Lallart M. (ed.) Ferroelectrics

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1442

Multifunctional Characteristics of B-site

Substituted BiFeO

Films

Hiroshi Naganuma

Department of Applied Physics, Graduate school of Engineering, Tohoku University

Japan

1. Introduction

In recent times, BiFeO

has been considered as an important material for the development of

multifunctional devices because of its distinctive ferroelectric, magnetic, piezoelectric, and

optical properties. These include a high Currie temperature of ferroelectricity (T

∼1100 K),

(Venevtsev et al., 1960), high Néel temperature of antiferromagnetism (T

∼650 K), (Kiselev et

al., 1963) lead-free piezoelectricity, and large flexibility in the wavelength of visible light

region. These features make BiFeO

particularly applicable in the fields of ferroelectrics,

magnetics, piezoelectrics, and optics; in addition, the cross correlation of these properties can

be expected above room temperature (RT). [Fig. 1] BiFeO

has a perovskite-type crystal

structure that is rhombohedrally distorted in the [111] direction and crystallized in the space

group R3c [Fig. 2]. (Kubel et al., 1990) The ferroelectric performance of BiFeO

is comparable to

that of conventional ferroelectric materials such as Pr(Zr,Ti)O

(PZT) because BiFeO

exhibits

excellent spontaneous polarization at RT. Theoretically, spontaneous polarization corresponds

to crystal symmetry, wherein the rhombohedral and tetragonal BiFeO

structures are expected

to show spontaneous polarizations of ≈100 μC/cm

in the [111] direction and ≈150 μC/cm

the [001] direction, respectively. [Fig. 3(a)] (Ederer et al., 2005) In fact, these theoretically

predicted large spontaneous polarizations in BiFeO

are almost consistent with the

experimental results [Figs. 3(b) and 3(c)] (Li et al., 2004, Yun et al., 2004), stating that BiFeO

favorable for use in ferroelectric random access memory (FeRAM) applications. However, the

practical application of BiFeO

thin films has been limited by their large leakage current

density and large coercive field at RT, (Naganuma et al., 2007, Pabst et al., 2007) as a result,

BiFeO

thin films easily undergo electrical breakdown when a large leakage current passes

through them before the polarization is switched. Therefore, in order for BiFeO

films to find

practical future application, the leakage current and/or coercive field of these films must be

reduced. In term of magnetic properties, BiFeO

is antiferromagnetic with a G-type spin

configuration; (Kubel et al., 1990, Ederer et al., 2005) that is, nearest neighbor Fe moments are

aligned antiparallel to each other, and there is a sixfold degeneracy, resulting in an effective

“easy magnetization plane” for the orientation of the magnetic moments within the (111)

plane. It should be noted that the antiferromagnetic (111) plane is orthogonal to the

ferroelectric polarization direction [111] in the rhombohedral structure.

[Fig. 4(a)] The

orientation of the antiferromagnetic sublattice magnetization is coupled through ferroelastic

strain due to crystal symmetry, and it should always be perpendicular to the ferroelectric

polarization [111] direction. Therefore, a polarization switch to either 71° or 109° should

Ferroelectrics – Physical Effects

374

change the orientation of the antiferromagnetic plane. This change in orientation would,

however, not occur in the case of 180

° to

180

ferroelectric polarization switching. [Fig. 4(b)-

4(d)] In fact, experimentally, (Zhao et al., 2006) the ferroelectric domain and antiferromagnetic

domain in BiFeO

(100) epitaxial films are strongly coupled (magneto-electric (ME) coupling)

in the orthogonal configuration, and the orientation of the antiferromagnetic plane is switched

by a 109

switch in the ferroelectric domain. In effect, the magnetization configuration can be

controlled by the application of an electric field through ferroelectric domain switching; by

means of this mechanism, it is possible to realize voltage control of magnetic random access

memory (V-MRAM). The use of V-MRAM can drastically reduce electrical consumption when

compared with spin-MRAM which is operated by spin-polarized current. In terms of “how to

detect the change of magnetization induced by ferroelectric domain switching”, it can be seen

that owing to Dzyaloshinskii-Moriya (DM) interaction, (Dzyaloshinskii, 1957, Moriya, 1960)

the symmetry permits a canting of the antiferromagnetic sublattices, resulting in a local weak

spontaneous magnetization. This magnetization is macroscopically canceled by a spiral spin

structure caused by the rotation of the antiferromagnetic axis through the crystal with an

incommensurately long-wavelength period of 62 nm. (Sosnowska et al., 1982) This spiral spin

structure might be suppressed in the film form of BiFeO

and the resulting magnetic moment

is caused by a weak ferromagnetism of ∼0.1 μ

/Fe atom. However, this small magnetic

moment is not suitable for application in devices such as spintronics because of the difficulty

associated with the direction of a weak magnetic moment using a magnetic sensor, as well as

the spin-filter effect. Therefore, in order to detect the change of magnetic states driven by

ferroelectric domain switching (i) introduction of ferrimagnetic spin order into BiFeO

having

a rombohedral structure or (ii) detection of the ferromagnetization change through exchange

coupling with BiFeO

(Chu et al., 2008) are the predominant candidates. In addition, the BiFeO

films show distinctive optical properties, as previously mentioned. BiFeO

has the highest

flexibility among the oxide materials: a flexibility of 3.22 at a wavelength of 600 nm; this

flexibility is expected to cross-correlate with the other physical properties. (Shima et al., 2009)

The details of the optical properties of BiFeO

films have been relegated to a subsequent

Fig. 1. Schematic illustration of cross correlation between ferroelectrcity (piezoelectrics),

ferroelasticity, optical properties and antiferromagnetism for BiFeO

Multifunctional Characteristics of B-site Substituted BiFeO

Films

375

Fig. 2. Schematic drawing of the crystal structure of perovskite BiFeO

(space group: R3c).

Two crystals along [111] direction are shown in the figure.

Fig. 3. (a) First-principle calculation of spontaneous polarization for BiFeO

with tetragonal

(P4mm) and rhombohedral (R3c) symmetry. ε

indicates the epitaxial strain. Symbols

represent directly calculated values. (Ederer et al., 2005) (b) Experimental result of BiFeO

with tetragonal structure, and (c) with rhombohedral structure.

discussion. Based on these background considerations, the main focus of this chapter is the

structure, ferroelectricity, and magnetism of BiFeO

films. In addition, we discuss the effect of

the substitution of various 3d transition metals into the B-site of BiFeO

films on the films’

structural, ferroelectric, and magnetic properties. We also propose a new multiferroic material

having a ferrimagnetic spin order and large spontaneous polarization at RT. This chapter

includes three sections. Section 1 presents the fundamental characteristics of pure BiFeO

films.

Section 2 describes substitution of various 3d transition metals for Fe in BiFeO

films, in small

amount (∼5 at.%), in order to achieve ferrimagnetic spin ordering together with low leakage

current density. Finally, in Section 3, we demonstrate the substitution of a large amount of Co

(∼58 at.%).

Ferroelectrics – Physical Effects

376

Fig. 4. Schematic illustration of cross correlation between ferroelectric polarization and

antiferromagnetic plane. Antiferromagnetic plane was defined as the direction of magnetic

moment of weak ferromagnetism due to DM interaction.

2. Structural, electronic, and magnetic properties of BiFeO

films

In order to elucidate the fundamental properties of BiFeO

films, here, we begin by

systematically investigating the effect of annealing temperature on the structural, electrical,

and magnetic properties of BiFeO

polycrystalline film. The results described in this section

is based on our recent work as follows; Naganuma et al. TMRSJ, 2007, Naganuma et al. MT,

2007, Naganuma et al. IF, 2007, Naganuma et al. TUFFC, 2008, Naganuma et al. JJAP, 2008,

Naganuma et al. APEX, 2008, Naganuma et al. JCSJ, 2010, Naganuma et al. JAP, 2011.

Polycrystalline BiFeO

films were prepared by a chemical solution deposition (CSD)

method. The preparation processes are shown in Fig. 5 and can be summarized as follows:

an enhanced-metal-organic-decomposition (E-MOD) solution (with stoichiometric

composition Bi:Fe=1:1, 0.2 mol/l) was used as the precursor solution. The precursor solution

was spin-coated onto Pt (150 nm)/Ti (5 nm)/SiO

/Si (100) substrates at a rotation speed

between 4000 and 6000 rpm for 50 s. Pt and Ti were deposited by r.f. magnetron sputtering

using Ar gas at RT. The spin-coated films were dried at 150°C for 1 min and calcined at

350°C for 5 min. The spin coating and calcination processes were repeated 4-5 times, after

which the films were sintered in air at 400 - 800°C for 10 min by rapid thermal annealing

(RTA; ULVAC mila-5000). The films had a thickness of approximately 200 nm. The surface

morphology of the films was observed by atomic force microscopy (AFM; SII SPI3800N) and

scanning electron microscopy (SEM; JEOL JSM-6380). The crystal structure and orientation

were confirmed by X-ray diffraction (XRD; PANalytical X’Pert MRD) with Cu-Kα radiation

and transmission electron microscopy (TEM; JEOL JEM-2100F, JEOL JEM-3000F, LEO-922)

working at 200 and 300 kV. The leakage current density was measured using a pico-ampere

Multifunctional Characteristics of B-site Substituted BiFeO

Films

377

meter (HP 4140B). The ferroelectric hysteresis (P-E) loop of the films was measured using a

ferroelectric tester (aixACCT TF-2000, TOYO FCE-1A) with a single triangular pulse.

Positive-Up-Negative-Down (PUND) measurement was also used for evaluating electrical

properties. The magnetic properties were measured with a vibrating sample magnetometer

(VSM; Tamakawa) at RT, and a superconducting quantum interference device (SQUID;

Quantum design MPMS) magnetometer was used for the in-plane direction.

Fig. 5. Schematic illustration of preparation process using CSD.

Figure 6 shows the XRD profiles of BiFeO

films deposited on Pt/Ti/SiO

/Si(100) substrates

annealed at various temperatures. For each annealing temperature, a strong diffraction peak

at 2θ = 39.8° was observed due to the Pt (111) plane. At an annealing temperature of 400°C, a

weak diffraction peak was observed in the region of 2θ = 46°; however, it was not clear

whether this diffraction peak originated from BiFeO

(024) at 2θ = 45.74° or Pt (200) at 2θ =

46.24°. Therefore, in order to clarify the origin of the diffraction peak in the region of 2θ =

46°, XRD analyses were undertaken for the Pt/Ti/SiO

/Si(100) substrate only. On the basis

of this experiment, the diffraction peak in the region of 2θ = 46° was identified as Pt (200) for

the sample annealed at 400°C. Therefore, the structure of the film annealed at 400°C was

amorphous or nanocrystalline. The formation of the polycrystalline BiFeO

films for

annealing temperature above 450°C was indicated by the appearance of numerous

diffraction peaks attributed to the BiFeO

structure above the stated temperature. The strong

[111] orientation of the bottom Pt layer did not affect BiFeO

crystal growth. At annealing

temperature above 700°C, secondary phases such as α-Fe

and BiPt were formed. The

observation of these phases indicates that the excess Fe formed α-Fe

due to diffusion of

Bi into the Pt electrode at high annealing temperatures. The indication is that the single

phase of randomly oriented polycrystalline BiFeO

film was formed at annealing

temperatures between 450 and 650°C.

Ferroelectrics – Physical Effects

378

Fig. 6. XRD profiles of BiFeO

films deposited on Pt/Ti/SiO

/Si(100) substrates annealed at

various temperatures

Fig. 7. Cross-sectional TEM images of polycrystalline BiFeO

films annealed at 550°C.

Lallart M. (ed.) Ferroelectrics - Physical Effects

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