Formation of bcc non-equilibrium La, Gd and Dy alloys and the magnetic structure of Mg-stabilized [beta] Gd and [beta] Dy

Herchenroeder, James
Major Professor
Karl A. Gschneidner, Jr.
Committee Member
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Materials Science and Engineering

The high temperature bcc allotrope of a rare earth metal has the potential for substantially different magnetic properties than the room temperature hexagonal (hcp or dhcp) counterpart because of its more symmetrical crystal field. The stabilization by alloying and quenching of this bcc phase was studied for La-M alloys where M is a non-rare earth metal from Group II or III. The factors influencing the stabilization, such as size of M and quench rate, are discussed. [gamma]La (bcc) could be retained over a composition range around the eutectoid composition by Mg or Cd alloying. A comparison of T[subscript] o curves of the various alloy systems suggest that the eutectoid temperature of the La-M system must be approximately equal to or less than a critical T[subscript] o temperature of 515°C if the bcc phase is to be retained by quenching. The thermal stability of [beta]Gd (bcc) was investigated by DTA and isothermal annealing. It was found to transform to an intermediate phase before reverting to the equilibrium phases in contrast to [gamma]La alloys which decompose directly on heating to the equilibrium phases;Bcc [beta]Gd and [beta]Dy stabilized by Mg additions exhibit spin glass-like behavior. Both systems show field cooling effects in the magnetic susceptibility which is indicative of spin freezing reactions. The [beta]Gd alloys order ferromagnetically (<80 K) first on cooling before undergoing a Gabay-Toulouse type disordering transition (<50 K) into a mixed ferromagnetic plus spin glass phase. Low field ac susceptibility measurements show both the Curie and spin freezing transitions. Low temperature heat capacity (down to 1.5 K) shows evidence of both ferromagnetic and spin glass excitations. A magnetic phase diagram predicts a pure spin glass phase for Gd concentrations up to 66 at.% Gd. The [beta]Dy alloys exhibit a cusp in the ac susceptibility characteristic of spin glass behavior. Field cooled magnetic susceptibility measurements suggest a close competition between antiferromagnetic and spin glass behavior. Retention of the susceptibility maximum to 1.4 T is evidence that the ordered magnetic state may be a mixture of antiferromagnetic and spin glass phases. A large linear heat capacity term which is probably due to both the electronic specific heat, [gamma], and a spin glass contribution plus the presence of a large T[superscript]2 and T[superscript]3 terms support the mixed state hypothesis.