Session L14: Focus Session: Patterned and High Anisotropy Films for Data Storage

Computational study of local meta-magnetic states in manganese doped silicon nano-crystals

Using a real-space electronic structure theory with ordinary pseudo-potentials, we show that manganese doped at the center of the nano-crystals (diameters 1-5 nm) and near the surface has local meta-magnetic states which differ from their respective ground states by few tens to hundreds of meV in energy but larger than $K_{B}T$ at room temperature so they are not switchable easily at room temperature. We discuss origin of such meta-magnetic states and argue about a possible switching between these meta-magnetic states for potential information storage applications   

Thickness dependence of magnetization switching of sub-100 nm magnetic nanodots.

Vortex and single domain states are found in sub-100 nm Fe planar nanodots, fabricated by electron-beam evaporation using self-ordered porous alumina masks.[1] The thickness dependence of the magnetization reversal is studied by in-plane and out-of-plane SQUID measurements, and the thickness is verified using low-angle X-ray diffraction. The hysteresis loops of 53 nm and 66nm diameter dots changes with the dot thickness. This is attributed to the magnetization reversal occurring via a combination of in-plane and out-of-plane single domain and vortex states, as obtained from micromagnetic simulation. The 66 nm diameter and thickness above 50 nm dots are expected to develop a funnel-like configuration with two in-plane vortices of opposite chiralities as an out-of-plane field is applied. [1] C.-P. Li \textit{et al}., J. Appl. Phys. \textbf{100}, 074318 (2006).   

Imaging Magnetic Nanostructures via Resonant Soft X-Ray Spectro Holography

We will present how to exploit the coherence and tunable polarization of soft X-ray synchrotron radiation for imaging magnetic nanostructures via Fourier Transform Holography. This new lensless imaging technique is based on the direct Fourier inversion of a holographically formed soft x-ray interference pattern. Our implementation is particularly simple and is based on placing the sample behind a lithographically manufactured mask with a micron-sized sample aperture and a nano-sized reference hole. By exploiting the magnetic dichroism in resonance at the L$_{3}$ edges of the magnetic transition metals, images of magnetic nanostructures have been obtained with a spatial resolution of 50 nm. Different examples will be presented. The technique is transferable to a wide variety of specimen, appears scalable to diffraction-limited resolution (about 2 nm), and is well suited for ultra-fast single-shot imaging with future X-ray free electron laser sources.   

Patterned Magnetic Media: Recording Properties and Fabrication Issues

As the onset of thermal instability with decreasing grain size makes the extension of conventional sputtered granular media to higher data recording densities increasingly difficult, the use of nanopatterning techniques to create well-ordered arrays of isolated, highly uniform magnetic islands may enable a new generation of recording media extendible to densities of 1 terabit per square inch and beyond. A workable patterned media recording system requires a highly uniform magnetic island array, both in terms of dimensional parameters (island size, shape, and placement tolerance within the array) and materials properties (magnetic moment and switching field). Selectively writing individual tracks of islands without affecting neighboring islands requires a write transducer with sufficiently high field gradient and peak field. The island size range of interest (15-25 nm diameter islands on 20-40 nm array periodicity) makes fabrication of patterned media particularly challenging. One strategy for media fabrication is to create a high resolution master pattern via e-beam lithography and/or self-assembly, and to replicate this pattern over thousands of media samples using UV-cure nanoimprint lithography. The imprinted pattern can serve as an etch mask for patterning either the media substrate or magnetic layer. Trenches between islands may be filled to create a smooth surface suitable for flying a read/write head over the media surface at a spacing of a few nm. Although a variety of magnetic materials may be used, multilayer Co-Pt or Co-Pd are preferred based on their perpendicular anisotropy, moment, switching field, and strong coupling between grains (necessary to ensure that islands switch as a single unit). Depending on the fabrication method used, magnetic material in the trenches between islands can generate unwanted magnetic flux which generates noise in the readback signal. Island nonuniformity (both dimensional and magnetic) also contributes to increased errors in writing and increased noise in readback. Write errors may be generated via imperfect synchronization of the switching of the write field as the write transducer passes over the media. Tight tolerance control is required both for write synchronization and positional tracking of the head over the island array.   

Origins of switching field distributions in nanopatterned Co/Pd multilayers

We studied the reversal properties of perpendicularly magnetized nanodot arrays down to 50 nm in diameter. When continuous films undergo nanopatterning the phenomenon of a switching field distribution (SFD) becomes significant whereby the reversal fields from nanostructure-to-nanostructure vary. In applications such as patterned storage media, a broad SFD is undesirable since all nanostructures will not reliably switch at the same applied field. The origin of this phenomenon has been attributed primarily to grain boundary variations within a nanostructure as well as lithographic variations that occur during processing. While these two factors will certainly contribute to a larger SFD, we find that the primary origin of a SFD is an intrinsic material property of the continuous film. We will present our results of nanostructured Co/Pd exchange coupled multilayers. By changing the material properties using various seed layers and growth conditions, we were able to reduce the SFD to below 5 {\%} of the average switching field. By studying both polycrystalline and epitaxial multilayers we isolated the effects of grain boundary variation since epitaxial nanostructures are all single grain with identical orientation.   

Intrinsic switching field distribution in perpendicular recording media: numerical study of the $\Delta H(M, \Delta M)$ method

We present a numerical study of the $\Delta H(M,\Delta M)$ method and its ability to accurately determine intrinsic switching field distributions in interacting granular magnetic materials such as perpendicular recording media. In particular, we study how this methodology fails for large ferromagnetic inter-granular interactions, at which point strongly correlated magnetization reversal sets in that cannot be properly represented by the $\Delta H(M,\Delta M)$ method, which is based on a mean-field approximation. In this study, we use a 2-dimensional array of square hysterons that have a distribution of intrinsic switching fields and ferromagnetic next-neighbor interactions $J$. We find the $\Delta H(M,\Delta M)$ method to be very accurate for small $J$ values, while substantial errors develop once the effective exchange field becomes comparable with the intrinsic switching field distribution width, corroborating earlier results from micromagnetic simulations. This failure is correlated with deviations from data set redundancy, which is a key property of the mean-field approximation. Thus, the $\Delta H(M,\Delta M)$ method fails in a well defined and quantifiable manner that can be easily assessed from the data sets alone.   

Optimizing Graded Recording Media

A key limitation of recording densities arises from the fact that maintaining high thermal stability requires high anisotropies, whereas writeability requires low anisotropies: yielding contradictory requirements. Recently, Victora and Shen proposed that the recording density of perpendicular media is increased in exchange spring media: a structure with a soft and a hard layer. A decrease in coercivity up to a factor of 4 has been predicted. Very recently Suess considered a tri-layer system, reporting further increase. In the present work, we report optimizing media where the anisotropy is a continuous function of the thickness. We performed extensive finite element simulations and optimized the media performance by minimizing the coercivity, while maintaining a high energy barrier against thermal decay and the squareness of the hysteresis loop. Simple analytic estimates suggest that a quadratic thickness dependence is optimal. We explore the role of anisotropy convexity, a hard capping layer, and the exchange interaction. This graded anisotropy media decouples minimizing the coercivity while maximizing the barrier height, promising efficient new ways to optimizing recording media.   

Figures of Merit for Magnetic Recording Media

Since the first nucleation-field calculations for hard-soft nanostructures with multilayered [1] and arbitrary [2] geometries, exchange-spring magnets have attracted much attention in various areas of magnetism, including magnetic recording. Ultrahigh storage densities correspond to the strong-coupling limit, realized on small length scales and described by volume-averaged anisotropies. Second-order perturbation theory yields finite-size corrections that describe a partial decoupling of the phases. Since soft phases reduce the nucleation field, nanostructuring can be used to reduce the coercivity $H_{c}$ while maintaining the energy barrier $E_{B}$. However, the ratio $E_{B}$/$H_{c}$ is an ill-defined figure of merit, because the comparison with the Stoner-Wohlfarth model requires the introduction of a particle volume, as contrasted to an area. By using elongated particles with a continuous anisotropy gradient, it is possible to reduce the coercivity by a factor scaling as the bit size divided by the domain-wall width of the hard phase. However, with decreasing bit size this effect becomes less pronounced. In the strong-coupling limit, thermal stability yields a maximum storage density of order \textit{$\gamma $}/$k_{B}T$, where \textit{$\gamma $} is the domain-wall energy of the hard phase. - This research is supported by NSF MRSEC, INSIC, and NCMN. [1] S. Nieber and H. Kronm\"{u}ller, phys. stat. sol. (b) \textbf{153}, 367 (1989). [2] R. Skomski and J. M. D. Coey, Phys. Rev. B \textbf{48}, 15812 (1993).   

Experimental Study of Perpendicular Exchange Spring Media

We have investigated the magnetic reversal and recording properties of perpendicular exchange spring media. These structures, which combine a soft and a hard layer material$^{1}$, have recently been proposed as suitable candidates for advanced perpendicular magnetic recording$^{2}$. Previous studies$^{3}$ have also shown that the magnetization reversal can be tuned by means of a suitable coupling layer. In our study, we have investigated structures that consist of two magnetic layers having different H$_{K}$-values and being separated by a coupling layer of adjustable thickness. Similar to the results of our previous work on longitudinal exchange spring media$^{4}$, we find that there is an optimum coupling layer thickness, at which the magnetic reversal field is minimized. We also observe in our magnetometry experiments that the most robust parameter to quantify this improved magnetization reversal is the closure field H$_{S}$. The anticipated writability improvements for exchange spring media with optimal interlayer coupling strength are corroborated by detailed recording studies. [1] E.E. Fullerton et al., Phys. Rev. \textbf{B} \textbf{58}, 12193 (1998); [2] R. Victora et al.$, $IEEE Trans. MAG \textbf{41}, 537 (2005); [3] K.C. Schuermann et al., J Appl. Phys. \textbf{99}, 08Q904 (2006); [4] N. Supper et al., IEEE Trans. MAG \textbf{41}, 3238 (2005)   

Texture development and magnetic properties of Ru-doped FePt films

L1$_{0}$ ordered FePt films are promising candidates for ultra-high density recording media due to their high magneto-crystalline anisotropy when grown with (001) texture. In this paper, the effects of Ru doping on the FePt L1$_{0}$ phase formation and development of (001) texture are studied systematically. Ru doping is realized by preparing Fe/Pt/Ru multilayers by magnetron sputtering on SiO$_{2}$ substrates, with subsequent annealing at 650 $^{o}$C for 5 minutes in forming gas or hydrogen gas. It appears that hydrogen gas annealing leads to improved (001) texture. For small Ru alloying (less than 5 at. {\%}), the L1$_{0}$ texture and degree of chemical ordering remain the same. X-ray diffraction analysis shows that the (001) and (002) peaks shift slightly to larger angles, indicating that the Ru is dissolved in the FePt L1$_{0}$ phase. Increasing the Ru concentration beyond 5 at. {\%} resulted in an increasing (111) texture and a steady decrease of both coercivity and saturation magnetization. The effects of Ru on the magnetization and the magnitude of the coercivity have been studied. The mechanism by which Ru doping influences the texture development also will be presented in this work.   

Real-time thermal annealing studies in FePt thin films and nanostructures

$L$1$_{0}$-ordered FePt thin films and nanostructures have been heavily studied for ultrahigh-density recording applications taking advantage of the very large perpendicular magnetic anisotropy this phase exhibits ($\sim $10$^{7}$ erg/cc). A high degree of $L$1$_{0}$ order can be achieved by optimizing deposition conditions and/or performing annealing treatments. Here, we report on recent real-time thermal annealing studies of Fe-implanted Pt thin films that begin to exhibit chemical ordering upon annealing at $\sim $400$^{o}$ C. Clusters of Fe are implanted onto a Pt thin film using the Toledo Heavy Ion Accelerator (THIA) in which their size and penetration depth can be tailored by modifying the implantation conditions. These annealing studies were partially performed at the MHATT/XOR beam line (Sector 7) at the Advanced Photon Source at Argonne National Laboratory.   

Exchanged-Coupled FePt Cluster Films

L1$_{0}$ structure FePt films with (001) texture have attracted much attention for potential application in high-density perpendicular recording. In an effort to fine-tune their magnetic properties, novel structures of continuous FePt coupled to a FePt nano-composite layer have been investigated. The FePt layer, called the Continuous Layer (CL), was magnetron sputtered from a Pt target partially covered with Fe chips. The nano-composite layer uses carbon as a matrix and was made by two different methods: magnetron sputtered multi-layers of FePt and C, and cluster deposited $\sim $5 nm FePt particles with the C sputtered on top. All films were deposited on thermally oxidized Si substrates and processing was done for 300 seconds at 600$^{o}$C in an Ar with 5{\%} H$_{2 }$environment. Characterization was done with SQUID magnetometry and XRD. It was found that for the sputtered bilayer films, fixing the thickness of the nano-composite layer (5, 10, 15, or 20 nm) and varying the CL from 2 to 14 nm gave an increase in the coercivity of the film. The films with cluster deposited FePt particles showed exchange coupling after annealing and a decrease in coercivity over the pure CL. Models of these systems will be discussed in the talk. This work was supported by NSF-MRSEC, NCMN, DOE, INSIC and NRI.