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II Materials. Chalcospinels Delafossite oxides Dilute oxide nanoparticles Al-doped Co:ZnO thin films Future work. Staff, Publications. M Venkatesan Senior postdoc Karsten Rode Postdoc Delphine Lebeugle Postdoc Jonathan Alaria Postgrad Marita O’Sullivan Postgrad

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Ii materials

II Materials

Chalcospinels

Delafossite oxides

Dilute oxide nanoparticles

Al-doped Co:ZnO thin films

Future work

MANSE Midterm Review


Staff publications

Staff, Publications

  • M VenkatesanSenior postdoc

  • Karsten Rode Postdoc

  • Delphine Lebeugle Postdoc

  • Jonathan AlariaPostgrad

  • Marita O’Sullivan Postgrad

  • Simone Alborgetti Postgrad

MANSE Midterm Review


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Publications:

—Oxide dilute magnetic semicondutors – Fact or Fiction? J.M.D. Coey, S.A. Chambers, MRS Bulletin 33 1063-8 (2009)

—Dilute magnetic oxides and nitrides, K. Rode and J. M. D. Coey, in Handbook of Magnetism and Advanced Magnetic Materials (H Kronmullar and S Parkin, editors), Vol 4, pp 2107 – 2121 (2007)

—Dilute magnetic oxides, J. M. D. Coey, Comments on Solid State and Materials Sciences 10 83-92 (2007)

—Magnetism in dilute magnetic oxide thin films based on SnO2, C. B. Fitzgerald, M. Venkatesan, L. S. Dorneles, R. Gunning, P. Stamenov, J. M. D. Coey, P. A. Stampe, R. J. Kennedy, E. C. Moreira and U. S. Sias, Physical Review B, 74, 115307 (2006)

— Giant moment and magnetic anisotropy in Co-doped ZnO films grown by pulse-injection metal organic chemical vapor deposition, A. Zukova, A. Teiserskis, S. van Dijken, Y. K. Gun’ko and V. Kazlauskiene, Applied Physics Letters, 89, 232503 (2006)

— Charge-transfer ferromagnetism in oxide nanoparticles, JMD Coey, Kwanruthai Wongsaprom, J. Alaria and M. Venkatesan, Journal of Physics D: Applied Physics, 41, 134012 (2008)

— Magnetic, magnetotransport and optical properties of Al-doped Co-doped ZnO thin films M. Venkatesan, P. Stamenov, L. S. Dorneles, R. D. Gunning and J. M. D. Coey, Applied Physics Letters 90 242508 (2007)

—Magnetic and structural properties of Co-doped ZnO thin films, L.S. Dorneles, M. Venkatesan, R. Gunning, P. Stamenov. J. Alaria, M. Rooney, J.G. Lunney, J.M.D. Coey, Journal of Magnetism and Magnetic Materials 310 2087-2088 (2007)

MANSE Midterm Review


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— Room temperature ferromagnetism in Mn- and Fe-doped indium tin oxide thin films, M. Venkatesan, R.D. Gunning, P. Stamenov, J.M.D. Coey, Journal of Applied Physics, 103, 07D135 (2008)

— Structural and magnetic properties of wurzite CoO thin films, J. Alaria, N. Cheval, K. Rode, M. Venkatesan and J.M.D. Coey, Journal of Physics D: Applied Physics, 41, 135004 (2008)

— Magnetism of ZnO nanoparticles doped with 3d cations prepared by a solvothermal Method, J. Alaria, M.Venkatesan and J.M.D. Coey, Journal of Applied Physics 103 07D123 (2008)

—Magnetism’s ticklish giant, Nature Materials 5 677-8 (2006)

—Magnetic properties of CNx whiskers. R. D. Gunning, M. Venkatesan, D. H. Grayson and J. M. D. Coey, Carbon, 44 3213-7 (2006)

—The origin of Magnetism of etched silicon. P. Grace, M. Venkatesan, J. Alaria and J.M.D. Coey, Advanced Materials (in press)

—Absence of toroidal moments in aromagnetic anthracene. S. Alborghetti, E. Puppin, M. Brenna, E. Pinotti, P. Zanni, J.M.D. Coey, New Journal of Physics 10 063019 (2008)

—Thin films of semiconducting lithium ferrite produced by pulsed laser deposition, R.D. Gunning, Karsten Rode, Sumesh R.G. Sophin, M. Venkatesan, JMD Coey, Igor V. Shvets, Applied Surface Science (in press)

—Half-metallic Ferromagnets, M. Venkatesan,in Handbook of Magnetism and Advanced Magnetic Materials (H Kronmullar and S Parkin, editors), Vol 4, pp 2133 – 2156 (2007)

MANSE Midterm Review


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— Ferromagnetic nanoparticles with strong surface anisotropy: Spin structures and magnetisation processes, L. Berger, Y. Labaye, M. Tamine, J.M.D. Coey, Physical Review B 77 104431 (2008)

— Magnetic anisotropy of ilmenite-hematite solid solution thin films grown by pulsed laser ablation, K. Rode, R.D. Gunning, R.G.S. Sofin, M. Venkatesan, J.G. Lunney, J.M.D. Coey and I.V. Shvets, Journal of Magnetism and Magnetic Materials, 320, 3238 (2008)

—Permanent Magnets, T. Ni Mhiochain and J. M. D. Coey, Encyclopedia of Life Support Systems Volume 3: Physical methods, instruments and measurements, Y. M. Tsipenyuk (editor),.Chapter 10 pp 203 – 258 EOLSS/UNESCO Paris (2007)

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Characterization

  • X-ray/Neutron diffraction

  • SEM/EDAX/RBS/AFM/MFM/HRTEM

  • SQUID magnetometry

  • Optical spectrometry

  • XAS/XES/XMCD

  • Transport measurements

MANSE Midterm Review


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I. Chalcospinels

Chalcospinels

Normal cubic spinel structure.

n-type magnetic semiconductors

CuCr2S4TC = 420 K 4.6 B/f.u

CuCr2Se4TC = 460 K 4.9 B/f.u

CdCr2Se4 TC = 130 K

Conduction electrons may be fully spin polarized - potential

half-metal?

A red shift (0.05 eV) of the absorption edge on passing the TC.

High room temperature magneto-optical Kerr effect (1.2º at 0.9 eV).

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CuCr2Se4 ceramic

Prepared at 550°C (below peritectic transition)

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High temperature synthesis

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PLD films

  • Deposition conditions

  • Ceramic target

    • Substrate c-Al2O3, MgO, MgAl2O4, RT-700°C

    • 1 J/cm2 5Hz

    • Pressure ~ 10-6 mbar

  • Metallic target

    • Substrate MgO 200°C

    • 1 J/cm2 5Hz

    • Pressure ~10-6 mbar

Growth of CuCr2Se4 thin films from ceramic target

Annealing process

500°C in Se Vapour (from elemental Se powder) in a vacuum sealed quartz tube for 48 hours

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Magnetizaton

Before Annealing

After Annealing

Films from metallic target

Polycrystalline samples, mixed phases

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CuCr2Se4-xBrx

  • Powders

    • Synthesis temperature is critical.

    • Saturation magnetic moment of 6 B/mol can be achieved in CuCr2Se4 made at 550 C. It is probably a half-metal.

  • Single crystals

    • Metallic (CuCr2Se4) or intrinsic semiconductor (CdCr2Se4) when undoped

    • Anomalous Hall effect and AMR

  • Thin films

    • ~ Single phase after annealing

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Next steps

  • Complete torque curves

  • Low-temperature heat capacity

  • IR optical conductivity (with Dimitri Basov, UCSD)

  • Thermal conductivity

  • Neutron diffraction (LLB April)

  • Andreev reflection

  • AC Squid magnetometry; Sensitivity 3 10-15 A m2 for dc fields < 1 T.

    If the mobility permits, demonstrate an all-ferromagnetic transistor.

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II. Delafossite oxides

Cu-delafossite is still considered to be a potential p-type semiconductor for transparent electronics.

CuAlO2

CuCrO2:Ca,Mg

CuInO2:Mg,Sn

Carrier density and mobility are the major factors that require to be improved.

MANSE Midterm Review


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CuCrO2

  • CuCrO2

  • p-type transparent conducting oxide (TCO)

    • Delafossite structure: A1+B3+O2

    • Crystal system: Rhombohedral

    • Space group: R-3m

    • Lattice parameters: a = 2.9761(2) Å, c = 17.102(1) Å

    • Bandgap: 3.2 eV

    • Antiferromagnetic: TN = 25K

  • Mg-doped CuCrO2

    • High conductivity for p-type TCO: 220 S/cm (5% Mg)

    • Thermopower +153 μV/K at 300K

    • 50% transparent to visible light (250 nm thick film)

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PLD films

10% Mg

5% Mg

2% Mg

Undoped

Growth Conditions

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10% Mg-CuCrO2/0.1% Al-ZnO/(0001)/Al2O3

10% Mg

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Summary

Growth of highly-crystalline native p-type delafossite oxide films CuCrO2, CuAlO2

Good quality n-type Al:ZnO films are also grown by PLD (mobility ~ 20 cm2 V-1 s)

Next steps: Make all-oxide heterostructures; pn junctions and pnp stacks. Use sapphire shadow masks.

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III. Dilute oxide nanoparticles

LSTO nanoparticle system

Tokura et al, PRL 1988

  • spd-band metal.

  • 0.5 electrons per formula

  •  = 5 mJ mol-1K-2

  • properties depend on oxygen stoichiometry

Systematic investigation of the magnetic properties of LSTO,

undoped and with transition metal doping (substitution for Ti at the

1.5 or 2.0 % level) for dopants ranging from Sc to Ni.

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Nanoparticle synthesis

Polymerized complex method, using Ti isopropoxide and nitrate precursors

Bulk ceramic samples of undoped LSTO, and LSTO with 2 % 57Fe doping were made by mixing and firing the components at 1000 C.

The pellet was placed in a ceramic boat and sintered at 1150 C for 24 h in air or flowing argon.

The nominal purity of the starting materials was 99.99 % or better.

X-ray diffraction

SEM/EDAX

TEM

SQUID magnetometry

Mössbauer spectrometry

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(La0.5Sr0.5)TiO3:Undoped

Paramagnetism due to S = 1/2 defects in the LSTO particles

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Magnetization

Magnetization

Nanocrystalline dia = -4.1 10-9 m3 kg-1

Ceramic dia = -1.2 10-9 m3 kg-1

The ceramics show a diamagnetic susceptibility that is

smaller by a factor of three than that of the nanoparticles.

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TM: LSTO

Co:LSTO

2% Co

Fe:LSTO Ceramic

Fe:LSTO Nanocrystalline

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LSTO summary

The nanocrystalline samples doped with the late transition

elements Fe, Co and Ni behave differently.

In addition to a temperature-dependent, Curie-Weiss term in

the susceptibility, they all show a nonlinear, ferromagnetic-like

component in their magnetization curves

The samples doped with cations from Sc – Mn all exhibit

linear magnetization curves and a Curie-Weiss susceptibility

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TM: ZnO nanoparticles

Phys Rev B 2007Many oxide nanoparticles exhibit a tiny magnetization < 0.1 A m2 kg-1

  • ZnO: 5% M = Sc - Cu

Solvo-/hydrothermal technique

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Characterization

Characterization

All the samples prepared in series A, except

for TM=Ni, are diamagnetic or paramagnetic

as expected for the dilution of the TM in the ZnO matrix.

MANSE Midterm Review


M ssbauer spectra

Mössbauer spectra

Sample A

No magnetic ordering of the iron,

Fe3+, with an isomer shift of 0.37 mm s-1 relative to α-Fe, and a quadrupole splitting of 0.46 mm s-1, as expected for substituted Fe3+ on tetrahedral site in ZnO.

A

Sample B

70% of the iron is a similar +3 state.

However, 30% of the iron appears in

a magnetically order form, identified

from the spectrum as magnetite and

hematite.

B

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Zno nanorods

ZnO nanorods

5%Co-doped ZnO nanorods

Hydrothermal, Zn acetate, Co acetate, NaOH,

120°C for 12h

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Summary

Summary

In two nanoparticle systems — ZnO;M and LSTO;M the TM dopants are usually paramagnetic. Ferromagnetic moments only apperar in some sample when M = Fe, Co or Ni.

Where it was possible to analyse the iron phases specifically, using Mossbauer spectroscopy, evidence of a ferromagnetic secondary phase (Fe or Fe3O4) was found.

It is likely that much or all of the ferromagnetism in these materials can be explained by ferromagnetic secondary phases.

The origin of the room temperature ferromagnetism in the Fe and Ni doped ZnO prepared with a non-homogeneous precursor is explained by the presence of a secondary phase magnetite and metallic Ni, respectively.

The evidence indicates that room temperature ferromagnetism in these doped ZnO nanoparticles has an extrinsic origin.

MANSE Midterm Review


Iv al doped co zno films

IV. Al-doped Co:ZnO films

Zn0.95Co0.05O + x at.% Al

x = 0.1, 0.2, 0.5, 0.7 and 1 at.% Al

Zn0.95Co0.05O 450C 6 min. 10 Hz C-Al2O3

Zn0.95Co0.05O + 0.2% Al 450C 6 min. 10 Hz C-Al2O3

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Band gap widening

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Summary

Larger moments for films on C-cut substrates compared to R-cut substrates.

Conductivity is enhanced significantly in films with low

Al doping (0.1-0.2 %), maintaining the magnetic moment.

Magnetic moment decreases with increasing Al content.

Band-gap shift (~ 0.5 eV), is observed with Al-doping.

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Collaboration

Detailed electronic structure calculations with theorists in TCD

- LDA and spin transport calculations - Stefano Sanvito’s group

- Electronic structure of oxides - Charles Patterson’s group

Dopants and defects control magnetic properties

- X-ray magnetic circular dichroism (ISRF, Grenoble)

- XAS and XES (Cormac McGuinness)

- Transmission electron microscopy (Peter Nellist)

Collaboration within SFI

Symposium on dilute magnetic oxides

MANSE Midterm Review


Future work

Future work

  • Chalcogenides

    • Detailed characterization on chalcogenide systems (Neutron, Andreev etc.) and synthesis of single crystals

Delafossite oxides

Make all-oxide heterostructures; pn junctions and pnp stacks.

Nanoparticle systems

Understanding of defects, interface magnetism and detailed theoretical calculations.

Dilute Oxides

Search for new and novel dilute magnetic oxides by suitable cation doping.

Heusler alloys

Exploit high Curie temperature Heusler alloys

Co2MnSi, Co2FeSi etc.

Materials developed will continue to be exploited for applications in MANSE.

MANSE Midterm Review


Outline

Outline

  • Background

  • TiO2:Fe

  • Magnetic silicon

  • Graphite

  • Anthracene

  • MgO:N

  • Au nanoparticles

  • A model — Charge-transfer ferromagnetism

MANSE Midterm Review


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