R.P. Pinto

Critical behaviour of transport and thermal properties in GdAl2 intermetallic compound

Abstract High-accuracy measurements of the electrical resistivity (ϱ), thermo-electric power (S), thermal conductivity (K), and the temperature derivatives dϱ/dT and dS/dT have been carried out in a high purity GdAl2 sample over the temperature range 70–320 K. Our results reveal for the first time the existence of a truly divergent behaviour in transport properties for GdAl2, with very sharp peaks both in dϱ/dT and dS/dT at the Curie point Tc = 164.8 K; a well developed minimum was also observed in K(T) at the transition point. The results are analyzed both within and outside the critical region. Using a high resolution differential thermal-analysis method, a very sharp anomaly was detected at the Curie point, indicative of a well defined λ-anomaly in the specific heat of GdAl2.

Electrical and thermoelectric transport properties of RZn and RCd compounds (R = Tb, Gd, Nd, Pr)

Abstract We present the results of an extended investigation of the transport properties of the CsCl structure intermetallic compounds GdZn, TbZn, GdCd, TbCd, PrCd and NdCd (all single crystals), using high resolution measurements of the electrical resistivity (ϱ, dϱ dT ) and the thermoelectric power (S, dS dT ) , from 4.2 up to 300 K. Both the resistivity and the thermoelectric power are separated in their magnetic and nonmagnetic contributions, and relevant information and parameters are obtained for the different scattering mechanisms. A detailed analysis of the data is presented in the vicinity of the different phase transitions present along the series: of magnetic ( T c , T N ), structural ( T ∗ , T ∗ L ) and spin reorientation types ( T sr ). A through comparison of the data with the available theoretical treatment to describe transport properties in magnetic metals is presented. For the case of the thermopower we show that the existing theories are inadequate to describe quantitatively, and in some cases even qualitatively, the main features associated with the magnetic transitions. Finally, new experimental evidence is given on the CsCl lattice instability in RCd intermetallic compounds.

Aspects on the critical behaviour of transport properties in ferromagnetic metals

Abstract New experimental results are presented to extend information into less well known aspects concerning the behaviour of transport properties in ferromagnetic systems. We study in particular the critical behaviour of the temperature derivative of the electrical resistivity (dϱ/d T ) in the vicinity of a spin-reorientation transition ( T s ). In addition, the extreme sensitivity of dϱ/d T is used to investigate the role of magnetic domains in the vicinity of a ferro-paramagnetic phase transition ( T c ).

Conditions for metastable thermal and magnetic domain structures and for observable oscillations in dρ/dT near an electrothermal instability

Experimental conditions for observing a new type of oscillations in the temperature derivative of the electrical resistivity near magnetic transition temperatures are discussed. An electrothermal instability arises due to a nonlinear electrical current‐voltage drop characteristics as in a ballast resistor. Inhomogeneous temperature distributions are found for various experimental conditions.

The onset of helimagnetism in Gd-Y alloys

Abstract We have measured the temperature dependence of the elastic constants, ultrasonic attenuation and resistivity in two single crystal Gd-Y alloys. An alloy with 70.3 at% Gd transforms to an ordered magnetic phase at 220 K and on further cooling to a ferromagnetic phase at 208 K. A second alloys with 68.9% Gd initially orders at 206 K before going ferromagnetic at 172 K. A study of the thermal hysteresis in the elastic properties indicates that the high temperature phase of the 70.3% Gd alloy is of ferromagnetic character while that of the 68.9% is a spiral spin antiferromagnetic. Similar conclusions have been reached from the transport property measurements. The anomalies at the Neel temperature in the 68.9% alloy are most unusual and indicate an interesting competition between the ferromagnetic and antiferromagnetic phases.

Transport properties and ultrasonic propagation in single crystal Gd65.4-Y34.6

Abstract Measurements have been carried out on the elastic constants, ultrasonic attenuation, resistivity, thermoelectric power and Hall effect of single crystals of a Gd 65.4 -Y 34.6 alloy over a temperature range from 300 down to 70 K. This temperature range allows the various magnetic phases of the alloy to be studied in detail. The alloy first orders at a Neel temperature of 200 K into a spiral spin antiferromagnetic phase which on further cooling transforms to a ferromagnetic phase at a Curie temperature of 132 K. The behaviour of both the elastic and the transport properties at the Neel temperature indicate the presence of unusual pretransitional effects.