Tuesday, 8 October 2019

QUANTUM SPIN HALL EFFECT

SPIN HALL EFFECT


Amount of polarization, thanks to spin-orbit coupling. Hence, with every scarring event, the carrier electron applies a small amount of torque to the recording ferromagnet. SOT replaces STT’s large one-time push with many small pushes to switch the recording magnetization.

ATOMIC-SCALE
FIGURE. ATOMIC-SCALE

solenoid. This fanciful solenoid generates a staggered magnetic field that points in different directions at different atoms in an antiferromagnet. (Blue and red arrows indicate the antiferromagnet’s antiparallel spins.) Effective fields created through spin-orbit torque can act in an equivalent way and thus provide the means for efficient manipulation of the antiferromagnetic moments. Experiments have already demonstrated that electrical writing pulses enable reliable switching between distinct antiferromagnetic memory states that can be read electrically.

With SOT, the recording magnet reverses its magnetic moment with no need for a reference ferromagnet. A coordinated sequence of angular momentum-conserving processes flips the bit. The magnet is thus like a falling cat, which manages angular momentum along its body to flip itself and land safely on its feet without violating a sacred conservation law.

Antiferromagnets are not so useless

The latest twist we have encountered in the intertwined academic and applied paths of the SHE and the ISGE points toward the prospect of making antiferromagnetic microelectronic memories a reality.

In his 1970 Nobel lecture, Louis Néel expressed the common perception that antiferromagnets, whose existence he had predicted, are interesting but useless. Antiferromagnets are magnetically ordered materials in which the spins alternate being up or down from one atom to the next; as a result, their total magnetization vanishes. That lack of magnetization is the key reason why, unlike for a ferromagnet, an antiferromagnet’s spin orientation cannot be easily manipulated by an external magnetic field and why Néel did not see antiferromagnets as being useful for applications. On the other hand, if one could manipulate them efficiently, antiferromagnets would have inherent advantages over ferromagnets. They would be natural materials for nonvolatile, radiation- and magnetic field insensitive technologies; neighboring bits would not disturb each other because of the absence of stray fringing fields; and the resonance frequencies seeing the limit to writing speed would be in the terahertz range, as opposed to the gigahertz frequencies relevant for ferromagnetics.

To efficiently reorient the spins in an antiferromagnet, an applied field would somehow have to alternately flip directions at an atomic scale. Figure 4 shows a playful depiction in which hypothetical atomic-scale solenoids wrap around atoms in an antiferromagnetic crystal and generate opposite magnetic fields on opposite spin sublattices. (A spin sublattice comprises spins that are all directed the same way.) Over the course of a nearly 100-year history of investigation, researchers did not imagine a feasible mechanism for generating the fanciful solenoids’ staggered fields. But lessons learned from the SHE and the ISGE have changed that.

It turns out that efficient SOTs generated by the SHE or the ISGE is not limited to magnets with ferromagnetic order. In 2014 we and colleagues proposed that in antiferromagnets with a particular symmetry, the effective fields induced by the SHE or the ISGE can flip the directions of the antiferromagnetic spin sublattices. Spin-orbit coupling thus provides a uniquely efficient means for the manipulation of antiferromagnetic moments.

Last year the proposal was demonstrated. Investigators working with a single-crystal copper manganese arsenic film showed they could write, store, and read information on an antiferromagnetic memory cell at room temperature. Moreover, the expected ability to use picosecond-long writing pulses has been verified. 

Those successes, combined with the structural and fabrication compatibility of the CuMnAs antiferromagnet with silicon and common microelectronic circuitry, have opened a new chapter in the R&D story of magnetic memories. This new research direction inspired by antiferromagnetic memory will be acknowledged in the upcoming 2017 Magnetism Roadmap.

Antiferromagnetic spin or bionics, just taking its first steps, is sure to open many new paths. Apart from memory logic devices, antiferromagnets have an unparalleled potential to facilitate synergies of spintronics with other highly active fields of condensed-matter physics, such as investigations of topological matter. he path that took us from the origin of the SHE to the present day is as inspiring as it was impossible to predict. What we can foresee with almost absolute certainty is that we have not seen the last of its twists and turns.

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