CẤU TRÚC VÀ TÍNH CHẤT TỪ CỦA VẬT LIỆU GỐM ĐA TINH THỂ Bi0.84La0.16Fe0.98Ti0.02O3

The presence of multiphase crystal structure was showed a novel effect such as the double hysteresis loop, unusual piezoelectric properties, and the exchange bias [r]

CRYSTAL STRUCTURES AND MAGNETIC PROPERTIES OF

University of Sciences - TNU

ABSTRACT

Sample Bi0.84La0.16Fe0.98Ti0.02O3 was prepared by the solid-state reaction method The analysis of

XRD pattern indicated the coexistence of multi-phase crystal structure of the R3c rhombohedral and the Imma orthorhombic The Rietveld refinement method was used to calculate the lattice

parameter, bonding angle Fe–O–Fe, and bonding length Fe–O Sample showed a typical of weak ferromagnetic The presence of multi-phase crystal structure affected strongly the magnetic properties of sample Especially, the vertical hysteresis shift (exchange bias) was observed in sample We believed that the origin of this effect come from the magnetic interaction at phase boundary of two crystal structures

Keywords: BiFeO 3 , Crystal structures, Magnetic properties; multiferroics materials; multiphase crystal

INTRODUCTION*

Recently, multiferroic BiFeO3 (BFO) has

attracted much attention because of the

coexistence of both ferroelectric and magnetic

properties at room temperature, making BFO

become a potential candidate for many

applications in magnetoelectric devices Bulk

BFO materials have a rhombohedrallly

distorted perovskite structure (space group

R3c) with a high ferroelectric Currie

temperature (TC 1100 K) and

antiferromagnetic (AFM) Néel temperature

(TN  643 K) [1] The magnetic order in BFO

is G-type AFM with a cycloid-type spatial

spin modulation The cycloidal spin spirals

with periodicity of ~62 nm risen from the

combination of exchange and spin-orbit

interactions produces spin canting away from

perfect AFM ordering [2] In addition, due to

the lack of inversion symmetry in the R3c

structure, BFO shows weak ferromagnetic

(wFM) order associated with

Dzyaloshinky-Moriya (DM) interactions [3] A disadvantage

of BFO is the existence of a high leakage

current due to impurity phases (such as

Bi2Fe4O9, Bi25FeO40) and oxygen vacancies

generated during the synthesis process The

improvement of this disadvantage can be

*

Tel: 0983 009975; Email: dangnv@tnus.edu.vn

based on the partial substitution of Bi by a rare-earth or alkaline-earth element

For the case of La-doped BFO, the structural change and distortion depend on La concentration that could significantly suppress impurity phases, reduce oxygen vacancies This breaks spiral spin structure, and lower leakage current Additionally, FM order would emerge in the AFM phase It has also been found that the crystal structure of La-doped BFO is strongly dependent on synthesis and processing conditions Using sol-gel method to prepare Bi1-yLayFeO3, one found cubic and orthorhombic structures in

the samples with y = 0.2 – 0.4 and x = 0.5,

respectively [4] The tetragonal structure was reported on Bi0.8La0.2FeO3 prepared by solid state reaction and sol-gel methods [5,6] Particularly, a phase separation was found in

Bi1-yLayFeO3 as y = 0.16 [7], resulting in

unusual magnetic and piezoelectric properties By substituting Fe by a transition metal, such as Mn or Ti, one can improve remarkably the magnetic property of BFO This is understood as an increase of supper-exchange interactions between Fe ions Doping Ti into the Fe site could break the cycloidal spin order and reduce oxygen vacancies, because Ti is formed in an oxidation state Ti4+ higher than Fe3+

However, no detailed study on the influence

of the crystal structure on magnetic properties

of (La, Ti) co-doped BFO materials,

particularly around the

rhombohedral-orthorhombic phase transfor-mation Dealing

with this issue, we prepared polycrystalline

Bi0.84La0.16Fe0.98Ti0.02O3 samp-les, and studied

their crystal structures and magnetic

properties by using an X-ray diffractometer,

and a magnetometer The results obtained

from above measurements indicated the

presence of phase separation between

rhombohedral-orthorhombic crystal structures

and a weak ferromagnetic due to the

structural transformation and doping effect

EXPERIMENTAL DETAILS

Polycrystalline sample of Bi0.84La0.16

-Fe0.98Ti0.02O3 (BLFTO) was prepared by

solid-state reaction, using high-purity oxides

of Bi2O3, La2O3, Fe2O3 and TiO2 as

precursors These powders in stoichiometric

masses were thoroughly mixed by using an

agate mortar and pestle, and then pressed into

disc-shaped pellets After several times of

calcining, the pellet was finally annealed at

900 oC for 20 h The crystal structure was

studied by using an X-ray diffractometer

(Miniflex Rigaku) equipped with a Cu-K

radiation ( = 1.5405Å) Magnetization

measurements were performed on a vibrating

sample magneto-meter (VSM) All the

investigations were carried out at room

temperature

RESULTS AND DISCUSSION

The Rietveld refinement of XRD data was

performed using GSAS-II program A

multi-phase models of R3c rhombohedral and

another crystallographic symmetry such as

Pbam orthorhombic, Imma orthorhombic, or

I4/mcm tetragonal were carried out Base on

the weighted profile residual (Rwp) and

good-ness of fit (G.O.F), the best fit was obtained

with the R3c + Imma models as shown in

Figure 1 The structural parameters obtained

with the help of the refinement are listed in

Table 1 The R3c phase has lattice parameters

of a = b = 5.573 Å, c = 13.812 Å, which is in agreement with the JCPDS card No 86-1518

Fig 1 Rietveld refined XRD pattern using two

phases model of R3c and Imma

Table.1 Refined structural parameter

(52%)

Imma

(48%)

Cell volume (Å3) 371.51 248.47

<d Fe-O >(Å) 2.053 2.020

<d Bi-O >(Å) 2.445 2.441

1.06

G.O.F The Imma phase has lattice parameters of a =

5.633 Å, b = 7.821 Å, and c = 5.640 Å The value of Fe – O bond length in octahedron is approximately equal to 2 Å And, the Fe – O – Fe differs from 1800

of the ideal perovskite structure, which is implication the distorted R3c symmetry The partially distorted crystal structure may affect the magnetic properties due to the suppression of cycloidal spin structure To further confirmed Rietveld results, we plotted the simulated patterns of

R3c and Imma phase with experimental XRD

pattern, as seen in Fig 2 It is clear that two

phases model of R3c and Imma phases were

fitted well with the experimental pattern Our results are consisted with previous reported

on Bi1-yLayFeO3 that the Imma occurred at y =

0.25 [8] The presence of small among of Ti

(~2 wt%) in Bi0.84La0.16FeO3 can stabilize

Imma phase The collinear spin structure of

Imma orthorhombic have been studied by

neutron diffraction on Bi0.8La0.2Fe0.8Mn0.2O3

compound [9] The presence of multiphase

crystal structure was showed a novel effect

such as the double hysteresis loop, unusual

piezoelectric properties, and the exchange

bias effect Therefore, the coexistence of R3c

and Imma phases with different spin structure,

and magnetic anisotropy may lead interesting

magnetic properties

simulated

Si Si

measured

2  (degree)

Imma

R3c

measured

simulated

Si

Fig 2 Experimental XRD pattern and the

simulated XRD pattern of R3c and Imma phases

Inset shows the representation of R3c

rhombohedral and Imma orthorhombic structures

Moreover, Figure 3shows the hysteresis loop

of BLFTO sample measured at 300 K with

two cycles hysteresis loop The hysteresis

loops are showed unsaturated magnetization

at 10 kOe with small remanent magnetization

(M r) confirmed wFM behavior of sample The

weak ferromagnetic behavior of BLFTO

sample has origin from the partial suppression

of cycloidal spin structure due to the

structural transformation from R3c to Imma

phase The opening hysteresis loop with large

M r value compared with pure BFO confirms

the improvement of magnetic properties by

co-doping La and Ti to BFO It is interesting

to observe the vertical hysteresis shift in

BLFTO sample The vertical hysteresis shift

is widely known as the minor loop effect,

which only present after cycling hysteresis

loop at high applied field, or cooling sample

in external field from high to low temperature [10] Therefore, the vertical hysteresis shift observed in BLFTO is unexpected and cannot come from the minor loop effect

-0.10 -0.05 0.00 0.05 0.10 0.15

H (kOe)

1st 2nd

Fig 3 The M(H) loops of BLFTO sample

measured with two cycles (1 st and 2 nd )

the minor loop is characteristic by the vertical shift and unclosed loop But, as seen in 2nd loop, the unclosed hysteresis loop can be solved in second loop cycle In BFO-based compounds, the previous studies also observed the vertical shift, but the origin of this effect is remain unclear [11] The inhomogeneous magnetic phase from two crystal structures could be the origin of this effect In second loop, the remanent magnetization and coercivity reduce strongly comparison with the first loop The critical reduction was only observed for second loop, whereas high order loop cycles their values are almost stable (Figure 4) The hysteresis loop in Figure 4 was measured after pre-applied -10 kOe on sample It is clear that the direction of hysteresis shift is depended on the sign of applied field From these results confirm that the spin pinning is the origin of the vertical hysteresis shift The spin pinning effect possibly originates from the magnetic interaction at phase boundary of two crystal

structure Depending on the sign of applied

field, the spin pining governs the hysteresis

shift behavior Performing hysteresis loop at

high applied field may destroy the pinned

spin to reveal a high symmetry of hysteresis

loop, as normally observed in previous

reports [12]

-0.10

-0.05

0.00

0.05

0.10

-1 0 1 2 3 -0.01

0.00

0.01

H (kOe)

1st

2nd

3th

H (kOe)

Fig 4 The multi-cycles hysteresis loops of BLFTO

sample measured after pre-applied -10 kOe

The spin pinning is the origin of the vertical

hysteresis shift and exchange bias effect

observed in sample The magnitude of the

exchange bias field can be calculated as HEB =

- (Hc1 + Hc2)/2, where Hc1 and Hc2 are the

negative and positive side fields, respectively

The exchange bias field, which showed in

Figure 3, decreases from 4.0 kOe to 3.2 kOe,

for the first and second hysteresis loops,

respectively It worth noting that the strength

of exchange bias field decreases with the

increase of applied field It also depends on

the magnetic coupling between two phases It

is implied that the exchange bias effect is

dependent on the phase ratio of two phases

CONCLUSIONS

We have successfully synthesized a

multiferroic Bi0.84La0.16Fe0.98Ti0.02O3 without

any impurity phase The crystal structure has

been studied in detail using Rietveld

refinement method revealed the multiphase

rhombohedral and orthorhombic presence in

sample The coexistence of multiphase crystal structure plays the important role on the magnetic properties of sample The magnetic properties were improved with co-doped La and Ti to BiFeO3 The vertical hysteresis shift and the exchange bias effect were observed in sample We proposed that the magnetic coupling at phase boundary is the origin of this effect However, further investigation with another method is necessary to fully understand the contribution of phase boundary to the magnetic properties in this compound

Acknowledgments

This work was supported by the ĐH2015 TN06-10 project of Thai Nguyen University

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Avdeev, A J Studer, F Klose, C Ulrich, and X

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origin of weak ferromagnetism in the disordered

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TÓM TẮT

CẤU TRÚC VÀ TÍNH CHẤT TỪ CỦA VẬT LIỆU GỐM ĐA TINH THỂ

Bi 0.84 La 0.16 Fe 0.98 Ti 0.02 O 3

Trường Đại học Khoa học – ĐH Thái Nguyên

Vật liệu đa pha điện từ Bi 0.84 La0.16Fe0.98Ti0.02O3 được chế tạo bằng phương pháp phản ứng pha rắn Kết quả phân tích giản đồ nhiễu xạ tia X cho thấy vật liệu đồng tồn tại hai pha cấu trúc

là R3c rhombohedral và Imma orthorhombic Phương pháp phân tích cấu trúc Rietveld được sử

dụng để xác định các tham số cấu trúc như hằng số mạng, góc liên kết Fe – O – Fe, và độ dài liên kết Fe – O Kết quả đo đường cong từ trễ cho thấy vật liệu có tính chất sắt từ yếu ở nhiệt độ phòng

Sự đồng tồn tại và cạnh tranh của hai pha cấu trúc ảnh hưởng mạnh lên tính chất từ của vật liệu Đặc biệt, chúng tôi quan sát thấy hiện tượng đường cong từ trễ bị dịch theo trục từ độ (hiệu ứng trao đổi hiệu dịch) Theo chúng tôi, hiệu ứng này có nguồn gốc bởi tương tác từ tại vùng biên pha cấu trúc

Từ khóa: BiFeO 3 , Cấu trúc tinh thể, Tính chất từ, Vật liệu đa pha điện từ; đa pha cấu trúc.

Ngày nhận bài: 25/8/2017; Ngày phản biện: 13/9/2017; Ngày duyệt đăng: 30/9/2017

*

Tel: 0983 009975; Email: dangnv@tnus.edu.vn