- Email: [email protected]
Coercive field behavior of permalloy antidot arrays based on self-assembled template fabrication
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238 www.elsevier.com/locate/jmmm
Coercive field behavior of permalloy antidot arrays based on self-assembled template fabrication K.R. Pirotaa,, P. Prietoc, A.M.J. Netob,d, J.M. Sanzc, M. Knobelb, M. Vazqueza a
Instituto de Ciencia de Materiales de Madrid—CSIC, Campus Cantoblanco, 28049 Madrid. Spain Instituto de Fı´sica ‘‘Gleb Wataghin’’, Universidade Estadual de Campinas—UNICAMP. C.P. 6165 Campinas, SP, Brazil cc Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid. Campus de Cantoblanco, 28049 Madrid. Spain d Departamento de Fı´sica, Universidad Federal do Para´, Campus universita´rio do Gama, 66075,110 Bele´m_PA. Brasil
b
Available online 29 February 2008
Abstract High-density magnetic antidot arrays have been fabricated by deposition of Fe20Ni80 thin films on self-assembled nanoporous alumina membranes (NAM) with high-order hexagonal symmetry. The magnetic properties induced by the size and the geometry configuration of the holes introduced in a Fe20Ni80 thin film are discussed based on hysteresis loops measured as a function of temperature. The precursor NAMs have pore diameters ranging between 35 and 95 nm (55 and 75 nm after the film deposition) and a lattice parameter of 105 nm. An enormous increase of coercitivity, as compared with the corresponding continuous films, was observed for temperatures between 2 and 300 K. This effect depends on the size and surface density of holes in the Fe20Ni80 antidot arrays. Rutherford backscattering spectrometry (RBS) measurements were performed in order to better clarify the magnetic material that was eventually deposited within the NAM pores. r 2008 Elsevier B.V. All rights reserved. Keywords: Magnetic antidots; Coercive field; Nanostructured materials; Alumina membrane
1. Introduction Recently, theoretical and experimental studies have demonstrated that the magnetic properties of thin films can be controlled by the artificial introduction of small holes on them [1–4]. This procedure results in a magnetic thin film with periodic nonmagnetic inclusions, which is usually referred to as magnetic antidot array. These nanostructures are promising candidates for a new generation of ultra-high-density magnetic storage media, mainly due to the absence of the superparamagnetic limit, once there is no isolated small magnetic entity. The holes introduce shape anisotropies that allow the nucleation and movement of domain walls. In those systems, properties such as magnetoresistance, coercivity, permeability and magnetization reversal can be controlled [1–5]. Most of the works performed on magnetic antidot arrays are on submicron or micron scales and are obtained by the use of e-beam Corresponding author.
E-mail address: [email protected] (K.R. Pirota). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.146
lithography [1–3]. An alternative technique that is being increasingly employed to produce antidot arrays with characteristic sizes around 100 nm makes use of nanoporous alumina membranes (NAM) as a precursor template. In this way, the nanostructure antidot array is obtained by growing magnetic thin films onto the NAM surface, which result in a magnetic antidot nanostructure with hole sizes as small as 20 nm [4,5]. In this work we report the use of NAM to fabricate magnetic antidot arrays, and we discuss the magnetic effect induced by the size and the geometry configuration of the holes introduced in a Fe20Ni80 thin film. We have mainly focused the work on the strong effect produced by the antidots in the coercive fields. We have demonstrated that by controlling the size and the density of holes, the coercive field can be systematically tailored. 2. Experimental The NAMs were fabricated using the so-called two-step anodization process [6] in oxalic acids. The obtained
ARTICLE IN PRESS e236
K.R. Pirota et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238
membranes show a porous diameter of 35 nm and have a distance between them of 105 nm, which can be modified by a heat treatment in phosphoric acid (5% in volume) at 35 1C. Such treatment increases the porous diameter in a rate of 2 nm/min of acid etching, keeping fixed the interporous distance [1]. The magnetic antidots were fabricated by deposition of Fe20Ni80 thin films by ion beam sputtering (using a Fe20Ni80 target) on the upper surface of the membranes, replicating the array of nanoholes of the substrate and resulting in films with an array of antidots. The energy and total current for the sputter ion gun were 500 eV and 10 mA, respectively, giving a deposition rate of 0.3 A˚/s controlled by a quartz crystal monitor. The base pressure before deposition was 2 10 7 Torr. For comparison we have also grown in the same deposition process continuous Fe20Ni80 thin films on Si substrates. During the deposition process, the substrate temperature was kept at 200 1C. The samples were capped with a 3 nm Cu layer to prevent the oxidation of the magnetic thin film and the antidot arrays for ‘‘ex situ’’ characterization. The morphology of the antidot arrays was determined by scanning electron mycroscopy (SEM) and Rutherford backscattering spectrometry (RBS). In the case of RBS experiments, 3.7 MeV 4 He+ ions have been used at 71 from normal incidence to obtain the backscattering depth profiles of the different samples with solid-state detectors in the horizontal plane at 170.51 and 1651. The magnetic properties of the samples were studied in a temperature range between 2 and 300 K using both vibrating sample magnetometers (VSM) and SQUID magnetometers. Magnetization loops were measured with the applied field parallel and perpendicular to the antidot arrays plane. In addition, the angular dependence of the hysteresis loops was measured by an in-plane angular rotation from 01 to 3601, every 101 with a precision of 0.51 in order to determine the magnetic inplane anisotropy distribution. 3. Result and discussion Fig. 1 shows the SEM images of the precursor NAM template with 95 nm pore diameter and 105 nm lattice
parameter (a) and after 80-nm-thick deposition of Fe80Ni20 onto its surface (b). The deposition reduced the antidot pore diameter to about 75 nm. Those images reveal a high hexagonal order degree in the antidot configuration that retains the same shape as the templates. The SEM images also indicate the high uniformity of the pore diameter size. The magnetization curves (not shown), with applied field parallel and perpendicular to the sample plane, for the magnetic continuous thin film and studied antidot arrays clearly show a magnetic anisotropy with an easy plane magnetization. The in-plane remanence and loop squareness are larger for the continuous film and decrease for the antidot arrays. These results agree with previous results obtained by Vovk et al. [5] that states there is a noncollinear spin configuration around the pores that promote incoherent magnetization reversal, which result in reduced remanence. In order to better investigate the in-plane anisotropy, hysteresis loops have been measured by rotating the sample with respect to the direction of the in-plane applied magnetic field. In Fig. 2, one can see the dependence of the coercive field (Hc) as a function of the direction of external in-plane applied magnetic field for the continuous film deposited onto a Si substrate and for the antidot arrays with 25, 55 and 75 nm pore diameter (fixed 105 nm of lattice parameter), respectively. The behavior of the coercive field (obtained from in-plane and out-of-plane hysteresis loops, not shown) indicates that, even for the largest antidot size, the characteristic in-plane anisotropy of the magnetic thin film (that originates from the deposition method) remains in the antidot configurations. Vavassori et al. [3] have found that on permalloy antidot arrays with characteristic sizes on the micrometer range, the anisotropy of the arrays is clearly related to the antidot lattice symmetry. In our case, as is well known, the hexagonal order degree depends on the first anodization time [6]. For longer first anodization process the pores in the NAM are better ordered. All the samples studied in this work are produced with 24 h of first anodization and present similar degrees of order. Further experiments with increasing and decreasing this time are still being performed. As one can see in the figure, the continuous
Fig. 1. SEM images of the NAM template with 95 nm pore diameter and 105 nm lattice parameter (a) and after 80-nm-thick deposition of Fe80Ni20 onto its surface (b).
ARTICLE IN PRESS
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -20 0
6.0 5.8 5.6 5.4
4%
5.2 5.0 4.8 -20 0
20 40 60 80 100 120 140 160 180 200
20 40 60 80 100 120 140 160 180 200
Applied field angle (°)
Applied field angle (°) 11.5
6.8
11.0
6.4 μ0Hc (mT)
20 % μ0Hc (mT)
e237
6.2
μ0Hc (mT)
μ0Hc (mT)
K.R. Pirota et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238
6.0 5.6 5.2
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8.5 0
20
40
60
0
80 100 120 140 160 180
20
40
60
80 100 120 140 160 180
Applied field angle (°)
Applied field angle (°)
Fig. 2. Hc as a function of the direction of external in-plane applied magnetic field for the continuous film (a) and for the antidot arrays with 25 (b), 55 (c) and 75 (d) nm pore diameter.
20 HL easy axes H // easy axes 16 μ0Hc (mT)
film presents a very strong in-plane anisotropy that is modified in the antidot array with 25 nm diameter. An inplane bi-axial-like anisotropy character is recovered for a nanopore diameter of 55 nm (20% of change of Hc between the hard and easy magnetic directions). For the sample with 75 nm pore diameter, an even stronger (42% of change in Hc between the hard and easy directions) almost uniaxial anisotropy is observed. Fig. 3 shows the easy axes coercive field as a function of the parameter 1/(D d), where D is the distance between the centers of the pores (fixed at 105 nm) and d is the precursor NAM pore diameter (ranging between 35 and 95 nm) and D d is the interpore distance. As expected, the coercivity increases as the pore size increases. In this figure, one can see the strong increase of Hc observed in both in-plane and out-of-plane configurations. If one compares the results with the continuous thin film, the Hc increases from 0.45 to 11 mT (in-plane configuration) and from 1.1 to 17.3 mT (out-of-plane configuration). As expected, as the antidots are getting closer, the pinning effects are stronger and the coercivity increases. The observed behavior is consistent with the predicted dependence between coercivity and packing fraction predicted by Hilzinger and Kronmu¨ller [7] and also with the dependence observed between Hc and the interpore distance in other systems of Fe [2] antidots.
12
8
4 0.02
0.04
0.06
0.08
0.10
1/(D-d) (nm-1)
Fig. 3. Easy axes coercive field as a function of the parameter 1/(D d).
The magnetic properties of permalloy antidot arrays obtained with NAM templates with final antidot pore diameters of 25, 35, 55 and 75 nm were also measured over a wide temperature range (2–300 K). The coercive fields of the antidot arrays as a function of the temperature are shown in Fig. 4. The observed behavior at room
ARTICLE IN PRESS K.R. Pirota et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238
e238
Fe
Si 35 nm 95 nm Intensity (a.u.)
15
μ0Ηc (mT)
3000
35nm 55nm 75nm 95nm
12
Ni
2000 Si 1000
9
Al
Cu
6 0
0
100
200
300
600
Temperature (K)
700
900 800 Channel (eV)
1000
1100
Fig. 4. Coercitive fields of the antidot arrays as a function of temperature. The pore diameter values indicated in the figure are those of the original NAM precursor.
Fig. 5. RBS performed for 80-nm-thick FeNi grown on Si (1 0 0), NAM with 35 nm pore size and NAM with 95 nm pore size (25 and 75 nm pore after film deposition).
temperature between the coercive field and the antidot size remain in all the studied temperature range. In addition, from this figure one can see that the coercive field is higher for larger antidot diameter and, for each sample, its value decreases when the temperature increases. Another interesting aspect is that all curves show an abrupt change of increasing slope for temperatures close to 30 K. This effect could be an indication of a presence of superparamagnetic phase in the antidot arrays. As also observed by Luis et al. [8] in Co-deposited clusters, for very low deposition time, the deposited material should be in form of small clusters with room temperature superparamagnetic behavior. We suspect that this effect could be happening with the magnetic material that is deposited within the pores, but further experiments are necessary to clarify this point. Finally, in order to better clarify the characteristic of the magnetic material inside the pores, RBS experiments were performed for 80-nm-thick FeNi grown on (a) Si (1 0 0), (b) antidot array with 25 nm pore size and 105 nm lattice parameter and (c) antidot array with 75 nm pore size and 105 nm of lattice parameter. The results are shown in Fig. 5. The surface energies of the elements are indicated by black arrows. A quantitative analysis of the magnetic thin film grown on the Si substrate indicates that the thickness and composition of the thin film are 80 nm and Fe20Ni80, respectively. In addition, the qualitative analysis of the shape of the Fe+Ni peak on the antidot samples, compared with the thin continuous film, allows us to determine whether the magnetic material was or not deposited inside the pores. The asymmetry of the peak reveals that the size of the antidots should be larger at the pore bottom and gradually decreases toward the top surface, indicating that indeed there is a deposition of material within the pores. This evidence was also observed by Xiao et al. [4], using SEM microscopy on Ni antidot arrays obtained with NAM templates.
4. Conclusions The results shown in this work demonstrate a highly controlled method to produce magnetic antidots and tailor its magnetic response as a function of the antidot geometric parameters. The utilization of self-assembled NAMs as templates for the fabrication of studied nanostructures seems to be a simple and controllable way to grow nanoscale magnetic antidot arrays. The enormous increase observed in the coercive field for of the antidot configuration for all the temperature ranges studied, when compared with the continuous film, together with the absence of the superparamagnetic limit, makes the magnetic nano-scaled antidots arrays very promising for ultra-high-density information storage.
References [1] L.J. Heyderman, F. Nolting, D. Backes, S. Czekaaj, L. Lopez-Diaz, M. Kla¨ui, U. Ru¨diger, C.A.F. vaz, J.A.C. Bland, R.J. Matelon, U.G. Volkmann, P. Fischer, Phys. Rev. B 73 (2006) 214429. [2] I. Ruiz-Feal, L. Lopez-Dı´ az, A. Hirohata, J. Rothman, C.M. Guertler, J.A.C. Bland, L.M. Garcia, J.M. Torres, J. Bartolome, F. Bartolome, M. Natali, D. Decanini, Y. Chen, J. Magn. Magn. Mater. 242–245 (2002) 597. [3] P. Vavassori, G. Gubbiotti, G. Zangani, C.T. Yu, H. Yin, H. Jiang, G.J. Mankey, J. Appl. Phys. 91 (10) (2002) 7992. [4] Z.L. Xiao, C.Y. Han, U. Welp, H.H. wang, V.K. Vlasko-Vlasko, W.K. Kwok, D.J. Millar, J.M. Hiller, R.E. Cook, G.A. Willing, G.W. Crabtree, Appl. Phys. Lett. 81 (15) (2002) 2869. [5] A. Vovk, L. Mlakinski, V. Golub, S. Whittenburg, C. O’ Connor, J.J. Jung, S.H. Min, J. Appl. Phys. 97 (2005) 101506. [6] M. Herna´ndez-Velez, K.R. Pirota, F. Paszti, M. Vazquez, Appl. Phys. A 80 (2005) 1701. [7] H.R. Hilzinger, H. Kronmu¨ller, J. Magn. Magn. Mater. 2 (1976) 11. [8] F. Luis, et al., Europhys. Lett. 76 (2006) 142.
Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238 www.elsevier.com/locate/jmmm
Coercive field behavior of permalloy antidot arrays based on self-assembled template fabrication K.R. Pirotaa,, P. Prietoc, A.M.J. Netob,d, J.M. Sanzc, M. Knobelb, M. Vazqueza a
Instituto de Ciencia de Materiales de Madrid—CSIC, Campus Cantoblanco, 28049 Madrid. Spain Instituto de Fı´sica ‘‘Gleb Wataghin’’, Universidade Estadual de Campinas—UNICAMP. C.P. 6165 Campinas, SP, Brazil cc Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid. Campus de Cantoblanco, 28049 Madrid. Spain d Departamento de Fı´sica, Universidad Federal do Para´, Campus universita´rio do Gama, 66075,110 Bele´m_PA. Brasil
b
Available online 29 February 2008
Abstract High-density magnetic antidot arrays have been fabricated by deposition of Fe20Ni80 thin films on self-assembled nanoporous alumina membranes (NAM) with high-order hexagonal symmetry. The magnetic properties induced by the size and the geometry configuration of the holes introduced in a Fe20Ni80 thin film are discussed based on hysteresis loops measured as a function of temperature. The precursor NAMs have pore diameters ranging between 35 and 95 nm (55 and 75 nm after the film deposition) and a lattice parameter of 105 nm. An enormous increase of coercitivity, as compared with the corresponding continuous films, was observed for temperatures between 2 and 300 K. This effect depends on the size and surface density of holes in the Fe20Ni80 antidot arrays. Rutherford backscattering spectrometry (RBS) measurements were performed in order to better clarify the magnetic material that was eventually deposited within the NAM pores. r 2008 Elsevier B.V. All rights reserved. Keywords: Magnetic antidots; Coercive field; Nanostructured materials; Alumina membrane
1. Introduction Recently, theoretical and experimental studies have demonstrated that the magnetic properties of thin films can be controlled by the artificial introduction of small holes on them [1–4]. This procedure results in a magnetic thin film with periodic nonmagnetic inclusions, which is usually referred to as magnetic antidot array. These nanostructures are promising candidates for a new generation of ultra-high-density magnetic storage media, mainly due to the absence of the superparamagnetic limit, once there is no isolated small magnetic entity. The holes introduce shape anisotropies that allow the nucleation and movement of domain walls. In those systems, properties such as magnetoresistance, coercivity, permeability and magnetization reversal can be controlled [1–5]. Most of the works performed on magnetic antidot arrays are on submicron or micron scales and are obtained by the use of e-beam Corresponding author.
E-mail address: [email protected] (K.R. Pirota). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.02.146
lithography [1–3]. An alternative technique that is being increasingly employed to produce antidot arrays with characteristic sizes around 100 nm makes use of nanoporous alumina membranes (NAM) as a precursor template. In this way, the nanostructure antidot array is obtained by growing magnetic thin films onto the NAM surface, which result in a magnetic antidot nanostructure with hole sizes as small as 20 nm [4,5]. In this work we report the use of NAM to fabricate magnetic antidot arrays, and we discuss the magnetic effect induced by the size and the geometry configuration of the holes introduced in a Fe20Ni80 thin film. We have mainly focused the work on the strong effect produced by the antidots in the coercive fields. We have demonstrated that by controlling the size and the density of holes, the coercive field can be systematically tailored. 2. Experimental The NAMs were fabricated using the so-called two-step anodization process [6] in oxalic acids. The obtained
ARTICLE IN PRESS e236
K.R. Pirota et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238
membranes show a porous diameter of 35 nm and have a distance between them of 105 nm, which can be modified by a heat treatment in phosphoric acid (5% in volume) at 35 1C. Such treatment increases the porous diameter in a rate of 2 nm/min of acid etching, keeping fixed the interporous distance [1]. The magnetic antidots were fabricated by deposition of Fe20Ni80 thin films by ion beam sputtering (using a Fe20Ni80 target) on the upper surface of the membranes, replicating the array of nanoholes of the substrate and resulting in films with an array of antidots. The energy and total current for the sputter ion gun were 500 eV and 10 mA, respectively, giving a deposition rate of 0.3 A˚/s controlled by a quartz crystal monitor. The base pressure before deposition was 2 10 7 Torr. For comparison we have also grown in the same deposition process continuous Fe20Ni80 thin films on Si substrates. During the deposition process, the substrate temperature was kept at 200 1C. The samples were capped with a 3 nm Cu layer to prevent the oxidation of the magnetic thin film and the antidot arrays for ‘‘ex situ’’ characterization. The morphology of the antidot arrays was determined by scanning electron mycroscopy (SEM) and Rutherford backscattering spectrometry (RBS). In the case of RBS experiments, 3.7 MeV 4 He+ ions have been used at 71 from normal incidence to obtain the backscattering depth profiles of the different samples with solid-state detectors in the horizontal plane at 170.51 and 1651. The magnetic properties of the samples were studied in a temperature range between 2 and 300 K using both vibrating sample magnetometers (VSM) and SQUID magnetometers. Magnetization loops were measured with the applied field parallel and perpendicular to the antidot arrays plane. In addition, the angular dependence of the hysteresis loops was measured by an in-plane angular rotation from 01 to 3601, every 101 with a precision of 0.51 in order to determine the magnetic inplane anisotropy distribution. 3. Result and discussion Fig. 1 shows the SEM images of the precursor NAM template with 95 nm pore diameter and 105 nm lattice
parameter (a) and after 80-nm-thick deposition of Fe80Ni20 onto its surface (b). The deposition reduced the antidot pore diameter to about 75 nm. Those images reveal a high hexagonal order degree in the antidot configuration that retains the same shape as the templates. The SEM images also indicate the high uniformity of the pore diameter size. The magnetization curves (not shown), with applied field parallel and perpendicular to the sample plane, for the magnetic continuous thin film and studied antidot arrays clearly show a magnetic anisotropy with an easy plane magnetization. The in-plane remanence and loop squareness are larger for the continuous film and decrease for the antidot arrays. These results agree with previous results obtained by Vovk et al. [5] that states there is a noncollinear spin configuration around the pores that promote incoherent magnetization reversal, which result in reduced remanence. In order to better investigate the in-plane anisotropy, hysteresis loops have been measured by rotating the sample with respect to the direction of the in-plane applied magnetic field. In Fig. 2, one can see the dependence of the coercive field (Hc) as a function of the direction of external in-plane applied magnetic field for the continuous film deposited onto a Si substrate and for the antidot arrays with 25, 55 and 75 nm pore diameter (fixed 105 nm of lattice parameter), respectively. The behavior of the coercive field (obtained from in-plane and out-of-plane hysteresis loops, not shown) indicates that, even for the largest antidot size, the characteristic in-plane anisotropy of the magnetic thin film (that originates from the deposition method) remains in the antidot configurations. Vavassori et al. [3] have found that on permalloy antidot arrays with characteristic sizes on the micrometer range, the anisotropy of the arrays is clearly related to the antidot lattice symmetry. In our case, as is well known, the hexagonal order degree depends on the first anodization time [6]. For longer first anodization process the pores in the NAM are better ordered. All the samples studied in this work are produced with 24 h of first anodization and present similar degrees of order. Further experiments with increasing and decreasing this time are still being performed. As one can see in the figure, the continuous
Fig. 1. SEM images of the NAM template with 95 nm pore diameter and 105 nm lattice parameter (a) and after 80-nm-thick deposition of Fe80Ni20 onto its surface (b).
ARTICLE IN PRESS
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -20 0
6.0 5.8 5.6 5.4
4%
5.2 5.0 4.8 -20 0
20 40 60 80 100 120 140 160 180 200
20 40 60 80 100 120 140 160 180 200
Applied field angle (°)
Applied field angle (°) 11.5
6.8
11.0
6.4 μ0Hc (mT)
20 % μ0Hc (mT)
e237
6.2
μ0Hc (mT)
μ0Hc (mT)
K.R. Pirota et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238
6.0 5.6 5.2
10.5 10.0 42 %
9.5 9.0
4.8
8.5 0
20
40
60
0
80 100 120 140 160 180
20
40
60
80 100 120 140 160 180
Applied field angle (°)
Applied field angle (°)
Fig. 2. Hc as a function of the direction of external in-plane applied magnetic field for the continuous film (a) and for the antidot arrays with 25 (b), 55 (c) and 75 (d) nm pore diameter.
20 HL easy axes H // easy axes 16 μ0Hc (mT)
film presents a very strong in-plane anisotropy that is modified in the antidot array with 25 nm diameter. An inplane bi-axial-like anisotropy character is recovered for a nanopore diameter of 55 nm (20% of change of Hc between the hard and easy magnetic directions). For the sample with 75 nm pore diameter, an even stronger (42% of change in Hc between the hard and easy directions) almost uniaxial anisotropy is observed. Fig. 3 shows the easy axes coercive field as a function of the parameter 1/(D d), where D is the distance between the centers of the pores (fixed at 105 nm) and d is the precursor NAM pore diameter (ranging between 35 and 95 nm) and D d is the interpore distance. As expected, the coercivity increases as the pore size increases. In this figure, one can see the strong increase of Hc observed in both in-plane and out-of-plane configurations. If one compares the results with the continuous thin film, the Hc increases from 0.45 to 11 mT (in-plane configuration) and from 1.1 to 17.3 mT (out-of-plane configuration). As expected, as the antidots are getting closer, the pinning effects are stronger and the coercivity increases. The observed behavior is consistent with the predicted dependence between coercivity and packing fraction predicted by Hilzinger and Kronmu¨ller [7] and also with the dependence observed between Hc and the interpore distance in other systems of Fe [2] antidots.
12
8
4 0.02
0.04
0.06
0.08
0.10
1/(D-d) (nm-1)
Fig. 3. Easy axes coercive field as a function of the parameter 1/(D d).
The magnetic properties of permalloy antidot arrays obtained with NAM templates with final antidot pore diameters of 25, 35, 55 and 75 nm were also measured over a wide temperature range (2–300 K). The coercive fields of the antidot arrays as a function of the temperature are shown in Fig. 4. The observed behavior at room
ARTICLE IN PRESS K.R. Pirota et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e235–e238
e238
Fe
Si 35 nm 95 nm Intensity (a.u.)
15
μ0Ηc (mT)
3000
35nm 55nm 75nm 95nm
12
Ni
2000 Si 1000
9
Al
Cu
6 0
0
100
200
300
600
Temperature (K)
700
900 800 Channel (eV)
1000
1100
Fig. 4. Coercitive fields of the antidot arrays as a function of temperature. The pore diameter values indicated in the figure are those of the original NAM precursor.
Fig. 5. RBS performed for 80-nm-thick FeNi grown on Si (1 0 0), NAM with 35 nm pore size and NAM with 95 nm pore size (25 and 75 nm pore after film deposition).
temperature between the coercive field and the antidot size remain in all the studied temperature range. In addition, from this figure one can see that the coercive field is higher for larger antidot diameter and, for each sample, its value decreases when the temperature increases. Another interesting aspect is that all curves show an abrupt change of increasing slope for temperatures close to 30 K. This effect could be an indication of a presence of superparamagnetic phase in the antidot arrays. As also observed by Luis et al. [8] in Co-deposited clusters, for very low deposition time, the deposited material should be in form of small clusters with room temperature superparamagnetic behavior. We suspect that this effect could be happening with the magnetic material that is deposited within the pores, but further experiments are necessary to clarify this point. Finally, in order to better clarify the characteristic of the magnetic material inside the pores, RBS experiments were performed for 80-nm-thick FeNi grown on (a) Si (1 0 0), (b) antidot array with 25 nm pore size and 105 nm lattice parameter and (c) antidot array with 75 nm pore size and 105 nm of lattice parameter. The results are shown in Fig. 5. The surface energies of the elements are indicated by black arrows. A quantitative analysis of the magnetic thin film grown on the Si substrate indicates that the thickness and composition of the thin film are 80 nm and Fe20Ni80, respectively. In addition, the qualitative analysis of the shape of the Fe+Ni peak on the antidot samples, compared with the thin continuous film, allows us to determine whether the magnetic material was or not deposited inside the pores. The asymmetry of the peak reveals that the size of the antidots should be larger at the pore bottom and gradually decreases toward the top surface, indicating that indeed there is a deposition of material within the pores. This evidence was also observed by Xiao et al. [4], using SEM microscopy on Ni antidot arrays obtained with NAM templates.
4. Conclusions The results shown in this work demonstrate a highly controlled method to produce magnetic antidots and tailor its magnetic response as a function of the antidot geometric parameters. The utilization of self-assembled NAMs as templates for the fabrication of studied nanostructures seems to be a simple and controllable way to grow nanoscale magnetic antidot arrays. The enormous increase observed in the coercive field for of the antidot configuration for all the temperature ranges studied, when compared with the continuous film, together with the absence of the superparamagnetic limit, makes the magnetic nano-scaled antidots arrays very promising for ultra-high-density information storage.
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