High Density Ordered Arrays of CoPt3 Nanoparticles with Individually

Subscriber access provided by University of Winnipeg Library

Article

High Density Ordered Arrays of CoPt3 Nanoparticles with Individually Addressable Out-of-Plane Magnetization Yong-Tae Kim, Hyunok Jung, U-Hwang Lee, Tae-Hoon Kim, Jaekyung Jang, Jong Bae Park, Joo Yull Rhee, Cheol-Woong Yang, Je-Geun Park, and Young-Uk Kwon ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02281 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

High Density Ordered Arrays of CoPt3 Nanoparticles with Individually Addressable Out-ofPlane Magnetization Yong-Tae Kim,a Hyunok Jung,b U-Hwang Lee,c Tae-Hoon Kim,d Jae Kyung Jang,e Jong Bae Park, Je-Geun Park,b,

g, h

f

Joo Yull Rhee,e Cheol-Woong Yang,d

Young-Uk Kwon*,a,c,i

a

SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 16419, Korea b

Center for Strongly Correlated Materials Research, Department of Physics & Astronomy, Seoul National University, Seoul 08826, Korea c

Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea d

e

f

g

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea

Department of Physics, Sungkyunkwan University, Suwon 16419, Korea Jeonju Center, Korea Basic Science Institute (KBSI), Jeonju 54907, Korea

Department of Physics and Astronomy, Seoul National University (SNU), Seoul 08826, Korea

ACS Paragon Plus Environment

1

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

h

Page 2 of 41

Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Korea i

School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China

ABSTRACT

The bit patterned media (BPM) technology is a promising approach for

developing

high

density

memory

devices.

Porous

templates

such as anodized aluminum oxide and self-assembled block copolymer films have been explored to be used in BPM. In this work, in

order

to

further

increase

the

pore

density,

we

use

a

mesoporous silica thin film (MSTF) with 8 nm sized regularly ordered pores and 4 nm-thick walls as a template to grow CoPt3 NPs into 2D hexagonal arrays. The use of Au (111)/SiO2 substrate induces epitaxial growth of single crystalline CoPt3 NPs in the face-centered

cubic

(fcc)

structure,

as

evidenced

by

high-

resolution transmission electron microscopy (HR-TEM) and grazing incident

X-ray

scattering

(GIXS)

data.

Direction-dependent

magnetic measurements data show that the CoPt3 NPs have out-ofplane

magnetic

data

indicate

independently.

polarization. that

Magnetic

individual

First-principles

calculations indicate that

force

CoPt3

microscopy

can

electronic

be

(MFM)

addressed structure

the observed out-of-plane magnetic


2

Page 3 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


polarization

of CoPt3 NPs originates from the

tensile

stress

induced by the lattice mismatch between CoPt3 NPs and Au (111). The array of CoPt3 NPs has a remarkably high density of 5 x 1012 NPs /in2.

KEYWORDS Magnetic memory, CoPt, template synthesis, nonoparticle array, out-of-plane polarization


3


Page 4 of 41

1. Introduction Magnetic recording is one of the most important research areas in the pursuit for ultrahigh density data storage.1-5 The generally accepted goal of areal density is 2 Tbit/in2 or higher, which requires extremely small magnetic bits (≤ 20 nm) and magnetic grains (≤ 6 nm) to be arranged in 2D with out-of-plane magnetization.5 The conventionally pursued approaches of forming thin films of high coercivity magnetic materials such as FePt and CoPt, the L10 phases,6-8 are facing difficulties in forming very small domains of a uniform size. The concept of bit patterned media (BPM) has emerged as an alternative in which individual nanoparticles (NPs) function as single bits.9-11 A recent theoretical study predicted that this approach could achieve a density as high as 10 Tbit/in2.12 Although this approach has a strong appeal by the ability, garnered over the past decades, to synthesize various NPs with high degrees of control over the size and shape, there are many problems in realizing workable magnetic memory devices based on it. The attempts to self-assemble pre-formed magnetic NPs into films13-21 suffer from the interparticle interactions which induce collective alignment of magnetic moments.22 In order to circumvent this problem, the NPs need to be separated by at least 2-5 nm,23 which has not been achieved by this method.


4

Page 5 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recently, a novel method of utilizing the Gd-Au hexagonal trigon phase formed on Au(111) surface as a template to grow Co NPs into arrays has been introduced.24 However, whether this method can lead to the development of high density memory devices is yet to be seen. The use of nanoporous templates with regularly ordered pores has appealed as a viable alternative approach for achieving BPM. There have been many promising reports on fabricating patterned magnetic materials by using anodized aluminum oxide (AAO)25-27 and self-assembled thin films of block copolymers (BCPs) as templates.28-33 Because the smallest pore sizes of these templates are a few nm, very high NP densities can be realized. For instance, Xiao et al. reported 4 Tbit/in2 of pore density by using a BCP.34 However, the typical pore-to-pore distances of these templates are rather large making the maximum pore density not very high. In this regard, mesoporous thin films (MTFs), thin films of inorganic mesoporous materials appear to be a potential solution to this issue.35,36 Mesoporous materials are formed through the self-assembly between surfactant and inorganic species.37 Because of the sharply contrasting chemical properties between the two constituents, MTFs tend to form ordered pore structures with much smaller pores (typically in the range of 2-10 nm) and pore-


5


Page 6 of 41

to-pore distances (as short as 2-3 nm), hence the possibility of increasing the memory density if used to fabricate magnetic recording media. However, little has been pursued in this line of research probably because of the difficulties in fabricating mesoporous thin films (MTFs) suitable for BPM. In order for a porous film to be used as a template in BPM technology, the pores not only need to be ordered into an array structure but also need to be open at the film surface so that active materials for memory can be introduced into the pores; the second requirement has been a big challenge for MTFs, because the involved interfacial energies in the self-assembly process strongly favors pore channels parallel over those perpendicular to the substrate plane. Attempts to align the pore channels of the 2D hexagonal structure of MTFs by applying an electric or a magnetic field produced not so satisfactory results.38 On the other hand, it has been demonstrated that MTFs with the Im3m (or cage-like) pore structure can allow well-ordered replicas with the 2D hexagonal symmetry when the cubic unit cell of the MTF is (111) oriented with respect to the substrate plane. The cagelike pores merge into perpendicular pore channels during the process of aging of the MTF (Supporting Information, Figures S1 and S2).39 By using such MTF templates, arrays of Pt nanoparticles (NPs) with various sizes ranging from 3 nm to 9 nm have been achieved.40


6

Page 7 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Another issue of the BPM approach is aligning the magnetic moment of the NPs out-of-plane to the substrate surface. In the case of thin films of L10 phases, several underlayer materials such as Ag41 and MgO42 are known to induce such preferential growth. However, in the cases using nanoporous templates, there has been little developed on controlling the magnetic directions of the NPs. In this study, we used a mesoporous silica thin film (MSTF) template with the above-mentioned pore alignment characteristic to fabricate 2D arrays of CoPt3 NP with 8 nm in diameter and 4 nm gap between the NPs to achieve a record breaking ultrahigh density of 5 Tbit/in2. We chose CoPt3 with the face-centered cubic (fcc) structure as the material for this demonstrative work to take advantage of its unusual properties of out-of-plane magnetization when (111) oriented with respect to the substrate plane.43-48

2. Experimental Section 2.1. Formation of MSTF on Au(111) coated quartz substrate The synthesis of the MSTF template was carried out following the method developed in our group using tetraethoxyorthosilicate (TEOS) as the silica source and a nonionic surfactant F-127


7


Page 8 of 41

((EO)106(PO)70(EO)106, EO = ethylene oxide, PO = propylene oxide) as the structure directing agent with some modification.49 A mixture solution with a molar ratio of TEOS : F-127 : HCl : H2O : EtOH = 1 : 6.60x10-3 : 6.66x10-3 : 4.62 : 22.6 was spin-coated on a Au(111) substrate in a relative humidity of 70% and temperature of 20–25 C. The resultant films were aged at 80 C for 20–24 h followed by calcination at 450 C for 5 h to obtain MSTF. 2.2. Formation of arrayed CoPt3 nanoparticles on Au(111) substrate The electrochemical experiments were carried out in a conventional three-electrode cell. A Au coated quartz substrate covered with a mesoporous thin film was used as the working electrode. We found an aqueous solution of 250 mM CoSO4· 7H2O and 5 mM H2PtCl6 in 0.5 M Boric acid was suitable as the electrolyte for the CoPt3 deposition. The CoPt3 NPs were deposited under a constant potential of -1.5 V at room temperature for 0.1 s by using a potentiostat/galvanostat station (Ivium Compactstat). After the deposition, the MSTF template was removed by dissolving in a 0.2 wt% HF aqueous solution 2.3. Characterization of morphological and magnetic properties


8

Page 9 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were performed on a JEOL 7100F (5~10kV). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) pattern images were obtained by a high-resolution TEM (HR-TEM; JSM-3011, 300 kV) and a high-voltage TEM (HVEM; JEM-ARM 1300S, 1250 kV). The surfaces of thin films were observed by atomic force microscopy (AFM) and magnetic force microscopy (MFM). AFM/MFM imaging was performed with a tapping mode (Burker Nanoscope IV multimode AFM). A cobalt-coated MFM cantilever (PPP-LM-MFMR, Coercivity 250 Oe, Magnetization 150 emu/cm3, Guaranteed tip radius < 30 nm, Park Systems) was used. The MFM image (scan size of 3 m) was obtained with a lift mode (height: 33 nm) with a scan rate of 5.33 μm/s. Magnetic characterization was carried out using a SQUID (Superconducting quantum interference device) magnetometer (MPMS 5XL, Quantum Design). The measurements were conducted using an applied field in-plane (//H) or out-of-plane (⊥H) geometry to the film plane at 5K and 300K. The sample was located in the center of the pick-up coil. The volume of CoPt3 NPs was estimated from the average dimension of CoPt3 NPs from the TEM images ( = 8 nm; height = 8 nm), the number density of NPs (5 x 1015/in2), and the external dimension of the sample (1 x 1 cm2). Grazing-incident X-ray scattering


9


Page 10 of 41

(GIXS) data were obtained from the 5D beam-line at Pohang Accelerator Laboratory (λ = 1.23984 Å). 2.4. Electronic-structure calculations WIEN2k package,50 in which an all-electron full-potential linearized-augmented-plane-wave method51 with local orbitals implemented, was used to calculate the electronic structures. The generalized-gradient approximation of Perdew, Burke, and Ernzerhof52 for the exchange-correlation functional was employed. An on-site-exact-exchange and hybrid functional were also included. All the calculations were spin-polarized ones with the spin-orbit interactions included in the self-consistent iterations. The atomic core density and, hence, the core levels were calculated fully relativistically at each iteration. Since we should consider two different cases of magnetization direction, and , we chose the unit cell containing 2 formula units, i.e., 2(CoPt). We varied the unit-cell volume and c/a ratio but set b/a = 1. Since we deal with a very small difference in the total energy, less than 1.0 mRyd, the number of k points in the primitive reciprocal unit cell was 20,000 to ensure the convergence of the Kohn-Sham eigenvalues for accurate total-energy calculations.


10

Page 11 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and Discussion 3.1 Morphological and structural characterization The fabrication of 2D hexagonal arrays of CoPt3 NPs was achieved in two steps (Figure 1). In the first step, we synthesized a MSTF onto a Au-coated quartz substrate by the previously reported method,49 which involved spin-coating of a precursor solution containing a silica precursor and a surfactant in ethanol, and aging and calcination of the film. The substrate was flame-treated to induce preferential orientation of Au with (111) plane parallel to the surface. We chose the condition for the MSTF which is composed of cylindrical pores of 8 nm in diameter and wall-thickness of 4 nm (Figure S2(b)). In the second step, we formed CoPt3 NPs inside the pores of the MSTF template by an electrochemical deposition technique. After the synthesis, the MSTF template was removed by etching with a dilute HF solution to reveal the array structure of CoPt3 NPs. Because the relative deposition rates the Co- and Pt-reagents depends not only on their reduction potentials but also their respective diffusion and migration rates into the pores, the control of the thickness of MSTF can be a means to control the composition of the deposited Co-Pt NPs (Supporting Information, Table S1). When thick MSTFs (~250 nm) were used,


11


Page 12 of 41

the composition of the NPs tends to be close to Co/Pt = 1/3 (Figure S3), whereas thin MSTFs (~100 nm) repeatedly produced Co/Pt = 1/1 by EDS (Figure S4). The CoPt3 NPs in this work are prepared by using thick MSTF templates.

Figure 1. Illustration of the process of forming an array of CoPt3 NPs on Au (111)/SiO2 substrate. (a) Formation of a MSTF on a Au(111) substrate by spin-casting, aging and calcination, (b) electrochemical deposition of CoPt3 NPs onto the Au(111) substrate through the pores of the MSTF template, and (c) removal of the MSTF template to reveal the array structure.

Figure 2a is the top-view FE-SEM image which shows uniform-sized NPs ordered in 2D hexagonal symmetry. The top-view HVEM image (Figure S5a) shows that the size of the NP is 8 nm and the spacing between them is 4 nm, demonstrating the high degree of fidelity of the replication process of the MSTF template. The calculated areal density based on these dimensions and the hexagonal array structure is about 5 x 1012 NPs/in2. The SAED


12

Page 13 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pattern on CoPt3 NPs scraped off the Au/SiO2 substrate (Figure S5b) shows rings with d = 0.22, 0.19, 0.14, and 0.12 nm corresponding to the (111), (200), (220), and (311) planes of the fcc structure with a = 0.383 nm, from which the Co/Pt atomic ratio of ~1/3 can be deduced,53 in agreement with the composition determined by EDS (Figure S3). In the phase diagram of Co-Pt,54 CoPt3 forms an alloy with Co and Pt atoms randomly distributed in the fcc lattice at temperatures higher than 950 K. Below this temperature, it transforms into an ordered (L12) structure, which would show weak superlattice peaks such as (100) and (110) with d = 0.385 and 0.275 nm, respectively.55 The absence of such superlattice peaks in the SAED pattern may be because their intensities are too weak or there is indeed no atomic ordering. Since the CoPt3 NPs in this study are formed at room temperature without any heat treatment, we think that the latter explanation is more likely. In the literature, CoPt3 NPs synthesized by a solution reaction at temperatures 40C or lower formed in the fcc structure56 but similarly prepared NPs transformed to the ordered L12 structure upon heat-treatment at 700C,57 consistent with our result. Further support for the fcc structure over the L12 structure is obtained from the magnetic data, which indicate that the CoPt3 NPs in our study are ferromagnetic at room temperature (below). According to the literature, the Curie


13


Page 14 of 41

temperature of the fcc structure is 483 K while that of the L12 structure is 273 K.58 In order to observe the orientational relationship between the CoPt3 NPs and the Au(111) substrate, we used GIXS (Figure 2b). A strong peak corresponding to Au (111) was detected (Figure S6), indicating that the substrate surface was preferentially (111) oriented. Although very weak in intensity because of the paucity of CoPt3 NPs compared with Au in the sample, we were able to observe their diffraction peak at 2 = 32.4 (d = 0.222 nm) that can be identified as the (111) diffraction peak of CoPt3. Other than these, there was no peak. Since this technique measures diffractions in the in-plane direction, the observation of the (111) CoPt3 peak suggests its preferential orientation with the direction perpendicular to the substrate plane. The peak width of q = 0.1 nm-1 corresponds to a crystallite size of 6.5 (= 2/q) nm in the lateral dimension, in good agreement with the size from TEM images (Figure S5a). A cross-section HR-TEM image of a CoPt3 NP grown on Au(111) is shown in Figure 2c. It should be noted that CoPt3 NPs can be grown much longer than shown in this figure; we chose a short NP for detailed structural analysis because longer ones tend to be deformed during sample preparation for TEM (Figure S5c). The lattice fringe patterns and the corresponding fast Fourier transform (FFT) images


14

Page 15 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(insets of Figure 2c) of both CoPt3 NP and Au substrate clearly show that they are single crystalline and that the NP is grown epitaxially on the substrate. However, the 1-D FFT image on the interfacial region of the HF-TEM image (Figure 2d) shows that there are edge dislocations in the CoPt3 NPs in the frequency of one dislocation per every 24-28 Au layers (Figure S7). The dislocations are located inside the NP or at the interface between the NP and the substrate. Contrarily, the Au substrate is almost free of dislocation. The dislocations in CoPt3 NPs are likely to be due to the lattice mismatch of 5.4 % between CoPt3 (a = 0.3854 nm; JCPDS 29-0499) and Au (a = 0.40640 nm; JCPDS 11174). With the observed density of dislocation in the CoPt3 NPs, the lattice mismatch would be reduced to less than 2 %.


15


Page 16 of 41

Figure 2. Structural properties of CoPt3 NPs grown on a Au(111)/SiO2 substrate. (a) Top view SEM image of CoPt3 NPs. (b) GIXS spectrum of the region for the (111) diffraction peak of CoPt3 NPs. (c) Cross-section TEM image of a CoPt3 NPs grown on a Au(111) substrate. Insets are the FFT patterns on the CoPt3 NP and the Au substrate parts showing that they have the same crystallographic orientation. (d) 1-D FFT pattern of the NP part of the image in (c). The red circles indicate the positions of edge dislocations on CoPt3.


16

Page 17 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2 Magnetic characterization The CoPt3 NP film was characterized for the magnetic properties. The volume of CoPt3 NPs in the sample was estimated from their average dimensions (ø = 8 nm; height = 8 nm) and their number density calculated from the TEM (in Figure S5) and AFM images (such as in Figure S8) for the calculation of the magnetization. Figure 3 shows the temperature dependent zerofield-cooled (ZFC) and field-cooled (FC) magnetization data of the CoPt3 NP film under the magnetic field of 100 Oe applied in in-plane and out-of-plane geometries. The discrepancy between the ZFC and FC plots in each geometry is a signature of superparamagnetism of the CoPt3 NPs. The sharp upturns at the low temperatures (< 15 K) are attributed to the paramagnetic impurities or to the parasitic paramagnetism due to lone Co atoms on the surfaces of the CoPt3 NPs. Besides the paramagnetic signal at low temperatures, the in-plane ZFC data are close to zero at all temperatures. On the contrary, the out-of-plane magnetization is much larger for both ZFC and FC sweeps, indicating that the magnetic easy axis of the NPs is along the out-of-plane direction. In principle, the CoPt3 NPs in our sample can interact with one another directly (through dipole-dipole interaction) or mediated by the conduction electrons of the Au layer underneath them, such as RKKY, spin diffusion, and spin


17


Page 18 of 41

torque transfer mechanisms. However, the almost flat ZFC curves throughout the entire temperature range for both in-plane and out-of plane geometries indicate negligible inter-NP interactions.

Apparently, these mechanisms are either very weak

or non-operative in our system. As for the dipole-dipole interaction mechanism, the inter-NP spacing of 4 nm appears to be large enough to suppress it. According to Frankamp et al. magnetic NPs with inter-NP distance of 2-5 nm do not show interNP dipolar interaction.23 As for the mechanisms through the Au layer, there can be many reasons why they do not work in our system, one of which may be related with the weak ferromagnetism of the CoPt3 NPs.


18

Page 19 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. ZFC (□) and FC (○) magnetization curves of CoPt3 NPs on Au(111)/SiO2 substrate under a small magnetic field of 100 Oe. The external field directions are in-plane (open) and out-ofplane (solid) to the film surface.

Figure 4 shows the field-dependent magnetization data taken at 5 K and 300 K for both in-plane and out-of-plane geometries. The data are corrected for the diamagnetism of the quartz substrate and paramagnetism. In the in-plane geometry, the sample shows no hysteresis at 300 K but very weak hysteresis at 5 K. On the contrary, the out-of-plane data show hysteresis, a signature of ferromagnetic ordering, at both 5 and 300 K. These are


19


Page 20 of 41

consistent with the data in Figure 3. The coercivity of the outof-plane plots is 614 Oe at 5 K and 28 Oe at 300 K, smaller than but close to those reported on CoPt3 NPs of 980 Oe at 1.85 K56 and 169 Oe at 298 K57 in the literature. Although the foregoing magnetic data prove that the CoPt3 NPs have out-of-plane magnetization, these are their averaged properties. On the other hand, one of the key features to be realized in BPM technology is the ability to address each magnetic NP individually. The absence of inter-NP interaction suggests that the NPs are individually addressable, but it cannot be a direct proof. For this, we studied the CoPt3 NP array with MFM. Unfortunately, however, because the tip size of typical MFM measurements is much larger than the size of a CoPt3 NP, there are some limitations to fully explore the behavior of individual NPs by this technique.

(a)

(b)


20

Page 21 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Hysteresis loops of CoPt3 NPs on Au(111)/SiO2 substrate (a) at 5 K and (b) at 300 K. Inset in (b) shows corresponding data without corrections for diamagnetism and paramagnetism.

In order to prevent superfluous signaling from the sample morphology, a morphology scan (AFM-mode) is performed before each MFM scan. The room temperature morphology scan and the MFM images of our CoPt3 NPs so-obtained are shown in Figure 5. The AFM image in Figure 5a shows that the surface is fully covered by NPs. The 3D plot of this image in Figure 5c shows that the Au substrate surface is rather rough with dips making the heights of the NPs vary from place to place. On the contrary, the corresponding MFM images (Figure 5b and 5d) show a flat surface because the topological information of AFM had been subtracted in obtaining them. In the MFM image, individual NPs can be discerned, a sharp contrast to the blurry MFM image on Co NPs formed into a Langmuir-Blodgett film, reported by Puntes et al.22 In their work, the Co NPs exert dipole interactions and form 100-200 nm sized magnetic domains. Therefore, their MFM image does not show the individual NPs and only the domains of the interacting NPs can be seen. Figure 5e compares the height profiles taken across the AFM and MFM images. While the two curves agree with each other in most of the places in terms of ups and downs, there are also many places where they show


21


Page 22 of 41

discrepancies, which can be attributed to the individually magnetized CoPt3 NPs.


22

Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. AFM/MFM data of CoPt3 NPs on Au(111)/SiO2 substrate measured at room temperature: (a) height AFM image, (b) height MFM image, (c) 3D AFM

image, (d) 3D MFM

image and (e) AFM and

MFM height profiles along the white lines in (a) and (b).


23


Page 24 of 41

3.3 On the origin of the out-of-plane magnetization of CoPt3 NPs In the literature, (111) epitaxially grown fcc CoPt3 thin films on substrates such as WSe2 (0001),45,47 Ru (0001),43,44 Pt (111), or Al2O3 (0001)48 by molecular beam epitaxy or sputtering show outof-plane magnetic polarization, which property has attracted considerable attention in relation with high density magnetic or magneto-optical memory devices. Also, the unidirectional polarization despite the isotropic cubic crystal structure has led many researchers to investigate the origin of this phenomenon. In this regard, it is noteworthy that the corresponding ordered cubic phase, L12, shows isotropic magnetic polarization.59 Several different mechanisms for the out-of-plane magnetization of fcc CoPt3 have been proposed including growthinduced heterogeneity60 and strain.43 More recent analysis results converge to the mechanism in which the formation of small inplane 2D Co clusters as the origin of magnetic anisotropy.44,47,48 The kinetics during the film growth has been discussed to be responsible for the formation of the Co clusters. Although the strain induced by the lattice mismatch with the substrate may also be involved in the formation of the Co clusters, this possibility has not been discussed. However, there are some discrepancies in magnetic properties between our CoPt3 NPs and the CoPt3 thin films in the literature.


24

Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Albrecht et al. reported that the out-of-plane magnetization of CoPt3 films was observed only when the films were thinner than 6 nm,45 whereas many of our CoPt3 NPs are longer than 10 nm (Figure S5c). Makarov et al. reported that out-of-plane magnetization was observed only when the CoPt3 films were grown at temperatures higher than 160C,46 contrary to our CoPt3 NPs which were grown at room temperature. It appears that some of the mechanisms proposed to explain the properties of CoPt3 thin films, especially those involving the kinetics of film growth, may not be applicable to our CoPt3 NPs because of the different synthesis methods. We, therefore, looked into the possibility of the strain effect as the origin of the out-of-plane magnetization in CoPt3 NPs. The lattice mismatch between CoPt3 and Au is 5.4%. Even if the edge dislocations observed in the HRTEM image (Figure 2d) are considered, there is still a tensile strain exerted on the CoPt3 NPs in the (111) plane. To verify the possibility of changing the easy axis due to the tensile strain, we performed the first-principles electronicstructure calculations with different orientations of the magnetization on CoPt. Since the WIEN2k package has a feature to specify the direction of magnetization, when the spin-orbit interactions are included in the self-consistent-field calculation, we do not need to change the symmetry of the unit


25


Page 26 of 41

cell depending on the direction of magnetization. Although the calculations were done for bulk CoPt, the results would give us valuable information for the effect of the tensile strain on the direction of easy axis. We considered two cases, one with the magnetization along the and direction and the other along the direction and the total energies (ETOT) were calculated by varying the c/a ratio and the volume of the unit cell. The < 111 > < 001 > contour plot of the total energy difference, ∆𝐸TOT ≡ 𝐸TOT ― 𝐸TOT

, between the cases with the magnetization along the and directions, is presented in Figure 6. If ∆𝐸TOT > 0 the direction is the easy axis, and the direction is the easy axis otherwise. The blue-square symbol represents the location of the bulk lattice constants. As shown in Figure 6, it is evident that there are some regions where ∆𝐸TOT < 0, implying that the direction can be the easy axis. Especially, the reddot symbol corresponds to the decreased d(001) and increased d(111) might be the case corresponds to the experimental results. Therefore, it is clear that the tensile stress can cause a rotation of easy axis.


26

Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Contour plot of the total-energy difference (ETOT) in the units of Ryd between the cases with the magnetization along the and directions. The thick-blue curve corresponds to ETOT = 0, i.e., phase boundary between easy axis along and directions. The blue-square symbol corresponds to the bulk lattice constants and the red circle to the NP’s lattice constants.

4. Conclusions We demonstrate that high density arrays of magnetic nanoparticles can be processed by using mesoporous thin films as


27


Page 28 of 41

templates. Mesoporous thin films have the advantages of smaller pore sizes and much thinner walls than other templates such as AAO and BCP. As a consequence of the geometric features of mesoporous thin films, much higher bit density than the other templates can be achieved. By using a (111) oriented Au film as the substrate, (111) epitaxial deposition of CoPt3 nanoparticles was achieved, which showed not only out-of-plane magnetization but also individual addressability. The density in the present study is 5 x 1012 NPs/in2, but even higher densities can be achieved when MTFs with smaller pores and/or thinner walls are used. The small pores are thin wall thickness of MTFs are originated from the sharp contrast in chemical nature between the constituent materials during the self-assembly process of the film formation. In order for the findings of the present work to be utilized for further advancement of BPM technology, methods to control the pore structures and/or pore-alignment of MTFs need to be developed. Alternatively, incorporation of inorganic entities into the present purely organic BCPs may be able to lead to higher bit densities. At the same time, methods to fabricate better magnetic materials than CoPt3 for magnetic memory need to be explored.


28

Page 29 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information. The formation mechanism of the pore arrays with 2D hexagonal symmetry by mesoporous thin films, SEM, TEM, EDS, GIXS data on CoPt3 NPs grown on Au(111)/SiO2 substrate.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through

the

National

Research

Foundation

of

Korea

(NRF-

20090081018,

NRF-2016R1A6A3A1193310

and

NRF-

2018R1A5A6075959).

Work at IBS CCES was supported by Institute

for Basic Science (IBS) in Korea (IBS-R009-G1).

REFERENCES 1. Guimarães,

A.

P.,

Principles

of

Nanomagnetism,

2nd

Ed.

Springer International Publishing, Switzerland, 2017. 2. Parkin, S.; Yang, S.-H., Memory on the Racetrack, Nat. Nanotech. 2015, 10, 195-198. 3. Natterer, F. D.; Yang, K.; Paul, W.; Willke, P.; Choi, T.; Greber,

T.;

Heinrich,

A.

J.;

Lutz,

C.

P.,

Reading

and

Writing Single-Atom Magnets, Nature, 2017, 543, 226-228. 4. Müller, J., Magnetic Skyrmions on a Two-Lane Racetrack, New J. Phys. 2017, 19, 025002.


29


Page 30 of 41

5. Hono, K.; Takahashi, Y. K., “Chapter 5. L10–FePt Granular Films

for

Heat-Assisted

“Ultra-High-Density

Magnetic

Magnetic

Recording

Recording

Storage

Media”

in

Materials

and Media Designs”, Gaspare Varvaro, Francesca Casoli Eds. Pan Stanford, New York, 2016. 6. Homma, T.; Wodarza, S.; Nishiiea, D.; Otania, T.; Geb, S.; Zangarib, G., Fabrication of FePt and CoPt Magnetic Nanodot Arrays by Electrodeposition Process, ECS Transactions, 2015, 64, 1-9. 7. Sui,

Y.;

Sellmyer

Yue, D.

J.

L.;

Skomski,

CoPt

Hard

R.;

Li,

Magnetic

X.

Z.;

Zhou,

Nanoparticle

J.;

Films

Synthesized by High Temperature Chemical Reduction, J. Appl. Phys. 2003, 93, 7571–7573. 8. Rozman, K. Z.; Krauseb, A.; Leistnerb, K.; Fahlerb, S.; Schultzb,

L.;

Schlorb,

H.

Electrodeposition

and

Hard

Magnetic Properties of Co–Pt Films in Comparison to Fe–Pt Films. J. Magn. Magn. Mater. 2007, 314, 116–121. 9. Griffiths, R. A.; Williams, A.; Oakland, C.; Roberts, J.; Vijayaraghavan, A.; Thomson, T., Directed Self-Assembly of Block Copolymers for Use in Bit Patterned Media Fabrication, J. Phys. D: Appl. Phys. 2013, 46, 503001.


30

Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.

Richter, H. J.; Dobin, A. Y.; Heinonen, O.; Gao, K. Z.;

v. d. Veerdonk, R. J. M.; Lynch, R. T.; Xue, J.; Weller, D.; Asselin, P.; Erden, M. F.; Brockie, R. M. Recording on BitPatterned Media at Densities of 1 Tb/in2 and Beyond, IEEE Trans. Mag. 2006, 42, 2255-2260. 11.

Richter, H. J.; Dobin, A. Y.; Lynch, R. T.; Weller, D.;

Brockie, R. M.; Heinonen, O.; Gao, K. Z.; Xue, J.; v. d. Veerdonk, R. J. M.; Asselin, P.; Erden, M. F. Recording Potential of Bit-Patterned Media, Appl. Phys. Lett. 2006, 88, 222512. 12.

Vogler,

Praetorius,

C.; D.

Abert,

C.;

Heat-Assisted

Bruckner, Magnetic

F.;

Suess,

Recording

of

D.; Bit-

Patterned Media Beyond 10 Tb/in2, Appl. Phys. Lett. 2016, 108, 102406. 13.

Sun,

Cobalt

S.;

Murray,

Nanocrystals

C. and

B.,

Synthesis

their

Assembly

of

Monodisperse

into

Magnetic

Superlattices, J. Appl. Phys. 1999, 85, 4325–4390. 14.

Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser,

A., Monodisperse FePt Nanoparticles and Ferromagnetic FeP Nanocrystal Superlattices, Science 2000, 287, 1989–1992.


31


15.

Page 32 of 41

Kang, S.; Harrell, J. W.; Nikles, D. E. Reduction of

the fcc to L10 Ordering Temperature for Self-Assembled FePt Nanoparticles Containing Ag, Nano Lett. 2002, 2, 1033–1036. 16.

Hou, Y.; Kondoh, H.; Kogure, T.; Ohta, T. Preparation

and Characterization of Monodisperse FePd Nanoparticles, Chem. Mater. 2004, 16, 5149–5152. 17.

Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S.

Spin-Dependent

Tunneling

in

Self-Assembled

Cobalt-

Nanocrystal Superlattices. Science 2000, 290, 1131–1134. 18. Rong, C. B.; Li, D.; Nandwana, V.; Poudyal, N.; Ding, Y.; Wang, Z. L.; Zeng, H.; Liu, J. P., Size-Dependent Chemical and Magnetic Ordering in L10-FePt Nanoparticles. Adv. Mater., 2006, 18, 2984–2988. 19. Dai, Q.; Berman, D.; Virwani, K.; Frommer, J.; Jubert, P.-O.; Lam, M.; Topuria, T.; Imaino, W.; Nelson, A. Self-Assembled Ferrimagnet−Polymer Composites for Magnetic Recording Media, Nano Lett., 2010, 10, 3216–3221 20.

Sundar, V.; Zhu, J.; Laughlin, D. E.; Zhu, J.-G. Novel Scheme for Producing

Nanoscale Uniform Grains Based on Templated Two-Phase Growth, Nano Lett., 2014, 14, 1609–1613. 21.

Wu, L.; Jubert, P.-O.; Berman, D.; Imaino, W.; Nelson,

A.; Zhu, H.; Zhang, S.; Sun, S. Monolayer Assembly of Ferrimagnetic CoxFe3–xO4 Nanocubes for Magnetic Recording, Nano Lett., 2014, 14, 3395–3399.

22.

Puntes, V. F.; Gorostiza, P.; Aruguete, D. M.; Bastus,

N. G.; Alivisatos, A. P., Collective Behaviour in Two-


32

Page 33 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dimensional

Cobalt

Nanoparticle

Assemblies

Observed

by

Magnetic Force Microscopy. Nat. Mater. 2004, 3, 263–268. 23.

Frankamp, B. L.; Boal, A. K.; Tuominen, M. T.; Rotello,

V. M., Direct Control of the Magnetic Interaction between Iron Oxide Nanoparticles through Dendrimer-Mediated SelfAssembly. J. Am. Chem. Soc. 2005, 127, 9731–9735. 24.

Fernández,

L.;

Ilyn,

M.;

Magaña,

A.;

Vitali,

L.;

Ortega, J. E.; Schiller, F., Growth of Co Nanomagnet Arrays with

Enhanced

Magnetic

Anisotropy,

Adv.

Sci.

2016,

3,

Electrodeposition

of

1600187. 25.

Yasui,

N.;

Imada,

A.;

Den,

T.

(001) Oriented CoPt L10 Columns into Anodic Alumina Films. Appl. Phys. Lett. 2003, 83, 3347–3349. 26.

Kim, C.; Loedding, T.; Jang, S.; Zeng, H.; Li, Z.; Sui,

Y.; Sellmyer, D. J., FePt Nanodot Arrays with Perpendicular Easy Axis, Large Coercivity, and Extremely High Density, Appl. Phys. Lett. 2007, 91, 172508. 27. X.

Cheng, W.; Zhou, Y.; Guan, X.; Hui, Y.; Wang, S.; Miao, Preparation

and

Magnetic

Properties

of

FePt

Nanodot

Arrays Sputtering on AAO Templates, Mater. Manufac. Proc., 2016, 31, 173-176.


33


28.

Thurn-Albrecht,

T.;

Page 34 of 41

Schotter,

J.;

Ka-stle,

G.

A.;

Emley, N.; Shibauchi, T.; Krusin-Elbaum L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. UltrahighDensity

Nanowire

Arrays

Grown

in

Self-Assembled

Diblock

Copolymer Templates. Science 2000, 290, 2126–2129. 29. J.;

Baruth, A.; Rodwogin, M. D.; Shankar, A.; Erickson, M. Hillmyer,,

M.

A.;

Leighton,

C.,

Non-lift-off

Block

Copolymer Lithography of 25 nm Magnetic Nanodot Arrays, ACS Appl. Mater. Interfaces 2011, 3, 3472–3481. 30.

Barrera,

Lupi,

F.

G.;

F.;

Celegato, F.;

Seguini,

G.;

Coïsson, M.; Boarino,

L.;

Manzin, A.; Aprile, G.;

Perego, M.; Tiberto, P., Magnetization Switching in HighDensity Magnetic Nanodots by a Fine-Tune Sputtering Process on a Large-Area Diblock Copolymer Mask, Nanoscale, 2017, 9, 16981-16992. 31.

Ghoshal, T.; Maity, T.; Senthamaraikannan, R.; Shaw, M.

T.; Carolan, P.; Holmes, J. D.; Roy, S.; Morris, M. A.; Size

and

Space

Controlled

Hexagonal

Arrays

of

Superparamagnetic Iiron Oxide Nanodots: Magnetic Studies and Application, Sci. Rep. 2013, 3, 2772. 32.

Lin, C.-H.; Polisetty, S.; O’Brien, L.; Baruth, A.;

Hillmyer, M. A.; Leighton, C.; Gladfelter, W. L., Size-


34

Page 35 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tuned

ZnO

Synthesis

Nanocrucible via

Atomic

Arrays Layer

for

Magnetic

Nanodot

Deposition-Assisted

Block

Polymer Lithography, ACS Nano, 2015, 9, 1379–1387. 33.

Yang, X.; Wan, L.; Xiao, S.; Xu, Y.; Weller, D. K.

Directed

Block

Copolymer

Assembly

versus

Electron

Beam

Lithography for Bit-Patterned Media with Areal Density of 1 Terabit/inch2 and Beyond, ACS Nano 2009, 3, 1844–1858. 34.

Xiao, S.; Yang, X.; Park, S.; Weller, D.; Russell, T.

P. A Novel Approach to Addressable 4 Teradot/in2 Patterned Media, Adv. Mater. 2009, 21, 2516–2509. 35.

Koh, C.-W.; Lee, U.-H.; Song, J.-K.; Lee, H.-R.; Kim,

M.-H.; Suh, M.; Kwon, Y.-U., “Mesoporous Titania Thin Film with Highly Ordered and Fully Accessible Vertical Pores and Crystalline Walls”, Chem.-Asian J. 2008, 3, 862-867 36.

Lee,

K.-R.;

Kwon,

Y.-U.,

Hard

Templates

for

Fabrication of Nanostructured Films, NANO, 2010, 5, 75-87. 37.

Alothman,

Z.

A.,

A

Review:

Fundamental

Aspects

of

Silicate Mesoporous Materials, Materials 2012, 5, 2874-2902. 38.

Yamauchi,

Y.;

Sawada,

M.;

Sugiyama,

A.;

Osaka,

T.;

Sakka, Y.; Kuroda, K. Magnetically Induced Orientation of Mesochannels in 2D-Hexagonal Mesoporous Silica Films, J. Mater. Chem. 2006 , 16, 3693-3700.


35


39.

Kim,

Y.-T.;

Han,

J.

H.;

Page 36 of 41

Hong,

B.

H.;

Kwon,

Y.-U.

Electrochemical Synthesis of CdSe Quantum-Dot Arrays on a Graphene

Basal

Plane

Using

Mesoporous

Silica

Thin-Film

Templates”, Adv. Mater., 2010, 22, 515-518. 40.

Lee,

K.-R.;

Kim,

M.-H.;

Hong,

J.-E.;

Kwon,

Y.-U.

Templated Syntheses of Pt Thin Films with Feature Sizes of 3, 6 and 9 nm and their Effects on SERS, J. Raman Spect. 2013, 44, 6–11. 41. Ag

Hsu, Y. N.; Jeong, S. K.; Laughlin, D. E. Effects of Underlayers

on

the

Microstructure

and

Magnetic

Properties of Epitaxial FePt Thin Films. J. Appl. Phys. 2001, 89, 7068–7070. 42.

Yu, M.; Ohguchi, H.; Zambanoa, A.; Takeuchi, I.; Liu,

J.P.;

Josell,

D.;

Bendersky,

L.

A.,

Orientation

and

Magnetic Properties of FePt and CoPt Films Grown on MgO(110) Single-Crystal

Substrate

by

Electron-Beam

Coevaporation,

Mater. Sci. Eng. B 2007, 142, 139–143. 43.

Maret, M.; Cadeville, M. C.; Poinsot, R.; Herr, A.;

Beaurepaire, E.; Monier, C. Structureal Order Related to the

Magnetic

Anisotropy

in

Epitaxial

(111)

CoPt3

Alloy

Films, J. Magn. Magn. Mater. 1997, 166, 45-52.


36

Page 37 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44.

Meneghini, C.; Maret, M.; Parasot, V.; Cadeville, M.

C.; Hazemann, J. L.; Cortes, R.; Colonna, S. Structural Origin of Magnetic Anisotropy in Co-Pt Alloy Films Probed by Polarized XAFS, Eur. Phys. J. B 1999, 7, 347-357. 45.

Albrecht,

Riedlinger,

M.; B.;

Perpendicular

Maret, Mazur,

Magnetic

M.;

Maiser,

U.;

Schatz,

Anisotropy

in

A.; G.; CoPt3

Treubel,

F.;

Anders,

S.

(111)

Films

Grown on a Low Energy Surface at Room Temperature, J. Appl. Phys. 2002, 91, 8153-8155. 46.

Makarov, D.; Klimenta, F.; Fischer, S.; Liscio, F.;

Schulze,

S.;

Hietschold,

M.;

Maret,

M.;

Albrecht,

M.

Nonepitaxially Grown Nanopatterned Co-Pt Alloys with Outof-Plane Magnetic Aanisotropy, J. Appl. Phys. 2009, 106, 114322. 47.

Liscio,

F.;

Maret,

M.;

Meneghini,

C.;

Mobilio,

S.;

Proux, O.; Makarov, D.; Albrecht, M.

Structural Origin of

Perpendicular

in

Magnetic

Anisotropy

Epitaxial

CoPt3

Nanostructures Grown on WSe2 (0001), Phys. Rev. B 2010, 81, 125417. 48.

Cross, J. O.; Newville, M.; Maranville, B. B.; Bordel,

C.; Hellman, F.; Harris, V. G. Evidence for Nanoscale TwoDimensional Co Clusters in CoPt3 Films with Perpendicular


37


Page 38 of 41

Magnetic Anisotropy, J. Phys.: Condens. Matter, 2010, 22, 146002. 49.

Lee, U-H.; Yang, J.-H.; Lee, H.-j.; Park, J.-Y.; Lee,

K.-R.; Kwon, Y.-U. Facile and Adaptable Synthesis Method of Mesostructured Silica Thin Films, J. Mater. Chem., 2008, 18, 1881–1888. 50. D.;

Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, Luitz

Orbitals

J.,

WIEN2k,

Program

for

An

Augmented

Calculating

Plane

Wave

Crystal

+

Local

Properties

(Karlheinz Schwarz, Techn. Universität Wien, Austria, 2001). 51.

Wimmer, E.; Krakauer, H.; Weinert, M.; Freeman, A., J.

Full-Potential Self-Consistent Linearized-Augmented-PlaneWave Method for Calculating the Electronic Structure of Molecules and Surfaces: O2 Molecule, Phys. Rev. B 1981, 24, 864-875. 52.

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized

Gradient Approximation Made Simple, Phys. Rev. Lett. 1996, 77, 3865-3868. 53.

Alloyeau, D.; Langlois, C.; Ricolleau, C.; Le Bouar,

Y.; Loiseau, A. A TEM in situ Experiment as a Guideline for the

Synthesis

of

As-Grown

Ordered

CoPt

Nanoparticles,

Nanotechnology 2007, 18, 375301.


38

Page 39 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


54.

Pearson,

W.

B.

A

Handbook

of

Lattice

Spacings

and

Structures of Metals and Alloys, (Oxford, Pergamon, 1964). 55.

Shapiro, A. L.; Rooney, P. W.; Tran, M. Q.; Hellman,

F.; Ring, K. M.; Kavanagh, K. L.; Rellinghaus, B.; Weller, D.

Growth-Induced

Magnetic

Anisotropy

and

Clustering

in

Vapor-Deposited Co-Pt Alloy Films, Phys. Rev. B 1999, 60, 12826. 56.

Du,

X.;

Inokuchi,

M.;

Toshima,

N.,

Silver-Induced

Enhancement of Magnetic Properties of CoPt3 Nanoparticles, Chem. Lett. 2006, 35, 1254-1255. 57.

Wang, Y.; Zhang, X.; Liu, Y.; Jiang, Y.; Zhang, Y.;

Wang, J.; Liu. Y.; Liu, H.; Sun, Y.; Beach, G. S. D.; Yang, J., Fabrication, Structure and Magnetic Properties of CoPt3, CoPt and Co3Pt Nanoparticles, J. Phys. D: Appl. Phys. 2012, 45, 485001. 58.

Berg, H.; Cohen, J. B. Long-Range Order and Ordering

Kinetics in CoPt3, Metall. Mater.

Trans. B 1972, 3, 1797-

1805. 59.

Albrecht,

Poinsot,

R.;

M.;

Maier,

Schatzr,

A.; G.

Treubel,

F.;

Self-Assembled

Maret,

M.;

Magnetic

Nanostructures of CoPt3 with Favoured Chemical Ordering, Europhys. Lett. 2001, 56, 884-890.


39


60.

Weller,

D.;

Brändle,

H.;

Chappert,

Page 40 of 41

C.

Relationship

between Kerr Effect and Perpendicular Magnetic Anisotropy in Co1-xPtx and Co1-xPdx Alloys, J. Magn. Magn. Mater. 1993, 121, 461-470.


40

Page 41 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic


41

High Density Ordered Arrays of CoPt3 Nanoparticles with Individually

Recommend Documents