Anisotropies of the Lower and Upper Critical Fields in [Formula presented] Single Crystals

The temperature dependence of the upper ([Formula presented]) and lower ([Formula presented]) critical fields has been deduced from Hall probe magnetization measurements of high quality [Formula presented] single crystals along the two main crystallographic directions. We show that [Formula presented] and [Formula presented] differ significantly at low temperature (being [Formula presented] and [Formula presented], respectively) and have opposite temperature dependencies. We suggest that [Formula presented] can be described by a single field dependent anisotropy parameter [Formula presented] ([Formula Presented]) that increases from [Formula presented] at low field to [Formula presented] at high field.