The rapid development of magnetic bubble technology has required growth to exacting specifications of a large number of magnetic films of garnets and some amorphous binary and ternary alloys of rare-earth and transition metals. The characterisation of these films is an essential part in the search for newer materials which hold out promise for better device performance and cost viability. Many methods of films characterisation have been reported from time to time. By and large, these methods can be divided into two groups: one, bulk measurements made on the film and two, measurements made on the domains. We have attempted to collate and briefly introduce various techniques to characterise magnetic bubble materials in this review.

Electron transport properties of a few hot-pressed garnets of the series Y_3−xGd_xFe₅O₁₂ (wherex=0, 1 and 2·4) have been measured. For comparison, a normal sintered YIG has been studied to see the effect of porosity and microstructure. The electron transport properties have been discussed on the basis of the model suggested by Austin and Mott keeping in view the distortion caused by the substitution.

Glasses in the system PbO-BaO-TiO₂-B₂O₃-SiO₂ with and without P₂O₅ as nucleant have been prepared. The glass samples were ceramized based ondta studies. The ferroelectric phase crystallizing out has been found to be BaTiO₃ fromxrd. The optical and scanning electron micrographs show the presence of BaTiO₃ as major phase. In these glass ceramic samples, dielectric constant and dissipation factor are approximately constant with temperature and frequency upto the glass transition temperatureT_g and thereafter increase sharply with temperature and finally level off. The addition of P₂O₅ as nucleant and molar ratio of (PbO + BaO) to TiO₂ has marked influence on the dielectric behaviour and composition of ferroelectric phase crystallizing out.

Glass ceramics prepared by controlled crystallization of glasses produce fine dispersion of crystallites in a glassy matrix. Glasses containing a mjor portion of constituents of a ferroelectric phase produce crystallites of ferroelectric phase in glass through a suitable heat treatment. The amount of network former in the initial glass has a profound influence on its crystallization behaviour and microstructure of the resulting ferroelectric glass ceramics. The value of dielectric constant and the nature of ferroelectric to paraelectric transition depend on the crystallite size and volume fraction of the ferroelectric phase. These glass ceramics are transparent for crystallite size less than 0·1µm and exhibit large quadratic-electro-optic effect.

Measurements of DC electrical resistivity and Seebeck coefficient on the perovskite system La_1−xPb_x−δ Co_1−xTi_xO_3−δ for 0·2≤x≤0·9 have been made in the temperature range 300–800 K. AC conductivity,σ_a.c, of all the samples were measured as a function of temperature (300–573 K) and frequency (1 kHz-1 MHz). DC resistivity behaviour of all the samples is similar. However, the resistivity value, which varies over 5–6 orders of magnitude, depends on both the compositionx and the structure of the samples. All the samples exhibitp-type electronic conduction. The value of Seebeck coefficient,α, for samples withx≤0·5 initially increases with temperature up to a particular temperature. Above this temperature, the behaviour ofα for samples withx≤0·5 and for samples withx>0·5 over the entire temperature range is similar to that of La CoO₃. All the samples exhibit frequency-dependent a.c. conductivity at low temperatures.

Dielectric behaviour of samples of the system Ca_{1 −x}La_xTi_{1 −x}Co_xO₃ withx ⩽ 0·20 has been studied in the temperature range 300–525 K as a function of frequency. The strong dispersion ofɛ andD observed in these materials indicate the significant contribution of interfacial polarisation to the observed dielectric properties. The interfacial polarisation arises due to the presence of microscopic chemical heterogeneities arising out of the slow diffusion-controlled solid state sintering process used for their preparation.

The spin-state equilibria of cobalt ions in the system La_1−xNa_xCo_1−xNb_xO₃ (x⩽0·40) has been studied by measuring its magnetic susceptibility as a function of temperature in the range 300–600 K. It is found that the behaviour of the samples withx⩽0·10 is similar to that of LaCoO₃, while compositions withx⩾0·20 behave quite differently, exhibiting simple paramagnetic behaviour.

Glasses in the system (65 −x) [SrO·TiO₂] − (35) [2SiO₂·B₂O₃] − (x) [Bi₂O₃] wherex = 1, 5, 10 (wt%) prepared by melting in alumina crucible (1375–1575 K), were subjected to different heat treatment schedules followed by DTA studies. Crystallization study showed the formation of Sr₂B₂O₅ as major phase at low temperature (≈950°C) heat treatment. At high temperatures, TiO₂ and SrTiO₃ with or without Sr₂B₂O₅ crystallize out depending on heat treatment. In this paper, the influence of variation in composition, thermal treatment on the nature of crystallizing phases as well as on the resulting microstructures are investigated through XRD, IR and SEM. Uniform crystallization was achieved by suitable addition of Bi₂O₃ and proper heat treatment.

The electrical behaviour of valence-compensated ceramic system Ba_1−xLa_xTi_1−xCo_xO₃ has been studied as a function of temperature (300–600 K) and composition (x ⩽ 0·20), using the method of impedance spectroscopy. The necessary equivalent circuit models that represent the data best have been obtained using impedance and modulus formalisms and grain and grain boundary contributions have been separated out. The compositionsx = 0·20 and 0·10 show a negative temperature coefficient of resistance (NTCR) behaviour whereas a very small variation of the grain and grain boundary resistance with temperature is observed forx = 0·05. A positive temperature coefficient of resistance (PTCR) behaviour having two ferroelectric components is observed forx = 0·01. These results reveal limitations in current theories of the PTCR effect.

The infrared spectra (IR) of various glass compositions in the glass system, [(Pb_𝑥Sr_1–𝑥)O.TiO₂]– [2SiO₂.B₂O₃]–[BaO.K₂O]–[La₂O₃], were recorded over a continuous spectral range (400–4000 cm^-1) to study their structure systematically. IR spectrum of each glass composition shows a number of absorption bands. These bands are strongly influenced by the increasing substitution of SrO for PbO. Various bands shift with composition. Absorption peaks occur due to the vibrational mode of the borate network in these glasses. The vibrational modes of the borate network are seen to be mainly due to the asymmetric stretching relaxation of the B–O bond of trigonal BO₃ units. More splitting is observed in strontium-rich composition.

Glasses were made by melt-quench method in the system [(Sr_1–𝑥Pb_𝑥)O.TiO₂]–[2SiO₂.B₂O₃]–5[K₂O–BaO] (0.0 ≤ 𝑥 ≤ 0.4) with addition of 1 mol% Nb₂O₅. Perovskite strontium lead titanate in solid solution phase has been crystallized in borosilicate glassy matrix with suitable choice of composition and heat treatment schedule. Addition of 1 mol% of Nb₂O₅ enhances the crystallization of lead strontium titanate phase in the glassy matrix. Scanning electron microscopy (SEM) is performed to study the surface morphology of the crystallites and crystalline interface to the glass. Dielectric properties of these glass ceramics were studied by measuring capacitance and dissipation factor as a function of temperature at a few selected frequencies. Nb₂O₅ doped strontium lead titanate glass ceramic shows a high value of dielectric constant. It is of the order of 10,000 while the dielectric constant of undoped glass ceramic sample is of the order of 500. Complex impedance and modulus spectroscopic techniques were used to find out the contributions of polarization of crystallites and glass crystal interfaces to the resulting dielectric behaviour.

The effect of Co⁺² doping on Cu⁺² and Ti⁺⁴ sites in calcium copper titanate, CaCu₃Ti₄O₁₂, has been examined. The doped compositions, CaCu_3−𝑥Co_𝑥Ti₄O₁₂ and CaCu₃Ti_4−𝑥Co_𝑥O₁₂ (𝑥 = 0.10) ceramics, were prepared by novel semi-wet route. In this method, calcium, copper and cobalt salts were taken in solution form and TiO₂ was used in solid form. XRD analysis confirmed the formation of single-phase materials. Structure of CaCu₃Ti₄O₁₂ does not change on doping with cobalt either on Cu-site or Ti-site and it remains cubic. Scanning electron micrographs (SEM) show average grain size of CaCu_2.9Co_0.1Ti₄O₁₂ to be larger than CaCu₃Ti_3.9Co_0.1O₁₂ ceramic. Energy dispersive X-ray spectroscopy (EDX) studies confined the purity of parent and Co-doped CaCu₃Ti₄O₁₂ ceramics. Dielectric constant (𝜀_r) and dielectric loss (tan 𝛿) of CaCu_2.9Co_0.1Ti₄O₁₂ is comparatively higher than that of CaCu₃Ti_3.9Co_0.1O₁₂ ceramic at all measured frequencies and temperatures.

The aim of this paper is to investigate the effect of sintering temperature and time on the properties of Fe–Al₂O₃ composite (5 wt% Al₂O₃; 95 wt% Fe) prepared by powder metallurgy process. X-ray diffraction, microstructure, density, hardness and compressive strength of prepared samples have been investigated. XRD studies show the presence of Fe and Al₂O₃ along with iron aluminate phase. Iron aluminate is formed as a result of reactive sintering between iron and alumina particles. Microstructural examination of the specimen showed a dense structure with nanosize dispersion of the reinforcement of ceramic phase. Density as well as hardness of specimens depend on the formation of iron aluminate phase, which in turn depends on sintering temperature and time.

The present paper reports the effect of height to diameter ($h/d$) ratio on the deformation behaviour of Fe–Al$_2$O$_3$ metal matrix nanocomposites (MMNCs) during bulk processing. Sintered compacts were machined to the required size with different $h/d$ ratios. Test specimens were subjected to deformation at room temperature under three different interfacial friction conditions such as dry, solid and liquid lubrications. Deformed specimensshow a significant improvement in the density and hardness. Results also revealed the formation of a nanosize iron aluminate phase due to reactive sintering, which in turn contributes to grain refinement. Experimental density of the specimens was also verified with the theoretical density using the standard equations. It is expected that the present work will be useful in designing and developing MMNC products with better quality at competitive cost.

MgO−Al$_2$O$_3$−SiO$_2$ (MAS)-based glass–ceramic system was prepared using very-low-cost raw materials, i.e.,talc, calcined alumina and calcined china clay with titanium dioxide as a nucleating agent. Glass–ceramics were preparedby a two-step process. In the first step, raw materials were mixed in the required proportion and melted at 1450$^{\circ}$C followedby water quench into a glassy frit. In the second step, powdered glass frit was uniaxially dry pressed into pellets followedby sintering at 1200$^{\circ}$C for 3h. X-ray diffraction pattern of the sintered compact shows well-defined peaks of cordieritealong with some anorthite and magnesium titanium oxide. The microstructure study of sample shows the presence of crystallineand glassy phases. Permittivity and permeability measurements were performed in the microwave frequency range12.4–17 GHz. The permittivity value of 5.7–6.0 and the permeability value of $\sim$1 were obtained. The reflection and transmission measurements show that the material possesses a shielding effectiveness in the range 2–10 dB over the frequencyrange 12.4–17 GHz.