T.D. Hien

Specific heat and thermal expansion in the heavy RECu2 compounds

Abstract Specific heat and thermal expansion measurements in the heavy RECu2 compounds reveal several magnetic transitions in most of the studied compounds. Broad Schottky-type anomalies are observed above and below the Neel temperature. For ErCu2 the magnetic entropy, the magnetic volume, the crystal field splitting and its relative volume dependence have been determined.

The magnetic behaviour of rare-earth—transition metal compounds

Abstract The basic exchange interactions will be discussed for a variety of (R, T) compounds, in particular RT 2 (R: rare earth, or Y; T: transition metal). A magnetic phase diagram for (R, Y)Co 2 compounds is derived. The combined effect of the volume and the molecular field caused by the rare-earth spins is demonstrated on R(Co, Fe) 2 , (R, Y)Co 2 , R(Co, Al) 2 and R(Co, Cu) 2 , with an emphasis on the metamagnetic transition in the 3d-subsystem.

Electronic and magnetic properties of ErxY1-xCo2 compounds

Ferromagnetic compounds of the series ErxY1-xCo2 (x=1, 0.8, 0.7, 0.6) have been investigated in specific-heat, thermal-expansion, magnetisation, AC susceptibility and resistivity experiments. The magnetic moment on the Co atoms decreases with decreasing Er content, as does the Curie temperature. The character of the magnetic transition changes from first order to second order around x=0.7. This change is compared with the similar one observed in other RExY1-xCO2 compounds with RE-Gd, Tb, Dy and is discussed in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d electrons. The spontaneous volume magnetostriction at zero temperature, the magnetic contribution to the specific heat and the magnetic entropy below the magnetic ordering temperature are determined. The experimental results for these quantities confirm the induced character of the cobalt magnetic moment in these compounds.

Magnetization and AC susceptibility of TbxY1-xCo2 compounds

Abstract Magnetization and ac susceptibility measurements have been performed on Tb x Y 1- x Co 2 compounds. Samples with x > 0.1 order ferromagnetically below room temperature. The cobalt magnetic moments in these compounds are induced by the internal magnetic field exerted by the terbium moments on the cobalt atoms. The ac susceptibility measurements indicate a change from second order to first order in the ferromagnetic transition for samples with x -values equal or smaller than 0.5.

The magnetic phase transitions in (Tb, Ho)Co2 and (Tb, Y)Co2 compounds

Abstract The type of the magnetic phase transitions in a series of TbxHo1−xCo2 and TbxY1−xCo2 compounds is studied by means of magnetization, electrical-resistivity and specific-heat measurements. In both systems the TC values decrease with decreasing Tb content. At about x = 0.7 the type of the magnetic transition changes from second order to first order. An explanation is offered in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d itinerant electrons.

The magnetic phase transitions in R(Co, Al)2 compounds (R: Dy, Ho, Er)

Abstract The type of the magnetic phase transition in the R(Co1-xAlx)2 (R=Dy, Ho, Er) compounds is studied by means of magnetization, thermal-expansion and electrical-resistivity measurements. With increasing Al content, a change from a first-order to a second-order transition is observed, at x≃0.075, 0.075 and 0.025 for Er, Ho and Dy, respectively. An explanation is offered in terms of the Inoue-Shimizu model by introducing a concentration dependence of T3, the temperature at which a3(T) changes sign (a3 is the coefficient of the M4d-term in the expansion of the free energy of the d-subsystem). In fact, T3 is taken proportional to Tm, the temperature at which the susceptibility of the corresponding Y(Co, Al)2 shows a maximum.

Specific heat and thermal expansion in GdxY1-xCu2

Specific heat and thermal expansion experiments have been performed on GdxY1-xCu2 compounds between 1.2K and 50K and from 1.4K up to 77K, respectively. The specific heat data are analysed in an electronic, lattice and magnetic part resulting in a gradual variation of the Debye temperature with composition and in a magnetic entropy proportional to the number of Gd atoms. From the thermal expansion data below TN a magnetic contribution to the volume at zero temperature of negative sign is deduced. The magnetic volume is roughly proportional to x2 and is attributed to the loss of short-range polarisation at the antiferromagnetic spin configuration. By comparing the specific heat and thermal expansion data, the electronic, lattice and magnetic Gruneisen parameters have been determined.

Spin reorientation phenomena in Dy1-xYxFe11Ti alloys

Abstract Spin reorientation phenomena in the Dy 1− x Y x Fe 11 Ti( x = 0, 0.2, 0.4, 0.6, 0.8 and 1) compounds have been studied by ac susceptibility and magnetization measurements on bulk and magnetically aligned powder samples. The observed concentration dependence of the spin reorientation temperatures and the temperature dependence of the cone structure of this series of compounds have been analyzed on the basis of the crystal field model. A set of crystal field parameters is proposed.

Crystal-field effects in ErCu2 studied in specific heat and thermal expansion measurements

Abstract Thermal expansion and specific heat measurements on ErCu 2 reveal a crystal-field splitting between the lowest two doublets of 76 K. Experimental values for the relative volume dependence of this energy splitting are compared with the results from crystal-field calculations.

Effect of pressure on the Néel temperature of YMn2: resistivity of Y(Mn1−xCux)2

Abstract Resistivity measurements have been carried out for Y(Mn1−xCux)2 compounds with values of x of 0, 0.04, 0.07, 0.10, 0.20 and 0.30 in the temperature range from 4.2 up to 300 K at zero pressure and for YMn2 under a pressure of 2 kbar. TN has been found to decrease rapidly by applying external pressure as well as by applying a chemical pressure caused by a partial substitution of Mn atoms by Cu atoms.