M.E. Braga

Temperature dependence and critical behaviour of the thermoelectric power in ferromagnetic TbZn compound

High-accuracy data on the thermoelectric power (S) are obtained for ferromagnetic single crystals of TbZn (CsCl structure), over a wide range of temperatures (80-300K). The general temperature dependence S(T) is analysed in the context of s-f scattering models. Information is obtained on such parameters as the s-f exchange constant G, the Fermi level EF, and the effect of simultaneous spin disorder and phonon scattering on the thermopower. Experiment suggests the possible existence of extra S contributions in TbZn, besides that arising from s-f scattering. Finally, the critical behaviour of dS/dT near the Curie point (Tc=199.6K) is investigated in detail, and related to that previously observed in the temperature derivative of the electrical resistivity (d rho /dT) in TbZn.

Thermopower behavior in the Gd5(Si0.1Ge0.9)4 magnetocaloric compound from 4 to 300 K

Recent studies on the orthorhombic Gd5(Si0.1Ge0.9)4 compound show, upon heating, a ferromagnetic to antiferromagnetic-like (AFM*) transition at TS=78 K, coupled with a first-order structural martensitic transformation keeping the orthorhombic symmetry but producing a large increase in the interlayer Si(Ge) distances leading to covalent bond-pair breaking. A second-order AFM*→(paramagnetic)PM transition occurs at TN=125 K. We report thermopower (S) measurements for the Gd5(SixGe1−x)4 series, performed on an x=0.1 sample, from 4 to 300 K, with increasing and decreasing temperatures through successive thermal cycling. Resistivity measurements show a systematic increase in the residual resistivity and a dramatic change in the ρ(T) behavior upon thermal cycling. In spite of this, the thermopower data show a common intrinsic behavior both in the ferromagnetic phase (T<TS=78 K) and above ∼230 K, i.e., independent of the number of thermal cycles, increasing or decreasing temperature, and of the particular residua...

Transport properties and magnetic phases of NdRu2Si2

The tetragonal NdRu2Si2 compound exhibits a sequence of different magnetic phases below TN = 23.5 K: (i) first, a purely sine-wave modulation with q = (2π/a)(0.13, 0.13, 0), (ii) below T∗ = 15 K this structure gradually squares-up (3 q-harmonics) and (iii) below Tc ≈ 9 K the ferro-magnetic phase sets in. Accurate measurements of the electrical resistivity (p, dp/dT), thermoelectric power (S, dS/dT) and high resolution differential thermal analysis (DTA) are reported. Sharp singularities occur in dp/dT and dS/dT at Tc and TN. The onset of the square-modulated phase (T < 15 K) produces a pronounced decrease in p(T). The DTA data show that the TN-anomaly is of 2nd order. Also, for the order-order transition at Tc (modulated-ferro) the DTA data give no evidence for a latent heat. A theoretical account is given on the dominant effects observed in each magnetic phase. The anomalous behaviour of p and S at high temperature are here associated to crystal field effects.

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.

Transport and magnetic study of the spin reorientation transition in the Tb5(Si0.5Ge0.5)4 magnetocaloric compound

Detailed measurements of the electrical resistivity ?(T), thermopower S(T) and magnetization of Tb5(Si0.5Ge0.5)4 in the vicinity of the spin reorientation transitions observed in this compound are reported. Our results indicate a complex spin reorientation process associated with three different lattice sites occupied by the Tb ions. We identify two critical transition temperatures: one at TSR1 = 57?K, as previously reported, and a new one at TSR2 = 40?K. A simple model based on an approximate magnetic anisotropy energy is presented; it gives a satisfactory qualitative description of the main features of the reorientation processes.

Temperature dependence of the electrical resistivity and thermopower of U2Ni2In and Nd2Ni2Sn

Accurate measurements of the electrical resistivity (ρ,dρ/dT) and of the thermoelectric power (S,dS/dT) were performed in the ternary compounds U2Ni2In and Nd2Ni2Sn from 4 K to 250 K, and the results are compared with those previously obtained in U2Ni2Sn. The pair U2Ni2In/U2Ni2Sn enables us to study the influence of the p‐electron character (and other band effects related to the elements In/Sn) on the transport properties, whereas the pair Nd2Ni2Sn/U2Ni2Sn provides direct information on the role played by the 4f/5f electrons. All these compounds order initially in the antiferromagnetic state, exhibiting a characteristic minimum in dρ/dT at TN. In the U2Ni2In compound a drastic reduction is observed in ρ(T) slightly below TN, suggesting the coexistence of the antiferromagnetic state with a coherent Kondo effect when T≤0.8 TN. In Nd2Ni2Sn we observe two distinct phase transitions below TN, of first‐order character and likely associated with order‐order magnetic transitions. The anomalous behavior of ρ(T) in...

Electrical and thermal transport properties of PrCd single crystals

The temperature dependences of the electrical resistivity ( rho ), thermopower (S), thermal conductivity (K) and temperature derivatives d rho /dT and dS/dT have been measured in single crystals of PrCd, with the electric field and temperature gradient applied along the (110) direction. Three phase transitions have been identified: a structural transition at Ts=125K, where the high-temperature CsCl structure ceases to exist, an antiferromagnetic-paramagnetic transition at TN=40K and an unknown phase transition at T*=20K. Particular attention is focused on the details of the structural transition at Ts, namely on the peculiar irreversible behaviour observed there. An analysis is presented to identify and estimate different contributions to the transport coefficients of PrCd. A brief comment is also made on the problem of the lattice stability in CsCl structure intermetallic compounds.

TRANSPORT AND MAGNETIC PROPERTIES OF GDAG

Abstract Measurements of the electrical resistivity (ρ), thermoelectric power ( S ), temperature derivatives d ρ /d T and d S /d T , thermal conductivity ( K ) and differential thermal analysis, have been made in GdAg single crystals over the temperature range 10–300 K. The electrical resistivity is analysed in terms of impurity, phonon and magnetic contributions. The behaviour of ρ and d ρ /d T near T N = 132 K indicates that the magnetic transition is not driven by an electronic (Fermi level) mechanism. Spin fluctuation effects are observed up to T ∼ 1.5 T N , in the transport, magnetic and thermal properties. However, they affect d ρ /d T and d S /d T differently; whereas d ρ /d T has a maximum at T N , followed by a minimum at T ∼ 1.15 T N , d S /d T has only a minimum at T N . The thermal conductivity exhibits a minimum at T N , resulting from the competition between a decreasing electron mean free path (as T increases) and a linearly increasing electronic specific heat. At low temperatures the transport properties indicate an anomalous behaviour, marked by a sharp decrease in d ρ /d T starting around 35 K, and anomalously large thermopower values going through a maximum around 20 K ( S ∼ 18.5 μ V K −1 ).

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.

Electrical resistivity singularities and spin reorientation transitions in Pr2(Co1-xFex)17

The temperature- and field-induced spin reorientation transitions in uniaxial ferromagnetic Pr2(Co1-xFex)17 pseudo-compounds are studied using high-accuracy measurements of the electrical resistivity derivatives d rho /dT and d rho /dH. This technique avoids the need for single crystal samples. For x=0.2, a first-order spin reorientation transition is precisely located at Tsr=190K, where a sharp peak occurs in d rho /dT. The spontaneous magnetisation Ms then rotates abruptly from the basal plane to the c axis. A minute discontinuity was found in the resistivity at Tsr, Delta rho / rho approximately=0.003. For x=0, no spin reorientation transition is observed, confirming previous (single crystal) information that Ms always sits in the basal plane. For x=0.2 and T=77K, a field-induced spin reorientation transition was observed for a critical value H.c=5.0+or-0.2 kG, with a pronounced anomaly in d rho /dH. This just corresponds to the first-order Ms rotations inside those particular crystallites with c parallel to H (hard direction). Theoretical considerations are developed to characterise the anomalies associated with the spin reorientations in polycrystalline samples.