Speaker
Description
Ludwigites are oxyborate compounds with the general formula $M_2^{2+}M^{\prime{\kern.5pt}3+}$BO$_5$. Their structure consists of low-dimensional zigzag walls with triangular motifs, making them an interesting playground for the realization of magnetic frustration on quasi-low-dimensional lattices. Of particular interest are copper ludwigites, in which the divalent transition-metal ion is Cu$^{2+}$, carrying a quantum spin 1/2, whereas the trivalent ion is nonmagnetic. Cu$_2$GaBO$_5$ and Cu$_2$AlBO$_5$ ludwigites have been careful characterized. Both compounds order antiferromagnetically with $T_\text{N}\approx4.1$ K and 3 K, respectively. Propagation vector for Cu$_2$GaBO$_5$ is $(0.45~0~-\!0.7)$, which was determined by diffraction measurement. We also collected $\mu$SR data as a function of temperature and weak longitudinal magnetic field. They indicate a decoupling in weak fields of about 2000 gauss, which suggests that the internal field experienced by the muon is unusually weak. On the other hand, magnetic field also induces a very fast depolarization of some small fraction of the muons, leading to a decrease in initial asymmetry, which is consistent with field-induced magnetic disorder. We also present inelastic neutron scattering measurements evidencing diffuse low-energy spin fluctuations associated with such a crossover. We suggest that these investigation help understand magnetic ordering and will be an additional step towards understanding the quantum spin system.
