Anisotropy Field Distribution Research Articles

Extremely low coercivity of powder Co-rich amorphous alloy obtained by gas atomisation

A novel Co-base soft magnetic powder with ultra-low coercivity was produced by inert gas atomisation. The as-atomised powder is fully amorphous owing to its high glass forming ability and the proper selection of atomisation conditions; it exhibits a very low coercivity (0.130 Oe) mainly due to the low magnetostriction coefficient of Co-based alloys. After production, the powder was annealed at different temperatures between 300 and 600 ºC for 30 minutes. Crystallisation started at around 575 ºC, which is the temperature of the first exothermic peak detected by differential scanning calorimetry. Annealing the powder at temperatures below 550 ºC develops only structural relaxation of the amorphous structure. The smallest coercivity (0.056 Oe) was found in the sample annealed at 400 ºC. On the other hand, an increment of the coercivity of four orders of magnitude occurred after full crystallisation at 600 ºC (394.320 Oe). From the analysis of the curves of anisotropy field distribution, it can be concluded that this Co-base alloy shows lower average anisotropy field, a more gaussian shape and a wider distribution than Fe-base alloys. This is explained by a lower magnetoelastic anisotropy and a more homogeneous distribution of the magnetic anisotropy. The powder exhibits a spherical shape that makes it very suitable as ferromagnetic phase to manufacture powder cores with extremely low hysteresis loss for medium-high frequency applications.

A novel coercivity enhancement method of Nd-Ce-Fe-B magnets by adjusting anisotropy field distribution and magnetostatic field distribution

In order to give full play to the potential of Nd-Ce-Fe-B magnets and increase the coercivity of magnets with a fixed average composition, the influence of the cooperation effect of anisotropy field and magnetostatic field on their coercivity was studied. Micromagnetic simulations revealed that neither the most heterogeneous composition distribution nor the most homogeneous composition resulted in the highest coercivity. This is due to the variation in a critical field H crit which quantifies the cooperation effect of anisotropy field and magnetostatic field. By subtly changing the composition distribution of the Nd-Ce-Fe-B magnets, this work provides a new perspective for further improvement of the coercivity. This adjustment modifies the anisotropy field distribution and the magnetostatic field distribution.

Thermal stability and magnetic anisotropy dispersion in high-density hard-disk media

Abstract A method to correct the anisotropy field distribution for granular recording media, measured by switching methods (isothermal remanent magnetization (IRM) and direct-current demagnetisation (DCD) processes were used as an example) for the effects of thermal relaxation, that supposes an a priori knowledge of the grain size distribution, is presented. For a hard-disk medium, it is shown that the anisotropy field distribution is much narrower (about one order of magnitude) with respect to the grain size distribution. The anisotropy field value estimated from the dependence of the remanence coercivity on the field application time is found in agreement (to within less than 10 %) with the most probable value of the anisotropy field distribution.

Magnetic hysteresis of blocked ferrihydrite nanoparticles

Using minor hysteresis loops in the Stoner–Wohlfarth model allows describing the experimental behavior of the coercive force of minor hysteresis loops in ferrihydrite nanoparticles with a change in the field amplitude. The description allows estimating the parameters of the distribution of the magnetic anisotropy field in nanoparticles. The best agreement of the anisotropy fields estimated by different approaches is achieved for the assumption of uniaxial anisotropy in ferrihydrite nanoparticles.

Structural, magnetic and electrical investigations of Fe1─XMnX (0 ≤ x ≤ 0.39) alloy nanowires via electrodeposition in AAO templates

The Fe1─XMnX (0 ≤ x ≤ 0.39) nanowire (NW) arrays were synthesized through anodic aluminum oxide (AAO) template-based electrodeposition method. The elemental composition of the NWs was varied by changing the Mn-ions concentration in the electrochemical bath during electrodeposition. Energy Dispersive X-rays Spectroscopy (EDX) spectrums confirmed the Mn contents, that was increased in the NWs from 0 to 39 with respect to increase in Mn-ion concentration in the electrolyte. The X-ray diffraction (XRD) patterns exhibited the NWs were crystallized into the body-centered cubic (bcc) structure. Furthermore, the lattice parameter of NWs decreased as Mn content increased in the NWs whereas the increasing trend in grain size was observed. The magnetic properties of NWs were determined by recording MH-loops at room temperature with applied magnetic field in parallel (∥) and perpendicular (⊥) direction to the long-axis of NWs. The MH-loops revealed the existence of strong magnetic anisotropy and anisotropy field (Hk) that was determined by the Anisotropy Field Distribution (AFD) method. A decreasing trend in the coercivity (Hc), saturation magnetization (Ms) and anisotropy field (Hk) was observed as a function of Mn contents in FeMn NWs. The ZFC-FC curves were also recorded with 1kOe applied magnetic field, which demonstrated the ferromagnetic behavior and exhibited the blocking temperature was increased (Fe NWs ~ 300 K, Fe0.82Mn0.18 NWs ~ 330 K, and Fe0.61Mn0.39 NWs > 350 K) with the increase of Mn content. Moreover, the conductivity and mobility of the NWs were also decreased that were analyzed through four-probe Hall techniques.

Composition-driven crystal structure transformation and magnetic properties of electrodeposited Co–W alloy nanowires

Abstract The cobalt (Co)–tungsten (W) alloys exhibit unique combinations of mechanical and magnetic properties, biocompatibility, resistance against corrosion, wear, and high-temperature, which makes them desirable materials for various practical applications. A nanoporous template with incorporated Co–W alloy nanowires is a soft magnetic composite, whose dielectric and magnetic properties can be tuned through the host material, pore distribution and size, Co–W composition and crystal structure, and geometry of the nanowires. Here, we report the composition-dependent structural and magnetic properties of Co–W alloy nanowires embedded in alumina templates by electrodeposition. The addition of W transforms cobalt from the crystalline hexagonal-close-packed (hcp) Co to a mixed nanocrystalline/amorphous-like Co(W) solid solution with ferromagnetic behavior and composition similar to that of the weakly magnetic Co3W compound. The combination of the approach to magnetic saturation, anisotropy field distribution method, micromagnetic simulations, and first-order reversal curve diagram identification method elucidates the structure-driven magnetization reversal processes in both individual nanowires and magnetostatically coupled array as a whole.

Effect of Mn (12 at%) substitution on magnetic anisotropy and magnetization reversal of equiatomic FeCo alloy nanorod arrays

The effect of manganese (Mn) substitution on magnetic properties of equiatomic FeCo (1:1) alloy nanorod arrays was investigated by comparatively analyzing Fe50Co50 and Fe45Co43Mn12 nanorod arrays having diameter and length about 50 nm and 250 nm respectively. The Fe45Co43Mn12 nanorod arrays were also annealed under vacuum at 400 °C, 500 °C, and 600 °C to further improve their properties. The Fe50Co50 and Fe45Co43Mn12 nanorod arrays showed body-centered cubic (bcc) structure while after annealing at 600 °C a hexagonal close pack (hcp) Co phase was also induced in Fe45Co43Mn12 nanorod arrays. The effective anisotropy field (Hk) was determined through anisotropy field distribution method and magnetization reversal was examined by angular dependence of coercivity (Hc). The angular dependence of Hc demonstrated that the magnetization reversal in Fe50Co50 nanorods occurred via coherent rotational mode while magnetization reversal in Fe45Co43Mn12 nanorods occurred by the combination of both coherent rotational and curling mode.

Tailoring the magnetization linearity of Finemet type nanocrystalline cores by stress induced anisotropies

In this paper a theoretical and experimental investigation will be presented on the linearity of the flat hysteresis loop obtained by continuous stress annealing of Finemet type nanocrystalline ribbon. The stability of the effective permeability versus the magnetizing field depend on the residual random distribution of local magnetization and can be characterized by i) the coefficient a in the expression of magnetization approaching the saturation: M/Ms = 1 − a/H, valid in a restricted region, for Hlin < H < Hsat, where Hlin is the limit of linearity and by ii) the distribution of the anisotropy field ΔHK. The theoretical upper limit of linearity is given by the anisotropy field HK = Bs/µo.µeff. The linearity limit, Hlin, can be expressed as a difference between the value HK and the half width of the anisotropy field distribution, ΔHK. The linearity will be measured by the ratio R = Hlin/HK. The parameter R is almost constant (around 0.72) for large applied stresses and carefully selected annealing parameters (furnace geometry, temperature distribution along the furnace and pulling velocity). For small applied stresses (i.e. for effective permeability’s above 4000) however this linearity parameter is reduced to zero and a potbellied loop appears. A possible explanation will be given for the stress dependence of linearity parameter based on the back stress model of stress annealing.

Influencing mechanisms of atomic diffusion and compositional distribution on the magnetic anisotropy of Cr/SmCo/(Cu)/Cr thin films

Rare earth (RE) permanent magnetic thin films, such as SmCo-based films, are promising candidates for future thermal-assisted magnetic recording media because of their supreme thermal stability and fine superparamagnetic critical size. Phase compositional distributions and crystallographic orientations can directly influence the magnetic domain evolutions and magnetic performance of SmCo films. However, these films often exist in multi-phases without well-defined distinct magnetic anisotropy, thereby causing difficulties in analyzing their functions. The effects of atomic diffusions induced by annealing on crystallization and magnetic anisotropy energy are poorly understood. In this work, the influence of doping Cu atoms via introducing Cu layer on magnetic anisotropy of Cr/SmCo/(Cu)/Cr films is investigated. Introducing a Cu layer and post-annealing lead to tunable magnetic anisotropy from isotropy to anisotropy and domain structure formation. Micromagnetic simulation is used to further analyze the effects of crystallization, anisotropy field and compositional distribution on the magnetization reversal behavior and coercivity variations in SmCo films. It reveals that the differences of anisotropy field of Co and amorphous phases could affect the domain wall motions and coercivity, which shows the analogous trend as an experimental result. Moreover, the elevated anisotropy constant of Sm(Co,Cu) alloy and the small fractions of SmCo5 boundary phases are also beneficial to the enhancement of coercivity derived by calculating domain wall energy. In-plane magnetic anisotropy is improved by the SmCo5 phase, which has an in-plane preferred orientation of the c axis. This work provides theoretical and experimental bases for the future fabrication of SmCo-based films by controlling atomic diffusion with optimized grain boundary phase distribution.

Effect of Recording Conditions on the Downtrack Thermal Gradient in Heat-Assisted Magnetic Recording

Downtrack thermal gradient (DTG) is calculated for heat-assisted magnetic recording (HAMR) systems in both spin-stand experiment and micromagnetic model. DTG is an important parameter in HAMR. It describes the change in temperature in the vicinity of the write position. Recording conditions and media properties affect the switching process, which in turn affects the measured DTG. Trends in DTG versus different recording conditions such as linear velocity and applied field showed very good agreement between experiment and model. The model is also used to quantify the effect on DTG from parameters that are difficult to measure, such as Curie temperature distributions, anisotropy field distributions, and curvature. Finally, a breakdown of the contribution of the different conditions and properties considered on DTG is presented.

Magnetic Properties of Annealed Amorphous Fe72.5Si12.5B15 Alloy Obtained by Gas Atomization Technique

In this paper, we report the effect of thermal treatment at different temperatures (523, 623, and 723 K) on the magnetic properties of an amorphous powder with composition Fe72.5Si12.5B15 obtained by gas atomization. Differential scanning calorimetry curves indicate that a significant structural rearrangement takes place during annealing before crystallization. As expected, the intrinsic coercivity decreases with the annealing temperature, being the most significant drop from 3.26 to 0.44 Oe for the powder with a particle size $ , non-annealed, and annealed at 723 K, respectively. An analysis of the effect of annealing on the anisotropy field distribution obtained directly from the magnetization curve is presented for both the non-annealed and the annealed sample with the lowest value of coercive field. The probability density function of the anisotropy field exhibits a lower applied field value at the maximum and is better fit by a Gaussian distribution after the heat treatment, which is a sign of better amorphicity after structural relaxation.

Systematic Evaluation of Microwave-Assisted Magnetic Recording

Microwave Assisted Magnetic Recording (MAMR) is a type of energy assisted recording technology that uses microwaves to assist in the recording process. This allows for the use of high anisotropy media to maintain good thermal stability at high areal densities [1]. In MAMR, a spin torque oscillator (STO) is placed between the main pole and trailing shield to generate a magnetic field at microwave frequencies. The magnetic recording trilemma [2] is overcome by reducing the coercive field of the media via this microwave field, thereby aiding in the switching of media grains. Appropriate media stack configurations for MAMR and STO optimization have been investigated by both analytical theory and micromagnetic simulation [3], [4], [5]. In this paper, MAMR performance is evaluated on a reference design using our MAMR micro-magnetic model and the results are reported using EWSNR metrics [6]. In this study, we compare performance metrics for MAMR with other recording technologies such as Perpendicular Magnetic Recording (PMR) and Heat Assisted Magnetic Recording (HAMR). Recording media dynamics are modeled using the Landau-Lifshitz-Gilbert (LLG) equation for PMR and MAMR, and utilizing the renormalized LLG method for HAMR [7]. The DC magnetic write field is a current generation PMR writer design with 55 nm physical pole width and 1.0 T peak field strength. The STO stack has an optimized geometry that applies a 3D circularly polarized AC field to the media. STO field strength (∼0.1Hk) and oscillating frequency (∼35GHz) are determined by the magnetic properties of the field generation layer (FGL) and the magnitude of the injected spin current. The magnetic head to media spacing (HMS) is fixed at 6.0 nm and head velocity is modeled to be 20 m/s. For MAMR, single layer media with varying anisotropy are considered. The PMR ECC multilayer media model, which is calibrated based on current PMR products, contains multiple magnetic layers and non-magnetic break layers that provide optimal exchange coupling. The HAMR media is a single layer L1 0 FePt media with a Curie temperature distribution of 3% and anisotropy field distribution of 10%. The down track thermal gradient used in the HAMR model is around 8K/nm, which is consistent with common near field transducer designs. The average media grain size is around 8.0 nm with 17% grain size distribution. A magnetoresistive reader with 30 nm width is used for obtaining the play back signal. Magnetic information is encoded using pseudo random bit sequences (PRBS), which mimic real user data. Figure 1 shows a comparison between PMR, HAMR and MAMR with thick free layer designs in terms of common performance metrics. Ensemble waveform analysis is used to calculate the total spatial SNR, the breakdown between transition and remanence SNR contributions, and the channel bit density (CBD) [6]. Bit error rate (BER) is calculated using a pattern dependent Viterbi detector [8]. The use of an NFT in HAMR allows the recording of much narrower tracks than PMR, with similar CBD and reasonable SNR and BER. In general, MAMR exhibits a large CBD, which may be related to intersymbol interference and track edge erasure caused by the demagnetization field. This high CBD results in much worse BER than both HAMR and PMR at all track width and linear density combinations considered. For the same ADC (shown in the last two table items in figure 1), SNR and BER are much better when the linear density is higher and the track width is larger. Since track edge demagnetization effects seem to have such a large effect in MAMR, this data suggests MAMR should aim to increase ADC via higher linear density and lower track density. For thick free layers, multi domains and multi-scattering contributions to electron’s propagation can greatly deteriorate STO performance. Therefore, SNR versus recording media Hk for MAMR with thin free layer designs below 15 nm and STO width of 40 nm is shown in Figure 2. The figure suggests an optimal for MAMR media of around 25 kOe. MAMR SNR degradation at 20 kOe is due to erasure effects for low Hk material [9]. For high anisotropy media ∼30 kOe, the transition SNR is reasonable (around 12 dB), but remanence noise is high, causing a 2 dB loss in total SNR.

Effect of Field Annealing Induced Magnetic Anisotropy on the Performance of Meander‐Core Orthogonal Fluxgate Sensor

The magnetic anisotropy field distribution is discussed for Co‐based amorphous ribbon annealed under different field conditions leading to different induced anisotropies: longitudinal field (LF) and transversal field (TF) annealing. In order to understand the relation between the field annealing‐induced anisotropy and the performance of meander‐core orthogonal fluxgate sensor, field‐annealed ribbons with different in‐plane anisotropies as cores of orthogonal fluxgate sensors are employed and both noise and sensitivity in fundamental mode are measured. Experimental results indicate that LF annealing‐induced anisotropy with easy axis perpendicular to the direction of excitation field can effectively improve the sensor sensitivity. On the other hand, TF annealing‐induced anisotropy with easy axis parallel to excitation field results in lower sensitivity, but the noise is also lower. The results obtained are useful for developing the high‐performance orthogonal fluxgate sensors based on amorphous ribbon cores.

High frequency permeability of Fe-Cu-Nb-Si-B nanocrystalline flakes with the distribution of shape anisotropy fields

Fe-Cu-Nb-Si-B flakes with multiphase nanostructures have been obtained by annealing the amorphous ribbon and subsequently ball milled for 30 h. The crystal structures have been examined by X-ray diffraction pattern and Mössbauer spectrum. The results show that the particles annealed at 900 °C are made up of amorphous ferromagnetic phase, α-Fe3Si ferromagnetic phase and Fe2B phase, and the average hyperfine magnetic field (HBhf) of particles is 24.02 T. Meanwhile, the relationships between the structure and the high frequency permeability have been studied. Compared with particles annealed at 600 °C, particles annealed at 900 °C exhibit higher saturation magnetization, which is evidenced by the larger HBhf. Also, three magnetic loss peaks in a permeability spectrum have been observed for the particles annealed at 900 °C. The natural resonance frequencies are calculated, which are in good agreement with the experimental resonance peaks. The origin of the multiple magnetic loss peaks can be explained from the perspective of the distribution of shape anisotropy fields which is caused by multiple phase structure.

Magnetic behavior of NiO nanoparticles determined by SQUID magnetometry

NiO nanoparticles (4 nm–80 nm diameter) are investigated by SQUID magnetometry. Hysteresis loops at various temperatures and susceptibility versus temperature measurements using field-cooled/zero-field cooled protocols at various magnetic fields are performed. The measurements give evidence for a strong increase of magnetic moments with decreasing size of NiO nanoparticles. From the dependence of the magnetic moment on temperature and external field, blocked superparamagnetism is observed superimposed to the core antiferromagnetism of NiO-particles. A quantitative estimate demonstrates that uncompensated surface spins contribute to a ferromagnetic-like hysteresis modelled by a broad distribution of anisotropy fields at the surface induced by the antiferromagnetic core.

The effect of film thickness on Curie temperature distribution and magnetization reversal mechanism for granular L10 FePt films

Structural and magnetic properties of granular L10 FePt films are investigated as a function of film thickness in the range 3–11 nm. Scanning transmission electron microscopy observations indicate an increasing contribution of in-plane variants to the L10 FePt crystalline structure with increasing film thickness, which may explain the noticed degradation of the perpendicular anisotropy. The temperature dependence of coercivity suggests a progressive deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm, corresponding to a domain-wall width. The anisotropy field distribution (〈HK〉, ) is estimated using Monte Carlo (MC) simulations of AC transverse susceptibility (ACTS). The average anisotropy field 〈HK〉 displays a similar temperature dependence to (MS)1.1, where MS is the film saturation magnetization, consistent with the theoretical predictions for L10 FePt. The Curie temperature distribution (〈TC〉, ) is evaluated using two methods: fast (ns) thermal erasure (ThEr) of remanent magnetization and MC modeling of the temperature dependence of AC susceptibility (ACS). The values evaluated by the two methods are in agreement, indicating an increase of in the range 2–6% with decreasing film thickness. The observed increase of 〈TC〉 in the range 620–680 K with increasing film thickness is explained using the finite-size scaling law with the critical exponent set to the theoretical value for L10 FePt. A general procedure to extract 〈TC〉 and using MC modeling of ACS, essentially independent of the magnetization reversal mechanism, is proposed. MC simulations of ACS confirm the deviation of the magnetization reversal mechanism from coherent rotations with increasing film thickness above ~6 nm.

Review Article: FePt heat assisted magnetic recording media

Heat-assisted magnetic recording (HAMR) media status, requirements, and challenges to extend the areal density (AD) of magnetic hard disk drives beyond current records of around 1.4 Tb/in.2 are updated. The structural properties of granular high anisotropy chemically ordered L10 FePtX-Y HAMR media by now are similar to perpendicular CoCrPt-based magnetic recording media. Reasonable average grain diameter ⟨D⟩ = 8–10 nm and distributions σD/D ∼ 18% are possible despite elevated growth temperatures TG = 650–670 °C. A 2× reduction of ⟨D⟩ down to 4–5 nm and lowering σD/D &amp;lt; 10%–15% are ongoing efforts to increase AD to ∼4 Tb/in.2. X = Cu ∼ 10 at. % reduces the Curie temperature TC by ∼100 K below TC,bulk = 750 K, thereby lowering the write head heat energy requirement. Multiple FePtX-Y granular layers with Y = 30–35 vol. % grain-to-grain segregants like carbides, oxides, and/or nitrides are used to fully exchange decouple the grains and achieve cylindrical shape. FePt is typically grown on fcc MgO (100) seedlayers to form well oriented FePt (002). A FePt lattice parameter ratio c/a ∼0.96 and high chemical order S &amp;gt; 0.90 result in magnetic anisotropy KU ∼ 4.5 × 107 erg/cm3, and only 25% below the FePt single crystal value KU = 6.6 × 107 erg/cm3 has been achieved in 7–8 nm diameter grains. Switching field distributions depend on anisotropy field (HK) distributions, which are currently of the order of ΔHK/HK ∼ 10% (ΔHK ∼ 10–12 kOe, HK ∼ 10–11 T) at room temperature. High thermal conductivity heat sink layers, including Ag, Au, Cu, and Cr, are used to optimize the cooling rate and maximize the down- and cross-track thermal gradient, which determines the achievable track density.

Surface magnetization and the role of pattern defects in various types of ripple patterned films

We present a detailed study of the magnetic properties of cobalt films with wide-area nanoscale ripple patterns, either on their surface only, or on both the film surface and substrate interface. Angular dependence vectorial-resolved magnetometry measurements and magnetic force microscopy with in situ magnetic field have been used to determine the magnetization reversal processes to correlate them to the different patterned nanostructures. All the samples show well-defined uniaxial magnetic anisotropy with the anisotropy axis lying along the ripple direction. Atomic force microscopy of the different types of pattern reveals various pattern defects: height corrugation and breaks of continuity along the ripple direction, and overlapping ripples and Y-shaped defects (pattern dislocation) across the pattern. In spite of the existence of such customary defects of erosive-regime patterns, the type of low-amplitude, surface-patterned films remarkably behave as a macrospin over almost the whole in-plane angular range (340°), with negligible spread of anisotropy axis or energy. In turn, it is found that high-amplitude surface-patterned films develop an angular distribution of anisotropy axes, probably related to the large distribution of amplitudes in a pattern of short ripples, and a significant distribution of anisotropy fields ΔHk/Hk up to 15%. On the other hand, films grow on pre-patterned silicon with a significantly longer mean ripple length, and develop a larger anisotropy energy with Hk up to 110 mT, probably because of the double interface effect. The switching fields close to the magnetization easy axis of all types of ripple pattern are not well reproduced by the macrospin approximation, but the observed pattern defects seem to be not responsible for the domain wall pinning that occurs with the field applied along the ripple direction.

Anisotropy field distribution in soft magnetic Hitperm alloys submitted to different field annealing processes

The magnetic anisotropy field distribution is discussed for Hitperm alloys annealed under different field conditions leading to different induced magnetic anisotropies: zero (ZF), transversal (TF), and longitudinal (LF) field annealing and compared to that of as-quenched (AQ) melt-spun amorphous ribbon. In order to accurately use the present method, the demagnetizing factor has been obtained by analyzing the field dependence of the inverse of the field derivative of the magnetization. The coherence of the analysis is supported by testing the normalization of the complete distribution of anisotropy fields. Independently of the composition, two groups can be distinguished among the studied samples: those with mainly perpendicular anisotropy field contributions (ZF and TF samples) and those with mainly longitudinal anisotropy field contributions (LF and AQ samples). Behavior of TF samples is well reproduced using Stoner–Wohlfarth model and, in the case of as-quenched amorphous samples, the anisotropy field depends almost linearly on the thickness of the ribbon.

Studies on the microwave permittivity and electromagnetic wave absorption properties of Fe-based nano-composite flakes in different sizes

The effective permittivity of composites containing Fe-Cu-Nb-Si-B nanocrystalline micro flakes has been studied within 0.5–10 GHz. Obvious differences in microwave permittivity have been observed for composites consisting of large flakes (size range: 23–111 μm, average thickness: 4.5 μm) and small flakes (size range: 3–21 μm, average thickness: 1.3 μm). Both the real part and imaginary part of permittivity of large flake composite are much larger than these small one in a given frequency. And faster decrease of permittivity with the increasing frequency can be observed for large flake composite than that of small one. These differences in permittivity spectra of different flakes have been explained from the perspective of interfacial polarization and ac conductivity. The assumption that more extensive ohmic contact interface between large flakes and matrix has been validated by the fittings and the calculated percolation threshold. Meanwhile, the permeability spectra of both composites also have been studied by Lorentzian dispersion law. The broadened spectra can be attributed to the distribution of magnetic anisotropy fields of two kinds of ferromagnetic phases in the particles. Finally, the composite containing the small flakes exhibits better electromagnetic wave absorption properties.