S. E. Lofland

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta).

The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

Elastic and electronic properties of select M2AX phases

In this letter we report on the low-temperature specific heat of several M2AX phases: Ti2AlC, V2AlC, V2AsC, Nb2SnC, Ti2AlN, Hf2InC, Nb2AlC, and Cr2AlC. The Debye temperatures are quite high. The density of states at the Fermi level, N(EF) varies from ≈1.4 (eV formula unit)−1 to 6 (eV formula unit)−1. Ab initio calculations show that N(EF) is dictated by the transition metal d–d bands; the A-group element has little effect. We also measured the velocity of sound in V2AlC, V2AsC, Ti2AlC, and Ti2AlN. The average bulk modulus of these materials is over 100 GPa, with a high of ≈140 GPa for Ti2AlN. Our theoretical calculations correctly predict the trend in both the density of states and the bulk modulus, although there is some disagreement in the actual values.

Thermal expansion of select Mn+1AXn (M=earlytransitionmetal, A=Agroupelement, X=C or N) phases measured by high temperature x-ray diffraction and dilatometry

Herein we report on a systematic investigation of the thermal expansion of select Mn+1AXn phases. The bulk dilatometric thermal expansion coefficient αdil was measured in the 25–1200u2009°C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25–800u2009°C temperature range. The coefficient of thermal expansion for the a axis αa ranged between (2.9±0.1)×10−6u2009°C−1 (Nb2AsC) and (12.9±0.1)×10−6u2009°C−1 (Cr2GeC) while the coefficient for the c axis (αc) ranged between (6.4±0.2)×10−6u2009°C−1 (Ta2AlC) and (17.6±0.2)×10−6u2009°C−1 (Cr2GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of αc/αa belonging to Nb2AsC. The Gruneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c13 and we conclude that the anisotropy in thermal expansion is relate...

Tunable multiferroic properties in nanocomposite PbTiO3–CoFe2O4 epitaxial thin films

We report on the synthesis of PbTiO3–CoFe2O4 multiferroic nanocomposites and continuous tuning of their ferroelectric and magnetic properties as a function of the average composition on thin-film composition spreads. The highest dielectric constant and nonlinear dielectric signal was observed at (PbTiO3)85–(CoFe2O4)15, where robust magnetism was also observed. Transmission electron microscopy revealed a pancake-shaped epitaxial nanostructure of PbTiO3 on the order of 30 nm embedded in the matrix of CoFe2O4 at this composition. Composition dependent ferroics properties observed here indicate that there is considerable interdiffusion of cations into each other.

Multimode quantitative scanning microwave microscopy of in situ grown epitaxial Ba1−xSrxTiO3 composition spreads

We have performed variable-temperature multimode quantitative microwave microscopy of in situ epitaxial Ba1−xSrxTiO3 thin-film composition spreads fabricated on (100) LaA1O3 substrates. Dielectric properties were mapped as a function of continuously varying composition from BaTiO3 to SrTiO3. We have demonstrated nondestructive temperature-dependent dielectric characterization of local thin-film regions. Measurements are simultaneously taken at multiple resonant frequencies of the microscope cavity. The multimode measurements allow frequency dispersion studies. We observe strong composition-dependent dielectric relaxation in Ba1−xSrxTiO3 at microwave frequencies.

Electrical, thermal, and elastic properties of the MAX-phase Ti2SC

We report on the electronic, thermal, and elastic properties of the layered ternary, Ti2SC. Resistivity, Hall effect, and magnetoresistance were measured as a function of temperature between 2 and 300 K and at fields up to 9 T. The Hall coefficient is negative and roughly temperature independent. The transport results were analyzed within a two-band framework, with electrons as the dominant charge carrier. The room-temperature thermal conductivity (≈60u2002W/mu2009K) is the highest of any MAX phase measured to date, with a substantial phonon contribution. The specific heat was measured from 2 to 300 K, yielding a Debye temperature of 765 K and in agreement with the Debye temperature of 745 K found from ultrasonic time-of-flight measurements. Young’s, shear, and bulk moduli from the latter measurements were 290, 125, and 145 GPa, respectively. The calculated values of the lattice parameters (a=3.2051u2002A and c=11.2636u2002A), and Young’s, shear, and bulk moduli (329, 138, and 179 GPa, respectively), based on the results...

Structural, electrical transport, magnetization, and 1∕f noise studies in 200MeV Ag ion irradiated La0.7Ce0.3MnO3 thin films

The effect of 200MeV Ag ion irradiation on structural, electrical transport, magnetization, and low-frequency conduction noise properties of electron-doped La0.7Ce0.3MnO3 thin films have been investigated. The as-grown thin films show c-axis epitaxial structure along with a small amount of unreacted CeO2 phase. After the irradiation, at the lowest fluence both the magnetization and metal-insulator transition temperature increase. Further increase in fluence reduces the metal-insulator transition temperature and leads to larger resistivity; however, the unreacted phase of CeO2 disappears in the x-ray diffraction pattern. On the other hand, the normalized electrical noise is greatly enhanced even at the lowest nonzero fluence. Surprisingly the conducting noise in the irradiated samples is much higher in the metallic state than in the semiconducting one. The observed modifications in structural, electrical, magnetic, and noise properties of 200MeV Ag ion irradiated La0.7Ce0.3MnO3 thin films have been explain...

Co-doped La0.5Sr0.5TiO3−δ: Diluted magnetic oxide system with high Curie temperature

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La0.5Sr0.5TiO3−δ). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (<∼2%), but develops inhomogeneity at higher cobalt concentrations. The films range from being opaque metallic to transparent semiconducting depending on the oxygen pressure during growth and are yet ferromagnetic.

Search for ferromagnetism in undoped and cobalt-doped HfO2−δ

We report on the search for ferromagnetism in undoped and cobalt-doped high-k dielectric HfO2 films. Over a broad range of growth conditions, we do not observe ferromagnetism in undoped HfO2 films. On the other hand, we do observe room temperature ferromagnetism in dilutely Co-doped HfO2 films, but the origin of the same appears extrinsic (a Co rich surface layer) at least for the regime of growth conditions explored.

Magnetic imaging of perovskite thin films by ferromagnetic resonance microscopy—La0.7Sr0.3MnO3

We present ferromagnetic resonance scans of epitaxial thin films of La0.7Sr0.3MnO3. In most films, the resonance field indicates the presence of large stresses, the line is wide and both the field and width vary significantly with position. However, in the perpendicular geometry (field normal to the film plane) the best films show narrow lines (<20u200aOe) which are relatively independent of position, indicative of magnetic homogeneity. The experiments show that this technique can be used as a contactless, nondestructive diagnostic tool for assessing the magnetic quality of colossal magnetoresistance thin films.