Magnetic And Electrical Transport Properties Of Single Crystal Transition Metal Oxide Thin Films And Heterostructures

Spintronics is a new discipline that jointly utilizes the spin and charge degrees of freedom to exhabit unique magnetic or electrical properties of materials or heterostructures.Since the discovery of the Giant magnetoresistance(GMR)in 1988,spintronics has rapidly developed in demand for information storage,computing,communication,biological neural network simulation,energy conversion,and high-sensitivity sensors.In the last 30 years,researchers have explored a wide variety of spintronic materials through upgraded preparation techniques,advanced micro-nano fabrication techniques and advanced characterization methods.Among these materials,transition metal oxides comprise a wide class of materials,exhibiting rich crystal structures and physical properties that make them ideal candidates for both theoretical and experimental studies.For example,the materials studied include antiferromagnetic materials for exchange bias effect,ferromagnetic insulators for spin pumping processes,high-temperature superconducting materials for commercial applications,perovskite manganese oxides with colossal magnetoresistance effect,magnetic semiconductor materials for semiconductor industry,and multiferroic materials with both ferroelectric and ferromagnetic features.In particular,these materials and their heterostructures have attracted attentions in the fields of magnetoresistance,spin pumping,metal-insulator transition and superconductivity,which have exhibited novel electrical or magnetic properties.Recently,theoretical and experimental researches of oxide antiferromagnets has made great breakthroughs.In addition to their intrinsic property with magnon frequencies extending into the terahertz range,the heterostructures consisting of ferromagnetic materials and oxide antiferromagnets exhibit excellent characteristics in the aspects of exchange bias,antiferromagnetic exchange spring effect,tunneling anisotropic magnetoresistance and enhanced injection of spin current.Furthermore,theoretical and experimental works have also revealed that manipulation of antiferromagnetic order can extend the functionality of antiferromagnetic materials,such as creating ferromagnetic surfaces by uncompensated magnetic moments,introducing localized conductive paths by dislocations,changing the spin-orbit coupling strength by changing the composition,and modulating the orientations of magnetic moments by external field and temperature,etc.The transition metal oxide CoO is a key antiferromagnetic material.Because the CoO bulk material has high magnetocrystalline anisotropy and experimentally accessible Neel temperature(-293 K).On the other hand,crystalline CoO has NaCl-type lattice structure and collinear spin structures,while the CoO(111)planes are both polar and uncompensated magnetic planes.Therefore,single crystal CoO is an ideal material for studying the antiferromagnetic properties and manipulating antiferromagnetic order.Works about manipulating antiferromagnetic order in high-quality single crystal films is still highly desirable in experiment.In this work,we reported that high-quality single crystal epitaxial CoO(111)thin films were grown.In Zn doped CoO(111)thin film,we found that both Zn composition and oxygen vacancies affect the coexistence of antiferromagnetism and ferromagnetism in ZnxCo1-xO films.While in ultrathin heterostructures the magnetic and electrical transport properties of[Co/CoO]n superlattices are modulated by the thickness of Co and CoO layers.Oxide ferromagnetic semiconductors has attracted attention for about twenty years.The main stream for ferromagnetic semiconductors is to intergrate magnetism with semiconductor within a material in semiconductor spintronics devices.From the application view,a ferromagnetic semiconductor with high Curie temperature and large saturation magnetization is required.For this goal,a variety of magnetic semiconductors have been experimentally studied,such as GaMnAs,MnGe,TiCoO,ZnCoO,etc.However,either lacking high Curie temperature and large saturation magnetization or the existence of extrinsic room-temperature ferromagnetic phases hinders the applications of magnetic semiconductors in practical spintronic devices.Up to date,high quality single crystal magnetic semiconductors with high transition metal concentrations are still highly desirable.In order to achieve this goal,we choose to investigate quaternary fcc-Cox(MgyZn1-y)1-xO1-v(CoMgZnO)epitaxial thin films.Because ternary MgxZn1-xO shows structural evolution from ZnO-based hexagonal structure to MgO-based face-centered-cubic structure with increasing Mg concentration and CoxMg1-xO shows fcc structure within the entire composition range.Hence,quaternary fcc-CoMgZnO provides an alternative way to break the low solubility limitation of transitional metal.Moreover,band gap engineering could be expected in CoMgZnO by tuning the composition ratio of Mg/Zn.Above all,magnetic property modulation could be achieved by tuning the Co and oxygen vacancy concentration in the CoMgZnO films,which makes CoMgZnO a promising candidate for future optical and spintronics applications.Here,we systematacially studied the magnetic properties of CoMgZnO single crystal film and the multiple magnetic phases modulated by the composition and oxygen vacancy concentration.This dissertation includes the following three parts,1.Modulated antiferromagnetism in ZnxCo1-xO single crystal epitaxial filmsHigh-quality single crystal epitaxial ZnxCo1-xO(111)thin films were grown by co-evaporating Zn and Co and simultaneously oxidizing in oxygen plasma.It is found that the exchange bias fields of Co/as-prepared ZnxCo1-xO heterostructures decrease with increasing Zn composition.When as-prepared ZnxCo1-xO layer was annealed in oxygen plasma to remove oxygen vacancies,the exchange bias field of Co/oxygen-plasma-annealed ZnxCo1-xO heterostructures can be further enhanced.Moreover,a weak ferromagnetism was observed at 300 K in the as-prepared antiferromagnetic ZnxCo1-xO film with oxygen vacancies,but it did not exist in oxygen-plasma-annealed ZnxCo1-xO film.This indicates that oxygen vacancies can simultaneously weaken the antiferromagnetism but enhance the ferromagnetism of ZnxCo1-xO layer.Therefore,we offer a method of manipulating the antiferromagnetism of ZnxCo1-xO films by changing Zn composition and oxygen vacancies,which is useful for designing antiferromagnetic spintronic devices.?.Structure,magnetism,and electrical transport properties of single crystal epitaxial[Co/CoO]n superlatticesCompared with bilayer heterostructures,it is very challenging in experiments to prepare single crystal epitaxial ultrathin superlattices consisting of ferromagnetic metal and antiferromagnetic oxide,where the interface exchange coupling,magnetic order,and electrical transport properties are expected to be greatly modulated due to the multiple interfaces and periodical structures.Here we prepared single crystal epitaxial[Co/CoO]n superlattices consisting of ferromagnetic metal Co and antiferromagnetic oxide CoO by molecular beam epitaxy.It is found that the saturation magnetization of[Co 0.6nm/CoO 1.2nm]5 superlattice is robust against temperature increase,which only shows a slight decrease of 1.5%from 5 K to 300 K.Moreover,we found that the longitudinal resistivity and anomalous Hall resistivity of[Co/CoO]n superlattices are modulated by the thickness of Co and CoO layers,which are distinguished from the electrical transport properties of Co bulk ferromagnetic metal.The[Co/CoO]n superlattices provide a new insight into heterostructures consisting of ferromagnetic metal and antiferromagnetic insulator,which are beneficial for designing future spintronics devices.?.Controlled multiple magnetic phases in epitaxial single crystal Cox(Mg0.55Zn0.45)1-xO-v thin filmsHigh quality single-crystal fcc-Cox(Mg0.55Z0.45)1-xO1-v epitaxial thin films with high Co concentration up to x=0.5 have been fabricated by molecular beam epitaxy.Systematic magnetic property characterization and soft X-ray absorption spectroscopy analysis indicate that the coexistence of ferromagnetic regions,superparamagnetic clusters,and non-magnetic boundaries in the as-prepared Cox(Mg0.55Zn0.45)1-xO1-v films is a consequence of the intrinsic inhomogeneous distribution of oxygen vacancies.Furthermore,the relative strength of multiple phases could be modulated by controlling the oxygen partial pressure during sample preparation.Armed with both controllable magnetic properties and tunable band-gap,Cox(MgyZn1-y)1-xO1-y films may have promising applications in future spintronics.