Perpendicular Nanomagnetic Logic Based on Low Anisotropy Co\Ni Multilayer

Journal Pre-proofs Perpendicular Nanomagnetic Logic Based on Low Anisotropy Co\Ni Multi‐ layer Simon Mendisch, Valentin Ahrens, Martina Kiechle, Adam Papp, Markus Becherer PII: DOI: Reference:

S0304-8853(19)33055-0 https://doi.org/10.1016/j.jmmm.2020.166626 MAGMA 166626

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Journal of Magnetism and Magnetic Materials

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24 January 2020 15 February 2020

Please cite this article as: S. Mendisch, V. Ahrens, M. Kiechle, A. Papp, M. Becherer, Perpendicular Nanomagnetic Logic Based on Low Anisotropy Co\Ni Multilayer, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/j.jmmm.2020.166626

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Perpendicular Nanomagnetic Logic Based on Low Anisotropy Co\Ni Multilayer Simon Mendisch1 , Valentin Ahrens1 , Martina Kiechle1 , Adam Papp1 and Markus Becherer1 1

Technical University of Munich, Department of Electrical and Computer Engineering, Chair for Nanoelectronics Arcisstraße 21, 80333 Munich, Germany, [email protected]

Abstract Cobalt\Nickel multilayers with low perpendicular magnetic anisotropy are optimized for the lowest achievable coercivity while retaining the highest possible total magnetic moment, still supporting single-domain states. This optimization is done to achieve a vital clock-field reduction in nanomagnetic logic devices with perpendicular magnetic anisotropy, enabling highly efficient onchip field clocking. It is shown that sub 10 mT coercivities are achievable utilizing a Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]x8 stack in combination with precise manipulation of the anisotropy landscape via highly localized Ga+ ion irradiation, in order to control the point of domain-wall nucleation. Statistical data is used to assess the Ga+ ion dose-dependent coercivity and provide a detailed insight into the overall switching-field-distributions. Nanosecond field pulsing is used to assess the time-evolution of the nucleation fields, confirmed to be following the Arrhenius model at least down to pulse lengths of 10 ns, resulting in nucleation fields more than twice as high. The obtained findings are then applied to demonstrate the first pNML logic elements fulfilling the requirements for on-chip clocking schemes. Keywords: pNML, Co\Ni, PMA, Perpendicular Magnetic Anisotropy, Ion Beam Irradiation, Low Coercivity, Nanomagnetic Logic.

1. Introduction

2. pNML Backround & Material Requirements

Perpendicular Nanomagnetic logic (pNML) is listed as a potential “Beyond-CMOS” candidate in the International Roadmap for Devices and Systems 2017 (IRDS) [1]. PNML thereby utilizes the anti-ferromagnetic dipole coupling of adjacent nanomagnets with perpendicular magnetic anisotropy to achieve complex logic operations [2]. Perpendicular Nanomagnetic logic is the most mature and promising form of domain-wall (DW) logic, as it operates with a single global clock field and is not affected by variations in shape anisotropy preventing further device scaling [3, 4]. In recent years we have demonstrated a comprehensive family of Boolean and non-Boolean logic elements on device-level and forecast a potential power dissipation of low single-digit Atto-Joules per NAND/NOR operation utilizing a soft-magnetic cladding enhanced on-chip clocking scheme [5, 6]. Power efficiency is the leading aspect of any more-than-Moore technology, which makes pNML, provided ultra-low-power switching, a prime candidate. The vast majority of the total energy required is thereby dissipated inside the field-coils, which drive the magnetization reversals. The minimally achievable clock field is, therefore, one of the defining figures of merit and should ideally fall well below 20 mT. In this work, we report on the recent progress in coercivity reductions for operational devices, achieved with near singledomain threshold PMA Co\Ni multilayer stacks. For that purpose, it was not only necessary to optimize the stack architecture but also to re-validate the working principles pNML is built on and, for the first time, provide relevant statistical data.

Perpendicular nanomagnetic logic is based on the strong dipole interaction between adjacent out-of-plane (OOP) magnetized nanomagnets. Every magnetic thin-film used for pNML therefore requires both, perpendicular magnetic anisotropy and a domain size larger than the respective feature size, guaranteeing single domain states, needed to encode the digital information into the magnetization direction. PMA requires the effective anisotropy (Keff ) of the film, to be > 0. It is given as

Preprint submitted to JMMM

1 Ks , Keff = Ku − µ0 Ms2 + 2 tFM

(1)

with Ku as uniaxial anisotropy term, Ms as the effective saturation magnetization, Ks as the mean surface /interface anisotropy and tFM as the effective thickness of ferromagnetic layer. The quintessence here is, that Keff scales ∝ −µ0 Ms2 . The critical (single) domain size Dcrit defines the maximum possible feature size, beyond which, a multi-domain configuration is energetically more stable. It is derived from DW-Theory and can be approximated by [7] √ 72 AKeff Dcrit ≈ , (2) µ0 Ms2 with A as the exchange stiffness of the material (≈ 1 × 10−11 J m−1 ). For research purposes, Dcrit is usually kept within the visible range to allow for optical probing of the magnetization (Dcrit ≥ 500 nm). Besides these two vital constraints, a variety of second-order factors determines the feasibility of a material. For reliable error-free operation, the coupling should January 24, 2020

be as high as possible. Its strength at a distance r from the magnet can be derived from the established point-dipole approximated and in its simplest form approximates to, C≈

Ms V 1 ∝ 3, 3 4πr r

their thicknesses given in nanometer are deposited at room temperature via confocal rf-magnetron sputtering onto thermally oxidized n− doped silicon (100) substrates. A brief overview of selected stack configurations is depicted in figure 1. The effective anisotropies are calculated from angle-dependent AHE measurements on film level, according to [10]. Promising stack configurations are further processed after the initial analysis. Therefore the films are patterned via focused ion beam (FIB) lithography (using PMMA as a positive E-beam resist). A T i hard mask deposition (with subsequent lift-off process) is combined with a final Ar+ ion-beam milling step to realize the designed test-structures on the samples. Laser, as well as widefield Kerr-microscopy (WMOKE), is then used to characterize the nanostructures and asses their respective switching field distributions (SFD). The SFDs for the disks are derived from WMOKE images, tracking the individual magnets via patternmatching and automated image recognition. The individual switching events are then detected via differential imaging between field-steps. To complement the analysis of the switching behavior with measurements in the time domain, pulsed on-chip coils are used to determine nucleation fields down to the single-digit ns regime, needed to test the magnets within realistic time scales. A custom-developed ns-range current-pulse generator is hereby used to drive currents up to 40 A through on-chip coils with a single winding, surrounding the devices under test (DUT) from two sides [11, 12]. To create the necessary nucleation sites, 50 keV Ga+ ions are used to destroy the crystal lattice locally and thus modify the anisotropy landscape of the test-structures. The effects of varying doses and areas on the domain-wall nucleation and SFD were analyzed and optimized to achieve the necessary coercivity reduction and control over the point of nucleation. For this purpose, the facilitation of a FIB-microscope (Micrion 9500e) is vital in achieving the necessary control over the irradiation location, area, and the implantation dosage in a research facility. Reposition accuracies of less than 200 nm and beam diameters of ≈ 5 nm allow hereby for the automated irradiation of sub µm structures across multiple writing fields with nm precision. Alignment accuracies even below 100 nm, necessary for logic blocks, can be achieved via manual re-alignment at the individual writing fields. Based on the obtained findings, inverter structures were fabricated and their scaling behavior according to r assessed. All experiments were conducted at room temperature.

(3)

with V as the volume of the magnet. The most effective parameter to tailor C is, therefore, the distance r, it is usually required to be ≤ 50 nm, thus posing high demands on the fabrication process. The efficiency of the entire system is determined by the amount of energy needed to switch the individual magnets, in combination with the on-chip infrastructure needed to provide this energy. Taking into account established fabrication technologies and the target efficiency results in an upper boundary for clocking fields that are realistically achievable of ≈ 20 mT [5]. The coercivity (Hc ) of single domain magnets in contrast to films is dominated by the nucleation field (Hnuc ) Hc ≈ Hnuc presuming that the domain wall (DW) depinning fields are much lower (Hdepin  Hnuc ). In theory (0 K), Hnuc is equal to the anisotropy field (Hnuc = µ2KMeff2 ). In reality, local fluctuations 0 s of the anisotropy field caused by inhomogeneities and defects inside the magnet lead to domain nucleation at the weakest point, followed by rapid domain wall propagation through the entire magnet. These nucleation centers define not only the energy needed but also the point where the magnetization reversal starts. By artificially creating regions with lowered Keff , it is possible to create so-called artificial nucleation centers (ANC) and tailor the switching field as well as the point of nucleation. For magnets with active ANC, the critical field can therefore be written as Hc = Hnuc = Hanc , (4) provided that Hnuc > Hdepin . The time and temperature dependence of this switching field can be estimated, facilitating the Sharrock formula, derived from the Arrhenius switching model of an ideal Stoner Wohlfarth particle, to be   !!  f0 tp (1/2)  k T B Hsw = Hs0 1 − ln (5)  , E0 ln(2) with Hs0 as the switching field at 0 K, f0 as the attempt frequency (≈ 2 × 109 Hz) and E0 as the energy barrier without field [8, 9]. Additional performance and application-specific parameters like the DW-velocity, defect density, or the curie-temperature are not part of this work and will, therefore, not be discussed.

A.. Coercivity behavior It is important to differentiate between coercivities measured on film-level and those measured on µm sized singledomain structures. On film level, the magnetization reversal process is mainly carried out via domain expansion and is thus, almost exclusively dependent on the depinning and demagnetizing fields of the material. For single-domain magnets, the reversal process takes place by nucleating and subsequently propagating a DW though the structure (tswitch ≈ tnuc + tprop ). For PMA materials, it usually holds that Hnuc  Hdepin , leading to nucleation rather than depinning dominated switching. The semi-log plot in figure 1 expresses this differentiation depending on the effective anisotropy. For Keff ≥ 1 × 105 J m−3 these

3. Experiment Co\Ni thin-film stacks with varying anisotropy and magnetic moment were fabricated and characterized on film level, to assess their feasibility in terms of coercivity and domain sizes (determined via local (µm) laser Kerr-microscopy and anomalous hall-effect (AHE) measurements). The multilayers with 2

a high uncertainty. The model clearly shows that while for low clock frequencies (broad pulse widths), the coercivities are well within the range for on-chip clocking, for higher frequencies ( fclk > 50 MHz) the threshold of ≈ 20 mT is increasingly exceeded. This underlines the need for further stack optimizations as well as the consideration of amorphous materials at the cost of reduced coupling strengths due to the limited number of possible multilayers.

ANC ≈ 1 × 1014 ions/cm2

Ta2 \Pt5 [Co0.2 \Ni0.7 ]6

Ta2 \Pt5 [Co0.2 \Ni0.4 ]5

10

Ta2 \Pt2 [Co0.2 \Ni0.4 ]8 Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]8 Ta2 \Pt1 [Co0.15 \Ni0.7 ]5 Ta2 \Pt1 [Co0.15 \Ni0.7 ]8

1

Film Coercivity Disks d = 0.8 µm Disks with ANC

Ta15 [Co0.2 \Ni0.4 ]5 (annealed)

0

0.5

25 µ0 Hnuc /mT

µ0 Hc /mT

100

1

1.5 −3

Keff /J m

2

Figure 1: Overview of the switching behavior of selected Co\Ni stacks with increasing Keff . The plot underline the separation of depinning dominated coercivity from nucleation determined switching by showing the coercivity differences between films and single domain nano-structures. The Ta15 [Co0.2 \Ni0.4 ]x5 stack was annealed post-deposition at 225 ◦C for 30 min to increase the PMA. The highlighted stack was used for the further experiments. The error bars indicate the respective FWHM SFDs.

Experimantal Data Sharrock Fit Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]x8

15 ANC ≈ 1 × 1014 ions/cm2

10 5

·105

On-Chip Clocking Threshold

20

d ≈ 0.8 µm

10−9

10−8

10−7

10−6 10−5 10−4 10−3 Pulse width tp /s

10−2

10−1

100

Figure 2: Measured nucleation fields (Hnuc ) as a function of the applied pulse width. The Data is supplemented by a fit, facilitating the Sharrock formalism BT (equation 5) with an attempt frequency of 2 GHz, E0 = k34.7 and Hs0 = 28.9 mT. The error-bars indicate the standard deviation of the 40 measured nano-disks. The inlet displays a schematic illustration of the DUT.

B.. Nucleation Centers & Switching Field Distribution To reduce the coercivity and control the point of DW nucleation, ANCs are of vital importance for pNML operation. Ion beam irradiation has proven to be an effective way to manipulate the anisotropy of ML stacks with extremely high spatial resolution. It is furthermore a widely adopted technology in industry, in contrast to other proposed techniques like nanoindenting or lift-off induced anisotropy gradients [13, 14]. Figure 3 (a) displays the ion-dose dependent coercivity development of circular nano-disks with a diameter of 0.8 µm and an ANC area of 50 × 50 nm (square-shaped). The size of the ANC plays a crucial role in the behavior of the magnet. Irradiation areas smaller than the minimum nucleation volume have no measurable effect, and beyond a certain point, the magnets lose their single domain behavior. The area of 50×50 nm was chosen as it is the proposed width of the nucleation fin (magnet area affected by the input fields) of scaled logic elements and lies within the tolerated area, roughly determined for this stack to be within 20 × 20 and 100 × 100 nm. The coercivity measurements reveal a steep initial decrease of Hc,mean , as the PMA decreases, followed by a local recovery, associated with the easy axis of the ANC turning inplane, before the effective loss of ferromagnetic behavior at doses > 0.3 × 1014 ions/cm2 . This behavior is also observed in high Keff materials like Pt\Co, thus strengthening the argument, that the principles, pNML relies on, in the high Keff regime still hold for near-threshold anisotropies [15, 16]. Figure 3 (b) depicts the detailed SFD data of 1260 nano-disks in the as-grown as well as the irradiated state (1 × 1014 ions/cm2 ). Switching fields of SD magnets are generally not Gaussian distributed, as the magnetization reversal is a thermally assisted process governed by Arrhenius statistics. The model nevertheless delivers a useful approximation. The position and width

coercivity discrepancies amount to more than one order of magnitude. Only for smaller values of Keff the coercivities start to converge, up to the point where the magnets, with our without ANC, break apart into multiple domains (e.g. the Ta2 \Pt1 [Co0.15 \Ni0.7 ]x8 after Ga+ ion irradiation or the Ta15 [Co0.2 \Ni0.4 ]x5 even in the as-grown state). Especially important is the difference in coercivity, before and after introducing the ANC. This difference also scales with the anisotropy and defines the ANC process window, within which the introduced defect is the dominant one, thus determining the point of nucleation. Overlapping SFDs can, therefore, have tremendous effects on the error rate of larger systems. After the extensive preliminary coercivity analysis, the best results are obtained for effective anisotropies around 0.5 × 104 J m−3 . Higher anisotropies not only increase the coercivity of the as-grown magnets but also Hc of the irradiated ones, due to the fact that the anisotropy gradient between the magnet and the ANC (the energy barrier that the DW needs to overcome) also increases. However, further decreasing the anisotropy, quickly closes the window in which the irradiated magnets of a given size still feature single domain behavior in the demagnetized state. This empirically found sweet-spot is, however, by no means universal, as it critically depends on the used materials and magnetic moment of the stack. Of the two stacks around this spot, the Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]x8 stack showed higher ANC impacts, narrower distributions and a larger magnetic moment. It was therefore selected for further, more detailed analyses. Figure 2 displays the measured time evolution of the nucleation fields, including the Sharrock fit, according to equation 5. With an attempt frequency of 2 GHz, E0 and Hs0 converge kB T BT to E0 = 34.7 ± k2.1 and Hs0 = (28.9 ± 7.3) mT, naturally with 3

10 0

(b)

0

0.2

0.4

0.6

0.8

+

1

Ga dose in ions/cm

500

400 Number of Magnets

IP peak

20

anti-parallel input state. Point-dipole simulations, taking into account the geometry of the inverter, were conducted and are compared to the experimental data, resulting in good agreement between simulation and experiment. The fin-width of the output magnet measures 100 nm (twice the size of the ANC) to ensure a sufficiently large target area for the aligned focused ion beam. State of the art fabrication processes, however, would allow the gap-size to be pushed well below 100 nm.

Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]8

Hc,mean = (9.4 ± 0.2) mT σFWHM = (6.4 ± 0.2) mT

1.2 2

1.4 ·1014

Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]x8

50

Hc,mean = (24.0 ± 0.7) mT σFWHM = (11.9 ± 0.5) mT

300

200

100

0

5

10

15

20 25 µ0 Hc /mT

30

35

40

Gap

30

ANC

OUT 20

Hclk < 20 mT

10

As-Grown Ga+ 1 × 1014 ions/cm2

0

IN

Scaled versions

Coupling in mT

µ0 Hc /mT

(a) 30

0

40

Figure 3: (a) Ga+ ion-dose dependent switching fields of circular nano-disks (d = 0.8 µm) with centered ANCs (50 nm × 50 nm). The error bars indicate the FWHM distribution of 100 magnets each. (b) Detailed switching field distribution histogram of 1260 nano-disks before and after irradiation with a dose of 1 × 1014 ions/cm2 .

Measurements Simulation

100

150

Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]x8

200 250 Gap Size in nm

300

350

Figure 4: Measured inverter coupling strength depending on the gap width of a fork structure made from a Ta2 \Pt1.5 [Co0.2 \Ni0.4 ]x8 stack. Simulation data as indicator for the expected scaling behavior beyond 200 nm. The inlet displays a SEM micrograph of a typical fork-inverter with positions of in- and output marked.

of the corresponding distributions, not only show the in figure 3 (a) displayed shift in Hc,mean but also a significant narrowing of the overall SFD (in this case, from 11.9 to 6.4 mT FWHM). This is caused by the introduction of a well-defined nucleation center, rather than nucleation at random defects. Applying automatic image-recognition and tracking allows for a verification of the ANC induced coercivity reductions for every single magnet (by measuring the delta in Hc ), thus rendering the overlap in SFDs to be considered uncritical. This would not be possible just considering the distribution data alone. The mechanisms contributing to the remaining SFD are, however, at this point not sufficiently understood and will, therefore, be the focus of future investigations. This is of tremendous importance, as the SFD is the defining parameter for error-free operation and coupling requirements.

4. Conclusion Co\Ni multilayer stacks with near single-domain threshold anisotropies were investigated and optimized to achieve relevant clock-field reductions in operational pNML logic devices. The target of sub-threshold (20 mT) nucleation fields was met up to pulse widths of 20 ns translating to frequencies of ≈ 50 MHz. These results were achieved by tailoring the anisotropy of the stacks to the absolute limit while preserving sufficient magnetic moment for reliable coupling. Additional investigations were made targeting the effects of local Ga+ ion irradiation in these low anisotropy systems, shaping the coercivity as well as achieving significant reductions in the absolute SFD. 5. Acknowledgment

C.. Inverter Scaling To validate the above-obtained findings, the output magnets of pNML fork-inverters were irradiated with a dose of ≈ 1 × 1014 ions/cm2 and the correct functionality of the devices checked accordingly. Figure 4 illustrates the coupling strength of the inverters, scaling with the total gap size of the fork-input (depicted in the inlet). The coupling strength is measured to be the total shift in coercivity between a parallel and an

The authors would like to thank Christian Schmid for the support in measurements. Furthermore, we would like to thank the IGSSE for financial support. Finally, we would like to acknowledge the support of the Central Electronics and Information Technology Laboratory – ZEITlab .

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