Fang Yikun

Magnetic domain structures of precipitation-hardened SmCo 2:17-type sintered magnets: Heat treatment effect

The typical magnetic domains of Sm(CobalFe0.25Cu0.07Zr0.02)7.4 magnets quenched through various heat-treatment steps have been revealed by using magnetic force microscopy (MFM). For the specimens in which the nominal c-axis is perpendicular to the imaging plane, the domain configurations change from plate-like for the as-sintered magnet to corrugation and spike-like for the homogenized one, and then to a coarse and finally to a finer domain structure when isothermally aged at 830°C and then annealed at 400°C. However, only plate-like domains can be detected on the surfaces with the nominal c-axis parallel to the imaging plane. The finer domain (so-called interaction domain) is a characteristic magnetic domain pattern of the Sm–Co 2:17-type magnets with high coercivities. Domain walls in a zigzag shape are revealed by means of MFM in final bulk Sm–Co 2:17-type sintered magnets.

Magnetic and Crystalline Microstructures of Fe–Pt–B Nanocomposite Ribbons

We investigate magnetic and crystalline microstructures of melt-spun (Fe0.675Pt0.325)(100-x)B-x (x = 12, 14, 16, 18, 20) nanocomposite ribbons after optimal thermal treatment using a magnetic force microscope. The magnetic microstructures are characterized by darker spots adjacent to brighter ones in a sub-micro scale and in random distribution. It is found that the strength of the exchange coupling interaction between the crystals in the 10-100nm scale, implied by the maximum value (delta M)(max) of the Henkel plot, could be roughly described by the ratio of the average width of the magnetic spots (w) over bar to the average crystal size (D) over bar for the ribbons. Moreover, we find that the intrinsic coercivity H-j(c) of the ribbons is sensitive to their crystal sizes, and the smaller (D) over bar, the higher jHc. Finally, by using roughness analysis, the curve of the root mean square values of the phase shift of the magnetic force images versus the boron content x is obtained, which is qualitatively consistent with that of the magnetization sigma(12)kOe of the ribbons versus x.

Magnetic Microstructure of Sintered Nd–Fe–B Magnets Made from Casting Strips

The magnetic microstructures of two Dy–Al substituted sintered Nd–Fe–B magnets with the different nominal compositions of Nd12.2Dy0.6Fe80.4Al0.7B6 (at.%) (composition-A, C-A) and Nd13.7Dy0.6Fe78.8Al0.7B6.2(at.%) (composition-B, C-B) prepared by strip casting technique have been revealed by using a magnetic force microscope. The magnetic properties of sintered C-B magnets are worse than that of C-A sintered magnets. In particular, the value of density products (BH)max for sintered C-A magnets is about 32% higher than that of C-B magnets, which is reflected by their quite different magnetic microstructures. We believe that for the C-B samples, the inappropriate composition and thus the redundant Nd2Fe17(B) phase of the casting strips make its final magnetic microstructures worse than the C-A, and then deteriorates the performance of the C-B magnets.

Magnetic microstructures of PryFe90-yB10 (y=8-11.76) nanocrystalline ribbons using magnetic force microscopy

Magnetic microstructure of melt-spun PryFe90−yB10 (y = 8, 8.5, 9, 9.5, 10 and 11.76) nanocrystalline ribbons in an as-grown state has been studied using a magnetic force microscope. The magnetic domains are characterized by dark areas adjacent to bright areas in a sub-micron scale and in random distribution. By comparing with the size of the micro-crystals measured from the TEM image, the exchange coupling effect was confirmed to exist in all the ribbons. Using the roughness analysis, the variation of the root mean-square values of the phase shifts obtained from the magnetic force images versus the content y of Pr were measured, which is very consistent with the curve of the residual induction Br versus the content y.

Magnetic and Crystalline Microstructures of the Sr–La–Co M-type Ferrites by Magnetic Force Microscopy

With proper erosion of sample surface and making a grid of crystal boundaries, the magnetic and crystalline microstructures of Sr–La–Co M-type ferrites of nominal composition of La0.2Sr08Fe11.8Co0.2O19 (at.%) have been investigated by using magnetic force microscopy. By calculating average diameter and thickness, and average alignment degree of the grains of the ferrites, recognizing their domain patterns and calculating the proportion of grains with single domain or multi-domains, the Sr–La–Co M-type ferrites with high magnetic performance have been characterized at microcrystalline and micromagnetic level. In addition, we has interpreted why the grains always present plate-like domains.

Effect of Sm Volatilization on Magnetic Microstructures of Sintered Sm(Co,Fe,Cu,Zr)z Magnets at High Temperatures

We present a magnetic force microscopy study of surface magnetic microstructure changes at high temperatures in 2:17-type Sm(Co,Fe,Cu,Zr)z (z~7.4) magnets. Surface magnetic microstructures are found to change greatly in parallel and vertical specimens after heat-treatment at 400°C for one hour in vacuum of 10−5 Torr with Ar gas as protecting atmosphere. Changes of microstructures are attributed to the formation of a soft-magnetic surfaces layer in the specimens, resulting from Sm volatilization due to high temperature. This hypothesis is further confirmed by the heat-treatment experiments at 400°C for 0.5 h and 2 h. Finally, the existence of the soft-magnetic layers, which consist primarily of Fe–Co compounds, is verified by the results of both XRD and XPS of the vertical specimens before and after heat-treatment.