Effect of off-stoichiometry and site disorder on the properties of Ni₃Al: II. Magnetics

Abstract

A detailed comparison between the magnetic behaviours of the 'as-prepared' ap-Ni_xAl_100-x alloys with x = 74.3, 74.8, 75.1 and 76.1 at.% (that have both compositional disorder and site disorder) and 'annealed' counterparts (that have only compositional disorder) over a wide range of temperatures and magnetic fields (H) permits us to draw the following conclusions about the role of disorder. Regardless of the type of disorder, Curie temperature, T_C, and spontaneous magnetization at 0 K, M0, decrease in accordance with the power laws T_C(x) = t_x(x-x_c)^t and M₀(x) = m_x(x-x_c)^ψ as x → x_c(the threshold Ni concentration below which the long-range ferromagnetic order ceases to exist). Site disorder lowers the value of xc by nearly 1 at.% Ni, enhances T_C for a given composition (more so as x → x_c) by increasing the number of Ni nearest neighbours for a given Ni atom, and leaves M0 essentially unaltered because site disorder has essentially no effect on the density of states, N(E_F), at the Fermi level, EF, and the shape of the density-of-states curve near EF (except for x≈ x_c, where site disorder tends to primarily enhance N(E_F) and thereby stabilize long-range ferromagnetic order for Ni concentrations below the threshold concentration,x_c ≅ 74.6 at.%, dictated by compositional disorder). At low and intermediate temperatures, spontaneous magnetization, M(T,H = 0), as well as the 'in-field' magnetization, M(T,H), exhibit non-Fermi liquid behaviour in the samples ap-Ni_74.3 and ap-Ni_74.8. As x_c is approached from above, i.e. as the compositional disorder increases, stronger deviations from the Fermi liquid behaviour occur and the temperature range over which the non-Fermi liquid behaviour persists widens. In contrast, the ap-Ni_75.1 and ap-Ni_76.1 alloys follow the behaviour that the self-consistent spin-fluctuation theory predicts for a weak itinerant-electron ferromagnet with no disorder. Both compositional disorder and site disorder have no effect on the critical behaviour of the alloys near the ferromagnetic-to-paramagnetic phase transition.