Successive spin-flop transitions of a Néel-type antiferromagnet ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ single crystal with a honeycomb lattice

We have carried out high magnetic field studies of single-crystalline ${\mathrm{Li}}_2{\mathrm{MnO}}_3$, a honeycomb lattice antiferromagnet. Its magnetic phase diagram was mapped out using magnetization measurements at applied fields up to 35 T. Our results show that it undergoes two successive meta-magnetic transitions around 9 T fields applied perpendicular to the ab plane (along the $c^{*}$ axis). These phase transitions are completely absent in the magnetization measured with the field applied along the ab plane. In order to understand this magnetic phase diagram, we developed a mean-field model starting from the correct Néel-type magnetic structure, consistent with our single crystal neutron diffraction data at zero field. Our model calculations succeeded in explaining the two meta-magnetic transitions that arise when ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ enters two different spin-flop phases from the zero field Néel phase.

Figure 1

(a) A polyhedral view of a ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ single crystal (viewed perpendicular to the ab plane). (b) The $C_{xz}$-type antiferromagnetic spin structure (unit cell) of ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ with only Mn atoms shown for better clarity. The nearest, next-nearest, and next-next-nearest neighbor exchange interactions ($J_1$, $J_2$, and $J_3$) between Mn atoms in the ab plane are shown by double headed solid, dashed, and dotted arrows, respectively. The possible exchange interaction along the $c$ axis ($J_c$) is shown by a double headed dashed-dotted arrow.

Figure 2

Result of magnetic structure refinements using neutron diffraction data of a ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ single crystal. The dashed lines in each graph are the reference lines of perfect match between the squared observed ($F_{\mathrm{obs}}^2$) and calculated ($F_{\mathrm{calc}}^2$) structure factors. Schematic diagrams are given for each magnetic structures used in the refinement.

Figure 3

Variation of high field magnetization of a ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ single crystal with magnetic field applied (a) parallel and (b) perpendicular to the ${\mathit{c}}^{*}$ axis measured (only for decreasing field) at different temperatures. (c) An expanded view of high field magnetization measured for both increasing and decreasing field applied parallel to the ${\mathit{c}}^{*}$ axis. The corresponding magnetization calculated using mean-field models (MFC) having spin $S=3/2$ and $S$ = 1 are shown by dashed lines and dashed-symbol lines, respectively.

Figure 4

Temperature dependence of magnetization of a ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ single crystal measured for different values of magnetic field applied both parallel and perpendicular to the ${\mathit{c}}^{*}$ axis. In this figure, unless specified otherwise, the direction of the magnetic field is parallel to the ${\mathit{c}}^{*}$ axis. The corresponding magnetization calculated using mean-field models (MFC) having spin $S=3/2$ and $S$ = 1 are shown by dashed lines and dashed-symbol lines, respectively.

Figure 5

Magnetic phase diagram of a ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ single crystal. The data points are derived from the experimental magnetization measured at either fixed temperatures (▲ and ▼ symbols) or fields (■ and $★$ symbols) while sweeping the other parameter. (The lines connecting the data points are guides to the eyes.) The phase diagram constructed from our mean-field model calculations (shaded area) is shown as the background for the phase diagram of experimental data. The magnetic spin structures of the ordered AFM (obtained from the analysis of experimental single crystal neutron diffraction data), spin flop (SF), and intermediate SF phases (obtained in the mean-field analysis) are also shown. The inset shows an expanded view of the phase diagrams with a trend of merging first-order $H_{\mathrm{SF}1}$ and $H_{\mathrm{SF}2}$ boundary lines and joining the second-order PM-AFM/SF phase boundary line.