Physicists in the US, Europe and Korea have produced a long-lasting light-driven magnetic state in an antiferromagnetic material for the first time. While their project started out as a fundamental study, they say the work could have applications for faster and more compact memory and processing devices.
Antiferromagnetic materials are promising candidates for future high-density memory devices. This is because in antiferromagnets, the spins used as the bits or data units flip quickly, at frequencies in the terahertz range. Such rapid spin flips are possible because, by definition, the spins in antiferromagnets align antiparallel to each other, leading to strong interactions among the spins. This is different from ferromagnets, which have parallel electron spins and are used in today’s memory devices such as computer hard drives.
Another advantage is that antiferromagnets display almost no macroscopic magnetization. This means that bits can be packed more densely onto a chip than is the case for the ferromagnets employed in conventional magnetic memory, which do have a net magnetization.
A further attraction is that the values of bits in antiferromagnetic memory devices are generally unaffected by the presence of stray magnetic fields. However, Nuh Gedik of the Massachusetts Institute of Technology (MIT), who led the latest research effort, notes that this robustness can be a double-edged sword: the fact that antiferromagnet spins are insensitive to weak magnetic fields also makes them difficult to control.
Antiferromagnetic state lasts for more than 2.5 milliseconds
In the new work, Gedik and colleagues studied FePS3, which becomes an antiferromagnet below a critical temperature of around 118 K. By applying intense pulses of terahertz-frequency light to this material, they were able to control this transition, placing the material in a metastable magnetic state that lasts for more than 2.5 milliseconds even after the light source is switched off. While such light-induced transitions have been observed before, Gedik notes that they typically only last for picoseconds.
The technique works because the terahertz source stimulates the atoms in the FePS3 at the same frequency at which the atoms collectively vibrate (the resonance frequency). When this happens, Gedik explains that the atomic lattice undergoes a unique form of stretching. This stretching cannot be achieved with external mechanical forces, and it pushes the spins of the atoms out of their magnetically alternating alignment.
The result is a state in which the spin in one direction is larger, transforming the originally antiferromagnetic material into a state with net magnetization. This metastable state becomes increasingly robust as the temperature of the material approaches the antiferromagnetic transition point. That is a sign that critical fluctuations near the phase transition point are a key factor in enhancing both the magnitude and lifetime of the new magnetic state, Gedik says.
A new experimental setup
The team, which includes researchers from the Max Planck Institute for the Structure and Dynamics of Matter in Germany, the University of the Basque Country in Spain, Seoul National University and the Flatiron Institute in New York, wasn’t originally aiming to produce long-lived magnetic states. Instead, its members were investigating nonlinear interactions among low-energy collective modes, such as phonons (vibrations of the atomic lattice) and spin excitations called magnons, in layered magnetic materials like FePS3. It was for this purpose that they developed a new experimental setup capable of generating strong terahertz pulses with a wide spectral bandwidth.
“Since nonlinear interactions are generally weak, we chose a family of materials known for their strong coupling between magnetic spins and phonons,” Gedik says. “We also suspected that, under such intense resonant excitation in these particular materials, something intriguing might occur – and indeed, we discovered a new magnetic state with an exceptionally long lifetime.”
While the researchers’ focus remains on fundamental questions, they say the new findings may enable a “significant step” toward practical applications for ultrafast science. “The antiferromagnetic nature of the material holds great potential for potentially enabling faster and more compact memory and processing devices,” says. Gedik’s MIT colleague Batyr Ilyas. He adds that the observed long lifetime of the induced state means that it can be explored further using conventional experimental probes used in spintronic technologies.
The team’s next step will be to study the nonlinear interactions between phonons and magnons more closely using two-dimensional spectroscopy experiments. “Second, we plan to demonstrate the feasibility of probing this metastable state through electrical transport experiments,” Ilyas tells Physics World. “Finally, we aim to investigate the generalizability of this phenomenon in other materials, particularly those exhibiting enhanced fluctuations near room temperature.”
The work is detailed in Nature.
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