Tunable giant magnetocaloric effect with very low hysteresis in Mn3CuN1−xCx

Accepted Manuscript Tunable giant magnetocaloric effect with very low hysteresis in Mn3CuN1-xCx N.-O. Born, L. Caron, F. Seeler, C. Felser PII:

S0925-8388(18)31188-5

DOI:

10.1016/j.jallcom.2018.03.311

Reference:

JALCOM 45541

To appear in:

Journal of Alloys and Compounds

Received Date: 6 November 2017 Revised Date:

6 February 2018

Accepted Date: 24 March 2018

Please cite this article as: N.-O. Born, L. Caron, F. Seeler, C. Felser, Tunable giant magnetocaloric effect with very low hysteresis in Mn3CuN1-xCx, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.03.311. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Tunable giant magnetocaloric effect with very low hysteresis in Mn3 CuN1-x Cx N. -O. Born,1,2 L. Caron,2 F. Seeler1 and C. Felser2 2

BASF SE, Robert-Bosch-Str. 38, 67056 Ludwigshafen, Germany Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany

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ABSTRACT

Mn3 CuN magnetically ordered materials magnetocaloric phase transitions

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antiperovskite

The structural, magnetic and magnetocaloric properties of the antiperovskite materials Mn3 CuN1-x Cx have been studied. Substituting N with C increases the temperature of the magnetostructural transition between a paramagnetic cubic high temperature phase and a ferrimagnetic tetragonal low temperature phase. Furthermore, the first order character of the phase transition is retained upon substitution with a hysteresis below 2 K for all compositions. The magnetostructural transition gives rise to giant magnetocaloric effects in a tunable 30 K temperature range with a maximum entropy change of 11.8 J⁄Kkg at a 2 T field change, making these compounds promising for low temperature magnetic refrigeration applications.

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Keywords:

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Highlights

The lattice parameter of the antiperovskite structure increases with C content. All compounds present first order magneto structural transitions. TC is tunable between 132 K and 165 K by substituting N with C. The observed thermal hysteresis of the transition remains below 2 K. ∆SM doubles from 5.8 J/Kkg for x = 0 to 11.8 J/Kkg for x = 0.25.

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1. Introduction

In recent years, refrigeration based on the magnetocaloric effect (MCE) has become a proven [1], environmentally friendly and efficient alternative to conventional gas-compression based technology. All magnetic materials show a magnetocaloric effect when exposed to an external magnetic field, that manifests as an adiabatic temperature change ∆Tad and magnetic entropy change ∆SM. Both property changes are intrinsically maximal around magnetic phase transitions, where changes in the magnetization concentrate in narrow temperature and field ranges. The effect is particularly high (giant) around first order phase transitions where the magnetism and structure change discontinuously, as is the case in materials presenting magneto-structural transitions [2]. Most materials which show giant magnetocaloric effects, such as Gd5 Si2 Ge2  [3], LaFe,Si13 [4], (Mn,Fe)2 (P,Si) [5] and MnCoGe-based compounds [6], undergo an order-disorder magnetic phase transition accompanied by a discontinuous structural change. Other compounds, such as Mn2-x Crx Sb [7] and most of the NiMnX (X = Ga, In, Sn,

ACCEPTED MANUSCRIPT Sb) Heusler compounds [8-11] experience order-order transitions, for example from a ferro-magnetic to an anti- or ferri-magnetic state, also accompanied by discontinuous structure changes.

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Mn-rich antiperovskite compounds with the composition Mn3 MX, where M is a metal and X a main group element like N or C, come in a huge variety and have a myriad of interesting properties [12]. For example materials based on Mn3 ZnN [13] or Mn3 Cu1-x Gex N [14] experience a giant negative thermal expansion (NTE) at low temperatures and materials based on Mn3 AgN[15] or Mn3 NiN [16] are known for their near-zero temperature coefficient of resistance (NZ-TCR). The structure of Mn-rich antiperovskites (Pm3 m) consists of a cubic primitive cell of the metal M. The faces of the cell are occupied with Mn atoms building an octahedron. The octahedral hole in the center of the cell is occupied by the main group element X [12].

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A number of Mn-rich antiperovskite materials show moderate to giant (inverse) magnetocaloric effects. Mn3 GaC has been found to undergo a first order phase transition from a ferromagnetic high temperature phase to an antiferromagnetic low temperature phase at ~160 K accompanied by an abrupt decrease in volume without a symmetry change [17]. The magnetic entropy change ∆SM calculated from isothermal magnetization curves using the Maxwell relation reaches up to 15 J/kgK at a 5 T field change. Substitution with Co can be utilized to tune the temperature of the first order transition from ferro- to antiferromagnetism [18] and N substitution for C reduces the hysteresis leading to higher reversibility [19].

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Mn3 SnC has a similar magnetic structure to Mn3 CuN and undergoes a first order phase transition from a paramagnetic high temperature phase to a ferrimagnetic low temperature phase at 279 K. However, the magnetic transition is not accompanied by a distortion of the structure from cubic to tetragonal and is isosymmetric showing only a discontinuous contraction of the cell [12]. At the transition temperature the magnetic entropy change -∆SM reported is 80.69 mJ/cm3 K 10.37 J/kgK at a field change of 2 T and 133 mJ/cm3 K 17.1 J/kgK at a field change of 4.8 T [20]. Substitution with Fe leads to a shift in the transition temperature to lower temperatures and to lower entropy changes [21, 22]. Mn3 SbN experiences a discontinuous distortion from a cubic to a tetragonal structure at 360 K accompanied by a transition from paramagnetic to ferromagnetic states upon cooling [22]. The magnetic entropy change is maximal at the transition temperature with 2.1 J/molK (7 J/kgK at a 5 T field change [23]. Mn3 AlC undergoes a transition from paramagnetism to ferromagnetism with a TC of 287 K. However, there is no change in structure at this temperature leading to a more second order-like phase transition. The resulting entropy change is fairly low with -∆SM = 3.28 J/kgK and the adiabatic temperature change ∆Tad has a maximum of 1.62 K both at a field change of 4.5 T [24].

Fig. 1. Structure of the tetragonal low temperature phase T 1.

Recently Yan et al. discovered a magnetic entropy change of 13.52 J/kgK under a field change of 5 T in Mn3 Cu0.89 N0.96 at 145 K [25]. At room temperature Mn3 CuN is paramagnetic and crystallizes in the cubic antiperovskite structure. At 143 K the structure changes from cubic Pm3 m symmetry to the tetragonal low-

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temperature phase T  1 with P4/mmm symmetry by contracting the c axis of the cell and expanding the a and b axes (see Fig. 1) [12]. The c/a ratio of the tetragonal cell is estimated to be 0.9853 and the volume of the cell is nearly conserved through the phase transition [26]. At the same temperature Mn3 CuN undergoes a magnetic phase transition from the high-T paramagnetic to the low-T ferrimagnetic phase. The magnetic unit cell of the tetragonal low-T phase consists of two moments located on the Cu atoms and six moments located on the Mn atoms. The moments located on the Mn atoms are sizably larger than those on the Cu atoms. The two Cu moments and two moments on the Mn1 atoms of the same layer are ferromagnetically coupled and align along the c axis while the other four Mn2 moments are canted from the c axis towards the [111] direction with a canting angle of Θ = ±72.2° [27]. There are many materials reported where Cu is substituted by another element like Co, Ni, Zn, Ge, Ga, Ge, Rh, Pd, In or Sn [28]. However, substitutions of Mn or N have been very little explored. Lin et al. reported the substitution of N by C and Mn by Co and found a tunable low TCR [29].

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In this paper we report the properties of the Mn3 CuN1-x Cx series with respect to the magnetocaloric effect associated with the magneto-structural transition from the cubic paramagnetic high-T phase to the tetragonal ferrimagnetic lowT phase.

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2. Experimental

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3. Results

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Polycrystalline samples of Mn3 CuN1-x Cx with x = 0, 0.05, 0.1, 0.2, 0.25, 0.3 and 0.45 where prepared by solid state reaction. First Mn3 N2+x powder was synthesized by flowing ammonia over Mn powder at 1037 K for 4 h. Then powders of Cu (≥99%), Mn (99.99%), activated charcoal (p. a.) and the previously synthesized Mn3 N2+x were thoroughly mixed and sealed in a quartz ampule under vacuum, heated to 1033 K for 5 days and then oven cooled. The product was ground, pressed into pills, sealed in quartz ampules under vacuum and annealed at 1073 K for 5 days, after which it was once more oven cooled. X-Ray powder diffraction (XRD) at room temperature was performed on a Bruker D8 Series 2 with a Cu Kα source. Back Scattering Electron (BSE) images and Energy Dispersive X-Ray Spectra were taken using a Scanning Electron Microscope with an attached Energy Dispersive XRay Spectroscopy Detector (SEM-EDXS) at 20 kV. Chemical analysis was performed using a combustion technique with heat conductivity detector. Magnetic measurements were performed using a Quantum Design Vibrating Sample Magnetometer (VSM). Differential Scanning Calorimetry (DSC) measurements were done using a TA Instruments DSC Q2000 model.

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Seven samples with x = 0, 0.05, 0.1, 0.2, 0.25, 0.3 and 0.45 were studied. Room temperature XRD shows that all samples crystallize in a cubic structure with space group Pm3 m (see Fig. 2). All samples show small amounts of a secondary antiferromagnetic Cu3 Mn (Fm3 m) phase and antiferromagnetic MnO (Fm3 m) impurities. Because of the Cu-rich secondary phase, the main phase is not stoichiometric but Cu deficient. The averaged composition of the main phase, derived from SEM/EDX measurements, therefore is Mn3 Cu0.81 N1-x Cx. Chemical analysis was performed to determine the N and C content of the bulk and results are presented in Table S1. However, the composition of the main phase might deviate from the bulk values as a secondary phase is present in all samples. For simplicity, we use the nominal stoichiometry Mn3 CuN1-x Cx throughout this manuscript. Lattice parameters were obtained using the Rietveld refinement method. At room temperature the sample with x = 0 presents a lattice parameter a = 3.9057(3) Å, in excellent agreement with literature [30, 31]. The cell size increases with carbon content up to x = 0.25 indicating that no additional carbon can be incorporated into the structure by adding more C.

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2θ [°] 40

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x

0,3

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a [Å]

Intensity (a. u.)

* *+ + Cu3Mn x=0.25 * MnO + x=0 3,912 3,910 3,908 3,906

80 (220)

(200)

(110)

A x=0.45

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(111)

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Fig. 2. A: X-ray diffraction patterns at room temperature of the Mn3 CuN(1-x) Cx compounds. B: Lattice parameters of said compounds.

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DSC was used to measure the phase transitions of all samples. A linear baseline was subtracted from each measurement. The transition temperature increases with increasing carbon content from 132 K for x = 0 up to 163 K for x = 0.45, upon cooling (see inset Fig. 3). However, above x = 0.25 the transition temperature does not increase significantly confirming that no more C is incorporated into the structure. Table 1 summarizes all relevant magnetic and thermodynamic quantities. Sharp peaks in the heat flow were observed at the phase transition revealing a strong first order character of the transition (see Fig. 3) and a large latent heat. A first order magneto-structural phase transition (FOMST) is desired to reach high entropy changes and thus large magnetocaloric effects.

0.0

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∆q [W/g]

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TT [K]

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-0.5 -0.6 100

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140

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160

x=0 x=0.05 x=0.1 x=0.2 x=0.25 x=0.3 x=0.45 180

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T [K] Fig. 3. DSC measurements of Mn3 CuN(1-x) Cx in cooling. Inset: Transition temperatures TT .

Magnetic field cooling (FC) and field heating (FH) curves were taken for all samples between 2 K and 300 K in a 0.01 T field (see Fig. 4). Very sharp magnetic phase transitions from a paramagnetic to a ferrimagnetic state are observed upon cooling for all compositions. For the x = 0.2 compound this first transition is followed by a smaller

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sharp decrease after which the magnetization remains in a stable ferrimagnetic state (see inset Fig. 4). In all cases except x = 0.2 the Curie temperatures obtained from magnetization measurements are in good agreement with the transition temperatures observed in the DSC measurements, underlining the first-order nature of the magnetostructural phase transition. The sample with x = 0.2 has a higher TC and the transition temperature obtained from the DSC measurement corresponds more closely to the temperature where the step-like decrease in magnetization occurs. In many regards the sample with x=0.2 presents itself as an outlier. Chemical analysis of said sample suggests that the N content of this sample is lower than expected (see Table S1) and the behavior described above is well known for Mn3 CuN with a N deficiency [32]. A very narrow thermal hysteresis is observed (less than 2 K for all studied compositions), indicating a low energy barrier at the phase transition. The low thermal hysteresis at the first order phase transition is essential to achieve reversibility upon thermo-magnetic cycling in magnetocaloricbased refrigeration [33].

x=0.2

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Fig. 4. Isofield curves of the magnetization of Mn3 CuN(1-x) Cx (x=0, 0.05, 0.1, 0.25, 0.3, 0.45) in cooling (dark symbols) heating (light symbols) at 0.01 T. Inset: Iso-field curve of the magnetization of Mn3 CuN0.8 C0.2 in cooling (dark symbols) heating (light symbols) at 0.01 T. Scale values can be found in Table S2.

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Isofield measurements at different fields up to 2 T were used to calculate the magnetic entropy changes ∆SM using the Maxwell relation: 

∆  =    

    

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where M is the magnetization, T the temperature, H the external magnetic field and µ0 the permeability of vacuum. High magnetic entropy changes (see Fig. 5) were calculated for all compositions, showing a strong conventional (or direct) magnetocaloric effect. -∆SM increases from 6 J/Kkg for Mn3 CuN to 12 J/Kkg for x = 0.25, for a 2 T field change. Higher C content (x > 0.25) leads to slightly lower magnetic entropy changes around 10 J/Kkg for a 2 T field change. The saturation magnetization (MS) at 2 K increases with increasing C content from 29 Am2 ⁄kg for Mn3 CuN up to 36 Am2 ⁄kg for x = 0.2, contributing to the increase in the maximum entropy change value. For x > 0.2 no further increase in MS is observed.

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x=0

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x=0.2

2T 1.5 T 1T 0.5 T

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Fig. 5. Magnetic entropy change ∆SM for Mn3 CuN(1-x) Cx.

The large magnetic entropy change of Mn3 CuN1-x Cx is comparable to that of materials like Mn3 GaC[17] or Heusler compounds [10]. As an example, the iso-field measurements used for the calculation of ∆SM of the sample with x = 0.25 are given in Fig. S1 in the supplementary. Additionally, isothermal cycling was performed on the same sample (Fig. S2). From this data set, one can observe some metamagnetic behavior, yet not very pronounced. For the x = 0.2 compound the calculated entropy change is markedly lower than that observed for the other compositions (∆SM = 5.5 J/Kkg). This also suggests a weaker magneto-structural coupling in this compound, already pointed out above.

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Table 1 Thermodynamic and magnetic quantities of Mn3 CuN(1-x) Cx .

∆SM

dTC dµ H 0

[K]

JKkg



Am kg 5T

0.17

29

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0

132

0-2T -5.8

MS 2

143

-8.1

0.23

30

0.1 0.2

149 153

-9.9 -5.5

0.24 0.13

30 36

0.25

160

-11.8

0.23

35

0.3

163

-105

0.22

32

0.45

165

-9.9

0.25

34

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0.05

Another important quantity to be considered is the rate at which the magneto-structural phase transition shifts with increasing magnetic field dTC ⁄dµ 0 H. To achieve a reversible effect upon thermomagnetic cycling dTC ⁄dµ 0 H must be larger than the thermal hysteresis upon a given applied field and the phase transition can be crossed back into the initial state when the magnetic field is removed. As such dTC ⁄dµ 0 H gives the upper bound of the adiabatic temperature change[34]. For C-substituted Mn3 CuN compounds, apart from x=0.2, values between 0.22 K⁄T and 0.25 K⁄T were observed (see Table 1). These are considerably lower compared to the best magnetocaloric materials [35, 36]. The small dTC ⁄dµ 0 H points to a weaker magneto-structural coupling such as the one observed in MnCoGe

ACCEPTED MANUSCRIPT [37]. The dTC⁄dµ 0 H value of Mn3 CuN0.8 C0.2 is significantly lower compared to the other compounds (0.13 K⁄T) confirming the low coupling between magnetic and structural transition in this sample due to a N deficiency, compared to the rest of the series. As dTC ⁄dµ 0 H for C-substituted Mn3 CuN compounds is lower than the observed thermal hysteresis further improvement is necessary. Increasing the alignment of magnetic moments of the ferrimagnetic low temperature phase by increasing the lattice parameter through simultaneous substitution on the Cu position could be used to increase coupling between magnetic and structural transition.

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4. Discussion

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Considering the nature of the magnetic interactions and the origin of the structural transition in Mn3 CuN helps explaining the order and temperature of the magneto-structural transition in compounds with C substitution for N. As the magnetism of Mn-rich antiperovskites is of itinerant character, changes in the electronic structure around the Fermi energy EF have a huge influence on the magnetic transitions. Furthermore, Jardin and Labbe [38] found that a singularity in the density of states close to the EF leads to an instability of the cubic phase and is the reason for the transition to the tetragonal structure. The energy difference between the singularity and the Fermi energy plays a vital role for the stability of the tetragonal phase and the order of the transition [38]. Mn3 CuN shows this feature just below EF [27].

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Fig. 6 shows the development of the transition temperature with respect to the valence electron number per formula unit N(VE) (left) and the lattice parameter a of the cubic phase. The substitution of N (p3 ) with C (p2 ) reduces the total number of valence electrons and thus lowers the Fermi level closer to the singularity. That stabilizes the tetragonal distortion and the transition temperature increases. At the same time the cell size increases as does the distance between the Mn atoms, narrowing the bandwidth of the d-bands and thus increasing the density of states close to the Fermi level and stabilizing the magnetic phase [39]. Hence, the low temperature phase is stabilized and the transition temperature increases with increasing cell size.

36.9 36.8

x=0.2 x=0.25

36.7

b

x=0.25 x=0.2 x=0.1 x=0.05

3.910 3.908

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140

150

160

TT [K]

Fig. 6. Transition temperatures (based on the maximum of -∆SM ) of Mn3 CuN(1-x) Cx with respect to the total valence electron number (a) and lattice parameter a (b).

5. Summary

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In summary, we present the discovery of the giant magnetocaloric effect in a series of antiperovskite materials based on Mn3 CuN where N is partially substituted with C. These compounds undergo a first-order magneto-structural phase transition from the cubic paramagnetic high temperature phase to a tetragonal ferrimagnetic low temperature phase. All compounds show sharp first order phase transitions with very low thermal hysteresis and entropy changes up to 12 J⁄kg K at a 2 T field change. By tuning the total number of valence electrons and the cell size through the substitution of N with C the transition temperature can be adjusted in a range of more than 30 K between 132 K and 165 K making these compounds promising for magnetic cryocooling applications like gas liquefication.

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