Magnetic Functionalization of Scanning Probes by Focused Electron Beam Induced Deposition Technology

: 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.


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][3][4][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][9][10][11][12][13][14][15][16][17][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][25][26][27][28][29][30], which has allowed a significant progress 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].

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 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. 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.  [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]. 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]. 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 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. 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 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.  On the other hand, in order to provide a quantitative characterization of the magneti stray fields, micromagnetic simulations were performed. Firstly, Figure 6a illustrates th variation of along the long axis of the nanowire depending on the nanostructure aspec ratio. Nanowires with 1 µm in length, diameters ranging from 50 nm to 95 nm and = T in the central region were modelled. An important difference in the magnetic signal ca be appreciated at the estimated MFM working distance-around 75 nm far away from th tip-reducing the magnetic induction 3.5 times when decreasing the diameter by hal Secondly, the stray fields of 50-nm-wide nanowires were calculated as a function of th length, using values between 300 nm and 1 µm. In this case, the impact of the length o the nanowire is negligible in the MFM working distance, as shown in Figure 6b. Therefor regarding the values of B for the MFM performance, the length of the nanowire is not a relevant as the diameter. This means that shorter nanowires can be used, which is an ad vantage 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 long tudinal axis direction for a nanowire of 750 nm in length and 50 nm in diameter is consid ered. As expected, the results indicate that, as decreases, the stray field value at th position of MFM operation decreases. More interestingly, at lower fields an increase o 30% of leads to an increase of the stray field by a factor of 2 in the MFM working area 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 µm 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 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 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 B leads to an increase of the stray field by a factor of 2 in the MFM working area. Magnetochemistry 2021, 7, x FOR PEER REVIEW 9 of 17

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.

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. 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.

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, 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.

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. These elements can act as contrast agents in nuclear magnetic resonance, heating elements in hyperthermia for cancer treatment, magnetic carriers for drug delivery, etc. Specifically, the magnetic properties which determine their effective implementation in biomedicine can be examined: magnetic domains configuration at remanence, magnetization reversal mechanism, stray fields, aggregate state of nanostructures in accordance with their size, level of material functionalization, and so on. For that purpose, the technological challenge of the observation and characterization of these magnetic samples in liquid media has been recently addressed using Co-and Fe-FEBID nanowire tips [41]. Figure 8 shows the results obtained on a hard disk in air and liquid conditions for commercial and FEBID tips. The use of a standard Nanosensors TM PPP-MFMR tip [47] demonstrates a clear deterioration of the signal-to-noise ratio in water environment, whereas the commercial Team Nanotec tip entails a stable, constant total signal-to-noise ratio in the liquid medium, with improved sensitivity and worse noise. and nanowires is reviewed. These elements can act as contrast agents in nuclear magnetic resonance, heating elements in hyperthermia for cancer treatment, magnetic carriers for drug delivery, etc. Specifically, the magnetic properties which determine their effective implementation in biomedicine can be examined: magnetic domains configuration at remanence, magnetization reversal mechanism, stray fields, aggregate state of nanostructures in accordance with their size, level of material functionalization, and so on. For that purpose, the technological challenge of the observation and characterization of these magnetic samples in liquid media has been recently addressed using Co-and Fe-FEBID nanowire tips [41]. Figure 8 shows the results obtained on a hard disk in air and liquid conditions for commercial and FEBID tips. The use of a standard Nanosensors TM PPP-MFMR tip [47] demonstrates a clear deterioration of the signal-to-noise ratio in water environment, whereas the commercial Team Nanotec tip entails a stable, constant total signal-to-noise ratio in the liquid medium, with improved sensitivity and worse noise. Given the performance constraints of the commercial probes, experiments with FEBID functionalized tips have been reported using dedicated cantilevers for working in liquid media. In particular, the Olympus BioLever mini AFM probe with a FEBID nanowire evidences a remarkable improvement of contrast and a strong noise reduction with respect to the standard commercial MFM tips in liquid. In fact, the image quality and sensitivity are approximately the same in both air and water, with the tip showing no aging effects after one year in storage.

Nanowire Magnetic Force Sensors
Magnetic nanowires have also been applied as scanning magnetic force microscopy sensors. Rossi et al. reported the use of GaAs nanowires with a single-crystalline MnAs magnetic tip [44]. The combination of the high force sensitivity and the small magnetic tip results in a potential tool for imaging weak magnetic field nano-patterns. More recently, Figure 8. Comparison of the MFM images of a hard disk acquired in air and water environments for the commercial Nanosensors TM PP-MFMR and Team Nanotec tips, and for the functionalized Olympus BioLever mini with an Fe-FEBID nanowire. Adapted from [61] and reproduced from [41] with permission from the Royal Society of Chemistry.
Given the performance constraints of the commercial probes, experiments with FEBID functionalized tips have been reported using dedicated cantilevers for working in liquid media. In particular, the Olympus BioLever mini AFM probe with a FEBID nanowire evidences a remarkable improvement of contrast and a strong noise reduction with respect to the standard commercial MFM tips in liquid. In fact, the image quality and sensitivity are approximately the same in both air and water, with the tip showing no aging effects after one year in storage.

Nanowire Magnetic Force Sensors
Magnetic nanowires have also been applied as scanning magnetic force microscopy sensors. Rossi et al. reported the use of GaAs nanowires with a single-crystalline MnAs magnetic tip [44]. The combination of the high force sensitivity and the small magnetic tip results in a potential tool for imaging weak magnetic field nano-patterns. More recently, Mattiat et al. showed a special type of MFM based on a transducer composed by a FEBID nanowire also acting as a local magnetic force sensor, with high-quality flexural mechanical modes, as shown in Figure 9. In particular, the growth of long Co-FEBID nanowires (~10 µm)-exhibiting 80 at. % of metallic purity and magnetic switching fields of around 40 mT-allows for very sensitive detection by means of dynamic cantilever magnetometry [43]. Due to the force interaction between the stray fields of the sample and the tip of the nanowire, different changes on the resonance frequency arise, which can be correlated to the flexural vibration modes of the long, mechanically stable nanowires. This design allows for reducing the invasiveness, favouring the mapping of magnetic fields of different magnetization textures with high resolution and sensitivity down to 3 nT·Hz 1/2 . This study also revealed that FEBID nanowires can be used as nanometer-scale force transducers, due to their high mechanical quality, and being sensitive to 2D lateral force and dissipation detection. In addition, this device has the potential capability for vectorial force sensing in 3D. Mattiat et al. showed a special type of MFM based on a transducer composed by a FEBID nanowire also acting as a local magnetic force sensor, with high-quality flexural mechanical modes, as shown in Figure 9. In particular, the growth of long Co-FEBID nanowires (~10 µm)-exhibiting 80 at. % of metallic purity and magnetic switching fields of around 40 mT-, allows for very sensitive detection by means of dynamic cantilever magnetometry [43]. Due to the force interaction between the stray fields of the sample and the tip of the nanowire, different changes on the resonance frequency arise, which can be correlated to the flexural vibration modes of the long, mechanically stable nanowires. This design allows for reducing the invasiveness, favouring the mapping of magnetic fields of different magnetization textures with high resolution and sensitivity down to 3 nT·Hz 1/2 . This study also revealed that FEBID nanowires can be used as nanometer-scale force transducers, due to their high mechanical quality, and being sensitive to 2D lateral force and dissipation detection. In addition, this device has the potential capability for vectorial force sensing in 3D.

Scanning Ferromagnetic Hall Sensors
Scanning probe microscopy has recently spread its scope through the use of advanced ferromagnetic Hall sensors [73], underpinning their potential for stray field characterization of magnetic samples minimizing the invasiveness. Gabureac et al. explored the performance of Hall sensors fabricated with Co-FEBID, with a lateral size of 50 × 50 nm 2 , reaching a magnetic field sensitivity of 1 Ω·T −1 and a resolution of 1 µT·Hz −1/2 , benefitting from the intergranular scattering of the microstructure [22]. Years later, FEBID granular ferromagnets were grown by M. Huth and co-workers to be used in scanning Hall probe microscopy, exhibiting an extremely high stray field sensitivity and a lateral resolution of 500 nm, limited due to the probe geometry [73]. As shown in Figure 10, the granular ferromagnet Hall sensors were grown in the insulating area located between the

Scanning Ferromagnetic Hall Sensors
Scanning probe microscopy has recently spread its scope through the use of advanced ferromagnetic Hall sensors [73], underpinning their potential for stray field characterization of magnetic samples minimizing the invasiveness. Gabureac et al. explored the performance of Hall sensors fabricated with Co-FEBID, with a lateral size of 50 × 50 nm 2 , reaching a magnetic field sensitivity of 1 Ω·T −1 and a resolution of 1 µT·Hz −1/2 , benefitting from the intergranular scattering of the microstructure [22]. Years later, FEBID granular ferromagnets were grown by M. Huth and co-workers to be used in scanning Hall probe microscopy, exhibiting an extremely high stray field sensitivity and a lateral resolution of 500 nm, limited due to the probe geometry [73]. As shown in Figure 10, the granular ferromagnet Hall sensors were grown in the insulating area located between the Cr/Au electrodes. This design allowed to acquire the first scanning Hall probe images of a magnetic tape where bits with different magnetization orientation can be seen with this type of sensor (see Figure 10c).
Cr/Au electrodes. This design allowed to acquire the first scanning Hall probe images of a magnetic tape where bits with different magnetization orientation can be seen with this type of sensor (see Figure 10c).

Magnetic Resonance Force Microscopy
MRFM encompasses the interaction between tip and sample to unveil the spin dynamics properties of a magnetic sample [74,75]. This imaging technique allows for exploring the spatial distribution of magnetic resonance modes of nano-objects. It should be highlighted the study of permalloy disks measured with 100 nm magnetic lateral resolution for the detection of spin waves modes localized in the structure [37]. Similarly, as shown in Figure 11a, Chia et al. reported the fabrication of magnetic Co nanocube tips by FEBID used for the generation and detection of localized spin wave precession modes in thin films [76]. In this example, an external magnetic field aligns the tip and sample magnetization in the same plane. A microwave field , perpendicular to , drives the spin precession near the ferromagnetic resonance frequency, eventually producing a cantilever deflection. More recently, Sangiao et al. reported the use of magnetic FEBID Co nanospheres in magnetic vortex state, see Figure 11b,c, aiming at minimizing the hysteresis effects and favouring quantitative analyses of the MRFM signals optimized by the high magnetization values of the functionalized probe [77].

Magnetic Resonance Force Microscopy
MRFM encompasses the interaction between tip and sample to unveil the spin dynamics properties of a magnetic sample [74,75]. This imaging technique allows for exploring the spatial distribution of magnetic resonance modes of nano-objects. It should be highlighted the study of permalloy disks measured with 100 nm magnetic lateral resolution for the detection of spin waves modes localized in the structure [37]. Similarly, as shown in Figure 11a, Chia et al. reported the fabrication of magnetic Co nanocube tips by FEBID used for the generation and detection of localized spin wave precession modes in thin films [76]. In this example, an external magnetic field H aligns the tip and sample magnetization in the same plane. A microwave field H 1 , perpendicular to H, drives the spin precession near the ferromagnetic resonance frequency, eventually producing a cantilever deflection. More recently, Sangiao et al. reported the use of magnetic FEBID Co nanospheres in magnetic vortex state, see Figure 11b,c, aiming at minimizing the hysteresis effects and favouring quantitative analyses of the MRFM signals optimized by the high magnetization values of the functionalized probe [77]. Cr/Au electrodes. This design allowed to acquire the first scanning Hall probe images of a magnetic tape where bits with different magnetization orientation can be seen with this type of sensor (see Figure 10c).

Magnetic Resonance Force Microscopy
MRFM encompasses the interaction between tip and sample to unveil the spin dynamics properties of a magnetic sample [74,75]. This imaging technique allows for exploring the spatial distribution of magnetic resonance modes of nano-objects. It should be highlighted the study of permalloy disks measured with 100 nm magnetic lateral resolution for the detection of spin waves modes localized in the structure [37]. Similarly, as shown in Figure 11a, Chia et al. reported the fabrication of magnetic Co nanocube tips by FEBID used for the generation and detection of localized spin wave precession modes in thin films [76]. In this example, an external magnetic field aligns the tip and sample magnetization in the same plane. A microwave field , perpendicular to , drives the spin precession near the ferromagnetic resonance frequency, eventually producing a cantilever deflection. More recently, Sangiao et al. reported the use of magnetic FEBID Co nanospheres in magnetic vortex state, see Figure 11b,c, aiming at minimizing the hysteresis effects and favouring quantitative analyses of the MRFM signals optimized by the high magnetization values of the functionalized probe [77].

Conclusions
The versatility of FEBID for the functionalization of AFM cantilevers with magnetic nanostructures has been discussed. Particularly, the reproducible shape tunability enables the fabrication of 3D ferromagnets, i.e., nanowires, nanocubes, and nanospheres, with accurate positioning onto the probes, as shown for some selected sensing devices in Figure 12. The geometrical customization has opened new alternatives for different operational modes in MFM, MRFM, Magnetic Force Sensing and Scanning ferromagnetic Hall sensors, giving rise to magnetic sensors with high quality performance in terms of resolution, sensitivity, and mechanical stability.

Conclusions
The versatility of FEBID for the functionalization of AFM cantilevers with magnetic nanostructures has been discussed. Particularly, the reproducible shape tunability enables the fabrication of 3D ferromagnets, i.e., nanowires, nanocubes, and nanospheres, with accurate positioning onto the probes, as shown for some selected sensing devices in Figure  12. The geometrical customization has opened new alternatives for different operational modes in MFM, MRFM, Magnetic Force Sensing and Scanning ferromagnetic Hall sensors, giving rise to magnetic sensors with high quality performance in terms of resolution, sensitivity, and mechanical stability. The use of alternative nanostructured materials on the probes for distinct functionalities, especially feasible due to the broad range of available FEBID precursor gases, the growth of more complex geometries, thanks to the application of computer-aided design programs, and the implementation of post-growth approaches for further tunability of the composition, crystallinity, and magnetic properties of the structures have a great potential for the future. This upcoming research may contribute, for instance, to the development of quantitative MFM, where the retrieval of magnetic information requires tips with welloptimized and well-defined properties.

Data Availability Statement:
The data presented in this study are available upon reasonable request from the corresponding author. The use of alternative nanostructured materials on the probes for distinct functionalities, especially feasible due to the broad range of available FEBID precursor gases, the growth of more complex geometries, thanks to the application of computer-aided design programs, and the implementation of post-growth approaches for further tunability of the composition, crystallinity, and magnetic properties of the structures have a great potential for the future. This upcoming research may contribute, for instance, to the development of quantitative MFM, where the retrieval of magnetic information requires tips with welloptimized and well-defined properties.

Data Availability Statement:
The data presented in this study are available upon reasonable request from the corresponding author.