Negative Magnetoresistance Behavior in Polymer Spin Valves Based on Donor−Acceptor Conjugated Molecules

Organic spin valves (OSVs) have become an essential building block of next‐generation memory devices which focus on spin degree of transporting carriers. Meanwhile, negative magnetoresistance (MR) effect in the OSV devices is increasingly observed which deserves further exploration for the rich spin physics behind. In this work, the negative MR response in ferromagnetic (FM) metal‐based OSVs using donor−acceptor (D−A) conjugated polymer based on the naphthalenediimide units as a spacer material is observed. The negative MR effect does not result from negative polarization at spin injection and detection interface as well as tunneling anisotropic magnetoresistance effect, but from the spin transport inside the D−A polymer spacer. Even the stacking sequence of the spin injection and detection electrodes is reversed or changed the polymer coating solvents, the D−A polymer contributed negative MR response still can be observed. For further identifying negative MR origin, the bottom FM metal polarizer is replaced into high‐polarization La0.7Sr0.3MnO3 thin film, negative MR feature is well reproduced. Based on these points, it is deduced that the spin‐orientation reversal inside D−A polymer spacer is the ultimate reason for such negative MR response. The filtering‐like spin reversal activity is also in positive proportion to intermolecular interaction. The positive magnetoresistance (MR) effect is observed in LSMO/Au/Co or NiFe/Au/Co spin valves. When introducing naphthalenediimide‐based donor−acceptor conjugated polymer PNVT‐CN‐8 between the Au and Co layers, original positive MR response is transformed into negative state. Such MR inversion is governed by spin orientation reversal inside the PNVT‐CN‐8 thin film instead of metal penetration or interfacial hybridization.