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Analyzing the spatial distribution of stray field from magnetic nanoparticles is a crucial step to design and optimize the magnetometric system for a clinical magnetic particle imaging (MPI) scanner. Here, we used a magnetoresistive (MR) sensor to probe the stray field directly from a commercial magnetic nanoparticle suspension conditioned under a static field. For a given 20 mT by a small permanent magnet, the stray field of a liquid sample with a 0.7 mgFe iron mass is in nanotesla order measured by the MR sensor at 50 mm apart from the sample, while the magnetization is comparable to several microtesla. This field decay demands picotesla sensitivity of the sensing system to record the stray field for a further distance or a smaller excitation field. Moreover, from a two-dimensional trajectory of sample and magnet movements relative to the sensor position, we confirmed that the spatial distribution of the stray field appeared to correlate with sample geometry. The distribution became broadening for low iron mass concentration of the sample. From this observation, an MR sensor proves its potential for locating the magnetic nanoparticles under a quasistatic field, which can be extensively implemented for a single-sided MPI scanner.

Despite the critical contribution of dipolar interparticle interactions to the hysteretic magnetization curve of superparamagnetic nanoclusters, the quantitative analysis of dipolar field remains a major challenge at room temperature. To investigate such demagnetizing effect due to a close core-to-core distance within a nanocluster, we evaluate the symmetry of ac minor magnetization hysteresis in the presence of static magnetic field. While the nanoclusters are immobilized in a gelatin matrix, we record their dynamic magnetizations for a given 100-kHz sinusoidal magnetic field perpendicular to the static field and further correlate the changes in the coercive fields and remanences with the dipolar field and its demagnetization ratio, respectively. In this work, we also show that the dipolar field-induced shift of the magnetization loop is size-dependent. Under high static bias field, larger core size proportionally enhances the resulting dipolar field. Meanwhile, the cluster size appears to be influential to the dipolar magnetism triggered at low bias field. This study conclusively highlights the distinguishable dynamics between randomly oriented and collaterally aligned magnetic moments.

In this study, we precisely calculated the specific loss powers (SLPs) of magnetic nanoparticles (MNPs) based on dynamic hysteresis measurements. The advantage of this evaluation method is that the intensity and frequency of the applied magnetic field can be varied over a wide range for samples of various condition. The results show that the coercive field and SLP of Resovist® increase by orienting the magnetic easy axes of the nanoparticles. The magnetic field was applied either parallel or perpendicular to the nanoparticle orientation. The area enclosed by the dynamic hysteresis curve was larger when the AC field was applied parallel to the nanoparticle orientation, indicating a greater increase in the hyperthermia temperature. This characteristic originated from the magnetic anisotropy energy of the nanoparticles and is in good agreement with our simulational results. The SLP of a solid sample with an aligned easy axis measured under an AC field of 4 kA/m, which was applied parallel to the axis, was more than two times that of a liquid sample. We also evaluated the SLPs of superparamagnetic 4-nm-diameter γ-Fe2O3 and ferromagnetic 20–30-nm-diameter Fe3O4 MNPs and compared them to that of Resovist®.

Magnetic particle imaging (MPI) is beneficial for bioanalytical cellular imaging, due to its lack of both attenuation and background signal, but it typically requires a very high field gradient to achieve submillimeter resolution. The authors use the N\'eel relaxation response of the tracers to encode the coordinates of the field-free point (FFP) while magnetically scanning the phantom image under a low field gradient. This approach identifies the FFP steering frequencies and trajectory density as important parameters for refining the spatial resolution, and unlocks the possibility for noninvasive submillimeter imaging of cells or small-animal models.

In this study, we evaluated the magnetization properties of a magnetic alloy with single-crystalline cubic nanostructures, in order to clarify its magnetocrystalline anisotropy. Upon applying a specific annealing treatment to the CuNiFe base material, the precipitated magnetic particles grew into cubic granules, resulting in the formation of nanometric cubic single crystals of magnetic CuNiFe in a nonmagnetic Cu-rich matrix. The cubic nanostructures of CuNiFe were oriented along their crystallographic axis, in the &lt, 100&gt, direction of the face-centered-cubic structure. We evaluated the static magnetization properties of the sample, which originated primarily from the CuNiFe nanocubes precipitated in the Cu-rich matrix, under an applied DC magnetic field. The magnetocrystalline anisotropy was readily observed in the magnetization curves. The &lt, 111&gt, axis of the CuNiFe was observed to be the easy axis of magnetization. We also investigated the dynamic magnetization properties of the sample under an AC magnetic field. By subtracting the magnetic signal induced by the eddy current from the magnetization curves of the sample, we could obtain the intrinsic AC magnetization properties of the CuNiFe nanocubes.

Size homogeneity is a critical factor in defining the effective room temperature magnetism of magnetic nanostructures yet a crucial issue toward their implementation in biomedics. Using field-modul...

Local hyperthermia using magnetic nanoparticles has attracted attention as a less invasive cancer therapy. Accurate estimation of heat dissipation of magnetic nanoparticles is important for hyperthermia treatment. In this study, the magnetization properties and heat dissipation of Feraheme®, Resovist®, and Synomag®-D commercial magnetic nanoparticles was measured. The effective core diameter and anisotropy constant were estimated by the Langevin function taking into consideration the effect of anisotropy. The required frequency and intensity of an applied magnetic field with respect to the weight and concentration of magnetic nanoparticles in an assumed spherical tumor were estimated based on measurement and theory of magnetic relaxation. A method to quantitate realistic heat dissipation of magnetic nanoparticles in tumor tissue by measurement of their magnetization properties is introduced. Because the physical rotation of magnetic nanoparticles is inhibited in tumor tissue, the realistic heat dissipation is estimated by solidifying a mass of nanoparticles in an experimental sample. The specific loss power and intrinsic loss power as the heat dissipations of magnetic nanoparticles were estimated from the area of the magnetization curves. The results indicated that only the linear component of magnetization was associated with the loss powers. Our method to quantitate loss powers and the index for applied magnetic field will facilitate the clinical application of hyperthermia treatment using magnetic nanoparticles.

We achieved a harmonic-rich signal from linear magnetization responses of magnetic nanoparticles under 40 μT/μ 0 excitation field to facilitate magnetic particle imaging (MPI). In contrast, large harmonic responses are typically attributed to the nonlinear field-dependent magnetization characteristics of the particles, thus questioning technical and clinical issues toward a human-sized MPI scanner. By using a magnetoresistive sensor, we propose a strategy to exploit the linear responses of the tracers at low field regime where the standard MPI may struggle with spatial signal decoding. The achieved high-contrast images of a solid ferucarbotran phantom bring new expectation toward clinical use of MPI.

The coexisting Neel and Brownian relaxation behaviors of magnetic nanoclusters in a viscous medium lead to a highly nonlinear field-dependent magnetization response, which can benefit magnetic particle imaging and hyperthermia. To empirically correlate the moment and particle dynamics with the core and cluster sizes, we performed spectroscopic susceptometry to assess frequency-dependent complex magnetic susceptibility of water-dispersed magnetic nanoclusters at very low field amplitude. The superparamagnetic core particles of nanoclusters should undergo fast moment dynamics. However, for the nanoclusters experiencing the field-driven Brownian relaxation, their constituent core particles appear to collectively behave as a large effective core with a long Neel relaxation time constant. We later numerically interpolated the phase-delay spectra of the immobilized nanoclusters to estimate the Neel relaxation time constant attributed to the intrinsic dipolar interparticle magnetism. From additional static magnetometry, the overlapping bimodal magnetic moment distribution predicts the secondary core sizes larger than the actual sizes from the electron microscopy images. The different estimates of the effective Neel relaxation time constant obtained from the (nearly field-free) frequency-dependent and (static) field-dependent magnetization responses further indicate the activation energies limiting the relaxation behavior of magnetic nanoclusters. This finding highlights the number of effective cores affecting the intracluster interaction energy.

The heat dissipated by magnetic nanoparticles may be used as a heat source for hyperthermic treatment of cancer and it can be estimated based on the magnetic susceptibility when an alternating current (AC) magnetic field is applied. Here, the frequency dependence of the magnetization of magnetic nanoparticles is measured for different AC fields. This is used to examine the AC susceptibility of linear and nonlinear magnetization responses. Two methods were used to evaluate the AC susceptibility based on measured magnetization properties. The first used the static susceptibility and phase delay associated with magnetic relaxation, and the second used the area under the AC magnetization curve to derive the imaginary part of the susceptibility. When the field intensity was low, the estimated AC susceptibilities were comparable. However, when the field intensity was high, the first method predicted a lower value because it did not consider the nonlinearity of the magnetization response. To optimize the material and the applied field conditions for hyperthermia it is important to understand the difference between the two methods. The intrinsic loss power and specific loss power of the magnetic nanoparticles were also evaluated. A liquid sample and a solid sample with an oriented easy axis provided high heat dissipation.

The magnetization induced by oscillatory magnetic-field in a polydispersive superparamagnetic-suspension shows a nonlinearity which strongly depends on the applied frequency. Referring to the initially-measured complex magnetic-susceptibility in function of field-strength, the highly-nonlinear curve of dynamic magnetization at low frequency tends to be linear at higher frequency. Likewise, the dynamic susceptibility appears to be less field-strength dependent at higher frequency, emphasizing a frequency-dependence of magnetically-induced particle-dynamics. This finding is attributable to the imaginary part of magnetization which saturates at lower field-strength of low-frequency magnetic-field. Hence, Brownian relaxation losses should be constant after the saturation due to a confinement of particle rotations.

Understanding magnetic interparticle interactions within a single hydrodynamic volume of polydispersed magnetic nanoparticles and the resulting nonlinear magnetization properties is critical for their implementation in magnetic theranostics. However, in general, the field-dependent static and dynamic magnetization measurements may only highlight polydispersity effects including magnetic moment and size distributions. Therefore, as a complement to such typical analysis of hysteretic magnetization curves, we spectroscopically examined the complex magnetization harmonics of magnetic nanoclusters either dispersed in a liquid medium or immobilized by a hydrocolloid polymer, later to emphasize the harmonic characteristics for different core sizes. In the case of superparamagnetic nanoclusters with a 4-nm primary size, particularly, we correlated the negative quadrature components of the third-harmonic susceptibility with an insignificant cluster rotation induced by the oscillatory field. Moreover, the field-dependent in-phase components appear to be frequency-independent, suggesting a weak damping effect on the moment dynamics. The characteristic of the N&eacute, el time constant further supports this argument by showing a smaller dependence on the applied dc bias field, in comparison to that of larger cores. These findings show that the complex harmonic components of the magnetization are important attributes to the interacting cores of a magnetic nanocluster.

Frequency-dependent magnetization characteristics potentially promote magnetic nanoparticles as prominent nanotracers for tomographic imaging. Owing to fast Neel dynamics relative to Brownian rotation of colloidal magnetic nanoparticles, we can demonstrate a two-dimensional image reconstruction from the Neel and Brownian relaxation responses modulated by external oscillatory field bursts with two distinctive frequencies. To realize a micrometer-scale resolution, a field-free-point (FFP) is manipulated to move in a dense Lissajous trajectory by low-frequency perpendicular sinusoidal fields across a steep field gradient between two permanent magnets. Instead of directly recording the resulting harmonic responses, we use an orthogonal high-frequency field to probe the time-varying FFP movement coupled with the moment relaxation. The proposed method achieves high spatial resolution, irrespective of the nonlinearity of the magnetization response, making it practical for bioanalytical imaging of cells and small animal models.

The stochastic Brownian particle-kinetics in a superparamagnetic ferrofluid at room temperature is of significance in nullifying total magnetization vectors of the suspended particles. Correspondingly, the apparent magnetization response observed under static magnetic field shows no hysteresis loop, but being linear at a given finite field-difference. Owing to this superparamagnetism, we propose a differential magnetometry to analyze the static field-induced particle-kinetics and further to identify the effective field-strength in reorienting particle-moments toward the applied field direction. A polydispersive ferrofluid containing iron-oxide nanoparticles, in practice, is subjected to a very-low oscillatory-field, immediately after applying the static-field. For a given frequency, we confirm a decreasing ac susceptibility as dc field-strength increases, which suggests a statistically less fluctuating magnetization-vectors. Via numerical integration of ac susceptibility recorded, we furthermore estimate ...

Magnetic hyperthermia is a promising application of magnetic nanoparticles (MNPs) in cancer therapy. It is important to consider and optimize the parameters that affect heat dissipation, such as particle diameters, structures, and surface coatings. In this study, we measured the magnetic properties of two superparamagnetic nanoparticles under DC and AC magnetic fields. Resovist is approved to be used as a magnetic resonance imaging contrast agent. CMEADM-033-02, with the blood-pooling property and biocompatibility, exhibits high magnetization. The blood-pooling property makes it easier for MNPs to accumulate in tumors and tissue. While preparing samples, we aligned the easy axis of the samples using a DC magnetic field to enhance heat dissipation. We discussed the magnetic property in terms of magnetic relaxation associated with anisotropy energy. We observed that the peak frequency of Neel relaxation was considerably shifted owing to effectively changed anisotropy by the alignment of the easy axis. However, the change in the peak frequency of Neel relaxation could not be directly confirmed. Furthermore, we calculated the intrinsic loss power (ILP) and specific loss power (SLP) for heat dissipation from the areas of AC magnetization curves and estimated the SLP at 1 MHz to compare with the high heating characteristic of ILP that has been reported in a conventional study. We achieved equivalent ILP for heat dissipation as that reported in the study by aligning the easy axis of the MNPs with the blood-pooling property under a therapeutic condition.

Some practical applications of magnetic nanoparticles in theranostics systems require a preliminary study on magnetically-induced particle behavior with regard to the frequency and field strength-dependent magnetic susceptibility, measurable through a complex magnetic susceptometer. A wide measurement range, however, is favorable to provide comprehensive understanding in relaxation dynamics of magnetic nanoparticles, as well as their nonlinear magnetization. Therefore, we particularly designed a coil system, thus achieving measurability of complex magnetic susceptibility up to 1 MHz with field strength more than 10 Oerms. Our design includes parallel-arranged multiturn-coils as an induction coil to minimize overall resistance without a significant decrease of coil inductance, thus improving current-to-field conversion efficiency with a minimized dimension. Additionally, a typical coil was inserted inside the induction coil to provide a spatially-controllable field-distribution, and a single-turn sheet-coil was used as a sensor to obtain minimum noise and improve sensitivity reaching a relatively-constant value regardless field strength and temperature.

We empirically uncovered a two-step process of a temporal magnetization response under a pulsed field, which was observed by delaying the particle kinetics in a viscous medium. The Brownian alignment of the magnetization easy-axis appeared to occur separately after the rotated moments reached equilibrium, instead of randomly mixed relaxation behavior. The effective cluster moment and anisotropic magnetopotential difference are two key factors for determining each contribution to the steady-state magnetization. This finding adds new insight to the understanding of colloidal magnetic nanoclusters as a semi-rigid dipole system, which has potential applications in biomedical platforms.

We describe the relationship between the excitation frequency dependence of harmonic signal intensity under alternating magnetic fields and the hydrodynamic size of magnetic particles in ferrofluids. The hydrodynamic size estimated from magnetic susceptibility increases with the increase in ion and particle concentrations in the ferrofluids. The excitation frequency dependence of the fifth-harmonic signal intensity exhibits a minimum point at a certain frequency. The frequency of the minimum point shifts to the lower-frequency range, and after reaching the minimum, it increases with the increase in ion and particle concentrations, reflecting the increase in hydrodynamic size owing to the aggregation of magnetic particles. These behaviors may be useful for applications in ion sensing by a magnetic technique.

The distinguishable Brownian relaxation dynamics of a clustered-particle system of superparamagnetic iron oxide nanoparticle suspension compared to that of a dispersed-particle system has been experimentally investigated through characterization of the frequency and field strength dependences of complex magnetic susceptibility. We confirmed that the application of low sinusoidal magnetic field strength enables cluster rotation instead of individual particle rotations. Furthermore, we found that the cluster rotation was altered to individual particle rotations in higher field strength, resulting in a shorter Brownian relaxation time, which suggests a change in the hydrodynamic volume. This evolutional relaxation behavior was associated with a change in the fitting parameter which satisfies the empirical model of relaxation and further represents the significance of interparticle interactions in defining the nonlinearity of the magnetization response.

The present paper describes a magnetic-sensing device to detect superparamagnetic beads for a sentinel lymph node biopsy. The sentinel lymph node biopsy is a promising method to improve quality of life of patients after surgery of cancer. The sensing device consists of a solenoid coil to excite magnetically the beads and two Hall devices placed on the both ends of the coil. The differential signal between the two Hall devices reflects the existence of magnetic beads near one of the Hall devices. A computer simulation indicates that the signal increases with the increase of magnetic fields from the exciting coil. A prototype is designed based on the simulation of the devices that effectively detect the beads up to the distance of 10 mm from the sensor top. The sensing device detected superparamagnetic iron-oxide beads containing Fe content of 0.5 mg, which are expected to accumulate in lymph nodes, at the distance of 8 mm; the estimated differential magnetic-field between the two Hall devices was 0.026 G.

Telah diaplikasikan magnet sebagai sistem isolasi vibrasi (shock absorber) pada kendaraan bermotor sebagai pengganti sistem pegas yang memanfaatkan gaya tolak antara dua buah magnet sejenis.. Dilakukan pemodelan active magneto-pneumatic suspension untuk mengetahui performansi magnet melalui pendekatan analisisinterpretasi respon suspensi dan distribusi densitas fluks magnetik. Pemodelan terdiri atas model geometrik dan model matematik, terbagi atas komponen suspensi (magnet permanen Nd-Fe-B dan elektromagnet tersusun dalam konfigurasi serial-vertikal sesuai prinsip levitasi magnet berdasar polarisasi), dan komponen kontrol vibrasi (piezoelektrik beserta charge amplifier, PID voltage controller, dan power amplifier). Berdasarkan hasil simulasi, diperoleh bahwa magnet permanen Nd-Fe-B memiliki performansi yang cukup baik dalam sistem isolasi vibrasi pada kendaraan bermotor, ditunjukkan berdasarkan kapabilitasnya dalam mereduksi vibrasi melalui evaluasi transmisibilitas yang relatif kecil dan stabilitas respon suspensi yang relatif stabil. Desain tekanan pneumatik 0,125 bar dan kontrol umpan maju pada active magneto-pneumatics suspension didapatkan nilai transmisibilitas untuk fungsi profil lintasan sinusoidal yaitu 0,9%-7%, untuk fungsi profil lintasan bumb berkisar 6,5%-25% dan untuk fungsi profil lintasan random mencapai 2,5%.