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Structural and magnetic phase diagrams of MnFe0.6Ni0.4(Si,Ge) alloys and their giant magnetocaloric effect probed by heat capacity measurements
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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Research articles
Structural and magnetic phase diagrams of MnFe0.6Ni0.4(Si,Ge) alloys and their giant magnetocaloric effect probed by heat capacity measurements ⁎
T
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W. Hanggai, O. Tegus , H. Yibole , F. Guillou Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, College of Physics and Electronic Information, Inner Mongolia Normal University, 81 Zhaowuda Road, Hohhot, Inner Mongolia 010022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magneto-structural phase transitions Phase diagram Magnetocaloric effect
The structural and magnetic phase diagrams of MnFe0.6Ni0.4Si1−xGex alloys (0 ≤ x ≤ 1) are explored using powder X-ray diffraction, magnetic and calorimetric measurements. At room temperature, the crystal structure evolves from the orthorhombic TiNiSi-type to the hexagonal Ni2In-type with the increase in Ge. A magnetostructural coupling leading to a first-order ferromagnetic transition is found at 204 K for x ~ 0.5. Differential scanning calorimetry reveals a recalescence-like event at the first cooling across the structural transition. In-field specific heat measurements are used to quantify the giant magnetocaloric effect and show that the latent heat of the magneto-structural transition is less than in closely related MnMX alloys, which leads to sizable transition sensitivity to the magnetic field and finite adiabatic temperature change.
1. Introduction MnMX manganese alloys where M is a 3d transition metal and X a metalloid or a non-metal form a rich material families with intriguing physics in general and magnetism in particular. They mainly belong to one of the four following crystal-type structures: Fe2P (P −62m ), Fe2As (P4/nmm), TiNiSi (Pnma), or Ni2In (P63/mmc) [1]. Transitions from one crystal-type to the other as a function of temperature are possible, and those that couple with a change in magnetic order may lead to the appearance of magneto-structural first-order transitions. Ternaries such as MnCoGe, MnNiGe or MnNiSi, which are orthorhombic TiNiSi- type near room temperature, are alloyed with a fourth element or vacancies in order to induce a TiNiSi-to-Ni2In structural transition near the magnetic ordering transition, leading to a magneto-structural coupling [2–8]. In recent years, MnMX alloys showing such simultaneous structural orthorhombic TiNiSi to hexagonal Ni2In and magnetic transitions have been most active as the associated large volume change and latent heat result in giant negative thermal expansion and magnetocaloric effect (MCE) [7–16]. The later effect has received much attention due to its potential use into a green and energy efficient magnetic refrigeration technique. But, in order to use this giant MCE in applications several challenges remain to be coped with, such as: large thermal hysteresis which brings irreversibility in applications, large magnetic field changes required to turn most of the latent heat into MCE, pulverization of the samples when crossing the transition, limited adiabatic
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temperature change, etc. Due to the high sensitivity of the structural and magnetic properties of MnMX materials to alloying, one can expect to limit these drawbacks by adjusting the chemical composition. (Mn,Fe)Ni(Si,Ge) or (Mn,Fe)Ni(Si,Al) alloys are known to present giant magnetocaloric effect [14–16], but their giant MCE is not yet fully optimized. First, in ferromagnetic orthorhombic ternaries such as MnCoGe or MnNiSi, Mn occupies the pyramidal site and carries most of the magnetic moment 2.6–2.9 µB, while Co or Ni carries only 0.5–0.9 µB or ~0 µB, respectively [17]. In order to maximize the magnetic density and total MCE, substitutions on Mn should be avoided (with the additional complexity of the preferential site occupancy of Mn). Second, in the series of alloys with Ni fixed to unity, the latent heat (L) appears particularly large, transition entropy changes ΔStr = L/Ttr as large as 71 J kg−1 K−1 were reported [14]. While a large ΔStr is favorable to reach a large isothermal entropy change at high magnetic field, it also brings limited sensitivity of the transition temperature (Ttr) to the magnetic field, which in turn lowers the adiabatic temperature change. On the other hand, the phase diagrams of Mn(Ni,Fe)(Si,Ge) alloys with Mn fixed to unity have received less attention. Accordingly, we explore here the structure and magnetic properties of MnFe0.6Ni0.4(Si,Ge) alloys, with a special emphasis on thermal measurements. As a preliminary step, we investigated the properties of Mn (Fe,Ni)Si alloys to select Fe and Ni contents for which Curie temperature is near room temperature. Then, Si is substituted by Ge to drive the ortho.-hexa. structural transition near TC. The vast majority of the MCE
Corresponding authors. E-mail addresses: [email protected] (O. Tegus), [email protected] (H. Yibole).
https://doi.org/10.1016/j.jmmm.2019.165785 Received 28 June 2019; Received in revised form 29 August 2019; Accepted 4 September 2019 Available online 04 September 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
W. Hanggai, et al.
studies on MnMX alloys rely on indirect measurements of ΔS from magnetization data, a method which is prone to artefact in case of firstorder transitions [18–22]. To avoid this issue, we show that in-field specific heat measurements using the semi-adiabatic technique are technically feasible on these materials. Differential scanning calorimetry measurements are used to confirm the latent heat estimation and, in addition, reveal an unconventional recalescence-like phenomenon at the first cooling of as-prepared samples. 2. Experimental methods Eleven MnFe0.6Ni0.4Si1−xGex alloys with x ranging between 0 and 1 were prepared by arc-melting in high-purity argon atmosphere. Elemental starting materials with purity 99.9% for Mn, Ni, Ge and Fe, and 98.9% for Si were used. The as-cast ingots were then sealed in evacuated quartz tubes and annealed at 1373 K for 4 h ending by a quenching in water. Except when especially stated, all samples were cycled in temperature down to 77 K prior to measurements. Powder Xray diffraction experiments were performed on a Panalytical Empyrean X-Ray Diffractometer using Cu-Kα radiation. The Fullprof software was used to perform Rietveld refinements [23]. Differential scanning calorimetry measurements were performed on a TA 2500 DSC. The vibrating sample magnetometer and heat capacity options of a Quantum Design VersaLab system were employed for magnetic and in-field calorimetric measurements. For in-field specific heat measurements, single heat pulses of ~8 K covering most of the transition were used to avoid underestimating the latent heat [24]. Tzero® aluminum crucibles are used for the sample mounting during both calorimetric experiments. An addenda taking into account the contribution of N grease and that of the crucible was measured prior to specific heat experiments. While DSC experiments were performed on an – initially – small bulk piece with a polished bottom, for the semi-adiabatic calorimetric measurements, we mounted rough pre-cooled powder. Pre-cooling in liquid N2 of the x = 0.5 sample leads to a broad distribution in particle size: 360–120 µm (~39 w%), 120–36 µm (~43 w%), and less than 36 µm (~18 w%). But, this distribution is found very sensitive to the heat treatment profile. In addition, further particle size reduction occurs during subsequent thermal cycles. In order for the specific heat to reflect average properties, we chose not to select specific particle sizes, but rather use a large amount (39.0 mg) of powder as it comes after selfpulverization during pre-cooling. The use of such powder in magnetocaloric heat pumps would require appropriate shaping, by, for instance, using binder.
Fig. 1. Room temperature X-ray diffraction patterns of MnFe0.6Ni0.4Si1−xGex (0 ≤ x ≤ 1) alloys, the asterix mark out the two most intense reflections of the hexagonal Ni2In-type structure.
3.2. Magnetic properties of MnFe0.6Ni0.4Si1−xGex alloys Magnetization data as a function of temperature and external magnetic field were systematically recorded for all samples. For clarity, Fig. 2 illustrates only the most representative samples, for the other compositions the transition temperatures defined as the dM/dT maximum are presented in the phase diagram (Fig. 4). For x ≤ 0.4, one observes a ferromagnetic behavior with the Curie temperature progressively decreasing with the increase in Ge content. In this range, the ferromagnetic transition is a second-order transition as indicated by a pronounced broadening of the transition when applying a field of 1 T (not shown) and in line with absence of thermal hysteresis. For x = 0.5, the Curie temperature lowers suddenly and shows a large thermal hysteresis of ~15 K, as often observed in this class of materials when structural and magnetic transitions occur simultaneously. For x > 0.5, a magnetization drop resembling the ferromagnetic transition of the samples with x ≤ 0.5 is still observed. But, in addition, one can note a low temperature feature looking like a transition toward a complex antiferromagnetic state in the sense that the magnetization remains substantial which is incompatible with a collinear arrangement, and can easily turn into ferromagnetism in intermediate magnetic fields. At first glance, such low temperature transition toward lower magnetization could be attributed to a reminiscence of the spiral antiferromagnetic ground state found in MnNiGe ternary [4], it has been observed in other quaternaries [7], but the exact magnetic structure is not known yet. Accordingly, we hereafter refer to this low temperature transition as a spin-reorientation (S.R.) transition. The saturation magnetization (or close to) at 50 K and 3 T decreases with Ge for Si substitution, but not linearly. Samples that remain orthorhombic down to low temperatures present higher magnetic saturation than hexagonal samples. This can be ascribed to the change in magnetic moments at the structural transition, already pointed out in MnCoGe or MnNiGe related materials by local probes such as neutron diffraction, Mössbauer or Xray magnetic circular dichroism spectroscopies [17,26–28]. We can also note that the saturation magnetization ~104 Am2 kg−1 for x = 0.5 appears significantly larger than that usually found in (Mn,Fe)Ni(Si,Ge) materials, ~70–80 Am2 kg−1 [8], confirming the interest of materials with Mn content fixed to unity to enhance the magnetization.
3. Results and discussion 3.1. Crystal structure as a function of Ge content Fig. 1 shows the room-temperature XRD patterns of MnFe0.6Ni0.4Si1−xGex (0 ≤ x ≤ 1) alloys and Table 1 lists the corresponding cell parameters, unit cell volumes and phase fraction. The alloys with x < 0.4 crystallize in the TiNiSi-type orthorhombic structure (space group Pnma). In addition, for x = 0.2–0.4 we can see the appearance of tiny (1 0 2) and (1 1 0) reflection peaks belonging to the hexagonal Ni2In-type structure (space group P63/mmc). At room temperature, the transformation from TiNiSi to Ni2In crystal type structures occurs between x = 0.4 and 0.5. For 0.5 ≤ x ≤ 1, the samples have a Ni2In-type hexagonal structure. In general, Ge substitution for Si leads to an expansion of the unit cell with all cell parameters increasing either within the orthorhombic structure or within the hexagonal structure. The structural change from ortho. to hexa. corresponds to a significant cell volume (V) contraction (anisotropic), for instance, for x = 0.4 which shows the coexistence of the two phases, the difference between Vortho. and 2Vhexa. is ~−2.8%. This observation is in line with the large negative thermal expansion when the structural transition occurs as a function of the temperature in a given composition [2,25].
3.3. Recalescence-like event detected by differential calorimetry Differential scanning experiments were carried out and the most representative examples are shown in Fig. 3. For x < 0.5, the magnetic transition detected on M(T) curves corresponds to a minor thermal 2
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
W. Hanggai, et al.
Table 1 Room temperature lattice parameters, unit cell volume and phase fractions of MnFe0.6Ni0.4Si1−xGex (0 ≤ x ≤ 1) alloys. x
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Orthorhombic TiNiSi-type
Hexagonal Ni2In-type 3
a (Å)
b (Å)
c (Å)
V (Å )
w%
a (Å)
c (Å)
V (Å3)
5.780(1) 5.808(1) 5.8126(5) 5.8321(9) 5.822(1)
3.6926(4) 3.7009(2) 3.7089(3) 3.7228(5) 3.7312(9)
6.978(1) 6.9824(4) 6.9889(6) 7.015(1) 7.011(1)
148.94(3) 150.09(2) 150.67(3) 152.31(4) 152.31(4)
100 100 97(2) 96(2) 82(2) 0 0 0 0 0 0
4.0418(6) 4.052(1) 4.063(1) 4.074(1) 4.087(1) 4.098(1) 4.1083(2)
5.230(1) 5.241(1) 5.254(1) 5.271(2) 5.290(2) 5.298(1) 5.3104(3)
73.99(2) 74.53(3) 75.14(3) 75.78(4) 76.56(4) 77.05(3) 77.62(1)
(or occurs at lower temperature than DSC could access). Instead, one can see two small thermal anomalies, which correspond to the spinreorientation and Curie temperatures detected on magnetization curve. The limited amplitude, the dissymmetrical shape and absence of hysteresis of these transitions are consistent with second-order transformations. 3.4. Phase diagram of MnFe0.6Ni0.4Si1−xGex Fig. 4 shows the structural and magnetic phase diagram built using XRD, magnetic and thermal data. From x = 0 to 0.4, the alloys are orthorhombic and exhibit Curie temperatures near room temperature with little dependence on the Ge content. The ortho.-hexa. structural transition is however strongly sensitive to Ge substitution as it decreases at about −17 K/at.% of Ge. When structural and magnetic transitions couple near x ~ 0.5, the Curie temperature suddenly drops as a function of Ge content, as fast as the temperature of the structural transition. For x > 0.6, the alloys have an hexagonal structure with an antiferromagnetic-like behavior at low temperature, then a ferromagnetic phase appears around 120–160 K until disappearing at TC between 200 and 220 K. The first-order magnetostructural transition thus appears as the result of crossing between structural and magnetic transition lines. Similar phase diagrams were reported for related Mn alloys [7–9].
Fig. 2. Temperature dependence of the magnetization for selected MnFe0.6Ni0.4Si1−xGex (x = 0–1) alloys in a magnetic field of 0.05 T upon heating (line) and cooling (open symbols). In inset, magnetization versus field curve at 50 K.
anomaly, in line with its second-order character. A stronger transition involving a sizable latent heat and thermal hysteresis is found at higher temperatures and correspond to the thermal signature of the ortho.hexa. structural transition. For x = 0.5, only one strong and hysteretic heat flow peak is observed marking the magneto-structural ferromagnetic transition. At TC upon heating ~204 K, the area of the peak corresponds to a latent heat of 6.4 J g−1 or transition entropy ΔStr = 31 J kg−1 K−1. The transition during the first cooling of as-prepared sample occurs at significantly lower temperature than subsequent cycles, marking a training effect sometime referred to as “virgin effect”. This phenomenon has already been pointed out in several studies on related materials [9,14,29]. More surprisingly, here, this virgin effect is associated with the opening of a loop on the heat flow versus temperature curve. When looking at this signal as a function of time, one can note that the loop originates from a significant temperature increase of the sample upon cooling, a jump of +0.8 K at 159 K superposed on the cooling rate of the experiment −15 K min−1. Such a recalescence-like event turns out to be rare around magnetic transitions, having been observed only in ultrapure Er and Dy [30], and in (Hf,Ta) Fe2 [31]. It however presents many similarities with that recently observed in MnFe(P,Si) [32], in particular as this recalescence occurs only at the first cooling (irreversible) and is linked to a strong evolution of the microstructure since bulk materials readily get pulverized across the transition. For x > 0.5, the large latent heat peak is no longer visible
3.5. In-field heat capacity and giant magnetocaloric effect in MnFe0.6Ni0.4Si0.5Ge0.5 In-field specific heat and magnetocaloric effect of MnFe0.6Ni0.4Si0.5Ge0.5 are shown in Fig. 5. An intense specific heat peak is found at ~201 K. The peak does not present a dissymmetrical λ-like shape of a second-order transition, but it does not show either the expected symmetrical Gaussian-like shape of a first-order one. It looks more like the superposition of at least 2 symmetrical first-order peaks. We do not think that this splitting originates from chemical inhomogeneity, since the sample has been annealed at high temperatures after casting, our laboratory XRD data do not reveal abnormal peak splitting, and the second cooling DSC curve shows only one single latent heat peak. We rather believe that this splitting involve the high sensitivity of these alloys to stress/strain because of the large volume change at the magnetostructural transition. Several studies on related systems have pointed out that particle size, fragment selection and sample training can strongly affect the shape of MCE and first-order magnetic transition [8,14,16]. Integrating the zero-field C/T curve using a linear background between 180 and 220 K leads to a transition entropy of 25.8 J kg−1 K−1. This value is reasonably close to that obtained from DSC measurements (31 J kg−1 K−1). Despite using external re-analysis of large heat pulse, an underestimation (−15%) of the latent heat by the semi-adiabatic calorimetric technique can however not be ruled 3
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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Fig. 3. DSC thermograms measured at 15 K/min for MnFe0.6Ni0.4Si1−xGex with x = 0.4, 0.5 and 0.8 in panels (a), (b) and (c), respectively. Panel (d) presents the first cooling curve of MnFe0.6Ni0.4Si0.5Ge0.5 from panel (b).
−11.0 J kg−1 K−1 and +2.0 K, respectively for a field change of 2 T. The isothermal entropy change is also calculated by applying the Maxwell relation to M(T) data [33]. Bearing in mind that either magnetic or calorimetric indirect determinations of the entropy change suffer from substantial uncertainties, typically of the order of ~10% [34], the results are in reasonable agreement with calorimetry data. In addition, the present in-field calorimetric measurements start from 140 K instead of 0 K, which adversely affects the accuracy of magnetocaloric data from calorimetry [35]. Table 2 summarizes magnetocaloric parameters and compare them to closely related MnMX alloys. Unfortunately, any comparison is made difficult, first, because ΔTad and thermal measurements are seldom reported, second, because most magnetocaloric studies rely on indirect magnetization M(B) data, a method prone to major spike artefacts [18–22]. When considering the maximum of ΔS for 2 T, MnFe0.6Ni0.4Si0.5Ge0.5 shows intermediate performances, partly due to the broadening of the ΔS(T) curve. When using a figure of merit such as Temperature averaged Entropy Change (TEC10) which consider the ΔS taken over a temperature range of 10 K [36], the profile of the ΔS(T) curve becomes less critical and the majority of MnMX alloys including MnFe0.6Ni0.4Si0.5Ge0.5 show similar performance with TEC10 in the range 8.5–9.5 J kg−1 K−1. The present MnFe0.6Ni0.4Si0.5Ge0.5 however stands out when considering the maximum of ΔTad in 2 T (+2.0 K) which is larger than other MnMX materials with ortho.-hexa. magnetostructural transition, such as (Mn,Fe)Ni (Si,Ge) with Ni fixed to unity or MnCoGe alloys. Incomplete and scattered parameters make an interpretation difficult. At first glance, this large ΔTad can be ascribed to a sizable dTC/dB, which itself is large due to modest transition entropy and relatively large magnetization. Both ΔTad and dTC/dB remain however significantly lower than that observed in La(Fe,Si)13 or MnFe(P,Si) prototypical magnetocaloric materials [37,38].
Fig. 4. Structural and magnetic phase diagram of MnFe0.6Ni0.4Si1−xGex alloys.
out. This probably originates from the relatively broad transition, which makes it unpractical to cross the full transition by a single heat pulse, so that the specific heat might still be underestimated in the wings of the peak. As a result, the MCE might also be slightly underestimated. When applying a magnetic field, the specific heat peak shifts to higher temperatures at a rate of ~+1.6 K/T, without obvious field induced broadening, in line with the results from M(T) curves measured at different magnetic field (dTC/dB ~ 1.7–1.8 K/T). The MCE from in-field calorimetry is calculated by a two steps process starting by the integration of S(T,B)-S(Ti,B) lines from C(T,B) data and followed by adiabatic or isothermal differences to get adiabatic temperature change or isothermal entropy change [33]. A relatively large magnetocaloric effect is observed with ΔS and ΔTad of 4
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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Fig. 5. Thermal and magnetocaloric properties of MnFe0.6Ni0.4Si0.5Ge0.5. (a) In-field specific heat measurements upon heating. (b) Field dependence of the transition temperature from specific heat and from M(T) data. (c) Adiabatic temperature change from calorimetry. (d) Isothermal entropy change from calorimetry. (e) M(T) curves recorded upon heating (line) and cooling (open symbols). (f) Isothermal entropy change from M(T) upon heating magnetic data.
Table 2 Summary of thermal properties and magnetocaloric parameters (for ΔB = 2 T) of MnFe0.6Ni0.4Si0.5Ge0.5 (calorimetric data) and a selection of closely related MnMX alloys with ortho.-hexa. magneto-structural transition considered as promising MCE materials. Composition
ΔStr (J kg−1 K−1)
dTC/dB (K/T)
ΔSmax (J kg−1 K−1)
TEC10 (J kg−1 K−1)
ΔTad-max (K)
Refs.
Mn(Fe0.6Ni0.4)(Si0.5Ge0.5) (Mn0.7Fe0.3)Ni(Si0.3Ge0.7) (Mn0.53Fe0.47)Ni(Si0.53Ge0.47) Mn(Ni0.73Fe0.27)Ge Mn(Ni0.77Fe0.23)Ge (Mn0.76Fe0.24)NiGe (Mn0.94Fe0.06)Ni0.99Ge1.01 Mn0.90Fe0.08Ni0.97Ge1.05 Mn0.74Fe0.26Ge0.7Si0.3 MnCo0.95Ge0.95 MnCoGeB0.02
25.7–31 60* 71.25 42.7 23* 30* 79.3 92.5 57 – 42.0
1.6–1.8 1.0 0.4 0.9* 2.2 2.0 – – 0.9* – 1.2
11.0 14.0 27.3# 12.8# 7.0#,* 12.3*,# – – 17# – 16.0
9.0 8.5* 8.6*,# 8.5* 7.0* 8.0* – – 9.2* – 9.3
2.0 1.3 (1.93 T) – – – – – – – 1.5 (1.93 T) 1.6
present [8] [14] [39] [7] [7] [40] [40] [10] [12] [9,41]
* Estimations based on graphical extrapolations or application of the Clausius-Clapeyron equation ΔStr = −ΔM/(dTC/dB). # Measured by indirect magnetization measurements using M(B) data. 5
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4. Conclusion
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Structural, magnetic and thermal properties of Ge-substituted MnFe0.6Ni0.4Si1−xGex alloys were explored in order to establish the phase diagram for this system. For x ≤ 0.4, the alloys crystallize in the orthorhombic TiNiSi-type structure at room temperature and are ferromagnetic with Curie temperature around 300 K. An orthorhombic to hexagonal Ni2In-type structural transition occurs above room temperature and this transition temperature decreases progressively with Ge for Si substitutions until coupling with the magnetic transition at x = 0.5. For x > 0.5, the alloys have a hexagonal structure and present an additional magnetic second-order transition at low temperatures. For x = 0.5, a first-order magnetostructural transition is observed at TC = 201 K. DSC measurements reveal that the first cooling of as-prepared samples is associated with a recalescence-like phenomenon, which is irreversible, and attributed to the evolution of the microstructure. In-field specific heat measurements have been performed to evaluate the magnetocaloric effect. While the isothermal entropy at intermediate field is on par with closely related MnMX alloys, the adiabatic temperature change is larger due to a stronger sensitivity of the transition to the magnetic field. Whereas many recent studies on giant magnetocaloric effect in alloys deriving from MnNiGe or MnNiSi focused on materials with Ni fixed to unity, here we show that samples with Mn fixed to unity should not be neglected in particular thanks to their larger magnetization. This work also shows that sizable adiabatic change can be achieved in MnMX alloys with ortho.-hexa. magnetostructural transition by alloying. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgments The work from H.Y. and F.G. is supported by Inner Mongolia Normal University (grant Nos. 2018YJRC002 and 2018YJRC003) and the Natural Science Foundation of China (grant Nos. 51850410514 for FG, 51961033 and 11904188 for H.Y.). References [1] A. Krumbugel-Nylund, D. Boursier, A. Rouault, J.P. Senateur, R. Fruchart, Mater. Res. Bull. 9 (1974) 21. [2] V. Johnson, Inorg. Chem. 14 (1975) 1117. [3] A. Szytula, A.T. Pedziwiatr, Z. Tomkowicz, W. Bażeła, J. Magn. Magn. Mater. 25 (1981) 176. [4] H. Fjellvåg, A.F. Andresen, J. Magn. Magn. Mater. 50 (1985) 291.
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Research articles
Structural and magnetic phase diagrams of MnFe0.6Ni0.4(Si,Ge) alloys and their giant magnetocaloric effect probed by heat capacity measurements ⁎
T
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W. Hanggai, O. Tegus , H. Yibole , F. Guillou Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, College of Physics and Electronic Information, Inner Mongolia Normal University, 81 Zhaowuda Road, Hohhot, Inner Mongolia 010022, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magneto-structural phase transitions Phase diagram Magnetocaloric effect
The structural and magnetic phase diagrams of MnFe0.6Ni0.4Si1−xGex alloys (0 ≤ x ≤ 1) are explored using powder X-ray diffraction, magnetic and calorimetric measurements. At room temperature, the crystal structure evolves from the orthorhombic TiNiSi-type to the hexagonal Ni2In-type with the increase in Ge. A magnetostructural coupling leading to a first-order ferromagnetic transition is found at 204 K for x ~ 0.5. Differential scanning calorimetry reveals a recalescence-like event at the first cooling across the structural transition. In-field specific heat measurements are used to quantify the giant magnetocaloric effect and show that the latent heat of the magneto-structural transition is less than in closely related MnMX alloys, which leads to sizable transition sensitivity to the magnetic field and finite adiabatic temperature change.
1. Introduction MnMX manganese alloys where M is a 3d transition metal and X a metalloid or a non-metal form a rich material families with intriguing physics in general and magnetism in particular. They mainly belong to one of the four following crystal-type structures: Fe2P (P −62m ), Fe2As (P4/nmm), TiNiSi (Pnma), or Ni2In (P63/mmc) [1]. Transitions from one crystal-type to the other as a function of temperature are possible, and those that couple with a change in magnetic order may lead to the appearance of magneto-structural first-order transitions. Ternaries such as MnCoGe, MnNiGe or MnNiSi, which are orthorhombic TiNiSi- type near room temperature, are alloyed with a fourth element or vacancies in order to induce a TiNiSi-to-Ni2In structural transition near the magnetic ordering transition, leading to a magneto-structural coupling [2–8]. In recent years, MnMX alloys showing such simultaneous structural orthorhombic TiNiSi to hexagonal Ni2In and magnetic transitions have been most active as the associated large volume change and latent heat result in giant negative thermal expansion and magnetocaloric effect (MCE) [7–16]. The later effect has received much attention due to its potential use into a green and energy efficient magnetic refrigeration technique. But, in order to use this giant MCE in applications several challenges remain to be coped with, such as: large thermal hysteresis which brings irreversibility in applications, large magnetic field changes required to turn most of the latent heat into MCE, pulverization of the samples when crossing the transition, limited adiabatic
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temperature change, etc. Due to the high sensitivity of the structural and magnetic properties of MnMX materials to alloying, one can expect to limit these drawbacks by adjusting the chemical composition. (Mn,Fe)Ni(Si,Ge) or (Mn,Fe)Ni(Si,Al) alloys are known to present giant magnetocaloric effect [14–16], but their giant MCE is not yet fully optimized. First, in ferromagnetic orthorhombic ternaries such as MnCoGe or MnNiSi, Mn occupies the pyramidal site and carries most of the magnetic moment 2.6–2.9 µB, while Co or Ni carries only 0.5–0.9 µB or ~0 µB, respectively [17]. In order to maximize the magnetic density and total MCE, substitutions on Mn should be avoided (with the additional complexity of the preferential site occupancy of Mn). Second, in the series of alloys with Ni fixed to unity, the latent heat (L) appears particularly large, transition entropy changes ΔStr = L/Ttr as large as 71 J kg−1 K−1 were reported [14]. While a large ΔStr is favorable to reach a large isothermal entropy change at high magnetic field, it also brings limited sensitivity of the transition temperature (Ttr) to the magnetic field, which in turn lowers the adiabatic temperature change. On the other hand, the phase diagrams of Mn(Ni,Fe)(Si,Ge) alloys with Mn fixed to unity have received less attention. Accordingly, we explore here the structure and magnetic properties of MnFe0.6Ni0.4(Si,Ge) alloys, with a special emphasis on thermal measurements. As a preliminary step, we investigated the properties of Mn (Fe,Ni)Si alloys to select Fe and Ni contents for which Curie temperature is near room temperature. Then, Si is substituted by Ge to drive the ortho.-hexa. structural transition near TC. The vast majority of the MCE
Corresponding authors. E-mail addresses: [email protected] (O. Tegus), [email protected] (H. Yibole).
https://doi.org/10.1016/j.jmmm.2019.165785 Received 28 June 2019; Received in revised form 29 August 2019; Accepted 4 September 2019 Available online 04 September 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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studies on MnMX alloys rely on indirect measurements of ΔS from magnetization data, a method which is prone to artefact in case of firstorder transitions [18–22]. To avoid this issue, we show that in-field specific heat measurements using the semi-adiabatic technique are technically feasible on these materials. Differential scanning calorimetry measurements are used to confirm the latent heat estimation and, in addition, reveal an unconventional recalescence-like phenomenon at the first cooling of as-prepared samples. 2. Experimental methods Eleven MnFe0.6Ni0.4Si1−xGex alloys with x ranging between 0 and 1 were prepared by arc-melting in high-purity argon atmosphere. Elemental starting materials with purity 99.9% for Mn, Ni, Ge and Fe, and 98.9% for Si were used. The as-cast ingots were then sealed in evacuated quartz tubes and annealed at 1373 K for 4 h ending by a quenching in water. Except when especially stated, all samples were cycled in temperature down to 77 K prior to measurements. Powder Xray diffraction experiments were performed on a Panalytical Empyrean X-Ray Diffractometer using Cu-Kα radiation. The Fullprof software was used to perform Rietveld refinements [23]. Differential scanning calorimetry measurements were performed on a TA 2500 DSC. The vibrating sample magnetometer and heat capacity options of a Quantum Design VersaLab system were employed for magnetic and in-field calorimetric measurements. For in-field specific heat measurements, single heat pulses of ~8 K covering most of the transition were used to avoid underestimating the latent heat [24]. Tzero® aluminum crucibles are used for the sample mounting during both calorimetric experiments. An addenda taking into account the contribution of N grease and that of the crucible was measured prior to specific heat experiments. While DSC experiments were performed on an – initially – small bulk piece with a polished bottom, for the semi-adiabatic calorimetric measurements, we mounted rough pre-cooled powder. Pre-cooling in liquid N2 of the x = 0.5 sample leads to a broad distribution in particle size: 360–120 µm (~39 w%), 120–36 µm (~43 w%), and less than 36 µm (~18 w%). But, this distribution is found very sensitive to the heat treatment profile. In addition, further particle size reduction occurs during subsequent thermal cycles. In order for the specific heat to reflect average properties, we chose not to select specific particle sizes, but rather use a large amount (39.0 mg) of powder as it comes after selfpulverization during pre-cooling. The use of such powder in magnetocaloric heat pumps would require appropriate shaping, by, for instance, using binder.
Fig. 1. Room temperature X-ray diffraction patterns of MnFe0.6Ni0.4Si1−xGex (0 ≤ x ≤ 1) alloys, the asterix mark out the two most intense reflections of the hexagonal Ni2In-type structure.
3.2. Magnetic properties of MnFe0.6Ni0.4Si1−xGex alloys Magnetization data as a function of temperature and external magnetic field were systematically recorded for all samples. For clarity, Fig. 2 illustrates only the most representative samples, for the other compositions the transition temperatures defined as the dM/dT maximum are presented in the phase diagram (Fig. 4). For x ≤ 0.4, one observes a ferromagnetic behavior with the Curie temperature progressively decreasing with the increase in Ge content. In this range, the ferromagnetic transition is a second-order transition as indicated by a pronounced broadening of the transition when applying a field of 1 T (not shown) and in line with absence of thermal hysteresis. For x = 0.5, the Curie temperature lowers suddenly and shows a large thermal hysteresis of ~15 K, as often observed in this class of materials when structural and magnetic transitions occur simultaneously. For x > 0.5, a magnetization drop resembling the ferromagnetic transition of the samples with x ≤ 0.5 is still observed. But, in addition, one can note a low temperature feature looking like a transition toward a complex antiferromagnetic state in the sense that the magnetization remains substantial which is incompatible with a collinear arrangement, and can easily turn into ferromagnetism in intermediate magnetic fields. At first glance, such low temperature transition toward lower magnetization could be attributed to a reminiscence of the spiral antiferromagnetic ground state found in MnNiGe ternary [4], it has been observed in other quaternaries [7], but the exact magnetic structure is not known yet. Accordingly, we hereafter refer to this low temperature transition as a spin-reorientation (S.R.) transition. The saturation magnetization (or close to) at 50 K and 3 T decreases with Ge for Si substitution, but not linearly. Samples that remain orthorhombic down to low temperatures present higher magnetic saturation than hexagonal samples. This can be ascribed to the change in magnetic moments at the structural transition, already pointed out in MnCoGe or MnNiGe related materials by local probes such as neutron diffraction, Mössbauer or Xray magnetic circular dichroism spectroscopies [17,26–28]. We can also note that the saturation magnetization ~104 Am2 kg−1 for x = 0.5 appears significantly larger than that usually found in (Mn,Fe)Ni(Si,Ge) materials, ~70–80 Am2 kg−1 [8], confirming the interest of materials with Mn content fixed to unity to enhance the magnetization.
3. Results and discussion 3.1. Crystal structure as a function of Ge content Fig. 1 shows the room-temperature XRD patterns of MnFe0.6Ni0.4Si1−xGex (0 ≤ x ≤ 1) alloys and Table 1 lists the corresponding cell parameters, unit cell volumes and phase fraction. The alloys with x < 0.4 crystallize in the TiNiSi-type orthorhombic structure (space group Pnma). In addition, for x = 0.2–0.4 we can see the appearance of tiny (1 0 2) and (1 1 0) reflection peaks belonging to the hexagonal Ni2In-type structure (space group P63/mmc). At room temperature, the transformation from TiNiSi to Ni2In crystal type structures occurs between x = 0.4 and 0.5. For 0.5 ≤ x ≤ 1, the samples have a Ni2In-type hexagonal structure. In general, Ge substitution for Si leads to an expansion of the unit cell with all cell parameters increasing either within the orthorhombic structure or within the hexagonal structure. The structural change from ortho. to hexa. corresponds to a significant cell volume (V) contraction (anisotropic), for instance, for x = 0.4 which shows the coexistence of the two phases, the difference between Vortho. and 2Vhexa. is ~−2.8%. This observation is in line with the large negative thermal expansion when the structural transition occurs as a function of the temperature in a given composition [2,25].
3.3. Recalescence-like event detected by differential calorimetry Differential scanning experiments were carried out and the most representative examples are shown in Fig. 3. For x < 0.5, the magnetic transition detected on M(T) curves corresponds to a minor thermal 2
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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Table 1 Room temperature lattice parameters, unit cell volume and phase fractions of MnFe0.6Ni0.4Si1−xGex (0 ≤ x ≤ 1) alloys. x
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Orthorhombic TiNiSi-type
Hexagonal Ni2In-type 3
a (Å)
b (Å)
c (Å)
V (Å )
w%
a (Å)
c (Å)
V (Å3)
5.780(1) 5.808(1) 5.8126(5) 5.8321(9) 5.822(1)
3.6926(4) 3.7009(2) 3.7089(3) 3.7228(5) 3.7312(9)
6.978(1) 6.9824(4) 6.9889(6) 7.015(1) 7.011(1)
148.94(3) 150.09(2) 150.67(3) 152.31(4) 152.31(4)
100 100 97(2) 96(2) 82(2) 0 0 0 0 0 0
4.0418(6) 4.052(1) 4.063(1) 4.074(1) 4.087(1) 4.098(1) 4.1083(2)
5.230(1) 5.241(1) 5.254(1) 5.271(2) 5.290(2) 5.298(1) 5.3104(3)
73.99(2) 74.53(3) 75.14(3) 75.78(4) 76.56(4) 77.05(3) 77.62(1)
(or occurs at lower temperature than DSC could access). Instead, one can see two small thermal anomalies, which correspond to the spinreorientation and Curie temperatures detected on magnetization curve. The limited amplitude, the dissymmetrical shape and absence of hysteresis of these transitions are consistent with second-order transformations. 3.4. Phase diagram of MnFe0.6Ni0.4Si1−xGex Fig. 4 shows the structural and magnetic phase diagram built using XRD, magnetic and thermal data. From x = 0 to 0.4, the alloys are orthorhombic and exhibit Curie temperatures near room temperature with little dependence on the Ge content. The ortho.-hexa. structural transition is however strongly sensitive to Ge substitution as it decreases at about −17 K/at.% of Ge. When structural and magnetic transitions couple near x ~ 0.5, the Curie temperature suddenly drops as a function of Ge content, as fast as the temperature of the structural transition. For x > 0.6, the alloys have an hexagonal structure with an antiferromagnetic-like behavior at low temperature, then a ferromagnetic phase appears around 120–160 K until disappearing at TC between 200 and 220 K. The first-order magnetostructural transition thus appears as the result of crossing between structural and magnetic transition lines. Similar phase diagrams were reported for related Mn alloys [7–9].
Fig. 2. Temperature dependence of the magnetization for selected MnFe0.6Ni0.4Si1−xGex (x = 0–1) alloys in a magnetic field of 0.05 T upon heating (line) and cooling (open symbols). In inset, magnetization versus field curve at 50 K.
anomaly, in line with its second-order character. A stronger transition involving a sizable latent heat and thermal hysteresis is found at higher temperatures and correspond to the thermal signature of the ortho.hexa. structural transition. For x = 0.5, only one strong and hysteretic heat flow peak is observed marking the magneto-structural ferromagnetic transition. At TC upon heating ~204 K, the area of the peak corresponds to a latent heat of 6.4 J g−1 or transition entropy ΔStr = 31 J kg−1 K−1. The transition during the first cooling of as-prepared sample occurs at significantly lower temperature than subsequent cycles, marking a training effect sometime referred to as “virgin effect”. This phenomenon has already been pointed out in several studies on related materials [9,14,29]. More surprisingly, here, this virgin effect is associated with the opening of a loop on the heat flow versus temperature curve. When looking at this signal as a function of time, one can note that the loop originates from a significant temperature increase of the sample upon cooling, a jump of +0.8 K at 159 K superposed on the cooling rate of the experiment −15 K min−1. Such a recalescence-like event turns out to be rare around magnetic transitions, having been observed only in ultrapure Er and Dy [30], and in (Hf,Ta) Fe2 [31]. It however presents many similarities with that recently observed in MnFe(P,Si) [32], in particular as this recalescence occurs only at the first cooling (irreversible) and is linked to a strong evolution of the microstructure since bulk materials readily get pulverized across the transition. For x > 0.5, the large latent heat peak is no longer visible
3.5. In-field heat capacity and giant magnetocaloric effect in MnFe0.6Ni0.4Si0.5Ge0.5 In-field specific heat and magnetocaloric effect of MnFe0.6Ni0.4Si0.5Ge0.5 are shown in Fig. 5. An intense specific heat peak is found at ~201 K. The peak does not present a dissymmetrical λ-like shape of a second-order transition, but it does not show either the expected symmetrical Gaussian-like shape of a first-order one. It looks more like the superposition of at least 2 symmetrical first-order peaks. We do not think that this splitting originates from chemical inhomogeneity, since the sample has been annealed at high temperatures after casting, our laboratory XRD data do not reveal abnormal peak splitting, and the second cooling DSC curve shows only one single latent heat peak. We rather believe that this splitting involve the high sensitivity of these alloys to stress/strain because of the large volume change at the magnetostructural transition. Several studies on related systems have pointed out that particle size, fragment selection and sample training can strongly affect the shape of MCE and first-order magnetic transition [8,14,16]. Integrating the zero-field C/T curve using a linear background between 180 and 220 K leads to a transition entropy of 25.8 J kg−1 K−1. This value is reasonably close to that obtained from DSC measurements (31 J kg−1 K−1). Despite using external re-analysis of large heat pulse, an underestimation (−15%) of the latent heat by the semi-adiabatic calorimetric technique can however not be ruled 3
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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Fig. 3. DSC thermograms measured at 15 K/min for MnFe0.6Ni0.4Si1−xGex with x = 0.4, 0.5 and 0.8 in panels (a), (b) and (c), respectively. Panel (d) presents the first cooling curve of MnFe0.6Ni0.4Si0.5Ge0.5 from panel (b).
−11.0 J kg−1 K−1 and +2.0 K, respectively for a field change of 2 T. The isothermal entropy change is also calculated by applying the Maxwell relation to M(T) data [33]. Bearing in mind that either magnetic or calorimetric indirect determinations of the entropy change suffer from substantial uncertainties, typically of the order of ~10% [34], the results are in reasonable agreement with calorimetry data. In addition, the present in-field calorimetric measurements start from 140 K instead of 0 K, which adversely affects the accuracy of magnetocaloric data from calorimetry [35]. Table 2 summarizes magnetocaloric parameters and compare them to closely related MnMX alloys. Unfortunately, any comparison is made difficult, first, because ΔTad and thermal measurements are seldom reported, second, because most magnetocaloric studies rely on indirect magnetization M(B) data, a method prone to major spike artefacts [18–22]. When considering the maximum of ΔS for 2 T, MnFe0.6Ni0.4Si0.5Ge0.5 shows intermediate performances, partly due to the broadening of the ΔS(T) curve. When using a figure of merit such as Temperature averaged Entropy Change (TEC10) which consider the ΔS taken over a temperature range of 10 K [36], the profile of the ΔS(T) curve becomes less critical and the majority of MnMX alloys including MnFe0.6Ni0.4Si0.5Ge0.5 show similar performance with TEC10 in the range 8.5–9.5 J kg−1 K−1. The present MnFe0.6Ni0.4Si0.5Ge0.5 however stands out when considering the maximum of ΔTad in 2 T (+2.0 K) which is larger than other MnMX materials with ortho.-hexa. magnetostructural transition, such as (Mn,Fe)Ni (Si,Ge) with Ni fixed to unity or MnCoGe alloys. Incomplete and scattered parameters make an interpretation difficult. At first glance, this large ΔTad can be ascribed to a sizable dTC/dB, which itself is large due to modest transition entropy and relatively large magnetization. Both ΔTad and dTC/dB remain however significantly lower than that observed in La(Fe,Si)13 or MnFe(P,Si) prototypical magnetocaloric materials [37,38].
Fig. 4. Structural and magnetic phase diagram of MnFe0.6Ni0.4Si1−xGex alloys.
out. This probably originates from the relatively broad transition, which makes it unpractical to cross the full transition by a single heat pulse, so that the specific heat might still be underestimated in the wings of the peak. As a result, the MCE might also be slightly underestimated. When applying a magnetic field, the specific heat peak shifts to higher temperatures at a rate of ~+1.6 K/T, without obvious field induced broadening, in line with the results from M(T) curves measured at different magnetic field (dTC/dB ~ 1.7–1.8 K/T). The MCE from in-field calorimetry is calculated by a two steps process starting by the integration of S(T,B)-S(Ti,B) lines from C(T,B) data and followed by adiabatic or isothermal differences to get adiabatic temperature change or isothermal entropy change [33]. A relatively large magnetocaloric effect is observed with ΔS and ΔTad of 4
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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Fig. 5. Thermal and magnetocaloric properties of MnFe0.6Ni0.4Si0.5Ge0.5. (a) In-field specific heat measurements upon heating. (b) Field dependence of the transition temperature from specific heat and from M(T) data. (c) Adiabatic temperature change from calorimetry. (d) Isothermal entropy change from calorimetry. (e) M(T) curves recorded upon heating (line) and cooling (open symbols). (f) Isothermal entropy change from M(T) upon heating magnetic data.
Table 2 Summary of thermal properties and magnetocaloric parameters (for ΔB = 2 T) of MnFe0.6Ni0.4Si0.5Ge0.5 (calorimetric data) and a selection of closely related MnMX alloys with ortho.-hexa. magneto-structural transition considered as promising MCE materials. Composition
ΔStr (J kg−1 K−1)
dTC/dB (K/T)
ΔSmax (J kg−1 K−1)
TEC10 (J kg−1 K−1)
ΔTad-max (K)
Refs.
Mn(Fe0.6Ni0.4)(Si0.5Ge0.5) (Mn0.7Fe0.3)Ni(Si0.3Ge0.7) (Mn0.53Fe0.47)Ni(Si0.53Ge0.47) Mn(Ni0.73Fe0.27)Ge Mn(Ni0.77Fe0.23)Ge (Mn0.76Fe0.24)NiGe (Mn0.94Fe0.06)Ni0.99Ge1.01 Mn0.90Fe0.08Ni0.97Ge1.05 Mn0.74Fe0.26Ge0.7Si0.3 MnCo0.95Ge0.95 MnCoGeB0.02
25.7–31 60* 71.25 42.7 23* 30* 79.3 92.5 57 – 42.0
1.6–1.8 1.0 0.4 0.9* 2.2 2.0 – – 0.9* – 1.2
11.0 14.0 27.3# 12.8# 7.0#,* 12.3*,# – – 17# – 16.0
9.0 8.5* 8.6*,# 8.5* 7.0* 8.0* – – 9.2* – 9.3
2.0 1.3 (1.93 T) – – – – – – – 1.5 (1.93 T) 1.6
present [8] [14] [39] [7] [7] [40] [40] [10] [12] [9,41]
* Estimations based on graphical extrapolations or application of the Clausius-Clapeyron equation ΔStr = −ΔM/(dTC/dB). # Measured by indirect magnetization measurements using M(B) data. 5
Journal of Magnetism and Magnetic Materials 494 (2020) 165785
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4. Conclusion
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Structural, magnetic and thermal properties of Ge-substituted MnFe0.6Ni0.4Si1−xGex alloys were explored in order to establish the phase diagram for this system. For x ≤ 0.4, the alloys crystallize in the orthorhombic TiNiSi-type structure at room temperature and are ferromagnetic with Curie temperature around 300 K. An orthorhombic to hexagonal Ni2In-type structural transition occurs above room temperature and this transition temperature decreases progressively with Ge for Si substitutions until coupling with the magnetic transition at x = 0.5. For x > 0.5, the alloys have a hexagonal structure and present an additional magnetic second-order transition at low temperatures. For x = 0.5, a first-order magnetostructural transition is observed at TC = 201 K. DSC measurements reveal that the first cooling of as-prepared samples is associated with a recalescence-like phenomenon, which is irreversible, and attributed to the evolution of the microstructure. In-field specific heat measurements have been performed to evaluate the magnetocaloric effect. While the isothermal entropy at intermediate field is on par with closely related MnMX alloys, the adiabatic temperature change is larger due to a stronger sensitivity of the transition to the magnetic field. Whereas many recent studies on giant magnetocaloric effect in alloys deriving from MnNiGe or MnNiSi focused on materials with Ni fixed to unity, here we show that samples with Mn fixed to unity should not be neglected in particular thanks to their larger magnetization. This work also shows that sizable adiabatic change can be achieved in MnMX alloys with ortho.-hexa. magnetostructural transition by alloying. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgments The work from H.Y. and F.G. is supported by Inner Mongolia Normal University (grant Nos. 2018YJRC002 and 2018YJRC003) and the Natural Science Foundation of China (grant Nos. 51850410514 for FG, 51961033 and 11904188 for H.Y.). References [1] A. Krumbugel-Nylund, D. Boursier, A. Rouault, J.P. Senateur, R. Fruchart, Mater. Res. Bull. 9 (1974) 21. [2] V. Johnson, Inorg. Chem. 14 (1975) 1117. [3] A. Szytula, A.T. Pedziwiatr, Z. Tomkowicz, W. Bażeła, J. Magn. Magn. Mater. 25 (1981) 176. [4] H. Fjellvåg, A.F. Andresen, J. Magn. Magn. Mater. 50 (1985) 291.
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