Spin Transport Properties And Tunability Of Perpendicular Magnetic Anisotropy Of YIG-based Heterostructures

Pure spin current and devices have become more and more important in the spintronics study.With a flow of angular momentum without accompanying charge currents,a pure spin current generates no Oersted field,and least Joule heating.A lot of means can be utilized to generate pure spin current,such as electrical means,spin Hall effect?SHE?,spin Seebeck effect?SSE?,spin pumping?SP?and nonlocal spin valve?NLSV?.According to the different ways of generating pure spin current,the spintronics can be classified into:spin orbitronics,spin caloritronics,magnonics.The advantage of using FMIs as opposed to ferromagnetic metals is that the flow of charge currents is avoided,thus preventing Ohmic losses or the emergence of undesired spurious effects.Ferromagnetic insulator yttrium iron garnets?YIG?is one of the most extensively studied materials in spintronics due to its insulating nature,long transmission length and extremely low damping constant.In addition,the pure spin current in YIG is generated and transferred in spin wave mode which makes YIG promising in magnonics study.This dissertation focus on the growth and spin transport properties of YIG-based heterostructure.It mainly includes the following three parts:?1?Nanometer-thick Y₃Fe₅O₁₂?YIG?films epitaxially grown on?111?Gd₃Ga₅O₁₂with a magnetic dead layer as thin as about 1.2nm are quantified by polarized neutron reflectivity and magnetization measurements.Vacuum annealing on YIG at 300–400?leads to substantial reduction in the anomalous Hall effect,spin Hall magnetoresistance,and spin pumping in YIG/Pt bilayers but causes large enhancement in the spin Seebeck effect.The structural,static,and dynamic magnetic measurements show that the annealing has no discernible influence on the global oxidization states and saturation magnetization of YIG films but introduces subtle defects possibly in the form of oxygen vacancies.This study suggests that subtle defects in thin YIG films have multiple effects on the spin transport properties,and caution should be taken in annealing YIG in vacuum.?2?The spin current efficiency in ferromagnet?FM?/nonmagnetic metal?NM?heterostructures is a key issue for many subcategories of spintronics,and it is determined by both spin Hall angle(₄₅)of NM and spin-mixing conductance(_??)related with the constituents and particularly with the interface.In this paper,we studied the spin transport in the Y₃Fe₅O₁₂?YIG?/W_1-xCr_x heterostructure over the full composition by means of spin Seebeck effect and spin pumping.Appreciably enhanced inverse spin Hall voltage was observed for the substitutional alloy of bcc-W and Cr with maximum obtained at the equiatomic composition.We found that the ₄₅ of bcc-WCr is 1.3 times as large as that of?-W,which mainly originates from intensive disorder atomic scattering.More significantly,YIG/bcc-WCr has a_??of 1.42×10¹⁸m^-2,more than twice of that for YIG/?-W(5.98×10¹⁷ m^-2).Meanwhile,a built-in negative interface potential was identified in YIG/?-W,but absence of such an interfacial potential in YIG/bcc-WCr,showing strong correlation between_??and interfacial potential.The present study not only acquired the bcc-WCr alloy with ₄₅exceeding that of the metastable?-W,but also revealed a promising method to manipulate_??by tuning interfacial potential in insulating FM/NM heterostructures,and specifically YIG/bcc-WCr with significantly improved spin current efficiency is favorable for YIG based spintronic devices.?3?We show tunable perpendicular magnetic anisotropy?PMA?induced by epitaxial strain and film composition in solid-solution garnets Y_3-XTm _XFe₅O₁₂?0?x?3?thin films over a wide range of thickness?10?30nm?by magnetron sputtering on(Gd_0.63Y_2.37)?Sc₂Ga₃?O₁₂?GYSGG?substrate.With increasing Tm concentration from x=0 to x=3,a continuous increase of effective PMA field(H_eff)ranging from 300Oe to2500Oe is observed in M-H hysteresis loops of single Y_3-XTm _XFe₅O₁₂ films.In bilayers consisting of Pt and Y_3-XTm _XFe₅O₁₂,the magnetotransport characterization results show that the surface of YTIG with perpendicular magnetization allows efficient equilibrium and/or nonequilibrium spin interexchange across the heterostructure interface.The large squared Hall hysteresis loop in Pt which resembled the out-of-plane magnetic hysteresis of the underlying YTIG films verify the persistence of PMA over the whole constitute films.The V_th-H loop of LSSE which resembled the in-plane magnetization component also confirm that considerable PMA(H_eff)ranging from300Oe to 2500Oe are readily formed in the solid-solution Y_3-XTm _XFe₅O₁₂ films.Furthermore,spin-orbit-torque?SOT?induced switching can be achieved in Y_3-XTm _XFe₅O₁₂/Pt?0?x?3?heterostructure with tunable critical current density ranging from 1.76×10⁽(??to 2.13×10⁽(??A·m^-2 with an external field as low as 10Oe.Our results demonstrated that solid-solution garnets as an effective way to tailor PMA and critical current density for SOT switching by modulating the crystal lattice constant mismatch between film and substrate.