Quantum Magnetism And Spin Transport Properties Of Metal-phthalocyanine

Spintronics has become a significant research direction in the development of information storage technologies,giving rise to novel storage technologies,such as spin-orbit torque magnetic random access memory(SOT-MRAM).Rooted in the utilization of spin angular momentum as the information carrier,spintronics encompasses three primary aspects:spin generation,transport,and detection.The generation or injection of spin predominantly relies on ferromagnetic materials(FM)endowed with spontaneous polarization.The detection of spin typically occurs in heavy metals(HM)or at interfaces exhibiting Rashba effect with spin-momentum locking.The core of spintronics lies in the efficient transmission of spin information.The longer spin lifetimes dictate the stability of information transmission and computational efficiency,serving as a key metric for the practical application of materials in spintronics devices.Organic semiconductor materials composed of lighter elements possess weaker spin-orbit coupling and reduced hyperfine interactions,leading to a significant reduction in spin momentum scattering.As a result,organic semiconductor materials exhibit longer spin diffusion distances and relaxation times,rendering them as ideal mediums for spin transport.Metal phthalocyanine(MPc)materials,as organic small molecules discovered over a century ago,possess quasi-one-dimensional chain arrangement characteristics and excellent optoelectronic properties.Moreover,they also offer numerous advantages such as rich variety,tunable structures,lightweight flexibility,ease of processing,and low production costs.These attributes make them a solid foundational platform for research into quantum magnetism as well as spin transport/detection.In this thesis,we selected the metal phthalocyanine family members H2Pc,CoPc,and FePc as our study subjects and investigated the low-dimensional magnetism of single crystals,spin transport properties of thin films,and the impact of interface effects on spin transport,aiming to reveal their underlying physical mechanisms.These findings hold significant implications for the design and development of spintronic devices.This thesis primarily comprises the following four aspects.(1)Quantum Magnetism of β-FePc Single CrystalLow-dimensional magnetism,including Haldane phase and large single ion anisotropy(Large-D)phase,has received ongoing attention due to their potential applications in quantum computing and molecular spintronics.However,the lack of materials possessing S=1 one-dimensional spin-chain with antiferromagnetic intrachain interaction(J)and single ion anisotropy(D)has hindered the investigation and practical implementation of this topic.In this study,FePc crystals in β-phase(β-FePc)hosting molecular chains along b-axis are prepared as revealed by X-ray diffraction(XRD).The molecular orientation is found to be almost upright within the natural cleaved bc plane as determined by synchrotron-based near-edge X-ray absorption fine structure spectroscopy(NEXAFS).β-FePc crystals exhibit clear magnetic anisotropy with the easy plane perpendicular to the b-axis,antiferromagnetic intrachain exchange interaction and a large D value.These results demonstrate experimentally that β-FePc crystal is a new antiferromagnetic spin-1 Large-D phase material.This discovery expands the scope of low-dimensional magnetism research in organic molecular crystals,providing a new platform to study the topological quantum phase transition in one-dimensional spin chains.(2)Spin Reorientation Induced Spin Memory Loss at Py/Pd InterfaceAchieving spin current switching functionality is crucial for the development next-generation low power information storage.In this study,the spin reorientation and temperature dependence of spin Hall angle θSH in the Permalloy(Py)/Pd bilayer were investigated by using ferromagnetic resonance(FMR),spin pumping,inverse spin Hall effect(ISHE)and quantum interference transport.The uniaxial ferromagnetic perpendicular magnetic anisotropy(PMA)induced by spin reorientation persists at the Py/Pd interface below 30 K.This PMA further enhances the interfacial spin scattering,leading to a reduction of injected spin current,as indicated by the underestimated θSH values.These experimental results demonstrate that the interfacial spin reorientation at the FM/HM interface,commonly employed in spintronic devices,causes a significant spin memory loss effect.Our findings provide valuable insights into the influence of interlayer spin configuration on spin transport,which can be utilized in the rational design of spintronic devices based on pure spin current.(3)Spin Transport Properties of MPcIn chapter five,we quantitatively analyzed the spin transport properties of MPc molecules and found that spin-orbit coupling is the dominant mechanism governing spin relaxation.Through measurements of inverse spin Hall voltage with different thickness of MPc,we quantified the spin diffusion length(Ls)and spin relaxation time of H2Pc,CoPc,and FePc thin films.At low temperatures,the temperature dependence of Ls is consistent with that of the average hopping distance of polarons.This correspondence suggests that the intermolecular spin transport occurs via polaron hopping,aligning with the theoretical framework of spin transport dominated by spin-orbit coupling.(4)Molecular Orientation and Rashba Effect at the MPc/Pd InterfaceIn Chapter Six,we investigated the modulation of spin detection by interface interactions.When MPc molecules are oriented vertically with respect to the Pd metal surface,the Rashba spin-orbit coupling at the interface is enhanced.By using PTCDA molecules to modulate the orientation of MPc molecules into a planar arrangement,the Rashba effect is diminished.These findings suggest that the amplified Rashba effect at the Pd/MPc interface likely stems from a larger interface electric dipole moment under the vertical molecular orientation.Through quantum coherent transport measurements,we also observed the Rashba spin-orbit coupling at the Pd/MPc interface,which enhanced weak antilocalization magnetoconductance of Pd.Utilizing Hikami-Larkin-Nagaoka theory,we estimated the Rashba coefficientαR at the interface to be approximately 2.61×10-12 eV m.This value is comparable to that of the oxide interface LaAlO3/SrTiO3,indicating the potential application of this structure in the design of organic spintronics devices.In summary,we have found that MPc crystals possess quasi-one-dimensional structures and low-dimensional quantum magnetism.β-FePc exhibits a one-dimensional spin Large-D phase with S=1,providing crucial experimental evidence for the study of quantum magnetic evolution.The interfacial magnetic anisotropy plays a pivotal role in spin functional devices.Modulating spin injection/transport through magnetic anisotropy represents an effective avenue for designing novel devices.The tunable spin-orbit coupling effect in MPc facilitates the investigation of spin relaxation in organic semiconductors.This phenomenon enables the targeted design of medium materials with long-range spin transport.At interfaces of MPc/HM,the orientation of organic molecules serves as a significant degree of freedom.The interfacial electric dipole moment exerts substantial control over Rashba spin-orbit coupling,highlighting the important role of interface in spintronics devices.These studies underscore the interdependence between magnetism and spin transport,both of which significantly rely on spin attribute of electrons.The exploration and application of spin-related properties in organic magnetic materials contribute to the advancement of next-generation information processing and storage technologies.