Nanoparticles of Spin Transition Compounds Made Electrochemically
Authors: Dominguez-Benetton Xochitl , Pozo Guillermo , Prato-Modestino Rafael , Marin Pilar , de la Presa Patricia Marcela , Fransaer Jan 
Published: February 2020

Materials exhibiting geometrically frustrated magnetism are at the forefront of the search for novel magnetic ground states (i.e., quantum magnets), best achieved in synthetic herbertsmithite [1]. A method for synthesizing crystalline Zn4-xCux(OH)6Cl2 (in which x=1 for herbertsmithite, x=0 for clinoatacamite and 0.33<x<1 for paratacamite) was first discovered in 2012 (while being first found in nature in 2004) [2]. These compounds are typically synthesized by hydrothermal or solvothermal methods (e.g., 185 - 200°C). To date, these methods have been limited to very low production rates and yields and focus on the production of macroscopic crystals, due to the need to obtain single crystals in order to reveal their novel magnetic properties. Here we introduce the electrochemically-driven synthesis of mondisperse nano-particles of herbertsmithite at room temperature (18 °C). The synthesis was carried out using a mixture of Cu2+ and Zn2+ ions as the metal precursors and O2 (in air) as the oxidant gas through a gas-diffusion cathode. XRD patterns obtained for different [Cu2+]0/[Zn2+]0 ratios are presented (Figure 1a). Zero-field-cooled (ZFC) and field-cooled (FC) mass magnetization (M) in a field of 7.98 kA m-1, over the temperature range of 2 to 300 K, showed a ferromagnetic response below Tc ~ 5 K that is accompanied by bifurcation of FC data (Figure 1b). Upon reaching Tc, the sample showed a spin glass (SG)-like state, in which the magnetic moments were frozen in random directions [3]. We believe that the extracted ferromagnetic hysteresis at T = 2 K was caused by an impurity phase (Figure 1b inset). As the purity of the herbertsmithite nanoparticles is increased, a clear distinction of the quantum spin liquid state is expected.
Our new synthesis method, namely gas-diffusion electrocrystallization (GDEx), also produces other magnetic nanoparticles with controllable properties on demand, such as a precise degree of magnetization for e.g., pure phase magnetite. Given the electrochemical nature of the synthesis and the nano-scale dimensions of the magnetic particles formed, GDEx can be easily coupled to other electrochemical methods to produce magnetic thin films either in aqueous or non-aqueous media. Yet, we can achieve solid powders or stable colloidal dispersions for processing by other methods.
References
[1] M.P. Shores, E. a Nytko, B.M. Bartlett, D.G. Nocera, A Structurally Perfect S = 1/2 Kagome Antiferromagnet, J. A. Chem. Soc. 127 (2005) 13462-13463.
[2] T.H. Han, J.S. Helton, S. Chu, A. Prodi, D.K. Singh, C. Mazzoli, P. Müller, D.G. Nocera, Y.S. Lee, Synthesis and characterization of single crystals of the spin-1/2 kagome-lattice antiferromagnets ZnxCu4−xOH6Cl2, Phys. Rev. B. 83 (2011) 100402.
[3] M. Schmidt, F.M. Zimmer, S.G. Magalhaes, Spin glass induced by infinitesimal disorder in geometrically frustrated kagome lattice, Phys. A Stat. Mech. Its Appl. 438 (2015) 416–423.
Acknowledgements: G. Pozo acknowledges the funding from the European Union’s Horizon 2020 research and innovation programme MSCA-IF-2017, under grant agreement no. 796320 (MAGDEx: Unmet MAGnetic properties in micro and nano-particles by synthesis through gas diffusion electrocrystallisation, (GDEx).
Figure 1. (a) XRD patterns (Co-Kα2 wavelength=1.7928 Å) obtained for different ratios of Zn2+ and Cu2+ in the electrolyte. (b) Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility (χ) for sample 1 (400 ppm Cu2+ + 130 ppm Zn2+) showing bifurcation during the transition at Tc= ~ 5K. (Inset b) Extracted ferromagnetic hysteresis from sample 1.

Figure 1