MCE_of_EuCu2P2-CPB-Revised

Magnetic transition and large reversible magnetocaloric effect

in EuCu1.75P2 compound

1,211,2Huo De-Xuan（霍德璇）, Liao Luo-Bing（廖罗兵）, Li Ling-Wei（李领伟）, Qian

Zheng-Hong（钱正洪）2

1

2Institute of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, China

China

Center for Integrated Spintronic Device, Hangzhou Dianzi University, Hangzhou 310018,

Abstract

The magnetocaloric effect (MCE) in EuCu1.75P2 compound has been studied by the magnetization and heat capacity measurements. Magnetization and modified Arrott plots indicate that the compound undergoes a second-order phase transition at TC~ 51 K.

A large reversible MCE was observed around TC. The values of maximum magnetic entropy change (- SMmax) reach 5.6 and 13.3 J Kg-1 K-1 for the field change of 2 and 7 T , respectively, with no obvious hysteresis loss in the vicinity of Curie temperature. The corresponding maximum adiabatic temperature changes ( Tadmax) are evaluated to be 2.1 and 5.0 K. The magnetic transition and the origin of large MCE in EuCu1.75P2 were also discussed.

PACS: 75.30.Sg; 75.30.Kz

Keywords: EuCu1.75P2 compound; Magnetocaloric effect; Magnetic transition.

Introduction

In recent years, the magnetocaloric effect (MCE) in various magnetic materials has been extensively studied experimentally and theoretically, not only because of their great potential for magnetic refrigeration applications but also for further understanding the fundamental physical properties of the materials. [1-13] The MCE manifests as an isothermal magnetic entropy change ( SM) or an adiabatic temperature change ( Tad) when the magnetic material is exposed to a varying magnetic field. Magnetic refrigeration based on the MCE is advantageous being an environment friendly and energy efficient refrigeration mechanism, which is expected to be an important future cooling technology. [1-3] A large value of MCE is considered to be the most important requirement of the application, and therefore it is desirable to find new materials with large MCE especially at low magnetic fields and with a wide temperature range. Recently, some rare-earth based compounds with an antiferromagnetic (AFM) or a ferromagnetic (FM) have been found to possess not only large magnetic entropy change but also a small hysteresis loss. [9-15]

The intermetallic compounds of the EuT2X2 (T = Fe, Co, Cu etc.; X = Si, P, B etc.) with the ThCr2Si2-type crystal structure have been extensively studied due to their interesting physical properties. Depending on the constituent element or composition, various properties like superconductivity, magnetic ordering, Kondo effect, heavy-fermion properties, valence fluctuation and large MCE, etc. are observed. [16-23] EuFe2As2 are attracting much attention due to the discovery of Fe-based superconductivity. [19, 20] EuFe2P2 is a rare Eu-containing dense Kondo lattice compound.

[21] Recently, a ferromagnetic transition around 51 K for EuCu1.75P2 was confirmed by examining the temperature dependence of magnetization and electrical resistivity. [22]

Very recently, Kim et al reported a giant reversible anisotropic MCE in antiferromagnetic EuFe2As2 single crystal. [23] To search new materials displaying large MCE and further understand the physical properties of EuT2P2 system, in this paper, the critical behaviour and MCE in EuCu1.75P2 were systematically studied.

Experimental

The samples of EuCu1.75P2 were synthesized by heating reactant of high purity Eu, Cu, and P in evacuated fused silica tube very slowly. Detail description of sample preparation and phase analysis is given in Ref. [22]. The magnetic measurement was performed on a vibrating sample magnetometer (VSM) which is an option on physical property measurement system (PPMS-9, Quantum Design). The specific heat measurements were carried out by the adiabatic heat relaxation method by using PPMS.

Results and discussion

Figure 1 shows the temperature dependence of magnetization (M H = 0.1 T) for EuCu1.75P2. A sharp paramagnetic to ferromagnetic (PM-FM) transition can be observed around 50 K. The temperature dependence of zero field specific heat results (C/T) for EuCu1.75P2 is also shown in Fig. 1. A clear -shape peak in specific heat results corresponds to the magnetic transition. These data provide a determination for TC = 51 K which was consistent with that deduced from M(T) curve and previous reported results. [22] A set of magnetic isothermals on increasing and decreasing field for EuCu1.75P2 were measured in a temperature range from 4 to 90 K up to 7 T. No obvious hysteresis can be observed at all temperature range. To ensure the readable of the figure, only several isotherms with increasing field are presented in Fig. 2. For low temperature

ones, the magnetization M tends to be saturated at low field. A large reversible MCE is expected around the transition temperature where the magnetization rapidly changes with varying temperature. Since the MCE have a strong correlation with the order of the corresponding magnetic phase transition, it is important to understand the nature of magnetic transition in EuCu1.75P2 compound. According to Banerjee criterion, [24] the magnetic transition is of a second order if all the H/M versus M2 curves (also named as Arrott plot) have positive slope. On the other hand, if some of the H/M versus M2 curves show negative slope at some point, the magnetic transition is of the first order. To understand the order of the magnetic transition in EuCu1.75P2, the Arrott-plots H/M vs. M2 at some selected temperatures for EuCu1.75P2 are plotted in Fig. 3. Neither the inflection point nor negative slopes can be observed, providing the occurrence of a second order magnetic transition for EuCu1.75P2 from paramagnetic to ferromagnetic.

The second-order magnetic phase transition around the Curie point is characterized by a set of critical exponents, (associated with the spontaneous magnetization Ms), associated with the initial magnetic susceptibility ), and (associated with the critical magnetization isotherm at TC). The mathematical definitions of those exponents from magnetization measurements are given below: [25]

Ms T lim M M0 , 0,T TC (1)

H 0

0 1 T lim H/M h0/M0 , 0,T TC (2) H 0

M XH1/ , 0,T TC (3)

where M0, ho, and X are the critical amplitudes, and =(T TC)/TC is the reduced temperature. The fitting procedure is as follows. By selecting initial values for and , we first plot M1/ versus (H/M)1/ . Then we determine Ms from the intersection of the

linearly extrapolated curve with the M1/ axis and plot Ms as a function of temperature, Ms(T). A similar procedure is also used for with the (H/M)1/ axis. Values for the critical exponents are obtained that are then reintroduced into the scaling of the modified Arrott plot. These procedures are repeated until the iterations converge and lead to the optimum fitting values. Figure 4 shows the modified Arrott plots of M1/ versus (H/M)1/ for EuCu1.75P2 with optimized = 0.45±0.03 and = 1.22±0.04 values, deduced from fitting Ms (T) and data using Equations (1) and (2), as shown in the inset of Fig. 4. The critical exponents from this static scaling analysis are related to the Widom scaling relationship, [26]

= 1 +

Using the above scaling relation and estimated values of and , the value of is determined to be 3.71±0.07. The value of is also determined using the Eq. (3) from two M(H) curves (shown in Fig. 5) with the nearest temperature (50 and 52 K) around TC. The value of is determined to be 3.93 and 3.58 for T = 50 and 52 K, respectively. These values are in agreement with each other indicated that the critical exponents obtained from the magnetization data are reliable. The critical values for EuCu1.75P2 are deviate from those of the mean field theory [27] which is probably related to the valence fluctuation of Eu contained compounds and/or the magnetic inhomogeneous in alloys.

According to Maxwell’s thermodynamic relation, the isothermal magnetic entropy changes associated with a magnetic field variation is given by

SM T, H Hmax

0 M H,T dH, (1) T H

where S, M, H, and T are the magnetic entropy, magnetization of the material, applied

magnetic field, and the temperature of the system, respectively. From the magnetization measurements made at discrete field and temperature intervals, SM can be approximately calculated by the following expression:

SM T, H iMi 1(Ti 1,H) Mi(Ti,H)H. (2) Ti 1 Ti

The temperature dependence of magnetic entropy change - SM for EuCu1.75P2 with magnetic field changes up to 7 T was calculated using equation (4) in the vicinity of its ordering temperature based on the results of magnetization isotherms, the results are shown in Fig. 6. A large magnetocaloric effect can be observed around 50 K. The maximum values of magnetic entropy change (- SMmax) reach 5.6, 10.7 and 13.3 J kg-1 K-1 for field changes of 2, 5 and 7 T, respectively. The observed large MCE in EuCu1.75P2 is believed to be related to the second order magnetic phase transition which is discussed above. The refrigerant capacity or relative cooling power (RCP) is a quality factor of a refrigerant material which is a measure of the amount of heat transfer between the cold and hot reservoirs in an ideal refrigeration cycle. The RCP is defined as the product of the maximum magnetic entropy change SMmax and full width at half maximum in SM (T) curve δTFWHM. The RCP values of EuCu1.75P2 are 101, 331 and 478 J/kg for field changes of 2, 5 and 7 T, respectively. Another important parameter for MCE materials is the temperature dependence of adiabatic temperature change Tad, which was also roughly evaluated using the SM(T) and zero-field specific heat results (Fig. 1). The temperature dependence of Tad for EuCu1.75P2 with various magnetic field changes up to 7 T are shown in Fig. 7. The overall nature of Tad as a function of temperature is remarkably similar to that of SM (T). The maximum values of adiabatic temperature change ( Tadmax) reach 2.1, 4.0 and 5.0 K for field changes of 2, 5

and 7 T, respectively. Despite of the fact that the values of MCE parameters ( SMmax, admax and RCP) for EuCu1.75P2 are smaller than those of which processed the first order magnetic transitions, the present values are comparable with those of which undergoing the second order magnetic transitions. The giant reversible anisotropic MCE in antiferromagnetic EuFe2As2 single crystal was attributed to the first order magnetic transitions in a-b plane and the second order magnetic transitions out-of-plane.[23] The maximum value of the out-of-plane magnetic entropy change for field changes of 5 T is 10.3 J kg-1 K-1, which is comparable with the value of 10.7 J kg-1 K-1 for our ferromagnetic EuCu1.75P2 polycrystalline sample. The present EuCu1.75P2 compound exhibits a high reversible MCE, which makes it to be competitive materials for active magnetic-refrigeration application.

Conclusions

In summary, the critical behaviour and magnetocaloric effect in EuCu1.75P2 were systematically investigated by determining the magnetization and heat capacity. The estimated values of , and is determined to be 0.45±0.03, 1.22±0.04, and 3.71±0.07, respectively. The observed reversible MCE around 51 K is related to a second-order magnetic phase transition. The maximum values of magnetic entropy change (- SMmax) reach 5.6 and 10.7 J kg-1 K-1 for the field change of 2 and 5 T, respectively. The corresponding values of relative cooling power are 101 and 331 J/kg. The present results may give some clue for searching new materials with large MCE.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of

China (Grant Nos. 11004044 and 50871036) and the Zhejiang Provincial Natural Science Foundation of China (Grant No.Y4110581).

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Figure captions

Figure 1.Temperature dependence of magnetization (M H = 0.1 T) and zero field

specific heat results (C/T) for EuCu1.75P2.

Figure 2. Magnetic field dependence of the magnetization for EuCu1.75P2 at some

selected temperatures up to 7 T.

Figure 3. The Arrot-plot of EuCu1.75P2 at some selected temperatures.

Figure 4. The modified Arrott plot isotherms for EuCu1.75P2 in the vicinity of TC. The

inset shows the temperature dependence of spontaneous magnetization Ms(T, 0) and the inverse initial susceptibility 0-1 (T, 0).

Figure 5. Isothermal M vs. H plots at 50 and 52 K; the inset shows the same plot in ln-ln

scale.

Figure 6. Magnetic entropy change - SM as a function of temperature for EuCu1.75P2. Figure 7. Temperature dependence of adiabatic temperature change Tad for EuCu1.75P2.

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