Magnetic properties and magnetocaloric effect in amorphous Co35Er65 ribbon

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Magnetic properties and magnetocaloric effect in amorphous Co 35 Er 65 ribbon

A. Boutahar

^a,n

, H. Lassri

^a

, K. Zehani

^b

, L. Bessais

^b

, E.K. Hlil

^c

aLPMMAT, Université Hassan II-Casablanca, Faculté des Sciences Ain Chock, BP 5366 Mâarif, Casablanca, Morocco

bICMPE-CMTR, UMR CNRS 7182, 2-8, rue H. Dunant, 94320 Thiais, France

cInstitut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France

a r t i c l e i n f o

Article history:

Received 11 January 2014 Received in revised form 21 May 2014

Available online 17 June 2014 Keywords:

Amorphous Co35Er65ribbon Magnetocaloric effect Low temperature Magnetic transition Arrott plot

a b s t r a c t

Amorphous Co35Er65ribbon was synthesized using melt spinning technique. Their magnetic properties and magnetocaloric effect (MCE) have been studied by the magnetization and isothermal magnetization of different temperature measurements. It is found that the sample obeys to theﬁrst-order magnetic transition (FOMT) from ferromagnetic (FM) to paramagnetic (PM) state at the Curie temperatures TC¼10 K. Using the thermodynamic Maxwell's relation, magnetic entropy changeðΔSMÞis estimated and discussed in terms of MCE. The ribbon presents a large magnetocaloric effect at low temperature reaching 5.97 J/K kg under magneticﬁeld of 0–5 T.

Published by Elsevier B.V.

1. Introduction

Amorphous Co

_100x

Er

x

ribbon exhibit many interesting physical phenomena such as competition between exchange interactions, large local random crystal

ﬁ

elds and random magnetic anisotropy [1

–

6].

Magnetic refrigeration based on the magnetocaloric effect (MCE) is advantageous being an environment friendly and energy ef

ﬁ

cient refrigeration mechanism. It is expected to be an important future cooling technology [7

–

10]. A large value of MCE is considered to be the most important requirement for industrial application near liquid helium temperature [11]. In addition, it is known that materials exhibiting large MCE below 70 K can be used in magnetic refrigeration or to liquefy hydrogen gas [12]. As a result, it is highly desirable to develop new magnetic materials applicable at a low temperature range with large MCE. Recently, some magnetic materials, in particular Er

2

Mn

2

O

7

[13], Er

2

In [14] and TbCo

3

B

2

[15] compounds showed a large reversible MCE at low temperature range.

In the present work, we have carried out further investigation on the magnetic properties and magnetocaloric effect of amor- phous Co

35

Er

65

ribbon.

2. Experimental details

The amorphous Co

35

Er

65

alloy was prepared by melt spinning technique. The X-ray diffraction (XRD) measurement was performed

at room temperature by using Co-K

_α

radiation. The sample was examined by XRD and was found to be amorphous (Fig. 1). The magnetic and magnetocaloric properties were determined using a physical property measurement system (PPMS) magnetometer from Quantum Design operating up to 5 T and a temperature range of 3

–

30 K.

3. Results and discussion

3.1. Magnetic properties

The temperature (T) dependence of the magnetization (M) (Fig. 2) reveals the presence of a ferromagnetic (FM) to paramag- netic (PM) transition at T

C¼

10 K, which is de

ﬁ

ned as the in

ﬂ

ection point of dM/dT (T) curve (inset Fig. 2). In order to better explain the change in the magnetic order.The metamagnetic behavior plays a critical role in determining the order of magnetic transition which in

ﬂ

uences directly the magnitude of the magnetocaloric effect.

For the present amorphous, the nature of the magnetic phase transition can be determined using the Inoue

–

Shimizu s

–

d model [16,17], which has been widely used to discuss behaviors of several types of magnetocaloric materials.

According to the spin

ﬂ

uctuation model based on the Landau theory [16,17], the magnetic free energy F (M, H) versus magne- tization and temperature can be expressed as follows:

F

¼

1 2 a

ð

T

Þ

M

²þ

1 4 b

ð

T

Þ

M

⁴þ

1 6 c

ð

T

Þ

M

⁶μ0

MH

ð

1

Þ

Contents lists available at ScienceDirect

journal homepage:www.elsevier.com/locate/jmmm

Journal of Magnetism and Magnetic Materials

http://dx.doi.org/10.1016/j.jmmm.2014.06.003 0304-8853/Published by Elsevier B.V.

nCorresponding author.

E-mail address:[email protected](A. Boutahar).

Journal of Magnetism and Magnetic Materials 369 (2014) 92–95

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The coef

ﬁ

cients a(T), b(T) and c(T) exhibit a temperature dependence according to the thermal variation of amplitude of spin

ﬂ

uctuation. The parameters of the F function are those of the Landau expansion corrected by the f

–

d exchange interaction. In this model, the Landau coef

ﬁ

cients express the magnetization dependence of the magnetic free energy. They can be determined through the equation of state linking magnetic

ﬁ

eld and magne- tization. In the thermal equilibrium conditions (dF/dM

¼

0), the equation of state is derived as:

a

ð

T

Þ

M

þ

b

ð

T

Þ

M

³þ

c

ð

T

Þ

M

⁵¼μ0

H

ð

2

Þ

Examination of the free energy demonstrates that the para- meter a(T) is always positive and would get a minimum value at Curie temperature corresponding to a maximum of the suscept- ibility. In the other hand, the order of magnetic transition is

Fig. 1.X-ray diffraction patterns of amorphous Co35Er65ribbon.

0 20 40 60 80 100 120

0.0 0.3 0.6

0 60 120

-0.04 -0.02 0.00

M (emu/g)

T (K)

dM/dT (emu/ g K)

T (K) Tc= 10 K

Fig. 2.Variation of the magnetization and thedM/dTas the function of temperature in an applied magneticﬁeld of 0.01 T for the amorphous Co35Er65ribbon.

0.3 0.6 0.9 1.2

-4 -2 0 2

0 10 20 30 0 10 20 30

0 10 20 30

1.0 1.5 2.0 2.5 T (K) a ( 10

^-1

T)

T (K) b ( 10

^-3

T)

T (K)

c ( 10

^-8

T)

Fig. 3.Temperature dependence of Landau coefﬁcients for the amorphous Co35Er65ribbon. The units fora(T),b(T) andc(T) are T²kg/J, T⁴kg³/J³, T⁶kg⁵/J⁵, respectively.

0 700 1400

0 1000 2000 3000

ΔT=3 K Co₃₅Er₆₅

M

2

(emu

2

/ g

-2

)

H/M (Oe g /emu)

3 K

29 K

Fig. 4. Arrott plots of the amorphous Co35Er65ribbon at different temperatures near closeTC.

A. Boutahar et al. / Journal of Magnetism and Magnetic Materials 369 (2014) 92–95 93

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governed by the sign of b(T) at the transition: a 1st transition takes place if b(T

C

)

o

0 while a 2nd order occurs when b(T

C

)

Z

0. Besides c(T) is positive at T

C

but otherwise it is negative or positive.

The values of Landau's coef

ﬁ

cients are determined

ﬁ

tting the magnetization traces to Eq. (2). Accordingly, b(T) was found to be negative for the Co

35

Er

65

ribbon studied here, indicating the 1st character to the magnetic transition for amorphous ribbon. Fig. 3 shows the temperature dependence of Landau's parameters for Co

35

Er

65

. As explained above, a(T) was found positive with a minimum close to T

_C

while b(T

_C

) was found negative indicating the occurrence of a FOMT. As shown in Fig. 3, the value of Curie temperature T

C

derived from thermomagnetic measurements is exactly that obtained from the a(T) behavior. These results were con

ﬁ

rmed from an analysis of Arrott's plots close to T

C

, as shown in Fig. 3. The in

ﬂ

ection point observed in Arrott's plots for the amorphous ribbon con

ﬁ

rms occurrence of a

ﬁ

eld-induced mag- netic transition, i.e. FOMT (Fig. 4).

3.2. Magnetocaloric effect

The magnetocaloric effect can be related to the magnetic properties of the material through the thermodynamics Maxwell's relations. It has been calculated in terms of isothermal magnetic entropy change using magnetization isotherms obtained at various temperatures (Fig. 5). According to the thermodynamically theory [18], the isothermal magnetic entropy changes associated with a magnetic

ﬁ

eld variation is given by

Δ

S

Mð

T

;Δ

H

Þ ¼

S

Mð

T

;

H

Þ

S

Mð

T

;

0

Þ ¼Zm0Hmax

0 ∂

S

ð

T

;

H

Þ=∂

H

T

H

ð

3

Þ

From Maxwell's thermodynamic relation

∂

S

ð

H

;

T

Þ

∂

H

T

¼ ∂

M

ð

H

;

T

Þ

∂

T

H

ð

4

Þ

One can obtain the following expression:

Δ

S

Mð

T

;Δ

H

ÞΔH¼Z _m₀Hmax

0 ∂

M

ð

H

;

T

Þ=∂

T

H

dH

ð

5

Þ

were

m0

H

max

is the maximum external

ﬁ

eld.

Fig. 6 shows the thermal variation of the magnetic entropy for Co

35

Er

65

ribbon. The (

Δ

S

M

) is found to increase with increasing temperature to reach a broad maximum at T

C

. It can be seen from Fig. 6 that the (

Δ

S

M

) of Co

35

Er

65

shows peaks around T

C

and the maximum values of (

Δ

S

M

) are found to be 2.76 J/kg K and 5.97 J/kg K for the

ﬁ

eld changes of 0

–

2 T and 0

–

5 T, respectively.

It is important to note that the peak values of (

Δ

S

M

) for Co

35

Er

65

can compete with the other popularly researched magnetic refri- gerant materials such as DyCoAl (16.3 J/kg K), GdPd

₂

Si (8.6 J/kg K), RNi

5

(8 J/kg K) and ErRu

2

Si

2

(17.6 J/kg K) in low temperature regime under the same

ﬁ

eld changes [19

–

22]. To compare differ- ent magnetocaloric materials, we calculate their relative cooling power (RCP) based on the magnetic entropy change. The RCP evaluated by considering the magnitude of (

Δ

S

M

) and its full width at half maximum

δ

T

^FWHM

was represented as follows [23]:

RCP

¼ Δ

S

^max_M δ

T

^FWHM ð

6

Þ

The (

Δ

S

max

) attained 5.97 J/kg K and the RCP is in the range of 131.34 J/kg for a

ﬁ

eld of 5 T (Table 1).

4. Conclusion

In summary, magnetic properties and magnetocaloric effect of the amorphous Co

35

Er

65

ribbon have been studied in detail. It is shown that the ribbon possesses a FM

–

PM transition at T

_C¼

10 K.

In addition, analysis of the magnetic ordering transition using the Landau theory and Arrott's plots reveals occurrence of a FOMT in the amorphous Co

35

Er

65

ribbon. For a magnetic

ﬁ

eld change of 0

–

5 T, a large value of

Δ

S

max

(5.97 J/kg K) is observed around T

C

of Co

35

Er

65

ribbon.

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0 2 4 6

0 20 40 60

M (emu/g)

µ

₀

H (T)

2 K 5 K 8 K 11 K 14 K 17 K 20 K 23 K 26 K 29 K

Fig. 5.Magnetization versus applied magneticﬁeldm0H, measured at different temperatures, for the amorphous Co35Er65ribbon.

0 3 6 9 12 15 18 21 24 27 30

0 3 6

- Δ S (J/ kg K)

5 T 4 T

3 T 2 T

T (K)

1 T

Fig. 6.Temperature dependence of the magnetic entropy change (ΔSM) under different magneticﬁeld for the amorphous Co35Er65ribbon.

Table 1

Summary of magnetocaloric properties of the amorphous Co35Er65ribbon com- pared with other magnetic materials.

Sample l0H(T) Torder(K) ΔSMmax(J/kg K) RCP (J/kg) Refs.

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