(Invited) Electrochemical Control of Epitaxial Growth and Magnetic Properties of Fe Nanostructures
Authors: Leistner Karin 
Published: February 2020

The control of interfacial properties offers genuine routes to tailor magnetism at the nanoscale. Epitaxial growth on suitable substrates is one approach to define the shape and orientation of nanobjects. We investigated the electrodeposition of iron nanoparticles on GaAs(001) to achieve epitaxial growth. The use of an electrolyte with low iron ion concentration (0.01 mol/l FeSO4) and a short deposition time (10 s) resulted in the formation and growth of individual Fe nuclei. The interplay between metal nuclei growth and hydrogen evolution is found to be decisive for the epitaxial interface formation. For electrochemical conditions with dominating hydrogen evolution, the deposited nanoparticles exhibit a faceted shape, crystallographic alignment and notable magnetic in-plane anisotropy. The beneficial role of the hydrogen evolution on the epitaxy is found to be related to the effect of hydrogen adsorption during the Fe/GaAs interface formation [1]. In consequence, we applied a compliance voltage during immersion of the substrate to boost the hydrogen evolution at the very start of the deposition. This lead to the formation of epitaxial nanocuboids that are aligned throughout the substrate [2]. Surface {100} facets predominate with a thin crystalline oxide shell that protects the nanoparticles during prolonged storage in air. The single crystallinity of the iron in combination with structural alignment leads to an in-plane magnetic anisotropy. These immobilized, oriented, and stable nanoparticles are promising for applications in nanoelectronic, sensor, and data storage technologies, as well as for the detailed analysis of the eﬀect of shape and size on magnetism at the nanoscale.
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, E
red and E
ox, respectively. The magneto-ionic changes were probed by in situ anomalous Hall Effect (AHE) measurements. The switching between E
red and E
ox 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.
[6] K. Duschek et al., Electrochem. Comm. 72 (2016) 153.

Figure 1