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Thickness dependence of surface roughness and magnetic properties of FeNiCr thin films
Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Thickness dependence of surface roughness and magnetic properties of FeNiCr thin films Yu Cao n, Chungen Zhou School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
a r t i c l e i n f o
abstract
Article history: Received 10 August 2012 Received in revised form 14 December 2012 Available online 31 December 2012
In order to apply FeNiCr ternary alloy thin films for magnetic machine-readable devices, the thickness effects on the surface roughness and magnetic properties of these films have been studied. The surface roughness of FeNiCr increased from 0.77 nm to a maximum value of 2.16 nm as the film thickness was increased from 33 to 103 nm, and then decreased to 1.75 nm as the film thickness was further increased to 189 nm. The coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. The in-plane coercivity of FeNiCr increased from 27.3 Oe to a maximum value of 60.1 Oe as the film thickness was increased from 33 to 103 nm, and then decreased to 10.4 Oe as the film thickness was further increased to 189 nm. & 2012 Elsevier B.V. All rights reserved.
Keywords: Thickness Surface roughness Magnetic property
1. Introduction The study of the magnetic thin films is one of the key features in magnetism today because of its enormous possibility of new functionalities being incorporated with other materials. Those functionalities include but not limited to magnetic sensors, ultrahigh-density magnetic storage media, spin polarized transistors and many more [1–4]. The FeNiCr ternary alloy system constitutes many commercially important materials such as stainless steels and the H-series and C-series heat- and corrosion-resistant casting alloys. This paper lays importance in the preparation of thin films of FeNiCr alloys by a web magnetron sputtering technique. The focus is on FeNiCr thin films to their magnetic properties. In this work, we studied surface roughness and magnetic properties as a function of the FeNiCr thin film thickness. The evolution of the surface with various film thicknesses was studied using atomic force microscopy. The in-plane magnetic properties of FeNiCr thin films were measured by VSM. The coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. It was found the film thickness of FeNiCr thin film has a significant influence on the surface roughness and magnetic properties such as coercivity, anisotropy and squareness. The effect of surface roughness on the magnetic and electrical properties such as coercivity, magnetic domain structure, and magnetoresistance has always been a very intriguing subject in the field of thin film magnetism [5]. A few years ago, in-plane surface roughness modulation had been successfully used to trap magnetic domain walls in thin Fe films [6]. From a technological
n
Corresponding author. Tel.: þ86 10 58055359; fax: þ86 10 58055601. E-mail address: [email protected] (Y. Cao).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.042
Fig. 1. The relationship between the film thickness and the applied power.
Table 1 Film thickness and composition of different layers obtained from XRF analysis. Power (kW)
Thickness (nm)
Fe (at%)
Ni (at%)
Cr (at%)
0.64 1.27 1.94 2.64 3.36
33 63 103 146 189
56.2 56.0 55.2 55.0 53.9
36.4 36.7 37.3 37.5 38.2
7.4 7.3 7.5 7.5 7.9
2
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
Fig. 2. AFM 3D images showing different surface roughness of FeNiCr thin films with different thickness: (a) 0 nm, (b) 33 nm, (c) 63 nm, (d) 103 nm, (e) 146 nm, and (f) 189 nm.
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
point of view, surface roughness effects are of importance because they strongly influence the way in which thin films can be applied for magnetic recording and magneto-electronics. Various works on the relationship between surface roughness and coercivity, of thin and ultrathin films, have been carried out. For examples, Li et al. [7] investigated the effect of surface roughness on magnetic properties of Co films on plasma etched Si(1 0 0) substrates. It was shown that the coercivity of Co films strongly correlated with the surface roughness parameter and increased with the etch time. Malyutin et al [8] investigated the effect of surface roughness on the coercivity of chemically-etched NiFeCo films (20 to 100 nm thick). The coercivity of the film increases with the increase of etch time. During the etching, the thickness of the magnetic film decreases and the surface roughness increases with etch time. Vilain et al [9] investigated the dependence of coercivity on the surface roughness for electrodeposited NiCo alloy films. For a 2-mm-thick film with 11% Ni, the coercivity increases from 37 to ˚ 100 Oe as the surface roughness increased from 200 to 350 A. As previously reported for various systems [10], the coercivity increased with increasing surface roughness. In this paper we present a systematic study of the influence of surface roughness on the magnetic properties of FeNiCr thin films of various film thicknesses. Attempts are made to correlate the observed magnetic properties with thin films thickness. Understanding the relationships between surface roughness and magnetic properties, especially the coercivity mechanism becomes an important issue in the process of optimizing the media.
2. Experiment A series of FeNiCr thin films were prepared by a roll coater and direct current (DC) magnetron sputtering system which was reported in our previous paper [11]onto a polyethylene terephthalate substrate (PET, 19 mm thick). PET polymer web was wound and rewound through a main roll with a constant tension. The element composition of FeNiCr alloy target was design as Fe54Ni38Cr8 (atomic%). The FeNiCr thin films were deposited onto PET substrate by using different strength of dc power (0.64, 1.27, 1.94, 2.64, 3.36 kW) to the target while the PET substrate rolling speed was kept constant (0.5 m/min). Besides varying thickness of the FeNiCr thin films, other deposition conditions of the samples were kept constant. Argon was used as the sputtering gas. The sputtering pressure was 0.6 Pa, and the base pressure was below 8.6 10 4 Pa. The substrate was water cooled during deposition, because high substrate temperatures (4100 1C) would damage or deform the PET substrates [12]. The composition and thicknesses of these films were measured by X-ray fluorescence (XRF). The root mean square (rms) roughness and grain sizes were measured by scanning a 1 1 mm area with atomic force microscope (AFM). Magnetic properties were measured by using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 300 Oe.
increasing, the Fe content slightly decreases with increase of film thickness, and Ni content slightly increases with increase of film thickness. The relationship between the film thickness and Fe, Ni and Cr content was listed in Table 1. The compositional changes slightly, which occur as film thickness increases for DC power increase, may be due to the difference of the affinity of sputtering atoms to the substrate at an initial deposition stage [13]. As the film thickness increasing, the composition ratio was closed to the composition of FeNiCr alloy target.
3.1.2. Surface roughness The AFM images (a–f) in Fig. 2 show different surface roughness of FeNiCr thin films with different thickness on PET substrates induced by different dc sputtering power applied to the substrate. We observed marked changes in the surface roughness as the film thickness was increased from 33 to 189 nm. The best known parameter in characterizing the morphology of a surface is the root mean square (rms) roughness [14]. The rms roughness of FeNiCr thin films as a function of film thickness were plotted in Fig. 3. According to the data obtained from AFM images, the surface roughness of FeNiCr increased from 0.77 nm to a maximum value of 2.16 nm as the film thickness was increased from 33 to 103 nm, and then decreased to 1.75 nm as the film thickness was further increased to 189 nm. It was found that the root mean square (rms) roughness increased sharply with the film thickness at thinner stage, but showed a slightly decreased at thicker stage. The growth of nano structures will be the resultant of the competition between roughening by self shadowing and the smoothening due to adatom surface diffusion. During the initial stages of growth, the microstructure is strongly affected by shadowing. The change in roughness is implying that a smoothening mechanism by surface diffusion. These results suggest that during initial stages of growth, self shadowing is dominating and as the film thickness increases surface diffusion is also playing an active role in the growth process [15–17]. The variation of surface roughness was also attributed to the change of sputtering conditions resulting from the dc sputtering power. As dc sputtering power increasing, the more energetic sputtered atoms thus migrate faster on the substrate surface, that is to say the surface mobility of sputtered atoms is increased. The increased atom mobility on the substrate encourages a more even deposition and decreases the surface roughness [18].
3. Result and discussion 3.1. Composition and structure 3.1.1. Composition The FeNiCr thin film thickness increases from 33 to 189 nm as the strength of dc power applied from 0.64 to 3.36 kW. The relationship between the film thickness and the applied power was showed in Fig. 1. A perfect linear relationship with R¼9.99 10 1 can be seen in Fig. 1. As the film thickness
3
Fig. 3. DC power effect on surface roughness RMS of FeNiCr thin films.
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Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
Fig. 4. AFM height sensor images showing different grains of FeNiCr thin films with different thickness: (a) 0 nm, (b) 33 nm, (c) 63 nm, (d) 103 nm, (e) 146 nm, and (f) 189 nm.
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
5
3.1.3. Grain size To understand the relationship between the grain size and FeNiCr film thickness, the grain sizes were measured and analyzed by scanning a 1 1 mm area with atomic force microscope (AFM). Separate AFM measurements within a surface area of 1 1 mm indicated that at this stage of the film growth, the films are not completely continuous and contain small isolated grains. Round caps of grains are seen clearly in Fig. 4, based on which their sizes could be estimated. As shown in Fig. 2, with increasing thickness, small grains appear and grow in the form of laterally expanding conical columns. Fig. 5 shows that there is a clear relation between grain size and film thickness, grain size increased from 25.7 to 30.6 nm as FeNiCr film thickness increased from 33 to 189 nm. The grain size of the FeNiCr film increased with increasing thickness, and grain growth is obvious in the thicker FeNiCr films as the thickness is more than 100 nm. Fig. 6. Coercivity (Hc) and surface roughness rms with different thickness.
3.2. Magnetic properties 3.2.1. Coercivity In order to investigate the relationship between the surface roughness and the magnetic properties, FeNiCr films with different thickness were prepared with different sputtering power. The in-plane magnetic properties of FeNiCr thin films were measured by using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 300 Oe. Magnetic hysteresis loops of the FeNiCr films were measured at room temperature in a magnetic field (H) applied parallel to the film plane. The coercivity and rms roughness of FeNiCr thin films as a function of film thickness were plotted in Fig. 6. According to the data obtained from VSM, the in-plane coercivity of FeNiCr increased from 27.3 Oe to a maximum value of 60.1 Oe as the film thickness was increased from 33 to 103 nm, and then decreased to 10.4 Oe as the film thickness was further increased to 189 nm. As shown in Fig. 6, the coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. A maximum value in coercivity with 103 nm film thickness was achieved. The change of coercivity of FeNiCr films was attributed to the changes in surface roughness. Following Soohoo and Middelhoek, the coercivity Hc for a single rough boundary and a straight wall moving parallel to itself were treated as [19–21]. 1 @EW EW Hc ¼ þ rrms ; 2M @d d
Fig. 5. Variation of the average grain size of FeNiCr thin film as a function of the film thickness.
( )ð1=2Þ Z E ð2pÞ4 Q c 2 D 2 rrms ¼ k 9hðkÞ9 dk A Qd where the domain energy is EW and the thickness fluctuations are ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rD E rh2 , Q c ¼ p=a0 represented by the local surface slope rrms ¼ with a0 being the order of an atomic spacing, and Q d ¼ 2p=d with d being the domain wall width. For a Neel wall, EW ¼ Aex p2 =d K in d=2 þ pM 2 dd=ðd þ dÞ, Aex ¼ JS2 =ae is the exchange constant (J ¼155 K and ae is an atomic length scale), and S the average spin ( o0.65 for d o10 nm). K in ¼ K v þ 2K s =d is the inplane anisotropy constant with K v and K s being the in-plane volume and surface anisotropy constants, respectively. These results show a close relationship between the microstructure of the film surface and the coercivity. Therefore, it is considered that the surface roughness affects largely the coercivity of FeNiCr thin films. This indicates that surface roughness and thin film growth mechanisms can have strong influence on magnetic properties of thin films. 3.2.2. Anisotropy The direction of easy magnetization is known to be an important factor for the magnetic thin films [22], and hence, the change in anisotropy is investigated as a function of the film thickness. The distribution of magnetic anisotropy is investigated by examining hysteresis loops measured in the in-plane of thin films. Fig. 7(a–e) showed the in-plane magnetic hysteresis loops of FeNiCr films with different film thickness. Both loops have been measured using a VSM. During the measurement process, the external field up to 300 Oe was parallel with the film plane in rolling direction (RD) and transverse direction (TD) of the PET substrate, respectively. There is a strong variation in the shape of the hysteresis curve with film thickness. The loops become broader as film thickness increased, and then become narrower as film thicker than 146 nm. No noticeable anisotropy is observed at film thickness thinner than 63 nm, and no obvious difference being noted in the shape of hysteresis loops measured in the in-plane rolling direction (RD) and transverse direction (TD). The magnetization curves show good magnetic squareness (Mr/Ms) for the samples deposited with higher power. From Fig. 7d and e, a square loop can be seen when the applied field is oriented along rolling direction of the film plane, which indicates that the easy magnetization direction is of easy-plane
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Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
Fig. 7. Magnetic hysteresis loops for different thickness: (a) 33 nm, (b) 63 nm, (c) 103 nm, (d) 146 nm, and (e) 189 nm.
type. It is indicated that the film increasingly possess a uniaxial magnetic anisotropy as film thickness increasing. The squareness (Mr/Ms) of the hysteresis loop with different film thickness was showed in Fig. 8. The squareness increases abruptly with increasing film thickness and tends to be saturated at a film thickness about 100 nm.
4. Conclusions In this work, we studied surface roughness and magnetic properties as a function of the FeNiCr thin film thickness. The evolution of the surface with various film thicknesses was studied using atomic force microscopy. The in-plane magnetic properties of FeNiCr thin films were measured by VSM. The coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. It was found the film thickness of FeNiCr thin film has a significant influence on the surface roughness and magnetic properties such as coercivity, anisotropy and squareness.
Fig. 8. Variation of the squareness (Mr/Ms) of FeNiCr thin film as a function of the thickness.
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
References [1] S.A. Haque, A. Matsuo, Y. Yamamoto, et al., Enhancement of coercivity of Ni/GaAs(001) and its thickness dependency [J], Physica B: Condensed Matter 325 (2003) 259–264. [2] J.M. Daughton, GMR and SDT sensor applications [J], IEEE Transactions on Magnetics 36 (2000) 2773–2778. [3] J.A.C. Bland, A. Hirohata, Yong-Bing Xu, et al., Spin-polarized electron transport processes at the ferromagnet/semiconductor interface [J], IEEE Transactions on Magnetics 36 (5) (2000) 2827–2831. ¨ [4] Peter Grunberg, Layered magnetic structures: history, highlights, applications [J], Physics Today 54 (5) (2001) 31. [5] J. Swerts, S. Vandezande, K. Temst, et al., Surface roughness effects on the magnetization reversal of polycrystalline Fe/Ag thin films [J], Solid State Communications 131 (2004) 359–363. [6] J. Swerts, K. Temst, M. Van Bael, et al., Magnetic domain wall trapping by in-plane surface roughness modulation [J], Applied Physics Letters 82 (8) (2003) 1239–1241. [7] M. Li, G.-C. Wang, H.-G. Min, Effect of surface roughness on magnetic properties of Co films on plasmaetched Si(100) substrates [J], Journal of Applied Physics 83 (1998) 5313–5320. [8] V.I. Malyutin, V.E. Osukhovskii, D. Vorobiev, et al., Structure and magnetic properties of etched NiFeCo flims [J], Physica Status Solidi A 65 (1) (1981) 45–52. [9] S. Vilain, J. Ebothe, M. Troyon, Surface roughness and composition effects on the magnetic properties of electradeposited NiCo alloys [J], Journal of Magnetism and Magnetic Materials 157 (1996) 274–275. [10] J. Swerts, K. Temst, N. Vandamme, et al., Correlation between structural and magnetic properties of thin FexCo1 x(110) films on sapphire [J], Thin Solid Films 413 (2002) 212–217. [11] Yu Cao, Chungen Zhou, Magnetic anisotropy properties of CoZrNb thin films deposited on PET substrate by magnetron sputtering [J], Journal of Magnetism and Magnetic Materials 324 (2012) 1832–1836.
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[12] T. Osaka, M. Takai, K. Hayashi, et al., New soft magnetic CoNiFe plated films with high Bs¼ 2.1–2.1T [J], IEEE Transactions on Magnetics 34 (1998) 1432–1434. [13] Y.M. Kim, S.H. Han, H.J. Kim, et al., Thickness effects on magnetic properties and ferromagnetic resonance in Co–Ni–Fe–N soft magnetic thin films [J], Journal of applied physics 91 (2002) 8462–8464. [14] Senoy Thomas, Fabrication of Thin Films and Nano Columnar Structures of Fe–Ni Morphous Alloys and Modification of its Surface Properties by Hermal Annealing and Swift Heavy Ion Irradiation for Tailoring the Magnetic Properties[D] (2009). [15] A. Yanguas-Gil, J. Cotrino, A. Walkiewicz-Pietrzykowska, et al., Scaling behavior and mechanism of formation of SiO2 thin films grown by plasmaenhanced chemical vapor deposition [J], Physical Review B: Condensed Matter 76 (2007) 075314. [16] A. Borra´s, A. Yanguas-Gil, A. Barranco, et al., Relationship between scaling behavior and porosity of plasma-deposited TiO2 thin films [J], Physical Review B: Condensed Matter 76 (2007) 235303. [17] M.A. Auger, L. Va´zquez, O. Sa´nchez, et al., Growth dynamics of reactivesputtering-deposited AlN films [J], Journal of Applied Physics 97 (2005) 123528. [18] V. Ng, J.F. Hu, A.O. Adeyeye, Factors affecting surface roughness and coercivity of Ni80Fe20 thin films [J], Journal of Applied Physics 91 (10) (2002) 7026–7028. [19] G. Palasantzas, Y.P. Zhao, G.C. Wang, et al., Roughness effects on magnetic properties of thin films [J], Physica B: Condensed Matter 283 (2000) 199–202. [20] R.F. Soohoo, Influence of particle interaction on coercivity and squareness of thin film recording media [J], Journal of Applied Physics 52 (3) (1981) 2459–2461. [21] George Palasantzas, Static and dynamic aspects of the rms local slope of growing random surfaces [J], Physical Review E: Statistical, Nonlinear, and Soft Matter Physics 56 (1997) 1254–1257. [22] S.H. Han, H.J. Kim, S.H. Lim, et al., Magnetic properties of amorphous Sm–Fe and Sm–Fe–B thin films fabricated by radio-frequency magnetron sputtering [J], Journal of Applied Physics 83 (11) (1998) 7270–7272.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Thickness dependence of surface roughness and magnetic properties of FeNiCr thin films Yu Cao n, Chungen Zhou School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
a r t i c l e i n f o
abstract
Article history: Received 10 August 2012 Received in revised form 14 December 2012 Available online 31 December 2012
In order to apply FeNiCr ternary alloy thin films for magnetic machine-readable devices, the thickness effects on the surface roughness and magnetic properties of these films have been studied. The surface roughness of FeNiCr increased from 0.77 nm to a maximum value of 2.16 nm as the film thickness was increased from 33 to 103 nm, and then decreased to 1.75 nm as the film thickness was further increased to 189 nm. The coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. The in-plane coercivity of FeNiCr increased from 27.3 Oe to a maximum value of 60.1 Oe as the film thickness was increased from 33 to 103 nm, and then decreased to 10.4 Oe as the film thickness was further increased to 189 nm. & 2012 Elsevier B.V. All rights reserved.
Keywords: Thickness Surface roughness Magnetic property
1. Introduction The study of the magnetic thin films is one of the key features in magnetism today because of its enormous possibility of new functionalities being incorporated with other materials. Those functionalities include but not limited to magnetic sensors, ultrahigh-density magnetic storage media, spin polarized transistors and many more [1–4]. The FeNiCr ternary alloy system constitutes many commercially important materials such as stainless steels and the H-series and C-series heat- and corrosion-resistant casting alloys. This paper lays importance in the preparation of thin films of FeNiCr alloys by a web magnetron sputtering technique. The focus is on FeNiCr thin films to their magnetic properties. In this work, we studied surface roughness and magnetic properties as a function of the FeNiCr thin film thickness. The evolution of the surface with various film thicknesses was studied using atomic force microscopy. The in-plane magnetic properties of FeNiCr thin films were measured by VSM. The coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. It was found the film thickness of FeNiCr thin film has a significant influence on the surface roughness and magnetic properties such as coercivity, anisotropy and squareness. The effect of surface roughness on the magnetic and electrical properties such as coercivity, magnetic domain structure, and magnetoresistance has always been a very intriguing subject in the field of thin film magnetism [5]. A few years ago, in-plane surface roughness modulation had been successfully used to trap magnetic domain walls in thin Fe films [6]. From a technological
n
Corresponding author. Tel.: þ86 10 58055359; fax: þ86 10 58055601. E-mail address: [email protected] (Y. Cao).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.12.042
Fig. 1. The relationship between the film thickness and the applied power.
Table 1 Film thickness and composition of different layers obtained from XRF analysis. Power (kW)
Thickness (nm)
Fe (at%)
Ni (at%)
Cr (at%)
0.64 1.27 1.94 2.64 3.36
33 63 103 146 189
56.2 56.0 55.2 55.0 53.9
36.4 36.7 37.3 37.5 38.2
7.4 7.3 7.5 7.5 7.9
2
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
Fig. 2. AFM 3D images showing different surface roughness of FeNiCr thin films with different thickness: (a) 0 nm, (b) 33 nm, (c) 63 nm, (d) 103 nm, (e) 146 nm, and (f) 189 nm.
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
point of view, surface roughness effects are of importance because they strongly influence the way in which thin films can be applied for magnetic recording and magneto-electronics. Various works on the relationship between surface roughness and coercivity, of thin and ultrathin films, have been carried out. For examples, Li et al. [7] investigated the effect of surface roughness on magnetic properties of Co films on plasma etched Si(1 0 0) substrates. It was shown that the coercivity of Co films strongly correlated with the surface roughness parameter and increased with the etch time. Malyutin et al [8] investigated the effect of surface roughness on the coercivity of chemically-etched NiFeCo films (20 to 100 nm thick). The coercivity of the film increases with the increase of etch time. During the etching, the thickness of the magnetic film decreases and the surface roughness increases with etch time. Vilain et al [9] investigated the dependence of coercivity on the surface roughness for electrodeposited NiCo alloy films. For a 2-mm-thick film with 11% Ni, the coercivity increases from 37 to ˚ 100 Oe as the surface roughness increased from 200 to 350 A. As previously reported for various systems [10], the coercivity increased with increasing surface roughness. In this paper we present a systematic study of the influence of surface roughness on the magnetic properties of FeNiCr thin films of various film thicknesses. Attempts are made to correlate the observed magnetic properties with thin films thickness. Understanding the relationships between surface roughness and magnetic properties, especially the coercivity mechanism becomes an important issue in the process of optimizing the media.
2. Experiment A series of FeNiCr thin films were prepared by a roll coater and direct current (DC) magnetron sputtering system which was reported in our previous paper [11]onto a polyethylene terephthalate substrate (PET, 19 mm thick). PET polymer web was wound and rewound through a main roll with a constant tension. The element composition of FeNiCr alloy target was design as Fe54Ni38Cr8 (atomic%). The FeNiCr thin films were deposited onto PET substrate by using different strength of dc power (0.64, 1.27, 1.94, 2.64, 3.36 kW) to the target while the PET substrate rolling speed was kept constant (0.5 m/min). Besides varying thickness of the FeNiCr thin films, other deposition conditions of the samples were kept constant. Argon was used as the sputtering gas. The sputtering pressure was 0.6 Pa, and the base pressure was below 8.6 10 4 Pa. The substrate was water cooled during deposition, because high substrate temperatures (4100 1C) would damage or deform the PET substrates [12]. The composition and thicknesses of these films were measured by X-ray fluorescence (XRF). The root mean square (rms) roughness and grain sizes were measured by scanning a 1 1 mm area with atomic force microscope (AFM). Magnetic properties were measured by using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 300 Oe.
increasing, the Fe content slightly decreases with increase of film thickness, and Ni content slightly increases with increase of film thickness. The relationship between the film thickness and Fe, Ni and Cr content was listed in Table 1. The compositional changes slightly, which occur as film thickness increases for DC power increase, may be due to the difference of the affinity of sputtering atoms to the substrate at an initial deposition stage [13]. As the film thickness increasing, the composition ratio was closed to the composition of FeNiCr alloy target.
3.1.2. Surface roughness The AFM images (a–f) in Fig. 2 show different surface roughness of FeNiCr thin films with different thickness on PET substrates induced by different dc sputtering power applied to the substrate. We observed marked changes in the surface roughness as the film thickness was increased from 33 to 189 nm. The best known parameter in characterizing the morphology of a surface is the root mean square (rms) roughness [14]. The rms roughness of FeNiCr thin films as a function of film thickness were plotted in Fig. 3. According to the data obtained from AFM images, the surface roughness of FeNiCr increased from 0.77 nm to a maximum value of 2.16 nm as the film thickness was increased from 33 to 103 nm, and then decreased to 1.75 nm as the film thickness was further increased to 189 nm. It was found that the root mean square (rms) roughness increased sharply with the film thickness at thinner stage, but showed a slightly decreased at thicker stage. The growth of nano structures will be the resultant of the competition between roughening by self shadowing and the smoothening due to adatom surface diffusion. During the initial stages of growth, the microstructure is strongly affected by shadowing. The change in roughness is implying that a smoothening mechanism by surface diffusion. These results suggest that during initial stages of growth, self shadowing is dominating and as the film thickness increases surface diffusion is also playing an active role in the growth process [15–17]. The variation of surface roughness was also attributed to the change of sputtering conditions resulting from the dc sputtering power. As dc sputtering power increasing, the more energetic sputtered atoms thus migrate faster on the substrate surface, that is to say the surface mobility of sputtered atoms is increased. The increased atom mobility on the substrate encourages a more even deposition and decreases the surface roughness [18].
3. Result and discussion 3.1. Composition and structure 3.1.1. Composition The FeNiCr thin film thickness increases from 33 to 189 nm as the strength of dc power applied from 0.64 to 3.36 kW. The relationship between the film thickness and the applied power was showed in Fig. 1. A perfect linear relationship with R¼9.99 10 1 can be seen in Fig. 1. As the film thickness
3
Fig. 3. DC power effect on surface roughness RMS of FeNiCr thin films.
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Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
Fig. 4. AFM height sensor images showing different grains of FeNiCr thin films with different thickness: (a) 0 nm, (b) 33 nm, (c) 63 nm, (d) 103 nm, (e) 146 nm, and (f) 189 nm.
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
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3.1.3. Grain size To understand the relationship between the grain size and FeNiCr film thickness, the grain sizes were measured and analyzed by scanning a 1 1 mm area with atomic force microscope (AFM). Separate AFM measurements within a surface area of 1 1 mm indicated that at this stage of the film growth, the films are not completely continuous and contain small isolated grains. Round caps of grains are seen clearly in Fig. 4, based on which their sizes could be estimated. As shown in Fig. 2, with increasing thickness, small grains appear and grow in the form of laterally expanding conical columns. Fig. 5 shows that there is a clear relation between grain size and film thickness, grain size increased from 25.7 to 30.6 nm as FeNiCr film thickness increased from 33 to 189 nm. The grain size of the FeNiCr film increased with increasing thickness, and grain growth is obvious in the thicker FeNiCr films as the thickness is more than 100 nm. Fig. 6. Coercivity (Hc) and surface roughness rms with different thickness.
3.2. Magnetic properties 3.2.1. Coercivity In order to investigate the relationship between the surface roughness and the magnetic properties, FeNiCr films with different thickness were prepared with different sputtering power. The in-plane magnetic properties of FeNiCr thin films were measured by using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 300 Oe. Magnetic hysteresis loops of the FeNiCr films were measured at room temperature in a magnetic field (H) applied parallel to the film plane. The coercivity and rms roughness of FeNiCr thin films as a function of film thickness were plotted in Fig. 6. According to the data obtained from VSM, the in-plane coercivity of FeNiCr increased from 27.3 Oe to a maximum value of 60.1 Oe as the film thickness was increased from 33 to 103 nm, and then decreased to 10.4 Oe as the film thickness was further increased to 189 nm. As shown in Fig. 6, the coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. A maximum value in coercivity with 103 nm film thickness was achieved. The change of coercivity of FeNiCr films was attributed to the changes in surface roughness. Following Soohoo and Middelhoek, the coercivity Hc for a single rough boundary and a straight wall moving parallel to itself were treated as [19–21]. 1 @EW EW Hc ¼ þ rrms ; 2M @d d
Fig. 5. Variation of the average grain size of FeNiCr thin film as a function of the film thickness.
( )ð1=2Þ Z E ð2pÞ4 Q c 2 D 2 rrms ¼ k 9hðkÞ9 dk A Qd where the domain energy is EW and the thickness fluctuations are ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rD E rh2 , Q c ¼ p=a0 represented by the local surface slope rrms ¼ with a0 being the order of an atomic spacing, and Q d ¼ 2p=d with d being the domain wall width. For a Neel wall, EW ¼ Aex p2 =d K in d=2 þ pM 2 dd=ðd þ dÞ, Aex ¼ JS2 =ae is the exchange constant (J ¼155 K and ae is an atomic length scale), and S the average spin ( o0.65 for d o10 nm). K in ¼ K v þ 2K s =d is the inplane anisotropy constant with K v and K s being the in-plane volume and surface anisotropy constants, respectively. These results show a close relationship between the microstructure of the film surface and the coercivity. Therefore, it is considered that the surface roughness affects largely the coercivity of FeNiCr thin films. This indicates that surface roughness and thin film growth mechanisms can have strong influence on magnetic properties of thin films. 3.2.2. Anisotropy The direction of easy magnetization is known to be an important factor for the magnetic thin films [22], and hence, the change in anisotropy is investigated as a function of the film thickness. The distribution of magnetic anisotropy is investigated by examining hysteresis loops measured in the in-plane of thin films. Fig. 7(a–e) showed the in-plane magnetic hysteresis loops of FeNiCr films with different film thickness. Both loops have been measured using a VSM. During the measurement process, the external field up to 300 Oe was parallel with the film plane in rolling direction (RD) and transverse direction (TD) of the PET substrate, respectively. There is a strong variation in the shape of the hysteresis curve with film thickness. The loops become broader as film thickness increased, and then become narrower as film thicker than 146 nm. No noticeable anisotropy is observed at film thickness thinner than 63 nm, and no obvious difference being noted in the shape of hysteresis loops measured in the in-plane rolling direction (RD) and transverse direction (TD). The magnetization curves show good magnetic squareness (Mr/Ms) for the samples deposited with higher power. From Fig. 7d and e, a square loop can be seen when the applied field is oriented along rolling direction of the film plane, which indicates that the easy magnetization direction is of easy-plane
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Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
Fig. 7. Magnetic hysteresis loops for different thickness: (a) 33 nm, (b) 63 nm, (c) 103 nm, (d) 146 nm, and (e) 189 nm.
type. It is indicated that the film increasingly possess a uniaxial magnetic anisotropy as film thickness increasing. The squareness (Mr/Ms) of the hysteresis loop with different film thickness was showed in Fig. 8. The squareness increases abruptly with increasing film thickness and tends to be saturated at a film thickness about 100 nm.
4. Conclusions In this work, we studied surface roughness and magnetic properties as a function of the FeNiCr thin film thickness. The evolution of the surface with various film thicknesses was studied using atomic force microscopy. The in-plane magnetic properties of FeNiCr thin films were measured by VSM. The coercivity of FeNiCr films changes with the film thickness in a similar way as surface roughness. It was found the film thickness of FeNiCr thin film has a significant influence on the surface roughness and magnetic properties such as coercivity, anisotropy and squareness.
Fig. 8. Variation of the squareness (Mr/Ms) of FeNiCr thin film as a function of the thickness.
Y. Cao, C. Zhou / Journal of Magnetism and Magnetic Materials 333 (2013) 1–7
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