Temperature-induced phase transition from cycloidal to collinear antiferromagnetism in multiferroic ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ driven by $f-d$ induced magnetic anisotropy

In multiferroic ${\mathrm{BiFeO}}_3$ a cycloidal antiferromagnetic structure is coupled to a large electric polarization at room temperature, giving rise to magnetoelectric functionality that may be exploited in novel multiferroic-based devices. In this paper, we demonstrate that substituting samarium for 10% of the bismuth ions increases the periodicity of the room-temperature cycloid, and upon cooling to below $\sim 15$ K the magnetic structure tends towards a simple G-type antiferromagnet, which is fully established at 1.5 K. We show that this transition results from $f\text{−}d$ exchange coupling, which induces a local anisotropy on the iron magnetic moments that destroys the cycloidal order—a result of general significance regarding the stability of noncollinear magnetic structures in the presence of multiple magnetic sublattices.

Figure 1

(a) Medium-resolution neutron powder diffraction data (WISH detector bank 2) measured from ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ at 1.5 K (black circles). The polar $R3c$ crystal structure (upper tick marks and red line) is fit to the data, showing excellent agreement; the difference pattern is shown as the solid black line at the bottom of the figure. The diffraction patterns calculated for antipolar (${\mathrm{PbZrO}}_3$-related) and nonpolar (${\mathrm{GdFeO}}_3$-type) crystal structures are also shown. The magnetic diffraction peaks (lower tick marks) at $d\simeq 4.57$ are labeled with an asterisk. (b, c) Temperature dependence of the $a$ and $c$ lattice parameters, respectively, plotted with the same $y$-axis scale.

Figure 2

High-resolution (WISH detector bank 5) magnetic neutron powder diffraction data, showing the peaks labeled by an asterisk in Fig. 1, measured at (a) 300 K, (b) 30 K, and (c) 1.5 K. A cartoon of the corresponding magnetic structures projected onto a single site, as found by fitting the data (solid red line), is shown on the right-hand side of each panel. The solid blue line in (c) shows the theoretical diffraction pattern for a G-type structure with in-plane magnetic moments.

Figure 3

G-type collinear antiferromagnetic structure of ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ at 1.5 K, drawn on a single ($R3c$) unit cell. Magnetic moments (green arrows) are inclined 32(2)${}^{\circ }$ from the $c$ axis and drawn here to lie within the $c$-glide plane of the magnetic space group $Cc$. Bismuth/samarium, blue spheres; iron, green spheres; and oxygen ions, red spheres. ${\mathrm{FeO}}_6$ octahedra are shaded gray.

Figure 4

Temperature dependence of (a) the magnetization measured upon field cooling in 5000 Oe, (b) the propagation vector component, $\delta$, (c) the spin rotation ellipticity, and (d) the average moment magnitude of the ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ magnetic structure. Note that the instrumental resolution was insufficient to determine whether or not a region of phase coexistence was present.

Figure 5

Specific heat capacity measured from both ${\mathrm{BiFeO}}_3$ and ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ polycrystalline sintered pellets (black points). The bottom panels show the specific heat divided by $T^2$, in order to accentuate low-temperature features. Data were fit with a Debye-Einstein model (red line).

Figure 6

Low-temperature specific heat capacity measured for both ${\mathrm{BiFeO}}_3$ (blue triangles) and ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ (black circles). Inset: ${\mathrm{Bi}}_{0.9}{\mathrm{Sm}}_{0.1}{\mathrm{FeO}}_3$ data divided by temperature, having subtracted the Debye-Einstein contribution.