The Magnetic Origin And Coupling Mechanism In Dilute Magnetic Semiconductors (DMSs) Studied By Positron Annihilation Techniques

Spintronic devices will bring great changes to the IT field in the future due to their outstanding properties such as high-speed operation, low power consumption, etc. Dilute magnetic semiconductors (DMSs) have attracted increasing attention in the past decade because of their promising technological applications in the field of spintronics. A lot of groups focus their efforts on the topic of magnetic origin, while no agreed mechanism of observed ferromagnetism (FM) was established clearly. This topic rigorously analyzed the relationship between the magnetic properties and microstructures in some of DMSs by using positron annihilation spectroscopy, which was proved to be a sensitive tool for study of microstructures and defects in materials. Based on the positron facilities in the State Key Laboratory of Particle Detection and Electronics of China, we studied the magnetic mechanisms in DMSs, including origin of DO magnetism, defect-induced FM, and the effect of microstructures on the coupling between magnetic ions. Moreover, the first-principle calculation (based on density functional theory) also was developed to study the spin-density states and magnetic interaction in different magnetic microstructures. We try to give original insight in magnetic origin and coupling mechanism in different magnetic systems, and resolve some controversial issues in the field of DMSs. Research on this topic will provide theoretical basis for spintronics application and benefit the development of information technology in the future. The main contents are as follows.1. We implanted virgin MgO and Al2O3single crystals with non-magnetic anions at room temperature to investigate the origin of DO magnetism in these systems. The annealing effects on the microstructures and induced defects of these samples were determined by positron annihilation spectroscopy, X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), etc. All samples with high-dose implantation show room temperature ferromagnetism, and the relevance of the ferromagnetism on microstructures was studied. Experimental results indicate that ferromagnetism can be introduced to metal oxides by cation vacancies. The implanted C/N ions played more effective role in ferromagnetic performance than VMg in MgO systems. However, the VMg coexistence with C or N ions may play coupling and inhibition role in magnetic performance in these MgO samples, respectively. The magnetic moment possibly occurred from the localized wave function of unpaired electrons and the exchange interaction formed a long-range magnetic order. The phenomenological bound magnetic polaron model (BMP) was employed to understand magnetic coupling of localized moments in these insulated metal oxides.2. ZnO-based DMSs were extensively investigated as a promising candidate for spintronics applications due to their outstanding characteristics such as wide-band gap, large solubility of transition metal ions and spin injection efficiency, etc. Co-and Mn-doped ZnO have aroused widespread interest as the most attractive DMS candidates, while the origin and mechanism of the observed ferromagnetism is far from being clearly understood, since experimental and theoretical studies on the magnetic properties of Co/Mn-doped ZnO show a number of contradictions. Further studies are required to understand the magnetic mechanism of this system in detail. We investigated the structural and magnetic properties of Co-and Mn-doped ZnO films deposited on sapphire substrates by pulsed-laser deposition. Also the pure ZnO film was prepared under identical growth conditions for comparison and further N-implantation. Positron annihilation spectroscopy with other characterized methods was used to investigate the dependence of Magnetic properties on doped-ions and crystal defects. Experimental results reveal the coexistence of ferromagnetism with super-paramagnetic behavior in Co-doped ZnO films. We confirm the ferromagnetism is intrinsic property of samples and oxygen vacancies play an important mediation role in Co-Co coupling. Super-paramagnetism probably arises from the nanosize effect or nano-scale aggregation of ferromagnetism. Only low temperature ferromagnetism (Curie temperature lower than50K) was observed in Mn-doped samples, which might be interpreted as p-d hybridization from indirect coupling of Mn ions (Mn-O-Mn). N-implantation implies that DO ferromagnetism in ZnO films may arise from VZn instead of neutral oxygen vacancies.3. We made use of ion implantation method to induce Zn ions in6H-SiC single crystal, certain defects were also produced in materials after implantation. Firstly we found that lattice defects can induce ferromagnetism in C-implanted6H-SiC sample. Following the increase of annealing temperature, magnetization in Zn-implanted6H-SiC sample reduced gradually until disappear at first stage and then remerged, and increased to a certain level at second stage. Experimental results indicate that ferromagnetic performance cannot be subscribed by only one coupling mechanism in Zn-doped6H-SiC samples. Based on the microstructural analysis, we consider that substitutional Zn ions contribute to ferromagnetism in the first stage of annealing sample, and the O-Si combined bond induced Si vacancies play an important role in magnetization increase in the second annealing stage.4. For further analysis the experimental results and study the magnetic mechanism of DMSs, we employed first-principle calculation (based on density functional theory) to calculate the magnetic structures of semiconductor systems mentioned in above experimental works. Density functional theory calculations were performed using the plane-wave pseudopotential method in the Vienna AB initio simulation package (VASP). Calculation results show that room-temperature ferromagnetism in MgO supercell originated in the electronic status around Mg vacancies due to the spin polarization of2p electrons of the oxygen atoms surrounding the Mg vacancies. Also it’s proved the VMg involve in complex interaction with N or C ions and play different role in magnetic performance in these samples. Zn vacancies can introduce localized magnetic moments in ZnO lattices and coupling interaction form long-range magnetic order. The magnetic interaction between Co-Co (Mn-Mn) in defect-free supercell of ZnO may tend to form antiferromagnetic ground state. Oxygen vacancies play an important mediation role in coupling of magnetic ions. Besides, we also calculated the defect induced magnetism in Al2O3systems and the Zn-doped SiC systems, the results are in good agreement with the experimental discussions.