University of California at Berkeley – Physics Department – Hellman Lab

1. Heat capacity measurements of disordered Cr films David W. Cooke, Zoe Boekelheide, Daniel R. Queen, Frances Hellman University of California, Berkeley University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

2. Motivation Nobel Prize Committee – 2007:“The discovery of giant magnetoresistance immediately opened the door to a wealth of new scientific and technological possibilities, including a tremendous influence on the technique of data storage and magnetic sensors.” FM NM FM University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

3. Motivation “Rough” “Smooth” Pierce, Stroscio, Unguris, Celotta, J.Mag.Mag.Mat200 (1999) • Spin density wave AFM sets short-range coupling in Fe/Cr MMLs • Roughness can lead to frustration • Alters competition between short- and long-range coupling terms Fullerton, Bader, Robertson, PRL77, 1382 (1996) University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

4. Cr Fermi Surface & SDW AF1 AF2 ISDW AF0 CSDW Kummamuru, Soh. Nature 452 (2008) University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

5. Stress, Disorder, & the SDW Boekelheide et al., PRB 76 (2007) University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

6. How to measure the DOS? Si Frame Thermometers Heater Membrane Thin Film Sample Denlinger, Abarra, Allen, Rooney, Messer, Watson, and Hellman. Rev. Sci. Instr.65, 946 (1994) University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

7. Mixed SDW CSDW ISDW CP Results • Different slopes equal different Debye temps. • High temperature values collapse • Increasing γ with increasing disorder ӨD = 405K γ =6.5 γ =1.55 ӨD = 585K University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

8. c E-P coupling and the DOS McMillan, Phys. Rev. 167 (1968) (a,b,c) Bulk data references: γ, ӨD from a) Heiniger et al. Phys. kondens. Materie5 (1966), N(ε) from b) Laurent, et al. PRB 23 (1981), and  from c) Brorson, et al. PRL64 (1990). University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

9. Disorder Broadening University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008

10. Conclusions • Within error, there is no observable difference in N between ISDW and CSDW • Increasing disorder leads to phonon softening, as expected • Strongly disordered samples show a strongly enhanced electron-phonon coupling • Disorder broadening accounts for the variation in N University of California at Berkeley – Physics Department – Hellman Lab MMM Conference (Austin, TX) – November 13, 2008