Terahertz emitters are widely used in medicine and technology: they are used by doctors to detect tumors, by airport employees to scan luggage and by art historians to check the authenticity of paintings. A subtype of these devices are spintronic emitters, which convert laser pulses into THz radiation by transferring the magnetic moment from the ferromagnetic layer of the material to a non-magnetic one, where it is converted into electric current.
The ferromagnetic layer consists of a metal (such as cobalt), which can be demagnetized very quickly under the influence of laser pulses. Demagnetization results in the so-called spin current, a phenomenon when, instead of the charge being transferred like with ordinary electric current, the magnetic moment is transferred between atoms. The spin current reaches the second, non-magnetic, layer, which consists of platinum. There, it is converted into an electric current that generates THz radiation.
As the power of THz emitters depends on how well the magnetic moment is transferred between the metal layers, scientists are trying to find a way to make this transfer as efficient as possible. A solution has been offered by the scientists from the Ioffe Physical-Technical Institute. They believe that an intermediate layer with a smooth gradient of cobalt and platinum is needed to form a bond between the metal layers. The scientists obtained an intermediate layer with a crystalline structure by using magnetron sputtering of cobalt and platinum alloys of varying concentrations.
After that, they conducted an experiment in which they applied ultrashort laser pulses and measured the time profile of the resulting THz radiation. They found that the gradient layer was able to double the efficiency of converting laser pulse energy into THz radiation compared to traditional structures that did not have a smooth transition between the layers. The scientists have concluded that the spin current, which occurs as a result of ultrafast demagnetization of cobalt under the influence of laser pulses, passes better through the gradient layer. Thanks to this, the platinum layer, which converts the spin current, emits more powerful THz radiation pulses.
“The structures of spintronic emitters with a smooth interface between cobalt and platinum have already been studied, and work has been carried out to calibrate the optimal layer thickness in such structures. However, as far as we know, there has been no explanation as to why THz radiation grows in the case of a smooth interface in such systems. Our conclusions about the mechanisms responsible for increased efficiency of the THz emitter will allow us to efficiently optimize the existing structures,” Leonid Shelukhin, a participant in the study, is quoted as saying by the Ioffe Physical-Technical Institute.
