Magnetic studies of vortices in YBaCuO crystals

Rotational Magnetization-vector (RMV) measurements (where a fixed field H is applied parallel to the sample disk plane which was rotated about the perpendicular to the plane in steps to 360{dollar}spcirc{dollar}) revealed that there is a rotational steady state that is achieved after an initial sample rotation of about 180{dollar}spcirc.{dollar} The vortex flux density vector B inside the superconductor typically was found to consist of two different components: a rotational component B{dollar}sb{lcub}rm R{rcub},{dollar} that rotates rigidly with the sample, and frictional component B{dollar}sb{lcub}rm F{rcub},{dollar} which stays at a fixed angle {dollar}(thetasb{lcub}rm F{rcub}){dollar} relative H, thus tuming frictionally relative to the sample. Moreover, B{dollar}sb{lcub}rm R{rcub}{dollar} diminishes while B{dollar}sb{lcub}rm F{rcub}{dollar} grows gradually in size with increasing H, which evidences a broad distribution in the strength of pinning torques on different vortices. The pinning torque per vortex {dollar}(tausb{lcub}rm P{rcub}){dollar} in the ab plane is found to have a broad distribution in strength, similar to previous findings for polycrystalline samples of YBCO and (Ba{dollar}sb{lcub}0.57{rcub}{dollar}K{dollar}sb{lcub}0.43{rcub}{dollar}BiO{dollar}sb3){dollar} BKBO. Simple modeling shows that the {dollar}tausb{lcub}rm P{rcub}{dollar} distribution function rises linearly with increasing {dollar}tausb{lcub}rm P{rcub}{dollar} and then drops rapidly to zero. The peak value of {dollar}tausb{lcub}rm P{rcub}/mu{dollar} ({dollar}mu{dollar} being the vortex moment) defines H{dollar}sb{lcub}rm P{rcub},{dollar} a characteristic pinning field per vortex, and it was found to decrease with increasing temperature (T) in a distinctly two-component manner. The strongly temperature dependent H{dollar}sb{lcub}rm P{rcub}{dollar} component in YBCO is suggested to derive from vortex pinning by oxygen vacancies.; Magnetization-vector measurements were also made on a YBCO crystal, initially cooled to 4.2 K in an external field H{dollar}sb{lcub}rm e{rcub}{dollar} parallel to the c axis. With H{dollar}sb{lcub}rm e{rcub}{dollar} fixed, the crystal was then tilted, such that the angle {dollar}theta{dollar} between the c axis and H{dollar}sb{lcub}rm e{rcub}{dollar} was gradually raised to 90{dollar}spcirc{dollar} and lowered back to zero (the TILT experiment). Our results revealed that all the vortex flux remains parallel to the c axis until {dollar}theta{dollar} reaches a threshold value (close to 15{dollar}spcirc{dollar} for H{dollar}sb{lcub}rm e{rcub} le{dollar} 500 Oe). As {dollar}theta{dollar} exceeds this value, the vortex flux in the ab plane rises rapidly from zero, and it appears to derive solely from a bending of the vortices initially directed along c. This whole process is fairly consistent with theoretical predictions, and is found to be essentially reversible with the cycling of {dollar}theta{dollar} at low H{dollar}sb{lcub}rm e{rcub}.{dollar} The hysteresis that develops at higher H{dollar}sb{lcub}rm e{rcub}{dollar} rises and approaches the conventional hysteresis measured along the ab plane after zero-field cooling. The remanent values of the flux density along the ab plane, B{dollar}sb{lcub}rm ab{rcub}sp{lcub}rm rem{rcub},{dollar} in the TILT experiment, which was determined by the values of B{dollar}sb{lcub}rm ab{rcub}{dollar} when {dollar}theta{dollar} was lowered to 0{dollar}spcirc,{dollar} were compared with those from ZFC and standard cross-flux (SCF) experiments.