Enhancement in magnetic properties of Ba-doped BiFeO₃ ceramics by mechanical activation

Highlights

• Ba-doped BiFeO₃ ceramics prepared by SSR reaction with and without ball milling.

• Nearly 50% increase in magnetization found in mechanically activated samples.

• Pre-alloying by mechanical activation leads to three folds increase in coercivity.

• Local magnetic behavior has been investigated by Mössbauer spectroscopic studies.

Abstract

Here, we report on the enhancement in the magnetic properties of polycrystalline Bi_1−xBa_xFeO₃ (x = 0.0, 0.05, 0.1 and 0.2) ceramic samples as a consequence of mechanical activated solid-state-reaction in comparison to standard solid-state-reaction processed samples. Our results suggest that mechanical activation has pronounced effect on the temperature of phase evolution as well as magnetic characteristics of Ba doped BiFeO₃. The temperature, at which perovskite phase crystallizes is lower for mechanical activation assisted samples than for standard solid-state-reaction processed samples, attributed to pre-alloying during mechanical activation process. All the samples crystallize in a distorted perovskite structure with space group R3c and no structural change is found up to highest Ba doping. Mechanical activation also leads to significantly altered magnetic properties, particularly in higher Ba-doping samples which show about 50% increase in the magnetization and more than three fold increase in the coercive field. Local magnetic behavior investigated by ⁵⁷Fe Mössbauer spectroscopy rules out any valence fluctuations of Fe and the hyperfine field corroborates the magnetization data.

Introduction

Multiferroic materials show more than one ferroic properties like ferromagnetism, ferroelectricity, ferroelasticity in a single phase and in particular, the coexistence of ferroelectric and magnetic ordering in single phase makes them very interesting for device applications [1], [2], [3]. However, there are very few materials which show magnetic ordering together with ferroeletricity at room temperature or above. The reasons of counterintuitive nature of coexistence of electrical and magnetic ordering in a single phase material are well discussed by Hill [4]. Among very few single-phase materials showing multiferroism, bismuth ferrite (BiFeO₃ or BFO) is the most exciting material, mainly due to its higher magnetic and ferroelectric transition temperatures. It crystallizes in a rhombohedrally distorted perovskite structure with space group R3c and undergoes ferroelectric transition at 1103 K. It is a G-type antiferromagnet with magnetic transition at the Neel temperature of 643 K [5], [6]. As both the transition temperatures are well above room temperature, it is a potential candidate for practical applications in novel magnetoelectric devices [7]. However, limitations such as formation of impurity phases due to Bi loss, presence of defects and very low magnetization due to modulated spin spiral structure with periodicity of 62 nm in addition to the G-type antiferromagnetism, restrict the use of bulk BFO in real applications [8].

A closer look at the Bi₂O₃–Fe₂O₃ pseudo-binary phase diagram shows that the BiFeO₃ perovskite phase forms at around 826 °C which is well above the melting point of Bi (≈271 °C) [9]. Iron rich impurity phases such as Bi₂Fe₄O₉ are often found to exist along with pure perovskite phase, mainly due to Bi loss during fabrication of BiFeO₃. To overcome these limitations, various strategies based on doping at both Bi and Fe-sites of BFO have been proposed and used successfully, at least to some extent as doping is suggested to stabilize the perovskite phase of BFO [10], [11]. For example, improved ferroelectric properties and increased magnetization have been reported in single-phase rare earth and alkaline metal substituted BiFeO₃ ceramics [12], [13], [14], [15], [16]. Single phase samples up to 25% Ba doping with increasing magnetic properties has been found in Ba-doped ceramics [16]. The Fe-site doping in the BFO by other transitions metals such as Cr, Mn and Ti has been reported to increase the net magnetization, attributed to the suppression of spin spiral structure along with departure from the crystalline correlation and introduction of oxygen vacancies [17], [18], [19]. In addition, other interesting features such as a cluster spin glass nature has been found in 5% Co-doped BFO ceramics [20]. Structural and magnetic properties of non-magnetic Zr⁴⁺ substituted BFO ceramics and thin films suggested structural distortion as well as impurity phase suppression [21]. Structural, optical and transport properties of Al³⁺ doped BiFeO₃ nanopowders synthesized by solution combustion method were also reported [22]. Similarly, cobalt doing of BFO ceramics has been shown to result in a significant increase in the magnetization as compared to pure BFO samples prepared by rapid sintering of sol–gel derived powders [23]. Further, detailed magnetic and ⁵⁷Fe Mössbauer spectroscopy of Mn-doped BFO ceramics showed no contributions from Fe²⁺ state [24].

In many reports on BFO, polycrystalline ceramic samples of undoped and doped BiFeO₃ are prepared by solid-state-reaction (SSR) method which requires higher temperature and impurity phases are often found due to Bi volatility. Sometimes two or three step solid-state-reaction method with intermediate grinding is required to obtain single-phase samples, often also requiring leaching in dilute acid to get rid of impurity phases. Mechanical activation assisted solid state reaction (MA-SSR) is an efficient method for preparing ceramic samples which requires comparatively low annealing temperature due to atomic level intermixing of species during high energy milling fastening the kinetics of phase formation. Concerns of contamination can be minimized by the use of new hard balls of nonmagnetic materials for grinding and repeated use with the same material. To the best of our knowledge, there are only a few reports on BFO samples prepared by MA-SSR method e.g. undoped and Sm-doped BFO samples [25], [26]. Single phase sample of Bi_1−xSm_xFeO₃ (x = 0.00, 0.1 and 0.2) prepared by MA-SSR at 700 °C and held for 1 h crystallized in the perovskite phase [26]. The magnetization and coercive field were highest for 10% Sm doping and then decreased with further doping (20%). However, there was no detailed comparison of the structural and magnetic properties of the doped BFO samples synthesized with and without MA-SSR method. To investigate the effects of mechanical activation on the structural and magnetic properties of Ba-doped BFO samples, we have prepared Bi_1−xBa_xFeO₃ (x = 0.00, 0.05, 0.1 and 0.2) ceramics by MA-SSR and SSR methods. The choice of Ba as dopant has specific reasons. Firstly, Bi ions are in 3+ states in the BFO matrix and Ba ions preferably exist in 2+ oxidation states. So, the substitution of Ba²⁺ can introduce Fe⁴⁺ or oxygen vacancies in BFO due to charge compensation whose effect can be studied on the magnetic behavior. Secondly, the ionic radius of Ba²⁺ (ionic radius ≈ 1.35 Å) is quite large as compared to that of Bi³⁺ (ionic radius ≈ 1.03 Å) which can change the Fe–O–Fe bond angle and can alter the magnetic behavior further.