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Induced magnetic anisotropy and strain in permalloy films deposited under magnetic field
Thin Solid Films 520 (2012) 5981–5984
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Induced magnetic anisotropy and strain in permalloy films deposited under magnetic field Eon Byeong Park a, b, Sung-Uk Jang a,⁎, Ji-Hong Kim a, Soon-Ju Kwon a,⁎ a b
Department of Materials Science and Engineeing, POSTECH, Pohang, Gyeongbuck 790-784, Republic of Korea New Metal Research Department, Research Institute of Industrial Science & Technnology, Pohang, Gyeongbuck 790-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 August 2011 Received in revised form 11 April 2012 Accepted 27 April 2012 Available online 4 May 2012 Keywords: Magnetic anisotropy Grazing Incidence X-ray Diffraction Permalloy Magnetic field Electron beam evaporation
a b s t r a c t Field induced magnetic-anisotropy is a very important but poorly understood property. There have been many hypotheses on the origin of the phenomenon, e.g. strain, atomic pair ordering, etc., but little experimental evidence exist. This study prepares 100 nm thick Permalloy films having the field-inducedmagnetic-anisotropy and carefully measure strains, i.e. interplanar distance of crystallographic (111) planes in various directions, using high power synchrotron radiation and precise Grazing Incidence X-Ray Diffraction method. The result delineates that the field-induced-magnetic-anisotropy has a strong correlation with the strain-anisotropy in the film. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Magnetic anisotropy is an important fundamental property of a magnetic material in many applications, including information storage devices, sensors [1] and Magnetoresistive Random Access Memory [2], etc. Out of many types of magnetic anisotropies (such as crystalline, shape, surface, exchange, and induced), induced magnetic anisotropy is very practical and important one, because a manufacturing process heavily affects the property. There has been known various means to induce magnetic anisotropy in a film; such as applying magnetic field during deposition, annealing and/or quenching [3,4], oblique angle incidence of evaporated atoms to the substrate [5,6]. Despite such extensive efforts to create and to take the advantage of the property, however, not much is understood on the atomic scale structural origin with clear experimental evidences, as pointed out by O'Handley [7]. Several hypotheses suggest that the origins of the induced magnetic anisotropy might be strain, atomic pair ordering, or shape irregularity. Also efforts have been put to detect any evidence of the origins, such as atomic pair ordering using linearly polarized Extended X-ray Absorption Fine Structure [8,9] and theoretical calculation of anisotropy constants [10]. On the other hand, as emphasized by Kittel [11], strain (though the magnitude is very small) has never negligible effects on the magnetization of a material. However, the effect of atomic scale strain has been hardly investigated experimentally.
⁎ Corresponding authors. Tel.: + 82 542792137; fax: + 82 542796629. E-mail addresses: [email protected] (S.-U. Jang), [email protected] (S.-J. Kwon) 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.04.077
Understanding the phenomenon requires a very careful crystal structure analysis of the material having induced magnetic anisotropy. Because the saturation magnetostriction coefficient, i.e., the magnitude of strain (λ = δL/L) at saturation magnetization, is order of 10 − 5–10 − 6, which is only measurable with a high precision instrument. Furthermore, the tiny scattering volume of a thin film requires a high power signal and a very sensitive detection system, which is not available with conventional instrumentation. However, the tremendous precision and power of current synchrotron radiation facility enable us to revisit the hypotheses on the origin of induced magnetic anisotropy in a film. In this paper, Grazing Incidence X-ray Diffraction (GIXRD) [12,13], using synchrotron X-ray as the source, studies atomic scale structure in Permalloy films with field induced anisotropy during deposition. An appropriate setting of the measurement system provides information on the atomic scale interplanar spacing between the planes, which are perpendicular to the film surface. Relationship between the displacement and the induced anisotropy is examined. 2. Experimental details Disk-shape Permalloy films of 100 nm thickness were grown on thermally oxidized SiO2/Si substrates using an electron-beam evaporator system. One has to eliminate the effect of shape anisotropy. This can be achievable with a disk shape sample, which is easy to make using a mask with a circular hole of 8 mm diameter. The composition of alloy target was 80% nickel (99.999) and 20% iron (99.999). A magnetic anisotropy in the film was induced by applying magnetic field of 0, 0.025, or 0.085 T during film deposition parallel to the substrate. The
E.B. Park et al. / Thin Solid Films 520 (2012) 5981–5984
base pressure was lower than 6.7 × 10− 5 Pa and the distance from the sample to the target was 0.7 m. Deposition rate was 0.3 Å/s (calibrated using a quartz microbalance). Some of the samples were post-annealed for 3 min at 150, 300 or 450 °C in argon atmosphere using a Rapid Thermal Annealing. To characterize induced anisotropy in the film, magnetic hysteresis curves were measured at every 10° step from ϕ = 0° to ϕ = 360° using a Vibrating Sample Magnetometer(Lakeshore's model 7300) with sweeping field between +/−1 × 10− 3 T, which was sufficient to fully magnetize the very soft magnetic Permalloy film. The crystallographic structure in the film was characterized by GIXRD, setup at Pohang Light Source (PLS) 10C1 beam line in Korea. After monochromatized by the fixed-exit type double crystal monochromator using Si(111) crystals, the photon beam is focused both horizontally and vertically by a toroidal mirror. The source (λ=1.5420 Å) was used as incident beam. After placing the deposited film on the stage with heater then aligns the beam and rotates the film to be χ=90°. χ is the angle between the rotated film plane and the horizontal plane as shown in the Fig. 1. At this point, the incident beam and detector must be parallel to each other. After that, beam half cut was carried out through the ThetaX scan within −2°–2°. The angle of the incident X-ray beam was μ=1°, and enables consistent θ–2θ scan by imply rotating the sample and the detector. This setup provides diffraction pattern of the planes, which are perpendicular (in reality 1° inclined from the perpendicular) to the substrate surface. Diffraction patterns were obtained for ϕ=0, 30, 60, and 90° under identical conditions (In this case ϕ is the angle between the applied field direction and the normal of planes constructing the diffraction patterns). Inter-planar spacing, d(111), along each direction of measurement (ϕ) was determined by fitting the relevant diffraction pattern with Pseudo-Voigt1 function. For comparison reason, d(111) normal to the substrate, i.e. distance between the (111) planes parallel to the film surface, was also measured by conventional θ–2θ scan geometry. The measurement provides precision of 10− 5 nm and 5 effective digits of interplanar spacing (e.g. 0.12345 nm). Fig. 1 shows the direction of measurements (magnetic and strain) with respect to the direction of applied magnetic field during deposition. ϕ is the angle between the direction of measurement and the direction of applied field during deposition (ϕ=0°) and ϕ increases in the counter-clockwise direction.
3. Results Magnetic hysteresis curves are shown in Fig. 2. The sample deposited at Happ = 0 T has almost identical curve shape in all directions
: 0 T, φ=0º : 0.085 T, φ =0º : 0.085 T, φ =90º
0.5
0.0
-0.5
-1.0 -800
-600
-400
-200
0
200
400
600
800
Applied field (A/m) Fig. 2. Hysteresis curves of Permalloy films deposited Happ = 0 T and Happ = 0.085 T.
(solid square), i.e. isotropic. Whereas, samples deposited at Happ ≠ 0 T indeed show anisotropic hysteresis curves. Typical curves are shown in Fig.2 (open square) taken from the sample deposited at Happ = 0.085 T. Along the direction of ϕ = 0° (parallel to the applied field direction during deposition), the sample has very high initial permeability and almost perfect squareness (Mr/Ms = 0.999–1), extremely narrow switching field distribution) with coercivity (Hc) of about 278.6 A/m. On the other hand, the measurement in ϕ = 90° (open circle) shows sheared-over linear hysteresis loop with much reduced Hc b79.6 A/m. It is clear that a strong uniaxial anisotropy has been developed along the applied field direction, which is well known as ‘field induced anisotropy’ during film deposition [7]. Such a set of hysteresis curves implies that the magnetic domain walls are mostly 180° parallel to the applied field direction [14]. And we also confirmed a uniaxial anisotropy behavior from torque curves obtained by rotating the sample from the axis of easy magnetization. GIXRD pattern of Fe20Ni80 film deposited without magnetic field is shown in Fig.3. (111) plane is the preferred orientation in the direction parallel to the film surface because the relative intensity ratio of (200) to (111) peaks is 0.268. But there is no the preferred orientation in the direction perpendicular to the film surface, because the intensities from π-scan are similar to each other. Fig. 4 compares the d(111) of the samples. The as-deposited film at Happ = 0 T has d(111) of 2.0586 Å along all the directions parallel to the film surface, whereas
=90° (111)
film surface // H (applied field)
1.0
Magnetization (M/Ms)
5982
(200)
Intensity (a. u.)
0 T, as-deposited
39
42
45
48
51
54
2θ (deg.) Fig. 1. Schematic diagram for GIXRD measurement.
Fig. 3. GIXRD pattern of Fe20Ni80 film deposited without magnetic field.
E.B. Park et al. / Thin Solid Films 520 (2012) 5981–5984
of perpendicular component. (Detailed field distribution analysis had not been carried yet.)
2.0610
(c) H=0.085 T, As-deposited 2.0605
4. Discussion
(b) H=0.025 T, As-deposited 2.0600 2.0595 2.0590
(a) H=0 T,
As-deposited
2.0585
d(111) (Å)
2.0580
d(111) d(111)along along the direction thefield field direction
by 450 annealing
2.0575 2.0570 2.0565
(d) 450
Annealed
2.0560
d(111) along the film normal direction H=0 T, As-deposited
2.0430
H=0.025 T, As-deposited
2.0425
H=0.085 T, As-deposited
2.0420 2.0415
0
10
20
30
40
5983
50
60
70
80
90
Φ (deg.) Fig. 4. Variations of d(111) spacing plotted as a function of the ϕ angle in GIXRD. (a) Happ = 0 T, as deposited, (b) Happ = 0.025 T, as deposited, (c) Happ = 0.085 T, as deposited and (d) after annealing at 450 °C.
2.0429 Å along the surface normal. This implies that the crystal expands (or contracts) in surface parallel (or normal) direction even without applied field. Though the origin of this expansion (or contraction) is not understood, constraint can be developed by the presence of residual gas pressure and the condition of the substrate, and the temperature of the substrate during evaporation, according to the explanation of Siegle [15] who said that stress is induced by such constraint. Therefore, we can understand that the value increased larger than the d(111) value after annealing by the stress generated during deposition. Also, Siegle explained that the strain, imparted on the thin film during deposition, can be eliminated by annealing and the deposition-induced anisotropic stress is related to the induced magnetic anisotropy. Notice that when the field increase to 0.025 and 0.085 T, d(111) along the field direction (ϕ = 0° on dot and dash lines) elongates to 2.0600 Å and 2.0606 Å from 2.0586 Å (case of Happ = 0 T), whereas d(111) along the film normal direction contracts to 2.0426 Å and 2.0420 Å from 2.0430 Å (case of Happ = 0 T), respectively. In the as-deposited films, the elongation of d(111) decreases as the plane normal deviates from the field direction, i.e. increase of ϕ. However, d(111)'s at ϕ = 90° (distance along perpendicular both to the field and the film normal) are still larger than that of the film made at Happ = 0 T. This raises a suspicion that the actual magnetic field vector might have a certain magnitude
Fig.5 is the polar plot of coercivity (Hc, in-plane measured at every ϕ = 10° step) to display variations of above directional characteristics due to post-annealing. The film deposited at Happ = 0 T sustains its isotropic character when the post-annealing temperature is lower than 300 °C. The film deposited at Happ = 0.085 T, as well, sustains its anisotropy up to the same temperature (300 °C), but suddenly change to isotropic at 450 °C with 2 times higher Hc. Above variation of field-induced-strain of d(111) in as-deposited films matches with the field-induced-magnetic-anisotropy in Fig. 5. In the as-deposited film at Happ = 0 T (or ≠ 0), the elongations are isotropic (or anisotropic) for d(111)'s along the directions parallel to the film surface. So are the isotropic (or anisotropic) coercivities. And the coercivity increases with the magnitude of lattice elongation and contraction. After post-annealing at 450 °C, the anisotropy of fieldinduced-strain disappears (Fig.5). So is the field-induced-magneticanisotropy [15]. Important point is that this field-induced-strain is not relevant to magnetostriction. Because the film has composition of Ni:Fe ~ 80:20, where the anisotropy constant (K1) and the magnetostriction coefficient (λ) vanish simultaneously (Bozorth 1993; Hall 1960). To verify the cause of the formation of induced magnetic anisotropy, we simulated two dimensional atoms ordering in Fe25Ni75 composition similar to our composition where 64 atoms comprising 16 and 48 Fe and Ni, respectively, were aligned (a) in random ordering, (b), (c) and (d) in directional pair ordering. When the direction of pair ordering is vertical, alignment (a) has 3.6% of Fe\Fe pairs, 46.4% of Ni\Ni pairs and 50% of Fe\Ni pairs along the vertical axis. Alignment (b) has 14.3% of Fe\Fe pairs, 57.1% of Ni\Ni pairs and 28.6% of Fe\Ni pairs along the vertical axis. Alignment (c) has 21.4% of Fe\Fe pairs, 64.3% of Ni\Ni pairs and 14.3% of Fe\Ni pairs along the vertical axis. Alignment (d) has 23.2% of Fe\Fe pairs, 66.1% of Ni\Ni pairs and 10.7% of Fe\Ni pairs along the vertical axis. Given that in Fe22.5Ni77.5 alloy, bond lengths of Fe\Fe, Ni\Ni and Fe\Ni are 2.5620 Å, 2.5328 Å and 2.5210 Å respectively [16], the strain((L −l0)/l) of (a) model(random ordering) is 6.35×10− 5, the strain of (b) model is 1.61×10− 3, the strain of (c) model is 2.94×10− 3, and the strain of (d) model is 3.47×10− 3. Strain by magnetostriction was around 10− 5–10− 6 and the strain calculated from deposition with magnetic field (case of Happ =0.085 T) was 1.359×10− 4. Therefore, it can be concluded that the induced magnetic anisotropy was formed by field-induced-strain rather than by directional pair ordering. The unexpected transition to isotropy with high Hc by postannealing at 450 °C seems to be closely related to the grain size. Fig. 6 plots the grain size deduced by applying Scherrer's equation to the width of XRD peaks. At below 300 °C, the grain size remains at 8–13 nm, but the 450 °C post-annealing suddenly coarsens the grain size to 30–40 nm. Reportedly, the coercivity of the Permalloy thin film increases with the grain size in nanometer region, which is smaller than the ferromagnetic exchange interaction length of about 270 nm [17,18]. Also, the results of Masahiro's work [19] show that coercivity increases greatly according to the growth of the crystal grain when the Permalloy is annealed at 400 °C, which coincides with the experimental results of Kao [20] saying annealing-induced transition occurs in the 350–400 °C of the Permalloy thin film and coercivity is significantly increased by recrystallization and grain growth. These results indicate that the magnetic anisotropy disappeared because the directional property control in the thin film completely disappeared by recrystallization. Another interesting feature in Fig. 5 is that post-annealing (at 150 °C or 300 °C) the sample with induced anisotropy (deposited at Happ = 0.085 T) further increases Hc along ϕ = 0° at the expense of
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E.B. Park et al. / Thin Solid Films 520 (2012) 5981–5984
90 600
120
0 T, as-deposited
60
0 T, 150 °C 0.085 T, as-deposited
400
0.085 T, 150 °C
30
150
Coercivity (A/m)
0.085 T 450 °C
200
0 180
0
200 330
210 400
600
240
300 270
Fig. 5. Polar plot of coercivity as a function of the ϕ angle.
Hc in the other direction. This implies that the post-annealing sharpens the pre-existing induced uniaxial anisotropy, which is very similar to the self-magnetic-annealing. However, any long range diffusion process (e.g. atomic ordering) should be hardly the mechanism, because the post-annealing had been performed only for 3 min at relatively low temperature (150 °C or 300 °C). But possibility is enhancement of crystallinity and removal of defect.
5. Conclusion In conclusion, GIXRD using high power synchrotron radiation measures interplanar spacing of (111) along various directions in Permalloy films, in which field-induced-magnetic-anisotropy has been developed. The result clearly tells that application of magnetic field in a direction induces strain anisotropy, and this field-inducedstrain-anisotropy has strong relationship with the field-inducedmagnetic-anisotropy in the Permalloy film. The physics of the fieldinduced-strain-anisotropy is not understood at this moment, and requires further research.
0 T in plane 0.085 T in plane 0 T out of plane 0.085 T out of plane
40 35
Grain-Size (nm)
This study made use of the 10C1 beamline at PLS, Korea. The authors gratefully acknowledge Dr. Choi, Dr. Lee and Mr. Ahn at the beamline, for their kind support and comments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
45
30 25 20 15 10 5 0
Acknowledgment
0
50
100 150 200 250 300 350 400 450 500
Annealing Temperature ( C) Fig. 6. Variation of grain as function of post annealing temperature.
J. McCord, J. Westwood, IEEE Trans. Magn. 37 (2001) 1755. J.Y. Son, S.G. Lee, S.H. Lee, S.H. Kim, J.I. Hong, J. Appl. Phys. 110 (2011) 53908. M. Takahashi, J. Appl. Phys. 33 (1962) 1101. S. Chikazumi, J. Phys. Soc. Jpn. 11 (1956) 551. E.I. Kondorsky, P.P. Denisov, IEEE Trans. Magn. MAG-6 (1970) 167. O. Konmoto, Y. Yoshitomi, H. Murakami, IEEE Trans. Magn. 41 (2005) 3434. R.C. O'Handley, Moden Magnetic Materials: Principle and Applications, John Wiley-Interscience, 1999 (ch. 14.). J.C. Cezar, M. Knobel, H.C.N. Tolentino, J. Synchrotron Radiat. 8 (2001) 877. J. Diaz, C. Quiros, L. Zarate, L.M. Alvarez-Prado, S.D. Panfilis, J.M. Alameda, J. Magn. Magn. Mater. 316 (2007) e390. J.H. Min, C.O. Kim, T.D. Lee, H.J. Kim, China Phys. Soc. 16 (2007) 3520. C. Kittel, Introduction to solid state physics, Wiley, 1995 (ch. 15.). M.S. Lee, J.Y. Kim, IEEE Trans. Magn. 43 (2007) 3149. B.D. Fulthorpe, T.P.A. Hase, B.K. Tanner, C.H. Marrows, B.J. Hickey, J. Magn. Magn. Mater. 226–230 (2001) 1733. L.C. Cullity, Introduction to magnetic materials, Addison-Wesley, 1972 (ch. 7.). W.T. Siegle, W.R. Beam, J. Appl. Phys. 36 (1965) 1711. X. Jing, G.E. Ice, C.J. Sparks, L. Robertson, Zschack, Phys. Rev. B 54 (1996) 3211. G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. H.L. Seet, S.P. Li, J.B. Yi, W.Y. Ooi, K.S. Lee, J. Alloys Compd. 449 (2008) 284. K. Masahiro, Y. Kasuhiro, J. Magn. Magn. Mater. 147 (1995) 214. A.S. Kao, P. Kasiraj, IEEE Trans. Magn. 27 (1991) 4452.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Induced magnetic anisotropy and strain in permalloy films deposited under magnetic field Eon Byeong Park a, b, Sung-Uk Jang a,⁎, Ji-Hong Kim a, Soon-Ju Kwon a,⁎ a b
Department of Materials Science and Engineeing, POSTECH, Pohang, Gyeongbuck 790-784, Republic of Korea New Metal Research Department, Research Institute of Industrial Science & Technnology, Pohang, Gyeongbuck 790-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 August 2011 Received in revised form 11 April 2012 Accepted 27 April 2012 Available online 4 May 2012 Keywords: Magnetic anisotropy Grazing Incidence X-ray Diffraction Permalloy Magnetic field Electron beam evaporation
a b s t r a c t Field induced magnetic-anisotropy is a very important but poorly understood property. There have been many hypotheses on the origin of the phenomenon, e.g. strain, atomic pair ordering, etc., but little experimental evidence exist. This study prepares 100 nm thick Permalloy films having the field-inducedmagnetic-anisotropy and carefully measure strains, i.e. interplanar distance of crystallographic (111) planes in various directions, using high power synchrotron radiation and precise Grazing Incidence X-Ray Diffraction method. The result delineates that the field-induced-magnetic-anisotropy has a strong correlation with the strain-anisotropy in the film. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Magnetic anisotropy is an important fundamental property of a magnetic material in many applications, including information storage devices, sensors [1] and Magnetoresistive Random Access Memory [2], etc. Out of many types of magnetic anisotropies (such as crystalline, shape, surface, exchange, and induced), induced magnetic anisotropy is very practical and important one, because a manufacturing process heavily affects the property. There has been known various means to induce magnetic anisotropy in a film; such as applying magnetic field during deposition, annealing and/or quenching [3,4], oblique angle incidence of evaporated atoms to the substrate [5,6]. Despite such extensive efforts to create and to take the advantage of the property, however, not much is understood on the atomic scale structural origin with clear experimental evidences, as pointed out by O'Handley [7]. Several hypotheses suggest that the origins of the induced magnetic anisotropy might be strain, atomic pair ordering, or shape irregularity. Also efforts have been put to detect any evidence of the origins, such as atomic pair ordering using linearly polarized Extended X-ray Absorption Fine Structure [8,9] and theoretical calculation of anisotropy constants [10]. On the other hand, as emphasized by Kittel [11], strain (though the magnitude is very small) has never negligible effects on the magnetization of a material. However, the effect of atomic scale strain has been hardly investigated experimentally.
⁎ Corresponding authors. Tel.: + 82 542792137; fax: + 82 542796629. E-mail addresses: [email protected] (S.-U. Jang), [email protected] (S.-J. Kwon) 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.04.077
Understanding the phenomenon requires a very careful crystal structure analysis of the material having induced magnetic anisotropy. Because the saturation magnetostriction coefficient, i.e., the magnitude of strain (λ = δL/L) at saturation magnetization, is order of 10 − 5–10 − 6, which is only measurable with a high precision instrument. Furthermore, the tiny scattering volume of a thin film requires a high power signal and a very sensitive detection system, which is not available with conventional instrumentation. However, the tremendous precision and power of current synchrotron radiation facility enable us to revisit the hypotheses on the origin of induced magnetic anisotropy in a film. In this paper, Grazing Incidence X-ray Diffraction (GIXRD) [12,13], using synchrotron X-ray as the source, studies atomic scale structure in Permalloy films with field induced anisotropy during deposition. An appropriate setting of the measurement system provides information on the atomic scale interplanar spacing between the planes, which are perpendicular to the film surface. Relationship between the displacement and the induced anisotropy is examined. 2. Experimental details Disk-shape Permalloy films of 100 nm thickness were grown on thermally oxidized SiO2/Si substrates using an electron-beam evaporator system. One has to eliminate the effect of shape anisotropy. This can be achievable with a disk shape sample, which is easy to make using a mask with a circular hole of 8 mm diameter. The composition of alloy target was 80% nickel (99.999) and 20% iron (99.999). A magnetic anisotropy in the film was induced by applying magnetic field of 0, 0.025, or 0.085 T during film deposition parallel to the substrate. The
E.B. Park et al. / Thin Solid Films 520 (2012) 5981–5984
base pressure was lower than 6.7 × 10− 5 Pa and the distance from the sample to the target was 0.7 m. Deposition rate was 0.3 Å/s (calibrated using a quartz microbalance). Some of the samples were post-annealed for 3 min at 150, 300 or 450 °C in argon atmosphere using a Rapid Thermal Annealing. To characterize induced anisotropy in the film, magnetic hysteresis curves were measured at every 10° step from ϕ = 0° to ϕ = 360° using a Vibrating Sample Magnetometer(Lakeshore's model 7300) with sweeping field between +/−1 × 10− 3 T, which was sufficient to fully magnetize the very soft magnetic Permalloy film. The crystallographic structure in the film was characterized by GIXRD, setup at Pohang Light Source (PLS) 10C1 beam line in Korea. After monochromatized by the fixed-exit type double crystal monochromator using Si(111) crystals, the photon beam is focused both horizontally and vertically by a toroidal mirror. The source (λ=1.5420 Å) was used as incident beam. After placing the deposited film on the stage with heater then aligns the beam and rotates the film to be χ=90°. χ is the angle between the rotated film plane and the horizontal plane as shown in the Fig. 1. At this point, the incident beam and detector must be parallel to each other. After that, beam half cut was carried out through the ThetaX scan within −2°–2°. The angle of the incident X-ray beam was μ=1°, and enables consistent θ–2θ scan by imply rotating the sample and the detector. This setup provides diffraction pattern of the planes, which are perpendicular (in reality 1° inclined from the perpendicular) to the substrate surface. Diffraction patterns were obtained for ϕ=0, 30, 60, and 90° under identical conditions (In this case ϕ is the angle between the applied field direction and the normal of planes constructing the diffraction patterns). Inter-planar spacing, d(111), along each direction of measurement (ϕ) was determined by fitting the relevant diffraction pattern with Pseudo-Voigt1 function. For comparison reason, d(111) normal to the substrate, i.e. distance between the (111) planes parallel to the film surface, was also measured by conventional θ–2θ scan geometry. The measurement provides precision of 10− 5 nm and 5 effective digits of interplanar spacing (e.g. 0.12345 nm). Fig. 1 shows the direction of measurements (magnetic and strain) with respect to the direction of applied magnetic field during deposition. ϕ is the angle between the direction of measurement and the direction of applied field during deposition (ϕ=0°) and ϕ increases in the counter-clockwise direction.
3. Results Magnetic hysteresis curves are shown in Fig. 2. The sample deposited at Happ = 0 T has almost identical curve shape in all directions
: 0 T, φ=0º : 0.085 T, φ =0º : 0.085 T, φ =90º
0.5
0.0
-0.5
-1.0 -800
-600
-400
-200
0
200
400
600
800
Applied field (A/m) Fig. 2. Hysteresis curves of Permalloy films deposited Happ = 0 T and Happ = 0.085 T.
(solid square), i.e. isotropic. Whereas, samples deposited at Happ ≠ 0 T indeed show anisotropic hysteresis curves. Typical curves are shown in Fig.2 (open square) taken from the sample deposited at Happ = 0.085 T. Along the direction of ϕ = 0° (parallel to the applied field direction during deposition), the sample has very high initial permeability and almost perfect squareness (Mr/Ms = 0.999–1), extremely narrow switching field distribution) with coercivity (Hc) of about 278.6 A/m. On the other hand, the measurement in ϕ = 90° (open circle) shows sheared-over linear hysteresis loop with much reduced Hc b79.6 A/m. It is clear that a strong uniaxial anisotropy has been developed along the applied field direction, which is well known as ‘field induced anisotropy’ during film deposition [7]. Such a set of hysteresis curves implies that the magnetic domain walls are mostly 180° parallel to the applied field direction [14]. And we also confirmed a uniaxial anisotropy behavior from torque curves obtained by rotating the sample from the axis of easy magnetization. GIXRD pattern of Fe20Ni80 film deposited without magnetic field is shown in Fig.3. (111) plane is the preferred orientation in the direction parallel to the film surface because the relative intensity ratio of (200) to (111) peaks is 0.268. But there is no the preferred orientation in the direction perpendicular to the film surface, because the intensities from π-scan are similar to each other. Fig. 4 compares the d(111) of the samples. The as-deposited film at Happ = 0 T has d(111) of 2.0586 Å along all the directions parallel to the film surface, whereas
=90° (111)
film surface // H (applied field)
1.0
Magnetization (M/Ms)
5982
(200)
Intensity (a. u.)
0 T, as-deposited
39
42
45
48
51
54
2θ (deg.) Fig. 1. Schematic diagram for GIXRD measurement.
Fig. 3. GIXRD pattern of Fe20Ni80 film deposited without magnetic field.
E.B. Park et al. / Thin Solid Films 520 (2012) 5981–5984
of perpendicular component. (Detailed field distribution analysis had not been carried yet.)
2.0610
(c) H=0.085 T, As-deposited 2.0605
4. Discussion
(b) H=0.025 T, As-deposited 2.0600 2.0595 2.0590
(a) H=0 T,
As-deposited
2.0585
d(111) (Å)
2.0580
d(111) d(111)along along the direction thefield field direction
by 450 annealing
2.0575 2.0570 2.0565
(d) 450
Annealed
2.0560
d(111) along the film normal direction H=0 T, As-deposited
2.0430
H=0.025 T, As-deposited
2.0425
H=0.085 T, As-deposited
2.0420 2.0415
0
10
20
30
40
5983
50
60
70
80
90
Φ (deg.) Fig. 4. Variations of d(111) spacing plotted as a function of the ϕ angle in GIXRD. (a) Happ = 0 T, as deposited, (b) Happ = 0.025 T, as deposited, (c) Happ = 0.085 T, as deposited and (d) after annealing at 450 °C.
2.0429 Å along the surface normal. This implies that the crystal expands (or contracts) in surface parallel (or normal) direction even without applied field. Though the origin of this expansion (or contraction) is not understood, constraint can be developed by the presence of residual gas pressure and the condition of the substrate, and the temperature of the substrate during evaporation, according to the explanation of Siegle [15] who said that stress is induced by such constraint. Therefore, we can understand that the value increased larger than the d(111) value after annealing by the stress generated during deposition. Also, Siegle explained that the strain, imparted on the thin film during deposition, can be eliminated by annealing and the deposition-induced anisotropic stress is related to the induced magnetic anisotropy. Notice that when the field increase to 0.025 and 0.085 T, d(111) along the field direction (ϕ = 0° on dot and dash lines) elongates to 2.0600 Å and 2.0606 Å from 2.0586 Å (case of Happ = 0 T), whereas d(111) along the film normal direction contracts to 2.0426 Å and 2.0420 Å from 2.0430 Å (case of Happ = 0 T), respectively. In the as-deposited films, the elongation of d(111) decreases as the plane normal deviates from the field direction, i.e. increase of ϕ. However, d(111)'s at ϕ = 90° (distance along perpendicular both to the field and the film normal) are still larger than that of the film made at Happ = 0 T. This raises a suspicion that the actual magnetic field vector might have a certain magnitude
Fig.5 is the polar plot of coercivity (Hc, in-plane measured at every ϕ = 10° step) to display variations of above directional characteristics due to post-annealing. The film deposited at Happ = 0 T sustains its isotropic character when the post-annealing temperature is lower than 300 °C. The film deposited at Happ = 0.085 T, as well, sustains its anisotropy up to the same temperature (300 °C), but suddenly change to isotropic at 450 °C with 2 times higher Hc. Above variation of field-induced-strain of d(111) in as-deposited films matches with the field-induced-magnetic-anisotropy in Fig. 5. In the as-deposited film at Happ = 0 T (or ≠ 0), the elongations are isotropic (or anisotropic) for d(111)'s along the directions parallel to the film surface. So are the isotropic (or anisotropic) coercivities. And the coercivity increases with the magnitude of lattice elongation and contraction. After post-annealing at 450 °C, the anisotropy of fieldinduced-strain disappears (Fig.5). So is the field-induced-magneticanisotropy [15]. Important point is that this field-induced-strain is not relevant to magnetostriction. Because the film has composition of Ni:Fe ~ 80:20, where the anisotropy constant (K1) and the magnetostriction coefficient (λ) vanish simultaneously (Bozorth 1993; Hall 1960). To verify the cause of the formation of induced magnetic anisotropy, we simulated two dimensional atoms ordering in Fe25Ni75 composition similar to our composition where 64 atoms comprising 16 and 48 Fe and Ni, respectively, were aligned (a) in random ordering, (b), (c) and (d) in directional pair ordering. When the direction of pair ordering is vertical, alignment (a) has 3.6% of Fe\Fe pairs, 46.4% of Ni\Ni pairs and 50% of Fe\Ni pairs along the vertical axis. Alignment (b) has 14.3% of Fe\Fe pairs, 57.1% of Ni\Ni pairs and 28.6% of Fe\Ni pairs along the vertical axis. Alignment (c) has 21.4% of Fe\Fe pairs, 64.3% of Ni\Ni pairs and 14.3% of Fe\Ni pairs along the vertical axis. Alignment (d) has 23.2% of Fe\Fe pairs, 66.1% of Ni\Ni pairs and 10.7% of Fe\Ni pairs along the vertical axis. Given that in Fe22.5Ni77.5 alloy, bond lengths of Fe\Fe, Ni\Ni and Fe\Ni are 2.5620 Å, 2.5328 Å and 2.5210 Å respectively [16], the strain((L −l0)/l) of (a) model(random ordering) is 6.35×10− 5, the strain of (b) model is 1.61×10− 3, the strain of (c) model is 2.94×10− 3, and the strain of (d) model is 3.47×10− 3. Strain by magnetostriction was around 10− 5–10− 6 and the strain calculated from deposition with magnetic field (case of Happ =0.085 T) was 1.359×10− 4. Therefore, it can be concluded that the induced magnetic anisotropy was formed by field-induced-strain rather than by directional pair ordering. The unexpected transition to isotropy with high Hc by postannealing at 450 °C seems to be closely related to the grain size. Fig. 6 plots the grain size deduced by applying Scherrer's equation to the width of XRD peaks. At below 300 °C, the grain size remains at 8–13 nm, but the 450 °C post-annealing suddenly coarsens the grain size to 30–40 nm. Reportedly, the coercivity of the Permalloy thin film increases with the grain size in nanometer region, which is smaller than the ferromagnetic exchange interaction length of about 270 nm [17,18]. Also, the results of Masahiro's work [19] show that coercivity increases greatly according to the growth of the crystal grain when the Permalloy is annealed at 400 °C, which coincides with the experimental results of Kao [20] saying annealing-induced transition occurs in the 350–400 °C of the Permalloy thin film and coercivity is significantly increased by recrystallization and grain growth. These results indicate that the magnetic anisotropy disappeared because the directional property control in the thin film completely disappeared by recrystallization. Another interesting feature in Fig. 5 is that post-annealing (at 150 °C or 300 °C) the sample with induced anisotropy (deposited at Happ = 0.085 T) further increases Hc along ϕ = 0° at the expense of
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Fig. 5. Polar plot of coercivity as a function of the ϕ angle.
Hc in the other direction. This implies that the post-annealing sharpens the pre-existing induced uniaxial anisotropy, which is very similar to the self-magnetic-annealing. However, any long range diffusion process (e.g. atomic ordering) should be hardly the mechanism, because the post-annealing had been performed only for 3 min at relatively low temperature (150 °C or 300 °C). But possibility is enhancement of crystallinity and removal of defect.
5. Conclusion In conclusion, GIXRD using high power synchrotron radiation measures interplanar spacing of (111) along various directions in Permalloy films, in which field-induced-magnetic-anisotropy has been developed. The result clearly tells that application of magnetic field in a direction induces strain anisotropy, and this field-inducedstrain-anisotropy has strong relationship with the field-inducedmagnetic-anisotropy in the Permalloy film. The physics of the fieldinduced-strain-anisotropy is not understood at this moment, and requires further research.
0 T in plane 0.085 T in plane 0 T out of plane 0.085 T out of plane
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This study made use of the 10C1 beamline at PLS, Korea. The authors gratefully acknowledge Dr. Choi, Dr. Lee and Mr. Ahn at the beamline, for their kind support and comments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Annealing Temperature ( C) Fig. 6. Variation of grain as function of post annealing temperature.
J. McCord, J. Westwood, IEEE Trans. Magn. 37 (2001) 1755. J.Y. Son, S.G. Lee, S.H. Lee, S.H. Kim, J.I. Hong, J. Appl. Phys. 110 (2011) 53908. M. Takahashi, J. Appl. Phys. 33 (1962) 1101. S. Chikazumi, J. Phys. Soc. Jpn. 11 (1956) 551. E.I. Kondorsky, P.P. Denisov, IEEE Trans. Magn. MAG-6 (1970) 167. O. Konmoto, Y. Yoshitomi, H. Murakami, IEEE Trans. Magn. 41 (2005) 3434. R.C. O'Handley, Moden Magnetic Materials: Principle and Applications, John Wiley-Interscience, 1999 (ch. 14.). J.C. Cezar, M. Knobel, H.C.N. Tolentino, J. Synchrotron Radiat. 8 (2001) 877. J. Diaz, C. Quiros, L. Zarate, L.M. Alvarez-Prado, S.D. Panfilis, J.M. Alameda, J. Magn. Magn. Mater. 316 (2007) e390. J.H. Min, C.O. Kim, T.D. Lee, H.J. Kim, China Phys. Soc. 16 (2007) 3520. C. Kittel, Introduction to solid state physics, Wiley, 1995 (ch. 15.). M.S. Lee, J.Y. Kim, IEEE Trans. Magn. 43 (2007) 3149. B.D. Fulthorpe, T.P.A. Hase, B.K. Tanner, C.H. Marrows, B.J. Hickey, J. Magn. Magn. Mater. 226–230 (2001) 1733. L.C. Cullity, Introduction to magnetic materials, Addison-Wesley, 1972 (ch. 7.). W.T. Siegle, W.R. Beam, J. Appl. Phys. 36 (1965) 1711. X. Jing, G.E. Ice, C.J. Sparks, L. Robertson, Zschack, Phys. Rev. B 54 (1996) 3211. G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. H.L. Seet, S.P. Li, J.B. Yi, W.Y. Ooi, K.S. Lee, J. Alloys Compd. 449 (2008) 284. K. Masahiro, Y. Kasuhiro, J. Magn. Magn. Mater. 147 (1995) 214. A.S. Kao, P. Kasiraj, IEEE Trans. Magn. 27 (1991) 4452.