Research and Publications - Department of Metallurgical and Materials Engineering

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Magnetic field control of charge excitations in CoFe2O4
(American Institute of Physics, 2018) Holinsworth, Brian S.; Harms, Nathan C.; Fan, Shiyu; Mazumdar, Dipanjan; Gupta, Arun; McGill, Stephen A.; Musfeldt, Janice L.; University of Tennessee Knoxville; University of Alabama Tuscaloosa; Florida State University; Southern Illinois University
We combine magnetic circular dichroism and photoconductivity with prior optical absorption and first principles calculations to unravel spin-charge interactions in the high Curie temperature magnet CoFe2O4 . In addition to revising the bandgap hierarchy, we reveal a broad set of charge transfer excitations in the spin down channel which are sensitive to the metamagnetic transition involving the spin state on Co centers. We also show photoconductivity that depends on an applied magnetic field. These findings open the door for the creation and control of spin-polarized electronic excitations from the minority channel charge transfer in spinel ferrites and other earth-abundant materials. (C) 2018 Author(s).
A Place-based Assessment of Flash Flood Hazard and Vulnerability in the Contiguous United States
(Nature Portfolio, 2020) Khajehei, Sepideh; Ahmadalipour, Ali; Shao, Wanyun; Moradkhani, Hamid; University of Alabama Tuscaloosa
Flash flood is among the most catastrophic natural hazards which causes disruption in the environment and societies. Flash flood is mainly initiated by intense rainfall, and due to its rapid onset (within six hours of rainfall), taking action for effective response is challenging. Building resilience to flash floods require understanding of the socio-economic characteristics of the societies and their vulnerability to these extreme events. This study provides a comprehensive assessment of socio-economic vulnerability to flash floods and investigates the main characteristics of flash flood hazard, i.e. frequency, duration, severity, and magnitude. A socio-economic vulnerability index is developed at the county level across the Contiguous United States (CONUS). For this purpose, an ensemble of social and economic variables from the US Census and the Bureau of Economic Analysis were analyzed. Then, the coincidence of socioeconomic vulnerability and flash flood hazard were investigated to identify the critical and non-critical regions. Results show that the southwest U.S. experienced severe flash flooding with high magnitude, whereas the Northern Great Plains experience lower severity and frequency. Critical counties (high-vulnerable-hotspot) are mostly located in the southern and southwestern parts of the U.S. The majority of counties in the Northern Great Plains indicate a non-critical status.
Scaling of contact networks for epidemic spreading in urban transit systems
(Nature Portfolio, 2021) Qian, Xinwu; Sun, Lijun; Ukkusuri, Satish V.; University of Alabama Tuscaloosa; McGill University; Purdue University; Purdue University West Lafayette Campus
Improved mobility not only contributes to more intensive human activities but also facilitates the spread of communicable disease, thus constituting a major threat to billions of urban commuters. In this study, we present a multi-city investigation of communicable diseases percolating among metro travelers. We use smart card data from three megacities in China to construct individual-level contact networks, based on which the spread of disease is modeled and studied. We observe that, though differing in urban forms, network layouts, and mobility patterns, the metro systems of the three cities share similar contact network structures. This motivates us to develop a universal generation model that captures the distributions of the number of contacts as well as the contact duration among individual travelers. This model explains how the structural properties of the metro contact network are associated with the risk level of communicable diseases. Our results highlight the vulnerability of urban mass transit systems during disease outbreaks and suggest important planning and operation strategies for mitigating the risk of communicable diseases.
In Situ Ag-MOF Growth on Pre-Grafted Zwitterions Imparts Outstanding Antifouling Properties to Forward Osmosis Membranes
(American Chemical Society, 2020) Pejman, Mehdi; Firouzjaei, Mostafa Dadashi; Aktij, Sadegh Aghapour; Das, Parnab; Zolghadr, Ehsan; Jafarian, Hesam; Shamsabadi, Ahmad Arabi; Elliott, Mark; Sadrzadeh, Mohtada; Sangermano, Marco; Rahimpour, Ahmad; Tiraferri, Alberto; University of Alabama Tuscaloosa; Polytechnic University of Turin; Babol Noshirvani University of Technology; University of Alberta; Amirkabir University of Technology; University of Pennsylvania
In this study, a polyamide forward osmosis membrane was functionalized with zwitterions followed by the in situ growth of metal-organic frameworks with silver as a metal core (Ag-MOFs) to improve its antibacterial and antifouling activity. First, 3-bromopropionic acid was grafted onto the membrane surface after its activation with NN-diethylethylenediamine. Then, the in situ growth of Ag-MOFs was achieved by a simple membrane immersion sequentially in a silver nitrate solution and in a ligand solution (2-methylimidazole), exploiting the underlying zwitterions as binding sites for the metal. The successful membrane functionalization and the enhanced surface wettability were verified through an array of characterization techniques. When evaluated in forward osmosis tests, the modified membranes exhibited high performance and improved permeability compared to pristine membranes. Static antibacterial experiments, evaluated by confocal microscopy and colony-forming unit plate count, resulted in a 77% increase in the bacterial inhibition rate due to the activity of the Ag-MOFs. Microscopy micrographs of the Escherichia coli bacteria suggested the deterioration of the biological cells. The antifouling properties of the functionalized membranes translated into a significantly lower flux decline in forward osmosis filtrations. These modified surfaces displayed negligible depletion of silver ions over 30 days, confirming the stable immobilization of Ag-MOFs on their surface.
Ultrasensitive electrochemical biosensors based on zinc sulfide/graphene hybrid for rapid detection of SARS-CoV-2
(Springer Nature, 2023) Sarwar, Shatila; Lin, Mao-Chia; Amezaga, Carolina; Wei, Zhen; Iyayi, Etinosa; Polk, Haseena; Wang, Ruigang; Wang, Honghe; Zhang, Xinyu; Auburn University; University of Alabama Tuscaloosa; Tuskegee University
The coronavirus disease 2019 (COVID-19) is a highly contagious and fatal disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In general, the diagnostic tests for COVID-19 are based on the detection of nucleic acid, antibodies, and protein. Among different analytes, the gold standard of the COVID-19 test is the viral nucleic acid detection performed by the quantitative reverse transcription polymerase chain reaction (qRT-PCR) method. However, the gold standard test is time-consuming and requires expensive instrumentation, as well as trained personnel. Herein, we report an ultrasensitive electrochemical biosensor based on zinc sulfide/graphene (ZnS/graphene) nanocomposite for rapid and direct nucleic acid detection of SARS-CoV-2. We demonstrated a simple one-step route for manufacturing ZnS/graphene by employing an ultrafast (90 s) microwave-based non-equilibrium heating approach. The biosensor assay involves the hybridization of target DNA or RNA samples with probes that are immersed into a redox active electrolyte, which are detectable by electrochemical measurements. In this study, we have performed the tests for synthetic DNA samples and, SARS-CoV-2 standard samples. Experimental results revealed that the proposed biosensor could detect low concentrations of all different SARS-CoV-2 samples, using such as S, ORF 1a, and ORF 1b gene sequences as targets. This microwave-synthesized ZnS/graphene-based biosensor could be reliably used as an on-site, real-time, and rapid diagnostic test for COVID-19.
Flexible and High Performance Supercapacitors Based on NiCo(2)O(4)for Wide Temperature Range Applications
(Nature Portfolio, 2015) Gupta, Ram K.; Candler, John; Palchoudhury, Soubantika; Ramasamy, Karthik; Gupta, Bipin Kumar; Pittsburg State University; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Los Alamos National Laboratory; Council of Scientific & Industrial Research (CSIR) - India; CSIR - National Physical Laboratory (NPL)
Binder free nanostructured NiCo2O4 were grown using a facile hydrothermal technique. X-ray diffraction patterns confirmed the phase purity of NiCo2O4. The surface morphology and microstructure of the NiCo2O4 analyzed by scanning electron microscopy (SEM) showed flower-like morphology composed of needle-like structures. The potential application of binder free NiCo2O4 as an electrode for supercapacitor devices was investigated using electrochemical methods. The cyclic voltammograms of NiCo2O4 electrode using alkaline aqueous electrolytes showed the presence of redox peaks suggesting pseudocapacitance behavior. Quasi-solid state supercapacitor device fabricated by sandwiching two NiCo2O4 electrodes and separating them by ion transporting layer. The performance of the device was tested using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The device showed excellent flexibility and cyclic stability. The temperature dependent charge storage capacity was measured for their variable temperature applications. Specific capacitance of the device was enhanced by similar to 150% on raising the temperature from 20 to 60 degrees C. Hence, the results suggest that NiCo2O4 grown under these conditions could be a suitable material for high performance supercapacitor devices that can be operated at variable temperatures.
Strain induced anisotropy in liquid phase epitaxy grown nickel ferrite on magnesium gallate substrates
(Nature Portfolio, 2022) Liu, Ying; Zhou, Peng; Regmi, Sudhir; Bidthanapally, Rao; Popov, Maksym; Zhang, Jitao; Zhang, Wei; Page, Michael R.; Zhang, Tianjin; Gupta, Arunava; Srinivasan, Gopalan; Oakland University; Hubei University; University of Alabama Tuscaloosa; Ministry of Education & Science of Ukraine; Taras Shevchenko National University Kiev; Zhengzhou University of Light Industry
This work focuses on the nature of magnetic anisotropy in 2.5-16 micron thick films of nickel ferrite (NFO) grown by liquid phase epitaxy (LPE). The technique, ideal for rapid growth of epitaxial oxide films, was utilized for films on (100) and (110) substrates of magnesium gallate (MGO). The motivation was to investigate the dependence of the growth induced anisotropy field on film thickness since submicron films of NFO were reported to show a very high anisotropy. The films grown at 850-875 C and subsequently annealed at 1000 C were found to be epitaxial, with the out-of-plane lattice constant showing unanticipated decrease with increasing film thickness and the estimated in-plane lattice constant increasing with the film thickness. The uniaxial anisotropy field H-sigma, estimated from X-ray diffraction data, ranged from 2.8-7.7 kOe with the films on (100) MGO having a higher H-sigma value than for the films on (110) MGO. Ferromagnetic resonance (FMR) measurements for in-plane and out-of-plane static magnetic field were utilized to determine both the magnetocrystalline the anisotropy field H-4 and the uniaxial anisotropy field H-a. Values of H-4 range from -0.24 to -0.86 kOe. The uniaxial anisotropy field H-a was an order of magnitude smaller than H-sigma and it decreased with increasing film thickness for NFO films on (100) MGO, but H-a increased with film thickness for films on (110) MGO substrates. These observations indicate that the origin of the induced anisotropy could be attributed to several factors including (i) strain due to mismatch in the film-substrate lattice constants, (ii) possible variations in the bond lengths and bond angles in NFO during the growth process, and (iii) the strain arising from mismatch in the thermal expansion coefficients of the film and the substrate due to the high growth and annealing temperatures involved in the LPE technique. The LPE films of NFO on MGO substrates studied in this work are of interest for use in high frequency devices.
Carbide Nanoparticle Dispersion Techniques for Metal Powder Metallurgy
(MDPI, 2021) Rocky, Bahrum Prang; Weinberger, Christopher R.; Daniewicz, Steven R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Colorado State University
Nanoparticles (NP) embedded into a matrix material have been shown to improve mechanical properties such as strength, hardness, and wear-resistance. However, the tendency of NPs to agglomerate in the powder mixing process is a major concern. This study investigates five different mechanochemical processing (MCP) routes to mitigate agglomeration to achieve a uniform dispersion of ZrC NPs in an Fe-based metal matrix composite. Our results suggest that MCP with only process controlling agents is ineffective in avoiding aggregation of these NPs. Instead, the uniformity of the carbide NP dispersion is achieved by pre-dispersing the NPs under ultrasonication using suitable surfactants followed by mechanically mixing of the NPs with iron powders in an alcohol solvent which is then dried. High-energy MCP is then used to embed the NPs within the powders. These collective steps resulted in a uniform dispersion of ZrC in the sintered (consolidated) Fe sample.
Composition-dependent apparent activation-energy and sluggish grain-growth in high entropy alloys
(Taylor & Francis, 2019) Gwalani, B.; Salloom, R.; Alam, T.; Valentin, S. G.; Zhou, X.; Thompson, G.; Srinivasan, S. G.; Banerjee, R.; University of North Texas System; University of North Texas Denton; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Pacific Northwest National Laboratory
Experimental results reveal that the apparent activation-energy for grain-growth in an fcc-based AlxCoCrFeNi high entropy alloy (HEA) system increases from 179 to 486kJ/mol when the Al content increases from x=0.1 to 0.3. These unexpectedly high apparent activation-energy values can be potentially attributed to solute clustering within the fcc solid-solution phase that develops with increasing Al content in this HEA. Detailed microstructural analysis using atom-probe tomography and density functional theory (DFT) calculations strongly indicate the presence of such nanoscale clusters. This phenomenon can change grain-growth from a classical solute-drag regime to a much more sluggish cluster-drag based mechanism in these HEAs. [GRAPHICS] IMPACT STATEMENTFirst report on a composition dependent change in apparent activation-energy for grain-growth in high entropy alloys. A novel cluster drag effect inhibiting grain-growth kinetics is suggested.
A computational investigation into the microstructures and stability of the zeta phase in transition metal carbides and nitrides
(Taylor & Francis, 2018) Weinberger, Christopher R.; Yu, Hang; Wang, Billie; Thompson, Gregory B.; Colorado State University; Drexel University; University of Alabama Tuscaloosa
A high-volume fraction of the zeta phase in multiphase group VB transition metal tantalum carbides has been shown to dramatically increase fracture toughness. This has been attributed to its unique nanoscale lath-based microstructure. However, what governs the microstructure and how it forms is still not well understood. In this paper, we propose a precipitation model for the formation of these phases and demonstrate that the anisotropic surface energies govern the observed zeta-phase morphology. The energetics and zeta-phase microstructure for other group VB carbides were found to be similar. In contrast, multiphase hafnium nitrides can form both thin-lath-based microstructure as well as large, single zeta-phase grains. The difference between hafnium nitride and the group VB carbides is attributed to the relative bulk free energies and low-temperature stability between the phases.
Chemical variation induced nanoscale spatial heterogeneity in metallic glasses
(Taylor & Francis, 2018) Wang, Neng; Ding, Jun; Luo, Peng; Liu, Yanhui; Li, Lin; Yan, Feng; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; Chinese Academy of Sciences; Institute of Physics, CAS
Metallic glasses possess amorphous structures with inherent heterogeneity at the nanoscale. A combined experimental and modeling investigation to elucidate the chemical effect on such nanoscale heterogeneity in a Cu-Zr-Al metallic glass system is conducted. By using the dynamic atomic force microscopy, we reveal a reduction of the nanoscale spatial heterogeneity in the local viscoelastic response after introducing Al into the Cu50Zr50 metallic glass. The change of such nanoscale heterogeneity can be contributed to the variation of local atomic structures. The addition of Al increases the population of the icosahedral short-range ordered clusters, thus reducing the structural heterogeneity at the nanoscale. IMPACT STATEMENTThis paper provides a combination between the nanoscale experimental and theoretical understanding of the chemical variation induced spatial heterogeneity in CuZrAl metallic glass and their impacts on the mechanical properties.
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.
Plasticity mechanisms in HfN at elevated and room temperature
(Nature Portfolio, 2016-10-06) Vinson, Katherine; Yu, Xiao-Xiang; De Leon, Nicholas; Weinberger, Christopher R.; Thompson, Gregory B.; University of Alabama Tuscaloosa; Drexel University
HfN specimens deformed via four-point bend tests at room temperature and at 2300 degrees C (similar to 0.7 T-m) showed increased plasticity response with temperature. Dynamic diffraction via transmission electron microscopy (TEM) revealed < 110 > {111} as the primary slip system in both temperature regimes and < 110 > {110} to be a secondary slip system activated at elevated temperature. Dislocation line lengths changed from a primarily linear to a curved morphology with increasing temperature suggestive of increased dislocation mobility being responsible for the brittle to ductile temperature transition. First principle generalized stacking fault energy calculations revealed an intrinsic stacking fault (ISF) along < 112 > {111}, which is the partial dislocation direction for slip on these close packed planes. Though B1 structures, such as NaCl and HfC predominately slip on < 110 > {110}, the ISF here is believed to facilitate slip on the {111} planes for this B1 HfN phase.
Influence of Grain Boundary Character and Annealing Time on Segregation in Commercially Pure Nickel
(2016) Welsh, Shery L.; Kapoor, Monica; Underwood, Olivia D.; Martens, Richard L.; Thompson, Gregory B.; Evans, Jeffrey L.; University of Alabama Tuscaloosa
Commercially pure nickel (Ni) was thermomechanically processed to promote an increase in Σ3 special grain boundaries. Engineering the character and chemistry of Σ3 grain boundaries in polycrystalline materials can help in improving physical, chemical, and mechanical properties leading to improved performance. Type-specific grain boundaries (special and random) were characterized using electron backscatter diffraction and the segregation behavior of elements such as Si, Al, C, O, P, Cr, Mg, Mn, B, and Fe, at the atomic level, was studied as a function of grain boundary character using atom probe tomography. These results showed that the random grain boundaries were enriched with impurities to include metal oxides, while Σ3 special grain boundaries showed little to no impurities at the grain boundaries. In addition, the influence of annealing time on the concentration of segregants on random grain boundaries was analyzed and showed clear evidence of increased concentration of segregants as annealing time was increased.
Grain Boundary Specific Segregation in Nanocrystalline Fe(Cr)
(2016-10-06) Zhou, Xuyang; Yu, Xiao-xiang; Kaub, Tyler; Martens, Richard L.; Thompson, Gregory B.; University of Alabama Tuscaloosa
A cross-correlative precession electron diffraction – atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.
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.
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.
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.

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