Peter R. Duncombe

Observation of large low‐field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current‐perpendicular‐to‐plane transport. These results show that low‐field spin‐dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

(Ba,Sr)TiO 3 dielectrics for future stacked- capacitor DRAM

Thin films of barium-strontium titanate (Ba,Sr)TiO3 (BSTO) have been investigated for use as a capacitor dielectric for future generations of dynamic random-access memory (DRAM). This paper describes progress made in the preparation of BSTO films by liquid-source metal-organic chemical vapor deposition (LS-MOCVD) and the issues related to integrating films of BSTO into a DRAM capacitor. Films of BSTO deposited on planar Pt electrodes meet the electrical requirements needed for future DRAM. The specific capacitance and charge loss are found to be strongly dependent on the details of the BSTO deposition, the choice of the lower electrode structure, the microstructure of the BSTO, the post-electrode thermal treatments, BSTO dopants, and thin-film stress. Films of BSTO deposited on patterned Pt electrodes with a feature size of 0.2 µm are found to have degraded properties compared to films on large planar structures, but functional bits have been achieved on a DRAM test site at 0.20-µm ground rules. Mechanisms influencing specific capacitance and charge loss of BSTO films are described, as are the requirements for the electrode and barrier materials used in stacked-capacitor structures, with emphasis given to the properties of the Pt/TaSi(N) electrode/barrier system. Major problems requiring additional investigation are outlined.

Enhanced magnetoresistance in sintered granular manganite/insulator systems

We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.

Dielectric relaxation of Ba0.7Sr0.3TiO3 thin films from 1 mHz to 20 GHz

The dielectric relaxation of Ba0.7Sr0.3TiO3 thin films was investigated up to K band (20 GHz) using time domain and frequency domain measurements. Our results show that from 1 mHz to 20 GHz, the dielectric relaxation of the complex capacitance of Ba0.7Sr0.3TiO3 thin films can be understood in terms of a power law dependence known as the Curie–von Schweidler law. The small dispersion (less than 7% decrease in capacitance from 1 mHz to 20 GHz) and low loss (loss angle less than 0.006 at 20 GHz) measured in Ba0.7Sr0.3TiO3 thin films indicate that these films are applicable to device application up to at least K band.

Growth and giant magnetoresistance properties of La‐deficient LaxMnO3−δ (0.67≤x≤1) films

Epitaxial thin films of lanthanum‐deficient LaxMnO3−δ (0.67≤x≤1) have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The as‐deposited films exhibit a ferromagnetic transition at temperatures ranging from 115 to 240 K, with the transition temperature (Tc) increasing with higher La deficiency. A sharp drop in resistivity and negative magnetoresistance is observed close to Tc, a behavior similar to that observed in divalent substituted La1−xMxMnO3−δ (M=Ba, Sr, Ca, Pb) films. Postannealing the films in O2 reduces the resistivity and raises the Tc to values close to room temperature. A magnetoresistance value of 130% (Δρ/ρH) has been obtained at 300 K at 4 T for a post‐annealed film with x=0.75.

TEMPERATURE DEPENDENT, NON-OHMIC MAGNETORESISTANCE IN DOPED PEROVSKITE MANGANATE TRILAYER JUNCTIONS

We report spin-dependent perpendicular transport in the magnetic trilayer junction structure La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3. Large (factor of 5) changes of magnetoresistance induced by a field of ∼200 Oe are observed at 4.2 K. Junction I–V characteristics at low temperatures are consistent with a metal–insulator–metal tunneling process with a large spin-polarization factor of 0.81 for the conduction electrons. Above 100 K, a variable range-hopping conduction shunts out the magnetoresistance contribution. This second conduction channel comes from the impurity states within SrTiO3 barrier and therefore is not an intrinsic limit to the magnetoresistance performance of the device at high temperatures.

METALLIZATION INDUCED BAND BENDING OF SRTIO3(100) AND BA0.7SR0.3TIO3

We have investigated the interaction of Pt with single-crystal SrTiO3(001) and polycrystalline Ba0.7Sr0.3TiO3 thin films using photoemission spectroscopies. Significant band bending is caused by interface formation, determining the Schottky barrier height. We have depth profiled the band bending for Ba0.7Sr0.3TiO3 thin films, giving a direct measurement of the depletion depth and built-in potential.

The effect of grain size on microstructure and stress relaxation in polycrystalline Y 1 Ba 2 Cu 3 O 7−δ

We have used transmission electron microscopy and optical microscopy to examine the effect that grain size and heat treatment have on twinning and microcracking in polycrystalline Y 1 Ba 2 Cu 3 O 7−δ . It is shown that isothermal oxygenation heat treatments produce twin structures consisting of parallel twins, with a characteristic spacing that increases with increasing grain size. Slow cooling through the temperature range where the orthorhombic-to-tetragonal transformation induces twinning, however, produces a structure consisting of a hierarchical arrangement of intersecting twins, the scale of which appears to be independent of grain size. It is also shown that the microcracking induced by anisotropic changes in grain dimensions on cooling or during oxygenation can be suppressed if the grain size of the material is kept below about 1 μm. The results are examined in the light of current models for transformation twinning and microcracking and the models used to access the effect other processing variables such as oxygen content, doping or heat treatment may have on the microstructure of Y 1 Ba 2 Cu 3 O 7−δ .

Magnetoresistance and Hall effect of chromium dioxide epitaxial thin films

Epitaxial CrO2 thin films have been grown on TiO2(100) and Al2O3(0001) substrates by atmospheric pressure chemical vapor deposition. The films have a Curie temperature (Tc) of around 393 K with the ones grown on TiO2 exhibiting in-plane uniaxial magnetic anisotropy. They also display metallic characteristics, with room temperature resistivity of about 285 μΩ cm, dropping by about two orders of magnitude upon cooling down to 5 K. Magnetoresistance (MR) properties of the films have been measured with the magnetic field in the plane. For a field of 40 kOe, a positive transverse MR of about 25% at 5 K and a negative MR of about 7% at near Tc have been observed. In addition, Hall resistivity has been measured with magnetic field up to 40 kOe. A positive ordinary Hall effect is found at low temperatures, indicating the conduction carriers are holes.

Compensation doping of Ba0.7Sr0.3TiO3 thin films

We have investigated the effects of Mn impurities on Ba0.7Sr0.3TiO3 thin films using x-ray photoemission spectroscopy. Mn acts as an electron acceptor, compensating for the charge density found in nominally undoped films. This causes a greatly increased depletion width in acceptor-doped films. We also present evidence that acceptor-doped films have an increased barrier to thermionic emission of electrons from Pt contacts into the dielectric. This may explain the decrease in leakage current observed in some acceptor-doped titanates.