2024 – Magnetic flux microconcentrators based on magnetic metasurfaces
Relevant to TAČR / CHIST-ERA project TH77010001 MetaMagIC: Magnetic Metasurfaces for sustainable Information and Communication technologies
Output n° TH77010001-V01
About the project
The MetaMagIC project aims to develop a novel approach to efficiently and homogeneously concentrate magnetic flux and collect energy from weak low-frequency magnetic fields, at small scales, that will radically improve the efficiency of the next-generation of sensors and magnetic harvesters. Over the period of project implementation, the goal is to design, test, and optimize configurations of magnetic flux concentrators (MFC) based on engineered metasurfaces.
About the functional sample
Here, we specifically focus on on-chip planar devices with extended applicable field range. In the first-generation of micro-MFCs with continuous concentrators (miniaturized version of macrodevices), the performance and reliability were hindered by the presence of magnetic domains with their configuration depending on local defects and previous magnetic history. The study (joint with partners) of such MFCs was published in Advanced Materials Technologies [DOI: 10.1002/admt.202300177]. In addition, the external field range giving a linear response (i.e., constant gain) was only a few mT, often less than 1 mT.
To address these issues, here we present micro-MFC formed from metasurfaces – coupled disks with each hosting a magnetic vortex, i.e., well-defined magnetic state.
The aim is to obtain on-chip micro-MFC with extended linear response to the external field while keeping reasonable gain (amplification). The sensing/useful area is designed to be 1 µm in diameter. Here, we focus on tuning the linearity range and gain via geometrical factors. Aside from more sensitive field sensing, such micro-MFC can be used for localized field amplification – useful for microscopy techniques where application of magnetic field at the sample level is challenging (low-energy electron microscopy, photoemission electron microscopy).
Figures 1-3 successively depict examples of micromagnetic modeling, fabricated structures, and experimental investigation of disk-based MFCs, respectively. The linear field range is significantly extended (Figure 1) and for large disk thickness and small gaps among disks, one can obtain gains on par with continuous MFCs (reaching ~1.5). The gain is larger for thicker structures. An additional tuning parameter is the disk diameter. Large disks have a higher susceptibility but saturate faster and thus reduce the usable field range. A good compromise was identified for diameters around 400 nm. A similar situation pertains to the disk spacing: smaller gaps lead to enhanced gain at the expense of reduced linear field range. Interestingly, the gain does not significantly increase for gaps below 40 nm, except for touching disks, where the response is highly non-linear with the applied field.
Figure 1 : Example of micromagnetic modeling of disk-based MFC. The total MFC size is 10 µm, central gap 1 µm. Disk diameter 400 nm, gap among disks 40 nm, and thickness 80 nm. Gain is ~1.44 up to 36 mT.
Figure 2 : Scanning electron microscopy micrographs of MFCs based on coupled nanodisks. While the constituent disk diameter is kept at 400 nm, the disk spacing varies (60 nm in the left panel and 100 nm for the right panel). The MFCs are 60-nm-thick and prepared with and without central sensing disks.
Figure 3 : (a) Lorentz transmission electron microscopy image of a planar MFC made from NiFe disks with the sensor disk in the center. Deflection of the vortex core (bright spot in the inset marked by yellow circle) is evaluated as a function of applied field along both x and y axes and compared with the reference disk. (b) Vortex core deflection as a function of applied field shows a linear response to the applied magnetic field up to +-40 mT. Comparison with a reference disk gives a gain of 1.2 along x and close to 1.0 along y. The gain along the concentrator is consistent with simulations.
Summary
We realized on-chip microscopic MFCs formed by metasurfaces with extended applicable magnetic field range – tens of mT instead of less than 1 mT for continuous planar concentrators of similar size – tens of microns. The design contains micro-MFC with various gaps among disks to provide different gains and field ranges. The smaller the gap, the higher the gain, and the smaller the field range. The same design was tested both on common Si substrates and electron-transparent SiN membranes that are useful for advanced magnetic microscopies. The presented sample is made of 60-nm-thick NiFe; using thicker structures and/or a material with a higher saturation magnetization, the gain can be increased further.