Toward Flexible Spintronics: Perpendicularly Magnetized Synthetic Antiferromagnetic Thin Films and Nanowires on Polyimide Substrates

The successful fabrication of ultra‐thin films of CoFeB/Pt with strong perpendicular magnetic anisotropy and antiferromagnetic interfacial interlayer coupling on flexible polyimide substrates is demonstrated. Despite an increased surface roughness and defect density on the polyimide substrate, magnetic single layers of CoFeB still show sharp coercive switching. Magnetic Kerr imaging shows that the magnetization reversal is dominated by a greater density of nucleation sites than the identical film grown on Si. These layers maintain their magnetic characteristics down to a radius of curvature of 350 ± µm. Further, antiferromagnetically (AF) Ruderman‐Kittel‐Kasuya‐Yoshida (RKKY) coupled bilayers of CoFeB were fabricated which are robust under bending and the coupling strength is successfully modulated via interlayer engineering. Finally, a perpendicular synthetic antiferromagnetic (SAF) thin film grown on a polyimide substrate is patterned into straight 10 µm long nanowires down to 210 nm in width that displayed the robust switching characteristics of the thin film. These are extremely promising results for the fabrication of robust, flexible, magneto‐electronic, non‐volatile memory, logic, and sensor devices.

Recent progress in the fabrication of electronic systems on fl exible substrates has led to research in applications ranging from electronic skins to bendable light emitting screens. [1][2][3] However, it is still a signifi cant challenge and an active area of research to fabricate fl exible, nonvolatile memory elements, and recent work has been focused primarily on memory devices based on fi eld effect transistor systems. [4][5][6] An alternative, technologically relevant solution is to use devices based on magnetic materials.
Flexible magnetoelectronic devices to date have generally been focused on sensors using giant magnetoresistance (GMR) elements with in-plane magnetized layers [ 7,8 ] and improving GMR performance on fl exible substrates is of technological interest. [ 9,10 ] However, perpendicular magnetic anisotropy (PMA) materials are generally of greater relevance with regards

Toward Flexible Spintronics: Perpendicularly Magnetized Synthetic Antiferromagnetic Thin Films and Nanowires on Polyimide Substrates
T. Vemulkar , * R. Mansell , A. Fernández-Pacheco , and R. P. Cowburn The successful fabrication of ultra-thin fi lms of CoFeB/Pt with strong perpendicular magnetic anisotropy and antiferromagnetic interfacial interlayer coupling on fl exible polyimide substrates is demonstrated. Despite an increased surface roughness and defect density on the polyimide substrate, magnetic single layers of CoFeB still show sharp coercive switching. Magnetic Kerr imaging shows that the magnetization reversal is dominated by a greater density of nucleation sites than the identical fi lm grown on Si. These layers maintain their magnetic characteristics down to a radius of curvature of 350 ± μm. Further, antiferromagnetically (AF) Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupled bilayers of CoFeB were fabricated which are robust under bending and the coupling strength is successfully modulated via interlayer engineering. Finally, a perpendicular synthetic antiferromagnetic (SAF) thin fi lm grown on a polyimide substrate is patterned into straight 10 μm long nanowires down to 210 nm in width that displayed the robust switching characteristics of the thin fi lm. These are extremely promising results for the fabrication of robust, fl exible, magneto-electronic, non-volatile memory, logic, and sensor devices. to nonvolatile memory applications and magnetic fi eld sensors. A particular challenge arises in fabricating PMA multilayer based magnetoelectronic devices on fl exible substrates since the quality of the interface between the various layers, which is often strongly dependent on the device substrate and buffer layers used, can signifi cantly alter key characteristics of these magnetic thin fi lm heterostructures such as the magnitude of the PMA [ 11,12 ] and interlayer exchange coupling effects. [ 12,13 ] Further, the thickness of the magnetic layers in PMA multilayer stacks are often in the sub-nanometer range. Hence, producing these devices on fl exible substrates is a signifi cant challenge due to substrate roughness and defect density considerations. It is therefore remarkable that we have fabricated thin fi lms and nanowires with both strong PMA as well as tunable interlayer exchange coupling effects on a polyimide substrate, and is a fi rst step toward fl exible PMA based magnetoelectronic devices.
PMA based multilayer thin fi lms are extremely interesting from the perspective of fundamental physics, as well as commercial memory and logic applications. Perpendicular magnetic tunnel junctions (p-MTJs) fabricated from multilayer heterostructures of perpendicularly magnetized thin fi lms are a prime candidate for tunneling magnetoresistance (TMR) based magnetic random access memory (MRAM)-the next generation of non-volatile memory devices. [ 14 ] They offer bit scalability and thermal stability down to sub-20 nm due to their strong PMA, and a low switching current due to their high spin transfer torque effi ciency. [ 15,16 ] Spin orbit interaction effects in such PMA multilayer heterostructures also offer the potential of fabricating ultralow power memory and logic devices [17][18][19] and can drive the motion [ 20 ] of characteristically narrow and fast moving domain walls. [ 21 ] Hence, they are also attractive for a variety of current induced domain wall motion (CIDWM) based magnetic memory and logic elements, [ 22 ] most notably enabling signifi cant progress in racetrack memory architectures. [ 17,23 ] Further, PMA materials in conjunction with antiferromagnetic (AF) coupling are of signifi cant technological relevance for MRAMs using spin transfer torque (STT) switching, [ 24,25 ] as well as sensors [ 26,27 ] and novel 3D logic applications. [ 28 ] Perpendicular synthetic antiferromagnetic (SAF) nanowires in particular are of great scientifi c interest and have recently been shown to have enabled a fundamental development in domain wall based memory systems. [ 29 ] Finally, TMR based MTJs fi nd applicability not only in memory applications, but are also of relevance for high sensitivity magnetic fi eld sensor devices. [ 30 ] We show here the fabrication of perpendicular CoFeB ultrathin fi lms on fl exible polyimide substrates. We also successfully fabricate tunable, perpendicular SAF thin fi lms via antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) interlayer coupling of CoFeB bilayers on fl exible polyimide substrates. We further pattern a perpendicular SAF thin fi lm into nanowires with widths down to 210 nm that maintain the magnetic characteristics of the thin fi lm. The nanopatterned perpendicular multilayer heterostructures with tunable exchange coupling effects on fl exible substrates fabricated in this work may open the way for new fl exible sensors, magnetic memory elements, and even new fl exible spintronic devices.  Figure 1 c summarizes the remanent magnetization and coercivity of such a fi lm when bent over a range of different radii. The thin fi lm retains its full remanent magnetization and coercivity under bending, and these characteristics are robust down to a radius of bending far less than the ≈1 cm bending radius of most envisioned fl exible electronics applications. [ 31 ] Figure 2 compares the magnetic properties of single CoFeB layers of increasing thickness on Si, fl at polyimide, and bent polyimide with a radius of curvature ( R C ) of 350 ± 50 µm. The magnetic multilayer stack used here is Ta(4)/Pt(6)/CoFeB(0.7-1.2)/Pt(2) with thicknesses in nm. Figure 2 a shows that fi lms fabricated on polyimide substrates generally displayed slightly lower anisotropy for the same CoFeB thickness due to the poorer substrate conditions. [ 32 ] We observe the expected 1/t dependence of anisotropy with increasing CoFeB thickness [ 33 ] on both the Si and polyimide substrate as measured by the hard axis saturation fi eld K H . [ 34 ] The decreasing PMA with increasing CoFeB thickness is because PMA is an interfacial effect that is dominant in ultrathin CoFeB/Pt multilayers. The total magnetic anisotropy can be separated into an interfacial contribution and a volume contribution. As the thickness of the magnetic layer is increased, it approaches the spin reorientation transition. At this point the magnetization direction will be dominated by the volume contribution to the magnetic anisotropy which arises from long range dipolar interactions in the thin fi lm and is described by a demagnetizing fi eld. This is known as the shape anisotropy effect, and it favors a magnetization orientation in the plane of the fi lm. This decreasing trend in anisotropy is accompanied by a decrease in the coercivity as seen in Figure 2 b since it becomes easier to nucleate a domain of contrary magnetization direction with decreasing anisotropy. Figure 2 c-h shows the polar MOKE hysteresis loops of perpendicular single layers of CoFeB on substrates of Si, fl at polyimide, and bent polyimide with a R C of 350 ± 50 µm. The magnetization reversal process between the two substrates is notably different. The CoFeB single layers on the Si substrates display rectangular hysteresis loops with sharp magnetic switching as seen in Figure 2 c-h (black curves) which is evidence of domain wall motion dominated magnetic reversal. This type of magnetic reversal process in PMA CoFeB is dominated by the nucleation of large, roughly circular domains with domain walls that rapidly expand through the thin fi lm. [ 35 ]  magnetic reversal of a 0.8 nm fi lm of CoFeB and demonstrates this reversal mechanism. This sharp magnetic switching is expected on the Si substrate, due to a low inherent root mean square (RMS) roughness of 0.431 nm that was measured in a 7 × 7 µm 2 area in Figure 3 c which was masked to minimize the contribution of point defects to the RMS roughness, as well as an extremely low defect population as seen in the atomic force microscopy (AFM) scan in Figure 3 c.
We next study the reversal process on the polyimide substrate. We notice quite square loops with robust magnetic switching despite the increased roughness of the polyimide substrate which is extremely promising for device characteristics. In considering the polar MOKE loops on the polyimide substrate in Figure 2 c-f, we observe a signifi cantly increased coercivity compared to the Si substrate. We attribute the increased coercivity to increased domain wall pinning due to the presence of a higher defect density in the form of peaks and pits, on the scale of tens and hundreds of nanometers in width and height as seen on the AFM scan in Figure 3 d. These defects can act to pin or trap domain walls due to local changes in anisotropy or fi lm thickness. [36][37][38] This is further evidenced by the domain imaging of a 0.8 nm CoFeB layer on the polyimide substrate in  The decreased anisotropy closer to the spin reorientation transition in these fi lms both leads to a higher number of reversal domains, and effectively reduces the anisotropy difference around defects that act to trap domain walls. These effects reduce the coercivity because the domain wall pinning effect is now less pronounced, and the coercive fi eld ( C H ) on polyimide approaches those of the identical fi lm grown on Si.
The buffer layers in magnetic multilayer thin fi lms are often crucial to obtaining robust PMA. This is especially true on the polyimide substrate here, where the substrate conditions are inherently poor as evidenced by a RMS roughness of 2.8 nm. This was measured from a 7 × 7 µm 2 area in Figure 3 d masked specifi cally to minimize the contribution of large point defects to the RMS roughness value. We characterize the roughness profi le of the polyimide substrate further in supplementary Figure 3 . We illustrate how the buffer layers reduce the amplitude specifically of high frequency, short wavelength spatial undulations on the bare polyimide substrate that have a cut-off wavelength of ≈20 nm or less. The reduction in the RMS roughness in this wavelength range is signifi cant, from 0.440 nm on a bare polyimide substrate, to 0.232 nm on a Ta(4)/Pt(6)/CoFeB(0.7)/Pt(2) magnetic thin fi lm (thicknesses in nm) that was grown on a polyimide substrate. The cut-off wavelength is comparable to Adv. Funct. Mater. 2016, 26, 4704-4711 www.afm-journal.de www.MaterialsViews.com the ≈10 nm domain wall widths typically observed in such PMA materials systems. [ 21 ] Hence a reduction in the roughness at this spatial length scale is expected to have a signifi cant effect on the nucleation and pinning of magnetic domain walls during the magnetic reversal process. [ 39 ] We evidence this in supplementary Figure 4 , where we demonstrate how signifi cantly the magnetic properties are affected by the smoothing effect of the Ta layer, [ 40,41 ] highlighting the role of the buffer in promoting PMA  in thin fi lms especially when using rough initial conditions as is expected on most fl exible substrates.
The robust magnetic characteristics are maintained under bending the polyimide substrates to a R C of 350 ± 50 µm. This is clearly evidenced by the red curves in Figure 2 c-h. Slight differences in the shape and coercivity of the transitions between the fl at and bent polyimide is attributed to the local topography around the MOKE laser spot which has a significant effect on the magnetization reversal process.
Having characterized the key properties of CoFeB single layers on polyimide substrates, we further successfully demonstrate RKKY coupled bilayers on a similar polyimide substrate, and compare the RKKY coupling to that of the identical bilayers grown on Si substrates. The RKKY coupled bilayer stack used here is Ta(2)/Pt(3)/CoFeB(0.8)/Pt(t)/Ru(0.9)/Pt(t)/ CoFeB(0.8)/ Pt (2) with thicknesses in nm. The thickness of the Ru was chosen to lie at the fi rst AF coupling peak [ 42 ] and the Ta/Pt buffer for this stack was optimized to maximize the RKKY Adv. Funct. Mater. 2016, 26, 4704-4711 www.afm-journal.de www.MaterialsViews.com www.afm-journal.de www.MaterialsViews.com coupling magnitude and is shown in supplementary Figure 5 . The Pt layers on either side of the Ru layer stabilize the PMA and can be used to exponentially increase the magnitude of the RKKY coupling by decreasing their thickness. [ 43 ] We successfully demonstrate this tuning of the magnitude of the RKKY coupling on a fl exible polyimide substrate, even under bending. We note that for fi lms grown on the polyimide substrate the thickness of the Pt layers on either side of the Ru coupling layer has a more signifi cant effect on the magnetic reversal than for the stacks grown on Si. At lower Pt thicknesses (Figure 4 a-c), we observe more slanted transitions with low coercivities on the polyimide. This is attributed to the reduced perpendicular anisotropy due to the relatively thin Pt interlayers on either side of the Ru that are less able to stabilize the magnetic anisotropy on the rougher polyimide substrate. We see sharper reversal transitions and increasing coercivity on the polyimide when the Pt interlayer thicknesses are increased (Figure 4 d-f), indicating increasing perpendicular magnetic anisotropy with increasing Pt interlayer thickness. [ 44 ] The increased coercivity in the transitions on Si substrates compared to the polyimide is attributed to the higher PMA on Si. For RKKY coupled magnetic bilayers on the polyimide substrate, the lower PMA due to the thin Pt insertions on either side of the Ru leads to reversals with lower coercivities, similar to the magnetic reversals seen for the single magnetic layer thin fi lms in Figure 2 g,h. Further we observe a slightly different shaped transition when going from the antiparallel state to the parallel state (toward magnetic saturation) as opposed to going from the parallel to antiparallel state (coming from magnetic saturation). The shape of these two transitions is closely linked to the population of specifi c types of defects which can locally increase or decrease both the magnitude of the RKKY coupling as well as the layer coercivity. [ 35 ] We also estimate the magnitude of the RKKY coupling with an effective RKKY coupling fi eld J H . [ 43,44 ] While strong, the RKKY coupling on the polyimide substrate (blue and red) is reduced compared to the fi lms grown on Si substrates (black). This is again likely due to the poorer interface quality between layers in the thin fi lm stack owing to the high substrate roughness and defect density. [ 12 ] We further see in Figure 4 g that bending the fi lm to a R C of 350 ± 50 µm results in no significant change in the RKKY coupling magnitude or the tunability of this coupling effect which is extremely promising for device applications on fl exible substrates.
Finally, we patterned a perpendicular SAF thin fi lm into nanowires via electron beam lithography of a resist hard mask, and subsequent Ar ion milling of the thin fi lm stack. The nanowires were magnetically characterized using polar MOKE. In Figure 5 a we see the hysteresis loop of the perpendicular SAF thin fi lm stack on a polyimide substrate. Figure 5 b,c shows the hysteresis loops of 370 and 210 nm wide and 10 µm long, straight nanowires that were patterned from the thin fi lm and Figure 5 d shows a representative array of the 210 nm nanowires with a single nanowire shown in the inset. The fi lm is described by Ta(2)/Pt(3)/CoFeB(0.8)/Pt(0.5)/Ru(0.9)/Pt(0.5)/ CoFeB(0.8)/Pt(2) with thicknesses in nm. In the thin fi lm we observe no remanent magnetic state, and strong AF RKKY coupling as evidenced by a J H of roughly 700 Oe. We also observe sharp, square magnetic switching processes. Similar behavior is seen in both the 370 and 210 nm wide nanowires. The signifi cant increase in the noise in the signal in Figure 5 b,c is expected due to the extremely low Kerr signal obtained from the switching of a single nanowire on the rough polyimide substrate. This demonstration of patterned nanoscale perpendicular SAF structures with robust magnetic characteristics is extremely promising for the fabrication of advanced spintronic devices on fl exible substrates. PMA multilayers are of great interest and technological relevance due to their high perpendicular anisotropy and tunable interfacial coupling effects which are leveraged in GMR, TMR, CIDWM, and other novel spintronic effects in devices. We demonstrate the successful fabrication of ultrathin perpendicularly magnetized CoFeB single layers on fl exible polyimide substrates over a wide range of CoFeB thicknesses. The magnetic layers display high perpendicular anisotropy and sharp magnetic switching. We further successfully fabricate a SAF thin fi lm on the fl exible substrates with tunable RKKY coupling strength, and demonstrate the robust magnetic switching of AF coupled nanowires. The fl exible magnetic multilayer stacks are magnetically stable even under bending to a radius of curvature of R C of 350 ± 50 µm which is signifi cantly lower than the expected fi gure of merit for most fl exible electronics applications. This work opens the way for non-volatile memory, sensor, and other advanced magnetoelectronic devices based on PMA and PMA SAFs to become a part of the fl exible electronics revolution.

Experimental Section
Polyimide Films : The polyimide fi lms used were of two different types due to processing considerations. The single magnetic layers were fabricated on 25 µm thick Kapton polyimide fi lm without an adhesive backing layer manufactured by Dupont. The SAF bilayer thin fi lms were fabricated on 65 µm thick polyimide fi lms manufactured by TESA with an adhesive backing layer. The adhesive backing layer allowed for ease of processing for the fabrication of the SAF nanowires during processes such as electron beam resist spinning and electron beam resist patterning.
Deposition of Magnetic Multilayers : The deposition of the magnetic thin fi lm multilayers was done using DC magnetron sputtering. The base pressure of the sputtering system was less than 1 10 7 × − mbar. Argon was used as the sputter gas with a sputter pressure of (7 8) 10 3 − × − mBar. The thicknesses of the individual layers are calibrated by growing a series of fi lms of each material at a specifi c power. The thickness of each fi lm is measured with an AFM, and a growth rate is determined from a plot of the fi lm thickness versus the growth time. The CoFeB was sputtered at a rate of 0.05 nm s −1 at a power of 60 W. The various thicknesses of CoFeB were grown by adjusting the growth time. The substrates of polyimide tape and Si were introduced in to the sputter chamber simultaneously when growing the identical multilayer stack on each substrate to minimize process variation between deposition runs.
Nanowire Fabrication: The electron beam lithography resist MA-N 2403 was used as the milling mask and was spun at 6500 rpm on top of the multilayer stack grown on a polyimide substrate. The exposure was done on a CABL 9000C 50 keV e-beam system. The pattern was developed in MA-D 533 developer for 10 s. The thin fi lm was then Ar ion milled at a milling current of 28 mA for 50 s to ensure complete removal of the surrounding fi lm, and the milling mask was subsequently removed by dissolution in acetone followed by a brief oxygen plasma ash.
Magnetic Characterization : Polar MOKE hysteresis loops and domain imaging were collected using a NanoMOKE system from Durham Magneto Optics. Measurements for the bent samples were carried out by adhesively attaching the samples around a wire of nominal diameter 0.25 mm which was subsequently mounted in the NanoMOKE 3 system, and the curvature was measured for each sample using Vernier calipers to ensure a R C of 350 ± 50 µm. This process was repeated for the bending measurements using metal posts for the higher diameters shown in Figure 1 c.
Magnetic Figures of Merit : Effective fi elds are used here to characterize the perpendicular anisotropy and RKKY coupling parameters. The hard axis saturation fi eld H K is a measure of the magnitude of the perpendicular anisotropy and is measured via a vibrating sample magnetometry (VSM) hard axis hysteresis loop. The effective anisotropy is defi ned as K HM ( )/2 eff K S = , where M S is the saturation magnetization of the CoFeB. The interfacial or surface perpendicular anisotropy K S is obtained from the effective anisotropy

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.