We have studied the oscillations of the magnetisation induced by commensurability between intervortex and CuO plane distances in YBa_2Cu_3O_y (h chemical bounds 6.97+0.02) single crystals using high angular resolution magnetometry. A sharp peak in the temperature dependence of the amplitude of oscillations was found at T_f approx 60K. It is followed by other changes in the behaviour of the amplitude and irreversible magnetisation. We suggest that T_f reflects the vortex freezing transition from smectic to solid phase; the angular phase diagram is constructed.
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