Two - dimensional carbon/ boron nitride-based materials for spintronics applications

Abstract

The desire to continuously reduce the size of spintronics stimulates the development of new spintronic materials in nanoscale. The appearance of two-dimensional materials with novel electronic and magnetic properties provides great opportunities for this purpose. Introducing transition metal embedded 2D materials provides another possibility to obtain magnetism in main group based 2D systems with high Curie temperature. Transition metal embedding significantly decreases the metal loading and induces magnetic character in the non-magnetic main group based atomically thin systems. The metal free main group based spintronic are an area of great interest for a decade since graphene has been explored. It is first obtained in experiment, partly due to its weak spin-orbit coupling for long lifetime spin transport and novel electronic properties. Meanwhile, metal-free 2D materials with only s and p electrons have also attracted intensive research interests. Besides, the p-electron based systems are important as they have a long spin relaxation time due to the presence of weak spin-orbit coupling. The electron and hole doping successfully induce magnetism by creating unpaired electron in metal free main group base 2D system. However, the Curie temperature in these metal-free 2D materials is still not as high as metal embedded systems. Though recently experiments shows that N doped graphene is half-metallic with very high Curie temperature (<600 K). So, research needs to be done in this field to improve the scope of main group based 2D materials in the field of nanospintronics. So, the joint ventures of both experimental and theoretical effort are still necessitated in near future for two dimensional main group based spintronic systems. The contents of each chapter included in the thesis are discussed briefly as follows:. Introduction In this chapter, a brief overview of the spintronics devices and their working principle has been discussed. However, we have focused mainly on two dimensional (2D) main group based materials. The benefit of low dimensional materials over bulk materials has also been discussed. The recent advantages and potential applications of different types of atomically thin low dimensional materials (2D, 1D, 0D) for spintronics applications have been thoroughly reviewed. Furthermore, the applicability of materials with half-metallic character and high Curie temperature and their possible applications as spintronics materials have been discussed. In addition, the role of holes/electrons towards magnetism has been discussed. The stability of the materials and the resistivity of half-metallicity from heat and strain have also been deliberated. We have used the density functional theory (DFT) to calculate the properties of the reported systems. Therefore, in the last part of this chapter, we discuss the basis of the density functional theory (DFT) and its importance towards spin related property calculations. This chapter also covers the basic theory behind the calculation of Curie temperature using mean field theory (MFT) and Monte Carlo simulation. Furthermore, we have discussed the basic theory behind phonon dispersions, molecular dynamics, and charge related calculations. 2. Transition-Metal Embedded Carbon Nitride Monolayers: High- Temperature Ferromagnetism and Half-Metallicity In this chapter, we report the potential applicability of first row transition metal (TM) embedded atomically thin carbon nitride (TM@gt-C3N4) systems for spintronics applications. This is done as such TM embedding creates unsaturated p/d orbitals and thus induces local magnetic moment. The triazine based graphitic carbon nitride (gt-C3N4) has been chosen, as it has been synthesized and found to be a stable system with band gap of1.59 eV. The electronic and magnetic properties are explored in detail to find out the origin of magnetism and half-metallicity in such TM@gt- C3N4 systems. The detailed investigation reveals that the transition metal embedding is the main reason behind the magnetic moments. This is due to partially filled TM 3d-orbitals. The d orbitals splitting and projected density of states (PDOS) also confirms that the unpaired electrons in the dorbitals are the solo reason behind the half-metallicity. Our stability related calculations show that they are quite stable up to 500 K. The Cr@gt-C3N4 system is a ferromagnetic half-metallic with a Curie temperature of ~450 K. The calculated Curie temperature is noticeably high compared to the planar 2D materials studied to date. The calculated magnetic anisotropy energy (MAE) in Cr@gt-C3N4 is as high as 137.26 μeV/Cr. Thereby, such atomically thin TM@gt-C3N4 are promising for high temperature spintronics devices. 3. Transition-Metal Embedded gt-C3N3 Monolayers: High Temperature Ferromagnetism and High Anisotropy In this chapter, a metallic phase of the carbon-nitride monolayer (gt-C3N3) has been chosen for possible spintronics applications. Therefore, the gt- C3N3 system is quite different from our earlier studied gt-C3N4 system as gt-C3N3 is metallic, whereas gt-C3N4 is semiconducting. For this, first-row transition metal (TM = Sc to Cu) embedded carbon nitride (TM@gt-C3N3) systems are thoroughly studied. Our electronic structures show that partially filled transition metal 3d orbitals are the origin of magnetism. The d-orbital splitting can explain further the geometry and magnetic moment of the studied systems. High temperature ferromagnetism and high magnetic anisotropy energy (MAE) found in Cr, Mn and Fe incorporated gt-C3N3 systems. The ferromagnetism in Cr@gt-C3N3 system can survive up to 325-338 K. Further the ferromagnetism in Cr@gt-C3N3 can survive up to 10% tensile strain. It also shows a MAE value of 4.02 meV/Cr, which can be increased to ~24 meV/Cr under external electricield of 0.1 V/Å. Furthermore, these materials show remarkable thermal and mechanical stability. Therefore, such material with remarkable stability and high MAE can be very promising for spintronics and memory device. 4. Metal-Free Half-Metallicity in a High Energy Phase C-doped gh- C3N4 System: A High Curie Temperature Planar System Metal free spintronics devices are very important due to weak spin-orbit coupling in p-electronic systems, which is important for long spin relaxation time. Here, the atomically thin graphitic heptazine carbon nitride (gh-C3N4) based systems are studied for possible metal free spintronics application. The gh-C3N4 is one of the most stable carbon nitride systems and easily synthesizable. It is a semiconductor with band gap of 2.67 eV. C doping at the N site of gh-C3N4 system induces a hole in the system. Such hole doping is an effective strategy to induce magnetism in metal free non-magnetic system. Interestingly, ferromagnetism is observed in all the C-doped gh–C3N4 systems. However, strong half– metallicity is achieved only in the high energy phase of C-doped systems (CN2@gh–C3N4). This could be due to high unsaturation of doped C atom at the edge of the large voids present in the gh-C3N4 framework, which lead to the localization of electronic states. Furthermore, the halfmetallicity is lost as the C-doping concentration decreases. Thus, not only the selective doping but also concentration plays an important role towards magnetism and half-metallicity. Furthermore, the stability of CN2@gh– C3N4 system is evaluated from energetic (formation and binding energy), mechanical (stress vs. strain), dynamical (phonon dispersion), and thermal (molecular dynamics simulation) studies. It is found that the CN2@gh– C3N4 is dynamically, thermodynamically and mechanically stable. We find that the ferromagnetism in CN2@gh–C3N4 can survive up to ~400 K based on our mean field theory and Monte Carlo simulations. Besides, the half–metallic in CN2@gh–C3N4 can sustain up to 3% tensile strain. Thus,he high-energy phase of C-doped gh–C3N4 (CN2@gh–C3N4) system is a high Curie temperature ferromagnetic half–metallic material system. 5. Ferromagnetism and Half-metallicity in Atomically Thin Holey Nitrogenated Graphene Based System In this chapter, atomically thin holey nitrogenated graphene (C2N) based systems are studied for possible metal-free spintronics applications. The atomically thin planar C2N is a recently synthesized carbon nitride structure with evenly distributed holes and nitrogen atoms [11]. Previous studies suggest that hole doping is an effective way to create unsaturation and induce magnetism in metal free non-magnetic systems. Hence, C doping (hole doping) is done at the N site of C2N. Ferromagnetism is observed in all C-doped C2N systems. Besides, a strong half–metallicity is observed after achieving a particular C-doping concentration (16.67%) (CN@C2N). The presence of half-metallicity can be explained based on the unsaturation on the doped C atom. Furthermore, such localization of electronic state is stabilized due to the presence of large voids in the C2N framework. However, the ferromagnetism depends on doping concentration. The stability of planar two-dimensional CN@C2N system is evaluated from energetic (total, formation, and binding energies), mechanical (stress vs. strain), dynamical (phonon dispersion), and thermal (molecular dynamics simulation) studies. It is found that the CN@C2N system is dynamically, thermodynamically and mechanically stable. Furthermore, the ferromagnetism in CN@C2N system can survive up to 297 K. Besides, the half–metallicity in CN@C2N shows remarkable strain sustainability. It can sustain up to 7% tensile uniaxial strain. 6. Ferromagnetism and Half-Metallicity in a High Band Gap h-BN System Like carbon nitrides, hexagonal boron nitride (h-BN) is another important material due to its stability, wide band gap and multifunctional properties.However, we wanted to see whether hole doping has similar effect in BN system as it’s a wide band gap system. In this chapter, different patterned (Pattern-A and Pattern-B) C-doped h-BN based systems are studied for possible spintronics devices. Ferromagnetism is observed in 3.125% and 9.375% C@h-BN systems. Besides, half–metallicity is observed after achieving a particular C-doping concentration (9.375%) irrespective of the doping site. The presence of half-metallicity can be explained based on the unpaired electron in the C 2p orbital. However, the half-metallicity is vanished as the C-doping concentration changes. Our molecular study in C doped borazine molecule shows that the gap between two spin states is maximum, when C is doped at B-site of borazine. This indicates that Cdoping at B-site stabilizes the singly occupied electron, which may slow down the spin relaxation process and thus promising for spintronic applications. The stability of planar two-dimensional CB/N@h-BN system is evaluated from energetic (total, formation, and binding energies), mechanical (stress vs. strain), dynamical (phonon dispersion), and thermal (molecular dynamics simulation) studies, which show that CB@h-BN is more stable compared to CN@h-BN. Besides, the ferromagnetism in the CB@h-BN system can survive up to 324 K. The half–metallicity in CB@h-BN shows remarkable strain sustainability. It can sustain up to 2% uniaxial and 3% biaxial tensile strains. Thus, it is predicted that a C-doped h-BN (CB@h-BN) system can be a promising ferromagnetic half–metallic material for memory and spintronics devices. 7. Conclusions The conclusions of the thesis can be described as follows: i) First row transition metal embedded carbon nitride (gt-C3N4) systems show ferromagnetism and half-metallicity. The Cr@gt-C3N4 shows Curie temperature as high as ~450 K, which is higher than that any 2D systems studied to date.ii) A new phase of the carbon-nitride monolayer (gt-C3N3) can be promising for magnetism. High temperature ferromagnetism and high magnetic anisotropy energy (MAE) observed in transition metal incorporated high-energy phase carbon nitride systems (TM@gt-C3N3). iii) A high-energy phase graphitic heptazine carbon nitride (gh- C3N4) based system show ferromagnetism and half–metallicity on hole doping. Such high-energy phase structures can be promising for spintronics applications. iv) Selective doping and concentration plays important role to induce ferromagnetism and half-metallicity in atomically thin holey nitrogenated graphene (C2N) based systems. A strong half–metallicity is observed after achieving a particular Cdoping concentration (16.67%) in C2N (CN@C2N). Concentration plays an important role. v) Different patterned C-doping at B and N site of h-BN shows that half–metallicity can be achieved in a high band gap material. Furthermore, ferromagnetism and half-metallicity can be achieved irrespective of the doping site.