Voltage-Switchable Superparamagnetism

Voltage-control of magnetism has attracted attention in the scientific community, as it promises an energy-efficient magnetic switching, with potential application in magnetic data storage. Nanoparticles below the superparamagnetic threshold are particularly interesting for that purpose, as they can be ‘pinned’ in their ferromagnetic state by increasing the particle’s magnetic anisotropy energy. Indirect voltage-control of superparamagnetism has already been demonstrated via an electrically generated strain in a substrate, which affects the magnetic particles anisotropy constant [1,2]. Here, we present a novel approach to voltage-switchable superparamagnetism, utilising magneto-ionic hydrogen intercalation as the key affecting the effective magnetic volume of the embedded superparamagnetic entities.
For that purpose, we prepared nanoporous palladium containing clusters of cobalt -npPd(Co)- via electrochemical dealloying from a CoPd alloy. Palladium as a high-susceptibility paramagnet is easily magnetically polarised, but also known for its high affinity for hydrogen intercalation in the crystal lattice. High-resolution TEM in combination with EDS mapping techniques revealed that Co clusters with an average size of 1.5-2 nm well below the superparamagnetic limit are embedded in the nanoporous Pd matrix. The main idea motivating our study was that changes in the electronic structure due to hydrogen intercalation can affect the magnetic properties of the buried Co clusters.
Hydrogen-intercalation into the Pd substrate was conducted in our customised in situ electrochemical cell in a SQUID-magnetometer [3], allowing the direct determination of changes in magnetisation. Voltage-induced electrochemical hydrogenation reversibly altered the magnetic moment by more than 600%, corresponding to a complete On- and Off-switching of magnetism. A new magneto-ionic concept, based on a RKKY-mediated superparamagnetic cluster growth upon hydrogen intercalation, is suggested to explain the giant changes in magnetisation upon hydrogenation [4]. Zero field cooled magnetisation curves before and after hydrogenation give strong support for the proposed cluster growth mechanism.
This work is financially supported by the Austrian Science Fund (FWF): P30070-N36.



References
[1] H. K. D. Kim, L. T. Schelhas, S. Keller, J. L. Hockel, S. H. Tolbert, Gregory P. Carman, Nano Lett. 13, 884, (2013)
[2] A. Arora, L. C. Phillips, P. Nukala, M. Ben Hassine, A. A. Ünal, B. Dkhil,
Ll. Balcells, O. Iglesias, A. Barthélémy, F. Kronast, M. Bibes, S. Valencia,
Phys. Rev. Mater. 3, 024403, (2019)
[3] S. Topolovec, H. Krenn, R. Würschum, Rev. Sci. Instrum. 86, 063903, (2015)
[4] M.Gößler, M. Albu, G. Klinser, E.-M. Steyskal, H. Krenn, R. Würschum,
Small 15, 1904523, (2019)