Magnetic Nanodot Arrays Research Articles

Nano-engineering the evolution of skyrmion crystal in synthetic antiferromagnets

The evolution of skyrmion crystals encapsulates skyrmion's critical behaviors, such as nucleation, deformation, and annihilation. Here, we achieve a tunable evolution of artificial skyrmion crystals in nanostructured synthetic antiferromagnet multilayers, which are composed of perpendicular magnetic multilayers and nanopatterned arrays of magnetic nanodots. The out-of-plane magnetization hysteresis loops and first-order reversal curves show that the nucleation and annihilation of the artificial skyrmion can be controlled by tuning the diameter of and spacing between the nanodots. Moreover, when the bottom layer thickness increases, the annihilation of skyrmion shifts from evolving into a ferromagnetic spin texture to evolving into an antiferromagnetic spin texture. Most significantly, nonvolatile multiple states are realized at zero magnetic field via controlling the proportion of the annihilated skyrmions in the skyrmion crystal. Our results demonstrate the tunability and flexibility of the artificial skyrmion platform, providing a promising route to achieve skyrmion-based multistate devices, such as neuromorphic spintronic devices.

Conical-shaped hexagonal barium ferrite nanodot arrays on an alumina substrate based on an ultrathin alumina mask method

Abstract In this work, conical-shaped M-type ferrite (BaFe12O19, BaM) magnetic nanodot arrays have been prepared on alumina substrate by pulsed laser deposition (PLD) through an ultrathin alumina mask (UTAM). The UTAM has periodically ordered cylindrical holes with a diameter of 350 nm and an interpore distance of 450 nm. Nanodots with a shape distribution in which the diameter is centred at 327 nm with a standard deviation of 23 nm have been fabricated. The microstructures, morphologies and magnetic properties of the nanodot arrays were investigated. Scanning electron microscopy and X-ray diffraction revealed that the prepared nanodots are c-axis-oriented conical crystals. The hysteresis loops indicated that the BaM nanodot arrays have a 63% squareness ratio, which is significantly higher than that of the film (45%). The first-order reversal curve (FORC) diagram showed the nanodots have small distributions of coercivity (Hc) and interactive field (Hu) between particles, with the centre at Hc = 1800 Oe and Hu = −280 Oe. Furthermore, the micromagnetic simulation revealed that this unique structure greatly impacts the distribution of demagnetizing field and effectively reduces the intensity of demagnetizing field.

Scanning Near Field Circular Polarization Optical Microscope for Measuring Magnetic Nanodot Arrays

Scanning Near Field Circular Polarization Optical Microscope for Measuring Magnetic Nanodot Arrays

In situ grazing-incidence small-angle X-ray scattering observation of block-copolymer templated formation of magnetic nanodot arrays and their magnetic properties

The fabrication of bit-patterned media (BPM) is crucial for new types of hard disk drives. The development of methods for the production of BPM is progressing rapidly. Conventional lithography reaches the limit regarding lateral resolution, and new routes are needed. In this study, we mainly focus on the dependence of the size and shape of magnetic nanodots on the Ar+-ion etching duration, using silica dots as masks. Two-dimensional (2D) arrays of magnetic nanostructures are created using silica-filled diblock-copolymer micelles as templates. After the self-assembly of the micelles into 2D hexagonal arrays, the polymer shell is removed, and the SiO2 cores are utilized to transform the morphology into a (Co/Pt)2-multilayer via ion etching under normal incidence. The number of preparation steps is kept as low as possible to simplify the formation of the nanostructure arrays. High-resolution in situ grazing-incidence small-angle X-ray scattering (GISAXS) investigations are performed during the Ar+-ion etching to monitor and control the fabrication process. The in situ investigation provides information on how the etching conditions can be improved for further ex situ experiments. The GISAXS patterns are compared with simulations. We observe that the dots change in shape from cylindrical to conical during the etching process. The magnetic behavior is studied by utilizing the magneto-optic Kerr effect. The Co/Pt dots exhibit different magnetic behaviors depending on their size, interparticle distance, and etching time. They show ferromagnetism with an easy axis of magnetization perpendicular to the film. A systematic dependence of the coercivity on the dot size is observed.

Binary information propagation in circular magnetic nanodot arrays using strain induced magnetic anisotropy

Nanomagnetic logic has emerged as a potential replacement for traditional Complementary Metal Oxide Semiconductor (CMOS) based logic because of superior energy-efficiency (Salahuddin and Datta 2007 Appl. Phys. Lett. 90 093503, Cowburn and Welland 2000 Science 287 1466–68). One implementation of nanomagnetic logic employs shape-anisotropic (e.g. elliptical) ferromagnets (with two stable magnetization orientations) as binary switches that rely on dipole−dipole interaction to communicate binary information (Cowburn and Welland 2000 Science 287 1466–8, Csaba et al 2002 IEEE Trans. Nanotechnol. 1 209–13, Carlton et al 2008 Nano Lett. 8 4173–8, Atulasimha and Bandyopadhyay 2010 Appl. Phys. Lett. 97 173105, Roy et al 2011 Appl. Phys. Lett. 99 063108, Fashami et al 2011 Nanotechnology 22 155201, Tiercelin et al 2011 Appl. Phys. Lett. 99, Alam et al 2010 IEEE Trans. Nanotechnol. 9 348–51 and Bhowmik et al 2013 Nat. Nanotechnol. 9 59–63). Normally, circular nanomagnets are incompatible with this approach since they lack distinct stable in-plane magnetization orientations to encode bits. However, circular magnetoelastic nanomagnets can be made bi-stable with a voltage induced anisotropic strain, which provides two significant advantages for nanomagnetic logic applications. First, the shape-anisotropy energy barrier is eliminated which reduces the amount of energy required to reorient the magnetization. Second, the in-plane size can be reduced (∼20 nm) which was previously not possible due to thermal stability issues. In circular magnetoelastic nanomagnets, a voltage induced strain stabilizes the magnetization even at this size overcoming the thermal stability issue. In this paper, we analytically demonstrate the feasibility of a binary ‘logic wire’ implemented with an array of circular nanomagnets that are clocked with voltage-induced strain applied by an underlying piezoelectric substrate. This leads to an energy-efficient logic paradigm orders of magnitude superior to existing CMOS-based logic that is scalable to dimensions substantially smaller than those for existing nanomagnetic logic approaches. The analytical approach is validated with experimental measurements conducted on dipole coupled Nickel (Ni) nanodots fabricated on a PMN-PT (Lead Magnesium Niobate-Lead Titanate) sample.

Theoretical formalism for collective spin-wave edge excitations in arrays of dipolarly interacting magnetic nanodots

A general theory of edge spin wave excitations in semi-infinite and finite periodic arrays of magnetic nanodots existing in a spatially uniform magnetization ground state is developed. The theory is formulated using a formalism of multi-vectors of magnetization dynamics, which allows one to study edge excitations in arrays having arbitrary complex primitive cells and lattice geometry. The developed formalism can describe edge excitations localized both at the physical edges of the array and at the internal "domain walls" separating array regions existing in different static magnetization states. Using a perturbation theory in the framework of the developed formalism it is possible to calculate damping of edge modes and their excitation by external variable magnetic fields. The theory is illustrated on the following practically important examples: (i) calculation of the FMR absorption in a finite nanodot array having the shape of a right triangle, (ii) calculation of nonreciprocal spin wave spectra of edge modes, including modes at the physical edges of an array and modes at the domain walls inside an array, (iii) study of the influence of the domain wall modes on the FMR spectrum of an array existing in a non-ideal chessboard antiferromagnetic ground state.

Coherent precession in arrays of dipolar-coupled soft magnetic nanodots

Precession modes of rectangular nanodot arrays with diameters of 90 and 150 nm and varied dipolar interaction are systematically studied by vector network analyzer ferromagnetic resonance, and micromagnetic simulations. The large dots reveal two dominant modes: a low frequency edge mode and a high frequency center mode, while in the smaller dots only the edge mode is observed. With increasing dipolar interaction, the precession-mode frequencies increase. The interaction has a stronger influence on the edge mode than on the center mode, resulting in different precession behaviors in different ranges of magnetic fields: well-separated modes in high fields and a merging of edge and center modes at low fields. At low fields and for strong dipolar interaction, coherent precession of the whole dot array is observed.

Prototype of Atomic Force Microscope with High Resolution Optical Microscope for Observing Magnetic Nanodot Arrays

This paper is dedicated to develop an atomic force microscope (AFM) system cou-pled with a high resolution optical microscope (OM), which serves to observe AFM image from a desired micro-area. The system employs through-the-lens optical path for detecting atomic force based on optical lever. By switching the objective lenses from low to high magni cation, a micro-area for obtaining AFM image can be easily found. AFM images of magnetic nanodotarrays with 300 nm and 150 nm pitches are obtained from two local micro-areas using the system. The results demonstrate the proposed prototype has the su cient function to nd out a micro-area for obtaining AFM image.

Observation of Magnetic Nanodot Arrays Using Near-Field Polarization Microscope

In this study, we tried to develop the scanning near-field polarization microscope (SNPM) and challenge to observe magnetization of the magnetic nanodot in the arrays. The CoPt dot arrays with a size of 25 nm and a pitch of 150 nm were adopted. The nanodots change contrast after changing the direction of the external magnetic field. This implied that our prototyped SNPM system had the resolution of less than 25 nm. So, it is demonstrated that SNPM is an effective tool for the observation of magnetization of the magnetic nanodot on the nanometer scale.

Fabrication of FePt and CoPt Magnetic Nanodot Arrays by Electrodeposition Process

Fabrication of Co-Pt and Fe-Pt ferromagnetic nanodot arrays for the application of ultra-high density bit patterned media (BPM) by electrochemical processes was described. In order to form patterned substrates for nanodot arrays with the density higher than 1 Tbit/inch2, electron beam lithography technique was employed. Nanopatterns with 25 nm and 18 nm pitch, which correspond to 1 and 2 Tbit/inch2 were successfully formed by optimizing the condition of the lithography process, and Co-Pt and Fe-Pt with relatively high coercivity of 4.8 kOe and 10.3 kOe were formed by electrodeposition. By utilizing the nanopatterned substrates, Co-Pt and Fe-Pt nanodot arrays with 25 nm and 35 nm pitch were uniformly formed. These results demonstrate capability of the electrochemical processes for fabricating magnetic nanodot arrays with Tbit-level density.

Fabrication of FePt and CoPt Magnetic Nanodot Arrays by Electrodeposition Process

Electrodeposition processes have been extensively used to fabricate various magnetic thin films and micro/nano structures. We have attempted to utilize these processes to form magnetic nanodot arrays for use in bit patterned media (BPM) applications [1,2] for ultra-high density magnetic recording. In this paper, we will discuss our recent results on the synthesis of magnetic nanodot arrays using electrochemical deposition, focusing on FePt and CoPt.Patterned substrates with nanopore arrays were prepared by electron beam lithography (EBL), which has the capability to form smaller-scale patterns, as well as by UV-nanoimprint lithography (UV-NIL), with larger size but high throughput. N-Si(100) wafer covered with sputter deposited Cu or Ru was used as a substrate. Electrodeposition of CoPt [3] was successfully carried out into the patterned nanopores to form arrays with 20 nm diameter dots, and fabrication of even smaller dots with <10 nm diameter has been performed. In order to achieve precise control of growth, the initial stage of the deposition process was investigated in detail, and correlation between deposition conditions such as applied potential and microstructure of the deposited dots were studied. Based upon these results, we attempted to fabricate nanodot arrays using electrodeposition of FePt, which is expected to exhibit higher perpendicular coercivity, Hc, due to the formation of the fct L10 structure. Application of FePt to BPM has been widely studied, mainly using arrays of discrete nanoparticles [2]. Based upon the bath described in previous studies [4,5], deposition conditions were optimized. It was found that layered deposition of Fe-rich and Pt-rich regions, followed by RTA (rapid thermal annealing) process was most effective in forming the L10 structure, resulting in higher Hc at thinner thickness region; in particular, a Hc of 12 kOe was obtained at < 20 nm thick films. By using this method, the patterned deposition of FePt was attempted and the array with 15 nm diameter dots was formed, the structure of which has been maintained after the annealing process.This work was financially supported in part by Grant-in-Aid for Scientific Research, MEXT, Japan and by The Storage Research Consortium.[1] D. Weller et. al, Appl. Phys. Lett. 88, 222512 (2006). [2] B. D. Terris, T. Thomson, J. Phys. D: Appl. Phys., 38, R199 (2005).[3] T. Ouchi, Y. Arikawa, Y. Konishi, T. Homma, Electrochim. Acta, 55, 8081 (2010).[4] D. Liang, J. J. Mallett, G. Zangari, J. Electrochem. Soc., 158,149 (2011).[5] S. Wodarz, Y. Maniwa, H. Hagiwara, T. Otani, D. Nishiie, G. Zangari, T. Homma, IEICE Tech. Rep., MR2013-20, 7 (2013).

Fabrication of CoPt Nanodot Array with a Pitch of 33 nm Using Pattern-Transfer Technique of PS-PDMS Self-Assembly

The progress of information technology has increased the demand of the capacity of storage media. Bit patterned media (BPM) has been known as a promising method to achieve the magnetic-data-storage capability of more than 1 Tb/in.2. In this work, we demonstrated fabrication of magnetic nanodot array of CoPt with a pitch of 33 nm using a pattern-transfer method of block copolymer (BCP) self-assembly. Carbon hard mask (CHM) was adopted as a mask to pattern-transfer self-assembled nanodot array formed from poly (styrene)-b-poly (dimethyl siloxane) (PS-PDMS) with a molecular weight of 30,000-7,500 mol/g. According to our experiment results, CHM showed its high selectivity against CoPt in Ar ion milling. Therefore, this result boosted the potential of BCP self-assembly technique to fabricate magnetic nanodot array for the next generation of hard disk drive (HDD) due to the ease of large-area fabrication, and low cost.

Fabrication of Carbon Nanodot Arrays with a Pitch of 20 nm for Pattern-Transfer of PDMS Self-Assembled Nanodots

We investigated the fabrication of self-assembled nanodot array using poly (styrene)-poly (dimethyl-siloxane) (PS-PDMS) block copolymer and its transfer technique as a promising method to fabricate magnetic nanodot arrays for ultrahigh density recording. A carbon (C) layer with a high etch-resistance was especially adopted for magnetic nanodot fabrication. We fabricated PDMS nanodot using PS-PDMS block copolymer with a molecular mass of 11,700-2,900 g/mol. The nanodots were first transferred into silicon (Si) layer and then into C layer on Si substrate by carbon tetrafluoride (CF4) and oxygen (O2) reactive ion etching (RIE), respectively. We succeeded in fabricating C nanodots with a diameter of 10 nm and an average pitch of 20 nm.

Spinovi khvyli u masyvakh mahnitnykh nanotochok, pov"yazanykh mahnitodypol'noyu vzayemodiyeyu

A general theory of collective spin-wave excitations in finite and infinite periodic arrays of magnetic nanodots with magnetodipolar coupling has been developed. Non-uniform profiles of static and dynamic magnetizations in a dot are taken into account. The theory allows the spectra of collective excitations, their damping rates, excitation efficiencies by an external microwave field, and so on to be calculated and the stability of a stationary magnetic array configuration to be analyzed. An efficient technique has been proposed to calculate the spin-wave spectra in periodic arrays using the method of projection onto the eigenmodes of a solitary nanodot. The results obtained are compared with experimental data.

Theory of ground-state switching in an array of magnetic nanodots by application of a short external magnetic field pulse

A theory of a ground-state switching in an array of axially magnetized cylindrical magnetic dots arranged in a square lattice is developed. An array can be switched into a quasiregular chessboard-antiferromagnetic state by the application of a short pulse of external in-plane magnetic field having a sufficiently long trailing front. The statistical properties of an array magnetization in its final (after switching) state are determined at the linear stage of growth of unstable collective spin-wave modes of the array under the action of a time-dependent magnetic field, and depend critically on the rate of the field decrease: the slower this decrease, the more regular is the final magnetization state. An analytical procedure is presented that allows one to relate the statistical properties of the final demagnetized state of the array and the linewidth of the array's microwave absorption to the parameters of the external switching pulse. The comparison of the developed analytic theory with the results of numerical simulations is presented and demonstrates good agreement between analytical and numerical results.

MAGNETO-OPTICAL MEASUREMENTS OF COLLECTIVE SPIN DYNAMICS OF TWO-DIMENSIONAL ARRAYS OF FERROMAGNETIC NANOELEMENTS

Magnetic nanodot arrays are interesting systems for future applications in nanotechnology including patterned magnetic media, magnonic crystals, magnetic logic, sensors, STNOs and biomedical applications. All applications require the knowledge base of magnetization processes of magnetic nanodot arrays at various time and length scales. Here, we review the present status of experimental studies of picosecond precessional magnetization dynamics in magnetic nanodot arrays. We discuss the fabrication methods of magnetic nanodot arrays and excitation and detection methods of precessional dynamics by optical means. We further discuss the all-optical excitation and detection of precessional dynamics in Ni80Fe20 (permalloy) nanodot arrays with width between 200 nm and 50 nm and with interdot separation between 50 nm and 400 nm. A transition from strongly collective dynamics to completely isolated dynamics through various weakly collective regimes, variation of precession frequency and damping with the interdot separation, effects of dipolar and quadrupolar interdot interaction, effects of the variation of dot size on the dynamics of single elements and arrays, and anisotropy of collective dynamics have been thoroughly studied by experimental and micromagnetic simulation results. Finally, we discuss the future directions in the research on the dynamics of magnetic nanodot arrays.

Localized Defect Modes in a Two-Dimensional Array of Magnetic Nanodots

The microwave properties of an array of magnetic nanodots in a ferromagnetic state having a point defect-a dot with inverted magnetization or different material parameters-are considered. The existence of a single-point defect in a dot array may lead to the appearance of several localized modes: one “defect eigenmode” and several “well modes,” the number and structure of which strongly depend on the magnetostatic interactions between the dots. By performing a ferromagnetic resonance experiment on an array of magnetic dots containing a small number of defects, it is possible to obtain information about the entire spin-wave spectrum of the array.

Fast switching of a ground state of a reconfigurable array of magnetic nano-dots

We show numerically that a ground state (ferromagnetic or chessboard antiferromagnetic) and microwave absorption frequency of a dipolarly coupled two-dimensional array of axially magnetized magnetic nano-dots can be switched by application of bias magnetic field pulses (duration 30–70 ns). Switching to the ferromagnetic state can be achieved by applying a rectangular field pulse along the dot axis while switching to the antiferromagnetic state requires the field pulse oriented in the dot plane and having a sufficiently long trailing edge (tail). Our results prove that arrays of magnetic nano-dots can be used as materials having rapidly reconfigurable magnetic and microwave properties.

Collective spin-wave excitations in a two-dimensional array of coupled magnetic nanodots

A general theory of collective spin-wave excitations in a two-dimensional array of magnetic nanodots coupled by magnetodipolar interaction is developed. The theory allows one to analytically calculate spectra, damping rates, excitation efficiencies, and other characteristics of spin waves in both periodic and aperiodic ground states of an array. It is demonstrated that all the properties of collective spin waves in an array existing in any spatially periodic ground state (e.g., ferromagnetic or chessboard antiferromagnetic) are determined by the same state-independent array's demagnetization tensor ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbit{F}}}_{\mathbit{k}}$, which is determined by the spin-wave wave vector $\mathbit{k}$, the size and shape of the array's elements (nanodots), and the geometry of the array's lattice. The applications of the developed general theory are illustrated on particular examples: (i) spin waves in ferromagnetic and chessboard antiferromagnetic states of a square array, and (ii) localized spin-wave excitations associated with an isolated ``defect'' in a uniform ferromagnetic ground state of a square array.

Non-lift-off Block Copolymer Lithography of 25 nm Magnetic Nanodot Arrays

Although nanolithographic techniques based on self-assembled block copolymer templates offer tremendous potential for fabrication of large-area nanostructure arrays, significant difficulties arise with both the lift-off and etch processes typically used for pattern transfer. These become progressively more important in the limit of extreme feature sizes. The few techniques that have been developed to avoid these issues are quite complex. Here, we demonstrate successful execution of a nanolithographic process based on solvent annealed, cylinder-forming, easily degradable, polystyrene-b-polylactide block copolymer films that completely avoids lift-off in addition to the most challenging aspects of etching. We report a "Damascene-type" process that overfills the polystyrene template with magnetic metal, employs ion beam milling to planarize the metal surface down to the underlying polystyrene template, then exploits the large etch rate contrast between polystyrene and typical metals to generate pattern reversal of the original template into the magnetic metal. The process is demonstrated via formation of a large-area array of 25 nm diameter ferromagnetic Ni(80)Fe(20) nanodots with hexagonally close-packed order. Extensive microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation. We argue that the approach is generalizable to a wide variety of materials, is scalable to smaller feature sizes, and critically, minimizes etch damage, thus preserving the essential functionality of the patterned material.