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Page 8 of 25 Lin et al. Soft Sci 2023;3:14 https://dx.doi.org/10.20517/ss.2023.05

frame of Figure 2G]. In this case, the electrons in the minority-spin sub-band of one FM layer tunnel into
the majority-spin sub-band of the other layer, and those in the majority-spin sub-band enter the opposite,
resulting in small tunneling current and high resistance. In another case where the two FM layers are in the
same magnetization direction, the electrons would pass into the consistent sub-band from one layer to the
other, thereby leading to large tunneling current and low resistance [right frame of Figure 2G]. Since there
is almost no interlayer coupling between the two FM layers, a small external magnetic field can change the
magnetization direction of one layer and induce considerable resistance variation. Such processes render the
high magnetic sensitivity of TMR sensors.

However, the fabrication of high-performance TMR materials usually requires an annealing process at
[101]
elevated temperatures (≥ 300℃) and high vacuum conditions , which impedes the formation of TMR
sensors on flexible polymer. The transfer printing process, which involves retrieving material from a
carrying substrate and delivering it onto other flexible or stretchable substrates, can solve the problem to
[104]
some extent [102,103] . Figure 2H shows an example of flexible TMR sensors . Here, a conventional deposition
process is used to form nanomembranes of CoFeB/MgO/CoFeB (thickness: 6/2/4 nm) on a thermally
oxidized silicon substrate. By etching away the underlying silicon, the nanomembranes can be released and
transferred onto various flexible substrates, including aluminum foil, silicone elastomer, nitrile glove, and
others [bottom frame of Figure 2H]. In this particular case, the transfer process enhances the MR ratios to
more than 200% (~1.38 times higher than the TMR prior to transfer), primarily due to the strain-induced
change on quantum tunneling, which enables the development of flexible TMR sensors.

Although the transfer printing method enables the fabrication of relatively high-quality flexible MTJs, it
complicates the manufacturing process. An alternative approach is to deposit TMR materials directly on
flexible substrates with high thermal tolerance [105,106] . Figure 2I demonstrates a highly sensitive thin-film
strain gauge based on an MTJ formed on a thin flexible polyimide substrate (thickness: 50 μm) . Under an
[107]
external magnetic field (intensity: 2 mT), the flexible MTJ with a pseudo-spin valve (SV) structure shows a
much larger gauge factor (~1000) compared with conventional metal-foil strain gauges (gauge factor: ~2).
Using strain-sensitive free layers and adding strain-insensitive exchange-biased pinned layers allow the MTJ
to obtain stable performance without external magnets, thereby making it more suitable for practical
applications.

Although transfer printing and direct deposition on high-temperature resistant materials provide effective
means to fabricate soft TMR sensors, the fragile and strain-sensitive natures of the inorganic
nanomembranes pose challenges in achieving stable electrical and mechanical performances. As a result,
most TMR sensors still possess rigid form factors, targeting applications in areas ranging from robotic
industrial control to consumer electronics and in vitro biosensing [108-111] .

SOFT ELECTRONICS BASED ON MAGNETIC COMPOSITES
Soft electronics based on magnetic nanomembranes/nanostructures have shown great potential in
applications of multimodal e-skins, wearable navigation devices, and flexible mechanical sensors [41,112,113] .
Such systems mainly rely on structural designs (e.g., ultrathin layers, serpentine and/or other patterns) to
achieve flexibility and stretchability. The materials themselves, especially those sensing elements (i.e., GMR,
AMR and TMR materials), exhibit high modulus and are not intrinsically stretchable. These properties
impede the conformal integration of magnetic sensors with soft biological tissues, and limit the degree of
freedom of deformations. Unlike nanomembranes, which only have small dimensions in one direction
(thickness), magnetic micro/nanoparticles and nanowires (NWs) possess small features in all directions
and, therefore, can be well dispersed in other mediums to form magnetic composites. Adjusting the