Previously, thermoelectric materials are a research hotspot, which can generate voltage in the presence of temperature differences. In terms of minimizing the use of fossil fuels and helping prevent the global energy crisis, thermoelectric energy harvesting technology is one of our best choices.

However, there are various types of thermoelectric mechanisms, and despite recent efforts, some of them are rarely understood. A recent study by Korean scientists aims to fill this knowledge gap.

One of the aforementioned mechanisms is the Spin Seebeck Effect (SSE), which was discovered in 2008 by a research team led by Professor Eiji Saitoh of the University of Tokyo, Japan. SSE is a phenomenon in which the temperature difference between non-magnetic and ferromagnetic materials generates a spin current.

Knowledge of spin transport in thermoelectric devices helps fill such gaps

In order to collect thermoelectric energy, inverse SSE is especially important. In some heterostructures, such as yttrium iron garnet-platinum (YIG/Pt), the spin current generated by the temperature difference is converted into a charged current, thereby providing a method of generating electricity from inverse SSE.

Because this spin-to-charge conversion is relatively inefficient in most known materials, researchers have tried to insert an atomically thin layer of molybdenum disulfide (MoS 2) between the YIG and Pt layers. Although this method improves the conversion efficiency, the underlying mechanism behind the role of the 2D MoS 2 layer in spin transport is still elusive.

In order to solve this knowledge gap, Professor Lee Sang-kwon from the Department of Physics, Chung’an University, South Korea recently led an in-depth study on the subject, which has been published in Nano Letters (“Enhanced Spin Seebeck Thermoelectric Pt / Holey MoS 2 / Y 3 Fe 5 O 12 hybrid structure”). In order to understand the influence of 2D MoS 2 on YIG/Pt thermoelectric potential, many colleagues from the University of China and Japan and Professor Saitoh attended the meeting.

To this end, the scientists prepared two YIG / MoS 2 / Pt samples with different morphologies in the MoS 2 layer, as well as a reference sample without MoS 2 at all.

They prepared a measurement platform in which a temperature gradient can be applied, a magnetic field can be applied, and the voltage difference caused by the subsequent spin current can be monitored. Interestingly, they found that the thermoelectric performance of the inverse SSE and the entire heterostructure can be increased or decreased depending on the size and type of MoS 2 used.

In particular, compared to YIG/Pt alone, the use of porous MoS 2 multilayers between the YIG and Pt layers generates a 60% increase in thermoelectric power.

Through careful theoretical and experimental analysis, the scientists determined that this apparent increase was caused by the promotion of two independent quantum phenomena, which together constitute the total SSE inverse. These are called the inverse spin Hall effect and the inverse Rashba-Edelstein effect, and they both produce spin accumulation, which is then converted into a charging current.

In addition, they studied how holes and defects in the MoS 2 layer change the magnetic properties of the heterostructure, thereby advantageously improving the thermoelectric effect.

Lee was excited about the results. He said: “Our research is the first to prove that the magnetic properties of the interface layer will cause spin fluctuations at the interface and eventually increase the spin accumulation, thereby generating higher voltage and thermal power through the inverse SSE. evidence.”

The results of this work represent a key part of the technical problem of thermoelectric materials, and may soon have practical significance, as Lee explained: “Our discovery reveals a large-area thermoelectric energy harvester with an intermediate layer in YIG/Pt The important opportunity system.

They also provided the necessary information to understand the physical principles of the combination of Rashba-Edelstein effect and SSE in spin transport. He added that their SSE measurement platform can greatly help the study of other types of quantum transport phenomena, such as valley-driven Hall and Nernst effects.

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