Cấu trúc và tính chất từ của vật liệu gốm đa tinh thể Bi0.84La0.16Fe0.98Ti0.02O3

Chu Thi Anh Xuan vd Dtg Tgp chi KHOA HOC & CONG NGHB 169(09)- 165-169

CRYSTAL STRUCTURES AND MAGNETIC PROPERTIES OF

Bio.84Lao,i6Feo.98Tio.o203 POLYCRYSTALLINE CERAMIC

Chu Thi Anh Xuan, Pham Truong Tho, Nguyen Van Dang*

University of Sciences - TNU

ABSTRACT

Sample Biog^LaoieFeoggTiooiOs was prepared by the solid-state reaction mediod The analysis of

XRD pattern indicated the coexistence of multi-phase crystal structure of the R3e rhombohedral

and the Imma orthorhombic The Rietveld refinement meUiod was used to calculate the lattice

parameter, bonding angle Fe-O-Fe, and bonding length Fe- 0 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 ongin of this effect come from the magnetic interaction at phase

boundary of two crystal structures

Keywords: BiFeOj, Crystal structures. Magnetic properties; multiferroics materials; multiphase

crystal

INTRODUCTION

Recentiy, multiferroic BiFeOs (BFO) has

attracted much attention because of the

coexistence of both ferroelectnc and magnetic

properties at room temperature, making BFO

become a potential candidate for many

applications m magnetoelectric devices. Bulk

BFO materials have a rhombohedrallly

distorted perovskite structure (space group

R3c) with a high ferroelectric Currie

temperature (TQ ~II0 0 K) and

antiferromagnetic (AFM) Neel temperature

(T^N ^ 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 frora

perfect AFM ordering [2]. In addition, due to

the lack of inversion symmetry in the R3c

structure, BFO shows weak ferromagnetic

(wFM) order associated witii Dzyaloshmky￾Moriya (DM) interactions [3], A disadvantage

of BFO is the existence of a high leakage

current due to impurity phases (such as

Bi2Fe409, Bi23Fe04o) and oxygen vacancies

generated during the synthesis process The

improveraent of this disadvantage can be

Tel: 0983 009975; Emad [email protected] v.

based on the parUal 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 sigmficantiy

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 Bii-yLayFeOa, 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 BioBLao2Fe03 prepared by solid

state reaction and sol-gel methods [5,6].

Particularly, a phase separation was found in

Bii,yLayFe03 as _v = 0,16 [7], resulting in

unusual raagnetic 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 Ti*^ higher than Fe^^

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