Browsing by Author "Joshi, D. C."

Now showing 1 - 2 of 2

Low-temperature anomalous magnetic behavior of Co2TiO4 and Co2SnO4
(2016-10-28)
Nayak, S.
;
Dasari, K.
;
Joshi, D. C.
;
Pramanik, P.
;
Palai, R.
We report the low-temperature anomalous magnetic behavior of ferrimagnetic spinels cobalt orthotitanate (Co2TiO4), which exhibits magnetic compensation behavior across 31.74 K, and cobalt orthostannate (Co2SnO4) exhibiting two sequential magnetic transitions, namely (i) ferrimagnetic to paramagnetic transition with Neel temperature T-N similar to 41 K and reentrant spin-glass behavior with glass transition temperature T-SG similar to 39 K. The Arrott plot (H/M versus M-2) criterion has been used to extricate the order of sequential magnetic transitions occurring below TN. Negative slopes of the Arrott plots below 32 K, metamagnetic-like character of the M-H isotherms, anomalies in the specific-heat (C-P T-1 versus T) below 15 K, and a zero-crossover of isothermal magnetic-entropychange (Delta S) signify the presence of pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5K <= T <= 32 K. The dc- and ac-susceptibilities of both Co2TiO4 and Co2SnO4 are interpreted in terms of frozen-spin-clusters, which are responsible for very large magnitudes of the coercivity H-C similar to 20 kOe and bipolar-exchange bias H-EB similar to -20 kOe observed below 10 K. Published by AIP Publishing.
Spectroscopic studies of Co2TiO4 and Co3O4 two-phase composites
(2016-11)
Nayak, S.
;
Dasari, K.
;
Joshi, D. C.
;
Pramanik, P.
;
Palai, R.
In this article, we report a comparative analysis of various spectroscopic studies including low-temperature (25T300K) Raman spectroscopy of cobalt-orthotitanate (Co2TiO4) and tricobalt-tetraoxide (Co3O4), and their solid solutions (1-x) Co3O4+x of Co2TiO4 (0x1 (100wt.%)). For all the lower and intermediate compositions, five Raman-active modes were recognized at 689,618,518,480, and 195cm-1 that are associated with A1g, Eg, and 3F2g phonon symmetries. Conversely, pure Co2TiO4 exhibits a broad spectrum of width approximate to 93.3cm-1 without any signatures of F2g(3) mode. At low-temperatures (down to 25K) the A1g and F2gpeaks of both Co2TiO4 and Co3O4 shift toward the high-frequency side with anomalies across the ferrimagnetic Neel temperature (TN approximate to 48 +/- 5K) and antiferromagnetic Neel temperature (TN approximate to 30 +/- 10 K), respectively. All the investigated samples exhibit two distinct bands at 576cm-1 (B1) and 665cm-1 (B2) in the Fourier transform infrared spectra recorded at 300 +/- 10 K, associated with the vibrational stretching of the metal-oxygen bonds of length approximate to 195.8pm (B-O) and approximate to 185.4pm (A-O), respectively. The intensity of these sharp bands gradually decreases as the crystal structure transforms from normal-spinel (a=8.07 angstrom) to inverse-spinel structure (a=8.45 angstrom). The X-ray photoelectron spectroscopy (XPS) studies revealed that the Ti was incorporated into the octahedral B-sites of inverse-spinel structure of Co2TiO4. Interestingly, the XPS spectra of Co2TiO4 provide evidence of the trivalent character of Ti instead of tetravalent cationic configuration together with a weak Co3+ character at the octahedral sites. These results are discussed in terms of the binding-energy (BE) difference between the O-1s and Ti-2p3/2 ([O-Ti-2p3/2]=BE(O-1s) -BE(Ti-2p3/2)) and the mean chemical bond length l[Ti-O]. The peculiarities of all these results in consonance with the crystal-structure (bond angles and bond lengths) and electron-spin-resonance studies are discussed in detail.