1. The Hall effect in hydrided rare earth films: removing bilayer effects D. W. Koon 1,2, D. E. Azofeifa 1, and N. Clark 1 1 CICIMA, Universidad de Costa Rica, CR 2 St. Lawrence University, Canton, NY, USA We describe two new techniques for measuring the Hall effect in capped rare-earth films during hydriding. In one, we simultaneously measure resistivity and Hall coefficient for a rare earth film covered with four different thicknesses of Pd, extrapolating to zero overlayer thickness. In the second technique, we replace Pd with Mn as the covering layer. We will present results from both techniques.

2. Acknowledgements Elián Conejo, Universidad de Costa Rica Associate Dean for Academic Affairs, St. Lawrence University Vicerectoría de Investigación, UCR Board of Trustees, SLU CICIMA, Escuela de Física de la UCR

3. Advantages of Pd overlayer in hydriding studies Catalyzes atomic hydrogen absorption (e.g. Al) Protects underlying metal from oxidation (e.g. rare earths) Maintains structural integrity of underlying metal Disadvantage of Pd overlayer: How does one remove the effects of the overlayer in resistivity, Hall, and optical transmission studies?

4. Resistivity and Hall effect in a bilayer: (1/R) total = (1/R) 1 + (1/R) 2 (t  ) total = (t  ) 1 + (t  ) 2 (t  H ) total = (t  H ) 1 + (t  H ) 2 where  = 1/ ,  H = HR H  2. A. Nedoluha and K. M. Koch, Z. Phys., 132, 608 (1952). D. W. Koon, D. E. Azofeifa, and N. Clark, submitted to Thin Solid Films (2001).

5. Effect of Pd overlayer on conductivities: tt  t  H at 0.6T (nm)     10 -6    Al501.8-1200 Dy3000.33-40 Gd3000.22-200 Pd200.19-200

6. Eliminating bilayer effects: I. The “Step Method” (a) Specimen, consisting of five connected crosses (b) Side view of the same specimen. The vertical scale is greatly exaggerated.

7. Eliminating bilayer effects: I. Step Method, continued Intercept:(t  metal (t  H  metal Slope: (  Pd (  H  Pd

8. Film deposition at CICIMA

9. Step Method: results for AlPd:H (50nm Al, 20nm Pd) Aluminum:  R/R = 2%,  R H /R H = 5%. Compare to Pd:  R/R = 7%,  R H /R H = 23% over the same range of H concentration.

10. Step Method: results for DyPd:H (300nm Dy, 20nm Pd) Conclusion: The Hall effect in Dy changes sign as the metal becomes insulator. (Or does it?)

11. Limits of the Step Method: “Si no temes a Dios, témele a los metales.”* * “Cien Años de Soledad”, Gabriel García Márquez Conclusion: The Hall conductivity of 20nm of Pd still dominates that of the 300nm of Dy, limits precision of Hall measurements for Dy.

12. Eliminating bilayer effects: II. “Alternative Overlayer Method” The alternative material must: Have low Hall conductivity (high resistivity) Dissociate molecular hydrogen (or at least be permeable to it) Be impermeable to oxygen (at least slow down its absorption)

13. Some candidate overlayer materials t(nm)t    t  H  10 -6    at 0.6T Pb200.095+6.8 Pu200.02+12 Bi200.017-13000 Mn200.015+1.5 Compare to…. Al501.8-1200 Dy3000.33-80 Gd3000.22-200 Pd200.19-200

14. Alternative Overlayer Method, DyMn bilayer (200nm Dy, 20nm Mn)

15. Disadvantages of Mn: 1) High film resistance means more noise, less signal. 2) Mn passes hydrogen very slowly (see Figure)

16. Conclusions We have developed two methods for reducing the effects of the Pd overlayer in hydriding studies: I. The Step Method works best for hydriding materials of high Hall conductivity (low resistivity). II. The Alternative Overlayer Method works best for materials with low Hall conductivity (high resistivity). Mn blocks oxygen absorption and allows hydriding, but only very slowly.

17. Inconclusions What affects the rate of hydrogen diffusion through the Mn overlayer? Does manganese dissociate molecular hydrogen into atomic form, or merely let it pass in molecular form? Does the Hall effect change sign in Dy:H as it absorbs hydrogen?