1

Recent developments in (Mn,Fe)2(P,Si) materials

H. Yibole

Fundamental Aspects of Materials and Energy, TU Delft, The Netherlands

DDMC 2015, 2nd Nov

2

OutlinePart I:

- Parameters influencing the cyclability of the MCE

in (Mn,Fe)2(P,Si,B) compounds

- Compositional mapping of magnetocaloric figure of merit

- Mechanical stability

Part II:

- Growth of single-crystalline (Mn,Fe)2(P,Si)

- Magnetocrystalline anisotropy

- Magnetic behaviour towards the ferromagnetic state

3

Fe2P-based manganese compounds

Tegus et al., Nature 415, 150 (2002)

Magneto-elasticFirst-order phase transition

Hexagonal (Mn,Fe)2(P,Si) compounds

N.H. Dung et al., Adv. Energy Mater. 1, 1215 (2011)

∆B = 2 T∆B = 1 T

∆B = 5 T∆B = 2 T

4

Cyclability of the MCE

- Thermal hysteresis

- Finite transition width

- Shift of the transition due to the field

....

cyclic= measured

Irreversible

...

∆Tad

t

“Initial” state

Cyclability is a complex question !

Real FOMT

B =0

B =Bt

∆Tad

S

T

dTtr/dB

280 290 300 310 3200.0

0.5

1.0

1.5

2.0

2.5

T cycl

ic (K

)

T (K)

B = 1.1 T

Mn1.25Fe0.70P0.50Si0.50

H. Yibole et al., J. Phys. D: Appl. Phys. 47, 075002(2014)

Direct ∆T method

5

30 40 50 60 70

277

278

279

280+B +B+B+B+B +B

-B-B-B

T (K

)

t (s)

-B -B

Boron doped (Mn,Fe)2(P,Si) materials

F. Guillou et al., Adv. Mater., 26, 2671 (2014)

- Mn/Fe = 1, optimization of saturation MPromising MCE properties

50 100 150 200 250 3000

20

40

60

80

100

120

140

160

M

(A m

2 kg-1

)

T (K)

1 T

MnFe0.95P2/3-xSi1/3Bx

x = 0x = 0.075

Other promising compositions in this system?

- Metalloid site, controlling the phase transition

6

Influence of B content on hysteresis and sensitivity of transition to field

MnFe0.95P0.67-xSi0.33Bx

x = 0.070

x = 0.065

x = 0.060

x = 0, but Si=0.45

0

20

40

60

80

100

120

M (A

m2 k

g-1)

(c)

250 260 270 280 2900

20

40

60

80

100

120

M (A

m2 k

g-1)

T (K)

(d)

0

20

40

60

80

100

120

M (A

m2 k

g-1)

(b)0

20

40

60

80

100

120

M (A

m2 k

g-1)

(a)

0.05 T

1 T

dTtr/dB = 4.2 K/TdTtr/dB (1T) > hysteresis

dTtr/dB = 4.1 K/TdTtr/dB (1T) > hysteresis

dTtr/dB = 3.6 K/TdTtr/dB (1T) ≈ hysteresis

dTtr/dB = 2.1 K/TdTtr/dB (1T) < hysteresis

7

Magnetocaloric effect in B ~1 T

∆S from Indirect method,MB(T) using Maxwell equation

∆Tcyclic from direct measurement

- dTtr/dB (1T) > hysteresis, ∆Tcyclic is ensured

- dTtr/dB (1T) ≈ hysteresis, ∆Tcyclic is reduced

- dTtr/dB (1T) < hysteresis, ∆Tcyclic disappears

250 260 270 280 290

-20

-16

-12

-8

-4

0

T (K)

Cool. ; Heat. ; B0.07

; B0.065

; B0.06

; Si0.45

S

(J k

g-1 K

-1)

250 260 270 280 2900.0

0.5

1.0

1.5

2.0

2.5

T cy

clic(K

)

T (K)

Sweeping modeB = 1.1 T

B = 1 T

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Threshold hysteresis for MCE cyclability

Direct method ∆Tcyclic = 1.1 K

Indirect method ∆Tad = 2.65 K

- Distinction between ∆Tad and ∆Tcyclic

for materials with hysteresis.

- dTtr/dB (1T) ≈ hysteresis:

threshold for MCE cyclability.

- In (Mn,Fe)2(P,Si,B ), dTtr/dB ≈ 4.2 K the

maximal hysteresis ≤ 4 K for B ~1 T.

MnFe0.95P0.61Si0.33B0.06

40 50 60 70 80

263

264

265

-B

T

(K)

t (s)

+B (b)

B = 1T

260 265 270 275

-70

-60

-50

-40

-30

-20

-10

B = 0 Cool. ; Heat.

B = 1 T Cool. ; Heat.

[S-S

(280

K)]

(J k

g-1 K

-1)

T (K)

9

Mapping of magnetocaloric figure of merit

Coefficient of refrigerant performance (CRP) (Wood and Potter 1985)

Type of ∆T CRP

MnFe0.95P0.595Si0.33B0.075

∆Tad 0.62

∆Tcyclic 0.62

MnFe0.95P0.61Si0.33B0.060

∆Tad 0.78

∆Tcyclic 0.37

Gd ∆T 0.17

0,8 0,9 1,0 1,1 1,20,00

0,05

0,10

0,15

B

Mn

0

0,10

0,21

0,31

0,41

0,52

0,62

CRPcyclic(1 T)(a)

0,8 0,9 1,0 1,1 1,20,25

0,30

0,35

0,40

0,45

Si

Mn

(b)

max

0

( , ) 'B

C

refrigerant capacity S TCRPpositive work on refrigerant

M T B dB

Si = 1/3

B = 0.07(hysteresis < 1 K)

(hysteresis = 3.5 K)

150 < T < 380 K.

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Mechanical stability of (Mn,Fe)2(P,Si,B)

- Sample A: MnFe0.95P0.582B0.078Si0.34 (boron doped)

- Sample B: Mn1.25Fe0.7P0.5Si0.5 (Mn-rich)

- Sample C: MnFe0.95P0.55Si0.45 (Mn/Fe = 1)

-150 -100 -50 0 50 100 150 200 250-2

-1

0

1

2

3

4

5

6

[(c/a

)-(c/

a)T C

] / (c

/a) T C

(%

)

T-TC (K)

A B C

(c)

0 5 10 15 20

255

260

265

270

275

T C (K

)

(b)

0 5 10 15 20210

220

230

240

250

260

270

280

290

T C (K

)

n (cycle)

0 5 10 15 20 25

264

268

272

276

280

284

288

T C (K

)

TC Heating TC Cooling

(a)A

B

C

The degradation of material may result from:

- The volumetric stress resulting from volume change

- Anisotropic internal stress

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0 5 10 15 20 25 301E-6

1E-5

1E-4

1E-3

0,01

0,1

1

0 5 10 150

20406080

100120140

(1

0-6

m)

n (cycle)

(

m)

n (cycle)

T = 340 K

Mechanical stability

200 220 240 260 280 300 320 3400

4

8

12

16

20

24

28

32

36

(1

0-6

m)

T (K)

200 220 240 260 280 300 320 3400

50

100

150

200

250

300

350

400

450

500

(1

0-6

m)

T (K)

(b)

(a)

200 220 240 260 280 300 320 340

0

2

4

6

8

10

12

14

T (K)

(1

0-3

m)

(c)

(A)

(B)

(C)

Electrical resistivity measurement

- Sample A: MnFe0.95P0.582B0.078Si0.34 (boron doped)

- Sample B: Mn1.25Fe0.7P0.5Si0.5 (Mn-rich)

- Sample C: MnFe0.95P0.55Si0.45 (Mn/Fe = 1)

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Mechanical stability

- MnFe0.95P0.585Si0.34B0.075

~ no ∆V, small ∆(c/a)

- Mn1.25Fe0.7P0.5Si0.5

limited ∆V, small ∆(c/a)

- MnFe0.95P0.55Si0.45

limited ∆V, large ∆(c/a)0 1 2 3 4 5 6 18 19 20 210

200

400

600

800

HV

n (cycle)

Vickers micro-hardness measurements

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Part I:

− Definition of a thermal hysteresis threshold ensuring the cyclic character of the MCE in the

MnFe(P,Si,B) system

− Compositional mapping of the magnetocaloric performances : identification of MnFe(P,Si,B)

compositions with promising performances on a broad temperature range 150 < T < 380 K.

− Quantification of mechanical ageing due to cycling across the transition by DSC, resistivity

micro-hardness. The mechanical degradation is highly dependent on the (c/a) change.

Conclusions

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Part II: Single-crystalline (Mn,Fe)2(P,Si)

15

(Mn,Fe)2(P,Si) Single Crystals

- Anisotropy

Needs for single crystals:

- Homogeneity

- Absence of grain boundaries

Viktor Hoglin, PhD Thesis, Uppsala Universitet.

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- Growth technique: Flux method

- Metallic flux: tin

- Crystal habit: prismatic

- Crystal surface: regular and homogeneous

- Average dimension: ~ 0.15×0.15×1.5 mm3

- Colour: metallic

(Mn,Fe)2(P,Si) Single Crystals

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Single crystal diffraction and refinement

- Chemical composition (EDS): Mn0.83Fe1.17P0.72Si0.28

- Crystal structure: Hexagonal Fe2P

- Space group: 62

- Refinement method: full-matrix least-squares on F2

Reciprocal-Space mapping

Bruker AXS Kappa APEX II DiffractometerAtomic position: 3g (x1, 0, 1/2); 3f (x2, 0, 0); 2c (1/3, 2/3, 0); 1b (0, 0, 1/2)

hk0 hk1

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0 100 200 3000

20

40

60

80

100

120

0 50 100 150 200 250 3000

20

40

60

80

100

120

M (A

m2 /k

g)

T (K)

Virgin curve

c // H

H = 1 T

M (A

m2 /k

g)

T (K)

FC FW

Magnetic properties of ferromagnetic crystal

Chemical composition: Mn0.83Fe1.17P0.72Si0.28

First-order ferromagnetic transition !

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0.0 0.5 1.0 1.5 2.00

20

40

60

80

100

120

140

160

180

c0H

c // 0H

After two cycles

M (A

m2 /k

g)

0H (T)

T = 5 K

Mangetocrystalline anisotropy in Mn0.83Fe1.17P0.72Si0.28 crystal

Corrected demagnetization field

20

0.0 0.5 1.0 1.5 2.00

1

2

3

H

int/( 0M

)

(M)2 (T2)

sin sin

Sucksmith-Thompson plot

0.28 10 /

0.22 10 /

The preferred magnetization direction --- along the c axis !

Mangetocrystalline anisotropy energy for hexagonal system

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Mangetocrystalline anisotropy of related polycrystalline compounds

L. Caron et al, PRB 88, 094440 (2013)

Fe2P single crystalNeutron powder diffraction

- Fe-rich Mn0.66Fe1.29P1−xSix (x = 0.42, 0.37,0.34)

angle with c-axis 67˚, 46˚, 29˚Ou et al, J. Mag. Magn. Mater. 340, 80 (2013)

- MnFeP0.5As0.5along c-axisBacmann et al, J. Mag. Magn. Mater. 134, 59 (1994)

- Mn-rich Mn1.3Fe0.64P0.5Si0.5 and Mn1.1Fe0.9P0.8Ge0.2(a,b) planeDung et al, PRB 86, 045134 (2012); Liu et al, PRB 79, 014435 (2009);

0.28 10 /

0.22 10 /

2.3 10 /Fe2P

Mn0.83Fe1.17P0.72Si0.28

22

0 1 2 3 4 50

30

60

90

120

150

180

0 1 2 3 4 50

30

60

90

120

150

180

M (A

m2 /k

g)

0H (T)

c // 0H

T = 5 K T = 5 K

M (A

m2 /k

g)

0H (T)

c 0H

Peculiar magnetic behaviour towards ferromagnetic state

(After ZFC)

23

0 1 2 3 4 50

30

60

90

120

150

180

M (A

m2 /k

g)

0H (T)

T = 5 K 0 100 200 300 400 500 600 700

156

158

160

162

164

166

M (A

m2 /k

g)

t (s)

B = 2.6 T

c // µ0H

Spontaneous magnetization jumps

- Phase transition behaviour- Not magnetic screening by eddy currents or domain wall movements

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Possible interpretations for magnetization jumps

(i) An underlying antiferromagnetic order

G. Li et al, Appl. Phys. Lett. 105, 262405 (2014).

- Prompted by chemical disorder on 3f and 3g site

(ii) A dynamical phase separation phenomenon

- General to a First-order phase transition (Manganites, Gd5Ge4, Heusler alloys, CeFe2, FeRh,etc.)

(iii) A combination of both

25

Conclusions

Part II:

− (Mn,Fe)2(P,Si) single crystals presenting FOMT have been grown for the first time.

− Crystal structure and magnetic properties support the findings of previous works on polycrystals.

− The weakening of the magnetocrystalline anisotropy from Fe2P to Mn0.83Fe1.17P0.72Si0.28

is beneficial for magnetocaloric application.

− The magnetization process toward the ferromagnetic state turns out to be complex.

26

Acknowledgements

Dr. YingKai Huang,Van der Waals-Zeeman Institute

Dr. Graeme R. Blake,Zernike Institute for Advanced Materials

Dr. Francois GuillouDr. Giacomo PorcariDr. Lian ZhangDr. Niels van DijkProf. Dr. Ekkes Brück

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More details:

H. Yibole et al., Appl. Phys. Lett. 107, 162403 (2015).

F. Guillou et al., Adv. Mater. 26, 2671 (2014).

H. Yibole et al., J. Phys. D: Appl. Phys. 47 075002 (2014).

F. Guillou et al., J. Appl. Phys. 116, 063903 (2014).

F. Guillou et al., J. Alloys & Compd. 617, 569 (2014).

F. Guillou et al., Phys. status solidi (c) 11, 1007 (2014).

F. Guillou et al., J. Alloys & Compd. 632, 717 (2015).

Thank you for your attention!