Multi-segmented nanowire (NW) arrays are strong candidates to improve the memory storage capacity due to their lower magnetic bit area [1]. But writing/reading information on such devices is still a challenge. Recently, micromagnetic simulations revealed that the integration of a soft magnetic segment (with lower coercivity) along the racetrack could ease the writing process by using external magnetic fields [2]. Here, we demonstrate the feasibility of using NiCu/FeCo magnetic NWs as in-line writing components in 3D racetrack memory devices. Using template-assisted DC electrodeposition [3-5], 4 samples of NW hexagonal arrays (50 nm-diameter, 100 nm of interwire distance) were produced: i) Ni90Cu10; ii) Fe20Co80; iii) bi-segmented NiCu/FeCo; and iv) multi-segmented NiCu/[FeCo/Au]3. Morphological, chemical and structural characterization revealed the formation of single, bi- and multi-segmented NW arrays with tuned dimensions and uniform compositions (Fig. 1). Magnetic hysteresis loops, ferromagnetic resonance (FMR) measurements and first-order reversal curve (FORC) diagram analysis confirmed the hard and soft magnetic properties of the FeCo and NiCu single arrays, respectively. In the interacting NiCu/FeCo bi- and multi-segmented systems, a significant reduction of the parallel coercivity occurred, while keeping the magnetization easy axis lying parallel to the NWs' axis. FORC diagrams also demonstrated a two-step reversal process in systems (Fig. 2), illustrating a FORC fingerprint typically present in soft/hard magnetic interacting bi-layered NWs [6]. Micromagnetic simulations confirmed such a two-step reversal process, revealing the presence of strong interactions at the soft/hard interface [7]. This work was supported by projects POCI-01-0145-FEDER-028676, POCI-01-0145-FEDER-031302, UIDB/04968/2020, EXPL/IF/00541/2015 and contract IF/01159/2015 from Portuguese FCT and COMPETE 2020 (FEDER); grant 02.2020/0261 from the National Research Foundation of Ukraine; and project PID2020-117024GB-C42 from the Spanish Ministerio de Ciencia e Innovacíon.

![](https://assets.underline.io/uploads/markdown_image/1/image/cd280bc8126f81abd395a71d877c76eb.jpg)

Fig. 1. Scanning electron microscopy images of the electrodeposited multi-segmented NWs.

![](https://assets.underline.io/uploads/markdown_image/1/image/22038b2a0a7bf508db8232151a5ff998.jpg)

Fig. 2. FORC diagram of the multi-segmented sample along the parallel direction.

Feasibility of Using NiCu/FeCo Coupled Bi-segmented Nanowires as In-line Writing Heads for 3D Racetrack Storage Devices

An interesting phenomenon occurs in submicron, magnetic vortex ground state particles as their thickness is increased, and the vortex becomes a fully-fledged three dimensional object. Although the ground state remains largely homogeneous through the thickness, one but needs to apply the smallest of excitation to see the emergent shearing of the magnetization texture and resonant modes associated with it [1,2]. Using the nanostencil fabrication technique[3] we obtained planar arrays of increasingly thick (up to 350 nm) magnetic particles, going beyond thicknesses studied previously. Not only do we observe the inversion of intensities of the modes, as high order modes become the most intense, we see also the complete disappearance of the homogeneous G0 mode, which marks a transition where the 2D dynamic solutions completely cease their existence, see the spectrum presented on Fig. 1. This new effect also goes toward a significant increase of the speed of dynamics in the system. In the course of study we place a special emphasis on taking the real, dome-like particle profiles into account, which, as we show, influence the spectra for thick particles significantly. The profiles used were attained with a help of atomic force microscopy. The realistic boundaries primarily tune the non-radiative standing spin wave modes[4] which in turn affect gyrational modes through hybridization. We observe this through the detailed, microscopic study of mode distributions such as the one on Fig. 2, by matching experimental spectra, and juxtaposing stationary trajectories (in the time domain).

![](https://assets.underline.io/uploads/markdown_image/1/image/8f782616bbfa5a82e7cffe9fbfba735f.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/fc47f4d3547c1babcc525b8c1162cc50.jpg)

High Order Gyromode Observation in Dome-like Nanoparticles

High Order Gyromode Observation in Dome-like Nanoparticles

Extending nanostructures into the third dimension has become a major research avenue in modern magnetism and spintronics because of geometry-, curvature-, and topology-induced phenomena. Here, we introduce nanovolcanoes as on-demand engineered nano-architectures for 3D magnon spintronics. 40 nm-thick Co-Fe nanovolcanoes with diameters down to 300 nm were fabricated by the focused electron beam-induced deposition (FEBID) technique (Fig. 1) and studied by perpendicular spin-wave resonance (PSWR) spectroscopy. The spin-wave eigenfrequencies of the nanovolcanoes are notably different from the eigenfrequencies of the nanodisks and nanorings they are built from, because of the strongly non-uniform internal magnetic field. The experimental findings were elucidated with the aid of micromagnetic simulations, which indicate that the rings encircling the volcano craters lead to an effective confinement of the lower-frequency eigenmodes under the volcano crater, while the highest-frequency eigenmodes are confined in the ring area (Fig. 2). Accordingly, extension of 2D nanodisks into 3D nanovolcanoes allows to obtain same response in low frequency part of the spectrum in elements with 30% smaller footprints [1]. The nanovolcanoes can be viewed as multi-mode resonators with potential applications in telecom industry. Also, we demonstrated the engineering of the magnetic properties of CoFe-based nanodisks fabricated by FEBID. The material composition was tuned in situ via the e-beam waiting time in the deposition process and their post-growth irradiation with Ga ions. The saturation magnetization Ms of the nanodisks was deduced from PSWR measurements. The achieved variation of Ms in the broad range from 720 to 1430 emu/cm3 continuously bridges the gap between the Ms values of widely used magnonic materials such as Permalloy and CoFeB [2].

![](https://assets.underline.io/uploads/markdown_image/1/image/ca3695146d0c42bbf0d0b907eb2d1eb1.jpg)

Fig. 1. Atomic force microscopy image of the nanovolcano with the outer diameter of 300 nm and the crater diameter of 200 nm. Inset: cross-sectional line scan.

![](https://assets.underline.io/uploads/markdown_image/1/image/355d13301bed6b9caa1885c2c3af1f2e.jpg)

Fig. 2. Calculated spatial dependences of the spin-wave eigenfunctions for (a) 40 nm-thick nanovolcano NV300/200, (b) 20 nm-thick nanodisk ND300, and (c) 20 nm-thick nanoring NR300/200 at the out-of-plane bias magnetic field of 1.2 T.

Spin-wave spectra of 3D nanovolcanoes fabricated by focused electron beam-induced deposition

Thin films with perpendicular magnetic anisotropy (PMA) have been widely studied for perpendicular recording media since the middle of the 1970s when Prof. Iwasaki suggested this approach [1]. Since them, a great effort has been done by the research community for the use of this kind of nanostructures with PMA in other technological applications such as in high-density spin-transfer torque magnetic random access memories (STT-MRAM) [2], skyrmion-based devices [3] or even in synthetic antiferromagnets for biomedical applications [4]. PMA in thin films can be usually achieved through different strategies such as by using materials with large magnetocrystalline anisotropy energy [5] as well as multilayer thin films with high surface anisotropy energy contributions [6]. Regarding the last alternative, Ikeda et al. [2] demonstrated in 2010 that amorphous CoFeB ultrathin films can show PMA and it is supported by the CoFeB/MgO interfacial anisotropy contribution. Since then, the development of non-magnetic/ferromagnetic superlattices with PMA, using CoFeB alloys as the ferromagnetic element, has received great attention. In particular, PMA in CoFeB/Pd was first demonstrated in bilayers by Fowley et al. [7] as well as in multilayers by Jung et al. [8]. Literature has reported that the effective anisotropy energy depends on the thickness of magnetic (tCoFeB) and non-magnetic (tPd) layers as well as on the number of bilayers (N). In this work, the dynamical response of [CoFeB (tCoFeB) / Pd (10 Å)]5 multilayered ultrathin films (1 Å ≤ tCoFeB ≤ 5 Å) were studied by using two complementary methods: broadband ferromagnetic resonance (Figure 1 a) and time-resolved magneto-optical Kerr effect (Figure 1 b). The perpendicular magnetization was confirmed for multilayers with tCoFeB ≤ 4 Å, while magnetization became in-plane oriented for tCoFeB ≥ 5 Å. This behaviour was explained by considering competing contributions from surface and magnetoelastic anisotropies. In addition, we have observed that the effective damping parameter is reduced with increasing tCoFeB (Figure 1 c), and the lower parameter, αeff ≈ (0.019 ± 0.001), was determined for the multilayer with 4 Å CoFeB thickness. We have suggested that for tCoFeB ≥ 4 Å the layer is continuous and the main contribution to effective damping is coming from spin-pumping when for lower thicknesses αeff is dominated by two-magnon scattering. Afterwards, we have studied the magnetic behaviour of ultrathin [CoFeB(tCoFeB)/Pd(10 Å)]N films. Intending to ensure that the system shows PMA, tCoFeB (3 or 4 Å) was fixed, while the number of bilayers (N) was ranged from 3 to 15. We observed that our samples show PMA and it is generally improved with N. In addition, we studied the dependence of αeff with the number of CoFeB/Pd repeats (N). Up today, literature in superlattices with PMA reported that αeff is proportional, inversely proportional or even, independent of N [9]. Therefore, the reported relationship between the damping parameter and N in superlattices with PMA is still unclear. We show that the evolution of αeff with N shows a complex behaviour that we have correlated with the magnetization process: larger αeff for samples with square hysteresis loops while αeff is independent of N for samples with bow-tie-shaped hysteresis loops.

**References:**

1. S. Iwasaki, Y. Nakamura and K. Ouchi, IEEE Trans. Magn., MAG-61, 1456 (1979). 2. S. Ikeda et al., Nat. Mater, 9, 721 (2010). 3. C. Moreau-Luchaire et al., Nature Nanotechnology, 11, 444 (2016). 4. R. Mansell et al., Sci. Rep,. 7, 4257 (2017). 5. N. Inaba , Y. Uesaka, and M. Futamoto , IEEE Trans. Magn., 36, 54 (2000). 6. C. J. Tatnall et al., J. Magn. Magn. Mater., 165, 391 (1997). 7. C. Fowley et al., IEEE Trans. Magn., 46, 2116 (2010). 8. J. H. Jung et al., Appl. Phys. Express, 3, 023001 (2010). 9. A. Barman et al., J. Appl. Phys., 101, 09D102 (2007); S. Mizukami et al., Appl. Phys. Express, 4, 013005 (2011); H.-S. Song et al., Appl. Phys. Lett., 102, 102401 (2013).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/dc5eba45a2ba94ed3193f118e071a51e.jpg)
Fig. 1. (a) VNA-FMR spectra of the [CoFeB (4 Å) / Pd (10 Å)]5 multilayer thin film with the external magnetic field applied perpendicularly to the sample plane. (b) Time-resolved magneto-optical Kerr effect (TR-MOKE) signals of a [CoFeB (2 Å) / Pd (10 Å)]5 multilayer thin film under different external applied fields (H = 5650, 5060, 4695, 4310 Oe and 3900 Oe) and when H was applied at θH = 78°. Theoretical curves (solid curves) are fits to the experimental data (open symbols). Data was moved along the vertical axis for clarity. (c) Evolution of the effective damping (αeff) vs tCoFeB and determined from the TR-MOKE measurements (Δ), the perpendicular VNA-FMR (●) and the in-plane VNA-FMR (○) measurements.

 Magnetization Dynamics of ultrathin [CoFeB (tCoFeB) / Pd]N films with perpendicular magnetic anisotropy

**Session chair:** Pavel Ripka, Czech Technical University



EOA - 3D Nanomagnetism and Nanowire Arrays

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Presentations

Feasibility of Using NiCu/FeCo Coupled Bi-segmented Nanowires as In-line Writing Heads for 3D Racetrack Storage Devices

High Order Gyromode Observation in Dome-like Nanoparticles

Spin-wave spectra of 3D nanovolcanoes fabricated by focused electron beam-induced deposition

Magnetization Dynamics of ultrathin [CoFeB (tCoFeB) / Pd]N films with perpendicular magnetic anisotropy

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