The structural response of pulsed laser deposited La0.7Ca0.3Mn03−δ films to low-level 10 MeV I+ irradiation is correlated with magnetic and electronic properties. A series of annealed pulsed laser deposited films were subjected to ion irradiation at doses of 0.5–2.0 × 1013 10 MeV iodine ions/cm2. X-ray diffraction measurements show no measurable increase in the full width at half maximum values of major reflections before and after irradiation. Extended x-ray absorption fine structure analyses (EXAFS), employing both a model independent fitting using expanded cumulants as well as a nonlinear least squares multiple scattering model based on FEFF generated theoretical EXAFS data, were employed to measure local properties relative to the Mn ions. Increasing irradiation doses lead to an unambiguous evolution of MnO6 octahedra from highly symmetric (i.e., undistorted) to a clear bimodal structure in which the MnO6 octahedra are measured to undergo a c-axis stretching bearing resemblance to a Jahn-Teller distortion. Concomitantly, the metal to insulator transition temperature decreases more than 270 K and the magnetoresistance increases from 102% to 106%.

Recent measurements of the thermoelectric transport properties of a series of the half- Heusler compound ZrNiSn are presented. These materials are known to be bandgap intermetallic compounds with relatively large Seebeck coefficients and semimetallic to semiconducting transport properties. This makes them attractive for study as potential candidates for thermoelectric applications. In this study, trends in the thermoelectric power, electrical conductivity and thermal conductivity are examined as a function of chemical substitution on the various fcc sub-lattices that comprise the half-Heusler crystal structure. These results suggest that the lattice contribution to the thermal conductivity may be reduced by increasing the phonon scattering via chemical substitution. The effects of these substitutions on the overall power factor and figure-of-merit will also be discussed.

MnZn ferrites have been produced via the high-energy ball milling of binary oxide precursors. The milled ferrites have a nonequilibrium cation site distribution, with an unusually high population of Zn cations on the octahedral sites. The particle size distribution drops precipitously with milling time from 60±1 to ∼14±1 nm at 10 h, but increases to 18.5±1 nm after long durations (20–40 h) concurrent with the formation of nearly pure ferrite. A 1 h anneal at 673 K facilitates a redistribution of cations to their near equilibrium sites. This processing approach circumvents the need for deleterious high-temperature heat treatments that often lead to nonstoichiometries in the resulting ferrites.

The structure and magnetic properties of sol-gel-synthesized, nanocrystalline K δ MnO 2 were investigated. The nanoparticles were determined by x-ray diffraction and high-resolution transmission electron microscopy to be single-crystal rods of the cryptomelane phase of MnO 2 , with a typical particle size of 6 nm × 20 nm. The field and temperature dependence of the magnetization indicates superparamagnetic behavior, with a blocking temperature of 15K. The dependence of the magnetic properties on particle size, surface layers and mixed valency is discussed.

Multilayer samples and structures composed of NiO(t)/Co(2.5 nm)/NiO(t)/Py(2.5 nm)/NiO(t) with t=6.6 and 13.2 nm exhibit ferromagnetic interlayer coupling below 400 K. A crossover is observed between the behavior near 300 K and at 10 K in ferromagnetic(F)/antiferromagnetic(AF) structures. Effects observed near 300 K, but not at 10 K, are an enhancement of Hc and the M–H loop squareness in F/AF multilayers as one increases the number of layers. A possible explanation for the crossover is given based on the domain wall thickness becoming less than the AF film thickness.

Magnetization and ferromagnetic resonance measurements provide evidence of ferromagnetic coupling between ferromagnetic (F) layers separated by an intervening layer of antiferromagnetic (AF) NiO of thickness, t(AF), where 7 nm < t(AF)< 13 nm. Near 300 K, due to this coupling, the coercivity H-c and M-H loop squareness in F/AF multilayers increases as one increases the number of layers. The increased H-c and squareness have potential application in thin film magnets. (C) 1999 American Institute of Physics. [S0003-6951(99)03225-8].

The structure and magnetic properties of sol-gel-synthesized, nanocrystalline KδMnO2 were investigated. The nanoparticles were determined by x-ray diffraction and high-resolution transmission electron microscopy to be single-crystal rods of the cryptomelane phase of MnO2, with a typical particle size of 6 nm × 20 nm. The field and temperature dependence of the magnetization indicates superparamagnetic behavior, with a blocking temperature of 15K. The dependence of the magnetic properties on particle size, surface layers and mixed valency is discussed.

A La0.67Ca0.33MnO3−δ alloy was produced via a low-temperature solid state reaction in a high-energy ball mill. The milled alloy is found to exhibit the paramagnetic-to-ferromagnetic (P–F) transition at 150 K. However, the widely reported insulator-to-metal (I–M) transition that usually accompanies the P–F transition is totally suppressed. Anneals at 500–1000 °C in air (t=8 h) are found to lower the resistivity but not restore the I–M transition. The suppression of the I–M transition is attributed to anion defects in the structure that have not been annihilated during heat treatments. Extended x-ray absorption fine structure is employed to track the evolution of the atomic structure around the Mn cations at various stages of processing.

The Curie temperature and x-ray photoemission spectra of thin films of La0.67(CaxSr1−x)0.33MnO3 have been studied as a function of the Ca/Sr ratio. The films were grown by off-axis cosputtering from individual targets of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) onto (100) oriented NdGaO3 substrates. The films grow with a (100) orientation, with no other orientations observed by x-ray diffraction. For the alloy mixtures, the Curie temperature, TC, varies slowly as the Ca/Sr is decreased, remaining ≈300 K, while for the LCMO and LSMO films TC is 260 and 330 K, respectively. The Mn-O valence structure is composed of two dominant peaks, whose positions undergo a change as the Ca fraction is decreased. The core lines behave as linear combinations of lines from pure LCMO and LSMO.

Single crystals of Ba6Fe45Ti17O106and BaFe11Ti3O23were obtained as major and minor coproducts, respectively, by slow-cooling an off-stoichiometric BaO:Fe2O3:TiO2melt. The former compound exhibits variable stoichiometry, Ba6Fe48−xTi14−xO106, with the Fe:Ti ratio dependent upon the partial pressure of oxygen. The value ofxcorresponds to the equivalents of reduction that occur to maintain electroneutrality as the Ti-content increases. When prepared in air, this phase occurs atx=3 with the stoichiometry Ba6Fe45Ti17O106, while in 100% oxygen thex-value approaches zero with the resulting stoichiometry Ba6Fe48Ti14O106(all Fe3+and Ti4+). The structures of Ba6Fe45Ti17O106and BaFe11Ti3O23were solved using single-crystal X-ray diffraction methods. Ba6Fe45Ti17O106was prepared in polycrystalline form, and further structural details, including accurate Fe/Ti occupancy factors, were determined by a combined refinement using neutron and synchrotron powder diffraction data. (Ba6Fe45Ti17O106: Space groupC2/m(No. 12);a=19.390(1) Å,b=20.260(1) Å,c=10.076(1) Å,β=105.27(1)°;V=3818.5(3)Å3;Z=2;ρcalc=5.08 g/cm3. Ba6Fe11Ti3O23= Space groupC2/c (No. 15);a=19.561(1) Å,b=8.6614(7) Å,c=10.120(1) Å,β=105.62(1)°;V=1651.1(3) Å3;Z=4;ρcalc=5.08 g/cm3.) Both compounds adopt eight-layer close-packed structures built from alternatingccpandhcp[O, (Ba, O)] layers stacked along thea-direction with a (ch)4repeat sequence. Both structures feature octahedral sites occupied by a mixture of Fe and Ti as well as tetrahedral sites occupied by Fe3+the structural formulas areXIIBa6IVFe6VI(Fe39Ti17)O106andXIIBaIVFe2VI(Fe9Ti3)O23. Both compounds are partially reduced; the former contains 3 moles of Fe2+(or Ti3+) per formula unit, and the latter contains 1mole. The formation of Fe2+is considered more likely than Ti3+, but could not be experimentally confirmed. BaFe11Ti3O23is apparently metastable in air when cooled from above the solidus and could not be prepared as a polycrystalline sample. Indexed experimental X-ray powder diffraction data for Ba6Fe45Ti17O106are given. Polycrystalline samples of this compound were used to measure its magnetic and electrical properties. The magnetic behavior of Ba6Fe45Ti17O106above room temperature up to 1073 K was found to obey the Curie–Weiss law, which indicated a small effective magnetic moment (34μBper mole Ba6Fe45Ti17O106) and a large negative temperature intercept (−806 K). Electrical resistivity measurements between room temperature and 120 K revealed nonmetallic behavior with an activation energy on the order of 0.17eV. At 347 MHz under ambient conditions, Ba6Fe45Ti17O106exhibited a relative permittivity of 24 and a dielectric loss tangent of 0.10.

We have grown thin films of (100) oriented La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} on (100) NdGaO_{3} substrates by off-axis sputtering. We have looked at the changes in the resistivity and magnetoresistance of the samples as the Ca/Sr ratio was varied. We find that as the calcium fraction is decreased, the lattice match to the substrate decreases, and the films become more disordered, as observed in transport measurements and the variation in Curie and peak resistance temperatures. We find a correlation between the temperature independent and T^2 terms to the low temperature resistivity. The room temperature magnetoresistance exhibits a maximum as the peak temperature is increased by the substitution of Sr for Ca, and a change in the field dependence to the resistivity at room temperature is observed.

We have produced MnZn ferrites via high-energy ball milling (HEBM) of elemental oxides MnO, ZnO, and Fe2O3. X-ray diffraction (XRD) indicates a pure phase spinel forms after 21 h of HEBM. Extended x-ray absorption fine structure (EXAFS) analysis shows a nonequilibrium cation distribution, with an unusually high population of Zn cations on the octahedral sublattice. We then used EXAFS modeling to study cation site occupancy in an equilibrium MnZn-ferrite standard subjected to HEBM. We found that HEBM produces an increased preference for octahedral-site occupation among all cations for milling durations up to 300 min. After an initial improvement in magnetic properties, the magnetization diminishes steadily throughout this interval. With further increases in milling duration these structural and magnetic trends reverse, possibly due to annealing effects evidenced by XRD.

The effects of disorder on the transport and magnetic properties of pulsed laser deposited La{sub 0.7}Ca{sub 0.3}MnO{sub 3} thin films were studied. Ion irradiation with 10 MeV I and 6 MeV Si ions was used to produce controlled levels of defects ranging from 0.006 to 0.024 displacements per atom. The peak resistance temperature of the I-irradiated films decreased from 264 K for the undamaged film to 0 K for the 0.016 dpa film. The magnetic ordering temperature decreased from 270 K (undamaged) to 130 K (0.011 dpa), and remained nearly constant at 130 K for higher damage levels. This demonstrates a decoupling of the magnetic and metal-insulator transitions. A characteristic relationship between the peak resistance temperature and activation energy of resistance with the form T{sub p} = 285 (1-{Delta}{sub {rho}}/115){sup 0.13}, was observed for all films. This characteristic relationship indicates that the range of resistivity and magnetoresistivity values for observed La{sub 0.7}Ca{sub 0.3}MnO{sub 3} can be explained in terms of disorder-limited polaron hopping.