Magnetostructural transformation and large magnetocaloric effect in Mn0.9Cu0.1CoGe1−xSix alloys

Intermetallics 91 (2017) 45–49

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Magnetostructural transformation and large magnetocaloric effect in Mn0.9Cu0.1CoGe1−xSix alloys

MARK

Kun Taoa, Hu Zhanga,∗, Ke Wen Longb,c, Yi Xu Wanga, Mei Ling Wua, Ya Ning Xiaoa, Cheng Fen Xinga, Li Chen Wangd, Yi Longa a

School of Materials Science and Engineering, University of Science and Technology of Beijing, Beijing, 100083, PR China ChuanDong Magnetic Electronic Co. Ltd., FoShan, GuangDong, 528513, PR China c ChengXian Technology Co. Ltd., FoShan, GuangDong, 528513, PR China d Department of Physics, Capital Normal University, Beijing, 100048, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Magnetic properties Magnetostructural transformation Magnetocaloric effect

The structural transition temperature Tt increases largely with the introduction of Si in Mn0.9Cu0.1CoGe1-xSix alloys due to the enhancement of degree of hexagonal distortion. Thus, the first-order magnetostructural transformation from ferromagnetic orthorhombic to paramagnetic hexagonal phase can be obtained in the range of 0.15 < x < 0.21. Large magnetocaloric effect (MCE) under low magnetic field change of 2 T is obtained due to the magnetostructural transformation, e.g., the maximum magnetic entropy change (-ΔSM) value for x = 0.16 is 10.3 J/kg K at 297 K, which is comparable to or even larger than those of some typical room-temperature magnetocaloric materials. The nature of magnetostructural transition has been studied by different methods, and it is found that the Arrott plots fail to determine the order of phase transition. In contrast, the universal curve of ΔSM is proved to be a more effective criterion to distinguish the order of phase transition. Besides, a linear relationship between -ΔSM and Δμ0H is found, and so the -ΔSM values for higher field changes can be estimated by linear fitting. Consequently, large MCE induced by magnetostructural transformation suggests that Mn0.9Cu0.1CoGe1-xSix alloys could be promising candidates for room-temperature magnetocaloric materials.

1. Introduction In recent years, intermetallic MM'X (M, M′ = transition metals, X = carbon or boron group elements) alloys that undergo magnetostructural transformation have drawn increasing attention due to various interesting magnetoresponsive properties, such as magnetic-volume effect [1,2], magnetoresistance [3], and magnetocaloric effect (MCE) [4,5]. Especially, the magnetostructural transition temperature can be tuned in a wide temperature range from liquid nitrogen temperature to above water-boiling temperature, making MM'X alloys of great potential for magnetic refrigeration, magnetic heat pump, and electric power generation in different temperature regions [6]. In this regard, extensive works have been made to investigate the regulatory mechanism of magnetostructural transition and related magnetoresponsive properties in many MM'X systems [1,5,7,8]. The stoichiometric MnCoGe alloy, as one of the important MM'X alloys, crystallizes in a Ni2In-type hexagonal structure (space group P63/mmc, 194) at high temperature, and it transforms to a low-temperature TiNiSi-type orthorhombic structure (space group Pnma, 62) at

∗

∼420 K [9]. It is noted that this structural transformation temperature (Tt) is much higher than the Curie temperatures of both hexagonal austenite (TCA ∼273 K) and orthorhombic martensite (TCM ∼355 K) [9,10]. It implies that the structural transformation occurs in paramagnetic (PM) state, resulting in the absence of magnetostructural coupling in stoichiometric MnCoGe. Interestingly, the structural transition is much more sensitive than magnetic ordering transitions in response to the change of many factors, such as chemical substitution [11,12], vacancy introduction [13], and the application of pressure [14]. Therefore, the Tt can be lowered to below TCM by appropriate adjustment, resulting in the magnetostructural transformation from PM hexagonal to ferromagnetic (FM) orthorhombic phase. Furthermore, it is found that the magnetostructural transformation will decouple when the Tt is even lower than TCA [5]. Therefore, it is also of importance to search suitable way to lift Tt in order to obtain the magnetostructural transformation again. In our previous work, we successfully enhanced the Tt and realized the magnetostructural coupling again in Mn0.95CoGe1-xSix by applying the isostructural alloying principle [5]. To further confirm the effectiveness of this method, in present work, we chose Mn0.9Cu0.1CoGe as a

Corresponding author. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail address: [email protected] (H. Zhang).

http://dx.doi.org/10.1016/j.intermet.2017.08.010 Received 24 January 2017; Received in revised form 2 July 2017; Accepted 12 August 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.

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corresponding to the typical characteristic of second-order phase transition (SOPT). For x = 0.16–0.20 samples, significant thermal hysteresis can be observed during the phase transition, suggesting the occurrence of first-order magnetostructural transformation from FM orthorhombic to PM hexagonal phase. With further increase of Si content, x = 0.22 and 0.24 samples with orthorhombic structure again exhibit a second-order FM-PM magnetic transition around TCM , implying the decoupling of magnetostructural transformation. Fig. 2(b) shows the M-T curves of x = 0.16 alloy under the magnetic fields of 0.05 T and 3 T, respectively. It is found that the Tt shifts by 2.7 K with the magnetic field increasing from 0.05 T to 3 T, which is related to the field-induced metamagnetic transition from PM to FM state. At 3 T, a large ΔM of −37.5 Am2/kg is observed during the magnetostructural transformation around Tt. This large ΔM results in a large Zeeman energy difference between FM orthorhombic and PM hexagonal phases, which would then lead to a large MCE [13,17]. By employing the ΔS ΔB Clausius-Clapeyron equation ΔM ≈ ΔT [18,19], the maximum entropy change ΔS value of x = 0.16 sample for a field change from 0.05 to 3 T is estimated to be −41.0 J/kg K, comparable to (MnNiSi)0.46(MnFeGe)0.54 (−42.0 J/kg K) using the same method [19]. On the basis of M-T measurements, a structural and magnetic phase diagram for Mn0.9Cu0.1CoGe1-xSix is constructed as shown in Fig. 3. It is clearly seen that the Tt increases largely with the introduction of Si as expected, and thus realizing the magnetostructural coupling in the range of 0.15 < x < 0.21. The temperature window ranges from 300 K to 365 K, which is suitable for room-temperature magnetic refrigeration. When Si content is higher than 0.21, the Tt is tuned to be higher than TCM , leading to the decoupling of magnetostructural transformation again. It has been reported that the Tt can be tuned by valence-electron concentration e/a ratio [20,21]. However, the e/a ratio of Mn0.9Cu0.1CoGe1-xSix does not change since Ge (4s24p2) and Si (3s23p2) atoms have the same valence electron number. This fact implies that the enhancement of Tt may be mainly affected by other factors. From the crystallographic point of view, the orthorhombic structure is regarded as a distortion of hexagonal structure, and the free energy of orthorhombic structure strongly depends on the degree of hexagonal distortion (chex/ahex or aort/bort) [22,23]. Therefore, recently the degree of hexagonal distortion has been considered as a key factor that affects the structural stability and transition temperature [5,17,23]. Here, it is found that the chex/ahex ratio increases from 1.3046 for x = 0.05 to 1.3121 for x = 0.18, and aort/bort ratio increases from 1.5589 for x = 0.16 to 1.5605 for x = 0.24. The increasing degree of hexagonal distortion would stabilize orthorhombic phase and then enhance the structural transition temperature Tt. Fig. 4(a) shows the magnetization isotherms of x = 0.16 alloy upon field ascending and descending modes in a wide temperature range. In

starting material since it exhibits the hexagonal structure at room temperature [15], and investigated the effects of Si-doping on the magnetostructural transformation and related MCE in Mn0.9Cu0.1CoGe1-xSix system. 2. Experimental details The polycrystalline alloys with the nominal compositions of Mn0.9Cu0.1CoGe1-xSix (x = 0–0.24) were prepared by arc-melting constituent elements of purity higher than 99.9 wt % in a high-purity argon atmosphere. The as-cast ingots were annealed in a high-vacuum quartz tube at 1123 K for 5 days, and then cooled slowly to room temperature to avoid stress in samples. The as-prepared samples are very brittle and the ones with x ≥ 0.16 even naturally cracked into powders due to the drastic structural transition during cooling. Powder X-ray diffraction (XRD) measurement was performed at room temperature by using Cu Kα radiation. The Rietveld refinement based on the XRD patterns was carried out to identify the crystal structure, fraction of different phases, and lattice parameters using the LHPM Rietica software [16]. Magnetization was measured using a cryogenfree cryocooler-based physical property measurement system (model VersaLab) from Quantum Design Inc. Powder samples (∼10 mg) were used for the magnetic measurements. 3. Results and discussion The powder XRD patterns of Mn0.9Cu0.1CoGe1-xSix (x = 0–0.24) alloys measured at room temperature are shown in Fig. 1(a). The XRD investigation reveals that Mn0.9Cu0.1CoGe crystallizes in the Ni2In-type hexagonal structure. With the introduction of Si, the TiNiSi-type orthorhombic phase appears and grows gradually, and finally dominates for the samples with x ≥ 0.20. Fig. 1(b) displays the fraction of hexagonal and orthorhombic phases at room temperature, determined from the Rietveld refinement based on the XRD patterns, as a function of Si content. There are three phase regions with increasing Si content: single hexagonal phase when x ≤ 0.1, coexistence of hexagonal and orthorhombic phases when 0.1 < x < 0.20, and nearly single orthorhombic phase when x ≥ 0.20, respectively. This result suggests that the substitution of Si for Ge in Mn0.9Cu0.1CoGe1-xSix could improve the stability of orthorhombic phase and enhance the Tt to higher than room temperature. The temperature (T) dependencies of zero-field-cooling (ZFC) and field-cooling (FC) magnetizations (M) were measured with a ramp rate of 1 K/min under 0.01 T as shown in Fig. 2(a). It is seen that x = 0.05 and 0.1 samples with hexagonal structure undergo a magnetic transition from FM to PM states around TCA without thermal hysteresis,

Fig. 1. (a) The powder XRD patterns of Mn0.9Cu0.1CoGe1-xSix (x = 0–0.24) alloys measured at room temperature. (b) The fraction of orthorhombic and hexagonal phases as a function of Si content.

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Fig. 2. (a) Temperature dependencies of ZFC and FC magnetizations for Mn0.9Cu0.1CoGe1-xSix (x = 0.05–0.24) alloys under 0.01 T. (b) Temperature dependencies of ZFC and FC magnetizations for x = 0.16 alloy under 0.05 T and 3 T.

large magnetic hysteresis can be clearly seen during the field ascending and descending cycles, corresponding to the typical characteristic of first-order field-induced metamagnetic transition from PM hexagonal to FM orthorhombic phase. The ΔSM values for different magnetic field changes were estimated from the M-H curves by using Maxwell relation [26],

ΔSM (T , H ) =

∫0

H

(∂M / ∂T ) H dH

(1)

and are shown in Fig. 4 (b). For a field change of 2 T, the maximum value of -ΔSM for x = 0.16 reaches as high as 10.3 J/kg K at 297 K, and this large ΔSM value is attributed to the magnetostructural coupling. Moreover, it is found that the ΔSM value for x = 0.16 is comparable to or even larger than those of some typical room-temperature magnetocaloric materials, such as Gd5Si2Ge2 (14.0 J/kg K at 278 K) [27], Gd (4.9 J/kg K at 293 K) [27], LaFe11Co0.9Si1.1 (7.4 J/kg K at 294 K) [28], and Ni50Mn37Sn13 (−6.7 J/kg K at 299 K) [29]. Since the magnetic field of 2 T can be simply provided by a permanent magnet, this large MCE under relatively low magnetic field change suggests that x = 0.16 alloy could be a promising candidate for room-temperature magnetocaloric materials. It is well known that magnetocaloric effect is closely related to the nature of phase transition, and so the nature of phase transition has been widely studied by different methods [30–32]. Generally, Arrott plot is often applied to investigate the nature of magnetic transition [30,33,34]. The magnetic transition is considered to be first-order when the Arrott plot exhibits negative slope or inflection point, while it is

Fig. 3. Structural and magnetic phase diagram for Mn0.9Cu0.1CoGe1-xSix alloys as a function of Si content.

order to avoid the residual effect generated in previous measurement which may lead to a spurious magnetic entropy change (ΔSM) [24,25], the M-H curves were measured in a loop process, in which the sample was first heated up to complete PM state and then cooled back to the target temperature before starting each M-H measurement [5,24]. A

Fig. 4. (a) Magnetization isotherms of x = 0.16 alloy upon field ascending and descending modes in a wide temperature range. (b) Temperature dependence of -ΔSM for x = 0.16 alloy under different magnetic field changes.

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ΔSM (T , Hmax )/ ΔSMpk (Hmax ) ; and (2) rescaling the temperature axis below and above Tpk by imposing that the positions of two reference points in the curve correspond to θ = ± 1, − (T − TCM )/(Tr1 − TCM ), T ≤ TCM θ=⎧ ⎨ (T − TCM )/(Tr 2 − TCM ), T > TCM ⎩

(2)

where Tr1 and Tr2 are the temperatures of two reference points that 1 have been chosen as those corresponding to 2 ΔSMpk . Fig. 5(b) shows the universal curve of ΔSM for x = 0.16 alloy under various magnetic field changes. It is seen that all the curves under different magnetic fields collapse onto the same universal curve within the range of −2 < θ < 2. However, the breakdown of universal behavior can be observed when θ is out of this range. It has been reported that the deviation of collapse for first-order phase transition (FOPT) may not be obvious around θ = 0 since the curves coincide by construction. Thus, whether the collapse of universal curve occurs below θ = −1 is crucial for determining the nature of phase transition [36]. Here, the breakdown of universal behavior away from θ = 0 suggests the nature of FOPT. Moreover, the breakdown can be quantified as dispersion from the vertical spread of points below θ = −1,

dispersion = 100 ×

W (θ = −3) ′ (θ = −3) ΔSave

(3)

where W is the width of vertical spreading for all scaled ΔS′ curves and ΔS′ave is the average value of ΔS′ at an arbitrary θ < −1, such as here θ = −3. It is pointed out that the dispersion for SOPT is usually lower than 30% [36]. Here, the dispersion is obtained to be 74.7%, which is much higher than 30%, further proving the nature of FOPT. Fig. 5(c) shows the magnetic field dependence of maximum -ΔSM value for x = 0.16 alloy. On the basis of mean field approach, the field dependence of -ΔSM value for materials with SOPT can be expressed as

ΔSM ∝ H n

(4)

and the local exponent n at TC is predicted to be 2/3 [31]. Here, the exponential fitting of -ΔSM-Δμ0H curve reveals that n at Tt is ∼1.0 with the adjusted R-squared factor of 0.99954. This notable deviation of n value also confirms the nature of FOPT. Furthermore, n = ∼1.0 suggests a linear relationship between ΔSM and Δμ0H, i.e., -ΔSM = aΔμ0H, where a is the slope factor which describes how strong the ΔSM depends on magnetic field change. Similar result has also been reported in Mn1yFeyNiGe1-xSix systems [7]. Hence, the ΔSM values for higher field changes up to 5 T can be estimated by linear fitting as shown in Fig. 5(c). For a magnetic field change of 5 T, the maximum -ΔSM value is estimated to be 26.0 J/kg K, which is also larger than those of some typical room-temperature magnetocaloric materials [27–29].

Fig. 5. (a) Arrott plots of x = 0.16 alloy derived from M-H isotherms. (b) Universal curve of ΔSM under various magnetic field changes. (c) Magnetic field dependence of maximum -ΔSM value for x = 0.16 alloy, and the fitting line to -ΔSM-Δμ0H curve.

4. Conclusions

expected to be second-order when the slope is positive. The Arrott plots of x = 0.16 alloy derived from M-H isotherms are displayed in Fig. 5(a). Since the M-T and M-H curves have proved that this sample exhibits a typical first-order magnetostructural transformation with distinct thermal and magnetic hysteresis, an obviously negative slope was expected in the Arrott plots. Contrary to our expectation, the Arrott plots show clearly positive slope, implying the failure of Arrott plots in determining the nature of phase transition in present work. Difficulties in determining the nature of phase transition using Arrott plots have also been reported in some other alloys, such as Mn0.98Fe0.02CoGe [12], Mn1.3Fe0.7P0.6Si0.25Ge0.15 [35], and DyCo2 [36]. Recently, Franco et al. proposed a universal behavior for ΔSM-T in materials with SOPT [32], and it has been suggested to be a more effective method to analyze the nature of phase transition [36]. The universal curve of ΔSM can be constructed by using the following phenomenological procedure: (1) normalizing all ΔSM curves with their respective peak entropy change as ΔS′ (T , Hmax ) =

To summarize, the substitution of Si for Ge in Mn0.9Cu0.1CoGe1-xSix could enhance the degree of hexagonal distortion, as well as raise the structural transition temperature Tt by stabilizing orthorhombic phase. Due to the increase of Tt, the first-order magnetostructural transformation from FM orthorhombic to PM hexagonal phase is observed around room temperature in the range of 0.15 < x < 0.21. For a relatively low field change of 2 T, the maximum -ΔSM value for x = 0.16 is 10.3 J/kg K at 297 K, and this large MCE under low field change is desirable for practical application of room-temperature magnetic refrigeration. In addition, the nature of phase transition for x = 0.16 has been investigated by different methods. It is found that the Arrott plots fail to determine the order of phase transition. On the contrary, the breakdown of universal behavior and the deviation of n value from 2/3 confirm the nature of FOPT. Moreover, a linear relationship between -ΔSM and Δμ0H is found, and then the -ΔSM values for higher field changes can be estimated by linear fitting. 48

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