Asymmetric Magnetization Research Articles

The proton resonance enhancement for CEST imaging and shift exchange (PRECISE) family of RF pulse shapes for CEST MRI.

To optimize a 100 ms pulse for producing CEST MRI contrast and evaluate in mice. A gradient ascent algorithm was employed to generate a family of 100 point, 100 ms pulses for use in CEST pulse trains (proton resonance enhancement for CEST imaging and shift exchange). Gradient ascent optimizations were performed for exchange rates = 500, 1500, 2500, 3500, and 4500 s-1; and labile proton offsets (Δω) = 9.6, 7.8, 4.2, and 2.0 ppm. Seven proton resonance enhancement for CEST imaging and shift exchange pulse shapes were tested on an 11.7 T scanner using a phantom containing three representative CEST agents with peak saturation B1,peak = 4 μT. The pulse producing the most contrast in phantoms was then evaluated for CEST MRI pH mapping of the kidneys in healthy mice after iopamidol administration. The most promising pulse in terms of contrast performance across all three phantoms was the 9.6 ppm, 2500 s-1 optimized pulse with ˜2.7 × increase in asymmetric magnetization transfer ratio (MTRasym) over Gaussian, and ˜ 1.3 times over Fermi pulses for the same B1,peak = 4 μT. This pulse also displayed a large improvement in contrast over the Gaussian pulse after administration of iopamidol in live mice. A new 100-ms pulse was developed based on gradient ascent optimizations, which produced better contrast compared to standard Gaussian and Fermi pulses in phantoms. This shape also showed a substantial improvement for CEST MRI pH mapping in live mice over the Gaussian shape and appears promising for a wide range of CEST applications.

Non-exchange bias hysteresis loop shifts in dense composites of soft-hard magnetic nanoparticles: New possibilities for simple reference layers in magnetic devices

Exchange bias has been extensively studied in both exchange-coupled thin films and nanoparticle composite systems. However, the role of non-exchange mechanisms in the overall hysteresis loop bias is far from being understood. Here, dense soft-hard binary nanoparticle composites are used not only as a novel tool to unravel the effect of dipolar interactions on the hysteresis loop shift but also as a new strategy to enhance the bias of any magnet exhibiting an asymmetric magnetization reversal. Mixtures of equally sized, 6.8 nm, soft maghemite (γ-Fe2O3) nanoparticles (no bias—symmetric reversal) and hard cobalt doped γ-Fe2O3 nanoparticles (large exchange bias—asymmetric reversal) reveal that, for certain fractions of soft particles, the loop shift of the composite can be significantly larger than the exchange-bias field of the hard particles in the mixture. Simple calculations indicate how this emerging phenomenon can be further enhanced by optimizing the parameters of the hard particles (coercivity and loop asymmetry). In addition, the existence of a dipolar-induced loop shift (“dipolar bias”) is demonstrated both experimentally and theoretically, where, for example, a bias is induced in the initially unbiased γ-Fe2O3 nanoparticles due to the dipolar interaction with the exchange-biased hard nanoparticles. These results open a new paradigm in the large field of hysteresis bias and pave the way for novel approaches to tune loop shifts in magnetic hybrid systems beyond interface exchange coupling.

Noncollinear spin texture-driven torque in deterministic spin–orbit torque-induced magnetization switching

To reveal the role of chirality on field-free spin–orbit torque (SOT) induced magnetization switching, we propose an existence of z-torque through the formation of noncollinear spin texture during SOT-induced magnetization switching in a laterally two-level perpendicular magnetic anisotropy (PMA) system. For the investigation of torque, we simulate magnetization dynamics in the two-level PMA system with SOT, which generates the noncollinear spin texture. From the spatial distribution of magnetic energy, we reveal the additional z-directional torque contribution in the noncollinear spin texture, which is unexpected in the conventional SOT-induced magnetization switching in collinear spin texture. The z-directional torque originates from the interaction between the chirality of the noncollinear spin texture and the interfacial Dzyaloshinskii-Moriya interaction of the system. Furthermore, the experimental observation of the asymmetric magnetization switching to the direction of the current flow in the two-level PMA system supports our theoretical expectation.

Efficient Josephson diode effect on a two-dimensional topological insulator with asymmetric magnetization

Efficient Josephson diode effect on a two-dimensional topological insulator with asymmetric magnetization

Asymmetrical magnetization processes induced by compositional gradients in ferromagnetic nanowires

Electrodeposited nanowires are an excellent scenario to study and control magnetic domain wall motion in nanostructures. In particular, the introduction of local changes in composition during the growth procedure has been proven to be very efficient for controlling the magnetization dynamics. In this work, we show the possibility of introducing compositional gradients in FeNi electrodeposited nanowires by gradually changing the Fe/Ni ratio along their axis. These compositional gradients produce an asymmetrical landscape for domain wall motion which is reflected in asymmetrical magnetization processes under an applied magnetic field. By studying nanowires with different compositional gradients we were able to correlate composition and magnetic asymmetry. Our results pave the way towards full control of the movement of domain walls along the nanowires.

The Value of Amide Proton Transfer MRI in the Diagnosis of Malignant and Benign Urinary Bladder Lesions: Comparison With Diffusion-Weighted Imaging.

Conventional magnetic resonance imaging (MRI) has certain limitations in distinguishing between malignant and benign urinary bladder (UB) lesions. Amide proton transfer (APT) imaging may provide more diagnostic information than diffusion-weighted imaging (DWI) to distinguish between malignant and benign UB. To investigate the potential of APT imaging in the diagnosis of malignant and benign UB lesions and to compare its diagnostic efficacy with that of conventional DWI. Prospective. Eighty patients with UB lesions. A 3.0 T/turbo spin echo (TSE) T1-weighted and T2-weighted imaging, single-shot echo planar DWI, and three-dimensional TSE APT imaging. Patients underwent radical cystectomy or transurethral resection of the bladder lesions within 2 weeks after CT urography and MRI examination. APT signal intensity in UB lesions was quantified by the asymmetric magnetization transfer ratio (MTRasym). MTRasym and apparent diffusion coefficient (ADC) values were measured and compared between malignant and benign UB lesions. Kolmogorov-Smirnov test, Student's t test or Mann-Whitney U test, Spearman rank correlation coefficient, area under the receiver operating characteristic (ROC) curve (AUC), Delong test, and intraclass correlation coefficient (ICC). The significance threshold was set at P < 0.05. Thirty-two patients had pathologically confirmed benign UB lesions, including 2 bladder leiomyomas, 1 submucosal amyloidosis, 1 inflammatory myofibroblastic tumor, and 28 inflammatory lesions, and 48 patients had pathologically confirmed urothelial carcinoma. Urothelial carcinomas showed significantly higher MTRasym values (1.53% [0.74%] vs. 0.85% [0.23%]) and significantly lower ADC values (1.24 ± 0.34 × 10-3 mm2/s vs. 1.43 ± 0.22 × 10-3 mm2/s) than benign UB lesions. The MTRasym value (AUC = 0.928) was significantly better in differentiating urothelial carcinoma from benign UB lesions than the ADC value (AUC = 0.722). APT imaging may have value in discriminating malignant from benign UB lesions and has better diagnostic performance than DWI. 3 TECHNICAL EFFICACY: Stage 2.

Multiparametric chemical exchange saturation transfer MRI detects metabolic changes in breast cancer following immunotherapy

BackgroundWith metabolic alterations of the tumor microenvironment (TME) contributing to cancer progression, metastatic spread and response to targeted therapies, non-invasive and repetitive imaging of tumor metabolism is of major importance. The purpose of this study was to investigate whether multiparametric chemical exchange saturation transfer magnetic resonance imaging (CEST-MRI) allows to detect differences in the metabolic profiles of the TME in murine breast cancer models with divergent degrees of malignancy and to assess their response to immunotherapy.MethodsTumor characteristics of highly malignant 4T1 and low malignant 67NR murine breast cancer models were investigated, and their changes during tumor progression and immune checkpoint inhibitor (ICI) treatment were evaluated. For simultaneous analysis of different metabolites, multiparametric CEST-MRI with calculation of asymmetric magnetization transfer ratio (MTRasym) at 1.2 to 2.0 ppm for glucose-weighted, 2.0 ppm for creatine-weighted and 3.2 to 3.6 ppm for amide proton transfer- (APT-) weighted CEST contrast was conducted. Ex vivo validation of MRI results was achieved by 1H nuclear magnetic resonance spectroscopy, matrix-assisted laser desorption/ionization mass spectrometry imaging with laser postionization and immunohistochemistry.ResultsDuring tumor progression, the two tumor models showed divergent trends for all examined CEST contrasts: While glucose- and APT-weighted CEST contrast decreased and creatine-weighted CEST contrast increased over time in the 4T1 model, 67NR tumors exhibited increased glucose- and APT-weighted CEST contrast during disease progression, accompanied by decreased creatine-weighted CEST contrast. Already three days after treatment initiation, CEST contrasts captured response to ICI therapy in both tumor models.ConclusionMultiparametric CEST-MRI enables non-invasive assessment of metabolic signatures of the TME, allowing both for estimation of the degree of tumor malignancy and for assessment of early response to immune checkpoint inhibition.

Hierarchical K-means clustering method for accelerated Lorentzian estimation (KALE) in chemical exchange saturation transfer-magnetic resonance imaging quantification.

Quantification of in vivo chemical exchange saturation transfer (CEST) magnetic resonance signals is challenging due to contamination from coexisting effects, including the direct water effect and asymmetric magnetization transfer. Fitting-based analysis allows the calculation of multiple types of signals from the line shape of Z-spectra. However, the conventional voxelwise method has several drawbacks, including its long computation time and its susceptibility to image noise and Z-spectra oscillations, and it is difficult to determine the initial fitting parameters. Herein, we propose a K-means clustering method for accelerated Lorentzian estimation (KALE) in CEST quantification. Briefly, voxels in CEST images are clustered into K groups according to their Z-spectra characteristics. A 'groupwise' fitting process is then performed with preset initial values, yielding a set of fitted spectra and fitted parameters for each group. With the updated initial values, each group is further clustered into subgroups, and groupwise fitting is performed again. This hierarchical K-means clustering and parameter updating process continues until the pixel number or intensity error meets the termination criteria. Voxelwise fitting could be further conducted to improve the quantification images (termed voxel-K) by utilizing the previous groupwise KALE results as the initial values (termed group-K). Incorporated with Lorentzian difference (LD) quantification, KALE was first optimized and evaluated on 5 healthy human brain datasets at 3 Tesla. Compared with traditional voxel-by-voxel LD quantification, the computation times of group-K and voxel-K were significantly reduced by ~85% and ~70%, respectively (P<0.001). Furthermore, the group-K images exhibited better denoising performance than traditional LD and voxel-K. KALE was further validated on six ischemic rat brains acquired at 7 Tesla, with both LD_group-K and LD_voxel-K displaying almost identical contrast maps with traditional voxelwise maps. When incorporated with the five-pool Lorentzian fitting (LF), KALE exhibited an improved contrast-to-noise ratio (CNR) for amplitude maps of each pool [P=0.003, 0.015, 0.047, and 0.047 for amide, nuclear Overhauser effect (NOE), magnetic transfer (MT) and guanidine amine, respectively] and improved fitting goodness (P=0.033). KALE quantification provides comparable or even superior contrast maps to traditional voxelwise fitting, with significantly reduced computation time. The 'smart' and hierarchical voxel-clustering and parameter updating process of KALE may facilitate more preclinical and clinical CEST applications.

Anisotropic Interlayer Dzyaloshinskii–Moriya Interaction in Synthetic Ferromagnetic/Antiferromagnetic Sandwiches

AbstractThe interfacial Dzyaloshinskii–Moriya interaction (DMI) in ferromagnetic/non‐magnetic‐metal bilayers is essential to stabilize chiral spin textures for potential applications. Recent works reveal that the interlayer DMI is beneficial to designing 3D chiral spin textures that possess fundamental importance and the associated technological promises. Here, the interlayer DM constants are determined quantitatively in synthetic ferromagnetic/antiferromagnetic Pt/Co/Pt/Ru/Pt/Co/Ta structures. The results demonstrate that the interlayer DMI shows uniaxial anisotropic characteristics. The first‐principles calculations elucidate that the anisotropic interlayer DMI is induced by the in‐plane symmetry breaking along two high symmetric directions, which favors the magnetization of adjacent ferromagnetic layers canting in different directions. The anisotropic interlayer DMI is also confirmed by spin‐orbit torque driven asymmetric magnetization switching. Moreover, the interlayer DMI can be tuned by the Ru‐layer‐thickness and beneficial to designing 3D spin textures for future spintronic devices.

A Novel High Quadratic Gain Boost Converter for Fuel Cell Electric Vehicle Applications

This work has adopted the concept of asymmetric inductor magnetization to develop a new quadratic converter with the advantages like a wide range quadratic voltage gain and low source current ripple. In addition, the proposed high quadratic converter has achieved reduced voltage stress due to the presence of switched capacitor cell at the output side. Initially, the operation of the converter and steady-state analysis, including efficiency calculations, are explained. In addition, the design and selection of converter components are presented along with a small-signal analysis. Further, a comprehensive comparison with recent high quadratic gain converters is presented. The comparison is made with regard to the total component count, voltage conversion ratio, effectiveness index, electric stress, current ripple, switching device power (SDP) rating, and input current ripple factor. Furthermore, the 150 W laboratory prototype is used to verify the performance of the proposed converter in terms of operation, dynamic response, and efficiency, and the corresponding experimental findings are reported. The wide range voltage gain, low current ripple, and low electric stress features of the proposed converter highlight its superiority in fuel cell electric vehicle applications.

Evaluation of amide proton transfer imaging for bladder cancer histopathologic features: A comparative study with diffusion- weighted imaging

PurposeTo assess the ability of amide proton transfer (APT) imaging, in comparison with diffusion-weighted imaging (DWI), to differentiate low-grade from high-grade bladder tumors and predict the aggressiveness of bladder cancer (BCa). MethodsForty-eight patients diagnosed with BCa confirmed by histopathological findings who underwent magnetic resonance (MR) imaging, including APT imaging and DWI (b = 0, 1000 sec/mm2), were enrolled in this study. The asymmetric magnetization transfer ratio (MTRasym) was defined as the magnetization transfer asymmetry at 3.5 ppm. MTRasym and apparent diffusion coefficients (ADCs) were compared between the low- and high-grade groups and between non-muscle-invasive bladder cancer (NMIBC) and muscle-invasive bladder cancer (MIBC) in terms of the areas under the receiver operating characteristic curves (AUCs). ResultsThe MTRasym values were significantly higher in patients with high-grade bladder tumors than in those with low-grade tumors (1.61 % [0.76 %], 1.12 ± 0.3 %; P = 0.000) and in MIBC than in NMIBC (2.53 ± 0.67 %, 1.38 % [0.35 %]; P = 0.000). The AUCs of MTRasym were significantly larger than those of ADC for differentiating MIBC from NMIBC (0.973, 0.771; P = 0.016). Adding APT imaging to DWI significantly improved the diagnostic accuracy for differentiating MIBC from NMIBC versus DWI alone (0.985, 0.876; P = 0.013). ConclusionsAPT imaging can predict tumor grade and aggressiveness in BCa. The diagnostic performance of APT imaging in predicting tumor aggressiveness was better than that of DWI, and adding APT imaging to DWI significantly improved the diagnostic accuracy of predicting tumor aggressiveness versus DWI alone.

Strengthened spin–valley polarization and negative magnetoresistance based on an asymmetrical double-ferromagnetic WSe2 junction

The spin–valley transport and tunneling magnetoresistance (TMR) are studied in an off-resonant circularly polarized light (CPL)-modulated asymmetrical double-ferromagnetic (FM) WSe 2 junction. Based on the results, we propose a device that can simultaneously operate as spin–valley filter and exhibit giant magnetoresistance in a single system. For the antiparallel configuration, the broken spin degeneracy and optical Stark effect in the first FM barrier produce 100% spin (valley)-polarized particles. Subsequently, the spin valve effect from the second FM barrier gives rise to a 100% spin (valley)-selective polarization in the strengthened polarized energy windows of the first CPL. Furthermore, increasing the asymmetrical magnetization of such a junction is found to substantially enhance the TMR, leading to a positive 100% TMR. The TMR can be tuned from −100% to 100% and is sensitively dependent on the chirality of the second CPL due to the coupling between the spin valve effect and the optical Stark effect. Our findings indicate that double-FM barriers combined with off-resonant CPL modulation can be an excellent option to improve the spin–valley polarization and magnetoresistance of transition metal dichalcogenide-based structures. • A model was proposed to improve the magnetoresistance response as well as obtain two well-defined spin–valley polarization states. • The structure gives rise to a 100% spin (valley)-selective polarization in strengthened polarized energy windows of light. • Increasing the asymmetrical magnetization of this junction is found to substantially enhance the tunnel magnetoresistance to 100%. • The magnetoresistance can be tuned from −100% to 100% and is sensitively dependent on the chirality of light.

Origin and avoidance of double peaks in the induced voltage of a thermomagnetic generator for harvesting low-grade waste heat

Thermomagnetic harvesting is an emerging approach used to convert low-grade waste heat to electricity, which recently obtained a boost due to the development of both more efficient functional materials and innovative device concepts. Here, we examine a thermomagnetic generator which utilizes gadolinium as the thermomagnetic material and report on the double peaks of the induced voltage. Using a combination of experiments and theory we show that these double peaks originate from the interaction between an asymmetric magnetization curve and a pretzel-like magnetic field topology. Double peaks are detrimental for the output power and can be avoided by matching the magnetization change by adjusting the cold and hot fluid flow.

Asymmetric magnetization reversal processes in amorphous composites (Fe40Co40B20)x(AL2O3)100-x

The amorphous composite films (Co40Fe40B20)x(Al2O3)100-x with different content of the metal (Co40Fe40B20)x “(CoFeB)x” and the dielectric (Al2O3)100-x components were obtained in the original ion-beam deposition unit. Sputtering of a composite target from an amorphous Co40Fe40B20 alloy plate with dielectric Al2O3 inserts was carried out in the argon atmosphere at an operating pressure of ∼5.10–4 Torr on glass-ceramic Sitall substrates. The presence of two regions in the IR spectra of composites with different values of х confirms the existence of the metal and dielectric components during the ion-plasma deposition of composites on a sitall substrate as a result of self-arrangement. The magnetic hysteresis was measured at room temperature for (CoFeB)x(Al2O3)100-x thin film samples for x = 1, 75 and from 9 to 69 with the step x = 10 at different angles (φ) when magnetic field applied out of a plane (φ = 90°) and in-plane (φ = 0°) of the films. The obtained results show that at low alloy (CoFeB)x contents the film composites with nominal x = 1, 9, and 19 behave as paramagnetic from the first glance. With an increase of the alloy (CoFeB)x content to nominal x = 29–75 amorphous composites films seems as superparamagnetic and ferromagnetic materials that are characterized as reversal magnetization with no coercivity and remanence. The existence of a magnetic vortex in the samples with x = 59–75 opens different remarkable applications like data storage.

Magnetic charge and geometry confluence for ultra-low forward voltage diode in artificial honeycomb lattice

Spin diode is important prerequisite to practical manifestation of spin electronics. Yet, a functioning magnetic diode at room temperature is still illusive. Here, we reveal diode-type phenomena due to magnetic charge mediated conduction in artificial honeycomb geometry, made of concave shape single domain permalloy element. We find that honeycomb lattice defies symmetry by populating vertices with low and high multiplicity magnetic charges, causing asymmetric magnetization, in applied current of opposite polarity. High multiplicity units create highly resistive network, thereby inhibiting magnetic charge dynamics propelled electrical conduction. However, practical realization of this effect requires modest demagnetization factor in constituting element. Concave structure fulfills the condition. Subsequently, magnetic diode behavior emerges across broad thermal range of T = 40 K–300 K. The finding is a departure from the prevailing notion of spin-charge interaction as the sole guiding principle behind spintronics. Consequently, a new vista, mediated by magnetic charge interaction, is envisaged for spintronic research.

Revealing hidden magnetoelectric multipoles using Compton scattering

Magneto-electric multipoles, which are odd under both space-inversion $\cal I$ and time-reversal $\cal T$ symmetries, are fundamental in understanding and characterizing magneto-electric materials. However, the detection of these magneto-electric multipoles is often not straightforward as they remain "hidden" in conventional experiments in part since many magneto-electrics exhibit combined $\cal IT$ symmetry. In the present work, we show that the anti-symmetric Compton profile is a unique signature for all the magneto-electric multipoles, since the asymmetric magnetization density of the magneto-electric multipoles couples to space via spin-orbit coupling, resulting in an anti-symmetric Compton profile. We develop the key physics of the anti-symmetric Compton scattering using symmetry analysis and demonstrate it using explicit first-principles calculations for two well-known representative materials with magneto-electric multipoles, insulating LiNiPO$_4$ and metallic Mn$_2$Au. Our work emphasizes the crucial roles of the orientation of the spin moments, the spin-orbit coupling, and the band structure in generating the anti-symmetric Compton profile in magneto-electric materials.

Unusual magnetization process and magnetocaloric effect in α-CoV2O6 driven by pulsed magnetic fields

In low-dimensional Ising spin systems, an interesting observation is the presence of step magnetization at low temperatures. Here we combine both DC and pulsed magnetic fields to study the 1/3 magnetization plateau and multiple steps in the Ising spin-chain material α-CoV2O6. Magnetization in pulsed fields is quite different from that in DC fields, showing multiple steps in an intermediate range of 4.2–6 K, inverted hysteresis below 4.2 K and asymmetric magnetization in negative fields below 11 K. We demonstrate that these unusual behaviors in magnetization are caused by the spin dynamics and the anomalous magnetocaloric effect (MCE) in α-CoV2O6, i.e., abrupt changes of sample temperature in adiabatic conditions. We successfully separate the influence between the intrinsic slow spin dynamics and the quasi-extrinsic temperature change. From the MCE, we find that some irreversible behavior is originated from the slow spin dynamics. Two different slow dynamics associated with the metastable steps are observed: one is sensitive to the slow field sweep rate at the order of ∼mT s−1 and weakly depends on temperature, while the other responds to the rapid field sweep rate of ∼kT s−1 and dominates at lowest temperature. We also distinguish that the metastable transition at H 4 is the first order and crucial for the ferrimagnetic to ferromagnetic transition. This study is useful to the understanding of multistep magnetization in α-CoV2O6 and sheds light on recent experimental findings of related compounds.

Annealing temperature dependence of non-collinear magnetic anisotropy in NiFe/NiO bilayers

The unidirectional anisotropy and uniaxial anisotropy in exchange bias systems determine the effective field that governs magnetization precession, which is critical to magnetic applications. Research has found that a misalignment between unidirectional and uniaxial anisotropies may exist, i.e., non-collinear anisotropy, where the intrinsic mechanism and contributing factors are poorly reported. In this work, exchange-biased NiFe/NiO bilayers were fabricated by using magnetron sputtering. We investigated the effects of different annealing temperatures on the non-collinear anisotropy of the NiFe/NiO bilayers by measuring the in-plane angular dependence of the magnetic hysteresis loops and the ferromagnetic resonance. We found that the non-collinear angle β varied gradually from 0° to 45° with the variation in annealing temperature, which showed an asymmetric magnetization phenomenon. The competition between unidirectional and uniaxial anisotropy and the interfacial magnetic frustration play a major role in the asymmetric reversal phenomena in an antiferromagnet/ferromagnet system. Our results provide insight into the exchange-biased field and promote its application in spintronic devices.

Chemical exchange saturation transfer for detection of antiretroviral drugs in brain tissue.

Antiretroviral drug theranostics facilitates the monitoring of biodistribution and efficacy of therapies designed to target HIV type-1 (HIV-1) reservoirs. To this end, we have now deployed intrinsic drug chemical exchange saturation transfer (CEST) contrasts to detect antiretroviral drugs within the central nervous system (CNS). CEST effects for lamivudine (3TC) and emtricitabine (FTC) were measured by asymmetric magnetization transfer ratio analyses. The biodistribution of 3TC in different brain sub-regions of C57BL/6 mice treated with lipopolysaccharides was determined using MRI. CEST effects of 3TC protons were quantitated by Lorentzian fitting analysis. 3TC levels in plasma and brain regions were measured using ultraperformance liquid chromatography tandem mass spectrometry to affirm the CEST test results. CEST effects of the hydroxyl and amino protons in 3TC and FTC linearly correlated to drug concentrations. 3TC was successfully detected in vivo in brain sub-regions by MRI. The imaging results were validated by measurements of CNS drug concentrations. CEST contrasts can be used to detect antiretroviral drugs using MRI. Such detection can be used to assess spatial--temporal drug biodistribution. This is most notable within the CNS where drug biodistribution may be more limited with the final goal of better understanding antiretroviral drug-associated efficacy and potential toxicity.

Magnetic Response of Magnetoactive Elastomers with Allowance for Slippage at the Particle‐Matrix Interfaces

AbstractA new model to describe magnetization of a magnetoactive elastomer (MAE) filled with magnetically hard powder is proposed. The magnetization process is treated as a result of a joint contribution of an assembly of single‐domain particles each of which dwells inside its own elastomer cavity. Magnetization of such a particle is a complex magnetomechanical process that combines intrinsic magnetization reversal and particle mechanical rotation. Possible slippage of the particles, in the course of which they disconnect from the matrix and, on their move, rub against the cavity wall, is explicitly taken into account. The influence of the slippage effect on the macroscopic magnetization of the composite is analyzed by means of computer simulation. Occurrence of asymmetrical magnetization loops of single‐domain particles located in an elastic medium is demonstrated. It is shown that by allowing for the slippage, it becomes possible to explain the experimentally observed anomalously low but non‐zero half‐width (coercivity) of the hysteresis loops of the MAE samples.