While in the case of nanoelectrodeposition, electrochemistry at the interface is exploited to irreversibly define the shape, structure and magnetism of the iron nanoparticles, reversible manipulation of magnetism solid/liquid electrolyte interfaces can be achieved by magneto-ionic reactions. The magneto-ionic effect relies on voltage-triggered charge transfer reactions in solid or liquid electrolyte-gated architectures [3,4]. For instance, a repeatable electrochemical transformation between metal and oxide can be exploited to manipulate magnetic metals at room temperature and via the application of only a few volts. This makes the magneto-ionic approach very competitive to many other magnetoelectric mechanisms such as multiferroics and magnetic semiconductors.
We showed that polarized Fe-O/Fe heterostructures in a liquid electrolyte, readily undergo repeatable oxidation and reduction reactions that give rise to enormous magnetic property changes [5,6]. Voltage-induced changes of the saturation magnetization and magnetic anisotropy were achieved with FeOx/Fe and FeOx/Fe/FePt thin film heterostructures prepared by vacuum methods [5]. Enhanced effects are expected when the interfacial area-to-volume ratio of the magnetic nanostructures is increased by, e.g., using nanogranular structures instead of continuous films. We show that ultrathin iron nanoislands suitable for magneto-ionic effects can be efficiently prepared by electrodeposition in ambient conditions. The 3D growth mode leads to a nanogranular morphology when the deposition is stopped prior to coalescence. Upon removal from the electrodeposition setup natural oxidation sets in and iron/iron oxide nanoislands are present as starting material. To achieve voltage-control of magnetism in these electrodeposited nanoislands, an aqueous electrolyte containing 1 mol/l KOH was chosen that was already proven to be suitable for magneto-ionic effects in sputter-deposited continuous FeOx/Fe films [5]. The magneto-ionic reactions are directly linked to the electrochemical processes at the solid/liquid interface [6]. The nanostructures were then repeatedly polarized in the electrolyte at suitable reduction and oxidation potentials, Ered and Eox, respectively. The magneto-ionic changes were probed by in situ anomalous Hall Effect (AHE) measurements. The switching between Ered and Eox leads to a repeatable reduction to the ferromagnetic metal iron and oxidation to a non-ferromagnetic oxide phase with significantly lower magnetization. Almost complete ON/OFF switching is achieved (see Figure 1). The effect is larger than in continuous sputtered films of similar nominal thickness [5], which can be seen as a direct result of the higher interface/volume ratio of the nanoisland structures. Thus, for the first time, the crucial impact of the morphology on the magneto-ionic effects is elucidated. The electrochemical synthesis of magneto-ionic starting material is especially favorable because tunable magnetic material can also be deposited in channel walls and recesses.
[1] K. Leistner et al., J. Electrochem. Soc. 165 (2018) H3076.
[2] K. Leistner et al., Nanoscale 9 (2017) 5315.
[3] K. Leistner et al., Phys. Rev. B 87 (2013) 224411.
[4] U. Bauer et al., Nat. Mater. 14 (2015) 174.
[5] K. Duschek et al., APL Mater. 4 (2016) 032301.