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Ultrathin Films and Some Cross Effect (1) Some properties in ultra thin films (a) surface and interface anisotropy (b) fcc Fe and fcc Co (c) surface magnetism.

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Presentation on theme: "Ultrathin Films and Some Cross Effect (1) Some properties in ultra thin films (a) surface and interface anisotropy (b) fcc Fe and fcc Co (c) surface magnetism."— Presentation transcript:

1 Ultrathin Films and Some Cross Effect (1) Some properties in ultra thin films (a) surface and interface anisotropy (b) fcc Fe and fcc Co (c) surface magnetism (d) M(T) in ultra thin film (2) Some cross effect (a) magneto-optical effect (b) dilute magnetic semiconductor (DMS)

2 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 ?

3 Spin polarization, p (in%), of photoelectrons from Fe(100) on Ag(100), versus magnetic field along the surface normal (Stam- panoni et al.,PRL 59(1987)2483). Perpendicular anisotropy

4 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. P=(N - N )/ ( N + N )

5 From the above two figures, the following facts are evident: (1) Films of 1ML or thicker of bcc Fe/Ag(001) are ferromagnetic (2) At T=30K, the remanence magnetization of 3.5 ML film is dirrected along the surface normal. The remanence equals practically the full saturation magnetization showing the film to be essentially single domain (3) The Curie temperature of films thicker than 5 ML equals the bulk bcc Fe

6 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).

7 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).

8 Total anisotropy Kt for evaporated (111) texturized polycrystalline Co/Pd multilayers versus thickness t of Co films. 50 C 200 C

9 Magnetic hysteresis loops at 20 o C. Co/Pt Multilatersa

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

11 The effective anisotropy energy measured for a film of thickness d may be described as = or writing as, (1) (2) (3) K eff d = 2k s + (k V -2πM s 2 )d

12 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) [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.

13 Fcc Co on Cu(001) 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.

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

15 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 perpen- dicular to the film plan. Fcc Fe on (001)Cu

16 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.

17 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.

18 Conclution: (1) 1ML of Co on Cu(001) is ferromagnetic; The magnetization being in plane; T c (for 1 ML) > room temperature. (2) Fcc Fe (1 ML)stabilized at the lattice constant of Cu (001) has been found to have a ferromagnetic ground state; Anisotropy and Curie temperature is dependent on the temperature and film thick- ness [1] D. Pescia et al., PRL 58(1987)933, [2] D.Pescia et al., PRL 58(1987)2126

19 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. Perpendicular M s Surface Magnetisation

20 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).

21 Spin-resoled density of state for 8 ML Ni(001) film on Cu(001) The interior, bulklike layers (layers 3-6 from Cu) The surface-like layers (layers 7 and 8 from Cu )

22 M s (T) behaviour 48Ni/52Fe (111) films on Cu(111)

23 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.)

24 Calculated spin distribution in a thinn sample containing A 180 O domain wall. Domain in ultrathin films

25 (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

26 Magneto-optical Effect The three types of geometries of the Kerr effect 1876 John Kerr

27 The arrangement of the magnetization M and wave vector k in the local coodination employed in the derivation of the p- MOKE equation for Normal incidence.

28 The dielectric tensor has the following form The normal model solution to the Fresnel Eq. and the corresponding electric field model are (1) (2) (3)

29 The definition of Kerr rotation and Kerr ellipticity

30 Kerr (Faraday) rotation and ellipticity are expressed by the component of conductivity sensor

31 Petros N. Argyres, Theory of the Faraday and Kerr effect in ferromagnets, PR 97 (1955)334, P.M. Oppeneer, Magneto-optical Kerr spectra in Handerbook of Magnetic Materials, Edited by Buschow (Vol.13), Physical Review B, 45(1992)10924.

32 Diluted Magnetic Semiconductors The charge of electrons in Semiconductor (Integrated circuits, devices); Spin of electrons in data storage (hard disc, tapes, magneto-optical disks) May we be able to use the capability of mass storage and processing of information at the same time ? If both the charge and spin of electrons can be used to further enhance the performance of devices.

33 Three types of semiconductors: (A) a magnetic semiconductor, (B) a diluted magnetic semiconductor, an alloy between nonmagnetic semiconductor and magnetic element; and (c) a nonmagnetic semi- conductor.

34 Lattice constant a vs Mn composition x in (Ga 1-x, Mn x )As films. a was determined by XRD at room temperature (Ohno et al., APL 69(1996)363. GaMnAs

35 Magnetic field dependence of magnetization M at 5K for a (Ga, Mn)As film with x Mn = The field was applied parallel to the sample surface (Ohno et al., APL 69(1996)363).

36 Room temperature longitudinal MOKE responses for ferromagnetic MnAs on ZnSe: (a) a single phase MnAs/ZnSe (b) a dual phase MnAs/ZnAs heterostructure (Berry et al., APL 77(2000)3812). GaAs(001)/200nmZnSe/170nmMnAs MnAs/ZnSe

37 ZnCoAl XRD patterns and VSM curves of the thin films deposited at 400 o C at oxygen pressure 5x10 -5 Pa (Yan et al., JAP 96(2004)508).

38 Co doped TiO 2 An XRD pattern of a Co doped TiO 2 film (x=0.08) showing (004) and (008) peaks of anatase without any impurity peaks. Atomic reslution TEM image. No segregation of impurity phase was observed. Matsumoto et al., Science 291(2001)854

39 Images taken at 3K for anatase thin films with different Co contents on a combinatorial chip. (a) x=0, (b) 0.02, (c) 0.03, (d) Magnetic domain were observed in all doped film. A series of scanning SQUID microscope images 200 µm x 200 µm

40 (a) an M-H curve of an x=0.07 film on SrTiO 3 taken at room temperature. (b) M-T curve in a field of 20 mT parallel to the surface. T c > 400K.

41 Source ? (1) RKKY interaction (H.Ohno Science 281(1998)951); (2) Forming resonant states (J.Inoue et al., PRL 85(2000) 4610; (3) Clusters of Co in Co-doped anatase TiO 2 thin film (J.K. Kim et al., PRL 90(2003)

42 In the absence of holes, the magnetic interaction among Mn has been shown to be antiferromagnetic in n-type (In,Mn)As and in fully carrier compensated (Ga.Mn)As. This results show that the ferromagnetic interaction is hole induced. RKKY Theory Origin of Ferromagnetism

43 Comparison between the experimental T c (open circles). The error bars for the calculated T c represent the erro involved in determining exchange and carrier concentration. (F.Matsukura et al., PRB 57(1998)R2037) GaMnAs

44 The RKKY exchange Hamiltonian between the Mn spins at sites i and J is expressed by H = -J i j s i s j, where J i j is given by Here r i j is the distance between I and j, and F(2k F r i j ) is the ordinary RKKY oscilation term, and l is the mean free path of carries. The exp(-r i j /l) represants the effect of a finite l following de Gennes. Form the ab ove equation, T c is given by, Where z r is the number of r-th nearest group-III sites.

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