Transport properties and point-contact spectra of RECu6 with RE=La,Ce and Pr

Measurements of the current-voltage characteristics on point-contacts, of the thermopower and the thermal conductivity between 1.3 and 300 K and of the electrical resistivity between 4.2 and 900 K of polycrystalline samples of ReCu₆ (RE=La, Ce, Pr, Gd) are presented. The anomalies of the heavy fermion system CeCu₆ are strong compared to the other systems, but the properties of LaCu₆ and PrCu₆ are also unusual, possibly caused by the complicated crystal structure and, for PrCu₆, by crystal field effects.     

Magnetic and related properties of the solid solution CeCuxGa4−x

Physical properties of polycrystalline samples of CeCu_xGa_4−x (x = 0.2–1.4), crystallizing in the tetragonal BaAl₄-type structure (space group I 4/mmm), were studied by means of X-ray powder diffraction, magnetization, specific heat, electrical resistivity and magnetoresistivity measurements in wide temperature and magnetic fields ranges. The unit-cell volume of the system was found to decrease with increasing x (in total by about 4%) but the magnetic moments of Ce³⁺ ions remain localized in the whole x-range studied. The alloys exhibit ferromagnetic order at low temperatures, which manifests itself as distinct and relatively sharp anomalies in all the temperature characteristics measured. The ordering temperature decreases with increasing the Cu content from 5.5(1) K for x = 0.2 down to 1.35(5) K for x = 1.4, and the electrical transport properties of the system show some features characteristic of Kondo lattices.     

Magnetic behavior of the alloys CeCuyGa4−y and Ce1−xLaxCuGa3

The results of magnetic susceptibility (x), electrical resistivity (ρ) and heat-capacity (C) measurements of the alloys CeCu_yGa_4−y (y = 0.5, 1.0, 1.25 and 1.5) and Ce_1−xLa_xCuGa₃ (x = 0.2 and 0.9), crystallizing in the BaAl₄-type tetragonal structure, are reported. The Kondo effect tends to dominate with increasing y. All the data for the y = 0.5 alloy are consistent with the onset of ferromagnetic ordering at 6 K. For y = 1.0 and 1.25, though ρ gradually drops below 6 K, the C data do not show the existence of magnetic ordering above 2 K; presumably, for y = 1.0, 1.25 and 1.5, long-range magnetic ordering sets in below 2 K. Above all, C tends to increase with decreasing temperature before the onset of long range magnetic ordering and the values of C/T per Ce mol are considerably diminished for x = 0.9. It is proposed that this C/T enhancement lies in the magnetic precursor effects and not on the heavy-fermion behaviour.     

Magnetism in quasibinary systems based on the valence-unstable compound CeNi

Magnetic ordering in solid solutions of Ce _x (Gd,Pr,Nd,La)_1-x Ni is studied by measuring the DC magnetization and the AC susceptibility in the temperature range of 1.8–300 K. The valence state of ceriumions in Ce _x (Gd,Pr,Nd,La)_1-x Ni quasibinary systems is studied based on X-ray absorption spectra measured at synchrotron-radiation sources in the temperature range of 5–300 K. It is shown that chemical pressure and lowering the temperature help heighten the degree of delocalization of the 4f electrons of cerium in Cex(Gd, Nd, Pr)_1-x Ni systems. It is found that the substitution of magnetic ions (Gd, Pr, and Nd) with cerium results in significantly weaker magnetic-ordering suppression than the substitution of these ions with lanthanum at equal concentrations. The obtained data reveal the strong influence of cerium electrons on localized magnetism in the studied compounds. This effect is most probably associated with the contribution of partially delocalized 4f electrons of cerium to the exchange interaction.     

Structural and magnetic properties of Ce(Cu,Ni)2 pseudobinary phases

The stability of the pseudobinary phases obtained by partial substitution of Ni with Cu, Ag or Au in the cubic Laves phase CeNi₂, or by partial substitution of Cu with Ni in the orthorhombic compound CeCu₂ has been investigated. No solid solubility of Cu, Ag and Au in the CeNi₂ cubic lattice could be detected. In contrast, pseudobinary orthorhombic phases, corresponding to the formula CeCus_2?xNi_x, exist over the range x = 0 to 1. For these phases, crystal structures and magnetic properties have been determined. In every case, the electronic configuration of cerium appears to be unaffected to the Ni content, and corresponds to the trivalent state. The Ni atoms are in a non-magnetic state. The magnetic susceptibility of CeCu₂ in the region from 530 to 900 K shows anomalous behaviour, which should be due to a metamagnetic transition.     

Magnetic and57Fe Mössbauer studies of intermetallic compounds R2 (Fe1−x−y Ni x Co y )17 (R=Y,Gd,Tb,x=0,0.10, 0.20,y=0,0.05, 0.10)

Mössbauer studies of R₂(Fe_1?x?y Ni _x Co _y )₁₇ showed that the transferred hyperfine field at Fe nuclei due to magnetic rare earth (R) atoms is about one Tesla. Magnetic moments of the R atoms were determined from magnetic measurements as μTb=8.52μB, μGd=6.22μB. The mixed substitution of Ni and Co for Fe leads to an increase of the ordering temperature. A slight preference occupancy for Fe was observed involving the dumbbell shaped f or c site. The substitution effects of Ni and Co on the hyperfine field of f or c site, the average hyperfine field and the average isomer shift were also discussed.     

Heavy fermion behaviour of the nonmagnetic CeCu4Al compound

We present measurements of the temperature dependence of the electrical resistivity, the thermopower and the specific heat of the hexagonal compound CeCu₄Al. At high temperatures, the electrical resistivity is characterized by a nearly temperature independent behaviour, followed by a continuous increase below 100K. No maximum has been found down to 1.7 K. The thermopower shows a positive maximum at about 30 K. As in CeCu₆ no negative values are observable in the range from 4.2 K up to a room temperature. The specific heat data between 7 and 15 K reveal a γ value around 280 mJ mol^-1 K^-2. Below this temperature range the specific heat c_p/T shows a rapid rise and crosses the value of 1 J mol^-1 K^-2 at about 1.45 K.     

Evolution of 4f heavy fermion state from 5f heavy fermion state in U1−xCexPd2Al3 system

We report the effect of Ce substitution for U in the heavy-electron antiferromagnetic superconductor UPd₂Al₃. CePd₂Al₃ is an established non-superconducting heavy-electron system which undergoes antiferromagnetic ordering below 2.8 K. Thus studies on U_1−xCe_xPd₂Al₃ system provide a unique opportunity to observe the evolution from a 4f heavy-electron state to a 5f heavy-electron state. We have measured the resistivity and magnetic susceptibility from 1.5 to 300 K and the heat capacity from 2 to 20 K for several U_1−xCe_xPd₂Al₃ samples. Our studies show that the antiferromagnetic (AF) ordering temperature (T_N) of U_1−xCe_xPd₂Al₃ does not decrease monotonically from T_N = 14 K for UPd₂Al₃ to T_N = 2.8 K for CePd₂Al₃ but rather shows a local maximum of 5 K near x = 0.4.     

Magnetic properties of off-stoichiometric R2Co3Zn14 (R=Y,Gd) single crystals

Single crystal X-ray diffraction data indicate that the R₂Co₃Zn₁₄ (R=Gd, Y) phase crystallizes non-stoichiometrically with a mixed occupancy of Co/Zn atoms on the 12-coordinated transition metal site and one of the three zinc sites. The crystals are rhombohedral with R-3m space group. Magnetization measurements provide no evidence of localized 3d electron moment in Y₂Co_2.3Zn_14.7 which is non-magnetic down to 1.8 K. Thermodynamic and transport measurements on two Gd₂Co_3+xZn_14−x crystals reveal that the extra cobalt influences temperature below which the samples enter into an antiferromagnetic state: T_N=31.5(3) K for Gd₂Co₃Zn₁₄ and 28(1) K for Gd₂Co_4.2Zn_12.8. A lower magnetic ordering temperature of T_mag=6.0(2) K is common in both Gd samples.     

Susceptibility measurements on some R5In3 compounds

The magnetic susceptibility of some rare earth-indium compounds has been measured in the temperature range 4.2–300 K under a constant field of 1 kOe. The compounds with R = Gd, Tb and Dy are antiferromagnetic, having Néel temperatures between 4.2 and 78 K ; those with R = Ho, Er and Tm seem to be ferromagnetic with ordering temperatures probably below 4.2 K. Y₅In₃ presents a temperature independent susceptibility.     

Electronic resistivity and deffect structure of β-LiAl as a function of temperature and composition

The electrical resistivity of β-Li_xAl_1?x has been measured from 10 K to 290 K in single crystal samples for four different concentrations, x=0.480, 0.486, 0.500 and 0.525. An anomaly at 100 K is observed for the three lowest Li concentrations. This anomaly is associated with an ordering of constitutional Li vacancies. Above 100 K the temperature dependent part of the resistivity is independent of the Li concentration. However, the residual resistivity increases very rapidly with increasing Li concentration, becoming ～ 75 μΩ-cm for x=0.525. These results suggest that the two defect model of β-LiAl may be incomplete.     

Magnetic properties of RNi5−xCux intermetallics

The magnetic properties have been studied for the series of RNi_5−xCu_x intermetallics with R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu; x  ?2.5. Compositional dependences of magnetic susceptibility for the Pauli paramagnets (R=Y, La, Ce, Lu) and the Curie temperature for ferromagnets (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm) have maximum at x=0.2–0.4$x=0.2–0.4$ and 1, respectively. The substitution of Cu for Ni is accompanied by decreasing spontaneous magnetic moment and increasing coercive force of all ferromagnetic RNi_5−xCu_x but GdNi_5−xCu_x. These results are explained in the frame of band magnetism, random local crystal field, and domain wall pinning theories.     

Magnetic ordering in CeMnCuSi2

Temperature and field-dependent magnetization measurements on polycrystalline CeMnCuSi₂ reveal that the Mn moments in this compound exhibit ordering with a ferromagnetic (FM) component ordered instead of the previously reported purely antiferromagnetic (AFM) ordering. The FM ordering temperature, T_c, is about 120 K and almost unchanged with external fields up to 50 kOe. Furthermore, an AFM component (such as in a canted spin structure) is observed to be present in this phase, and its orientation is modified rapidly by the external magnetic field. The Ce L₃-edge X-ray absorption result shows that the Ce ions in this compound are nearly trivalent, very similar to that in the heavy fermion system CeCu₂Si₂. Large thermomagnetic irreversibility is observed between the zero-field-cooled (ZFC) and field-cooled (FC) M(T) curves below T_c indicating strong magnetocrystalline anisotropy in the ordered phase. At 5 K, a metamagnetic-type transition is observed to occur at a critical field of about 8 kOe, and this critical field decreases with increasing temperature. The FM ordering of the Mn moments in CeMnCuSi₂ is consistent with the value of the intralayer Mn–Mn distance R^a_Mn–Mn=2.890 Å, which is greater than the critical value 2.865 Å for FM ordering. Finally, a magnetic phase diagram is constructed for CeMnCuSi₂.     

Ferromagnetic rare-earth and diamagnetic iron sublattices in ternary RFe0.67Ge1.33 type compounds

The magnetic properties of RM_2?xSi_x and RM_2?xGe_x where R = La, Ce, Nd, Sm, Eu and Gd and M = Fe, Co, Ni and Ag have been measured. It has been found that for samples of x = 0.4 both R and M sublattices are paramagnetically above 4.2δ K in the high spin state, whereas, if x = 0.67, R is magnetically ordered whereas M is diamagnetic.     

Magnetic and electrical properties of amorphous PdCoP alloys

Magnetic susceptibility and electrical resistivity measurements were performed (Pd_100?xCo_x)₈₀P₂₀ alloys where 15 < x < 50. The magnetic properties show that these alloys undergo a ferromagnetic transition between 272 and 399 K as the cobalt concentration increases from 15 to 50 atomic %. Below 20 atomic % Co the short-range exchange interactions which produce the ferromagnetism are unable to establish a long-range magnetic order and a peak in the magnetization shows up at the lowest temperature range under an applied field of 6.0 kOe. The electrical resistivity of these alloys has been measured from 4.2 K up to the vinicity of the melting point (900 K). The electrical resistivity data could be interpreted by the coexistence fo a Kondo-like minimum and ferromagnetism. The minimum becomes less important as the transition metal concentration increases. The coefficients of In T and T² become smaller and concentration dependent. The spin ordering in such alloys can be simulated as either the ordering due to an applied “external field” or as an increase in “internal fields”. These are due to an increase in transition metal concentration. The negative magnetoresistivity is a strong indication of the existence of localized moment.     

Magnetic properties of the R1+eFe4B4 compounds (R = rare earth) from magnetization and Mössbauer measurements

The magnetic properties of rare earth-iron-boron alloys with composition R_1+eFe₄B₄ have been determined using Mössbauer and magnetization measurements. Magnetic ordering occurs at temperatures between 4.2 and 25 K for the compounds with R = Pr, Nd, Sm, Gd, Tb, Dy, Ho. The Curie temperature scales very well with the de Gennes factor for the heavy rare earth members of the series, while significant deviations are observed for the light rare earths indicating the presence of strong CEF effects. The absence of magnetic hyperfine splitting even at 4.2 K indicates that the Fe ion has a zero magnetic moment. This is confirmed by Mössbauer spectra in an applied magnetic field.     

Magnetic properties of some RPd2Si2 compounds (R=Gd,Tb, Dy,Ho and Er)

The magnetic susceptibility of the ternary compounds, RPd₂Si₂ (where R=Gd, Tb, Dy, Ho and Er) has been measured. GdPd₂Si₂ order antiferromagnetically at 13 and 20 K respectively; the rest of the compounds do not show clear ordering down to 4.2 K. Palladium carries no moment in these compounds. The De Gennes formula is not obeyed indicating that the exchange interaction between the 4f moments via conduction electrons is not isotropic     

Magnetic susceptibility and electrical resistivity of CexY1−xAl2

The magnetic susceptibility and the electrical resistivity have been measured for Ce_xY_1−xAl₂ (x = 0, 0.05, 0.15, 0.25) between 4.2°K and 300°K. _x3kT vanishes at low temperature. We find a minimum in resistivity at 60°K (x = 5) and 68°K (x = 0.25).     

Influence of V substitution for Fe on the transport and magnetic properties of Sr2FeMoO6

We have investigated the crystal structure, magnetization and magnetoresistance of the double perovskite compounds Sr₂(Fe_1−xV_x)MoO₆ (0≤x≤0.1). The lattice constants and the cation ordering decrease monotonously with the V content. The Curie temperature, saturation magnetization and low field magnetoresistance of the compounds decrease with increasing x due to the reduced degree of ordering. The resistivity of Sr₂FeMoO₆ and lightly doped samples shows semiconductive behavior, while the samples with higher doping levels exhibit a semiconductor-metal transition around 80 K.