J.J.M. Franse

Magnetic properties of ternary rare-earth compounds of the type R2Fe14B

Abstract Ternary tetragonal compounds of the composition R 2 Fe 14 B were observed for R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The lattice constants and the X-ray density of these compounds were determined. Also determined were the magnetic properties, comprising the temperature dependence of the magnetization in the range 4.2–700 K and the field dependence of the magnetization at 4.2 K in fields up to 20 T. These latter measurements were made in two mutually perpendicular directions, making it possible to determine the anisotropy fields. The magnetocrystalline anisotropy was found to consist of contributions due to the Fe and rare-earth sublattice, respectively.

Magnetic properties of UxPty compounds

Abstract Magnetization, magnetic susceptibility and specific heat studies have been performed on a series of uranium-platinum compounds. The magnetic ordr of UPt is confirmed, whereas in UPt 2 and UPt 5 no magnetic order is observed. An antiferromagnetic type of order, suggested for UPt 3 by a peak in the susceptibility near 19 K and by the magnetic transition in the magnetization curve at 4.2 K near 19 T is not supported by the specific heat data in which no anomaly near 19 K was found. Additional magnetization and susceptibility measurements on a single-crystalline sample exclude a simple type of antiferromagnetic order. The low-temperature specific heat of UPt 3 is rather similar to that of the “spin-fluctuation” compound UAl 2 .

Magnetic and crystallographic properties of ternary rare earth compounds of the type R2Co14B

Abstract Ternary tetragonal compounds of the composition R 2 Fe 14 C were observed for R = Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu. The lattice constants of these compounds were determined. Also determined were the magnetic properties, comprising the temperature dependence of the magnetization in the range 4.2–700 K and the field dependence of the magnetization at 4.2 K in fields up to 35 T. These latter measurements were made on magnetically-a ligned powders with the field parallel and perpendicular to the alignment direction, making it possible to determine the anisotropy fields. The magnetocrystalline anisotropy, consisting of contributions due to the Fe and rare-earth sublattice, was found to be of comparable magnitude in R 2 Fe 14 C and R 2 Fe 14 B.

Strength of the rare-earth-transition-metal exchange coupling in hard magnetic materials, an experimental approach based on high-field magnetisation measurements: Application to Er2Fe14B

Abstract A new experimental technique is presented to determine the intersublattice molecular-field coefficient, n RT , in heavy rare-earth-transition-metal intermetallic compounds. The technique is based on high-field magnetisation measurements on finely powdered polycrystalline material that is free to rotate in the sampleholder. Experimental results are reported for a number of Er 2 Fe 14− x Mn x B compounds. The strength of the RT exchange coupling is not affected by the Mn substitution, and a value of 0.445 Tkg/Am 2 has been deduced for the coefficient n RT .

Heavy fermions in Kondo lattice compounds (invited)

Even in the absence of both magnetic order and superconductivity, Ce‐based Kondo lattice systems show distinct anomalies in the low‐temperature specific heat and thermopower. These features can be understood, in the frame of a microscopic treatment of the asymmetric Anderson model, by taking into account the periodicity of the Ce sites. A substantial variation in the observed phenomena between CeAl3 and normal (n) state CeCu2Si2 on the one hand as well as CeRu2Si2 and CeCu6 on the other highlight the importance of realistic band structure calculations for the quasiparticles (‘‘heavy fermions’’) in these different compounds. The temperature dependence of the upper critical magnetic field Bc2(T) for the heavy‐fermion superconductor CeCu2Si2 (Tc≂0.6 K) shows a flat maximum at ∼0.2 Tc, which is ascribed to coherence‐derived structure in the n‐state quasiparticle density of states. A dc Josephson effect with a critical pair current of ordinary size is observed for the first time on a weak link between CeCu2Si2...

UPt3, heavy fermions and superconductivity

Abstract This review considers studies of the normal and superconducting state of the heavy-fermion superconductor UPt 3 ( γ = 422 mJ/K 2 molU, T c = 0.5 K). We discuss a large variety of experiments, most of them performed on good quality single crystals. Concerning the normal state we concentrate on the magnetic, thermal, elastic and transport properties. These properties can be fairly described in a spin-fluctuation model. Experiments concerning the superconducting phase reveal several unusual features. The possibility of an unconventional pairing mechanism is discussed. On alloying UPt 3 with Pd, additional anomalies are observed, suggesting that UPt 3 is close to an antiferromagnetic instability.

Systematics across the UTX series (T = Ru, Co, Ni; X = Al,Ga, Sn) of high-field and low-temperature properties of non-ferromagnetic compounds

Abstract Results are presented of magnetic and specific-heat studies performed on some equiatomic compounds of the series U TX with a special emphasis on non-ferromagnetic UNiAl, UCoAl, URuA1 and URuGa. The occurrence of magnetism in these compounds is attributed mainly to the 5f electrons of uranium. The systematics of the ground-state properties across the series is discussed with respect to delocalization mechanisms of the 5f electrons. The possible role of spin fluctuations in the low-temperature behaviour of UCoAl, URuAl and URuGa is indicated by magnetic measurements. The compound with the highest γ value, UNiAl (γ = 167 mJ/mol K 2 ), behaves as an itinerant antiferromagnet.

Resistivity of single-crystalline UPt3 and its pressure dependence; Interpretation by a spin-fluctuation model

Abstract Resistivity measurements on a series of uranium-platinum compounds were carried out in the range 1.3 up to 300 K and pressures up to 5 kbar. The resistivity of single-crystalline UPt3 is strongly anisotropic and points to pronounced spin-fluctuation effects that are partly suppressed at a pressure of 4 kbar. In paramagnetic UPt2 and UPt5 spin-fluctuation effects are less pronounced, whereas in ferromagnetic UPt the spin-disorder scatter is dominant above 27 K.

Exchange interactions in rare earth—transition metal compounds

Abstract Using the strenght of R—R interactions deduced from the ordering temperatures of RNi2 compounds, the R—T exchange-coupling parameter (ART) in RmTn ( R = rare earth, T = Co or Fe, m / n = 1 2 ; 1 3 ; 6 23 ; 1 5 ; 2 17 ; 1 12 ) and RmTnBk (m / n / k = 1/4/1; 2/14/1; 1/12/6) systems has been evaluated from analyses of Curie temperatures. Whereas ARR, as deduced from the RNi2 compounds, increases by almost an order of magnitude in a given series, ART is enhanced by a factor of only about three from the heavy to the light rare earth compounds. These phenomena, already reported in literature, are based on 4f–5d exchange interactions at R sites. In addition, for a given rare earth element, going from T-poor to T-rich compounds, a tendency to decrease is found for both ARCo and ARFe; this variation is discussed in terms of 3d–5d hybridization.

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.