Manoj Raama Varma

Microwave Dielectric Properties of RETiTaO6 (RE = La, Ce,Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) Ceramics

Microwave dielectric ceramics based on RETiTaO 6 (RE = La, Cc, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) were prepared using a conventional solid-state ceramic route. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscopy techniques. The sintered samples were characterized in the microwave frequency region. The ceramics based on Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy, which crystallize in orthorhombic aeschynite structure, had a relatively high dielectric constant and positive τ f while those based on Ho, Er, and Yb, with orthorhombic euxenite structure, had a low dielectric constant and negative τf. The RETiT a O 6 ceramics had a high-quality factor. The dielectric constant and unit cell volume of the ceramics increased with an increase in ionic radius of the rare-earth ions, but density decreased with it. The value of τf increased with an increase in RE ionic radii, and a change in the sign of τf occurred when the ionic radius was between 0.90 and 0.92 A. The results indicated that the boundary of the aeschynite to euxenite morphotropic phase change lay between DyTiTaO 6 and HoTiTaO 6 . Low-loss ceramics like ErTiTaO 6 (e r = 20.6, Q u xf = 85,500), EuTiTaO 6 ( r = 41.3, Q u xf = 59,500), and YtiTaO 6 (e r = 22.1, Q u xf = 51,400) are potential candidates for dielectric resonator applications.

Magnetism and magnetocaloric effect in nanocrystalline La0.67Ca0.33Mn0.9V0.1O3 synthesized by nebulized spray pyrolysis

Vanadium (V) substituted La0.67Ca0.33MnO3 was synthesized in the nanocrystalline form using nebulized spray pyrolysis. Highly uniform nanoparticles of average size in the range 30–40 nm as revealed by transmission electron microscopic studies were prepared. Rietveld refinement of XRD patterns shows that the sample adopts an orthorhombic structure with the Pnma space group. The systematic change in lattice parameters and magnetic phase transition (TC = 252 K) as compared with those of nanocrystalline La0.67Ca0.33MnO3 (TC = 260 K) indicate the substitutional effect of V. M–H curves in a certain temperature range reflect the superparamagnetic nature of the sample. The average particle size (~28 nm) of the superparamagnetic clusters obtained from the Langevin fit is coincident with that of the nanocrystalline clusters revealed by the high-resolution transmission electron microscopy images. The Arrott plots reveal the first order character of magnetic phase transition of the sample. The peak magnetic entropy change (ΔSmax) of these nanopowders is estimated to be about 4.5 J kg−1 K−1 for a magnetic field of 5 T at 252 K. Compared with the bulk La0.67Ca0.33Mn0.9V0.1O3, the entropy change of the nanoparticles is found to be smaller, but the entropy change occurs over a broad temperature range, giving rise to a large relative cooling power.

Effect of dopants on microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics

Ba(Zn1/3Ta2/3)O3 (BZT) has been prepared with various amounts of different dopants such as oxides of divalent, trivalent, tetravalent, pentavalent and hexavalent elements. Effect of these dopants on microwave dielectric properties of BZT is investigated. Some of the dopants are found to increase quality factor (Q). Most of the dopants increase the temperature coefficient of resonant frequency (τf). Annealing undoped BZT increased the order parameter and quality factor. Small amounts of dopants such as that of oxides of Zr, Ga, Cr, Ce, Sn, In, Mn and Sb increased the quality factor. The doped ions substitute for the ordered B ions decreasing the order parameter. Annealing Ga and In doped BZT decreased the Q factor where as it increased for Zr, Cr, Ce, Sn, Mn, Sb doped samples. Doping BZT with Zr, Ga, Cr, Ce, Sn, In, Mn, Sb decreased the order parameter but at the same time increased the quality factor indicating that order parameter alone is a poor indicator of quality factor. The quality factor is found to depend on the dopant ionic radii and dopant concentration. The quality factor increased when the ionic radius of the dopant is close to the ionic radius of the B site ions Zn or Ta.

Co-existence of magnetocaloric effect and magnetoresistance in Co substituted La0.67Sr0.33MnO3 at room temperature

Magnetization (M), Magnetocaloric effect (MCE), and Magnetoresistance (MR) in Co substituted La0.67Sr0.33Mn1−xCoxO3 (x = 0.03, 0.05, and 0.1) samples have been investigated. All the studied samples were crystallized into a single phase rhombohedral structure with R3¯c space group. The Curie temperature (TC) is found to decrease with the increase of Co content and for the composition x = 0.1, a cluster glass-like nature is observed. Temperature variation of magnetic entropy change (ΔSM) shows a positive peak at TC for the composition x = 0.1 with ΔSM = 3.1 J/kg K and relative cooling power, RCP = 155 J/kg at a magnetic field of 50 kOe. Again, the critical analysis of the paramagnetic to ferromagnetic transition and the scaling behaviour of field dependence of MCE for x = 0.1 composition is also reported. Furthermore, the temperature dependence of electrical resistivity under the magnetic fields up to 50 kOe is reported along with the values of 20% enhancement in MR at room temperature for the Co substitute...

Magnetocaloric properties, exchange bias, and critical behavior of Ge substituted Ni50Mn36Sn14 Heusler alloys

The effect of Ge substitution on the magnetic, magnetocaloric, and exchange bias properties of Heusler alloy system Ni50Mn36Sn14-xGex (x = 1, 2) has been investigated. With the increase of Ge content, the cell volume decreases due to the smaller Ge radius and the martensitic transition temperature increases, while the Curie temperature of the austenite phase shows a small decrease. Metamagnetic behavior is observed in the low temperature magnetization isotherms for x = 1, whereas it is less pronounced in x = 2. The maximum magnetic entropy changes associated with the martensitic transition are 7.8 J/kg K and 2.3 J/kg K for x = 1 and 2, respectively, for a field change of 5 T. Relative cooling power is found to be more in the vicinity of the Curie temperature of the austenite phase, compared to that at the martensitic transition temperature in both x = 1 and 2. At low temperatures, both the samples exhibit exchange bias effect, with x = 2 showing higher value of exchange bias field. This is ascribed to the...

Structural properties, magnetic interactions, magnetocaloric effect and critical behaviour of cobalt doped La0.7Te0.3MnO3

The effect of cobalt doping on the structural, magnetic and magnetocaloric properties of electron-doped manganite La0.7Te0.3Mn1−xCoxO3 (x = 0, 0.1, 0.2, 0.25, 0.3 and 0.5) has been investigated. The parent compound La0.7Te0.3MnO3 crystallizes in a rhombohedral structure with Rc space group. With the increase in Co concentration to x = 0.2, a structural transition from rhombohedral (Rc space group) to orthorhombic (Pbnm space group) is observed. X-ray photoelectron spectroscopy (XPS) indicates that the structural transition is due to the disordered distribution of Mn2+/Mn3+ and Co2+/Co3+ ions. All the samples undergo a paramagnetic–ferromagnetic (PM–FM) phase transition. With the increase in Co content to x = 0.1, the unit cell volume increases with a decrease in both Mn–O–Mn bond angle and Tc indicating a weakening of the double exchange interaction. However, with further increase in Co concentration, Tc increases. The presence of competing ferromagnetic and antiferromagnetic interactions leads to a glassy behaviour at low temperatures for low Co doping concentrations. However, for higher Co concentrations, no such behaviour is observed. Arrott plots reveal a second order nature of magnetic transition for all the samples. The magnetic exchange interactions for x = 0.3 and 0.5 follow the mean-field model. Magnetization results show that the magnetocaloric property of the electron-doped manganite is affected by the substitution of Co at Mn sites. Relatively large values of relative cooling power and broad temperature interval of the magnetocaloric effect make the present compounds promising for sub-room temperature magnetic refrigeration applications.

Evidence of reduced antiferromagnetic transition in mesocrystals of CuO synthesized by a surfactant-free solution phase method

We explore the finite size effect on the magnetic properties of CuO mesocrystals synthesized by a surfactant-free solution phase method. Significant evidence for the evolution of 3D oval, elliptical and 2D sheet-like CuO mesocrystals driven by oriented aggregation of CuO nano-subunits is presented. The dimensions of the CuO nano-subunits constituting the mesocrystals, as well as their final morphology, are tailored by the NaOH concentration in the precursor solution. Changes in the mesocrystal morphology reflect on the position and intensity of the Raman vibrational modes. DC magnetization curves reveal distinct features of antiferromagnetic transitions where the magnetization behaviour of the mesocrystal is influenced by the dimensions of the CuO nano-subunits. The magnetic ordering temperature (TN) is reduced from the bulk value of 230 K to 58 K as the size of the CuO nano-subunit decreases to 7 nm. Moreover, all nanostructures exhibit linear field-dependent magnetization below the transition temperature similar to an antiferromagnetic system. However, below TN field cooled isothermal magnetization displays an anomalous behaviour as a shift along the vertical axis whose origin is related to surface uncompensated spins. The vertical shift decreases as the CuO nano-subunit dimensions increase.

Re-entrant spin glass behaviour and magneto-dielectric effect in insulating Sm2NiMnO6 double perovskite

The structure and magneto-dielectric properties of Sm2NiMnO6 double perovskite have been investigated. The pure monoclinic bulk phase of Sm2NiMnO6 is an intrinsic B-site disordered ferromagnet showing a second order magnetic phase transition ∼157 K, along with a low temperature re-entrant spin glass-like transition (∼20 K) due to the inherent coupling of Ni–Mn network with Sm3+ ion. Moreover, it is an insulator with room temperature dc resistivity ∼108 Ω cm. Furthermore, the high dielectric permittivity (∼103) coupled with a shift in dielectric maxima towards higher temperature with frequency, analysis of the phenomenological parameters (Tm, TB, γ, ΔTrelax), and the validity of non-linear Vogel–Fulcher relation, we define this ferromagnetic insulator as a relaxor-type dielectric material. Importantly, it exhibits a positive intrinsic magneto-dielectric (MD) response ∼2% at 100 kHz under a low magnetic field of 0.5 T close to 150 K. The intrinsic MD effect indicates a correlation between spin orientations on the two magnetic sublattices (Ni and Mn) and the magneto-dielectric coupling in the ferromagnetic regime. The insulating ferromagnetic behaviour coupled with MD effect make Sm2NiMnO6 an attractive spintronic material.

Multiple magnetic transitions, Griffiths-like phase, and magnetoresistance in La2CrMnO6

DC and AC magnetic measurements indicate the presence of multiple magnetic transitions arising from the competing magnetic interaction between Cr and Mn in the perovskite La2CrMnO6. Ferromagnetic and spin glass transitions are observed in La2CrMnO6. The material also has a Griffith-like phase with the occurrence of ferromagnetic short range correlations above TC. The system ultimately turns into paramagnetic at the Griffiths temperature 180 K. A combination of variable range hopping, and the nearest neighbor small polaron hopping governs the conduction mechanism in the material. A negative magnetoresistance of 22% at 105 K is observed for the material at 90 kOe which increases to 29% near 110 K and reduces gradually to zero on further increase in the temperature.

Observation of high-temperature magnetic transition and existence of ferromagnetic short-range correlations above transition in double perovskite La2FeMnO6

We present the observation of a high-temperature magnetic transition along with ferromagnetic short-range correlations (FSCs) in La2FeMnO6 perovskite system. XPS analysis confirmed the presence of Fe3+ and Mn3+ cations. Magnetization vs. temperature curves show two distinct transitions at TC1 ∼ 60 K and TC ∼ 425 K. Coercivity values of ∼1140 Oe and ∼35 Oe are observed at 2 K and 300 K respectively. Thermomagnetic analysis reveals the presence of FSCs in La2FeMnO6 up to T* = 570 K, well above the transition point, similar to a Griffiths-like phase (GP). The presence of ferromagnetic clusters in the paramagnetic region might be due to the intrinsic inhomogeneities associated with the structure, the quenched disorder related to the B-site cations and the antisite boundaries. The coefficient of lower temperature electronic specific heat is as high as 59.5 mJ mol−1 K−2. The electron spin resonance spectra show ferromagnetic resonance signals that point to the possibility of the presence of FSCs at room temperature. The material seems to be quite a promising candidate for some room temperature applications due to the possibility of the coexistence of functionalities like ferromagnetism, ferrimagnetism, GP, magnetotransport coupling, etc. in a single material.