The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(ε). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

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A very small capacitive sensor for measuring thermal expansion and magnetostriction of small and irregular shaped samples has been developed. A capacitive method with tilted plates is used. The tilted plate capacitance formula is used for the calculation of the capacitor gap, the calibration is performed by measuring the signal of a standard material. The active length of the sample can be less than 1 mm. The absolute resolution is about 1 Å. All mechanical connections of the dilatometer are carried out by tiny CuBe springs, enabling the small force on the sample to be adjusted ͑50-500 mN͒ and no additional sample fixing is necessary. The cell has been tested in the temperature range 0.3-200 K and in static magnetic fields up to 15 T. The zero signal of the dilatometer has been determined by measuring a silver sample. The correct operation and reproducibility has been verified by measuring the thermal expansion of Cu. The thermal expansion and magnetostriction of a DyCu 2 single crystal has been determined. The advantage of this method compared to specific heat measurements is that a large temperature range can be covered with one equipment. This high static and dynamic range of sample length, temperature, and magnetic field suggests a number of possible applications, like the investigation of crystal field effects on the magnetoelastic properties of single crystals or structural phase transitions.

The hansport properties of intermetallic compounds RCo2 (R = Pr, Nd, Sm, Gd-Lu, Sc and U) in the temperahue range f" 4.2 up to 1000 K were studied and different contributions were determined. In the non-magnetic compounds the spin fluchlation conhibution to the elechical resistivity, p,f U ( T / Z f ) z , and themal resistivity. Wsf c( (T/Trf). were found to follow Ihe theoretical predictions. The spin disorder wnuibution arising f" the scattering of conduction elecImns on 4f moments was deduced and found to be proportional to the de Gennes factor. It is shown that the spin fluctuations m responsible for the low-temperame features of the kmperature-dependent thermopower, whereas the high-temperarure behaviour is mainly determined by the density of states function.In the paramagnetic region near the Curie tempen", the critical fluctuations cause a considerable enhancement of the electrical resistivity of the magnetic RCq compounds, which is manifested by pronounced upturns in p versus T dependences.[9, 101. In the following the most important results are summarized.Low temperatures, T c l;f:Electrical resistivity: psf = AT2, with A proportional to TX7'. Magnetoresistance: ( A p / p ) s f is negative and proportional to the square of the appliedThermal resistivity: Wsf = cT, with c proportional to q:'.Thennopower: the situation is not yet clear. In some calculations a minimum below Elevated temperatures:Electrical resistivity: psj saturates somewhat above G,.. According to [9], for high values of the Stoner enhancement factor the resistivity can go through a maximum above i " , followed by a negative slope towards the high-temperature region.Thermal resistivity: Wsf passes a maximum around Tsf and decreases proportional to T-' at higher temperatures. magnetic field. cf has been predicted 1111.. .

The thermal expansion and anisotropic magnetomiction of the R C q Laves phases were studied in the temperature range 4-500 K using the x-ray powder diffraction methcd.In the heavy RCo2 the magnetic moment of the itinerant d electron subsystem derived imm the magnetovolume effect was found to fit well with the magnetization curve of YCo2. A pronounced paraprocess above the metamagnetic m i t i o n has been observed when increasing the f-d exchange field. "%e type and temperature variation of the distortion of the njbic unit cell of the t h ~ compounds PrCol, NdCo2 and SmCoz have been studied in detail. The corresponding magnetostriction ~nslants 1111 or AIM were calculated. At 4 K the following values have been obtained: R c 0 2 . temgonai distortion. easy axes (100). AIM = -3.4 x SmCq, rhombohedral distortion, easy axes (Ill), A l l , r -4.6 x NdC4, teuagonal distortion and eaxy axes (100) for 42 K c I e T,. orthorhombic distortion and easy axes (110) for T c 42 K with 111111 = 1.9 x aid AIM = -4.0 x IO-' .

Studies of the electrical resistivity were carried out under applied hydrostatic pressures up to 20 kbar and in the temperature range from 1.5 K to room temperature. The pressure dependence of the magnetic-ordering temperatures and the suppression of spin fluctuations in the paramagnetic temperature regime have been deduced from the variation of characteristic features in (T, P). Grüneisen parameters of the magnetic-ordering temperature T C and the spin-fluctuation temperature T SF have been obtained and discussed with respect to a pressure-induced destabilization of the itinerant d subsystem. Finally, as the measurements are performed under hydrostatic conditions, a possible change in the character of the magnetic phase transition from first towards second order has been investigated. ͓S0163-1829͑98͒02405-9͔

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