Letter

Fabrication of high valence Sr(Fe0.65Co0.35)O3 film by a pulsed laserablation technique.

Hidekazu Tanakaa,*, Nobumichi Matsuokab, Sachio Okib, Susumu Gohdab, Tomoji Kawaia

aInstitute of Scientific and Industrial Research, Osaka University., 8-1 Mihogaoka, Ibaraki, Osaka 567, JapanbDepartment of Industrial Engineering Faculty of Science and Technology,

Kinki University, 3-4-1 Kowakae, Higashi-osaka, Osaka 577, Japan

Received 5 January 1998; accepted 20 March 1998

Abstract

The formation of Sr(Fe0.65Co0.35)O3 thin films was performed by pulsed laser deposition for the first time. The growth conditions wereexplained by the substrate temperature and oxygen pressure diagram by using thermodynamical equilibrium. An oxygen deficient phasewas formed at the substrate temperature of 630°C. At low substrate temperature of 500°C, Sr(Fe0.65Co0.35)O3 perovskite structure is formed.This result can be interpreted as the lower substrate temperature stabilizing the tetravalent ion thermodynamically. This Sr(Fe0.65Co0.35)O3

film shows semiconductive-like behavior in resistivity with an activation energy of 0.15 eV and ferromagnetic property with Curietemperature of 370 K. 1998 Elsevier Science S.A. All rights reserved

Keywords:Sr(Fe,Co)O3 film; Perovskite oxide; Laser ablation; Film formation diagram; Oxidation equilibrium

1. Introduction

Perovskite type oxide, A(B,B′)O3 exhibits various inter-esting magnetic and electrical properties. Combinationamong transition metal ions has the potential to create inter-esting magnetic properties by exchange interactions viatransition metal ion–oxygen–transition metal ion. Kawa-saki et al. reported preparation of SrFe1−xCoxO3 (SFCO)bulk materials by using a high-pressure technique andtheir ferromagnetism withTC above 300 K for 0.4≤x ≤ 0.9 [1]. SrFeO3 is an antiferromagnet with Ne´el tem-perature (TN) of 134 K [2]. SrCoO3 is a ferromagnet withCurie temperature (TC) of 280 K [2]. It is interesting thatcombination between Fe ion and Co ion brings ferromag-netism with higher Curie temperature. Both SrFeO3 andSrCoO3 contain transition metal ions with a high valencestate (Fe4+ and Co4+) so that using a high-pressure techni-que is needed to control stoichiometry. Therefore there is

few investigations of perovskite oxide with tetravalent ionscompared with trivalent ones. In film formation, controllingoxidation states can be achieved by arranging the atmo-sphere and substrate temperature in the chamber. Pulsedlaser deposition (PLD) has provided high quality oxidehigh TC superconductor films, magnetic oxide films andferroelectric oxide films [3–8]. The advantage of this tech-nique is the ability to control the film formation conditions,such as atmosphere and substrate temperature easily. Theaim in this study is to form an oxygen deficiency minimizedSFCO thin film and to clarify the correlation between tem-perature, oxygen pressure and phases in the SFCO film for-mation using the PLD technique.

2. Experimental

The PLD technique used in the present study has beendescribed previously [8,9]. An ArF excimer laser (wave-length: 193 nm) was used for the ablation, and the targetswere sintered pellets which were placed at the center of avacuum chamber. These Sr(Fe0.65Co0.35)O3 targets were pre-

Thin Solid Films 326 (1998) 51–55

0040-6090/98/$19.00 1998 Elsevier Science S.A. All rights reservedPII S0040-6090(98)00600-2

* Corresponding author. Tel.: +81 6 8798446; fax: +81 6 8752440;e-mail: h.-tanaka@sanken.osaka-u.ac.jp

pared by mixing SrCO3 (99.9%), Fe2O3 (99.9%) and CoO(99.9%) powders in desirable compositions, followed bypressing at 2.0 ton/cm2 and sintering at 1000°C.

An O2 gas mixed with 8% O3 was introduced into thechamber to oxidize the transition metal effectively. Thepulsed laser beam was focused on the target with an energydensity of about 1 J/cm2 at a frequency of 10 Hz. Thin filmformation was carried out under the following conditions.Substrate: SrTiO3(100) single crystal; substrate temperature(Ts): from 500 to 630°C; deposition rate: 10–15 A˚ /min;oxygen pressure(PO2

) including 8% O3: 1.0 × 10−3–1.0 × 10−2 mbar; film thickness: 1000 A˚ . After the filmformation, samples were slowly cooled to room temperature(about 200°C/10 min) Film thickness was measured by aquartz crystal oscillator during the film formation and itwas checked by a multiple beam interferometry methodafter the formation. The crystal structures of the filmswere characterized by X-ray diffraction (XRD) measure-ment. The properties of them were evaluated by magnetiza-tion–temperature (M–T) measurement by the SQUIDmagnetometer (Quantum Design, MPMS2), and resistiv-ity–temperature (R–T) measurement by use of a fourprobe method.

3. Results and discussion.

Fig. 1 shows typical two types of X-ray diffraction patternfor Sr(Fe,Co)Oy. In Fig. 1a, Sr(Fe,Co)Oy film was formed at

a substrate temperature of 650°C and an oxygen pressure of3.0 × 10−3 mbar and crystallizes in preferential orientationwith lattice parameter,a630°C, of 7.954 Aperpendicular toSrTiO3 (100) substrate. In Fig. 1b Sr(Fe,Co)Oy film wasformed at a substrate temperature of 500°C at an oxygenpressure of 3.0× 10−3 mbar. The (001) and (002) peaksindicate preferential orientation of the primitive perovskitestructure along the direction perpendicular to SrTiO3 (100)substrate with lattice constants,a500°C, of 3.840 A. Thisvalue is close to one reported by Kawasaki [1] about oxygendeficiency minimized Sr(Fe0.6Co0.4)O3. The lattice constantof SFCO films formed at 630°C is twice as long as thatformed at 500°C, a630°C ≈ 2a500°C. The former has twice aslong a period as the latter. Fig. 2 shows annealing tempera-ture dependence of X-ray diffraction patterns in an O2 (1atm) atmosphere for oxygen deficient phase formed at630°C. With increasing annealing temperature from 400to 600°C, the X-ray intensities coming from oxygen defi-cient phase become weaker and disappeared at 800°C.Furthermore, the lattice constant become shorter drasticallyfrom 3.988 to 3.840A˚ as shown in the inset in Fig. 2c.Therefore, twice as long a period structure in the filmsformed at the higher temperature originates from the oxygendeficient structure.

Fig. 1. X-ray diffraction patterns of strontium iron cobalt complex oxidefilms formed under typical conditions. (a) Oxygen deficient phase, Sr(Fe,-Co)O3−d formed at 650°C, 3.0× 10−3 mbar. (b) Perovskite Sr(Fe,Co)O3

formed at 500°C, 3.0× 10−3 mbar.

Fig. 2. Annealing temperature dependence of X-ray pattern of Sr(Fe,-Co)O3−d films in an O2 atmosphere. (W) Primitive perovskite unit, (O)twice perovskite unit. (a) As-deposition at a substrate temperature of630°C, (b) 400°C for 2 h, (c) 600°C for 2 h, (d) 800°C for 2 h. Theinset shows lattice constants for various annealing temperature. (W) Cal-culated from perovskite (100) peak, (O) calculated from oxygen deficientphase, Sr(Fe,Co)O3−d, (100) peak.

52 H. Tanaka et al. / Thin Solid Films 326 (1998) 51–55

In the case of parent substance, SrFeOx, it is reportedthat there are four types of crystal structures for the oxy-gen content,x, i.e. cubic perovskite phase for 2.9, x , 3.0,tetragonal perovskite phase forx~2.86 with lattice para-meters, at ≈ 2 × 2l/2ac, ct ≈ 2ac, orthorhombic perovskitephase for x~2.75 with ao ≈ 2 × 2l/2ac, bo ≈ 2ac, co ≈ 21/2ac and orthorhombic brownmillerite structure forx = 2.5(c, t, o indicating the cubic, tetragonal and orthorhombicphases respectively) [10]. Compared with SrFeOx, the filmformed at a substrate temperature of 630°C indicates anoxygen deficient perovskite structure (Sr(Fe,Co)O3−d) andthe films formed at a substrate temperature of 500°C havealmost a primitive perovskite lattice (Sr(Fe,Co)O3).

Fig. 3 shows the X-ray diffraction patterns of La(Cr0.5-

Fe0.5)O3 and La(Mn0.5Co0.5)O3 complex perovskite oxidehaving trivalent transition metal ions at a substrate tem-perature of 630°C and an oxygen pressure of 3.0× 10−3

mbar. All patterns indicatec-axis orientation perovskitehaving a primitive perovskite unit cell with lattice constantsof 3.920 and 3.867A˚ , respectively. No oxygen deficientphase appeared unlike Sr(Fe,Co)Oy. Fig. 4 summarizesthe Sr(Fe,Co)O3 (see Fig. 4a and La(Mn,Co)O3 (see Fig.4b films formation diagram, deposited under variousPO2

(including 8%O3) andTs on the SrTiO3 (100) substrate. Theformed Sr(Fe,Co)O3 perovskite structure is located in alower temperature region than the La(Mn,Co)O3 perovskiteone. The formation diagram shows that critical lines toobtain crystallized films strongly depend on substrate tem-perature.

Thermodynamically, the interaction of the transitionmetal ion and oxygen is given by:

Ma+ → Mb+ (b . a) (1)

(MOa=2 +O2 → MOb=2) (2)

The condition which stabilizes metal oxides with high oxi-dation number is given by:

ln(PO2) ≥ DG0

T=(RT) (3)

where,PO2, R, T andDG0

T, correspond to oxygen pressure,the gas constant, substrate temperature and the Gibbs’ for-mation free energy, respectively. In the thin film formation,Hammond et al. reported that the region near the upperlimit temperature in Eq. (3) is a suitable condition to obtainthe crystallized films [11].

Fig. 5 shows the phase diagram for Mn, Fe, Co oxidationevaluated from thermodynamical data [12]. The line (A)indicates that the Mn3+ ion is stabilized at the higher oxygenpressure and lower substrate temperature region than thisline (4Mn3O4 + O2 = 6Mn2O3). The line (B) shows theregion where Co3+ and Co2+ ions are coexistent (6CoO + O2 = 2Co3O4) [13]. The lines (A)′ and (B)′ are the sta-bility lines of Mn3+ and (Co2+, Co3+) for ozone (O3) in thepresence of oxygen gas including 8% ozone (O3), respec-tively. The stability line shifts to the higher temperature byusing ozone gas [13]. The formation condition of La(Mn,-Co)O3 is located in the region near the line (A)′ as shown inFig. 5. It is considered that the formation condition shiftsfrom line (A) to line (A)′ by oxygen atmosphere including8% O3. Therefore, the film formation condition of La(Mn,-Co)O3 is dominated by the stability of Mn3+ ions. In the caseof Sr(Fe,Co)O3, there is no thermodynamical data for Co4+

Fig. 3. X-ray diffraction patterns of (a) La(Cr0.5Fe0.5)O3 and (b) La(Mn0.5-

Co0.5)O3 formed at a substrate temperature of 630°C with an oxygen pres-sure of 3× 10−3 mbar.

Fig. 4. Oxygen (including 8% O3) pressure (PO2) versus substrate tempera-ture (Ts) diagram for perovskite oxide films. (a) Diagram for Sr(Fe0.65-

Co0.35)O3: (W) perovskite phase is crystallized, (K) oxygen deficientperovskite phase is crystallized. (b) Diagram for La(Co0.5Mn0.5)O3: (W)perovskite phase is crystallized. (×) Amorphous phase is obtained.

53H. Tanaka et al. / Thin Solid Films 326 (1998) 51–55

ion because of its large instability. The tetravalent ion is lessstable than the trivalent one usually, i.e.:

DG0T(M4+ ) . DG0

T(M3+ ) (4)

From Eq. (3) and (4), the negative small Gibbs’ formationfree energy of the tetravalent ion system makes their sui-table substrate temperature lower than that of the trivalention.

Furthermore, transition metal ions become less stablewith increasing atomic number,n, because of the largerpositive nuclear charge, i.e.:

DG0T(M(n)a+ ) . DG0

T(M(n′)a+ ), n . n′ (5)

The stability of Co ion is important because Co ion is lessstable than Fe ion as shown in Fig. 5. Furthermore, thestabilized line of Co4+ ion is expected to be located inthe lower temperature region than line (A)′ in the presenceof oxygen including 8% O3 and from Eqs. (4) and (5).

Namely, lower substrate temperature plays an importantrole in stabilizing the high valence state in Sr(Fe,Co)O3

so that oxygen deficiency is suppressed drastically (espe-cially, Co3+ → Co4+). From the film formation regions forSr(Fe,Co)O3 and Sr(Fe,Co)O3−d, the stabilized line forCo4+ is located around the line (D) for the oxygen gasincluding 8% ozone. Oxygen deficiency of SFCO is mini-mized under this condition.

Finally, the electrical and magnetic property measure-ments were carried out for oxygen deficiency minimizedSr(Fe,Co)O3 films formed at 500°C. Fig. 6 shows the tem-perature dependence of resistivity. It shows high resistivity( ≈ 10−1 Q cm at room temperature) and their temperaturedependence shows semiconductive-like behavior. The insetin Fig. 6 is the Arrhenius plot with an activation energy of0.15 eV. Fig. 7 shows temperature dependence of magneti-zation. It shows ferromagnetic behavior with a Curie tem-perature of 370 K. Nevertheless, their magnetization is notsaturated at a magnetic field of 1000 Oe because their mag-netization value is much smaller than that expected from themagnetic moments of Co4+ (S = 1/2: low spin) and Fe4+

(S = 2: high spin). It is considered that there are magneticdomains in this film to cancel their macroscopic magnetiza-tion.

4. Conclusion

Sr(Fe0.65Co0.35)O3 thin films were prepared by controllingoxygen pressure and substrate temperature using a pulsedlaser ablation technique. Lower substrate temperature(Ts = 500°C) is very effective in stabilizing tetravalentCo4+ and Fe4+ ions to form perovskite Sr(Fe0.65Co0.35)O3

structure without oxygen deficient phase than that used toprepare perovskite structure film with trivalent ions. Theformed Sr(Fe0.65Co0.35)O3 films show a semiconductive-like transport property with activation energy of 0.1 5 eVand ferromagnetic property with Curie temperature of 370K, respectively.

Fig. 5. Calculated equilibrium lines between Mn, Fe, Co ions havingvarious oxidation numbers via oxygen (O2) gas (open mark) and via oxy-gen gas including 8% ozone (O3) (closed mark). The shaded areas indicateexperimental film formation regions for La(Mn0.5Co0.5)O3, Sr(Fe0.65Co0.35)O3 and Sr(Fe0.65Co0.35)O3−d.

Fig. 6. Temperature dependence of resistivity for Sr(Fe,Co)O3.

Fig. 7. Temperature dependence of magnetization for Sr(Fe,Co)O3 filmmeasured in Field coolingH = 1000 (Oe).

54 H. Tanaka et al. / Thin Solid Films 326 (1998) 51–55

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