From antiferromagnetic insulator to ferromagnetic metal: Effects of hydrogen substitution in LaMnAsO

Manganese oxyarsenide hydrides LaMnAsO${}_{1-x}$H${}_x$ ($x=0-0.73$) were synthesized by a solid-state reaction under a high pressure of 2 GPa. The hydride ions H${}^{-}$ in O${}^{2-}$ sites induced an insulator-to-metal conversion and suppressed the antiferromagnetic ordering of Mn spins. Instead, ferromagnetism with negative magnetoresistance was observed. The Curie temperature and saturated moment per Mn increased with $x$ up to 264 K and 1.5μ${}_{\mathrm{B}}$, respectively, at $x=0.73$. These results indicate that H${}^{-}$ donate conduction electrons to the MnAs layer, mediating a direct ferromagnetic interaction between Mn atoms.

Figure 1

(a) XRD patterns of LaMnAsO${}_{1-x}$H${}_x$ ($x=0$, 0.36, and 0.73). The inset shows the crystal structure, and (b) the analyzed $x$ value in LaMnAsO${}_{1-x}$(H/D)${}_x$ as a function of nominal $x$ value ($x_{\mathrm{nom}}$) in the starting mixture. The value of $x$ was determined by EPMA and TDS. (c) Variation of lattice parameters $a$ and $c$ of LaMnAsO${}_{1-x}$H${}_x$ (open symbols) and LaMnAsO${}_{1-x}$D${}_x$ (solid symbols).

Figure 2

Neutron powder diffraction patterns of LaMnAsO${}_{1-x}$D${}_x$ ($x=0$, 0.36, and 0.68) at room temperature. Note that magnetic reflection peaks disappear as a result of antiferromagnetic ordering of the Mn spin.

Figure 3

Magnetic properties of LaMnAsO${}_{1-x}$H${}_x$ ($0\le x\le 0.73$). (a) Temperature dependence of magnetization at 5 kOe, (b) magnetization curves at 2 K, (c) $T_{\mathrm{C}}$ vs hydrogen concentration $x$, and (d) saturated magnetic moment per Mn atom (μ${}_{\mathrm{Mn}}$).

Figure 4

Temperature dependence of dc resistivity and MR. (a) dc resistivity in LaMnAsO${}_{1-x}$H${}_x$ ($0.00\le x\le 0.73$). The arrows indicate the Curie temperature ($T_{\mathrm{C}}$) estimated by Curie-Weiss fitting, (b) MR ratios of LaMnAsO${}_{1-x}$H${}_x$ ($x=0.08$ and 0.14), and (c) maximum MR value of each sample ($M{R}_{\max }$) as a function of $x$ and MR peak temperatures ($T_{\mathrm{MR}}^{\max }$)

Figure 5

Electronic phase diagram of La${}_{1-x}$Ca${}_x$MnO${}_3$ (Ref. 3) and LaMnAsO${}_{1-x}$H${}_x$. PMM: paramagnetic metal; PMI: paramagnetic insulator; FMM: ferromagnetic metal; FMI: ferromagnetic insulator; AFI: antiferromagnetic insulator; CAF: canted antiferromagnetic insulator; CO: charge-ordered insulator.