Crossover from magnetostatic to exchange coupling in La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3 heterostructures

The influence of YBa2Cu3O4 (YBCO) superconductor layer (S-layer) with varying thickness d-YBCO = 20 to 50 nm on the magnetic coupling between two La0.67Ca0.33MnO3 (LCMO) ferromagnet layers (F-layer, thickness d-LCMO = 50 nm) in F/S/F heterostructures (HSs) was investigated by measuring global magnetization (M) in a temperature (T) range = 2 - 300 K and magnetic field (H) range = 0 - 10 kOe. All the HSs were superconducting with critical temperature (Tc) decreasing from = 78 to 36 K with decrease in d-YBCO, whereas the ferromagnetic ordering temperature Tm = 250 K did not change much. Systematically measured M-H loops of all HSs at both T > Tc and T < Tc show three main results- (a) the two step magnetic reversal above Tc converts into a four step reversal below Tc in HSs with d-YBCO >= 30 nm, (b) the magnetic field corresponding to the additional two switching steps and their magnitude show characteristic evolution with T and d-YBCO and (c) the HS with d-YBCO = 20 nm shows radically different behaviour, where the two step magnetic reversal above Tc continues to persist below Tc and converts into a single step reversal at T << Tc. The first two results indicate magnetostatic coupling between the magnetic domains and the vortices across the two F/S interfaces resulting in reversal dynamics different from that deep within the LCMO layers. Whereas, the result c reveals indirect exchange coupling between LCMO layers through the superconducting YBCO layer, which is a clear experimental evidence of coexistence of ferromagnetism and superconductivity in nm scale F/S/F HSs expected theoretically by C.A.R. Sa de Melo (Physica C 387, 17-25 (2003)).