United States

Magnetic nanoparticles (MNPs) have become a relevant technological topic as a result of their broad use in different fields, biomedicine, environmental applications, 3D printing, etc [1], [2], [3]. This wide use, together with the low MNPs concentrations used in most applications, requires the development of low-cost and high sensitivity detectors. So, this work explores the design of a Giant Magnetoimpedance effect (GMI) based detection platform for &lt;i&gt;contactless&lt;/i&gt; MNPs detection, enabling its reuse in multiple applications.&lt;br/&gt;The GMI effect, based on huge changes in the high-frequency electrical impedance under external magnetic field, has revealed as a useful tool in the design of high sensitivity magnetic sensors [4]. The proposed device is formed by two soft magnetic amorphous wires (Co&lt;sub&gt;66&lt;/sub&gt;Fe&lt;sub&gt;12&lt;/sub&gt;Si&lt;sub&gt;13&lt;/sub&gt;B&lt;sub&gt;15&lt;/sub&gt;Cr&lt;sub&gt;4&lt;/sub&gt;, length 2 cm) placed in parallel and connected electrically in series to increase the effective detection area [5]. Its working principle relies on the GMI response under the effect of the remnant magnetic field generated by Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; MNPs (diameter 140 nm). The detection target (sample) was prepared by depositing different MNPs masses, &lt;i&gt;m&lt;/i&gt;, from 1 to 15 mg, on different glass substrates. The sample to detect was placed over the sensor surface at different distances, &lt;i&gt;d&lt;/i&gt;, (0.31 - 13 mm) avoiding physical contact with the sensing wires. Using a voltage divider and a lock-in amplifier (current frequency&lt;i&gt; f&lt;/i&gt; = 100 kHz and peak to peak current&lt;i&gt; I&lt;/i&gt; = 15 mA.), the changes of the first &lt;i&gt;V1&lt;sub&gt;f&lt;/sub&gt;(H)&lt;/i&gt;, and second harmonic &lt;i&gt;V&lt;sub&gt;2f&lt;/sub&gt;(H)&lt;/i&gt;, components of the GMI voltage were studied as a function of &lt;i&gt;m&lt;/i&gt; and &lt;i&gt;d&lt;/i&gt;. The larger detection capacity (&lt;i&gt;|(V&lt;sub&gt;if&lt;/sub&gt;(x)-V&lt;sub&gt;if&lt;/sub&gt;(x&lt;sub&gt;max&lt;/sub&gt;))/( V&lt;sub&gt;if&lt;/sub&gt;(x&lt;sub&gt;max)&lt;/sub&gt;)|; i=1,2; x=m,d&lt;/i&gt;) under a similar amount of MNPs found for &lt;i&gt;V&lt;sub&gt;2f&lt;/sub&gt;&lt;/i&gt; (70%) respecting &lt;i&gt;V&lt;sub&gt;1f&lt;/sub&gt;&lt;/i&gt; (10 %), led to the selection of &lt;i&gt;V&lt;sub&gt;2f&lt;/sub&gt;&lt;/i&gt; in the final sensor prototype. A homemade electronics for sensor interfacing and signal conditioning was developed to improve both the sensitivity and spatial resolution where MNPs were detectable.&lt;br/&gt;The results show how the designed detection platform permits the quantification of low amounts of MNPs at larger distances (order of mm) than other detectors reported in the bibliography under the principles of low-cost, simple design, single-step measurement and reusability.

2022 Joint MMM-INTERMAG

Non-Linear GMI Based Detection Platform for an Enhanced Contactless and Reusable Detection of Magnetic Nanoparticles

## Dr. Nishikant (Nish) Sonwalkar: Academician, Scientist, Musician

![](https://assets.underline.io/uploads/markdown_image/1/image/0f9b63a0b3deba4a355d98e2c87af68a.png)

Dr. Sonwalkar is regarded as a leading expert in the application of computers in education. He has extensive research and development experience in hypermedia authoring, mobile learning, adaptive learning, and brain-based synaptic learning, advanced scientific visualization, database management systems, and computer simulation.

After receiving doctoral degree from MIT in the area of Molecular Dynamics of Nano-interfaces, he joined MIT as faculty in Mechanical Engineering, but his passion for technology enabled education led him become the founding director of Hypermedia Teaching Facility (Hypermedia Lab) at MIT and later he was appointed as the Principal Educational Architect of MIT.

<iframe width="560" height="315" src="https://www.youtube.com/embed/dgGdFmr9kgs" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

NISH 2021

Magnetic nanoparticles (MNPs) have become a relevant technological topic as a result of their broad use in different fields, biomedicine, environmental applications, 3D printing, etc [1], [2], [3]. This wide use, together with the low MNPs concentrations used in most applications, requires the development of low-cost and high sensitivity detectors. So, this work explores the design of a Giant Magnetoimpedance effect (GMI) based detection platform for contactless MNPs detection, enabling its reuse in multiple applications. The GMI effect, based on huge changes in the high-frequency electrical impedance under external magnetic field, has revealed as a useful tool in the design of high sensitivity magnetic sensors [4]. The proposed device is formed by two soft magnetic amorphous wires (Co66Fe12Si13B15Cr4, length 2 cm) placed in parallel and connected electrically in series to increase the effective detection area [5]. Its working principle relies on the GMI response under the effect of the remnant magnetic field generated by Fe3O4 MNPs (diameter 140 nm). The detection target (sample) was prepared by depositing different MNPs masses, m, from 1 to 15 mg, on different glass substrates. The sample to detect was placed over the sensor surface at different distances, d, (0.31 - 13 mm) avoiding physical contact with the sensing wires. Using a voltage divider and a lock-in amplifier (current frequency f = 100 kHz and peak to peak current I = 15 mA.), the changes of the first V1f(H), and second harmonic V2f(H), components of the GMI voltage were studied as a function of m and d. The larger detection capacity (|(Vif(x)-Vif(xmax))/( Vif(xmax))|; i=1,2; x=m,d) under a similar amount of MNPs found for V2f (70%) respecting V1f (10 %), led to the selection of V2f in the final sensor prototype. A homemade electronics for sensor interfacing and signal conditioning was developed to improve both the sensitivity and spatial resolution where MNPs were detectable. The results show how the designed detection platform permits the quantification of low amounts of MNPs at larger distances (order of mm) than other detectors reported in the bibliography under the principles of low-cost, simple design, single-step measurement and reusability.

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) 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 plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

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In granular FePt-L10 HAMR media. sufficient grain volume is critical for achieving expected areal density capability (ADC) [1][2]. Grain height needs to be increased as the grain size decreases. We present an experimental study on the fabrication of HAMR media with tall/high aspect ratio FePt-L10 grains. The underlayer stack consists of Ta(5nm)/Cr(40nm)/MgO(8nm) prior to the magnetic layer deposition. A 0.5 nm FePt layer is deposited on the MgO underlayer for initial grain nucleation, followed by the deposition of 2 nm FePt-BN (35% boron nitride) both at 700 oC. Over the FePt/FePt-BN stack, FePt-SiOx of 10nm thickness is deposited in the following manner: The first 3.5nm are deposited at 600 oC followed by immediately turning-off of the substrate heating and the rest of the layer is then deposited while the substrate undergoes naturally cooling. The ending temperature is 430 oC . The volumetric SiOx concentration of the FePt-SiOx layer is also graded from 40% at the bottom to 18% at the top. The L10 order parameter is S=0.72, calculated based the XRD measurement. Figure 1 shows the plan view HAADF TEM image (left) and cross-section view TEM image (right). The plan view image clearly shows well separated grains with average grain size around 6nm. The cross-section image shows columnar grains with well-defined grain boundaries and a grain aspect ratio ranging from 1.7 to 2. In this film stack, the SiOx grain boundaries are vertically matched with boron-nitride grain boundaries at the interface of the two layers. If the deposition temperature exceeds 650 oC, the majority of the adjacent FePt grains could become laterally connected over the entire thickness of the FePt-SiOx layer whereas the BN grain boundaries remains intact. While we vary the deposition conditions of the FePt-SiOx layer, the FePt/FePt-BN stack acts as a fixed template layer for the grain growth on top. For the FePt-SiOx layer, the graded SiOx composition combined with the graded deposition temperature serves the key to (1) ensure columnar grains that are well separated by SiOx grain boundaries and (2) avoid the formation of 2nd layer FePt grains.

![](https://assets.underline.io/uploads/markdown_image/1/image/89ab165f3824e2641ad4a23daad4429d.jpg)

Fig. 1. Planview HAADF (Left) and Cross-section (Right) TEM image of the entire medium stack.

Fabrication of Granular FePt-L10 HAMR Media with High Grain Aspect Ratio

Transition curvature limits the performance of Heat-assisted Magnetic Recording (HAMR), especially when bit length is reduced below 10nm. The dependence of this transition curvature on bit length was obtained micromagnetically in our previous work [1]. In the current study, the dependence of signal-to-noise ratios (SNRs) and bit error rates (BERs) on HAMR transition curvatures is further investigated. More than 20,000 bits are written ideally (with an infinite thermal gradient) with various bit lengths and realistic transition curvatures. A pseudorandom binary sequence is used. The width of the read head is 20nm, shield to shield distance is 22nm, and element thickness is 4nm. Magnetic fly height is 6nm. A Viterbi algorithm is applied to decode the readback signals and calculate BERs. Fig. 1 and 2 show how SNRs and BERs change with transition curvatures (characterized by the coefficient to the quadratic term of the defining parabola) for a minimum bit length of 7.5nm. Clearly, larger transition curvatures result in lower SNRs and higher BERs, thus negatively affecting HAMR performance. Although the results in the figures represent very small bits compared to the readback resolution, our calculations for longer bits and/or narrower shield-shield spacing show similar results, albeit with improved SNR and BER, as will be shown in the presentation.

![](https://assets.underline.io/uploads/markdown_image/1/image/16d047464ce2c11df0f3338173afaea4.jpg)

Fig. 1 Dependence of SNRs on HAMR transition curvatures with a bit length of 7.5nm.

![](https://assets.underline.io/uploads/markdown_image/1/image/53a95ed559b573a5cfb00cb65785b952.jpg)

Fig. 2 Dependence of BERs on HAMR transition curvatures with a bit length of 7.5nm.

Dependence of Bit Error Rates on HAMR Transition Curvatures

Acoustically-driven ferromagnetic resonance (ADFMR) (1,2) is a powerful technique used to transmit RF surface acoustic waves (SAWs) and tune their transmission using a thin magnetic film and a static magnetic field HDC. We explore tunability of ADFMR devices for their potential in agile RF communications applications. Two examples are nonreciprocal ADFMR devices, and devices with curved interdigital transducers (IDTs), which focus SAWs with enhanced intensity in the magnetic film. Nonreciprocity of RF transmission has an important role in critical components like isolators and circulators. We demonstrate that a thin synthetic antiferromagnet using FeGaB (3) can selectively absorb waves travelling in one direction with 48.4 dB isolation- performance that can compete with state-of-the-art commercial devices despite much smaller package size and power draw. Another tuning knob which can have device impacts is nonlinearity. With curved interdigitated transducers (IDTs), we focus SAW energy to a small, intense region near the center of the magnet. We then study the dependence of the magnetoacoustic interaction on input power. Reaching moderate input powers of 10 dBm or less allows access to a nonlinear regime previously unavailable in these devices. Using a scanning acoustic-wave interferometer, we can observe the change in amplitude of the acoustic waves as they travel across the device surface. The power dependence is studied in depth, and an applied magnetic field modifies the amplitude wherever the magnet is present. Using these tools we gain new insight into how the magnetoelastic coupling works in ADFMR, which guides our understanding in the pursuit of new tunable RF communications device types.

![](https://assets.underline.io/uploads/markdown_image/1/image/2590b73047f4f871dd45b187b9cd8f14.jpg)

Operation of a magnetoacoustic isolator. With external magnetic field applied in one direction, the device transmits RF unidirectionally with 48.4 dB isolation. Reversing the external field switches the direction of RF transmission.

![](https://assets.underline.io/uploads/markdown_image/1/image/bd4fddee8bbe14c38d434b0fda5a4473.jpg)

A) Image of a focused IDT ADFMR device. Curved IDTs transmit and receive SAWs, which travel across a 20-nm-thick Ni film on a y-cut LiNbO3 substrate. B) ADFMR pattern showing output with 860-MHz 27-dBm input as a function of HDC angle and magnitude. C) SAW amplitude measured by interferometry.

Tuning RF Acoustic Waves via Magnetism

Since the advent of spin-torque nano-oscillators (STNOs) {1}, mutual synchronization of two or more STNOs has been of intense interest as it not only improves both the microwave signal power and the signal quality factor (Q-factor) appealing to communication technology but can also be used directly for neuromorphic computing due to the tunable magnetic nature of the interaction between STNOs. Thanks to the spin Hall effect, a new class of spintronic oscillators known as spin Hall nano-oscillators (SHNOs) has emerged. Compared to STNOs, they rely on the current flowing in-plane, which makes their fabrication easier and allow for a large number of SHNOs to synchronize in both chains and arrays. {5,6} In this work, we explore mutual synchronization in nano-constriction (NC) based SHNO chains and demonstrate robust synchronization of SHNOs for longer chains with up to 21 Oscillators [Fig. 1(a&b)]. We investigate both NiFe(4 nm)/Pt(5 nm) {6,7} and W(5 mm)/CoFeB(1.4 nm)/MgO(2 nm) {8,9} SHNOs. We report that the robust mutual synchronization can deliver an enhanced output power and significantly lower linewidth (Fig. 2(a&b)). A sub-MHz linewidth [Fig. 2(b), as low as 300 kHz for NiFe/Pt] can be achieved for 21 synchronized oscillators. The high-frequency operation results in a very high quality factor (Q=f/△f) of &gt;30,000 for NiFe/Pt and &gt;25000 for W/CoFeB/MgO, which is the highest reported in chains. A linear decrease in linewidth and increase in output power (up to in-phase synchronization) is observed with the number of oscillators [Fig. 2(c)]. The low current and low field operation of these oscillators along with wide frequency tunability (2-28 GHz) with both current and magnetic fields, make these oscillators ideal candidates for various spintronic applications.

![](https://assets.underline.io/uploads/markdown_image/1/image/f0e099e3c61e0350cd61b894af51a975.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/c1278063a10ddeee4cc0d0ca490c6969.jpg)

Robust mutual synchronization of spin Hall nano-oscillator chains

Robust mutual synchronization of spin Hall nano-oscillator chains

A noise-suppression sheet (NSS), which is composed of very thin soft magnetic flake particles with organic binder, is indispensable to suppress the electromagnetic compatibility problem for recent mobile communication devices. NSS is mounted on the circuit board and absorbs extra-electromagnetic waves inside the mobile device. This absorption property is characterized as a peak of imaginary complex permeability μ’’ owing to the ferromagnetic resonance [1]. Although a commercial NSS with flake particles of Sendust (Fe-Si-Al alloy) generally exhibits a bimodal μ’’ spectrum, the physical origin of this behavior has not been understood so far. This situation would be a bottleneck for the next generation mobile communication. In this study, we investigated the μ’’ spectra of NSSs with various magnetic materials (Fe74.8Al25.2, Fe50.2Co47.6V2.2, and Fe) and compared with those of constituent single flake particles. μ’’ spectrum measurements of single flake particles are realized by means of TC-PERM which is the newly developed complex permeability measurement method with very high sensitivity [2]. Figure 1 shows the complex permeability of NSS of Fe and the constituent single flake grain of 180 μm in diameter and 1 μm in thickness. As well as the commercial NSS of Sendust, the bimodal μ’’ spectrum is clearly observed for NSS of Fe. Moreover, quite similar μ’’ spectrum is also observed for the single flake grain. This fact strongly indicates that the bimodal μ’’ spectrum of NSS is attributed to the nature of a constituent single flake. We found that the magnetic vortex model can reasonably explain this bimodal μ’’ spectrum, that is, the low- and high-frequency peaks are owing to the resonances of vortex core gyration motion and flux closure magnetization state, respectively. The calculated low- and high-frequency peaks quantitatively agree with the experimental results as marked in Fig. 1(b). This good agreement can be also confirmed for other materials. This work is supported by MIC SCOPE JP195003002.

![](https://assets.underline.io/uploads/markdown_image/1/image/f99cbc0953a7d799ac41a9fe1edbc7b8.jpg)

Fig. 1 Complex permeability spectra of (a) NSS and (b) a single constituent flake grain of Fe.

Physical origin of bimodal complex permeability spectrum of a noise-suppression sheet

The patch antenna is advantageous in many applications because of its simple and inexpensive structure. However, one disadvantage is its large size. One of the methods to reduce the size of an antenna is to utilize ferrite material [1]. While this does effectively miniaturize the antenna, the peak realized gain (PRG) is decreased by 8.3 dBic due to the magnetic loss of the ferrite core. In this paper, we take the multi-layer ferrite and dielectric approach to reduce the patch antenna size. All antennas were simulated using ANSYS HFSS for their performance. Fig. 1 shows the specifications and designs of the patch antennas. The baseline patch antenna has the dimensions given in [2] and uses a 1.4 mm thick FR-4 substrate (permeability (μr) = 1, permittivity (εr) = 4.4). The dielectric and magnetic loss tangents (tanδe and tanδu) are 0.02 and 0.00, respectively, at UHF band frequency, and its volume (V) is 9800 mm3. To realize the multi-layer concept in the antenna size reduction, 0.8 mm thick layers of ferrite (μr = 6, εr =13, tanδu = 0.05, tanδe = 0.02, and V = 2720 mm3) are inserted into the FR-4 substrate. Three topologies are investigated: one ferrite layer, two ferrite layers, and three ferrite layers. For each topology, the volume of the ferrite remains the same. For the cases with more than one ferrite layer, the inner-most layer has a constant V = 600 mm3, and the remaining volume is divided equally among the outer layers. Antenna performance results are summarized in Table 1. For one ferrite layer, 16.7% miniaturization is achieved, but the PRG decreases by 14.1 dBi. However, as the number of layers increases to two, 49.8% miniaturization is achieved with a decrease in PRG of 5.6 dBi. For three layers, 49.3% miniaturization is observed with a 5.6 dBi decrease in PRG. Compared to the case with the all-ferrite substrate (V = 9800 mm3), the patch antenna with two ferrite layers has a 4.4 dBi higher PRG. Ferrite and dielectric substrate contributes remarkably to antenna size reduction.

![](https://assets.underline.io/uploads/markdown_image/1/image/e68b9a4e3e72ffd5c9f56c3ee81cfc08.jpg)

Fig. 1. Design and specifications of (a) baseline patch antenna, (b) ferrite-dielectric layering, and (c) multi-layer patches

![](https://assets.underline.io/uploads/markdown_image/1/image/74ed72c7f2f5ae5991e9a4a0fc105818.jpg)

Table 1. Comparison of antenna performance results

Miniaturizing Patch Antenna with Multi-layer Soft Ferrite and Dielectric Substrate Structure

Recently, 45 μm thick Ti3C2Tx (MXene) showed high total shielding effectiveness (SE) of 92 dB between 8 and 12 GHz [1]. However, MXene mitigates the unwanted EM signal via reflection, and the reflect signal generates another spurious signal, harming neighboring electronic components or systems. This is because the high conductivity and paramagnetic properties of MXene cause impedance mismatch. To improve the impedance matching and absorption intensity, many researchers investigated the MXenes with magnetic material and reported a high SE via absorption but mostly in X-band (8-12 GHz) [2-3]. In this paper, we investigated MXene and M-type hexaferrite (BaFe12O19) composite to absorb EM wave occurring in Ka Band (26.5-40 GHz) for mmWave 5G application. We chose BaFe12O19 because it possesses a ferrimagnetic resonance (FMR) frequency of 42.5 GHz [4]. MXene was synthesized by the LiF-HCl etching method reported in [2], and BaFe12O19 nanopowder, purchased from Sigma Aldrich, was used. The composite was obtained by dispersing 0.25 g of MXene into 50 mL of deionized water and then ultrasonicated with 2.5 g polyvinylpyrrolidone (PVP) for 30 minutes. The sample was mixed with 0.25 g of BaFe12O19 powders and stirred for 12 hours. Finally, the composite was centrifuged and dried in an oven for 12 h. First, the phase of the synthesized composite was characterized by X-ray diffraction (XRD: Bruker D8 Discover) using Cu kα radiation, as shown in Fig. 1. Both MXene and BaFe12O19 was confirmed in the composite. Fig. 2 shows the static magnetic property of the composite, which is measured by a vibrating sample magnetometer (VSM: MicroSense EV9). Compositing MXene with BaFe12O19 increased saturation magnetization (Ms) from 0 for MXene to 10.5 emu/g and intrinsic coercivity (Hci) from 4.8 to 889 Oe. We will discuss the dynamic properties (complex permittivity, complex permeability, and reflection loss) of the MXene/BaFe12O19 composite in the paper. This work was supported in part by NSF-IUCRC under Grant No. 1650564.

![](https://assets.underline.io/uploads/markdown_image/1/image/70aed31acce356eea2293fb5d5eec926.jpg)

Fig. 1. XRD pattern of MXene, BaFe12O19, and MXene/BaFe12O19 composite.

![](https://assets.underline.io/uploads/markdown_image/1/image/d728a89e771a010b4d9419ef20265a20.jpg)

Fig. 2. Magnetic hysteresis loop of MXene, BaFe12O19, and MXene/BaFe12O19 composite.

Microwave Absorption Performance of M-type Hexagonal Ferrite and MXene Composite

This paper discusses magnetic material technology to improve receiver sensitivity in an IC chip. The idea is to use FMR losses to supress conduction noise in receiver circuit. We studied conventional NiZn ferrite particle composite and thin plate. While their materials’ FMR frequency is below 1 GHz, strong demagnetization given by the narrowness of current-carrying wires on interposer may shift up the FMR frequency to GHz range and hence useful to suppress electromagnetic noise in the GHz range. While Y and Z-type hexagonal ferrite [1] composite materials are good candidates for this application [2], this idea may widen the range of material selection NiCuZn spinel ferrite particles (commercially available) were embedded in epoxy resin to form composite sheet. Sintered plate was also prepared. Y and Z type hexagonal ferrites [2] are tested for comparison. Average diameter of the particle is 0.1~3.6 μm. The particles are mixed with a polymer binder to form a powder composite. Volume ratio of magnetic particle is nominally 50 %. Thickness of each power composite is about 50 μm to fit the space between interposer and IC chip die, as shown in Fig. 1. Noise suppression performance is proportional to the parameter 2*pai*f* ur”, It was 73@ 1 GHz for NiCuZn plate. This peak intensity was doubly higher than hexagonal ferrite composites. The composites and plate was mounted to an IC chip interposer asshown in Fig. 1 A magnetic near field probe (Langer XF-R 3-1) was placed on the IC chip package and measured radiated emission noise from the test IC chip driven by 100-200 MHz clock. The 4.8 GHz component of magnetic near filed was successfully suppressed by the proposed sintered magnetic plate. Degree of suppression counts 4-17 dB, comparing IC only and NiCuZn(17 dBmax), BaZn-Y hexagonal ferrite composite(10 dBmax) at 4.8 GHz, which is higher than material’s FMR frequency of NiCuZn ferrite. 3D RF EM simulation (HFSS Ver.14.0) predicts that current crowding at narrow current-carrying wires on chip or interposer could shifted the FMR toward higher frequency and it effected to yield high noise suppression beyond material’s FMR frequency.

![](https://assets.underline.io/uploads/markdown_image/1/image/a3cea1678471facb6553aa572b174f36.jpg)

Fig. 1. Cross section of an IC chip interposer.

NiZn ferrite noise suppressor embedded in IC interposer

To cope with the increasingly serious microwave radiation pollution, an extensive search for efficient microwave absorption materials (MAMs) has attracted great attention recently [1-3]. We have already reported the lightweight ferrite nano-hollow spheres (NHS) to exhibit enhanced electromagnetic (EM) wave absorption [4]. Herein, the deposition of the dielectric SiO2 layer on MnFe2O4 (MnFO) NHSs is found to be an effective strategy to enhance EM wave attenuation for a larger bandwidth. EM wave absorption properties of as-synthesized bare and SiO2 coated MnFe2O4 NHSs with different coating thicknesses are investigated within a widely-used frequency range. Larger interfacial area, higher magnetic anisotropy, internal reflections and scattering from NHSs along with higher magnetic permeability of MnFO are responsible for superior absorption properties of MnFO NHS. The higher dielectric permittivity of SiO2 coated on the surfaces of MnFO NHSs further improves microwave absorption for a large frequency bandwidth. The MnFO (~ 450 nm)/SiO2 (~ 68 nm) bi-layered NHSs (TEM micrograph shown in Fig.1) result in a sufficiently high RL ~ - 30.0 dB with a composite absorber of thickness only 3mm and filler concentration of 20% as shown in Fig.2. The above study on the variation of SiO2 thickness over MnFO NHSs helps us to optimize for maximum bandwidth and absorption with minimum composite absorber thickness. Analysis from the λ/4 model for best matching thickness (tm) displays a good agreement between experimental and simulated tm values. This study demonstrates the optimized MnFO/SiO2 NHS as a highly promising low-cost and lightweight EM wave absorber suitable for high-frequency filter applications.

![](https://assets.underline.io/uploads/markdown_image/1/image/8eeeb9d1dedd928bf661d78bf5d3079c.jpg)

Fig. 1 : Transmission Electron Micrograph of a MnFO/SiO2 NHS

![](https://assets.underline.io/uploads/markdown_image/1/image/7e2a9f7c39566099f818789ae491f7c2.jpg)

Fig. 2 : Three dimensional plot of reflection loss versus frequency and composite absorber thickness for MnFO (~ 450 nm)/SiO2 (~ 68 nm) bi-layered NHSs

Enhanced Electromagnetic Wave Absorption by Bi-layered Nano-hollow Spheres

We present an implementation of sequential Bayesian experiment design[1] for efficient Ramsey sequence magnetometry using NV centers in diamond. In the Ramsey sequence, the magnetic field is encoded as a phase difference between two spin states, where the phase accumulates over a selected precession time τ, and the experimental signal oscillates as a function of τ with precession frequency ω = γ B. The challenge is to choose values of τ that make efficient use of measurement time tm to reduce the field uncertainty σB. We use the equivalent noise sensitivity η = σB tm1/2 as a figure of merit. Sequential Bayesian experiment design chooses measurement settings (τ) by forecasting measurement outcomes and the effects they would have on the parameter distributions. The measurement benefit is quantified as the change in the information entropy of the parameter distribution and the measurement cost is the time required. With each measurement, more precise parameter distributions are inferred from new measurement data, improving forecasts and τ choices for later iterations. Results plotted in Fig. 1 compare the performance of sequential Bayesian experiment design with three other protocols: a non-adaptive variant of the quantum phase estimation algorithm (QPEA) [2], nonadaptive random selection of τ (Random), and an adaptive heuristic that chooses τ inversely proportional to the frequency uncertainty (Tau) [3]. For consistency, all measurement data is interpreted using Bayesian inference. The main figure plots the evolution of η against combined computation time and simulated measurement time. The vertical axis allows comparison of average uncertainty at a fixed measurement time. The inset plots η2, allowing comparison of time required to achieve a given uncertainty. The SBED protocol is faster than the Tau, Random, and QPEA protocols by factors of 2, 4 and 5, respectively.

![](https://assets.underline.io/uploads/markdown_image/1/image/5112480a93aec4b2523b3d94617abfa6.jpg)

Fig. 1. Comparison of equivalent noise sensitivity η = σB tm1/2 achieved by different measurement protocols. Initial measurements have small effects on the field uncertainty σB as tm increases. After 1 s of measurment, all protocols have converged to aymptotic σB∝ tm-1/2 behavior. The sequential Bayesian experiment design olffers the best performance.

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