Presentation on theme: "Ultrathin films and preparation of films 1.Ultrathin films Surface anisotropy, Magnetisation, M(T) behavior, Domains 1.Film preparation (a) Vacuum evaporation."— Presentation transcript:
Ultrathin films and preparation of films 1.Ultrathin films Surface anisotropy, Magnetisation, M(T) behavior, Domains 1.Film preparation (a) Vacuum evaporation (b) Magnetron sputtering (c) Laser abrasion (d) Molecular beam epitaxy (e) Self-assembled monolayer 3. 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 ML thick 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 Fe films.
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/P o for 1, 3, and 5 ML films of fcc Fe on Cu(001). P o is the saturation polarization at low temperature. The Curie temperature are 230K for 1ML film, 390 K for 3 and 5 ML films.
Normalized polarization P/P o as a function of the externally applied field perpendicular to the film plane. Data are given for five film thickness at T=300 K. Fcc Co on Cu(001) PRL 58(1987)933
Temperature dependence of the spin polarization for a 1 ML film measured in saturation. Applied field 15 KOe
PRB 70(2004) 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 Matt 86(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) 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 the field 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 o C.
Effective anisotropy times Co thickness versus cobalt thickness for [Co/Pt] multilayers (Engle PRL 67(1990)1910). K eff d = 2k s + (k V -2πM s 2 )d
Surface Magnetic Anisotropy ? The reduced symmetry at the surface (Neel 1954); The ratio of L z 2 / (L x 2 + L y 2 ) is increased near the surface Interface anisotropy (LS coupling)  J.G.Gay and Roy Richter, PRL 56(1986)2728,  G.H.O. Daalderop et al., PRB 41(1990)11919,  D.S.Wang et al., PRL 70(1993)869.
Ferromagnetism in fcc Fe(111) on CuAu (111). Magnetic moment µ Fe in the Fe film versus the mean lattice parameter a CuAu or Au concentration c Au 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 µ B /Ni and 0.74 µ B /Ni (bulk moment 0.6 µ B /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)
M s (T) behaviour 48Ni/52Fe (111) films on Cu(111)
Curie temperature of 48Ni/52Fe(111) versus number of atomic layers D M. 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 180 O domain wall.
(a)Domain pattern as measured by MFM above the surface of an eiptaix Cu/200nmNi/Cu(100) film. (b) Vibrating sample magnetometry M-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 dZ A / dZ B = (M B /M A ) 1/2 exp[-(ΔH A - ΔH B )/RT] (C A /C B ) = K (C A /C B ). M A, M B are the mass of A and B element; ΔH A and ΔH B the evaporation heat, C A : C B 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 laser width ms, Density 1-5 J/cm 2
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/cm 2 Ceramic target The distance between the target and substrate is 55 mm 3x10 -5 mTorr before introducing pure O 2 O 2 gass flow of 60 sccm at a pressure of 75 mTorr After deposition, the amorphous film is post annealed for 2 minutes at 650 o C in air
Bi 2.0 Dy 1.0 Fe 3.5 Ga 1.0 O 12
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, 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 with The deposition ML Substrate deposite
Two arrangements for four deposited atoms in the same phase 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 -4u AB – 12u AA -8u AB -10u AA If u AA > 2u AB the case of (a) is beneficial for the reduction of energy (a) (b)
The condition for double layers arrangement (Island): (a) N=8, u AA >2u AB, (b) N=18, u AA >1.5u AB, (c) N=32, u AA >1.33u AB, (d) N=50, u AA >1.25u AB, (e) N=72, u AA > 1.24 u AB.
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 KL 1 L 2,3 transition
High resolution AES spectrum of Ge LMM for 5KV incident energy. The strongest peaks, within the L 2 M 4,5 M 4,5 series at 1145 and L 3 M 4,5 M 4,5.
The integrating spectroscopy, N(E), of the surface AES, and N (E)=dN(E)/dE. The surface AES of 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, E k = hv – E b, where, hv photon energy, UPS uses ultra-violet radiation as the probe and collects electrons directly from the valence band, XPS excites a core hole 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 observe surface 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 changes 0.01nm. 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 TiO 2 film. No segregation of impurity phases was obser- ved in the film.