Tailoring The Magnetoelectric Properties Of Srfe12o19 Via Partial Substitution Of Bi And Mn

Abstract

There has been a huge demand nowadays for the magnetoelectric materials for multifunctional devices. The research activity on magnetoelectric multiferroic materials has increased significantly by realizing its potential for meeting the demand for technological need. In this regard, strontium hexaferrite and its derivatives are prepared. In the present thesis, various physical properties (such as structural, surface morphology, electrical, magnetic, etc) along with the magnetoelectric property have been studied for the synthesized compounds. The strontium hexaferrite (SrFe12O19), belongs to the family of M-type hexaferrite. It is a magnetoelectric multiferroic compound having high Curie temperature (TC ~743 K), high electrical resistivity (in the megaohm range at room temperature (RT)), strong net magnetization (20 𝜇𝐵𝑓.𝑢.⁄) and a very hard (~ 6 GPa) compound. The SrFe12O19 possesses giant coercivity value (~ 4.5 kOe at room temperature) and high magnetocrystalline anisotropy (~105 erg/cm3). The strontium hexaferrite is extremely flexible to customize its magnetic property or other physical properties in order to meet the desired application requirement. Here, the SrFe12O19 and bismuth (Bi) and manganese (Mn) substituted SrFe12O19 compounds (bismuth (Bi) and manganese (Mn) are substituted in place of strontium (Sr) and iron (Fe) respectively) are prepared using sol-gel auto combustion method. All the prepared samples are confirmed to be phase pure via X-ray diffraction (XRD). The lattice parameters, unit cell volume, lattice strain, etc. are obtained by the refinement of the XRD data via Rietveld technique. The phase purity of the compound SrFe12O19 (SrM), Sr0.98Bi0.02Fe12O19 (SBFO2), SrFe9Mn3O19 (SrM3), SrFe7Mn5O19 (SrM5), and SrFe5Mn7O19 (SrM7) are also confirmed by the neutron diffraction (ND). Both the refinement data confirm the hexagonal crystal structure of the compounds having space group P63/mmc. The bond valence calculation from ND data shows the presence of Fe2+ at 12k crystallographic sites and it indicates an increase of Fe2+ content at 12k-site with increasing Bi in the sample. The distortion calculation from ND suggests an increase in strain at the 2b, 12k-sites due to Bi and Mn substitution. The ND for Mn substituted compounds reveal that Mn ions mostly prefer 2a and 12k- sites. The total magnetic moment (calculated from the respective site occupations) for the Mn substituted compounds, decreases with Mn substitution at room temperature (RT). Similarly, for SBFO2, the total magnetic moment increases as temperature drops. The surface morphology study is carried out via field emission scanning electron microscopy (FESEM) and all samples grain size are in the range ~0.5 μm - 1 μm order. The FESEM micrographs show grains-shape transformation from hexagonal to rhombohedral for SBFO2 and SrM7. The highly strained compound Sr0.99Bi0.01Fe12O19 (SBFO1) and SrM5 grains are most irregular-shaped grains as well as irregular sizes and grain arrangements are also different from rest of the compounds. The strain created in the systems is also observed in the Raman spectra, where Raman spectra changes drastically for Mn substituted compounds and in case of SBFO1, a new Raman mode evolves. The X-ray photoelectron spectroscopy (XPS) measurement is also performed for selected compounds and it confirms the existence of desired elements in the respective compounds. Respective element’s valence states are also deduced from high-resolution XPS spectrum. The XPS data stands by the bond valence calculations. The iron ion exists in both 2+ and 3+ states in SBFO2, SrM5, and SrM7. Similarly, the Mn exists in different states (2+, 3+, and 4+) in SrM5 and SrM7. The Mn3+, Mn4+ increases and Fe3+ content decreases with increasing Mn content. The magnetization variation as a function of temperature (MT-curve) for all the compounds show irreversibility in zero field cooled (ZFC) and field cooled (FC) data right below the Curie temperature (TC) and the Hopkinson peak is observed in the ZFC data. The TC varies in a short temperature range (733 K-739 K) in the Bi substituted compounds whereas in the case of Mn substituted compounds, the TC drops drastically (~620 K for SrM3, ~517 K for SrM5, and ~429 K for SrM7). The TC shifts to higher temperature under the application of magnetic field. The temperature dependence of saturation magnetization of all the compounds follows the Bloch relation and the exponent obtained is close to the theoretical value (1.5). All compound’s saturation magnetization decreases with substitution except for SBFO1, where slight increase is observed. However, the coercive field shows an unusual behaviour with temperature and this has been fitted using an equation devised by taking into consideration of all the three anisotropies (i.e. magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy). The critical exponents are calculated for SrM, SBFO1, SBFO2, SrM3, and SrM5 compounds at the ferromagnetic-paramagnetic phase transition (second-order phase transition) boundary by using modified Arrott plots. These critical exponent values are slightly overvalued as per mean-field theory. Interestingly, for the Mn substituted SrM5 and SrM7, the field varied magnetization data (MH loop) has contribution of two magnetic phases (FiM1 and FiM2) and it is confirmed through the Derivative - Deconvolution – Selective Integration (D-D-SI) method. The presence of the second magnetic phase is also seen in the ZFC curve of SrM5. From SrM5 onwards, the growth of another magnetic phase (FiM2) of lower coercivity apart from the parent phase (FiM1) of higher coercivity is seen. The FiM2 phase is found to be increasing with the Mn content in the sample (16% for SrM5 but 66% for SrM7). Although the magnetization for both FiM1 and FiM2 decreases with increasing temperature, for SrM7 the FiM2 phase is found to persist till higher temperatures compared to the FiM1 phase, whereas just the opposite behavior is observed for SrM5 sample. It thus suggests a transformation of compound from high magnetic anisotropy (SrM) to low magnetic anisotropy (SrM7). The FiM2 phase 𝑀𝑠2(𝑇) and 𝐻𝑐2(𝑇) decreases with rising temperature. The presence of Fe2+ at the 12k site and the rise of Fe2+ percentage by Bi are supported by the Mössbauer data of SrM, SBFO1, and SBFO2. The impedance and modulus study of parent and the Bi substituted compounds suggest the presence of two relaxation mechanisms and the two contributions are coming due to grain and grain boundary. A transition in the relaxation mechanism appears from grain dominated (below 150 K) to grain boundary dominated (above 150 K). In the case of the Mn substituted, the presence of only one relaxation mechanism is dominating (i.e. grain). The two anomalies that appeared in the output of impedance and modulus for the parent and Bi substituted compounds are coming from the magnetic blocking temperature i.e. 75 K and another one due to the grain-grain boundary relaxation transition. The extracted resistance value (of grain and grain boundary) for SrM, SBFO1, and SBFO2 decreases with temperature. The grain related capacitance for SBFO1 is found to be ∼50 nF, and this value is ∼10 times higher than SrM, SBFO2, whereas the grain resistance has dropped by 10 times for SBFO1. Similarly, grain resistance obtained from impedance shows that Mn substitution decreases the resistance by two order as compared to SrM. The SrM, SBFO1, and SBFO2 resistivity data follow the nearest neighboring hopping model, but the resistivity data of Mn substituted compound confirms the conduction via variable range hopping and nearest neighboring hopping. The magnetoresistance measured at various sub-room temperatures for Bi substituted compounds (i.e. SrM, SBFO1, and SBFO2) shows the interplay of anisotropy magnetoresistance (AMR) and giant magnetoresistance (GMR). Low temperature data are dominated by GMR and gradual participation of AMR increases as room temperature is approached. The linear magnetoelectric coefficient αd⁄ (in mV∙cm−1∙Oe−1) for SrM is found to be 0.33(2) at 125 K, and this value decreases gradually to 0.27(1) at 300 K. The SBFO1 sample displayed the highest (even 10% higher than the SrM sample) value of αd⁄ at low temperature. Unfortunately, the increased value of αd⁄ is also accompanied by a drastic reduction in its magnitude for temperatures higher than 200 K, due to the increased electrical conduction which in SBFO1 is ∼94% higher than the parent. Similarly, in the Mn substituted compound, SrM5 compound shows maximum ‘αd⁄’ value, 0.83(2) mV∙cm−1∙Oe−1 and this ‘αd⁄’ value is ~2.5 times higher than that of the parent sample at low temperature. This enhancement is due to the strain produced in the SrM5 system. The influence of strain is also seen in the quadratic magnetoelectric coefficient (2βd⁄). The ‘2βd⁄’ value is also highest for SrM5 (i.e. 2.83(2) mV∙cm−1∙Oe−2) as compared to the other compounds.