Multi-technique Microscopy and Synchrotron Radiation Study of

Novel Magnetic Alloys

Gavin Bell

University of Warwick

J. D. Aldous 1, T. Decoster 2, W. Theis 2, C. Sánchez-Hanke 3, A. Quesada 4, A. K. Schmid 4, T. P. A. Hase 1, J. A. Duffy 1, C. Burrows 1, A. Sanchez 1 and R. Beanland 1

1 Department of Physics, University of Warwick, UK 2 School of Physics and Astronomy, University of Birmingham, UK3 National Synchrotron Light Source, USA4 National Center for Electron Microscopy, USA

Outline● Transition metal pnictides

– Motivation, band structures, spin injection

– Properties and epitaxial growth of MnSb

● Surfaces

– Why do we care?

– Structure and magnetism of MnSb● Polymorphism

– MnSb polymorphs: theory and experiment● Next steps

– Spatially resolved analysis

Motivation: spintronics● Inject / manipulate spin

polarised current in non-magnetic semiconductors?

From: http://www.nims.go.jp/apfim/gif/SpinFET.gif

Good spin injection material:

● Ferromagnetic

● Compatibility with III-Vs or Si

● High P

● High Tc

● Epitaxy

● Processing

Band structures

First HMF discovered theoretically in 1980s – NiMnSb.

● Is P really 100%?

● How robust is the minority spin gap?

Spin polarisation & injection

Which “spin polarisation” is relevant?

Spin transport in semiconductor: basic idea is parallel conduction channels for ↑ and ↓ (Mott).

Solve conductivity mismatch to metallic FM contact with Schottky or tunnel barrier, or P = 100%, or RFM ~ Rsc.

R↑

R↓

V

Rsc

Rsc

MnAs, NiMnSb, etc.

Our materials: antimonides● Binary pnictides

– MnSb (FM)

– CrSb (AFM)

– NiSb (PM)

● Heusler alloys– NiMnSb (HMF)

– NiCrSb (nHMF)

– Ni2MnIn (nHMF)

MnSb: basic properties● Soft ferromagnet, TC = 587 K.

● High magneto-optical activity.

● Weak p-type metal (p ~ 1021 cm-3, S ~ 107 ohm-1 m-1).

● Stable crystal structure: dhcp ('niccolite', ABAC).

VSM XMCD

Molecular beam epitaxy● MnSb: III-V-like growth.

● Well defined surfaces and interfaces.

● Can grow multilayers.

MnSb: quenched STM-MBE

(2x2) td(1x4)

MnSb(0001) / GaAs(111)

c and c / 2 step heights (dhcp)

threading dislocations ~1E8 / sq. cm

Surfaces and interfaces?Theoretical structure of NiMnSb(111)-CdS(111), preserving half-metallicityG. de Wijs and R. de Groot, PHYSICAL REVIEW B 64 020402(R)

Surface reconstruction or interface structure may close minority spin gap...

'The strong sensitivity of the tunneling spin polarization and TMR to the interface atomic and electronic structure dramatically expands the possibilities for engineering optimal MTJ properties for device applications.'

S(k||) for majority, minority and antiparallel alignment.E. Tsymbal et al. Prog. Mat. Sci. 52 (2007) 401

STM ≠ quantitative structure!

Reciprocal space: low energy electron diffraction

(hopefully SXRD too)

Real space: medium energy ion scattering

No quantitative surface structure solutions for this entire class of materials...

Hatfield and Bell, Surf. Sci. 601 (2007), 5368

Getting a good surface

Surfaces are delicate with nasty native oxides.

Thick (~ 6 nm) Mn oxides: how to re-prepare surface outside MBE chamber? Etch in HCl, then vacuum anneal.

Hatfield, Aldous and Bell, Appl. Surf. Sci. 255 (2009), 3567

Must start with a flat epilayer (we get Rrms = 1nm).

Away from ideal MBE conditions: trench/mesa morphology.

Etching and annealing

HCl etch + 10h anneal @ 750K

HCl etch + 10h anneal @ 700K

Etch / anneal

AFM: Rrms a few nm

Etch / anneal

STM: increased step and double-step density

Surface magnetism: XMCD

Red curves – semi-crystalline Sb cap grown on MnSbBlack and green curves – oxidised MnSb (untreated or etched)

2 nm Sb cap suppresses Mn oxidation effectively.

Total electron yield Reflected intensity

Flipping ratio

LEEM and SPLEEM

Aex=∣P0∣ I −I / I I

LEEM● Ultra-high vacuum.● Electron energy at surface 1 – 100 eV.● Contrast:

● chemical composition● topography, atomic steps● reconstruction domains● exchange scattering

asymmetry (small)

SPLEEM image is the difference between two LEEM images obtained with opposite electron polarisations (magnitude P0).

R. J. Phaneuf and A.K Schmidt, Physics Today 56 (2003) 50.

Surface damage: LEEM

LEEM8 micron FOV

During UHV anneal: surface defects grow at high temp.

LEEDseparate optics

crystallinity improves at high temp.

T = 645 K T = 672 K

T = 520 K T = 677 K

Surface magnetism: SPLEEM

8 micron FOV

beam energy 6.2 eV

zero-field-cooled MnSb

etch / anneal 520 K

large FM domains

polarised in-plane

No magnetic contrast on untreated samples.

Polymorphism in MnSb?● Density functional

theory (DFT)

● Predictions of half-metallicity in cubic CrSb, etc.

● Ultra-thin cubic CrSb can be stabilised on GaAs

● Thicker films? MnSb? Other polymorphs? Munich SPR-KKR DFT code

The Munich SPR-KKR package, version 5.4, H.Ebert et al, http://olymp.cup.uni-muenchen.de/ak/ebert/SPRKKR; H Ebert, Fully relativistic band structure calculations for magnetic solids – Formalism and Application, in Electronic Structureand Physical Properties of Solids, editor: H. Dreyss´e, Lecture Notes in Physics, vol. 535, p. 191, Springer Berlin.

n-MnSb

w-MnSb c-MnSb

MnSb polymorph properties

n-MnSb

w-MnSb c-MnSb

Structure aSADP

(Å)

aDFT

(Å)

cSADP

(Å)

μS

(μB)

EG (eV)

P

n - MnSb

4.115 4.0 5.769 2.8 n/a 18%

w - MnSb

4.291 4.3 7.003 3.9 0.35 90%

c - MnSb

6.502 6.3 n/a 4.1 0.34 91%

Wurtzite and cubic MnSb:

Predict robust minority spin gap, high Fermi level spin polarisation and nearly integer spin moments: characteristic of half-metallicity.

MnSb polymorphs: TEM

Bright-field TEM of MnSb film

Threading dislocation density < 1E7 / sq. cm HRTEM of epitaxial cubic /

niccolite MnSb boundary

MnSb polymorphs: SADP

Diffraction pattern of whole structure: GaAs, three MnSb polymorphs.

Cubic (111), wurtzite and niccolite (0001): same basal plane so may not see in RHEED during MBE.

MnSb polymorph properties

n-MnSb

w-MnSb c-MnSb

Structure aSADP

(Å)

aDFT

(Å)

cSADP

(Å)

μS

(μB)

EG (eV)

P

n - MnSb

4.115 4.0 5.769 2.8 n/a 18%

w - MnSb

4.291 4.3 7.003 3.9 0.35 90%

c - MnSb

6.502 6.3 n/a 4.1 0.34 91%

Good agreement with DFT.Confirmed by synchrotron XRD (wurtzite even in thin films).Need to clarify dislocation behaviour and formation mechanism.

Summary● Multi-imaging techniques on MnSb thin films

– STM in MBE vacuum system

– Ex situ AFM, SEM, TEM, SPLEEM● Can re-prepare excellent surfaces outside MBE

● Surfaces are nonmagnetic without optimised re-preparation (SPLEEM and XMCD TEY)

● Observed first cubic and wurtzite MnSb polymorphs

– 'big' epitaxial clusters (50 – 100 nm)

– promising properties for spin injection!

– need spatially resolved analysis of structure, magnetism and electronic properties

AcknowledgmentsMBE & UHV science

J. D. Aldous, C. Burrows, R. Johnston(University of Warwick)

SPLEEM

T. Decoster, W. Theis (University of Birmingham)

A. Quesada, A. K. Schmid(National Center for Electron Microscopy, USA)

X-ray measurements

C. Sánchez-Hanke (National Synchrotron Light Source, Brookhaven National Laboratory, USA)

T. P. A. Hase, J. A. Duffy(University of Warwick)

TEM

A. Sanchez and R. Beanland(University of Warwick)

DFT

I. Maskery(thanks to J. Staunton & M. Dias too)(University of Warwick)