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John O | October 2017

Scientists transform ambient heat into motion in nanoscale devices


A team of researchers from the University of Glasgow (Scotland), the University of Exeter (England) and ETH Zurich and the Paul Scherrer Institute in Switzerland have demonstrated a magnetic system that extracts thermal energy on the nanoscale and using magnetic energy to direct rotation of the magnetization.

 


Researchers turn thermal energy into motion at nanoscale.
(University of Glasgow)

 

According to a report from the University of Glasgow, “The thermal ratchet was realized in a material known as ‘artificial spin ice’, made of an assembly of tiny nanomagnets of Permalloy, a nickel–iron alloy. The individual nanomagnets are just 470 nanometers long (or about 200 times smaller than the diameter of a human hair) and 170 nanometers wide, with only a single magnetic domain; that is, the magnetization can only point in one of two directions along the long axis of the magnet.”

 

Using an external magnetic field to tune the magnetization in a specified direction, the researchers observed that magnetization rotated in one of only two directions.

 

Through numerical modeling, the researchers sought to explain what was causing this unexpected behavior. As one of the researchers noted, the edges created an “asymmetric energy potential.”

 

The article continued, “This asymmetry is reflected in the distribution of the magnetic field at the boundaries of the nanomagnet array and causes the magnetization to rotate in a preferred direction.”

 

Scientists used X-rays and the X-ray magnetic circular dichroic effect to monitor the magnetic state fo the system at the synchrotron light source at the Paul Scherrer Institute and the Lawrence Berkeley National Laboratory (Berkeley, Calif.).

 

“These findings establish an unexpected route to transforming magnetic energy into the directed motion of magnetization,” the article said. “The effect now found in the two-dimensional magnetic structures comes with the promise that it will be of practical use in nanoscale devices, such as magnetic nanomotors, actuators, or sensors. Indeed, because angular momentum is conserved and spin is a type of angular momentum, the change in the magnetic moment of the system can in principle induce a physical rotation of the system (through the Einstein–de Haas effect).”

 

The research was recently published in Nature Materials. The abstract read:

 

“Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice can lead to specific collective behaviour, including emergent magnetic monopoles, charge screening and transport, as well as magnonic response.

 

“Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation.

 

“Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization.

 

“This opens the possibility of implementing a magnetic Brownian ratchet, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.”

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