University of California at Berkeley – Physics Department

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