Ultrasonic investigation of ³He-A vortices in low magnetic fields

We report ultrasonic measurements on rotating ³He-A at 26.7 bar pressure and at magnetic fields below 12.0 mT. We observed two distinct types of vortices: the attenuation of zero sound by vortices which were created at small fields is higher than the attenuation by vortices created at high fields. The first order transition between the two vortex states was observed at H_c∼H_D ({all equal to}3 mT). The low field structure can be identified with type-I vortices proposed by Fujita et al. and the high field one with that of Seppälä and Volovik and initially discovered in NMR experiments. The structural change may be viewed as a topological transition, because the d̂-vector distribution is different for the dipole-locked vortices in low fields from that for the dipole-unlocked vortices in high fields. In addition, a continuous change between coreless vortices and vortices with a well-defined core was observed at low enough fields H′ ≪ H_c. The vortices in ³He-A can, therefore, be characterized by three magnetic fields: H′, H_c, and H_cl. The last is the catastrophe field at which the dipole-locked vortices become unstable.