Analysis of magnetization and magnetocaloric effect in amorphous fezrmn ribbons

The peak of magnetic entropy change was observed at the Curie temperature.. INTRODUCTION The temperature change of a magnetic material, associ – ated with an external magnetic field cha

Analysis of magnetization and magnetocaloric effect in amorphous FeZrMn ribbons

S.G Min, K, S Kim, and S C Yua)

Department of Physics, Chungbuk National University, Cheongju 361 – 763, Korea

H S Suh

Korea lnstiute of Energy Research, Daejeon 305 – 343, Korea

S W Lee

Department of Metallugry of Engineering, Chungnam National University, Daejeon 305 – 764, Korea

(Presented on 9 November 2004;b Published online 16 May 2005)

The magnetization behaviors have been measured for amorphous Fe90-xMnxZr10 (x=8 and 10) alloys The curie temperature is decreased from 210 K to 185 K with increasing Mn concentration (x=8 to x=10) The magnetization measurements were conducted at temperatures above the Curie temperature in the paramagnetic region In both samples, the magnetic entropy change ∆S M of Fe82Mn8Zr10 is 2.87 J/Kg K at 210 K for an applied field of 5 T The peak of magnetic entropy change was observed at the Curie temperature The ∆S M decreases with increasing Mn concentration to 2.33 J/ Kg K 2005 American

Institute of Physics [ DOI: 10.1063/1.1853193]

I INTRODUCTION

The temperature change of a magnetic material, associ – ated with an external magnetic field change in an adiabatic process, is defined as the magnetocaloric effect (MCE) The thermal effect was discovered in 1881 by Warburg when he applied varying magnetic field to metal iron.1 Debye and Giauque explained the nature of MCE later and suggested achieving an ultralow temperature by adiabatic demagnetiza – tion cooling.2,3 MCE is intrinsic to magnetic solids and is induced via the coupling of the magnetic sublattice with the magnetic field, which alters the magnetic part of the total entropy due to a corresponding change in the magnetic filed It can be measured and/or calculated as the adiabatic tem-perature change ∆S M(T,∆H ).4 – 6 The MCE is a

function of both temperature T and the magnetic field change ∆Hand is usually recorded as a function of temperature at a constant ∆H

Resently, a search for new magnetic materials, which exhibit a significant change in the magnetic entropy in re-sponse ti the change of magnetic field inder isothermal con-ditions, has become an important task in applied physics Traditionally, diluted paramagnetic slats and rare earth inter-metallic conpounds that display sifnificant MCE were con-sidered as attractive materials for cryogenic applications.4,5

In our work, magnetization and MCE of Fe90-xMnxZr10 (x=8 and 10) compounds were investigated These kind of amorphous materials with low Curie temperature have many useful properties that are attractive for application

as mag-netic refrigerants

II EXPERIMENTS

Amorphous Fe90-xMnxZr10 (x=8 and 10) alloys were pre-pared by are melting the high – purity elemental constiuents under argon gas atmosphere and

by single roller melt spin-ning in the from of long ribbons of 1 – 2 mm width and 20 – 40 µm thickness The amorphous nature of the samples was confirmed through x-ray diffraction studies using Cu-Kα radiation The compositions of the samples were veri-fied through energy dispersive x-ray analysis (EDAX) The magnetization measurements as a function of temperature and field were carried out on a single long ribbon sample using a Quantum Design superconducting quantum interfer-ence device (MPMS mode) magnetometer (VSM)

According to thermodynamic theory, the magnetic en-tropy change caused by the variation of the external mag-netic filed from 0 to Hmax is given by

max

0

H M

T

S

H

δ

 

δ

 

From Maxwell’s thermodynamic relationship:

=

Equation (1) can be rewritten as follows:

max

0

H M

H

M

T

δ

δ

Numerical evaluation of the magnetic entropy change was carried out from formula (3) using isothermal magnetization measurements at small discrete

field and temperature inter-vals ∆S M can be computed approximately from Eq (3) by

1 1

i i M

i

T T

+ +

−

−

Thus, the magnetic entropy changes associated with applied field variations can be calculated from Eq (4)

III RESULTS AND DISCUSSION

The nature of the reentrant spin glass transition behavior in Fe – Zr amorphous alloys has been investigated extensively by means of various techniques Intense efforts have been carried out to understand the nature of the magnetic phase diagram from various magnetic measurements in the amor-phous Fe – Zr – Mn alloy system.6-8 Especially, these FeZrMn materials with low Curie temperature and high magnetization as magnetic refrigerants For this reason we studied the amorphous Fe80Mn10Zr10 and Fe82Mn8Zr10 alloys It is sug-gested that in some cases there exists a temperature inrerval in which the magnetic system consists of ferromagnetic (FM) grains separated by paramagnetic interlayers Thus the role of two magnetic phases in the intergrain magnetic cou-pling can possibly be taken apart in a sufficiently broad tem-perature range and investigated separately Particular materi-als with competing magnetic exchange interactions show characteristics of enhanced magnetoresistance and softer magnetic properties when magnetic nanocrystals are dis-persed in an amorphous matrix

Figure 1 shows the temperature dependence of low-field magnetization for the samples The Curie temperature, Tc was found to be 210 K and 195 K for x=8 and x=10 of Fe90-xMnxZr10, respectively With an increase of the concen-tration of Mn for Fe90-xMnxZr10 systems, the Curie tempera-ture decreases almost linearly and then the re-entrant behav-ior is observed in both samples The magnetization data measured as a function of temperature show that the shape of the magnetization (M) vs T curve is quite sensitive to the applied magnetic field and M decreases as Mn concentration increases It is believed that the inhomogeneities of amor-phour nature (frequently referred as clusters) exist in the as-quenched state

Figure 2 shows isothermal M – H curves, which have been measured at various temperature in the vicinity of Tc in Fig 2, superparamagnetic behavior is

observed above the Tc, but the mean magnetic moment of the superparamangetic spin clusters decreased with increasing temperature.8-10

To determine the type of phase transition for x=8 and x=10 of Fe90-xMnxZr10, the measured data of the M – H iso-therms were transferred into H/M

vs M2 isotherms should give a set of straight lines [known as Arrott-Kouvel plots] just below and above Tc.8 The absence of linear behavior in Fig 3 suggests that the mean-field theory is not valid for our present case

In evaluating the magnetocaloric properties of the Fe90-xMnxZr10 (x=8 and 10) samples, the magnetic entropy change, a function of temperature, and magnetic field, pro-duced by the variation of the magnetic field from 0 to Hmax is calculated by Eq (4),11 ∆S M vs T for the two samples, was plotted in Fig 4 As can be seen in Fig 4, with a magnetic field varying from 0 to 5 T, the magnetic entropy change ∆S M reaches a maximum value of about 2.78 J/Kg K fox x=8 at

210 K, while ∆S M is about 2.33 J/Kg K for x=10 at 195 K (the Curie temperature)

Though the maximum ∆S M is less than that for pure Gd metal (∆S M

10.2 J/Kg K at ∆H =5T), it is much more uni-from, which is desirable for an Ericson-cycle magnetic refrigerator.12 In comparison with pure Gd metal,13 these rib-bon samples are much cheaper; their Curie temperature can be easily adjusted by tuning the Mn concentration And they are much more chemically stable than pure Gd metal

IV CONCLUSIONS

The magnetic properties and entropy changes of FeMnZr amorphous alloys were investigated The Curie temperature decreases with increasing Mn concentration, and the peaks of entropy change appear at the Curie temperature region In comparison With Gd metal, the peaks are broader around the Curie temperature Our results indicate that these ribbon samples are very useful for wideband temperature

V ACKNOWLEDGMENT

This work was supported by Korea Science and Engi-neering Foundation through the Research center for Ad-vanced Magnetic Materials at Chungnam National Univer-sity