Ferromagnets are among the most common forms of magnetism and a well-researched phenomenon in physics. Their properties have been harnessed a series of inventions which, in turn, have led to a series of further ground-breaking inventions, including electromagnets and generators. Ferromagnets have a positive susceptibility and magnetism that align parallel with an applied external field. This can enhance the total amount of magnetism and maintain it even when the applied field is removed.
Antiferromagnets behave in a similar fashion as ferromagnets, the only exception being that their magnetic moments align in an antiparallel manner to the neighbouring moments. This happens spontaneously below a critical temperature known as the Neel temperature, named after Louis Néel who discovered the phenomenon of antiferromagnetics in the 60s. Above the Neel temperature the material becomes paramagnetic, which means that some materials are weakly attracted by an externally applied magnetic field.
Now (2022), scientists at Brookhaven National Laboratory Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have taken research in antiferromagnets one step further by discovering a magnetic state of matter called an “antiferromagnetic excitonic insulator.” In this state, strong magnetic attraction between electrons in a layered material bring the electrons want to arrange their magnetic moments into an up-down “antiferromagnetic” pattern. That such antiferromagnetism could be caused by electron coupling in an insulating material was first discussed in the 1960s when physicists explored the differing properties of metals, semiconductors, and insulators. Then, the scientists already suspected that, under certain conditions, you could make electrons align their magnetic moments in an antiferromagnetic manner, the very phenomenon which the Brookhaven team has just discovered.
The other component of the new material, excitons, are created when under certain conditions electrons move around and interact with one another to form bonds. Electrons can also form bonds with “holes,” with the vacancies forming when the electrons move to a different position or energy level in a material. In the case of electron-electron interactions, the bonding is caused by magnetic attractions that are strong enough to overcome the repulsive force between the two particles with the same charge. Concerning electron-hole interactions, the attraction must be strong enough to overcome the “energy gap” of the material, which is the characteristic e.g. of an insulator. In very special circumstances, the energy gain from magnetic electron-hole interactions can outweigh the energy cost of electrons jumping across the energy gap.
The scientists started the experiment at high temperatures and then gradually cooled the material. This narrowed the energy gap. At 11,85 C°, electrons started moving between the magnetic layers of the material but immediately formed bound pairs with the holes they had left behind, simultaneously initiating the antiferromagnetic alignment of neighbouring electron spins. Also, calculations were performed to develop a model using the concept of the predicted antiferromagnetic excitonic insulator, which showed that this model comprehensively explained the experimental results. Using x-rays, the scientists found that the binding triggered by the attraction between electrons and holes actually returned more energy than when the electron jumped over the band gap. Because energy was saved by this process, all the electrons were preparing to undergo this process. Then, after all electrons had finished the transition, the material was different from the high-temperature state in terms of the overall arrangement of electrons and spins. The new configuration was made up of the electron spins ordered in an antiferromagnetic pattern while the bound pairs created a ‘locked-in’ insulating state.
Image: Antiferromagnetic excitonic insulator phase diagram.
Source: D. G. Mazzone, Y. Shen, H. Suwa, G. Fabbris, J. Yang, S.-S. Zhang, H. Miao, J. Sears, Ke Jia, Y. G. Shi, M. H. Upton, D. M. Casa, X. Liu, Jian Liu, C. D. Batista & M. P. M. Dean/ Antiferromagnetic excitonic insulator state in Sr3Ir2O7/ Nature Communications volume 13, Article number: 913 (2022), 17 February 2022/ doi.org/10.1038/s41467-022-28207-w/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License
Scientists have long tried to harness the potential of antiferromagnets. In 2020, scientists proved that ultrafast damping was important for antiferromagnets because of their strongly elliptical spin movement. In time-resolved measurements, they found that ultrafast damping resulted in an immediate spin along the short precession axis. The interplay between antiferromagnetic exchange and magnetic anisotropy increased this movement massively towards large-amplitude modulations of the antiferromagnetic order parameter. This leverage effect was highly beneficial for the ultrafast manipulation of magnetism in antiferromagnetic spintronics.
Image: Experimental geometry: a Projection of the YMnO3 crystal structure onto the basal plane. Mn3+ ions (violet) in grey and white areas are located in planes at z = 0 and z = c/2, respectively. The three sublattice magnetisations Mi point along equivalent x axes xi as indicated in the coordinate system. Note that spins in HoMnO3 point along equivalent y axes for TSR < T < TN (Supplementary Fig. 1). b Schematic of the setup. The pump and probe pulses are circularly and linearly polarised, respectively. P polariser, λ/4 quarter-wave plate, WP Wollaston prism, BPD balanced photodiode. c Visualisation of optical Z-mode excitation with field-like and damping-like torques TFL and TDL exerted by the effective field HIFE of the IFE on the magnetisation Mi∥x^i (x^i denotes the unit vector in the direction xi). Black lines illustrate the ensuing strongly elliptical spin precession. Dashed ellipses show the expected trajectory without spin excitation via TDL. Precession amplitudes are significantly reduced.
Source: Christian Tzschaschel, Takuya Satoh & Manfred Fiebig/ Efficient spin excitation via ultrafast damping-like torques in antiferromagnets/ Nature Communications volume 11, Article number: 6142 (2020), 01 December 2020/ doi.org/10.1038/s41467-020-19749-y/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License
In 2021, scientists proved that the velocity of domain walls in SAF (synthetic antiferromagnetic) nanowires could be reversibly tuned by several hundred m/s in a non-volatile manner by ionic liquid gating. Ionic liquid gating was responsible for reversible changes in oxidation of the upper magnetic layer of the SAF over a wide gate-voltage window. This caused a change in the magnetic properties of the SAF and, therefore, led to noticeable changes in the exchange coupling torque and the current-induced domain wall velocity. Furthermore, an example of an ionitronic-based spintronic switch was demonstrated as a component of a potential logic technology towards energy-efficient, all electrical, memory-in-logic.
Image: ILG-induced magnetization changes in SAF structures: a Schematic illustration of ionic liquid gating of the pristine SAF films. b Magnetic hysteresis loops obtained from ex situ SQUID measurements for several VG applied in the following sequence: 0 V (pristine state), +2 V, +4 V, −2 V, and −3 V. c VG dependence of MR/MS (left axis) and MSt (right axis). d DW velocity at a fixed current density (1.2 × 108 A cm−2) plotted as a function MR/MS. The dotted line corresponds to simulation results based on a 1D analytical model. The error bars in d correspond to 1 SD.
Source: Yicheng Guan, Xilin Zhou, Fan Li, Tianping Ma, See-Hun Yang & Stuart S. P. Parkin/ Ionitronic manipulation of current-induced domain wall motion in synthetic antiferromagnets/ Nature Communications volume 12, Article number: 5002 (2021), 18 August 2021/ doi.org/10.1038/s41467-021-25292-1/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License
There are many potential benefits to antiferromagnetic excitonic insulators: in an antiferromagnet, the axes of magnetic polarization (spins) of electrons on adjacent atoms are aligned in alternating directions. Throughout the entire material those alternating internal magnetic orientations annihilate one another, resulting in no net magnetism of the overall material. Therefore, such materials can be switched quickly between different states. Antiferromagnetic excitonic insulators are also resistant to information being lost due to interference from external magnetic fields. These properties make antiferromagnetic materials attractive for different kinds of modern technologies.
This discovery puts an end to decades of research to research of many decades focusing on electron arrangement in materials. Now that scientists are finally one step closer to understanding the connections between spin and charge, new technologies can emerge making use of their findings.