temperature magnetocaloric effect in antipervoskite

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Nov 11, 2010 - We report the structural, magnetic, electrical transport properties, and magnetocaloric effect MCE of antipervoskite compound AlCMn3.
JOURNAL OF APPLIED PHYSICS 108, 093925 共2010兲

Structural, magnetic, electrical transport properties, and reversible roomtemperature magnetocaloric effect in antipervoskite compound AlCMn3 B. S. Wang (王铂森兲,1 J. C. Lin,1 P. Tong,1,a兲 L. Zhang,2 W. J. Lu,1 X. B. Zhu,1 Z. R. Yang,1 W. H. Song,1 J. M. Dai,1 and Y. P. Sun1,2,a兲 1

Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei 230031, People’s Republic of China 2 High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

共Received 24 January 2010; accepted 25 September 2010; published online 11 November 2010兲 We report the structural, magnetic, electrical transport properties, and magnetocaloric effect 共MCE兲 of antipervoskite compound AlCMn3. It exhibits a second-order ferromagnetic–paramagnetic phase transition around 共TC兲 287 K. The electronic resistivity 共␳兲 shows a good metallic behavior except for a slope change around TC. At lower temperatures 共below 130 K兲, ␳ ⬀ T2 indicates that the electron-electron scatterings domain. At evaluated temperatures 共130–270 K兲, ␳ is linear dependence on temperature, implying that the phonon scatterings boost up greatly. Furthermore, a broad distribution of the magnetic entropy change 共−⌬SM兲 peak is found to about 100 K with the magnetic field change ⌬H = 45 kOe. The relative cooling power are ⬃137 J / kg and ⬃328 J / kg 共or ⬃68 K2 and ⬃162 K2兲 with ⌬H = 20 kOe and 45 kOe, respectively. All these values are comparable with the typical MCE associated with a second-order transition. It suggests that AlCMn3 may be considered as a candidate material for near room-temperature magnetic refrigeration because of: 共i兲 the large full width at half peak of the −⌬SM-T curve, 共ii兲 no hysteresis losses, 共iii兲 the near room-temperature working temperature, and 共iv兲 the low-cost and innoxious raw materials. Moreover, it is found that the simple theoretical model which only considering the magnetoelastic and magnetoelectronic couplings couldn’t account well for the observed MCE in antiperovskite AlCMn3. © 2010 American Institute of Physics. 关doi:10.1063/1.3505753兴 I. INTRODUCTION

Magnetic refrigeration based on the magnetocaloric effect 共MCE兲 has attracted much interest due to its energyefficient and environment-friendly features compared with the traditional gas-compression refrigeration technology. In the past decades, many magnetic materials exhibiting large MCEs around both first-order and/or second-order magnetic phase transitions have been extensively investigated experimentally and theoretically.1–3 Generally, the giant MCE is considered to be associated with a first-order magnetic transition owing to the large and/or sharp changes in magnetizations between the two adjacent magnetic phases. However, the practical application of MCE around a first-order magnetic phase transition is quite restricted because of the existence of the relevant hysteresis under magnetization/ demagnetization processes, which makes the magnetic refrigeration less efficient.3–5 Furthermore, most of the promising magnetic refrigerant materials contain expensive rare earth and/or poisonous chemical elements. Consequently, for the actual applicability, how to reduce or even eliminate the magnetic hysteresis loss is becoming a challenging topic. In a word, high refrigerant capability, innocuity, low-cost production, and close to room-temperature are becoming significant targets. Recently, Mn-based antiperovskite structural compounds a兲

Electronic addresses: [email protected] and [email protected].

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AXMn3 共A = Al, Ga, Sn, In, Zn, etc.; X = C, N兲 have attracted considerable attention due to their interesting properties, such as magnetoresistance 共MR兲,6,7 MCE,8–10 and giant negative thermal expansion.11–14 AlCMn3 has been investigated for several decades. As reported previously, AlCMn3 is a ferromagnetic 共FM兲 material and undergoes a magnetic transition to paramagnetic 共PM兲 phase around the Curie temperature 共TC兲 with different values from 290 to 310 K reported by different researchers.15,16 Much recently, the influences of carbon deficiency on the structural, magnetic, and electrical transport properties of AlCxMn3 have been also studied and indicated that all the physical properties are highly sensitive to the carbon content.17 However, more work is desirable in order to further understand the nature of AlCMn3. In this work, we investigate systematically the structure, magnetic, electrical transport properties, and MCE of antipervoskite AlCMn3. Variational temperature x-ray diffraction 共XRD兲 analyses show that AlCMn3 remains the cubic antiperovskite structure covering the whole temperature ranges and no structural transition appears accompanying the FM–PM transition. The measurements of electrical resistivity indicate a Fermi liquid behavior at lower temperatures and electron-phonon scatterings dominates at elevated temperatures. Meanwhile, a reversible MCE is found around the second-order FM–PM transition and the working temperature ranges can reach 100 K under a magnetic field change of 45 kOe. Furthermore, the simple theoretical model which

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only considering the magnetoelastic and magnetoelectronic couplings could not account well for the temperature dependence of magnetic entropy change 共−⌬SM兲. II. EXPERIMENTAL DETAILS

Polycrystalline AlCMn3 was prepared from powders of Al 共4N兲, graphite 共3N兲, and Mn 共4N兲 powders directly. The detailed preparation technism has been reported elsewhere.6–10 The starting materials were weighed in the desired proportions, mixed uniformly, pressed into pellets, wrapped in a Mo foil, sealed in evacuated quartz tubes and then annealed at 973–1053 K for about eight days. After quenching the tubes to room temperature, the products were pulverized, mixed, pressed, and annealed again under the same conditions in order to obtain the homogeneous samples. During the process of preparation, excess graphite was added in the starting composition to avoid the carbon deficiency. XRD measurements on powder samples were performed by using Cu K␣ radiation to identify the phase purity and the crystal structure. The magnetic measurements were performed on a Quantum Design superconducting quantum interference device 共SQUID5T兲 magnetometer 共1.8 K ⱕ T ⱕ 400 K , 0 ⱕ H ⱕ 50 kOe兲. The measurements of the electrical transport, the specific heat, and the thermopower coefficient were carried out on a Physical Property Measurement System 共PPMS9T兲 共1.8 K ⱕ T ⱕ 400 K , 0 ⱕ H ⱕ 90 kOe兲 from Quantum Design. III. RESULTS AND DISCUSSION

Figure 1 presents the Rietveld refined room-temperature powder XRD patterns of AlCMn3. All the diffraction peaks could be indexed to the cubic antiperovskite structure 共space group Pm3m兲. Inset of Fig. 1共a兲 shows the sketch map of the crystal structure for AlCMn3. The refined lattice parameter 共a兲 obtained by using Rietveld refinement technique is 0.3873 nm, which almost matches well with those of previous reports,15–17 indicating little or no carbon deficiency exists in our sample. As shown in Fig. 1共b兲, the XRD patterns were also carried out at several selected temperatures 共100, 200, 250, 275, 285, 290, and 295 K兲. Based on the refined results, the temperature dependent a is plotted 关see inset of Fig. 1共b兲兴. Obviously, AlCMn3 remains the cubic antiperovskite structure covering the whole temperature ranges and the value of a increases with increasing the temperature. Figure 2 displays the temperature dependent magnetization M共T兲 of AlCMn3 measured at 100 Oe under both the zero-field-cooled 共ZFC兲 and field-cooled 共FC兲 processes. Obviously, a magnetic phase transition takes place around TC = 287 K 共defined as the maximum slope of M-T curve兲, which is basically in accord with previous reports.15–17 As shown in the inset of Fig. 2, the isotherm magnetization curve M共H兲 of AlCMn3 was measured at 5 K with magnetic fields up to 45 kOe. With increasing the magnetic field, the magnetization increases sharply at low fields, and then tends to saturation above 10 kOe. The saturated magnetization ␮S = 1.15共9兲␮B / Mn is obtained from an extrapolation of the high field M共H兲 curve to the zero field, and the magnitude of ␮S is close to that of other previous reports 共1.2 ␮B / Mn兲.15

FIG. 1. 共Color online兲 共a兲 Room temperature x-ray powder diffraction pattern 共circle兲 and Rietveld refinement pattern 共solid line兲 of AlCMn3. The vertical marks indicate the position of Bragg peaks, and the solid line at the bottom corresponding to the difference between observed and calculated intensities. Inset shows the sketch map of the crystal structure of antiperovskite compound; 共b兲 XRD patterns of AlCMn3 at several selected temperatures 共100, 200, 250, 275, 285, 290, and 295 K兲; inset shows the temperature dependent lattice constant. The Bragg peaks of Cu impurity come from the substrate materials.

However, the saturated magnetic moment is quite small when compared with that of the localized Mn moment in the Mn-based perovskite oxides 共3 – 4 ␮B / Mn兲, indicating an itinerant character of the carriers in AlCMn3. Figure 3共a兲 illustrates the temperature dependence of the resistivity ␳共T兲 for AlCMn3 from 5 to 350 K at zero magnetic field and 50 kOe, respectively. The lower residual resistivity at 5 K 关␳共5 K兲 = 107 ␮⍀ cm兴 and higher residual resistivity ratio 关RRR= ␳共300 K兲 / ␳共5 K兲 = 22.03兴 may indicate a good quality of our sample. During the whole temperature ranges, AlCMn3 exhibits a good metallic behavior except for a slope change around 287 K, which is closely related to the FM-PM magnetic transition. As shown in the

FIG. 2. 共Color online兲 Temperature dependence of magnetization M共T兲 in ZFC and FC processes at 100 Oe from 5 to 380 K. Inset: isotherm magnetization curve M共H兲 at 5 K with field up to 45 kOe.

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FIG. 3. 共Color online兲 共a兲 Temperature dependent of resistivity ␳共T兲 for AlCMn3 at 0 and 50 kOe. Inset shows the values of MR vs T at 50 kOe. 共b兲 ␳共T兲 vs T2 at zero field and its linear fitting to lower temperature data; Inset show the linear fitting between 130 and 270 K 共in the left hand兲 and the enlargement of Fig. 3共b兲 below 130 K 共in the right hand兲.

inset of Fig. 3共a兲, the MR about 3% 关defined as MR= 共␳H − ␳0兲 / ␳0兴 can be found around TC. Such a smaller MR behavior is similar to that of GaCMn3 around TC where the magnetic scatterings is dominant and the spin-fluctuations are suppressed at higher magnetic fields.6,17 As shown in Fig. 3共b兲, the low temperature resistivity was well fitted by the formula ␳ = ␳0 + AT2 up to about 130 K, indicating a Fermi liquid behavior for AlCMn3. Namely, below 130 K, the electron-electron scatterings are dominant. However, with increasing the temperature, the number of phonon increases sharply and the phonon scatterings enhance accordingly. Based on the fitting results, ␳共T兲 curves are almost linearly dependent on the temperature at the elevated temperature 共from 130 to 270 K兲, indicating that the electron-phonon scatterings exceeds the electron-electron scatterings in this temperature range. All these results are basically consistent with previous reports.17 In order to further study the nature of FM–PM transition, related magnetic measurements around TC were carried out. Figure 4共a兲 displays the isothermal magnetization curves M共H兲 measured in a temperature range of 145–340 K under the magnetic fields H up to 45 kOe for AlCMn3. For each measurement, firstly, the sample was heated to a certain temperature well above TC, and then cooled down under zero magnetic field to the temperature where the M共H兲 curve was measured. The Arrott plots derived from M共H兲 curves covering a broad temperature range around TC are displayed in Fig. 4共b兲. For AlCMn3, it is evident that the slopes of H / M versus M 2 curves at high fields are positive for all the temperatures, confirming a second-order magnetic transition.18 Furthermore, at several selected temperatures close to TC, M共H兲 curves were also measured under increasing/

FIG. 4. 共Color online兲 共a兲 Isotherm magnetization curves M共H兲 for AlCMn3 covering a broad temperature range of 145–340 K with external magnetic fields up to 45 kOe; 共b兲 Arrott plots deduced from M共H兲 curves in Fig. 3共a兲. Inset of Fig. 3共b兲: M共H兲 curves under increasing/decreasing magnetic field at several selected temperatures 共280, 290, and 300 K兲.

decreasing field processes. As shown in the inset of Fig. 4共b兲, the M共H兲 curves at 280, 290, and 300 K are reversible during increasing/decreasing field processes without any hysteresis, indicating a second-order character of the FM–PM transition. Such a nonhysteresis behavior is significant for practical application of magnetic refrigeration.3–5 Based on the classical thermodynamical theory and Maxwell’s relation, −⌬SM induced by the variation in a magnetic field from 0 to H is given by5,19

冕冉 冊 H

⌬SM共T,H兲 =

0

⳵M ⳵T

共1兲

dH. H

For the magnetization measured at small discrete field and temperature intervals, −⌬SM can be approximated as5,19

冏 冉 ⌬SM

Ti + Ti+1 2

冊冏 兺 冋 =

共M i − M i+1兲Hi Ti+1 − Ti



⌬Hi ,

共2兲

where M i and M i+1 represent the experimental values of the magnetizations at Ti and Ti+1 under the same magnetic fields, respectively. From Eq. 共2兲, −⌬SM associated with the magnetic field and/or temperature variation has been calculated from the measured M共H兲 curves shown in Fig. 4共a兲. In Fig. 5共a兲, −⌬SM versus T is plotted under different magnetic field variations. Generally, the maximum value of −⌬SM occurs around TC where the value of dM / dT is the sharpest as expected from Eq. 共2兲. As shown in Fig. 5共b兲, the values of −⌬SM cover larger temperature spans and the maximum max magnetic entropy change 共−⌬SM 兲 is about 3.28 J/kg K for ⌬H = 45 kOe, which is close to those of many roomtemperature MCE materials around the second-order phase transitions.1–3,5

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FIG. 6. 共Color online兲 Temperature dependence of the adiabatic temperature change for AlCMn3 under different magnetic fields of 10 kOe, 20 kOe, 30 kOe, 40 kOe, and 45 kOe, respectively. The inset shows the zero-field heat capacity of AlCMn3.

FIG. 5. 共Color online兲 共a兲 Magnetic entropy change −⌬SM as a function of temperature at different magnetic field changes in ⌬H = 1, 5, 10, 20, 30, 40, and 45 kOe for AlCMn3. Inset shows the maximum magnetic entropy max at TC as a function of H2/3 for AlCMn3. The solid line shows change −⌬SM the liner fit to the experimental data; 共b兲 the −⌬SM-T curve for magnetic field change ⌬H = 45 kOe. The shaded area is the RCP. The inset of Fig. 4共b兲 shows the magnetic field dependence of the relative power.

In the framework of the mean field theory, the relation max between −⌬SM and the magnetic field near TC can be described as follow:20 max ⌬SM ⬇ − 1.07qR



g␮BJH kTC



2/3

with

M S = g␮ BJ,

共3兲

where q is the number of magnetic ions, R is the gas constant, and g is the Lande factor. As shown in the inset of Fig. max max versus H2/3 is plotted. Obviously, −⌬SM is 5共a兲, −⌬SM 2/3 linearly dependent on H for AlCMn3, indicating the second-order character of magnetic transition under the framework of mean field theory, consistent with the above results of M共T兲 and M共H兲 curves.21 Another parameter representing the MCEs of magnetic refrigerant materials is the adiabatic temperature change ⌬Tad. As shown in the inset of Fig. 6, the specific heat C P共H , T兲 as a function of temperature is plotted at zero magnetic field for AlCMn3. Apparently, a broad peak around TC is observed in the zero-field specific heat curve, which is corresponding to FM–PM transition. The peak shape is similar to “␭” type, indicating a second-order phase transition.22 Using the formula ⌬Tad = −⌬SM共T , H兲T / C P共T , H兲,1,2 where C P共T , H兲 is zero-field specific heat, ⌬Tad are calculated as functions of temperature and/or magnetic field. Figure 6 shows the temperature dependences of ⌬Tad of AlCMn3 with the field changes ⌬H = 10 kOe, 20 kOe, 30 kOe, 40 kOe, and 45 kOe, respectively. Obviously, with increasing ⌬H, the value of ⌬Tad increases gradually and reaches the maximum value 共about 1.62 K兲 at ⌬H = 45 kOe. Meanwhile, the values

of ⌬Tad cover broader temperature spans 共see Fig. 6兲. Such behavior is much superior to those MCE materials around first-order transitions with larger hysteresis losses and narrower temperature spans in actual magnetic refrigerants.1,3,23,24 The relative cooling power 共RCP兲 is an important parameter for selecting potential substances for magnetic refrigerants, providing a measure of how much heat can be transferred between the cold and hot sinks in an ideal refrigerant cycle.1–3,5 Different methods have been adopted to estimate the RCP in the literature.3,5 As suggested by Gschneidner, Jr. and Pecharsky,5 the value of RCP could be determined from two different methods based on the temperature dependent magnetic entropy change and/or adiabatic temperature change, respectively. One method is the RCP共S兲, which could be obtained from the product of max and the full width at half maximum ␦TFWHM, −⌬SM max RCP共S兲 = − ⌬SM ␦TFWHM .

共4兲

The other method is the RCP共T兲, which is given by the product of the ⌬Tad maximum and the value of ␦TFWHM, max ␦TFWHM . RCP共T兲 = − ⌬Tad

共5兲

For AlCMn3, the value of ␦TFWHM increases with increasing the external field H 共see Fig. 5兲. At 20 kOe and 45 kOe, ␦TFWHM reaches ⬃79 K and ⬃100 K, respectively. These values are much larger than those of the most promising MCE materials such as Gd 共⬃60 K, ⌬H = 50 kOe兲.23 For comparison, both the RCP共S兲 and RCP共T兲 are obtained from Eqs. 共4兲 and 共5兲, respectively. As indicated in Ref. 5, the values of ␦TFWHM are almost equivalent in both cases. So, in this work, we adopt the estimated value of ␦TFWHM from −⌬SM versus T plots since the value of heat capacity at higher temperature 共higher than 300 K兲 couldn’t be obtained under our present experimental conditions. As shown in the inset of the Fig. 5共b兲, the RCP共S兲 increases almost linearly with increasing field. The values of RCP共S兲 for AlCMn3 are about 137 J/kg and 328 J/kg for ⌬H = 20 kOe and 45 kOe, respectively. The value at 45 kOe is 1.5 times that of isostructural antiperovskite compound GaCMn3 共⬃218 J / kg, ⌬H = 45 kOe, at 250 K兲,24 and about 2.2 times that of Mn3Sn2 共⬃147 J / kg for ⌬H = 50 kOe, at 220–280 K兲,25,26 and close to those of Gd 共⬃410 J / kg, ⌬H = 50 kOe, at 295

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K兲 and Gd5Si2Ge2 共⬃535 J / kg, ⌬H = 50 kOe, at 276 K兲,1,3,5,23 Meanwhile, the values of RCP共T兲 are also obtained. For AlCMn3, the RCP共T兲 values are 68 K2 and 162 K2 共for ⌬H = 20 kOe and 45 kOe, respectively兲, which is about 1.6 times that of Mn3Sn2 共⬃100 K2 for ⌬H = 50 kOe兲.25,26 Meanwhile, these values are ⬃24.5% and ⬃19.5% that of Gd 共⬃277 K2 for ⌬H = 20 kOe; ⬃830 K2 for ⌬H = 50 kOe, respectively兲,1,5 the highest among the room-temperature magnetic refrigerants. Generally, MCE materials around a first-order transition have significantly lower RCP共T兲 values since their peak is definitely narrower such as Gd5Si2Ge2 共⬃315 K2, ⌬H = 50 kOe兲, MnAs 共⬃315 K2, ⌬H = 50 kOe兲 and LaFe11.7Si1.3 共⬃21 K2, ⌬H = 14 kOe兲.3 In addition, considering the fact that the magnetic magnetizations are much less than that of Gd, the observed RCP in AlCMn3 is considerably large. As discussed above, the near room-temperature MCE in AlCMn3 is reversible due to its second-order character of magnetic phase transition. Thus, detrimental effects for fast-cycling refrigerators of hysteresis losses and slow kinetics vanish in AlCMn3. Furthermore, AlCMn3 is additionally made from abundant, low-cost, and nontoxic elements, by contrast with most of the giant MCE materials. Therefore, the antiperovskite compound AlCMn3 may be an alternative magnetic refrigerant around room temperature. Recently, in order to discuss the origin of MCE around a second-order magnetic transition, a simple theoretical model based on the magnetoelastic couplings and electron interaction was introduced in many magnetocaloric compounds.27–29 Herewith, this model was adopted to investigate the MCE around the FM–PM transition of AlCMn3. In generally, the Gibbs free energy G can be expressed as a function of the order parameter M as follows: G共T,M兲 = G0 + 21 AM 2 + 41 BM 4 − MH,

共6兲

where the coefficients A and B are temperature-dependent parameters containing the magnetoelastic couplings and electron interaction. From the condition of equilibrium ⳵G / ⳵ M = 0, the so-called magnetic equation of state can be obtained as H/M = A + BM 2 .

共7兲

The nature of the magnetic transition of AlCMn3 can be checked based on the Banerjee criterion.18 From the differential of the Gibbs free energy as a function of temperature, the magnetic entropy can be obtained from following equation: S M 共T,H兲 = −

1 ⳵A 2 1 ⳵B 4 M − M . 2 ⳵T 4 ⳵T

共8兲

The temperature dependence of parameters A and B are obtained from the linear fitting regions of the Arrott plots of H / M versus M 2 关see Fig. 4共b兲兴. The sign of parameter A varies from negative to positive with increasing the temperature, and the temperature point corresponding to the zero value of A is identical to the value of TC in AlCMn3 关see Fig. 7共a兲兴. However, the parameter B remains positive for each temperature point, which is common for a ferromagnet with a second-order transition.27,30 In this model, the parameter A

FIG. 7. 共Color online兲 共a兲 Temperature dependence of parameters A 共in the left hand兲 and B 共in the right hand兲 where the parameters A and B are the magnetoelastic coupling part and electron interaction part of the Gibbs free energy, respectively; the dashed line indicates A = 0 and the crossing point is corresponding to TC; 共b兲 the calculated 共in the left hand兲 and experimental values 共in the right hand兲 of magnetic entropy change as a function of temperature under field change ⌬H = 20 kOe and 45 kOe, respectively; 共c兲 temperature dependent thermopower S共T兲 for AlCMn3 between 140 K and 360 K at zero field. Inset shows the S共T兲 from 5 to 360 K. The red dashed line indicates S = 0.

represents the electron interaction part of the Gibbs free energy and the parameter B represents the magnetoelastic part, which plays an important role in determining −⌬SM.31 From Eq. 共8兲, the temperature dependent −⌬SM is obtained with ⌬H = 20 kOe and 45 kOe, respectively. Figure 7共b兲 displays the calculated and the experimental values of −⌬SM for AlCMn3. Clearly, the calculated value 关deduced from Eq. 共8兲兴 is much larger than that of the experimental one and the peak temperature of calculated value much lower than that of the experimental one. In a word, the theoretical and the experimental results are of much discrepant in the case of antiperovskite AlCMn3 and similar behaviors have been observed in several other MCE compounds such as La0.9Te0.1MnO3 and GaCMn3.27,32 However, the origin of this behavior is unclear up to now. In electron-doped compound La0.9Te0.1MnO3, this difference was attributed to degradation linearity in the H / M versus M 2 plot below TC which is related with the sign change in the thermopower coefficient below TC for some manganites and intermetallic compounds.27,32,33 As shown in Fig. 7共c兲, the temperature dependence of the thermopower coefficient S共T兲 of AlCMn3 was measured at zero magnetic field from 5 to 360 K. Below 185 K, the value of S共T兲 is negative, indicating that the electron-type carriers are dominant. Around 185 K, the sign

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of S共T兲 changes into positive with increasing the temperature, which may be attributed to the enhancement of phonon scatterings. This behavior of S共T兲 is consistent with the results of the electrical resistivity measurement. More interestingly, this temperature 共185 K兲 is identical to the peak temperature of calculated magnetic entropy change, indicating that the calculated values of magnetic entropy change may be valid and much closely related with the sign change in S共T兲 or the phonon scatterings. So, the large discrepant between the experimental and calculated values may originate from the excessive simplification of this theoretical model which only considering the magnetoelastic couplings and electron–electron interaction since the electron-phonon scatterings domains at elevated temperature. Therefore, other theoretical models considering more comprehensive factors on the basis of reducing the proportion of magnetoelastic couplings and electron–electron interaction contribution for antiperovskite AlCMn3 need to be further investigated. IV. CONCLUSIONS

In summary, the structure, magnetic properties, electrical transport properties, and MCE of antipervoskite compound AlCMn3 have been investigated systematically. The study of electronic resistivity illustrates a Fermi liquid behavior at lower temperatures and electron-phonon scatterings dominate at higher temperatures in AlCMn3. A large reversible MCE is found around FM–PM transition. A large RCP value 共⬃328 J / kg or ⬃162 K2 for ⌬H = 45 kOe兲 is observed in antiperovskite AlCMn3. A large full width at half peak of the −⌬SM-T curves and no hysteresis loss are the advantages of this compound. Furthermore, the MCE in antiperovskite AlCMn3 could not be well understood based on the simple theoretical model which only considering the magnetoelastic couplings and electron–electron interaction. ACKNOWLEDGMENTS

This work was supported by the National Key Basic Research under contract No. 2007CB925002, the National Natural Science Foundation of China under Contract Nos. 50701042, 51001094, 10774146, and 10804111, and Director’s Fund of Hefei Institutes of Physical Science, Chinese Academy of Sciences under Contract No. O84N3A1133. 1

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