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Anomalous Hall effect in Antiferromagnetic Crystal May Enable Computation with Atomic Spin

By: William Brown, Biophysicist at the Resonance Science Foundation

Digital Computations are based on the ability to read, write, and erase an on/off state in a material, representing the ‘0’ and ‘1’ of binary data. In today’s integrated circuits, this is achieved via transistors, which are semiconductor materials— like silicon or germanium (tetrahedral elements)— that can switch electrical signals to an “on” or “off” state and therefore function as the binary state, or logic gate in a digital computation.

In this way, the metal-oxide-silicon transistors in integrated circuits forms the memory cells of the chips, and because of the relative ease of fabrication, scalability, and low-power consumption such chips are found in nearly all digital electronic devices, from smartphones to TVs. The civilization-scale effect of this functional material with easily controlled binary state cannot be overstated, as even the US Patent and Trademark Office calls it a "groundbreaking invention that transformed life and culture around the world" [1].

The ability to miniaturize transistors and pack more and more of them onto a silicon wafer has been one of the major driving forces in increasing computational power and capabilities. However, a certain threshold is fast-approaching, in which the continued miniaturization will soon require binary state control (the on/off – “0” and “1” state) of single atoms or even subatomic particles. This introduces a host of considerable challenges as scientists try to figure out how to achieve binary computations using the quantum states of matter.

One exciting possibility is to access the spin state of the atom or electron, a form of computation called spintronics—in which the chirality or “handedness” (spin-up or spin-down) of an electron and the associated spin-charge coupling in a material can be used as an additional state, in addition to charge state, for read/write operations. Spintronic systems have promising implications for advancements in quantum computing, neuromorphic computing (see our recent article Novel Material Found to Contain Electronically Accessible Continuous Memory), and massively powerful data storage. Like the continuous memory accession discussed in the previous article, spintronics is a technique that is closer to how the master of nanoengineering—the biological system—processes information; see RSF physicist Dr. Olivier Alirol’s article on Nature’s Effective Way of Conducting Electrons to learn more about spintronics and chiral-induced spin selective filtering by macromolecules in the living organism.

For technological realization of spintronic-based systems, a class of unique materials called antiferromagnets have exciting potential applications. In contrast to ordinary ferromagnets that have an externally accessible magnetic field, antiferromagnets have an intrinsic magnetic order but only a weak or negligible interacting external magnetic field. Such a property is highly useful in high-density memory storage, as external interacting quantum fields become a significant problem with miniaturization of bit components as adjacent memory cells can interfere.

Antiferromagnets thus offer very compelling technological functionalities, such as:

  • insensitivity to data-damaging perturbations by stray fields due to zero net external magnetization; [2]  
  • no effect on near particles, implying that antiferromagnetic device elements would not magnetically disturb its neighboring elements;  
  • far shorter switching times (antiferromagnetic resonance frequency is in the THz range compared to GHz ferromagnetic resonance frequency); [3]
  • broad range of commonly available antiferromagnetic materials including insulators, semiconductors, semimetals, metals, and superconductors.

A remarkable material that may one day enable such charge-spin coupled memory is the semimetal Mn3Sn, an antiferromagnetic crystal that is known for its nearly magnetization-free anomalous Hall effect. The Hall effect is a fundamental effect in physics in which a charge carrier experiences a drift transverse to the direction of electrical conduction and perpendicular to an external magnetic field. In the anomalous Hall effect the same orthogonal drift occurs but in the absence of an external magnetic field, instead it arises from the intrinsic magnetic field arising in the lattice structure of the conducting material. The anomalous Hall effect is therefore a powerful modality for probing the unique properties and behaviors of antiferromagnets, like piezomagnetism.

Like its electrical counterpart the piezoelectric effect, in which electrical currents are induced from mechanical strain or vice-versa—the most technologically relevant material of which is quartz crystal, in which the piezoelectric properties are utilized to form the time-gate in central processing units and the high precision of atomic clocks—piezomagnetism couples mechanical strain in antiferromagnetic crystals with the spontaneous induction of a magnetic moment, or conversely a physical deformation by applying a magnetic field.

Previously, experimentation with piezomagnetism was restricted to antiferromagnetic insulators at cryogenic temperatures (as we discussed in the article DNA-guided Construction of Superconductive Carbon Nanotubes, phenomenal material properties that only occur at ultracold temperatures have limited technological applicability).  Now, an international team of researchers at the University of Tokyo, the University of Birmingham, and Max Planck Institute for Chemical Physics of Solids have reported in the journal Nature Physics the discovery of a large piezomagnetism in Mn3Sn at room temperature [4].

The international team of researchers discovered that the anomalous Hall effect can be finely tuned by applying small uniaxial strain to the antiferromagnetic crystal, such that the piezomagnetism can be utilized to control the anomalous Hall effect in Mn3Sn distinctly from the magnetization via uniaxial strain (conventionally functional control of the anomalous Hall effect is achieved by applying an external magnetic field).

Because Mn3Sn is not a perfect antiferromagnet it retains a weak external magnetic field, by applying strain to the crystal and increasing the external magnetic field the researchers were able to show that there was no corresponding effect on the voltage across the material, and therefore it is the arrangement of spinning electrons within the material that is responsible for the anomalous Hall effect.

As the co-author of the paper Dr. Clifford Hicks states it: “These experiments prove that the Hall effect is caused by the quantum interactions between conduction electrons and their spins. The findings are important for understanding – and improving – magnetic memory technology.” [Scientists unravel ‘Hall effect’ mystery in search for next generation memory storage devices].

The ability to continue to pack more and more memory cells into smaller and smaller integrated chips will require a fine level of control over the quantum properties of materials and understanding how to control the anomalous Hall effect and piezomagnetic switching, as demonstrated in this latest discovery, will offer significant advancements to the field of magnetic memory technology and spintronics. We now know that the anomalous Hall effect can be controlled, both its sign and magnitude, via strain-induced lattice displacements and resulting electron anisotropy in certain materials, like the antiferromagnetic crystal Mn3Sn.

Professor Satoru Nakatsuji and Project Associate Professor Tomoya Higo from the Department of Physics at the University of Tokyo, co-authors on the Nature Physics paper, commented further on the remarkable discovery by their team:
Like ferromagnets, antiferromagnets’ magnetic properties arise from the collective behavior of their component particles, in particular the spins of their electrons, something analogous to angular momentum. Both materials can be used to encode information by changing localized groups of constituent particles. However, antiferromagnets have a distinct advantage in the high speed at which these changes to the information-storing spin states can be made, at the cost of increased complexity.”

Some spintronic memory devices already exist. MRAM (magnetoresistive random access memory) has been commercialized and can replace electronic memory in some situations, but it is based on ferromagnetic switching. After considerable trial and error, I believe we are the first to report the successful switching of spin states in antiferromagnetic material Mn3Sn by using the same method as that used for ferromagnets in the MRAM, meaning we have coaxed the antiferromagnetic substance into acting as a simple memory device.” [Magnetic memory milestone: Developments in the field of spintronics promise faster, more efficient devices]

According to the research team, the strain-control of the anomalous Hall effect engenders additional means to control antiferromagnets, complementary to control utilizing magnetic field and electrical current. As the international research team states in their paper: “given the recent report on the gigantic THz optical enhancement [5], as well as the perspective of antiferromagnetic spintronics, the piezomagnetic effect may become useful in facilitating the ultrafast operation of antiferromagnets.”

References

[1] "Remarks by Director Iancu at the 2019 International Intellectual Property Conference"United States Patent and Trademark Office. June 10, 2019. Retrieved August 29, 2022.

[2] Jungwirth, T.; Marti, X.; Wadley, P.; Wunderlich, J. (2016). "Antiferromagnetic spintronics". Nature Nanotechnology. Springer Nature. 11 (3): 231–241.  doi:10.1038/nnano.2016.18

[3] Gomonay, O.; Jungwirth, T.; Sinova, J. (21 February 2017). "Concepts of antiferromagnetic spintronics". Physica Status Solidi RRL. Wiley. 11 (4): 1700022. arXiv:1701.06556

[4] M. Ikhlas et al., “Piezomagnetic switching of the anomalous Hall effect in an antiferromagnet at room temperature,” Nat. Phys., pp. 1–8, Aug. 2022, doi: 10.1038/s41567-022-01645-5

[5] Ankit S Disa et al. “Polarizing an antiferromagnet by optical engineering of the crystal field”. In: Nat. Phys. 16.9 (2020), pp. 937–941.

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