University of California at Berkeley – Physics Department – Hellman Lab

MMM Conference (Austin, TX) – November 13, 2008

Heat capacity measurements ofdisordered Cr films

David W. Cooke, Zoe Boekelheide, Daniel R. Queen, Frances HellmanUniversity of California, Berkeley

University of California at Berkeley – Physics Department – Hellman Lab

MotivationNobel 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 FMNM

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

Motivation2

212211 )ˆˆ()ˆˆ( mmJmmJE )(cos)cos( 2

21 JJ

Pierce, Stroscio, Unguris, Celotta, J.Mag.Mag.Mat 200 (1999)

“Rough” “Smooth”

•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, PRL 77, 1382 (1996)

tJtJ LS

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

Cr Fermi Surface & SDW

Kummamuru, Soh. Nature 452 (2008)

ISDW

001 Q

0

CSDW0

AF1

AF2

AF0

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

Stress, Disorder, & the SDW

Boekelheide et al., PRB 76 (2007)

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

How to measure the DOS?

Denlinger, Abarra, Allen, Rooney, Messer, Watson, and Hellman. Rev. Sci. Instr. 65, 946 (1994)

T

x

x

DP

D

e

dxxeTTC

/

02

4

3

3

1

2TT

CP

20

DT

13

22

FB Nk

34

5

12 D

R

Si Frame

Heater

Thermometers

Thin Film Sample

Membrane

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

CP Results

ISDW

CSDW

Mixed SDW

•Different slopes equal different Debye temps.

•High temperature values collapse

•Increasing γ with increasing disorder

ӨD = 585K

ӨD = 405K

γ = 6.5

γ =1.55

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

E-P coupling and the DOS

2

2

M

N F

McMillan, Phys. Rev. 167 (1968)

13

22

FB Nk

22D

M2 c

N (ε ) ΔN (ε ) γ Δγ Ө ΔӨ Δ

Bulk 0.49 1.55 0.05 585 15 0.13

Bulk-Like 0.49 0.04 1.60 0.10 500 30 0.18 0.03.75 mTorr 0.50 0.04 1.70 0.10 460 30 0.21 0.034 mTorr 0.69 0.08 2.60 0.20 423 30 0.35 0.068 mTorr 1.34 0.13 6.50 0.30 405 20 0.74 0.10

Bulk data references: γ, ӨD from a) Heiniger et al. Phys. kondens. Materie 5 (1966), N(ε) from b) Laurent, et al. PRB 23 (1981), and from c) Brorson, et al. PRL 64 (1990).

(a,b,c)

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

Disorder Broadening

tE

02

2 1

ne

mt

m

ne e

MMM Conference (Austin, TX) – November 13, 2008

University of California at Berkeley – Physics Department – Hellman Lab

ConclusionsBroadened Measured γ Δγ Ө ΔӨ ρ0 Density Δt ΔE

N (ε) N (ε) ΔN (ε) (mJ/mol.K) Δ (μΩcm) (g/cc) (fs) (eV)

Bulk 0.45 0.49 1.55 0.05 585 15 0.13 0.02 0.55 7.14 244.9 0.003Bulk-Like 0.46 0.49 0.04 1.60 0.10 500 30 0.18 0.03 6.1 7.14 23.0 0.029.75 mTorr 0.50 0.50 0.04 1.70 0.10 460 30 0.21 0.03 15.6 7.14 9.3 0.0714 mTorr 0.70 0.69 0.08 2.60 0.20 423 30 0.35 0.06 98.6 6.15 1.9 0.3488 mTorr 1.09 1.34 0.13 6.50 0.30 405 20 0.74 0.10 381.6 5.53 0.7 0.938

(K)

•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

MMM Conference (Austin, TX) – November 13, 2008