  With dc magnetisation and polarized neutron reflectometry we studied the
ferromagnetic response of YBa$_2$Cu$_3$O$_7$/La$_{2/3}$Ca$_{1/3}$MnO$_3$
(YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found
that whereas for certain growth conditions (denoted as A-type) the
ferromagnetic moment of the LCMO layer is strongly dependent on the structural
details of the YBCO layer on which it is deposited, for others (B-type) the
ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of
similar structural quality, but electron energy-loss spectroscopy (EELS)
studies with a scanning transmission electron microscope reveal an enhanced
average Mn oxidation state of +3.5 for the A-type as opposed to the B-type
samples for which it is close to the nominal value of +3.33. The related,
additional hole doping of the A-type LCMO layers, which likely originates from
La and/or Mn vacancies, can explain their fragile ferromagnetic order since it
places them close to the boundary of the ferromagnetic order at which even weak
perturbations can induce an antiferromagnetic or glassy state. On the other
hand, we show that the B-type samples allow one to obtain YBCO/LCMO
heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic
LCMO layers.
