Maria R. Hossu

Enhanced piezoresistive characteristics of Nb2O5 modified La0.8Sr0.2MnO3 ceramics

This letter reports the giant piezoresistance phenomenon in Nb2O5 modified La0.8Sr0.2MnO3 (LSMO) polycrystalline ceramics. LSMO modified with 2mol% Nb exhibited ∼2% change in resistivity at 19.2MPa pressure as compared to 0.5% for the pure LSMO. A dramatic improvement was obtained for 5mol% Nb-modified LSMO composition, which exhibited a linear change in resistivity with uniaxial stress and the fractional change was of the order of 3% at 19.2MPa stress. The origin of the giant piezoresistance was found to be related to Nb substitution on the Mn sites which distorts the Mn–O bond, lowering the magnitude of stress required for the manganite lattice transformation.

Self-assembled periodic nanoporous network in multifunctional ZrO2–CeO2–(La0.8Sr0.2)MnO3 composites

This letter reports the structural characteristics of the self-assembled surface nanostructure in the 0.75wt% (Zr0.88Ce0.12)O2–0.25wt% (La0.8Sr0.2)MnO3 ceramics. The surface consists of periodic porous network with islandlike structures occurring at the triple boundary junctions. The pores were found to have the diameter in the range of 150–200nm with the thickness of the porous surface layer of the order of 100nm. The pore distribution was quasiperiodic with the spacing in the range of 50–150nm. The formation of the nanoporous network is correlated to the structural transformation occurring in ZrO2 system.

Field controlled magnetic thermal hysteresis in Co∕Tb multilayers

Experimental results proving that Co∕Tb multilayers show thermal hysteresis in the total magnetic moment are presented. [Co(30A)∕Tb(45A)]8 was synthesized using dc magnetron sputtering. Due to antiferromagnetic coupling between the Co and Tb layers this system behaves like an artificial ferrimagnet. The measurement of magnetic moment (M) as a function of temperature (T) shows that magnetic phase transition occurs at different temperatures during heating (superheating) and cooling (supercooling) cycles resulting in a characteristic bow-tie shaped thermal hysteresis curve for M(T). The width of the thermal hysteresis was measured to be around 90 K in an external magnetic field of 2000 Oe. Increasing the field to 4000 Oe reduces the thermal hysteresis width to about 40 K and above 6000 Oe the width disappears. Co∕Tb multilayers with thicker layers such as [Co(40A)∕Tb(60A)]8 show the same trend. However, the width of the thermal hysteresis is generally smaller at a given magnetic field.

Entropy change for magnetic phase transitions in CoNi/Gd nanolayers

First-order magnetic phase transitions showing thermal hysteresis have been investigated in CoNi/Gd nanolayers. The phase transitions between two coexisting ordered phases are controlled by external magnetic fields (50–500 Oe). The magnetic entropy change has been calculated from isothermal hysteresis data showing two discontinuities with opposite signs as the temperature is cycled in the 20–300 K temperature interval.