The effect of Ni-substitution on the magnetic properties of Ni2MnGe Heusler alloys

The effect of Ni-substitution on the magnetic properties of Ni2MnGe Heusler alloys

Journal of Alloys and Compounds 462 (2008) 1–3 The effect of Ni-substitution on the magnetic properties of Ni2MnGe Heusler alloys P.Z. Si a,∗ , J.J. ...

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Journal of Alloys and Compounds 462 (2008) 1–3

The effect of Ni-substitution on the magnetic properties of Ni2MnGe Heusler alloys P.Z. Si a,∗ , J.J. Liu b , C.Q. Chen a , Q. Wu a , Z.W. Jiao a , H.L. Ge a a b

College of Materials Science & Engineering, China Jiliang University, Hangzhou 310018, China Faculty of Materials Science & Chemical Engineering, Ningbo University, Ningbo 315211, China Received 12 July 2007; received in revised form 7 August 2007; accepted 7 August 2007 Available online 11 August 2007

Abstract The magnetic properties of Ni-rich polycrystalline Ni2+x Mn1−x Ge Heusler alloys prepared by arc melting method have been investigated. The Curie temperature Tc , the total magnetic moment, the lattice constants, and the magnetic entropy change S of Ni2+x Mn1−x Ge alloys decrease with increasing Ni-substitution to Mn. In a field of 1 T, the S in Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge is 0.43 J kg−1 K−1 (at Tc = 246 K) and 0.36 J kg−1 K−1 (at Tc = 151 K), respectively. In comparison with Ni2 MnGe, our sample with Ni-substitution shows simple magnetic structure and zero coercivity. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Magnetocaloric; Magnetic measurements

1. Introduction Magnetic refrigeration has long been utilized to produce ultra-low temperatures (<1 K) since its development in 1926 [1,2]. In recent years, room temperature magnetic refrigeration as an efficient and environmentally friendly technology compared with compressor-based refrigeration received great attention for its high commercial potential [3]. Materials with high magnetic entropy change are critical for the development of room temperature magnetic refrigeration. Up to now, most of magnetocaloric effect studies have been performed on ferromagnetic materials near their Curie temperatures [4]. Among all these studies, Gd5 Si2 Ge2 [5] and MnAs-based compounds [6,7] had a great impact on this field for their large magnetic entropy change. For Heusler alloys, Ni54.8 Mn20.2 Ga25.0 has been reported to show an extremely large S (−121 mJ/cm3 K) at its Tc (351 K) for a 0–18 kOe field change [8], and thus Ni–Mn–Ga was regarded as a promising new system for magnetic refrigeration [9]. As a member of Heusler alloys, Ni2 MnGe is quite similar to Ni2 MnGa in many aspects, including structural and magnetic properties. However, no reports on magnetocaloric effects in Ni–Mn–Ge system were found up to now. The Curie

temperature of Ni2 MnGe is a little bit above or near room temperature [10]. In general the magnetic properties and electronic structure of Ni2 MnGe have not been fully recognized yet [10]. In this study, the magnetic properties of the Ni-rich polycrystalline Ni2+x Mn1−x Ge Heusler alloys prepared by arc melting method has been investigated systematically. 2. Experimental Bulk Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge were prepared by melting Ni, Mn, and Ge pieces of 99.99% purity in an arc furnace with a watercooled Cu hearth under an Ar atmosphere at the pressure of 0.3 atm. Each sample was inverted and remelted three times to promote homogeneity prior to being sealed in vacuum quartz tube and annealed at 1223 K for 70 h. The magnetic properties of the samples were measured by using a vibrating sample magnetometer (VSM) equipped with liquid nitrogen cooling systems. The temperature dependence of magnetization (M) of the sample was obtained with increasing temperature and in an applied field of 0.05 T while the isothermal magnetization curves were measured in an applied field up to 1 T with certain temperature intervals. The magnetic entropy change, S, was evaluated by integrating the fundamental Maxwell relation:

 S = 0



Corresponding author. Tel.: +86 137 38091690. E-mail address: [email protected] (P.Z. Si).

0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.012

H

 ∂M  ∂T

H

dH

where H is the magnetic field, M the magnetization and T is the absolute temperature. By using the X-ray diffraction measurements with Cu K␣ radiation, the lattice parameters of the samples were calculated.

2

P.Z. Si et al. / Journal of Alloys and Compounds 462 (2008) 1–3

3. Results and discussion Our investigation begins with measurements of the magnetization versus temperature in a constant magnetic field of 0.05 T, as shown in Fig. 1, where M versus T is plotted for Ni2+x Mn1−x Ge with x = 0.1 and 0.2. The Curie temperature Tc , at which the M–T curve shows greatest slope, for Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge is determined to be 246 and 151 K, respectively. For stoichiometric Ni2 MnGe, the Tc seems vary in different samples; depending on the annealing conditions or sample state (bulk or film for instance) [10,11]. The Tc for Ni2 MnGe thin films grown on GaAs(0 0 1) by molecular-beam epitaxy was reported to be 320 K [11] while the Tc in the bulk annealed Ni2 MnGe was 257 K [10]. Thus we can conclude that the Curie temperature decreases with increasing nickel substitution to manganese for Ni2+x Mn1−x Ge. Oksenenko et al.’s work reported three distinct magnetic transitions in bulk Ni2 MnGe alloys. The origin of these transitions was not well understood yet [10]. Several magnetic transformations have also been reported in various Ni–Mn–Ge alloys due to the complicated magnetic structure ([10] and references therein). However, as seen in Fig. 1, only one magnetic transition was found in both Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge alloys, indicating a more simple magnetic structure in our samples. Ni2 MnGe consists of four interpenetrating fcc sublattices at the origin base in (0, 0, 0) A position, (1/4, 1/4, 1/4) B position, (1/2, 1/2, 1/2) C position and (3/4, 3/4, 3/4) D position, which are occupied by Mn, Ni, Ge and Ni atoms, respectively [12]. Since Ni2 MnGe alloy usually exhibits structural instability because of fairly broad free energy minimum as a function of the c/a ratio [11,13]. We speculate that the additional Ni atom might have stabilization ability on the structure and thus the Ni-rich samples show simple magnetic structure. Although both curves were measured under the same applied magnetic field, Fig. 1 shows that at 80 K, the magnetization of Ni2.2 Mn0.8 Ge is lower than that of Ni2.1 Mn0.9 Ge, indicating the decrease of the total magnetic moments with increasing

Fig. 1. The magnetization as a function of temperature measured during heating for Ni2+x Mn1−x Ge with x = 0.1 and 0.2, respectively.

Ni content. This phenomenon can be understood by relating the magnetic moments to the lattice constants. Maria found that the total magnetic moment of Ni2 MnGe decrease linearly with increasing pressure [12]. Higher pressure indicates smaller lattice parameters. The lattice constant of Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge is 0.572 and 0.570 nm, respectively, which are somewhat smaller than the 0.576 nm reported in Ni2 MnGe alloys [10]. Thus we can conclude that the lattice constant for Ni2+x Mn1−x Ge decreases with increasing Ni-substitution while decreased lattice constant results in decreased total magnetic moments. On the other hand, the strong dependence of Tc on the Ni/Mn ratio in Ni2+x Mn1−x Ge alloy, which essentially alters the interatomic distances, indicates a strong distance dependence of the exchange interactions. Fig. 2 plots the data on which the results of the magnetic entropy change is based, namely the variation of the magnetization with field at various fixed temperatures for Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge in the vicinity of their ordering temperatures. Before each isotherm (field sweep at constant temperature) was recorded, the sample was heated above Tc to produce a demagnetized state before zero-field cooling to below Tc and measuring

Fig. 2. The isothermal magnetization curves for Ni2.1 Mn0.9 Ge (from top 222 K to bottom 280 K in steps of 4 K) and Ni2.2 Mn0.8 Ge (from top 120 K to bottom 180 K in steps of 3 K) measured at various temperatures.

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decrease of the S was ascribed to the decrease of the total magnetic moments with increasing Ni as discussed above. 4. Conclusions

Fig. 3. The temperature dependence of the magnetic entropy change for Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge at fields up to 1 T, calculated from the magnetization data.

at a new temperature. To ensure equilibrium was maintained during each measuring sequence, a sufficiently slow sweep rate was adopted. The data in Fig. 2 indicate that both samples are ferromagnetic and easy to saturate below Tc . We have also measured the magnetic hysteresis loops for Ni2.2 Mn0.8 Ge at 80 K and found that the sample show zero coercivity, which is different from Ni2 MnGe films in which a small coercivity was observed [11]. The magnetic entropy change calculated based on the above Maxwell relation and the data in Fig. 2 are reproduced in Fig. 3 for a field of 1 T. As is evident in figure, the S reaches its maximum value, 0.43 J kg−1 K−1 for Ni2.1 Mn0.9 Ge at 246 K and 0.36 J kg−1 K−1 for Ni2.2 Mn0.8 Ge at 151 K, respectively. Thus we can conclude that the magnetic entropy change in Ni2+x Mn1−x Ge alloys decreases with increasing Ni content. The

In summary, the polycrystalline Ni2+x Mn1−x Ge Heusler alloy prepared by arc melting method have been investigated systematically. The Curie temperature, the total magnetic moment, the lattice constants, and the magnetic entropy change of Ni2+x Mn1−x Ge alloys decrease with increasing x value. The Curie temperature for Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge is 246 and 151 K, respectively. In a field of 1 T, the magnetic entropy change at Tc in Ni2+x Mn1−x Ge with x = 0.1 and 0.2 is 0.43 and 0.36 J kg−1 K−1 , respectively. In comparison with Ni2 MnGe, our sample Ni2.1 Mn0.9 Ge and Ni2.2 Mn0.8 Ge show zero coercivity and more simple sharp magnetic transition, indicating possible structural stability in Ni-rich samples. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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