Spin Transport In YIG-based Multilayers

With the development of modern society and the advancement of information technology,traditional microelectronic devices have begun to approach their performance limits.How to further realize the miniaturization of devices and avoid the failure caused by thermal effects is a new round of challenges faced by scientific research and industry.In 2010,scientists discovered that the transmission of electrical signals via magnons in magnetic insulators Yttrium Iron Garnet(Y3Fe5O12,YIG)was possible,triggering a research boom in insulator-based spintronics.A new generation of the magnon-based device is expected to realize the circuit without heat loss,thereby breaking the bottleneck on traditional electronic devices.This thesis will focus on the YIG films with nano-thickness which are prepared by pulsed laser deposition and systematically study the spin transport in YIG-based multilayers(YIG with different materials including heavy metals,antiferromagnetic metals,antiferromagnetic insulators)by magnetic and electrical measurements.Primary experiments and results are as follows:(1)The magnon transport in non-local YIG/Pt structures is studied.The inverse spin Hall voltages induced by electrically and thermally generated magnons(namely magnon-dragged magnetoresistance and spin Seebeck effect)are separated by the current reversal method.With the current increasing,a nonlinear relationship between the magnon-dragged magnetoresistance and the current is observed.We suppose that when the current density is close to YIG damping compensation point,its magnetization will be driven far away from the thermal equilibrium state,and then multiple magnon modes are excited in YIG.The nonlinear scatterings between different modes will cause the system to a thermalized redistribution.During this process,the long-wavelength magnons with smaller damping are easier to be excited,so the frequency of the dominant magnons will shift from high frequency to low frequency,resulting in a nonlinear curve.This speculation is confirmed by the stronger suppression of non-local signals by the external field at larger current.(2)Comparative measurements of local and nonlocal spin Seebeck effect in YIG/Pt nano-thick films are performed.Through the qualitative and quantitative comparisons of these two effects under different current,external field and temperature,it is found that both are caused by the current-induced Joule heating.However,the local spin Seebeck effect is generated by the interfacial temperature gradient,and influenced by the magnon-phonon coupling.While the non-local Seebeck effect is caused by the transport of magnons in the magnetic layer,which is mainly determined by the bulk transport property and not sensitive to the interface.Besides,the frequency of the dominant magnons excited in these two spin Seebeck effects is different.(3)The magnon-dragged magnetoresistance and the spin Seebeck effect in the non-local YIG/IrMn structures are studied.The antiferromagnetic metal is used simultaneously as the magnon injector and detector.The dependence of the non-local signal on current,magnetic field and channel distance is studied systematically.The magnon diffusion length in YIG is obtained by the IrMn detector and the result is similar to that found by Pt contacts.The voltage in IrMn is larger,which confirms that the antiferromagnetic metal is very effective to be the contacts in the nonlocal structures.In addition,it is found in the experiment that the one-dimensional diffusion model is not applicable for the short-distance spin Seebeck effect.(4)The effect of antiferromagnetic insulator Cr2O3 on the spin transport in YIG/Pt structure is studied,herein Cr2O3 grown on YIG is amorphous.The spin Hall magnetoresistance,magnon-dragged magnetoresistance,spin Seebeck effect in the YIG/Cr2O3(t nm)/Pt structure is investigated by electrical measurement.The experimental results show that the spin Hall magnetoresistance is mainly caused by the spin absorption and reflection at the Cr203/Pt interface,not related to the YIG magnetization.The magnon-dragged magnetoresistance decreases to zero with the insertion of Cr203.The spin Seebeck effect is mainly generated by the thermally excited magnons in YIG,which can be transported through the Cr203 layer.Our results confirm that even without long-range ordered magnetic moments,the Cr203 can transmit magnons through short-range magnetic correlation.The magnon diffusion length in Cr203 is measured to be 1.6 nm.It can be inferred from the three effects that the spin information cannot be transmitted from the Pt interface to the YIG via Cr2O3,but it can be transmitted to the Pt from the YIG interface via Cr2O3.This demonstrates that the spin conversion efficiency at the interface for the electrically and the thermally excited magnon is not the same.