Small and Powerful: Pushing the Boundaries of Nano-Magnets

Researchers have created extremely small, thermally stable magnetic particles. These CoFe₂C nanoparticles have magnetic properties comparable to some rare earth magnets, the strongest permanent magnets ever created, at sizes as small as 5 nanometers, a million times smaller than an ant.

The Impact

The next generation of thermally stable data storage devices demands materials that are highly magnetic in a specific direction at small particle sizes. The new CoFe₂C nanoparticles accomplish this goal and can lead to nano-magnets that work at room temperature.

Summary

Van Vleck’s Nobel-prize winning explanation of the quantum origin of magnetism dates back to 1937. However it was not until 1999 that research, supported by the Office of Basic Energy Sciences within the U.S. Department of Energy, demonstrated that density-functional-theory could accurately predict the magnetic strength of molecular-scale systems. By 2007, several groups had confirmed these developments, and today researchers can computationally ask: How chemically and electromagnetically durable could such nanoscale memory devices be? Physical laws impose limits. The reduction in size of ordinary iron-based magnets, the foundation of computer memory, decreases the temperature at which such particles can store information. One of the greatest problems hindering the field of nano-magnetism is that small particle sizes tend to mean small magnetic anisotropy (directional dependence of magnetic properties). A large magnetic anisotropy is absolutely crucial to these nanoparticles because it prevents fluctuations of the magnetic moment, a phenomenon that limits the use of these particles in memory storage and many other applications. To become technologically relevant, nano-magnets must be small, have a large magnetic anisotropy, and be thermally stable. Researchers at Virginia Commonwealth University have computationally investigated CoFe₂C nanoparticles with mixed Co_xC and Fe_xC carbide phases that fit this exact description. After promising theoretical results, the researchers successfully synthesized the CoFe₂C particles with the properties that were computationally expected. The newly synthesized particles have been proven thermally stable (and thus store information) up to 790K at sizes as small as 5 nanometers. Additionally, these particles have a magnetic anisotropy of 4.6 ± 2 x 10⁶ J/m³, which is ten times larger than cobalt nanoparticles, and magnetic properties comparable to some rare earth magnets, the strongest permanent magnets ever created. These CoFe₂C nanoparticles possess the unique characteristics of both small size and a large anisotropy and could represent the future of data storage devices.

Funding

This long-term developmental work was supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Award Number DE-SC0006420 and DE-FG02-96ER45579.

Publications

M.R. Pederson and S.N. Khanna, “Magnetic anisotropy barrier for spin tunneling in Mn₁₂O₁₂ molecules.” Physical Review B 60, 9566 (1999). [DOI: 10.1103/PhysRevB.60.9566].

F. Islam and S. N. Khanna, “Stable magnetic order and charge-induced rotation of magnetization in nano-clusters.” Applied Physics Letters 105, 152409 (2014). [DOI: 10.1063/1.4898670].

A. A. El-Gendy, M. Bertino, D. Clifford, M. Qian, S. N. Khanna, and E. E. Carpenter, “Experimental evidence for the formation of CoFe₂C phase with colossal magnetocrystalline-anisotropy.” Applied Physics Letters 106, 213109 (2015). [DOI: 10.1063/1.4921789].