- Lian Zhang Search for magnetic refrigerant materials WZI group meeting May 28 th 2003, UvA

Outline What & why is MR? Search for candidate materials  Gd 5 (Si,Ge) 4  Fe 2 Mn(Si,Ge)  La(Fe,Si) 13  MnFe(P,As) Conclusion

Fleet FR de Boer, KHJ Buschow E Brück, A de Visser, J Klaasse, YK Huang O Tegus AL Wolf

Why magnetic refrigeration?  Conventional gas expansion cooling Ozone depletion Global warming  Green MR Higher efficiency Less noisy More compact Low temperature capability

Magnetic Refrigeration Magnetic-materials change in temperature if subjected to magnetic-field change Magnetocaloric effect S T B0B0 B=0 Ericsson Q Q Carnot

T1T1 T2T2 S2S2 S1S1 SmSm  T ad Refrigerant capacity Table-like SS T Integration from isothermal magnetization curves Directly measure or compute from specific heat measurements

T SS B M B 1 > B 2 T1T2T3T4::TnT1T2T3T4::Tn Composite material TCTC Field-induced or not, behaves differently.

Gd 5 (Si,Ge) 4 Magnetostructural transition M T Hysteresis Misleading message

M. Nazih Solid State Comunications 126 (2003) O. Tegus Physica B 319 (2002)

Fe 2 Mn(Si 1-x Ge x ) Fe 3 Si:  T C > 800 K Fe 2 MnSi:  T C ~ 250 K  Cubic D0 3  Heusler-type Fe 3 Ge:  D0 19 (HT)  L1 2 (LT)

L. Zhang J. Alloys Comp. 352 (2003) D0 3  D0 19

Lattice change with Ge content T C - Ge content

L. Zhang Physica B 328 (2003)

Disappointed!

La(Fe,Si) 13 The highest 3d metal concentration (1:13) in RT intermetallic compounds Failed to be a good permanent magnet Cubic: weak anisotropy Palstra 1983

Fe Fe Hypothetical LaFe 13 structure: Space group = Fm-3c Unit cell = 8 f.u. = 112 atoms = 8 La + 8 Fe I + 96 Fe II Al, Si Moze 1999

Modification Substitution of Fe by other transition metals Mn: failed to get single phase LaFe Mn 0.65 Si 1.43 Co: LaFe Co 0.65 Si 1.43 has a 2 nd transition at T C =265K Doping with interstitial atoms B:  - Fe emerges in small amount of doping (B=0.2) N: difficulty in diffusion makes sample inhomogeneous and broadens the transition enormously C: saturated at C=0.5, while T C =250K

Field step 0.1T Field step 0.02T Field step 0.005T Be cautious with the height of  S peak when it is a 1 st order field-induced transition.

For the field-induced 1 st order transition, the field-up  S-T show point rotation symmetry with the field-down  S-T curve.

MnFe(P,As)

MnFeP MnFeAs T(K) Orth AF TNTNTNTN TNTNTNTN AF TCTCTCTC F TetrHex TNTNTNTN  s (  B /f.u.) ssss AF Beckman-Lundgren, 1991

O. Tegus Physica B 319 (2002)

Virgin effect

c b a a b c

X=0.4 X=0.2

X=0.2 X=0.4

Conclusion Materials for MR are ready Much room for improvement Behaviors vary: rich ingredients Theories are called Accommodate engineering challenges