# Presentation: 502983

## Si-based Half-Metallic Transition Metal Heterostructures

- Michael Shaughnessy
- UC Davis Department of Physics
- Collaborators:	R. Snow, C.Y. Fong, J. E. Pask, L. H. Yang.
**                                                 **
- Work supported by NSF, DOE
- California APS meeting 2007

**Notes:**
Si based half metallic transition metal heterostructures

## Outline

- Method of Calculation
- Results and Discussion
- Ideal DFH (Digital Ferromagnetic      Heterostructure)
- Substitutional Defects
- Vacancy
- Summary

## Motivation: Why Half-Metals?

- Julliere Formula for Magnetoresistance:  MR = 2P2/(1-P2)
- where P is the spin polarization at EF.
- P = (DOS↑- DOS↓)/(DOS↑+ DOS↓).
- For a half-metal, P =1 and MR  ∞ (As a reference P for iron is 0.45)
- Consequently half-metals are ideal candidates for spintronic devices making use
- of magnetoresistance properties

## Why Si-based half-metals?

- Other materials suffer significant drawbacks:
- Heusler alloys (NiMnSb, Co2MnSi) (R. A. de Groot et al., PRL **50**, 2024 (1983)),
- (2)	Transition metal oxides (Fe3O4, CrO2, etc.) (K. Inomata et al. J. Phys. D: Appl. Phys. **39**, 816 (2006)),
- Transition metal pnictides, chalcogenides (CrAs, VTe) (W. H. Xie et al. PRL **91**, 037204 (2003), J. Pask, et al. PRB **67**, 224420 (2003)).
- The above materials have difficulties in growth due to complicated stoichiometry and multiple crystal phases as well as complete or partial spin-depolarization at high temperatures; we are led to consider Si-based half-metals.

**Notes:**
Heusller alloys with defect problems
Color “complicate ..” and “multiple crys...”

## Encouraging results: digital ferromagnetic heterostructure.

- We have recently designed a -layer doped Mn in Si, a Si-based digital ferromagnetic heterostructure (Si-DFH). It is a two dimensional half metal and is unlikely to have a T* (88 K) (at which HM properties disappear in Heusler alloys)  high Tc.
- M. C. Qian et al. Phys . Rev. Lett. **96**, 027211 (2006).

**Encouraging results****: ****digital ferromagnetic heterostructure****.**

## Method of Calculation

-          Some details are:
- The ultrasoft pseudopotentials for the elements are determined 	by using the normal valence electronic configurations. 	For example, Mn has (3d)5(4s)2.
- The Perdew 91 version of generalized gradient approximation, 	GGA, is used for the exchange and correlation - 		characterizing the electron-electron interactions in 		addition to the Coulomb interaction.

**Notes:**
Indent configurations

## Results and Discussion

## The model show at left has one Si atom fixed and the rest are relaxed to minimize the energy. The optimal lattice constant is 

- The model show at left has one Si atom fixed and the rest are relaxed to minimize the energy. The optimal lattice constant is
- a = 5.47 Å. This is a half-metal: the up spin states are metallic and there is a gap for the down spin states. Egap= 0.25 eV.The magnetic moment is 12 μB/cell (four Mn atoms/cell).

## Defects: Substitional (two types) and Vacancy

- Extra Si                             Extra Mn                             Vacancy

## Ideal DFH

- Extra Si

**Notes:**
Move up results...

## The presence of 25% vacancies in the -layer destroys half-metallicity;

- The presence of 25% vacancies in the -layer destroys half-metallicity;
- there is non-zero DOS for both spins at EF. We hypothesize that this is
- due to the reduced d-p hybridization between the Mn in the layer and
- the adjacent Si’s.

## Subtracted Charge Densities

- Ideal - Vacancy
- Isosurface at ∆ = 0.00092

- Ideal - Extra Si
- Isosurface at ∆ = -0.00026

## Summary

|  | Half-Metal? | μB/unit-cell
 | gap (eV) |
| --- | --- | --- | --- |
| Ideal | yes | 12.0 | 0.25 |
| Vacancy (25%) | no | 10.02 | -- |
| Extra Si | yes | 9.0 | 0.29 |
| Extra Mn | yes | 11.0 | 0.13 |

**Notes:**
Decimal on 12

## 	 The exchange interaction (molecular field) determines	the relative energy of the spin up and down states.

- 	 The **exchange interaction** (molecular field) determines	the relative energy of the spin up and down states.

**Notes:**
Move table before as “summary”