Macroscopic Quantum Tunnelling of Josephson Vortex in Antiferromagnetic Superconductor

Abstract

The specific feature caused by the long-range antiferromagnetic order in the mixed state of superconductor might be the creation of spin-flop (or metamagnetic) domain along each vortex core. Let us consider an antiferromagnet with two magnetic sublattices as an example. An infinitesimal magnetic field applied perpendicular to the easy axis makes the ground antiferromagnetic state unstable against the phase transformation to the canted phase (spin-flop, SF). On the contrary, if the magnetic field is applied parallel to the easy axis, the antiferromagnetic configuration is stable up to the thermodynamic critical field HT. When the field is further increased a canted phase develops in the system. Let us assume that in the antiferromagnetic superconductor the lower critical field fulfils the relation Hc1 < 1 2HT and that the external field, Hc1 < H < 1 2HT, is applied parallel to the easy axis. Then the superconducting vortices appear in the ground antiferromagnetic state. If the field is increased above 12HT, the phase transition to the canted phase originates in the vortex core because the field intensity in the core doubles the external one. The spatial distribution of the field around the vortex is a decreasing function of the distance from its centre. Hence the magnetic field intensity in the neighbourhood of the core is less than HT. Therefore, the rest of the vortex remains in the antiferromagnetic configuration. The radius of spin-flop domain grows as the field is increased. Thus, in the considered model there are two distinct types of vortices. Possible candidate of such system might be ErBa2Cu3O7 [1, 2]. This compound has tetragonal unit cell with small orthorhombic distortion