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Full-Text Articles in Physics

Shape-Memory Transformations Of Niti: Minimum-Energy Pathways Between Austenite, Martensites, And Kinetically Limited Intermediate States, Nikolai A. Zarkevich, Duane D. Johnson Dec 2014

Shape-Memory Transformations Of Niti: Minimum-Energy Pathways Between Austenite, Martensites, And Kinetically Limited Intermediate States, Nikolai A. Zarkevich, Duane D. Johnson

Ames Laboratory Publications

NiTi is the most used shape-memory alloy; nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudged elastic band method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between a body-centered orthorhombic (bco) ground state and a newly identified stable austenite (“glassy” B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited Rphase), and between martensite variants (bco orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed ...


Crossover In The Magnetic Response Of Single-Crystalline Ba1−Xkxfe2as2 And Lifshitz Critical Point Evidenced By Hall Effect Measurements, Yong Liu, Thomas A. Lograsso Dec 2014

Crossover In The Magnetic Response Of Single-Crystalline Ba1−Xkxfe2as2 And Lifshitz Critical Point Evidenced By Hall Effect Measurements, Yong Liu, Thomas A. Lograsso

Ames Laboratory Publications

We report on the doping evolution of magnetic susceptibility χ(T) and Hall coefficient RH in high-quality Ba1−xKxFe2As2 (0.13≤x≤1) single crystals. It is found that the normal-state magnetic susceptibility of Ba1−xKxFe2As2 compounds undergoes a crossover from linear-T dependence in the undoped and underdoped samples into KFe2As2-type magnetic response in the overdoped samples with increasing K content. Although magnetic susceptibility χ(T) of optimally doped samples (0.34≤x≤0.47) still follows a monotonic increase with increasing temperature, a big hump around 300 K emerges. As x exceeds 0.53, a broad peak forms in ...


Configurational Thermodynamics Of Alloyed Nanoparticles With Adsorbates, Lin-Lin Wang, Teck L. Tan, Duane D. Johnson Nov 2014

Configurational Thermodynamics Of Alloyed Nanoparticles With Adsorbates, Lin-Lin Wang, Teck L. Tan, Duane D. Johnson

Ames Laboratory Publications

Changes in the chemical configuration of alloyed nanoparticle (NP) catalysts induced by adsorbates under working conditions, such as reversal in core–shell preference, are crucial to understand and design NP functionality. We extend the cluster expansion method to predict the configurational thermodynamics of alloyed NPs with adsorbates based on density functional theory data. Exemplified with PdRh NPs having O-coverage up to a monolayer, we fully detail the core–shell behavior across the entire range of NP composition and O-coverage with quantitative agreement to in situ experimental data. Optimally fitted cluster interactions in the heterogeneous system are the key to enable ...


Green's Function Multiple-Scattering Theory With A Truncated Basis Set: An Augmented-Kkr Formalism, Aftab Alam, Suffian N. Khan, Andrei V. Smirnov, D. M. Nicholson, Duane D. Johnson Nov 2014

Green's Function Multiple-Scattering Theory With A Truncated Basis Set: An Augmented-Kkr Formalism, Aftab Alam, Suffian N. Khan, Andrei V. Smirnov, D. M. Nicholson, Duane D. Johnson

Ames Laboratory Publications

The Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient site-centered, electronic-structure technique for addressing an assembly of N scatterers. Wave functions are expanded in a spherical-wave basis on each scattering center and indexed up to a maximum orbital and azimuthal number Lmax=(l,m)max, while scattering matrices, which determine spectral properties, are truncated at Ltr=(l,m)tr where phase shifts δl>ltr are negligible. Historically, Lmax is set equal to Ltr, which is correct for large enough Lmax but not computationally expedient; a better procedure retains higher-order (free-electron and single-site) contributions for Lmax>Ltr with δl ...


Effect Of Electron Irradiation On Superconductivity In Single Crystals Of Ba(Fe1−Xrux)2as2 (X=0.24), Ruslan Prozorov, M. Kończykowski, Makariy A. Tanatar, Alexander N. Thaler, Sergey L. Bud'ko, Paul C. Canfield, V. Mishra, P. J. Hirschfeld Oct 2014

Effect Of Electron Irradiation On Superconductivity In Single Crystals Of Ba(Fe1−Xrux)2as2 (X=0.24), Ruslan Prozorov, M. Kończykowski, Makariy A. Tanatar, Alexander N. Thaler, Sergey L. Bud'ko, Paul C. Canfield, V. Mishra, P. J. Hirschfeld

Ames Laboratory Publications

A single crystal of isovalently substituted Ba(Fe1−xRux)2As2 (x=0.24) is sequentially irradiated with 2.5 MeV electrons up to a maximum dose of 2.1×1019 e−/cm2. The electrical resistivity is measuredin situ at T=22  K during the irradiation and ex situ as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature Tc decreases and the residual resistivity ρ0 increases. We find that electron irradiation leads to the fastest suppression of Tccompared to other types of artificially introduced disorder, probably due to the strong short-range potential of the ...


Complex Magnetic Ordering In Cege1.76 Studied By Neutron Diffraction, Wageesha T. Jayasekara, W. Tian, Halyna Hodovanets, Paul C. Canfield, Andreas Kreyssig, Alan I. Goldman Oct 2014

Complex Magnetic Ordering In Cege1.76 Studied By Neutron Diffraction, Wageesha T. Jayasekara, W. Tian, Halyna Hodovanets, Paul C. Canfield, Andreas Kreyssig, Alan I. Goldman

Ames Laboratory Publications

Neutron diffraction measurements on a single crystal of CeGe1.76 reveal a complex series of magnetic transitions at low temperature. At TN≈7 K, there is a transition from a paramagnetic state at higher temperature to an incommensurate magnetic structure characterized by a magnetic propagation vector (0 0 τ) with τ≈14 and the magnetic moment along the a axis of the orthorhombic unit cell. Below TLI≈5 K, the magnetic structure locks in to a commensurate structure with τ=14 and the magnetic moment remains along the a axis. Below T☆≈4 K, we find additional half-integer and integer ...


Influence Of Composition And Heat Treatment On Damping And Magnetostrictive Properties Of Fe–18%(Ga + Al) Alloys, I. S. Golovin, V. V. Palacheva, V. Yu. Zadorozhnyy, J. Zhu, H. Jiang, J. Cifre, Thomas A. Lograsso Oct 2014

Influence Of Composition And Heat Treatment On Damping And Magnetostrictive Properties Of Fe–18%(Ga + Al) Alloys, I. S. Golovin, V. V. Palacheva, V. Yu. Zadorozhnyy, J. Zhu, H. Jiang, J. Cifre, Thomas A. Lograsso

Ames Laboratory Publications

The structure, magnetostriction and damping properties of Fe82Ga(18−x)Alx (x = 0, 5, 8, 12) alloys were analyzed. The anelastic response of Fe–18(Ga + Al) alloys was studied as a function of temperature (from 0 to 600 °C), frequency (from 0.01 to 200 Hz) and amplitude (from 0.0004% to 0.2%) of forced vibrations. The origin of the relatively high damping capacity of Fe–Ga–Al alloy at room temperature was determined by applying a magnetic field and different heat treatment regimes. The substitution of Ga by Al in Fe–18% Ga alloys was found to ...


Unoccupied Electronic States Of Icosahedral Al-Pd-Mn Quasicrystals: Evidence Of Image Potential Resonance And Pseudogap, M. Maniraj, Abhishek Rai, S. R. Barman, M. Krajčí, Deborah L. Schlagel, Thomas A. Lograsso, K. Horn Sep 2014

Unoccupied Electronic States Of Icosahedral Al-Pd-Mn Quasicrystals: Evidence Of Image Potential Resonance And Pseudogap, M. Maniraj, Abhishek Rai, S. R. Barman, M. Krajčí, Deborah L. Schlagel, Thomas A. Lograsso, K. Horn

Ames Laboratory Publications

We study the unoccupied region of the electronic structure of the fivefold symmetric surface of an icosahedral (i) Al-Pd-Mn quasicrystal. A feature that exhibits parabolic dispersion with an effective mass of (1.15±0.1)me and tracks the change in the work function is assigned to an image potential resonance because our density functional calculation shows an absence of band gap in the respective energy region. We show that Sn grows pseudomorphically on i−Al−Pd−Mn as predicted by density functional theory calculations, and the energy of the image potential resonance tracks the change in the work function ...


Raman Studies Of Nearly Half-Metallic Ferromagnetic Cos2, S. G. Lyapin, A. N. Utyuzh, A. E. Petrova, A. P. Novikov, Thomas A. Lograsso, S. M. Stishov Sep 2014

Raman Studies Of Nearly Half-Metallic Ferromagnetic Cos2, S. G. Lyapin, A. N. Utyuzh, A. E. Petrova, A. P. Novikov, Thomas A. Lograsso, S. M. Stishov

Ames Laboratory Publications

We measured the Raman spectra of ferromagnetic, nearly half-metallic, CoS2over a broad temperature range. All five Raman active modes Ag, Eg, Tg(1), Tg(2) and Tg(3) were observed. The magnetic ordering is indicated by a change of the temperature dependences of the frequency and the line width of Ag and Tg(2) modes at the Curie point. The temperature dependence of the frequencies and line widths of the Ag, Eg, Tg(1), Tg(2) modes in the paramagnetic phase can be described in the framework of the ...


Stable Atomic Structure Of Niti Austenite, Nikolai A. Zarkevich, Duane D. Johnson Aug 2014

Stable Atomic Structure Of Niti Austenite, Nikolai A. Zarkevich, Duane D. Johnson

Ames Laboratory Publications

Nitinol (NiTi), the most widely used shape-memory alloy, exhibits an austenite phase that has yet to be identified. The usually assumed austenitic structure is cubic B2, which has imaginary phonon modes, hence it is unstable. We suggest a stable austenitic structure that “on average” has B2 symmetry (observed by x-ray and neutron diffraction), but it exhibitsfinite atomic displacements from the ideal B2 sites. The proposed structure has a phonon spectrum that agrees with that from neutron scattering, has diffraction spectra in agreement with x-ray diffraction, and has an energy relative to the ground state that agrees with calorimetry data.


Fermi Surface Reconstruction In (Ba1−Xkx)Fe2as2 (0.44≤X≤1) Probed By Thermoelectric Power Measurements, Halyna Hodovanets, Yong Liu, Anton Jesche, Sheng Ran, Eun Deok Mun, Thomas A. Lograsso, Sergey L. Bud'ko, Paul C. Canfield Jun 2014

Fermi Surface Reconstruction In (Ba1−Xkx)Fe2as2 (0.44≤X≤1) Probed By Thermoelectric Power Measurements, Halyna Hodovanets, Yong Liu, Anton Jesche, Sheng Ran, Eun Deok Mun, Thomas A. Lograsso, Sergey L. Bud'ko, Paul C. Canfield

Ames Laboratory Publications

We report in-plane thermoelectric power measurements on single crystals of (Ba1−xKx)Fe2As2(0.44≤x≤1). We observe a minimum in the S|T=const versus x at x∼0.55 that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of M point crossing the Fermi level. This feature is clearly observable below ∼75 K. Thermoelectric power also shows a change in the x∼0.8–0.9 range, where the maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition ...


Mixed Valency And Site-Preference Chemistry For Cerium And Its Compounds: A Predictive Density-Functional Theory Study, Aftab Alam, Duane D. Johnson Jun 2014

Mixed Valency And Site-Preference Chemistry For Cerium And Its Compounds: A Predictive Density-Functional Theory Study, Aftab Alam, Duane D. Johnson

Ames Laboratory Publications

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states—itinerant Ce4+ and localized Ce3+. Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an “alloy” problem involving two valences with competing and numerous site-occupancy configurations. We use density-functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling the valence by pseudoternary alloying. For Ce and its compounds, such as (Ce,La)2(Fe,Co)14B permanent magnets, we find a stable mixed-valent α state ...


Magnetic Excitations And Anomalous Spin-Wave Broadening In Multiferroic Fev2o4, Qiang Zhang, Mehmet Ramazanoglu, Songxue Chi, Yong Liu, Thomas A. Lograsso, David Vaknin Jun 2014

Magnetic Excitations And Anomalous Spin-Wave Broadening In Multiferroic Fev2o4, Qiang Zhang, Mehmet Ramazanoglu, Songxue Chi, Yong Liu, Thomas A. Lograsso, David Vaknin

Ames Laboratory Publications

We report on the different roles of two orbital-active Fe2+ at the A site and V3+ at the B site in the magnetic excitations and on the anomalous spin-wave broadening in FeV2O4. FeV2O4 exhibits three structural transitions and successive paramagnetic (PM)–collinear ferrimagnetic (CFI)–noncollinear ferrimagnetic (NCFI)/ferroelectric transitions. The high-temperature tetragonal/PM–orthorhombic/CFI transition is accompanied by the appearance of a large energy gap in the magnetic excitations due to strong spin-orbit-coupling-induced anisotropy at the Fe2+ site. While there is no measurable increase in the energy gap from the orbital ordering of V3+ at the orthorhombic/CFI–tetragonal ...


Fermi Surfaces And Phase Stability Of Ba(Fe1−Xmx)2as2 (M=Co,Ni,Cu,Zn), Suffian N. Khan, Aftab Alam, Duane D. Johnson May 2014

Fermi Surfaces And Phase Stability Of Ba(Fe1−Xmx)2as2 (M=Co,Ni,Cu,Zn), Suffian N. Khan, Aftab Alam, Duane D. Johnson

Ames Laboratory Publications

BaFe2As2 with transition-metal doping exhibits a variety of rich phenomena from the coupling of structure, magnetism, and superconductivity. Using density functional theory, we systematically compare the Fermi surfaces (FSs), formation energies (ΔEf), and densities of states (DOSs) of electron-doped Ba(Fe1−xMx)2As2 with M={Co,Ni,Cu,Zn} in tetragonal (I4/mmm) and orthorhombic (Fmmm) structures in nonmagnetic, antiferromagnetic, and paramagnetic (disordered local moment) states. We explain changes to the phase stability (ΔEf) and Fermi surfaces (and nesting) due to chemical and magnetic disorder. We compare our results to observed/assessed properties and contrast alloy theory with the results ...


Fragile Antiferromagnetism In The Heavy-Fermion Compound Ybbipt, Benjamin G. Ueland, Andreas Kreyssig, K. Prokeš, J. W. Lynn, L. W. Harriger, D. K. Pratt, D. K. Singh, T. W. Heitmann, S. Sauerbrei, Scott M. Saunders, E. D. Mun, Sergey L. Bud'ko, Robert J. Mcqueeney, Paul C. Canfield, Alan I. Goldman May 2014

Fragile Antiferromagnetism In The Heavy-Fermion Compound Ybbipt, Benjamin G. Ueland, Andreas Kreyssig, K. Prokeš, J. W. Lynn, L. W. Harriger, D. K. Pratt, D. K. Singh, T. W. Heitmann, S. Sauerbrei, Scott M. Saunders, E. D. Mun, Sergey L. Bud'ko, Robert J. Mcqueeney, Paul C. Canfield, Alan I. Goldman

Ames Laboratory Publications

We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, τAFM = (121212), and ordered moments that align along the [1 1 1] direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below TN≈0.4 K and corresponds to a magnetic correlation length of ξn≈ 80 Å, and a broad component that persists up to T∗≈ 0.7 K and corresponds to antiferromagnetic correlations extending over ξb≈ 20 Å. Our results illustrate the ...


Evolution Of London Penetration Depth With Scattering In Single Crystals Of K1-Xnaxfe2as2, H. Kim, Makariy A. Tanatar, Yong Liu, Zachary Cole Sims, Chenglin Zhang, Pengcheng Dai, Thomas A. Lograsso, Ruslan Prozorov May 2014

Evolution Of London Penetration Depth With Scattering In Single Crystals Of K1-Xnaxfe2as2, H. Kim, Makariy A. Tanatar, Yong Liu, Zachary Cole Sims, Chenglin Zhang, Pengcheng Dai, Thomas A. Lograsso, Ruslan Prozorov

Ames Laboratory Publications

London penetration depth, lambda(T), was measured in single crystals of K1-xNaxFe2As2, x = 0 and 0.07, down to temperatures of 50 mK, similar to T-c/50. Isovalent substitution of Na for K significantly increases impurity scattering, with rho(T-c) rising from 0.2 to 2.2 mu Omega cm, and leads to a suppression of T-c from 3.5 to 2.8 K. At the same time, a close to T-linear Delta lambda(T) in pure samples changes to almost T-2 in the substituted samples. The behavior never becomes exponential as expected for the accidental nodes, as opposed to ...


Anisotropic Thermal Expansion In Molecular Solids: Theory And Experiment On Libh4, Nikolai A. Zarkevich, E. H. Majzoub, Duane D. Johnson Apr 2014

Anisotropic Thermal Expansion In Molecular Solids: Theory And Experiment On Libh4, Nikolai A. Zarkevich, E. H. Majzoub, Duane D. Johnson

Ames Laboratory Publications

We propose a reliable and efficient computational method for predicting elastic and thermal expansion properties in crystals, particularly complex anisotropic molecular solids, and we apply it to the room-temperature orthorhombic Pnmaphase of LiBH4. Using density-functional theory, we find thermal expansion coefficients at finite temperature, and we confirm them by temperature-dependent, in situ x-ray diffraction measurements. We also consider the effects of volume and pressure, as well as energy barriers for BH4− rotations and collective motions. Our combined study validates the theory and provides a better understanding of the structural behavior of LiBH4.


Lifshitz Transition And Chemical Instabilities In Ba1−Xkxfe2as2 Superconductors, Suffian N. Khan, Duane D. Johnson Apr 2014

Lifshitz Transition And Chemical Instabilities In Ba1−Xkxfe2as2 Superconductors, Suffian N. Khan, Duane D. Johnson

Ames Laboratory Publications

For solid-solution Ba1−xKxFe2As2 Fermi surface evolution is mapped via Bloch spectral functions calculated using density functional theory implemented in Korringa-Kohn-Rostoker multiple scattering theory with the coherent-potential approximation. Spectral functions reveal electronic dispersion, topology, orbital character, and broadening (electron-lifetime effects) due to chemical disorder. Dissolution of electron cylinders occurs near x∼0.9 with a nonuniform, topological (Lifshitz) transition, reducing the interband interactions; yet the dispersion maintains its dxz or dyz character. Formation energies indicate alloying at x=0.35, as observed, and a tendency for segregation on the K-rich (x>0.6) side, explaining the difficulty of controlling sample ...


Comprehensive Scenario For Single-Crystal Growth And Doping Dependence Of Resistivity And Anisotropic Upper Critical Fields In(Ba1-Xkx)Fe2as2 (0.22 <= X <= 1), Yong Liu, Makariy A. Tanatar, Warren E. Straszheim, Brandt Jensen, Kevin W. Dennis, R. W. Mccallum, Vladimir G. Kogan, Ruslan Prozorov, Thomas A. Lograsso Apr 2014

Comprehensive Scenario For Single-Crystal Growth And Doping Dependence Of Resistivity And Anisotropic Upper Critical Fields In(Ba1-Xkx)Fe2as2 (0.22 <= X <= 1), Yong Liu, Makariy A. Tanatar, Warren E. Straszheim, Brandt Jensen, Kevin W. Dennis, R. W. Mccallum, Vladimir G. Kogan, Ruslan Prozorov, Thomas A. Lograsso

Ames Laboratory Publications

Large high-quality single crystals of hole-doped iron-based superconductor (Ba1-xKx)Fe2As2 were grown over a broad composition range 0.22 <= x <= 1 by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals (0.65 <= x <= 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the c-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity rho(T) found doping-independent, constant within error bars, resistivity at room temperature, rho(300 K), in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe2As2 based compositions. The shape of the temperature-dependent resistivity, rho(T), shows systematic doping-evolution, being close to T-2 in overdoped and revealing significant contribution of the T-linear component at optimum doping. The slope of the upper critical field, d H-c2/dT, scales linearly with T-c for both H parallel to c, H-c2,(c), and H parallel to ab, H-c2,H-ab. The anisotropy of the upper critical field. equivalent to Hc(2,ab)/H-c2,H-c determined near zero-field T-c increases from similar to 2 to 4-5 with increasing K doping level from optimal x similar to 0.4 to strongly overdoped x = 1.


Suppression Of Electron Correlations In The Collapsed Tetragonal Phase Of Cafe2as2 Under Ambient Pressure Demonstrated By As75 Nmr/Nqr Measurements, Yuji Furukawa, B. Roy, S. Ran, Sergey L. Bud'ko, Paul C. Canfield Mar 2014

Suppression Of Electron Correlations In The Collapsed Tetragonal Phase Of Cafe2as2 Under Ambient Pressure Demonstrated By As75 Nmr/Nqr Measurements, Yuji Furukawa, B. Roy, S. Ran, Sergey L. Bud'ko, Paul C. Canfield

Ames Laboratory Publications

The static and the dynamic spin correlations in the low-temperature collapsed tetragonal and the high-temperature tetragonal phase in CaFe2As2 have been investigated by As75 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. Through the temperature (T) dependence of the nuclear spin lattice relaxation rates (1/T1) and the Knight shifts, although stripe-type antiferromagnetic (AFM) spin correlations are realized in the high-temperature tetragonal phase, no trace of the AFM spin correlations can be found in the nonsuperconducting, low-temperature, collapsed tetragonal (cT) phase. Given that there is no magnetic broadening in As75 NMR spectra, together with the T-independent behavior of ...


Development Of Mnbi Permanent Magnet: Neutron Diffraction Of Mnbi Powder, J. Cui, J. P. Choi, G. Li, E. Polikarpov, J. Darsell, Matthew J. Kramer, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson, M. Marinescu, Q. Z. Huang, N. V. Vuong, J. Ping Liu Mar 2014

Development Of Mnbi Permanent Magnet: Neutron Diffraction Of Mnbi Powder, J. Cui, J. P. Choi, G. Li, E. Polikarpov, J. Darsell, Matthew J. Kramer, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson, M. Marinescu, Q. Z. Huang, N. V. Vuong, J. Ping Liu

Ames Laboratory Publications

MnBi attracts great attention in recent years for its great potential as permanent magnetmaterials. MnBi phase is difficult to obtain because of the rather drastic peritectic reaction between Mn and Bi. In this paper, we report our effort on synthesizing high purity MnBi compound using conventional powder metallurgical approaches. Neutron diffraction was carried out to investigate the crystal and nuclear structure of the obtained powder. The result shows that the purity of the obtained powder is about 91 wt. % at 300 K, and the magnetic moment of the Mn atom in MnBi lattice is 4.424 and 4.013 μ ...


Thermodynamics Of The Ferromagnetic Phase Transition In Nearly Half Metallic Cos2 At High Pressures, F. S. Elkin, I. P. Zibrov, A. P. Novikov, S. S. Khasanov, V. A. Sidorov, A. E. Petrova, Thomas A. Lograsso, J. D. Thompson, S. M. Stishov Mar 2014

Thermodynamics Of The Ferromagnetic Phase Transition In Nearly Half Metallic Cos2 At High Pressures, F. S. Elkin, I. P. Zibrov, A. P. Novikov, S. S. Khasanov, V. A. Sidorov, A. E. Petrova, Thomas A. Lograsso, J. D. Thompson, S. M. Stishov

Ames Laboratory Publications

The volume change and heat capacity at the ferromagnetic phase transition in CoS2 were measured at high pressures using X-rays generated by the Argonne synchrotron light source and by ac-calorimetry, respectively. The transition entropy, calculated on the basis of these experimental data, drops along the transition line due to quantum degradation, as required by Nernst's law. The volume change increases strongly along the transition line, which is explained by specifics of the compressibility difference of coexisting phases that results from nearly half metallic nature of the ferromagnetic phase of CoS2.


Anomalous Magneto-Structural Behavior Of Mnbi Explained: A Path Towards An Improved Permanent Magnet, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson Jan 2014

Anomalous Magneto-Structural Behavior Of Mnbi Explained: A Path Towards An Improved Permanent Magnet, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson

Ames Laboratory Publications

Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation and magnetic susceptibilitymaximum near 90 K, and, importantly for high-temperature magnetic applications, an increasingcoercivity (unique to MnBi) above 180  K. We calculate the total energy and magneto-anisotropyenergy (MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above anomalies. We predict that coercivity and MAE increase due to increasing a, suggesting means to improve MnBi permanent magnets.


Thermal Stability Of Mnbi Magnetic Materials, J. Cui, J. P. Choi, G. Li, E. Polikarpov, J. Darsell, N. Overman, M. Olszta, D. Schreiber, M. Bowden, T. Droubay, Matthew J. Kramer, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson, M. Marinescu, I. Takeuchi, Q. Z. Huang, H. Wu, H. Reeve, N. V. Vuong, J. P. Liu Jan 2014

Thermal Stability Of Mnbi Magnetic Materials, J. Cui, J. P. Choi, G. Li, E. Polikarpov, J. Darsell, N. Overman, M. Olszta, D. Schreiber, M. Bowden, T. Droubay, Matthew J. Kramer, Nikolai A. Zarkevich, Linlin Wang, Duane D. Johnson, M. Marinescu, I. Takeuchi, Q. Z. Huang, H. Wu, H. Reeve, N. V. Vuong, J. P. Liu

Ames Laboratory Publications

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The ...