Investigations of Phase Transitions and Magneto-electric Properties in Single Phase and Composite Multiferroic Systems

Abstract

The research and development on magneto-electric (ME) multiferroic systems received significant attention among the scientific community since its discovery due to the enigmatic theory behind the ME coupling as well as tremendous technological applications in multifunctional devices. Multiferroics are those materials, which simultaneously exhibit two or more primary ferroic ordering like ferroelectric (FE), ferromagnetic (FM), and ferroelastic. The term magneto-electric is associated with the coupling between ferroelectric and magnetic order parameters. The magneto-electric multiferroics are available in single phase as well as in composite form. Single phase multiferroics are very rare in nature due to the chemical incompatibility and mutual exclusiveness for the occurrence of ferroelectricity and magnetism. However, few single phase multiferroics exist in nature by the consequences of several phenomena such as mixed perovskites with d0 and dn ions, ferroelectricity due to lone pair effect, ferroelectricity due to charge ordering, geometric driven ferroelectricity and spin driven mechanisms. The single phase multiferroics further classified in to type-I and type-II depending upon the origin of ferroelectricity in these classes of materials. Type-II multiferroic materials display stronger magneto-electric properties (because of the magnetic origin of ferroelectricity) as compared to type-I multiferroics systems. DyMnO3 (DMO) belongs to the perovskite structure is one of the promising candidate among the rare earth manganite; RMnO3 (R = rare earth ion) systems which comes under type-II multiferroic systems. It lies on the phase boundary between hexagonal and orthorhombic phase and can be crystalized either in orthorhombic or hexagonal phase depending upon the synthesis condition. Orthorhombic phase of DMO undergoes three magnetic phase transitions: antiferromagnetic ordering of Mn ion around 38-43 K, lock-in temperature below which ferroelectricity is observed around 18-23 K and antiferromagnetic ordering of Dy ion below 10 K. Orthorhomic DyMnO3 (single crystal) exhibits giant magneto-capacitance behaviour ( 500%) around 18 K and it shows larger ferroelectric polarization value which is much higher compared to other RMnO3 systems like TbMnO3. So, orthorhombic phase of DyMnO3 is the ultimate choice for investigations on different aspects. In the present study, rare earth element Gd which has comparatively larger ionic radius than Dy is substituted on the Dy-site of DyMnO3 to stabilize the orthorhombic phase. Acrylamide based gel template method has been adopted to synthesize single phase Dy1-xGdxMnO3 (DGMO, x = 0, 0.05, 0.1, 0.15, 0.2) ceramics. The Rietveld refinement of X-ray diffraction (XRD) patterns recorded at room temperature suggest that DGMO ceramics stabilized in orthorhombic crystal structure with Pnma space group. Gd3+cation plays an important role in stabilizing the orthorhombic phase and the unit cell volume increases with x. The magnetic transition temperatures corresponding to TN(Dy) and Tlock decrease with increase in Gd concentration as inferred from the temperature dependent magnetization data (M-T curve). The magnetization value increases with increase in Gd concentration below TN(Dy) as observed from the M-T plot. The magnetic hysteresis loop is fitted to extract ferromagnetic (FM), antiferromagnetic/paramagnetic (AFM/PM) contributions for each composition x. Fittings of M-H loops indicate the increase in magnetic contribution below TN(Dy) upon increasing the Gd concentrations, while that decreases above TN(Dy). The temperature dependent specific heat data identifies these magnetic transition temperatures TN(Dy), Tlock and TN(Mn). Furthermore, magnetic field dependencies on TN(Dy), Tlock and TN(Mn) are studied in an elaborate manner. Both DMO and DGMO samples are found to be viable candidates for magnetic refrigeration applications in cryogenic temperature region. The true dielectric response of the material is observed up to 160 K and thereafter large increase in dielectric constant may be due to the extrinsic contribution of polarization phenomena as observed from the temperature dependence dielectric constant ( vs. T) plot. For all compositions, vs. T plot shows peak around Tlock which shifted to 17 K for x = 0.2 (19 K for x = 0). Further the magnetic field dependence of Tlock is studied and observed that the peak becomes broader by the application of magnetic field. The magneto-dielectric study reveals that the dielectric constant increases by the application of magnetic field for all the compositions. Multiferroic composites are promising candidates for magnetic field sensors, next-generation low power memory and spintronic devices, as they exhibit much higher magneto-electric coupling at room temperature (RT) compared to the single-phase multiferroics. Hence, the 3-0 type particulate multiferroic composites having general formula (1-Φ) [PbFe0.5Nb0.5O3]-Φ [Co0.6Zn0.4Fe1.7Mn0.3O4] (Φ = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, (1-Φ) PFN-Φ CZFMO) are prepared using a hybrid synthesis technique. Preliminary structural and microstructural analysis are carried out using XRD and FESEM techniques, which suggest the formation of 3-0 type particulate composites without the presence of any impurity phases. The multiferroic behaviour of the composites are studied by measuring polarization versus electric field (P-E) and magnetization versus magnetic field (M-H) characteristics at room temperature. The temperature dependent dielectric behaviour of the composites (Φ = 0.3, 0.4, 0.5) show anomalies around ferroelectric phase transition (Tm) for the PFN phase along with broad relaxation peak arising due to CZFMO phase. For lower compositions (Φ = 0.0, 0.1, 0.2), the dielectric properties as a function of temperature show anomaly around Tm due to dominant contribution from PFN phase. However, the parental phase; PFN shows ferroelectric to paraelectric phase transition around 380 K and the CZFMO shows broad relaxation peak around 455 K. In order to examine the magneto-dielectric effect in PFN-CZFMO composites, frequency dependent dielectric measurement is carried out at various magnetic fields at RT. The nature of magneto-electric coupling is investigated elaborately by employing the Landau’s free energy equation along with the magneto-capacitance measurement. This investigation suggests the existence of biquadratic nature of magneto-electric coupling (P2M2). The magneto-electric coupling measurement also suggests that strain mediated domain coupling between the ferroelectric and magnetic ordering is responsible for the magneto-electric behaviour. The obtained value of direct ME coefficient 26.78 mV/cm-Oe for the optimum concentration Φ = 0.3, found to be higher than the well-known single-phase materials and polycrystalline composites. Solid solution of Pb-based relaxor magneto-electric multiferroics as matrix and ferrite as dispersive phase offer a new way for the fabrication of multiferroic composite. The solid solution of PbZr0.53Ti0.47O3 (PZT) and PbFe0.5Ta0.5O3 (PFT) (0.6PZT-0.4PFT, PZTFT) is a new RT single phase multiferroics material with magneto-electric coupling behaviour. The multiferroic composites of (1-Φ) PZTFT-Φ CZFMO (Φ = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0) are prepared using hybrid synthesis technique. The preliminary structural study from the XRD data conveys the formation of particulate composites and the generation of strain in the composites due to the mismatch of lattice size between the parental phases. The FESEM micrograph showed well dispersion of the CZFMO phase in the matrix of the PZTFT phase. The measured P-E loops for the composites indicate the existence of ferroelectric nature of the composites at RT. The soft magnetic nature is observed from the M-H loops for the composites at RT. Thus, the prepared composites exhibit multiferroic behaviour at RT. The vs. T plots show anomaly across Tm due to the PZTFT phase for all values of Φ. For higher composition, Φ = 0.3, 0.4, 0.5, along with Tm another relaxation peaks are observed which is arising due to CZFMO phase. The magneto-dielectric study at RT suggests change of the dielectric parameters by the application of magnetic field suggesting magneto-dielectric behaviour in the composites. The nature of magneto-electric coupling is investigated and found to be biquadratic in nature. The biquadratic nature of ME coupling for the composites arising from the interface coupling between the constituent phases. The composition Φ = 0.3, display direct magneto-electric coefficient of 20.72 mV/cm-Oe which is found to be the optimum composition among the prepared (1-Φ) PZTFT-Φ CZFMO composites. These composites (1-Φ) PFN-Φ CZFMO and (1-Φ) PZTFT-Φ CZFMO with highest magneto-dielectric properties might be suitable for next-generation low power memory, magnetic field sensors and other multi-functional devices.