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Bringing nanomagnetism into the third dimension is of growing interest due to the many advantages that 3D structures provide. The possibility for larger volumes and higher surface-to-volume ratios, as well as the introduction of new topologies, chirality and curvature into the structure, lead to numerous opportunities for new physics and applications.[1] Combining this promising field of 3D nanomagnetism with the well-established field of spintronics offers huge opportunities, a key example being the proposal to use 3D nanomagnets in a 3D racetrack architecture that promises the delivery of non-volatile and ultra-high-density information storage[2]. However, the integration of 3D nanomagnets into 2D microelectronics is challenging and has so far impeded the realisation of technological applications such as sensing, computing or memory. Now, with 3D Nano-printing capabilities, we have developed a method to integrate complex 3D magnetic nanostructures [3] into 2D microelectronic circuits and to measure the magneto-transport properties of our 3D nanomagnetic structures. Here, we demonstrate the direct integration of a complex 3D magnetic nanostructure in a microelectronic circuit (Figure 1). In particular, we exploit the recent advances of Focused Electron Beam Induced Deposition, a 3D nano-printing technique, to directly print a high-quality 3D Cobalt nanobridge on pre-synthesized electrical contacts. This 3D nanomagnetic circuit allows for the determination of the bridge’s magnetic and transport properties by performing magnetotransport measurements for different geometries of the field with respect to the 3D nanostructure (Figure 2). In addition, we infer the field-induced switching process of the 3D nanostructure from the transport measurement, that includes the nucleation, propagation and pinning of domain walls. [4]. This methodology can be extended to more complicated geometries and materials, and so this work represents the first step towards the realisation of 3D nanomagnetic devices and will facilitate further exploration of 3D nanomagnetism for both fundamental and device-based studies.References: 1 A. Fernandez-Pacheco, R.Streubel, O. Fruchart, Nature Communications., Vol. 8, p.15756 (2017) 2 S. Parkin, M. Hayashi, L.Thomas, Science., Vol. 320, p.190 (2008) 3 L.Skoric, D. Sanz-hernandez, F. Meng., Vol. 20, p.184 (2020) 4 F.Meng, C. Donnelly, C. Abert (In preparation).

MMM 2020

Creating 3D nanomagnetic circuits for Spintronics applications

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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technical paper

Magnetic domain walls (DWs) in ferromagnetic nanowires are candidates for both Boolean [1–3] and non-Boolean computing architectures such as neuromorphic/stochastic computing [4]. In this context, interconnected nanorings are particularly interesting as they show emergent behaviour which make them interesting for reservoir computing [5-6]. Since these approaches use magnetic DW motion for logic operations, it is crucial to understand the dynamics of DWs in these structures for device realisation. We used micromagnetic simulations (using MuMax3 [7]) to simulate DWs passing through junctions of 2 μm diameter, 5 nm-thick Permalloy bi-ring structures with different ring widths and degrees of ring overlap. An example structure is shown in the inset of Fig. 1. The system was initialized with DWs in the upper and lower halves of the left and right ring. A rotating field was then applied to move the DWs towards the junction following previous experimental approaches [8]. The field amplitude was varied to record the depinning field HDP when the DWs separated from the junction. The variation of HDP for different overlap and wire widths is shown in Fig. 1. A minimum in HDP is observed for most widths indicating that competing mechanisms are involved in the depinning mechanism. In order to better understand these competing mechanisms, we look at the magnetic energy density [7] E=-½M.Beff (1) for a wire width of 100 nm with 12 (Fig. 2a) and 100 nm (Fig. 2b) overlap . In Fig. 2 (a), the DWs form a ‘diamond’ state which forms an energy minima requiring a high HDP to depin the DW. For the largest overlap in Fig. 2 (b), the DWs get ‘pinned’ before and after the junction and a high HDP is required to overcome this pinned energy threshold. Thus the intermediate overlap states (of 25 & 50 nm) require lower HDP due to competition between these states. We expect that the understanding of these dynamics is crucial for the development of ring based computing applications.References: [1] S. S. P. Parkin, M. Hayashi, L. Thomas, Science 320, 190 (2008). [2] D. A. Allwood et. al., Science 309, 1688 (2005). [3] K. A. Omari et. al., Adv. Func. Mater. 29, 1807282 (2019). [4] J. Grollier, et. al., Nat. Electron. 1-11 (2020). [5] R. Dawidek et al., MMM conference (2017). [6] R. Dawidek et al., submitted [7] A. Vansteenkiste et. al., AIP Adv. 4 (10), 107133 (2014). [8] M. Negoita et. al., J. Appl. Phys. 114.1, 013904 (2013).

Domain Wall Dynamics in Junctions of Bi-Ring Structures

Magnetic skyrmions are topological spin textures that can be stabilized by the Dzyaloshinskii-Moriya interaction [1,2]. There are particularly suitable for next generation spintronics devices, like the skyrmion-based racetrack memory [3]. Recent studies confirmed their current-driven skyrmion dynamics in ultrathin ferromagnets via spin-orbit torques [4], where the chirality and spin texture of the skyrmions are key. However, the topological Magnus effect leads to a transverse motion of ferromagnetic skyrmions due to their non-zero topological charge [5], which is disadvantageous for devices. Antiferromagnetically exchange-coupled skyrmions or compensated ferrimagnets could suppress this effect owing to an overall zero topological charge [6]. Especially at the angular momentum compensation temperature skyrmion propagation is predicted to be collinear with the current [7]. Here we explore a GdFeCo-based ferrimagnet system with perpendicular magnetic anisotropy. We demonstrate that in this system chiral worm domains as well as magnetic skyrmions can be observed using scanning electron microscopy with polarization analysis (SEMPA) (Figure 1). The high spatial resolution magnetic imaging technique reveals the domain wall spin structure as a function of temperature, with the observed structures found to be pure Néel-type spin textures, which is promising for devices. From the images we extract the domain wall width, which allows us to calculate the exchange stiffness for the system across the full temperature range.References: [1] I. Dzyaloshinsky, J. Phys. Chem. Solids 4, 241 (1958) [2] T. Moriya, Phys. Rev. 120, 91 (1960) [3] R. Tomasello et al., Sci. Rep. 4, 6784 (2015) [4] S. Woo et al., Nat. Mater. 15, 501 (2016) [5] K. Litzius et al., Nat. Phys. 13, 170 (2017) [6] Y. Hirata et al., Nat. Nanotechnol. 14, 232 (2019) [7] J. Barker et al., Phys. Rev. Lett. 116, 147203 (2016)

Direct imaging of chiral domain walls due to the Dzyaloshinskii-Moriya interaction in ferrimagnetic alloys at different temperatures

We present Ir-based Synthetic Antiferromagnets (SAFs) composed of asymmetric building blocks of Pt/[CoNi]x/Ir (PCI) and Ir/[CoNi]x/Pt (ICP). The number of repeats ‘x’ and thickness of the Ir spacer layer allow, respectively, for simultaneous control of the interfacial Dzyaloshinskii-Moriya interaction (iDMI).1 and interlayer exchange coupling (IEC).2 CoNi layers show strong iDMI that reverses in sign when Pt and Ir stacking sequence are reversed (PCI or ICP), which induces Néel (over Bloch) DWs of opposite chirality. Using Lorentz Transmission Electron Microscopy (LTEM), we explore configurations of coupled SAF DWs with different magnitudes and sign of iDMI. We demonstrate conditions to directly observe these DWs with LTEM and study their “untangling” under in-situ perpendicular magnetic field (see Fig 1). Possible 1D excitations confined to the SAF DWs in these films are presented and compared to micromagnetic and LTEM simulations. The prospects for thin film SAFs with DMI will be discussed in the context of domain wall devices, where high velocity and a low critical current are desirable.References: 1. Lau, D., et al. “Disentangling Factors Governing Dzyaloshinskii Domain-Wall Creep in Co/Ni Thin Films Using PtxIr1−x Seed Layers.” Physical Review B, vol. 98, no. 18, 2018, doi:10.1103/physrevb.98.184410. 2. Prudnikov, Aleksandr, et al. “Simultaneous Control of Interlayer Exchange Coupling and the Interfacial Dzyaloshinskii–Moriya Interaction in Ru-Based Synthetic Antiferromagnets.” IEEE Magnetics Letters, vol. 10, 2019, pp. 1–4., doi:10.1109/lmag.2018.2882163.

Experimental Observation and Control of Coupled Chiral Domain Walls in Synthetic Antiferromagnets using In-Situ Lorentz Transmission Electron Microscopy

The efficiency of manipulation of domain walls and skyrmions in ferromagnetic racetracks with perpendicular anisotropy determines perspectives of development of data storage and logic devices relying on spintronic and spin-orbitronic concepts [1, 2]. The domain wall dynamics is dependent on its orientation with respect to the racetrack axis. In-plane fields [3], edge roughness [4] and current [5] result in the domain wall tilt in samples, possessing Dzyaloshinskii-Moriya interaction (DMI). Here, we show theoretically, that the tilt can appear in equilibrium and describe the domain wall dynamics under the action of external field. We consider a thin biaxial stripe with DMI of interfacial type [6]. The main easy axis of anisotropy is perpendicular to the plane, and the direction of the second easy axis lies in the stripe plane under the angle α to the stripe axis. While the shape anisotropy results in α = 0, a general case α ≠ 0 can appear under the influence of other effects, e.g crystalline structure [7]. While the second easy axis defines the preferable in-plane magnetization within the domain wall, the DMI forces the domain wall being perpendicular to the magnetization gradient. The competition between these two energy contributions and the domain wall tension results in the unidirectional tilt of the whole domain wall. If the DMI is weak enough, there is an additional metastable domain wall state, tilted into the opposite direction. The symmetry break is observed not only for static magnetization texture, but also in the domain wall dynamics under the action of external magnetic field. The domain wall reveals fast and slow motion regimes for the opposite signs of A. The maximum of the Walker field and Walker velicities is determined by the angle A of the second easy axis anisotropy and does not coincide with a shape-induced anisotropy direction A=0. The domain wall possesses the switch of the magnetization direction inside the domain wall in the slow motion regime, which results in the faster motion.References: [1] K.-S. Ryu, L. Thomas, S.-H. Yang et al., Nat. Nanotech., Vol. 8, 527 (2013) [2] O. Pylypovskyi, D. Sheka, V. Kravchuk et al., Sci. Rep. Vol. 6, 23316 (2016) [3] C. Muratov, V. Slastikov, A. Kolesnikov et al., Phys. Rev. B. Vol. 96, 134417 (2017) [4] E. Martinez, S. Emori, N. Perez et al. J. Appl. Phys. Vol. 115, 213909 (2014) [5] O. Boulle, S. Rohart, L. Buda-Prejbeanu et al., Phys. Rev. Lett. Vol. 111, 217203 (2013) [6] O. Pylypovskyi, V. Kravchuk, O. Volkov et al., J. Phys. D. (2020), DOI:10.1088/1361-6463/ab95bd [7] M. Heide, G. Bihlmayer, S. Blügel, Pys. Rev. B, Vol. 78, p. 140403 (2008).

Domain Wall Tilt and Enhancement of the Walker Limit in Stripes with Dzyaloshinskii-Moriya Interaction and Perpendicular Anisotropy

Chiral magnetic domain walls stabilised by the interfacial Dzyaloshinskii-Moriya interaction are of great technological interest since the latter favors a given Néel wall with fast current-driven domain wall propagation [1]. We study experimentally by Kerr microscopy, numerically and analytically chiral magnetic domain wall motion in the creep regime under easy-axis driving fields in materials with a perpendicular magnetic anisotropy with an in plane magnetic field. We study two samples: one asymmetric [Pt(3nm)/Co(0.6nm)/Ta(3nm)] and one symmetric [Pt(3nm)/Co(0.6nm)/Pt(3nm)]. Brillouin light scattering has been used to determine the DMi constant: D = -1.2mJ/m2 and D = 0 mJ/m2 in the asymmetric and symmetric case respectively. As expected from the DMi constant measured by BLS, the DW velocity curves as a function of the in-plane magnetic field are qualitatively different. In the asymmetric system, we could not reach a minimum of the velocity v within our experimental in plane field range (Fig 1a), consistently with the DM field = 220 mT estimated from the BLS measurements. For the symmetric system, a slight asymmetry is observed with a minimum around µ0Hip = 0 mT (Fig 1c). We detail the creep regime analysis by studying the evolution of the depinning field from the creep regime to the flow regime for different µ0Hip by studying the linear variation of ln(v) vs (μ0Hz)−1/4. In the asymmetric case the slope is constant (Fig 1b) unlike in the symmetric case (Fig 1d): when |µ0Hip| < 50 mT the DW goes from left-handed Néel to Bloch to right-handed Néel wall and the slope varies while for larger fields the slope is constant. We can extrapolate the evolution of the depinning field which is strongly influenced by the in-plane biasing field. In the asymmetric system, we observe an excess of velocity with respect to the creep law close to the depinning field (Fig 1d, explained by the increase of the size of the deterministic relaxation close to the depinning transition [2]. The stronger the DMi field, the more this excess of velocity occurs at larger out-of-plane field values.References: [1] A. Thiaville, et al. EPL 100, 57002 (2012) [2] N. B. Caballero, et al., Phys. Rev. B 96, 224422 (2017).

Chiral magnetic domain walls dynamics in creep regime

Spintronics has developed into a growing field with the fabrication of magnetic memories and logic devices. Domain wall motion is a major part of the research in spintronics where spin polarized current is used to move the domain walls by two mechanisms spin tranfer torque(STT) and spin orbit torque(SOT). Compensated ferrimagnets have been in focus recently for their ability to reach the magnetic and angular compensation points. At the angular compensation point, the precessional motion of the local magnetic moment becomes negligible which massively enhances the domain wall velocity near this point[1,2]. In our previous studies, we had shown epitaxially grown ferrimagnet Manganese Nitride(Mn4N) having an anti-perovskite crystal structure with Mn atoms at the corner and the face centre sites while Nitrogen atom in the centre[3]. It has a very low magnetization (71kA/m) and a high perpendicular magnetic anisotropy(PMA) with mm sized domains[3]. Such small magnetization and high PMA is necessary for faster domain wall motion. Such properties led us to observe a very high domain wall velocity of 900 m/s at 1.2*1012 A/m2 [4] using pure STT. Here we report an even larger domain wall velocity of more than 2000 m/s at 1.16*1012 A/m2 using pure adiabatic STT near the magnetic and angular compensation point in epitaxially grown Ni substituted Mn4N nanowires (MnNiN). We also show a very small thresold current density of 1*1011 A/m2. This high speed is comparable to the best velocities obtained by SOT with the assitance of an external in-plane field. In our case, such domain wall velocity is achieved without any external in-plane field, with the sole action of pure current in this single layer system. Addtionally, we also show a reversal in the direction of the domain wall motion after the magnetic compensation point, where the domain walls move in the same direction as the current. We discuss this reversal of the domain wall motion phenomena with the help of DFT calculations.References: 1. K.J. Kim, S.K. Kim, Y Hirata, Nature Materials., Vol 16, p1187 (2017) 2. L. Caretta, M. Mann, F. Buttner, Nature Nanotechnology., Vol 13, p1154 (2018) 3. T. Gushi, L. Vila, O. Fruchart., Jpn J. Appl. Phys. Vol 57, p120310 (2018) 4. T. Gushi, M. J. Klug, J.P. Garcia, Nano Letters, Vol 19, p8716 (2019)

Current induced domain wall motion at the magnetic compensation point in Manganese Nickel Nitride (MnNiN) thin films

Cell Sorting has become an indispensable technique for a variety of biological and biomedical applications. Moreover, with the development of fields such as personalized medicine and new breakthroughs in cancer treatments, cell sorting devices that have high throughput and single-cell manipulation capabilities are needed now more than ever1. Magnetically activated cell sorting (MACS) has quickly become one of the most widely used techniques due to its extremely high performance in throughput and ease of use, unfortunately these systems lack the much needed single-cell resolution that modern biomedical problems require. To address this, there has been much research and development aimed at creating MACS devices in the microfluidic regime where microscale fabrication can allow for single-cell sorting mechanisms to be implemented. In addition to increased resolution, these microfluidic MACS devices have other benefits that include low cost, the ability to run fast low volume experiments, and the ability to capture and release cells while simultaneously culturing them within a device. In this paper we present a novel method of capturing and releasing magnetically activated cells using Spin Orbit Torque (SOT) to change the magnetization of micromagnet structures patterned within a microfluidic channel. SOT has been shown to be a reliable and fast method of manipulating the magnetization of magnetic memory devices2 and for this reason it was chosen as the method of operation. This device is a significant improvement on current cell sorting technologies because it has both single cell manipulation capabilities and high throughput whereas most current cell sorting technologies suffer a tradeoff between these two properties. This paper includes both modeling and experimental data to verify the theoretical mechanism proposed.References: 1X. Wang, S. Chen, D. Sun. Lab on a Chip., Vol. 11, p. 3656 (2011) 2W. Kwak, J. Kwon, B. K. Cho. Scientific Reports.,Vol 8 (2018)

High Throughput Magnetic Single-Cell Sorting Using Spin Orbit Torque

Complex oxide perovskites are attractive for spintronics applications due to the ease of integration into functional devices through epitaxial, isostructural growth. However, this epitaxial integration may cause unexpected changes and interfacial phenomena. Here, we report on modifications to the magnetic properties of ferromagnetic (La2/3Sr1/3)MnO3 (LSMO) films on LSAT substrates by an epitaxial layer of CaRuO3 (CRO). Using ferromagnetic resonance (FMR) as a sensitive magnetic probe, we find that the presence of CRO can modify the magnetic anisotropy of the LSMO. By increasing CRO thickness, we find a reduction of the out-of-plane anisotropy and simultaneous rotation of the easy axis within the plane, from the <110> to <100> axes (see Fig. 1a). The anisotropy strength determined by FMR disagrees with bulk SQUID magnetometry. This is consistent with a depth-dependent structural change, such as oxygen octahedral rotations imposed on rhombohedral LSMO [1] by orthorhombic CRO. FMR is also used to study spin pumping across this LSMO/CRO interface. Spin pumping manifests as an increase in Gilbert damping with CRO thickness. The frequency dependence of the FMR linewidth with an in-plane field indicates two-magnon scattering is significant. By varying the field direction in-plane, we find the in-plane anisotropy of the FMR linewidth – and therefore strength of two-magnon scattering – increases as a function of increasing CRO thickness (see Fig. 1b). These results suggest that while LSMO and CRO remain promising candidates for efficient pure spin current generation and detection, respectively, the epitaxial integration of perovskites for spintronics applications will cause additional changes which must be accounted for.References: [1] Z. Liao, M. Huijben, Z. Zhong et al., Nature Materials, Vol. 15, p.425 (2016) [2] H. K. Lee, I. Barsukov, A. G. Swartz et al., AIP Advances, Vol. 6, p.055212 (2016)

Magnetic Anisotropy and Spin Scattering in (La2/3Sr1/3)MnO3/CaRuO3 Bilayers

The La-based double layered (DL) managnites La2-2xSr2+2xMn2O7 have gained importance due to colossal magnetoresistance (CMR) [1] analogous to 3D perovskite manganite. However, In incongruity with the three-dimensional (3D) half doped perovskite manganites (La0.5Sr0.5MnO3), which shows charge exchange (CE) spin/charge ordering (CO) [2], the A-type of antiferromagnetic (AFM) state is dominant in Ruddlesden –Propper half doped bilayered manganites (La1Sr2Mn207) [3]. The MR observed in the layered system is much higher than observed in 3D perovskites manganites because of the high spin polarisation of conduction electron. This abstract reports on the structural and magneto-transport properties of La2-2xSr2+2xMn2O7 (0.42≤x≤0.52) synthesized by solid state reaction method. The Rietveld refined X-ray diffraction pattern suggests that the samples crystallizes into tetragonal structure (I4/mmm) without any trace of impurity phase. The temperature dependent resistivity plot of the samples in the magnetic field 0-5T is shown in Fig 1. At 0T, the samples 0.42≤x≤0.48 show insulator to metal (TMI) transition and later a resistivity mimima (Tmin) with the lowering of temperature, whereas x=0.50 and 0.52 shows CO with trace of metal-insulator transition. The application of magnetic field reduces the resistivity to a large order and TMI shifts towards the higher temperature side suppressing the resistivity minima. This can be understood due to reduction of the spin fluctuation by the field [4]. The resistivity of the samples increases with Sr subsitution upto x=0.48, and later decreases for higher Sr substitution, which may be due to phase transformation from canted AFM to the CO insulator [5]. The conduction mechanism at high temperature follows hopping polaron model. At 5K and 8T, 0.46≤x≤0.50, shows nearly similar MR (58%) and decreases for (x=0.42, 0.52). MR is dominated by the spin-polarised tunnelling across the interface or grain separated by the insulating barrier. We intend to study the magnetic behaviour of all the samples. This work is supported by UGC-DAE-CSR, Indore (grant No. CSR-IC/CRS-89), Ames Lab and MRF(MITM)References: 1. Moritomo Y., Asamitsu A., Tokura Y., Nature, 380 , 141.(1996) 2. C. N. R. Rao and B. Raveau, Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, edited by (World Scientific, Singapore, (1998). 3. M. Kubota, H. Yoshizawa, Y. Moritomo, Y. Endoh, J. Phys. Soc. Jpn. 68, 2202 (1999). 4. C. Zener, Phys. Rev. 82 403 (1951) 5. Mitchell J F, Ling C D, Millburn Medarde M J. Appl. Phys. 89, 6618 (2001)

Structural and Magneto Transport Properties of Ruddlesden –Propper La2-2xSr2+2xMn2O7 (0.42≤x≤0.52) Layered Manganites

The considerable spin diffusion length and high electron mobility of graphene at room temperature makes it a suitable candidate to improve the spin transport properties of magnetic semiconductors when incorporated in a suitable stoichiometry. Here, we report observation of room temperature magnetoresistance properties of reduced graphene oxide (rGO) / cobalt ferrite (CFO) nanocomposite, which were synthesized by chemical methods followed by mixing in different ratios. The structural properties and morphology of rGO-CFO mixed states have been investigated from the x-ray diffraction and TEM imaging. The Raman spectroscopy shows the presence of various vibrational bands of CFO in addition to the D and G bands at ~ 1354 and 1591 cm-1 respectively, which indicates the formation of rGO. The magnetic field dependent electrical resistivity shows a decrease in resistivity with the increase in magnetic field from 0 to 0.5 T, representing a negative magnetoresistance in our samples. The highest MR of around – 1.4 % was observed in the 1:1 ratio CFO and rGO nanocomposite, whereas the samples with 2:1 and 3:1 ratio show the MR of maximum value about 1%. In the rGO-CFO nanocomposite, the charge transport takes place mainly through the rGO flakes due to the insulating nature of the CFO nanoparticles. However, the ferromagnetic moments of the CFO nanoparticles induce electron-spin scattering during the charge transport. The reduction in electron-spin scattering due to the parallel alignment of the magnetic moments results in negative magnetoresistance in our sample. The alignment of ferromagnetic moments under the external magnetic field has been further confirmed from the observation of the magnetocapacitance effect of CFO nanoparticles. Our results show the possibility to utilize the wide band gap magnetic oxides for room temperature magnetotransport applications when incorporated with the 2D materials like graphene.

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