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Strain and Interface Effects on Magnetocrystalline Anisotropy of MnN

Robert A. Lawrence, Matt I. J. Probert

Abstract

Thin film effects on the Magnetocrystalline Anisotropy Energy (MAE) of MnN were studied using density functional theory (DFT). Initially, strain effects on bulk MnN were considered as a proxy for lattice-matching induced strain and a linear relationship between the $c/a$ ratio and the MAE was found. A fundamental explanation for this relationship in terms of the underlying point-group symmetry is given, which we show is applicable to all uniaxial magnetic materials. Strain and charge-transfer effects were then considered for an ultra-thin film. It was found that a Ta seed-layer suppresses the net spin moment on the Mn ions, leading to a reduction of the MAE. Charge transfer is shown to be the cause of this, and hence similar effects may be expected at any magnetic heterostructure interface.

Strain and Interface Effects on Magnetocrystalline Anisotropy of MnN

Abstract

Thin film effects on the Magnetocrystalline Anisotropy Energy (MAE) of MnN were studied using density functional theory (DFT). Initially, strain effects on bulk MnN were considered as a proxy for lattice-matching induced strain and a linear relationship between the ratio and the MAE was found. A fundamental explanation for this relationship in terms of the underlying point-group symmetry is given, which we show is applicable to all uniaxial magnetic materials. Strain and charge-transfer effects were then considered for an ultra-thin film. It was found that a Ta seed-layer suppresses the net spin moment on the Mn ions, leading to a reduction of the MAE. Charge transfer is shown to be the cause of this, and hence similar effects may be expected at any magnetic heterostructure interface.

Paper Structure

This paper contains 16 sections, 13 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Background Theory
  3. Non-van der Waals Heterostructures
  4. Disorder
  5. Lattice Matching and Strain
  6. Fermi Level Equilibration and Charge Transfer
  7. Shape Anisotropy
  8. Computational Methods
  9. Surface Reconstruction
  10. Results and Discussion
  11. Strain effects on Bulk MnN
  12. Applicability to Other Magnetic Systems
  13. Strain Effects in the Ultra-thin Film Limit
  14. Non-Strain Effects of a Seed Layer
  15. Conclusions
  16. ...and 1 more sections

Figures (5)

  • Figure 1: Schematic of changing crystal field splitting against relative uniaxial elongation / compression of an octahedral point group. The magnitude of the splitting depends on the magnitude of the charge asphericity that cause the splitting.
  • Figure 2: a) H (light pink) -passivated Mn (purple) N (light blue) bilayer on Ta (bronze), b) H-passivated MnN bilayer, c) MnN bilayer, d) Bulk MnN. Spins are represented by red arrows, with the length of the arrow representing the per-site projection of the spin density. Spin magnitudes are reported in table \ref{['tab:Spin-length']}
  • Figure 3: MAE (in meV/ formula unit) against the $c/a$ ratio for bulk phases of MnN. Both the unit cells with relaxed $\vec{c}$ (highlighted by blue circles) and unrelaxed $\vec{c}$ lie on the same line. The vertical dashed line indicate the $c/a$ ratio for bulk MnN, and the dashed horizontal line the cubic anisotropy case.
  • Figure 4: Change in quadrupole moment with application of a biaxial strain. The symmetry of the change in bond lengths with lattice parameters ensures no net dipole moment about the Mn-site, and the change in effective monopole moment with any displacement that is not totally symmetric gives rise to a change in the quadrupole moment that is linear with the $c/a$ ratio. Note, in a real system, the effective charge, q, need not be an integer multiple of the electron charge.
  • Figure 5: MAE for the ultrathin film case of MnN shown in panel b of figure \ref{['fig:MnN-Prog']}. The vertical black dotted line indicates the inverse lattice parameter for MnN on a Ta seed layer.