Magnetic anisotropy in the frustrated spin-ladder system BiCu$_{2}$PO$_{6}$ from magnetostriction in pulsed fields

The spin interactions in BiCu$_{2}$PO$_{6}$ have been studied by inelastic neutron scattering, magnetic susceptibility, and numerical calculation. There is strong frustration between magnetic interactions along the ladder leg, $J_{Leg}$ and $J_{NNN}$, and it has been pointed out that a spin gap persists in this frustrated system. Both $J_{leg}$ and $J_{NNN}$ are intra-ladder and two-leg ladder is always gapped. Longitudinal magnetostriction (MS) measurements were performed using a fiber optic strain gauge in a 60 T pulsed magnet [1] . Specific heat ($C_{p})$ and magnetocaloric effect (MCE) measurements were performed in a 35 T DC magnet. $C_{p}$ vs $T$ was obtained using both a thermal relaxation time and dual slope techniques. We have in this way determined the (H,T) phase diagram of BiCu$_{2}$PO$_{6}$ up to 45 T. The 3D character of phase transitions is suggested by the observation of sharp anomalies in CM and \textit{$\Delta $}L/L. Our MCE and MS data provides direct evidence of first-order phase transitions for H//c, while phase transitions for H//a and H//b are characterized as second-order phase transitions. The anisotropic and complex phase boundaries will be discussed. \\[4pt] [1] Daou R et al., \textit{Rev. Sci. Instrum}. \textbf{81}, 033909 (2010).