GROWTH AND INVESTIGATION OF HALF-METALLIC Fe 3 O 4 THIN FILMS B. Vengalis, V. Lisauskas, A. Lisauskas, K.Šliužienė, V. Jasutis Semiconductor Physics Institute, Vilnius Lithuania M. A. Bari, J.J. Versluijis, J. M. D. Coey Physics Department, Trinity College, Dublin 2, Ireland MAGNETIC DEVICES BASED ON THIN FILM MULTILAYERS July 2002, Dublin, Ireland

 Magnetite as promising material for magnetoelectronics  Fe 3 O 4 thin films and related technological problems  Growth of Fe 3 O 4 thin films by magnetron sputtering  Characterization of crystalline structure  Electrical and magnetic properties  Conclusions Short outline of this report

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

Phase diagram of Fe-O FeO+Fe 3 O 4 Fe 2 O 3 +Fe 3 O 4 FeO Fe 3 O 4  -Fe + FeO Fe 2 O 3 Liquid oxide Oxygen (wt %) Fe 3 O 4  -Fe+Fe 3 O 4 Magnetite: phase diagram, technology problems High T C value compared to other HM oxides La 2/3 Sr 1/3 MnO 3, Sr 2 FeMoO 6, CrO 2 Simple structure, one element Low deposition temperature Advantages : Iron Fe (Cubic) Maghemite  - Fe 2 O 3 (Rhomohedr) Magnetite - Fe 3 O 4 (Cubic) Wuestite FeO (Rhombohedral) 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 Fe 3 O 4 in various oxygen ambient needs to be studied Stability of interfaces needs to be studied Problems:

TechnologyTargetP(O 2 ), Pa Ts,  C SubstrateFilm quality References PLD  -Fe 2 O 3 Fe 3 O 4 3x10 -1 <1x x Si MgO(100) SrTiO 3 (100)  -Al 2 O 3 SrTiO 3 MgO(100) P E (0.3%) E ( 8 % ) E JAP 83 (1998) 7049 PR(B)57(1998)7823 PR(B)64(2001) DC-MSFe2x MgO(100) MgAl 2 O 4 E E (4 % ) PR(B) 53(1996)9175 RF-MSFe5x , 400  -SiO 2 PJ.A.P.75(1994) 431 RF-MSFe 3 O MgOEPR(B)80(2002)823 DC-MSFe1.5x MgO(100)EThis work Preparation of Fe 3 O 4 thin films by various authors

Target: Fe disk, 35 mm diam (h=0.5 mm) Substrates: Cleaved MgO(100) (a MgO =0.42 nm  ½ a Fe3O4 ) Glass Temperature: T s =300, 400, 450  C Gas ambient: Ar:O 2 30:1, (p  5 Pa) Film thickness: (d=50  600 nm) Preparation of Fe 3 O 4 thin films in this work MgO Glass x Fe Deposition rate versus substrate to target distance at I disch = 95 mA. DC Magnetron sputtering.

Reflected High Energy Electron Diffraction (RHEED) Microstucture of the grown Fe 3 O 4 thin films Regions of deposition rate resulting growth of single phase, epitaxial (E) and policrystalline (P) Fe 3 O 4 at p(O 2 )  0.15 Pa as found from XRD, RHEED and resistivity measurements Fe 3 O 4 Fe 3 O 4 + Fe Fe 2 O 3 + Fe 3 O T,  C E P P Fe 3 O 4 / Glass Fe 3 O 4 / MgO

T=I / I s  (h )  - ln T / d I0I0 I IsIs d , 10 4 cm d, (  ) = Fe 3 O 4 MgO Fe 3 O 4 thin films on MgO and Glass. Optical absorption E, eV

 (T) =   exp ( -E a /kT )  (T) = A exp ( B /T )1/4 T >T V T <T V Variable range hopping (Motts low ) Activation R(T) behaviour Resistance versus temperature of Fe 3 O 4 thin films grown epitaxially on MgO(100) at 400  C

T s =350  C T s =450  C Resistance versus temperature of Fe 3 O 4 /MgO thin films Resistance anomaly at T Vv was only seen for Fe 3 O 4 /MgO films grown at 350 and 400  C Activation energy of R(T) behavior at T>T V for epitaxial Fe 3 O 4 films depends sensitively on crystalline quality DR, nm/min DR, d 1/T

Stability of Fe 3 O 4 thin film during heating (dT/dt=7deg/min) Fe 3 O 4 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(O 2 )=10 5 Pa and 0,16 Pa, respectively T,  C R,  ,15 P(O 2 ), Pa =10 5 Fe 3 O 4 /MgO d=0.35 

1. Magnetite is realy an intersting material! 2. It likes vacuum and doesn’t like oxygen 3. High quality Fe 3 O 4 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/O 2 ratio (sputtering rate at a fixed oxygen pressure) of key importance for growth of single phase films. Conclusions