Observed magnetism and its field dependence in c-axis-oriented YBCO vortex states.

Muon-spin-resonance ($\mu $SR) data of c-axis-oriented YBCO [1] vortex states are analyzed to determine the field dependence of observed AF magnetism. Field distributions are obtained from $\mu $SR data using Maximum-Entropy (ME). We found [2] that well below T$_{c}$ YBCO vortex signals are best fitted by a Gaussian and a Lorentzian; the latter indicating AF in and near the vortex cores. The field dependence of the AF Lorentzian width is about linear. [2] ME-$\mu $SR analysis of c-axis-oriented YBCO data also suggests a field \textit{direction} dependence, pointing toward 3-dim magnetism. Our results show contradictions to curve fitting and FFT results. [1] An LSCO neutron study agrees with 3-dim field-induced AF. [3] An AF presence in and near vortex cores supports theories predicting a magnetic origin for cuprate superconductivity. [3, 4] Research supported by NSF-REU, DOE-LANL and WiSE@SJSU. [1] Lichti \textit{et al}, Hpf Int's 63 (1990) 73; [2] Boekema \textit{et al}, Physica C460-462 (2007) 1255 and ref's therein; [3] Lake \textit{et al}, Nature Materials 4 (2005) 658; [4] Chen, Zhang \textit{et al}, Phys Rev B 67 (2003) 22051.