Crossover from magnetostatic to exchange coupling in La_0.67Ca_0.33MnO₃/YBa₂Cu₃O₇/La_0.67Ca_0.33MnO₃ heterostructures

The influence of YBa₂Cu₃O₇ (YBCO) superconductor layer (S-layer) with a varying thickness d _YBCO  =  20-50 nm on the magnetic coupling between two La_0.67Ca_0.33MnO₃ (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 a magnetic field (H) range  =  0-10 kOe. All the HSs were superconducting with the critical temperature (T _C) decreasing from  =  78 to 36 K with decrease in d _YBCO, whereas the ferromagnetic ordering temperature T _M  =  250 K did not change much. Systematically measured M-H loops of all HSs at both T  >  T _C and T  <  T _C show three main results (a) the two step magnetic reversal above T _C converts into a four step reversal below T _C in HSs with d _YBCO  ⩾  30 nm, (b) the magnitude and magnetic field corresponding to the additional two switching steps 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 T _C continues to persist below T _C and converts into a single step reversal at T [Formula: see text] T _C. 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 Sa de Melo (2003 Physica C 387 17-25).