United States

All of the polymorphs of pure ammonium iodide NH4I have space groups that are non-polar. However, we identify that when a fraction of the hydrogen is replaced by deuterons or when small percentages of iodide are replaced by bromide or potassium, polar phases can be established. In particular, the electric polarization of the following compounds can be switched by an applied electric field: (NH4)0.67(ND4)0.33I (where D is deuteron), NH4I0.91Br0.09 and NH4I0.94K0.06 according to pyroelectric current measurements. Magnetic susceptibility measurements indicate that the polar phases may be magnetic in origin. We explain our results by taking into account the notion that every reorienting NH4+ possesses a molecular magnetic moment that can couple with that of neighboring NH4+. The magnetic moments arise from the protons (H+) having to orbit in concert about the central nitrogen atom when the ion reorients. In the room-temperature phases, enough orbits are available to avoid mutual resonance, but below a critical temperature, a subset of these orbits become energetically inaccessible so neighboring NH4+ become mutually resonant. To avoid lattice instabilities from resonant forces, the NH4+ become ordered to establish a spiral modulation of its magnetic moments. A replacement of a fraction of H with D or I with Br or K breaks the spatial-inversion symmetry of the modulating vector so an electric polarization emerges. Our findings provide the potential of adding an extra ferroelectric dimension to hydrogen-bonded materials.&lt;br&gt;

**References:**&lt;br&gt;
F. Yen, Z. Zheng and Z. Si, Angewandte Chemie, 129(44), 13863-13866 (2017)&lt;br&gt;
L. Meng, C. He and F. Yen, The Journal of Physical Chemistry C, 124, 17255−17261 (2020)&lt;br&gt;

MMM 2022

Inducing ferroelectricity in NH4I via doping of deuterons, bromide and potassium

All of the polymorphs of pure ammonium iodide NH4I have space groups that are non-polar. However, we identify that when a fraction of the hydrogen is replaced by deuterons or when small percentages of iodide are replaced by bromide or potassium, polar phases can be established. In particular, the electric polarization of the following compounds can be switched by an applied electric field: (NH4)0.67(ND4)0.33I (where D is deuteron), NH4I0.91Br0.09 and NH4I0.94K0.06 according to pyroelectric current measurements. Magnetic susceptibility measurements indicate that the polar phases may be magnetic in origin. We explain our results by taking into account the notion that every reorienting NH4+ possesses a molecular magnetic moment that can couple with that of neighboring NH4+. The magnetic moments arise from the protons (H+) having to orbit in concert about the central nitrogen atom when the ion reorients. In the room-temperature phases, enough orbits are available to avoid mutual resonance, but below a critical temperature, a subset of these orbits become energetically inaccessible so neighboring NH4+ become mutually resonant. To avoid lattice instabilities from resonant forces, the NH4+ become ordered to establish a spiral modulation of its magnetic moments. A replacement of a fraction of H with D or I with Br or K breaks the spatial-inversion symmetry of the modulating vector so an electric polarization emerges. Our findings provide the potential of adding an extra ferroelectric dimension to hydrogen-bonded materials. 

**References:** 
F. Yen, Z. Zheng and Z. Si, Angewandte Chemie, 129(44), 13863-13866 (2017) 
L. Meng, C. He and F. Yen, The Journal of Physical Chemistry C, 124, 17255−17261 (2020)

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Pt has been considered as a system close to ferromagnetic state because Pt has a large density of states near the Fermi energy. Magnetic moment in Pt ultrathin film was reported to be induced by a magnetic proximity effect through a spinel ferrimagnet CoFe2O4(CFO) at room temperature, and a saturation value of the anomalous Hall effect (AHE), which is proportional to magnetic moment, was tuned by an electric field effect (1). In this report, we induced a ferromagnetic hysteresis in ultrathin Pt (5.9 nm) by electric field-effect at low temperature. Both the remanence and the coercive field were tuned from zero to a finite value.
A ferrimagnetic insulator CFO thin film (~ 50 nm) with TC above 300 K were prepared on a SrTiO3(STO) substrate by a pulse laser deposition. With a hall-bar shaped photoresist mask a 5.9 nm Pt film was fabricated by sputtering (Pt/CFO sample). Another Pt (5.9 nm) film was also prepared on a STO substrate as a sample for comparison (Pt/STO sample). Electric double layer transistors (EDLT) were fabricated on these samples using an ionic liquid electrolyte (DEME-TFSI) and a Pt gate electrode.
Figures show a magnetic field dependence of an anomalous Hall resistivity ρAH at 2 K for the Pt/CFO (Fig. 1) and Pt/STO (Fig. 2) samples, respectively. An ordinary Hall coefficient RH was estimated at 250 K for samples before the fabrication of the EDLT, and ρAH was estimated by ρAH = ρxy-RHB. For a gate bias (VG) of 0 V, both samples showed no hysteresis loop, while AHE was observed. By applying VG = -3 V and +3 V, the Pt/CFO sample showed clear hysteresis loop, indicating ferromagnetism induced by the VG . On the other hand, no such behavior was observed in Pt/STO. These results suggest that the ferromagnetism of Pt is induced both by the MPE from the adjacent CFO layer and tuning of Fermi level by gating. The remanence for VG = +3 V decreased with increasing temperature, and disappeared at around 19.5 K, probably corresponding to the TC of Pt. In the presentation, we will discuss the origin of the AHE. 

**References:** 
(1) S. Nodo, S. Ono, and Y. Toshihiro, Appl. Phys. Exp., Vol. 13, p.063004 (2020) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/43877251e1893af665a7fe4d881d5148.png)

Ferromagnetic hysteresis loop induced by electric field effect on Pt/CoFe2O4

Physically unclonable function (PUF), harnessing inherent stochastic variation of physical properties originating from the manufacturing process, is a crucial part of hardware security primitives [1]. PUF offers more robust information security than the conventional software-based ones. To date, owing to the compatibility to complementary-metal-oxide-semiconductor (CMOS) technology, silicon-based PUFs have been widely investigated [2]. However, their reliability with environmental fluctuations and susceptibility to external machine learning attacks are yet to be ascertained. Recently, graphene- or memristor-based PUFs were proposed and found out to be effective in resolving such issues [3,4]. However, their analog output inevitably involves additional circuit modules or analogue-to-digital converter, requiring significant power consumption and area overhead.
Here, we demonstrate highly reliable spintronic PUFs based on field-free spin-orbit torque (SOT) switching in an IrMn/CoFeB/Ta/CoFeB structure. We show that the switching polarity of the perpendicular magnetization of the top CoFeB can be randomly distributed by manipulating the exchange bias directions of the bottom IrMn/CoFeB. Figure 1 shows stochastic field-free SOT switching polarity after the randomization process of the bottom exchange-biased layers. Ideal PUF properties are found in the spintronic PUFs: high entropy close to unity, uniqueness with an inter-Hamming distance of 0.5 and reconfigurability. Furthermore, the spintronic PUFs generate binary digital outputs, potentially eliminating the need for analog-to-digital converters and error correction codes. We observed a zero bit-error rate in 5×104 repetitive measurements which demonstrates the high reliability. Due to the exchange-biased bottom layers, robustness against external magnetic fields are secured. These, when integrated with magnetic random-access memory in particular, are expected to promote scalable and energy-efficient hardware information security 
**References** [1] Y. Gao, S. F. Al-Sarawi, D. Abbott, Nat. Electron., 3, 81 (2020)
[2] J. L. Zhang, G. Qu, Y. Q. Lv, Q. Zhou, J. Comput. Sci. Technol., 29, 664 (2014)
[3] A. Dodda, S. Subbulakshmi Radhakrishnan, S. Das, Nat. Electron., 4, 364 (2021)
[4] R. A. John, N. Shah, N. Mathews, Nat. Commun., 12, 3681 (2021) 

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Spintronic Physical Unclonable Functions in an Exchange biased Trilayer Structure

The prospect of controlling topologically protected spin textures such as skyrmions lattices in high-density race-track memories offers an alternative to conventional domain wall based spintronic devices (1, 2). A variety of skyrmion spin structures have been observed in non-centrosymmetric compounds and understanding the phase field stability and size of the magnetic texture is of fundamental importance to control the desired functionality in the materials.
The lacunar spinel family with the net formula AB4X8 (A = Al, Ga, Ge; B = V, Mo; X = S, Se) has gained much attention in this respect as several compounds crystallize in the non-centrosymmetric polar rhombohedral structure (R3m, C3v) at low temperature and are magnetically ordered, with skyrmion lattice observed in GaV4S8 (3), GaV4Se8 (4) and GaMo4S8 (5). So far only polycrystalline samples of GaMo4Se8 have been studied (6).
We have performed extensive structural and magnetic characterisation on both single and poly-crystals of GaMo4Se8. Using high resolution powder neutron diffraction we confirms the phase separation below the phase transition from cubic to mixture of rhombohedral and orthorhombic phases and a significant magneto-structural coupling. Among lacunar spinels, the phenomenon of phase separation is unique to GaMo4Se8 and likely to result from the high compressive strain relaxation through the formation of the minority phase (Fig. 1a). Our small angle neutron scattering study on single crystals reveals a cycloid spin structure with a period of 15.4 nm (Fig. 1b) and upon the application of a magnetic field along the polar axis, pattern compatible with a skyrmion lattice is observed (Fig. 1c). The temperature-magnetic field phase field is established and shows that the spin texture is preserved down to the lowest temperature and under a large magnetic field of 1 T. 

**References:** 
(1) S. S. P. Parkin, M. Hayashi, L. Thomas, Science, 2008, 320, 190. 
(2) A. Fert, V. Cros, J. Sampaio, Nature Nanotechnology, 2013, 8, 152. 
(3) I. Kezsmarki et al., Nature Materials, 2015, 14, 1116. 
(4) S. Bordacs et al., Scientific Reports, 2017, 7, 7584 
(5) A. Butykai et al., npj Quantum Materials, 2022, 7, 26 
(6) E. C. Schueller et al., Phys. Rev. Materials, 2020, 4, 064402 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/be289371f992bffd20a155c6bf3b2882.png)

Magneto structural coupling and skyrmions lattice phase field in the lacunar spinel GaMo4Se8

Ultrathin ferromagnetic strips, sandwiched between a heavy metal and an oxide, have been suggested as potential platforms for recording devices where the information is coded in perpendicular magnetized domains (up or down) separated domain walls (DWs). Due to the Dzyaloshinskii-Moriya interaction, such DWs depict a homochiral Néel configuration [1,2] and are displaced with high velocity (~500 m/s) by current pulses (J~1 TA/m2) [3]. However, the writing mechanism for such DW-based devices, that is, a controlled nucleation of domains and DWs, is still challenging. Nucleation is conventionally performed by injecting an electrical current pulse along an orthogonal conducting wire [1]. However, the resulting Oersted field that nucleates a new domain also annihilates an already shifted one unless an intricate device layout is used [4]. Although some ideas have been proposed to solve this issue [5,6], a simple nucleation scheme remains elusive.
Here we propose a simple geometrical design where both the nucleation and the shifting can be purely achieved by current pulses (Fig. 1(a)). A DW is initially located in the bottom-left arm of the two symmetrical arms close to its junction with the longitudinal strip (LS), where the bits will be coded and shifted. By applying a proper voltage pulse (V2 in Fig. 1(b)), the DW is pushed towards the LS, and split in two new DWs: one at the top-left arm and other entering in the LS. A second pulse along the same bottom-left arm (V2) will shift the new DW along the LS. If injected along the top-left arm (V1), another DW and domain will be nucleated whereas the former DW will be also driven along the LS. Based on this method, new domains (bits) and DWs can be simultaneously nucleated (written) and driven by alternatively applying voltage pulses between one of the two left arms and the LS of the structure. In the talk, we will show by means of realistic simulations the potential of this idea to perform the writing operation in DW-based recording devices. 
**References** [1] S. Emori et al. Nat. Mater. 12, 611 (2013).
[2] K.-S. Ryu et al. Nat. Nanotechnol. 8, 527 (2013).
[3] I. M. Miron et al. Nat. Mater. 10, 419 (2011).
[4] O. Alejos et al. Scientific Reports, 7, 11909 (2017).
[5] Z. Luo et al. Appl. Phys. Lett. 111, 162404 (2017).
[6] T. Phuong Dao et al. Nano Lett. 19, 9, 5930 (2019).
 
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Purely current driven writing mechanism for domain wall

Wood's anomaly is a phenomenon discovered in 1902 [1], manifesting as a decrease of the reflected waves amplitude due to the excitation of a surface mode that is still investigated in photonics [2]. Another curious effect observed in the reflection of obliquely incident waves is the Goos-Hanchen effect (GHE) [3,4], which causes a spatial shift of the reflected waves. One of the configurations suitable for observing both phenomena for spin waves (SWs) is the Gires-Tournois interferometer (GTI) proposed in Ref. [5,6], made of a magnetic stripe placed above the edge of a magnetic layer. We use micromagnetic simulations to study the reflection of an oblique incident SW beam at such a GTI. We identified the resonance conditions required to efficiently excite SW modes in the GTI by the incident SW beam, which give rise to magnonic Wood's anomaly in our system. The amplitude of these modes is confined in the stripe and emits SWs back to the layer; therefore, these modes can be classified as 'leaky-modes'. The consequence of the leaky-mode is the creation of multiple spatially shifted SW beams in the layer parallel to one another. Furthermore, the excitation of the leaky-modes is accompanied by a significant GHE for the primary reflected beam with values spanning between -175 nm and 225 nm with respect to the non-resonant case. In Fig. 1a and 1b we show the distributions of SW intensity for the case with the resonant excitation of a leaky-mode and without the excitation, respectively. In Fig. 1b multiple reflected SW beams are evident. Our findings contribute to understanding and utilizing the interaction of propagating SWs in thin films with the localized leaky-modes. Additionally, an incident beam's excitation of modes in GTI provides a platform for investigations of SWs inelastic scattering on interferometer's modes. We acknowledge the funding from the Polish National Science Centre project No. UMO-2019/33/B/ST5/02013.

Magnonic interferometer’s leaky mode influence on spin

It is important to understand physically how to reliably control the domain wall (DW), because the promising applications such as solid-state spintronic memories utilizing
DW displacements along ferromagnetic nanowires (NWs) [1] are directly connected to the reproducibility of repeatable DW motion. As those are based on operations at room temperature,
extensive studies on how the thermal effect on DW depinning can be controlled by various geometrical and methodological considerations should be required for enhancing controllabilty
of the DW motion. In the previous study, we have found that their depinning fields from artificial constraints (notches) on ferromagnetic NWs can be experimentally oscillated under a
magnetic field HT transverse to the nanowire [2]. Interestingly, this study is focused on the thermal fluctuation of the 2D magnetization (M) vector, m(r,t)=mx
(r,t)x+my
(r,t)y [3]. Figure
1(a) shows noise spectra in magnetization components at specific frequencies under HT. Figure 1(b) depicts schematics of thermal M fluctuation at each frequency corresponding to Fig.
1(a), which means the red region indicates the highest magnetization fluctuation at each mode. It is clearly seen that the specific modes of thermal M fluctuations in the DWs exist and the
trend of tilting-angle difference between magnetizations at mode 4 of Fig. 2(a) is consistent with that of depinning field from notches as shown in Fig. 2(b). This depinning behavior is
related to the thermal M fluctuation in the DWs pinned at the notches, as confirmed by a micromagnetic simulation. 
**References** [1] S. Parkin and S. –H. Yang, Nature Nanotech. 10, 195 (2015).
[2] S.-M. Ahn et al., J. Nanosci. Nanotechnol. 11, 6472 (2011).
[3] N. Smith, J. Appl. Phys. 90, 5768 (2001). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a4d93e2ed8785583d2cd80fc9529fafc.png) 

Fig. 1 (a) Noise spectra in magnetization components (mx and my
). (b) Schematic maps of thermal M fluctuations at specific frequencies, (1) 1.52 GHz, (2) 2.28 GHz, (3) 3.58 GHz, and
(4) 4.52 GHz. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/a181122f90dc2dfdb65d9bbac50bf50a.png) 
Fig. 2 (a) Tilting-angle difference for the depinning field with respect to HT (simulation). (b) The depinning field as a function of HT. Inset : High resolution image of U-shaped 350-nm- wide Permalloy nanowire structure with double notches.

Role of the thermal magnetization fluctuation in depinning of domain wall from constraints on the ferromagnetic nanowire

Scaling TMR reader down to sub 20 nm for magnetic storage technology beyond 4 Tb/in2 is challenging, due to its complex film stack and increasing noise at smaller size. Recently, a spin-orbit torque (SOT) reader based on the inverse spin Hall effect was proposed as an alternative candidate [1]. The SOT reader consists of a SOT layer and a ferromagnetic layer (FM). A spin-polarized current is injected from the FM layer to the SOT layer, which generates an output voltage via the inverse spin Hall effect. However, the output voltage and signal-to-noise ratio (SNR) of SOT reader using traditional heavy metals are insufficient due to the relatively small spin Hall angle (SHA) and low sheet resistance [2]. Hence, a SOT material with large SHA is essential to generate a large output and sufficient SNR. The topological insulator (TI) BiSb emerges as potential candidate for SOT reader thanks to its giant SHA [3] and larger sheet resistance. In this work, we demonstrate a proof-of-concept for a BiSb-based SOT reader with large output voltage and SNR. Figure 1 shows the schematic structure of our device. The SOT reader is a 20 μm × 20 μm pillar of CoFe (5 nm)/MgO (2 nm)/BiSb (10 nm) on top of a 20 nm-thick Ta/Pt bottom current electrode deposited on an oxidized silicon substrate. Then, a top current electrode was deposited on top of the pillar. A perpendicular bias current was applied to the pillar, and the output voltage of the inverse spin Hall effect was read out during sweeping an in-plane magnetic field. Figure 2(a) shows a representative data at Japp = 6.25 kA/cm2 for the inverse spin Hall resistance RISHE = VISHE/Iapp of the reader. Figure 2(b) shows the characteristics of the reader. The output is as large as 15 mV, which is 3 orders of magnitude larger than that of Pt-based devices [2]. We project a giant inverse spin Hall angle of 24 for this device, which demonstrates the potential of BiSb for SOT reader application. 
**References** 
[1] P. A. V. Heijden, Q. Le, K. S. Ho, US patent US9947347B1 (2016). 
[2] V. T. Pham, I. Groen, S. Manipatruni, Nat. Elec. 3, 309 (2020). 
[3] N. H. D. Khang, Y. Ueda, and P. N. Hai, Nat. Mater. 17, 808 (2018). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/876d0c64ad5b253f6d4185bcb1e6882f.png) 
Schematic device structure of our BiSb-based SOT reader. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/caa4f1c75de584c6a6127f7610c871a9.png) 
(a) Inverse spin Hall resistance at Japp = 6.25 kA/cm2. (b) Output voltage at various applied current density.

Giant inverse spin Hall effect in BiSb topological insulator for SOT reader

2D bi-material artificial spin ice systems have been well studied [1-4] but restricting spins to 2D limits functionality. Extending into 3D provides an extra degree of
freedom, but the underlying physics need deeper understanding. We show how spin stability can be controlled via introduction of mixed materials and overlap layer (OL) in 3D
nanostructures (NS), with potential applications in computation and memory [5].
Self-aligned hexagonally arranged Py and Co NS were fabricated via stepwise nanosphere lithography [6] via a two-step deposition at different orientation. A mixture of Co NS, Py NS and
3D overlapped Co/Py NS (OLNS) comprising of a Co parallelogram on top of a Co/Py base layer is formed (Fig 1a&b).
Magnetization reversal studies were done via MFM. Fig 1c-f show remanant domain configurations at different field values. After +3 kOe saturation (Fig 1c), most Py NS (circled)
reversed at -300 Oe (Fig 1d). At -700 Oe (Fig 1e), a Co NS reversed (circled) while OLNS exhibited stable transition states (arrows). OOMMF [7] simulations showed a Néel wall (circled)
in Fig 2a but none in Fig 2b for NS without OL. Shape anisotropy causes spins in the OL to lie along the geometrical easy axis. Vertical spins of the Néel wall also connect spins in the base
layer and OL via strong exchange coupling, causing in-plane spins to remain unperturbed over large fields.
The in-plane spins were seen in Fig 2a to also pin nearby spins in the base layer outside the OL in the same direction (light blue), resulting in the stable transition state in Fig 1e. In contrast,
reversal begins at the constituent NS' corners without any OL, as seen in Fig 2b (circled).
The tunability of spin stability via OL's shape and area, material and size will be further discussed via experiments and simulations. These interesting static observations in 3D show
promise for new dynamic spin wave propagation behavior in 3D. This study aims to extend well-established static and dynamic magnetic behavior of 2D structures into 3D. 


**References:** 

[1] A. Rodríguez, L. Heyderman, F. Nolting, Appl. Phys. Lett., vol. 89, p. 142508 (2006) 
[2] S. Choudhury, S. Saha, R. Mandal, ACS Appl. Mater. Interfaces, vol. 8, p. 18339 – 18346 (2016) 
[3] A. O. Adeyeye, S. Jain, and Y. Ren, IEEE Trans. Magn., vol. 47, p. 1639 – 1643 (2011) 
[4] S. Choudhury, S. Pan, S. Barman, J. Magn. Magn. Mater., vol. 490, p. 165484 (2019) 
[5] S. Sahoo, A. May, A. van Den Berg, Nano Letters, vol. 21, no. 11, p. 4629-4635 (2021) 
[6] B. Myint, D. Yap and V. Ng, Nano Express, vol. 1, p. 020029 (2020) 
[7] M. Donahue and D. Porter, OOMMF User’s Guide, Version 1.0 Interagency Report NISTIR 6376 (1999) 

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Controlling Spin Stability in Three Dimensional Bi

The development of functional printable materials enables the production of electronic components in unconventional materials and different form factors[1]. Here, we
show magnetically sensitive inks/pastes based on magnetoresistive powder that can be printed via stencil, dispenser, or screen printing. We employ bismuth microparticles[2] as well as
[Co/Cu], [Py/Cu],[3] and permalloy [4] flakes showing giant, anisotropic, or non-saturating large magnetoresistance, respectively.
We demonstrate that magnetic field sensors based on various types of magnetoresistive flakes can be printed onto rigid, flexible, and deformable substrates. We employed blockcopolymers
and elastic binders to enable mechanical resilience of these sensors: they can withstand bending down to 16 μm, 100% of stretching, and bending for hundreds of cycles
without losing functionality.[3]
Additionally, by automatizing the dispenser printing process of bismuth-based pastes, we demonstrate the production of fully printed magnetic field sensors. The use of a micro-optically
optimized high-power diode laser array provided a versatile approach for selective sintering of sensors over flexible foils in areas exceeding several square centimeters. In this work we
experimentally confirm that such sensors retain their non-saturating magnetoresistive performance (MR = 146%) in high field conditions, allowing operation above 5 T. [2]
Our printed magnetic field sensors can be used to create interfaces that are responsive to magnetic fields through remote human input. Being flexible, they can be laminated on the skin, or
stuck onto any object, from a desk to a house wall. We demonstrate the capabilities of printed magnetic field sensors to work as an interface to navigate through digital maps, as input
panels for smart home applications, and as interactive wallpapers. 

**References:** 

[1] A. Kamyshny, S. Magdassi. Chem. Soc. Rev. Vol. 48, p.1712 (2019) 
[2] E.S. Oliveros-Mata, G.S. Cañón Bermúdez, M. Ha, et al. Appl. Phys. A Vol. 127, p.280 (2021) 
[3] M. Ha, G.S. Cañón Bermúdez, T. Kosub, et al. Adv. Mater. Vol. 33, p.2005521 (2021) 
[4] E.S. Oliveros-Mata, C. Voigt, G.S. Cañón Bermúdez, et al. Adv. Mater. Technol. p.2200227 (2022)

Detecting Magnetic Fields with Printed Magnetoresistive Sensors on Rigid and Flexible Substrates

Geometrical transformations, such as rolling a 2D ferromagnetic film into a 3D cylinder provide means to tune its magnetic properties generating different magnetic ground states [1]. Rolled-up magnetic membranes with azimuthal magnetic anisotropy are very attractive due to expected much higher domain wall velocity (compare to their planar counterparts) [2] and for applications as impedance-based field sensors [1]. However, a clear recipe for acquiring highly mobile azimuthal domains in a soft ferromagnetic tubular geometry is unclear. State of the art studies report the rolling of an extended ferromagnetic film (of hundreds of micro-meters), which after rolling converts into tubular geometry with 2-3 windings [1]. Changes in the magnetic domain configuration in such tubular geometries may arise from modifications of the shape anisotropy in addition to stress-induced anisotropy due to rolling. In our work, we report on rolling-induced azimuthal anisotropy in Ni78Fe22 stripes (as shown in figure 1a) purely due to strain, considering that curvature induced changes in shape anisotropy can be neglected due to reduced dimensions of our magnetic stripe in the azimuthal direction. For that, we employed a self-assembly rolling technology based on a polymeric platform, which allows choosing the shape and size of the magnetic structure willingly. Magnetic structures patterned on the polymeric platform can be bent controllably and hence the sign and magnitude of strain on the magnetic structure can be adjusted. We quantify the induced azimuthal magnetic anisotropy (figure 1c) by electrical measurements (figure 1b) capable of providing magnetic properties of magnetic structures hidden under the 3D polymer architecture.

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Inducing ferroelectricity in NH4I via doping of deuterons, bromide and potassium

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