.2 at.% Al. From 2 to 4 at.% Al no T_N value could be defined from the resistivity measurements. Finally, at concentrations higher than 4 at.% Al, T_N was observed to increase with increasing Al content. The variation of dT_N/dp with Al concentration was found to be similar to the T_N curve in the first region. Exceptionally high values of dT_N/dp were found.     

Formation andstability of magnetic moment on isolated Rh impurities in dilute PtFe alloys

The local susceptibilities (T) of isolated Rh impurities in paramagnetic dilute Pt(Fe) alloys have been measured using the TDPAC method. The results show existence of 4d magnetic moment for Rh with reduced spin fluctuation temperatures (T _sf). It is suggested that moment stability scaled byT _sf is governed mainly by the interatomic d-d exchange interaction caused by induced ferromagnetic polarization of host d band electrons.     

Effect of pressure on the spin-Peierls temperature

Experiments have shown that a hydrostatic pressure increases the spin-Peierls transition temperature (T_c) in MEM-(TCNQ)₂ while a lowering is observed in TTF-CuBDT. A mean field theory, involving an anharmonic coupling between the distortion and the strain and a possibility for both structural and spin-Peierls instabilities is presented. From this theory, we conclude that, depending on the sign of the coupling energy between the dimerization soft mode and the strain, T_c can increase or decrease with pressure. Without this anharmonic coupling, T_c decreases with pressure.     

Effect of a random crystalline field on the Curie temperature of an anisotropic crystalline ferromagnet

We examine how substitution of nonmagnetic atoms of one species by another that leads to the emergence of a random crystalline field affects the Curie temperature T _C of an anisotropic crystalline ferromagnet. We study the case of low concentrations, in which individual substitutional impurities create isolated clusters of perturbed magnetic ions with additional easy-or hard-magnetization axes. Finally, we analyze the various sign relations among the parameters D and d of regular and impurity second-order anisotropies with Kramers (J=1) and non-Kramers (J=3/2) angular momenta, and show that usually the effects of a random crystalline field lower the Curie temperature as the concentration of the nonmagnetic impurity atoms increases. Zh. éksp. Teor. Fiz. 111, 127–143 (January 1997)