Intergranular Magnetic Coupling Research Articles

In situ construction of complex spinel ferrimagnet in multi-elemental alloy for modulating natural resonance and highly efficient electromagnetic absorption

The contradiction between the demand for highly efficient low-frequency electromagnetic wave (EMW) absorption materials and the relatively poor permeability of existing absorbers has become increasingly prominent. However, the intrinsic natural resonance in magnetic material hinders the modulation and enhancement of permeability. Herein, (FeCoNiCu)xCr3−xO4 complex spinel ferrimagnets (CSFMs) are constructed in situ in multi-elemental alloy powders (MEAs) for the first time, via annealing treatment. The microstructure and composition of the CSFMs are regulated by adjusting the annealing temperature, aiming to modulate the natural resonance frequency and achieve highly efficient megahertz (MHz) electromagnetic absorption. Simultaneously, annealing treatment improves the crystal integrity, which is conducive to enhancing intergranular magnetic coupling to elevate permeability temperature stability and the Curie temperature. The minimum reflection loss value of the MEAs/CSFMs composites is less than −7 dB at 3 mm and the effective EMW absorption bandwidth maintains between 425 and 470 MHz at 5 mm from −50 to 150 °C. In addition, the MEAs/CSFMs composites also exhibit excellent oxidation resistance and corrosion resistance due to the oxide layer on the surface. This work offers a new and tunable strategy toward modulating the natural resonance frequency for efficient temperature-stable broadband EMW absorption at specific frequency bands.

FeCoNiCr0.4CuX High-Entropy Alloys with Strong Intergranular Magnetic Coupling for Stable Megahertz Electromagnetic Absorption in a Wide Temperature Spectrum.

Electromagnetic (EM) absorbers serving in the megahertz (MHz) band and a wide temperature range (from -50 to 150 °C) require high and temperature-stable permeability for outstanding EM absorption performance. Herein, FeCoNiCr0.4CuX high-entropy alloy (HEA) powders with a unique nanocrystalline structure separated by a thin amorphous layer (NTA) are designed to improve permeability and enhance intergranular coupling. Simultaneously, the long-range anisotropy is introduced via devising the preparation process and tuning the chemical composition, such that the intergranular exchange interaction is further strengthened for stable permeability and EM wave absorption in a wide temperature range. FeCoNiCr0.4Cu0.2 HEAs exhibit a near-zero permeability temperature coefficient (5.7 × 10-7 °C-1) a in wide temperature range. The maximum reflection loss (RL) of FeCoNiCr0.4Cu0.2 HEAs is higher than -7 dB with 5 mm thickness at -50-150 °C, and the absorption bandwidth (RL < -7 dB) can almost cover 400-1000 MHz. Furthermore, FeCoNiCr0.4Cu0.2 HEAs also have a high Curie temperature (770 °C) and distinguished oxidation resistance. The permeability temperature dependence of FeCoNiCr0.4CuX HEAs is investigated in-depth in light of the microstructural change induced by tuning the chemical composition, and a new inspiration is provided for the design of magnetic applications serving in wide temperature, such as transformers, sensors, and EM absorbers.

Influence of magnetic domain wall orientation on Barkhausen noise and magneto-mechanical behavior in electrical steel

We investigate the influence of magnetic domain configurations on the magnetic Barkhausen noise (MBN) and magneto-mechanical behavior of electrical steel. Micro-macro magnetic properties from individual crystallographic grains with different domain wall (DW) orientations under different tensile stresses are measured. We show a strong dependency of the MBN activity and magneto-mechanical behavior on the DW orientation. Time evolution and statics of magnetic domain states are elaborated for the analysis of the magnetization processes, revealing an interaction between the stress-induced demagnetizing effect, the magneto-elastic behavior and the DW dynamics. The variation of supplementary domains under tensile stress is directly connected to the demagnetizing effect, further affecting the MBN activity. In addition, the influence of magnetic domain configurations in the neighboring grains on the DW dynamics and MBN jumps is studied. From this, the dipolar-dipolar and intergranular magnetic coupling interactions are derived. The results provide substantial microscopic insight for MBN and magneto-mechanical behavior analysis.

ROLE OF MICROSTRUCTURE AND DOMAIN STRUCTURE ON THE SOFT MAGNETIC PROPERTIES OF MAGNETIC FIELD ANNEALED Fe89-xZr11Bx ALLOYS

We report the evolution of microstructure, domain structure, and soft magnetic properties of amorphous and nanocrystalline Fe 89-x Zr 11 B x(x = 0 - 10) alloys. High-resolution electron microscopy observations reveal that the annealed alloys exhibit a two-phase microstructure. Addition of B enhances the ferromagnetic properties of Fe – Zr amorphous phase in the two-phase structured microstructure, resulting in good soft magnetic properties. Large-sized domains with smooth domain walls are observed in the alloys annealed below 873 K, which exhibit excellent magnetic softness. On the other hand, in the alloys annealed above 873 K, small-sized domains with irregular domain walls and domain wall pinning by Fe – Zr compound are seen. The soft magnetic properties in Fe – Zr – B alloys not only depend on mean grain size, but also on the strength of the intergranular magnetic coupling and structural inhomogeneities.

Nanocap arrays of granular CoCrPt:SiO2 films on silica particles: tailoring of the magnetic properties byCo + irradiation

An approach for tailoring the magnetic properties by ion irradiation of granular perpendicularCoCrPt:SiO2 films grown on silica particles with sizes down to 10 nm was investigated. The as-preparedsamples reveal an intriguing scaling dependence of the coercive field and remnantmagnetization: both parameters are found to decrease with decreasing particle size. However,Co + irradiation at alow fluence of 0.5 × 1014 cm − 2 already results in an opposite scaling behavior. It is assumed that this modification is dueto the enhancement of the intergranular magnetic exchange coupling of the granularCoCrPt:SiO2 filminitiated by Co + irradiation resulting in a modified reversal behavior. Further increase of the irradiation fluence beyond1.6 × 1014 ions cm − 2 leads to a degradation of the magnetic layer properties, lowering the remnantmagnetization and the coercive field in the easy-axis direction. Moreover, the localmagnetic properties of the samples were analyzed by magnetic force microscopy revealingmagnetic multi-domain cap structures.

Preliminary study on (CoPtCr/NiFe)-SiO/sub 2/ hard/soft-stacked perpendicular recording media

Hard/soft-stacked perpendicular recording media were proposed in order to improve recording writability without notable changes in thermal stability and noise performance. A preliminary study on stacked media consisting of a 10-nm magnetically hard CoPtCr-SiO/sub 2/ layer underneath a soft NiFe-SiO/sub 2/ layer (0-4 nm) was performed. Transmission electron microscope images revealed that NiFe/CoPtCr stacked grains were well isolated by SiO/sub 2/ at grain boundaries, similar to the isolation of single layer CoPtCr-SiO/sub 2/ media. No change was observed in the slope of the hysteresis loop on stacking the NiFe-SiO/sub 2/ layer, suggesting there was no significant change in intergranular magnetic coupling. The switching field decreased by 2 kOe as the NiFe-SiO/sub 2/ layer thickness increased from 0 to 3 nm, without a notable reduction in the stabilizing thermal energy barrier. Moreover, the angular dependence of remanence coercivity indicates that magnetization reversal occurs by coherent rotation in (CoPtCr/NiFe)-SiO/sub 2/ stacked media as predicted theoretically, and no significant difference in switching mechanism from single layer CoPtCr-SiO/sub 2/ media was observed. We successfully demonstrated that the hard/soft-stacked media show a better recording writability with no notable change in signal-to-media-noise ratio or thermal stability.

Growth and characterization of BSTO/hexaferrite composite thin films

Ferroelectric/ferrite composite films are excellent systems in which the electromagnetic material properties can be tuned through changes in composition as well as microstructure. We report on our studies of the structural and magnetic properties of Ba0.5Sr0.5TiO3/BaFe12O19 (BSTO/BaM) composite thin films deposited on Al2O3 substrates using magnetron sputtering. Optimizing the sputtering conditions and postdeposition annealing lead to high-quality films without any impurity phases. Magnetic measurements yield saturation magnetization values in the range of 100 to 200 emu/cm3. Hysteresis loops exhibit a double transition that is observed only in the composite films and not in pure hexaferrite films grown under identical deposition conditions. The unusual feature in the M–H data is ascribed to the possibility of magnetodielectric effects associated with intergranular magnetic coupling mediated by the BSTO polarization layer.

Small-angle neutron scattering measurements of magnetic cluster sizes in magnetic recording disks

We describe small-angle neutron scattering measurements of the magnetic cluster size distributions for several longitudinal magnetic recording media. We find that the average magnetic cluster size is slightly larger than the average physical grain size, that there is a broad distribution of cluster sizes, and that the cluster size is inversely correlated to the media signal-to-noise ratio. These results show that intergranular magnetic coupling in these media is small and they provide empirical data for the cluster-size distribution that can be incorporated into models of magnetic recording.

Effect of intergranular magnetic coupling on soft magnetic and magnetotransport properties in nanocrystalline materials

The soft magnetic and magnetotransport properties of nanocrystalline Fe–Zr–B–Cu–Ge and Fe–Zr–Cu–Ru alloys have been studied. Our study demonstrates that modification of the chemistry of the residual amorphous phase is an effective approach to adjusting the stiffness of the intergranular magnetic coupling and thereby tailoring the soft magnetic and magnetotransport properties.

Thermodynamic calculations of the effect of B and Ta on magnetically induced phase separation in Co–Cr–Pt alloys

In order to clarify the relation between the magnetically induced phase separation and the recording media characteristics, the thermodynamic calculations of Co–Cr–Pt–B and Co–Cr–Pt–Ta systems have been carried out by using the available binary assessment data and Miedema’s semiempirical values. B is segregated to the boundary in a similar manner as Cr, which makes the boundary region paramagnetic. This result is consistent with available data that B weakens the intergranular magnetic coupling and increases the magnetic anisotropy in Co–Cr–Pt recording media. By adding Ta, the Cr content in the paramagnetic phase is also increased, reducing the intergranular magnetic coupling. However, the Ta content in the ferromagnetic phase is higher than in the paramagnetic phase, decreasing the magnetic anisotropy. Accordingly, the thermodynamic calculations successfully explain experimental magnetic data for Co–Cr–Pt–B and Co–Cr–Pt–Ta recording media.

Magnetoresistance of nanocrystallized amorphous Fe–Zr–(Ru) alloys

We have studied the magnetic and magnetotransport properties of melt-spun Fe 91Zr 8Cu 1 and Fe 81Zr 8Cu 1Ru 10. The Fe 91Zr 8Cu 1 alloy after nanocrystallization at 753 K for 14.4 ks shows conventional anisotropic magnetoresistance (MR). On the other hand, spin-dependent MR (Δ ρ≈−0.04 μΩ cm) is confirmed for the Fe 81Zr 8Cu 1Ru 10 alloy after the same nanocrystallization treatment. Our 57Fe Mössbauer results indicate that the residual amorphous phase in the Ru-free alloy is ferromagnetic, whereas that in the Ru-doped alloy is paramagnetic. The spin-dependent MR in Fe 81Zr 8Cu 1Ru 10 is considered to be due to the inhibition of the intergranular magnetic coupling by this paramagnetic amorphous phase. Our study shows that the magnetotransport in BCC-Fe/amorphous two-phase nanocrystalline systems can be spin-dependent.

Intergranular Magnetic Coupling and Coercivity in Two-Phase Nanocrystalline Materials.

Bcc/amorphous two-phase nanocrystalline samples with various volume fractions of the amorphous phase were prepared from a melt-spun precursor amorphous Fe91Zr7B2 alloy. The coercivity of the sample at 77 K, well below the Curie temperature of the intergranular amorphous phase (Tcam ≈ 370 K), shows a tendency to decrease with increasing the volume fraction of the amorphous phase (Vam). This suggests that the magnetocrystalline anisotropy energy of the sample is diluted by the volume of the residual amorphous phase when the intergranular coupling is stiff. On the other hand, the coercivity at 450 K, well above Tcam, increases dramatically with increasing Vam. This indicates that the intergranular magnetic coupling above Tcam depends on the intergranular distance. The stiffness of ferromagnetic coupling in the intergranular region above Tcam was estimated to be ∼ 10-14 to 10-13 J/m2. The coercivity at T ∼ Tcam appears to vary as the - 6th power of the spontaneous magnetization of the intergranular amorphous phase when Vam is relatively large.

Role of the residual amorphous phase in the intergranular magnetic coupling in nanocrystalline magnetic alloys

The temperature dependence of the coercivity ( H c) in nanocrystalline Fe 91Zr 7B 2 samples with various fractions of the residual amorphous phase ( V am) has been investigated with particular attention paid to the role of the residual amorphous phase in determining the intergranular magnetic coupling. H c in the temperature range above the Curie temperature of the residual amorphous phase ( T am C) decreases significantly with decreasing V am. On the contrary, H c in the temperature range ≪ T am c increases slightly with decreasing V am. These result suggest that the intergranular magnetic coupling is dominated by the intergranular distance in the temperature range > T am C whereas it is independent of the intergranular distance well below T am C.

Effect of Pt on the Intergranular Exchange Coupling in CoCrTaPt Thin Film Media Prepared under Ultra Clean Sputtering Process

The effect of Pt addition on magnetic properties for CoCrTaPt media with extremely thin Cr underlayer of 2.5 nm fabricated under ultra clean sputtering process are connected with the magnetic anisotropy field of grains, the microsturucture, and intergranular magnetic coupling. As a results, (1)Pt addition more than 8 at% decreases the magnetrocrystalline anisotropy field of the grains. (2)Pt addition more than 8 at% disturbs a formation of Cr segregated rain boundary structure, inducing intergranular exchange coupling. (3) The upper limit of Pt addition is proposed to be 5-6 at%.

Effects of ECR ion shower treatment on the magnetic and recording properties of CoCrTaPt thin film media

The electron-cyclotron-resonance (ECR) microwave plasma nitrogen or oxygen ions shower treatment on the NiP/Al substrates before sputtering leads to an increase in coercivity of CoCrTaPt/Cr isotropic media and a decrease in coercivity and OR of oriented media. And also we found that the medium noise and NLTS of the media become lower by using the substrates with ion shower treatment. The study of magnetic properties and microstructure of the media with and without ion shower treatment suggested that the improvement of the magnetic and recording properties of the CoCrTaPt/Cr films owing likely to the decrease in the intergranular magnetic coupling and the uniformity of the magnetic anisotropy. X-ray photoelectron spectroscopy (XPS) analysis indicated that the increase of the structural feature of N-H and N-O contributed to the improvement of them.

Quantitative MFM study on partial erasure behavior of longitudinal recording

In this paper, we propose a new MFM (magnetic force microscopy) method to analyze the probability and size of partial erasure (PE). An analysis classifying the linear and nonlinear partial erasure is also proposed. It was realized that the nonlinear PE is correlated with the inter-granular magnetic coupling in thin film media. The isolation of the magnetic coupling was found effective to reduce the nonlinear PE.

The dependence of media noise on the magnetic cluster size for Co based thin film media fabricated under ultra clean sputtering process

The media noise performance is discussed quantitatively in connection with magnetic microstructure for Co based thin film media fabricated under Ultra Clean (UC) process. Results are summarized as follows: (1) The media noise decreases by reducing the dispersion of magnetic field strength at transition, (2) The dispersion of magnetic field strength at transition is suppressed by the reduction of magnetic cluster size, and (3) The grain size reduction and the decrement of intergranular magnetic coupling play an important role for the improvement of recording performance though the reduction of magnetic cluster size.

Effects of CrNiN pre-layer on magnetic properties and medium noise of CoCrTa longitudinal media

The effects of the CrNiN pre-layer between the underlayer of Cr and the NiP/Al substrate on the microstructure and the micromagnetics of CoCrTa/Cr films were investigated. The CrNiN pre-layer with an amorphous-like phase enhanced H c of CoCrTa/Cr films, especially for the media with Cr 54Ni 36N 10 pre-layer. Measurements of the magnetic anisotropy field and δM indicated that the increase in coercivity is mainly due to a large increase of the anisotropy field as well as an induced magnetostatic coupling and a decrease of the intergranular magnetic coupling in CoCrTa/Cr films. The MFM revealed that the lower noise media with the Cr 52Ni 34N 14 pre-layer have finer magnetic clusters than the higher noise media, which contain the Cr 54Ni 36N 10 pre-layer.

MR recording media: microstructure and magnetic properties

To achieve high recording areal density, it is required that future MR media have high coercivity (H/sub c/), high coercive and remanent squareness (S/sup */ and S), low media noise and narrow track recording performance. The microstructure, which strongly affects magnetic properties, should be fully controlled in media fabrication process. The microstructural characteristics like grain size, grain separation, orientation, lattice mismatch, second phases, and stacking faults can affect intergranular magnetic coupling and media noise. The composition of ternary and quaternary cobalt alloy systems, e.g. CoCrTa, CoCrPt, and CoCrPtTa, has significant effects on magnetic performances. The underlayer design, e.g. Cr, CrV/sub x/, and CrTi/sub x/, can affect lattice mismatch, stacking fault density, and other microstructural features. Stress, shadowing, preferred orientation, and c-axis alignment are four mechanisms which control orientation ratio (OR). Oriented medium has more stress-induced noise than isotropic medium has. Thin film medium with bicrystal structure has 5 dB better SNR than non-bicrystal medium. Ultra clean sputtering process (UC-process) can enhance coercivity and reduce noise. For GMR recording media, it requires Hc>3000 Oe and M/sub r/t <0.5 memu/cm/sup 2/. Media with CoCrPtTa, and CoSm/Cr design will be compared for areal density upto 10 Gb/in/sup 2/. Mechanisms of high coercive force and low noise in MR media are reviewed in connection with microstructure and magnetic properties.

The role of alloying elements in Cu-free nanocrystalline Fe-Nb-B soft magnetic alloys

In order to clarify the role of alloying elements in Cu-free nanocrystalline Fe-Nb-B soft magnetic alloys, we have examined the kinetics of primary crystallization, the microstructure after primary crystallization, the changes in Curie temperature ( T c) and the hyperfine interactions of the amorphous phase upon primary crystallization in Fe 86 − xNb x B 14( x = 0, 2, 4, 5, 6 and 8) alloys. The mean grain size after primary crystallization decreased drastically in the x range 4–6 at.%. The kinetic analysis of the primary crystallization reaction showed that the decrease in the grain size can be attributed to the commencement of homogeneous nucleation. T c of the amorphous phase increased with the B enrichment during primary crystallization. T c decreased with Nb content and the hyperfine field distribution of the residual amorphous phase after primary crystallization for the x = 6 sample showed the presence of a non-magnetic component. These results suggest that Nb plays a role in the formation of the nanostructure by accelerating nucleation, as does B in the generation of a strong intergranular magnetic coupling, through increasing the T c of the residual amorphous phase.