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Tuning the Magnetocaloric Response in Half-Heusler/Heusler Mnni1+Xsb Solid Solutions

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Tuning the Magnetocaloric Response in Half-Heusler/Heusler Mnni1+Xsb Solid Solutions Tuning the magnetocaloric response in half-Heusler/Heusler MnNi1+xSb solid solutions Emily E. Levin,∗ Joshua D. Bocarsly, Kira E. Wyckoff, Tresa M. Pollock, and Ram Seshadri Materials Department, University of California, Santa Barbara, California 93106, USA and Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA (Dated: November 30, 2017) Materials with a large magnetocaloric response are associated with a temperature change upon the application of a magnetic field, and are of interest for applications in magnetic refrigeration and ther- momagnetic power generation. The usual metric of this response is the gravimetric isothermal entropy change ∆SM . The use of a simple proxy for the ∆SM that is based on density functional theory (DFT) calculations of the magnetic electronic structure, suggests that half-Heusler MnNiSb should be a bet- ter magnetocaloric than the corresponding Heusler compound MnNi2Sb. Guided by this observation, we present a study of MnNi1+xSb (x = 0, 0.25, 0.5, 0.75, 1.0) to evaluate relevant structural and magnetic properties. DFT stability calculations suggest that the addition of Ni takes place at a symmet- rically distinct Ni site in the half-Heusler structure, and support the observation using synchrotron X-ray diffraction of a homogeneous solid solution between the half-Heusler and Heusler endmembers. There is a maximum in the saturation magnetization at x = 0.5, and the Curie temperature systematically decreases with increasing x. ∆SM for a maximum magnetic field change of ∆H = 5 T monotonically decreases in magnitude from −2.93 J kg−1 K−1 in the half-Heusler to −1.35 J kg−1 K−1 in the Heusler compound. The concurrent broadening of the magnetic transition results in a maximum in the re- frigerant capacity at x = 0.75. The Curie temperature of this system is highly tunable between 350 K and 750 K, making it ideal for low grade waste heat recovery via thermomagnetic power generation. The increase in ∆SM with decreasing x may be extendable to other MnNi2Z Heusler systems that are currently under investigation for use in magnetocaloric refrigeration applications. INTRODUCTION is a hindrance to the study of thermomagnetic generators for waste heat recovery. The temperature change ∆Tad(H; T ) in a material The need for energy-efficient and environmentally upon adiabatic application of a magnetic field H, and the friendly technologies has prompted research into ma- isothermal change in entropy ∆SM (H; T ) upon applica- terials for efficient magnetic refrigeration and thermo- tion of a field, are used to describe the magnetocaloric magnetic power generation. These technologies rely on effect in a specific material. While measuring ∆Tad re- the magnetocaloric effect (MCE), which is the reversible quires specialized equipment, ∆SM can be calculated change in temperature of a material upon a change in from straightforward magnetic measurements, discussed applied magnetic field. In a typical ferromagnet near its later. magnetic transition temperature, the adiabatic applica- ∆S is dependent on the slope of the magnetization tion of a magnetic field causes the entropy of the spin M vs. temperature curve, and so a peak in ∆SM will oc- system to decrease as spins order in the direction of the cur at the ferromagnetic to paramagnetic transition, or applied field. Because entropy must be conserved in an the Curie temperature (TC ). The magnitude of ∆SM is adiabatic process, the lattice entropy, and therefore tem- dependent both on the magnetization of the material, perature, increases, causing the magnetocaloric effect. and on the breadth of the transition. Materials with first- By alternating adiabatic and isothermal magnetization order magnetic phase transitions5 tend to have a very and demagnetization, this phenomenon can be be used 1 sharp transition, and therefore a high ∆SM over a small to drive a thermodynamic cycle. Magnetic refrigeration temperature range, but thermal and magnetic hysteresis technology is expected to be very energy-efficient, and can cause intrinsic irreversibilities which reduce cooling eliminates the need for harmful refrigerants, such as hy- efficiency.6,7 Materials with standard second-order mag- drochlorofluorocarbons, further proving to be an envi- netic ferromagnetic to paramagnetic transitions, on the ronmentally friendly option. other hand, tend to have lower values of ∆SM over a The magnetic transition temperature of a magne- larger temperature range, and are free from these irre- tocaloric material determines its applicability. Of primary versibility losses. interest are materials with transitions near room temper- Heusler and half-Heusler compounds, which crys- ature, which can be used for residential and commer- tallize in the F m3m and F 43m structures respec- cial refrigeration.1,2 Materials displaying a large magne- tively, as shown in Figure 1, are well-known for tocaloric effect at high temperatures may have uses in a wide variety of exotic functional magnetic proper- recovery of waste heat via thermomagnetic power gener- ties, including half-metallic ferromagnetism,8,9 magnetic ation, while low temperature materials may be used for shape memory,10–12 and magnetocaloric behavior.13–16 3,4 gas liquefaction. There is currently a lack of informa- Heuslers, which have the stoichiometry XY2Z, are com- tion on the magnetocaloric properties of materials with prised of four interpenetrating fcc sublattices, such that transition temperatures above room temperature, which the X and Z atoms form a rock salt lattice and the Y 2 are two techniques to increase net magnetization and yield larger ∆SM . However, the Mn:Ni anti-site disor- der present in Heuslers implies a preference of Mn to sit on the Y site, which will likely not allow both techniques to be used simultaneously.23,24 The magnetic deformation ΣM , a density functional theory (DFT) based proxy developed by Bocarsly et al., is a measure of the unit cell deformation between re- laxed structures with and without the inclusion of spin 25 polarization. ΣM is an indication of the coupling be- tween magnetism and structure, and it correlates well with ∆SM . In the solid solution between MnNiSb and MnNi2Sb, the half-Heusler has a larger ΣM (1.61%) than the full-Heusler (1.21%), and so we expect to have a larger j∆SM j as Ni content decreases. Experimental re- sults in this study agree, supporting the importance of FIG. 1. Crystal structure of the solid solution MnNi1+xSb. The the coupling between magnetism and structure to the half-Heusler XYZ with the space group F 43m corresponds to magnetocaloric effect. one of the Ni sublattices being vacant, while the Heusler XY Z 2 The half-Heusler MnNiSb is well known as the first in the F m3m space group corresponds to completely occupied 8 sublattices, with only one symmetrically distinct nickel site. As predicted half metal by de Groot et al. in 1989. It nickel is added to the half-Heusler, the second Ni sublattice possesses 100% spin polarization at the Fermi level due becomes partially occupied, as shown. to a gap in the minority spin density of states. Its high Curie temperature (754 K), large magnetization (3.93 µB/f.u.), and spin polarization make it an ideal 26,27 atoms occupy the tetrahedral voids in the rock salt lat- candidate for spintronics. However, the Curie tem- tice. In half-Heuslers, one of the Y sublattices is vacant, perature is too high to be useful as a room temperature leaving only half of the tetrahedral sites occupied. magnetic refrigeration material, and is above the range needed for the conversion of low grade waste heat by The magnetocaloric effect in MnNi2Z (Z = Sb, Sn, In, Ga) Heusler compounds has been studied extensively thermomagnetic generation. due to the coupled structural and magnetic transitions It has been shown in previous studies that a maxi- that can be found in materials with off-stoichiometric mum in the saturation magnetization exists at an inter- compositions.16–19 These first order magnetostructural mediate composition in MnNi1+xSb (0 ≤ x ≤ 1), and the Curie temperature can be varied between 750 K and transitions lead to a large entropy change, but face prob- 27,28 lems with the intrinsic irreversibility and thermal hys- 350 K. The maximum in MS suggests that removing teresis. For this reason, much research has focused on nickel from the Heusler could increase j∆SM j while al- narrowing thermal hysteresis and operating within a mi- lowing for a tunable transition temperature useful for thermomagnetic power generation using low (≤ 505 K) nor loop of the hysteresis, where only a partial conver- 3 sion between phases is performed.6,19–21 The phase tran- and medium grade (505 K to 923 K) waste heat. In the sition involves a significant volume change, which poses present contribution, the magnetocaloric effect in the se- a challenge for engineering a device that must withstand ries MnNi1+xSb is characterized. The magnitude of ∆SM continuous, rapid cycling.2,22 decreases with increasing nickel content, while the re- RC This challenge was acknowledged by Singh et al., who frigerant capacity ( FWHM ), a measure of the heat explored the system Mn Ni In (x < 0.4) aiming that may be transferred between the hot and cold reser- 1+x 2 1−x x 2 to increase ∆S by increasing magnetization without voirs, is maximized for the = 0.75 composition. Here, M ∆S driving a first-order transition. As x increases, some of we demonstrate a materials system with significant M the Z site is replaced by Mn, increasing the overall mo- over a large range of transition temperatures useful for 7 thermomagnetic power generation. These results may ment and leading to an improved ∆SM . In MnNi2Z Heusler systems, ferromagnetic exchange between Mn prove useful in other well-studied Heusler systems which atoms, which provide the most significant contribution behave similarly.
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