RESISTIVITY OF THE REENTRANT SYSTEMS NiMn AND a-FeZr NEAR THE FERROMAGNETIC PHASE TRANSITION
نویسندگان
چکیده
We report on the temperature derivative of the resistivity close to the Curie temperature of the reentrant systems mMn 22 at % and a,&Zr 8 at % . Magnetic susceptibility results for the mMn system are also presented. Critical exponents could be deduced from the data. The critical behaviour of systems presenting quenched magnetic disorder is a topic of considerable current interest. In particular, the problem of the existence or not of a spin glass phase transition is still not completely elucidated [I]. Also, the nature of phase transitions in amorphous ferromagnets is noy yet well understood [2]. Another interesting case is represented by the so called magnetic reentrant systems, which first undergo a para-ferromagnetic transition followed by a transition to a spin glass-like state at lower temperatures. In this communication we focus on the critical behaviour at the Curie point of two reentrant-like systems, namely polycrystalline E M n 22 at % and amorphous FeZr 8 at % . Primarily, we study the temperature derivative of the resistivity, dp / dT, in the vicinity of T,. It is known that dp / dT presents the same critical behaviour as the magnetic specific heat [3]. For the NiMn system, we also report on detailed AC susceptiMlity memcrements close to Tc and on DC magnetization results between 4 K and 300 K. To obtain the NMn alloy in the reentrant state, the sample was submitted to an annealing of one hour at 900 OC, followed by 2 hours at 600 OC, then by a rapid quench to room temperature. The &Zr amorphous ribbon was prepared by melt-spinning as described earlier [4]. We perform accurate resistivity measurements for both samples using a standard AC technique. During the measurements, the temperature was allowed to drift very slowly. Around T, drift rates were typically of 100 mK/min, either for cooling or heating the samples. By measuring a large number of points, we could numerically determine the temperature derivative of the resistivity using a procedure described in reference [5]. Because of the weakness of the temperature dependence of the resistivity for both systems, the dp / dT points were obtained within relative uncertainties smaller than 2 % for XMn and 5 % for &Zr. Close to T,, the AC susceptibility results are accurate enough for critical behaviour studies [6]. The dp / dT results for E M n are shown in figure 1. I I I I I I 180 200 220 240 260 TEMPERATURE (K) Fig. 1. -temperature derivative of the resistivity for EMn 22 at % . The solid line corresponds to a fit obtained from equation (la) with a = -0.81 and T, = 228.6 K. The usual power law which describes the divergence of d p l dT in the critical region is given by [2]: -=A ' ( E . 1) + B+ (T > T,) dp (la) dT a * = A(b1-e' 1) + B(T < T,) (lb) dT a' where A and B are constants, E = (T T,) / Tc and the critical exponents a, a' are expected to be the same as those for the singular part of the specific heat [3]. However, we observe in figure 1 that for T < T, a complex feature is apparent in dp / dT, which prevents the analysis of these data with equation (lb). We are thus left with the paramagnetic regime to study the critical behaviour of dp / dT in EMn. A non linear least-square-fit of equation (la) with the points in the interval 0.005 < E < 0.13 gives T, = 228.6 f 0.3 K and = -0.81 f 0.05. In figure 2, we show AC susceptibility results close to T, of the same E M n sample. These results were analysed with xo = (E > 0). Noting that this formula, which represents the leading divergence in the suscepArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888518
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