Magnetic ground state and two-dimensional behavior in pseudo-kagome layered system Cu${}_3$Bi(SeO${}_3$)${}_2$O${}_2$Br

Anisotropic magnetic properties of a layered kagome-like system Cu${}_3$Bi(SeO${}_3$)${}_2$O${}_2$Br have been studied by bulk magnetization and magnetic susceptibility measurements as well as powder and single-crystal neutron diffraction. At $T_N=27.4$ K the system develops an alternating antiferromagnetic order of ($ab$) layers, which individually exhibit canted ferrimagnetic moment arrangement, resulting from the competing ferro- and antiferro-magnetic intralayer exchange interactions. A magnetic field $B_C\sim 0.8$ T applied along the $c$ axis (perpendicular to the layers) triggers a metamagnetic transition, when every second layer flips, i.e., resulting in a ferrimagnetic structure. Significantly higher fields are required to rotate the ferromagnetic component towards the $b$ axis ($\sim$7 T) or towards the $a$ axis ($\sim$15 T). The estimates of the exchange coupling constants and features indicative of an $\mathit{XY}$ character of this quasi-2D system are presented.

Figure 1

(a) $ab$ and (b) $bc$ projections of the Cu${}_3$Bi(SeO${}_3$)${}_2$O${}_2$Br crystal layer. For clarity the $ab$ projection is slightly canted. Here the magnetic Cu1 and Cu2 ions are indicated as small blue and red spheres, respectively; O ions are the smallest violet spheres in the corners of CuO${}_4$ plackets, Bi ions are intermediate light pink spheres, Se ions are larger light green spheres, and Br ions are the largest dark green spheres.

Figure 2

(a) Magnetic susceptibility at 0.01 T along all three crystallographic axes. Inset: The inverse susceptibilities (symbols) and fits (solid lines) to the Curie-Weiss laws. (b) Magnetic susceptibility measured in different external magnetic fields $B\parallel c$ between 0.01 and 5 T. Inset: Magnetization at 2 K measured along all three crystallographic directions.

Figure 3

The enlarged part of the neutron powder diffraction pattern, in which the two most pronounced magnetic reflections appear, measured at several temperatures. Inset: The measured (black line) and the calculated (red line) powder neutron diffraction patterns at 60 K, whereas the green line shows the difference between the two, and the markers show the calculated positions of the Bragg reflections.

Figure 4

Refined magnetic structure at (a) zero applied field and (b) $B_C=1$ T. Magnetic moments at the Cu1 sites are blue arrows, while Cu2 are red. For clarity only the magnetic lattice is shown, i.e., $J_1\approx J_3$ (violet) and $J_2$ (cyan) exchange interactions.

Figure 5

Temperature dependencies of the intensities of the (2 2 $\frac{1}{2}$) and (2 1 0) reflections at (a) 0 T, (b) 0.65 T, and (c) 1 T. The black dashed line indicates the magnetic transition at $T_N$, while the red one indicates the temperature at which field dependencies were measured. The solid red lines represents a fit to $I\sim {(T_N-T)}^{2\beta }$, with $\beta =0.30\left(2\right)$ and $T_N=27.4$ K. (d) Intensity of the (2 2 $\frac{1}{2}$) magnetic reflection as a function of ($1-T/T_N$), where solid and dashed lines denote two distinct $I\sim {(T_N-T)}^{2\beta }$ regimes.

Figure 6

Field dependence of the intensities of the (2 2 $\frac{1}{2}$) and (2 1 0) reflections at 1.5 K and 20 K.