P. Winotai

Cation distribution in bismuth-doped M-type barium hexaferrite

Mossbauer studies were conducted on Bi-doped barium ferrites BaFe12–xBixO19 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.6) with special attention paid to the behavior of the line widths in the Mossbauer sextets. Assuming that the broadening of the line widths in the various sextets is due to increases in the fluctuations in the hyperfine field (acting on a Fe ion in given site) caused by the introduction of the Bi ions into the neighboring Fe sites, the observed broadening of the lines belonging to the 4fvi and 4e patterns indicated that the impurity ions were entering into the 4e and 4fvi sites. The initial decrease in Ms would arise if the Bi ions first substituted in the 4fvi and then into the 4e sites as more Bi is added.

SURFACE MORPHOLOGY OF SUBMICRON CRYSTALS IN ALUMINUM NITRIDE FILMS GROWN BY DC MAGNETRON SPUTTERING

Aluminium nitride (AlN) thin films were fabricated on a glass substrate by reactive magnetron sputtering. Raman microscopy was then employed to follow the characteristics of their optical and acoustic phonon modes. At the optimal sputtering time of 30 minutes, the defect-induced first and second order Raman spectra were observed in 400–800 cm-1 band which were mostly related to the coating compositions. However, at the 30-, 60- and 90-minute sputtering, crystals of submicron size order of AlN were achieved. This could be clearly identified by the presence of Raman peak at 658–662 cm-1. Powder X-ray diffraction (PXRD) patterns revealed the development of (002) and (101) planes of hexagonal wurtzite AlN phase. The optimal average grain size measured by atomic force microscopy (AFM) is at 330 nm. It was found that the hardness was strongly dependent on roughness of the film, the maximum of which was achieved at 20.00 GPa. The presence of F-type defects in AlN films was investigated by X-band (~9.44 GHz) ESR spectrometer at 295 K. The ESR experiments were carried out by applying magnetic field perpendicular to AlN film, which showed the ESR six-peak multiplet signal at ~290 mT arising from superhyperfine interactions between nuclear spin I = 5/2 of 27Al and electron spins trapped in nitrogen vacancies. The ESR signals are simulated and the ESR parameters are calculated. The vacancies are clearly randomly distributed as the ESR signals are independent of rotation angle (φ) about the normal of the film. All these results were analyzed and presented as a function of the deposition parameters and composition, and crystalline phases existed in the films.

PROPERTIES OF GADOLINIUM-DOPED PZT (Zr:Ti=52:48)

PZT(Zr:Ti=52:48) ceramics can be modified by Gd3+, giving compositions Pb1-xGdx- (Zr0.52Ti0.48)1-x/4 O3 with x = 0.00, 0.01, 0.5, 1, 2, 6 and 10 mole percent. X-ray diffraction patterns show that all PGZT samples are of tetragonal structure and the highest doping should be no more than 2 mole percent Gd at which impurity unreacted oxides start to appear. ESR spectra of PGZTs indicate that Gd3+ can enter both A and B sites of the perovskite structure instead of only A site as widely believed. As a result, the tetragonality (c/a ratio) first increases and reaches a maximum at x ≅ 0.8 mole percent and drops as Gd3+ enters more A sites. The doping should, however be limited to one mole percent as overdoping proves detrimental to the dielectric and piezoelectric properties of PGZT. The dielectric constant (1 kHz) of the poled samples reaches a maximum value of 1150 at x = 0.02 while the largest d33 value of 160 pC/N is achieved at only lightly doped 0.01 mole percent. Furthermore, the Gd doping has a drastic effect on Qm and the dissipation factor DF — the former drops sharply while the latter increases with x.

Crystallization of amorphous Fe78B13Si9 ribbons as monitored by Mössbauer spectroscopy

Abstract The crystallization of amorphous Fe 78 B 13 Si 9 ribbons is monitored through the evolution of the hyperfine fields at the Fe sites as the ribbons are annealed between 300°C and 550°C for two hours at intervals of 50°. Mossbauer sextets belonging to α-Fe(Si), Fe 3 B, Fe 2 B and α-Fe compounds begin to be seen as the annealing temperature is raised past 400°C. The hyperfine field distributions responsible for the broadening of the Mossbauer sextet associated with the amorphous phase appear to be bimodal in nature. It is seen that the range of hyperfine field values becomes narrower as the annealing temperature is raised, pointing to a lessening of the inhomogeneity of the amorphous phase.

Enhancement of Tc and the re-entrant spin glass transition in La0.86Ca0.14Mn1−yCryO3 (y = 0, 0.1 and 0.2)

We report on structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La0.86Ca0.14Mn1−yCryO3 (y = 0, 0.1 and 0.2) prepared by the sol–gel technique. For y = 0, a paramagnetic to ferromagnetic transition was observed at Tc = 185 K.For y = 0.1, the value of Tc = 200 K, an increase of 15 K, and for y = 0.2, Tc = 195 K,an increase of 10 K. The imaginary part of the ac susceptibility for all three samples shows a secondary transition at Tf< Tc. For y = 0, there is no definite law accounting for the frequency dependence of Tf and it is attributed to a transition arising out of a canted structure. However, in the cases of y = 0.1 and 0.2, the frequency dependence indicates the presence of a re-entrant spin glass transition at Tf. Although all three samples show a semiconducting behaviour between 300 and 5 K, a negative MR was observed corresponding to Tc and Tf. The value of MR decreased for the Cr substituted samples.

OPTIMIZATION OF HEAT TREATMENTS OF AFRICAN GREEN SAPPHIRES

Under suitable heating conditions designed for a particular sapphire it is possible to change the color in order to increase its value. It was found that heating African green sapphires in an oxygen atmosphere could improve their quality and thus increase their prices. The color of green sapphires is due to the presence of trace amounts of Fe3+, Ni3+, Fe2+ and Ti4+ ions. It is known that Fe3+ yields pale yellow and Ni3+ a golden color while the charge transfer mechanism between Fe2+ and Ti4+ gives sapphire its blue color. It is obvious that the green color is due to a high ratio of Fe2+ to Ti4+. By using an X-ray diffractometer, the maximum decrease of the c/a ratio of the hexagonal structure is found to occur after heat treatment at 1200°C. These results are also confirmed by the shift of emission spectra toward shorter wavelengths. Furthermore, the number of Fe2+ ions converted to Fe3+ was detected by electron spin resonance spectrometry and found to increase with temperature. The sapphire with trace Ti4+ ions, appeared slightly bluish green after heating at 1500°C. The optimal temperature is therefore at 1300°C, where we can achieve yellowish green sapphires instead.

Structural changes and superconducting properties of Gd1−2xPrxCaxBa2Cu3O7−δ HTSCs

We report on structural changes and the properties of Gd1−2xPrxCaxBa2Cu3O7−δ (0≤x≤0.3) high temperature superconductors which were prepared and characterized. The transition temperature (Tc) was found to decrease roughly linearly with respect to the Pr and Ca concentration x. 65Cu2+(3d9) and 63Cu2+(3d9) ions from impurity phase BaCuO2 give rise to X-band electron spin resonant (ESR) two-quartet absorption peaks centred at ~310 mT (g~2.020) which are superposed by the broad ESR peak of Gd3+(4f7) at g~2.000 in the undoped Gd123. Both sets of ESR peaks shift towards low fields as the temperature is lowered, which is due to the change in crystal field experienced by the ions. It should be noted here that even at 77 K there still is no magnetic ordering from Gd3+ ions. However, at x = 0.025 doping, a new six-peak hyperfine multiplet ESR signal arising from 141Pr4+ (4f1,S = 1/2,I = 5/2) appears at ~323 mT (g~2.029). Typical simulated ESR parameters of Gd3+ in Gd1−2xPrxCaxBa2Cu3O7−δ at x = 0.025, are gx = 2.4879, gy = 2.3921 and gz = 2.2590, Axx = 12.50, Ayy = 23.50, Azz = 32.00 G and the zero-field splitting parameter (D) of 66.0 G, respectively. This signal arises from some Pr ions entering the Gd site with a valence of four, thus lending support to the hole-filling and pair-breaking models.

HEAT TREATMENTS OF TANZANIA RUBY AS MONITORED BY ESR SPECTROSCOPY

The main purpose of heat treatment is to find the optimum condition to either enchance or reduce the color in order to increase value of ruby. It was found that by heating a slightly bluish Tanzania ruby in oxygen gas atmosphere could lead to a more perfect ruby. While X-ray diffractometer found the maximum decrease in c/a ratio of the hexagonal structure after the heat treatment at 1200°C, the number of Fe2+ ions converted to Fe3+ was detected by electron spin resonance spectrometer and found to increase with heating temperatures. These were clearly two factors that reduced blue color in the Tanzania ruby. The best heat treating temperature to enhance the quality of the ruby was at 1300°C.

Liquid nitrogen quenching of lead-doped “2212” bismuth superconductors

Abstract The effects of lead substitution in Bi 4 Sr 4 Ca 2 Cu 4 O y conductors are studied. Liquid nitrogen quenching of the compounds Bi 4− x Pb x Sr 4 Ca 2 Cu 4 O y ( x =0.7, 0.9, 1.0, 1.1 and 1.2) is required to produce superconductors having T c s well above the transition temperature of the host “2212” superconductor. The X-ray diffraction patterns of these compounds are those of the “2212” structure (with a few weak lines of the “2223” structure appearing). The resistivity curve for the lighter doped compounds ( x =0.7 and 0.9) indicates the existence of two super-conducting phase transitions, one at around 105 K and another around 89 K. For the heavier doped compounds ( x =1.1 and 1.2), the resistivity curves show only a single transition. In general, the zero resistivity temperatures are seen to decrease with increased Pb doping.

FLUCTUATIONS IN THE LOCAL FIELDS DUE TO AL3+ IONS SUBSTITUTION IN THE M-TYPE BARIUM HEXAFERRITES, BAFE12-XALXO12

The effects of aluminum substitution into the M-type Ba-hexaferrites on the saturation magnetization and Mossbauer spectrum of these ferrites are studied. The saturation magnetization Ms of the BaFe12-xAlxO12 is seen to decrease linearly with increasing Al substitution. The Mossbauer spectrums for the x = 0.1, 0.2 and 0.6 specimens were all resolved as superpositions of four sub-spectrums (12k, 4fiv+2a, 4fvi and 4e). The half width of the Mossbauer lines of the 4fiv+2a sextets is seen to broadening as Al ions are substituted into the ferrite. No broadening is seen in the lines of the other sextets. These behaviors (the line broadening and the linear decrease in Ms) are consistent with the Al3+ ions substituting preferentially into the 2a sites.