Pseudogap in the density of states and the highest Néel temperature of the L1₀-type MnIr alloy system

The Néel temperature TN, electronic specific heat coefficient γ_e, band structures and the effective exchange constant J₀ of the L1₀-type MnIr alloy system have been investigated. The value of T_N is highest among Mn alloy systems, being 1145 K for the alloy with 50.1% Ir. The temperature dependence of electrical resistivity exhibits a hump just below T_N and the experimental electronic specific heat coefficient γ_e is small of 2-3mJmol^-1K^-2, characteristic to pseudo-gap-type antiferromagnets, though γ_e is rather larger than that of other L1₀-type Mn alloy systems. In addition, the concentration dependences of TN and γ_e are not so sensitive to the Ir concentration. These behaviors are well explained by the theoretical calculations, that is, a pseudogap is formed around the Fermi level EF and the total density of states of the equiatomic MnIr alloy in the antiferromagnetic state is about 6 states Ry^-1atom^-1spin^-1, corresponding to about 1mJmol^-1K^-2 of the calculated electronic specific heat coefficient γcale. The Néel temperature TcalN calculated from J₀ in the molecular field approximation is 1495 K, highest among several kinds of the L1₀-type equiatomic Mn alloy systems. From the calculated results of T^cal_N under the different additive element and/or the lattice constants, it is concluded that the magnitude of the Néel temperature TN of the L1₀-type equiatomic Mn alloy systems is explained by the J₀ curve which reflects the difference in the number of electrons at the Mn site.