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Al2O3 polycrystalline 100-nm – 2-μm films with Ni- and Ga-excess
Journal of Alloys and Compounds 767 (2018) 538e543
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The evolution of martensitic transformation in Ni-Mn-Ga/Al2O3 polycrystalline 100-nm e 2-mm films with Ni- and Ga-excess S. Shevyrtalov a, *, H. Miki b, M. Ohtsuka c, V. Khovaylo d, e, V. Rodionova a, d a
Center for Functionalized Magnetic Materials (FunMagMa), Immanuel Kant Baltic Federal University, 236041, Kaliningrad, Russian Federation Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Sendai, 980-8578, Japan c Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan d National University of Science and Technology MISiS, Moscow, 119049, Russian Federation e National Research South Ural State University, Chelyabinsk, 454080, Russian Federation b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2018 Received in revised form 10 July 2018 Accepted 11 July 2018
Polycrystalline Ni54Mn18Ga28 films with thicknesses ranging from 100-nm to 2-mm were fabricated by deposition on alumina ceramic substrates; bulk and surface properties were then characterized by structural and magnetic methods. The evolution of martensitic transformation of the films was studied. Both martensitic transformation and Curie temperature were found to be above room temperature, with the former overlapping the latter when films thicker than 100-nm were used. Static magnetic properties revealed the presence of strong internal stresses in the films, thus influencing coercive force value which changes with the increase in thickness. A distinct presence of martensitic twinning at room temperature was revealed, which started with the 400-nm film and increased with the 1- and 2-mm films. A maze-like domain structure (without correlation to the surface features), was found in the 1- and 2-mm films. A strong dependence between crystallites and magnetic domains was discovered in films ranging from 100- to 400-nm. A nonlinear dependence of the domain width on the film thickness was also observed, which converges with the coercive force dependence. © 2018 Elsevier B.V. All rights reserved.
Keywords: Heusler alloys Ni-Mn-Ga film Thin films Martensitic transformation Magnetic properties
1. Introduction Ferromagnetic shape memory Ni-Mn-Ga alloys have a great potential for applications in sensors and actuators [1,2], due to their high magnetostrain and magnetic shape memory effect [3]. Compared to the conventional shape memory effect, where temperature change is required, high strains of Ni-Mn-Ga alloys can be achieved by applying an external magnetic field [4]. There are two mechanisms responsible for these effects, which use interplay between martensitic transformation (MT) and magnetism. The first mechanism involves variation of magnetization at the transformation; the second mechanism is a favorable combination of a high magnetocrystalline anisotropy and a low elastic shear modulus of a martensite, enabling twin boundary displacement in response to the applied magnetic field [5]. However, this effect can only be observed in a low symmetric, low temperature martensite phase; thus, the transformation temperatures should be above
* Corresponding author. E-mail address: [email protected] (S. Shevyrtalov). https://doi.org/10.1016/j.jallcom.2018.07.144 0925-8388/© 2018 Elsevier B.V. All rights reserved.
room temperature for practical applications. In turn, transformation temperatures, observed in non-stoichiometric Ni-Mn-Ga single crystals and polycrystalline alloys, are strongly dependent on chemical composition [6,7]. While many of the functional properties of bulk alloys have been studied, a deeper understanding of thin films on solid substrates in a broad spectrum of thicknesses and chemical compositions is needed. It was shown that film thickness and substrate type influence the properties of Ni-Mn-Ga films [8e12]. The main factor affecting the properties of thin films deposited on solid substrates, is residual stress; this is mainly due to the mismatch between thermal expansion coefficients of the films and the substrate. Residual stress can influence lattice parameters, martensitic transformation temperatures and twinning [8]. Recently, we showed the difference in MT temperatures and magnetic properties in 2-mm film on alumina ceramic substrate and freestanding film [13]. Chernenko et al. investigated the structural, magnetic properties and martensitic transformation temperatures of Ni49.5Mn28Ga22.5 and Ni52Mn24Ga24 films specifically relating to the alumina ceramic substrate and thickness of the films [12]. It has been found that MT temperatures change differently depending on chemical
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composition. For Mn-excess films, the MT temperatures drop with increasing the thickness up to 400-nm, and then increase slowly. In turn, for Ni-excess films, the MT temperatures increase with increasing thicknesses up to 1 mm, and then slowly decrease. In the present work, the evolution of structural, magnetic properties, and MT of Ni-Mn-Ga polycrystalline films with excess Ni and Ga, (thicknesses ranging from 100 nm to 2 mm deposited on polycrystalline alumina ceramic substrate) is analyzed. These magnetic and structural properties at low temperatures, and across MT, were further studied. Investigated films were found to exhibit MT above room temperature, and overlapped Curie temperature (TC) in the case of films with a thickness more than 100-nm. The influence of the substrate, in the terms of surface and magnetic domain structure, magnetic properties and MT parameters are presented. 2. Experiments Ni-Mn-Ga thin films were deposited on a polycrystalline Al2O3 ceramic plate by radio-frequency magnetron sputtering setup (Shibaura, CFS-4ES) using a target with a nominal composition of Ni54Mn20Ga26 (at. %). During the deposition process the temperature of the substrates was kept at 320 K by a cooling water bath, the sputtering power was maintained at 200 W. The deposition rate was 0.3 nm/s, and the thickness of the films was controlled by the deposition time. After deposition, the thin films were stacked between ceramic plates and annealed at 1073 K for 1 h in a high vacuum furnace at the pressure of 2 104 Pa, and slowly cooled to room temperature. The chemical composition of the films, which was determined using energy dispersive X-ray spectroscopy (EDX, Thermo Scientific), was Ni54±0.1Mn18±0.1Ga28±0.2 (at. %). The structural characterization was performed at room temperature by X-ray diffractometer with a CuKa source (Bruker D8 Discovery). To evaluate the magnetic and martensitic phase transition parameters, ZFC-FC thermomagnetic curves with applied magnetic field of 10 Oe (SQUID, Quantum Design) and resistivity curves (4point contact mode, homemade setup) were collected at temperatures ranging from 100 to 420 K. The in-plane and out-of-plane hysteresis loops were measured at room temperature in a magnetic field up to 11 kOe (VSM, Lakeshore 7400). The atomic force microscopy (Aist-NT, SPM-1000) and magnetic force microscopy modes were used to derive information about the surface morphology and magnetic domain structure. Magnetic images were acquired using a two-pass technique in tapping mode, 90-nm above the surface with a second pass scan rate of 1 Hz. 3. Results and discussion 3.1. Crystal structure Crystal structure of annealed films was characterized by X-Ray diffraction (XRD) at room temperature in q-2q geometry. For complete comparison, diffraction patterns were acquired for the stand-alone substrate, and 1-mm as-deposited Ni-Mn-Ga film. Fig. 1(a) displays XRD pattern in the 2q range of 5e100 , showing the presence of the crystal structure and the absence of crystal structure for the 1-mm as-deposited film. In Fig. 1(b) one can see a shift of (220) austenite peak (44.5 ) to the (202) martensite (44.6 ) with the increase of film thickness. Peak splitting onto (220)a and (202)m becomes visible at the 200-nm thin film, and continues to increase to the 2-mm film, where only the (202)m peak remains; additional (220)m and (022)m peaks appear, which can correspond to the 14 M orthorhombic martensite phase [14]. The difference between these peaks corresponding to the austenite and martensite phases, can be highlighted by their intensity change.
539
The (220)a peak having a higher intensity in the 100-nm film is seen to split into two with a smaller intensity in the 200- and 400-nm films.
3.2. Transformation behavior MT temperatures were derived using a tangential method from low-field (10 Oe) thermomagnetic curves, and from resistivity curves measured within the temperatures ranging from 150 to 400 K. Curie temperature, TC, was found to be above room temperature, and varies from 344 K for the 200-nm film, to 361 K for the 2-mm film. Fig. 2 demonstrates thermomagnetic curves and resistivity curves. It can be seen that in the 100-nm film, the direct and reverse MT occurs in the ferromagnetic region. For the other films, MT starts in the ferromagnetic state and is completed in the paramagnetic state. As a result, first and second order phase transitions overlap. However, it is fair to conclude that this does not lead to the coupled magnetostructural transition. Temperatures of the phase transitions obtained from the resistivity curves, correspond to that in thermomagnetic curves, except in the 400-nm film, where the discrepancy between resistivity and thermomagnetic curves reaches about 11 K (Fig. 2(c)). Equilibrium MT temperature is defined as TM ¼ (MsþMf)/2, where Ms and Mf are the martensitic start and finish temperatures, respectively. The equilibrium MT temperature shifts with the increase of the film thickness from 322 K for the 100-nm film, to 370 K for the 1-mm film. Further increase in the thickness results in a small drop of the equilibrium MT temperature to 365 K for the 2mm film. These results can be attributed to residual stresses in the films which are produced by an interaction with the substrate [15]. The width of the MT, W, was calculated using the following formula:
W¼
Ms Mf
þ As Af 2
;
Where Ms, Mf, As and Af are the martensitic and austenitic start and finish temperatures, respectively. The temperature hysteresis of the MT, DTm, was calculated using the following formula:
DTM ¼
Ms Af
þ As Mf 2
:
The thin films with a 100-nm thickness show the smallest width of transformation (W ¼ 20.5 K), while the largest transformation width of W ¼ 30 K, was observed in the 200-nm films. The opposite results can be seen for the temperature hysteresis, DTM; these values decreased with the increasing thickness. The highest value of DTM ¼ 8.5 K, was observed for the 100-nm film; the lowest value of DTM ¼ 3.5 K, was observed in the 1- and 2-mm films. Dependence of the phase transition temperature on film thickness is shown in Fig. 2(f). In comparison with results obtained by Chernenko et al. for similar Ni-Mn-Ga films, our results demonstrate behaviors similar to the Ni49.5Mn28.0Ga22.5 films, with reference to increase of MT temperatures up to 1-mm films and decrease in the thicker films [12]. However, it should be noted that increase of the MT temperatures in correlation to increasing film thickness, is significantly higher in the studied films, showing a value of 52 K (Ms(100nm)Ms(1mm)) versus 30 K reported in Ref. [12]. Aforementioned results are presented in Table 1. The increase of Curie temperature and MT temperatures, along with the increase of film thickness, can be attributed to growing internal stresses. The positive shift of phase transitions temperatures was also observed by Kamarad et al., in off-stoichiometric Ni-
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Fig. 1. (a) X-Ray diffraction patterns collected at room temperature. As-deposited (AD) film with a 1-mm thickness was used to demonstrate the absence of crystal structure. (b) Enlarged patterns in a 2q range of 42e47. Red dotted lines indicate the difference between (220)a and (202)m peaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. (aee) ZFC-FC curves measured in a magnetic field of 10 Oe together with resistivity curves; (f) phase transitions temperatures as a function of the film thickness. As, Af and Ms, Mf are martensitic and austenitic starting and finishing temperatures, respectively.
Mn-Ga alloys when applying hydrostatic pressure [16]. The change in MT temperatures under these pressures achieves the value of dTM/dp ¼ 6 K/GPa for Ni53.75Mn21.25Ga25, and remains almost unchanged in the stoichiometric Ni2MnGa alloy. Curie temperature changes were observed in both cases, with a rate of dTC/dp ¼ 4.7 K/ GPa and 5.9 K/Gpa for Ni53.75Mn21.25Ga25 and Ni2MnGa alloys, correspondingly. It should be noted that the effect of pressure on magnetic properties differs significantly depending on the Ni
amount and substituted element [17], and can be explained by the increase of exchange interactions with the decreasing of interatomic distances [18]. 3.3. Static magnetic properties In-plane and out-of-plane hysteresis loops were measured at room temperature. For the 100- and 200-nm films, hysteresis loops
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541
Fig. 3. In-plane (a) and out-of-plane (b) low-field part of hysteresis loops measured at room temperature for the films with differing thicknesses. (Hysteresis loops measured in magnetic fields of ±11 kOe are shown in the insets).
looked similar, replicating the shape of each other (Fig. 3). Similar behavior was also observed in the 1- and 2-mm films, with a minor change in the slope of the loop. Hysteresis loop of the film with the thickness of 400-nm took an intermediate position between the 100e200-nm and 1e2-mm loops. Here it should be noted that angle dependence shows the absence of in-plane anisotropy for all films. Coercive force, HC, increases with the increase of thickness from 64 Oe in the 100-nm film to 100 Oe in the 2-mm film. The widening of the in-plane hysteresis loops was observed, which causes an increase of coercive force HC. The blue dashed line in Fig. 3(a) is a guide for the eye, which shows the possible magnetization process for the 400-nm film. The value of the magnetic field in which this effect vanishes, becomes higher with an increase in film thickness. For example, a low magnetic field of 180 Oe is required to suppress the widening of the 100-nm Ni-MnGa films, whereas 240 Oe is required for the 2-mm films. Such an effect has also been observed in numerous other experiments, as a result of applied compressive stresses to the material [16,17,19,20]. By using the values of HС , derived from the out-of-plane hysteresis loops (Fig. 3(b)), and comparing them with values from the in-plane curves, the coercive force difference, DHС, related to the internal stresses in plane of the film, and film thickness was determined. Larger values of DHС relates to the larger internal stresses. Blue dashed lines shown for the 400-nm films (Fig. 3(a)), indicate a possible direction for the magnetization process, in the absence of stresses. The coercive force change (DHC ¼ 6 Oe) is slight for the 100-nm films, and rises significantly with the increase in the film thicknesses, up to 48 Oe for the 1-mm film. This difference indicates that the highest internal stress is present in the 1-mm film. Chernenko et al. also demonstrated this similar behavior of in-plane stresses for Ni-Mn-Ga/Al2O3 films, in thicknesses ranging from 100- nm up to 5-mm, of which varied from 0.2 GPa to 0.32 GPa, correspondingly. It was explained by relaxation mechanisms available in these films due to a high surface roughness of alumina ceramics [8]. Miyagi et al. observed hysteresis loop widening in laminated steel, resulting in a DHС ¼ 180 Oe, under the compressive stress of 0.15 GPa [20]. The same behaviors were observed by Zhu et al., when using micromagnetic modeling for Ni film as a reference object [21], in addition to Han et al. when using NiFe/NiO bilayer thin films [22]. The origin of compressive stresses in polycrystalline films could also be related to the slow deposition rate (0.3 nm/s), high diffusivity and recrystallization during annealing [23]. All obtained values of HС and DHС are presented in Table 2.
3.4. Morphology and magnetic domain structure The surface morphology and magnetic domain structure were investigated using an atomic force microscope in tapping mode with CoCr-coated commercial tips. Measurements were made on the samples in a remanence magnetization state. Fig. 4(aee) shows the correlation between surface structure and magnetic domain structure for the films with differing thicknesses. The microstructure of the films with 100- and 200-nm thickness is polycrystalline, with a large crystallite size distribution. The minimal size of crystallites is d ¼ (250 ± 20) nm, and the maximum size is d ¼ (1100 ± 90) nm. Small grains, with d ¼ (100 ± 10) nm, can also be visible at the surface of these films. The origin of these grains could be attributed to the growth-mode feature, or could be related to a thin oxide layer on the surface. The surface of the 400-nm film consists of non-spherical faceted crystallites, with a size range of 300 nm ÷ 1.3 mm. On the 1-mm films one can see the similar lineament, but with a strong twinning effect. At the thickness of 2-mm, the twinning effect is still present, but the grain boundaries become indistinguishable. Magnetic domain structure, presented and summarized in Fig. 4(f), demonstrates alternating bright and dark regions, which correspond to the magnetization directed up and down to the film's surface. One can associate it with the formation of the (220) texture, where martensitic twin boundaries are situated parallel to the film plane or inclined by some angle to it, as was obtained previously by Chernenko et al. for the similar films [15]. The interconnection between the domain and surface structures can be observed in Ni-Mn-Ga films with thicknesses ranging from 100- to 400-nm. Alternating magnetic domain patterns can be observed, which are different for each separate crystallite. This relationship becomes blurred in the 1- and 2-mm films, where a maze-like domain structure is formed. The width of the domains (d) increases with the thickness from 270 to 890 nm in the 100-nm and 2-mm films, respectively. Obtained dependence has a nonlinear character, and can be related to both the growth method and the different types of internal stresses in the films themselves.
4. Summary In summary, polycrystalline Ni54Mn18Ga28 films with excess Ni and Ga and with thicknesses ranging from 100-nm to 2-mm were deposited on alumina ceramic substrates. Evolution of the
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Table 1 First- and second-order phase transition temperatures derived from the thermomagnetic curves, M(T), and resistivity curves, R(T). Ms,f and As, f are the martensitic and austenitic start and finish temperatures, respectively. TC and TM denotes Curie temperature and equilibrium MT temperature, respectively. DTM is temperature hysteresis of MT and W is the width of transformation. t, nm
M(T)
R(T)
Ms, K
Mf, K
As, K
Af, K
Ms, K
Mf, K
As, K
Af, K
100 200 400 1000 2000
331 e 338 e e
311 323 311 352 353
319 328 316 353 351
334 e 339 e e
328 347 350 380 375
308 317 322 357 351
316 324 327 359 352
337 354 356 385 381
Table 2 Coercive force HC and its difference, DHC, for in-plane and out-of-plane hysteresis loops. t, nm
100 200 400 1000 2000
DHC, Oe
HC, Oe in-plane
out-of-plane
65 73 87 95 100
59 54 58 47 57
6 19 29 48 43
TC, K
TM, K
DTM, K
W, K
350 344 353 365 361
322 335 339 370 365
8.5 7 5.5 3.5 3.5
20.5 30 28.5 24.5 26.5
structural, surface and bulk magnetic properties and MT transformation were shown. MT occurs above room temperature for all films. Curie temperature was found to be above room temperature as well, which overlapped with the MT temperatures for film thicknesses ranging from 200-nm ‒ 2-mm. The non-linear growth of phase transition temperatures was established up to the 1-mm film. Ni-Mn-Ga film with a 2-mm thickness shows a small decrease in phase transition temperatures, that could be related to the internal stress relaxation present in the films with a thickness of more than 1-mm. The
Fig. 4. Surface morphology (left) and corresponding magnetic domains image (right) for thin films with 100-nm (a), 200-nm (b), 400-nm (c), 1-mm (d) and 2-mm (e) thicknesses. Figure (f) shows the visual evolution of domain size in relation to thickness, as (g) shows a graphical dependence of domain width on film thickness.
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evidence of the internal stress influence was also found in the inplane hysteresis loops, where the growth of the coercive force was observed. Magnetic domain structure indicates the presence of perpendicular magnetic anisotropy at the surface of the films, which can be connected with the martensitic twin boundaries situated parallel to the film plane or inclined by some angle to it. A correlation between domain and the surface structure was observed in Ni-MnGa films with a thickness ranging from 100- to 400-nm; in turn, the 1- and 2-mm films had this relationship blurred. The width of magnetic domains increases with the thickness increase, showing non-linear dependence. Acknowledgments Helpful discussions with Prof. V.A. Chernenko are greatly acknowledged. VK acknowledges grant No.16-42-02035, provided by the Russian Science Foundation (for measurements of transport properties). Part of this work received financial support from the Act 211, in partnership with the Government of the Russian Federation, contract No.02.A03.21.0011 and is partially supported by 5 top 100 Russian Academic Excellence Project at the Immanuel Kant Baltic Federal University. VR acknowledges financial support from the Ministry of Education and Science of the Russian Federation in the framework of Government assignments 3.9002.2017/ 6.7. References €hler, M. Kohl, Freely movable ferromagnetic shape [1] M. Schmitt, A. Backen, S. Fa memory nanostructures for actuation, Microelectron. Eng. 98 (2012) 536e539, https://doi.org/10.1016/j.mee.2012.07.045. [2] M. Schmitt, A. Backen, S. Fahler, M. Kohl, Development of ferromagnetic shape memory nanoactuators, Proc. IEEE Conf. Nanotechnol. (2012), https://doi.org/ 10.1109/NANO.2012.6322098. €derberg, S.-P. Hannula, High-cycle fatigue of [3] I. Aaltio, A. Soroka, Y. Ge, O. So 10M NieMneGa magnetic shape memory alloy in reversed mechanical loading, Smart Mater. Struct. 19 (2010), 075014, https://doi.org/10.1088/ 0964-1726/19/7/075014. [4] O. Heczko, Magnetic shape memory effect and magnetization reversal, J. Magn. Magn. Mater. 290e291 PA (2005) 787e794, https://doi.org/10.1016/ j.jmmm.2004.11.397. [5] R.C. O'Handley, Model for strain and magnetization in magnetic shapememory alloys, J. Appl. Phys. 83 (1998) 3263e3270, https://doi.org/10.1063/ 1.367094. [6] F. Albertini, L. Morellon, P.A. Algarabel, M.R. Ibarra, L. Pareti, Z. Arnold, G. Calestani, Magnetoelastic effects and magnetic anisotropy in Ni2MnGa polycrystals, J. Appl. Phys. 89 (2001) 5614e5617, https://doi.org/10.1063/ 1.1350630. nchez-Alarcos, J.I. Pe rez-Landaza bal, V. Recarte, J. a Rodríguez[7] V. Sa n, V. a Chernenko, Effect of atomic order on the martensitic and Velamaza magnetic transformations in NieMneGa ferromagnetic shape memory alloys, J. Phys. Condens. Matter 22 (2010), 166001, https://doi.org/10.1088/0953-
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The evolution of martensitic transformation in Ni-Mn-Ga/Al2O3 polycrystalline 100-nm e 2-mm films with Ni- and Ga-excess S. Shevyrtalov a, *, H. Miki b, M. Ohtsuka c, V. Khovaylo d, e, V. Rodionova a, d a
Center for Functionalized Magnetic Materials (FunMagMa), Immanuel Kant Baltic Federal University, 236041, Kaliningrad, Russian Federation Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Sendai, 980-8578, Japan c Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan d National University of Science and Technology MISiS, Moscow, 119049, Russian Federation e National Research South Ural State University, Chelyabinsk, 454080, Russian Federation b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2018 Received in revised form 10 July 2018 Accepted 11 July 2018
Polycrystalline Ni54Mn18Ga28 films with thicknesses ranging from 100-nm to 2-mm were fabricated by deposition on alumina ceramic substrates; bulk and surface properties were then characterized by structural and magnetic methods. The evolution of martensitic transformation of the films was studied. Both martensitic transformation and Curie temperature were found to be above room temperature, with the former overlapping the latter when films thicker than 100-nm were used. Static magnetic properties revealed the presence of strong internal stresses in the films, thus influencing coercive force value which changes with the increase in thickness. A distinct presence of martensitic twinning at room temperature was revealed, which started with the 400-nm film and increased with the 1- and 2-mm films. A maze-like domain structure (without correlation to the surface features), was found in the 1- and 2-mm films. A strong dependence between crystallites and magnetic domains was discovered in films ranging from 100- to 400-nm. A nonlinear dependence of the domain width on the film thickness was also observed, which converges with the coercive force dependence. © 2018 Elsevier B.V. All rights reserved.
Keywords: Heusler alloys Ni-Mn-Ga film Thin films Martensitic transformation Magnetic properties
1. Introduction Ferromagnetic shape memory Ni-Mn-Ga alloys have a great potential for applications in sensors and actuators [1,2], due to their high magnetostrain and magnetic shape memory effect [3]. Compared to the conventional shape memory effect, where temperature change is required, high strains of Ni-Mn-Ga alloys can be achieved by applying an external magnetic field [4]. There are two mechanisms responsible for these effects, which use interplay between martensitic transformation (MT) and magnetism. The first mechanism involves variation of magnetization at the transformation; the second mechanism is a favorable combination of a high magnetocrystalline anisotropy and a low elastic shear modulus of a martensite, enabling twin boundary displacement in response to the applied magnetic field [5]. However, this effect can only be observed in a low symmetric, low temperature martensite phase; thus, the transformation temperatures should be above
* Corresponding author. E-mail address: [email protected] (S. Shevyrtalov). https://doi.org/10.1016/j.jallcom.2018.07.144 0925-8388/© 2018 Elsevier B.V. All rights reserved.
room temperature for practical applications. In turn, transformation temperatures, observed in non-stoichiometric Ni-Mn-Ga single crystals and polycrystalline alloys, are strongly dependent on chemical composition [6,7]. While many of the functional properties of bulk alloys have been studied, a deeper understanding of thin films on solid substrates in a broad spectrum of thicknesses and chemical compositions is needed. It was shown that film thickness and substrate type influence the properties of Ni-Mn-Ga films [8e12]. The main factor affecting the properties of thin films deposited on solid substrates, is residual stress; this is mainly due to the mismatch between thermal expansion coefficients of the films and the substrate. Residual stress can influence lattice parameters, martensitic transformation temperatures and twinning [8]. Recently, we showed the difference in MT temperatures and magnetic properties in 2-mm film on alumina ceramic substrate and freestanding film [13]. Chernenko et al. investigated the structural, magnetic properties and martensitic transformation temperatures of Ni49.5Mn28Ga22.5 and Ni52Mn24Ga24 films specifically relating to the alumina ceramic substrate and thickness of the films [12]. It has been found that MT temperatures change differently depending on chemical
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composition. For Mn-excess films, the MT temperatures drop with increasing the thickness up to 400-nm, and then increase slowly. In turn, for Ni-excess films, the MT temperatures increase with increasing thicknesses up to 1 mm, and then slowly decrease. In the present work, the evolution of structural, magnetic properties, and MT of Ni-Mn-Ga polycrystalline films with excess Ni and Ga, (thicknesses ranging from 100 nm to 2 mm deposited on polycrystalline alumina ceramic substrate) is analyzed. These magnetic and structural properties at low temperatures, and across MT, were further studied. Investigated films were found to exhibit MT above room temperature, and overlapped Curie temperature (TC) in the case of films with a thickness more than 100-nm. The influence of the substrate, in the terms of surface and magnetic domain structure, magnetic properties and MT parameters are presented. 2. Experiments Ni-Mn-Ga thin films were deposited on a polycrystalline Al2O3 ceramic plate by radio-frequency magnetron sputtering setup (Shibaura, CFS-4ES) using a target with a nominal composition of Ni54Mn20Ga26 (at. %). During the deposition process the temperature of the substrates was kept at 320 K by a cooling water bath, the sputtering power was maintained at 200 W. The deposition rate was 0.3 nm/s, and the thickness of the films was controlled by the deposition time. After deposition, the thin films were stacked between ceramic plates and annealed at 1073 K for 1 h in a high vacuum furnace at the pressure of 2 104 Pa, and slowly cooled to room temperature. The chemical composition of the films, which was determined using energy dispersive X-ray spectroscopy (EDX, Thermo Scientific), was Ni54±0.1Mn18±0.1Ga28±0.2 (at. %). The structural characterization was performed at room temperature by X-ray diffractometer with a CuKa source (Bruker D8 Discovery). To evaluate the magnetic and martensitic phase transition parameters, ZFC-FC thermomagnetic curves with applied magnetic field of 10 Oe (SQUID, Quantum Design) and resistivity curves (4point contact mode, homemade setup) were collected at temperatures ranging from 100 to 420 K. The in-plane and out-of-plane hysteresis loops were measured at room temperature in a magnetic field up to 11 kOe (VSM, Lakeshore 7400). The atomic force microscopy (Aist-NT, SPM-1000) and magnetic force microscopy modes were used to derive information about the surface morphology and magnetic domain structure. Magnetic images were acquired using a two-pass technique in tapping mode, 90-nm above the surface with a second pass scan rate of 1 Hz. 3. Results and discussion 3.1. Crystal structure Crystal structure of annealed films was characterized by X-Ray diffraction (XRD) at room temperature in q-2q geometry. For complete comparison, diffraction patterns were acquired for the stand-alone substrate, and 1-mm as-deposited Ni-Mn-Ga film. Fig. 1(a) displays XRD pattern in the 2q range of 5e100 , showing the presence of the crystal structure and the absence of crystal structure for the 1-mm as-deposited film. In Fig. 1(b) one can see a shift of (220) austenite peak (44.5 ) to the (202) martensite (44.6 ) with the increase of film thickness. Peak splitting onto (220)a and (202)m becomes visible at the 200-nm thin film, and continues to increase to the 2-mm film, where only the (202)m peak remains; additional (220)m and (022)m peaks appear, which can correspond to the 14 M orthorhombic martensite phase [14]. The difference between these peaks corresponding to the austenite and martensite phases, can be highlighted by their intensity change.
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The (220)a peak having a higher intensity in the 100-nm film is seen to split into two with a smaller intensity in the 200- and 400-nm films.
3.2. Transformation behavior MT temperatures were derived using a tangential method from low-field (10 Oe) thermomagnetic curves, and from resistivity curves measured within the temperatures ranging from 150 to 400 K. Curie temperature, TC, was found to be above room temperature, and varies from 344 K for the 200-nm film, to 361 K for the 2-mm film. Fig. 2 demonstrates thermomagnetic curves and resistivity curves. It can be seen that in the 100-nm film, the direct and reverse MT occurs in the ferromagnetic region. For the other films, MT starts in the ferromagnetic state and is completed in the paramagnetic state. As a result, first and second order phase transitions overlap. However, it is fair to conclude that this does not lead to the coupled magnetostructural transition. Temperatures of the phase transitions obtained from the resistivity curves, correspond to that in thermomagnetic curves, except in the 400-nm film, where the discrepancy between resistivity and thermomagnetic curves reaches about 11 K (Fig. 2(c)). Equilibrium MT temperature is defined as TM ¼ (MsþMf)/2, where Ms and Mf are the martensitic start and finish temperatures, respectively. The equilibrium MT temperature shifts with the increase of the film thickness from 322 K for the 100-nm film, to 370 K for the 1-mm film. Further increase in the thickness results in a small drop of the equilibrium MT temperature to 365 K for the 2mm film. These results can be attributed to residual stresses in the films which are produced by an interaction with the substrate [15]. The width of the MT, W, was calculated using the following formula:
W¼
Ms Mf
þ As Af 2
;
Where Ms, Mf, As and Af are the martensitic and austenitic start and finish temperatures, respectively. The temperature hysteresis of the MT, DTm, was calculated using the following formula:
DTM ¼
Ms Af
þ As Mf 2
:
The thin films with a 100-nm thickness show the smallest width of transformation (W ¼ 20.5 K), while the largest transformation width of W ¼ 30 K, was observed in the 200-nm films. The opposite results can be seen for the temperature hysteresis, DTM; these values decreased with the increasing thickness. The highest value of DTM ¼ 8.5 K, was observed for the 100-nm film; the lowest value of DTM ¼ 3.5 K, was observed in the 1- and 2-mm films. Dependence of the phase transition temperature on film thickness is shown in Fig. 2(f). In comparison with results obtained by Chernenko et al. for similar Ni-Mn-Ga films, our results demonstrate behaviors similar to the Ni49.5Mn28.0Ga22.5 films, with reference to increase of MT temperatures up to 1-mm films and decrease in the thicker films [12]. However, it should be noted that increase of the MT temperatures in correlation to increasing film thickness, is significantly higher in the studied films, showing a value of 52 K (Ms(100nm)Ms(1mm)) versus 30 K reported in Ref. [12]. Aforementioned results are presented in Table 1. The increase of Curie temperature and MT temperatures, along with the increase of film thickness, can be attributed to growing internal stresses. The positive shift of phase transitions temperatures was also observed by Kamarad et al., in off-stoichiometric Ni-
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Fig. 1. (a) X-Ray diffraction patterns collected at room temperature. As-deposited (AD) film with a 1-mm thickness was used to demonstrate the absence of crystal structure. (b) Enlarged patterns in a 2q range of 42e47. Red dotted lines indicate the difference between (220)a and (202)m peaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. (aee) ZFC-FC curves measured in a magnetic field of 10 Oe together with resistivity curves; (f) phase transitions temperatures as a function of the film thickness. As, Af and Ms, Mf are martensitic and austenitic starting and finishing temperatures, respectively.
Mn-Ga alloys when applying hydrostatic pressure [16]. The change in MT temperatures under these pressures achieves the value of dTM/dp ¼ 6 K/GPa for Ni53.75Mn21.25Ga25, and remains almost unchanged in the stoichiometric Ni2MnGa alloy. Curie temperature changes were observed in both cases, with a rate of dTC/dp ¼ 4.7 K/ GPa and 5.9 K/Gpa for Ni53.75Mn21.25Ga25 and Ni2MnGa alloys, correspondingly. It should be noted that the effect of pressure on magnetic properties differs significantly depending on the Ni
amount and substituted element [17], and can be explained by the increase of exchange interactions with the decreasing of interatomic distances [18]. 3.3. Static magnetic properties In-plane and out-of-plane hysteresis loops were measured at room temperature. For the 100- and 200-nm films, hysteresis loops
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Fig. 3. In-plane (a) and out-of-plane (b) low-field part of hysteresis loops measured at room temperature for the films with differing thicknesses. (Hysteresis loops measured in magnetic fields of ±11 kOe are shown in the insets).
looked similar, replicating the shape of each other (Fig. 3). Similar behavior was also observed in the 1- and 2-mm films, with a minor change in the slope of the loop. Hysteresis loop of the film with the thickness of 400-nm took an intermediate position between the 100e200-nm and 1e2-mm loops. Here it should be noted that angle dependence shows the absence of in-plane anisotropy for all films. Coercive force, HC, increases with the increase of thickness from 64 Oe in the 100-nm film to 100 Oe in the 2-mm film. The widening of the in-plane hysteresis loops was observed, which causes an increase of coercive force HC. The blue dashed line in Fig. 3(a) is a guide for the eye, which shows the possible magnetization process for the 400-nm film. The value of the magnetic field in which this effect vanishes, becomes higher with an increase in film thickness. For example, a low magnetic field of 180 Oe is required to suppress the widening of the 100-nm Ni-MnGa films, whereas 240 Oe is required for the 2-mm films. Such an effect has also been observed in numerous other experiments, as a result of applied compressive stresses to the material [16,17,19,20]. By using the values of HС , derived from the out-of-plane hysteresis loops (Fig. 3(b)), and comparing them with values from the in-plane curves, the coercive force difference, DHС, related to the internal stresses in plane of the film, and film thickness was determined. Larger values of DHС relates to the larger internal stresses. Blue dashed lines shown for the 400-nm films (Fig. 3(a)), indicate a possible direction for the magnetization process, in the absence of stresses. The coercive force change (DHC ¼ 6 Oe) is slight for the 100-nm films, and rises significantly with the increase in the film thicknesses, up to 48 Oe for the 1-mm film. This difference indicates that the highest internal stress is present in the 1-mm film. Chernenko et al. also demonstrated this similar behavior of in-plane stresses for Ni-Mn-Ga/Al2O3 films, in thicknesses ranging from 100- nm up to 5-mm, of which varied from 0.2 GPa to 0.32 GPa, correspondingly. It was explained by relaxation mechanisms available in these films due to a high surface roughness of alumina ceramics [8]. Miyagi et al. observed hysteresis loop widening in laminated steel, resulting in a DHС ¼ 180 Oe, under the compressive stress of 0.15 GPa [20]. The same behaviors were observed by Zhu et al., when using micromagnetic modeling for Ni film as a reference object [21], in addition to Han et al. when using NiFe/NiO bilayer thin films [22]. The origin of compressive stresses in polycrystalline films could also be related to the slow deposition rate (0.3 nm/s), high diffusivity and recrystallization during annealing [23]. All obtained values of HС and DHС are presented in Table 2.
3.4. Morphology and magnetic domain structure The surface morphology and magnetic domain structure were investigated using an atomic force microscope in tapping mode with CoCr-coated commercial tips. Measurements were made on the samples in a remanence magnetization state. Fig. 4(aee) shows the correlation between surface structure and magnetic domain structure for the films with differing thicknesses. The microstructure of the films with 100- and 200-nm thickness is polycrystalline, with a large crystallite size distribution. The minimal size of crystallites is d ¼ (250 ± 20) nm, and the maximum size is d ¼ (1100 ± 90) nm. Small grains, with d ¼ (100 ± 10) nm, can also be visible at the surface of these films. The origin of these grains could be attributed to the growth-mode feature, or could be related to a thin oxide layer on the surface. The surface of the 400-nm film consists of non-spherical faceted crystallites, with a size range of 300 nm ÷ 1.3 mm. On the 1-mm films one can see the similar lineament, but with a strong twinning effect. At the thickness of 2-mm, the twinning effect is still present, but the grain boundaries become indistinguishable. Magnetic domain structure, presented and summarized in Fig. 4(f), demonstrates alternating bright and dark regions, which correspond to the magnetization directed up and down to the film's surface. One can associate it with the formation of the (220) texture, where martensitic twin boundaries are situated parallel to the film plane or inclined by some angle to it, as was obtained previously by Chernenko et al. for the similar films [15]. The interconnection between the domain and surface structures can be observed in Ni-Mn-Ga films with thicknesses ranging from 100- to 400-nm. Alternating magnetic domain patterns can be observed, which are different for each separate crystallite. This relationship becomes blurred in the 1- and 2-mm films, where a maze-like domain structure is formed. The width of the domains (d) increases with the thickness from 270 to 890 nm in the 100-nm and 2-mm films, respectively. Obtained dependence has a nonlinear character, and can be related to both the growth method and the different types of internal stresses in the films themselves.
4. Summary In summary, polycrystalline Ni54Mn18Ga28 films with excess Ni and Ga and with thicknesses ranging from 100-nm to 2-mm were deposited on alumina ceramic substrates. Evolution of the
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Table 1 First- and second-order phase transition temperatures derived from the thermomagnetic curves, M(T), and resistivity curves, R(T). Ms,f and As, f are the martensitic and austenitic start and finish temperatures, respectively. TC and TM denotes Curie temperature and equilibrium MT temperature, respectively. DTM is temperature hysteresis of MT and W is the width of transformation. t, nm
M(T)
R(T)
Ms, K
Mf, K
As, K
Af, K
Ms, K
Mf, K
As, K
Af, K
100 200 400 1000 2000
331 e 338 e e
311 323 311 352 353
319 328 316 353 351
334 e 339 e e
328 347 350 380 375
308 317 322 357 351
316 324 327 359 352
337 354 356 385 381
Table 2 Coercive force HC and its difference, DHC, for in-plane and out-of-plane hysteresis loops. t, nm
100 200 400 1000 2000
DHC, Oe
HC, Oe in-plane
out-of-plane
65 73 87 95 100
59 54 58 47 57
6 19 29 48 43
TC, K
TM, K
DTM, K
W, K
350 344 353 365 361
322 335 339 370 365
8.5 7 5.5 3.5 3.5
20.5 30 28.5 24.5 26.5
structural, surface and bulk magnetic properties and MT transformation were shown. MT occurs above room temperature for all films. Curie temperature was found to be above room temperature as well, which overlapped with the MT temperatures for film thicknesses ranging from 200-nm ‒ 2-mm. The non-linear growth of phase transition temperatures was established up to the 1-mm film. Ni-Mn-Ga film with a 2-mm thickness shows a small decrease in phase transition temperatures, that could be related to the internal stress relaxation present in the films with a thickness of more than 1-mm. The
Fig. 4. Surface morphology (left) and corresponding magnetic domains image (right) for thin films with 100-nm (a), 200-nm (b), 400-nm (c), 1-mm (d) and 2-mm (e) thicknesses. Figure (f) shows the visual evolution of domain size in relation to thickness, as (g) shows a graphical dependence of domain width on film thickness.
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evidence of the internal stress influence was also found in the inplane hysteresis loops, where the growth of the coercive force was observed. Magnetic domain structure indicates the presence of perpendicular magnetic anisotropy at the surface of the films, which can be connected with the martensitic twin boundaries situated parallel to the film plane or inclined by some angle to it. A correlation between domain and the surface structure was observed in Ni-MnGa films with a thickness ranging from 100- to 400-nm; in turn, the 1- and 2-mm films had this relationship blurred. The width of magnetic domains increases with the thickness increase, showing non-linear dependence. Acknowledgments Helpful discussions with Prof. V.A. Chernenko are greatly acknowledged. VK acknowledges grant No.16-42-02035, provided by the Russian Science Foundation (for measurements of transport properties). Part of this work received financial support from the Act 211, in partnership with the Government of the Russian Federation, contract No.02.A03.21.0011 and is partially supported by 5 top 100 Russian Academic Excellence Project at the Immanuel Kant Baltic Federal University. VR acknowledges financial support from the Ministry of Education and Science of the Russian Federation in the framework of Government assignments 3.9002.2017/ 6.7. References €hler, M. Kohl, Freely movable ferromagnetic shape [1] M. Schmitt, A. Backen, S. Fa memory nanostructures for actuation, Microelectron. Eng. 98 (2012) 536e539, https://doi.org/10.1016/j.mee.2012.07.045. [2] M. Schmitt, A. Backen, S. Fahler, M. Kohl, Development of ferromagnetic shape memory nanoactuators, Proc. IEEE Conf. Nanotechnol. (2012), https://doi.org/ 10.1109/NANO.2012.6322098. €derberg, S.-P. Hannula, High-cycle fatigue of [3] I. Aaltio, A. Soroka, Y. Ge, O. So 10M NieMneGa magnetic shape memory alloy in reversed mechanical loading, Smart Mater. Struct. 19 (2010), 075014, https://doi.org/10.1088/ 0964-1726/19/7/075014. [4] O. Heczko, Magnetic shape memory effect and magnetization reversal, J. Magn. Magn. Mater. 290e291 PA (2005) 787e794, https://doi.org/10.1016/ j.jmmm.2004.11.397. [5] R.C. O'Handley, Model for strain and magnetization in magnetic shapememory alloys, J. Appl. Phys. 83 (1998) 3263e3270, https://doi.org/10.1063/ 1.367094. [6] F. Albertini, L. Morellon, P.A. Algarabel, M.R. Ibarra, L. Pareti, Z. Arnold, G. Calestani, Magnetoelastic effects and magnetic anisotropy in Ni2MnGa polycrystals, J. Appl. Phys. 89 (2001) 5614e5617, https://doi.org/10.1063/ 1.1350630. nchez-Alarcos, J.I. Pe rez-Landaza bal, V. Recarte, J. a Rodríguez[7] V. Sa n, V. a Chernenko, Effect of atomic order on the martensitic and Velamaza magnetic transformations in NieMneGa ferromagnetic shape memory alloys, J. Phys. Condens. Matter 22 (2010), 166001, https://doi.org/10.1088/0953-
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