Magnetic structure and spin-flop transition in the $A$-site columnar-ordered quadruple perovskite ${\mathrm{TmMn}}_3{\mathrm{O}}_6$

We present the magnetic structure of ${\mathrm{TmMn}}_3{\mathrm{O}}_6$, solved via neutron powder diffraction. We demonstrate that long-range magnetic order develops below 74 K, and at 28 K a spin-flop transition occurs driven by $f-d$ exchange and rare-earth single-ion anisotropy. In both magnetic phases, the magnetic structure may be described as a collinear ferrimagnet, contrary to conventional theories of magnetic order in the manganite perovskites. Instead, we show that these magnetic structures can be understood to arise due to ferro-orbital order, the $A, A^{\prime }$, and $A^{\prime \prime }$ site point symmetry, $mm2$, and the dominance of $A-B$ exchange over both $A-A$ and $B-B$ exchange, which together are unique to the $R{\mathrm{Mn}}_3{\mathrm{O}}_6$ perovskites.

Figure 1

The crystal structure of $R{\mathrm{Mn}}_3{\mathrm{O}}_6$ in the $Pmmn$ space group. (a)–(d) A projection of the crystal structure as seen from the $a$ axis. (a) The Mn1 ($A^{\prime }$) and Mn2 ($A^{\prime \prime }$) sites in their square planar and tetrahedral oxygen coordinations, respectively. (b) The 10-fold-coordinates $R$ ($A$) sites. (c) The unit cell with ${\mathrm{MnO}}_6$ octahedra drawn to illustrate the $a^{+}{a}^{+}{c}^{-}$ tilting pattern. (d) The Mn3 and Mn4 ions ($B$ sites) with the occupied $d_{3z^2-r^2}$ orbitals drawn in purple. (e) A section of the crystal structure as viewed from the $c$ axis showing a plane of Mn4 ions and a single Mn2 site. The orange dashed lines represent the $A-B$ exchange interactions, which are equivalent by the $mm2$ symmetry of the Mn2 site.

Figure 2

Temperature dependence of (a) ZFC and FCC magnetization measurements under an applied DC field of 100 Oe, and (b) single-ion moment magnitudes given for each of the magnetic sublattices. N.B. refined standard errors are smaller than the data points. (c) The direction of magnetization, where $\mathrm{\Omega }$ is the angle that describes a rotation from the $c$ axis to the $b$ axis. The black dashed lines are drawn at $T_{\mathrm{c}}=74\mathrm{K}$ and at $T_{\mathrm{flop}}=28\mathrm{K}$. The inset in (a) shows the temperature dependence of $H/M$ up to 400 K.

Figure 3

Neutron powder diffraction data taken on WISH at temperatures of (a) 85 K, (b) 40 K, and (c) 1.5 K. The refinement is given by the black line and the raw data by the red circles; peaks labeled with an asterisk are from the impurity phases. The green tick marks indicate, from top to bottom, the nuclear and magnetic reflections from ${\mathrm{TmMn}}_3{\mathrm{O}}_6$, labeled (1), the ${\mathrm{TmMnO}}_3$-related impurity, labeled (2), and the ${\mathrm{MnCO}}_3$ impurity, labeled (3).

Figure 4

The refined magnetic structure of ${\mathrm{TmMn}}_3{\mathrm{O}}_6$ at (a) 1.5 K and (b) 40 K. The crystallographic unit cell is drawn in black.

Figure 5

Stereographic projections of the Tm1 and Tm2 magnetic susceptibility calculated in the limit of small applied magnetic fields. The CEF-induced Ising-like single-ion anisotropy of Tm1 and Tm2 lies parallel to $a$ and $b$, respectively.