title: Crossover from magnetostatic to exchange coupling in La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3heterostructures.
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  >  TCand T  <  TCshow three main results (a) the two step magnetic reversal above TCconverts into a four step reversal below TCin 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 TCcontinues to persist below TCand converts into a single step reversal at T [Formula: see text] 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).
authors: Academy of Scientific and Innovative Research, CSIR-National Physical Laboratory, New Delhi-110012, India. CSIR-National Physical Laboratory, New Delhi-110012, India.
authors: Rajni
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