Research and Publications - Department of Metallurgical and Materials Engineering

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Lattice Expansion in Nanocrystalline Niobium Thin Films
(2003-04-09) Banerjee, R.; Sperling, E. A.; Thompson, G. B.; Fraser, H. L.; Bose, S.; Ayyub, P.; University of Alabama Tuscaloosa
High-purity nanocrystalline niobium (Nb) thin films have been deposited using high-pressure magnetron sputter deposition. Increasing the pressure of the sputtering gas during deposition has systematically led to reduced crystallite sizes in these films. Based on x-ray and electron diffraction results, it is observed that the nanocrystalline Nb films exhibit a significantly large lattice expansion with reduction in crystallite size. There is however, no change in the bcc crystal structure on reduction in crystallite size to below 5 nm. The lattice expansion in nanocrystalline Nb has been simulated by employing a recently proposed model based on linear elasticity and by appropriately modifying it to incorporate a crystallite-size-dependent width of the grain boundary.
Tuning phase stability in nanocomposite multilayers
(American Institute of Physics, 2003-08-26) Thompson, GB; Banerjee, R; Fraser, HL; University System of Ohio; Ohio State University; University of Alabama Tuscaloosa
As thin-film layers in a multilayered stack are reduced in thickness, changes in phase stability can result within the individual layers. These changes in phase are expected to have a significant influence upon the functional properties of the nanostructured composite. The ability to engineer, or tune, phase stability at this nanometer length scale is of significant importance in order to maximize the functional properties of these materials. We report the prediction and experimental conformation of tuning the hcp to bcc phase stability in Ti for Ti/Nb multilayered nanocomposites. The prediction was based upon selective alloying of Ti with a bcc beta stabilizing element using a new form of a thermodynamic phase diagram for predicting phase stability in thin-film multilayers. (C) 2003 American Institute of Physics.
Predicting pseudomorphic phases in multilayers: Hexagonal-closed-packed Nb in Nb/Zr
(American Institute of Physics, 2003-12-17) Thompson, GB; Banerjee, R; Fraser, HL; University of Alabama Tuscaloosa; University System of Ohio; Ohio State University
As the dimensions of materials are reduced to the nanometer scale, changes in phase stability, referred to as pseudomorphism, are being reported. Such changes in phase stability are often serendipitously discovered in multilayered thin films. In this letter, we use a classical thermodynamic treatment to model and predict phase stability in Nb/Zr multilayers. An outcome of this letter is the development of a biphase stability diagram that represents the interrelationship of phase stability to volume fraction and length scale. Using this methodology, an hcp Nb phase stability field was empirically postulated and subsequently confirmed by x-ray and electron diffraction. The successful prediction of this phase, based upon classical thermodynamics quantities, suggests that other types of phase stabilities in other multilayers could be proposed using the biphase diagram. (C) 2004 American Institute of Physics.
Size Effect Ordering in [FePt]100-xCrx Nanoparticles
(2006-03-02) Thompson, G. B.; Srivastava, C.; Harrell, J. W.; Nikles, D. E.; University of Alabama Tuscaloosa
A series of [FePt]100−xCrx nanoparticles (x=5, 10, and 16at.%) was chemically synthesized by two different techniques. In one method, the simultaneous chemical reduction of FeCl2∙4H2O, Pt-acetylacetonate, and Cr-acetylacetonate was used with 2, 4 hexadecanediol as the reducing agent and phenyl ether as the solvent. The as-prepared particles had a mean size of 1.5nm. In the second method, the simultaneous chemical reduction of Pt-acetylacetonate and Cr-acetylacetonate and the thermal reduction of Fe(CO)5 were used with adamantanecarboxylic acid as the reducing agent and hexadecylamine as the solvent. These as-prepared particles were 3.5nm in size. X-ray diffraction confirmed that the Cr formed a solid solution within the A1 FePt phase for both processes. Upon annealing, the Cr hindered sintered grain growth of FePt nanoparticle arrays. Consequently, we were able to use Cr as a means to tune the ordering temperature as a function of the size effect in FePt nanoparticles. The presence of Cr in the ordered FePt reduced the magnetic coercivity of the transformed nanoparticles.
Sintering behavior of spin-coated FePt and FePtAu nanoparticles
(American Institute of Physics, 2006-04-19) Kang, SS; Jia, Z; Zoto, I; Reed, D; Nikles, DE; Harrell, JW; Thompson, G; Mankey, G; Krishnamurthy, VV; Porcar, L; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Oak Ridge National Laboratory; National Institute of Standards & Technology (NIST) - USA
FePt and [FePt](95)Au-5 nanoparticles with an average size of about 4 nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250 to 500 degrees C for 30 min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt](95)Au-5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3 nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0 nm. The ratio of c/a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss of positional order with increasing annealing temperature. The effect is more pronounced for FePtAu than for FePt. Dynamic coercivity measurements show that the FePtAu nanoparticles have both higher intrinsic coercivity and higher switching volume at the same annealing temperature. These results are consistent with previous studies that show that additive Au both lowers the chemical ordering temperature and promotes sintering. (C) 2006 American Institute of Physics.
In Situ Transmission Electron Microscopy of Ion Irradiated Fe-Pt Alloy Thin Films
(2006-12-21) Thompson, G. B.; Morgan, N. W.; Birtcher, R. C.; University of Alabama Tuscaloosa
We report the microstructural evolution during irradiation of FePt and FePt 25at.% thin films sputter deposited onto electron transparent silicon monoxide substrates. The films were studied in situ for 500keV Kr+ irradiation up to a fluence of 1015ions/cm2 or 4displacements∕atom (dpa). Upon irradiation to approximately 1dpa, the initial disconnected granular morphology became continuous. In particular, for FePt, accelerated grain growth was observed once the continuous morphology was achieved during ambient temperature irradiation. No atomistic (chemical) ordering from the as-deposited A1 phase into either the L10 FePt or L12 Fe3Pt phases was observed during ambient temperature irradiation. After irradiation, the specimens were then in situ annealed. The intermetallic ordering temperature, compared to that of an unirradiated film, was lowered by ≈200°C for FePt 25at.%. No decrease in the ordering temperature was observed for irradiated FePt. The rate of FePt grain growth during annealing was very similar for both irradiated and unirradiated films over the 25–650°C temperature range investigated.
Microstructures and Magnetic Alignment of L10 FePt Nanoparticles
(2007-05-09) Kang, Shishou; Shi, Shifan; Jia, Zhiyong; Thompson, G. B.; Nikles, David. E.; Harrell, J. W.; University of Alabama Tuscaloosa
Chemically ordered FePt nanoparticles were obtained by high temperature annealing a mixture of FePt particles with NaCl. After the NaCl was removed with de-ionized water, the transformed FePt nanoparticles were redispersed in cyclohexanone. X-ray diffraction patterns clearly show the L10 phase. Scherrer analysis indicates that the average particle size is about 8 nm, which is close to the transmission electron microscopy TEM statistical results. The coercivity ranges from 16 kOe to more than 34 kOe from room temperature down to 10 K. High resolution TEM images reveal that most of the FePt particles were fully transformed into the L10 phase, except for a small fraction of particles which were partially chemically ordered. Nano-energy dispersive spectroscopy measurements on the individual particles show that the composition of the fully transformed particles is close to 50/ 50, while the composition of the partially transformed particles is far from equiatomic. TEM images and electron diffraction patterns indicate c-axis alignment for a monolayer of L10 FePt particles formed by drying a dilute dispersion on copper grids under a magnetic field. For thick samples dried under a magnetic field, the degree of easy axis alignment is not as high as predicted due to strong interactions between particles.
Formation of FePt Nanoparticles by Organometallic Synthesis
(2007-05-23) Bagaria, H. G.; Johnson, D. T.; Srivastava, C.; Thompson, G. B.; Shamsuzzoha, M.; Nikles, D. E.; University of Alabama Tuscaloosa
Our interest in determining the mechanism of FePt nanoparticle formation has led to this study of the evolution of particle size and composition during synthesis. FePt nanoparticles were prepared by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl. During the course of the reaction, samples were removed and the particle structure, size, and composition were determined using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy–energy dispersive x-ray spectrometry. Early in the reaction the particles were Pt rich (greater than 95at.% Pt) and as the reaction proceeded the Fe content increased to the target of 50%. The particle diameter increased from 3.1to4.6nm during the reaction. Energy dispersive x-ray spectrometry measurements of individual particle compositions using a high resolution TEM showed a broad distribution of particle compositions with a standard deviation greater than 15% of the average composition.
Formation Mechanism and Composition Distribution of FePt Nanoparticles
(2007-11-27) Srivastava, Chandan; Balasubramanian, Jayendra; Turner, C. Heath; Wiest, John M.; Bagaria, Hitesh G.; Thompson, Gregory B.; University of Alabama Tuscaloosa
Self-assembled FePt nanoparticle arrays are candidate structures for ultrahigh density magnetic storage media. One of the factors limiting their application to this technology is particle-to-particle compositional variation. This variation will affect the A1 to L10 transformation as well as the magnetic properties of the nanoparticles. In the present study, an analysis is provided for the formation mechanism of these nanoparticles when synthesized by the superhydride reduction method. Additionally, a comparison is provided of the composition distributions of nanoparticles synthesized by the thermal decomposition of Fe(CO)5 and the reduction of FeCl2 by superhydride. The latter process produced a much narrower composition distribution. A thermodynamic analysis of the mechanism is described in terms of free energy perturbation Monte Carlo simulations.
Selective Area Synthesis of Magnesium Oxide Nanowires
(2007-11-28) Kim, G.; Martens, R. L.; Thompson, G. B.; Kim, B. C.; Gupta, A.; University of Alabama Tuscaloosa
Single crystalline magnesium oxide MgO nanowires exhibiting a square cross section have been grown on 001-oriented MgO and Si substrates using the vapor-liquid-solid growth mechanism. While the nanowires grow vertically aligned on MgO, they display random orientations on the silicon substrate. For growth on MgO substrates, the selective placement and density of the nanowires can be controlled by using electron beam lithography for prepatterning the gold catalyst layer. The nanowire samples have been characterized using field-emission scanning electron microscopy and transmission electron microscopy. The described process for selective placement of the nanowires is attractive for their use as templates for coaxial coatings and also for their manipulation for potential device fabrication.
Wetting of Al2O3 by molten aluminum: The influence of BaSO4 additions
(Hindawi, 2008) Aguilar-Santillan, Joaquin; University of Alabama Tuscaloosa
The effects of BaSO4 additions on the wetting of alumina by molten aluminum were studied by the sessile drop technique. To study the effect of BaSO4 decomposition (1100-1150 degrees C), the additions were treated at two temperatures 700 degrees C (973 K) and 1450 degrees C (1723 K), respectively. BaSO4 additions at low and high temperatures did not improve the nonwetting character of these compositions. However, at higher firing temperature, the formation of BA(6) (BaO center dot 6Al(2)O(3)) has a nonwetting trend with increasing its content. To address the BA(6) specifically a pure BaO center dot 6Al(2)O(3) was produced and tested. It was more nonwetting than the pure alumina. After the analysis of the contact angles for the BaSO4 and the BA(6) (BaO center dot 6Al(2)O(3)), it was concluded that these additions to alumina do not inhibit wetting by molten aluminum. In fact, at the addition levels common for refractories, the wetting tendency of molten aluminum is enhanced. Alternative explanations for the effectiveness of BaSO4 additions to alumina refractories are discussed. Copyright (C) 2008 Joaquin Aguilar-Santillan.
Compositional Evolution During the Synthesis of FePt Nanoparticles
(2008-09-25) Thompson, G. B.; Srivastava, Chandan; Nikles, David E.; University of Alabama Tuscaloosa
A series of FePt nanoparticles was synthesized by the thermal decomposition of iron pentacarbonyl and reduction in platinum acetylacetonate in phenyl ether solvent. A range of precursor molar ratios of 2, 1.5, and 1 between iron pentacarbonyl and platinum acetylacetonate was studied. After 30 min of reflux, the synthesis method produced a wide distribution in composition and size for the nanoparticles. Given 200 min of reflux, it was observed that the particle-to-particle composition and size narrowed, and the atomic ratio of Fe to Pt, for the majority of nanoparticles, approached the initial precursor molar ratios except for the molar ratio of 1. It is speculated that the compositional variability may be a result of the slow kinetics of iron pentacarbonyl’s decomposition in the reaction.
Tailoring Nucleation and Growth Conditions for Narrow Compositional Distributions in Colloidal Synthesized FePt Nanoparticles
(2008-11-21) Thompson, Gregory B.; Srivastava, Chandan; Nikles, David E.; University of Alabama Tuscaloosa
To eliminate compositional and size variabilities between individual binary nanoparticles, it is essential to control the mechanistic steps involved in nanoparticle synthesis. A common method for synthesizing FePt nanoparticles involves the simultaneous decomposition and reduction in iron and platinum precursors, respectively. This simultaneous nucleation and growth method yields wide composition and size distributions. This paper describes and experimentally validates a methodology needed to tighten composition and size distributions for this process. By engineering the surfactant chemistry with tertiary phosphines to tightly bind the iron atoms in the iron precursor, uniform platinum rich seeds form during the initial stages of the synthesis. A thermodynamically preferred heterogeneous nucleation of iron atoms into these uniform platinum seeds in the subsequent stages produces a final dispersion with uniform particle-to-particle compositions. The paper addresses the understanding for optimizing the nucleation and growth sequences for compositional control in FePt nanoparticles.
L10 Ordering of FePt Thin Films using Sub-10 ms Laser Pulses
(2009-03-12) Thompson, G. B.; Inaba, Y.; Kang, S.; Izatt, J. R.; Harrell, J. W.; Kubota, Y.; Klemmer, T. J.; University of Alabama Tuscaloosa
The structural and magnetic properties of 10 nm FePt thin films annealed using a 1064 nm wavelength laser with 10, 7.5, 5.0, and 2.5 ms pulses have been examined. The A1 to L10 phase transformation was confirmed by x-ray diffraction (XRD). The maximum order parameter of 0.53 and coercivity of 5.36 kOe can be obtained with 10 ms pulse width laser annealing at a laser energy fluence of 10 J/cm2. The order parameter of the furnace annealed samples was approximately 1.0 suggesting that 10 ms is insufficient to obtain a fully ordered phase. The laser annealed grain size, as measured by in-plane XRD analysis, is 24 % smaller than that of furnace annealed sample for an equivalent order parameter demonstrating the merit of short time annealing.
Magnetic, ferroelectric, and dielectric properties of Bi(Sc0.5Fe0.5)O-3-PbTiO3 thin films
(American Institute of Physics, 2009-04-01) Yan, F.; Sterianou, I.; Miao, S.; Reaney, I. M.; Lai, M. O.; Lu, L.; National University of Singapore; University of Sheffield; University of Alabama Tuscaloosa
Bi(Sc0.5Fe0.5)O-3-PbTiO3 (BSF-PT) thin films with a composition in the vicinity of a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases have been grown on LaNiO3/SiO2/Si substrates at 550 degrees C and 150 mTorr by pulsed laser deposition. The dielectric properties of the film were enhanced due to high crystallinity, low porosity, and the vicinity of the MPB. A saturated ferroelectric hysteresis loop was obtained with 2P(r)=76 mu C/cm, and the leakage current was minimized by cooling the samples, postdeposition in O-2. Magnetic measurements revealed that BSF-PT thin films were antiferromagnetic confirming their anticipated multiferroic nature. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3093691]
The FePt L10 Phase Transformation in Thin Films using Multiple Laser Pulsing
(2010-03-04) Inaba, Yuki; Thompson, Gregory B.; Harrell, J. W.; Klemmer, Tim; Kubota, Yukiko; University of Alabama Tuscaloosa
A series of ≈12 nm thick FePt thin films deposited onto glass substrates have been annealed with multiple 1064 nm wavelength laser pulses. The fluence was varied using pulse widths of 10.0, 5.0, and 2.5 ms. The peak temperature for each individual pulse was kept near 700 °C. The A1 to L10 phase transformation was confirmed by x-ray diffraction. A single pulse was not sufficient to obtain a fully ordered state. A maximum order parameter of 0.89 and coercivity of 10.6 kOe was obtained after 5×10 ms pulses. This particular annealed film showed the greatest amount of grain growth with a mean size of 55.1 nm. This grain size is 20% smaller than that of a furnace annealed sample which was annealed for 60 s and yielded an approximately equivalent order parameter. Similar order parameters, grain sizes, and coercivity values were observed for films that had equivalent total annealing times regardless of pulse widths.
Compositional Dependent Thin Film Stress States
(2010-08-23) Thompson, G. B.; Fu, B.; University of Alabama Tuscaloosa
This paper addresses in situ stress evolution of two-component FexPt1−x, where x spanned 0 to 1, alloy thin films. The stresses of the high-temperature, quenched-in, solid solution phase was determined by in situ wafer curvature measurements during ambient temperature growth. The measured stresses were shown to be compositional dependent and spanned both compressive and tensile stress states. Under specific growth conditions, a “zero-stress” state could be achieved. The alloy stress states did not show any significant stress recovery upon ceasing the deposition, i.e. the stress state during growth was retained in the film. X-ray diffraction, transmission electron microscopy, and atom probe tomography were used to characterize the microstructures of each thin film. The evolution of the stress state with composition is described in terms of a chemical potential term for preferential segregation of one species in the alloy to the grain boundaries.
Time-Temperature-Transformation Measurements of FePt Thin Films in the Millisecond Regime Using Pulse Laser Processing
(2010-11-19) Inaba, Yuki; Zana, Iulica; Swartz, Caleb; Kubota, Yukiko; Klemmer, Tim; Harrell, J. W.; Thompson, Gregory B.; University of Alabama Tuscaloosa
A section of the time-temperature-transformation TTT curve has been measured in the millisecond regime to describe the A1 to L10 transformation of 10 nm FePt thin films. Short time annealing was accomplished using a pulsed neodymium-doped yttrium aluminum garnet laser operating at a wavelength of 1064 nm. The temperature-time profile of the films was measured using an optical pyrometer and a platinum thin film resistor and it was numerically modeled. Effective thermal pulse widths were determined from the time dependence of the atomic diffusion coefficient calculated from the measured temperature profile. The measured TTT diagram involving average order parameter is consistent with theoretical predictions of TTT diagrams involving ordered volume fraction and shows that partial ordering can be obtained with a single effective thermal pulse as short as 1.1 ms.
Ta clustering and Microstructural Evolution in the A1 To L10Fe52PtX(Ta1−X) Phase Transformation
(2010-11-29) Thompson, Gregory B.; Means, D.; Wang, B.; University of Alabama Tuscaloosa
A series of Fe52Pt48,Fe52.3Pt46.3Ta1.4 and Fe52Pt40.7Ta7.3 thin films were sputter deposited and subsequently annealed at 550 and 750 °C for 30 min. The as-deposited films, which adopted the A1 phase, had a change from a predominate (111) fiber texture to (200) with the Ta additions. This has been explained in terms of the competition between the surface energy and strain energy. Annealing at 550 °C facilitated the L10 order in Fe52Pt48 and Fe52.3Pt46.3Ta1.4. Upon annealing at 750 °C, all three composition films phase transformed into L10. Atom probe tomography revealed nanoscale clustering in the annealed Ta containing films. The formation of these clusters appeared to be a necessary initial step to allow the L10 ordering reaction to occur but clustering in of itself is not sufficient for order. For the Fe52Pt40.7Ta7.3 film, the Ta must be depleted within the matrix to a sufficient level to allow the binary Fe–Pt to order. For the Fe52.3Pt46.3Ta1.4 film, these clusters were qualitatively observed within the grains at 550 °C and in the grain boundaries at 750 °C. The Fe52Pt40.7Ta7.3 film had clusters within and near grain boundaries. The clustering in the grain boundaries deterred grain growth through a Zener-based pinning mechanism.
Effect of bottom electrodes on nanoscale switching characteristics and piezoelectric response in polycrystalline BiFeO3 thin films
(American Institute of Physics, 2011-10-17) Yan, F.; Zhu, T. J.; Lai, M. O.; Lu, L.; National University of Singapore; Zhejiang University; University of Alabama Tuscaloosa
We have investigated the nanoscale switching characteristics and piezoelectric response based on polycrystalline BiFeO3 (BFO) thin films with different orientations deposited on different oxide bottom electrodes. The BFO film deposited on the LaNiO3 (LNO)-coated Si substrate shows a (001) preferred orientation and higher ferroelectric properties, while the BFO film grown on the SrRuO3 (SRO) buffered Si substrate shows a random orientation. The domain structures have been determined via piezoresponse force microscopy (PFM) for both films, predicting that the BFO film with the LNO bottom electrode has a larger piezoelectricity property corresponding to the ferroelastic domain. Through local switching spectroscopy measurements, the evidence of ferroelectric switching and the origin of the enhanced piezoresponse properties have been provided. A greatly improved piezoelectric response has been demonstrated using PFM that is 66.8 pm V-1 for the BFO with a SRO bottom electrode, while we obtain a value of 348.2 pm V-1 for the BFO with a LNO bottom electrode due to the increased density of the polarization vectors along the external electrical field. (C) 2011 American Institute of Physics. [doi:10.1063/1.3651383]

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