Field-induced magnetic form factor ofLa2CuO4−y

Polarized-neutron-scattering techniques have been used to study the spatial distribution of the induced magnetization (by a magnetic field of 50 kG) in a single crystal of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4-y}$. The induced-moment magnetic form factor was obtained by measuring at $T=330\mathrm{}$ K ($>T_N=304\mathrm{}$ K) the polarization ratios of eight Bragg reflections. The measured induced-moment magnetic form factor was found to be in good agreement with the $3d$ magnetic form factor of the ${\mathrm{Cu}}^{2+}$ free ion. This observation implies that if any moment is induced on the in-plane oxygen sites, it is certainly much smaller than that predicted from recent band-theoretical calculations.