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Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy
Accepted Manuscript Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy A. Hannour, L. Bardotti, B. Prével, F. Tournus, D. Mailly, J.-P. Bucher, A. Nafidi PII: DOI: Reference:
S0167-577X(17)30139-8 http://dx.doi.org/10.1016/j.matlet.2017.01.114 MLBLUE 22067
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 November 2016 12 January 2017 24 January 2017
Please cite this article as: A. Hannour, L. Bardotti, B. Prével, F. Tournus, D. Mailly, J.-P. Bucher, A. Nafidi, Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.114
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy
A .Hannour a,*, L. Bardotti b, B. Prével b, F. Tournus b, D. Mailly c, J.-P. Bucher d, A. Nafidi a a
Laboratory of Condensed Matter Physics and Nanomaterials for Renewable Energy
Faculty of Sciences, Ibn Zohr University, Agadir, Morocco b
Institut Lumière Matière, UMR 5306 Université Lyon 1-CNRS, Université de Lyon 69622
Villeurbanne, France c
Laboratoire de Photonique et de Nanostructures, CNRS-LPN, Route de Nozay
91460 Marcoussis, France d
Institut de Physique et Chimie des Matériaux, UMR 7504, Université Louis Pasteur
23 rue du Loess, 67037 Strasbourg, France
Abstract In this work, we have focused on the elaboration and characterization of nanostructured L10CoPt magnetic dot arrays prepared by depositing Co50Pt50 nanoparticles preformed in the gas phase on Si substrates patterned by electron beam lithography (EBL). The MFM observations have revealed an out-of-plane single domain state. The VSM measurements have indicated a correlated super-spin glass state (CSSG) collective behavior. Consequently, the magnetic properties are mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions. Keywords: Co 50Pt50 nanoparticles; L10 phase; nanostructured dot array; correlated super-spin glass (CSSG); dipolar interaction. 1. Introduction *Corresponding author Tel.: +212 6 25 80 08 37; E-mail address: [email protected] (A. Hannour).
1
In recent years, ordered magnetic nanodot arrays have aroused particular interest due to their potential applications in various emerging devices and technologies such as spin torque nano-oscillators [1] and magnetic data storage [2]. This bit patterned media has recently been proposed to be among the future storage technologies. Such new promising technology would revolutionize the magnetic recording devices if the nanodot was in an out-of-plane single domain state [3]. However, the magnetization direction of a nanoparticle usually oscillates at room temperature, leading to a paramagnetic behavior with a vanishing average remanent magnetization. This phenomenon, called superparamagnetism, must be bypassed for practical applications [4]. In order to overcome this limit, L10-Co50Pt50 ordered alloy nanoparticles have recently attracted increased interest [5]. Here, we investigate a new approach based on the direct deposition of functionalized nanoparticles onto EBL-patterned Si substrates. Contrary to atomic deposition where dense dots can be achieved, the present approach based on the deposition of preformed nanoparticles allows the formation of sub-50 nm nanostructured L10-Co50Pt50 dots composed of a random packing of incident nanoparticles. The morphology and magnetic properties of the samples were characterized by tapping mode atomic force microscopy (TMAFM), scanning tunneling microscopy (STM), magnetic force microscopy (MFM) and vibrating sample magnetometer (VSM). 2. Experimental procedure EBL-patterned Si substrates were obtained using a LEICA 5000+ electron beam nanowriter operating at a voltage of 100 kV [6]. The Co50Pt 50 nanoparticles are produced by low energy cluster beam deposition (LECBD) using a combined laser vaporization-inert gas condensation source [7, 8]. A 20 nm Co50Pt50 layer was deposited on a Si substrate covered by a single resist layer in which an array of submicron holes was defined by electron beam lithography. Next, a 15 nm Au layer (capping layer to prevent oxidation of the magnetic material) was evaporated from a Knudsen cell. After the deposition of the layer, the remaining
2
resist was removed in 70 °C acetone, leaving arrays of nanostructured Co50Pt50 dots on the substrate. 3. Results and discussion First, in order to investigate the intrinsic properties of isolated supported Co50Pt50 nanoparticles before and after an annealing period of 2 hours at 650 °C under high vacuum, we deposited at room temperature a submonolayer Co 50Pt50 thin film (e ≈ 0.5 Å) on amorphous carbon coated grids. As shown in Fig. 1, HRTEM image revealed the occurrence of phase transformation from chemically disordered A1 fcc phase to chemically ordered L10 face centered tetragonal (fct) phase upon annealing [8]. Next, following nanoparticle deposition on EBL-patterned Si substrate, well-organized arrays of nanostructured Co50Pt 50 dots (lateral dimension D ≈ 50 nm and height h ≈ 40 nm) were prepared by lift-off process (Fig. 2(a) and (d)). We investigated the local magnetic properties of nanostructured Co50Pt50 dot arrays using a digital instruments DIM 3100 apparatus working in tapping/lift MFM mode. The tip was magnetized prior the experiments in the z-direction (down) parallel to the tip axis. However, in our case, the MFM measurements were made without external magnetic fields and at a lift height of 50 nm. Fig. 2(a) and (b) show the topography and corresponding MFM image of an array of nanostructured Co 50Pt50 dots (period 100 nm). The MFM image showed that the dots have either dark (attractive interaction) or bright (repulsive attraction) contrast (Fig. 2(b)). This suggests that they are single domains with the easy direction of magnetization perpendicular to the film plane [9]. Moreover, the nanodots show different contrast intensities. This intensity fluctuation can be explained by size distribution [10]. Fig. 2(c) shows the hysteresis loop for nanostructured Co50Pt50 dot arrays. The M-H curve was measured at 300 K for the applied field perpendicular to the surface plane. From
3
the point of view of collective magnetic behavior, the nanodot arrays exhibit a correlated super-spin glass (CSSG) phase. The lack of hysteresis is evident and the saturation state is reached at an applied field of ≈ 0.7 . According to the high density of nanoparticles inside the dots (deposited thickness ≈ 20 nm), the magnetic interactions between nanoparticles cannot be neglected (exchange interaction). Once the magnetic interactions are strong enough, the CSSG phase emerges. Consequently, the collective magnetic behavior of the CoPt dot arrays is essentially governed by CSSG phenomenon [11]. In order to promote L10 chemical order in nanostructured Co50Pt50 dots, we have annealed the sample in UHV at 650 °C for two hours. The MFM image, made without sample saturation, indicates that all the dots have a single domain configuration and reveals only a bright contrast, which represents the one type of magnetization state in the sample (Fig. 3(b)). Compared to the MFM image of the as-prepared sample (Fig. 2(b)), the lack of down state of the magnetization (dark contrast) is a proof that the dark dots turn their contrast to bright, indicating that the magnetization direction has switched upon annealing. Moreover, the nanodots present different contrast intensities that originated from size distribution [10]. Fig. 3(c) shows the magnetization curve at 300 K of the annealed sample for the applied field perpendicular to the surface plane. In this case, the data as compared to the 300 K curve of as-prepared sample (Fig. 2(c)) demonstrates a CSSG state with a notable difference in the approach to saturation. Interestingly, the competition between different effects (i.e. dipolar interaction, exchange interaction, intrinsic uniaxial anisotropy…) may produce such magnetic behavior of nanostructured magnetic dot arrays [10]. The magnetic behavior of nanostructured Co 50Pt50 dots can be analyzed using a random anisotropy (RA) model [12]. According to this model, the magnetic ground state is the result of the competition between RA (Hr) and exchange (Hex) fields. The key parameter to determine their relative strength is:
4
= =
(1)
where Ms is the saturation magnetization of the CoPt nanoparticles, Ku is their magnetic anisotropy constant, A is the exchange stiffness constant and Da represents the size of nanoregion over which the local anisotropy axis is correlated. Consequently, for > 1, the magnetic correlation length at zero field is (Da ~ nanoparticle diameter ≈ 2 nm in this work) and the magnetic moment in each nanoparticle is oriented along the local uniaxial anisotropy axis. On the other hand, by increasing the density ( < 1) the configuration evolves to CSSG state in which the magnetic moments in closely adjacent nanoparticles are nearly aligned in the same direction [11]. In our case, according to the previous work done by F. Tournus et al. on the isolated supported Co50Pt50 nanoparticles [8], the magnetic anisotropy constant (Ku) was estimated as ≈ 193 !⁄"# before annealing (A1 fcc phase) and ≈ 385 !⁄"# after annealing (L10 fct phase). From the VSM measurements, the saturation magnetization (Ms) was evaluated as ~ 600 )⁄" for as-prepared and annealed samples. In principle, the exchange constant A must be much smaller than the bulk value of ~10*++ !⁄" for both samples. For ) ≈ 7 × 10*+- !⁄" [13], we can write ≈ 0.11 ≪ 1 before annealing and ≈ 0.22 ≪ 1 after annealing. Consequently, for both cases and at 300 K, the exchange interaction was sufficiently high to produce a CSSG state in which the moments in neighboring nanoparticles are closely aligned in the same direction. These results indicate the excellent agreement between the data and the RA model at ambient temperature for our nanostructured CoPt dot arrays. However, careful analysis of the VSM curves before and after annealing revealed that the approach to saturation is slightly slower for as-prepared sample. This behavior is essentially originated from strong dipole-dipole interaction between the nanodots. Thus the effective magnetic field ( 122 ) acting on nanostructured CoPt dot
5
arrays is the sum of the external applied magnetic field and the demagnetizing field (i.e. magnetostatic field):
122 = − 4 5
(2)
where Np is the demagnetization factor of isolated CoPt nanodot and is about 0.36 under the assumption that the nanodot has a cylinder shape with 40 nm thick and 50 nm length [14] and M is the magnetization of patterned media. The maximum value (at the saturation state) of the effective magnetic field was therefore evaluated as 122 ≈ 0.3 before and after annealing. Indeed, the strength of dipolar interaction after annealing may be counterbalanced by the high magnetic anisotropy (uniaxial magnetic anisotropy perpendicular to the plane) offered by L10 phase. 4. Conclusion In summary, the arrays of nanostructured L10-Co50Pt50 dots were fabricated using an EBL-patterned Si substrate. These nanodots were observed as a single domain with the easy axis of the magnetization perpendicular to the plane. The collective magnetic behavior is mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions.
6
References [1] R. A. van Mourik, T. Phung, S. S. P. Parkin, and B. Koopmans, Phys. Rev. B 93 (2016) 014435-014441. [2] T. R Albrecht, H. Arora, V. Ayanoor-Vitikkate et al. IEEE Trans. Magn. 51 (2015) 1-42. [3] C. Vogler, C. Abert, F. Bruckner, D. Suess, and D. Praetorius, Appl. Phys. Lett. 108 (2016) 102406.1-102406.4. [4] S. Kralj, and D. Makovec, ACS Nano 9(10) (2015) 9700–9707. [5] P. Andreazza, V. Pierron-Bohnes, F. Tournus, C.Andreazza-Vignolle, V. Dupuis, Surf. Sci. Rep. 70(2) (2015) 188-258. [6] Y. Cheng, Microelectronic Engineering 135 (2015) 57-72. [7] A. Perez, P. Melinon, V. Dupuis, P. Jensen, B. Prével, J. Tuaillon, L. Bardotti, C. Martet, M. Treilleux, M. Broyer, M. Pellarin, J. L. Vaille, B. Palpantand, J. Lerme, J. Phys. D Appl. Phys. 30 (1997) 709-721. [8] F. Tournus, A. Tamion, N. Blanc, A. Hannour, L. Bardotti, B. Prével, P. Ohresser, E. Bonet, T. Epicier, V. Dupuis, Phys. Rev. B 77 (2008) 144411-144421. [9] S.S. Kamblea et al., Materials Letters 167 (2016) 61-64. [10] W. I. Pei, G.W. Qin, Y.P. Ren, S. Li, T. Wang, H. Hasegawa, S. Ishio, H. Yamane, Acta Materialia 59 (2011) 4818-4824. [11] C. Binns, M.J. Maher, New J. Phys. 4 (2002) 85.1-85.15. [12] E. M. Chudnovsky, J. Magn. Magn. Mater. 40 (1983) 21-30. [13] S. V. Komogortsev, R. S. Iskhakov, A. A. Zimin, E. Yu. Filatov, S. V. Korenev, Yu. V. Shubin, N. A. Chizhik, G. Yu. Yurkin, E. V. Eremin, J. Magn. Magn. Mater. 401 (2016) 236241. [14] S. Chikazumi, Physics of ferromagnetism, Oxford Science Publications, 2 nd edition (1997).
7
Figure captions
Fig. 1. HRTEM images and their corresponding Fourier transforms of as-prepared (a, b) and post-annealed (c, d) Co50P50 nanoparticles. The contrast periodicity is a direct signature of L10 chemically ordered phase.
Fig. 2. (a) Height (topography), (b) phase (magnetic signal) images (1 µm x 1 µm) and (c) perpendicular hysteresis loop measured at 300 K by VSM of an array of nanostructured Co 50P50 (20 nm) dots covered with a 15 nm Au layer. (d) STM image of a single dot.
Fig. 3. (a) Height (topography), (b) phase (magnetic signal) images (1 µm x 1 µm) and (c) perpendicular hysteresis loop measured at 300 K by VSM of an array of nanostructured Co 50P50 (20 nm) dots covered with a 15 nm Au layer after annealing at 650 °C for 2 hours.
8
Figure1
Figure2
Figure3
Highlights •
Nanostructured magnetic dot arrays were fabricated by depositing CoPt clusters.
•
The MFM observations have revealed an out-of-plane single domain state.
•
The VSM measurements have indicated a correlated super-spin glass (CSSG) state.
•
The magnetic behavior is governed by a CSSG state with strong dipolar interaction.
9
AFM: nanostructured Co50Pt50 dot array A1 Co50Pt50 nanoparticle
L10 Co50Pt50 nanoparticle
After annealing
15 nm (Au layer) 20 nm (randomly stacked CoPt nanoclusters)
300K
VSM: correlated super-spin glass (CSSG) collective behaviour with strong Data 21 dipolar interaction 1 10
-5
5 10
-6
Moment (emu)
Magnetic moment (emu)
MFM: single domain Co50Pt50 dot
0
-5 10
-6
-1 10
-5
-6 10
4
-4 10
4
-2 10
4
0
2 10
4
Champ appliqué Applied field(Oe) (Oe)
4 10
4
6 10
4
S0167-577X(17)30139-8 http://dx.doi.org/10.1016/j.matlet.2017.01.114 MLBLUE 22067
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 November 2016 12 January 2017 24 January 2017
Please cite this article as: A. Hannour, L. Bardotti, B. Prével, F. Tournus, D. Mailly, J.-P. Bucher, A. Nafidi, Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy, Materials Letters (2017), doi: http:// dx.doi.org/10.1016/j.matlet.2017.01.114
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanostructured L10-CoPt dot arrays with perpendicular magnetic anisotropy
A .Hannour a,*, L. Bardotti b, B. Prével b, F. Tournus b, D. Mailly c, J.-P. Bucher d, A. Nafidi a a
Laboratory of Condensed Matter Physics and Nanomaterials for Renewable Energy
Faculty of Sciences, Ibn Zohr University, Agadir, Morocco b
Institut Lumière Matière, UMR 5306 Université Lyon 1-CNRS, Université de Lyon 69622
Villeurbanne, France c
Laboratoire de Photonique et de Nanostructures, CNRS-LPN, Route de Nozay
91460 Marcoussis, France d
Institut de Physique et Chimie des Matériaux, UMR 7504, Université Louis Pasteur
23 rue du Loess, 67037 Strasbourg, France
Abstract In this work, we have focused on the elaboration and characterization of nanostructured L10CoPt magnetic dot arrays prepared by depositing Co50Pt50 nanoparticles preformed in the gas phase on Si substrates patterned by electron beam lithography (EBL). The MFM observations have revealed an out-of-plane single domain state. The VSM measurements have indicated a correlated super-spin glass state (CSSG) collective behavior. Consequently, the magnetic properties are mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions. Keywords: Co 50Pt50 nanoparticles; L10 phase; nanostructured dot array; correlated super-spin glass (CSSG); dipolar interaction. 1. Introduction *Corresponding author Tel.: +212 6 25 80 08 37; E-mail address: [email protected] (A. Hannour).
1
In recent years, ordered magnetic nanodot arrays have aroused particular interest due to their potential applications in various emerging devices and technologies such as spin torque nano-oscillators [1] and magnetic data storage [2]. This bit patterned media has recently been proposed to be among the future storage technologies. Such new promising technology would revolutionize the magnetic recording devices if the nanodot was in an out-of-plane single domain state [3]. However, the magnetization direction of a nanoparticle usually oscillates at room temperature, leading to a paramagnetic behavior with a vanishing average remanent magnetization. This phenomenon, called superparamagnetism, must be bypassed for practical applications [4]. In order to overcome this limit, L10-Co50Pt50 ordered alloy nanoparticles have recently attracted increased interest [5]. Here, we investigate a new approach based on the direct deposition of functionalized nanoparticles onto EBL-patterned Si substrates. Contrary to atomic deposition where dense dots can be achieved, the present approach based on the deposition of preformed nanoparticles allows the formation of sub-50 nm nanostructured L10-Co50Pt50 dots composed of a random packing of incident nanoparticles. The morphology and magnetic properties of the samples were characterized by tapping mode atomic force microscopy (TMAFM), scanning tunneling microscopy (STM), magnetic force microscopy (MFM) and vibrating sample magnetometer (VSM). 2. Experimental procedure EBL-patterned Si substrates were obtained using a LEICA 5000+ electron beam nanowriter operating at a voltage of 100 kV [6]. The Co50Pt 50 nanoparticles are produced by low energy cluster beam deposition (LECBD) using a combined laser vaporization-inert gas condensation source [7, 8]. A 20 nm Co50Pt50 layer was deposited on a Si substrate covered by a single resist layer in which an array of submicron holes was defined by electron beam lithography. Next, a 15 nm Au layer (capping layer to prevent oxidation of the magnetic material) was evaporated from a Knudsen cell. After the deposition of the layer, the remaining
2
resist was removed in 70 °C acetone, leaving arrays of nanostructured Co50Pt50 dots on the substrate. 3. Results and discussion First, in order to investigate the intrinsic properties of isolated supported Co50Pt50 nanoparticles before and after an annealing period of 2 hours at 650 °C under high vacuum, we deposited at room temperature a submonolayer Co 50Pt50 thin film (e ≈ 0.5 Å) on amorphous carbon coated grids. As shown in Fig. 1, HRTEM image revealed the occurrence of phase transformation from chemically disordered A1 fcc phase to chemically ordered L10 face centered tetragonal (fct) phase upon annealing [8]. Next, following nanoparticle deposition on EBL-patterned Si substrate, well-organized arrays of nanostructured Co50Pt 50 dots (lateral dimension D ≈ 50 nm and height h ≈ 40 nm) were prepared by lift-off process (Fig. 2(a) and (d)). We investigated the local magnetic properties of nanostructured Co50Pt50 dot arrays using a digital instruments DIM 3100 apparatus working in tapping/lift MFM mode. The tip was magnetized prior the experiments in the z-direction (down) parallel to the tip axis. However, in our case, the MFM measurements were made without external magnetic fields and at a lift height of 50 nm. Fig. 2(a) and (b) show the topography and corresponding MFM image of an array of nanostructured Co 50Pt50 dots (period 100 nm). The MFM image showed that the dots have either dark (attractive interaction) or bright (repulsive attraction) contrast (Fig. 2(b)). This suggests that they are single domains with the easy direction of magnetization perpendicular to the film plane [9]. Moreover, the nanodots show different contrast intensities. This intensity fluctuation can be explained by size distribution [10]. Fig. 2(c) shows the hysteresis loop for nanostructured Co50Pt50 dot arrays. The M-H curve was measured at 300 K for the applied field perpendicular to the surface plane. From
3
the point of view of collective magnetic behavior, the nanodot arrays exhibit a correlated super-spin glass (CSSG) phase. The lack of hysteresis is evident and the saturation state is reached at an applied field of ≈ 0.7 . According to the high density of nanoparticles inside the dots (deposited thickness ≈ 20 nm), the magnetic interactions between nanoparticles cannot be neglected (exchange interaction). Once the magnetic interactions are strong enough, the CSSG phase emerges. Consequently, the collective magnetic behavior of the CoPt dot arrays is essentially governed by CSSG phenomenon [11]. In order to promote L10 chemical order in nanostructured Co50Pt50 dots, we have annealed the sample in UHV at 650 °C for two hours. The MFM image, made without sample saturation, indicates that all the dots have a single domain configuration and reveals only a bright contrast, which represents the one type of magnetization state in the sample (Fig. 3(b)). Compared to the MFM image of the as-prepared sample (Fig. 2(b)), the lack of down state of the magnetization (dark contrast) is a proof that the dark dots turn their contrast to bright, indicating that the magnetization direction has switched upon annealing. Moreover, the nanodots present different contrast intensities that originated from size distribution [10]. Fig. 3(c) shows the magnetization curve at 300 K of the annealed sample for the applied field perpendicular to the surface plane. In this case, the data as compared to the 300 K curve of as-prepared sample (Fig. 2(c)) demonstrates a CSSG state with a notable difference in the approach to saturation. Interestingly, the competition between different effects (i.e. dipolar interaction, exchange interaction, intrinsic uniaxial anisotropy…) may produce such magnetic behavior of nanostructured magnetic dot arrays [10]. The magnetic behavior of nanostructured Co 50Pt50 dots can be analyzed using a random anisotropy (RA) model [12]. According to this model, the magnetic ground state is the result of the competition between RA (Hr) and exchange (Hex) fields. The key parameter to determine their relative strength is:
4
= =
(1)
where Ms is the saturation magnetization of the CoPt nanoparticles, Ku is their magnetic anisotropy constant, A is the exchange stiffness constant and Da represents the size of nanoregion over which the local anisotropy axis is correlated. Consequently, for > 1, the magnetic correlation length at zero field is (Da ~ nanoparticle diameter ≈ 2 nm in this work) and the magnetic moment in each nanoparticle is oriented along the local uniaxial anisotropy axis. On the other hand, by increasing the density ( < 1) the configuration evolves to CSSG state in which the magnetic moments in closely adjacent nanoparticles are nearly aligned in the same direction [11]. In our case, according to the previous work done by F. Tournus et al. on the isolated supported Co50Pt50 nanoparticles [8], the magnetic anisotropy constant (Ku) was estimated as ≈ 193 !⁄"# before annealing (A1 fcc phase) and ≈ 385 !⁄"# after annealing (L10 fct phase). From the VSM measurements, the saturation magnetization (Ms) was evaluated as ~ 600 )⁄" for as-prepared and annealed samples. In principle, the exchange constant A must be much smaller than the bulk value of ~10*++ !⁄" for both samples. For ) ≈ 7 × 10*+- !⁄" [13], we can write ≈ 0.11 ≪ 1 before annealing and ≈ 0.22 ≪ 1 after annealing. Consequently, for both cases and at 300 K, the exchange interaction was sufficiently high to produce a CSSG state in which the moments in neighboring nanoparticles are closely aligned in the same direction. These results indicate the excellent agreement between the data and the RA model at ambient temperature for our nanostructured CoPt dot arrays. However, careful analysis of the VSM curves before and after annealing revealed that the approach to saturation is slightly slower for as-prepared sample. This behavior is essentially originated from strong dipole-dipole interaction between the nanodots. Thus the effective magnetic field ( 122 ) acting on nanostructured CoPt dot
5
arrays is the sum of the external applied magnetic field and the demagnetizing field (i.e. magnetostatic field):
122 = − 4 5
(2)
where Np is the demagnetization factor of isolated CoPt nanodot and is about 0.36 under the assumption that the nanodot has a cylinder shape with 40 nm thick and 50 nm length [14] and M is the magnetization of patterned media. The maximum value (at the saturation state) of the effective magnetic field was therefore evaluated as 122 ≈ 0.3 before and after annealing. Indeed, the strength of dipolar interaction after annealing may be counterbalanced by the high magnetic anisotropy (uniaxial magnetic anisotropy perpendicular to the plane) offered by L10 phase. 4. Conclusion In summary, the arrays of nanostructured L10-Co50Pt50 dots were fabricated using an EBL-patterned Si substrate. These nanodots were observed as a single domain with the easy axis of the magnetization perpendicular to the plane. The collective magnetic behavior is mainly governed by a correlated super-spin glass state with strong inter-dot dipolar interactions.
6
References [1] R. A. van Mourik, T. Phung, S. S. P. Parkin, and B. Koopmans, Phys. Rev. B 93 (2016) 014435-014441. [2] T. R Albrecht, H. Arora, V. Ayanoor-Vitikkate et al. IEEE Trans. Magn. 51 (2015) 1-42. [3] C. Vogler, C. Abert, F. Bruckner, D. Suess, and D. Praetorius, Appl. Phys. Lett. 108 (2016) 102406.1-102406.4. [4] S. Kralj, and D. Makovec, ACS Nano 9(10) (2015) 9700–9707. [5] P. Andreazza, V. Pierron-Bohnes, F. Tournus, C.Andreazza-Vignolle, V. Dupuis, Surf. Sci. Rep. 70(2) (2015) 188-258. [6] Y. Cheng, Microelectronic Engineering 135 (2015) 57-72. [7] A. Perez, P. Melinon, V. Dupuis, P. Jensen, B. Prével, J. Tuaillon, L. Bardotti, C. Martet, M. Treilleux, M. Broyer, M. Pellarin, J. L. Vaille, B. Palpantand, J. Lerme, J. Phys. D Appl. Phys. 30 (1997) 709-721. [8] F. Tournus, A. Tamion, N. Blanc, A. Hannour, L. Bardotti, B. Prével, P. Ohresser, E. Bonet, T. Epicier, V. Dupuis, Phys. Rev. B 77 (2008) 144411-144421. [9] S.S. Kamblea et al., Materials Letters 167 (2016) 61-64. [10] W. I. Pei, G.W. Qin, Y.P. Ren, S. Li, T. Wang, H. Hasegawa, S. Ishio, H. Yamane, Acta Materialia 59 (2011) 4818-4824. [11] C. Binns, M.J. Maher, New J. Phys. 4 (2002) 85.1-85.15. [12] E. M. Chudnovsky, J. Magn. Magn. Mater. 40 (1983) 21-30. [13] S. V. Komogortsev, R. S. Iskhakov, A. A. Zimin, E. Yu. Filatov, S. V. Korenev, Yu. V. Shubin, N. A. Chizhik, G. Yu. Yurkin, E. V. Eremin, J. Magn. Magn. Mater. 401 (2016) 236241. [14] S. Chikazumi, Physics of ferromagnetism, Oxford Science Publications, 2 nd edition (1997).
7
Figure captions
Fig. 1. HRTEM images and their corresponding Fourier transforms of as-prepared (a, b) and post-annealed (c, d) Co50P50 nanoparticles. The contrast periodicity is a direct signature of L10 chemically ordered phase.
Fig. 2. (a) Height (topography), (b) phase (magnetic signal) images (1 µm x 1 µm) and (c) perpendicular hysteresis loop measured at 300 K by VSM of an array of nanostructured Co 50P50 (20 nm) dots covered with a 15 nm Au layer. (d) STM image of a single dot.
Fig. 3. (a) Height (topography), (b) phase (magnetic signal) images (1 µm x 1 µm) and (c) perpendicular hysteresis loop measured at 300 K by VSM of an array of nanostructured Co 50P50 (20 nm) dots covered with a 15 nm Au layer after annealing at 650 °C for 2 hours.
8
Figure1
Figure2
Figure3
Highlights •
Nanostructured magnetic dot arrays were fabricated by depositing CoPt clusters.
•
The MFM observations have revealed an out-of-plane single domain state.
•
The VSM measurements have indicated a correlated super-spin glass (CSSG) state.
•
The magnetic behavior is governed by a CSSG state with strong dipolar interaction.
9
AFM: nanostructured Co50Pt50 dot array A1 Co50Pt50 nanoparticle
L10 Co50Pt50 nanoparticle
After annealing
15 nm (Au layer) 20 nm (randomly stacked CoPt nanoclusters)
300K
VSM: correlated super-spin glass (CSSG) collective behaviour with strong Data 21 dipolar interaction 1 10
-5
5 10
-6
Moment (emu)
Magnetic moment (emu)
MFM: single domain Co50Pt50 dot
0
-5 10
-6
-1 10
-5
-6 10
4
-4 10
4
-2 10
4
0
2 10
4
Champ appliqué Applied field(Oe) (Oe)
4 10
4
6 10
4