Spin Current Enhancement by Inserting Ultra-thin Magnetic Layer at Interface between YIG and Pt INVITED

Abstract

Spin current control has attracted increasing attention from the viewpoint of fundamental research and application owing to their attractive performances; zero Joule heating, nano scale devices, and expansion of dimension. As typical phenomena we investigated the spin Seebeck effect (SSE) and the spin Hall magnetoresistance (SMR) in the bilayer consisting of a magnetic insulator Y3Fe5O12 (YIG) and a heavy metal (Pt). Since the spin current is originated from the diffusion phenomenon, the signals, that is, spin Seebeck coefficient and MRxx of SMR, are generally small. Therefore, it is important to increase the efficiencies of spin current generation and propagation for practical use.We observed significant enhancement of SSE and SMR values by inserting 0.3nm or 0.6 nm layers of magnetic materials with different composition (Cr, Ni80Fe20, Co90Fe10, Fe50Co50, and Fe) at the YIG/Pt interface [1]. The SSE and SMR showed the mostly same dependences on the insertion layer composition and thickness as shown in Fig. 1, indicating that there is the common reason of SSE and SMR enhancement. To find the reason, we investigated the magnetism of Pt on the insertion layers and the insertion layers themselves.It is known that the magnetization is induced in the Pt layer when Pt contacts the ferromagnetic metals including Fe or Co. However, it has not been clarified whether Pt on the sub-nm-thick magnetic layer is magnetized. In order to investigate the Pt magnetic property, we carried out X-ray magnetic circular dichroism (XMCD) at the BL39XU of SPring-8. The Pt thickness was decreased to 1 nm in order to emphasize the signal from Pt near the interface with the insertion layers. As a result, it was confirmed that there is merely negligible moment in Pt for all samples with and without insertion layers by XMCD.Next, we investigated the magnetism of the sub-nm insertion layers by generalized magneto-optical ellipsometry (GME). Although the magnetic materials with sub-nm thickness does not show magnetization at room temperature in general, there is the possibility that magnetization is induced when ultra-thin magnetic layer is deposited on the ferrimagnetic YIG. However, it is difficult to measure the magnetization of sub-nm insertion layers deposited on the YIG by using macroscopic measurement such as VSM or SQUID because the magnetization signal of the insertion layers is hidden by that of YIG. Compared to this, the surface magnetic information can be extracted by GME analysis. It was found that the magnetic insertion layers of 0.3 nm and 0.6 nm did not exhibit a measurable magnetization. On the contrary, when the thickness of insertion layers is increased to 1 nm, the magnetization becomes obvious and SSE and SMR were decreased. Taking it into consideration, the state of losing magnetic order in the insertion layers seems to increase the spin current propagation from YIG to Pt.In summary, the SSE and SMR was enhanced by the insertion of thin magnetic material layers, which did not result from a net interface magnetization in Pt and the insertion layers. On the contrary, the paramagnetic state is considered to have an important role to enhance the spin current related phenomena. **