Research

Quantum magnonics: When magnon spintronics meets quantum information science

Quantum magnonics: When magnon spintronics meets quantum information science
H. Y. Yuan, Y. Cao, A. Kamra, R. A. Duine, and P. Yan
Physics Reports (2022). DOI: 10.1016/j.physrep.2022.03.002

Tiny magnets have played an instrumental role in the modern data-heavy information technology based on the digital computers that we use everyday. Intense research efforts are currently underway in creating a new class of computers that would exploit the “spooky” nature of quantum mechanics to solve problems deemed impossible otherwise. Can the tiny nanomagnets be as fruitful for this new class of quantum computers as they have been for data storage in digital computers? A growing community of researchers asks and tries to answer this and related questions. This review article presents the state of the art in this young field of inquiry, highlighting some opportunities and challenges that lie ahead.

Providing a direction to particles gone astray

Control of Nonlocal Magnon Spin Transport via Magnon Drift Currents
R. Schlitz, S. Vélez, A. Kamra, C.-H. Lambert, M. Lammel, S. T. B. Goennenwein, and P. Gambardella
Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.257201

Theory of drift-enabled control in nonlocal magnon transport
S. de-la-Peña, R. Schlitz, S. Vélez, J. C. Cuevas, and A. Kamra
Journal of Physics: Condensed Matter (2022). DOI: 10.1088/1361-648X/ac6d9a

The flow of electrons in metals underlies electricity and electric currents thereby forming the foundation of modern technology. The negative charge of electrons means that they are attracted towards the positive terminal of a battery. Using this property, they can be steered in any chosen direction. But what if we had uncharged particles in our devices? The emerging paradigm of magnonics is based on such chargeless particles because they manifest unique bosonic features, not admitted by electrons. Will it be possible to steer these uncharged particles? These articles answer the question with a resounding “yes” and provides the necessary theory as well as the experimental demonstration.

A switch to turn up dissipation-free supercurrents

Large Enhancement of Critical Current in Superconducting Devices by Gate Voltage
M. Rocci, D. Suri, A. Kamra, G. Vilela, Y. Takamura, N. M. Nemes, J. L. Martinez, M. G. Hernandez, and J. S. Moodera
Nano Letters (2021). DOI: 10.1021/acs.nanolett.0c03547

Interfacial control of vortex-limited critical current in type II superconductor films
M. K. Hope, M. Amundsen, D. Suri, J. S. Moodera, and A. Kamra
Physical Review B (2021). DOI: 10.1103/PhysRevB.104.184512

Our contemporary digital computers and electronics use as a basic building block a "switch" that connects or disconnects a circuit depending on the voltage applied at a gate electrode. This is achieved by realizing the circuit with silicon constrictions, which are electrically insulating. Application of a gate voltage creates charge carriers in these constrictions and makes them conducting thereby closing the circuit. Using similar devices made out of superconducting films, our collaborators at MIT, Cambridge, USA have succeeded in increasing the maximum supercurrent that can flow through the constrictions. Besides presenting fresh opportunities for applications based on the control, and especially an increase, of the dissipationless supercurrent, this achievement defies decades of research suggesting that such a gate voltage should not affect a good conductor. In an oversimplified and intuitive comparison, the observed effect seems like turning the water of an entire lake orange by adding a bucketful of orange juice.

Controlling the spin of an antiferromagnetic magnon

Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle Effect
T. Wimmer, A. Kamra, J. Gückelhorn, M. Opel, S. Geprägs, R. Gross, H. Huebl, and M. Althammer
Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.247204

Antiferromagnetic magnon pseudospin: Dynamics and diffusive transport
A. Kamra, T. Wimmer, H. Huebl, and M. Althammer
Physical Review B (2020). DOI: 10.1103/PhysRevB.102.174445

Our electronics relies on information being encoded in the electronic charge. An emerging paradigm based on magnets relies on the spin angular momentum carried by emergent quasiparticles - magnons. These quasiparticles are different from and much more flexible than elementary particles such as electrons. Experimentalist colleagues at the Technical University of Munich and the Walther-Meissner Institute have succeeded in exploiting this flexibility and managed to bend the spin of antiferromagnetic magnons to their will. For further description, check out the article Information transport in antiferromagnets via pseudospin-magnons published by various news outlets.

Magnon-mediated superconductivity manifests the squeezing advantage

Enhancement of superconductivity mediated by antiferromagnetic squeezed magnons
Eirik Erlandsen, Akashdeep Kamra, Arne Brataas, and Asle Sudbø
Phys. Rev. B 100, 100503(R) (2019) (Editors' suggestion)

In a bilayer comprising an antiferromagnetic insulator and a normal metal, the conduction electrons in the latter layer experience attractive interaction via exchange of magnons residing in the former layer. Such an attraction may result in emergence of superconductivity up to a critical temperature that depends on the electron-magnon coupling strength. Being an interfacial effect, it is usually small and has to compete with a similar contribution arising from the exchange of phonons in the bulk of the metallic layer. Our study shows that exploiting the intrinsically squeezed nature of antiferromagnetic magnons, the electron-magnon coupling in such bilayers can be greatly amplified via smart interface engineering, accessible to contemporary fabrication technologies. The critical temperatures evaluated employing a simple model and experimentally determined material parameters are encouraging.

Emulating quantum optics with magnons

Spin current cross-correlations as a probe of magnon coherence
Scott A. Bender, Akashdeep Kamra, Wolfgang Belzig, and Rembert A. Duine
Phys. Rev. Lett. 122, 187701 (2019) (Editors' suggestion)

"A classic experiment with photons inspires a proposed method of measuring the coherence of spin waves." See the Physics - Viewpoint: Revealing the Coherence of Magnons and feature article on Phys.org: A method to determine magnon coherence in solid-state devices.

Is a ferrimagnet more than the sum of two magnets? Or less?

Spin pumping and shot noise in ferrimagnets: Bridging ferro- and antiferromagnets
Akashdeep Kamra and Wolfgang Belzig
Phys. Rev. Lett. 119, 197201 (2017)

A ferrimagnet constitutes of two interpenetrating sublattices, both effectively representing a magnet, coupled via antiferromagnetic exchange. We demonstrate that the spin injected by a driven ferrimagnet into an adjacent conductor is not simply given by the sum of the spin injected by the two constituent sublattices (magnets). We show that the net spin current is the addition of "independent" contributions from the two sublattices and a contribution due to the interaction between the two sublattices, which we call the cross-sublattice contribution. We also find that this latter contribution may cancel the former in several systems of practical interest leading to a vanishing spin pumping current. Our findings further suggest tuning the interface properties as a pathway to controlling and enhancing the spin pumping efficiency of magnetic insulators.

Probing noninteger-spin magnons via noise

Super-Poissonian Shot Noise of Squeezed-Magnon Mediated Spin Transport
Akashdeep Kamra and Wolfgang Belzig
Phys. Rev. Lett. 116, 146601 (2016)

Microscopic particles called electrons carry the charge currents which underlie the modern electronic devices. A novel technological paradigm based on magnets relies upon spin currents carried by fundamentally different particles - magnons, carrying a quantum of spin. In this paper, we show that interactions lead to a new particle called squeezed-magnon, consisting of a quantum conglomerate of several magnons and, thus, carrying an angular momentum different from the fundamental quantum. To test such a property we suggest to generalize the observation initially made by Schottky in 1918 that the electronic charge can be measured through the current fluctuations. Spin current fluctuations are predicted to carry the information about the squeezed-magnons. Hence, the experimentally feasible investigation of the fluctuations of spin current in a spin pumping setup might be the key to reveal the quantum nature of exotic quasiparticles like the squeezed magnon.