Perpendicular magnetic anisotropy mechanism on commercial CD substrate

Accepted Manuscript Perpendicular Magnetic Anisotropy mechanism on commercial CD substrate S. Karamanou, M. Vasilakaki, G. Giannopoulos, V. Psycharis, K. Trohidou, D. Niarchos PII: DOI: Reference:

S0304-8853(17)33692-2 https://doi.org/10.1016/j.jmmm.2018.02.089 MAGMA 63766

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

24 November 2017 25 February 2018 28 February 2018

Please cite this article as: S. Karamanou, M. Vasilakaki, G. Giannopoulos, V. Psycharis, K. Trohidou, D. Niarchos, Perpendicular Magnetic Anisotropy mechanism on commercial CD substrate, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.02.089

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Perpendicular

Magnetic

Anisotropy

mechanism

on

commercial CD substrate S. Karamanou1,*, M. Vasilakaki1, G. Giannopoulos1, V. Psycharis1, K. Trohidou1, and D. Niarchos1 1

Institute of Nanoscience and Nanotechnology, NCSR ‘Demokritos’, Aghia Paraskevi, 15341, Greece *Corresponding author e-mail: [email protected] Abstract Fe thin films were deposited on commercial CD substrates using the magnetron sputtering technique, to examine the effect of the patterned underlayer on their structural and magnetic properties. We report that the coercive field and magnetic anisotropy of these iron grained thin films are strongly depended on the patterned substrate, in accordance with Monte Carlo simulations performed using the Metropolis algorithm. The model developed well explains the role of the exchange and dipole-dipole interparticle magnetic interactions between the Fe grains, and predicts the optimum conditions for advanced magnetic sensor applications. Key words: iron clusters, commercial CD, magnetron sputtering, magnetic properties, Monte Carlo simulations 1. Introduction Iron-based magnetic thin films have attracted a lot of attention for many decades

[1] due to their broad range of physical properties. Magnetic anisotropy, magnetization switching processes and giant magnetoresistance are examples of the most investigated properties of single thin films or multilayers. High saturation magnetization, along with induced magnetocrystalline anisotropy through suitable underlayer selection, make Fe-based films one of the most promising candidates for magnetic sensor and recording applications [2], depending on the substrate or underlayer, as well as on the methods and conditions of preparation [3,4]. Inexpensive and widely available substrates that have been exploited in several research fields such as in material science, surface chemistry, and biomedical diagnosis are the commercial compact disks (CD). CD-derived surfaces have been proposed as effective electrodes for electroanalytical chemistry and as biomedical sensors [5]. More specifically, their metal reflective films are suitable for the preparation of high-quality self-assembled monolayers (SAMs) and for electrochemical analysis. The pre-grooved polycarbonate base is ideal for the customized fabrication of material micro/nanostructures and the immobilization of biomolecules. In conjunction with a conventional CD drive, this could evolve to be an inexpensive tool for biomedical diagnosis and gene analysis [6-8]. The Magnetron sputtering technique has become the most widely used deposition process for a wide range of coatings, making a significant impact in 1

applications including hard or wear-resistant coatings, low-friction coatings, corrosion-resistant coatings and coatings with specific magnetic, optical, or electrical properties [9]. Magnetic properties of grained thin films such as coercive field and magnetic anisotropy are found to depend strongly on substrate and deposition conditions [1].

Fig. 1. Basic characteristic lengths of the commercial CD substrates In this work we study the magnetic properties of iron clusters deposited on a prepatterned CD substrate as thin films using magnetron sputtering technique in two distinct cases: 1) when the iron grains are placed over the whole substrate and 2) after a polishing procedure the remaining iron grains are placed in the pits of the CD. We compare the magnetic experimental results with Monte Carlo simulations to understand the effect of the substrate on the magnetic properties of the system, aiming towards developing techniques using inexpensive substrates for various applications. 2. Experimental CD substrates (polycarbonate) were used as received from Digital Press Hellas. An AJA, Inc. ATC 2200 V high vacuum magnetron sputtering system was used to deposit 200 nm thick Fe film on the CD prepatterned substrates with base pressures on the order of 5 x 10-9 Torr at an argon pressure of 5 mTorr, and deposition rate was ~1.1 nm/min. The substrate temperature was kept constant (T = 25 oC) during deposition. After sputtering, the deposited iron film was polished using an OP-S non-drying colloidal silica suspension, with a mean particle diameter of 450 nm. The polishing is carried out to selectively remove the iron from the surface but not from the CD pits. The crystalline structure of the Fe film was determined by X-ray Diffraction (XRD) analysis using a Siemens D5000 diffractometer with CuKa radiation and scan rate of 0.02 degrees per second in the 2θ range 2o - 80o. The morphology of the deposited film was examined using a FEI Quanta 200 Scanning Electron Microscope (SEM) with simultaneous Energy Dispersive X-ray Analysis (EDS), as well as by Atomic Force Microscopy (AFM) using a NT-MDT Smena Instrument. 2

Magnetic measurements were performed using a Quantum Design Superconducting Quantum Interference Device (SQUID) magnetometer MPMS-5T. Hysteresis loops were measured at temperatures of 5, 50, 100 and 300 K. 3. Results and discussion 3.1 Structural Characterization of the films Given that the structure of the deposited iron inside the CD pits does not differ from the deposited iron film, and due to the low amount of the iron inside the CD pits after polishing, the structural characterization measurements presented below concerns the iron film. Figure 2 shows the XRD patterns of iron film coated on CD substrate before (a) and after (c) polishing, and the pattern of CD substrate (b) as a reference.

Lin (Counts)

2880

1740 300

2θ (ο)

42

47

(c)

1170

Fe (110)

Lin (Counts)

2310

(b)

CD

Fe-CD polished

(a)

600

Fe-CD

30 2

12

22

32

42

52

62

72

2θ (ο)

Fig. 2. XRD pattern of : (a) iron film coated on CD substrate, (b) CD substrate and (c) iron film coated on CD substrate after polishing. The inset graph (up and right side) shows a more detailed analysis of the curves, with 2θ = 42 - 47o. In the X-ray diffractograms, Fig. 2(a), the peak at 2θ = 44ο is attributed to the iron crystallographic plane (110) [10] which disappears after polishing (Fig.2c). The absence of the Fe peak is attributed to the very small amount of Fe present within the pits of the polished substrate not detectable with our XRD system. The XRD pattern of the CD substrate presents a small peak at 2θ = 9ο, the main peak at 2θ = 10o-23o, a broad peak at 2θ = 25ο characteristic of the amorphous polycarbonate material that CDs are made. [11-13]. In Fig. 3, the microstructure of the iron film is shown using SEM.

3

Fig. 3. SEM images of : (a) CD substrate, (b) Top view of the iron film coated on CD substrate, and (c) Cross section of the iron film coated on CD substrate. According to SEM analysis in Fig. 3(a) the track spacing of the CD substrate was determined to be on the order of 1.5 μm, the width of the pits equal to 0.55 μm, and the length of the pits ranged from 1.15 μm to 3.85 μm. The thickness of the deposited iron film on CD substrate was nominally 200 nm, Fig.3(c). SEM images reveal a highly uniform layer. Based on the EDS elemental analysis, besides the iron, the elements carbon and oxygen were detected arising from the polycarbonate substrate. Atomic Force Microscopy (AFM) was also used to determine the track spacing and the depth and their width of the CD pits before and after the iron deposition. Figure 4 shows the AFM images (a,c,e,g), the height image (f) and the image profile (b,d,h) of the CD substrate (a,b) and the iron film coated CD substrate before (c-f) and after polishing (g,h). In Fig. 4(a) an AFM image of the CD substrate, is shown, where the height (z) varies between 5 and 126.8 nm. The dark regions reflects correspond to the CD pits. Scanning along the direction of the arrow, the corresponding image profiles (Figs. 4b, 4d, 4h) are formed, indicating the depth of the pits. The distance B-E, between the beginning (B) and the end (E) of the curve (Fig.4b) corresponds to the width (w) of the pits in the direction of the arrow, and the distance (s) (Fig.4b) corresponds to the track spacing of the CD. From the image profiles the depth of the pits (d), the width of the pits (w) and the track spacing are d ≈ 140 nm, w ≈ 0.6 μm, and s ≈ 1.5 μm respectively, the latter two values in agreement with the SEM results. Fig.4(c) shows the AFM image of the CD substrate coated with iron, with z axis values ranging from 25 to 295.75 nm. The depth of the CD pits after deposition has been reduced to approximately 55 nm, Fig 4(d). In Fig. 4(e) and 4(f) the topography of the iron film is presented. The Fe, in the form of aggregates, shows spherical grain topography with mean diameter of ≈ 340 nm. Fig. 4(g) shows the AFM image of CD substrate coated with iron after polishing, and showing sample heights from 10 to 243.52 nm. From the image profile, Fig. 4h, the depth of the pits has been reduced after polishing to approximately 35 nm.

4

(b)

(a)

2.0 μm 295.75

(c)

(d)

4.0µm 0.00

(f)

(e)

243.52 nm

(h)

(g)

40 35 30

Z[nm]

25 20 15 10 5

1.0 μm

0 0

200

400

600

800

X[nm]

0.00 nm

Fig. 4. AFM images (a,c,e,g), height image (f) and image profile (b,d,h) of : CD substrate (a,b), iron film coated on CD substrate (c-f) and iron film coated on CD substrate after polishing (g,h). 5

3.2 Magnetic behavior 3.2.1 Experimental results In order to examine the effect of the CD substrate, magnetic measurements were performed with maximum applied fields of 2 T in two cases: 1) for Fe film deposited on the CD substrate and 2) for Fe clusters inside the pits of the CD. Figure 5(a) shows the hysteresis loops of the Fe film at four different temperatures 5, 50, 100 and 300 K, with the external magnetic field perpendicular to film plane, and Fig. 5(b) shows the corresponding loops with the applied field parallel to the film plane. The corresponding data for the Fe film after polishing –that is for the iron clusters remaining only in the pits- are presented in Figs. 5(c) and 5(d) respectively. In the first case, the coercive field of the Fe film at 300K is 289 Oe with the field applied perpendicular to the film plane and 152 Oe for the in-plane measurement. The corresponding values for the saturation magnetization are 1174 emu/cc and 1046 emu/cc respectively. In the second case, a different magnetic behaviour is observed. The coercive field of the Fe film deposited on CD substrate after polishing at 300K is Hc = 258 Oe for the external field perpendicular to the film plane and 220 Oe parallel to the film plane. The corresponding values for the saturation magnetization are 652 emu/cc and 450 emu/cc respectively. The hysteresis loop shapes for perpendicular field in the two cases Fig. 5 (a,c) are different and depend mainly on the surface morphology of the substrate [14]. The magnetization of the Fe coated film is in the film plane direction Fig. 5(a,b) while after polishing the shape of the magnetic hysteresis loop Fig. 5(c) indicates perpendicular to film plane growth.

6

1000 500

(b) 1500

Fe-CD, perpendicular T=5K T = 50 K T = 100 K T = 300 K

Magnetization (Emu / cc)

Magnetization (Emu / cc)

(a)1500

0 1

-500

0 -1

-1000

-2

-1500 -20000

-600

-10000

0

0

10000

600

20000

1000

T=5K T = 50 K T = 100 K T = 300 K

500 0

50 -500

0 -1000 -1500 -20000

Magnetic Field (Oe)

(c)

Fe-CD, parallel

-50 -300 -10000

0

0 300 10000 20000

Magnetic Field (Oe)

(d)

Fig. 5. Hysteresis loops of Fe films before (a,b) and after polishing (c,d), with external magnetic field perpendicular and parallel to the film plane at 5, 50, 100 and 300 K respectively.

7

3.2.2 Monte Carlo Modelling

(b)

(a)

1500nm

550nm

220nm

125nm 70nm

(c)

Fig. 6. 2D schematic representation of the quasi-2D sample of the assembly of random (a) and ordered (b) assemblies of ferromagnetic clusters of nanoparticles onto the corresponding AFM picture and 3D representation of Fe clusters inside two neighbouring pits (c) that are periodically repeated in the Monte Carlo calculations. In order to explain the magnetization data we have used Monte Carlo Modelling with the implementation of the Metropolis algorithm. We use mesoscopic modelling for the two cases of Fe clusters deposited on the CD substrate namely a nanogranular film (random assembly Fig. 6(a)) and Fe clusters in the pits of the CD supposing ideally that are in equal distances (ordered assembly Fig. 6(b,c)). The Stoner-Wohlfarth model has been employed to model the grains in random or ordered assemblies of N Fe nanograins placed on the nodes of a simple quasi-2D cubic lattice with a lattice constant ‘a’. We have to note here that the magnetic structure of the system does not follow necessarily the structure of the patterned CD so what is important here is to take into account the main characteristics of the system based on the AFM pictures. These characteristics are the absence of the exchange interparticle interactions and the lower particle concentration in the case of ordered arrays. The Fe clusters of particles are considered to be spherical according to the AFM picture that suggests a spherical morphology with a spherical median diameter D = 70 nm. In the ordered assembly each spherical grain is placed at a constant distance without touching its neighbours. In both cases the Fe clusters have random anisotropy axes and bulk anisotropy constant K = 2.4 105 erg/cc.

8

We represent each grain with a macro spin as a three dimensional unit vector. The magnitude of its magnetic moment is m=MsV, where Ms is the saturation magnetization per unit volume and V=πD3/6 is the particle volume. The magnetic particles have uniaxial anisotropy and interact via long-range dipolar forces and short range exchange forces when they are in contact in the case of a random assembly. The total energy of the system [15] is: N

E

 (

ˆh ) 0 mH(si e

ˆ i )2   KV(si e

i 1

0 m 2 4a 3

s D i

i j

ijs j



 Js s ) i j

 i, j

The first term in eq. 1 is the Zeeman energy term, the second term gives the random anisotropy energy, the dipole-dipole interaction energy is given in the third term and the fourth term describes the Heisenberg nearest neighbor exchange interaction energy term for nanoparticles in direct contact in the case of the random assembly. êh and êi are the directions of the magnetic field and the anisotropy axis of ith particle respectively, Dij is the dipolar interaction tensor for Ewald summation with periodic boundary conditions in the xy plane and free in the xz plane between the ith and jth nanoparticle, denotes summation over nearest neighbors only [16]. In our simulations, we use reduced (dimensionless) energy parameters so the magnetic field is expressed as h = (μ0Ms/k)H, where k is the effective anisotropy constant k = KV and the effective exchange energy j = J/k, the dipolar interaction strength is expressed as g = μ0Ms2(D/a)3/24K and the temperature t = kB T/k. In these reduced units, the anisotropy constant is taken as k = 1. We consider D/a = 1. The dipolar interaction strength is calculated from the experimental values of the saturation magnetization Ms of the system given in Section 3.2.1. Since g~Ms2 considering K constant for the ordered arrays, using the in plane and out of plane experimental Ms values, we set g paralell= 0.54 and g perpendicular = 1.06. For the corresponding random array Ms data, we set g paralell= gperpendicular= 3.12. Little information is available for the value of the effective exchange constant j, so it is treated as a free parameter and we set j =1. For the energy minimization we use the standard Metropolis Monte Carlo algorithm [15]. The Monte Carlo simulations results for a given temperature and applied field were averaged over 20 samples with various spin configurations, realizations of the easy-axes distribution and different spatial configurations for the nanoparticles. For every field and temperature value, the first 500 steps per spin are used for equilibration, and the subsequent 5000MC steps are used to obtain thermal averages. 3.2.2.1 Monte Carlo Simulation Results Figures 7(a) and 7(b) give the coercive field versus temperature for the Fe-CD before and after polishing (Fig.7a) and the Monte Carlo simulation for random and ordered Fe assemblies (Fig.7b) respectively, for applied external magnetic field parallel and perpendicular to the film plane.

9

Coercive Field, Hc (Oe)

700

(a)

Fe-CD, perpendicular Fe-CD polished, perpendicular Fe-CD, parallel Fe-CD polished, parallel

600 500 400 300 200 0

50

2.4 (b)

150 200 T (K)

250

300

random Fe assemblies h out of plane ordered Fe arrays h out of plane random Fe assemblies h in plane ordered Fe arrays h in plane

2.2

HC(a.u.)

100

2.0 0.4 0.2 0.0

0

50

100

150

T(K)

200

250

300

Fig. 7. Coercive field versus temperature from (a) experimental data of Fe-CD before and after polishing and (b) Monte Carlo simulation for random and ordered Fe assemblies, with external magnetic field applied parallel (blue and red symbols) and perpendicular (green and orange symbols) to the film plane respectively.

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From Figure 7 we see that Monte Carlo simulations are in qualitative agreement with the experimental results of the coercive field as a function of temperature indicating that the simulated model depicts well the main characteristics of the experimental system. We have to note here that in our model we have considered a monodispersed assembly of nanoparticles. We expect that the coercive field values are affected from the size and interparticle distance distribution in the experimentally studied system, as it has been demonstrated in [16]. Therefore the values of the coercive field given by the simulations are different from the measured ones, but the main magnetic characteristics of the system are clearly demonstrated with our model. Monte Carlo simulations show that the co-existence of the dipolar and the exchange coupling among the particles in contact in Fe random assemblies decreases the coercive field along the parallel axis to the plane in agreement with the experimental findings while the competition between anisotropy and dipolar interactions out of the plane increases the coercive field along the perpendicular axis. In addition, our results for the ordered arrays show that the out of plane coercivity is lower than that of the random case because of the lower concentration of the grains (increased interparticle distance) and the in-plane dipolar field competes with the anisotropy field resulting in a slight increase in the coercivity in comparison with the random assembly. Finally, the out of plane coercive field is larger than the in plane in the case of the ordered arrays as in the case of random assemblies, confirming that the same perpendicular anisotropy mechanism exists in both cases, in agreement with the experimental observations. The fact that in the case of the MC simulations the Hc is much bigger in the random assemblies for out of plane applied field than in the case of in plane applied field is due to the fact that our calculation for a monodispersed assembly gave a very big value for the dipolar strength. Since this strong dipolar interaction induces a strong in plane anisotropy that results in a big resistance in the change of the spin direction by applying the out of plane field, it results in a very big Hc; much bigger than the H c in the case of the random assembly when we applied in plane field. Conclusions Fe thin films were deposited on commercial CD substrates using sputtering technique and then we selectively removed the Fe film by polishing in order to remain only within the pits of the CD. According to SEM analysis the track spacing of the CD substrate was determined in the order of 1.5 μm, the width of the pits equal to 0.55 μm, whereas the length of the pits ranged from 1.15 μm to 3.85 μm, values that are in line with the AFM results. The films show spherical grain topography and after polishing procedure, the depth of the CD pits has been reduced. We report that the coercive field and magnetic anisotropy are strongly dependent on patterned substrate, in accordance with the Monte Carlo simulations performed using the Metropolis algorithm. The transition from the 2D film morphology of the large scale interparticle interactions to the 0D pits morphology where the exchange interparticle interactions and the dipolar fields are lower due to lower number of neighbors enhancing the role of the anisotropy, is the key issue to the observed different magnetic behaviour. In case of the patterned arrays the perpendicular anisotropy mechanism is the same as in the granular film case making them very promising for the magnetic recording applications.

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Acknowledgements This research has been co financed by the European Social Fund (EU) and Greek national funds through the Operational Program “Education and Lifelong Learning” in the framework of ARISTEIA I (Project No. COMANA/22). The authors thank the company Digital Press Hellas, for kindly helping by offering the commercial CDs and Dr. Eamonn Devlin for kindly helping reviewing the manuscript. References [1]B. Ghebouli, A. Layadi, A. Guittoum, L. Kerkache, M. Benkerri, A. Klimov, V. Preobrazhensky, and P. Pernod, Electrical properties and Kerr effect study of evaporated Fe/Si and Fe/glass thin films, Eur. Phys. J. Appl. Phys 48 (2009) 30503. [2] Z.H. Wang, K. Chen, Y. Zhou, H.Z. Zeng, MFM studies of microstructure and magnetic properties of iron thin films prepared by sputtering, Ultramicroscopy. 105 (2005) 343-346. [3] M. Mebarki, A. Layadi, A. Guittoum, A. Benabbas, B. Ghebouli, M. Saad, N. Menni, Structural and electrical properties of evaporated Fe thin films, Appl Surf Sci. 257 (2011) 7025-7029. [4] A. Tayal, M. Gupta, A. Gupta, V. Ganesan, L. Behera, S. Singh, S. Basu, Study of magnetic iron nitride thin films deposited by high power impulse magnetron sputtering, Surf Coat Technol. 275 (2015) 264-269. [5] H.Z. Yu, New chemistry on old CDs, Chem. Commun. (2004) 2633–2636. [6] C. Gellini, M. Muniz-Miranda, M. Innocenti, F. Carla, F. Loglio, M.L. Foresti and P.R. Salvi, Nanopatterned Ag substrates for SERS spectroscopy, Phys Chem Chem Phys. 10 (2008) 4555–4558. [7]F. Pena-Pereira, R.M.B.O. Duarte, A.C. Duarte, Immobilization strategies and analytical applications for metallic and metal-oxide nanomaterials on surfaces, Trends Analyt Chem. 40 (2012) 90-105. [8] G. Giallongo, R. Pilot, C. Durante, G.A. Rizzi, R. Signorini, R. Bozio, A. Gennaro, G. Granozzi, Silver Nanoparticle Arrays on a DVD-Derived Template: An easy&cheap SERS Substrate, Plasmonics. 6 (2011) 725–733. [9] P.J. Kelly, R.D. Arnell, Magnetron sputtering: a review of recent developments and applications, Vacuum. 56 (2000) 159-172. [10] G. Sun, B. Dong, M. Cao, B. Wei, C. Hu, Hierarchical Dendrite-Like Magnetic Materials of Fe3O4, γ-Fe2O3, and Fe with High Performance of Microwave Absorption, Chem Mater. 23 (2011) 1587–1593. [11]Z. Tang, C. Youshuang, W. Mouhua, Specific properties improvement of polycarbonate induced by irradiation at elevated particular temperature, Radiat Phys Chem. 96 (2014) 171–175. [12] E.A. Soliman, A. Samir, A.M.A. Hassan, M.S. Mohy-Eldin, G. A. El-Naim, Limiting the Migration of Bisphenol A from Polycarbonate Using Dielectric Barrier Discharge, Open Journal of Synthesis Theory and Applications. 3 (2014) 27-36. [13] H.R. Schubach, B. Heise, Structure and anisotropy in polycarbonate. I. Short range order of amorphous polycarbonate revealed by WAXS*, Colloid Polym Sci. 264 (1986) 335-342. [14] M. Tofizur Rahman, C.H. Lai, D. Vokoun, N.N. Shams, A Simple Route to Fabricate Percolated Perpendicular Magnetic Recording Media, IEEE Trans Magn. 43 (2007) 2133.

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[15] K.N. Trohidou, M. Vasilakaki, Magnetic Behaviour of Core/Shell Nanoparticle Assemblies: Interparticle Interactions Effects, Acta Phys Pol A. 117 (2010) 374. [16] K. Trohidou and M. Vasilakaki (2011) “Monte Carlo Studies of Magnetic Nanoparticles”, Applications of Monte Carlo Method in Science and Engineering (ed. Prof. S.Mordechai) InTech, Croatia Ch.6, 513-538.

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Highlights    



The novelty of this work is the exploitation of commercial compact disks (CD) as an inexpensive and widely available substrate in several research fields and industrial applications. Fe thin films were deposited on commercial CD substrates using sputtering technique and then we selectively removed the Fe film by polishing in order to remain only within the pits of the CD. We report that the coercive field and magnetic anisotropy of our magnetron sputtered thin films are strongly depended on substrate selection, fact that is in line with the modeling results. The transition from the 2D film morphology of the large scale interparticle interactions to the 0D pits morphology where the exchange interparticle interactions and the dipolar fields are lower due to lower number of neighbors enhancing the role of the anisotropy, is the key issue to the observed different magnetic behaviour. In case of the patterned arrays the perpendicular anisotropy mechanism is the same as in the granular film case making them very promising for the magnetic recording applications.

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