Magnetic Cluster Size Research Articles

Magneto-mechanical coupling effect of ferromagnetic materials: Test characteristics and theoretical model

The magneto-mechanical coupling effect of ferromagnetic materials is the result of macroscopic influence of mechanical properties on magnetic properties under the combined action of stress field and applied magnetic field, which essentially can be attributed to the change of material properties caused by the transformation of microscopic magnetic domain structures. The effect of applied magnetic field is mainly manifested as the change of magnetic moment cluster (direction and size) inside microscopic magnetic domain; The stress field mainly causes the movement and rotation of the domain walls inside the ferromagnetic material, and further causes the change of magnetic moment cluster. Taking Q235 low carbon steel as an example, the effects of tensile stress σt on some important magnetic parameters of ferromagnetic materials under different excitation intensities (i.e., H0), such as the maximum value of applied magnetic field Hmax, intrinsic coercive field Hcj, hysteresis loss power per unit volume ph and anhysteretic magnetization Man, were obtained by setting macroscopic magneto-mechanical coupling test and fully considering the influence of demagnetizing field Hd (The standard ring specimen was used for demagnetization correction of the non-standard strip specimen). Meanwhile, the basic magnetization characteristics of ferromagnetic materials were discussed by introducing the basic magnetization curve F(H0, Ban) and considering the effect of σt as the equivalent stress field Hσ based on the theory of microscopic magnetic domain and the principle of thermodynamic. In the range of 635.4 A/m ≤ H0 ≤ 928.6 A/m and 0 MPa ≤ σt ≤ 350 MPa, the theoretical calculation models Man(H0, σt) and Hσ(H0, σt) in the magneto-mechanical coupling effect were obtained, which are in good agreement with the experimental results.

Preparation, phase stability, and magnetization behavior of high entropy hexaferrites

The polycrystalline SrFe12O19 samples deeply substituted up to at.67% by Al3+, Ga3+, In3+, Co3+, and Cr3+ cations with a high configurational mixing entropy were prepared by solid-phase synthesis. Phase purity and unit cell parameters were obtained from XRD and analyzed versus the average ionic radius of the iron sublattice. The crystallite size varied around ∼4.5μm. A comprehensive study of the magnetization was realized in various fields and temperatures. The saturation magnetization was calculated using the Law of Approach to Saturation. The accompanying magnetic parameters were determined. The magnetic crystallographic anisotropy coefficient and the anisotropy field were calculated. All investigated magnetization curves turned out to be nonmonotonic. The magnetic ordering and freezing temperatures were extracted from the ZFC and FC curves. The average size of magnetic clusters varied around ∼350nm. The high values of the configurational mixing entropy and the phenomenon of magnetic dilution were taken into account.

Magnetism of Co-rich clusters embedded in nanoporous Pd prepared by electrochemical dealloying — Influence of thermal annealing

Nanoporous palladium samples containing superparamagnetic Co-rich clusters were prepared by electrochemical dealloying and the magnetic properties were subsequently varied by thermal annealing at different temperatures. It is shown that in this way porous magnetic nanomaterials exhibiting various magnetic characteristics can be obtained. The as-dealloyed, unannealed sample is ferromagnetic at 4.2 K and superparamagnetic at 300 K, with a blocking temperature TB of 100 K. The observed general decrease of the magnetization with increased annealing temperature is explained by the redistribution of Co from the Co-rich superparamagnetic clusters into the Pd matrix, leading to a shrinkage of the clusters and the formation of a diluted Co–Pd alloy phase. As a consequence of the smaller magnetic cluster size a down-shift of TB occurs for annealing temperatures of 373 K and 573 K. After annealing at 773 K the stronger dissolution of Co from the clusters into the Pd-matrix reduces TB further down to 35 K. Additionally, a second blocked magnetic phase with much higher TB emerges, which leads to ferromagnetic behavior at room temperature. The occurrence of this phase is attributed to the formation of larger magnetic entities consisting of Co-rich clusters lying close together and a Co–Pd alloy phase with relatively high Co concentration in between these clusters. After high temperature annealing at 973 K a weakly ferromagnetic phase is obtained at 4.2 K and at 300 K, which can be explained by the nearly completed dissolution of the Co in the Pd matrix.

Novel ferromagnetism and negative magnetoresistance in BaBiO3 nanoparticles

Materials showing semiconducting and magnetic properties have significant commercial value presently, particularly in the field of spintronics. We report a novel ferromagnetic behavior and a negative magnetoresistance in the non-magnetic semiconductor perovskite material BaBiO3 at room temperature. We also observe an associated exchange spring like magnetic behavior in the sample annealed at a higher temperature. We discuss these effects based on a model of defect induced magnetic centers and magnetic domains in the material. The enhanced magnetization and exchange spring like behavior observed in the sample annealed at a higher temperature of 800 °C is attributed to higher number of magnetic defects and two dominant sizes of magnetic clusters. Photoluminescence and energy dispersive X-ray spectra corroborate the explanation based on charge induced defects in the material. Interplay of Bi3+ and Bi5+ states in BaBiO3, leads to a semiconducting behavior at room temperature. From the temperature dependence of the resistivity two distinct activation energies of 0.21 eV and 0.14 eV above and below 256 K, respectively, have been measured.

Spin-lattice dynamics of surface vs core magnetization in Fe nanoparticles

Magnetization of clusters is often simulated using atomistic spin dynamics for a fixed lattice. Coupled spin-lattice dynamics simulations of the magnetization of nanoparticles have, to date, neglected the change in the size of the atomic magnetic moments near surfaces. We show that the introduction of variable magnetic moments leads to a better description of experimental data for the magnetization of small Fe nanoparticles. To this end, we divide atoms into a surface-near shell and a core with bulk properties. It is demonstrated that both the magnitude of the shell magnetic moment and the exchange interactions need to be modified to obtain a fair representation of the experimental data. This allows for a reasonable description of the average magnetic moment vs cluster size, and also the cluster magnetization vs temperature.

Temperature Dependence and Microstructure Effects on Magnetic Properties of FePt(B, Ag, C) Film.

A FePt(B, Ag, C) granular film was formed from post-annealed B4C(1.0 nm)/FePt(Ag, C) layers at a substrate temperature of 470 °C for 2 min. The 6 nm thick FePt(B, Ag, C) film demonstrates high perpendicular magnetic anisotropy (Ku = 2.83 × 107 erg/cm3 at 100 K) and out-of-plane coercivity (Hc = 38.0 kOe at 100 K). The Ku and out-of-plane Hc are respectively increased from 38% and 46% between 350 K and 50 K. The sample with a thickness of 8 nm also shows a similar trend for magnetic properties; however, the tiny magnetization kink which may come from rare Fe-B or disordered FePt grains was observed in the easy axis loop. The intrinsic (ΔHint = 12.6 kOe) and extrinsic switching field distribution (ΔHext = 1.62 kOe) were characterized by major and minor loops to correlate the microstructural grains. The coupled FePt grains grown on a single MgTiON grain were observed in a high-resolution transmission electron microstructure (HRTEM) image. This small intergranular exchange coupling was defined by estimating the magnetic cluster size (46.6 nm) from ΔHext and the average grains size (28.2 nm) from TEM images. The temperature dependence of coercivity was fitted to further understand the magnetization reversal process. The lower microstructural parameter was evidenced in the imperfect grain morphology.

Evolution of superparamagnetism in the electrochemical dealloying process

In situ superconducting quantum interference device magnetometry provides insights into the electrochemical dealloying mechanism of a CoPd alloy. Charge-dependent measurements of magnetic moment allow the separation of primary and secondary dealloying contributions. Coercivity evolution revealed the transition from collective ferromagnetism to superparamagnetism of small alloy clusters evolving in the dealloying process, which is interpreted as an “inverse” magnetic percolation problem. Temperature-dependent magnetization curves enable a qualitative comparison of magnetic cluster size distributions in the nanoporous Pd framework, which are found to be strongly influenced by dealloying potential. The study underlines the potential of electrochemical dealloying as a promising method for the preparation of tailor-made magnetic nanostructures.

Local magnetic cluster size identified by neutron total scattering in site-diluted spin glass Sn x Fe4−x N for x = 0.88

Local magnetic cluster size identified by neutron total scattering in site-diluted spin glass Sn_<i>x</i>Fe_4−<i>x</i>N for <i>x</i> = 0.88

Design of anti-site disorder for tunable spontaneous exchange bias: Mn-Ni-Al alloys as a case

A reasonable design not only accelerates the discovery of materials but also optimizes the performance of existing materials. Here, we report that microscopically the active design of the degree of anti-site disorder from 20% to 27% can be realized by tuning the Al content in Mn50Ni50-xAlx. Mesoscopically, the control of the pinning interface and the magnetic cluster size of the spin glass required for spontaneous exchange bias (SEB) are achieved. The P value reflecting the magnetic cluster size can be changed from 0.0067 to 0.0437. Finally, macroscopically, SEB in Mn50Ni50-xAlx (x = 10–15) is achieved, and the largest SEB field (1370 Oe) is obtained in Mn50Ni37Al13 with the largest degree of anti-site disorder and the smallest magnetic cluster size. Our results demonstrate that anti-site disorder can be not only an effective method for obtaining SEB but also a tool for stabilizing SEB in a wide elemental content window.

Magnetic study of Fe3O4/Ag nanoparticles

Nanocomposites of Fe3O4nanoparticles (NPs) impregnated with silver NPs display antibacterial properties and may be used in water treatment as disinfection agent. Three samples were synthesized: Fe3O4NPs obtained by the precipitation method and additionally two samples with added silver NPs with mass ratio of Ag:Fe3O4equal to 1:100 and 2:100. Magnetic properties of these samples were studied by SQUID magnetometry (in temperature range 2–300 K and magnetic fields up to 70 kG) and magnetic resonance technique at RT. Temperature dependence of dc susceptibility revealed the blocking temperature close to RT in all three samples and allowed to determine the presence of single or multi-mode distribution of NP sizes in a particular sample. Isothermal magnetisation measurements showed that the presence of silver NPs, especially those with smaller sizes, decreases the saturation magnetisation. The shape of ferromagnetic loop registered atT = 2 K was used to discuss the sizes of NP magnetic clusters in our samples. Conclusions obtained from analysis of the ferromagnetic resonance spectra were consistent with the propositions based on the magnetometric studies.

Preparation and characterization of (CuInTe2)1-x(TaTe)x solid solutions (0

Polycrystalline samples of the (CuInTe2)1-x(TaTe)x alloys system were prepared by the melt and anneal technique in the composition range 0 < x < 1. Products were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Thermal Analysis (DTA) and SQUID techniques. From XRD and SEM, it was found that the solubility of TaTe in CuInTe2 is around 10%; however, samples up to x = 2/3 are composed by a mean tetragonal CuInTe2-like phase with traces of Ta2Te3. Samples with x > 2/3 show diffraction patterns with several phases with poor crystallization. In addition, the tetragonal CuInTe2-like phase shows spinodal decomposition i.e. there are two phases, one Ta-rich and the other Ta-poor, with the same crystal structure and very close lattice parameters. SEM measurements also show up to six different phases, although they do not produce observed diffraction line in powder XRD patterns.DC magnetic susceptibility measurements as a function of temperature using the ZFC-FC protocol were performed in samples with x ≤ 2/3. With the exception of x = 2/3, all samples show a similar behavior in the ZFC and FC curves which are typical of a system with a distribution of magnetic cluster sizes. Sample x = 2/3 shows ferromagnetic behavior with Tc = 50 K. For sample x = 1/4, measurements of the magnetization as a function of the applied magnetic field and temperature were performed. From the analysis of the curves it was found that the clusters contain 105 Ta-atoms and a coercitive field of 0.22 kOe at T = 5 K.

Experimental determination of reader resolution using PRS patterns and application to media cluster size measurement

A technique to determine experimentally the reader resolution in magnetic recording by using pseudo-random sequence (PRS) patterns written on the disc is presented. The spectrum of the PRS signal is analyzed to extract simultaneously the width of the magnetic transitions and the reader sensitivity as a function of frequency. When playback spacing is increased, reader sensitivity rolls off faster with frequency, in good quantitative agreement with modeling results involving micromagnetic simulation of the reader. The reader sensitivity is further employed to de-convolve the magnetic cluster size from measured spectra of media noise.

Experimental Correlation between Nonlinear Optical and Magnetotransport Properties Observed in Au-Co Thin Films

Magnetic materials where at least one dimension is in the nanometer scale typically exhibit different magnetic, magnetotransport, and magnetooptical properties compared to bulk materials. Composite magnetic thin films where the matrix composition, magnetic cluster size, and overall composite film thickness can be experimentally tailored via adequate processing or growth parameters offer a viable nanoscale platform to investigate possible correlations between nonlinear magnetooptical and magnetotransport properties, since both types of properties are sensitive to the local magnetization landscape. It has been shown that the local magnetization contrast affects the nonlinear magnetooptical properties as well as the magnetotransport properties in magnetic-metal/nonmagnetic metal multilayers; thus, nanocomposite films showcase another path to investigate possible correlations between these distinct properties which may prove useful for sensing applications.

Manipulating cluster size of polyanion-stabilized Fe3O4 magnetic nanoparticle clusters via electrostatic-mediated assembly for tunable magnetophoresis behavior

We report in this article an approach for manipulating the size of magnetic nanoparticle clusters (MNCs) via electrostatic-mediated assembly technique using an electrolyte as a clustering agent. The clusters were surface-tethered with poly(sodium 4-styrenesulfonate) (PSS) through electrostatic compensation to enhance their colloidal stability. Dynamic light scattering was employed to trace the evolution of cluster size. Simultaneously, electrophoretic mobility and Fourier transform infrared spectroscopy analyses were conducted to investigate the possible schemes involved in both cluster formation and PSS grafting. Results showed that the average hydrodynamic cluster size of the PSS/MNCs and their corresponding size distributions were successfully shifted by means of manipulating the suspension pH, the ionic nature of the electrolyte, and the electrolyte concentration. More specifically, the electrokinetic behavior of the particles upon interaction with the electrolyte plays a profound role in the formation of the PSS/MNCs. Nonetheless, the solubility of the polymer in electrolyte solution and the purification of the particles from residual ions should not be omitted in determining the effectiveness of this clustering approach. The PSS adlayer makes the resultant entities highly water-dispersible and provides electrosteric stabilization to shield the PSS/MNCs from aggregation. In this study, the experimental observations were analyzed and discussed on the basis of existing fundamental colloidal theories. The strategy of cluster size manipulation proposed here is simple and convenient to implement. Furthermore, manipulating the size of the MNCs also facilitates the tuning of magnetophoresis kinetics on exposure to low magnetic field gradient, which makes this nano-entity useful for engineering applications, specifically in separation processes.

Extracting magnetic cluster size and its distributions in advanced perpendicular recording media with shrinking grain size using small angle x-ray scattering

We analyze the magnetic cluster size (MCS) and magnetic cluster size distribution (MCSD) in a variety of perpendicular magnetic recording (PMR) media designs using resonant small angle x-ray scattering at the Co L3 absorption edge. The different PMR media flavors considered here vary in grain size between 7.5 and 9.5 nm as well as in lateral inter-granular exchange strength, which is controlled via the segregant amount. While for high inter-granular exchange, the MCS increases rapidly for grain sizes below 8.5 nm, we show that for increased amount of segregant with less exchange the MCS remains relatively small, even for grain sizes of 7.5 and 8 nm. However, the MCSD still increases sharply when shrinking grains from 8 to 7.5 nm. We show evidence that recording performance such as signal-to-noise-ratio on the spin stand correlates well with the product of magnetic cluster size and magnetic cluster size distribution.

Direct Measurement of the Magnetic Cluster Size Using High-Resolution Magnetic Force Microscopy

High-resolution magnetic force microscopy was used to image high linear density tracks and measure the magnetic cluster size of the ac-erased state of perpendicular magnetic recording media. The full-width at half-maximum (FWHM) of the auto-correlation function corresponding to an ac-erased region determines the correlation length, i.e., the magnetic cluster size. This is demonstrated by comparing the calculated FWHM of auto-correlation function obtained on simulated Voronoi granular structures with the expected value for magnetically decoupled grains. The cluster size we obtained using the auto-correlation procedure is about four grain sizes. The impact of the finite cluster size on the recording performance was assessed by following the trend of tracks' auto-correlation function with increasing the linear density.

Dynamics of magnetic particles near a surface: Model and experiments on field-induced disaggregation

Magnetic particles are widely used in biological research and bioanalytical applications. As the corresponding tools are progressively being miniaturized and integrated, the understanding of particle dynamics and the control of particles down to the level of single particles become important. Here, we describe a numerical model to simulate the dynamic behavior of ensembles of magnetic particles, taking account of magnetic interparticle interactions, interactions with the liquid medium and solid surfaces, as well as thermal diffusive motion of the particles. The model is verified using experimental data of magnetic field-induced disaggregation of magnetic particle clusters near a physical surface, wherein the magnetic field properties, particle size, cluster size, and cluster geometry were varied. Furthermore, the model clarifies how the cluster configuration, cluster alignment, magnitude of the field gradient, and the field repetition rate play a role in the particle disaggregation process. The simulation model will be very useful for further in silico studies on magnetic particle dynamics in biotechnological tools.

Analysis of unswitched grains in thermally assisted magnetic recording

A simplified model of thermally assisted magnetic recording was used to study the influence of thermal gradients and the average grain size in recording media. The signal to noise ratio (SNR) peaked for thermal gradients between 10 and 20 K/nm and increased as the average grain size was reduced. However, the number of unswitched grains in written bits increased for smaller grains, limiting the increase in SNR. Exchange coupling decreased the probability of a grain remaining unswitched, but increased the magnetic cluster size, although the transition noise remained unaffected until the exchange coupling strength exceeded 0.8 erg/cm2.

Magnetic design evolution in perpendicular magnetic recording media as revealed by resonant small angle x-ray scattering

We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.

Magnetic Cluster Size “Knee” Analysis for Small Grain Continuous Media

By analyzing the magnetic cluster size of various media, we have investigated the variation of inter-granular exchange coupling with grain size. With the reduction of the grain size below 8.0 nm, higher inter-granular exchange coupled media showed a large increase in magnetic cluster size, while lower exchange media did not show as dramatic a cluster size change. This cluster size “knee” position was consistent with reduction in signal-to-noise ratio. To utilize small grain media for high density recording, it is desirable to push this “knee” position toward smaller grain size.