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Large magnetocaloric effect near room temperature in Mn–Fe–P–Ge nanostructured powders
Journal of Alloys and Compounds 652 (2015) 393e399
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Large magnetocaloric effect near room temperature in MneFeePeGe nanostructured powders X. Chen, R.V. Ramanujan* School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2015 Received in revised form 28 August 2015 Accepted 29 August 2015 Available online 31 August 2015
MneFeePeGe alloys are attractive candidates for affordable and high performance magnetocaloric materials (MCM). The magnetocaloric effect (MCE), crystal structure and magnetic transition of MneFe ePeGe nanostructured powders were investigated with the aim of obtaining both high relative cooling power and large entropy change. In this work, the Ge content was tuned to obtain high DSM near room temperature in powder samples. With increasing Ge content, the magnetic transition temperature increases. Interestingly, large relative cooling power (RC) together with high DSM were obtained near room temperature in Mn1.1Fe0.9P0.79Ge0.21 powders, the RC value is much larger than previously reported values in MneFeePeGe alloys. To model the magnetic transition and magnetocaloric behavior, Landau theory and a modified Arrott plot were utilized, good agreement was obtained between the theoretical model and the experimental results. Our results suggest that Mn1.1Fe0.9P0.79Ge0.21 powders possess attractive magnetocaloric properties for commercial applications. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetocaloric materials Magnetic and structural transition MneFeePeGe based alloys
1. Introduction Magnetic cooling technology, based on the Magnetocaloric Effect (MCE), has attracted intense fundamental research as well as commercial interest as an alternative thermal management technology. Due to its high energy efficiency and environmental friendliness [1e3], the magnetocaloric effect has been used in heat pumps, energy harvesting from low grade waste heat and a variety of cooling systems. The urgent requirement for such “green” energy technologies has led to several key developments in magnetic cooling materials and systems [4,5]. MCE can be understood as the induced temperature change when a magnetocaloric material (MCM) is adiabatically subjected to a varying magnetic field [6]. It is characterized by the entropy change DSM (T, H), relative cooling power (RC), temperature and field induced hysteresis losses, as well as the adiabatic temperature change (DTad) [7,8]. Large scale commercialization of this solid state cooling technology has been hampered by the high cost of most MCM, which are usually composed of expensive, corrosion and oxidation prone rare earth based materials. Therefore, low cost and high performance MCM are urgently needed, e.g., Fe based alloys
* Corresponding author. E-mail address: [email protected] (R.V. Ramanujan). http://dx.doi.org/10.1016/j.jallcom.2015.08.245 0925-8388/© 2015 Elsevier B.V. All rights reserved.
and Mn based alloys [9,10]. The discovery of “Giant” MCE in Gd5(SixGe1x)4 [11,12] compounds and other materials with a first order magnetostructural transition, such as La(Fe1xSix)13 [13], Heusler alloys [14,15] and MnFeP0.45As0.55 [16] has been reported. However, although these giant magnetocaloric materials possess high DSM, they also exhibit relatively low RC due to the narrow working temperature range. High magnetic entropy change (DSM) and RC are expected for a high performance magnetocaloric material. RC is the heat that can be transferred from the cold end to the hot end in a thermodynamic cycle, which is a key metric to evaluate the magnetocaloric performance of a MCM. It can be calculated by the relationship RC ¼ DSM(T, H)·dTFWHM (dTFWHM refers to the peak width at halfheight of the DSM curve). Thus, RC is related to both the peak value of entropy change and the temperature range in which the MCM can operate [17]. Generally, however, higher DSM can only be obtained in a first order transition, while broad peaks of the entropy change curve, corresponding to large RC, are usually observed in a second order transition. Hence, a crucial challenge to obtain attractive RC, DSM and tunable Tc. By replacing the toxic element As in MneFeePeAs alloys by Ge, low cost MneFeePeGe alloys were found to exhibit attractive magnetocaloric properties [18,19]. These MneFeePeGe alloys can exhibit either a first order or second order magnetic phase transformation from the ferromagnetic (FM) to the paramagnetic (PM)
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state, depending on the Ge content of the alloy. X-ray diffraction €ssbauer spectroscopy [20] and neutron diffraction anal(XRD), Mo ysis were employed to determine the crystal structure. The BeaneRodbell [21,22] model was applied to quantitatively model the magnetic transition. In our previous work [23], the magnetocaloric effect and magnetic transition in MneFeePeGe melt spun ribbons were studied, a change from first order to second order transition was observed with increasing Ge content, this effect was modeled using the BeaneRodbell model [24e28]. However, due to grain alignment and domains in ribbon samples, sharp entropy change peaks were observed, indicating a narrow operating temperature range of these MCM and hence, low RC. To increase RC, the present work concentrated on powder samples [29]. Powders can be used as a suspension in a suitable carrier fluid for active cooling devices and systems. In this work, we studied the magnetic transition and magnetocaloric behavior of Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) powder samples by experimental methods and modeling. Interestingly, large RC (1138 JKg1) at 5 T magnetic field was obtained in a Mn1.1Fe0.9P0.79Ge0.21 nanostructured powders, which is considerably higher than in ribbon samples. We observed much higher RC, and similar DSM, compared with previous work on these alloys [24,30]. The magnetic transition temperature increases with increasing Ge content and a second order magnetic transition occurs when Ge content increases to x ¼ 0.32, consistent with the results of ribbon samples.
2. Experimental Polycrystalline Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) alloys were prepared by powder metallurgy techniques. They are designated as Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32. The alloys were prepared by arc melting, followed by ball milling in argon atmosphere. Subsequently, the alloys were annealed in vacuum at 950 C for 12 h, resulting in an alloy with the Fe2P hexagonal structure [31]. The grain size is ~40 nm as calculated by XRD results using Scherrer equation. The field dependence of magnetization
was measured by a commercial superconducting cryostat PPMS (Physical Property Measuring System, Quantum Design, USA), equipped with a vibrating sample magnetometer. Magnetization values were recorded at various temperatures under a maximum magnetic field of 5 T. The isothermal magnetic entropy change (DSM) was calculated from the isothermal magnetization curves [32].
3. Results and discussion 3.1. Magnetic property and isothermal magnetic entropy change The magnetocaloric properties of Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) powder samples were measured. All these samples undergo a magnetic transition from the ferromagnetic to the paramagnetic state during heating. The temperature dependence of magnetization in 1 T applied magnetic field and the magnetic entropy change (DSM) for Dm0H ¼ 5 T are shown (Fig. 1 (a) and Fig. 1(b)) for Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 powder samples. The magnetic transition temperature of Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 powder samples in 1 T external magnetic field was determined (Fig. 1(a)). Thermal hysteresis was observed in Ge0.13, Ge0.19, Ge0.21 and Ge0.26 samples, indicating a first order magnetic transition. The magnetic entropy change (DSM) in a 5 T magnetic field of these samples are shown in Fig. 1(b). Maximum magnetic entropy change (DSM) of ~26.4 Jkg1K1 at 281.5 K and a maximum relative cooling power (RC) of ~1138 Jkg1 for an applied magnetic field of 5 T were obtained in the Mn1.1Fe0.9P0.79Ge0.21 alloy. The Ge0.21 powders exhibited attractive magnetocaloric properties near room temperature; hence, this alloy composition was selected for further evaluation. In Fig. 1(c) and (d), comparison of the magnetic transition temperature, magnetic entropy change and thermal hysteresis of Ge0.21 powder and ribbon samples is shown. The entropy change of the powder sample is greater than the corresponding value in the ribbon samples. In Table 1, the magnetic transition temperature (Tc), magnetic entropy change (DSM) and relative cooling power (RC) of these
Fig. 1. (a) FCC and FCW M(T) curves in 1 T applied field and (b) DSM of Ge0.13, Ge0.19, Ge0.21 Ge0.26 and Ge0.32 powder alloys in 5 T applied magnetic field. (c) FCC and FCW M(T) curves of Ge0.21 powder and ribbon samples in 1 T applied field and (d) DSM of Ge0.21 powder and ribbon samples in 5 T applied magnetic field.
X. Chen, R.V. Ramanujan / Journal of Alloys and Compounds 652 (2015) 393e399 Table 1 Magnetic transition temperature (TC), magnetic entropy change (DSM) and relative cooling power (RC) of Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 alloys.
Tc (K) DSM (Jkg1K1) RC (Jkg1)
Ge0.13
Ge0.19
Ge0.21
248.7 9.6 129
255 27.5 482
281.5 26.4 1138
283 (ribbon) 23.3 (ribbon) 524 (ribbon)
Ge0.26
Ge0.32
345 18.2 483
401.7 9.5 598.5
powders are listed. It can be seen that Tc, DSM and RC can be tuned in a wide temperature range by selecting Ge content. For example, Tc can vary from 248.7 K to 401.7 K, which encompasses a very useful range of near room temperature thermal management
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applications. RC was seen to be highest for Ge0.21 alloys, this alloy composition also exhibits high DSM (26.4 Jkg1K1). As mentioned previously, MneFeePeGe ribbon samples show higher DSM but narrow operating temperature range, resulting in lower RC. For example, as listed in Table 1, RC of Ge0.26 powder is found to be ~480 Jkg1, higher than the maximum RC obtained in Ge0.26 ribbon samples [23] (~400 Jkg1). The value of RC for the Ge0.21 powder (1138 Jkg1) is much higher than the RC of ~500 Jkg1 bulk samples for MneFeePeGe alloys obtained by other researchers [25] and ~600 Jkg1 for gadolinium at 5 T applied field; Gd is the benchmark material for MCE applications. The values of RC for the Ge0.21 powder at 1 T and 2 T fields are 207 Jkg1 and 442 Jkg1, respectively. Thus,
Fig. 2. TEM analysis (a) BF micrograph and (b) ð5143Þ zone of Ge0.13 powder. (c) BF micrograph and (d) ð0112Þ zone of Ge0.19 powder. (e) BF micrograph and (f) ð5143Þ zone of Ge0.21 powder. (g) BF micrograph and (h) ð0111Þ zone of Ge0.26 powder. (i) BF micrograph and (j) ð1211Þ zone of Ge0.32 powder.
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Fig. 2. (continued).
Mn1.1Fe0.9P0.79Ge0.21 powder samples are promising candidates for solid state magnetic cooling systems. The higher DSM and wider working temperature range of the powder can be attributed to the enhanced spin disorder at surfaces when the particle size decreases to the same order as the magnetic domain size as well as Neel surface anisotropy, due to the broken symmetry of the particle surroundings [29,33,34]. Higher DSM and wider working temperature range was observed in Ge0.21 powder sample. Moreover, thermal hysteresis during heating and cooling processes in the Ge0.21 powder sample (8 K) is smaller than the hysteresis of the Ge0.21 ribbon sample (17 K). Thus, compared with ribbon samples, Ge0.21 powders exhibit higher DSM near room temperature, wider working temperature range as well as less thermal hysteresis, making them attractive materials for magnetic thermal management systems.
pattern (SADP). Both the high temperature phase and low temperature phase exhibit a hexagonal structure, which indicates the magnetic transition in these alloys is not related to symmetry change. The crystal structure of the MneFeePeGe powders at room temperature was studied by X-ray diffraction pattern (Fig. 3). MneFeePeGe alloys show P62m hexagonal structures.
3.2. Microstructure The microstructure of the MneFeePeGe powders at room temperature was studied by transmission electron microscopy. The change in microstructure with Ge content is shown in Fig. 2. Consistent with the results of the magnetic properties, we find that the Ge0.13, Ge0.19 and Ge0.21 samples exhibit the high temperature paramagnetic phase, while Ge0.26 and Ge0.32 samples exhibit the low temperature ferromagnetic phase at room temperature. Fig. 2(a), (c), (e), (g) and (i) show the microstructure of the Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 powder samples, respectively, revealing individual powder particles. Fig. 2(b), (d), (f), (h) and (j) are the corresponding selected area diffraction
Fig. 3. X-Ray diffraction pattern of Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) powder at room temperature. Fe2P type hexagonal structure can be observed.
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Fig. 4. Temperature dependence of Landau coefficients a1, a3 and a5 for (a) Mn1.1Fe0.9P0.87Ge0.13 powder, (b) Mn1.1Fe0.9P0.81Ge0.19 powder, (c) Mn1.1Fe0.9P0.79Ge0.21 powder, (d) Mn1.1Fe0.9P0.79Ge0.21 ribbon, (e) Mn1.1Fe0.9P0.74Ge0.26 powder and (f) Mn1.1Fe0.9P0.68Ge0.32 powder alloys.
the value of the Landau coefficients a1, a3 and a5 can be determined as a function of temperature.
3.3. Modeling 3.3.1. Landau theory The magnetic free energy F (M, T) can be expressed as a Landau expansion with respect to magnetization (M) [23]. Assuming that the resulting splitting of the subbands is much smaller than their width, the Landau expansion of magnetic free energy F (M, T) can be used to describe the magnetic behavior near Tc in MneFeePeGe alloys [23]. The Landau expansion is expressed in Eq. (1):
F ðM; T Þ ¼
a1 ðT Þ 2 a3 ðT Þ 4 a5 ðT Þ 6 M þ M þ M þ … m0 HM 2 4 6
(1)
By fitting the M v/s H curves at different temperatures in Eq. (2),
m0 H ¼ a1 ðT ÞM þ a3 ðT ÞM3 þ a5 ðT ÞM 5
(2)
The temperature dependence of the Landau coefficients a1 (T), a3 (T) and a5 (T) can be used to identify the magnetic transition temperatures. Based on the nature of the Landau expansion, the positive minimum in a1 (T) is the value of Tc. T0 is the temperature at which a3 is zero. When a5 > 0 and T0 > Tc, a first-order transition can be expected [35]. Fig. 4 shows the plots of a1, a3 and a5 as a function of temperature for Ge0.13, Ge0.19, Ge0.21 (powder and ribbon), Ge0.26 and Ge0.32 powder samples. In the Ge0.13 powder sample, there are two minimum values in the a1 curve, indicating two transitions,
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which is consistent with the M v/s. T results. Both the values of T0 are larger than Tc and a5 is positive in the vicinity of these two temperatures, indicating that these two transitions are first-order (Fig. 4(a)). In Fig. 4(b)e(d). The T0 values are larger than Tc, while a5 is positive in the vicinity of these two temperatures, verifying that the magnetic transition in Ge0.19, Ge0.21 powder and Ge0.21 ribbon alloys are also first order. In Fig. 4(e), T0 and Tc are quite similar. Therefore, the Arrott plot was applied to identify the order of magnetic transition in Ge0.26 powders. The Landau coefficients of Ge0.32 powders are shown in Fig. 4(f), T0 is smaller than Tc, indicating a second order transition. 3.3.2. Arrott plot M2 versus H/M isotherms (known as Arrott plot) were plotted to identify the order of the magnetic transition in the Mn1.1Fe0.9P0.74Ge0.26 and Mn1.1Fe0.9P0.68Ge0.32 powder samples. Fig. 5(a) shows the Arrott plot curves at various temperatures of Ge0.26 powder sample, indicating a first order magnetic transition. In the vicinity of Tc, the Arrott plot curves exhibit negative slopes and inflection points. This indicates the existence of metamagnetic states and a field-induced first-order magnetic transition from the paramagnetic to the ferromagnetic phase [23]. In Fig. 5(b), the positive slope of the Arrott plot suggests that the paramagnetic to ferromagnetic transition in the Ge0.32 powder sample is second order. 3.3.3. Magnetic entropy change by Landau theory At a first-order phase transition, the first derivatives of the free energy and the order parameter undergo discontinuous changes.
Fig. 5. Arrott plots of (a) Mn1.1Fe0.9P0.74Ge0.26 and (b) Mn1.1Fe0.9P0.68Ge0.32 samples in the vicinity of Tc.
Landau theory can be applied to describe the magnetocaloric effect in ferromagnetic systems with magnetoelastic coupling. The sign of the Landau coefficients a1, a3 and a5 determine the order of the phase transition. The corresponding magnetic entropy can be obtained from differentiation of the magnetic part of the free energy with respect to temperature (Eq. (3)).
1 1 1 SM ðT; HÞ ¼ a1 0ðTÞM 2 a3 0ðTÞM 4 a5 0ðTÞM 6 2 4 6
(3)
a1 0 (T), a3 0 (T) and a5 0 (T) are the temperature derivatives of the expansion coefficients. The magnetic entropy change DSM(T, H) ¼ SM(T, H) SM(T, 0) was calculated using the temperature dependence of the expansion coefficients determined by fitting the measured magnetization to the equation of state (Eq. (2)). Fig. 6 shows the calculated magnetic entropy change of Ge0.21 powder and ribbon alloys in Dm0H ¼ 5 T applied field. The magnetic entropy change of Ge0.21 ribbon alloy, calculated by Landau theory, was higher than the experimental results, the calculated RC of Ge0.21 ribbon was similar to the experimental value of Ge0.21 powder, which can be due to the sharp transition induced by the highly uniform of grain and domain size in ribbon samples. A maximum entropy change of ~28 Jkg1K1 was obtained in the Ge0.21 powder by modeling, which is consistent with the experimental data (~26 Jkg1K1). In these MneFeePeGe powder alloys, the critical composition range for the cross over from first order to second order magnetic transition was determined to be in the range of 0.3 < x < 0.32, consistent with our previous study in ribbons [23]. For magnetocaloric materials, the MCE performance is related to the order of
Fig. 6. Temperature dependence of DSM calculated by Landau theory. (a) Mn1.1Fe0.9P0.79Ge0.21 powder and (b) Mn1.1Fe0.9P0.79Ge0.21 ribbon samples in the vicinity of Tc.
X. Chen, R.V. Ramanujan / Journal of Alloys and Compounds 652 (2015) 393e399
phase transition. The advantages of a first order transition are high DSM due to the sharp transition, while the disadvantages are narrow operating temperature range and hysteresis. Materials exhibiting second order transition show advantages such as absence of thermal and magnetic hysteresis, higher operating frequency and wider operating temperature range, on the other hand, DSM is typically much lower. In this work, the order of the transition was tuned by changing Ge content. It was found that Mn1.1Fe0.9P0.79Ge0.21 nanostructured powders yielded high DSM near room temperature. Wide temperature range was also observed in these powders, resulting in a large RC. A good combination of properties with advantages of both first and second order transitions was thus achieved, which can be used in active cooling systems [36]. 4. Conclusions MneFeePeGe alloys are attractive materials for magnetic cooling and thermal management applications due to their high magnetic entropy change and tunable working temperature. However melt spun samples show relatively low relative cooling power (RC). Hence, with the aim of increasing the relative cooling power, the giant magnetocaloric effect was investigated in MneFeePeGe powder samples. Experimental methods (magnetometry and TEM) and modeling (Arrott plot and Landau theory) were employed. The magnetic transition temperature increases with increasing Ge content. By appropriately tuning Ge content, DSM of ~26.4 Jkg1K1 and large maximum relative cooling power (RC) of ~1138 JKg1 were obtained near room temperature (281.7 K) in Mn1.1Fe0.9P0.79Ge0.21 powder samples. MneFeePeGe powder samples show both high DSM and wider working temperatures. These results can be useful to develop magnetocaloric materials suspended in carrier fluid for active magnetic cooling and energy harvesting applications. Acknowledgments This research is conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister's Office, Singapore. References [1] K.A. Gschneidner Jr., Y. Mudryk, V.K. Pecharsky, Scr. Mater. 67 (2012) 572.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Large magnetocaloric effect near room temperature in MneFeePeGe nanostructured powders X. Chen, R.V. Ramanujan* School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2015 Received in revised form 28 August 2015 Accepted 29 August 2015 Available online 31 August 2015
MneFeePeGe alloys are attractive candidates for affordable and high performance magnetocaloric materials (MCM). The magnetocaloric effect (MCE), crystal structure and magnetic transition of MneFe ePeGe nanostructured powders were investigated with the aim of obtaining both high relative cooling power and large entropy change. In this work, the Ge content was tuned to obtain high DSM near room temperature in powder samples. With increasing Ge content, the magnetic transition temperature increases. Interestingly, large relative cooling power (RC) together with high DSM were obtained near room temperature in Mn1.1Fe0.9P0.79Ge0.21 powders, the RC value is much larger than previously reported values in MneFeePeGe alloys. To model the magnetic transition and magnetocaloric behavior, Landau theory and a modified Arrott plot were utilized, good agreement was obtained between the theoretical model and the experimental results. Our results suggest that Mn1.1Fe0.9P0.79Ge0.21 powders possess attractive magnetocaloric properties for commercial applications. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetocaloric materials Magnetic and structural transition MneFeePeGe based alloys
1. Introduction Magnetic cooling technology, based on the Magnetocaloric Effect (MCE), has attracted intense fundamental research as well as commercial interest as an alternative thermal management technology. Due to its high energy efficiency and environmental friendliness [1e3], the magnetocaloric effect has been used in heat pumps, energy harvesting from low grade waste heat and a variety of cooling systems. The urgent requirement for such “green” energy technologies has led to several key developments in magnetic cooling materials and systems [4,5]. MCE can be understood as the induced temperature change when a magnetocaloric material (MCM) is adiabatically subjected to a varying magnetic field [6]. It is characterized by the entropy change DSM (T, H), relative cooling power (RC), temperature and field induced hysteresis losses, as well as the adiabatic temperature change (DTad) [7,8]. Large scale commercialization of this solid state cooling technology has been hampered by the high cost of most MCM, which are usually composed of expensive, corrosion and oxidation prone rare earth based materials. Therefore, low cost and high performance MCM are urgently needed, e.g., Fe based alloys
* Corresponding author. E-mail address: [email protected] (R.V. Ramanujan). http://dx.doi.org/10.1016/j.jallcom.2015.08.245 0925-8388/© 2015 Elsevier B.V. All rights reserved.
and Mn based alloys [9,10]. The discovery of “Giant” MCE in Gd5(SixGe1x)4 [11,12] compounds and other materials with a first order magnetostructural transition, such as La(Fe1xSix)13 [13], Heusler alloys [14,15] and MnFeP0.45As0.55 [16] has been reported. However, although these giant magnetocaloric materials possess high DSM, they also exhibit relatively low RC due to the narrow working temperature range. High magnetic entropy change (DSM) and RC are expected for a high performance magnetocaloric material. RC is the heat that can be transferred from the cold end to the hot end in a thermodynamic cycle, which is a key metric to evaluate the magnetocaloric performance of a MCM. It can be calculated by the relationship RC ¼ DSM(T, H)·dTFWHM (dTFWHM refers to the peak width at halfheight of the DSM curve). Thus, RC is related to both the peak value of entropy change and the temperature range in which the MCM can operate [17]. Generally, however, higher DSM can only be obtained in a first order transition, while broad peaks of the entropy change curve, corresponding to large RC, are usually observed in a second order transition. Hence, a crucial challenge to obtain attractive RC, DSM and tunable Tc. By replacing the toxic element As in MneFeePeAs alloys by Ge, low cost MneFeePeGe alloys were found to exhibit attractive magnetocaloric properties [18,19]. These MneFeePeGe alloys can exhibit either a first order or second order magnetic phase transformation from the ferromagnetic (FM) to the paramagnetic (PM)
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state, depending on the Ge content of the alloy. X-ray diffraction €ssbauer spectroscopy [20] and neutron diffraction anal(XRD), Mo ysis were employed to determine the crystal structure. The BeaneRodbell [21,22] model was applied to quantitatively model the magnetic transition. In our previous work [23], the magnetocaloric effect and magnetic transition in MneFeePeGe melt spun ribbons were studied, a change from first order to second order transition was observed with increasing Ge content, this effect was modeled using the BeaneRodbell model [24e28]. However, due to grain alignment and domains in ribbon samples, sharp entropy change peaks were observed, indicating a narrow operating temperature range of these MCM and hence, low RC. To increase RC, the present work concentrated on powder samples [29]. Powders can be used as a suspension in a suitable carrier fluid for active cooling devices and systems. In this work, we studied the magnetic transition and magnetocaloric behavior of Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) powder samples by experimental methods and modeling. Interestingly, large RC (1138 JKg1) at 5 T magnetic field was obtained in a Mn1.1Fe0.9P0.79Ge0.21 nanostructured powders, which is considerably higher than in ribbon samples. We observed much higher RC, and similar DSM, compared with previous work on these alloys [24,30]. The magnetic transition temperature increases with increasing Ge content and a second order magnetic transition occurs when Ge content increases to x ¼ 0.32, consistent with the results of ribbon samples.
2. Experimental Polycrystalline Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) alloys were prepared by powder metallurgy techniques. They are designated as Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32. The alloys were prepared by arc melting, followed by ball milling in argon atmosphere. Subsequently, the alloys were annealed in vacuum at 950 C for 12 h, resulting in an alloy with the Fe2P hexagonal structure [31]. The grain size is ~40 nm as calculated by XRD results using Scherrer equation. The field dependence of magnetization
was measured by a commercial superconducting cryostat PPMS (Physical Property Measuring System, Quantum Design, USA), equipped with a vibrating sample magnetometer. Magnetization values were recorded at various temperatures under a maximum magnetic field of 5 T. The isothermal magnetic entropy change (DSM) was calculated from the isothermal magnetization curves [32].
3. Results and discussion 3.1. Magnetic property and isothermal magnetic entropy change The magnetocaloric properties of Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) powder samples were measured. All these samples undergo a magnetic transition from the ferromagnetic to the paramagnetic state during heating. The temperature dependence of magnetization in 1 T applied magnetic field and the magnetic entropy change (DSM) for Dm0H ¼ 5 T are shown (Fig. 1 (a) and Fig. 1(b)) for Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 powder samples. The magnetic transition temperature of Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 powder samples in 1 T external magnetic field was determined (Fig. 1(a)). Thermal hysteresis was observed in Ge0.13, Ge0.19, Ge0.21 and Ge0.26 samples, indicating a first order magnetic transition. The magnetic entropy change (DSM) in a 5 T magnetic field of these samples are shown in Fig. 1(b). Maximum magnetic entropy change (DSM) of ~26.4 Jkg1K1 at 281.5 K and a maximum relative cooling power (RC) of ~1138 Jkg1 for an applied magnetic field of 5 T were obtained in the Mn1.1Fe0.9P0.79Ge0.21 alloy. The Ge0.21 powders exhibited attractive magnetocaloric properties near room temperature; hence, this alloy composition was selected for further evaluation. In Fig. 1(c) and (d), comparison of the magnetic transition temperature, magnetic entropy change and thermal hysteresis of Ge0.21 powder and ribbon samples is shown. The entropy change of the powder sample is greater than the corresponding value in the ribbon samples. In Table 1, the magnetic transition temperature (Tc), magnetic entropy change (DSM) and relative cooling power (RC) of these
Fig. 1. (a) FCC and FCW M(T) curves in 1 T applied field and (b) DSM of Ge0.13, Ge0.19, Ge0.21 Ge0.26 and Ge0.32 powder alloys in 5 T applied magnetic field. (c) FCC and FCW M(T) curves of Ge0.21 powder and ribbon samples in 1 T applied field and (d) DSM of Ge0.21 powder and ribbon samples in 5 T applied magnetic field.
X. Chen, R.V. Ramanujan / Journal of Alloys and Compounds 652 (2015) 393e399 Table 1 Magnetic transition temperature (TC), magnetic entropy change (DSM) and relative cooling power (RC) of Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 alloys.
Tc (K) DSM (Jkg1K1) RC (Jkg1)
Ge0.13
Ge0.19
Ge0.21
248.7 9.6 129
255 27.5 482
281.5 26.4 1138
283 (ribbon) 23.3 (ribbon) 524 (ribbon)
Ge0.26
Ge0.32
345 18.2 483
401.7 9.5 598.5
powders are listed. It can be seen that Tc, DSM and RC can be tuned in a wide temperature range by selecting Ge content. For example, Tc can vary from 248.7 K to 401.7 K, which encompasses a very useful range of near room temperature thermal management
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applications. RC was seen to be highest for Ge0.21 alloys, this alloy composition also exhibits high DSM (26.4 Jkg1K1). As mentioned previously, MneFeePeGe ribbon samples show higher DSM but narrow operating temperature range, resulting in lower RC. For example, as listed in Table 1, RC of Ge0.26 powder is found to be ~480 Jkg1, higher than the maximum RC obtained in Ge0.26 ribbon samples [23] (~400 Jkg1). The value of RC for the Ge0.21 powder (1138 Jkg1) is much higher than the RC of ~500 Jkg1 bulk samples for MneFeePeGe alloys obtained by other researchers [25] and ~600 Jkg1 for gadolinium at 5 T applied field; Gd is the benchmark material for MCE applications. The values of RC for the Ge0.21 powder at 1 T and 2 T fields are 207 Jkg1 and 442 Jkg1, respectively. Thus,
Fig. 2. TEM analysis (a) BF micrograph and (b) ð5143Þ zone of Ge0.13 powder. (c) BF micrograph and (d) ð0112Þ zone of Ge0.19 powder. (e) BF micrograph and (f) ð5143Þ zone of Ge0.21 powder. (g) BF micrograph and (h) ð0111Þ zone of Ge0.26 powder. (i) BF micrograph and (j) ð1211Þ zone of Ge0.32 powder.
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Fig. 2. (continued).
Mn1.1Fe0.9P0.79Ge0.21 powder samples are promising candidates for solid state magnetic cooling systems. The higher DSM and wider working temperature range of the powder can be attributed to the enhanced spin disorder at surfaces when the particle size decreases to the same order as the magnetic domain size as well as Neel surface anisotropy, due to the broken symmetry of the particle surroundings [29,33,34]. Higher DSM and wider working temperature range was observed in Ge0.21 powder sample. Moreover, thermal hysteresis during heating and cooling processes in the Ge0.21 powder sample (8 K) is smaller than the hysteresis of the Ge0.21 ribbon sample (17 K). Thus, compared with ribbon samples, Ge0.21 powders exhibit higher DSM near room temperature, wider working temperature range as well as less thermal hysteresis, making them attractive materials for magnetic thermal management systems.
pattern (SADP). Both the high temperature phase and low temperature phase exhibit a hexagonal structure, which indicates the magnetic transition in these alloys is not related to symmetry change. The crystal structure of the MneFeePeGe powders at room temperature was studied by X-ray diffraction pattern (Fig. 3). MneFeePeGe alloys show P62m hexagonal structures.
3.2. Microstructure The microstructure of the MneFeePeGe powders at room temperature was studied by transmission electron microscopy. The change in microstructure with Ge content is shown in Fig. 2. Consistent with the results of the magnetic properties, we find that the Ge0.13, Ge0.19 and Ge0.21 samples exhibit the high temperature paramagnetic phase, while Ge0.26 and Ge0.32 samples exhibit the low temperature ferromagnetic phase at room temperature. Fig. 2(a), (c), (e), (g) and (i) show the microstructure of the Ge0.13, Ge0.19, Ge0.21, Ge0.26 and Ge0.32 powder samples, respectively, revealing individual powder particles. Fig. 2(b), (d), (f), (h) and (j) are the corresponding selected area diffraction
Fig. 3. X-Ray diffraction pattern of Mn1.1Fe0.9P1xGex (x ¼ 0.13, 0.19, 0.21, 0.26 and 0.32) powder at room temperature. Fe2P type hexagonal structure can be observed.
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Fig. 4. Temperature dependence of Landau coefficients a1, a3 and a5 for (a) Mn1.1Fe0.9P0.87Ge0.13 powder, (b) Mn1.1Fe0.9P0.81Ge0.19 powder, (c) Mn1.1Fe0.9P0.79Ge0.21 powder, (d) Mn1.1Fe0.9P0.79Ge0.21 ribbon, (e) Mn1.1Fe0.9P0.74Ge0.26 powder and (f) Mn1.1Fe0.9P0.68Ge0.32 powder alloys.
the value of the Landau coefficients a1, a3 and a5 can be determined as a function of temperature.
3.3. Modeling 3.3.1. Landau theory The magnetic free energy F (M, T) can be expressed as a Landau expansion with respect to magnetization (M) [23]. Assuming that the resulting splitting of the subbands is much smaller than their width, the Landau expansion of magnetic free energy F (M, T) can be used to describe the magnetic behavior near Tc in MneFeePeGe alloys [23]. The Landau expansion is expressed in Eq. (1):
F ðM; T Þ ¼
a1 ðT Þ 2 a3 ðT Þ 4 a5 ðT Þ 6 M þ M þ M þ … m0 HM 2 4 6
(1)
By fitting the M v/s H curves at different temperatures in Eq. (2),
m0 H ¼ a1 ðT ÞM þ a3 ðT ÞM3 þ a5 ðT ÞM 5
(2)
The temperature dependence of the Landau coefficients a1 (T), a3 (T) and a5 (T) can be used to identify the magnetic transition temperatures. Based on the nature of the Landau expansion, the positive minimum in a1 (T) is the value of Tc. T0 is the temperature at which a3 is zero. When a5 > 0 and T0 > Tc, a first-order transition can be expected [35]. Fig. 4 shows the plots of a1, a3 and a5 as a function of temperature for Ge0.13, Ge0.19, Ge0.21 (powder and ribbon), Ge0.26 and Ge0.32 powder samples. In the Ge0.13 powder sample, there are two minimum values in the a1 curve, indicating two transitions,
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which is consistent with the M v/s. T results. Both the values of T0 are larger than Tc and a5 is positive in the vicinity of these two temperatures, indicating that these two transitions are first-order (Fig. 4(a)). In Fig. 4(b)e(d). The T0 values are larger than Tc, while a5 is positive in the vicinity of these two temperatures, verifying that the magnetic transition in Ge0.19, Ge0.21 powder and Ge0.21 ribbon alloys are also first order. In Fig. 4(e), T0 and Tc are quite similar. Therefore, the Arrott plot was applied to identify the order of magnetic transition in Ge0.26 powders. The Landau coefficients of Ge0.32 powders are shown in Fig. 4(f), T0 is smaller than Tc, indicating a second order transition. 3.3.2. Arrott plot M2 versus H/M isotherms (known as Arrott plot) were plotted to identify the order of the magnetic transition in the Mn1.1Fe0.9P0.74Ge0.26 and Mn1.1Fe0.9P0.68Ge0.32 powder samples. Fig. 5(a) shows the Arrott plot curves at various temperatures of Ge0.26 powder sample, indicating a first order magnetic transition. In the vicinity of Tc, the Arrott plot curves exhibit negative slopes and inflection points. This indicates the existence of metamagnetic states and a field-induced first-order magnetic transition from the paramagnetic to the ferromagnetic phase [23]. In Fig. 5(b), the positive slope of the Arrott plot suggests that the paramagnetic to ferromagnetic transition in the Ge0.32 powder sample is second order. 3.3.3. Magnetic entropy change by Landau theory At a first-order phase transition, the first derivatives of the free energy and the order parameter undergo discontinuous changes.
Fig. 5. Arrott plots of (a) Mn1.1Fe0.9P0.74Ge0.26 and (b) Mn1.1Fe0.9P0.68Ge0.32 samples in the vicinity of Tc.
Landau theory can be applied to describe the magnetocaloric effect in ferromagnetic systems with magnetoelastic coupling. The sign of the Landau coefficients a1, a3 and a5 determine the order of the phase transition. The corresponding magnetic entropy can be obtained from differentiation of the magnetic part of the free energy with respect to temperature (Eq. (3)).
1 1 1 SM ðT; HÞ ¼ a1 0ðTÞM 2 a3 0ðTÞM 4 a5 0ðTÞM 6 2 4 6
(3)
a1 0 (T), a3 0 (T) and a5 0 (T) are the temperature derivatives of the expansion coefficients. The magnetic entropy change DSM(T, H) ¼ SM(T, H) SM(T, 0) was calculated using the temperature dependence of the expansion coefficients determined by fitting the measured magnetization to the equation of state (Eq. (2)). Fig. 6 shows the calculated magnetic entropy change of Ge0.21 powder and ribbon alloys in Dm0H ¼ 5 T applied field. The magnetic entropy change of Ge0.21 ribbon alloy, calculated by Landau theory, was higher than the experimental results, the calculated RC of Ge0.21 ribbon was similar to the experimental value of Ge0.21 powder, which can be due to the sharp transition induced by the highly uniform of grain and domain size in ribbon samples. A maximum entropy change of ~28 Jkg1K1 was obtained in the Ge0.21 powder by modeling, which is consistent with the experimental data (~26 Jkg1K1). In these MneFeePeGe powder alloys, the critical composition range for the cross over from first order to second order magnetic transition was determined to be in the range of 0.3 < x < 0.32, consistent with our previous study in ribbons [23]. For magnetocaloric materials, the MCE performance is related to the order of
Fig. 6. Temperature dependence of DSM calculated by Landau theory. (a) Mn1.1Fe0.9P0.79Ge0.21 powder and (b) Mn1.1Fe0.9P0.79Ge0.21 ribbon samples in the vicinity of Tc.
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phase transition. The advantages of a first order transition are high DSM due to the sharp transition, while the disadvantages are narrow operating temperature range and hysteresis. Materials exhibiting second order transition show advantages such as absence of thermal and magnetic hysteresis, higher operating frequency and wider operating temperature range, on the other hand, DSM is typically much lower. In this work, the order of the transition was tuned by changing Ge content. It was found that Mn1.1Fe0.9P0.79Ge0.21 nanostructured powders yielded high DSM near room temperature. Wide temperature range was also observed in these powders, resulting in a large RC. A good combination of properties with advantages of both first and second order transitions was thus achieved, which can be used in active cooling systems [36]. 4. Conclusions MneFeePeGe alloys are attractive materials for magnetic cooling and thermal management applications due to their high magnetic entropy change and tunable working temperature. However melt spun samples show relatively low relative cooling power (RC). Hence, with the aim of increasing the relative cooling power, the giant magnetocaloric effect was investigated in MneFeePeGe powder samples. Experimental methods (magnetometry and TEM) and modeling (Arrott plot and Landau theory) were employed. The magnetic transition temperature increases with increasing Ge content. By appropriately tuning Ge content, DSM of ~26.4 Jkg1K1 and large maximum relative cooling power (RC) of ~1138 JKg1 were obtained near room temperature (281.7 K) in Mn1.1Fe0.9P0.79Ge0.21 powder samples. MneFeePeGe powder samples show both high DSM and wider working temperatures. These results can be useful to develop magnetocaloric materials suspended in carrier fluid for active magnetic cooling and energy harvesting applications. Acknowledgments This research is conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister's Office, Singapore. References [1] K.A. Gschneidner Jr., Y. Mudryk, V.K. Pecharsky, Scr. Mater. 67 (2012) 572.
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