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Induced moment antiferromagnet in an external magnetic field. I. Molecular field theory
ABSTRACTS OF PAPERS TO APPEAR IN I. PHYS. CHEM. SOLIDS
viii
and ThV04 can be studied in detail, experimentally, by applying an appropriate stress to the crystals. Based on the previous measurements, it is shown to be feasible to apply a stress to vary the crystal-field splitting, and actually induce a magnetic phase transition, even at T =0 and in the absence of the enhancement effects of a possible further cooperative J—T interaction. The possible cooperative J—T interaction can also be ascertamed from such a study.
Vol. 17, No.4
formation is probably associated with the presence of various tripositive cation and uninegative anion species depending upon the impurity concentrations of the sample. This implied impurity-controlled cation vacancy concentration is consistent with the earlier observed extrinsic nature of cation diffusion in SrO at 1200°C. Received 14 August 1974 Revised 12 March 1975
Received 2 April 1975 13. 12.
THE INFLUENCE OF EXCESS OXYGEN ON THE ELASTIC PROPERTIES OF NON-STOICHIOMETRIC SrO CRYSTALS S..Y. Pal and J.M. Sivertsen, Dept. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, U.S.A.
Measurements have been carried out of the elastic constants of SrO in the virgin undoped state and of the changes produced in them by equilibrium doping with oxygen at 1200°Cand oxygen partial pressure of 0.95 atm. The method used was Papadakis’ pulseecho overlap technique on conjunction with thermogravimetric analysis (T.G.A.) to determine mass and density changes due to oxygen doping.
A two level two sublattice antiferromagnetic induced moment system is examined in the Green’s function theory in the random phase approximation. Attentions are drawn to the effects of an externally applied magnetic field. It is shown that a “soft mode” behavior occurs at the transition field similar to that observed at the transition temperature in the absence of a field. The transition becomes first order in the RPA. The sublattice magnetizations and the transition field are significantly lowered compared to the molecular field theory results.
The values obtained for C11, C12 and C~of the
virgin crystal at 23 C are C11 C 12
=
INDUCED MOMENT ANTIFERROMAGNET IN AN EXTERNAL MAGNETIC FIELD. II. GREEN’S FUNCTION THEORY Bappanadu N. Rao and Yung-li Wang, Department of Physics, Florida State University, Tallahassee, FL 32306, U.S.A.
Received 7 April 1975
l7.60±0.O3xlo
11dyne/cm2 ii
2
4.808 ±0.007 x 10 dyne/cm2. 5.577 ±0.008 x iO’~dyne/cm (These values are in very good agreement with those of Son and Bartels.) = =
14.
INDUCED MOMENT ANTIFERROMAGNET IN AN EXTERNAL MAGNETIC I. MOLECULAR FIELD THEORYFIELD. Bappanadu N. Rao and Yung-li Wang, Department of Physics, Florida State University, Tallahassee, FL 32306, U.S.A.
Values for 6C
11/C11 and 6C~/C~ were found to be 1.74% and —0.86% respectively. Accurate values for 6C12/C12 could not be obtained because of sample size limitations after quenching. However, C12 was shown to defmitely increase due to doping. —
Analysis of the results indicate that the elastic modulus changes can only be attributed to the formation of cation vacancies during doping. Analysis of the T.G.A. behavior indicates that this cation vacancy
The behavior of a two-level induced moment antiferromagnet in an external magnetic field is investigated in the molecular field approximation. A significant reduction in the critical field and in the sublattice magnetizations is shown. However, the total magnetization rises more rapidly with field and can remain at large value in an external field even at T = 0. The magnetic susceptibility also remains finite at T = 0 in contrast to the case of a permanent moment Ising
Vol. 17, No.4
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
antiferromagnet. The effects of a ferromagnetic nextnearest neighbor interaction are thex~examined. It is shown that, in contrast to the usual Ising antiferromagnets, the ferromagnetic coupling has to exceed a certain value depending on the crystal field strength and the antiferromagnetic interaction, to allow for a first order phase transition in a field to occur even at zero temperature.
ix
form illumination. Steady state solutions have been computed, and used to assess the nature of the recovery process after cessation of the light. It is concluded that measurements of photoconductive decay do not in general lead to reliable lifetime estimates because, besides recombmation, the movement of space charges is involved in the recovery. Received 30 December 1974
Received 1Apr11 1975 17. 15.
PROPRIETES SUPERFICIELLES DE
-
w.s. Glaunsinger, Department of Chemistry,
L’HEXAMETHYLENE ADIPAM1DE ET CONTACTS METAL POLYMERE N. Rosenberg, G. Guillaud, R. Michel et M. Maitrot, Laboratoire de Physique Electronique 2, Université Claude Bernard Lyon 69100 Villeurbanne, France. The dielectric properties in the low and ultra low frequency range of hexamethylene adipamide samples in a “dry” or wet atmosphere or in vacuum are studied with some metallic contacts; in the range of temperatures above the glassy transition, the complex dielectric permittivity can be accounted for by a recent and sophisticated calculation of J.R. MacDonald. Water seems to be a protonic donor, and in a wet sample protons are majority carriers, and are blocked at the electrodes. In very dry samples two kinds of carriers are to be used, the negative ones being less mobile than the protons; their density seems to be a function of the electrode metal. The effect of a direct superposed voltage, and the nature-electronic or ionic of the negative carriers is discussed. Received 23 January 1975 Revised 17 March 1975
16.
MINORITY CARRIER INJECTION IN RELAXATION SEMICONDUCTORS UNDER ILLUMINATION C. Popescu and H.K. Henisch, Materials Research Laboratory and Department of Physics, The Pennsylvania State University, University Park, PA 16802, U.S.A.
The paper considers the injection of minority carriers into a relaxation semiconductor under urn-
EPR STUDY OF THE RKKY INTERACTION IN METALLIC Eu~Gd1 B6 Arizona State University, Tempe, AZ 85281, U.S.A. .
The RKKY interaction m metallic Eu~Gd1 B6 compounds has been studied by the EPR technique for x = 0.00, 0.20, 0.40, and 0.50. In the region of the absorption maximum the experimental spectra can be fitted to a lineshape function consisting of both Lorentzian absorption and dispersion components. Weiss constants, exchange fields, unbottlenecked g-shifts, and thermal broadening are calculated on the basis of the RKKY model and compared to the experimental results. The predicted Weiss constants and exchange fields are in good agreement with experiment, indicating the validity of the RKKY model. However, the predicted g-shifts and thermal broadening are much greater than those measured experimentally. The reduced g-shifts and thermal broadening are interpreted in terms of a magnetic resonance bottleneck, and the relaxation rates due to the interaction of the conduction electrons with the localized moments and lattice are evaluated. - ~
Received 20 September 1974 Revised 4 April 1975
18.
DONOR-ACCEPTOR PAIR RECOMBTh1ATION AND PHON REPLICA IN GaS~Se1 -x A. Mercier and J.-P. Voitchovsky, Laboratoire de Physique Appliqu [e,Ecole Polytechnique Fédérale, Lausanne, Switzerland. The photoluminescence of a series of mixed
viii
and ThV04 can be studied in detail, experimentally, by applying an appropriate stress to the crystals. Based on the previous measurements, it is shown to be feasible to apply a stress to vary the crystal-field splitting, and actually induce a magnetic phase transition, even at T =0 and in the absence of the enhancement effects of a possible further cooperative J—T interaction. The possible cooperative J—T interaction can also be ascertamed from such a study.
Vol. 17, No.4
formation is probably associated with the presence of various tripositive cation and uninegative anion species depending upon the impurity concentrations of the sample. This implied impurity-controlled cation vacancy concentration is consistent with the earlier observed extrinsic nature of cation diffusion in SrO at 1200°C. Received 14 August 1974 Revised 12 March 1975
Received 2 April 1975 13. 12.
THE INFLUENCE OF EXCESS OXYGEN ON THE ELASTIC PROPERTIES OF NON-STOICHIOMETRIC SrO CRYSTALS S..Y. Pal and J.M. Sivertsen, Dept. of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, U.S.A.
Measurements have been carried out of the elastic constants of SrO in the virgin undoped state and of the changes produced in them by equilibrium doping with oxygen at 1200°Cand oxygen partial pressure of 0.95 atm. The method used was Papadakis’ pulseecho overlap technique on conjunction with thermogravimetric analysis (T.G.A.) to determine mass and density changes due to oxygen doping.
A two level two sublattice antiferromagnetic induced moment system is examined in the Green’s function theory in the random phase approximation. Attentions are drawn to the effects of an externally applied magnetic field. It is shown that a “soft mode” behavior occurs at the transition field similar to that observed at the transition temperature in the absence of a field. The transition becomes first order in the RPA. The sublattice magnetizations and the transition field are significantly lowered compared to the molecular field theory results.
The values obtained for C11, C12 and C~of the
virgin crystal at 23 C are C11 C 12
=
INDUCED MOMENT ANTIFERROMAGNET IN AN EXTERNAL MAGNETIC FIELD. II. GREEN’S FUNCTION THEORY Bappanadu N. Rao and Yung-li Wang, Department of Physics, Florida State University, Tallahassee, FL 32306, U.S.A.
Received 7 April 1975
l7.60±0.O3xlo
11dyne/cm2 ii
2
4.808 ±0.007 x 10 dyne/cm2. 5.577 ±0.008 x iO’~dyne/cm (These values are in very good agreement with those of Son and Bartels.) = =
14.
INDUCED MOMENT ANTIFERROMAGNET IN AN EXTERNAL MAGNETIC I. MOLECULAR FIELD THEORYFIELD. Bappanadu N. Rao and Yung-li Wang, Department of Physics, Florida State University, Tallahassee, FL 32306, U.S.A.
Values for 6C
11/C11 and 6C~/C~ were found to be 1.74% and —0.86% respectively. Accurate values for 6C12/C12 could not be obtained because of sample size limitations after quenching. However, C12 was shown to defmitely increase due to doping. —
Analysis of the results indicate that the elastic modulus changes can only be attributed to the formation of cation vacancies during doping. Analysis of the T.G.A. behavior indicates that this cation vacancy
The behavior of a two-level induced moment antiferromagnet in an external magnetic field is investigated in the molecular field approximation. A significant reduction in the critical field and in the sublattice magnetizations is shown. However, the total magnetization rises more rapidly with field and can remain at large value in an external field even at T = 0. The magnetic susceptibility also remains finite at T = 0 in contrast to the case of a permanent moment Ising
Vol. 17, No.4
ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS
antiferromagnet. The effects of a ferromagnetic nextnearest neighbor interaction are thex~examined. It is shown that, in contrast to the usual Ising antiferromagnets, the ferromagnetic coupling has to exceed a certain value depending on the crystal field strength and the antiferromagnetic interaction, to allow for a first order phase transition in a field to occur even at zero temperature.
ix
form illumination. Steady state solutions have been computed, and used to assess the nature of the recovery process after cessation of the light. It is concluded that measurements of photoconductive decay do not in general lead to reliable lifetime estimates because, besides recombmation, the movement of space charges is involved in the recovery. Received 30 December 1974
Received 1Apr11 1975 17. 15.
PROPRIETES SUPERFICIELLES DE
-
w.s. Glaunsinger, Department of Chemistry,
L’HEXAMETHYLENE ADIPAM1DE ET CONTACTS METAL POLYMERE N. Rosenberg, G. Guillaud, R. Michel et M. Maitrot, Laboratoire de Physique Electronique 2, Université Claude Bernard Lyon 69100 Villeurbanne, France. The dielectric properties in the low and ultra low frequency range of hexamethylene adipamide samples in a “dry” or wet atmosphere or in vacuum are studied with some metallic contacts; in the range of temperatures above the glassy transition, the complex dielectric permittivity can be accounted for by a recent and sophisticated calculation of J.R. MacDonald. Water seems to be a protonic donor, and in a wet sample protons are majority carriers, and are blocked at the electrodes. In very dry samples two kinds of carriers are to be used, the negative ones being less mobile than the protons; their density seems to be a function of the electrode metal. The effect of a direct superposed voltage, and the nature-electronic or ionic of the negative carriers is discussed. Received 23 January 1975 Revised 17 March 1975
16.
MINORITY CARRIER INJECTION IN RELAXATION SEMICONDUCTORS UNDER ILLUMINATION C. Popescu and H.K. Henisch, Materials Research Laboratory and Department of Physics, The Pennsylvania State University, University Park, PA 16802, U.S.A.
The paper considers the injection of minority carriers into a relaxation semiconductor under urn-
EPR STUDY OF THE RKKY INTERACTION IN METALLIC Eu~Gd1 B6 Arizona State University, Tempe, AZ 85281, U.S.A. .
The RKKY interaction m metallic Eu~Gd1 B6 compounds has been studied by the EPR technique for x = 0.00, 0.20, 0.40, and 0.50. In the region of the absorption maximum the experimental spectra can be fitted to a lineshape function consisting of both Lorentzian absorption and dispersion components. Weiss constants, exchange fields, unbottlenecked g-shifts, and thermal broadening are calculated on the basis of the RKKY model and compared to the experimental results. The predicted Weiss constants and exchange fields are in good agreement with experiment, indicating the validity of the RKKY model. However, the predicted g-shifts and thermal broadening are much greater than those measured experimentally. The reduced g-shifts and thermal broadening are interpreted in terms of a magnetic resonance bottleneck, and the relaxation rates due to the interaction of the conduction electrons with the localized moments and lattice are evaluated. - ~
Received 20 September 1974 Revised 4 April 1975
18.
DONOR-ACCEPTOR PAIR RECOMBTh1ATION AND PHON REPLICA IN GaS~Se1 -x A. Mercier and J.-P. Voitchovsky, Laboratoire de Physique Appliqu [e,Ecole Polytechnique Fédérale, Lausanne, Switzerland. The photoluminescence of a series of mixed