Pseudogap in the density of states and the highest Néel temperature of theL10-type MnIr alloy system

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The N\'eel temperature ${T}_{N},$ electronic specific heat coefficient ${\ensuremath{\gamma}}_{e},$ band structures and the effective exchange constant ${J}_{0}$ of the ${L1}_{0}$-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 ${\ensuremath{\gamma}}_{e}$ is small of $2--3{\mathrm{mJ}\mathrm{}\mathrm{mol}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}2},$ characteristic to pseudo-gap-type antiferromagnets, though ${\ensuremath{\gamma}}_{e}$ is rather larger than that of other ${L1}_{0}$-type Mn alloy systems. In addition, the concentration dependences of ${T}_{N}$ and ${\ensuremath{\gamma}}_{e}$ 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 ${E}_{F}$ and the total density of states of the equiatomic MnIr alloy in the antiferromagnetic state is about 6 states ${\mathrm{Ry}}^{\ensuremath{-}1}{\mathrm{atom}}^{\ensuremath{-}1}{\mathrm{spin}}^{\ensuremath{-}1},$ corresponding to about $1{\mathrm{mJ}\mathrm{}\mathrm{mol}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}2}$ of the calculated electronic specific heat coefficient ${\ensuremath{\gamma}}_{e}^{\mathrm{cal}}.$ The N\'eel temperature ${T}_{N}^{\mathrm{cal}}$ calculated from ${J}_{0}$ in the molecular field approximation is 1495 K, highest among several kinds of the ${L1}_{0}$-type equiatomic Mn alloy systems. From the calculated results of ${T}_{N}^{\mathrm{cal}}$ under the different additive element and/or the lattice constants, it is concluded that the magnitude of the N\'eel temperature ${T}_{N}$ of the ${L1}_{0}$-type equiatomic Mn alloy systems is explained by the ${J}_{0}$ curve which reflects the difference in the number of electrons at the Mn site.