Single-domain Particles Research Articles

Detailed palaeomagnetic record at Rose Cottage Cave, South Africa: Implications for the Holocene geomagnetic field behaviour and chronostratigraphy

Palaeomagnetic data from a sedimentary section spanning the Holocene and terminal Pleistocene (~13 kya) from Rose Cottage Cave, eastern Free State (South Africa), are reported. The palaeomagnetic analysis took into account rock magnetism and directional analysis. The former reveals that most samples show stable single domain and superparamagnetic particles of Ti-poor magnetite and haematite. Natural remanent magnetisation directions were determined by progressive alternating field demagnetisation methodology. Directional analysis shows normal directions between samples 18 to 39 and 85 to 92; however, during the Early and Late Holocene in samples principally from RC40 to 84 ‘anomalous’ directions occurred. There is a significant westward shift in declination of ~80°, and a conspicuous fluctuating inclination in the lower part of the section during the Early Holocene at ≥9.5 kya and before ~12.0/13.0 kya. This palaeomagnetic record might become a chronostratigraphical marker for latest Pleistocene/Holocene sedimentary deposits in South Africa. Our two new accelerator mass spectrometry radiocarbon dates for the sampled deposit are 9500±50 BP and 1115±30 BP.
 Significance
 
 The study provides new accelerator mass spectrometry dates on the chronological sequence existing in Rose Cottage Cave.
 The findings contribute to the knowledge of the geomagnetic field behaviour since the terminal Pleistocene to the Late Holocene in a period spanning the last 13 000/12 000 years.
 This palaeomagnetic record might become a chronostratigraphical marker for the latest Pleistocene/ Holocene sedimentary deposits in South Africa.

Magnetic Granulometry and Mössbauer Spectroscopy of Synthetic FemOn–TiO2 Composites

FemOn–TiO2 particle aggregates have been obtained using the sol–gel method and hydrothermal treatment. It is shown that the synthesis conditions favor forming composites that contain titanomagnetite in very low concentrations. Hysteresis loops and demagnetization curves of anhysteretic remanent magnetization were measured with a vibration sample and a SQUID magnetometer, respectively. Mossbauer spectroscopy, bulk magnetic properties, scanning electron microscopy (SEM) observations, coupled with the theoretical analysis of magnetostatic interaction between finely dispersed particles lead to a conclusion that the studied composites can be viewed as groups of interacting clusters 2– $20~\mu \text{m}$ in size, consisting of chemically heterogeneous, mostly single-domain or close to single-domain particles with an average size of about 100 nm.

Simulation of Remanent, Transient, and Induced FORC Diagrams for Interacting Particles With Uniaxial, Cubic, and Hexagonal Anisotropy

AbstractThe diagnostic power of first‐order reversal curve (FORC) diagrams has recently been enhanced by an extended measurement protocol that yields three additional FORC‐like diagrams: the remanent (remFORC), induced (iFORC), and transient (tFORC) diagrams. Here, we present micromagnetic simulations using this extended protocol, including numerical predictions of remFORC, iFORC, and tFORC signatures for particle ensembles relevant to rock magnetism. Simulations are presented for randomly packed single‐domain (SD) particles with uniaxial, cubic, and hexagonal anisotropy, and for chains of uniaxial SD particles. Noninteracting particles have zero tFORC, but distinct remFORC and iFORC signals, that provide enhanced discrimination between uniaxial, cubic, and hexagonal anisotropy types. Increasing interactions lessen the ability to discriminate between uniaxial and cubic anisotropy but reproduces a change in the pattern of positive and negative iFORC signals observed for SD‐dominated versus vortex‐dominated samples. Interactions in SD particles lead to the emergence of a bi‐lobate tFORC distribution, which is related to formation of flux‐closure in super‐vortex states. A predicted iFORC signal associated with collapsed chains is observed in experimental data and may aid magnetofossil identification in sediments. Asymmetric FORC and FORC‐like distributions for hexagonal anisotropy are explained by the availability of multiple easy axes within the basal plane. A transition to uniaxial switching occurs below a critical value of the out‐of‐plane/in‐plane anisotropy ratio, which may allow FORC diagrams to provide insight into the stress state of hexagonal minerals, such as hematite.

High-temperature magnetic coercivity of CNTs filled with multi-phase Fe-based nanoparticles

A system of Fe-filled multiwall carbon nanotubes (MWCNT) was grown by aerosol chemical vapor deposition with three different concentrations of ferrocene as precursor catalyst. The obtained samples were thoroughly analyzed from the point of view of their morphological, microstructural and magnetic properties. It was found that ferrocene concentration did not change the MWCNTs morphology. However, it increases the number of MWCNTs containing Fe and the Fe amount in each MWCNT. The Fe is located either in form of nanorods within the inner channel of the MWCNTs or as nanoparticles of various sizes, sometimes aggregated in clusters. The magnetic results indicate the presence of different phases, identified as α-Fe, Fe3C and Fe3O4, mostly in form of single-domain particles. A simple model of single domain non-interacting particles allows explaining the observed temperature behavior of coercivity and remanence over a wide temperature range, up to 950 K. The different ferrocene concentrations affect only the saturation magnetization but not the coercivity of the samples. As a further result, MWCNT/epoxy nanocomposites, with possible application as electromagnetic shields, were prepared by the “two factor mechanical method”. The nanocomposites exhibit the same magnetic properties of the Fe-filled MWCNTs, in terms of both coercivity and hysteresis loop shape.

Magnetic phase transition of mechanically alloyed single sample Co0.5Ni0.5Fe2O4

The parallel evolutional relationship between microstructural properties and magnetic and electrical properties was elucidated through this study. A Co0.5Ni0.5Fe2O4 rod sample was prepared via high energy ball milling and subsequent moulding into a nano-sized compacted powder. This single sample was sintered through 10 cycles at different sintering temperatures in the range of 500 °C–1400 °C. After each sintering, the sample was characterized for its phase, microstructural, density, magnetic and electrical properties using XRD, SEM, B-H tracer, Curie temperature measurement and two probes method. An integrated study of microstructural properties with elevating sintering temperature would point to the existence of three stages of sintering, which involved atomic, interfaces (lattice and boundaries), and volume diffusions respectively. Three distinct shape-differentiated groups of B–H hysteresis loops were observed. The existence of these groups was associated with microstructural properties such as phase purity, volume fraction of disordered phase or grain boundaries, and grain size. In terms of average grain size, from 48.25 nm to 71.93 nm, a weak paramagnetic behaviour was observed; while from 83.65 nm to 374.79 nm, a relatively square-shaped hysteresis loops with moderate ferromagnetic behaviours were observed. The occurrences of erect and well-defined sigmoid-shape were observable when there were sufficiently high single-phase purity and crystallinity, where the average grain size was in the range of 964.73 nm–11215.91 nm. The critical grain size of 186.75 nm was found by plotting average grain size against coercivity, suggesting the number of single-domain particles was reduced, and the number of multi-domain particles was increased by increasing sintering temperature. The electrical resistivity variations were strongly related to the microstructural properties.

Machine Learning and Micromagnetic Studies of Magnetization Switching

Magnetization switching is one of the most fundamental topics in the field of magnetism. Machine learning (ML) models of random forest (RF), support vector machine (SVM), deep neural network (DNN) methods are built and trained to classify the magnetization reversal and non-reversal cases of single-domain particle, and the classification performances are evaluated by comparison with micromagnetic simulations. The results show that the ML models have achieved great accuracy and the DNN model reaches the best area under curve (AUC) of 0.997, even with a small training dataset, and RF and SVM models have lower AUCs of 0.964 and 0.836, respectively. This work validates the potential of ML applications in studies of magnetization switching and provides the benchmark for further ML studies in magnetization switching.

Shaping Magnetite with Poly-l-arginine and pH: From Small Single Crystals to Large Mesocrystals.

Control over particle size, size distribution, and colloidal stability are central aims in producing functional nanomaterials. Recently, biomimetic approaches have been successfully used to enhance control over properties in the synthesis of those materials. Magnetotactic bacteria produce protein-stabilized magnetite away from its thermodynamic equilibrium structure. Mimicking the bacteria’s proteins using poly-l-arginine we show that by simply increasing the pH, the dimensions of magnetite increase and a single- to mesocrystal transformation is induced. Using synchrotron X-ray diffraction and transmission electron microscopy, we show that magnetite nanoparticles with narrow size distributions and average diameters of 10 ± 2 nm for pH 9, 20 ± 2 nm for pH 10, and up to 40 ± 4 nm for pH 11 can be synthesized. We thus selectively produce superparamagnetic and stable single-domain particles merely by controlling the pH. Remarkably, while an increase in pH brings about a thermodynamically driven decrease in size for magnetite without additives, this dependency on pH is inverted when poly-l-arginine is present.

Dielectric and magnetic properties of rare-earth-doped cobalt ferrites and their first-order reversal curve analysis

The present study focuses on the effects of rare-earth (R) ion substitutions namely Y, Sm and Pr ions on the structural, magnetic and electrical properties of cobalt ferrite (CFO) and the nature of their magnetic domains using the first-order reversal curve (FORC) analysis. These samples were synthesized using the sol–gel auto-combustion method. The powder X-ray diffraction analysis reveals that the Pr-substituted CFO crystallizes in a single-phase spinel structure, while a few traces of RFeO3 appear as minor phases in Sm- and Y-doped CFO. The scanning electron micrographs show that the substitution of R3+ ions causes a considerable reduction in the grain size. The studies on magnetic properties reveal that the saturation magnetization of R-substituted CFO decreased. The FORC analysis was done to know the domain state of magnetization and the nature of magnetic interactions among the grains. All the FORC diagrams depict a single peak with a single contour suggesting that all the synthesized samples have single domain particles and are comprised only of mono-magnetic phase. The electrical properties of the R-doped CFO compounds exhibit high values of dielectric constant at room temperature with the highest value for Pr-doped CFO. The activation energy determined using electrical properties is found to decrease with R doping.

Domain structure of volcanics’ ferromagnetic grains: A case study of the Saatly ultradeep well, Azerbaijan

Various magnetic properties of magnetic minerals show strong grain-size depen-dence. This dependency occurs because these parameters are influenced by the magnetic grain-size (domain state) of the samples. Natural rocks often contain a mixture of different magnetic minerals with different grain-size (domain states). Thus determination of the domain state is important. The Saatly ultradeep well’s volcanics were used as a research material. In the depth intervals of 3540—8126 m the volcanics had been revealed by the borehole. According to data of the petrographic and microprobe analysis the volcanogenics of the ultradeep borehole is conditionally divided into a two parts: 1) the upper part (an interval of depths of 3540—5500 m); 2) the bottom part (an interval of depths of 5500—8126 m). In the upper part of section according to microprobe analysis of volcanogenic rocks ore grains are presented by two generations: 1) large grain of titanomagnetites (Ti-poor), homogeneous or with a decay structures; 2) small-sized usually well faced grain of practically stoichiometric magnetite. So, homogeneous small-sized grains or fragments of decay large grains (which are strongly changed chemically) can have properties of single-domain particles. Here the ferromagnetic grains (especially in the depth interval 3540—5000 m) are often noted as a pseudo-single domain. In the bottom part of the section the presence of multidomain grains of the Ti-poor titano-magnetite is noted. And since depths of 7000 m the oxidized magnetite is also observed. The pseudo-single-domain grains are fixed much less than in the upper section. And, since a depth mark of 7300 m and deeper, they are absent wholly. In the result of rese-arch it turned out that the ferromagnetic is presented by basically multidomain grains. It should be noted that Lauri—Fuller’s Criterion is not applicable at identification of a domain structure of the complex ensemble of the ferromagnetic grains.

Numerical solution of Fokker-Planck equation for single domain particles

The Fokker-Planck equation proposed by Brown to describe the evolution of the probability distribution density of the orientation of the magnetic moment of a single-domain particle is usually solved by expanding the unknown function in a series of spherical harmonics. In this paper, we use a different method to solve the Fokker-Planck equation, namely, the finite element method. We describe the procedure for constructing a triangular grid on the surface of a sphere and give formulas for calculating the coefficients of the equations for the probability density values at the grid nodes. As an example, the results of calculating the dynamic magnetic hysteresis for particles with cubic anisotropy are demonstrated.

Simulation of Uniaxial Deformation of a Ferrogel Sample Exposed by the External Magnetic Field

Ferrogels are magnetoactive composite materials with very soft hydrogel matrix, filled by ferromagnetic particles. This work is devoted to a numerical investigation of magneto-mechanical behaviour of a small sample of ferrogel. Our model is based on a coarse grained molecular dynamics approach: polymeric matrix is emulated by lattice of bead-strings chains and magnetic single-domain particles are considered as rigid spheres and placed in a nodes of the lattice. This model was previously successfully used for a research of equilibrium magnetization of the material. Here we examine of the reaction of ferrogel sample on uniaxial tension in presence of an external magnetic field. Modelling proves that key role in a changes of mechanical response during the magnetization plays an internal structure of material, specifically, the existence of particles aggregates, induced by forces of dipolar magnetic interaction. Also the influence of magnetic anisotropy on a process is considered.

AF demagnetization and ARM acquisition at elevated temperatures in natural titanomagnetite bearing rocks

SUMMARY Understanding the temporal changes of the Earth’s magnetic field intensity is one of the main goals of modern palaeomagnetism. For most palaeointensity methods to yield reliable results, the magnetic minerals must obey a set of rules. One of these rules is the additivity of partial thermal (TRM) or anhysteretic remanent magnetizations (ARM). Additivity was previously shown for partial TRM in single-domain particles and more generally for ARMs. Additivity between these two low-field remanences, however, has not been investigated, yet. This paper presents a series of rock magnetic experiments on natural low Ti titanomagnetites (Curie temperature between 534 °C and 561 °C) examining the effects of high temperatures on alternating field (AF) demagnetization and acquisition of an ARM. One of our sample sets comes from a borehole drilled through the impact melt sheet of the Manicouagan crater (Canada), the other from the Rocche Rosse lava flow on the island of Lipari (Italy). Hysteresis parameters indicate the magnetic carriers in the pseudo-single-domain range showing no evidence for oxidation. Thermal demagnetization at 300 °C and 500 °C before AF demagnetization shifts the coercivity spectra towards higher fields. AF demagnetization experiments at 500 °C show a significant (by a factor between 1.4 and >7.6) reduction in median destructive field and a shift towards lower coercivities. A linear relationship was found between the peak magnetic field required to demagnetize a fraction of a full TRM of a sample at a specific temperature and the one necessary to demagnetize the same fraction at room temperature after heating to that temperature. The comparison of full ARM and partial TRM at successively higher temperatures with a hybrid hTARM reveals that combined additivity between the two kinds of remanences is fulfilled. These results open the possibility to demagnetize highly coercive minerals, such as hematite and goethite, which is often not achievable at elevated temperatures. Furthermore, the additivity of TRM and ARM remanences may be used to develop novel hybrid TRM/ARM palaeointensity methods for samples, where heating is problematic (e.g. in meteorites).

Structural, magnetic and ESR studies of [formula omitted] ([formula omitted]) nanoparticles synthesized by a sol-gel method

Fabrication of garnet nanoparticles as per the device requirement is still a big challenge and technological problem. In order to fulfil the requirement of microwave device engineer, we have synthesized nanoparticles of aluminium (Al) doped YIG by a citrate sol-gel method using metal nitrates, citric acid, ethylene glycol and ammonia. The XRD and FTIR study confirmed the single phase cubic garnet structure. The crystallite size was increased from 55 to 60 nm and the particle size decreased from 87 nm to 59 nm suggesting a departure from multi-domain to single-domain particles. The saturation magnetization (Ms) decreased from 21.06 emu/g for x = 0.0 to 1.06 emu/g for sample x = 1.2. The peak to peak linewidth (ΔHp) values increased from 110 G to 620 G due to a reduction in superexchange interaction by Al ions. The properties of Al-YIG obtained in this study will be useful for phase shifter and circulator applications.

Dating of tsunami boulders from Ishigaki Island, Japan, with a modified viscous remanent magnetization approach

Reworked boulders are expected to acquire a viscous remanent magnetization (VRM) approximately parallel to Earth's modern magnetic field. The magnitude of such a VRM depends on several factors, including the time since reworking and ambient temperature, for which there are well-known theoretical relationships. VRM unblocking temperature can, therefore, be a powerful tool for determining the reworking age of boulders and can be used to assess the timing of geological hazards such as landslides and tsunami events. In this study, VRM unblocking temperatures for twenty-seven samples from four coral tsunami boulders on Ishigaki Island, Japan, are compared with three candidate time–temperature relationships for VRM acquisition. For the Pullaiah nomogram, which is applicable to single-domain magnetite particle assemblages, nineteen samples from four boulders agree well with the expected unblocking temperature derived from previously reported 14C ages. Two samples have low unblocking temperatures and six samples appear to have anomalously high VRM unblocking. The Walton nomogram, which is used for lognormal grain-size distributions, relates acquisition temperatures required to produce equal magnetic (paleo)intensities and cannot explain the high unblocking temperatures; it produces younger predicted ages than the youngest boulder 14C age. We find that an alternative time–temperature relationship, defined by a stretched exponential law, has the potential to yield reworking ages for the anomalous boulders that are consistent with the 14C ages. We suggest that future VRM dating can be undertaken using a combination of the Pullaiah nomogram and the stretched exponential law.

Magnetic Nanoparticles in Human Cervical Skin.

Magnetic iron oxide nanoparticles, magnetite/maghemite, have been identified in human tissues, including the brain, meninges, heart, liver, and spleen. As these nanoparticles may play a role in the pathogenesis of neurodegenerative diseases, a pilot study explored the occurrence of these particles in the cervical (neck) skin of 10 patients with Parkinson's disease and 10 healthy controls. Magnetometry and transmission electron microscopy analyses revealed magnetite/maghemite nanoparticles in the skin samples of every study participant. Regarding magnetite/maghemite concentrations of the single-domain particles, no significant between-group difference was emerged. In low-temperature magnetic measurement, a magnetic anomaly at ~50 K was evident mainly in the dermal samples of the Parkinson group. This anomaly was larger than the effect related to the magnetic ordering of molecular oxygen. The temperature range of the anomaly, and the size-range of magnetite/maghemite, both refute the idea of magnetic ordering of any iron phase other than magnetite. We propose that the explanation for the finding is interaction between clusters of superparamagnetic and single-domain-sized nanoparticles. The source and significance of these particles remains speculative.

An overview on nucleation theories and models

Several different models for coercivity are discussed. There are two main situations: i) nanocrystalline magnets, with grain size bellow the single domain particle size, and ii) magnets with grain size above single domain particle size. The described theories and models are general, and can be applied in either NdFeB magnets, SmFeCoCuZr or strontium ferrite magnets. The spring effect observed in isotropic nanocrystalline magnets can be explained with the Stoner-Wohlfarth model. Modifications of the Stoner-Wohlfarth model are necessary to take into account the effect of interaction between grains. When the grain size is above the single domain size, energy considerations show that nucleation should occur at the surface of grains. Nucleation is interpreted as a two-step process, where domain wall displacement occurs for grain size above single domain size, after a nucleus is first formed. The effect of grain size on the coercive field is discussed.

The relation between the composition, structure and absorption properties of ultra-high frequency radio waves of poly(vinylidene fluoride)/magnetite nanocomposites

In the presented work, nanocomposites based on poly (vinylidene fluoride) (PVDF) and magnetite Fe3O4 nanoparticles were prepared. The structure and content of nanocomposite materials were studied by using scanning electron microscope (SEM), atomic-force microscope (AFM) and X-Ray diffraction (XRD). Magnetic properties of PVDF[Formula: see text]+[Formula: see text]Fe3O4 nanocomposites have been studied upon increasing nanoparticle content in polymer matrix upto 20%, revealing superparamagnetic behavior as Fe3O4 nanoparticles in polymer matrix act out like single-domain particles. It has also been observed that PVDF[Formula: see text]+[Formula: see text]Fe3O4-based nanocomposites can absorb the electromagnetic waves in the high frequency range 0.1–30 GHz. It has been shown that the absorption of high frequency radio waves by PVDF[Formula: see text]+[Formula: see text]Fe3O4 nanocomposites can be explained by the different molecular structures and also by the scattering of the radio waves at the boundary of nanoparticle-polymer matrix.

New Insights about CuO Nanoparticles from Inelastic Neutron Scattering.

Inelastic Neutron Scattering (INS) spectroscopy has provided a unique insight into the magnetodymanics of nanoscale copper (II) oxide (CuO). We present evidence for the propagation of magnons in the directions of the ordering vectors of both the commensurate and helically modulated incommensurate antiferromagnetic phases of CuO. The temperature dependency of the magnon spin-wave intensity (in the accessible energy-range of the experiment) conforms to the Bose population of states at low temperatures (T ≤ 100 K), as expected for bosons, then intensity significantly increases, with maximum at about 225 K (close to TN), and decreases at higher temperatures. The obtained results can be related to gradual softening of the dispersion curves of magnon spin-waves and decreasing the spin gap with temperature approaching TN on heating, and slow dissipation of the short-range dynamic spin correlations at higher temperatures. However, the intensity of the magnon signal was found to be particle size dependent, and increases with decreasing particle size. This “reverse size effect” is believed to be related to either creation of single-domain particles at the nanoscale, or “superferromagnetism effect” and the formation of collective particle states.

Nanomagnets for ultra-high field MRI: Magnetic properties and transverse relaxivity of silica-coated ε-Fe2O3

Abstract Magnetic nanoparticles of iron oxides have received extensive attention in the biomedical research, e.g. as prospective contrast agents for T2-weighted magnetic resonance imaging (MRI). The ability of a contrast agent to enhance the relaxation rate of 1H nuclei in its vicinity is quantitatively described by its relaxivity. Among the polymorphs of the iron(III) oxide, the nanoparticulate e-Fe2O3, distinguished by its high magnetocrystalline anisotropy resulting in a blocked state of single-domain particles up to the Curie temperature of ∼500 K, has never been studied as a contrast agent for MRI before. We analyzed r1 and r2 relaxivities of e-Fe2O3 nanoparticles coated with amorphous silica, particularly with the aim to determine their dependences on the external magnetic field, temperature, and thickness of the silica coating. The r2 relaxivity was interpreted within the motional averaging and static dephasing regimes. MRI images at 11.75 T confirmed high applicability of e-Fe2O3-based contrast agents in ultra-high fields. We present the first case study considering e-Fe2O3 nanomagnets for prospective application in biomedicine, in particular for MRI in ultra-high fields. The study is complemented by a thorough investigation of magnetic properties of the nanoparticles, revealing some interesting anomalies.

Transient Chaos in Nanomagnets Subject to Elliptically Polarized AC Applied Fields

Magnetization dynamics in single-domain particles driven by elliptically polarized ac external fields is studied. It is shown that such dynamics exhibits chaotic and fractal nature at a moderate power level. This is due to the phenomenon of heteroclinic tangle produced by the combined action of ac excitations and saddle-type dynamics. By using the special perturbation technique based on the Melnikov function, analytical formulas for the threshold ac excitation amplitudes necessary to create the heteroclinic tangle are derived as a function of frequency and general elliptic polarization of the applied ac field. Numerical simulations confirm that the heteroclinic tangle implies the erosion of the safe basin around stable magnetization states.