Spin-polarized electrons Spin-polarized electrons from Fe films coated from Fe films coated

single crystal W(100) tips single crystal W(100) tips by field emissionby field emission

Department of Physics Department of Physics Hong Kong University of Science and Hong Kong University of Science and

Technology Technology

Y. Niu and M. S. AltmanY. Niu and M. S. Altman

OutlineOutline

1. Introduction1. Introduction 2. Experiments2. Experiments 3. Results and discussion3. Results and discussion 4. Summary and expectations4. Summary and expectations

1. Introduction1. Introduction

Atom physicsAtom physics High energy physicsHigh energy physics Solid state physics (Solid state physics (SPLEED, SPSEE, SPEELS, SPAES, SPLEED, SPSEE, SPEELS, SPAES,

SPLEEM, SEMPA, SPSTM, SARPES and SPIPES)SPLEEM, SEMPA, SPSTM, SARPES and SPIPES)

Spin-polarized electrons are extensively used in fields of

Specifically, Spin-polarized electron microscopies, such as Spin polarized low energy electron microscope (SPLEEM) desire high polarization, high brightness and long lifetime polarized electron beam. The bulk GaAs crystal is almost the only spin polarized electrons source

(PES) for such instruments. SPLEEM at ASU

PES of GaAs and GaAs-based PES of GaAs and GaAs-based materialsmaterials Advantages:Advantages: moderate even high polarization: moderate even high polarization: P ~90% @ QE 0.5% high brightness: 10high brightness: 1055 A/cm A/cm22∙∙sr sr good beam quality: small energy spreading etc.good beam quality: small energy spreading etc. polarization direction can be easily changed by reversing the polarization direction can be easily changed by reversing the

helicity of the incident lighthelicity of the incident light

Disadvantages:Disadvantages: PP is only 20─ 35% for bulk GaAs is only 20─ 35% for bulk GaAs low QE for strained superlattice GaAs-based material low QE for strained superlattice GaAs-based material Cs and OCs and O22 activation to get activation to get “negative electron affinity” (NEA)“negative electron affinity” (NEA) every every

hundreds of hours (1 days only for our SPLEEM)hundreds of hours (1 days only for our SPLEEM) Brightness is lower compared with FEM source(10Brightness is lower compared with FEM source(1077 A/cm A/cm22∙∙sr)sr)

Is it possible to generate spin-polarized electrons by Is it possible to generate spin-polarized electrons by

field emission?field emission? Spin-polarized field emissionSpin-polarized field emission

Φ

Spin-down Spin-up

Density of States

3d

ε

EF Effective potential barrier after applying electric field

The FEM electrons gun provides the highest brightness and the best The FEM electrons gun provides the highest brightness and the best beam quality and makes the best resolution for TEM/SEM, Sobeam quality and makes the best resolution for TEM/SEM, So

Fowler-NordheimFowler-Nordheim equationequation

Normally, field- emitted electrons’ polarization reflect the magnetic properties of Normally, field- emitted electrons’ polarization reflect the magnetic properties of ferromagnetic films on the tip’s apex. For example, the direction of polarization is ferromagnetic films on the tip’s apex. For example, the direction of polarization is parallel or antiparallel to the magnetization of films, which are dependent on majority parallel or antiparallel to the magnetization of films, which are dependent on majority and minority electrons population. and minority electrons population.

V

αkRφ106.8exp

(kR)

V

φ)(μα

(μμ/φ106.2i

3/27

2

2

2

1/26

Etched W tip

Earlier work on spin-Earlier work on spin-polarized field emissionpolarized field emission

Transition metal tips or transition metal and rare earth metals (Gd, Transition metal tips or transition metal and rare earth metals (Gd, Td, Dy, Ho, Er and Tm) coated W(110) tips (Chrokok et al.,1977 ) Td, Dy, Ho, Er and Tm) coated W(110) tips (Chrokok et al.,1977 )

Low-index planes of Ni and Fe tips (Landolt et al., 1977)Low-index planes of Ni and Fe tips (Landolt et al., 1977)

EuS thin film coated W tip (Müller et al., 1972)EuS thin film coated W tip (Müller et al., 1972)

Co-coated W(111) tips (Bryl et al., 2003)Co-coated W(111) tips (Bryl et al., 2003)

typical 10─25% while the highest of 48% (spontaneous or remanent Magnetization)typical 10─25% while the highest of 48% (spontaneous or remanent Magnetization)

Ni(100)<5%, Fe(100)=(+25±5)%, Fe(111)=(+20±5)%, Fe(110)= (−5±10) % (too soft)Ni(100)<5%, Fe(100)=(+25±5)%, Fe(111)=(+20±5)%, Fe(110)= (−5±10) % (too soft)

80% from massive Fe tips with electron bombardment treatment (bad reproducibility)80% from massive Fe tips with electron bombardment treatment (bad reproducibility)

(89±7) % was observed at T~ 10 K (dropped dramatically if temperature was raised a (89±7) % was observed at T~ 10 K (dropped dramatically if temperature was raised a few K, strong M field)few K, strong M field)

Why ultrathin Fe films on the Why ultrathin Fe films on the W(100) tip?W(100) tip?

W(100) face has relative low work function and W(100) face has relative low work function and can be sharpened in the vacuum. can be sharpened in the vacuum.

Interesting magnetic properties of Fe/W(100):Interesting magnetic properties of Fe/W(100): AF(AF(θ≤θ≤1 ML) , easy magnetization axes of 2─ 6 ML 1 ML) , easy magnetization axes of 2─ 6 ML Fe/W(100) are along <110> directions in plane and at Fe/W(100) are along <110> directions in plane and at higher coverage it shows a <100> easy axis in plane.higher coverage it shows a <100> easy axis in plane.

Theorists predict that almost 100% polarized Theorists predict that almost 100% polarized electrons can be field-emitted from electrons can be field-emitted from pseudomorphic Fe ultrathin films (4ML) on pseudomorphic Fe ultrathin films (4ML) on W(100) surface W(100) surface (Li B. et al. ,2006).(Li B. et al. ,2006).

E-beam Evaporator

2. Experiments

Pbase≤1×10-10 torr

Experiment procedure

Clean and sharpen tip Measure the asymmetry of the clean W tip Fe deposition Measure the Polarization of Fe coated tip Flash off the Fe film to get back clean tip Measure the asymmetry of the clean tip again

Cleaning and sharpening of the W(100) tipCleaning and sharpening of the W(100) tip Polarizations vs. Fe thicknessPolarizations vs. Fe thickness Stability of polarizations and emission Stability of polarizations and emission

currentscurrents Spin reorientation and magnetic anisotropySpin reorientation and magnetic anisotropy

3. Results and discussion3. Results and discussion

The builtup process of a The builtup process of a W(100) tipW(100) tip

a b c

d e f

(111)

(110)

(100)

<110>

<100>

(110)

(100)

g h

(a) the pattern after O(a) the pattern after O22 annealing, (b) a relatively annealing, (b) a relatively blunt W(100) oriented tip blunt W(100) oriented tip after removing covered after removing covered O2, (c)-(e) flashing at O2, (c)-(e) flashing at 1900 K in a strong 1900 K in a strong electric field, (f) a sharp electric field, (f) a sharp tip ready for tip ready for experiments. As shown in experiments. As shown in (g) and (h), a bcc crystal (g) and (h), a bcc crystal has lower surface energy has lower surface energy on the (110) faces, a on the (110) faces, a <100> oriented wire can <100> oriented wire can be sharpened into a be sharpened into a pyramid formed by four pyramid formed by four (110) faces via the (110) faces via the application of the proper application of the proper field and temperature field and temperature

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Fe thickness

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a b c d e f g h i

a b c

d e f

g h i

k l mj

* * It should be noted panels (a)-(i) and (j)-(m) are taken in two different experimentsIt should be noted panels (a)-(i) and (j)-(m) are taken in two different experiments..

FEM patterns FEM patterns and polarizations and polarizations for different Fe for different Fe coveragescoverages

The Fe coverages The Fe coverages (ML) in (a)-(m) are (ML) in (a)-(m) are 0, 4.9, 5.5, 6.1, 6.7, 0, 4.9, 5.5, 6.1, 6.7, 7.3, 7.9, 8.5, 0, 9.8, 7.3, 7.9, 8.5, 0, 9.8, 12.2, 14.6 and 17.1 12.2, 14.6 and 17.1 respectively. respectively.

Time stability of polarization and Time stability of polarization and emission currentemission current

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4.00E-09

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Hz)

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a

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P=25±5%

Total time= 22 hours, max is 3 days and no more days for testingTotal time= 22 hours, max is 3 days and no more days for testing

0. 00E+00

2. 00E-10

4. 00E-10

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1. 80E-09

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total emi ssi on currenttotal count i ng rate

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(minute)

Spontaneous and heat-driven Spontaneous and heat-driven reorientation of Polarizationsreorientation of Polarizations

Pmax=35±5%

<100><100>

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Heating circles

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<100>

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A rotation form <100> to <110> and A rotation form <100> to <110> and polarizations are along <110>polarizations are along <110>

<110>

<100>45º0

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heating circle

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<100> to <110> 45º rotation

<100>

<110>

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Time (total time=20 hours)

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)Superparamagnetic fluctuation of a Superparamagnetic fluctuation of a

single domain Fe filmsingle domain Fe film

Such Fe Such Fe nanostructures with nanostructures with diameter and height diameter and height smaller than 100nm smaller than 100nm and 8 nm and 8 nm respectively should respectively should be single domainbe single domain

1 nm

120nm

NNééel formula for a single el formula for a single domain particledomain particle

Tk

KV

B

exp0

wherewhereνν00 is the attempt frequency of the order of is the attempt frequency of the order of

10101010─10─101313Hz.Hz. UU==KVKV is the anisotropy energy barrier, is the anisotropy energy barrier, where where KK is anisotropy constant, and is anisotropy constant, and VV is a volume. is a volume. kkBB is is

boltzmann constant, and boltzmann constant, and TT is the temperature in Kelvin is the temperature in Kelvin

Two-level jump process model for Two-level jump process model for superparamagnetic fluctuationsuperparamagnetic fluctuation

txtxx exp)()0( 2 Autocorrelation function for TJPAutocorrelation function for TJP

x(t)

x0

x0

t10 t2 t3 t5t4 t6

txxxtxx exp)()()0( 222

And, multi-level jump processes (MJP) with equal probability And, multi-level jump processes (MJP) with equal probability and the unequal probability Kubo-Anderson Process (KAP)and the unequal probability Kubo-Anderson Process (KAP)

So,So,ν ν can be looked as a characteristic number to estimate the can be looked as a characteristic number to estimate the

degree of the superparamagnetic fluctuationdegree of the superparamagnetic fluctuation

0.01

0.1

1

0 2 4 6 8 10Time (s)A

utoc

orre

latio

n (a

rb. u

nit)

397K

415K

427K

442K

459K

467K

476K

486K

497K

508K

516K

527K

v=1010.01±0.58 exp[−(11700±2600)/T]

v=1011.32±0.66 exp[−(12700±976)/T]

The autocorrelation The autocorrelation functions of a 11ML Fe film functions of a 11ML Fe film on the W(100) tip at on the W(100) tip at different temperatures.different temperatures.

Flipping rates, ν, at Flipping rates, ν, at different temperatures different temperatures vsvs. . reciprocal of temperaturereciprocal of temperature

Tk

KV

B

exp0

v=1011.86±0.75exp[−(3.80±0.21)θ]

For a multiaxial magnetocrystalline For a multiaxial magnetocrystalline cubic crystal, its energy barrier in a cubic crystal, its energy barrier in a zero applied field is equal to zero applied field is equal to KVKV/4 /4 when when KK>0, where >0, where KK is the first order is the first order magnetocrystalline anisotropy energy magnetocrystalline anisotropy energy and and VV is the volume of the magnetic is the volume of the magnetic particle. particle.

For Fe bulk, K= 4.2×10For Fe bulk, K= 4.2×104 4 J/mJ/m33 and 1 and 1 ML Fe/W(100)= 1.176 Å, it is easy to ML Fe/W(100)= 1.176 Å, it is easy to get an effective radius of Fe film to be get an effective radius of Fe film to be 62±6 nm, which is very close to the tip 62±6 nm, which is very close to the tip radius calculated by Fowler-Nordheim radius calculated by Fowler-Nordheim equation. In another hand, K for thin equation. In another hand, K for thin film can be 2 order larger than bulk film can be 2 order larger than bulk and gives radius for 6nm. So the and gives radius for 6nm. So the radius of the magnetic nanostructure radius of the magnetic nanostructure on the apex are from 6-62nm and it is on the apex are from 6-62nm and it is a single domain. a single domain.

Hint to flip the polarization direction: Heat the tip to an elevated temperature (500-550K) and apply a small magnetic field to flip polarization to desired direction. Cool down the tip before removing magnetic field and the polarization will be stable at that direction. This process can be easy and fast controlled automatically.

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Coverage

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a b=0 c=0.33 d=0.66 e=1 f

For a certain Ag coverages lower than 1 ML, polarizations For a certain Ag coverages lower than 1 ML, polarizations of 20- 30% usually can be obtained which are almost same of 20- 30% usually can be obtained which are almost same as polarization magnitudes of the original Fe/W(100) tip. as polarization magnitudes of the original Fe/W(100) tip.

there are two consequent advantages decrease of the work there are two consequent advantages decrease of the work function of the field emission tip up to 20% and function of the field emission tip up to 20% and improvement of the collimation of the electrons beam.improvement of the collimation of the electrons beam.

Before Ag deposition After Ag deposition

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.70

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Ag coverage (ML)

Extraction voltage drops from 950 to 650v for the same counting rate (100) emission current

Work function vs. Ag thickness

Ag/Fe/W(100) tipsAg/Fe/W(100) tips

4. Summary and 4. Summary and expectationexpectation P=20-35%

stable for days without the magnitude decrease and the direction reorientation. Study on polarization directions gives evidences that easy magnetization axes of Fe/W(100) tips are along <100> or <110> directions. The effective emission area is a single domain nanostructure. The polarization of the tip can be easily flipped by both thermal energy and magnetic field. Sub-monolayer Ag film on the Fe/W(100) tip can improve the beam collimation and lower the work function without reduce the polarization.

Test higher emission current (Test higher emission current (μμA)A) Higher polarizationHigher polarization Improve the polarization and current stabilityImprove the polarization and current stability