ON- and OFF-Switching of Ferromagnetism in Nanoporous Pd(Co)

In the present work the particular features of dealloyed, nanoporous (np) metals have been exploited to switch magnetism. Besides the pore and ligament morphology itself, residues of the initial alloy affect the overall properties of np metals. During the dealloying process, parts of the base alloy are buried under nobler alloy atoms inside the ligaments. Ultimately, this leads to a cluster distribution of the remnant base alloy inside the np network, which sensitively depends on the initial dealloying parameters [1, 2].
In this work, residual clusters of cobalt in np palladium are utilized for the magnetic functionalization of the entire structure. Palladium as a d-band metal is on the verge of ferromagnetism, considering its electronic structure. Low amounts of magnetic impurities, such as 0.1 atomic percent of (dispersed) cobalt atoms were found to be enough to turn the cobalt-palladium alloy into a ferromagnet at low temperatures[3]. Dealloying of CoPd (initial composition Co75Pd25) allows to adjust the cobalt concentration and distribution, so that the materials get prone to ferromagnetism at ambient temperature. High-resolution TEM in combination with elemental mapping techniques revealed a residual cobalt concentration of about 8 at% agglomerated in clusters with an average size of 1.5-2 nm.
Our motivation was to start in the paramagnetic state of np Pd(Co) and to drive it into ferromag-netism by electrochemical stimuli. For that purpose an in situ electrochemical cell for the use in a SQUID-magnetometer was constructed. Hydrogenation of palladium in that cell proved to be an ideal reaction to achieve a reversible on- and off-switching of ferromagnetism. The high surface-to-volume ratios of np palladium were particularly advantageous for fast reaction kinetics. Magnetic hysteresis measurements revealed a substantial enhancement of both saturation magnetization and coercivity upon hydrogen treatment, which clearly indicates ferromagnetic behavior. Constant field measurements upon voltammetric cycling support these experimental findings by changes in the net magnetization of more than 800%. Our results are discussed in terms of magnetic interactions between the superparamagnetic cobalt clusters and a magneto-elastic coupling in the expanded palladium lattice upon hydrogenation.
This work is financially supported by the Austrian Science Fund (FWF): P30070-N36.
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