Controlling magnetic waves in a spin liquid

Spins carried by atoms are the building blocks of magnetism. In a ferromagnet, they all align in the same direction, a feature used to store data on hard drives. In antiferromagnets, they form an antiparallel alignment. Like the surface of a stormy sea where water mountains build up here and there, disappearing as fast as they come, the spins in a spin liquid are fluctuating and form no ordered magnetic state despite local interactions.  

Scientists at PSI have shown that the electromagnetic field of short THz pulses imprints its coherence onto the orbital wavefunctions of Terbium atoms of a Tb₂Ti₂O₇ crystal, causing the spins of 10¹⁵ Tb excited ions in the material to appear and move synchronized, reminiscent of how wind can create highly periodic patterns of waves. This created state is called an ensemble of coherent quantum states. 

The spins align faster than a picosecond causing a magnetic field inside the material, which is detected using ultrashort x-ray pulses at the Bernina experimental station of SwissFEL. The temporal shape and profile of the magnetic field is defined by the THz pulse. For linearly polarized THz fields, the created magnetic field oscillates until the coherence is lost. Circularly left and right polarized THz fields are predicted to create magnetic field with opposite field directions, respectively.  

As the fields can be induced on ultrashort femtosecond to picosecond timescales, they could find applications in new, fast data storage or more generally in the control of material properties such as its electrical conductivity or its magnetic state. The measured magnetic field further holds information about the coupling of the Tb spins in their spin liquid phase. 

Text: Paul Scherrer Institute / Roman Mankowsky

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