Magnetic anisotropy, first-order-like metamagnetic transitions and large negative magnetoresistance in the single crystal of Gd2_{2}PdSi3_3

Electrical resistivity ($\rho$), magnetoresistance (MR), magnetization, thermopower and Hall effect measurements on the single crystal Gd$_{2}$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure are reported. The well-defined minimum in $\rho$ at a temperature above N\'eel temperature (T$_N$= 21 K) and large negative MR below $\sim$ 3T$_N$, reported earlier for the polycrystals, are reproducible even in single crystals. Such features are generally uncharacteristic of Gd alloys. In addition, we also found interesting features in other data, e.g., two-step first-order-like metamagnetic transitions for the magnetic field along [0001] direction. The alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.Comment: RevTeX, 5 pages, 6 encapsulated postscript figures; scheduled to be published in Phy. Rev. B (01 November 1999, B1

Magnetic behaviour of Eu_2CuSi_3: Large negative magnetoresistance above Curie temperature

We report here the results of magnetic susceptibility, electrical-resistivity, magnetoresistance (MR), heat-capacity and ^{151}Eu Mossbauer effect measurements on the compound, Eu_2CuSi_3, crystallizing in an AlB_2-derived hexagonal structure. The results establish that Eu ions are divalent, undergoing long-range ferromagnetic-ordering below (T_C=) 37 K. An interesting observation is that the sign of MR is negative even at temperatures close to 3T_C, with increasing magnitude with decreasing temperature exhibiting a peak at T_C. This observation, being made for a Cu containing magnetic rare-earth compound for the first time, is of relevance to the field of collosal magnetoresistance.Comment: To appear in PRB, RevTex, 4 pages text + 6 psFigs. Related to our earlier work on Gd systems (see cond-mat/9811382, cond-mat/9811387, cond-mat/9812069, cond-mat/9812365

Observation of a temperature dependent electrical resistance minimum above the magnetic ordering temperature in Gd2_2PdSi3_3

Results on electrical resistivity, magnetoresistance, magnetic Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat capacity and Gd Mossbauer measurements on a Gd-based intermetallic compound, Gd$_{2}$PdSi$_{3}$ are reported. A finding of interest is that the resistivity unexpectedly shows a well-defined minimum at about 45 K, well above the long range magnetic ordering temperature (21 K), a feature which gets suppressed by the application of a magnetic field. This observation in a Gd alloy presents an interesting scenario. On the basis of our results, we propose electron localization induced by s-f (or d-f) exchange interaction prior to long range magnetic order as a mechanism for the electrical resistance minimum.Comment: 4 pages, 4 figure

Large Low Temperature Magnetoresistance and Magnetic Anomalies in Tb2_2PdSi3_3 and Dy2_2PdSi3_3

The results of heat-capacity, magnetic susceptibility, electrical resistivity and magnetoresistance $(\Delta \rho/\rho)$ measurements on the compounds Tb$_2$PdSi$_3$ and Dy$_2$PdSi$_3$, are reported. The results establish that these compounds undergo long-range magnetic ordering (presumably with a complex magnetic structure) below (Tc=) 23 and 8 K respectively. The $\Delta \rho/\rho$ is negative in the vicinity of Tc and the magnitude grows as Tc is approached from higher temperature as in the case of well-known giant magnetoresistance systems (La manganite based perovskites); this is attributed to the formation of some kind of magnetic polarons. The magnitude of magnetoresistance at low temperatures is quite large, for instance, about 30% in the presence of 60 kOe field at 5 K in the Dy sample.Comment: 4 pages, 3 figures, RevTe

Pion parameters in nuclear medium from chiral perturbation theory and virial expansion

We consider two methods to find the effective parameters of the pion traversing a nuclear medium. One is the first order chiral perturbation theoretic evaluation of the pion pole contribution to the two-point function of the axial-vector current. The other is the exact, first order virial expansion of the pion self-energy. We find that, although the results of chiral perturbation theory are not valid at normal nuclear density, those from the virial expansion may be reliable at such density. The latter predicts both the mass-shift and the in-medium decay width of the pion to be small, of about a few MeV.Comment: 9 Pages RevTex, 3 eps figure

Controlled Release of Drug to the Intestine from pH-responsive Chitosan-Poly (vinyl alcohol) Interpenetrating Network Hydrogels

Hydrogels are polymeric networks with three-dimensional configuration capable of imbibing high amounts of water or biological fluids. A truly amazing class of hydrogels that has found potential use for a wide variety of applications is the class of “smart” or “intelligent” hydrogels. Among all, pH-sensitive smart hydrogels have garnered special interest. Chitosan, a polycationic bioplomer, has received a great deal of attention in the fabrication of pH-responsive hydrogels. The interest in chitosan as intestinal drug delivery carriers arises from the fact that it is degraded by the microflora of the colon and is not digested in the upper gastrointestinal tract. Chitosan hydrogels, however, suffer from certain limitations such as poor mechanical strength, high porosity and tendency to absorb moisture. One of the most common methods to alleviate this problem is to incorporate chitosan into another polymeric network by the formation of an interpenetrating network. The focus of the present project is to synthesize pH-responsive chitosan- poly (vinyl alcohol) interpenetrating network hydrogels for controlled delivery of drug to the intestine

The structural and electrical properties of thermally grown TiO2 thin films

We studied the structural and electrical properties of TiO2 thin films grown by thermal oxidation of e-beam evaporated Ti layers on Si substrates. Time of flight secondary ion mass spectroscopy (TOF-SIMS) was used to analyse the interfacial and chemical composition of the TiO2 thin films. Metal oxide semiconductor (MOS) capacitors with Pt or Al as the top electrode were fabricated to analyse electrical properties of the TiO2 thin films. We show that the reactivity of the Al top contact affects electrical properties of the oxide layers. The current transport mechanism in the TiO2 thin films is shown to be Poole–Frenkel (P–F) emission at room temperature. At 84 K, Fowler– Nordheim (F–N) tunnelling and trap-assisted tunnelling are observed. By comparing the electrical characteristics of thermally grown TiO2 thin films with the properties of those grown by other techniques reported in the literature, we suggest that, irrespective of the deposition technique, annealing of as-deposited TiO2 in O2 is a similar process to thermal oxidation of Ti thin films

La substitution induced linear temperature dependence of electrical resistivity and Kondo behavior in the alloys, Ce_{2-x}La_{x}CoSi_{3}

The results of electrical resistivity, heat capacity and magnetic susceptibility behavior of new class of alloys, Ce_{2-x}La_{x}CoSi_{3}, are reported. The x= 0.0 alloy is mixed valent and La substitution for Ce (x= 0.25) induces linear temperature dependence of resistivity at low temperatures, an observation of relevance to the topic of non-Fermi liquid behavior. The modifications of Kondo effect for all the alloys are also presented.Comment: Accepted for publication in Solid State Communication

Residual resistivity ratio and its relation to the positive magnetoresistance behavior in natural multilayer LaMn2Ge2; relevance to artificial multilayer physics

Results of low temperature magnetoresistance ($\Delta\rho/\rho$) and isothermal magnetization (M) measurements on polycrystalline ferromagnetic (T_C close to 300 K) natural multilayers, LaMn_{2+x}Ge_{2-y}Si_y, are reported. It is found that the samples with large residual resistivity ratio, $\rho(300K)/\rho(4.2K)$, exhibit large positive magnetoresistance at high magnetic fields. The Kohler's rule is not obeyed in these alloys. In addition, at 4.5 K, there is a tendency towards linear variation of $\Delta\rho/\rho$ with magnetic field with increasing $\rho(300K)/\rho(4.2K$); however, the field dependence of $\Delta\rho/\rho$ does not track that of M, thereby suggesting that the magnetoresistance originates from non-magnetic layers. It is interesting that these experimental findings on bulk polycrystals are qualitatively similar to what is seen in artificially grown multilayer systems recently.Comment: 5 pages, 3 figures, separate figures. This work is a follow-up of our earlier paper in APL, Ref. : APL Vol 71, pp 2385 (1997