Magnetoelectric Control of Superparamagnetism Hyungsuk K. D. Kim, †,∥ Laura T. Schelhas, ‡,∥ Scott Keller, § Joshua L. Hockel, § Sarah H. Tolbert,* ,†,‡,# and Gregory P. Carman* ,†,§ † Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095, United States ‡ Department of Chemistry and Biochemistry, UCLA, Los Angeles, California 90095, United States § Department of Mechanical and Aerospace Engineering, UCLA, Los Angeles, California 90095, United States # California NanoSystems Institute, University of California, Los Angeles, California 90095, United States * S Supporting Information ABSTRACT: Here we demonstrate electric-field induced magnetic anisotropy in a multiferroic composite containing nickel nanocrystals strain coupled to a piezoelectric substrate. This system can be switched between a superparamagnetic state and a single-domain ferromagnetic state at room temperature. The nanocrystals show a shift in the blocking temperature of 40 K upon electric poling. We believe this is the first example of a system where an electric field can be used to switch on and off a permanent magnetic moment. KEYWORDS: Magnetoelectric, superparamagnetic, multiferroic, PMN-PT, Ni nanocrystal E lectromagnetic devices, including antennas, motors, and memory, rely on extrinsic coupling produced by passing an electrical current through a wire to generate a magnetic field, a discovery made by Oersted and further developed by Ampere in 1820. 1 While extremely successful in the large scale, this approach suffers significant problems in the small scale where resistive losses are preventing further device miniaturization. Since Curie’s first attempt in 1894, 2 researchers have searched for an intrinsic approach to electrically control magnetization, and some minor progress has been made during the past decade using electric field induced strain to modulate magnetization in multiferroic composite materials. 3−5 While promising, these “bulk” multiferroic materials contain multi- domain magnetic structures that produce marginal magnet- ization changes with the application of an electric field. 6−10 During the last five years, a handful of researchers have focused on nanoscale elements, using electric field induced strain to control a single magnetic domain. 11−15 These studies show more dramatic magnetization changes, but the electric fields only reorient the magnetization state and do not change its magnitude. Therefore, what is critically needed is an approach to intrinsically control the net observed magnetization state. Superparamagnetism, which occurs in nanoscale ferromagnetic crystals when the ambient thermal noise is larger than the magnetic ansisotropy resulting in a zero magnetization state, may hold the solution to this problem. 16−18 Here we report experimental results demonstrating that an electric-field- induced anisotropy in a multiferroic system is capable of electrically switching between a superparamagnetic state and a single-domain ferromagnetic state at constant temperature and thus represents an intrinsic approach to turn on and off a net magnetic field. This electrical modulation of magnetism is achieved via an electric-field-induced strain in a magnetoelectric composite composed of Ni nanocrystals mechanically coupled to a (011) oriented PMN-PT single crystal. To our knowledge, this is the first example of a system where an electric field is used to turn on and off a permanent magnetic moment, and thus this work marks a significant advance in the field of electromagnetic devices. The magnetoelectric composites used in this work were composed of ferromagnetic 16 nm diameter Ni nanocrystals mechanically coupled to (011) [Pb(Mg 1/3 Nb 2/3 )O 3 ] (1−x) − [PbTiO 3 ] x (PMN-PT, x ≈ 0.32) ferroelectric single crystal substrates. Nickel was chosen for its superior magnetoelastic properties as well as its stability in comparison to other pure metal nanocrystals. The nanocrystals were synthesized via thermal decomposition of 1 mmol nickel acetylacetonate in the presence of oleyamine (7 mL), oleic acid (2 mmol), and triocylphosphine (2 mmol). 19 For this work, optimized conditions for the synthesis varied slightly from ref 19 and are thus summarized here. The solution was stirred at room temperature for 20 min under gentle Ar flow before heating first to 130 °C for 30 min and then to 240 °C (reflux) for 30 min. The solution was then cooled, and the particles were precipitated with ethanol and centrifuged. Two further washings were done with ethanol and hexane followed by Received: September 17, 2012 Revised: February 1, 2013 Published: February 11, 2013 Letter pubs.acs.org/NanoLett © 2013 American Chemical Society 884 dx.doi.org/10.1021/nl3034637 | Nano Lett. 2013, 13, 884−888