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Spin waves in the spin-flop phase of an antiferromagnet, and metastability of the spin-flop transition
Vol. 2, No. ‘7
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
MAGNETOACOUSTICRESONANCES. L. Eriksson, 0. Beckman and S. Hijrnfeldt (Department of Physics, Uppsala University, Uppsala, Sweden). Geometric resonances and deHaas-van Alphen type oscillations in ultrasonic attenuation at 150 Mc/sec give a Fermi surface of antimony consisting of three electron ellipsoids tilted by 350 each containing 1.40. lo25 electrons/m3 and three hole ellipsoids each containing 1.46.1025 holes/m3. Comparison with available information in the literature from cyclotron resonance give Fermi energies of 18.10-21 J and 13.10-21 J respectively. (Received 23 April 1964) 6. SPIN WAVES IN THE SPIN-FLOP PHASE OF AN ANTIFERROMAGNET, AND METASTABILITY OF THE SPIN-FLOP TRANSITION. Yung-Li WangB and H. B. Callen l! (Department of Physics and Laboratory for Research of Matter, University of Pennsylvania, Philadelphia 4, Pennsylvania, U. S. A. ). The spin-flop transition in a uniaxial antiferromagnet defines three critical fields; that of the true thermodynamic transition and those limiting the local stability of antiferromagnetic and spin-flop phases. The latter two are calculated by a spin-wave analysis. The results are in qualitative agreement with the hysteresis observed by Schelleng and Friedberg in the spinflop transition in MnBr2.4H20. The spin-wave spectrum of the spin-flop phase is given, and the magnetization in the spin-flop phase is found to increase more rapidly than linearly with field because of quantum corrections absent in molecular field theory. (Received 23 April 1964) ?.REIATION OF BONDINGAND ELECTRONIC BAND STRUCTURE TO THE CREATION OF LATTICE VACANCIES IN TiO. S. P. Denker (Columbia University, New York, N.Y. 10027, U.S.A.). The relationship between bonding and electronic band structure in TiO is examined and it is found that a high equiIibrium lattice B Supported by the Advanced Research Project Agency. 7 Supported by the U.S. Office of Naval Research.
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vacancy concentration is energetically favorable. Lattice vacancies in TiO allows crystallization as a defective rock-salt type structure, emptying high-energy orbitals by reducing the effective number of valence electrons, which lowers the Fermi energy. Structural stability and mechanical properties of TiO, TIN and TiC are related to the position of the Fermi level, which for 100 per cent dense crystals of TiN and TiO would be forced to lie across antibonding levels. Consideration of energy band calculations by Bilz shows that lattice vacancies lower the Fermi energy by an amount (1.75 eV for TiO) which exceeds the total energy required to create a single titanium or oxygen vacancy (0.31 eV). In this process primarily antibonding orbitals are emptied giving for stoichiometric TiO a net reduction of ca. 1.1 eV in the free energy at absolute zero. (Received 28 April 1964) 8 THE EFFECT OF ELECTRIC FIELDS ON THE LUMINESCENCEAND CONDUCTIVITY OF ZnS SINGLE CRYSTALS. V. Bar, E. Alexander, J. Brada and I. T. Steinberger (Department of Physics, The Hebrew University, Jerusalem, Israel). Application of a direct electric field (of the order of lo4 volt/cm) causes a temporary increase of the luminescence of ZnS crystals (“Gudden-Pohl effect”). It is demonstrated that the light pulse is always accompanied by a current pulse. Studies of the effect showed that both pulses appear only if the crystal has been optically excited. The sizes and shapes of the pulses were systematically studied during the excitation, afterglow and thermal glow. Kinetic equations were set up, governing the free and trapped electron concentrations in the crystal. Most coefficients of these equations were experimentally determined by conventional luminescence and photoconductivity methods. Three different ways by which the field might cause release of electrons from traps, were considered: (a) tunneling from traps, (b) diminish ing the effective trap depths by the field and thereb enhancing thermal release, and (c) impact ionization of traps by accelerated conduction-band electrons. The kinetic equations showed that with either of these mechanisms current and light pulse appear, as observed in the experiments. However tunneling (a) would yield completely negligible effects at even much higher fields than actually applied and is in disagreement with the experimenl al results on the dependence of the pulse heights on the state of excitation of the crystal. Mechanis (b) can account for the order of magnitude of the
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
MAGNETOACOUSTICRESONANCES. L. Eriksson, 0. Beckman and S. Hijrnfeldt (Department of Physics, Uppsala University, Uppsala, Sweden). Geometric resonances and deHaas-van Alphen type oscillations in ultrasonic attenuation at 150 Mc/sec give a Fermi surface of antimony consisting of three electron ellipsoids tilted by 350 each containing 1.40. lo25 electrons/m3 and three hole ellipsoids each containing 1.46.1025 holes/m3. Comparison with available information in the literature from cyclotron resonance give Fermi energies of 18.10-21 J and 13.10-21 J respectively. (Received 23 April 1964) 6. SPIN WAVES IN THE SPIN-FLOP PHASE OF AN ANTIFERROMAGNET, AND METASTABILITY OF THE SPIN-FLOP TRANSITION. Yung-Li WangB and H. B. Callen l! (Department of Physics and Laboratory for Research of Matter, University of Pennsylvania, Philadelphia 4, Pennsylvania, U. S. A. ). The spin-flop transition in a uniaxial antiferromagnet defines three critical fields; that of the true thermodynamic transition and those limiting the local stability of antiferromagnetic and spin-flop phases. The latter two are calculated by a spin-wave analysis. The results are in qualitative agreement with the hysteresis observed by Schelleng and Friedberg in the spinflop transition in MnBr2.4H20. The spin-wave spectrum of the spin-flop phase is given, and the magnetization in the spin-flop phase is found to increase more rapidly than linearly with field because of quantum corrections absent in molecular field theory. (Received 23 April 1964) ?.REIATION OF BONDINGAND ELECTRONIC BAND STRUCTURE TO THE CREATION OF LATTICE VACANCIES IN TiO. S. P. Denker (Columbia University, New York, N.Y. 10027, U.S.A.). The relationship between bonding and electronic band structure in TiO is examined and it is found that a high equiIibrium lattice B Supported by the Advanced Research Project Agency. 7 Supported by the U.S. Office of Naval Research.
ix
vacancy concentration is energetically favorable. Lattice vacancies in TiO allows crystallization as a defective rock-salt type structure, emptying high-energy orbitals by reducing the effective number of valence electrons, which lowers the Fermi energy. Structural stability and mechanical properties of TiO, TIN and TiC are related to the position of the Fermi level, which for 100 per cent dense crystals of TiN and TiO would be forced to lie across antibonding levels. Consideration of energy band calculations by Bilz shows that lattice vacancies lower the Fermi energy by an amount (1.75 eV for TiO) which exceeds the total energy required to create a single titanium or oxygen vacancy (0.31 eV). In this process primarily antibonding orbitals are emptied giving for stoichiometric TiO a net reduction of ca. 1.1 eV in the free energy at absolute zero. (Received 28 April 1964) 8 THE EFFECT OF ELECTRIC FIELDS ON THE LUMINESCENCEAND CONDUCTIVITY OF ZnS SINGLE CRYSTALS. V. Bar, E. Alexander, J. Brada and I. T. Steinberger (Department of Physics, The Hebrew University, Jerusalem, Israel). Application of a direct electric field (of the order of lo4 volt/cm) causes a temporary increase of the luminescence of ZnS crystals (“Gudden-Pohl effect”). It is demonstrated that the light pulse is always accompanied by a current pulse. Studies of the effect showed that both pulses appear only if the crystal has been optically excited. The sizes and shapes of the pulses were systematically studied during the excitation, afterglow and thermal glow. Kinetic equations were set up, governing the free and trapped electron concentrations in the crystal. Most coefficients of these equations were experimentally determined by conventional luminescence and photoconductivity methods. Three different ways by which the field might cause release of electrons from traps, were considered: (a) tunneling from traps, (b) diminish ing the effective trap depths by the field and thereb enhancing thermal release, and (c) impact ionization of traps by accelerated conduction-band electrons. The kinetic equations showed that with either of these mechanisms current and light pulse appear, as observed in the experiments. However tunneling (a) would yield completely negligible effects at even much higher fields than actually applied and is in disagreement with the experimenl al results on the dependence of the pulse heights on the state of excitation of the crystal. Mechanis (b) can account for the order of magnitude of the