Martin C. Robson

Stress-induced effects in epitaxial (La0.7Sr0.3)MnO3 films

Abstract The transport and magnetic properties of epitaxial (La0.7Sr0.3)MnO3 (LSMO) films deposited on LaAlO3 (Sample L, under a compressive stress) and SrTiO3 (Sample S, under a tensile stress) are compared. The magnetic force microscope image of Sample L shows a ‘maze-like’ pattern indicating a sizable out-of-plane magnetization, while the magnetic image of Sample S shows a ‘feather-like’ pattern indicative of in-plane magnetization. The hysteresis loop and ferromagnetic resonance measurements indicate that the lattice mismatch between the film and the substrate is the origin of the differences in the magnetic anisotropy of the two films. Sample L has a smaller ‘peak’ resistivity and a larger saturation magnetization at room temperature than Sample S. Our results can be understood by the internal ‘pressure’ consequent upon lattice mismatch and agree qualitatively with the recent hydrostatic pressure studies on the doped manganites.

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.

Growth of colossal magnetoresistance thin films on silicon

We are able to grow high quality La0.67Sr0.33MnO3(LSMO) colossal magnetoresistive (CMR) thin films on Y‐stabilized zirconia (YSZ) buffered (100) Si substrates using a Bi4Ti3O12 texturing and lattice matching layer. The CMR films have very high structural perfection and show excellent transport and ferromagnetic properties, including the almost full saturation magnetization values and narrow ferromagnetic resonance peaks (15 Oe at 290 K). The lattice matching template/buffer layer approach is suitable for the high quality CMR films on Si. A close correlation between the magnetic hysteresis loop and the field dependence of MR is observed at lower temperatures.

Low‐frequency optical response in epitaxial thin films of La0.67Ca0.33MnO3 exhibiting colossal magnetoresistance

We report measurements of the low‐frequency optical response in epitaxial thin films of La0.67Ca0.33MnO3 in the temperature range covering both the ferromagnetic metallic state and the paramagnetic insulating state. We observe a bolometric optical response in fully oxygenated films of the above composition as well as in oxygen deficient films. In both types of samples, the optical response is thermal in nature as indicated by its proportionality with the temperature derivative of the resistance dR/dT.

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.

Desirable magnetotransport properties in doped Mn-oxide-based superlattices

Superlattice structures based on doped Mn-oxides and SrTiO3 exhibit desirable magnetotransport properties, such as an enhanced magnetoresistance (MR) and a broadened MR transition. An La0.7Ca0.3MnO3 (55 A)/SrTiO3 (160 A) superlattice shows 61% MR at 0.5 T and 109 K, defined by [R(0 T)−R(0.5 T)]/R(0 T), compared to 38% MR for a single layer La0.7Ca0.3MnO3 film at 125 K. MR of larger than 85% at 5 T has been observed in a broad temperature range (from 150 to 10 K) for this superlattice. A systematic study of La0.7Ba0.3MnO3/SrTiO3 superlattices shows that the decrease of the La0.7Ba0.3MnO3 layer thickness results in the broadening of the MR peak in temperature.

Low‐field microwave magnetoabsorption in manganites

We report the magnetic field dependence of microwave absorption at low fields (200 Oe), in thin film and bulk samples of La1−xBaxMnO3−z. We demonstrate, using a very simple cavity configuration, that the magnetic field sensitivity of the microwave response of the giant magnetoresistance materials offers possibilities of using such materials in devices operating at low fields. Low‐field dc magnetization and ac resistivity measurements are also presented.

1/F ELECTRICAL NOISE IN EPITAXIAL THIN FILMS OF THE MANGANITE OXIDES LA0.67CA0.33MNO3 AND PR0.67SR0.33MNO3

We report measurements of 1/f electrical noise in two hole doped manganite perovskite oxides, La0.67Ca0.33MnO3 and Pr0.67Sr0.33MnO3, which exhibit colossal magnetoresistance. The noise magnitude represented by the Hooge parameter is nearly 8 orders of magnitude larger than that observed in ordinary metals (and semiconductors) and nearly 5–6 orders of magnitude larger than that observed in epitaxial films of the perovskite oxide YBa2Cu3O7 in the normal state. The normalized noise spectral density increases with decreasing temperature below the resistivity peak, suggestive of the presence of additional low energy noise processes in the ferromagnetic metallic state.

Standing spin wave resonances in manganite films

Abstract We present first measurements of the spin wave stiffness in the giant magnetoresistance material La 0.67 Ba 0.33 MnO 3 by observation of standing spin wave resonances in about 100 nm thick films.

Surface resistance of thin Perovskite films — high-temperature superconductors and giant magnetoresistance manganites

Abstract We present a systematic study of the surface resistance of perovskite thin films and bulk samples at microwave frequencies. The d.c. resistivity ϱ of the various samples spans three orders of magnitude. For bulk samples, we obtain the usual ϱ 1 2 dependence of Rs. For ϱ 1 μΩ cm, Rs is not proportional to ϱ but to the conductivity σ. Using this fact, we have made the first direct observation of a peak in σt, just below Tc in a-axis films of YBa2Cu3O7.