287 research outputs found
Theory of the ferromagnetism in Ti1-xCrxN solid solutions
No full textFirst-principles calculations are used to investigate the magnetic properties of Ti1‑xCrxN solid solutions. We show that the magnetic interactions between Cr spins that favor antiferromagnetism in CrN is changed upon alloying with TiN leading to the appearance of ferromagnetism in the system at approximately x≤0.50 in agreement with experimental reports. Furthermore we suggest that this effect originates in an electron density redistribution from Ti to Cr that decreases the polarization of Crd states with t2g symmetry while it increases the polarization of Crd states with eg symmetry, both changes working in favor of ferromagnetism.Original Publication: Björn Alling, Theory of the ferromagnetism in Ti1-xCrxN solid solutions, 2010, Physical Review B Condensed Matter, (82), 5, 054408. http://dx.doi.org/10.1103/PhysRevB.82.054408 Copyright: American Physical Society http://www.aps.org
Stability trends of MAX phases from first principles
No full textWe have developed a systematic method to investigate the phase stability of M(n+1)AX(n) phases, here applied for M=Sc, Ti, V, Cr, or Mn, A=Al, and X=C or N. Through a linear optimization procedure including all known competing phases, we identify the set of most competitive phases for n=1-3 in each system. Our calculations completely reproduce experimental occurrences of stable MAX phases. We also identify and suggest an explanation for the trend in stability as the transition metal is changed across the 3d series for both carbon- and nitrogen-based systems. Based on our results, the method can be used to predict stability of potentially existing undiscovered phases.Original Publication:Martin Dahlqvist, Björn Alling and Johanna Rosén, Stability trends of MAX phases from first principles, 2010, Physical Review B. Condensed Matter and Materials Physics, (81), 22, 220102.http://dx.doi.org/10.1103/PhysRevB.81.220102Copyright: American Physical Societyhttp://www.aps.org
Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys
No full textWe demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1-xCuxMnSb alloys as a case study. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase T-C in Cu-rich samples.Original Publication:Björn Alling, A V Ruban and Igor Abrikosov, Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys, 2009, PHYSICAL REVIEW B, (79), 13, 134417.http://dx.doi.org/10.1103/PhysRevB.79.134417Copyright: American Physical Societyhttp://www.aps.org
Stability of the ternary perovskites Sc3EN (E=B,Al,Ga,In) from first principles
No full textMechanical and thermodynamic stability of the isoelectronic ternary inverse perovskites Sc3EN (E=B,Al,Ga,In) has been studied from first principles. We confirm stability of recently synthesized cubic phases Sc3AlN and Sc3InN, and predict the stability of cubic Sc3GaN and a triclinic phase aP20-Sc3BN. Substantial phonon softening in Sc3AlN and Sc3GaN is observed indicating a possibility that structural defects could form readily. In accord, our experiments show that magnetron sputter deposited films contain regions with high density of nonperiodic stacking faults along the < 111 > growth direction. We suggest that defect-free crystals may exhibit anomalies in the carrier properties, promising for electronic applications.Original Publication:Arkady Mikhaylushkin, Carina Höglund, Jens Birch, Zs Czigany, Lars Hultman, Sergey Simak, Björn Alling, Ferenc Tasnadi and Igor Abrikosov , Stability of the ternary perovskites Sc3EN (E=B,Al,Ga,In) from first principles, 2009, PHYSICAL REVIEW B, (79), 13, 134107.http://dx.doi.org/10.1103/PhysRevB.79.134107Copyright: American Physical Societyhttp://www.aps.org
House Unpassed Legislation 1912, H 1407 committee on legal affairs - leave to withdraw, SC1/series 230, Petition of B.B. Alling [Benjamin B. Alling]
No full textPetition subject: Racial discrimination Original: http://nrs.harvard.edu/urn-3:FHCL:25763576 Date of creation: (unknown) Petition location: Boston Legislator, committee, or address that the petition was sent to: James A. Hart, Boston; committee on legal affairs Selected signatures:B.B. Alling [Benjamin B. Alling] Actions taken on dates: 1912-01-18,1912-01-29 Legislative action: Received in the House on January 18, 1912 and referred to the committee on legal affairs and sent for concurrence and received in the Senate on January 29, 1912 and concurred Total signatures: 1 Legislative action summary: Received, referred, sent, received, concurred Legal voter signatures (males not identified as non-legal): 1 Female only signatures: No Identifications of signatories: citizen Prayer format was printed vs. manuscript: Manuscript Additional non-petition or unrelated documents available at archive: additional documents available Additional archivist notes: no organization shall be formed or maintained which is calculated to advance the interest of any particular race or religious sect, includes address Location of the petition at the Massachusetts Archives of the Commonwealth: House Unpassed 1912, H 1407 committee on legal affairs - leave to withdraw Acknowledgements: Supported by the National Endowment for the Humanities (PW-5105612), Massachusetts Archives of the Commonwealth, Radcliffe Institute for Advanced Study at Harvard University, Center for American Political Studies at Harvard University, Institutional Development Initiative at Harvard University, and Harvard University Library.</p
House Unpassed Legislation 1912, H 1407 committee on legal affairs - leave to withdraw, SC1/series 230, Petition of B.B. Alling [Benjamin B. Alling]
No full textPetition subject: Racial discrimination Original: http://nrs.harvard.edu/urn-3:FHCL:25763576 Date of creation: (unknown) Petition location: Boston Legislator, committee, or address that the petition was sent to: James A. Hart, Boston; committee on legal affairs Selected signatures:B.B. Alling [Benjamin B. Alling] Actions taken on dates: 1912-01-18,1912-01-29 Legislative action: Received in the House on January 18, 1912 and referred to the committee on legal affairs and sent for concurrence and received in the Senate on January 29, 1912 and concurred Total signatures: 1 Legislative action summary: Received, referred, sent, received, concurred Legal voter signatures (males not identified as non-legal): 1 Female only signatures: No Identifications of signatories: citizen Prayer format was printed vs. manuscript: Manuscript Additional non-petition or unrelated documents available at archive: additional documents available Additional archivist notes: no organization shall be formed or maintained which is calculated to advance the interest of any particular race or religious sect, includes address Location of the petition at the Massachusetts Archives of the Commonwealth: House Unpassed 1912, H 1407 committee on legal affairs - leave to withdraw Acknowledgements: Supported by the National Endowment for the Humanities (PW-5105612), Massachusetts Archives of the Commonwealth, Radcliffe Institute for Advanced Study at Harvard University, Center for American Political Studies at Harvard University, Institutional Development Initiative at Harvard University, and Harvard University Library.</p
Role of stoichiometric and nonstoichiometric defects on the magnetic properties of the half-metallic ferromagnet NiMnSb
No full textThe first material to be predicted from first-principles calculations as half-metallic was NiMnSb, and the research on this material has been intense due to its possible applications in spintronics devices. The failure of many experiments to measure spin polarization to more than a fraction of the predicted 100% has partly been blamed on structural defects. In this work a complete first-principles treatise of point defects, including nonstoichiometric antisites, interstitial and vacancy defects, as well as stoichiometric atomic swap defects in NiMnSb, is presented. We find that the formation energies of the defects span a large scale from 0.2 to 14.4 eV. The defects with low formation energies preserve the half-metallic character of the material. We also find that some of the defects increase the magnetic moment and thus can explain the experimentally observed increase of magnetic moments in some samples of NiMnSb. Most interesting in this respect are Mn interstitials which increase the magnetic moment, have a low formation energy, and keep the half-metallic character of the material.Original Publication:Björn Alling, Sam Shallcross and Igor Abrikosov, Role of stoichiometric and nonstoichiometric defects on the magnetic properties of the half-metallic ferromagnet NiMnSb, 2006, Physical Review B. Condensed Matter and Materials Physics, (73), 6, 064418.http://dx.doi.org/10.1103/PhysRevB.73.064418Copyright: American Physical Societyhttp://www.aps.org
Equation of state of paramagnetic CrN from ab initio molecular dynamics
No full textThe equation of state for chromium nitride has been debated in the literature in connection with a proposed collapse of its bulk modulus following the pressure-induced transition from the paramagnetic cubic phase to the antiferromagnetic orthorhombic phase [F. Rivadulla et al., Nature Mater. 8, 947 (2009); B. Alling et al., ibid. 9, 283 (2010)]. Experimentally the measurements are complicated due to the low transition pressure, while theoretically the simulation of magnetic disorder represents a major challenge. Here a first-principles method is suggested for the calculation of thermodynamic properties of magnetic materials in their high-temperature paramagnetic phase. It is based on ab initio molecular dynamics and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moments molecular dynamics method to the case of CrN and simulate its equation of state. In particular the debated bulk modulus is calculated in the paramagnetic cubic phase and is shown to be very similar to that of the antiferromagnetic orthorhombic CrN phase for all considered temperatures.Funding Agencies|Swedish Research Council (VR)|621-2011-4426621-2011-4417|Swedish Foundation for Strategic Research (SSF)|10-0026
First-principles prediction of stabilities and instabilities of compounds and alloys in the ternary B-As-P system
No full textWe examine the thermodynamic stability of compounds and alloys in the ternary B-As-P system theoretically using first-principles calculations. We demonstrate that the icosahedral B12As2 is the only stable compound in the binary B-As system, while the zinc-blende BAs is thermodynamically unstable with respect to B12As2 and the pure arsenic phase at 0 K, and increasingly so at higher temperature, suggesting that BAs may merely exist as a metastable phase. On the contrary, in the binary B-P system, both zinc-blende BP and icosahedral B12P2 are predicted to be stable. As for the binary As-P system, As1-xPx disordered alloys are predicted at elevated temperature-for example, a disordered solid solution of up to similar to 75 at.% As in black phosphorus as well as a small solubility of similar to 1 at.% P in gray arsenic at T = 750 K, together with the presence of miscibility gaps. The calculated large solubility of As in black phosphorus explains the experimental syntheses of black-phosphorus-type As1-xPx alloys with tunable compositions, recently reported in the literature. We investigate the phase stabilities in the ternary B-As-P system and demonstrate a high tendency for a formation of alloys in the icosahedral B-12(As1-xPx)(2) structure by intermixing of As and P atoms at the diatomic chain sites. The phase diagram displays noticeable mutual solubility of the icosahedral subpnictides in each other even at room temperature as well as a closure of a pseudobinary miscibility gap around 900 K. As for pseudobinary BAs1-xPx alloys, only a tiny amount of BAs is predicted to be able to dissolve in BP to form the BAs1-xPx disordered alloys at elevated temperature. For example, less than 5% of BAs can dissolve in BP at T = 1000 K. The small solubility limit of BAs in BP is attributed to the thermodynamic instability of BAs with respect to B12As2 and As
A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN
No full textWe use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.Original Publication:Björn Alling, A. V. Ruban, A Karimi, Lars Hultman and Igor Abrikosov, A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN, 2011, Physical Review B. Condensed Matter and Materials Physics, (83), 10, 104203.http://dx.doi.org/10.1103/PhysRevB.83.104203Copyright: American Physical Societyhttp://www.aps.org
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