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The current flows inside magnetic insulators

​​​​​​​​​​​​In the field of spintronics, researchers at IRIG have been studying yttrium iron garnet (YIG) for several years. This advanced magnetic material should reach a regime of superfluidity, where spin transport takes place without any energy loss. Are these extraordinary capabilities really possible?

Published on 8 February 2024

​​Spintronics concerns the transport of spin for electronics, supported by electrons in metals; but in magnetic insulators, spin is transported by quasi-particles called magnons. Compared with metals, magnetic insulators are better at conducting spin currents. 

This field is attracting interest because of the hope that the formation of a quantum condensa-tion of magnons, known as a Bose-Einstein condensation, will lead to a strong increase in con-ductivity. Researchers therefore seek to control and enhance spin currents in magnetic insula-tors. Inspired by the analogous electric diode they believe that a spin diode would be perfect for transporting spin currents without friction in analogy to a superconductor that transports electric current without resistance. This would make it possible to produce more efficient, energy-saving non-linear components such as microwave signal amplifiers or rectifiers. ​

A consortium (CEA-IRIG, CEA-IRAMIS and CNRS/Albert Fert Laboratory)​ has studied magnetic yttrium iron gar-net (YIG), a material with non-linear current characteristics. The device consists of a YIG film on which two adjacent platinum wires are deposited as magnon emitter and collector, in order to electrically control the chemical potential of the magnons and with very low magnetic damping. However, although the measurements showed a diode-like current-voltage characteristic curve, due to the non-linear increase in the population of low-energy magnons, the gain obtained was small, several orders of magnitude lower than expected (cf. Figure). 

Figure: illustration of the current-voltage characteristic curve I (V) of a spin diode made from YIG material

In the first article [1] the researchers explain this weakness by a rapid saturation of the population of low-energy magnons, which limits the spin diode effect to such an extent that the YIG material is unable to reach a state of infinite spin concentration. The whole material behaves more like a classical liquid of spins, without the quantum effect. 

A second article [2] explains why the non-linear diode effect can only be observed for large distanc-es between the electrodes. At short distances, spin transport is dominated by high-energy ther-mal magnons, which produce only a linear response as a function of the voltage applied between the electrodes. However, as their influence decays rapidly with distance, low-energy magnons are able to produce the spin diode effect once the distance between the electrodes exceeds a few micrometers. Furthermore, these experimental observations are corroborated by an analytical model that inte-grates all the effects of low-energy magnons and thermal magnons. 

The results of this study show that it is not possible to obtain the formation of a Bose-Einstein condensate in extended YIG films. Nevertheless, in future studies, it would be interesting to de-termine whether the saturation regime makes it possible to obtain a new condensed state, of the liquid type.

Bose-Einstein condensation: at very low temperatures elements of condensed matter behave as if they occupied a single lower-energy quantum state.

Collaboration: University Grenoble Alpes, University of Paris-Saclay, University of Lorraine and University of Bretagne Occidentale.

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