Experimental observation of the lock-in transition in a Bi2Sr2CaCu2O8+δ crystal near the lower critical field

Isothermal remanent magnetization of a Bi2Sr2CaCu2O8+δ crystal has been studied as a function of the applied field intensity and orientation. The modulus and direction of the magnetization vector have been determined. At the lower critical field (Hc1) boundary, the magnetization vector is directed nearly parallel to the a-b plane for an extended angular range. We attribute this effect to the lock-in transition of vortices. Field-induced reorientation of the magnetization vector and hence the vortex locked-unlocked transition has been detected. The determined angular dependence of Hc1 gives the effective mass ratio γ2≅70. We describe our results in terms of the three-dimensional London theory. copyright 1996 The American Physical Society