L10 FePt-based thin films for future perpendicular magnetic recording media

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L10 FePt-based thin films for future perpendicular magnetic recording media G. Varvaro, S. Laureti, D. Fiorani n Istituto di Struttura della Materia, ISM—CNR, 00015 Monterotondo Scalo, Roma, Italy

art ic l e i nf o

a b s t r a c t

Article history: Received 18 February 2014 Received in revised form 23 April 2014 Accepted 23 April 2014

Current magnetic recording media using perpendicular CoCrPt-Oxide granular films are reaching their physical limit (approx 750 Gbit/in2 density) due to thermal fluctuations that hinder a further reduction of grain size (o 6–7 nm) needed to scale down the bit size. L10-FePt alloy is currently considered the most promising candidate for future recording media with areal densities above 1 Tbit/in2 thanks to its high magneto-crystalline anisotropy (K ¼6–10 MJ/m3), which enables it to be thermally stable even at grain sizes down to 3 nm. However, its huge anisotropy implies an increase of the switching field, which cannot be afforded by current available write heads. To simultaneously address the writability and thermal stability requirements, exchange coupled composite media, combining two or multiphase hard and soft materials, where the hard phase provides thermal stability and the soft phase reduces the switching field, have been recently proposed. This paper briefly reviews the fundamental aspects as well as both experimental approaches and magnetic properties of L10 FePt-based single phase films and exchange coupled systems for future perpendicular magnetic recording media. & 2014 Published by Elsevier B.V.

Keywords: Perpendicular recording media FePt thin film Exchange coupled composite media Fe/L10-FePt film FePt graded film

1. Introduction The demand for digital data storage devices is growing continuously and it seems to be endless, coming from every sector of the modern society. A number of different storage technologies are currently available, such as hard disk drives (HDDs), optical drives, magnetic tapes, flash and solid state memories and novel technologies are being developed, such as spintronic and race-track memories, among others [1–6]. Each of these technologies has advantages and disadvantages making it more suitable for specific end-market applications in terms of capacity, performance, reliability and cost. For massive data storage, magnetic recording is still the dominant technology and the HDD [1,2], which is the key component of this technology, is the most diffuse device with 577 millions of units sold in 2012. The main advantage of the HDD with respect to its competitors is the combination of a very large density (above 700 Gbit/in2) and a very low cost of bit information (cost/Gbyte  0.05 $). Despite solid state memories have gained momentum in the global markets as computers and electronic devices have become increasingly mobile, the recent progress of the cloud storage technology, which involves storing huge amounts of data on multiple servers, has given new life to HDDs market, thus further stimulating the research towards the

n

Corresponding author. E-mail address: dino.fi[email protected] (D. Fiorani).

development of new hard disk devices with recording densities overcoming 1 Tbit/in2. In currently available HDDs, the recording layer of the medium consists of a granular thin film, with ideally fully decoupled grains, exchange coupled with a continuous layer, this structure assuring a high thermal stability and a large signal-to-noise ratio [1]. The digital information is physically hold in the granular layer, which consists of a ferromagnetic CoCrPt-Oxide thin film, made of columnar single-domain Co-rich magnetic grains (about 7 nm in diameter) with perpendicular uniaxial anisotropy (K 0.5 MJ/m3) separated by non magnetic Cr and oxide grain boundaries [1,2,7,8]. A cluster of multiple grains is used to record a single bit, which is approximately 50 nm long and 50 nm wide in the current devices. Increasing recording density above 1 Tbit/in2 requires to scale down the recording bit size, facing at the same time the so called trilemma, i.e. the need of simultaneously satisfying three conflicting requirements: low noise, writability and high thermal stability [9,10]. As the number of grains per bit must be kept constant to minimize the statistical noise and then ensure an adequate signalto-noise ratio (25–30 dB), the only way to reduce the bit dimension consists in decreasing the in plane grain size, which also leads to a decrease of the grain volume (V). Moreover, the thickness of the recording layer cannot exceed a threshold value (around 10–15 nm), above which the signal-to-noise ratio rapidly decreases [11]. For a given material, there is a lower limit in the grain size, called superpamagnetic limit, below which the energy barrier at zero external field ΔE0 ¼KV between the two magnetic states

http://dx.doi.org/10.1016/j.jmmm.2014.04.058 0304-8853/& 2014 Published by Elsevier B.V.

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(magnetization up and down), representing the binary number “0” and “1”, becomes smaller than the thermal energy kBT at room temperature, making the stored information unstable due to thermal fluctuations. For CoCrPt-Oxide this physical limit has been almost reached, and therefore new materials and novel recording solutions are necessary. The thermal stability issue, which requires ΔE0 440–60 kBT300 for ten years data retention, could be in principle overcome by using materials with very high anisotropy. Among the possible magnetic materials for recording media, L10 FePt alloy is currently considered the most promising candidate, thanks to its high magneto-crystalline anisotropy (K¼ 6–10 MJ/m3) [12], which enables it to be thermally stable even at grain sizes down to 3 nm. While the L10 FePt alloy would allow satisfying the criteria of thermal stability, its extremely high anisotropy results in an increase of switching field, which far exceeds the writing fields (μ0Hw) of currently available write heads (μ0Hw oBs with Bs,max ¼2.4 T). To overcome the problem of writing field limitations, novel approaches and media architectures were proposed, which allow reducing the switching field without compromising thermal stability. The two most promising solutions are energy assisted recording [13,14] and exchange coupled composite (ECC) media [15–22]. In energy assisted recording media, a local source of energy, either by heat (HARM) [13] or microwave (MARM) [14], is used to temporary lower the switching field during the writing process; as the external source is removed, the high anisotropy of the medium guarantees the retention of data over time against thermal fluctuations. In ECC media [15–22], the switching field of the hard magnetic layer is reduced by the exchange field of a neighboring softer magnetic layer, without compromising thermal stability being mainly determined by the anisotropy of the hardest magnetic phase. Soft and hard magnetic phases can be separated either by a sharp interface (exchange spring media) [15–19] or by a region where the anisotropy gradually varies from high to low values (graded media) [15,20–22]; the latter was proposed to yield a larger reduction of the switching field for similar values of the thermal stability. A different approach to overcome the thermal stability issue involves the use of so-called bit patterned media (BPM), which consist of a ordered two-dimensional array of individual magnetic nanostructures with perpendicular anisotropy, each of them representing one bit of information, obtained by nanolithography and/or selfassembly techniques [23–29]. Each element is exchange isolated from the others and behaves as a single-domain unit, which may consist of a few strongly coupled grains. The energy barrier is thus no longer governed by the grain volume, as in granular media, but rather by the entire bit volume. The larger magnetic switching volume ensures high thermal stability without needing high anisotropy materials for recording densities up to 10 Tbit/in2. The future evolution of the magnetic recording data storage towards its ultimate limit is expected to involve a combination of the different approaches (e.g. ECC-BPM, BPM-HARM) in order to avail the benefits of each technology [24,29–32]. This paper is intended to be a short review on the progress in the investigation of single phase and exchange coupled FePt-based thin films for future magnetic recording media. Section 2 will report on the fundamental properties of L10 FePt-based films and the techniques used to achieve suitable magnetic and microstructural properties. Section 3 will illustrate the basics of ECC media and will report on the preparation approaches and magnetic properties of FePt-based ECC systems.

which has a chemically disordered fcc structure, with a random distribution of the Fe and Pt atoms in the cubic cell (a ¼3.82 Å), and the L10 phase, with a chemical ordered fct structure (a¼ 3.85 Å and c ¼3.71 Å), where the two atoms are arranged into a superstructure consisting of monoatomic layers of pure Fe and Pt alternating along the [0 0 1] direction, i.e. c-axis of the tetragonal unit cell (Fig. 1). The chemical ordering in the L10 phase gives rise to a very strong uniaxial anisotropy (in excess of several MJ/m3) along the stacking direction, due to the large spin–orbit coupling on the paramagnetic atoms (Pt) and the strong hybridization of their 5d bands with the high polarized 3d bands of the ferromagnetic atoms (Fe) [33,34]. On the contrary, the lack of chemical order in the A1 FePt alloy makes this phase magnetically soft ( 0.1 MJ/m3). Although the L10 phase is thermodynamically stable at room temperature, high temperature processes (substrate heating and/ or post-deposition annealing treatments till to 600–700 1C) are required to overcome the energy barrier for atomic diffusion, which enables the chemical arrangement and the subsequent formation of the L10 FePt phase. The chemical ordering can be quantified by the order parameter S, which is equal to 1 for fully ordered alloys, and to 0 when the atomic arrangement is completely random [33]. The order parameter rises with the increase of temperature and has a maximum value around a FePt equiatomic composition [35]. The magnetocrystalline anisotropy of the FePt alloy strictly depends on the order parameter, i.e. the larger is S the higher is K, although it is also affected by the stoichiometry, being larger for a composition with a slight excess of Fe [36]. In order to use FePt films for future perpendicular recording media, many technical challenges are being addressed: enhancing the order parameter at low temperatures, improving (0 0 1) texture and obtaining a granular morphology consisting of small grains with a narrow size distribution, among others. To promote the ordering process at lower temperatures (down to approx 300 1C), different approaches have been investigated. It has been reported that, starting with a multilayer precursor obtained by alternate deposition of very thin Fe and Pt layers, the ordering process can be significantly enhanced, then reducing the temperature for the formation of the fct phase, due to the rapid interdiffusion at the Fe/Pt interfaces [37–40]. Being the transformation of the FePt from the cubic to the tetragonal phase accompanied by a variation of the crystalline structure (increase of a and decrease of c), the chemical ordering can be also promoted by the application of a strain or stress by using either a high Ar pressure during the sputtering process [41] or a suitable underlayer (e.g. CrX, X¼Ru, Mo, W, Ti) with a lattice constant larger than that of FePt [42]. Moreover, it was reported that doping the FePt film with a third element such as Au, Ag, and Cu can enhance the chemical ordering of the alloy, thus enabling the formation of the L10 phase at relative low process temperatures [43,44]. As an example, it was found that adding Cu as dopant allows reducing the ordering temperature down to 300 1C [43]. The induced order was

2. Hard FePt-based thin films FePt binary alloy in the composition range between Fe45Pt55 and Fe65Pt35 can exist in two different forms [33], the A1 phase,

Fig. 1. Crystal structure of the chemically disordered (A1 fcc) and chemically ordered (L10 fct) FePt phases.

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demonstrated to be related to the formation of a ternary FePtCu alloy where Fe atoms in the lattice are substituted by Cu [45–47]. The role of the Cu addition on the chemical and crystallographic features was carefully investigated by means of a detailed EXAFS analysis of FePtCu films with different Cu content [47]. The analysis carried out at the Pt-edge revealed that the chemical order around the Pt atoms is strongly influenced by the addition of Cu atoms, consistently with the variation of the magnetocrystalline anisotropy. On the other hand, the effect of Cu atoms in the alloy is a lattice distortion with a linear reduction of the c/a ratio, although this effect has a minor influence on the magnetocrystalline anisotropy when compared to the effect of the chemical ordering. To obtain FePt films with a strong (0 0 1) texture, single crystal MgO(1 0 0) substrates were used in the early studies [37,38,48]. However, such a substrate is not feasible for industrial applications because of its cost and many efforts have been devoted to the fabrication of ordered L10(0 0 1) films on more suitable substrates, like glass or Si by using different underlayers such as MgO, CrRu, RuAl and TiN [49–51]. Although MgO resulted a good underlayer for growing L10 FePt(0 0 1) films, industrial requirements restrict the use of such underlayer due to the need of RF sputtering. For this purpose, it has been recently proposed a new material, i.e. (Mg0.2Ti0.8)0, which can be DC sputtered and was demonstrated to promote a strong (0 0 1) texture [52]. To control the size and shape of grains as well as to minimize the exchange interactions among them, different segregant materials (e.g. SiOx, TiOx, MgO, B and C, among others) were proposed [53]. In order to reach the desired requirements no one of the investigated segregants has still shown the desired properties. For example, B, SiOx and TaOx promote columnar fine grains (the TiOx leading to smaller sizes and a better isolation), but produce a larger reduction of the chemical order with respect to carbon, which, on the other side, promotes ball-like structures. Optimization of deposition parameters, combinations of various segregants, or investigation into other segregant materials are needed to develop granular L10 FePt films for future recording media.

3. FePt-based exchange coupled composite films 3.1. Basics of ECC media ECC media (Fig. 2) allow tailoring the switching field and thermal stability independently thanks to the inhomogeneous reversal process that develop in such systems, which is schematically described by the domain wall assisted switching model [15–22]. For a soft/hard exchange spring system, the magnetization reversal under the action of an external field is a two-step process. In the first step a nucleation forms in the soft part at a critical field Hn; in the second step, the formed nucleation propagates to the soft/hard interface where it becomes pinned. The system completely reverses when the external field is larger than the pinning

Fig. 2. Schematic illustration of a magnetic grain with three different structures: single phase, exchange spring and graded. KS and KH are the anisotropy constants of the soft and hard phases, respectively.

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field Hp. The switching field Hsw is determined by the maximum value between Hn and Hp. Under the assumption that the soft and hard regions are thick enough to fit a full domain wall in each phase, and for the particular case when the spontaneous magnetization (Ms) and the exchange constants (A) are the same in both phases, Hn and Hp can be determined analytically and are written as: Hn ¼

2K S Aπ 2 þ μ0 Ms 2t 2S μ0 M s

ð1Þ

Hp ¼

KH KS  2μ0 M s 2μ0 M s

ð2Þ

where tS is the thickness of the soft part and, KS and KH are the anisotropy constants of the soft and hard phases, respectively. If the conditions are such that Hp 4Hn, the switching field equals the pinning field and it assumes a value lower than that of the single hard phase (KH/2μ0MS), in agreement with Eq. (2). Despite the reduction of the switching field, thermal stability is only determined by the magnetic properties in the hardest magnetic region. Indeed, simplified somewhat, pffiffiffiffiffiffiffiffiffi the energy barrier at zero external field ΔE0 is proportional to 4 AK H (i.e. the domain wall energy in the hard phase); this means that, for the special case that KS ¼ 0, a single phase with a anisotropy constant K has the same switching field of a soft/hard structure with KH ¼4K and thus a gain in the energy barrier of a factor of 2 can be achieved; for higher values of Ks the gain can be further improved. A larger reduction of the switching field for similar values of the thermal stability can be achieved in the graded media, where the anisotropy is continuously varied along the thickness from high to low values. Assuming that Hp 4Hn, the switching field is equal to the pinning field, which can be approximated, for a pffiffiffiffiffiffiffiffiffi quadratic anisotropy profile, by H p ¼ 2=μ0 M s AK H =t G ; where tG is the thickness of the graded region. Increasing tG allows obtaining much smaller switching fields with respect to conventional soft/hard exchange spring systems, while the energy barrier at zero external field is, to a first approximation, proportional to t G ; hence, in principle, the gain in energy barrier of graded media goes to infinity for t G -1: For practical applications, the maximum available thickness is tied to several factors, and a maximum gain in the energy barrier of a factor 4 compared to a single phase was theorized. 3.2. Fabrication and properties of FePt-based ECC media Since the concept of ECC media has been proposed, L10 FePtbased ECC systems either with a sharp or a graded phase boundary have been widely investigated with different softer regions designs. A lot of attention has been devoted to the exchange spring Fe/ L10-FePt(0 0 1) system, due to the high saturation polarization of the soft magnetic Fe phase (μ0Ms ¼2.15 T), which would ensure a significant reduction of the switching field and a good lattice match between Fe and L10-FePt phases. Extensive work has been carried-out to study the effect of intrinsic (e.g. saturation magnetization and anisotropy of the soft and hard phases) and extrinsic properties (e.g. interface structure, thickness and microstructure of the soft and hard phases) on the switching behaviour and reversal mechanism [16,54–60]. Among the different parameters, the thickness of the soft Fe phase was found to play a fundamental role in the switching field reduction and magnetization reversal [16,19,54–58]. It has been reported that when the thickness of the soft phase (tS) is smaller than the effective interlayer exchange ffi pffiffiffiffiffiffiffiffiffiffiffi theor coupling length λex ( r λex;FePt ¼ A=K H  2 nm, where A¼1  10  11 J/m is the FePt exchange stiffness constant, and KH the anisotropy of the hard phase), due to the strong coupling effect, soft and hard moments remain collinear and the composite system exhibits a rigid magnet behaviour, i.e. the two phases reverse

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coherently; by increasing the soft layer thickness (till to tS 4 λex), the magnetostatic effects become predominant forcing the Fe moment towards the in-plane direction and a transition to the exchange spring regime occurs, which is characterized by a gradual reduction of both perpendicular remanence and coercivity with the increase of the Fe layer thickness. As an example, Fig. 3 reports the out-of-plane angular remanence curves (i.e. the trend of remanence as a function of the ϕ angle between the applied field and the normal to the film plane; see inset of Fig. 3) of a set of Fe/L10-FePt(0 0 1) bilayers with a soft layer thickness varying from 0 to 5 nm [54]. In a simplified model, the total remanent moment along the field direction in a multilayer system is the sum  vector  of moment of each layer, i.e. mrem φ ¼ ∑ mirem  the  remanent  i  cos φ  φi , where mi rem is the remanent moment of the ith layer and ϕi is the angle between the normal to the film surface and the easy axis of the ith layer. Hence, the shape of the remanence curve (i.e. number and positions of minima and maxima), gives a rough indication of the easy axis arrangement in a multilayer system [61]. In the case of the single FePt layer, the curve shows the two-fold symmetry typical of a uniaxial system with a strong perpendicular anisotropy (i.e. mrem is max at ϕ ¼ 01); such symmetry is still retained at the lowest Fe layer thickness (2 nm), suggesting a good out-of-plane alignment of hard and soft moments (rigid magnet behavior). For soft layer thicknesses exceeding the effective interface exchange coupling length (i.e. tS ¼3.5 and 5 nm), a more complex figure is observed, which can be explained as the result of the combination of the angular remanence curves of two layers with the easy axis at ϕ ¼01 and 901, the different position of maxima being related to the different FePt mFe rem =mrem ratio in the two samples. The measurements clearly indicate that for tS 4 λex the magnetostatic effects are predominant thus forcing the Fe moments towards the film plane, and suggest the transition to the exchange-spring regime as confirmed by other experimental evidences reported in details in [54]. Although a significant reduction of the switching field was demonstrated for the Fe/L10-FePt(0 0 1) ECC structure, the inplane anisotropy of the Fe layer was suggested to be disadvantageous for thermal stability [15]. Better performance can be obtained by using as the softer layer, [Co/X]n films (X¼ Pt, Pd, Ni), with perpendicular magnetic anisotropy (originating form the Co/X interface), whose magnitude can be modulated varying the thickness and the number n of layers [62]. To date, only a few experimental works have been carried-out to prepare exchange coupled composite systems with a perpendicular configuration [63,64]. They show a larger thermal stability with respect to that achievable with the in-plane Fe/FePt structure, although a smaller reduction of switching field is observed because of the reduced saturation magnetization of the softer phase [15]. Since graded media were predicted to provide additional gain in writability compared to exchange spring composites, while maintaining similar thermal stability, several experimental approaches have been investigated in the last few years to design

and fabricate FePt-based ECC media with a controlled anisotropy graded interface. The anisotropy profile can be controlled by tuning either the atomic FePt composition or the chemical ordering along the film thickness. FePt films with a compositional gradient were fabricated by thermally inducing atomic diffusion in Fe/FePt and FePt/Pt systems during deposition at high temperatures or by post-deposition annealing treatments [25,65–68]. The chemical ordering was found to be gradually tuned by ion irradiation of a L10-FePt film [69], or by varying the percentage of dopant and segregant materials, like Cr, C and TiO2, all of them playing a key role in hindering FePt transformation from the A1 low K to the L10 high K phase [70–72]. Alternatively, a control of the chemical ordering can be achieved by continuously decreasing the deposition temperature [73–76]. This approach was successfully used to prepare FePt(A1CL10)graded/L10-FePt films with variable graded layer thicknesses (tG), which correspond to different final deposition temperature values (Tf); i.e. the larger was tG the smaller was Tf [73]. Using this procedure, thin films with a graded layer consisting of pure L10 and A1 phases separated by a rough interface were obtained instead of crystallographically homogenous layers with a gradually reduced chemical order, as supposed for conventional graded media [74,75]. Composite systems with such a microstructure, called phase graded media, show the same features of conventional graded systems, i.e. improved writability and high thermal stability [74,75]. Static magnetic measurements confirmed a significant reduction of the out-ofplane coercivity with the increase of the graded layer thickness (from  3.1 to  0.3 T), thus indicating that the writability can be greatly improved. It is worth noting that as prepared graded/hard

Fig. 4. FePt(A1CL10)graded/L10-FePt films. Room temperature normalized perpendicular (– ● – ) and in-plane (– ○ – ) hysteresis loops for tG ¼ 14 nm. (from Ref. [73]).

Fig. 3. Out-of-plane angular remanence curves (i.e. Mr/Ms vs. ϕ) of Fe/L10-FePt(0 0 1) bilayers for different soft layer thicknesses: (a) 0, (b) 2, (c) 3.5 and (d) 5 nm. (from Ref. [54]).

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sat Fig. 5. Dynamic measurements of FePt(A1CL10)graded/L10-FePt films with tG ¼14 nm. (a) Room temperature normalized remanence curves (M r =M sat is the r ; where M r saturation remanence) around the remanence coercivity Hrc (  Hsw) for different waiting times (10 ot o 10,000 s); the field was applied at an angle of 451 with respect to the easy axis, which is normal to the film surface. (b) Switching field as a function of waiting time (– ● –) together with Sharrock's fitting curve (solid line).

films maintain the perpendicular anisotropy also for larger graded layer thicknesses, as shown in Fig. 4 for tG ¼14 nm. Dynamic measurements were also performed to determine the so-called stability factor ΔE0/kBT300 and then the thermal stability of the system. A series of remanence curves were measured using field waiting times t between 10 and 10,000 s, i.e. a period t was waited before measuring the remanence for each reverse field (Fig. 5a). From each of these curves, the values of switching field (defined as the point where the remanence is equal to zero) were extracted and plotted as a function of the waiting time (Fig. 5b). To determine the stability factor, the experimental data were fitted by the Sharrock's equation, i.e. Hsw ¼Hsw0{1 [(kBT/ΔE0)ln(At)]1/n} [77], where A is the frequency factor (assumed to be 109 Hz), kB is the Boltzmann constant, Hsw0 is the time independent switching field (i.e. the switching field at 0 K) and n is an exponent that was set to 1 because of the linear field dependence of the energy barrier in graded media [21,22]. Note that the remanence curves were measured with the external field applied at 451 with respect to easy axis as this method allows to accurately measure the energy barrier ΔE0, being the exponent n almost independent on the strength of the applied field for measurements under 451 [78,79]. For samples with a graded layer thickness of 14 nm, a stability factor of about 250 (4 40–60) was calculated, confirming that such FePt-based phase graded systems are promising for future ultra high density magnetic recording, as they combine good writablity and high thermal stability. Although many approaches have been demonstrated to be effective in tuning the anisotropy in the graded layer, the industrial constraints will actually select the most suitable one for the next generation of recording media.

4. Summary In the last few years, a significant progress in HDD's technology has been achieved by developing new materials as well as novel media architectures and recording designs, which allow overcoming the limits imposed by signal-to-noise ratio, thermally stability and writability requirements. The fundamental properties, the experimental approaches as well as the magnetic properties of both single phase and ECC FePt-based systems were reviewed. FePt-based ECC media are currently considered a

promising candidate to push the recording density in the Tbit/ in2 regime. Despite the significant progress, a lot of issues still need to be addressed and further improvements are necessary to produce a new generation of recording media. Beside ECC media, other solutions are under investigation for future recording media, such as energy assisted recording and bit patterned media; the future evolution of the magnetic recording data storage toward its ultimate limit is expected to involve a combination of the different approaches in order to avail the benefits of each technology.

Acknowledgment This work was financially supported by the European Commission FP7 project TERAMAGSTOR (contract N. FP7-ICT-2007-2-224001). References [1] S.N. Piramanayagam, T.C. Chong, Developments in Data Storage, John Wiley & Sons, 2011. [2] S.N. Piramanayagam, K. Srinivasan, J. Magn. Magn. Mater. 321 (2009) 485–494. [3] R.E. Fontana, S.R. Helzler, G. Decad, IEEE Trans. Magn. 48 (2012) 1692–1696. [4] F. Tiziani, M. Iaculo, Memory Mass Storage, Springer, 2011. [5] D.D. Tang, Y.-J. Lee, Magnetic Memory: Fundamentals and Technolgy, Cambridge University Press, 2010. [6] S.S.P. Parkin, M. Hayashi, L. Thomas, Science 320 (2008) 190–194. [7] G. Varvaro, A.M. Testa, E. Agostinelli., D. Fiorani, S. Laureti, F. Springer., C. Brombacher, M. Albrecht, L. Del Bianco, G. Barucca, P. Mengucci, D. Rinaldi, Mater. Chem. Phys. 141 (2013) 790–796. [8] K. Srinivasan, S.N. Piramanayagam, R. Sbiaa, R.W. Chantrell, J. Magn. Magn. Mater. 320 (2008) 3041–3045. [9] H.J. Richter, J. Phys. D: Appl. Phys. 40 (2007) R149–R177. [10] H.J. Richter, S.D. Harkness, MRS Bull. 31 (2006) 384–388. [11] U. Kwon, H.S. Jung, M. Kuo, E.M.T. Velu, S.S. Malhotra, W. Jiang, G. Bertero, R. Sinclair, IEEE Trans. Magn. 42 (2006) 2330–2332. [12] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10–15. [13] M.H. Kryder, E.C. Gage, T.W. McDaniel, W.A. Challener, R.E. Rottmayer, G. Ju, Y.-T. Hsia, M.F. Erden, Proc. IEEE 96 (2008) 1810–1835. [14] J.-G. Zhu, X. Zhu, Y. Tang, IEEE Trans. Magn. 44 (2008) 125–131. [15] D. Suess, J. Lee, J. Fidler, T. Schrefl, J. Magn. Magn. Mater. 321 (2009) 545–554. [16] G. Asti, M. Ghidini, R. Pellicelli, C. Pernechele, M. Solzi, F. Albertini, F. Casoli, S. Fabbrici, L. Pareti, Phys. Rev. B: Condens. Matter 73 (2006) 094406 (16 pp). [17] D. Suess, J. Magn. Magn. Mater. 308 (2009) 183–197. [18] A.Y.u. Dobin, H.J. Richter, Appl. Phys. Lett. 89 (2006) 062512 (3pp). [19] H. Kronmuller, D. Goll, Phys. Status Solidi B 248 (2011) 2361–2367. [20] D. Suess, Appl. Phys. Lett. 89 (2006) 113105 (3pp). [21] D. Suess, J. Fidler, G. Zimanyi, T. Schrefl, P. Visscher, Appl. Phys. Lett. 92 (2008) 173111 (3pp).

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