Room-temperature electrical control of ferromagnetic ordering in cobalt demonstrated
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ime of 1.45 ps. They associated the fast decay with collision between charge carriers lying in the same band together with the emission of phonons, while the slow component was associated with relaxation of electrons and holes lying in different bands and the decrease of energy of long lifetime phonons (cooling of hot phonons). The researchers also estimated other important parameters for monolayer and bilayer graphene SAs necessary to generate ultrashort laser pulses (laser mode-locking). These included saturation fluences that determine the pulse energy required for extracting most of the energy stored in the gain medium of the laser; modulation depths that represent the maximum change in absorption which can be induced by the incident light at a particular wavelength; and nonsaturable losses that are the unwanted part of the losses. The researchers considered that the values they measured for graphene
were well suited to achieve stable mode-locking of bulk solid-state lasers. They demonstrated laser mode-locking with graphene SAs in a Cr:forsterite laser, delivering 94-fs pulses with a spectral bandwidth of 20 nm near 1.24 μm. These results yielded a time-bandwidth product of 0.37, which is close to the Fourier limit. The researchers achieved stable mode-locked operation for hours with an average output power up to 230 mW at 75 MHz, without the appearance of multiple pulsing and Q-switching instabilities, and without visible damage to the absorber. The researchers consider that graphene can be further applied for other bulk solid-state lasers in the wide spectral region due to its unique band structure and superior nonlinear optical properties with modulation depth being tailored through appropriate layer-bylayer stacking of monolayer graphene. Joan J. Carvajal
Room-temperature electrical control of ferromagnetic ordering in cobalt demonstrated
D
esigners 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). 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 re-
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.
Curie temperature (~320 K) they show that it is even possible to switch the material from a ferromagnetic respon
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