Magnetic phase diagram and structure of the magnetic phases in the quasi-one-dimensional antiferromagnet ${\text{BaCu}}_2{\text{Si}}_2{\mathrm{O}}_7$: A symmetry analysis

We have performed a symmetry analysis of the properties of the recently discovered quasi-one-dimensional compound ${\text{BaCu}}_2{\text{Si}}_2{\mathrm{O}}_7$. The unusual spin-reorientation transitions are explained to result from relativistic interactions, which are decisive due to the fact that the interchain Heisenberg-exchange interaction is very weak. The structure of the magnetic phases is determined.

Figure 1

Position of the magnetic ions in the unit cell. The fractional cell coordinates of the ${\text{Cu}}^{2+}$ ion (1) are given by $\left(\frac{1}{4}-0.028,0.004,\frac{3}{4}+0.044\right)$.

Figure 2

Proposed structures for all phases (field-induced distortions are omitted); phase I: collinear structure $l_6^z$ (the direction of the spins is shown by the signs); phase II: weakly noncollinear antiferromagnet $l_6^y$ and $l_4^x$; phase III: weakly noncollinear structure $l_6^x$ and $l_4^y$. Next layer of spins aligned antiferromagnetically.