Low Field Ac Research Articles

Tau-borides (Mnx{Ru,Os,Ir}1-x)23B6: X-ray single crystal and TEM data, physical properties

Investigation (electron microprobe and X-ray powder and single crystal diffraction analyses) of the phase relations in the Mn-rich corner (> 45 at% Mn) of the systems Mn-{Ru,Os,Ir}-B, prompted in all three systems a ternary compound with formula (Mnx{Ru,Os,Ir}1-x)23B6. For the systems Mn-Ru-B, Mn-Ir-B phase equilibria have been determined at 950°C (Ru), 900°C (Ir) revealing in both cases a small homogeneity region at constant B-content (Mn1-xRux)23B6 (0.29 < x < 0.37); (Mn1-xOsx)23B6 (0.33 < x < 0.37), as well as (Mn1-xIrx)23B6 (0.29 < x < 0.34). The crystal structure of the three compounds was determined from single crystal and X-ray powder intensity data analyses to be isotypic with the Cr23B6-type (so-called tau-phase, space group Fm3¯m, No. 225). In all cases Mn atoms fully occupy the 4a site (0,0,0) at the origin of the unit cell. For the 8c site (¼,¼,¼) we observed a random distribution of Mn1-x(PM)x with decreasing PM content (PM stands for a platinum group metal atom) going from Ru to Os and Ir, where Mn atoms fully occupy 8c. Whereas the remaining sites (48h, 32f) show various ratios of the two metal species, boron atoms fully occupy the centers (24e site) of the Archimedean metal atom antiprisms. A transmission electron microscopic study confirms the absence of superstructures related to these metal atom disorder, thus Mn and PM atoms randomly share their sites. The latter is well reflected in the behavior of the electrical resistivity of these compounds, which is dominated by disorder scattering. For (Mn0.6{Ru,Os}0.4)23B6, temperature dependent dc magnetization studies reveal distinct antiferromagnetic-like anomalies at TN ≅ 72 K and 114 K, respectively. Low field dc and ac magnetic susceptibility data of (Mn0.7Ir0.3)23B6 display a distinct ferromagnetic-like transition at TC = 280 K, consistent with a pronounced specific heat anomaly and a rather continuous temperature dependent evolution of magnetic anisotropy effects. Mechanical properties (hardness) characterize the tau phases among rather hard and brittle intermetallics (about 5–6 GPa).

Rapid millisecond heating via ferromagnetic resonance in MnFe2O4 nanoparticles

This study undertakes an exhaustive analysis of rapid heat generation in MnFe2O4 nanoparticles through ferromagnetic resonance within an ultra-fast timeframe of 1 ms. Real-time monitoring of temperature during single-field-pulse excitations provided detailed insights into the temperature rise profiles. By integrating micromagnetic simulations with analytical modeling—taking into account both convective and radiative losses—we have deepened our understanding of the heat transfer dynamics at play. Adjusting the analytical model to align with experimental temperature profiles enabled us to determine the efficiency of converting spin dissipation energy into heat, which stands at 17%. This figure reflects not only the surface area of the nanoparticles but also includes considerations for radiative and convective losses. Notably, employing a low AC-field strength of 17.6 Oe facilitated a rapid temperature increase of up to 90 K in just 0.5 s, showcasing a peak initial temperature rise rate of approximately 680 K/s. This research advances the frontiers of high-power heat generation driven by spin dynamics and provides a comprehensive exploration of heat transfer mechanisms over exceptionally short pulse durations. These findings could revolutionize precise and rapid temperature management at the nanoscale, unlocking prospects in bio applications, accelerated material processing, and inducing color and phase shifts in polymer matrices.

Magnetic carbon nanocomposites via the graphitization of glucose and their induction heating

Carbon nanocomposites containing iron-based nanoparticles are attractive materials for the catalyst supports used for magnetic (induction) heating catalysis. The metallic, soft-magnetic iron nanoparticles provide local heating of the support in an alternating magnetic field and ensure rapid magnetic separation of the nanocomposite particles from reaction suspensions. In this work, magnetic carbon nanocomposites were prepared by annealing the precursor particles consisting of iron-oxide nanoparticles dispersed in a carbohydrate matrix. The annealing was conducted at 600 °C and 750 °C in an Ar atmosphere. At both temperatures the carbothermal reduction of iron oxide to Fe/Fe3C was observed; however, at the lower temperature the rate of reduction and the growth of the nanoparticles were considerably slower. The Fe3C was formed in negligible amounts only after a prolonged period of annealing at 600 °C. A detailed structural analysis showed that the Fe/Fe3C nanoparticles catalyze the graphitization of the carbonaceous precursor material already at 600 °C, resulting in the formation of a graphitic shell that surrounds them. This shell is tight enough to prevent the areal oxidation of the encapsulated Fe nanoparticles; their magnetic properties remained unchanged even after 1 year of storage under ambient conditions. At the higher annealing temperature, the growth of the Fe/Fe3C nanoparticles caused bursting of the graphitic shell and thus partially exposed their surfaces to the atmosphere. All the nanocomposites exhibited ferromagnetic behavior in accordance with their compositions. The nanocomposite that was predominantly composed of a graphitic shell, encapsulated Fe nanoparticles and a negligible amount of Fe3C, showed the highest specific absorption rate (760 W/gFe at 274 kHz), even at a relatively low AC-field amplitude (88 mT).

Modeling of core-shell magneto-electric nanoparticles for biomedical applications: Effect of composition, dimension, and magnetic field features on magnetoelectric response.

The recent development of core-shell nanoparticles which combine strain coupled magnetostrictive and piezoelectric phases, has attracted a lot of attention due to their ability to yield strong magnetoelectric effect even at room temperature, thus making them a promising tool to enable biomedical applications. To fully exploit their potentialities and to adapt their use to in vivo applications, this study analyzes, through a numerical approach, their magnetoelectric behavior, shortly quantified by the magnetoelectric coupling coefficient (αME), thus providing an important milestone for the characterization of the magnetoelectric effect at the nanoscale. In view of recent evidence showing that αME is strongly affected by both the applied magnetic field DC bias and AC frequency, this study implements a nonlinear model, based on magnetic hysteresis, to describe the responses of two different core-shell nanoparticles to various magnetic field excitation stimuli. The proposed model is also used to evaluate to which extent realistic variables such as core diameter and shell thickness affect the electric output. Results prove that αME of 80 nm cobalt ferrite-barium titanate (CFO-BTO) nanoparticles with a 60:40 ratio is equal to about 0.28 V/cm∙Oe corresponding to electric fields up to about 1000 V/cm when a strong DC bias is applied. However, the same electric output can be obtained even in absence of DC field with very low AC fields, by exploiting the hysteretic characteristics of the same composites. The analysis of core and shell dimension is as such to indicate that, to maximize αME, larger core diameter and thinner shell nanoparticles should be preferred. These results, taken together, suggest that it is possible to tune magnetoelectric nanoparticles electric responses by controlling their composition and their size, thus opening the opportunity to adapt their structure on the specific application to pursue.

Entropic signatures of the skyrmion lattice phase in MnSi1–xAlx and Fe1–yCoySi

The entropic signatures of magnetic phase transitions in the skyrmion lattice host compounds MnSi0.962Al0.038 and Fe0.7Co0.3Si were investigated through low field magnetization and ac susceptibility measurements. These data indicate that the conical to skyrmion transition that occurs with the application of magnetic field in MnSi0.962Al0.038 is characterized by clear discontinuity in the magnetic entropy as expected for first order topological phase transition. These same magnetoentropic features are negligibly small in isostructural Fe0.7Co0.3Si due to the level of chemical substitution related disorder and differences in the spin dynamics (range and timescales). Despite the obvious similarities in the magnetic structures of these two compounds, the transitions between these phases is substantially different indicating a surprising non-universality to the magnetic phase transitions in this class of materials.

Bulk magnetization, temperature - Dependent ac susceptibility and dc resistivity study on Mn0.7+xZn0.3SixFe2-2xO4 (x = 0.0–0.3) spinel ferrites

Polycrystalline samples of spinel ferrite series, Mn0.7+x Zn0.3SixFe2-2xO4 (x = 0.0, 0.1, 0.2, 0.3), were characterized by X-ray powder diffractometry, bulk magnetization at 300 K and 80 K, temperature dependence of low field ac susceptibility (χac (T)) (T = 300–525 K) and dc electrical resistivity (ρdc (T)) (T = 300–772 K) measurements. The variation of intrinsic and extrinsic magnetic parameters as a function of Mn–Si concentration (x) has been discussed. It is found that the system possesses a non-collinear spin arrangement and the canting angle increases with x. The Néel temperatures determined experimentally from susceptibility measurement adequately reflected the values calculated theoretically. The χac (T) curves revealed normal ferrimagnetic behavior while ρdc (T) depicts characteristic semiconducting behavior. The activation energy is found to increase with x that can be explained in light of the structural and microstructural parameters.

Room temperature magnetoelectric coupling in Fe-doped sodium bismuth titanate ceramics

Multiferroic ceramics with switchable magnetization as well as high polarization at room temperature have remarkable technological impact. Herein, we report that a small concentration of Fe doping in the B-site of sodium bismuth titanate (NBT) lead to room temperature multiferroic nature with fascinating properties. The key factor responsible for the embellished multiferroic behavior at room temperature was electric field induced non-180° domain alignment bridged with dynamic exchange interaction of trapped electrons in ordered polarons. The optimum doping of 0.02 mole fractions Fe in NBT favored ferromagnetic loop with highest remanence of 198 memu/g and coercive field 133 Oe along with improved ferroelectric polarization of 31.8 μC/cm2 and a strong self-biased direct magnetoelectric coupling coefficient of 7.38 mV/cmOe. Strong coupling at low ac field, high reliability, easy control by an electric signal and fast switching behavior add importance to this lead-free Fe-doped sodium bismuth titanate ceramic as potential material, applicable in biocompatible magnetic sensors.

Demulsification of crude oil emulsions tracked by pulsed field gradient NMR. Part II: Influence of chemical demulsifiers in external AC electric field

Crude oil emulsions can reach stability levels that pose severe challenges for the processing industry. Certain conventional methods are used to deal with these challenges, such as chemical demulsification. In this article, two chemicals were mixed with crude oil emulsions to study their separation capabilities in the presence and absence of external AC fields. Experiments were conducted at 65 ⁰C to lower crude oil viscosity, which in turn affects separation efficiency. Effects from the chemical and electrical constituents on the crude oil emulsion was tracked by a pulsed field gradient NMR method, developed in a previous study. The NMR measured emulsified water content for 2 h, continuously, as a function of sample height. Given the water content, droplet sedimentation and free water layer kinetics could be quantified. Droplet size distributions from the top half oil phase were obtained at the beginning and end of each experiment, with the intent of mapping coalescence and sedimentation development of droplets. However, tracking droplet size evolution proved challenging due to rapid changes within the emulsion. The main focus with the NMR technique was therefore on sedimentation and free water layer kinetics. A synergy was observed between the two chemicals and low AC fields. The two chemicals showed varied degrees of separation efficiency, where limited amounts of water was resolved by chemical 1 without electric field, though it improved separation past the limit of electrical treatment when used in combination with AC field. Above a given electric field, sedimentation by electrocoalscence dominates, and the effect of chemical demulsifier is marginal. Chemical 2 achieved promising results as a lone separation method, while further enhancing separation in collaboration with AC fields.

Laser powder bed fusion of Ni-Mn-Ga magnetic shape memory alloy

Additive manufacturing (AM) has gone through major developments in the past decade, enabling the rapid manufacture of complex geometries from traditional engineering materials. This study aims to facilitate the development and additive manufacturing of a new generation of fast and simple digital components with integrated magnetic shape memory (MSM) alloy sections that can be actuated by an external magnetic field. Here, we employ a systematic design of experiments (DoE) approach for investigating laser powder bed fusion (L-PBF) of a Ni-Mn-Ga based MSM alloy. The effects of the applied process parameters on the chemical composition and relative density are determined, and detailed investigations are conducted on the microstructural properties of the as-deposited material obtained using optimized parameters. The results show that although the L-PBF of Ni-Mn-Ga is characterized by an ever-present loss of Mn, deposition of Ni-Mn-Ga with a high relative density of 98.3% and a minimal loss of Mn at ∼1.1 at.% is feasible. The material produced in this manner was compositionally near homogenous and, in as-deposited condition, consisted of a mixture of 14 M and non-modulated (NM) martensites. However, combined measurements by the low-field ac magnetic susceptibility method (LFMS) and DSC revealed that the phase transformation of the as-deposited material from martensite to austenite, and vice versa, was broad and occurred in a paramagnetic state. Inspection by SEM revealed a layered microstructure with a stripe-like surface relief that originated from the presence of martensitic twins within the sample. Additionally, AFM and MFM measurements showed that in as-deposited Ni-Mn-Ga, there exists a weak MFM contrast that can be attributed to the twinned martensite having magnetic anisotropy. Overall, L-PBF shows high potential for the production of functional Ni-Mn-Ga based MSM alloys.

A flow through device for simultaneous dielectrophoretic cell trapping and AC electroporation

Isolation of cells and their transfection in a controlled manner is an integral step in cell biotechnology. Electric field approaches such as dielectrophoresis (DEP) offers a more viable method for targeted immobilization of cells without any labels. For transfection of cells to incorporate exogenous materials, electrical methods such as electroporation, are preferred over chemical and viral delivery methods since they minimally affect cell viability and can target many types. However prior approaches to both methods required multiple excitation sources, an AC source for DEP-based trapping and another DC source for electroporation. In this paper, we present a first of its kind flow through lab-on-chip platform using a single AC excitation source for combined trapping using negative dielectrophoresis (nDEP) and AC electroporation. Use of AC fields for electroporation eliminates the unwanted side effects of electrolysis or joule heating at electrodes compared to DC electroporation. Adjusting the flow rate and the electrical parameters of the incident AC field precisely controls the operation (trap, trap with electroporation and release). The platform has been validated through trapping and simultaneous transfection of HEK-293 embryonic kidney cells with a plasmid vector containing a fluorescent protein tag. Numerical scaling analysis is provided that indicates promise for individual cell trapping and electroporation using low voltage AC fields.

Magnetization reversal, magnetic anisotropy and ordering studies on quenched copper-ferri-aluminates

The magnetic properties of thermally quenched spinel ferrite system, CuAlxFe2-xO4 (x = 0.0, 0.2, 0.4 and 0.6), have been investigated by means of DC magnetization, hysteresis loop characteristics and thermal variation of low field ac susceptibility measurements. The system possesses a tetragonally distorted spinel structure and exhibits a non-collinear spin arrangement. The magnetic characteristics suggest that the ferrites are low anisotropy compounds, the negative magnetization observed for x = 0.6 composition at the compensation temperature T = 22 K is due to the copper ion moment with a net effect antiparallel to that of the Fe3+ ions with different temperature dependence giving rise to a compensation effect. The low-temperature anomalies on the zero field cooled curves are due to the coexistence and interplay between the ferrimagnetic long-range ordering and short-range ordering in the system.

Effect of process parameters on non-modulated Ni-Mn-Ga alloy manufactured using powder bed fusion

Increasingly, metal parts made by additive manufacturing are produced using powder bed fusion (PBF). In this paper we report upon the combined effects of PBF parameters, including power and scan speed, in layer-by-layer manufacturing of gas atomized non-modulated (NM) Ni-Mn-Ga alloy. The effects of process parameters upon PBF is studied by applying nine different parameter sets in the as-printed state and after homogenization and ordering. The chemical composition of the samples is analyzed using EDX attached to an SEM, and the crystal structures are determined by X-ray diffraction. The phase transformation temperatures are measured using a low-field ac susceptibility measurement system and the magnetic properties are measured with a vibrating sample magnetometer (VSM). Before the heat-treatment, all as-printed samples showed paramagnetic behavior with low magnetization and no phase transformations could be observed in the susceptibility measurements. After annealing, the samples recovered the ferromagnetic behavior with comparable magnetization to annealed gas atomized powder. The as-printed samples were composed of a mixture of different crystal structures. However, after annealing the original NM structure with a = b = 5.47 Å and c = 6.66 Å with a c/a -ratio of 1.22 was recovered and crystallographic twins could be observed in an SEM.

Magnetoelastic coupling at spin-glass-like transition in Sr3NiSb2O9

We report a characteristic spin-glass-like behavior in Sr3NiSb2O9, as confirmed from the low-field dc magnetometry and ac susceptibility measurements. The memory effects in the dc magnetometry exhibit similar characteristic features of the spin-glass systems. The ac susceptibility measurements provide that the frequency dependent peak-shift follows the conventional Vogel-Fulcher and dynamical scaling laws with the characteristic relaxation time to be ∼ 10−14 s. This relaxation time fits in the recommended range of ∼10−12−10−14 s for the classical spin-glasses. Low temperature synchrotron diffraction studies point to the strong magnetoelastic coupling close to the spin-glass-like transition. The transition is found to be associated with the step-like anomalous lattice contraction. Possible origin of magnetic frustration leading to the spin-glass ground state is discussed by correlating the microstructural results, as obtained from the analysis of synchrotron diffraction studies.

Glassy behavior of diluted Cu-Zn ferrites

The magnetic behavior of Zn substituted Cu-Zn spinel ferrites having chemical formula Cu1−xZnxFe2O4 (x = 0.7, 0.8, 0.9 and 1.0) has been studied by SQUID magnetometry, by means of magnetic hysteresis, field-cooled (FC) and zero-field-cooled (ZFC) magnetization, memory effect and low field ac susceptibility measurements. These measurements suggest that the ferrimagnetic phase of the x ≤ 0.8 samples is gradually turned into a spin glass (x ≥ 0.9). The compound with x = 0.9 exhibits the typical dynamical behavior of spin glasses, with indication of aging, rejuvenation and memory effects. The evolution of the magnetic properties of Cu-Zn spinel ferrites with substitution of Zn for Cu is discussed.

Study of superparamagnetic nano particles of MnxCo1−xFe2O4 ferrite system prepared by co-precipitation technique

The ferrites with magnetic nano-particles of MnxCo1−xFe2O4 (with x = 0.0 to 1.0, step 0.2) system are prepared by the chemical co-precipitation method. Structural and magnetic properties of Mn2+ doped cobalt nano ferrites have been studied by x-ray diffraction (XRD), scanning electron microscopy (SEM), saturation magnetization and low field AC susceptibility. The crystallite size is determined from the FWMH of the higher angle Bragg line (4 0 0) using the Scherer’s formula. The saturation magnetization at 300 K in the peak field of 5 kOe is measured using B → H loop technique. The observed saturation magnetic moment is found very low compared to the Neel’s magnetic moment. The lower value of observed magnetic moment is explained on the basis of high magneto-crystalline anisotropy and exchange disorder. The Curie temperatures are determined through thermal variation of low field (0.5 Oe) AC susceptibility. All the plots of thermal variation AC susceptibility exhibit large hump due to single domain to superparamagnetic transition which is the characteristic of high anisotropic ion present in the system. The ZFC-FC curves exhibit thermo-magnetic irreversibility. The magnetic behavior observed through Mossbauer spectra also supports the concept of fine particle superparamagnetism.

Room Temperature Sensing of Magnetic Fields Using Magnetoelectric Thin Films

We present the design, characterization, and fabrication of a sensing device for measuring low-frequency ac magnetic field variations using a single-phase magnetoelectric (ME) thin film. This ME device is fabricated using SrCo 2 Ti 2 Fe 8 O 19 and deposited on a 500-nm-thick Indium Tin Oxide layer. The ME film is then deposited on a silicon substrate using a pulsed laser deposition technique. The device size (volume) is 0.02 cm 3 , and its overall power consumption is 14 μW. We also show that this sensing device can be used to detect low ac and dc magnetic fields. In this experiment, we employ a modified Wheatstone bridge circuit as an alternative approach to detect the change in capacitance of the ME film when an external field H ext is applied. The circuit uses a combination of an ac input source and a variable capacitor to balance the bridge when the external field is zero. The experimental results demonstrate that the ME device can detect both ac and dc magnetic fields (up to 0.3 T) and is particularly sensitive for ac field sensing. Since the ME device employs a single-phase material, it is not only relatively simple to fabricate but also capable of operating at very low voltages and, therefore, suited for planar integration and compatibility with other integrated circuits.

Polaron-mediated low-frequency dielectric anomaly in reduced LiNbO3:Ti

Measurements of the real and imaginary parts of the dielectric permittivity of reduced Ti-doped lithium niobate reveal a low-frequency dielectric relaxation as well as a strong increase in the conductivity compared to undoped samples. The low-frequency permittivity is increased by about an order of magnitude due to the relaxation. The analysis of the temperature-dependence of the relaxation frequency reveals a thermally activated behavior with an activation energy of Ea = 0.65 eV, which is in agreement with a transition between NbLi4+ small bound polarons and NbNb4+ free polarons. Both low-field ac conductivity and high-field dc conductivity indicate a Poole-Frenkel-type electron hopping mechanism between NbLi4+ sites at low temperature, turning into similar hopping between NbNb4+ at high temperatures.

Domain wall motions in BST ferroelectric thin films in the microwave frequency range

The existence of domain wall motion at microwave frequencies and its contribution to the ferroelectric complex permittivity is shown by evaluating the dielectric properties of BaSrTiO3 (BST) thin films as a function of the incident power. Even at low AC field amplitudes, the presence of the domain walls and the correlated motions (vibration and jumps) result in sensitivity of the dielectric properties to the incident field amplitude. Although the contribution of domain wall motion to the real part of the permittivity is not preponderant (less than 10%), it represents more than 50% of the material's global dielectric losses. This illustrates the importance to consider domain wall motion even in the microwave frequency region and the necessity to take into account the applied AC field amplitude (and thus the incident power) when characterizing ferroelectric materials. The present study has been realized on BST thin films, elaborated by pulsed laser deposition on MgO/Ir substrates.

Synthesis and characterization of Fe0.6Zn0.4Fe2O4 ferrite magnetic nanoclusters using simple thermal decomposition method

This paper presents experimental results regarding the effect of the quantity of solvent on formation of the Fe–Zn ferrite nanoparticles during thermal decomposition. A ternary system of Fe0.6Zn0.4Fe2O4 has been synthesized by a thermal decomposition method using metal acetylacetonate in high temperature boiling point solvent and oleic acid. The X-ray diffraction study was used to determine phase purity, crystal structure, and average crystallite size of iron–zinc ferrite nanoparticles. The average crystallite size of nanoparticles was increased from 13nm to 37nm as a result of reducing the solvent from 30ml to 10ml in a synthesis batch. The diameter of particles and morphology of the particles were determined by transmission electron microscopy (TEM) and field emission scanning electron microscope (FESEM). Mid and far Fourier transform infrared (FT-IR) measurement confirmed monophasic spinel structure of ferrite. Furthermore, the DC magnetic properties of the samples were studied using the vibrating sample magnetometer (VSM). The largest Fe–Zn ferrite nanoparticles exhibited a relatively high saturation magnetization of 96emu/g. Moreover, Low-field AC susceptibility measurement indicated blocking temperature of nanoparticles around 170–200K.

Enhanced magnetic anisotropy and heating efficiency in multi-functional manganese ferrite/graphene oxide nanostructures

A promising nanocomposite material composed of MnFe2O4 (MFO) nanoparticles of ∼17 nm diameter deposited onto graphene oxide (GO) nanosheets was successfully synthesized using a modified co-precipitation method. X-ray diffraction, transmission electron microscopy, and selected area electron diffraction confirmed the quality of the synthesized samples. Fourier transform infrared measurements and analysis evidenced that the MFO nanoparticles were attached to the GO surface. Magnetic measurements and analysis using the modified Langevin model evidenced the superparamagnetic characteristic of both the bare MFO nanoparticles and the MFO–GO nanocomposite at room temperature, and an appreciable increase of the effective anisotropy for the MFO–GO sample. Magnetic hyperthermia experiments performed by both calorimetric and ac magnetometry methods indicated that relative to the bare MFO nanoparticles, the heating efficiency of the MFO–GO nanocomposite was similar at low ac fields (0–300 Oe) but became progressively larger with increasing ac fields (>300 Oe). This has been related to the higher effective anisotropy of the MFO–GO nanocomposite. In comparison with the bare MFO nanoparticles, a smaller reduction in the heating efficiency was observed in the MFO–GO composites when embedded in agar or when their concentration was increased, indicating that the GO helped minimize the physical rotation and aggregation of the MFO nanoparticles. These findings can be of practical importance in exploiting this type of nanocomposite for advanced hyperthermia. Magnetoimpedance-based biodetection studies also indicated that the MFO–GO nanocomposite could be used as a promising magnetic biomarker in biosensing applications.