  Naturally occurring spin-valve-type magnetoresistance (SVMR), recently
observed in Sr2FeMoO6 samples, suggests the possibility of decoupling the
maximal resistance from the coercivity of the sample. Here we present the
evidence that SVMR can be engineered in specifically designed and fabricated
core-shell nanoparticle systems, realized here in terms of soft magnetic Fe3O4
as the core and hard magnetic insulator CoFe2O4 as the shell materials. We show
that this provides a magnetically switchable tunnel barrier that controls the
magnetoresistance of the system, instead of the magnetic properties of the
magnetic grain material, Fe3O4, and thus establishing the feasibility of
engineered SVMR structures.
