A magnet can be seen as a composition of elementary “needles of a compass”, called "spins", typically depicted as arrows showing the direction from North to South poles. In magnets all spins are aligned along the same direction by the force called exchange interaction. The exchange interaction is one of the strongest quantum effects which is responsible for the very existence of magnetic materials. The strength of the exchange interaction can be appreciated from the fact that it generates magnetic fields 10,000 times stronger than the Earth’s magnetic field. Another manifestation of its strength is the fact that it can drive spins to rotate with a period of one trillionth of a second and even faster.
The ever-growing demand for efficient magnetic data processing calls for novel means to manipulate the magnetic state and manipulating the exchange interaction would be the most efficient and ultimately fastest way to control magnetism. In the course of the MAGSHAKE project we have demonstrated the ultrafast control of the exchange interaction using three different mechanisms and experimental methods.
Firstly we developed an ultrafast magnetometer – a device, which is able to trace the dynamics of spins with less that a trillionth of a second resolution. This is much faster than the temporal resolution of modern electronics. By systematically varying the colour of the excitation laser pulses from red to blue, we were able to identify the light wavelength, for which the effect of light on magnetism is the strongest.
In the second experiment, we used light to optically stimulate specific atomic vibrations of the magnet’s crystal lattice, which extensively disturbed and distorted the structure of the material. After shaking the crystal for a very short period of time, we measured how the strength of the exchange interaction evolve directly in time. We were able to switch the spin alignment of the magnet by 90 degrees.
In the third experiment we looked at the common magnetic alloy of iron and rhodium (FeRh) which exhibits a transition in both its structure and magnetism when heated just above room temperature. Upon the interaction with the material, the laser pulse raised the temperature by a few hundred degrees Celsius at timescales shorter than a billionth of a second. We used the novel double pump time-resolved spectroscopy technique. We employed two laser pulses for double pumping: while the first laser pulse serves as ultrafast heater, the second one helps in generating electric field. By detecting this field at multiple time-lapses between the two laser pulses, we were able to look how fast the magnetization emerges in the material.
Rather unexpectedly, we discovered the novel way to generate and detect magnetic waves that propagate through the material at a speed much faster than the speed of sound. These so-called spin waves produce a lot less heat than conventional electric currents, making them promising candidates for future computation devices with significantly reduced power consumption. To achieve modulation of spin waves, a prerequisite of information transfer, we used the fact that dynamics of them is intrinsically nonlinear, meaning that the waves with different frequencies and wavelengths can convert into each other.
We have reported the results in the most prestigious journals Nature, Nature Physics, Nature Materials and Nature Communications and presented them to scientific community at major conferences.
