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Spin-polarized electrons from Fe films coated single crystal W(100) tips by field emission

Spin-polarized electrons from Fe films coated single crystal W(100) tips by field emission. Department of Physics Hong Kong University of Science and Technology. Y. Niu and M. S. Altman. Outline. 1. Introduction 2. Experiments 3. Results and discussion 4. Summary and expectations.

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Spin-polarized electrons from Fe films coated single crystal W(100) tips by field emission

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  1. Spin-polarized electrons from Fe films coated single crystal W(100) tips by field emission Department of Physics Hong Kong University of Science and Technology Y. Niu and M. S. Altman

  2. Outline • 1. Introduction • 2. Experiments • 3. Results and discussion • 4. Summary and expectations

  3. 1. Introduction Spin-polarized electrons are extensively used in fields of • Atom physics • High energy physics • Solid state physics (SPLEED, SPSEE, SPEELS, SPAES, SPLEEM, SEMPA, SPSTM, SARPES and SPIPES) • Specifically, Spin-polarized electron microscopies, such as Spin polarized low energy electron microscope (SPLEEM) desirehigh polarization, high brightnessand long lifetime polarized electron beam. The bulk GaAs crystal is almost the only spin polarized electrons source (PES) for such instruments. SPLEEM at ASU

  4. PES of GaAs and GaAs-based materials • Advantages: • moderate even high polarization: P ~90% @ QE 0.5% • high brightness: 105 A/cm2∙sr • good beam quality: small energy spreading etc. • polarization direction can be easily changed by reversing the helicity of the incident light • Disadvantages: • P is only 20─ 35% for bulk GaAs • low QE for strained superlattice GaAs-based material • Cs and O2 activation to get “negative electron affinity” (NEA) every hundreds of hours (1 days only for our SPLEEM) • Brightness is lower compared with FEM source(107 A/cm2∙sr)

  5. Spin-polarized field emission ε Φ EF 3d Spin-up Spin-down Density of States Effective potential barrier after applying electric field Is it possible to generate spin-polarized electrons by field emission? The FEM electrons gun provides the highest brightness and the best beam quality and makes the best resolution for TEM/SEM, So EtchedW tip Fowler-Nordheimequation • Normally, field- emitted electrons’ polarization reflect the magnetic properties of 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 and minority electrons population.

  6. Earlier work on spin-polarized field emission • Low-index planes of Ni and Fe tips (Landolt et al., 1977) Ni(100)<5%, Fe(100)=(+25±5)%, Fe(111)=(+20±5)%, Fe(110)= (−5±10) % (too soft) • 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 ) 80% from massive Fe tips with electron bombardment treatment (bad reproducibility) • EuS thin film coated W tip (Müller et al., 1972) (89±7) % was observed at T~ 10 K (dropped dramatically if temperature was raised a few K, strong M field) • Co-coated W(111) tips (Bryl et al., 2003) typical 10─25% while the highest of 48% (spontaneous or remanent Magnetization)

  7. Why ultrathin Fe films on the W(100) tip? • W(100) face has relative low work function and can be sharpened in the vacuum. • Interesting magnetic properties of Fe/W(100):AF(θ≤1 ML) , easy magnetization axes of 2─ 6 ML Fe/W(100) are along <110> directions in plane and at higher coverage it shows a <100> easy axis in plane. • Theorists predict that almost 100% polarized electrons can be field-emitted from pseudomorphic Fe ultrathin films (4ML) on W(100) surface (Li B. et al. ,2006).

  8. 2. Experiments E-beam Evaporator Pbase≤1×10-10 torr

  9. 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

  10. 3. Results and discussion • Cleaning and sharpening of the W(100) tip • Polarizations vs. Fe thickness • Stability of polarizations and emission currents • Spin reorientation and magnetic anisotropy

  11. g h (100) (110) c b a e f d (110) (100) <100> (111) <110> The builtup process of a W(100) tip (a) the pattern after O2 annealing, (b) a relatively blunt W(100) oriented tip after removing covered O2, (c)-(e) flashing at 1900 K in a strong electric field, (f) a sharp tip ready for experiments. As shown in (g) and (h), a bcc crystal has lower surface energy on the (110) faces, a <100> oriented wire can be sharpened into a pyramid formed by four (110) faces via the application of the proper field and temperature

  12. a b c d e f g h i j c a b e f d g i h l m k FEM patterns and polarizations for different Fe coverages • The Fe coverages (ML) in (a)-(m) are 0, 4.9, 5.5, 6.1, 6.7, 7.3, 7.9, 8.5, 0, 9.8, 12.2, 14.6 and 17.1 respectively. * It should be noted panels (a)-(i) and (j)-(m) are taken in two different experiments.

  13. a b 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 22 Time stability of polarization and emission current P=25±5% Total time= 22 hours, max is 3 days and no more days for testing

  14. (minute)

  15. 50 180 120 40 60 30 Polarization direction (deg) Polarization magnitude (%) 0 20 -60 10 -120 0 -180 0 20 40 60 80 100 120 140 Time (min) <100> <100> Spontaneous and heat-driven reorientation of Polarizations Pmax=35±5%

  16. <100> to <110> 45º rotation 45º <100> <110> <100> <110> A rotation form <100> to <110> and polarizations are along <110>

  17. 1 nm 120nm Superparamagnetic fluctuation of a single domain Fe film Such Fe nanostructures with diameter and height smaller than 100nm and 8 nm respectively should be single domain

  18. Néel formula for a single domain particle • whereν0 is the attempt frequency of the order of 1010─1013Hz.U=KV is the anisotropy energy barrier, where K is anisotropy constant, and V is a volume. kBis boltzmann constant, and T is the temperature in Kelvin

  19. x(t) x0 0 t1 t2 t6 t3 t4 t5 x0 Two-level jump process model for superparamagnetic fluctuation Autocorrelation function for TJP And, multi-level jump processes (MJP) with equal probability and the unequal probability Kubo-Anderson Process (KAP) • So,ν can be looked as a characteristic number to estimate the degree of the superparamagnetic fluctuation

  20. v=1011.32±0.66 exp[−(12700±976)/T] v=1010.01±0.58 exp[−(11700±2600)/T] The autocorrelation functions of a 11ML Fe film on the W(100) tip at different temperatures. Flipping rates, ν, at different temperatures vs. reciprocal of temperature

  21. v=1011.86±0.75exp[−(3.80±0.21)θ] • For a multiaxial magnetocrystalline cubic crystal, its energy barrier in a zero applied field is equal to KV/4 when K>0, where K is the first order magnetocrystalline anisotropy energy and V is the volume of the magnetic particle. • For Fe bulk, K= 4.2×104 J/m3 and 1 ML Fe/W(100)= 1.176 Å, it is easy to get an effective radius of Fe film to be 62±6 nm, which is very close to the tip radius calculated by Fowler-Nordheim equation. In another hand, K for thin film can be 2 order larger than bulk and gives radius for 6nm. So the radius of the magnetic nanostructure on the apex are from 6-62nm and it is 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.

  22. a b=0 c=0.33 d=0.66 e=1 f Before Ag deposition After Ag deposition Ag/Fe/W(100) tips Work function vs. Ag thickness • For a certain Ag coverages lower than 1 ML, polarizations of 20- 30% usually can be obtained which are almost same as polarization magnitudes of the original Fe/W(100) tip. • there are two consequent advantages decrease of the work function of the field emission tip up to 20% and improvement of the collimation of the electrons beam. Extraction voltage drops from 950 to 650v for the same counting rate (100) emission current

  23. 4. Summary and expectation • 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 (μA) • Higher polarization • Improve the polarization and current stability

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