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European School on Magnetism 2009 Inhomogeneities in magnetic systems Alberta Bonanni Institute for Semiconductor and Solid State Physics, Johannes Kepler University, Linz Austria Time-table 04.09.2009 J. Mike D. Coey Magnetism of

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Inhomogeneities in magnetic systems

Alberta Bonanni

Institute for Semiconductor and Solid State Physics, Johannes Kepler University, Linz – Austria

European School on Magnetism 2009

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Time-table

09.09.2009

  • A. Bonanni

Inhomogeneous magnetic systems: introduction and examples Wiktor Stefanowicz Measurements issue Bogdan Faina Materials issue Samaresh Guchhait Amorphous GeC:Mn Discussion

  • A. Bonanni

Inhomogeneous magnetic systems: control and applications Discussion

  • G. Irina Groza

AntiferromagneticCoO Coffee break 04.09.2009

  • J. Mike D. Coey

Magnetism of dilute oxides

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Before applying any model

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Before applying any model

We must gain knowledge about the distribution of magnetic ions

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Before applying any model

We must gain knowledge about the distribution of magnetic ions \ at surfaces \ at interfaces \ in the bulk

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Two examples

Tunneling magnetoresistance [TMR]:

magnetic ions distribution at the interface

Diluted Magnetic Semiconductors [DMS]:

magnetic ions distribution in the volume

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TMR

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Julliere‘s model

Jullière‘s model:

\ constant tunneling matrix elements \ electrons tunnel without spin-flip

TM films TMR ~ 2\3 ~ 67%

  • cfr. B. Dieny
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Jullière‘s model:

\ constant tunneling matrix elements \ electrons tunnel without spin-flip

TM films TMR ~ 2\3 ~ 67% TMR > 350%

  • S. Yuasa et al., Nature Materials 3, 868 [2004]

Julliere‘s model

–

Fe/MgO/Fe

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Jullière‘s model:

\ constant tunneling matrix elements \ electrons tunnel without spin-flip

TM films TMR ~ 2\3 ~ 67% TMR > 350%

  • S. Yuasa et al., Nature Materials 3, 868 [2004]

Julliere‘s model

–

Fe/MgO/Fe

▪ k || not conserved ▪ average density of states ▪ no quantum mechanical matrix elements appropriate for disordered interfaces

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Fe/MgO/Fe

  • S. Yuasa et al., Nature

Materials 3, 868 [2004]

High-quality interface Quantum effects important Fabry-Perot interferences

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Quantum mechanical model

  • J. Mathon et al., Phys. Rev. B 63,

220403R [2001]

  • W. H. Butler et al., Phys. Rev. B 63,

054416 [2001]

TMR ~ 1000% for:

\ ideal interface

\ T = 0 K \ thickness = 20 ML

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SLIDE 13
  • S. Ikeda et al., Appl. Phys. Lett 93,

082508 [2008]

300 K

Quantum mechanical model –CoFeB/MgO/CoFeB

  • J. Mathon et al., Phys. Rev. B 63,

220403R [2001]

  • W. H. Butler et al., Phys. Rev. B 63,

054416 [2001]

TMR ~ 1000% for:

\ ideal interface

\ T = 0 K \ thickness = 20 ML

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SLIDE 14
  • S. Ikeda et al., Appl. Phys. Lett 93,

082508 [2008]

300 K

Quantum mechanical model – CoFeB/MgO/CoFeB

  • J. Mathon et al., Phys. Rev. B 63,

220403R [2001]

  • W. H. Butler et al., Phys. Rev. B 63,

054416 [2001]

TMR ~ 1000% for:

\ ideal interface

\ T = 0 K \ thickness = 20 ML

TMR = 1100% at 5 K for ~ 22 ML

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SLIDE 15

DMS

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unpaired electrons magnetic behaviour

Diluted Magnetic Semiconductors

\ semi-conducting materials \ in which a fraction of the host cations \ is substitutionally and randomly replaced \ by transition metals or rare earths transition metals

  • partially filled d-states

rare earths

  • partially filled f-states

DMS – recalling the basics

B

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unpaired electrons magnetic behaviour

Diluted Magnetic Semiconductors

\ semi-conducting materials \ in which a fraction of the host cations \ is substitutionally and randomly replaced \ by transition metals or rare earths transition metals

  • partially filled d-states

rare earths

  • partially filled f-states

Challenges: \ ferromagnetism \ TC above RT B

DMS – recalling the basics

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Challenge

RT ferromagnetism in DMS theoretically requires: 1] holes 2] magnetic ions

T.Dietl et al., Science 287, 1019 [2000]

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Ferromagnetic DMS: status

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  • Y. Fukuma et al. Appl.Phys.Lett. 93, 252502 [2008]

State-of-the-art for (Ge,Mn)Te

TC = 190 K

8% (Ge,Mn)Te 1.57 x 1021 holes cm-3 : MBE

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State-of-the-art for (Ga,Mn)As

K.Olejník et al. Phys. Rev. B 78, 054403 [2008]

TC = 185 K

8% (Ga,Mn)As: annealed/etched/annealed Situation 16 years (!) after the discovery of carrier-mediated mechanism in III-V

  • M. Wang et al. Appl. Phys. Lett. 93, 132103 [2008]
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High TC: most promising DMS

p-d Zener model prediction of TC 5% Mn d5, p = 3.5 × 1020 cm-3

T.Dietl et al., Science 287, 1019 [2000]

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High TC: most promising DMS

T.Dietl et al., Science 287, 1019 [2000]

p-d Zener model prediction of TC 5% Mn d5, p = 3.5 × 1020 cm-3 GaN & ZnO:

small lattice constant a0 strong p-d hybridization \ increased N0β \ large TC

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Challenge

Magnetization above room temperature reported even: 1] without valence band holes 2] without magnetic ions (!) RT ferromagnetism in DMS theoretically requires: 1] holes 2] magnetic ions

T.Dietl et al., Science 287, 1019 [2000]

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With spontaneous magnetization at 300 K

wz-c-(Ga,Mn)N, (In,Mn)N, (Ga,Cr)N, (Al,Cr)N, (Ga,Gd)N, (Ga,Fe)N

(Ga,Mn)As, (In,Mn)As, (Ga,Mn)Sb, (Ga,Mn)P:C (Zn,Mn)O, (Zn,Ni)O, (Zn,Co)O, (Zn,V)O, (Zn,Fe,Cu)O (Zn,Cr)Te (Ti,Co)O2, (Sn,Co)O2, (Sn,Fe)O2, (Hf,Co)O2 (Cd,Ge,Mn)P2, (Zn,Ge,Mn)P2, (Zn,Sn,Mn)As2 (Ge,Mn) (La,Ca)B6,C, C60, HfO2…

Magnetically doped materials

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With spontaneous magnetization at 300 K

wz-c-(Ga,Mn)N, (In,Mn)N, (Ga,Cr)N, (Al,Cr)N, (Ga,Gd)N, (Ga,Fe)N

(Ga,Mn)As, (In,Mn)As, (Ga,Mn)Sb, (Ga,Mn)P:C (Zn,Mn)O, (Zn,Ni)O, (Zn,Co)O, (Zn,V)O, (Zn,Fe,Cu)O (Zn,Cr)Te (Ti,Co)O2, (Sn,Co)O2, (Sn,Fe)O2, (Hf,Co)O2 (Cd,Ge,Mn)P2, (Zn,Ge,Mn)P2, (Zn,Sn,Mn)As2 (Ge,Mn) (La,Ca)B6,C, C60, HfO2…

Magnetically doped materials

Origin of ferromagnetism

“The most challenging issue in nowadays physics of magnetism“

Mike Coey 04.09.2009 EMC

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ferromagnetism

P.J. Grace et al. Adv.Mat. 21, 71 [2009]

On account of the difficulty encountered and the sometimes ephemeral nature

  • f the magnetic moment, we will refer to

the phenomenon as phantom ferromagnetism

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a) random distribution b) crystallographic or chemical phase separation c) aggregation at surfaces or interfaces d) aggregation in grain boundaries

  • M. Coey seminar 04.09.2009

Inhomogeneous FM

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To elucidate origin of ferromagnetism: combine controlled epitaxy with comprehensive

nanocharacterization for each material

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Extended characterization – already online during growth

\ to elucidate correlation between fabrication conditions and ▪ structural [synchrotron XRD, HRTEM, EDS] ▪ magnetic [SQUID, EPR] ▪ optical [PL, magneto-optics] ▪ electrical [(magneto-)transport] ▪ chemical [EDS, SIMS] properties

New paradigm

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Extended characterization – already online during growth

\ to elucidate correlation between fabrication conditions and ▪ structural [synchrotron XRD, HRTEM, EDS]

  • advanced microscopic characterization -

▪ magnetic [SQUID, EPR] ▪ optical [PL, magneto-optics] standard ▪ electrical [(magneto-)transport] macroscopic ▪ chemical [EDS, SIMS] characteriz. properties

New paradigm

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▪ MOVPE [in-situ: ellipsometry, laser reflectometry] reactor: AIXTRON 200 ▪ c-plane Al2O3 substrates ▪ Precursors: TMGa, NH3, Cp2Fe, Cp2Mg, SiH4, Cp2Mn ▪ Growth procedure: 1] substrate nitridation 2] LT (540 °C) GaN nucl. layer 3] annealing/recrystallisation 4] 1 µm HT (1050 °C) GaN 5] 0.5 – 1 µm (Ga,Fe)N:Si(Mg) a] 800 – 950 °C

b] 50 – 400 sccm Cp2Fe

Growth – (Ga,Fe)N:Si(Mg)

(0001)Al2O3 GaN (Ga,Fe)N:Si(Mg)

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lattice constant (Å)

III-Nitrides

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[Ferro]magnetic coupling without magnetic ions?

  • P. Dev et al. Phys.Rev.Lett. 100, 117204 [2008]

DFT [LSDA]:

\ cation vacancies \ promote local magnetic moments \ long-range magnetic coupling between intrinsic defects

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Not in high-quality samples - GaN

GaN(:Mg,Si) without Fe no ferromagnetism

  • A. Bonanni et al. Phys.Rev.Lett. 101, 135502 [2008]
  • 60
  • 40
  • 20

20 40 60

  • 3
  • 2
  • 1

1 2 3

GaN:Si GaN:Mg GaN

M ( emu/cm

3 )

H ( kOe )

T = 5 K

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Not in high-quality samples - GaN

(Ga,Fe)N paramagnetism + ferromagnetism

A.Bonanni et al. Phys.Rev.B 75, 125210 [2007]

GaN(:Mg,Si) without Fe no ferromagnetism

  • Phys. Rev.Lett. 101, 135502 [2008]
  • 60
  • 40
  • 20

20 40 60

  • 3
  • 2
  • 1

1 2 3

GaN:Si GaN:Mg GaN

M ( emu/cm

3 )

H ( kOe )

T = 5 K

  • A. Bonanni et al. Phys.Rev.Lett. 101, 135502 [2008]
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Beyond the solubility limit – (Ga,Fe)N

  • A. Bonanni et al. Phys.Rev. B 75, 125210 [2007]

Superposition of paramagnetic and ferromagnetic response

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  • 2

2 4 6 8 10

  • 1.0
  • 0.5

0.0 0.5 1.0 T = 005K T = 100K T = 200K T = 320K T = 380K

(Ga,Fe)N

Magnetisation ( emu/cm

3 )

H ( kOe)

5K

▪ values of spontaneous magnetization MS

from high field measurements

▪ MS vs T Brillouin-like function TC

Evaluation of TC

Ferromagnetic response persisting at room-temperature

100 200 300 400 500 600 0.0 0.2 0.4 0.6 0.8

(Ga,Fe)N

MS ( emu/cm

3 )

Temperature ( K ) TC

  • 20

20 40 60

  • 1.0
  • 0.5

0.0 0.5 1.0

T = 005K T = 100K T = 200K T = 320K T = 380K

M ( emu/cm

3 )

H ( kOe )

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  • 2

2 4 6 8 10

  • 1.0
  • 0.5

0.0 0.5 1.0 T = 005K T = 100K T = 200K T = 320K T = 380K

(Ga,Fe)N

Magnetisation ( emu/cm

3 )

H ( kOe)

5K

▪ values of spontaneous magnetization MS

from high field measurements

▪ MS vs T Brillouin-like function TC

Evaluation of TC

Ferromagnetic response persisting at room-temperature

100 200 300 400 500 600 0.0 0.2 0.4 0.6 0.8

(Ga,Fe)N

MS ( emu/cm

3 )

Temperature ( K ) TC

  • 20

20 40 60

  • 1.0
  • 0.5

0.0 0.5 1.0

T = 005K T = 100K T = 200K T = 320K T = 380K

M ( emu/cm

3 )

H ( kOe )

573 K

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5K

Evaluation of TC

Brillouin function dependence [e.g. GaMnAs]

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100 200 300 400 500 600 20 40 60

M [G] T [ K ]

brillouin_D Arr408CLin_sqrB

40 80 120 1000 2000 3000 4000

M

2 [G 2]

H

/ M

T/K 5/100/180 250 322

5K

Evaluation of TC

Brillouin function dependence [e.g. GaMnAs]

  • A. Arrott, Phys. Rev. 108, 3194 [1957]

Otherwise: Arrott plot M2 vs. H/M extrapolated to 0 field

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Origin of ferromagnetism

Open question

Review: A. Bonanni, Semicond.Sci.Technol. 22, R41 [2007]

  • 50

50 100 150 200 250 300 350 400 220 240 260 280 300

  • ur standard

GaN FWHM ["] Cp2Fe flux [sccm]

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ESRF – European Synchrotron Radiation Facility

Grenoble - France

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Synchrotron powder diffraction

\ ID31 beamline ESRF [Grenoble – France] \ Powder diffraction \ E = 15.5 keV Above the solubility limit of Fe into GaN: formation of nanocrystals confirmed by synchrotron XRD

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Synchrotron powder diffraction

\ ID31 beamline ESRF [Grenoble – France] \ Powder diffraction \ E = 15.5 keV Above the solubility limit of Fe into GaN: formation of nanocrystals confirmed by synchrotron XRD ε-Fe3N hexagonal TC = 575 K

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Synchrotron powder diffraction

\ ID31 beamline ESRF [Grenoble – France] \ Powder diffraction \ E = 15.5 keV Above the solubility limit of Fe into GaN: formation of nanocrystals confirmed by synchrotron XRD ε-Fe3N hexagonal TC = 575 K Confirmed by EXAFS

  • M. Rovezzi, ..AB, ...Phys.Rev. B 79, 195209 [2009]
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Synchrotron powder diffraction

\ ID31 beamline ESRF [Grenoble – France] \ Powder diffraction \ E = 15.5 keV Above the solubility limit of Fe into GaN: formation of nanocrystals confirmed by synchrotron XRD ε-Fe3N hexagonal TC = 575 K Confirmed by EXAFS

  • M. Rovezzi, ..AB, ...Phys.Rev. B 79, 195209 [2009]
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ε- Fe3N

This is not the whole story

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ε- Fe3N

This is not the whole story

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ε- Fe3N

Related system (Ga,Mn)N M. Zając et al., J.Appl.Phys 93, 4715 [2003]

  • A. Martínez-Criado et al., Appl.Phys.Lett. 86, 131927 [2005]

This is not the whole story

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ε- Fe3N

This is not the whole story

Buried condensed magnetic semiconductors [CMS] source of high temperature ferromagnetism

Related system (Ga,Mn)N M. Zając et al., J.Appl.Phys 93, 4715 [2003]

  • A. Martínez-Criado et al., Appl.Phys.Lett. 86, 131927 [2005]
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Phase separation: (Ga,Mn)As

hex MnAs

GaAs TC ≈ 320 K

H (Oe) zb MnAs

GaAs TC ≈ 350 K crystallographic decomposition chemical decomposition

Moreno et al. (Berlin) JAP [2002]

control magnetic properties

De Boeck et al. APL [1996]

enhanced magnetooptical effects [MCD]

Akinaga et al. APL [2000] Shimizu et al. APL [2001] Yokoyama et al. JAP [2005]

affect conductance and Hall effect

Heimbrodt et al. PRB [2004]

Mn-rich regions

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SQUID total magnetic moment How do we setermine the saturation magnetisation Ms? from full volume or from the volume of the particle? Anisotropy field Ha = (2 Keff)/(µ0 Ms) Keff = K1 – µ0(N|| - N┴) Ms

From the box

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SQUID total magnetic moment How do we setermine the saturation magnetisation Ms? from full volume or from the volume of the particle? Anisotropy field Ha = (2 Keff)/(µ0 Ms) Keff = K1 – µ0(N|| - N┴) Ms volume of the ferromagnetic particle

From the box

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Fe incorporation into GaN host

chemical decomposition

nano-scale chemical phase separation / regions with high magnetic ions concentration / novel magnetic phases stabilized

crystallographic decomposition

  • f known ferromagnetic,

ferrimagnetic, antiferromagnetic compounds in semiconductor matrix

diluted material

paramagnetic behavior

  • f Fe3+ ions
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SLIDE 56

Magnetic nanotubes – courtesy of:

  • E. Simpson, T. Kasama, R. Dunin-Borkowski [DTU

Copenhagen];

  • Y. Hayashi [Cambridge, Engineering Department]

Electron holography

\ TEM technique recording the phase of an electron wave \ phase affected by a magnetic field \ quantitative information on: Field Magnetic moment

Advanced microscopy

  • R. Dunin-Borkowski et al. Magnetic Microscopy
  • f Nanostructures Ch.5 Springer, Berlin [2006]
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3D - atom probe

  • M. Kodzuka et al. Ultramicroscopy 109, 644 [2009]

(Ga,Mn)As/GaAs

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3D - atom probe

  • M. Kodzuka et al. Ultramicroscopy 109, 644 [2009]

Compositional analysis at the nm- scale

(Ga,Mn)As/GaAs

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