An extension in magnetoelectric effects is proposed to include reversible chemistry-controlled magnetization variations. This ion-intercalation-driven magnetic control can be fully reversible and pertinent to bulk material volumes. The concept is demonstrated for ferromagnetic iron oxide where the intercalated lithium ions cause valence change and partial redistribution of Fe(3+) cations yielding a large and fully reversible change in magnetization at room temperature.

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The magnetoelectric effect, i.e., electric-fi eld control of magnetism in artifi cial heterostructures is usually limited to surface/interface atoms of the magnetic materials. In order to attain electrical control of magnetism in bulk ferromagnets, this study proposes to extend the defi nition of magnetoelectric phenomena to include reversible, chemistry-controlled magnetization switching. A large and reversible change in the room temperature magnetization in strong ferromagnets is reported, with electrochemistry-driven Li-ion exchange; carefully chosen spinel ferrites demonstrate a reversible magnetization variation up to 50% for CuFe 2 O 4 and 70% for ZnFe 2 O 4 . In case of CuFe 2 O 4 , the magnetization variation is predominantly associated with the preferential reduction of Cu 2+ to Cu + ions, and, hence, abides a nearly one-to-one relationship with the amount of injected Li-ions. In addition, the reduction of Cu 2+ also annihilates the Fe 3+  O  Cu 2+ magnetic interaction, resulting in a marked decrease in the Neél temperature of CuFe 2 O 4 . In contrast, the electrical tuning of superexchange interactions is found to play the decisive role in ZnFe 2 O 4 , where the simple electrochemical reduction model of magnetic cations can only explain a nominal fraction of the total magnetization variation, and indeed an electrochemically controlled reversible change in transition temperature is found necessary to account for the large magnetization variation observed.

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