Peculiarities of the magnetic phase diagram in small-size untwinned YBa2Cu3Oy crystal constructed by highly sensitive OFC-magnetometer

We applied OFC-magnetometer (open-flat coil-based tunnel diode oscillator) to study fine peculiarities of vortex flow and magnetic phase (NIP) transitions between different phases of the vortex-matter in a high quality YBa2Cu3Oy crystal. The method is based on a detection of the inductance and Q-factor value changes of a single-layer flat coil, which is placed at a face of flat sample. These finally lead to frequency and/or amplitude changes of the oscillator. Simultaneous frequency and amplitude tests enabled to construct the vortex MP-diagram for untwinned YBa2Cu3Oy crystal with (0.25 x 0.6) mm(2) area and similar to50 mum-thick in the temperature range of 65 K to T-c and the field of 0-12 T. In outlines, the plotted diagram is similar to the one defined by several other methods such as SQUID and cantilever magnetometers, etc. for the same crystal. However, our technique revealed clear separation of the lines on a diagram below the upper critical point H-ucp i.e., no kink point was observed in our method. The amplitude tests revealed also some new lines on a diagram below the lower critical point H-lcp (close to T-c). The obtained results may help us to understand the behavior of the vortex-mater and its dynamics in HTS materials. The problem how to improve resolution of the method for very small-size HTS objects is also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.