Large reversible magnetocaloric effect in spinel MnV2O4 with minimal Al substitution

Journal of Magnetism and Magnetic Materials 324 (2012) 766–769

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Large reversible magnetocaloric effect in spinel MnV2O4 with minimal Al substitution X. Luo a,n, W.J. Lu a, Z.H. Huang b, X.B. Hu a, L. Hu a, X.B. Zhu a, Z.R. Yang a, W.H. Song a, J.M. Dai a, Y.P. Sun a,b,nn a b

Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 December 2010 Received in revised form 3 September 2011 Available online 29 September 2011

Magnetocaloric effect of MnV1.95Al0.05O4 was studied by the magnetization and heat capacity measurements. MnV1.95Al0.05O4 is a cubic spinel structure with ferromagnetism of second order in nature and performs reversible magnetic entropy around the magnetic transition temperature. The large magnetic entropy changes  DSM  5.2 and 8.2 J/kg K and the adiabatic temperature changes DTad  1.5 and 2.6 K are revealed for the magnetic field changes of 2 and 4 T near the Curie temperature (TC) of 59.6 K, respectively. The relative cooling power (RCP) are about 82.2 and 177.2 J/kg K for magnetic field changes 2 and 4 T, respectively. Compared with the parent compound, although the  DSM and DTad become smaller, the refrigeration working temperature span and the RCP have been improved. Crown Copyright & 2011 Published by Elsevier B.V. All rights reserved.

Keywords: Spinel vanadate Reversible magnetocaloric effect Substitution effect

1. Introduction Magnetic materials with large magnetocaloric effect (MCE) have been extensively investigated experimentally and theoretically, not only because of their great potential for magnetic refrigeration applications but also for fundamental interest [1–3]. The MCE manifests as an isothermal magnetic entropy (DSM) or an adiabatic temperature change (DTad) 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. The key problem in the application of magnetic refrigeration is to seek proper materials to produce a large entropy change and a good magnetic reversibility at low magnetic fields and with a wide temperature range. Up to now a number of materials with giant-MCE have been observed, for example, Gd5Si4  xGex [1–3], MnFePxAs1  x [4], La(Fe, Si)13 [5], shape memory alloys [6] and polycrystalline perovskite manganese oxides [7]. Most of the giant MCE materials experience a first-order phase transition around the magnetic order temperature, where the magnetic or thermal hysteresis is not favorable for applications. Compared to

the first-order magnetic phase systems, the magnetic or thermal hysteresis in the second-order phase systems is quite small, which makes them more suitable for industrial application from the aspect of refrigerant efficiency and energy conservation. Recently, the spinel vanadium oxides, with formula AV2O4 (A¼Mn and Zn), have been extensively studied due to their interesting physical properties [8–18]. MnV2O4 is a typical example. The ferrimagnetic (FIM) order temperature is about 57 K and the orbital-order (OO) phase transition (TOO) is around 53 K [13,15]. The couplings among spin, lattice and orbital of freedom play important roles in MnV2O4. In previous investigations, we observed the large MCE in spinel vanadates resulting from the change of the V ion’s orbital states [17,18]. However, due to the magnetic phase transition is of first order in MnV2O4, the magnetic or thermal hysteresis exist around TC, which is not desirable for magnetic refrigeration applications. In order to investigate the possible large reversibility MCE in vanadates, in this paper, the MCE tuned by the Al3 þ ions minimal substitution of V3þ ones in MnV2O4 was explored by magnetic and caloric measurements. A large MCE with magnetic reversibility was revealed in MnV1.95Al0.05O4. The relative cooling power has also been improved, the minimal Al substitution can meliorate the refrigerant capacity of spinel MnV2O4. 2. Experimental details

n

Corresponding author. Tel.: þ86 551 559 2757; fax: þ 86 551 559 1434. Corresponding author at: Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China Tel.: þ 86 551 559 2757; fax: þ86 551 559 1434. E-mail addresses: [email protected] (X. Luo), [email protected] (Y.P. Sun). nn

Polycrystalline samples of MnV1.95Al0.05O4 were prepared by the solid state reaction. Stoichiometric mixture of MnO (99%), Al2O3 (99.9%) and V2O3 (99.7%) from Alfa Aesar Co. were carefully

0304-8853/$ - see front matter Crown Copyright & 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.09.013

X. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 766–769

7 H=0.002 T ZFC FC

6 5

0.0 dM/dT (emu/g K)

M (emu/g)

ground, pressed in the form of bars. These bars were placed in a Pt crucible that was sealed into evacuated quartz tubes (  10  5 Torr). The tubes were heated to 950 1C for 40 h to obtain a single phase. The phase purity was examined by powder X-ray diffraction (XRD) using Cu-Ka radiation at room temperature. The magnetization measurements were performed with a Quantum Design (QD) superconducting quantum interference device (SQUID) system (1.9 K rTr400 K, 0 T rHr5 T). The specific heat measurements were taken with a QD Physical Property Measurement System (PPMS) (1.8 KrTr400 K, 0 T rHr9 T).

4 3 2

3. Results and discussion Fig. 1 displays the experimental XRD and the Rietveld analysis patterns of MnV1.95Al0.05O4 with 2y from 20 to 80 degrees at room temperature. All diffraction peaks can be indexed into a face-centered cubic cell with the lattice parameter a¼8.517 A˚ obtained from the Rietveld analysis. It agrees with a normal spinel structure of space group Fd3m with Al substitution of V sites (see Fig. 1). The lattice parameter with Al substitution is smaller than ˚ which is consistent with the smaller that of MnV2O4 (8.523 A), ˚ as compared V3 þ (0.640 A). ˚ size of Al3 þ (0.535 A) Fig. 2 shows the temperature dependence of magnetization in the zero-field-cooled (ZFC) and field-cooled (FC) modes at an external magnetic field H¼0.002 T. The magnetic transition temperature is about 59.6 K taken at the minimum of dM/dT showed in the inset of Fig. 2. For MnV2O4, where the FC curve increases steeply and then shows a dip around the structural phase transition temperature and finally increases monotonically as approaching from high temperature, the dip appearing in FC curve might be related to the OO present in system [11,17,18]. In contrast, the FC curve in MnV1.95Al0.05O4 exhibits a monotonic increase upon cooling from TC. In other words, the long range OO might disappear in Al minimal substitution compounds. Such a substantial effect of a small amount of impurities indicates that the OO in MnV2O4 might be a cooperative phenomenon dominated by the coupling among spin, orbital and lattice freedoms [11]. Fig. 3(a) shows the magnetization curves recorded in the temperature range of 20–90 K under the fields up to 4.5 T. A sharp change in magnetization is clearly observed in Fig. 3(a) as the temperature nears and eventually crosses over TC from FIM to

767

-0.5 -1.0 -1.5 40

TC = 59.6 K 50

60

70

T (K)

1 0 0

10

20

30

40

60

70

Fig. 2. Temperature dependence of magnetization measured in the ZFC, and FC modes under applied magnetic field H¼ 0.002 T. The inset plots dM/dT vs. T.

paramagnetic (PM) states. A noticeable feature in Fig. 3(a) is that a large proportion of the change in magnetization occurs below 2 T. This is beneficial for practical applications of MCE materials at modest fields [3]. The square mark plots show the reversible isothermal MT(H) recorded around TC shown in Fig. 3(a), each isothermal plots show a reversible behavior for the field ascending and descending processes. Compared to typical giant-MCE materials, such as Gd5Ge4  xSix [1], the small magnetic hysteresis near the magnetic transition temperature is advantageous for applications. Since the magnitude of MCE and its dependence on temperature and magnetic field are strongly related to the nature of the corresponding magnetic phase transition, it is essential to analyze the magnetic transition feature in this material. The measured data of the M–H isotherms were converted into M2 vs. H/M plots, which are shown in Fig. 3(b). According to the Banerjee criterion [19], the slope of M2 vs. H/M curves in the critical region can denote whether a magnetic transition is of first (negative slope) or second order (positive slope). Fig. 3(b) shows the Arrott plots of MnV1.95Al0.05O4 in which neither the inflection point nor the negative slope is observed above TC, confirming the occurrence of second-order magnetic transition. The magnetic entropy change is given by Z H  @M DSM ðT,HÞ ¼ SM ðT,HÞSM ðT,0Þ ¼ dH, ð1Þ @T H 0 and can be evaluated by the expression X M i M i þ 1 9DSM 9 ¼ DHi , T i þ 1 T i i

Fig. 1. X-ray powder diffraction pattern (plus marks) and Rietveld refinement pattern (solid line) of MnV1.95Al0.05O4 at room temperature. The vertical marks indicate the position of Bragg peaks, and the solid line at the bottom corresponding to the difference between experimental and calculated intensities. The inset shows the crystal structure of MnV1.95Al0.05O4.

50

T (K)

ð2Þ

where Mi is the magnetization at temperature Ti. The magnetic entropy change vs. temperature derived from Eq. (2) is shown in Fig. 3(c). Near the TC, the maximum entropy changes are about 5.2 and 8.2 J/kg K for the field changes of 2 and 4 T, respectively. The maximum temperatures of  DSM are nearly same at the low and high magnetic fields. The  DSM spans in a wide temperature range, the full width of half maxima (dT) approach to 16 and 21 K in the magnetic field changes from 0 to 2 and to 4 T, respectively. Consequently, the relative cooling power (RCP), evaluated by RCP¼  DSM  dT, reaches, respectively, to  82.2 and 177.2 J/kg K. Although, the values of  DSM at applied fields become smaller, the working temperature spans and the RCP are improved compared with those of the parent

768

X. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 766–769

70

Table 1 The Curie temperature (TC), the maximum values of magnetocaloric parameters ( DSM and DTad as well as RCP) under 2 and 4 T for MnV1.95Al0.05O4 and parent compound MnV2O4.

20 K

60

Material

TC (K)

M (emu/g)

50 MnV1.95Al0.05O4 MnV2O4

40

RCP (J/cm3)

DTad (K)

2T

4T

2T

4T

2T

4T

5.2 14.8

8.2 24.0

82.2 46.7

177.2 86.1

1.5 2.9

2.6 —

30 transition is described as [20]   g mB JH 2=3 DSM ffi 1:07qR , kT C

20 90 K 10 0 0

3000

M2 (emu/g)2

61 57

 DSM (J/kg K)

1

2

3 H (T)

4

5

6

90 K

ð3Þ

where q is the number of magnetic ions, R is the gas constant and g is the Lande factor. The inset of Fig. 3(c) shows the H2/3 dependence of DSM for MnV1.95Al0.05O4. It is found that DSM has a good linear dependence on H2/3 up to 4.5 T, implying a secondorder character for the magnetic transition, which is consistent with the result of Arrott plots. According to thermodynamics, the DTad at an arbitrary temperature T0 can be expressed as T0 , C p ðT 0 ,HÞ

2500

DT ffi DSM ðT 0 ,HÞ

2000

where Cp is the specific heat. We further performed the measurements of the heat capacity in the fields of H¼0, 2 and 4 T shown in the Fig. 4(a). An obvious thermal anomaly near 59 K in zero field corresponds to the magnetic transition. An applied field broadens this peak and rounds it off in high field, which further indicates a second-order phase transition [2]. The isothermal magnetic entropy change can be obtained from the heat capacity RT data DSC p ðTÞ ¼ 0 ½C p ð2TÞC p ð0ÞðdT=TÞ. As shown in the inset of Fig. 4(a) by open circles, the entropy change  DSCp exhibits a similar behavior to  DSM around magnetic transition temperature, implying that the two techniques yield consistent results. The reason why the data deviating from each other at low temperatures is still needed to be investigated in future. Based on the Eq. (3), the calculated DTad is presented in Fig. 4(b). The maximum DTad are about 1.5 and 2.6 K for magnetic field changes of 2 and 4 T, respectively. Although  DSM and DTad of MnV1.95Al0.05O4 are smaller than those of MnV2O4, the  DSM span and the RCP are improved (the RCP of MnV2O4 are about 46.7 J/kg K under the magnetic field change of 2 T). In other words, the working temperature spans and the refrigeration capacity are enhanced after the minimal substitution Al3 þ ions of V3 þ ones in MnV1.95Al0.05O4. As is well known, the RCP is related to the magnetic entropy DSM and the full width of half maxima (dT). For our studied system, after Al3 þ substitution, although the  DSM decreases, however, the dT increases, which results to the larger RCP. We try to explore the possible reasons. We turn to the MCE of MnV2O4, where the large MCE might be related to the contribution of spin entropy and orbital entropy. For MnV2O4, the OO is sensitive to the applied magnetic field, which induces a large MCE. It is known that the Mn2 þ (S¼ 5/2) ions are aligned in one direction below the TM in MnV2O4, due to the strong coupling between the V3 þ ions, the system shows spin frustration. The ground state is noncollinear FIM. The OO disappears in MnV1.95Al0.05O4, which might be due to that the coupling between the V3 þ ions becomes weaker after Al substitution. However, the weaker coupling of V3 þ ions might be helpful to improve the coupling between Mn2 þ ones. The stronger magnetic coupling is good for the MCE in magnetic system. So the minimal Al3 þ substation is useful to increase the spin entropy and depress the orbital one (the OO

1500 20 K

1000 500

0.1

0.2 0.3 H/M (T g/emu)

Experimental Calculated

8

8 6 4

-ΔSM (J/kg K)

6

0.4

0.5

-ΔSM (J/kg K)

0 0.0

2 1

4

2 H

2/3

3

(T2/3)

H=1 T H=2 T H=3 T H=4 T

2

0 10

20

30

40

50 T (K)

60

70

80

90

Fig. 3. (a) Magnetization as a function of applied field of MnV1.95Al0.05O4 between 20 and 90 K. The square mark plots show the magnetic field dependence of reversible magnetization around TC. (b) Arrott plots of MnV1.95Al0.05O4. (c) Temperature dependence of magnetic entropy change  DSM of MnV1.95Al0.05O4 for various magnetic field changes up to 4 T. Inset shows the relation between maximum  DSM and H2/3 near TC. The straight line gives the line fitted to data.

compound MnV2O4, the related parameters can be found in the Table 1. According to the mean field theory, the relation between magnetic entropy and magnetic field near the magnetic phase

ð4Þ

X. Luo et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 766–769

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4. Conclusion

4 In conclusion, MnV1.95Al0.05O4 shows a large reversible magnetic entropy change with its second-order magnetic transition. After the Al3 þ ions substitution of V3 þ ones, the refrigerant capacity of the spinel vanadate MnV2O4 is meliorated. It seems to suggest that the spinel vanadates might be a promising candidate as working materials in magnetic refrigeration technology because of (1) large and reversible MCE, (2) wide temperature span of  DSM and large RCP at low field and (3) relatively low cost of component and fabrication.

H=0 T H=2 T H=4 T 6

2

-ΔS (J/kg K)

Cp/T (J/kg K2)

3

1

4

H=2 T -ΔSCp -ΔSM

2 0 -2

Acknowledgments 0

20

0 0

30

60 T (K)

40

60 80 T (K)

100 120

90

120

3 H=2 T H=4 T

References

ΔTad (K)

2

1

0

20

This work was supported by the National Key Basic Research under Contract no. 2007CB925002, the National Science Foundation of China under Contract nos. 11004193, 10974205 and 10804111 and Director’s Fund of Hefei Institutes of Physical Science, Chinese Academy of Sciences.

40

60

80

T (K) Fig.4. (a) Heat capacity of MnV1.95Al0.05O4 measured under the fields of H¼ 0, 2 and 4 T. The inset shows the entropy changes extracted from magnetization and heat capacity (  DSCp) measurements with the field change from 0–2 T. (b) Temperature dependence of adiabatic temperature rise DTad in MnV1.95Al0.05O4 induced by a magnetic field change of 0–2 and 0–4 T.

disappears). Because the large orbital entropy exists around TC in MnV2O4, where the total entropy abruptly changes, which results to the smaller dT. For MnV1.95V0.05O4, the larger dT might be due to the  DSM mainly from the spin entropy. Therefore, from the aspect of MCE, the Al substitution decreases the lost of magnetic or thermal hysteresis and increase the refrigeration capacity of spinel MnV2O4 system.

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