CPM Seminar

Exotic and Unexpected Magnetic Phenomena in Intra-Lanthanide Phases in Low Magnetic Fields (<1 kOe) and Low Temperature (<100 K)

Karl Gschneidner

AMES Laboratory and Iowa State University

We have studied several (R_1-xR'_x)M, where R and R' are different lanthanides and M is generally Al, at low temperatures (~2 to 50 K) and in low magnetic fields (0 to 1000 Oe). The alloys were studied by adiabatic calorimetry, dc magnetization and ac magnetic susceptibility. Among the (R, R')Al₂ systems studied to date are Er-Dy, Er-Tb, Tm-Tb, Ho-Tb, Ho-Dy and Ho-Er. In these systems unexpected first order phase transitions in the magnetically ordered state are observed near the magic composition (R_0.75R'_0.25)Al₂ when R and R' have opposite signs of the second order Stevens coefficient, α_J, and R' has a negative value of α_J. The sign of the Stevens coefficient is a reflection of the shape of the 4f electron density: it is negative when the shape is oblate and positive when it is prolate. Preliminary calculations suggest that when the average quadrupole moment of R and R' is zero, the other competing effects such as crystal field splitting, and exchange interactions may dominate giving rise to the observed first order transitions. When the quadrupole moment is non-zero the three different energy contributions compete with each other, giving rise to conventional magnetic and thermal behaviors in compositions that are more than �15 at.% away from the magic concentration.

We have also observed Griffith-like behavior (short range ferromagnetic clustering) in the paramagnetic state ~50 K above the Curie temperature. These will also be discussed.

This work is supported by the Office of Basic Energy Science, Division of Materials Sciences and Engineering of the U. S. Department of Energy under Contract No. DE-AC02-07CH11358 with Iowa State University.