The ferro-paramagnetic phase transition in dilute Cr-Ni base alloys is investigated through a qualitative study of their thermal conductivity as a function of temperature in the near vicinity of the transition. A single apparatus was used to measure the thermal and electrical conductivities and the specific heat of three thin rod samples having Cr concentrations of 0.12, 0.51 and 1.13 at. %.A voltage-temperature (V-0) realign was applied to measure the ratio between the thermal and electrical conductivities. The thermal conductivity data were then evaluated by using the electrical resistivity results obtained from the measured current-voltage (I-V) characteristic curves. A model describing the effect of Cr concentration on the I-V curve is proposed and tested. The effect of Cr concentration on specific heat is also presented.There have recently been several experimental and theoretical investigation on the nature of the magnetic phase transition in ferromagnetic metals and alloys. The study of transport properties such as electrical and thermal eonductivities and thermoelectric power has a good deal of attention [1][2][3][4][5]. In the present work the ratio between thermal and electrical conductivities (K/o) has been measured by the direct passage of an electric current through the specimen and by making use of V-0 relation.The electrical resistivity (p) is evaluated directly from the I-V characteristic curves. The obtained data agree quite well with those determined previously for the same samples by using the four-probe method [4]. In order to understand the effect of Cr concentration on the I-V curve and consequently on the electrical resistivity, the following relation is proposed:

Accurate data are presented on the behaviour of the thermal conductivity K as a function of temperature for a pure Ni sample near its Curie point. Previous results on the electrical resistivity (Q, dQ/dT) are used to explain the temperature-dependence of K(T). The results are analysed in terms of electron-phonon and s-d exchange interactions. The critical behaviour of the thermal resistivity W(= K-~) has also been investigated.Study of the transport properties of magnetic phase transitions provides a sensitive and often rather simple means of investigation details of the microscopic interactions. The electrical resistivity Q in particular has received a good deal of attention [1][2][3][4][5][6][7]. The scanty experimental information available on the thermal conductivity K of magnetic materials and on the complex behaviour which occurs in the transition region earlier precluded any discussion on such matters as the values of the critical exponents. Accordingly, only general features were considered [8][9][10][11][12]. The critical exponents are of interest because many different kinds of physical systems behave in a similar way near the critical point To. This work reports for the first time the critical exponents of the thermal resistivity of pure Ni both below and above To. The universality concept [4,5,13] is also tested.In ferromagnetic metals and alloys, the most characteristic interaction is the s-d interaction, i.e. the spin exchange interaction between the conduction (s) and unfilled shell (d) electrons. According to Kasuya [1], this exchange interaction depends on the relative orientation of the spins of both electrons. Therefore, at T= 0all the spins of d-electrons being in order, there is no electrical resistance, while at a finite temperature this order is disturbed and thus a resistance appears and increases with temperature. Above To, the directions of the d-electron spins become perfectly random, and the electrical resistance remains constant. The resistivity caused by such a process is called the spin-fluctuation [3] or spin-disorder contribution [10].

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The method of determining the thermal conductivity depends upon a relation between the maximum temperature (0,,) attained for a given current and potential difference (V) in a current carrying specimen. Heat is assumed to enter and leave specimen only through the surfaces through which electric current enters and leaves, other surfaces being insulated against flow of both heat and electricity. The plane ends of the rod were taken to be isothermal and equipotential surfaces held at a constant temperature.For the measurement of the specific heat, a homogeneous constant Joule heating is imposed. The initia ! slope of (0m) as a function of time is inversely proportional of heat capacity regardless of heat losses. Measurements on dilute Ni base Cr alloy sample up to 360 ~ Direct measurement of thermal conductivity "K" is difficult and in most cases inaccurate [1]. The reasons for this are heat losses and imperfect thermal contact. In an attempt to avoid such difficulties, measurements on ferromagnetic metals and alloys were made by an indirect method to investigate the variation of thermal conductivity with temperature in the region around the Curie temperature [2--4]. The method depends upon a relation between the temperature and the potential distributions in a current-carrying conductor kept under vacuum.In the present work, measurements were performed on dilute Ni base Cr alloy sample up to 640 K. The electrical resistivity "Q" was accurately measured [5], and it has been found that the temperature coefficient of resistivity \~ d-~j diverges logarithmically at the Curie temperature Tc. The quantity "K" can be obtained by plotting the potential difference (V) across the sample against "0.,", where "0,," is the maximum temperature attainedfor a given current in the conductor the ends of 3*

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