Effect of off-stoichiometry and site disorder on the properties of Ni₃Al: I. Electrical and magneto-transport

Abstract

Electrical resistivity, ρ(T), and longitudinal magnetoresistance, Δρ_||/ρ = [ρ_|| (T,H) - ρ(T, H = 0)]/ρ(T,H = 0), of 'as-prepared' Ni_xAl_100-x alloys with x = 74.3, 74.8, 75.1 and 76.1 at.% and 'annealed' Ni_75.1Al_24.9 alloy, measured over wide ranges of temperature and external magnetic field (H), are discussed in the light of existing theoretical models. ρ(T) exhibits a non-Fermi liquid (NFL) behaviour at low temperatures in the range 1.7 K≤T≤T_x (where T_x decreases from 25 to 21 K as the Ni concentration x increases from 74.3 to 75.1 at.%) in the alloys with x<76.1 at.%. Compositional disorder (particularly under-stoichiometry) gives rise to stronger deviations from the Fermi liquid behaviour and widens the temperature range over which the NFL behaviour persists whereas site disorder makes the NFL behaviour more prominent, particularly in the stoichiometric composition (Ni₃Al), and stabilizes the NFL behaviour in any given composition over a much wider temperature range. The main contributions to ρ(T) and Δρ_||/ρ arise from the scattering of conduction electrons from the 'unconventional' spin waves (exchange-enhanced non-propagating spin-density fluctuations) at low temperatures (intermediate temperatures and temperatures close to the Curie point, T_C). The self-consistent spin fluctuation theory correctly predicts that H leaves the functional dependence of ρ on temperature unaltered and quantitatively describes the suppression of the spin-wave and spin-density fluctuation contributions to ρ by H in different temperature regimes.