K. Ghosh

Effect of crystallinity on the magnetoresistance in perovskite manganese oxide thin films

We report our study of the effect of crystallinity on the magnetoresistance in epitaxial and polycrystalline La2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 thin films. Magnetoresistance in epitaxial films exhibits field dependence and temperature dependence similar to bulk single crystals and sintered bulk ceramics. The polycrystalline films exhibit a markedly different behavior. The magnetoresistance in this case shows either a monotonic increase or saturation with decreasing temperature in contrast to that of epitaxial films in which the magnetoresistance peaks close to the ferromagnetic transition temperature. The field dependence in the polycrystalline films is also remarkably different. At low fields, we observe a sharp drop in resistance followed by a more gradual decrease at higher fields. Our data suggest that in addition to the intrinsic magnetoresistance, grain-boundary transport contributes significantly to the magnetoresistance in polycrystalline films.

Positive giant magnetoresistance in a Fe3O4/SrTiO3/La0.7Sr0.3MnO3 heterostructure

A very large positive magnetoresistance (MR) has been discovered in a Fe3O4/SrTiO3/ La0.7Sr0.3MnO3 heterostructure for the transport perpendicular to the layer planes and applied magnetic field in the film plane. The observed MR features do not show any obvious correlation with the hysteresis behavior of the ferromagnetic bilayers. A possible explanation of these results is given in terms of the relative differences in the majority and minority spin bands of the two ferromagnetic layers and the field induced modifications of domain structures therein.

Correlation between magnetic homogeneity, oxygen content, and electrical and magnetic properties of perovskite manganite thin films

Perovskite manganese oxide materials known for the phenomenon of colossal magnetoresistance often exhibit anomalously large 1/f noise and large, temperature-dependent ferromagnetic resonance (FMR) linewidths. We show that in epitaxial films, these anomalies are very sensitive to oxygen partial pressure during film growth and to postdeposition thermal processing in oxygen, suggesting that oxygen stoichiometry plays a key role. We find that the temperature coefficient of resistance (TCR) at the metal–insulator transition increases and the FMR linewidth decreases as we increase the oxygen partial pressure during growth. Postdeposition heat treatment in oxygen leads to further increase in TCR and decrease in FMR linewidth, accompanied by a dramatic reduction in 1/f noise magnitudes.

Growth and properties of c-axis textured La0.7Sr0.3MnO3−δ films on SiO2/Si substrates with a Bi4Ti3O12 template layer

c-axis textured La0.7Sr0.3MnO3−δ (LSMO) films were fabricated on SiO2/Si(001) substrates using a Bi4Ti3O12 (BTO) template layer. Electrical and magnetic properties of LSMO were investigated. The LSMO/BTO layer of this structure has no in-plane alignment. Even though a ferromagnetic transition temperature, TC, is as high as that of the epitaxial LSMO film (360 K), a resistivity peak temperature, TP, is about 140 K lower than TC. The resistivity behavior as a function of temperature for LSMO/BTO/SiO2/Si films is found to be dominated by grain boundary effects. Low field sensitive magnetoresistance which suggests spin tunneling through the grain boundaries is also observed at room temperature.

Ferromagnetism at room temperature in La0.8Ca0.2MnO3 thin films

Anomalously high metal–insulator transition temperature Tp and ferromagnetic transition temperature Tc have been observed in thin films of La0.8Ca0.2MnO3. Ferromagnetic resonance signals from microwave studies are observed at room temperature (well above Tc) suggesting the presence of multiple magnetic spin systems. The Tp and Tc values are higher than that of La0.67Ca0.33MnO3 which have the highest Tp and Tc according to the bulk phase diagram. These results suggest that properties well beyond those seen in the bulk phase diagram may be achievable in thin films of the manganites.

Magnetic homogeneity of colossal-magnetoresistance thin films determined by alternating current magnetic susceptibility

We report measurements of the alternating current (ac) magnetic susceptibility, χac=χ′+iχ″, performed on colossal-magnetoresistance (CMR) materials. We have studied thin film samples of La0.67Ca0.33MnO3 and Nd0.7Sr0.3MnO3. For homogeneous samples, the temperature of the peak observed in χ″(T) is in agreement with the temperature of peak resistivity (TP) obtained from transport measurements. This agreement is not found for inhomogeneous samples, where χ″(T) shows multiple peaks. The analysis of χ″(T) enables one to determine the quality of the CMR materials. The results obtained in thin films of La0.67Ca0.33MnO3 and Nd0.7Sr0.3MnO3, are consistent with those obtained from an homogeneous single crystal of La0.80Sr0.20MnO3. We show that the contactless ac magnetic susceptibility technique is a quick method to reveal inhomogeneities which are not directly evident in direct current transport measurements.

Temperature-tuned natural ferromagnetic resonances in

We present the results of an investigation into the zero-field microwave absorption in for frequencies ranging from 2 to 35 GHz. The temperature dependence of the microwave loss at zero applied field displays two peaks for frequencies less than 15 GHz. These peaks can be ascribed to temperature tuning of the internal anisotropy field to satisfy the conditions for ferromagnetic resonance.

Colossal oxygen isotope shift of the charge-ordering transition in Nd0.5Sr0.5MnO3

A `colossal oxygen isotope shift of the charge-ordering temperature was discovered in . Upon replacing with , the charge-ordering temperature increases by 21 K under zero magnetic field. With an increase of the magnetic field, the oxygen isotope shift increases rapidly and reaches a magnitude of about 43 K under a magnetic field of 5.4 T. The very strong magnetic field dependence of the oxygen isotope effect is difficult to explain within the existing theories of the charge-ordering transition in manganites.