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MAGNETIC DEVICES BASED ON THIN FILM MULTILAYERS 11-12 July 2002, Dublin, Ireland

MAGNETIC DEVICES BASED ON THIN FILM MULTILAYERS 11-12 July 2002, Dublin, Ireland.

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MAGNETIC DEVICES BASED ON THIN FILM MULTILAYERS 11-12 July 2002, Dublin, Ireland

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  1. MAGNETIC DEVICES BASED ON THIN FILM MULTILAYERS 11-12 July 2002, Dublin, Ireland GROWTH AND INVESTIGATION OF HALF-METALLIC Fe3O4 THIN FILMSB. Vengalis, V. Lisauskas, A. Lisauskas, K.Šliužienė, V. JasutisSemiconductor Physics Institute, Vilnius LithuaniaM. A. Bari, J.J. Versluijis, J. M. D. CoeyPhysics Department, Trinity College, Dublin 2, Ireland

  2. Short outline of this report • Magnetite as promising material for magnetoelectronics • Fe3O4thin films and related technological problems • Growth of Fe3O4thin films by magnetron sputtering • Characterization of crystalline structure • Electrical and magnetic properties • Conclusions

  3. 3d 4s A Fe3+ Fe3+ B Fe2+ A B a/2 O Magnetite: crystalline structure, attractive properties Ferrimagnetic ordering at T<TC 860 K(M = 4 B) Charge ordering at T<TV=120 K (Verwey transition) Crystalline structure Cubic inverse spinel structure Fd3m : O2-ions form frame of face centered cubic lattice, a = 0,8398 nm Electrical conductivity: ( 300 K) 10 mcmdue to hopping of spin-polarized electrons betweenmagnetically ordered Fe3+ ir Fe2+ states in B positions Ionic model:[Fe3+]A [Fe3+Fe2+]B O2- Fe3+occupies 1/8 tetrahedral positions(A) Fe3+and the same amount of Fe2+occupy 1/2 possible Bpositions

  4. Liquid oxide 1600 Fe3O4 1400 1200 Fe2O3 +Fe3O4 1000 FeO+Fe3O4 FeO 800 -Fe+FeO Fe3O4 600 Fe2O3 -Fe+Fe3O4 400 0.20 0.22 0.24 0.26 0.28 0.30 Oxygen (wt %) Magnetite: phase diagram, technology problems Advantages : Phase diagram of Fe-O • High TC value compared to other HM oxides La2/3Sr1/3MnO3 , Sr2FeMoO6, CrO2 • Simple structure, one element • Low deposition temperature IronFe (Cubic) Maghemite - Fe2O3(Rhomohedr) Magnetite - Fe3O4 (Cubic) WuestiteFeO (Rhombohedral) Problems: • Presence of isostructural phases in Ph. D. • Limited choice of lattice-matched substrate materials • There is a need in suitable isolating and conducting materials for heterostructures • Stability of Fe3O4 in various oxygen ambient needs to be studied • Stability of interfaces needs to be studied

  5. Preparation of Fe3O4 thin films by various authors

  6. Fe MgO Glass x Preparation of Fe3O4 thin films in this work DC Magnetron sputtering. Target:Fe disk, 35 mm diam (h=0.5 mm) Substrates: Cleaved MgO(100) (aMgO=0.42 nm ½ aFe3O4) Glass Temperature:Ts =300, 400, 450C Gas ambient: Ar:O2 30:1, (p  5 Pa) Film thickness:(d=50600 nm) Deposition rate versus substrate to target distance at Idisch= 95 mA.

  7. Fe2O3+ Fe3O4 Fe3O4 Fe3O4+ Fe T, C 450 E P P 400 350 Microstucture of the grown Fe3O4 thin films Regions of deposition rate resulting growth of single phase, epitaxial (E) and policrystalline (P) Fe3O4 at p(O2)0.15 Pa as found from XRD, RHEED and resistivity measurements Fe3O4 / MgO Fe3O4 / MgO Fe3O4 / Glass Reflected High Energy Electron Diffraction (RHEED)

  8. I0 d Is I Fe3O4 thin films on MgO and Glass. Optical absorption , 104 cm-1 Fe3O4 Fe3O4 d, () = 0.16 0.27 0.42 5.5 MgO 4.5 3.5 T=I / Is (h) - ln T / d 2.5 E, eV

  9. Resistance versus temperature of Fe3O4 thin films grown epitaxially on MgO(100) at 400C T >TV Activation R(T) behaviour (T) =  exp(-Ea /kT) T <TV Variable range hopping (Motts low) (T) = A exp(B/T)1/4

  10. 100 150 200 250 Ts=350C Resistance versus temperature of Fe3O4/MgO thin films 1 1 27 4 34 4 1/T 1/T DR, nm/min 27 DR, d 34 42 Ts=450C Resistance anomaly at TVv was only seen for Fe3O4/MgO films grown at 350 and 400 C Activation energy of R(T) behavior at T>TV for epitaxial Fe3O4 films depends sensitively on crystalline quality

  11. R,  Fe3O4/MgO d=0.35  10-4 106 105 104 0,15 P(O2), Pa =105 10-4 103 102 T,C Stability of Fe3O4 thin film during heating (dT/dt=7deg/min) • Fe3O4 thin film is stable during heating in vacuum up to 650 C. • Nonreversible resistance change appears at 200 and 400C during heating in oxygen at P(O2)=105 Pa and 0,16 Pa, respectively

  12. Conclusions 1. Magnetite is realy an intersting material! 2. It likes vacuum and doesn’t like oxygen 3. High quality Fe3O4 thin films exhibiting resistance anomaly in the vicinity of Verway transition point were grown heteroepitaxially at 350 and 400 C on lattice-matched MgO(100) substrates by a reactive DC magnetron using metallic Fe target. You can try also. 3. We point out the Fe/O2 ratio (sputtering rate at a fixed oxygen pressure) of key importance for growth of single phase films.

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