Crystal Structure and Electric/Magnetic Properties of Bi-related Perovskite Oxide thin Films

Grown by Pulsed Laser Deposition Method

○Takaaki Inaba1, Yuta Watabe1, Takahiro Oikawa1, Keisuke Oshima1, Chun Wang1, Huaping Song1,

Tomoko Nagata1, Takuya Hashimoto2, Kouichi Takase1, Hiroshi Yamamoto1, Nobuyuki Iwata1,

Abstract: BiFe1-xMnxO3 thin films are grown by pulsed laser deposition (PLD) method on SrTiO3(STO)(100) and (110)

substrates. The observation of the the surface and the result of the reciprocal space mappings (RSMs) suggest the different

crystal structure between the surface and the closer part to the interface. The surface of BFMO on STO(100) exhibits a

sawtooth-like structure with the angle of 60 and 120 degrees between grains, suggesting the hexagonal surface structure. The

results of RSMs on the other hand, indicate the orthorombic structure having a tilt of 0.2 degrees from the substrate crystal plane.

The results of the surface morphology, RSMs, and the current-voltage curve suggest that the leakage current of BFMO films is

caused by Poole-Frenkel model conduction.

1．Introduction

The aim of our study is to synthesize the novel multi-functional materials which show ferromagnetic (FM) and ferroelectric (FE)

properties with a giant magnetoelectric effect at room temperature. It is well known that the two-dimensional (2D) electron gas at

the LaAlO3(LAO)/SrTiO3(STO)(100) interface results in superconductivity and ferromagnetic ordering, even though the both

materials are nonmagnetic insulator. In the [CaFeO3/BiFe1-xMnxO3(BFMO)] superlattices, we expect the charge transfer through

the interface, like LAO/STO interface, with an electric field applied to realize and control the 3d 5-3d 4 state, which shows

ferromagnetic superexchange interaction around the interface according to the Kanamori-Goodenough rules. The atoms of Bi and,

Fe and Mn in the superlattices are to induce the FE property and FM interaction between 3d 5 and 3d 4, respectively. In order to

realize the superlattices with quite flat interface at an atomic level, growth condition have to be precisely controlled. In this study,

we report the fabrication process, crystal structures, and electric/magnetic properties of the BFMO films. All films were deposited

by pulsed laser deposition (PLD) method.

2．Experimental

For films growth, STO(100) and STO(110) substrate were used. The STO(100) and STO(110) substrates was ultrasonically

cleaned in acetone and ethanol. The cleaned substrates was soaked in pure water for 30min. The substrates surface was etched by

a Buffered HF (BHF) (Daikin Industries, Ltd, pH=5.0) for 45 sec, immediately rinsed by pure water. The etched STO(100)

substrate was annealed at 920 ºC for 6 h in air. The etched STO(110) substrate was annealed at 1100 ºC for 2 h in air. All films were

grown by the PLD method using excimer laser of KrF 248 nm. Typical ablation conditions were as follows; the substrate

temperature, energy density, repetition rate, oxygen partical pressure, and the distance between substrate and target were 670~700 ºC,

2.4~2.7 J/cm2, 2~4 Hz, 20 Pa, 55 mm, respectively. All films were post-annealed at 0.1 MPa oxygen atmosphere with the rate of

10 ºC/min down to room temperature. The targets were prepared by traditional solid solution method and Pechini method.

3．Results and Discussion

3.1 BFMO on STO(100)

Fig. 1 shows the surface morphology of the (a) annealed STO(100) substrate and (b) BFMO thin film grown on STO(100). The

step-terraces structure was observed on the surface treated STO(100). On the other hand, the surface of BFMO film exhibited a

sawtooth-like structure with the angle of 60 and 120 degrees between grains, suggesting the hexagonal structure of the BFMO film.

Fig. 2 shows the reciprocal space mappings (RSMs) around (a) STO(003), (b) STO(103), and (c) STO(113). Nothing changed in

the position and the intensity of the peaks in all RSMs when rotating the substrate along the ϕ direction. From the result of (a), two

1 : College of Science and Technology, Nihon University, Japan,

2 : College of Humanities and Sciences, Nihon University, Japan

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split peaks were observed, indicating that the BFMO film

grew with the tilting angle of 0.20º from the substrate

crystal plane. Two split peaks appeared in (b) and (c) as

well. The crystal structure is expected to be orthorombic

from the results of RSMs.

The observation of the surface and the result of the

RSMs suggest the different crystal structure between the

surface and the closer part to the interface. This

difference can be explained by the degree of the stress

from the substrate. We expect that the structure of

BFMO thin film was orthorombic at an initial stage of the

crystal growth due to the stress from substrate. On the

other hand, with increasing the film thickness, the grains

become a hexagonal due to the relaxing.

The electric property was also investigated. In

ferroelectric perovskite oxides, leakage mechanisms are

commonly classified into three mechanisms; (i)

space-charge-limited conduction (SCLC), (ii) Schottky

emission, and (iii) Poole-Frenkel emission. The

mechanisms can be estimated from current-voltage

curves.

Fig. 3 shows the leakage current characteristics of BFMO on STO(100). If SCLC were the dominant leakage mechanism, a

straight having a slope 2 fit to the data in log J vs log E. The slope of our film, 4.3 and 3.3 indicate the presence of other leakage

mechanisms. The Schottky and Poole-Frenkel emission can be investigated by a straight fit to the data in log J vs E1/2 and log vs

E1/2, respectively. Both models can explain our data as shown in Fig. 3 (ii) and (iii). Schottky emission is interface-limited

conduction. This conduction mechanism arises from a difference in Fermi levels between a metal (electrode) and an insulator or

semiconductor (films). Therefore, we expect that our BFMO film is Poole-Frenkel conduction because the film has a lot of grain

boundaries from the results of the surface and RSMs.

4．Summary

The BFMO thin film were grown on STO(100) and STO(110) substrates using the PLD method. From the surface and RSMs, the

structure of BFMO grown on STO(100) was orthorombic at an initial stage of the crystal growth due to the stress from substrate. On

the other hand, with increasing the film thickness, the grains become a hexagonal due to the relaxing. From the leakage current

characteristics, we expect that our BFMO film is Poole-Frenkel conduction because The film has a lot of grain boundaries from the

results of the surface and RSMs.

0.0 4.7 nm 17.0 nm 0.0

Fig. 1 The surface morphology

60º 120º

(a) STO(100) substrate

(2µm×2µm)

(b) BFMO thin films

(2µm×2µm)

-0.1 0.0 0.17.30

7.35

7.40

7.45

7.50

7.55

7.60

7.65

7.70

7.75

7.80

q[0

01

] (n

m-1

)

q[100]

(nm-1)

7.00

11.2

18.1

29.0

46.6

74.8

120

193

310

498

800

2.4 2.5 2.6 2.77.30

7.35

7.40

7.45

7.50

7.55

7.60

7.65

7.70

7.75

7.80

7.00

11.3

18.3

29.5

47.7

77.1

125

201

326

526

850

q[100]

(nm-1)

3.5 3.6 3.77.30

7.35

7.40

7.45

7.50

7.55

7.60

7.65

7.70

7.75

7.80

q[110]

(nm-1)

7.00

10.4

15.3

22.6

33.5

49.5

73.2

108

160

237

350

q[0

01](nm

-1)

q[100]

(nm-1) q

[100](nm

-1) q

[110](nm

-1)

(b) (a) (c)

Fig. 2 The RSMs around (a)STO(003), (b)STO(103), (c)STO(103)

0 5 10 15 20 25 30 35 4010

-5

10-4

10-3

10-2

10-1

100

101

102

103

E1/2

(kV1/2

/m1/2

)

logJ(

A/m

2)

108

109

0.01

0.1

1

10

100

1000

4.3

3.3

logJ(

A/m

2)

logE(V/m)

(i)

10 15 20 25 30 35 4010

-7

10-6

10-5

10-4

10-3

log

(1/

m)

E1/2

(kV1/2

/m1/2

)

(ii) (iii)

For SCLC For Poole-Frenkel For Schottky

Fig. 3 Leakage current characteristics (i) log J vs log E (ii) log J vs E

1/2 (iii) log vs E

1/2

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