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

Abstract

The influence of YBa2Cu3O7 (YBCO) superconductor layer (S-layer) with a varying thickness dYBCO = 20–50 nm on the magnetic coupling between two La0.67Ca0.33MnO3 (LCMO) ferromagnet layers (F-layer, thickness dLCMO = 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 (TC) decreasing from = 78 to 36 K with decrease in dYBCO, 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 dYBCO ⩾ 30 nm, (b) the magnitude and magnetic field corresponding to the additional two switching steps show characteristic evolution with T and dYBCO; and (c) the HS with dYBCO = 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 Sa de Melo (2003 Physica C 387 17–25).