&Bullet; physics 14, 92

A symmetry-breaking mechanism allows researchers to create a directed stream of magnons in a magnetic isolator and observe them, opening new possibilities in magnon-based electronics.

The demand for miniaturized, energy-efficient and ultrafast information processing devices continues to grow, but manufacturers are reaching the fundamental limits of the prevailing technology based on complementary metal oxide semiconductors (CMOS). One of the main obstacles to progress is Joule heating: the faster and smaller CMOS components become, the more they suffer from the heat that builds up due to the flow of electronic charge. Researchers have suggested a workaround for this problem by foregoing fee postponement altogether. Instead, information processing could be done by manipulating “magnons” – quasi-particles of electron spin excitations – in a magnetic medium. Now Richard Schlitz from the Eidgenössische Technische Hochschule (ETH), Zurich, and colleagues have taken an important step towards such a technology by showing that a magnon drift current can be induced in a magnetic heterostructure to the flux [1] .

Just as phonons represent the coherent propagation of lattice vibrations, magnons represent the collective precession of the magnetic moments of the electrons. In both cases, these quasiparticles move through a material, although the excitations that carry them remain fixed within the lattice. The possibility of on-chip computing based on magnons stimulates a new frontier in physics called “magnons,” where a magnon current replaces the spin or charge current used in spintronics or electronic devices [2, 3] . However, this is not an easy replacement. While an electron drift current is the physical movement of charges, a magnon current only represents the propagation of the phase of collective spin precession. This difference means that the interaction of magnons with a magnetic field does not correspond to the interaction of electrons with an electric field. Instead of driving magnons through a circuit, a magnetic field just changes their frequency. Instead, magnon drift currents must be manipulated by mechanisms that result from breaking the inversion symmetry of the magnetic medium.

One such mechanism is the Dzyaloshinskii-Moriya interaction (DMI), which can occur when a ferromagnetic layer comes into contact with a non-magnetic layer with a large spin-orbit coupling. This interface DMI (iDMI) is an antisymmetric three-point exchange interaction in which the spins of two ferromagnetic atoms interact via a non-magnetic atom of the non-magnetic layer on the other side of the interface. As a result, the DMI vector lies in the plane of the interface, creating an asymmetric magnon dispersion that can be examined directly with optical magnon spectroscopy [4] . However, this probing technique cannot distinguish “pure” magnon drift currents induced by the iDMI from diffusive magnon currents driven by the magnon chemical potential. As a result, no clean observations of magnon drift currents have been reported.

In a new study, Schlitz and his colleagues propose a theory of magnon transport in which the contribution of the drift current to the diffusive magnon current is added by adding an additional asymmetric term to the equation that describes the system. The effect of this additional term is to create an anisotropy in magnon velocity, which the team uses to experimentally untangle the two contributions. They sputter-deposited a

${\text{Yes}}_{3}{\text{Fe}}_{5}{\text{Ö}}_{12}$

(YIG) thin film on a (111) -oriented

${\text{God}}_{3}{\text{Ga}}_{5}{\text{Ö}}_{12}$

(GGG) substrate. YIG is a popular ferrimagnetic oxide for such studies because it allows long-range spin wave propagation, while the YIG-GGG interface has been shown to be iDMI. generated [5] . The structure of the team differed from previous experiments, namely in its innovative non-local measurement technology. Typically, such non-local electrical measurements of a material are made using two physically separate contact pads. Current is passed into the material through one of the contact pads and the resistance of the material is calculated by measuring the voltage on the other contact pad. Since the voltage is measured away from the current-carrying contact pad, the calculated resistance provides information about the transport properties of the material on which the contact pads are produced. The problem is that the non-local resistance, as it is described by a simplified magnon transport model, arises from the combination of diffusion and drift magnon currents in the material. Therefore, there is no way to pull the two effects apart in experiments.

To get around the limitations of this traditional two-contact approach, Schlitz and his colleagues fabricated three equally spaced, parallel platinum wires on top of the YIG film (Fig. 1). By driving an oscillating current into the central wire, they induced a magnon current in the lower YIG layer through the Spin Hall Effect (SHE). The SHE generates a pure spin current through the flow of charge current in materials with a large spin-orbit coupling, such as platinum and other heavy metals. Its inverse effect – known as ISHE – is the generation of a voltage due to the conversion of spin current into charging current. In the absence of a magnon drift current, the diffusive magnon current would generate an equal voltage on each wire, the magnitude of the voltage depending on the magnetic field strength and orientation. However, a drift current would create voltage asymmetry. In fact, the team found that the ISHE-induced voltage was different on each wire, and that these voltages varied asymmetrically as the orientation of the magnetic field was changed. From this asymmetry, the researchers were able to calculate the drift current contribution in isolation.

The clear demonstration of a magnon drift current by Schlitz et al. Is evidence of an important phenomenon in magnonics. As such, the result opens up new possibilities, such as improved magnon-based logic and communication devices. A goal for the future will be to find material combinations that produce a stronger DMI effect than the YIG-GGG interface, which would improve the magnon drift current. Such materials must also have low Gilbert attenuation – a phenomenon that causes spin excitation to be resolved – to allow significant magnon propagation. But DMI may not be the only game in town: Other mechanisms of inversion symmetry breaking – like an asymmetrical or uneven field [6] and the Rashba Effect [7] - Large Magnondrift currents can also be realized, and we look forward to demonstrating such ideas.

References

1. R. Schlitz et al., “Control of the non-local magnon spin transport via magnon drift currents”, Phys. Rev. Lett.126, 257201 (2021).
2. AV Chumak et al., “Magnon spintronics”, Nat. Phys.11, 453 (2015).
3. A barman et al., “The Magnonics Roadmap 2021”, J.Phys. Condensed. matter (2021).
4. HT Nembach et al., “Linear relationship between Heisenberg exchange and interface Dzyaloshinskii-Moriya interaction in metal films”, Nat. Phys.11, 825 (2015).
5. H. Wang et al., “Chiral Spin-Wave Velocities Induced by All-Garnet Interface Dzyaloshinskii-Moriya Interaction in Ultra-Thin Yttrium-Iron-Garnet Films”, Phys. Rev. Lett.124, 027203 (2020).
6. JH Kwon et al., “Huge non-reciprocal emission of spin waves in Ta / Py bilayers”, Science note2, e1501892 (2016).
7. K.-W. Kim et al., “Prediction of the huge spin motive force due to the Rashba-spin-orbit coupling”, Phys. Rev. Lett.108, 217202 (2012).