university of california at berkeley – physics department
DESCRIPTION
Thermodynamic measurements of iron-rhodium alloys. David W. Cooke, Frances Hellman Physics Department, University of California, Berkeley. Stephanie Moyerman , Eric E. Fullerton Physics Department, University of California, San Diego. - PowerPoint PPT PresentationTRANSCRIPT
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Thermodynamic measurements of iron-rhodium alloys
David W. Cooke, Frances HellmanPhysics Department, University of California, Berkeley
Stephanie Moyerman, Eric E. FullertonPhysics Department, University of California, San Diego
Why Fe-Rh?
Superparamagnetic limit – KUV ~ kBT
Large K?
Alternative: FePt / FeRh bi-layerThiele, J.-U., Maat, S., and Fullerton, E.E. APL 82, 2859 (2003)
FeRh undergoes an AFM>FM transition at Tcrit ~ 50ºC
RT < T < Tcrit: AFM FeRh; large K fixes FePt momentTcrit < T < TC: FM FeRh reduces HC to flip FePt via coupling
→ Large H or T ~ TC
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
MgO (100)
FePt (111)
FeRh (100)
m
m
FeRh Magnetic Phases
AFM IIT < Tcrit
FMTcrit < T
Tcrit
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Electrons in AF/FM States
Tu, P. et al. J. Appl. Phys. 40, 1368 (1969)
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
AFM
FM
222
3
1BF kDT
T
C Koenig, C. J. Phys. F 12, 1123 (1982)
εF
εF
Electrons in AF/FM States
%6
'
,0,0
'
0
FMAFM
critAFF
T
F
UU
TS
TDdTT
CS
Tu, P. et al. J. Appl. Phys. 40, 1368 (1969)
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
AFM
FM
222
3
1BF kDT
T
C
Alloys allow for tuning of Tcrit , eventually pushing Tcrit = 0 yielding a FM ground state
Electron-driven modelPros of electron-driven model:• Difference in N(EF) seen in DFT for AF/FM states because of
splitting of d-bands leaving gap at EF
• Assuming fixed N(EF) of T, matches experimental ΔS at Tcrit for a number of alloys
Cons of electron-driven model:• Fe49.5Rh45.5Ir5 has higher Tcrit but γAFM ~2γFM!
Outstanding questions:• No model of field/alloy dependence of Tcrit
• No DFT work explaining lack of Tcrit in certain alloys
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Thermal Fluctuation Model
nnn
iknnnnn
iknni nnnnn
kiikikii rVrVSSrJSDU,
2
• Note the shoulder at ~200K• Two-state system (Schottky)
• FM – competition between non/magnetization of Rh• AFM – no such competition because Fe AFM cancels
Gruner, M.E., et al. Phys. Rev. B 67, 064415 (2003)
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Tcrit Tcurie
“Calorimeter on a Chip”
• Specific heat of thin films– 30nm-200nm– 2K - 500K– 0T - 8T
2006 APS KeithleyInstrumentation Award
t
e
C
T
P
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
IBAD MgO Calorimeter
45º
MgO Target
Substrate
TargetIon Source
SubstrateIon Source
Figure adapted from L.S. Yu, et. Al., J. Vac. Sci. A 4, 443 (1986)
IBAD MgO(100) On Devices
• Current calorimeters limited to amorphous/polycrystalline films
• Use IBAD MgO on SiNX as template to grow biaxially-oriented films!
• MgO grows (100) out of plane
• 45º to substrate yields (110) in-plane due to channeling
• Provides biaxially oriented substrate
• Well-studied for high-Tc materials
• Can use as template for STO, etc.
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
FeRh Magnetization Data
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
FeRh XRD on IBAD MgO
• Similarly high crystal quality with some relaxation on IBAD
• Slightly more mosaicity than MgO sample
• Maintains four-fold symmetry
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
(100) out-of-plane Azimuthal scan
Specific Heat of Fe.49Rh.51
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Specific Heat of Fe.49Rh.51
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Future Work
University of California at Berkeley – Physics Department
March APS Meeting, Portland, OR – March 17, 2010
Conclusions:
• We have grown IBAD MgO on our unique a-SiNx-based microcalorimeters
• We have confirmed growth of high quality Fe.49Rh.51 films on IBAD MgO
• Specific heat data obtained on Fe.49Rh.51 matches that in the literature
• Growth of an Fe-rich alloy on IBAD MgO was confirmed to be FM down to 2K
Future work:
• Measure CP of this FM FeRh as a function of H, T to examine two-state Rh theory
• Ongoing collaboration to examine domain formation during AF>FM transition in using our devices as an in-situ heater stage in magnetic soft x-ray transmission microscope at the Advanced Light Source (LBNL)
• DFT calculations of non-stoichiometric FeRh alloys