# Presentation: 674573

## Ferromagnetic semiconductors for spintronics

***Kevin Edmonds, Kaiyou Wang, Richard Campion, Devin Giddings, Nicola Farley, Tom Foxon, Bryan Gallagher, Tomas Jungwirth***
***School of Physics & Astronomy, University of Nottingham***
***Mike Sawicki, Tomasz Dietl***
***IFPAN, Warsaw, Poland***
***Tarnjit Johal, Gerrit van der Laan***
***Daresbury Laboratory***

## Semiconductor spintronics

**Semiconductor spintronics**

**Benefits: ****	****Fast, small, low dissipation devices**
**	****	****Quantum computation?**
**	****	****New physics**

## (Ga,Mn)As

***H. Ohno et al.***** (1996): ferromagnetism in GaAs thin films doped ~5% with Mn**

**Growth by low temperature MBE to beat equilibrium solubility limit**

## Carrier-mediated ferromagnetism

**Carrier-mediated ferromagnetism**

**Substitutional Mn is an acceptor and a J=5/2 magnetic moment.**** **

**Ferromagnetism driven by antiferromagnetic exchange coupling **
***J*****p-d ****S****.****s**
**between Mn moments and spin-polarised GaAs valence electrons**

- 	**Carrier density determines the key magnetic ****	****properties of (Ga,Mn)As (e.g. T****C****, H****C****,...)**

**Mn: [Ar] 3*****d*****5**** 4*****s*****2**
**Ga: [Ar] 3*****d*****10**** 4*****s*****2**** 4*****p*****1**

**Mn**

## Carrier-mediated ferromagnetism

**Spin-FET**
*H. Ohno et al., Nature (2000)*

- Vgate

- InMnAs

**Photogenerated magnetism          ***Koshihara PRL (1997)*

- InMnAs

- GaSb

***B***** (mT)**

***ħ*********

## Curie temperatures

**Max. T****C****=****172K****  (so far...)**
*	**	**	**	**	**Wang et al., JAP ‘04*

## Interstitial Mn:                    a magnetism killer

*Yu et al., PRB ’02*:
** ****~10-20% of total Mn concentration is incorporated as interstitials**
**Increased T****C**** on annealing corresponds to removal of these defects.**

**Mn**

**As**

**Negative effects on magnetic order:**
******compensating donor – reduces hole density**
******antiferromagnetic coupling between interstitial and substitutional Mn predicted   ***Blinowski PRB ‘03*

## 1D diffusion  process

**1D diffusion  process**

**Diffusion to free surface**
** ****- activation energy ******** 0.7eV**

*Edmonds, Bogus**ławski** et al., PRL 92, 037201 (2004)*

**T=190****o****C**

## Magnetic moment and antiferromagnetic coupling

**XMCD asymmetry ******** 55%**
**Magnetic moment ******** 4.5**** ********B**

**XMCD asymmetry ******** 30%**
**Magnetic moment ******** 2.3**** ********B**

- X-ray absorption measurements, ALS line 4.0.2 and ESRF line ID8:

## Ferromagnetic moment vs. field in unannealed film at 6K:

- B=2T
- B=5T

- annealed

- as-grown

- B5/2(6K)

- B5/2(28K)

**Ferromagnetic moment vs. field in unannealed film at 6K:**

**AF coupling described by**
***T******eff****** = T + T******AF****** = *****(6+22)K**

## Ferromagnetic semiconductor heterostructures

**Protocols for growth of semiconductor heterostructures are well-established**
**Addition of spin gives a new degree of freedom**

**e.g. tunnelling structure**

**(Ga,Mn)As**
**AlAs**
**(Ga,Mn)As**

- Tanaka *et al.* (2001) 70% TMR
- Chiba *et al.* (2003) 400%
- Rüster *et al.* (2004) >100,000% !!_* *_

## Tunnelling Anisotropic Magnetoresistance

**(Ga,Mn)As**

**Au**

**AlO****x**

*Gould et al., PRL (2004)*
**TMR-like signal with in control sample with only *****one***** ferromagnetic contact**
**Tunnelling probability depends on magnetisation direction of single layer   (two step reversal process)**

**[110]**

**[100]**

**[100]**

## Anisotropic magnetoresistance

**Anisotropic magnetoresistance**

**Magnetoresistance on rotating M away from ‘*****x*****’ direction**
**- strong function of Mn concentration, well described by mean-field model**

*Jungwirth et al. APL ‘03*

## TAMR in Nanoconstrictions

***TAMR***** in Nanoconstrictions**

**5nm (Ga,Mn)As film with 30nm wide constrictions**
**Giant anisotropic magnetoresistance ~100%****	****	****	****	****           in tunnelling regime**
*Giddings et al., cond-mat/0409209*

## Prospects for room temperature ferromagnetism

*T. Dietl, Science ’00; JVSTB ‘03*

**Predicted T****C**** in (III,Mn)V semiconductors, ****	****	****  **
***if***** Mn is a shallow acceptor**

## Ga1-xMnxN

**Ga****1-x****Mn****x****N**

**Small RT ferromagnetic signal superimposed on larger paramagnetic part**
*(Sonoda ’01; Reed ’01; Thaler ’02; Biquard ’03 etc.)*

**Several Mn****x****N****y**** magnetic phases exist**
- Zajac *et al.* ‘03

**Most are *****n*****-type  ****	****	****  ******** results are inconsistent with carrier-mediated ferromagnetism**
- Dietl Science ‘00

**Phase segregation?**

## Cubic (Ga,Mn)N: a key to p-type conductivity

**Wurtzite (Ga,Mn)N is usually *****n*****-type; Mn ionisation energy ~1.4eV**
- 							(Graf *et al* APL (2002))
**But in *****zincblende***** (Ga,Mn)N/GaAs we observe robust *****p-*****type behaviour**

******E****a****~50meV**
**Evidence for collective magnetic effects at low T:**

*Novikov et al. Semicond. Sci. Tech. (2004)*

## Conclusions

****** ****GaAs doped with ~% Mn is ferromagnetic – a model system for investigating magnetic phenomena in semiconductors** 		- gate controlled magnetism
- 				- tunnelling magnetoresistance
- 				- new tunnelling effects
**** **prospects for semiconductors with room temperature ferromagnetism – but phase segregation may be an issue**

## Magnetic anisotropy

**Strong cubic anisotropy with <100> easy axes,****	****	**** reduced to biaxial (in-plane) or uniaxial (perpendicular) due to strain.**
**Weaker uniaxial anisotropy between in-plane [110] and [110] orientations,           origin unknown.**

**B******

**B****//**

## Magnetic anisotropy rotation

**easy axis [110]**