Surface Review and Letters, Vol. 9, Nos. 5 & 6 (2002) 1721-1724 @ World Scientific Publishing Company THE HALL EFFECT IN HYDRIDED RARE EARTH FILMS: REMOVING BILAYER EFFECTS D. W. KOON Physics Department, St. Lawrence University, Canton, NY 1361 7, USA dkoon@stlawu.edu D. E. AZOFEIFA and N. CLARK CICIMA and Escuela de Fisica, Universidad de Costa Rica, San Jose', Costa Rica We describe two new techniques for measuring the Hall effect in capped rare earth films during hydrid- ing. In one, we simultaneously measure resistivity and the Hall coefficient for a rare earth film covered with four different thicknesses of Pd, recovering the charge transport quantities for both materials. In the second technique, we replace Pd with Mn as the covering layer. We will present results from both techniques. 1. Introduction Palladium overlayers have been used in a number of studies of hydriding in The Pd overlayer can help stabilize the under- lying material during hydriding , preventing it from crumbling into powder during the process. It can also protect the underlying material from corrosion by oxygen after deposition, as would occur for the highly reactive rare earths. Finally, Pd can catalyze the absorption of hydrogen by converting molecular hydrogen into atomic hydrogen, in the case of-mate- rials like a l ~ m i n u m , ~ converting the hydrogen into a form that can be absorbed by the material. The presence of this overlayer, however, compli- cates the analysis of charge transport data - re- sistivity and the Hall coefficient - taken during the hydriding process. For such studies, it becomes necessary to subtract off the effect of the overlayer from the rest of the bilayer. To determine whether this overlayer significantly alters charge transport in the bilayer, one needs to know how the resistivity and Hall coefficient of the bilayer depend on the charge transport of each layer. The two films behave like two parallel resistors. That is, their inverse resistances, 1/R, add. In terms of the material properties of the films, where t is the thickness, a the conductivity, and the subscripts 1 and 2 refer to the two films. In the pres- ence of a magnetic field, the conductivity becomes a nondiagonal tensor, with all of its components be- having as in Eq. (1). This includes the off-diagonal component^^^^ as well, where OH is the Hall conductivity, for modest magnetic fields, H (such that the product of H, the Hall coefficient, RH, and the conductivity, a, is sufficiently small: H R H a << 1). Since the Hall conductivity is inversely propor- tional to the square of the resistivity, the resistivity of each constituent layer is the most important pa- rameter in determining the composite Hall coefficient of the bilayer. For rare earth films covered with much thinner layers of Pd, the majority of the Hall conduc- tion often takes place within the Pd layer, as shown