Remote Fermi level induced ferromagnetic transition in GdAl₂–Ag system

Abstract

We utilize an electrical set-up consisting of GdAl 2 ferromagnetic sample connected to a silver contact electrode where the a.c. voltage is applied only to the latter one. By measuring the a.c. resistivity of Ag electrode vs temperature in the vicinity of the Curie point we observe Fermi level induced phase transition in the Ag electron gas in the form of a kink. This measurement confirms the results anticipated earlier theoretically and obtained also experimentally for Cr -Cu, Gd -Ag, Cr -Ag, TiNi -Ag and CuZnSn -Ag systems.It is presently well established that the chemical potential of a solid possesses critical behaviour and in the case of second order phase transitions it manifests itself as a change in the slope at the critical point (cf. [1,2]). Such a behaviour of the chemical potential can practically be used to detect phase transitions in solids what has recently been shown in Refs [3,4]. According to this new method an investigated sample (exhibiting phase transition) is brought into an electrical contact with a small metallic (reference) electrode and placed into a thermostat. The reference electrode has to be selected in such a way that there are no phase transitions in the prescribed temperature range. The contact between the sample and the electrode causes a flux of electrons and after a thermal equilibrium is reached the chemical potentials of both constituents should be equal sampleWe apply a voltage U to the reference electrode in such a way that the electric current flows only through the latter one. We measure the resistivity of the reference electrode as function of temperature. The possibility to register kinks in the temperature dependence of the resistivity of the contact electrode connected with the investigated sample is simply a consequence of the chemical potential critical behaviour [1,2]. This is due to the fact that chemical potential enters the Kubo formula [5] for the electrical resistivity. Thus, the critical behaviour of the chemical potential (change in the slope) should influence the tempearture dependence of the reference electrode resistivity at the critical point in the form of a kink which can be registered. Using the contact electrode method [3,4] phase transitions (transformations) in Gd, Cr, TiNi and in CuZnSn have been investigated. Because of the novelty of this method further experiments are necessary to probe its applicability. This short communication is devoted to ferromagnetic-paramagnetic phase transition in the GdAl 2 sample.Crystalline GdAl 2 is metallic ferromagnet with MgCu 2 fcc structure and Curie temperature of about 170 K (see Ref.[6] for a review and papers cited therein). The GdAl 2 sample has been used in our experiment in a very similar way to Refs [3] and [4] (see Fig. 1 for details) where the contact electrode (Ag) was glued to the investigated sample. The a.c. voltage (100 Hz) has been applied to the reference

Chemical potential induced phase transition in the metallic electrode attached to the ferroelectric Pb5Ge3O11 single crystal

“…Experimentally, the kink effect is small but measurable and it occurs in each heating and cooling measurement cycle since the crossover in the resistance slope is repeatable. These features are quite similar to those obtained earlier for metallic samples (Gd, Cr, Gd 5 Si 4 , TiNi, CuAlNiTiMn) when copper or silver was used as the contact electrodes [15,17,18].…”

“…Till now, Refs. [15,17,18], the effect of the induced phase transition has been confirmed experimentally for metallic samples only i.e. for magnetic transitions in Gd, Cr, GdAl 2 and structural transitions in NiTi, CuZnSn when using various contact electrodes (Cu, Ag).…”

“…The results of the papers [15,17,18] indicate that the critical temperatures of the metallic samples can be detected in this way. However, this conclusion asks another question concerning the applicability of the method to a broader class of materials like non-metallic solids.…”

“…It is worthwhile to notice that the observed relative effect of the resistance deviation from the linear dependence is small, in accordance with Refs. [15,17,18].…”

Thermodynamical relations at characteristic points

“…A special attention should, however, be paid to the method, which allows measurement of the temperature dependence of the chemical potential of the electronic subsystem of solids by using the work function method [62][63][64][65], or the method of the galvanic cell [66,67], which works well for metallic samples. Another interesting method is the contact electrode method, which uses the chemical potential induced phase transition in the electron gas of the metallic contact electrode attached to the investigated sample [68][69][70][71][72]. The papers [62][63][64][65][66][67][68][69][70][71][72] experimentally demonstrate that the temperature behaviour of the chemical potential can successfully be applied to detect phase transitions in solids, and therefore, it is necessary to include it into the calculations presented in this article.…”