Local probe (170Yb3+) measurements of magnetic fields in YBa2Cu3Ox

We introduce the technique of studying the field dependence of the electro-nuclear energy levels of a rare earth to measure the magnetic field present at the rare earth/yttrium site in YBa2Cu3Ox. Measurements were made by 170Yb Moessbauer spectroscopy. The hyperfine spectrum of the ground state Kramers doublet for Yb3+ ions diluted into this matrix is sensitive to fields in the range 100 to 2000G. Flux penetration and trapping at the local site level have been measured in superconducting samples. A molecular field exists on the rare earth site in non superconducting samples suggesting that the ordered Cu2 magnetic moments are intrinsically non-colinear. (orig.)