First-principles calculations of electronics and magnetic properties of L10 binary alloys in spintronics

Abstract

Magnetism being one of the exotic phenomena holds a distinct position with the ad-vancement in the scientific technology. Living in the era of information and social net-working, enormous information is generated and stored which is only made possible due to growth in storage and memory technology. Due to the limitations of semiconductor technology, a search for non-volatile memory has been rigorously pursued and magnetic random access memory (MRAM) which works on the principles of spintronics turns out to be a potential candidate. Storing information relies on the magnetic retention property arising from the magnetic anisotropy of the storage layer where for writing magnetization switching need to be realized.

This dissertation is structured as follow. In chapter 1, we introduce magnetism and the magnetic anisotropy. We provide a brief history of spintronics and explain in detail the work principle of MRAM. We shed light on the physical phenomenon that provide the basics of reading, writing and storing information. Finally, we discuss the development of memory devices that somehow overcome the drawbacks of spin-transfer torque (STT) MRAM. In chapter 2, we recapitulate the foundation of density functional theory (DFT)

4

and discuss the general problems due to approximation in the exchange correlation energy functional. In chapter 3, the voltage control of magnetic anisotropy via the magnetoelectric effect that offer great potential for ultra low power, highly scalable and high density magneto-electric random access memory (MeRAM) is discussed. Here, by using first principles cal-culations, we investigate a potential hetrostructure of FePt/MgO and predict a huge voltage control magnetic anisotropy (VCMA) coefficient 1.77 (-1.36) pJ=(V m) for Fe- (Pt) interface. Fe-interface FePt/MgO film exhibits strain-induced magnetization reorientation instigated through second order magnetoelastic coupling. We predict a significant effect of external field on the induced electric dipole that can be understood in terms of charge accu-mulation or depletion at the metal/dielectric interface. Finally, we demonstrate that VCMA behavior is extremely sensitive to the applied strain due to induced orbital reconstruction. These results are a step forward to achieve a full potential of MeRAM with write voltage below 1 V and switching bit energy below 1 fJ.

In chapter 4, we study the magneto-electric effect in ferromagnetic-ferroelectric het-rostructure based on first principles calculations. Using FePt/BaTiO3 hetrostructure as a representative model, we find different bond lengths between atoms at the interface under polarization reversal that lead to sizable difference in magnetic moments. We predict a record value of magneto-electric coefficient surf = 4:37 10 9 G cm2 which decreases V

linearly under tensilr strain. Also we anticipate magnetization switching from in-plane to out-of-plane upon polarization reversal at 1.75% strain. In chapter 5, we investigated the magnetocrystalline anisotropy (MCA) of MnPt (001) film and MnPt/MgO (001) using ab initio electronic structure calculations. We found that

5

the magnetic ground state of the MnPt film strongly depends on thickness (n). In bulk and in film with n 7 monolayers; AFM-II is the magnetic ground state. However, as the film thickness reduces, the magnetic ground state shifts from AFM-II to AFM-III. We employ layer-by-layer Heisenberg model to explain thickness dependent ground state transition. AFM-III state in the Mn- terminated film shows large perpendicular MCA(PMCA), which further enhances on a MgO(001) substrate. Interface plays a key role for the enhancement of PMCA on the substrate.

In chapter 6, the controversial magnetic structures of fcc Fe/Cu(001), we carry out non-collinear-spin density-fucntional-theory calculations for epitaxial Fe films on Cu(001) substrate. We consider various magnetic configurations including FM, stripe AFM (AFM-A), single-layer AFM (AFM-G), double-layer AFM (AFM-2G), and non-collinear spin spiral (SS) states. The top most two surface layers of Fe are always found to be ferro-magnetically coupled regardless of the film thickness, while the magnetic coupling among other Fe layers varies depending on the layer thickness (n). Also our total energy results exclude a non-collinear ordering in the ultrathin fcc Fe film on Cu (001). Finally a conclusive note is given at the end of each chapter. Some derivation is de-ferred to the appendix.