Substitution Induced Modifications in Structural, Magnetic and
Magnetodielectric Properties of Bi2Fe4O9

Abstract

In the last few decades, magnetoelectric (ME) multiferroics have experienced rejuvenation in the scientific community for their novel potential applications. It offers additional degrees of freedom in device designing where magnetic and electric polarization can be tailored, and this could be very subtle to electric and magnetic fields respectively. In spite of remarkable technological applications, the natural occurrence of single phase ME multiferroics are very rare. This is due to their mutual exclusive characteristics and incompatibility between the order parameters. Generally, most of the ME multiferroic systems known so far are functional below the cryogenic temperature. So, current efforts address the quest for exploring new classes of near room temperature ME materials, which could exhibit strong ME coupling at ambient temperature, to meet the necessity of practical applications. In the search for new ME materials, Bi2Fe4O9 (BFO), a material proto-type of Cairo-spin lattice and a well-known antiferromagnetically ordered system has drawn considerable attention in the past few years. BFO shows substantial ME coupling close to room temperature with TN 260 K. Moreover, BFO shows a unique pentagon spin frustration arising due to the competing exchange interactions among different kinds of Fe3+ ions, thus leading to a non-collinear magnetic structure. So far, various research groups have studied to enhance the
multiferroic features of BFO by suitable chemical substitution. But till date, the microscopic understanding of ME effect and magnetodielectric (MD) study have not been investigated. Apart from this, BFO encounters with few other open challenges which we have addressed in the present thesis. These includes: synthesis of single phase BFO, to tune AFM transition (TN) towards room temperature by external perturbations, role of spin frustration and its correlation with MD effect and to tune MD coupling by chemical substitution as well as study the variation in MD effect. To address the above raised problems, single phase BFO is prepared using conventional solid state reaction route using ambient conditions and the phase purity is confirmed from room temperature X-ray diffraction (XRD) study. To comprehend the microscopic origin of ME coupling in BFO, temperature dependent neutron diffraction (ND) and polarized neutron scattering (PNS) techniques are implemented. Next, we intend to enhance the observed physical properties of BFO and make it a viable room temperature ME material by suitable chemical substitution at both Bi3+ and Fe3+ site of BFO. Subsequently, in the present thesis, we have substituted Co3+ at Fe3+ site, Gd3+ & Ho3+ at Bi3+ site and co-substituted both Ho3+ (at Bi3+ site) & Co3+ (at Fe3+ site) in BFO. All the substituted samples are prepared using solid state reaction route and their phase formation is confirmed from XRD study. Substitution induced changes in magnetic properties, TN, frustration parameter (f ) are studied using temperature dependent magnetic susceptibility measurement. Evidence of MD coupling in all the studied samples are confirmed from temperature and field dependent MD study. Strong evidence of near room temperature MD coupling consequently led to investigate the MD effect in all the samples both at room temperature as well as near the vicinity of magnetic transition (TN). This feature is studied from temperature, frequency and magnetic field dependent MD effect. An in-depth analysis of the experimental data is carried out. Temperature dependent ND result at 300 K revealed a broad diffuse scattering hump around 2θ 17°, indicating short range magnetic ordering. The evidence of magnetic nature of the diffuse scattering are extensively studied using PNS. The Rietveld refinement of the XRD data revealed the orthorhombic phase (space group ‘Pbam’) of the synthesized samples. The bond angles are found to vary due to substitution which is expected to have a substantial effect in governing the magnetic and magnetodielectric properties of the substituted samples. The temperature dependent susceptibility measurement showed decrease in TN for Co3+ substituted samples(152 K) whereas, TN is found to increase for Gd3+ (256 K), Ho3+ (266 K) and co-substituted samples (288 K). The magnitude of magnetization is found to increase in all the substituted samples which are attributed to d-d, f-d and f-f interactions. Gd3+ and Ho3+ substituted samples revealed a sharp rise in magnetization at lower temperature (around 50 - 70 K) indicating magnetic ordering of Gd3+ and Ho3+ ions. The temperature dependent dielectric permittivity of all the samples confirmed the coupling of dielectric and magnetic order parameters. This is seen as a signature of concomitant anomaly at the magnetic transition in the dielectric data. The true nature of MD coupling is later confirmed from the evidence of nil magnetoresistance in the samples. Lastly, MD effect was quantitatively calculated for all the samples both at room temperature and at/near the vicinity of TN. It is worth to note that, even at room temperature, MD effect is seen for all the samples under present investigation. Among all the substitutions, Co3+ substituted BFO for the highest substituted sample displayed highest value of MD effect both at TN ( -2.5%) and at room temperature ( -0.9%). Hence, the above interesting features such as enhanced magnetic and magnetodielectric properties along with very near room temperature TN would pave way for further scientific research and make the above studied materials a promising candidate for varied multifunctional device applications.