Magnetic, Magnetodielectric and Lattice Dynamic Effect of Chemically Substituted Y Type Ba2Mg2Fe12O22 Hexaferrites

In the last few decades, multiferroic (MF) materials have attracted interesting attention due to their mutual control of electric and magnetic properties termed as magnetoelectric (ME) effect. This area of research is important in terms of potential technological applications such as spintronic devices, electronic devices, medical drug delivery, and ME sensors. Most MF materials exhibit high field control ME coupling below liquid nitrogen temperature, which is not appropriate for effective devices based on the ME effect. To realize useful devices based on the ME effect, it requires a low field control ME effect at RT. Amongst MF materials, hexaferrite is one such class of materials that exhibit low field control ME effect near or above RT. Interestingly, Y-type hexaferrite possesses a significantly low field control ME effect at RT, which has an advantage over several other single-phase MF materials. However, direct measurement of ME coupling is not an easy task as most MF materials are poor insulators. Hence, an alternative route to investigate ME coupling is to measure the influence of dielectric properties with an application of magnetic field termed as magnetodielectric (MD) effect. The Y-type Ba2Mg2Fe12O22 (BMF) hexaferrite has drawn significant attention recently due to its tuneable magnetic and ME properties upon suitable substitution at magnetic and non-magnetic sites. In this present thesis, a systematic investigation was carried out on magnetic and MD properties along with lattice dynamic effect of polycrystalline BMF sample. For the first time, a robust MD effect is reported in the BMF sample. The effect of Mn and Ni substitution following Ba2Mg2(Fe1-xMnx)12O22 (0 ≤ x ≤ 0.12) and Ba2Mg2-xNixFe12O22 (0 ≤ x ≤ 2.0) system respectively on magnetic, MD, and lattice dynamics properties were investigated in detail. Further, the effect of Sr and Mn co-doped Ba2-xSrxMg2Fe11.48Mn0.52O22 (0 ≤ x ≤ 1) on magnetic, MD, and lattice dynamics properties is also studied. All the samples were prepared using a conventional solid-state reaction route. The BMF exhibits irreversibility between ZFC and FC of magnetization below 270 K, and magnetization value decreases with lowering temperature following an anomaly at TII (170 K) and TI (25 K). The anomaly around 170 K indicates ferrimagnetic (FIM) to proper screw transition, while at 25 K denotes proper screw to longitudinal conical spin transition. Surprisingly, two distinct dielectric anomalies, near 25 K (TI) and 170 K (TII) are observed in the vicinity of magnetic spin phase transition, suggesting a strong coupling between dielectric and magnetic properties. The MD% vs. H data suggest the colossal MD% value at T > 200 K is mainly due to the Maxwell-Wagner (M-W) effect, while the butterfly loop at T < 200 K indicates the intrinsic MD effect in the sample. Further, the temperature-dependent Raman study confirms the absence of structural phase transition and the existence of possible spin-phonon coupling (SPC) in the sample. The Mn substitution modulates not only the superexchange angle near the boundary of magnetic blocks but also the magnetic transition temperature. The transition temperature TII increases from 170 K to 208 K, while TI decreases from 25 K to 15 K. The value of loss tangent decreases with increasing doping concentration at 300 K, i.e., ~60% and 180% in a decrease in 4% (BMFM4) and 8% (BMFM8) Mn-doped sample respectively as compared to BMF, suggesting the evolution of intrinsic feature. The presence of substantial intrinsic MD% (~6%) at 1.3 T at 300 K for a 4% Mn-doped sample is observed. The nature and strength of magnetoelectric coupling in BMFM4 and BMFM8 samples at 300 K is found to be biquadratic (P2M2), and the maximum strength of coupling is 3.09×10-4 emu2/g2 and 2.34×10-4 emu2/g2, respectively. Ni substitution in Ba2Mg2-xNixFe12O22 (0 ≤ x ≤ 2.0) enhances temperature TII from 170 K for x = 0 to 230 K for x = 2.0 suggesting Ni substitution progressively stabilizes the screw order. While TI increases from 25 K to 40 K with an increase doping up to x ≤ 1.0 and a sharp decrease is seen for x ≥ 1.0. The K value increases with an increase in doping up to x ≤ 1.5, and a sharp decrease is seen for x ≥ 1.5, which is due to the substitution of larger magnetic anisotropic Ni2+ ions in place of non-magnetic Mg2+. The observed contraction in few Raman modes (704 and 507 cm-1) is due to distortion produced by doping of Ni ions which is well confirmed by the decrement of lattice parameters (a, c) with an increase in Ni content. The frequency-dependent dielectric ɛrˊ´ data suggest that the M-W type conduction mechanism dominates near RT. In contrast, the Warburg-type conduction mechanism is prevalent in the low-temperature regime. The intrinsic MD effect for the x = 1.0 sample is ~ 2 times greater than the x = 0 sample, and the range of frequency over which it is observed increases from 105 - 106 Hz to 103 - 106 Hz. Finally, the Sr and Mn co-substitution in Ba2 xSrxMg2Fe11.48Mn0.52O22 (0 ≤ x ≤ 1) enhances spin ordering transition temperature TI (TII) from 52 K (210 K) to 73 K (315 K). The presence of single cluster-glass transition at Tf1 (45 K) in the x = 0.25 sample. Interestingly, for the x > 0.25 sample, double cluster class transition is observed, one around Tf1 and the other at Tf2. The freezing temperature Tf1 decreases with an increase in x, while Tf2 increases substantially with x. The Ms value found to be decreased from 28.88 emu/g for x = 0.25 to 26.28 emu/g for x = 0.75 at 300 K. However, for x > 0.75 this value increases. Raman modes at 78, 135, 335, and 694 cm-1 show a gradual increase in Raman shift up to x = 0.75, but a drastic decrease is seen for x > 0.75. Along with enhanced magnetic properties, Sr doped sample shows better MD properties. The MD% value at RT increases with doping concentration up to x = 0.75 (MD% ~1.2); beyond that, its value decreases. The anomalous response of MD% associated with SPC is confirmed by the anomalous behavior of different Raman modes across the spin ordering transition temperatures. The H-dependent MD exhibits symmetric anomalies close to the H-induced transition indicating an exchange-striction phenomenon in the samples. Hence, the enhanced magnetic and low field control MD behavior at RT is exciting for exploring the ME devices based on hexaferrite