Magnetic functionalization of scanning probes by focused electron beam induced deposition technology

magnetochemistry Review Magnetic Functionalization of Scanning Probes by Focused Electron Beam Induced Deposition Technology Javier Pablo-Navarro 1,2,3,* , Soraya Sangiao 2,3,4 , César Magén2,3,4 and JoséMaría de Teresa 2,3,4   Citation: Pablo-Navarro, J.; Sangiao, S.; Magén, C.; de Teresa, J.M. Magnetic Functionalization of Scanning Probes by Focused Electron Beam Induced Deposition Technology. Magnetochemistry 2021,7, 140. https://doi.org/10.3390/ magnetochemistry7100140 Academic Editor: Vladimir I. Tsifrinovich Received: 11 September 2021 Accepted: 9 October 2021 Published: 13 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 2Instituto de Nanociencia y Materiales de Aragón (INMA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain; [email protected] (S.S.); [email protected] (C.M.); [email protected] (J.M.d.T.) 3Laboratorio de Microscopías Avanzadas (LMA), Universidad de Zaragoza, 50018 Zaragoza, Spain 4Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain *Correspondence: [email protected] Abstract: The fabrication of nanostructures with high resolution and precise control of the deposition site makes Focused Electron Beam Induced Deposition (FEBID) a unique nanolithography process. In the case of magnetic materials, apart from the FEBID potential in standard substrates for multiple applications in data storage and logic, the use of this technology for the growth of nanomagnets on different types of scanning probes opens new paths in magnetic sensing, becoming a benchmark for magnetic functionalization. This work reviews the recent advances in the integration of FEBID magnetic nanostructures onto cantilevers to produce advanced magnetic sensing devices with unprecedented performance. Keywords: nanomagnetism; focused electron beam induced deposition; nanofabrication; nanolithography; magnetic nanowires; three-dimensional 1. Introduction to FEBID and MFM Focused Electron Beam Induced Deposition (FEBID) is one of the most versatile and promising technologies for the fabrication of nanostructures, emerging as a complementary technique to existing fabrication methods. This single-step nanolithography process, first introduced by S. Matsui in 1984 [ 1 ], relies on the decomposition by a finely-focused electron beam of the molecules of a precursor gas on a substrate, eventually producing a solid deposit [ 2 – 5 ]. The precursor molecules are delivered locally over the substrate by a gas injection system (GIS) and adsorbed on the surface, some of them being dissociated by the electron beam into volatile and non-volatile parts. The non-volatile part of the gas is deposited whereas the volatile one is pumped out of the working chamber. The shape of the deposit is determined by the electron beam scan pattern, and the interactions between the electron beam, substrate, precursor gas molecules, and the growing structure [ 6 , 7 ]. The diffusion, adsorption, and desorption phenomena of the gas molecules on top of the substrate and the probability of the secondary electrons generated to break the molecules bonds are crucial in this nanofabrication process. FEBID has demonstrated to be remarkably successful in growing nanometer-scale objects and devices with 2D and 3D geometries [ 8 – 18 ], providing the possibility to pattern architectures with high lateral resolution and on unconventional surfaces. The technique is nurtured by an extensive catalogue of precursors that enable the deposition of an ever-growing number of materials and compounds, with a great variety of physical properties, such as metallicity and magnetoresistance [ 2 ], insulating behavior [ 19 ], superconductivity [ 20 ], ferromagnetism and superparamagnetism [21,22], or plasmonic behavior [23], among others. Specifically, it is worth stressing the use of precursor gas molecules containing magnetic elements, such as Co, Fe and Ni [ 21 , 24 – 30 ], which has allowed a significant progress Magnetochemistry 2021,7, 140. https://doi.org/10.3390/magnetochemistry7100140 https://www.mdpi.com/journal/magnetochemistry Magnetochemistry 2021,7, 140 2 of 16 towards the growth of complex-shaped magnetic deposits with high metallic content and magnetization, as recently reviewed [ 31 , 32 ]. This development has been focused on the optimization of thin magnetic patterns in 2D. More recently, the investigation has been extended to 3D FEBID magnetic deposits [ 33 – 35 ], spurred on by promising applications in scanning probe techniques, such as Magnetic Force Microscopy (MFM) [ 36 ] and Magnetic Resonance Force Microscopy (MRFM) [ 37 ], racetrack-type magnetic memories [ 14 ], Hall sensors [ 11 , 22 ], nanomagnetic logic circuits [ 36 , 38 ], superconducting vortex lattice pinning [ 39 ], remote magneto-mechanical actuation [ 40 ], etc. However, whereas FEBID flexibility allows the production of challenging structures with sophisticated geometries [ 7 , 10 , 12 ], many applications can be based on the simplest objects, such as vertical straight nanowires. MFM is one of the most common techniques used in research and industry for the characterization of magnetic properties at the nanoscale. To show the potential for the integration of 3D FEBID ferromagnetic nanostructures in MFM probes, the use of vertical magnetic nanowires [ 41 , 42 ] to functionalize Atomic Force Microscopy (AFM) tips is reviewed here. Specifically, this combination has been found to improve the performance of the standard magnetic tips and further progress could have a significant impact in this field. Based on this concept, the utilization of nanometer-scale force transducers to map with high resolution and sensitivity the magnetic fields of the sample (rather than magnetic field gradients) in scanning nanowire magnetic force sensing [ 43 , 44 ] is also discussed. Finally, the growth of magnetic nanocubes and nanospheres on AFM tips is analyzed as an application to enhance the performance of MRFM measurements, taking advantage of the tip–sample interaction for the investigation of spin dynamics [45]. MFM relies on the long-range magnetostatic force between a magnetic sample and a magnetic probe [ 46 ]. The MFM contrast is proportional to the magnetic pole density at the surface of the magnetic specimen. Therefore, samples with either perpendicular or in-plane anisotropy can be studied, where the poles are located at the domains or at the domain walls, respectively [ 46 ]. The sensitivity, defined by the minimum detectable change of the frequency shift, ∆f , [ 47 ] and the spatial resolution, which can reach values of ~10 nm under optimum conditions [ 48 , 49 ], are limited by different factors such as the tip–sample distance, the magnetized volume of the tip, the type of cantilever, the tip magnetic moment, etc. To enhance the MFM performance, some technical limitations, such as the non-magnetic interactions between the tip and the sample, should be considered in order to ensure a high magnetic signal-to-noise ratio. Similarly, the characterization could be improved by reducing the magnetic invasiveness. Additionally, improving the lateral resolution obtained with the current MFM tips would lead to cutting-edge investigations in magnetism. Diverse approaches have been taken to improve the performance of MFM tips [50–52] . The coating with magnetic material of nano-objects located in the apex, such as C nanotubes [ 53 ], the growth of magnetic nanowires by electrodeposition [ 54 ], or the use of nanoscale magnets sharpened by Focused Ion Beam milling [ 55 ] are strategies to reduce the magnetic material of the probe and consequently optimize the MFM signal. However, these procedures have not resulted in an improvement of the technique. In view of this, the development of new systems or refinements of the current sensing devices [ 21 , 36 ] are required to provide an upgraded and robust measurement method. This situation has triggered the growth of materials in the form of vertical nanowires by FEBID as magnetic probes onto AFM cantilevers. A proof-of-concept was demonstrated for the first time by growing high-aspect-ratio ferromagnetic Co nanowires and analyzing the tip performance in hard disks reference samples [ 21 , 56 ]. A few years later, the prevailing spatial magnetic resolution of ~40 nm was improved down to 10 nm by growing higher purity ferromagnetic Co-FEBID nanowires with thinner diameters [ 48 ]. Additionally, Febased deposits were grown on AFM tips correcting the angle formed with respect to the target sample, inducing an accurate MFM measurement of 3D nanomagnet logic arrays [ 36 ]. These pioneering studies pointed out that, due to the high aspect ratio, high coercivity, Magnetochemistry 2021,7, 140 3 of 16 small lateral dimensions, and the precise control of the deposition location, magnetic FEBID nanowires grown on standard AFM probes are perfect candidates for working as MFM tips [54,57]. Further efforts to optimize their performance have been reported [ 58 ]. In particular, 3D Co and Fe nanowires were grown on various kinds of tips and under different growth conditions, characterized magnetically, and tested in different environments to compare their operational performance with that of commercial MFM probes. It was recently shown that tailored FEBID tips are superior to standard MFM ones in certain applications and can give rise to a new generation of commercial MFM tips [ 41 ]. Furthermore, the use of these refined MFM tips for studies in non-standard environments such as liquid media could actively prompt the detection of new phenomena [ 47 ]. Moreover, FEBID tips may even be used in self-sensing cantilevers, which integrate piezoresistors for non-optical detection that would suffer from electrical shortcut due to magnetic coating [59]. 2. Magnetic Force Microscopy Using Tips Grown by FEBID 2.1. Growth and Properties of MFM Tips Grown by FEBID The different geometries of the commercial probes—the most typical ones being pyramidal and cone-shaped— require a very sharp tip, i.e., a small radius of curvature at the apex, to ensure a good spatial resolution both in AFM and MFM measurements. Although the sensitivity and resolution are also limited by the mechanical properties of the cantilever and the control electronics, in the MFM case, the magnetic stray field of the tip is a key parameter. This can be controlled by the modification of the final architecture of the probes, customizing their magnetic behavior. Particularly, the modulation of the general aspect ratio and the diameter and shape of the 3D ferromagnetic nanowires at the very end of the nanowire tip can be carried out by FEBID. This possibility breaks new ground in the degree of control in the functionalization of MFM probes. The growth of FEBID nano-objects on the apex of AFM tips requires an accurate electron beam aberration correction, and a proper charge dissipation during FEBID growth on the tips would be advisable. For instance, Nanoworld Arrow TM EFM,Budget Sensors ® ElectriMulti75-G, and Bruker probes, as well as tips with strong charging effect such as Olympus BioLever mini have been proven to be useful, as reported by Jaafar et al. [41]. The small deposition surface compared to conventional flat substrates reduces the number of molecules available for decomposition. Under this situation, the angle of the GIS needle with respect to the horizontal axis becomes increasingly important as it determines the gas flow quantity [ 60 ]. Furthermore, the probe geometry also has a negative impact on heat dissipation, promoting greater nanowire diameters as the temperature increases, especially when working at low voltages (~3–5 kV). Nonetheless, this factor also contributes to increase the metallic purity up to 80 at. % in Co and Fe deposits. Likewise, the control of the length and the diameter is entirely possible while keeping the voltage constant, mainly through the deposition time and the electron beam current. Specifically, the reduction of the nanowire diameter is linked to a decrease of the beam current. Figure 1d–f illustrates the growth of 3D Co and Fe nanowires at low voltages on different types of probes with diameters ranging from 40 nm to 90 nm. Even though Scanning Electron Microscopy (SEM) lateral resolution is below 1 nm, FEBID deposits with such dimensions cannot be fabricated due to the much larger interaction volume generated by the electron beam. As a result, nanostructures with sub-100 nm resolution are typically achieved. The coercive field of the FEBID tips reaches 55–60 mT, under external magnetic fields applied parallel to the nanowire long axis. In comparison, the commercial MFM tips show experimental values of 20 mT (Team Nanotec) and 35 mT (Budget Sensors ® ). Hence, FEBID tips are more robust upon contrast inversion due to magnetization reversal of the tip caused by the stray magnetic field generated by the sample. On the other hand, the sensitivity and image contrast can be compared using a highdensity hard disk as a reference sample. Figure 2shows similar performance for commercial Magnetochemistry 2021,7, 140 4 of 16 and FEBID tips [ 41 ]. However, it should be emphasized that the possibility of tuning the magnetic properties of FEBID tips is an advantage for high-performance MFM operation. Magnetochemistry 2021, 7, x FOR PEER REVIEW 4 of 17 Figure 1. SEM images of (a) Nanoworld ArrowTM EFM, (b) Budget Sensors® and (c) Olympus BioLever mini AFM probes, with their corresponding (d,f) Fe and (e) Co nanowires. (a–c,f) adapted from [61] and (d,e) reproduced from [41] with permission from the Royal Society of Chemistry. The coercive field of the FEBID tips reaches 55–60 mT, under external magnetic fields applied parallel to the nanowire long axis. In comparison, the commercial MFM tips show experimental values of 20 mT (Team Nanotec) and 35 mT (Budget Sensors®). Hence, FEBID tips are more robust upon contrast inversion due to magnetization reversal of the tip caused by the stray magnetic field generated by the sample. On the other hand, the sensitivity and image contrast can be compared using a highdensity hard disk as a reference sample. Figure 2 shows similar performance for commercial and FEBID tips [41]. However, it should be emphasized that the possibility of tuning the magnetic properties of FEBID tips is an advantage for high-performance MFM operation. Figure 1. SEM images of ( a )Nanoworld Arrow TM EFM, ( b )Budget Sensors ® and ( c )Olympus BioLever mini AFM probes, with their corresponding ( d , f ) Fe and ( e ) Co nanowires. ( a – c,f ) adapted from [ 61 ] and (d,e) reproduced from [41] with permission from the Royal Society of Chemistry. As illustrated in Figure 3, FEBID nanowires with different properties can be grown onto AFM probes. For example, either nanowires with extremely thin diameter or very sharp tip ends [ 42 ] or core-shell architectures to avoid the surface oxidation of the magnetic core to a non-ferromagnetic material, which degrades the magnetic properties [ 62 ]. Nonetheless, achieving the narrowest diameters with high metallic content (and magnetization) is not possible, being necessary to reach a trade-off between dimensions and composition [ 63 ]. Indeed, some studies have been devoted to determining the relationship between FEBID chemical composition, especially in Co, and the electron beam voltage and current [ 24 , 64 ], precursor gas flux [ 11 ], patterning parameters [ 65 ], substrate temperature [ 64 ], etc. In the case of Fe, even with a precise control of the growth parameters, the 3D as-grown materials present a moderate purity which can be increased by annealing treatments [ 25 , 66 , 67 ]. Moreover, performing future quantitative MFM studies is a potential advantage of FEBID tips, since the extraction of magnetic quantitative information is one of the major challenges for the future progress in MFM, which requires tips with well-defined physical properties [68]. One of the fundamental principles of SEM image resolution consists of the fact that the higher the accelerating voltage is, the better the resolution of the image becomes. This also works for the FEBID fabrication process, where the thinnest nanowires can be grown at the highest accelerating voltages. According to Figure 3d, the shape of the tip end replicates the top part of the electron beam interaction volume in the deposit [ 69 ]. Therefore, the shape of the nanowire tip end can be modulated by tuning the accelerating voltage. The upper nanowire part presents a blunt shape when using voltage values in the range 3–5 kV, as illustrated in Figure 3a; however, it exhibits a smaller diameter and an extremely pointy end at 30 kV. These features can be appreciated in the thinnest ferromagnetic nanowire ever grown by FEBID (~22 nm in diameter) [ 61 ] imaged in Figure 3b and, more precisely, in the example of the nanowire routinely grown under these conditions, shown in Figure 3c. In this case, an overall diameter of around 35 nm as well as a ~8 nm-wide tip were obtained with high reproducibility. This novel shape configuration is very useful to perform very high resolution MFM imaging as well as to obtain relatively low tip–sample interaction, minimizing the influence of the tip on the magnetic state of the sample structures [42]. Magnetochemistry 2021,7, 140 5 of 16 Magnetochemistry 2021, 7, x FOR PEER REVIEW 5 of 17 Figure 2. MFM imaging of a high-density hard disk with (a) Budget Sensors Multi-M tip with 50 nm of CoCr coating, (b) Co-FEBID tip and (c) Fe-FEBID tip. (d–f) Topographic images obtained by the selected MFM probes and (g–i) their corresponding magnetic images, respectively. Reproduced from [41] with permission from the Royal Society of Chemistry. As illustrated in Figure 3, FEBID nanowires with different properties can be grown onto AFM probes. For example, either nanowires with extremely thin diameter or very sharp tip ends [42] or core-shell architectures to avoid the surface oxidation of the magnetic core to a non-ferromagnetic material, which degrades the magnetic properties [62]. Nonetheless, achieving the narrowest diameters with high metallic content (and magnetization) is not possible, being necessary to reach a trade-off between dimensions and composition [63]. Indeed, some studies have been devoted to determining the relationship between FEBID chemical composition, especially in Co, and the electron beam voltage and current [24,64], precursor gas flux [11], patterning parameters [65], substrate temperature [64], etc. In the case of Fe, even with a precise control of the growth parameters, the 3D as-grown materials present a moderate purity which can be increased by annealing treatments [25,66,67]. Moreover, performing future quantitative MFM studies is a potential advantage of FEBID tips, since the extraction of magnetic quantitative information is one of the major challenges for the future progress in MFM, which requires tips with welldefined physical properties [68]. Figure 2. MFM imaging of a high-density hard disk with ( a )Budget Sensors Multi-M tip with 50 nm of CoCr coating, ( b ) Co-FEBID tip and ( c ) Fe-FEBID tip. ( d – f ) Topographic images obtained by the selected MFM probes and ( g – i ) their corresponding magnetic images, respectively. Reproduced from [41] with permission from the Royal Society of Chemistry. A local characterization of these tips was recently performed by Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS) chemical analyses. A metallic content of ~80 at. % was obtained in the FEBID probes with the best signal-tonoise ratio [ 42 ]. As illustrated in Figure 4a, the compositional profile along the length of the nanowire as a function of the distance to the tip end reveals that the Fe content decreases as the tip end is approached. As expected, the higher contribution of the oxidation layer at the tip end justifies the Fe drop from ~70 at. % in the central region down to ~35 at. % at 3.5 nm from the tip, which corresponds with the thickness of the oxidation shell. The magnetic characterization by electron holography, illustrated in Figure 4b, allows for mapping not only the magnetic induction ( B ) inside of the specimen (~1 T in the central region) but also the value of the magnetic stray field as a function of the distance to the tip end. This is particularly useful for the development of quantitative MFM measurements [ 70 ]. The magnetic flux lines indicate that the magnetization is high in the thick central part of the nanowire and decreases close to the narrow tip, giving rise to small stray fields. Therefore, the tip shape will have a significant impact in reducing the dipole magnetic interactions between the tip and the specimen. Jaafar et al. analyzed how the customization of the stray field by accurately controlling the nanowire tip dimensions impacts MFM experimental conditions [ 41 ]. As shown in Figure 5, the standard commercial MFM probes usually present frequency shifts around 55 Hz, which can be used to calculate B when using a calibration sample. For comparison, two different FEBID nanowires with 1 µ m in length and different diameters and tip end shapes were fabricated. The nanostructures with 50 nm in diameter and blunt shape showed frequencies shifts of ~40 Hz, Magnetochemistry 2021,7, 140 6 of 16 whereas the nanowires with a sharp tip end of 7 nm exhibited ~15 Hz. This shows that the tailoring of the nanowire geometry can be used to adjust the tip–sample interaction. Magnetochemistry 2021, 7, x FOR PEER REVIEW 6 of 17 Figure 3. SEM images of (a) a core-shell Fe@Pt nanowire with a core diameter of 54 nm grown at 5 kV, (b) the thinnest 3D Fe nanowire ever achieved by us, grown at 30 kV, and (c) an Fe nanowire fabricated with an extremely sharp tip end with only ~8 nm in diameter grown at 30 kV, reproduced from [61]. (d) Study of the tip end morphology as a function of the voltage, adapted from [59]. One of the fundamental principles of SEM image resolution consists of the fact that the higher the accelerating voltage is, the better the resolution of the image becomes. This also works for the FEBID fabrication process, where the thinnest nanowires can be grown at the highest accelerating voltages. According to Figure 3d, the shape of the tip end replicates the top part of the electron beam interaction volume in the deposit [69]. Therefore, the shape of the nanowire tip end can be modulated by tuning the accelerating voltage. The upper nanowire part presents a blunt shape when using voltage values in the range 3–5 kV, as illustrated in Figure 3a; however, it exhibits a smaller diameter and an extremely pointy end at 30 kV. These features can be appreciated in the thinnest ferromagnetic nanowire ever grown by FEBID (~22 nm in diameter) [61] imaged in Figure 3b and, more precisely, in the example of the nanowire routinely grown under these conditions, shown in Figure 3c. In this case, an overall diameter of around 35 nm as well as a ~8 nmwide tip were obtained with high reproducibility. This novel shape configuration is very useful to perform very high resolution MFM imaging as well as to obtain relatively low tip–sample interaction, minimizing the influence of the tip on the magnetic state of the sample structures [42]. A local characterization of these tips was recently performed by Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS) chemical analyses. A metallic content of ~80 at. % was obtained in the FEBID probes with the best signal-to-noise ratio [42]. As illustrated in Figure 4a, the compositional profile along the length of the nanowire as a function of the distance to the tip end reveals that the Fe content decreases as the tip end is approached. As expected, the higher contribution of the oxidation layer at the tip end justifies the Fe drop from ~70 at. % in the central region down to ⁓35 at. % at 3.5 nm from the tip, which corresponds with the thickness of the oxidation shell. The magnetic characterization by electron holography, illustrated in Figure 4b, allows for mapping not only the magnetic induction () inside of the specimen (~1 T in the Figure 3. SEM images of ( a ) a core-shell Fe@Pt nanowire with a core diameter of 54 nm grown at 5 kV, ( b ) the thinnest 3D Fe nanowire ever achieved by us, grown at 30 kV, and ( c ) an Fe nanowire fabricated with an extremely sharp tip end with only ~8 nm in diameter grown at 30 kV, reproduced from [61]. (d) Study of the tip end morphology as a function of the voltage, adapted from [59]. Magnetochemistry 2021, 7, x FOR PEER REVIEW 7 of 17 central region) but also the value of the magnetic stray field as a function of the distance to the tip end. This is particularly useful for the development of quantitative MFM measurements [70]. The magnetic flux lines indicate that the magnetization is high in the thick central part of the nanowire and decreases close to the narrow tip, giving rise to small stray fields. Therefore, the tip shape will have a significant impact in reducing the dipole magnetic interactions between the tip and the specimen. Jaafar et al. analyzed how the customization of the stray field by accurately controlling the nanowire tip dimensions impacts MFM experimental conditions [41]. As shown in Figure 5, the standard commercial MFM probes usually present frequency shifts around 55 Hz, which can be used to calculate  when using a calibration sample. For comparison, two different FEBID nanowires with 1 µm in length and different diameters and tip end shapes were fabricated. The nanostructures with 50 nm in diameter and blunt shape showed frequencies shifts of ~40 Hz, whereas the nanowires with a sharp tip end of 7 nm exhibited ~15 Hz. This shows that the tailoring of the nanowire geometry can be used to adjust the tip–sample interaction. Figure 4. (a) Longitudinal chemical profile of an Fe nanowire grown on top of an AFM probe, obtained by STEM-EELS. The vertical short dash dot line represents the nanowire apex. (b) Representation of the magnetic flux lines illustrating the stray field distribution, obtained experimentally by electron holography, including insets with the TEM image (bottom), and the chemical map showing the relative composition of Fe, C, and O contents in green, blue, and red, respectively (top). Adapted from [61] and reproduced from [41] with permission from the Royal Society of Chemistry. Figure 4. ( a ) Longitudinal chemical profile of an Fe nanowire grown on top of an AFM probe, obtained by STEM-EELS. The vertical short dash dot line represents the nanowire apex. ( b ) Representation of the magnetic flux lines illustrating the stray field distribution, obtained experimentally by electron holography, including insets with the TEM image (bottom), and the chemical map showing the relative composition of Fe, C, and O contents in green, blue, and red, respectively (top). Adapted from [61] and reproduced from [41] with permission from the Royal Society of Chemistry. Magnetochemistry 2021,7, 140 7 of 16 Magnetochemistry 2021, 7, x FOR PEER REVIEW 8 of 17 Figure 5. MFM contrast, related to the stray field values, as a function of the magnetic tip section, and their corresponding MFM images of a standard commercial MFM probe (red), a tip with an FeFEBID nanowire with a blunt tip end of 50 nm (green), and with a sharp end of 7 nm (purple). Adapted from [61] and reproduced from [41] with permission from the Royal Society of Chemistry. On the other hand, in order to provide a quantitative characterization of the magnetic stray fields, micromagnetic simulations were performed. Firstly, Figure 6a illustrates the variation of  along the long axis of the nanowire depending on the nanostructure aspect ratio. Nanowires with 1 µm in length, diameters ranging from 50 nm to 95 nm and = 1 T in the central region were modelled. An important difference in the magnetic signal can be appreciated at the estimated MFM working distance—around 75 nm far away from the tip— reducing the magnetic induction 3.5 times when decreasing the diameter by half. Secondly, the stray fields of 50-nm-wide nanowires were calculated as a function of the length, using values between 300 nm and 1 µm. In this case, the impact of the length of the nanowire is negligible in the MFM working distance, as shown in Figure 6b. Therefore, regarding the values of B for the MFM performance, the length of the nanowire is not as relevant as the diameter. This means that shorter nanowires can be used, which is an advantage concerning the fabrication time and their stability during MFM operation. Finally, in Figure 6c the evolution of  as a function of the distance to the apex along the longitudinal axis direction for a nanowire of 750 nm in length and 50 nm in diameter is considered. As expected, the results indicate that, as  decreases, the stray field value at the position of MFM operation decreases. More interestingly, at lower fields an increase of 30% of  leads to an increase of the stray field by a factor of 2 in the MFM working area. Figure 5. MFM contrast, related to the stray field values, as a function of the magnetic tip section, and their corresponding MFM images of a standard commercial MFM probe (red), a tip with an Fe-FEBID nanowire with a blunt tip end of 50 nm (green), and with a sharp end of 7 nm (purple). Adapted from [61] and reproduced from [41] with permission from the Royal Society of Chemistry. On the other hand, in order to provide a quantitative characterization of the magnetic stray fields, micromagnetic simulations were performed. Firstly, Figure 6a illustrates the variation of B along the long axis of the nanowire depending on the nanostructure aspect ratio. Nanowires with 1 µ m in length, diameters ranging from 50 nm to 95 nm and B= 1 T in the central region were modelled. An important difference in the magnetic signal can be appreciated at the estimated MFM working distance—around 75 nm far away from the tip— reducing the magnetic induction 3.5 times when decreasing the diameter by half. Secondly, the stray fields of 50-nm-wide nanowires were calculated as a function of the length, using values between 300 nm and 1 µ m. In this case, the impact of the length of the nanowire is negligible in the MFM working distance, as shown in Figure 6b. Therefore, regarding the values of B for the MFM performance, the length of the nanowire is not as relevant as the diameter. This means that shorter nanowires can be used, which is an advantage concerning the fabrication time and their stability during MFM operation. Finally, in Figure 6c the evolution of B as a function of the distance to the apex along the longitudinal axis direction for a nanowire of 750 nm in length and 50 nm in diameter is considered. As expected, the results indicate that, as B decreases, the stray field value at the position of MFM operation decreases. More interestingly, at lower fields an increase of 30% of Bleads to an increase of the stray field by a factor of 2 in the MFM working area. Magnetochemistry 2021,7, 140 8 of 16 Magnetochemistry 2021, 7, x FOR PEER REVIEW 9 of 17 Figure 6. Calculation of the magnetization as a function of the distance to the apex along the nanowire longitudinal axis direction for different (a) diameter, (b) length, and (c)  values in the central region of the nanostructure. The QuickField software package was used. Reproduced from [61]. Figure 6. Calculation of the magnetization as a function of the distance to the apex along the nanowire longitudinal axis direction for different ( a ) diameter, ( b ) length, and ( c ) B values in the central region of the nanostructure. The QuickField software package was used. Reproduced from [61]. Magnetochemistry 2021,7, 140 9 of 16 2.2. Detection of Soft Magnetic Textures Berganza et al. recently reported the observation and stabilization of magnetic skyrmions by the magnetic field arisen from these functionalized tips [ 42 ]. In this study, the exploration of magnetic Néel half-hedgehog skyrmions was performed in soft magnetic polycrystalline permalloy nanodots with diameters below 100 nm by field-dependent MFM experiments with different magnetic probes [ 42 ]. As shown in Figure 7, an in-plane applied magnetic field changes the magnetic state of the nanostructure from the skyrmion configuration to the saturated one, but stability of the skyrmion state depends on the MFM tip used for imaging the process. The magnetic field required for saturation was the highest with the commercial Nanosensors TM probe, the Co-coated tip by sputtering offered an intermediate value, and the Fe-FEBID nanowire tip exhibited the lowest saturation field. This can be explained by the fact that the stray field coming from the MFM tip contributes to the stabilization of the skyrmions [ 71 ], being larger for the commercial probes. Consequently, the saturating field decreases as the generated stray field is reduced. Magnetochemistry 2021, 7, x FOR PEER REVIEW 10 of 17 2.2. Detection of Soft Magnetic Textures Berganza et al. recently reported the observation and stabilization of magnetic skyrmions by the magnetic field arisen from these functionalized tips [42]. In this study, the exploration of magnetic Néel half-hedgehog skyrmions was performed in soft magnetic polycrystalline permalloy nanodots with diameters below 100 nm by field-dependent MFM experiments with different magnetic probes [42]. As shown in Figure 7, an in-plane applied magnetic field changes the magnetic state of the nanostructure from the skyrmion configuration to the saturated one, but stability of the skyrmion state depends on the MFM tip used for imaging the process. The magnetic field required for saturation was the highest with the commercial NanosensorsTM probe, the Co-coated tip by sputtering offered an intermediate value, and the Fe-FEBID nanowire tip exhibited the lowest saturation field. This can be explained by the fact that the stray field coming from the MFM tip contributes to the stabilization of the skyrmions [71], being larger for the commercial probes. Consequently, the saturating field decreases as the generated stray field is reduced. Figure 7. MFM images of permalloy nanodots as a function of the in-plane magnetic field, obtained with a commercial magnetic tip by NanosensorsTM, a sputtered Co-coated tip and an Fe-FEBID nanowire tip. All image sizes are 250 × 250 nm2. Reproduced from [42] with permission from the Royal Society of Chemistry. FEBID tips allow for imaging the skyrmions with better resolution and with the lowest invasiveness, maximizing the out-of-plane/in-plane (OOP/IP) stray field ratio. Since the application of OOP magnetic fields serves to tune the stability of the skyrmions, the FEBID nanolithography technique can modulate this stabilization by tuning the stray field values. Therefore, the use of FEBID functionalized probes is beneficial to analyze soft magnetic textures such as skyrmions without perturbating their magnetic state and to explore their magnetization dynamics. 2.3. Applications in Liquid Media Many research lines rely on the investigation of samples which must remain in a liquid environment to be stabilized, e.g., culture media or dilutions. In these environments, the sensitivity is inversely proportional to the effective spring constant, , and heavily dependent on the quality factor, , which is linked to the damping coefficient and ultimately to the density of the liquid [47]. As a result, the signal-to-noise ratio is much worse in liquid media than in air conditions for the same cantilever. In order to improve the sensitivity, high resonance frequency and low  are required [72], but a  decrease implies a noise increment, proportional to 󰇛󰇜  ⁄. Hereafter, an improved technology devoted to analyzing biological samples (cells, tissues, proteins, virus, bacteria, nucleic acids, etc.) with magnetic traces, nanoparticles, Figure 7. MFM images of permalloy nanodots as a function of the in-plane magnetic field, obtained with a commercial magnetic tip by Nanosensors TM , a sputtered Co-coated tip and an Fe-FEBID nanowire tip. All image sizes are 250 × 250 nm 2 . Reproduced from [ 42 ] with permission from the Royal Society of Chemistry. FEBID tips allow for imaging the skyrmions with better resolution and with the lowest invasiveness, maximizing the out-of-plane/in-plane (OOP/IP) stray field ratio. Since the application of OOP magnetic fields serves to tune the stability of the skyrmions, the FEBID nanolithography technique can modulate this stabilization by tuning the stray field values. Therefore, the use of FEBID functionalized probes is beneficial to analyze soft magnetic textures such as skyrmions without perturbating their magnetic state and to explore their magnetization dynamics. 2.3. Applications in Liquid Media Many research lines rely on the investigation of samples which must remain in a liquid environment to be stabilized, e.g., culture media or dilutions. In these environments, the sensitivity is inversely proportional to the effective spring constant, K , and heavily dependent on the quality factor, Q, which is linked to the damping coefficient and ultimately to the density of the liquid [ 47 ]. As a result, the signal-to-noise ratio is much worse in liquid media than in air conditions for the same cantilever. In order to improve the sensitivity, high resonance frequency and low K are required [ 72 ], but a K decrease implies a noise increment, proportional to (KQ)−1/2. Hereafter, an improved technology devoted to analyzing biological samples (cells, tissues, proteins, virus, bacteria, nucleic acids, etc.) with magnetic traces, nanoparticles, and nanowires is reviewed. 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