  Superconducting correlations which are long-ranged in magnetic systems have
attracted much attention due to their spin-polarization properties and
potential use in spintronic devices. Whereas experiments have demonstrated the
slow decay of such correlations, it has proven more difficult to obtain a
smoking gun signature of their odd-frequency character which is responsible,
e.g. for their gapless behavior. We here demonstrate that the magnetic
susceptibility response of a normal metal in contact with a superconducting
spin-valve provides precisely this signature, namely in form of an anomalous
"positive" Meissner effect which may be tuned back to a conventional "negative"
Meissner response simply by altering the magnetization configuration of the
spin-valve.
