Ultrathin films and preparation of films

1.Ultrathin films Surface anisotropy, Magnetisation, M(T) behavior, Domains1.Film preparation (a) Vacuum evaporation (b) Magnetron sputtering (c) Laser abrasion (d) Molecular beam epitaxy (e) Self-assembled monolayer3. Measurements

E.Bauer, Growth of thin films, J.Phys: Condens. Matter 11(1999)9365

Interesting aspects:(1) For studying new phases of materials

such as, fcc cobalt and fcc iron grown on Cu (001)

(2) Two dimensional features which not encountered in bulk specimens.

(3) Whether any dead layer appears ?

Spin polarization, p (in%), of photoelectrons from Fe(100) on Ag(100),

versus magnetic field along the surface normal (Stampanoni et al.,PRL

59(1987)2483).

(1)Perpendicular anisotropy

N=(P↑-P↓)/ (P↑+P↓)

Temperature dependence of the saturation polarization of a 3.5 MLthick epitaxial bcc Fe film on Ag(001) and a 3 ML fcc Fe on Cu(001).Insert: thickness dependence of the Cuire temperature of the bcc Fefilms.

Polarization P(H) of a 5 ML fcc Fe film on Cu(001), (a) for sample temperature T=215, 267, and 375 K, (b) at T=30 K, H is perpendicular to the film plan.

Fcc Fe on (001)Cu

Polarization P(H) measured at T=30 K, (a) for 3 ML fcc Fe on Cu(001), (b) for 1 ML. H is perpendicular to the film plane.

Temperature dependence of the reduced polarization P/Po for 1, 3,and 5 ML films of fcc Fe on Cu(001). Po is the saturation polarizationat low temperature. The Curie temperature are 230K for 1ML film, 390 K for 3 and 5 ML films.

Normalized polarization P/Po as a function of the externally applied field perpendicular to the film plane. Data are given for five film thickness atT=300 K.

Fcc Co on Cu(001) PRL 58(1987)933

Temperature dependence of the spin polarization for a 1 ML filmmeasured in saturation. Applied field 15 KOe

3M 286(2005)405

PRB 70(2004)214406

Fe(100) on V(100)

Magnetisation loop of 2.2 nm thick Ni(111) on Cu(111), coated by Cu(111), measured by TOM. The films show a perpendicular aniso-tropy (PA) between 1.0 – 2.5 nm (Gradmann, Ann. Phys.17(1966)91).

J Phys: Condens Matt86(2005)s573

From the above two figures, the following facts are evident:

(1) Films of 1ML or thicker of bcc Fe/Ag(001) is ferromagnetic, Tc ~400K;

For >5ML bcc Fe/Ag(001) Its Tc approaches to bulk;

3.5ML and 5ML layer thickness Fe(100) on Ag(100) show perpendicular anisotropy.

(2) For fccFe/Cu(001) 1, 3 and 5 ML Tc=220, 360 and 410K, respectively, 3ML Fe/Cu appears P anisotropy at 30K.

(3) Fcc Co/Cu(001) 1ML is F and Tc ~400K, Perpendicular anisotropy appears at room T.

(4) 1-2.5 ML Ni(111)/Cu(111) appears perpendicular anisotropy.

(5) Orbital Moment appears in ultra-thin Co film (<2ML).

In short : > 1ML F appears, Perpendicular anisotropy appears in ultra-thin films, Orbital Moment appears in ultra-thin Co film.

Magnetisation loops of Pd/Co multilayers, taken at 300 K, with thefield in the film plane (dashed curves) or along the surface normal(full lines) (Carcia APL 47(1985)178).

Total anisotropy Kt for evaporated (111) texturized polycrystalline Co/Pd multilayers versus thickness t of Co films.

Co/Pt Multilatersa

Magnetic hysteresis loops at 20 oC.

Effective anisotropy times Co thickness versus cobaltthickness for [Co/Pt] multilayers (Engle PRL 67(1990)1910).

Keff d = 2ks + (kV -2πMs2)d

Surface Magnetic Anisotropy ?

•The reduced symmetry at the surface (Neel 1954);•The ratio of Lz

2 / (Lx2 + Ly

2) is increased near the surface•Interface anisotropy (LS coupling)

[1] J.G.Gay and Roy Richter, PRL 56(1986)2728, [2] G.H.O. Daalderop et al.,PRB 41(1990)11919, [3] D.S.Wang et al., PRL 70(1993)869.

PRL 88(2002)217202

Ferromagnetism in fcc Fe(111) on CuAu (111). Magnetic momentµFe in the Fe film versus the mean lattice parameter aCuAu or Au concentration cAu in the substrate.

Surface Magnetisation

Variation of magnetic moment calculated by layer in an 8 ML Ni/Cu(001) film. The calculated bulk and surface moments are 0.56 µB/Niand 0.74 µB/Ni (bulk moment 0.6 µB/Ni) (Tersoff PRB 26(1982)6186).

The interior, bulklike layers (layers 3-6 from Cu)

The surface-like layers(layers 7 and 8 from Cu)

Spin-resoled density of state for 8 ML Ni(001) film on Cu(001)

Ms (T) behaviour

48Ni/52Fe (111) filmson Cu(111)

Curie temperature of 48Ni/52Fe(111) versus number of atomic

layers DM. The experiments is from Gradmann (Phys. Status Solidi 27(1968)313. Green-function theory from Brodkorb 16(1966)225.)

Domain in ultrathin films

Calculated spin distribution in a thinn sample containing A 180O domain wall.

(a) Domain pattern as measured byMFM above the surface of an eiptaixCu/200nmNi/Cu(100) film.

(b) Vibrating sample magnetometryM-H loop of the sample

Schematic drawing of the evaporation chamber.

Thermal evaporation and the uniformity of deposits

• The simplest technology, raising the temperature of the source materials;• An open boat, suspended on a wire;• The boat or wire is a high temperature material, such as W or Mo and must not react with the evaporant;• The substrate hold should be rotated in order to get uniform deposits;• The deposition rate is determined by the source area temperature and the distance between the source and substrate as well as the evaporant itself;• Electron beam deposition for high temperature materials or materials which interact with the crucible.

Binary alloy evaporation

dZA / dZB = (MB/MA )1/2 exp[-(ΔHA- ΔHB)/RT] (CA/CB) = K (CA/CB).

MA, MB are the mass of A and B element; ΔHA and ΔHB the evaporation heat, CA : CB is the atomic ratio.

The variation of the ratio of evaporated A and B element in binary alloy with time

(K)

(the ratio of A:B)

Sputtering and ion beam assisted deposition

Sputtering (Ion beam assisted deposition (IBAD), Ion beam sputter deposition (IBSD)) provides the better quality deposits: • at low substrate temperature, thus avoiding large scale interdiffusion,• adhere well to the substrate,• to realize a reactive sputtering.

Schematic picture of magnetron sputtering

The sputtering rate for the different element(using 500 eV Ar+).

Schematic picture of Laser ablation

Pulsed laserwidth 10-20 ms,Density 1-5 J/cm2

• A continuum NY81-C Nd:YIG laser• The wavelength, pulse frequency and pulse width are 355 nm, 10Hz and 10ns, respectively• The focused laser beam with the energy density of 3-4 J/cm2

• Ceramic target• The distance between the target and substrate is 55 mm• 3x10-5mTorr before introducing pure O2

• O2 gass flow of 60 sccm at a pressure of 75 mTorr• After deposition, the amorphous film is post annealed for 2 minutes at 650oC in air

Bi2.0Dy1.0Fe3.5Ga1.0O12

Summary

(1)The chemical composition of the film is the same as that of target(2) The polycrystalline films on ceramic glass substrate have easy magnetization axis normal To the film surface, nanometer size grain and very smooth surface(3) The film shows high squareness of Faraday hysteresis loop(4) Magnetization of the film at temperature range from 240K to 340K is almost temperature independent.

The special points of Pulse Laser Ablation

The advantages• The ablated sample with the same composition as the target composition;• High energy particles is beneficial for the film growth and realizing a chemical reaction on substrate;• Reaction deposition;• Multilayers growth and thickness control precisely. The disadvantages• Forming small particle, 0.1-10 µ m,• thickness deposited is not uniform

(1) Growth under controlled and monitored conditions with in situ analysis of film structure and composition (RHEED, LHEED, XPS, AES).

(2) A key advantage of MBE is that it enables growth of the layered structure along specific crystalline direction;

(3) Lattice-matching between the seed film (prelayer) and substrate can be achieved by appropriate choice of materials and the growth axis of the magnetic structure selected.

Advantages of MBE

Magnetic hysteresis loops for oriented Co-Pt super-lattice recorded by MO effect.

Schematic representation of the three growth modes (a) Island (b) layer-plus-island (c) layer by layer.

The change of AES peak withThe deposition

ML

Substrate

deposite

Two arrangements for four deposited atoms in the samephase epitaxy

7 AA bonds

8 AA bonds (stable state)

In the case of the same phase epitaxy, the stable state is one-Layer-arrangement, namely, two demitional growth.

For the different phase epitaxy

-4uAB – 12uAA

-8uAB-10uAA

If uAA > 2uAB the case of (a) is beneficial for the reduction ofenergy

(a)

(b)

The condition for double layers arrangement (Island):(a) N=8, uAA>2uAB, (b) N=18, uAA>1.5uAB, (c) N=32,uAA>1.33uAB, (d) N=50, uAA>1.25uAB, (e) N=72, uAA>1.24 uAB.

Other factors should be considered

(a) The size of the epitaxy atoms If the size of A atom (epitaxy) is larger than that of B (substrate), a compressive strain appears in the epitaxy layers, conversely, tensile force appears;

(b) The strain increases with the increase of epitaxy thickness and finally dislocation could exist;

(c) Island appears if the size A atom is largely different from B atom (substrate).

Electron-based techniques for examining surface and thin film process

AES (Auger electron spectroscopy)LEED (Low energy electron diffraction)RHEED (Reflection high energy electron microscopy)TEM (Transmission electron microscopy)REM (Reflection electron microscopy)STM (Scanning tunneling microscopy)AFM (Atomic force microscopy)PEEM (Photoemission microscopy)SEM (Scanning electron microscopy)SNOM (Scanning near field optical microscopy)XPS (X-ray photoemission spectroscopy)UPS (Ultra-violet photoemission spectroscopy)

Auger Electron Spectoscopy (AES)

Si KLL Auger scheme (Chang Surface Sci., 25(1974)53).

Si KL1L2,3 transition

High resolution AES spectrum of Ge LMM for 5KV incident energy.The strongest peaks, within the L2M4,5M4,5 series at 1145 and L3M4,5M4,5.

The integrating spectroscopy, N(E), of the surface AES, and N’(E)=dN(E)/dE.

The surface AESof Fe

Photoelectron Spectroscopies: XPS and UPS

After the electron at inner shell or valence electron absorb photon energy, they leave atom and become photo-electron, Ek = hv – Eb, where, hv photon energy,

UPS uses ultra-violet radiation as the probe and collectselectrons directly from the valence band, XPS excites a corehole with X-rays and collect binding energy of the electrons at the inner cells.

The electron energy spectrum on Ni obtained by bombardment of 1.25Kev photon.

XPS

Scanning Tunneling Microscopy(STM)

• The tunneling current is measured by W needle• The distance between the tip and sample surface is below 1 nm; resolution along vertical is 0.01nm and in transverse is 0.1nm• The tip is applied a few voltage and the tunneling current is 0.1 to 1.0 nA• The current is related not only to the height of atom on the surface, but also to the atomic density (density state)

Atomic Force Microscopy (AFM)

STM is only applied to observesurface for conductor or semi-conductor, while AFM is an appropriate tool for all samples.

The reflect light place is 3-10nm after the height of tip changes0.01nm.

Three operation models of AFM:(1) contact (2) non-contact (3)tapping model.

Transmission electron microscopy (TEM)

(1)With TEM one can obtain diffraction patterns and images of the sample, revealing microstractural defects such as dislocation, grain-, twin- and antiphase boundaries

(2) In order for the electrons to pass through the specimen, it has to be electron transparent (hundreds of nm)

(3) High resolution than a light microscope

Atoimic resolution TEM image of a Co doped TiO2

film. No segregation of impurity phases was obser-ved in the film.