Electrical properties of ferromagnetic Ni 2 MnGa and Co 2 CrGa Heusler alloys
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Electrical Properties of Ferromagnetic Ni2MnGa and Co2CrGa Heusler Alloys N. I. Kourov*, V. V. Marchenkov, V. G. Pushin, and K. A. Belozerova Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, ul. S. Kovalevskoi 18, Yekaterinburg, 620990 Russia *email: [email protected] Received November 26, 2012
Abstract—The electrical properties of ferromagnetic Ni2MnGa and Co2CrGa Heusler alloys are measured in the temperature range 4–900 K. The effect of the energy gap near the Fermi level in the electronic spec trum on the behavior of electrical resistivity and absolute differential thermopower is discussed. DOI: 10.1134/S1063776113080074
INTRODUCTION Ferromagnetic Heusler alloys (structure L21 and formula X2YZ, where X and Y are transition metals and Z is a group III–V element of the periodic table) can be conventionally divided into two subclasses hav ing different characters of the electronic band struc ture near the Fermi level with energy EF. Some of them are called semimetallic ferromagnets (SMFs) and have an energy gap at EF in the electronic spectrum of one of the subbands, which have different directions of the spins of conduction electrons with respect to the spon taneous magnetization vector [1]. The energy gap depth and width in different SMFs can be strongly dif ferent. In particular, a Co2CrGa alloy in the magneti cally ordered state has an energy gap of the limiting depth exactly at EF in its electronic spectrum for elec trons with spins directed opposite to the magnetiza tion vector: the gap depth is 0.1–0.5 eV [2, 3]. The second subclass of the ferromagnetic Heusler alloys does not have a pronounced “gap” specific fea ture in their electronic spectra near EF. Ni2MnGa is a representative of these alloys. This was shown in one of the first works dealing with an ab initio calculation of the electronic band structure of the Ni2MnGa alloy in both the ferromagnetic and paramagnetic states [4]. Obviously, the specific features of the electronic band structure should manifest themselves in the behavior of the electrical properties and result in a radical dif ference in this behavior for the discussed subclasses of Heusler alloys.
phonon (ρph) contributions. Figure 1a shows the gen eral view of the ρ(T) dependence for the Ni2MnGa alloy and the main additive components of the electri cal resistivity. We determined contribution ρph(T) using the tabulated Bloch–Grüneisen function [9], ρ ph
T 5 = B ⎛ ⎞ ⎝ θ D⎠
θ D /T
∫ 0
5
x dx dx. x –x (e – 1)(1 – e )
In Eq. (1), the Debye temperature (θD = 319 K) was chosen according to the results of measuring the low temperature specific heat [10], and coefficient B was chosen with allowance for the slope of the experimen tal linear dependence ρ(T) at T > θD , TC. Component ρm(T) was calculated as the difference between the measured values of ρ(T) and phonon contribution 1
ρph(T) calculated by Eq. (1). It is seen that ρm(T) increases gradually with temper
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