Ppp fmr

Crystal sizes were determined using atomic force microscopy (AFM: Park NX10, Seoul, Korea), applying the no contact method and using an aluminum-coated silicone tip PPP-FMR (Nanosensors, PointProbe, Neuchatel, Switzerland) with a resonance frequency of 286–362 kHz and a spring constant of 20–80 N m⁻¹. Samples of 0.5 × 0.5 cm were analyzed, and three 5 × 5 μm areas were scanned at a speed of 1 Hz with a resolution of 256 × 256 pixels [47 (link)]. A particle size of 20 particles was determined for each area of 5 μm × 5 μm, and the measurements were made on 5 different films and at 3 different areas of each one.

A series of ML-MFM probes were fabricated using magnetron sputtering (AJA International Aurora, ATC-2200) in Ar atmosphere. Commercial Si cantilevers (PPP-FMR, Nanosensors™) with typical resonance frequency f₀ = 70–80 kHz, force constant = 2–3 Nm⁻¹ and curvature radius of ~10 nm were chosen for coating. The coating was deposited on two faces of the pyramidal probe (Fig. 2a) and the ML-MFM probe was comprised of two Co layers separated by a Si interlayer (Fig. 1b). Two coating thicknesses were considered, i.e., Co(30 nm)/Si(10 nm)/Co(30 nm) for thick and Co(15 nm)/Si(10 nm)/Co(15 nm) for thin ML-MFM probe. The film thicknesses were estimated using SEM and material deposition rates measured on a flat surface. The final curvature radii were ~20 nm and ~35 nm for thin and thick ML-MFM probes respectively. For comparison, the curvature radius of commercial MFM probes (PPP-MFMR, Nanosensors™²⁵ ) is ~30 nm. Detailed SEM investigations of custom-made ML-MFM probes revealed that the outer magnetic layer is longer, i.e., geometrically closer to the sample’s surface, than the inner one, see schematics in Fig. 1b. Furthermore, the orientation of the ML-MFM probe faces was within 2° of being perpendicular to the sample surface during scanning.

AFM measurements were performed with a Bruker Dimension 3,100 V instrument in a tapping mode with a tip resonance frequency of ∼350 kHz. EFM measurements were carried out using a direct current (DC) bias voltage applied between doped silicon tip (Nanosensors PPP-FMR, 0.5–9.5 N m⁻¹) and the underlying doped silicon substrate.

All DNA and protein samples were preincubated for 20 min at room temperature, diluted 10× in AFM buffer (25 mM NaOAc, 25 mM Hepes–KOH (pH 7.5), and 10 mM Mg(OAc)₂), and deposited onto a freshly cleaved mica surface (SPI Supply). The samples were washed with purified water (MilliQ) and dried with nitrogen gas. The final concentration of substrates deposited onto mica was ∼0.5 ng/μl and 30 nM for DNA and protein, respectively. All images were obtained using the tapping-mode in air on an MFP-3D-Bio AFM (Asylum Research). We used cantilevers (PPP-FMR, Nanosensors) with spring constants at ∼2.8 N/m and collected images at a scan size of 3 μm x 3 μm, a scan rate of 1 to 2 Hz, and a resolution of 512 × 512 pixels. The DNA-bending angle was analyzed using Asylum software.