Study On Magnetic-insulator-based Magnonic Devices And Their Magnon Transport Properties

Spintronics devices with spin as the information unit are expected to be ideal for high-speed processing and storage in the post-Moore era due to their low power consumption,high thermal stability,high signal-to-noise ratio,and high semiconductor process compatibility.At present,the functional characteristics of various spintronic devices are completely dependent on spin-polarized electrons(current),which are similar to charges(current)in semiconductor and are inevitably scattered by impurities and crystal lattices.For devices based on electron current,the Joule heat and related thermal energy consumption is inevitable,so the development of core components and circuits based on pure spin control has become the focus of future research in spintronics.The successful applications of giant magnetoresistance(GMR)effect and tunnelling magnetoresistance(TMR)effect have facilitated the rapid development of the microelectronics industry.Magnetics and magnon-type storage and logic devices based on electrically neutral magnons as spin carriers can avoid the interference of external Joule heat,and are theoretically expected to greatly reduce the power consumption of the devices.In this thesis,we systematically investigate the magnonic devices based on ferromagnetic/antiferromagnetic insulators.After rational design,we finally realize the reading and writing operations of magnon information and other control functions in such devices.The contents are as follows:(1)Taking the YIG/NiO/YIG all-insulating magnon junction(MJ)as the research object,the spin Seebeck effect is used to realize the magnon valve effect(MVE)in the MJ.That is,the relative orientation(parallel or anti-parallel)of the two magnetic layers of the magnon valve(MV)corresponds to the detection voltage(high voltage or low voltage)in the Pt layer.By adjusting the thickness of NiO,a high on-off ratio of the magnon junction near room temperature can be achieved.By measuring the spin Seebeck voltage of the MJ at different temperatures with different NiO thicknesses,it is concluded that the top and bottom of the magnon junction have different contributions to the output signal in different temperature ranges and the thickness of NiO also affects the transport properties of magnons.It is calculated that the decay length of NiO magnon is about 3.5 nm～4.5 nm when the bath temperature is 100 K～200 K.Field cooling experiments show that the transport properties of magnets are independent of exchange bias field.(2)The spin Hall magnetoresistance(SMR)effect is studied in Pt of the YIG/NiO/YIG all-insulating magnon junction.The SMR of the magnon junction is measured by the four-terminal method,and the anomalous signal of the MJ is separated from the normal SMR signal.It is found that the anomalous partial resistance state of the SMR is related to the magnetization of the MJ.The device shows low-resistance and high-resistance state when the magnetization of top and bottom YIG is aligned parallel and anti-parallel,respectively.The anomalous SMR is named as magnonic nonlocal spin Hall magnetoresistance(MNSMR).Control experiments,macro-spin models and micro-magnetic simulations are used to rule out the possibility of MNSMR caused by interlayer magnetic coupling.The MNSMR effect can also be observed in the sandwich structure with CoO and Au instead of NiO as the intermediate layer.The MNSMR phenomenon is further explained through theoretical models as following:The relative magnetization state of the magnetic moment in the magnon junction/valve affects the effective magnon conductance of the magnon junction,and the effective magnon conductance ultimately leads to the change of the Pt resistance.(3)Through the post-annealing process,the lattice mismatch between the YIG and the substrate is utilized to create the stress-induced perpendicular magnetic anisotropy of the YIG.The quality of the YIG/Pt interface also changes the anisotropy of the film.The secondary annealing can reduce the roughness of the YIG/Pt interface,so that the perpendicular magnetic anisotropy of the double-layer film can be restored.Under the external magnetic field,the YIG magnetic moment was successfully switched deterministically by current.The effective spin Hall angle of Pt is calculated to be about0.039 with the critical switching current.The SOT efficiency of the YIG/Pt system is measured by the second harmonic technology.The damping-like field and the field-like field of the unit current density are 0.98 Oe/(MA/cm~2)and 0.057 Oe/(MA/cm~2),respectively.The spin Hall angle of Pt calculated by the effective field obtained by second harmonic technology is 0.026.(4)The YIG/NiO/Pt heterojunction is obtained through thin film deposition and post-annealing.We first test the perpendicular magnetic anisotropy of YIG through the PPMS-VSM option.The transmission electron microscopy is employed to prove the good film quality and clear interface.It is verified that YIG and NiO films obtained by thin film deposition and post-annealing have quasi-single crystal and good insulation.The angle-dependent measurement of the longitudinal voltage of the heterojunction is carried out to verify that the AHE of the sample is induced by SMR.The magnetic moment switching process of YIG/NiO system was studied.The spin current was transformed into pure magnon current through antiferromagnetic NiO.The magnon current entered the YIG film to induce the magnon transfer torque,which made the YIG magnetic moment be reversed deterministically.We have successfully reversed the magnetic moment of YIG by magnon transfer torque(MTT)in heterostructures with thin NiO intercalation.The spin Hall angles of YIG/Pt and YIG/NiO(1.5 nm)/Pt were calculated to be about 0.032 and 0.027,and the MTT efficiency of YIG/NiO(1.5 nm)/Pt was estimated.The blocking temperature measured by field cooling experiment is about175 K.The interference of Oersted field,magnetic proximity effect and direct/indirect electron tunneling effect is eliminated by the control experiment.The above results deepen the understanding of the transport properties of thermal magnon in magnetic insulator and its heterojunction,demonstrate the pure magnon transport properties of the magnon junction,and realize the deterministic switching of the perpendicular magnetic moment by the magnon transfer torque,which provides a feasible physical basis for the information reading and writing of the magnon device,namely,the MNSMR effect and the MTT effect.