Search

This work presents a comprehensive analysis of the magnetic properties of pristine and citric acid-coated cobalt ferrite (${\rm{CoF}}{{\rm{e}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}$ CoF e 2 O 4 ) nanoparticles (NPs) dispersed in the host nematic $4{^{\prime}}$ 4 ′ -heptyl-4-biphenylcarbonnitrile (7CB) system. Our results shed light on the promising potential of combining liquid crystals (LC) with magnetic nanoparticles for cancer treatment. This opens up exciting avenues for further exploration and possible applications in the field of oncology. The study focuses on examining the magnetic behaviour of LC dispersed systems with two concentrations of magnetic nanoparticles (0.03 and 0.04 wt%). Notably, intriguing temperature-dependent magnetisation plots reveal a distinctive cusp, which can be attributed to the freezing phenomenon of the liquid crystal. This observation offers valuable insights into the system's magnetic properties and interactions, deepening our understanding of its behavior. [ABSTRACT FROM AUTHOR]

The magnetic characteristics of a system of triply charged gadolinium ions Gd3+ chelated with carboxyls on the surface of detonation nanodiamond (DND) particles have been studied. Gd3+ ions demonstrate almost perfect spin (S = 7/2) paramagnetism with negligible antiferromagnetic interaction between spins (Weiss temperature about −0.35 K) for a wide range of concentrations up to ~18 ions per 5 nm particle. The study of the concentration dependence of the electron paramagnetic resonance signal for DND intrinsic defects with spin ½ (g = 2.0027) shows that Gd3+ ions are located on average at a distance of no more than 1.4 nm from shallow subsurface defects with spin 1/2. At the same time, they are located (according to density functional theory calculations) at a distance of about or at least 0.28 nm from the particle surface. Magnetic studies also confirm the isolated nature of the gadolinium chelate complexes on the surface of DND particles. DND particles turn out to be an optimal carrier for high-spin 4f- ions (gadolinium) in a highly concentrated isolated state. This property makes DND-Gd particles a candidate for the role of a contrast agent for magnetic resonance imaging. [ABSTRACT FROM AUTHOR]

Superparamagnetic iron oxide nanoparticles (SPIONs) coated with the synthetic hydrophilic biocompatible polymer polyvinyl alcohol were synthesized using the aqueous method. Static and dynamic magnetization processes were investigated for surface-modified SPIONs by analyzing the magnetization study at constant and varying magnetic fields. The magnetization on the applied magnetic field (M–H) and the magnetization dependent on temperature (M–T) were investigated. The temperature dependence of the complex susceptibility of SPIONs was investigated by measuring the in-phase (natural) and out-of-phase (imaginary) components of the susceptibility value at a frequency of 10 Hz and a very low magnetizing field. The XRD study shows diffraction peaks consistent with the magnetite (Fe3O4) phase of SPIONPs. FTIR, DSC, and TGA studies confirm the functional groups and stability of the coated nanoparticles. The magnetizing field cycle study at various constant temperatures (10, 100, and 300 K) shows the high magnetization value of polyvinyl alcohol-coated SPIONs with superparamagnetic states at and above 300 K. The effect of interparticle interaction on blocking temperature has been interpreted from FC/ZFC curves drawn at different DC magnetizing field values by varying temperature between 10 and 300 K. [ABSTRACT FROM AUTHOR]

Two‐line ferrihydrite particles are synthesized. Structural characterization of the sample is done with an X‐ray diffractometer, transmission electron microscope, Fourier‐transform infrared spectroscope, and thermogravimetric analyzer. Average particle size is 2 nm. Temperature variation (10–300 K) of zero‐field‐cooled and field‐cooled susceptibility in 500 G applied magnetic field bifurcates at 50 K. The field‐cooled susceptibility below this temperature is almost temperature independent. Magnetization versus applied magnetic field (0–120 kG) data at 300 K are analyzed considering particle size distribution. [ABSTRACT FROM AUTHOR]

Heterostructured magnetic nanoparticles show great potential for numerous applications in biomedicine due to their ability to express multiple functionalities in a single structure. Magnetic properties are generally determined by the morphological characteristics of nanoparticles, such as the size/shape, and composition of the nanocrystals. These in turn are highly dependent on the synthetic conditions applied. Additionally, incorporation of a non-magnetic heterometal influences the final magnetic behavior. Therefore, construction of multifunctional hybrid nanoparticles with preserved magnetic properties represents a certain nanotechnological challenge. Here, we focus on palladium/iron oxide nanoparticles designed for combined brachytherapy, the internal form of radiotherapy, and MRI-guided hyperthermia of tumors. The choice of palladium forming the nanoparticle core is envisioned for the eventual radiolabeling with 103Pd to enable the combination of hyperthermia with brachytherapy, the latter being beyond the scope of the present study. At this stage, we investigated the synthetic mechanisms and their effects on the final magnetic properties of the hybrid nanoparticles. Thermal decomposition was applied for the synthesis of Pd/Fe-oxide nanoparticles via both, one-pot and seed-mediated processes. The latter method was found to provide better control over morphology of the nanoparticles and was therefore examined closely by varying reaction conditions. This resulted in several batches of Pd/Fe-oxide nanoparticles, whose magnetic properties were evaluated, revealing the most relevant synthetic parameters leading to promising performance in hyperthermia and MRI. [ABSTRACT FROM AUTHOR]

Imaging of magnetic nanoparticles with ultrasound utilizes ultrasound excitation and electromagnetic receiving. At present, the size of magnetic nanoparticles (MNPs) for the imaging method is required to be no less than 100 nm, which limits its application in human bodies. To overcome the limitation of particle size, imaging of MNPs with ultrasound based on saturation magnetization is proposed. The relationship between the concentration of superparamagnetic pear-shaped MNPs and the induced voltage under saturation magnetization is derived based on the classical Langevin theory of paramagnetism. A 3D simulation model is constructed and numerical research is carried out under the saturated magnetic field provided by the Helmholtz coil. The concentration image is reconstructed by point-by-point scanning. Numerical studies show that the induced voltage is rich in the concentration information of superparamagnetic pear-shaped MNPs, proving the feasibility of the method. The maximum induced voltage, which enhances the imaging effect, occurs under the minimum radius of the detection coil containing the region of interest. MNPs under 20 nm still meet the imaging requirements in the proposed method. This research pushes the imaging of MNPs with ultrasound based on saturation magnetization a further step forward to practical applications in the biomedical field. [ABSTRACT FROM AUTHOR]

We present a theoretical study on the anisotropic magnetocaloric effect and the size-dependent magnetic properties of Fe particles of radii in the range 25–150 Å. An observable increase has been found in the magnetization, of the low radii (25–75 Å) particles, by reducing the temperature to 4 K. The anisotropic isothermal change in entropy ΔSm has been calculated by taking the difference between maximum ΔSm along the easy [100] and hard [111] directions. The maximum anisotropic ΔSm is 0.015 J/kg K for a field change of 500 Oe along the [100] direction. The ΔSm temperature dependence exhibits a table-like plateau for small radii (25–75 Å) and in low fields below 300Oe. This enhances the relative cooling power (RCP) of the Fe element to be 8.11 J/kg for particles of 25 Å radius. Also, the calculation of anisotropic ΔTad was performed along the easy axis and showed an increase in the maximum value around 37% relative to the experimental conventional value. [ABSTRACT FROM AUTHOR]

Mn x Zn 1-x Fe 2 O 4 (x = 0–1) solid solution was prepared using the TREG-mediated solvothermal method. 10–12 nm spherical superparamagnetic particles (except for ZnFe 2 O 4) were obtained. At room temperature, the ferrites have lower magnetic saturation (Ms) than magnetite. However, at 10 K, it is precisely the opposite. DRX and EXAFS showed that incorporating Zn2+ ions in tetrahedral A-sites provokes a redistribution of Mn and Fe ions in octahedral B-sites of the mixed spinel structure. For Zn concentrations from x = 0.5 and below, the Mn and Fe ions are almost equally distributed between A and B-sites. Therefore, at low Zn concentrations, the small migration of Fe and Mn ions to B-sites increases the magnetic moment of B sub-lattices and, consequently, the total magnetic saturation. In addition, Mn2+ oxidized to Mn3+ and, simultaneously, Fe3+ reduced to Fe2+, affecting the magnetic properties. The increment of Zn concentration severely reduces the magnetization. [ABSTRACT FROM AUTHOR]

Ferritin is an important biological system and found in blood of living organisms. Inorganic cores of this material are 8 nm in size. The cores are surrounded by protein shells which reduce interparticle interaction. Particle size distribution present in ferritin is determined from transmission electron micrograph. This distribution can be described by a log normal distribution function. The magnetic field (0–70 kG) dependence of the magnetization at different temperatures are analyzed considering the estimated particle size distribution. The resulting mean particle magnetic moment is temperature independent and equal to 310 μ B. Saturation magnetization and antiferromagnetic susceptibility of ferritin decrease with increasing temperature. • Ferritin contains an antiferromagnetic inorganic core surrounded by a protein shell. • Langevin function assumes that all particles have same magnetic moment. • Particle size distribution present in a sample of ferritin is determined from transmission electron micrograph. • Magnetization data on ferritin in superparamagnetic region are analyzed considering a distribution in particle size. [ABSTRACT FROM AUTHOR]

A high degree of spin polarization in half-metallic double perovskites is a prerequisite for several applications in spintronics, which depends crucially on the cationic order of the systems. This paper reports a study on tailoring the structure and morphology of nano-sized Sr 2 Fe 1- x Zn x MoO 6 (x = 0.05, 0.1, 0.15) materials to improve their spin polarization. The combined analysis of synchrotron X-ray diffraction and magnetization data shows that Zn replaces Fe in the B sites. Although the majority of particles have lateral dimensions in the range 30–60 nm as observed by scanning electron microscope, the samples with x = 0.1 and 0.15 show finite-size effects with superparamagnetism below room temperature and a reduced Curie temperature (from 410 K for x = 0.05–390 K for x = 0.15). The results are due to the formation of networks of insulating Mo–O–Zn–O–Mo linkages and anti-phase boundaries, which divide the particles into smaller domains with a mean diameter of ∼11 nm as determined via a Langevin fit. The almost perfectly ordered structure in the nanodomains is responsible for a high magnetoresistance ratio. A value of -42% at 5 K in 50 kOe is recorded for the sample x = 0.15. Via fitting the magnetoresistance curve using the Inoue-Mekagawa theory, the spin polarization of 99% is determined. [ABSTRACT FROM AUTHOR]

A new pentanuclear {Co5} cluster motif is found to have a D3h oblate trigonal bipyramidal geometry, which is extended into a 3D triangle network, forming a unique capped breathing kagomé lattice. Magnetic results confirmed a paramagnetism down to 2 K, accompanying the disappearance of two-thirds of the spin moment at low-temperature and the appearance of a 1/5 magnetization plateau. [ABSTRACT FROM AUTHOR]

Magnetic fabric analyses of rocks by anisotropy of magnetic susceptibility (AMS) are a robust petrofabric tool that has been used in varied geological environments and tectonic settings. A fundamental difficulty of this method is to define the dominant magnetic phases and their resulting geological interpretation. We study the magnetic behavior of rocks by simulating data of mixed magnetic phases (i.e., diamagnetic, paramagnetic, and ferromagnetic). We show that it is possible to recognize the dominant magnetic phases by measuring the mean susceptibility at room temperature (kmRT) and at low temperature (kmLT). Distinct regions of magnetic phase dominancy are demonstrated in kmLT/kmRT versus kmRT and kmLT versus kmRT plots. We present a comprehensive approach by coupling the magnetic phase dominancy with possible magnetic fabrics, which are obtained from AMS measured at room and low temperatures (RT‐AMS and LT‐AMS) and anisotropy of anhysteretic remanent magnetization, into a scenario table. Application of this table allows a robust procedure for determining which magnetic phases are dominant, and permits a fast and reliable geological interpretation in complex settings. Key Points: We present a robust procedure for determining the anisotropy of magnetic susceptibility (AMS) carriers (ferro, para, and diamagnetic phases)Mean susceptibility at room and low temperatures defines the respective contributions of the magnetic phases to the rock's susceptibilityA novel scenario table, coupling respective contributions and associated magnetic fabrics, allows reliable interpretation of AMS data [ABSTRACT FROM AUTHOR]

We study and compare magnetic and electron paramagnetic resonance behaviors of bulk and nanoparticles of Nd1-xCaxMnO3 in hole doped (x=0.4;NCMOH) and electron doped (x=0.6;NCMOE) samples. NCMOH in bulk form shows a complex temperature dependence of magnetization M(T), with a charge ordering transition at ∼250 K, an antiferromagnetic (AFM) transition at ∼150 K, and a transition to a canted AFM phase/mixed phase at ∼80 K. Bulk NCMOE behaves quite differently with just a charge ordering transition at ∼280 K, thus providing a striking example of the so called electron-hole asymmetry. While our magnetization data on bulk samples are consistent with the earlier reports, the new results on the nanoparticles bring out drastic effects of size reduction. They show that M(T) behaviors of the two nanosamples are essentially similar in addition to the absence of the charge order in them thus providing strong evidence for vanishing of the electron-hole asymmetry in nanomanganites. This conclusion is further corroborated by electron paramagnetic resonance studies which show that the large difference in the "g" values and their temperature dependences found for the two bulk samples disappears as they approach a common behavior in the corresponding nanosamples. [ABSTRACT FROM AUTHOR]

Magnetoacoustic concentration tomography of magnetic nanoparticles (MNPs) with magnetic induction (MACT-MI) is a method that has been propsed recently for MNP concentration tomography for use in targeted therapy. This procedure has the advantages of being noninvasive, having high-resolution and providing quantitative estimation of MNPs. In order to further fundamentally increase the signal-to-noise ratio of the magnetoacoustic signal, the forward problem of MACT-MI in the saturation magnetization state was studied, especially the magnetization characteristics of MNPs, including the derivation of the force formula of MNPs in the saturation magnetization state and the relationships among the gradient of the external magnetic field, magnetic force, number concentration and sound pressure. Firstly, the magnetic force formula of MNPs in the saturation magnetization state was derived according to the classical Langevin theory of paramagnetism. Then, a 2D model was constructed to calculate the magnetic flux density, magnetic force and sound pressure. Finally, the 2D distribution diagram of magnetic force and sound pressure and its corresponding 1D curve were obtained. The simulation results showed that the saturation magnetic field produced by the Maxwell–Helmholtz coils made the MNPs magnetically saturated. Compared with the MNPs in an unsaturated magnetization state, the magnetic force of MNPs in the saturation magnetization state was stronger, and the stronger magnetic force could generate stronger magnetoacoustic signals, which made it possible to measure magnetoacoustic signals with higher signal-to-noise ratio. The research results can provide a research basis for the design of equipment for further MACT-MI experiment and clinical application. [ABSTRACT FROM AUTHOR]

Co0.1Fe0.9S2 nanoparticles were synthesized by solvothermal method and studied by Mössbauer spectroscopy in the cooling process. Magnetic phase transition can be testified by Mössbauer investigation. The spectrum recorded at room temperature consists of a simple quadrupole doublet, the values of isomer shift and quadrupole splitting of doublet increase gradually with the decrease of temperature. However, the spectrum consists of a sextet when the temperature drops further to 25 K, indicating that the sample changed from paramagnetism to ferromagnetism. The magnetic field at low temperature is probably super-transferred field from the magnetically ordered cobalt atoms through a super-exchange mechanism. [ABSTRACT FROM AUTHOR]

Six lines ferrihydrite nanoparticles are synthesized following a chemical route. The sample is characterized using x-ray diffractometry, Fourier transform infrared spectroscopy, transmission electron microscopy and vibrating sample magnetometry. Average crystallite size is 5 nm. The sample exhibits superparamagnetic behavior. Magnetization versus magnetic field data in the superparamagnetic region are analyzed considering a distribution in particle size. Temperature dependence of saturation magnetization originating from surface spins and antiferromagnetic susceptibility of particle cores are studied. • Saturation magnetization decreases with increasing temperature. • Magnetization of antiferromagnetic particles is due to uncompensated surface spins. • Surface magnetization of crystals is expected to decrease linearly with temperature. • Temperature variation of saturation magnetization of six lines ferrihydrite is studied. [ABSTRACT FROM AUTHOR]

This article gives a review on some important experimental studies on the magnetization curves using different theoretical models. Applications of the law of approach to saturation magnetization and Langevin function to various kinds of magnetic materials belonging to different groups such as ferromagnetic, ferrimagnetic, antiferromagnetic, superparamagnetic materials at various experimental conditions, applied field ranges, and their dependencies on the applied field are collectively reviewed. Extraction of important magnetic parameters by fitting magnetization data with suitable models and their variation with composition, morphologies, size etc. along with the strategies adopted are also discussed. The purpose and outcomes of such studies along with future scope are also highlighted in this review. [ABSTRACT FROM AUTHOR]

The aim of this work consists in the use of cashew gum (Anacardium occidentale), a naturally occurring tropical specie from the Brazilian northeastern coast, for the synthesis of CoFe 2 O 4 (CF) and NiFe 2 O 4 (NF) nanoparticles. The structural, morphological and vibrational properties of nanoparticles were characterized by analytical and spectroscopic techniques such as X-ray diffraction (XRD), FTIR, Raman spectroscopy, TEM, SAED and TG. Magnetic properties were investigated through Mössbauer spectroscopy and DC magnetometry. The XRD results showed single phase nanoparticles with space group Fd-3m and crystallite size of 7.4 and 6.0 for CF and NF, respectively. TEM images showed agglomerated particles with mode sizes of 5.0 and 6.5 nm for CF and NF. SAED confirmed the crystalline spinel structure. The TGA and FTIR showed the presence of a carbonaceous material in the samples. FTIR and Raman spectroscopy demonstrated vibrational modes characteristic of metal‑oxygen bonds in the tetrahedral and octahedral sites. Magnetization measurements showed that both samples are superparamagnetic at 300 K. The Mössbauer spectra at 90 K showed the presence of single-phase CF and NF. • Cashew gum (Anacardium occidentale) is used for sol-gel synthesis. • Green synthesis of nanostructured single phases of Ni and Co ferrites • Structural, thermal, magnetic and spectroscopy characterization of ferrites [ABSTRACT FROM AUTHOR]

We have carried out a systematic study on the effect of high-temperature annealing on the structural and magnetic properties of ultra-small crystalline cobalt ferrite nanoparticles prepared by the hydrothermal method. The structural and the magnetic characterizations of nanoparticles were investigated by XRD, TEM, FTIR, and VSM. The results of Rietveld refinement revealed that the nanoparticles have a cubic single-phase spinel structure. High-temperature annealing was found to increase the lattice parameter and average crystallite size of the cobalt ferrite nanoparticles. The TEM measurements showed that the nanoparticles were monodisperse and spherical in shape. The FTIR results confirmed the single-phase nature of the prepared nanoparticles. The magnetic measurements showed that the nanoparticles were ferromagnetic over a wide temperature range. We have observed that the high-temperature annealing increased both the saturation magnetization and magnetocrystalline anisotropy and decreased the coercivity. We demonstrated that magnetic field induced superparamagnetism can be induced by applying stronger magnetic fields that were able to shift the blocking temperature (TB) down to below room temperature, resulting in a superparamagnetic behavior above TB. [ABSTRACT FROM AUTHOR]

We study spin excitations of the fully aligned state for three-dimensional nanocomposites of exchange coupled hard (SmFeN) and soft (FeCo) phases. The dipolar interaction lowers the magnon energy and controls the spin wave gap at k = 0, which closes when the amount of soft phase exceeds a critical value. With the addition of soft phase or increasing temperature the system moves to another ground state characterized by a tilting of the magnetization at the boundaries between spins of the hard and the soft phases. [ABSTRACT FROM AUTHOR]

We have investigated magnetization (M), magnetocaloric effect (MCE), and magnetothermopower (MTEP) in polycrystalline Pr0.6Sr0.4MnO3, which shows a second-order paramagnetic to ferromagnetic transition near room temperature (TC = 305 K). However, field-cooled M(T) within the long range ferromagnetic state shows an abrupt decrease at TS = 86 K for μ0H < 3 T. The low temperature transition is first-order in nature as suggested by the hysteresis in M(T) and exothermic/endothermic peaks in differential thermal analysis for cooling and warming cycles. The anomaly at TS is attributed to a structural transition from orthorhombic to monoclinic phase. The magnetic entropy change (ΔSm = Sm(μ0 H)-Sm(0)) shows a negative peak at TC (normal MCE) and a positive spike (inverse MCE) at TS. ΔSm = -2.185 J/kg K (-3.416 J/kg K) with refrigeration capacity RC = 43.4 J/kg (103.324 J/kg) for field change of μ0ΔH = 1.5 T (3 T) at TC = 304 K is one of the largest values reported in manganites near room temperature. Thermopower (Q) is negative from 350 K to 20 K, which shows a rapid decrease at TC and a small cusp around TS in zero field. The MTEP [ΔQ/Q(0)] reaches a maximum value of 25% for μ0ΔH = 3 T around TC, which is much higher than 15% dc magnetoresistance for the same field change. A linear relation between MTEP and magnetoresistance and between ΔSm and ΔQ are found near TC. Further, ac magnetotransport in low dc magnetic fields (μ0 H ≤ 0.1 T), critical analysis of the paramagnetic to ferromagnetic transition, and scaling behavior of ΔSm versus a reduced temperature under different magnetic fields are also reported. Coexistence of large magnetic entropy change and magnetothermopower around room temperature makes this compound interesting for applications. [ABSTRACT FROM AUTHOR]

It is proven that the common origin gauge proposed by Chan and Das for the calculation of molecular magnetizabilities within the conventional approach using gaugeless basis sets, i.e., the center of electronic charge, does not minimize the van Vleck paramagnetic contribution to the diagonal tensor components, unless the Arrighini-Maestro-Moccia sum rules for gauge invariance, and charge conservation are satisfied. Additional quantum mechanical constraints for extremum values of formally diamagnetic and paramagnetic contributions have also been obtained for magnetizabilities calculated via procedures allowing for a continuous translation of the origin of the current density. [ABSTRACT FROM AUTHOR]

[Co0.4 nm/Pd0.7 nm]50 multilayers with Pd film thicknesses in the first ferromagnetic maximum of interlayer exchange coupling display almost purely perpendicular-to-plane anisotropy and labyrinth stripe domain patterns in remanence. Their magnetization reversal is characterized by domain nucleation starting at a defined field HN and domain wall movement in a defined magnetic field range. The modification of the magnetization reversal by 10 keV He ion bombardment due to the reduced magnetic anisotropy has been investigated by polar magneto-optical Kerr effect, by vibrating sample magnetometry, and by magnetic force microscopy at room temperature. It is shown that the ion bombardment creates and increases areas with ferromagnetic in-plane anisotropy and proportions of the sample showing superparamagnetism, the latter predominantly in the deeper layers. [ABSTRACT FROM AUTHOR]

We have performed a systematic study on the effect of Zn substitution on the structural and magnetic properties of crystalline zinc-substituted cobalt ferrite nanoparticles, Zn x Co 1 - x Fe 2 O 4 with x = 0, 0.25, 0.5, 0.75, and 1, prepared by hydrothermal method. The structural and magnetic properties of these nanoparticles were investigated by XRD, TEM, FTIR, and VSM. All the ferrite nanoparticles were prepared with sizes smaller than 20 nm, thus lying within the range of single-domain regime. The results of Rietveld refinement revealed that all prepared nanoparticles were cubic and single phase, and the increase in Zn concentration resulted in an increase in the lattice constant, x-ray density, and the average bond length on tetrahedral sites. The TEM measurements showed that the nanoparticles were monodisperse and spherical in shape. All FTIR spectra of the prepared ferrites showed two dominant absorption bands, thus confirming the formation of single-phase spinel structure with two sub-lattices: tetrahedral (A-site) and octahedral (B-site). The room temperature M versus H magnetization measurements revealed that the ferrite nanoparticles were ferromagnetic for x = 0 and superparamagnetic for x ≥ 0.25 . At 10 K, all ferrite nanoparticles showed ferrimagnetic behavior that is weakened by Zn substitution. The saturation magnetization and the first anisotropy constant were observed to decrease with increasing Zn concentration. The zero field cooled and field cooled magnetization data revealed that both superparamagnetic and spin-glass like states may coexist together depending on amount of Zn concentration and temperature. [ABSTRACT FROM AUTHOR]

In real paramagnets, there is always a subtle many-body contribution to the system's energy, which can be regarded as a small effective local magnetic field (Bloc). Usually, it is neglected, since it is very small when compared with thermal fluctuations and/or external magnetic fields (B). Nevertheless, as both the temperature (T) → 0 K and B → 0 T, such many-body contributions become ubiquitous. Here, employing the magnetic Grüneisen parameter (Γmag) and entropy arguments, we report on the pivotal role played by the mutual interactions in the regime of ultra-low-T and vanishing B. Our key results are: i) absence of a genuine zero-field quantum phase transition due to the presence of Bloc; ii) connection between the canonical definition of temperature and Γmag; and iii) possibility of performing adiabatic magnetization by only manipulating the mutual interactions. Our findings unveil unprecedented aspects emerging from the mutual interactions. [ABSTRACT FROM AUTHOR]

Role of particle size distribution on magnetization of two polydispersed samples of antiferromagnetic NiO particles is investigated. Zero field-cooled and field-cooled susceptibility as a function of temperature (10–350 K) data indicate that samples are superparamagnetic. This behavior is again confirmed from M–H loops (up to ± 70 kG) at 10 and 350 K. The magnetization as a function of applied magnetic field data at 350 K is analyzed considering a lognormal distribution in particle size. The estimated particle size distributions are compared with those determined from transmission electron micrographs. We find that the particle size distribution is overestimated if magnetic anisotropy is ignored while analyzing the magnetization data. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide (RGO) decorated with superparamagnetic iron oxide nanoparticles (SPION) is a novel composite nanomaterial with a myriad of promising applications. However, processes such as the fast and simple synthesis of non-agglomerated monodispersed SPION on RGO and the accurate characterization of particle size distributions remain challenging. Here we present how to solve these two problems. Firstly, we introduce a new microwave-assisted synthesis of stabilized SPION on RGO which is fast, simple and up-scalable but at the same time renders well dispersed SPION with narrow size distributions. The coverage of the RGO flakes with SPION is extensively tuned and the results are compared with a non-stabilized microwave-assisted method. Secondly, we implement an accurate method for the determination of particle size distributions from magnetization curves in RGO-SPION composite nanomaterials. This method is applied to the prepared composites with different particle size distributions, degrees of particle agglomeration and coverage of the RGO flakes. The influence of sample characteristics in the size determination method is discussed and the results are compared with the values obtained from transmission electron microscopy (TEM) and X-ray diffraction (XRD), showing that the method is well suited for these and potentially other types of superparamagnetic composite nanomaterials. [ABSTRACT FROM AUTHOR]

This paper firstly studied an obvious magnetic transformation from paramagnetism to superparamagnetism in hydrogenated ZnFe2O4 nanoparticles. The zinc ferrite nanoparticles were successfully prepared by a simple solid-state reaction followed by a ball milling method. Moderate thermal treatments under hydrogen atmosphere were carried out, and a phenomenon was observed that the magnetic behavior of zinc ferrite nanoparticles transformed from paramagnetism to superparamagnetism. Various characterizations were carried out to confirm the creation of the hydrogen doping induced defects under post-treatments, which may be crucial for the magnetic transformation. By studying the effect of the defects caused by hydrogenation on the magnetization, we found that the oxygen vacancies could alter the magnetic ordering of materials and therefore modulate the magnetic behaviors. In addition, the saturation magnetization decreased after reheating the hydrogenated Zn-ferrite samples in the air, which indicated that the oxygen vacancies and Fe2+ ions directly modulated the magnetization of the Zn-ferrite materials. Notably, the residual saturation magnetization was still higher than raw Zn-ferrite, indicating that the changes of cation distribution could also modulate the magnetization and certain changes were irreversible. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental investigation of the magnetization relaxation processes in an ensemble of magnetic nanoparticles. The experimental results show a nonlinear dependence of the magnetic susceptibility on the volume concentration of the magnetic nanoparticles. Dependences of the magnetization relaxation time on the concentration and temperature of the magnetic nanoparticles are determined from an analysis of the frequency and temperature dependences of the magnetic susceptibility. A complex nonmonotonic dependence of the magnetization relaxation time on the magnetic particle concentration and temperature dependence of the magnetic susceptibility close to exponential one characteristic for a system of interacting dipole particles in the state of the so-called dipole glass have been established. The special features of the magnetization relaxation are caused by the interaction of particles and the change of the structure of the magnetic particle ensemble. [ABSTRACT FROM AUTHOR]

Magnetic properties of antiferromagnetic nanoparticles are not as straight forward as for ferromagnetic and ferrimagnetic nanoparticles. We analyzed the data of antiferromagnetic six-line ferrihydrite nanoparticles by considering distribution in particle magnetic moment. The fit of magnetization data, using non-linear least square fit procedure, considering Langevin function with and without distribution in particle magnetic moment, is presented. Magnetization of the system is measured as function of temperature and applied magnetic field. [ABSTRACT FROM AUTHOR]

The effects of single-ion anisotropies and an external magnetic field on the magnetization of the mixed-spin (7/2, 3/2) ferrimagnetic Ising system are investigated within Monte Carlo simulation. Under certain values of the physical parameters, multiple hysteresis loop behaviors such as double, triple, and quintuple hysteresis cycles have been observed. Particularly, the superparamagnetic phase has been shown. The ground-state phase diagrams are presented. [ABSTRACT FROM AUTHOR]

In this work, we have studied the suppression of the charge-ordered (CO) state in Nd0.8Na0.2MnO3 compound by varying the particle size and the applied magnetic field. The bulk samples with a particle size of D ≥ 140 nm exhibit different magnetic ground states, such as paramagnetic (PM), charge-ordering, ferromagnetic-cluster-glass (FM-CG), and reentrant-spin-glass-like (RSG) behavior. The complete suppression of charge ordering could be observed by reducing the particle size down to 50 nm. The field variation of magnetization curves at different temperatures show field-induced reversible transition from the antiferromagnetic (AFM) CO state to the FM state beyond a threshold field (HC). The M-H loops recorded at 5 K, show a distinct field-induced transition from a spin-frozen state to a FM state, especially on bulk samples with particle sizes of D ≥ 140 nm. T-H phase diagrams were constructed for samples with different particle sizes. The phase boundary between CO-AFM and FM states was found to shift toward higher temperature and lower magnetic fields with a decrease in particle size. Moreover, the phase-separation region was also found to narrow down with a decrease in particle size. The boundary of the spin-frozen state is found to shift toward lower temperature and magnetic field with decrease in particle size. The observed result could be explained based on the core-shell model. [ABSTRACT FROM AUTHOR]

Long range dipole-dipole interactions in the thermal fluctuations of the magnetization of an assembly of single-domain ferromagnetic particles are considered, ignoring orientational correlations between the particles, so that the evolution of the magnetization orientations may be described by a nonlinear Fokker-Planck equation (FPE) reducing to the usual linear one in the limit of infinite dilution [W. F. Brown Jr., Phys. Rev. 130, 1677 (1963)]. The thermally activated relaxation time scale of the assembly is estimated, leading to a simple and transparent modification of the axially symmetric asymptotes for the superparamagnetic relaxation time. [ABSTRACT FROM AUTHOR]

This letter reports a demonstration of microwave assisted magnetization reversal (MAMR) in a CoFeB film and the damping dependence in MAMR through the measurement of ferromagnetic resonance (FMR). Spin-pumping in non-ferromagnetic/ferromagnetic films provides a large range variation of Gilbert damping constants in magnetic samples when changing the thickness of non-ferromagnetic layers without changing the ferromagnetic film. An evident dependence of switching fields on the damping constant is observed in the presence of microwaves. The trend of the experimental data is well reproduced by a numerical simulation based on the Landau-Lifshitz-Gilbert equation. The result indicates that the large damping decreases the efficiency of microwaves in reducing the magnetization switching field. [ABSTRACT FROM AUTHOR]

In this paper we report high magnetic field (- 140 ≤ H ≤ 140 kOe) magnetization data for cobalt ferrites (crystallites size ∼ 42 nm) for temperatures (T) varying from 5 to 340 K. The T-dependence for the cubic magnetocrystalline anisotropy constant K1 was determined by using the 'law of approach' (LA) to saturation. The values of K1 were found to be substantially different from previously reported values obtained using the same procedure but with H up to 50 kOe. By properly choosing the quantum parameters we found a very good agreement between the values calculated by using the model proposed by Tachiki [Prog. Theor. Phys. 23, 1055 (1960)] and the K1 versus T data. For instance, the values of K1 measured (calculated) for 5 K and 340 K were 28.3 × 106 erg/cm3 (27.7 × 106 erg/cm3) and 7.4 × 106 erg/cm3 (6.8 × 106 erg/cm3), respectively. The values of the parameters used to fit the data in both magnetic field regimes were chosen based on cation distribution over the A and B-sites on the spinel structure of the nanoparticles. [ABSTRACT FROM AUTHOR]

We have studied the coexistence of exchange bias (EB) effect and spin-dependent magnetotransport in a Ni/NiO nanogranular sample by measuring the magnetization (M) and the magnetoresistance (MR) versus the magnetic field (H) in the 5-250 K temperature (T) range, both in zero-field-cooling (ZFC) and field-cooling (FC) conditions. The sample consisted of Ni nanocrystallites (mean size ∼13 nm) dispersed in a NiO matrix; the Ni volume fraction was ∼33%, above the percolation threshold for electrical conductivity, as revealed by the low resistivity (order of 10-3 Ωm) and by its growth with increasing T. The EB and magnetotransport phenomena appear strictly intertwined: the FC M(H) and MR(H) loops exhibit a similar horizontal shift, corresponding to an exchange field of ∼460 Oe at T = 5 K, which decreases with increasing T and disappears at ∼200 K. Both the EB and the magnetotransport properties have been explained, considering the presence of a structurally disordered component of the NiO matrix around the Ni nanocrystallites, whose spin-glass-like magnetic character rules the interface exchange interaction with the Ni phase and the spin-dependent conductivity. [ABSTRACT FROM AUTHOR]

In this paper, we present an analytical method to study the magnetization processes of two magnetic particles with ferromagnetic or antiferromagnetic coupling by using a modified expression for the anisotropy free energy. By considering a system consisting of two uniaxial particles, coupled by a simple exchange interaction, we describe analytically: the particles orientations, switching points and hysteresis loops for different ferromagnetic or antiferromagnetic couplings and for different orientations of the applied field. [ABSTRACT FROM AUTHOR]

We report DC magnetization, detailed systematic linear and nonlinear AC magnetic susceptibility and transport for a single phase RuSr2Eu1.4Ce0.6Cu2O10-δ (EuRu-1222) magneto-superconductor. The studied sample is synthesized through standard solid state reaction route, which is crystallized in single phase tetragonal structure with space group I4/mmm. DC magnetic susceptibility measurements revealed that the studied EuRu-1222 is a magneto-superconductor with Ru spins ordering at around 110 K and superconductivity in the Cu-O2 planes below ≈ 30 K. Temperature dependence of AC susceptibility with different frequency and amplitude variations confirms spin-glass behavior with cluster ferromagnetism of the system. Change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted feature for a spin-glass (SG) system with ferromagnetic clusters. The third harmonic of AC susceptibility (χ3) shows that the system undergoes a spin glass transition below 80 K. Superconducting transition temperature (Tc) onset and ρ = 0 are seen at around 30 and 18 K without any applied field and the same decreases to 10 and 2 K under 130 kOe applied field. Also low fields isothermal (MH) suggests that ferromagnetic clusters are embedded in spin-glass (SG) matrix. The magnetization versus applied field (MH) loops exhibited ferromagnetic (FM) like behavior with rather small coercive fields. Detailed AC magnetic susceptibility measurements are carried out to unearth the short range magnetic correlations. These results support the spin-glass (SG) formation followed by ferromagnetic clustering effects at low temperatures. Our detailed magnetization and magneto transport results will undoubtedly contribute to current understanding of the complex magnetism of the EuRu-1222 system. [ABSTRACT FROM AUTHOR]

We have investigated the magnetization dynamics of sputtered Co40Fe40B20 thin films in a wide range of thicknesses used as free layers in MgO-based magnetic tunnel junctions, with the technique of broadband ferromagnetic resonance (FMR). We have observed a large interface-induced magnetic perpendicular anisotropy in the thin film limit. The out-of-plane angular dependence of the FMR measurement revealed the contributions of two different damping mechanisms in thick and thin film limits. In thinner films (< 2 nm), two-magnon scattering and inhomogeneous broadening are significant for the FMR linewidth, while the Gilbert damping dominates the linewidth in thicker films (≥ 4 nm). Lastly, we have observed an inverse scaling of Gilbert damping constant with film thickness, and an intrinsic damping constant of 0.004 in the CoFeB alloy film is determined. [ABSTRACT FROM AUTHOR]

The nonlinear stationary ac response of the magnetization of assemblies consisting of (i) noninteracting uniaxial superparamagnetic nanoparticles with aligned easy axes and (ii) randomly oriented nanoparticles subjected to superimposed ac and dc bias magnetic fields of arbitrary strength and orientation is calculated by averaging Gilbert's equation augmented by a random field. The magnetization dynamics of uniaxial particles driven by a strong ac field applied at an angle to the easy axis of the particle (so that the axial symmetry is broken) alters drastically leading to new nonlinear effects due to coupling of the thermally activated magnetization reversal mode with the precessional modes via the driving ac field. In particular, the high frequency response reveals significant nonlinear effects in the precessional motion with significant consequences for the dynamic hysteresis and ultra-fast switching of the magnetization following an ultrafast change in the applied field. [ABSTRACT FROM AUTHOR]

Traditional magnetic/stress coupling models are based on the magnetoelastic theory, which cannot explain magnetic phenomena when ferromagnetic materials are subjected to a plastic deformation. To promote the development of magnetomechanical theories, a new physical-based magnetic/stress coupling model was proposed in this paper, where the effects of the applied magnetic field, elastic and plastic deformations on the magnetization are, respectively, considered in terms of different acting mechanisms. Moreover, the influences of variants on the strength in different magnetization stages are also calculated. [ABSTRACT FROM AUTHOR]

The superparamagnetic limit imposes a restriction on how far the miniaturization of electronic devices can reach. Recently it was shown that magnetic thin films with nanoscale dimensions can exhibit a vortex as its ground state. The vortex can lower its energy by developing an out-of-plane magnetization perpendicular to the plane of the film, the z direction, which can be 'up' or 'down.' Because the vortex structure is very stable this twofold degeneracy opens up the possibility of using a magnetic nanodisk as a bit of memory in electronic devices. The manipulation of the vortex and a way to control the core magnetization is a subject of paramount importance. Recent results have suggested that the polarity of a vortex core could be switched by applying a pulsed magnetic field in the plane of the disk. Another important effect induced by an external magnetic field due to the component out-of-plane in vortex-core is the gyrotropic mode. The gyrotropic mode is the elliptical movement around the disk center executed by the vortex-core under the influence of a magnetic field. In the present work we used numerical simulations to study the ground state as well as the dynamical behavior of magnetic vortices in thin nanodisks. We have considered a model where the magnetic moments interact through exchange (-J∑Svector i·Svector j) and dipolar potentials {D∑[Svector i·Svector j-3(Svector i·r⁁ij)×(Svector j·r⁁ij)]/rij3}. We have investigated the conditions for the formation of the vortex-core with and without an out-of-plane magnetization as a function of the strength of the dipole interaction D and of the size and thickness of the magnetic nanodisk. Our results were consistent with the existence of two vortex phases separated by a crossover line [(Dc-D)α]. We have observed that Dc does not depend on the radius of nanodisk but depends on its thickness. The exponent α was found to be α≈0.55(2). The gyrotropic motion is studied by applying an external magnetic field parallel to the plane of the magnetic nanodisk. Our results show that there is a minimum value for the modulus of the out-of-plane vortex-core magnetization, from which we can excite the gyrotropic mode. This minimum value depends on the thickness of the nanodisk. This result suggest that an experimental way to improve the stability of the process of switching may be through the thickness control. We also observed that the gyrotropic mode frequency increases with the aspect ratio, which is in qualitatively accordance with theoretical and experimental results. Finally, we present theoretical results for Permalloy nanodisks obtained from our model, which are also in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

The effect of multiple defects on the energy barrier of magnetization reversal in a one-dimensional magnetic nanowire is studied. By numerically solving the double Sine-Gordon equation via the finite-difference method, we obtain the energy saddle point through which the initial magnetic state jumps to the final ground states. The energy barrier which is defined as the difference between the energies of the saddle point and the initial states is calculated for various defect parameters. It is found that the energy barrier strongly depend on the number and distribution of the defects. For given defects, there exists an optimal distribution for a minimum energy barrier. [ABSTRACT FROM AUTHOR]

A simple kinetic approach based on approximate solution of the Fokker–Planck equation for magnetic moment orientations is developed for the calculation of the hysteresis loop of a superparamagnetic nanoparticle assembly with predominantly uniaxial magnetic anisotropy. The hysteresis loops of the oriented assembly have been obtained in the intermediate to high damping limit as a function of temperature at various angles that the applied magnetic field makes with the particle easy anisotropy axis. An analytic approximation is given for the effective energy barriers separating energy wells. The evolution of the hysteresis loop as a function of temperature is shown to take place between the ultimate Stoner–Wohlfarth loop and the equilibrium magnetization curve. Analytical estimates for the coercive force and the blocking temperature are obtained both for ordered and randomly oriented assemblies of uniaxial particles. [ABSTRACT FROM AUTHOR]

The size distribution of superparamagnetic iron oxide nanoparticles in two powder samples is determined from the measured magnetization curve fitted by a core-shell model and a uniform model, both based on the Langevin function with a log-normal particle volume distribution. Different average sizes are evaluated from the fitting parameters and compared to those determined by other techniques. Conceptual discussions are presented on different models and approaches, from which the core-shell model fitting is recommended for the magnetic size determination of superparamagnetic nanoparticles. [ABSTRACT FROM AUTHOR]

We investigate the magnetic properties of the nanoparticles of Co50Ni50 alloy embedded in the amorphous SiO2 host with volume fractions, [lowercase_phi_synonym]=5% and 10%. The static and dynamic aspects of the magnetic properties are investigated by temperature and time dependence of low-field dc magnetization and frequency dependence of ac susceptibilities. The experimental results and analyses suggest that the dilution of the Co50Ni50 nanoparticles belongs to the crossover regime from superparamagnetic to superspin glass states, which are distinguished from the atomic canonical spin glasses. [ABSTRACT FROM AUTHOR]

The reversal time of the magnetization of a single-domain ferromagnetic particle with a cubic anisotropy in the presence of a uniform magnetic field is evaluated using the Kramers escape rate theory for intermediate-to-high damping as extended by Brown [W. F. Brown, Jr., IEEE Trans. Magn. 15, 1196 (1979)] to the analogous magnetic spin problem. It is shown that this formula is in complete agreement with the relaxation time calculated numerically from the solution of the stochastic Landau-Lifshitz-Gilbert equation describing the dynamics of the magnetization of the particles. It is also shown that knowledge of the reversal time τ allows one to predict accurately the spectrum of the longitudinal magnetic susceptibility in the low-frequency range. [ABSTRACT FROM AUTHOR]

We investigate the magnetotransport properties of La0.7Ca0.3-xAgxMnO3 (0<=x<=0.3) polycrystalline films prepared by spray pyrolysis. The virgin La0.7Ca0.3MnO3 (LCMO) (x=0.0) film shows paramagnetic-ferromagnetic [insulator-metal (IM)] transition at TC∼260 K (TIM∼245 K). Ag doping induces appreciable enhancement in TC/TIM and interestingly TIM of Ag doped films is higher than the corresponding TC value. In the 30% Ag doped film IM transition occurs at ∼325 K which is ∼10 K higher than its TC∼315 K. Strong irreversibility is observed in zero field cooled–field cooled magnetization of all the films and the irreversibility temperature Tirr>TC increases with Ag doping. This suggests the formation of magnetic clusters above TC. The pure LCMO film exhibits small magnetoresistance (MR) ∼20% and rather poor temperature coefficient of resistance (TCR) in the vicinity of the room temperature. In contrast, around room temperature the Ag doped films show large MR which is ∼60% for x=0.15 and decreases to ∼30% for the pure Ag doped film. Ag doped films also have good TCR ∼4%/K in the vicinity of room temperature. [ABSTRACT FROM AUTHOR]

Magnetization, electrical resistivity, and Hall coefficient of single crystals Sb2-xFexTe3 (0

Magnetization reversal phenomena (MRP) along magnetic order-order transitions have recently been reported on impurity-doped magnetic systems. Because imperfect long-range magnetic order exists in these systems, it is expected that a systematic investigation of MRP will give physical information on thermomagnetic processes of magnetic systems in the range from the micro- to nanoscales. As a typical order-order transition (a state doubly modulated by helical and canting orders to a collinear ferrimagnetic state) has been known to occur on Cr5S6 at a transition temperature Tt, we investigate the magnetizations of (Cr0.70Ti0.30)5S6 on heating and cooling runs in various magnetic fields. At 20 Oe, the field-cooled magnetization just below the Curie temperature has a positive sign; the sign turns negative below the compensation temperature TCM (first step) and finally returns to positive below Tt (second step). The first-step MRP observed in this system is explained by the potential barriers resulting from anisotropy energy when the preferred direction of collinear ferrimagnetic moment reverses. The proposed mechanism for second-step MRP is the pinning effect caused by the impurity atoms (Ti) in the helical long-range-order chains. Comparing other examples of MRPs, we discuss the roles of local impurity centers in the thermomagnetic process in magnetic order-order transitions. [ABSTRACT FROM AUTHOR]