Magnetic properties and Mossbauer spectroscopy of novel alloys

Abstract

This thesis is an experimental study of structural and magnetic properties, and hyperfine interactions in crystalline alloys Cu2GdIn, CrNiP, CrNiAs, EuCu2Si2, Cr2FeSe4 and icosahedral quasicrystals Al60Cr19.9Fe0.1Ge 20, Ag50In36Gd14, Zn77Fe 7Sc16. The Heusler alloy Cu2GdIn is shown to crystallize in the L21 crystal structure (space group Fm3¯ m) with a lattice constant of 6.6643(3) A. It is an antiferromagnet with the Neel temperature of 9.6(1) K, the effective magnetic moment of 7.98(4) muB per Gd atom, and the paramagnetic Curie temperature of --41.2(9) K. Its Debye temperature is 229(5) K. The alloys CrNiP and CrNiAs crystallize, respectively, in the Co 2P-type structure (space group Pnma) and the Fe 2P-type structure (space group P6¯2 m), with the lattice parameters a = 5.7965(1) A, b = 3.5337(1) A, c = 6.8123(2) A, and a = 6.1128(2) A, c = 3.6585(1) A. CrNiP is shown to be a three-dimensional Heisenberg ferromagnetic with the Curie temperature of 142.9(6) K, whereas CrNiAs is demonstrated to be a mean-field ferromagnet with the Curie temperature of 171.9(1) K. A long standing controversy concerning the Ni magnetic moment in these alloys is solved unequivocally: Ni atoms carry a magnetic moment of 0.14 muB in CrNiP and 0.15(3) mu B in CrNiAs. The Debye temperatures of CrNiP and CrNiAs are, respectively, 261(3) K and 221(1) K. EuCu2Si2 is one of the first Eu-based alloys in which intermediate valence behavior of Eu was observed. It is demonstrated here that there are no valence fluctuations of Eu in this alloy: Eu atoms are divalent in the temperature range 2-300 K. The Debye temperature of EuCu 2Si2 is shown to be 236(4) K. Cr2FeSe4 is found to have antiferromagnetism with weak ferrimagnetism due to the two different distortions of the octahedral Cr3+ and the octahedral Fe2+. A negative magnetization is found in a magnetic field of 5 De and below due to the coercive-field magnetization reversal. The magnetization reversal disappears in a magnetic field larger than 65 Oe. The icosahedral quasicrystals Al60Cr19.9Fe 0.1Ge20, Ag50In36Gd14, and Zn77Fe7Sc16 are shown to have a primitive six-dimensional Bravais lattice with a six-dimensional lattice constant of 6.558(2) A, 7.805(2) A, and 8.087(1) A, respectively. Al 60Cr19.9Fe0.1Ge20 is a paramagnet with the effective magnetic moment of 0.312(3) muB per Cr/Fe atom. Its Debye temperature is 463(15) K. The temperature dependence of the electrical conductivity of Al60Cr19.9Fe0.1Ge20 is well accounted for quantitatively by theories of quantum interference effects. Ag50In36Gd14 and Zn77Fe 7Sc16 are shown to be spin glasses with the spin freezing temperature of 4.25(5) K and 7.75(2) K, respectively. The hyperfine magnetic fields at 155Gd and 57Fe nuclei in g50 In36Gd14 and Zn77Fe7Sc 16 are shown to set in at temperatures larger than the corresponding freezing temperatures. The frequency dependence of the freezing temperature in Zn77Fe7Sc16 is shown to be equally well accounted for by the Vogel-Fulcher law and the power law. Analysis of the aging effects observed in Zn77Fe7Sc16 leads to a major finding that the nature of the spin-glass state in this quasicrystal is fundamentally different from that of a canonical spin glass. The Debye temperatures of Ag50In36Gd14 and Zn 77Fe7Sc16 are, respectively, 199(2) K and 443(8) K.