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Effect of Mn substitution on Structural and Magnetic Properties of Ferromagnetic Shape Memory alloys ab

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C. Mahalakshmi , S. Vinodh Kumar , M. Muthuraman , S. Seenithurai & M. Mahendran

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Smart Materials Lab, Department of Physics, Thiagarajar College of Engineering, Madurai 625015, India b

Department of Physics, V.V. Vanniaperumal College for Women, Virudhunagar 626001, India Accepted author version posted online: 01 Jun 2015.

Click for updates To cite this article: C. Mahalakshmi, S. Vinodh Kumar, M. Muthuraman, S. Seenithurai & M. Mahendran (2015): Effect of Mn substitution on Structural and Magnetic Properties of Ferromagnetic Shape Memory alloys, Mechanics of Advanced Materials and Structures, DOI: 10.1080/15376494.2015.1022638 To link to this article: http://dx.doi.org/10.1080/15376494.2015.1022638

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ACCEPTED MANUSCRIPT Effect of Mn substitution on Structural and Magnetic Properties of Ferromagnetic Shape Memory alloys

C. Mahalakshmi1,2, S. Vinodh Kumar1, M. Muthuraman1, S. Seenithurai1, and M. Mahendran1*

Downloaded by [New York University] at 23:59 18 June 2015

1

Smart Materials Lab, Department of Physics, Thiagarajar College of Engineering, Madurai 625015, India

2

Department of Physics, V.V. Vanniaperumal College for Women, Virudhunagar 626001, India

ABSTRACT

The structure and the magnetic transitions have been investigated as a function of Mn in stoichiometric Ni2MnGa heusler alloys. Particular attention is paid to examine the linear increase of martensite transformation temperature on substituting Mn for Ga. It is observed that the martensite temperature increases and Curie temperature decreases with the effect of Mn content. Room-temperature magnetic measurements show the composition-dependent characteristics with decreasing magnetic saturation values and increasing coercivity values due to decrease in the magnetic exchange interaction strength with increasing Mn in place of Ga. The SEM image confirms that the Mn rich alloys have the martensitic plates.

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ACCEPTED MANUSCRIPT Keywords: Ferromagnetic Shape Memory Alloys, Ni-Mn-Ga alloys, Phase Transformations, Magnetization, Martensite, Microstructure

*

Corresponding author. Tel.: + 9 1 452 2482240x709; fax: + 9 1 452 2483427.


E-mail address: [email protected] (M. Mahendran).

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1. Introduction

Ferromagnetic Shape Memory Alloys (FSMAs), which exhibit giant magnetic field-induced strains (MFIS’s) in the martensitic phase, have attracted research interest due to their potential application in new generation actuation and transduction devices. The most interesting FSMA’s


are Ni-Mn-Ga [1, 2], Fe-Pd [3], Co-Ni-Al [4], Co-Ni-Ga [5], and Ni-Mn-Al [6]. Among these, Ni-Mn-Ga single crystal alloys can show large strain (up to 10%) in a magnetic field due to twin boundary motion in the martensitic phase [1, 7]. The large strain was only observed in Ni-Mn-Ga single crystal, which are difficult to prepare and also very expensive. From a technological point of view, there is a great interest in polycrystals which are easier to synthesis and the cost is low. In the high temperature austenitic phase, the structure of Ni2MnGa has been found to be cubic L21 ordered structure with a = 5.825 Å and also known as the Heusler structure [8]. In this particular composition, the martensitic transformation temperature and the Curie temperature are reported to be around 202 K and 370 K respectively [9]. The composition of the alloy should be tailored so that the austenite to martensite transformation in Ni-Mn-Ga is higher than the operating temperature for practical applications [10]. Hence, the off stoichiometric alloys are used to rise the martensitic transformation temperature above the room temperature. The lot studies have been reported that Ni rich alloy can increase the martensitic transformation temperatures [10, 11]. Jiang et al reported that the substitution of Ni in place of Mn/Ga, the saturation magnetization and Curie temperature decreases with the increase in Ni content. Enkovara et al studied through rigid-band approximation that the variation of Ni moment affects

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ACCEPTED MANUSCRIPT slightly in the total moment and whose variation is determined by Mn atom [12]. Moreover, the ferromagnetic properties of Ni2 MnGa alloy depend on the localized moment of Mn atom. A few studies have been reported about the composition dependence of Martensitic transformation temperatures and Curie temperatures in Mn-rich FSMA alloys [13]. Liu et al reported that the martensitic transformation varies non monotic and the Curie temperature increases while increasing the Mn substitution [14]. On the contrary, jiang et al reported that excess of Mn atom


increases martensitic transformation where as the Curie temperature decreases [15]. Hence, in the present investigation, it is attempting to examine how the martensitic transformation temperatures and crystal structure of Ni-Mn-Ga FSMA is influenced by the increase of Mn content in place of Ga . The results are shown that the martensite temperature increases with respect to Mn composition whereas Curie temperature decreases. The structural transition was changed from cubic to tetragonal and orthorhombic.

2. Experimental details

Ni-Mn-Ga polycrystalline ingots were produced using a conventional arc-melting furnace under argon atmosphere. High purity (>99.9%) raw elements Nickel, Manganese and Gallium were used to prepare the alloy. In order to ensure better homogeneity, the ingots were inverted and melted again and the process was repeated four times. The surface morphology was observed using scanning electron microscope and chemical composition was determined by energy dispersive analysis of X-rays (SEM-EDS, JEOL Model JSM - 6390LV). The powder X-ray diffraction measurement has been carried out to analyze the

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ACCEPTED MANUSCRIPT crystal structures of both austenite and martensite phases by using Cu-Kα radiation at room temperature (XRD, Bruker AXS D8 Advance). The transformation temperatures have been measured by using a differential scanning calorimetry (DSC, PERKIN-ELMER). Magnetic measurements were performed in a vibrating sample magnetometer (VSM-5, TOEI Industries) at room temperature.


3. Results and Discussion 3.1. Structural Studies

Powder XRD pattern of Alloy-A, B and C are shown in Fig. 1 and corresponding composition, and lattice parameter are summarized in the table 1. Fig. 1a indicates an oriented growth with preferred orientation along (220) plane and other high intensity planes that are observed in the spectra are characteristic peaks for Ni-Mn-Ga alloy [16, 17]. All the peaks in Fig. 1a are shifted towards a lower angle side by 0.5 to 10 degrees. It is mainly attributed to the presence of internal stress in the alloy. The XRD pattern of Alloy-A depicts the excellent polycrystalline growth of cubic L21 structure. The patterns are compared to the JCPDS data (JCPDS file no. 65-5328). In Fig. 1a, the superlattice peaks (111), (200) and (311) are clearly observed along with the fundamental reflections. Normally, the atomic scattering factor of the elements Ni, Mn and Ga are quite close to each other. Therefore, the second neighbor order may not be visible in XRD. Nevertheless, the peaks related to the second neighbor orders (111), (200) and (311) are clearly present, suggesting the observed pattern is cubic L21 structure. Observation of two different superlattice peaks in addition to the fundamental reflections is also observed by Vishnoi et al in

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ACCEPTED MANUSCRIPT L21 cubic structure [18]. The typically ordered L21 structure confirming the austenite phase with the lattice parameter a= 0.5899 nm using the powder cell software. The small change in the lattice parameter is mainly attributed to the shifting of peaks to the lower angle side. From Fig. 1b, diffraction lines observed are identified as non modulated martensite tetragonal phase. Two XRD reflections observed at 44.3° and 64.2° are corresponding to (222) and (400) respectively which infer that the alloy is in martensite phase with tetragonal structure a=b= 0.41543 and


c=0.5660 nm [19]. The XRD pattern of Alloy-B can be well indexed according to the non modulated tetragonal structure (c/a=1.36) with the space group of I4/mmm (no.139) and Ni atoms occupying 4d (0, 0.5, 0.25) wyckoff position, while Mn and Ga atoms occupy 2b (0, 0, 0.5) and 2a (0, 0, 0) positions, respectively. There is no modulation occurring in this type of martensite. In Fig 1c, the cubic austenite peak (220) is split into two peaks (220) and (202) and the nature is interpreted in terms of orthorhombic structure. The crystal structure of the 7M martensite in Alloy-C is nearly orthorhombic and has lattice parameters of a=0.6135 nm, b=0.5885 nm and c=0.5607 nm, these values are consistent with the earlier report [20]. The space group of this structure is Fmmm (69), with Ni atoms occupying 8f (0.25, 0.25, 0.25) wyckoff position, while Mn and Ga atoms occupying 4b (0, 0, 0.5) and 4a (0, 0, 0) positions, respectively. The stoichiometric Ni2 MnGa alloy shows that (220) austenite peak is narrow and relatively strong. As the Mn composition is increased, the peak gradually broadens and its intensity decreases slightly. On further addition of Mn in place of Ga, the reflection (220) cubic peak becomes broader and splits into two peaks, which correspond to (220) and (202) respectively. It reveals while substituting Mn in-place of Ga, the structural transition was found from cubic to

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ACCEPTED MANUSCRIPT tetragonal and orthorhombic [15]. Moreover, the peak splitting is mainly attributed to the structural change, at first the peak width should be increased and then splitting occurred [21]. The change in crystal structure is reflected in the unit cell volume reduction. Jiang et al reported that shrinkage of unit cell volumes accompanied with the substitution of Mn for Ga [15]. The atomic radius for Ni, Mn and Ga are 0.125, 0.127 and 0.141 nm respectively. Hence, while increasing the Mn substitution in place of Ga, a axis elongates, where as b and c axes contract


resulting in the martensite structure transition from tetragonal to orthorhombic [15]. The similar effect has also been observed when Ni is substituted in place of Ga, which was earlier reported [22].

3.3. Micro-Structural Studies The microstructure of Ni-Mn-Ga polycrystal using high resolution scanning electron microscope at room temperature is shown in Fig 2. To exhibit the magnetic shape memory effect, the martensite plates should be thin [23]. It is understood from the literature that thin plate martensite plates have lower martensitic transformation temperature. Most of the martensitic plates are parallel to each other and flows diagonally. Few martensitic plates are also found to cross the diagonal [24]. One martensite plate varies from the other one, which leads to rearrangement of the magnetic structure from a single domain to complex multiple magnetic domains. The domain width varies from 25-65 µm for Alloy-C [25]. From the SEM micrograph the thicker martensite plates are observed, which confirms the martensitic phase at room temperature.

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ACCEPTED MANUSCRIPT 3.4. Thermal Studies The transformation temperature for Alloy-A is not observed in Fig 3, because the transformation temperature for stoichiometric composition is reported to be around 210 K [8]. The Curie temperature of this alloy was measured as 372 K, which is in good agreement with the earlier reported values [8, 26]. Fig 4 and 5 show the typical DSC curve for Alloy-B and Alloy-C.


The results show that the martensite start (MS) and austenite start (AS) temperatures of Ni50Mn26.95Ga23.05 alloy increases monotonically from 303 K to 306 K and 305 K to 306 K respectively when composition changes from Ni50Mn28.75Ga21.25 to Ni50Mn31.45Ga18.55 [27,28]. The martensitic transformation temperature (Tm) can be calculated using the following equation Tm =

As + M f 2

K

(1)

The Curie temperature (Tc) and martensitic transformation temperature (Tm) of these alloys slightly varies due to the composition changes in the alloys is shown in table 2. When e/a ratio increase, Tm increases and Tc decreases. The change in Tm and Tc value is mainly attributed to the change in composition, exchange interaction between the atoms and size of the substitution atom. Vallal et al reported that the e/a ratio shifts the Tm to higher temperature (above room temperature) and slightly decrease the Tc [29]. Jin et al reported that Tm increases while Mn atom replaces the Ga atoms. Hence, Ga strongly influences on Tm because its e/a ratio differs more. The number of valence electrons per atom for Ni, Mn and Ga atoms are 10 (3d8, 4s2), 7 (3d5, 4s2) and 3 (4s2, 4p1), respectively [11]. Pons et al reported that the Tm is mainly depends upon the composition while Tc is not depends upon the composition of the alloy [30]. In addition to that the size factor and electronic concentration is the also the factors to determine the

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ACCEPTED MANUSCRIPT Tm and Tc [11, 22]. In case the substitution of Mn for Ga, the Tc decreases because the size of Ga is larger than Mn. Previous literature reported that the size of the substitution atom is smaller than the present atom, then Tc must be monotonically decreasing [22]. However, the Tc is not only depending on the size of the atom, it also depends on the interaction between the neighboring atoms. The excess of Mn atom induces the anti-ferromagnetic coupling [29]. Hence,


the phase transformation temperature values are consistent with the earlier reported values.

3.5. Magnetic Studies

The M-H loop of the polycrystalline Ni-Mn-Ga alloy is given in Fig. 6. The hysteresis loop confirms that all the polycrystalline alloys exhibit the ferromagnetic behaviors at room temperature. The measured values of saturation magnetization (Ms), coercivity (Hc) and remanent magnetization (Mr) for three samples are listed in table. 3. The decrease in saturation magnetization can be attributed to strong anti-ferromagnetic coupling in Mn rich alloy due to the substitution of Mn in place of Ga, which decreases the TC and enhance the transformation temperature. Usually, the ferromagnetism mainly originates from the localized moment of Mn atoms, 4.0 μB per Mn ions and the contribution of Ni ion is less than 0.6 μB [31]. However, for Mn rich alloy (Alloy-B and Alloy-C), the saturation magnetization value is decreasing, because the excess Mn atoms that occupy the Ga sites reduce the Mn-Mn distance which increase the antiferromagnetic coupling [29]. Enkovaara et al also reported about the variation of magnetization with the composition through the rigid-band approximation

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ACCEPTED MANUSCRIPT model. The substitutions of every Mn atom at the Ga sites are coupled antiferromagnetically with the neighboring Mn atoms at Mn sites and reduce the total magnetic moment [13]. In contrast to the saturation magnetization, the coercivity increases as the Mn content increases, it reveals due to the phase structure [32]. In the austenite state the coercivity displays a very low value (92 Oe) and is mainly attributed to the magnetic anisotropy [33]. When the structure transforms from austenite to martensite, the shape of the magnetization loops changes drastically.


The value of coercivity increases from 92 Oe to 325 Oe which confirms the presence of martensite structure is present at room temperature measurement. In accordance with the structural changes and phase transformation, the magnetic properties also vary with the effect of Mn substitution on Ga site. 4. Conclusion The effect of Mn substitution on both the martensitic and magnetic transition of stoichiometric Ni2MnGa alloys has been studied. The structural studies confirm that the Mn substitution induces the structure from highly ordered austenite cubic to martensite orthorhombic structure. The martensitic transformation temperatures increase and Curie transition temperature decreases with the increase of Mn content in place of Ga. The obtained martensitic transformation temperature is above the room temperature, it seems to be a promising candidate for practical application.

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Acknowledgement One of the authors (MM) thanks Dr. Robert C. O’ Handley for introducing him to the subject and acknowledges the UGC – DAE CSR , UGC (38-243/2009 (SR)) and UGC (RA-2012-14-OB-


TAM-1843) for financial support.

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Table 1: Lattice parameter and e/a ratio for Ni-Mn-Ga alloy at different composition

Sample ID

Composition

Lattice parameters a b c (nm) (nm) (nm) 0.5899 0.5899 0.5899

Alloy-A

Ni50Mn25Ga25

Alloy-B

Ni50Mn28.75Ga21.25

0.4154

0.4154

0.5660

7.65

Alloy-C

Ni50Mn31.45Ga18.55

0.6135

0.5885

0.5607

7.75

17

e/a ratio

7.5

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Table 2: Effect of composition on Martensitic transformation and Curie temperature


Sample Id Alloy-A

Ms (K) ---

Mf (K) ----

As (K) ----

Af (K) ---

Tc (K) 372

Tm= (Ms+Af)/2 (K) --

Alloy-B

303

302

303

305

369

304

Alloy-C

306

305

306

308

365

307

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Table 3: Calculation of magnetic properties of Ni-Mn-Ga alloy

Sample Id

Saturation Magnetization (emu/g)

Coercivity (Oe)

Alloy-A

61

92

Alloy-B

52

118

Alloy-C

40

338

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ACCEPTED MANUSCRIPT Figure 1. X-ray diffraction pattern of Ni50Mn30Ga20, Ni50Mn28.75Ga21.25, Ni50Mn31.45Ga18.55 alloy at room temperature. The substitution Mn in place of Ga induces the structure from


highly ordered L21 cubic structure to 7M martensite orthorhombic structure.

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Figure 2. Micrograph of Alloy-C shows that existence of Martensitic plate. The insert shows that enlargement of Twins variants

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Figure 3. DSC heating/cooling curve for as-cast Ni50Mn25Ga25 alloy at room temperature

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Figure 4.

DSC thermograms on cooling and heating of Ni50 Mn28.75Ga21.25 alloy studied

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Figure 5. DSC curves of Ni50Mn31.45Ga18.55 showing the forward (on cooling) and reverse (on heating) martensitic transformations.

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Figure 6.

M-H curve for Ni50Mn30Ga20, Ni50Mn28.75Ga21.25, Ni50Mn31.45Ga18.55 alloy. It shows the effect of Mn concentration on saturation magnetization and coercivity of the alloys.

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