Physical properties and crystal structure analysis of double-perovskite ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ by using single crystals

We have succeeded in growing large single crystals of double-perovskite ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ with the $c$ axis aligned, and carried out a crystal structure analysis and measurements of resistivity and magnetization. A first-order metal-insulator transition at $T_{MI}\simeq 290$ K is accompanied by a large jump of magnetization and lattice constants. The distortion of ${\mathrm{MnO}}_6$ octahedra is consistent with a ferroic orbital order of $x^2-y^2$ type, although other orbital orders cannot be completely discarded. Néel temperature $T_N$ is determined to be 235 K, which is fairly lower than $T_{MI}$, on the basis of the onset of a large magnetic anisotropy. Another structural phase transition from tetragonal to orthorhombic is found at 370 K, which arises from staggered tilting of ${\mathrm{MnO}}_6$ octahedra and little affects the transport and magnetic properties.

Figure 1

Temperature dependence of (a) magnetic susceptibility in $H\perp c$, (b) magnetic anisotropy, (c) lattice constants, and (d) electrical resistivity in ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$. The magnetic susceptibilities were measured in a magnetic field of 10 kOe. $T_S$ and $T_{MI}$ are structural phase transition temperatures. $T_N$ denotes Néel temperature. A broken line in (d) is a visual guide.

Figure 2

Temperature dependence of anisotropic magnetic susceptibilities (${\chi }_{obs}$) after subtracting the Curie-Weiss term (${\chi }_{CW}$) in ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ compared with theoretical curves using a simple molecular-field approximation (see text).

Figure 3

X-ray oscillation photographs of ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ and 8 0 0 peak profiles at (a, e) 400 K ($T_S<T$), (b, f) 300 K ($T_{MI}<T<T_S$), (c, g) 270 K ($T_N<T<T_{MI}$), and (d, h) 200 K ($T<T_N$). Dotted circles on the left panels indicate superlattice reflections.

Figure 4

Crystal structures of (a) HT, (b) MT, and (c) LT phases of ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$. Bond angles relevant to distortion of the ${\mathrm{MnO}}_6$ octahedron are also shown. (d, e, f) Projections of the crystal structures onto the $ab$ plane in HT, MT, and LT phases. Arrows in the projections (e) and (f) denote the oxygen displacements from the ideal double-perovskite structure. A large arrow on the projection (f) shows the polarization direction.

Figure 5

(a) Schematic drawings of $Q_2$ and $Q_3$ distortion modes which directly couple with the orbital occupation in doubly degenerate $e_g$ states. (b) $Q_2-Q_3$ mode analysis of the ${\mathrm{MnO}}_6$ octahedron in ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ at 100 K, 300 K, and 450 K. A stabilized orbital is also attached in each distorted octahedron.

Figure 6

Mn-O-Mn bond angles and lengths in the ${\mathrm{MnO}}_2$ layer in LT phase.