Nonlinear Magnetic Response Research Articles

Combined rotational diffusion of a superparamagnetic particle and its magnetic moment: Solution of the kinetic equation

This paper develops a theory of rotational diffusion of a uniaxial superparamagnetic particle, suspended in a fluid. A kinetic equation for the joint distribution function of orientations of anisotropy axis and magnetic moment of the particle is examined and a consistent method of its solution is proposed. The approach involves the introduction of a kinetic operator that generates the time evolution of the distribution function, and the usage of quantum-mechanical formalism, particularly, elements of the representation theory and the theory of additions of angular momenta. A convenient representation, in which matrix of the kinetic operator is close to diagonal, is specified. In this representation the evolution equation takes a form of linear recurrence–differential relations for statistical moments of the distribution function. Their numerical integration can be performed by one of the standard methods, giving the average (observed) magnetization of the system for any instant of time. The presented scheme assumes that frequency of an applied magnetic field is well below ferromagnetic resonance range (that is typical for majority of experiments) but it does not impose any restrictions on the field amplitude, material parameters of the particle, viscosity of fluid or temperature. It can serve as a theoretical basis for a consistent description of relaxation spectrum, dynamic magnetic susceptibility and nonlinear magnetic response of a dilute magnetic fluid with considering the interplay between mechanical and magnetic degrees of freedom of suspended nanoparticles. Also, it can be used for cross-checking of approximate models.

Magnetic particle imaging signal acquisition using second harmonic detection of magnetic nanoparticles

<p class="Abstract">This paper presents an approach for acquiring a magnetic particle imaging (MPI) signal, by utilizing the second harmonic detection of the magnetic nanoparticles tracers. An MPI signal with high signal-to-noise ratio (SNR) is crucial for high spatial resolution images that reveals the distribution of the tracers in a target area. Samples of Resovist and Perimag nanoparticles tracers were prepared in liquid and immobilised form, which were placed at some distances under the receiver coil of the single-sided MPI scanner. The samples were exposed to the excitation magnetic field generated at 22.8 kHz and a static gradient field generated with a direct current of 2 A. The non-linear magnetization response of the tracers for each spatial position is recorded in the form of voltage signal by a gradiometer pickup coil, with the second harmonic signal being extracted by a resonance circuit. The results obtained revealed that a sufficient signal from the tracers is recorded at up to 25 mm under the pickup coil, with Perimag samples inducing higher signals as compared to Resovist. The dependence of the DC gradient field on the second harmonic signal shows that the peak signal amplitude for Resovist and Perimag particles as ±5 mT and ±6 mT respectively. Additionally, the second harmonic signal amplitude increases exponentially with an increase in the excitation magnetic field. Thus, the results obtained shows the potential of this approach in acquiring high SNR MPI signals at low excitation frequency, which could be vital in reconstructing the contour images of the tracers, particularly in sentinel lymph node biopsy (SLNB) for breast cancer diagnosis.</p>

Universal superconducting precursor in three classes of unconventional superconductors

A pivotal challenge posed by unconventional superconductors is to unravel how superconductivity emerges upon cooling from the generally complex normal state. Here, we use nonlinear magnetic response, a probe that is uniquely sensitive to the superconducting precursor, to uncover remarkable universal behaviour in three distinct classes of oxide superconductors: strontium titanate, strontium ruthenate, and the cuprate high-Tc materials. We find unusual exponential temperature dependence of the diamagnetic response above the transition temperature Tc, with a characteristic temperature scale that strongly varies with Tc. We correlate this scale with the sensitivity of Tc to local stress and show that it is influenced by intentionally-induced structural disorder. The universal behaviour is therefore caused by intrinsic, self-organized structural inhomogeneity, inherent to the oxides’ perovskite-based structure. The prevalence of such inhomogeneity has far-reaching implications for the interpretation of electronic properties of perovskite-related oxides in general.

Comprehensive characterization of magnetite-based colloid for biomedical applications

An aqueous colloidal solution of dextran-coated magnetite nanoparticles was studied by nonlinear second-harmonic magnetic response (M2), transmission electron microscopy (TEM), dynamic light scattering (DLS) and electron magnetic resonance (EMR). Nanoparticles were found to aggregate. A set of magnetic parameters of the aggregates, such as the mean magnetic moment, the magnetization damping constant, the longitudinal relaxation time, the field and energy of magnetic anisotropy, and others were evaluated from M2 measurements with the data processing formalism based on the Gilbert–Landau–Lifshitz (GLL) equation for the stochastic dynamics of superparamagnetic (SP) particles. Combined with TEM and DLS, the M2 technique additionally enabled the differentiation between magnetic and nonmagnetic components of the colloid. To achieve full numerical consistency between the parameters obtained from the M2 and TEM data, magnetic correlations of nanoparticles inside the aggregates were taken into account and their correlation radius was evaluated. The observed crossover in the magnetic field dependence of the EMR signal occurring due to the break of the dipole–dipole (d–d) coupling between nanoparticles in the aggregates was described using the M2 and TEM data.

Multiphysics modeling of an MSMA-based clamped–clamped inertial energy harvester

In this paper, an alternative way of harvesting energy from ambient vibration is investigated through proposing a novel inertial energy harvester using magnetic shape memory alloys (MSMAs). To this end, a clamped–clamped beam is coupled with MSMA units which are attached to its roots. A shock load is applied to a proof mass in the middle of the beam. The beam vibration causes longitudinal strain in the MSMAs and as a result the magnetic flux alters in the coils wounding around the MSMA units and produce an AC voltage. To have a reversible strain in MSMAs, a bias magnetic field is applied in transverse direction of the MSMA units. The large scale vibration of Euler–Bernoulli beam is modeled with the help of von Kármán theory and a thermodynamics-based constitutive model is used to predict the nonlinear strain and magnetization response of the MSMAs. In order to predict the output voltage during martensite variant reorientation in MSMAs is studied with the help of Faraday’s law of induction. After showing the results for a clamped–clamped energy harvester, its performance is compared with cantilever one. The results show that this novel energy harvester has the capability of using as an alternative to the current piezoelectric and magnetostrictive ones for harvesting energy from ambient vibrations. Comparing the performance of clamped–clamped MSMA based energy harvester with cantilever one state that it can have some advantages over the cantilever energy harvester.

Mechanism of magnetic heating in Mn-doped magnetite nanoparticles and the role of intertwined structural and magnetic properties.

We study the mechanism of heat generation, induced by an alternating magnetic field, in magnetite nanoparticles doped with manganese, produced by thermal decomposition from organometallic precursors. We investigate a set of four samples obtained by varying the duration of the reflux treatment carried out at a temperature of 300 °C during the synthetic procedure. On increasing this parameter from 60 to 180 minutes, the mean size of the nanoparticles increases, though remaining below 10 nm, as well as the saturation magnetization, which in all the samples, thanks to the Mn doping, is higher than that in magnetite nanoparticles taken as a reference. The combination of these two events has two main consequences. First, it determines the intensity of dipolar interactions between the nanoparticles, thus influencing their magnetic relaxing behavior, which, in turn, is closely related to the heating efficiency. Secondly, in a heating test, it is possible to operate in the regime of non-linear magnetic response of the nanoparticles at values of amplitude and frequency of the alternating field usually employed for biomedical applications. We show that, in this regime, the Specific Absorption Rate (SAR) in each sample depends linearly on the fraction of nanoparticles that are not superparamagnetic. This opens the possibility of modulating the heating capacity of the produced nanoparticles, so as to match specific needs, changing only a single synthesis parameter and opportunely exploiting the strict connection between structural features, magnetic properties and measurement conditions.

Effects of size and anisotropy of magnetic nanoparticles associated with dynamics of easy axis for magnetic particle imaging

Abstract The structure of magnetic nanoparticles affects the signal intensity and resolution of magnetic particle imaging, which is derived from the harmonics caused by the nonlinear response of magnetization. To understand the key effects of particle structures on the magnetization harmonics, the dependence of the harmonics on the size and anisotropy of different structures was investigated. We measured the harmonic signals with respect to different magnetic nanoparticle structures by applying an AC field with a gradient field for magnetic particle imaging, which was compared with the numerically simulated magnetization properties. In addition, the dynamics of the easy axis of magnetic nanoparticles in the liquid state were evaluated. The difference between the harmonics in the solid and liquid states indicates the effective core size and anisotropy due to particle structures such as single-core, chainlike, and multicore particles. In the case of the chainlike structure, the difference between the harmonics in the solid and liquid states was larger than other structures. In the numerical simulations, core diameters and anisotropy constants were considered as the effective values, such as the increase in anisotropy in the chainlike structure due to dipole interaction. The multicore particles showed high harmonics owing to their large effective core diameters. The superparamagnetic regime in the multicore structure despite the large effective core diameter was derived from the small effective anisotropy. The effective core size and the effective anisotropy of each particle structure and their impacts on the harmonic signals were revealed.

Estimating saturation magnetization of superparamagnetic nanoparticles in liquid phase

Abstract Superparamagnetic nanoparticles (SPMNPs), with unique physical and magnetic properties that differentiate them from their bulk magnetic materials, have been widely studied for potential applications in biomedical areas. With proper surface chemical functionalization, SPMNPs have found their applications in magnetic hyperthermia therapy, magnetic bioassays, drug delivery, magnetic manipulation, etc. These applications require elaborate tuning of the physical and magnetic properties of the SPMNPs such as saturation magnetization M s and magnetic core size D . In this work, we present a search coil-based method to directly characterize the M s of SPMNPs in the liquid phase. The nonlinear magnetic responses of SPMNPs under oscillating magnetic fields are exploited and the induced harmonic signals are used to analyze their M s and D . Different combinations of M s and D are assumed and their harmonic ratios R are summarized. Curve fitting shows that the harmonic ratio R = 0.74 + 2.85 × 10 9 ∙ D - 4.41 ∙ M s - 1.44 , with the coefficient of determination R-square = 0.98.

On the unusual magnetic response of cryomilled BiFeO3 polycrystals

In the present work the influence of milling process below 173 K (cryomilling) on the nanostructuration of BiFeO3 powders, synthesized by fast-firing thermal treatment allied to high-energy ball milling, is evaluated as function of milling time as well as balls size/mass ratio. The influence of the cryomilling process on the structural and magnetic properties of BiFeO3 powders is investigated and the key role of the crystallite size on the magnetic response is evidenced. The results show a strong nonlinear magnetic response, which is governed by the crystallite size while induced strain does not play an important role.

On the degree of ill-posedness of multi-dimensional magnetic particle imaging

Magnetic particle imaging is an imaging modality of relatively recent origin, and it exploits the nonlinear magnetization response for reconstructing the concentration of nanoparticles. Since being first invented in 2005, it has received much interest in the literature. In this work, we study one prototypical mathematical model in multi-dimension, i.e. the equilibrium model, which formulates the problem as a linear Fredholm integral equation of the first kind. We analyze the degree of ill-posedness of the associated linear integral operator by means of the singular value decay estimate for Sobolev smooth bivariate functions, and discuss the influence of various experimental parameters on the decay rate. In particular, applied magnetic fields with a field free point and a field free line are distinguished. The study is complemented with extensive numerical experiments.

PO-506 Targeting membrane-bound Hsp70 on cancer cells with functionalized superparamagnetic nanoparticles: new perspectives for early diagnosis and therapy

IntroductionThe stress-inducible 72 kDa heat shock protein (mHsp70) is known to be expressed on the plasma membrane of tumour cells, but not on the corresponding normal cells. Targeting of the mHsp70 might provide a promising strategy for theranostics. Functionalized superparamagnetic iron oxide nanoparticles (SPIONs) have emerged as a promising contrast agents for the molecular magnetic resonance imagning (MRI). Coating of nanoparticles surface with therapeutic ligands could provide an anti-cancer activity.Material and methodsSynthesised nanocomplexes were conjugated with anti-Hsp70 tools (including monoclonal cmHsp70.1 antibodies (cmHsp70.1-SPIONs), granzyme B (GrB-SPIONs), tumour penetrating peptide TPP (TPP-SPIONs)) and in vitroanalysed for specific targeting of mHsp70 in cancer cells. Diagnostic potential of nanoparticles for MRI was assessed in the orthotopic models of U87 glioblastoma in NMRI nu/nu mice, C6 glioma in rats, orthotopic H1339 lung cancer in immunodeficient nu/nu mice, GL261 glioma in C57/Bl6 mice. To increase the tumour accumulation of particles additionally an external magnetic field was applied. Theranostic potential of GrB-SPIONs (that have a pro-apoptotic activity) was assessed as a monotherapy or in combination with radiotherapy.Results and discussionsSuperparamagentic conjugates had a high contrast enhancing properties (NMR relaxivity studies) and specifically targeted Hsp70-positive tumour cells (as shown by confocal and electron microscopies, flow cytometry) in a dose-dependent manner. High-resolution MRI (11 T) on T2-weighted images demonstrated the retention of conjugates in the tumour site. Biodistribution analysis employing highly sensitive non-linear magnetic response measurements (NLR-M2) confirmed the retention of particles in the tumour (that was further confirmed by histological studies). Application of the external magnetic field further enhanced accumulation of the nanocarriers in the tumour. Due to the pro-apoptotic potency of GrB-SPIONs conjugates the latter, when being systemically administered in tumor-bearing animals, not only targeted the Hsp70-positive tumours but also induced the delay of cancer progression (MRI volumometry) and increased the survival. Combination of GrB-SPIONs with radiotherapy further enhanced the anti-cancer activity of the conjugates.ConclusionOverall, the data showed that Hsp70-targeted superparamagnetic nanoparticles have a potential to be used for the early detection and therapy of the tumours.

PO-411 Combinatorial therapy with immune checkpoint blockade and theranostic tumor-targeted nanoparticles in eradication of tumours

IntroductionTargeted superparamagnetic iron oxide nanoparticles (SPIONs) have emerged as a promising detection tool for the molecular magnetic resonance imaging (MRI). Decoration of the surface with specific moieties that have anti-tumour immunotherapeutic activity could provide a theranostic potential for the nanocomplexes. Combination of functionalized particles with blocking of immune checkpoint inhibitors could further enhance the therapeutic benefit.Material and methodsSynthesised nanoparticles conjugated with granzyme B (GrB-SPIONs) were in vitro assessed (employing confocal and electron microscopies, flow cytometry) for specific targeting of the membrane-bound Hsp70 in cancer cells. Application of GrB-SPIONs as a monotherapy or in combination with single dose (10Gy) radiotherapy (SARRP, X-strahl) was performed in the orthotopic models of human U87 glioblastoma in NMRI nu/nu mice, GL261 glioma in C57/Bl6 mice, C6 glioma in rats. Combinatorial therapy of anti-CTLA-4 and anti-PD-1 anitbodies with GrB-SPIONs and radiotherapy was performed.Results and discussionsInternalised GrB-SPIONs in Hsp70-positive tumour cells induced apoptotic death in dose- and time-dependent manner (as shown by flow cytometry). Monotherapy with GrB-SPIONs in animal models resulted in delayed tumour progression (MRI volumometry) and increased survival. Biodistribution analysis employing highly sensitive non-linear magnetic response measurements (NLR-M2) confirmed the retention of particles in the tumour (that was further proved by histological studies). Addition of radiotherapy further promoted therapeutic benefit of systemically administered nanoparticles. Subsequent therapy with anti-CTLA-4 and anti-PD-1 antibodies synergistically improved the efficacy of GrB-SPIONs nanocomplexes with radiotherapy. Inhibitory immune checkpoint blockade significantly enhanced inflitration of tumour tissue with activated CD8 +and natural killer (NK1.1) cells and decreased suppressive PD-1 +cells.ConclusionThese findings reveal that GrB-SPIONs can specifically target Hsp70-positive cancer cells that could be used simultaneously for nonivasive MRI diagnosis of tumours and therapy. Combinatorial regimen of targeted nanocomplexes and immune checkpoint inhibitors has a high theraputic potency that could be translated into clinial trials.

Magnetic field dependence of nonlinear magnetic response and tricritical point in the monoaxial chiral helimagnet Cr1/3NbS2

We present a comprehensive study of the magnetization dynamics and phase evolution in the chiral helimagnet ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$, which realizes a chiral soliton lattice (CSL). The magnetic field dependence of the ac magnetic response is analyzed for the first five harmonic components, ${M}_{\mathrm{n}\ensuremath{\omega}}(H)\phantom{\rule{0.16em}{0ex}}(n=1--5)$, using a phase sensitive measurement over a frequency range, $f=11--10\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The behavior is indicative of the formation of a spatially coherent array of large ferromagnetic domains, which relax on macroscopic time scales. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time scales with increasing temperature. We experimentally resolve the tricritical point at ${T}_{\mathrm{TCP}}$ in a temperature regime above the ferromagnetic Curie temperature which separates the linear and nonlinear magnetic regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarizes the features of the field and temperature dependence of the magnetic crossovers and phase transitions in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$.

Recovery from Errors Due to Domain Truncation in Magnetic Particle Imaging: Approximation Error Modeling Approach

In magnetic particle imaging, the concentration of magnetic nanoparticles is imaged based on their nonlinear magnetization response to an applied magnetic field. Because of security limitations regarding the magnetic field, the field of view does not always cover the entire body being imaged. However, nanoparticles outside the field of view can also be excited by the applied magnetic field and therefore contribute to the measured signal. This leads to domain truncation artifacts, which can often be observed in the reconstructed particle concentrations. In this paper, a computational method for reducing such truncation artifacts is proposed. For this aim, an approximative statistical model for the modeling errors caused by domain truncation is constructed, and the reconstruction problem in magnetic particle imaging is solved in the Bayesian framework of inverse problems. Several numerical test examples illustrate the successful recovery from truncation artifacts.

Nonlinear magnetic responses at the phase boundaries around helimagnetic and skyrmion lattice phases in MnSi: Evaluation of robustness of noncollinear spin texture

The phase diagram of a cubic chiral magnet MnSi with multiple Dzyaloshinskii-Moriya (DM) vectors as a function of temperature $T$ and dc magnetic field ${H}_{\mathrm{dc}}$ was investigated using intensity mapping of the odd-harmonic responses of ac magnetization $({M}_{1\ensuremath{\omega}}$ and ${M}_{3\ensuremath{\omega}})$, and the responses at phase boundaries were evaluated according to a prescription [J. Phys. Soc. Jpn. 84, 104707 (2015)]. By evaluating ${M}_{3\ensuremath{\omega}}/{M}_{1\ensuremath{\omega}}$ appearing at phase boundaries, the robustness of noncollinear spin texture in both the helimagnetic (HM) and the skyrmion lattice (SkL) phases of MnSi was discussed. The robustness of vortices-type solitonic texture SkL in MnSi is smaller than those of both the single DM HM and chiral soliton lattice phases of a monoaxial chiral magnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$, and furthermore the robustness of the multiple DM HM phase in MnSi is smaller than that of its SkL. Through magnetic diagnostics over the wide $T\ensuremath{-}{H}_{\mathrm{dc}}$ range, we found a new paramagnetic (PM) region with ac magnetic hysteresis, where spin fluctuations have been observed via electrical magnetochiral effect. The anomalies observed in the previous ultrasonic attenuation measurement correspond to the peak positions of out-of-phase ${M}_{1\ensuremath{\omega}}$. The appearance of a new PM region occurs at a characteristic magnetic field, above which indeed the SkL phase appears. It has us suppose that the new PM region could be a phase with spin fluctuation like the skyrmion gas phase.

Targeting experimental orthotopic glioblastoma with chitosan-based superparamagnetic iron oxide nanoparticles (CS-DX-SPIONs).

BackgroundGlioblastoma is the most devastating primary brain tumor of the central nervous system in adults. Magnetic nanocarriers may help not only for a targeted delivery of chemotherapeutic agents into the tumor site but also provide contrast enhancing properties for diagnostics using magnetic resonance imaging (MRI).MethodsSynthesized hybrid chitosan-dextran superparamagnetic nanoparticles (CS-DX-SPIONs) were characterized using transmission electron microscopy (TEM) and relaxometry studies. Nonlinear magnetic response measurements were employed for confirming the superparamagnetic state of particles. Following in vitro analysis of nanoparticles cellular uptake tumor targeting was assessed in the model of the orthotopic glioma in rodents.ResultsCS-DX-SPIONs nanoparticles showed a uniform diameter of 55 nm under TEM and superparamagentic characteristics as determined by T1 (spin-lattice relaxation time) and T2 (spin-spin relaxation time) proton relaxation times. Application of the chitosan increased the charge from +8.9 to +19.3 mV of the dextran-based SPIONs. The nonlinear magnetic response at second harmonic of CS-DX-SPIONs following the slow change of stationary magnetic fields with very low hysteresis evidenced superparamagnetic state of particles at ambient temperatures. Confocal microscopy and flow cytometry studies showed an enhanced internalization of the chitosan-based nanoparticles in U87, C6 glioma and HeLa cells as compared to dextran-coated particles. Cytotoxicity assay demonstrated acceptable toxicity profile of the synthesized nanoparticles up to a concentration of 10 μg/ml. Intravenously administered CS-DX-SPIONs in orthotopic C6 gliomas in rats accumulated in the tumor site as shown by high-resolution MRI (11.0 T). Retention of nanoparticles resulted in a significant contrast enhancement of the tumor image that was accompanied with a dramatic drop in T2 values (P<0.001). Subsequent histological studies proved the accumulation of the nanoparticles inside glioblastoma cells.ConclusionHybrid chitosan-dextran magnetic particles demonstrated high MR contrast enhancing properties for the delineation of the brain tumor. Due to a significant retention of the particles in the tumor an application of the CS-DX-SPIONs could not only improve the tumor imaging but also could allow a targeted delivery of chemotherapeutic agents.

Selective Third-Harmonic Generation by Structured Light in Mie-Resonant Nanoparticles

We study the third-harmonic generation by structured light in subwavelength silicon nanoparticles which support both electric and magnetic multipolar Mie resonances. By tailoring the vectorial structure of the pumping light, we may control both strength and polarization composition of the excited harmonic fields. In this way, we generate nonlinear fields with radial or azimuthal polarizations by addressing selectively a different type of multipolar Mie resonances, also enhancing or suppressing the optically induced nonlinear magnetic response.

Modeling and Simulation Study for Solid Particles Detection in Gas–Oil Pipelines Using Field Free Line in Magnetic Particle Imaging and Its Hardware Realization

This paper suggests a new magnetic particle imaging (MPI) technique for 2-D tomographic imaging to detect the presence of flowing solid particles (i.e., black powder, which consist of paramagnetic nanoparticles) passing through gas-oil pipelines. The hardware consists of a ring of a modified Halbach array of 24 permanent magnets, in addition to two surrounding Helmholtz coil pairs, and a receiving coil, which are evenly distributed across a cross section of the probe. With the application of static and dynamically moving drive fields, the MPI utilizes the full benefit of superparamagnetic iron oxide nanoparticles (SPIONs) by providing a linear response when they are exposed to a relatively low magnetic field and no response when they are saturated. The scanning of region of interest with the field-free line (FFL) instead of the field-free point (FFP) is motivated by the fact that a high-intensity dc current is required for FFP, which is not tolerated in oil-gas pipelines where the maximal current should not exceed few amperes. With the presence of an oscillatory magnetic field, SPIONs react with a nonlinear magnetization response, which is further measured by the receiving coil. A 2-D image reconstruction is then performed using the frequency-based image reconstruction process. The hardware design, specifically the sizing of permanent magnets with regard to their relative position and dimensions, was refined following an optimization technique based on the particle swarm optimization technique. The assessment of the system using finite-element method indicates that the system can reconstruct the 2-D profile of the SPION with a mean absolute error of prediction of less than 4% and 12% using FFL and FFP methods, respectively.

2D-MPI System using second harmonic with HTS-SQUID

Magnetic Particle Imaging (MPI) is an imaging technique, with high sensitivity and high speed imaging, utilizing non-linear magnetization response for the detection of superparamagnetic iron oxide nanoparticles (MNP). MPI measures the magnetization change in the MNP under the AC magnetic field. Since the signal of the magnetization change is much smaller than the signal generated by the AC magnetic field, the signal response at the fundamental frequency cannot be used. Accordingly, it is the third harmonic of the response that is generally measured. However this method has disadvantages that the power of the AC magnetic field is large and as a result the system increased in size. In this study, we investigated the 2D imaging using a second harmonic, which can be theoretically obtained with a greater signal than in the case of the third harmonic. Moreover, an HTS-SQUID (Superconducting Quantum Interference Device) magnetometer was employed in the study to enhance the peak signal to noise ratio (PSNR). A 2D-MPI system, which enables an imaging by scanning a DC magnetic field under a gradient magnetic field, was constructed. As a result, PSNR was increased by 1.5 times using SQUID with a Flux Transformer.

Switchable metamaterial for enhancing and localizing electromagnetic field at terahertz band.

In this article, a novel metamaterial is designed aimed at generating a single electromagnetic hot spot, in order to realize the localization of the incident electromagnetic field at terahertz band, and this kind of metastructure is an ideal candidate for many research fields such as spintronics, nonlinear magnetic response, near-field optics, and optical antenna, etc. The specially tailored metamaterial takes the shape of diabolo with a metal triangle pair connected by a cubic gallium arsenide (GaAs) gap. We demonstrated by simulation that both electric- and magnetic-field of incident THz pulse can be confined in the small GaAs gap when a synchronized femtosecond laser pulse is illuminated. The numerical simulation results show that 2 orders of magnitude of field enhancement can be obtained for a 1-by-1 μm GaAs gap, and the field enhancement factor can also be further improved by tailoring the GaAs gap down to nanometer scale.