Designers of magnetic memories have long sought to control the ferromagnetic ordering temperature with the application of an electric field. Such control would enable the design of more efficient, multifunctional memory technologies, but coupled magnetic and electrical order is only observed in a handful of compounds and typically only at very low temperatures. Now, D. Chiba of Kyoto University and the Japan Science and Technology Agency, S. Fukami of NEC Corporation, and their colleagues have demonstrated room-temperature control of the ferromagnetic Curie temperature of cobalt, as reported in the November issue of Nature Materials (DOI: 10.1038/nmat3130; p. 853).
Magnetization curves under applied electrical bias for a Co/Pt multilayer. A switch from ferromagnetic (+10 V) to linear behavior (-10 V) is clearly visible. Reproduced with permission from Nature Mater. (DOI: 10.1038/nmat3130). © 2011 Macmillan Publishers Ltd.
The team applied a ±2 MVcm−1 electric field across a MgO/Co/Pt/Ta heterostructure—with an ultrathin 0.4 nm Co layer—and measured the resulting magnetic hysteresis using the anomalous Hall effect. The researchers found that it is possible to tune the coercivity of the Co layer at ~20 K below the Curie temperature by applying a positive or negative bias. Closer to the Curie temperature (~320 K) they show that it is even possible to switch the material from a ferromagnetic response to a linear response with no coercivity by reversing the polarity of the applied bias. The researchers said that this change in Curie temperature could be related to the local density of states on the surface of the Co layer. Assuming that the surface is perfectly (111)-ori ented, first principles calculations have shown that a charge modulation equal to approximately ±0.012 electrons/Co atom can be induced by the applied gate voltage. However, these calculations also predict that the Curie temperature should decrease with increasing electron number, which is in disagreement with the observed results.
The research team therefore offers a number of alternative explanations for their experimental observations, including the possible intermixing of the Co and Pt layers, as well as the formation of a two-dimensional ferromagnet. The latter case is particularly significant, as it implies that the ferromagnetic ordering is tied to the dimensionality of the system.
These results may lead to a better understanding of the mechanisms of thin- film magnetism and could even lead to the design of an electrically switched “field effect” magnet.
