Magnetocaloric effects in intermetallic compounds

1
Magnetocaloric effects in intermetallic compounds
Introduction Experimental results
discussion Conclusions

• Magnetic phase transitions
• Magnetocaloric effects Magnetic refrigeration
• - Magnetic-refrigerant materials

• 2nd order phase transition MCE
• 1st order phase transition MCE

2
Introduction Magnetic phase transitions
PM
FM
Tc
3
TN
TN
4
Magnetic field-induced transition
5
First-order phase transition
Magnetization


M
Entropy
Volume
6
Second-order phase transition



TC
7
Magneto-caloric effect Magnetic refrigeration
Adiabatic ?Tad Isothermal ?Sm
T
T?T
S
N
?Q
?Q
Absorb heat
T
T-?T
N
S
Cooling effect
8
Thermodynamics
Large ?B
Large
Small CB,p
9
Metal Gd sphere 3 kg Energy efficiency
20-60 Cooling power 200 W-600 W C.O.P
2-9 ?T 4.5 K for 1.5 T ?T 11K for 5 T
Magnetic field
Superconducting magnet
Gd
10
Permanent magnetic field
Space 114 x 128 x 12.7 mm3 Field strength ? 2 T
Nd2Fe14B magnet
Lee et al. JAP (2002)
11
Magnetic refrigerant materials
12
Adiabatic temperature change
13
Ordering T TC 295 K Field change
?B 5 T FWHM dTFWHM 65 K MAX
entropy change -?Sm(max) 8.5
J/kgK Relative cooling power RCP(S)
-?Sm(max)dTFWHM 552 J/kg Cooling
power
What are important for MR?
14
Experimental results discussion
Second order magnetic phase transition MCE
Gd
Sth(max) RLn(2J1)17.3 J/molK Sth(max) 110
J/kgK lt10
15
TC 298 K ?B 2 T ?Tad 1.7 K
Hashimoto et al (1982)
16
First-order magnetic phase transition MCE
Orthorhombic
Orthorhombic
Pecharsky et al (1997)
17
What makes Gd5Ge4-xSix have giant MCE?
Single crystal Gd5Si1.7Ge2.3 Monoclinic
(P1121/a) a 7.585 Å b 14.800 Å c 7.777 Å
ß 93.290
TC240.41 K
0.05 T
18
B-T phase diagram
19
Magnetization
Field-induced magnetic phase transition PM
FM Field hysteresis 1 T
20
Magnetic entropy changes
TC 240 K ?B 5 T ?S(max)
30.5J/kgK dTFWHM 18K RCP(S) 549
J/kgK Effect of magnetic anisotropy is small
21
Specific heat capacity
Gd5Si1.7Ge2.3


at TC ?S 11.0 0.5
J/molK Latent heat L 2.63 0.12 kJ/mol
22
?Tad Tc?Sm/Cp gt 15 K
23
Thermal expansion ?L/L (L(T)-L(T 5 K))/L(T
5 K)
Transition at TC 240.0 1.0 K TC 236.0 1.0
K Thermal hysteresis ?T 4 K ?La/La 6.8x10-3
gt0 ?Lb/Lb -2.0x10-3 lt0 ?Lc/Lc -2.1x10-3
lt0 Relative volume change ?V/V
2.7x10-3 Clausius-Clapeyron relation dTC/dp
3.2 0.2 K/kbar
M. Nazih et al. 2002
24
Transition-metal based compound MnFeP1-xAsx
Crystal structure (0.15 ? x ? 0.65) Fe2P-type
Hexagonal Space group P-62m
At transition ?c/c gt 0 ?a/a lt
0 ?V/V lt 0 There is no
crystallographic symmetry change. Magnetic
moment ?4 µB/f.u.
Fe-layer
Mn-layer
Fe-layer
3g
1b/2c
3f
25
X-T phase diagram
Composition dependence of TC
H
PM
AF
FM
T
O
X
Bacmann et al. JMMM(1994)
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Magnetization
160-330 K
Field hysteresis 0.5 T
Thermal hysteresis 3.4 K
27
B T phase diagram of MnFeP0.45As0.55
Ordering T TC 306 K TC 302.2 K Thermal
hysteresis 3.8 K ?TC/?B 4.2 K/T
First order phase transition
28
Specific heat capacity
Tp 296 K Latent heat L 526 J/mol Cp 550
J/kgK (T gt 300 K)
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Magnetic entropy changes
TC 306 K ?B 5 T -?S(max) 18.3 J/kgK dT
21.3 K RCP(S) 390 J/kg ?Tad Tc?SM/Cp ?Tad
10 K (?B5 T)
30
Magnetic entropy change in different compositions
MnFeP1-xAsx
Isothermal magnetic entropy changes
31

• Conclusions
• MCE is closely related to the critical behavior
  of magnetic
• phase transition.
• Second order transition gives broad MCE peak. MCE
  is small.
• First order transition gives sharp MCE peak. MCE
  can be large.
• 2. Gd5Si1.7Ge2.3 has a simultaneous structural
  and magnetic phase
• transition at 239 K. This transition is a first
  order transition with
• thermal hysteresis ? 7.4 K and with field
  hysteresis ?1 T.
• The MCE related with first order phase
  transition is quite large.
• Effect of magnetic anisotropy on MCE in this
  material is negligible.
• MnFeP1-xAsx (0.25ltxlt0.65) has a first order phase
  transition
• with thermal hysteresis ? 3.4 K and field
  hysteresis ? 0.5 T.
• The MCE related with this transition is also
  quite large.

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4. Advantages of MnFeP1-xAsx as a magnetic
refrigerant 1. Large MCE 2. Tunable ordering
temperature( between 168 and 332 K) 3. Small
hysteresis 4. Lower cost MnFe(P,As)
Mn,Fe,P,As(99,
150/kg) Gd-Si-Ge
Gd Gd(4N)
4000 /kg. Fe-Rh
Rh
12000/kg
33
Acknowledgment This work is supervised by E.
Brück, J.H.K. Buschow, F.R. de Boer. Collaborator
s L. Zhang, W. Dagula, X.W. Li Financially
supported by the STW.
34
Bean-Rodbell model Gibbs free energy G
Gex GH Gdist Gentr Gpress Volume
change is due to the effect of magnetization.
Tc Curie temperature T0 Curie temperature (not
compressible) V volume V0 volume(absent of
exchange interaction)
N number of atoms/V0 K compressibility s
relative magnetization
(J 1/2)
35
Bean et al. PR(1962)
2
1
?
0
s
J1/2
Set P 0 ? 0 s 0 TC T0 ?
lt 1 corresponds to 2nd order phase transition ? gt
1 corresponds to 1st order phase transition
For MnFeP0.5As0.5 ? 1.62, J 2, T0 250
K R. Zach et al. JAP (1998)
36
Heat capacity in field
Adiabatic T change