$L{1}_0$ rare-earth-free permanent magnets: The effects of twinning versus dislocations in Mn-Al magnets

Defects of various kinds strongly affect local magnetic anisotropy and thus play a crucial role in coercivity in high-performance permanent magnets. In particular, in rare-earth-free permanent magnetic alloys with $L{1}_0$ structure microstructural defects deserve special attention. In this work, we report on the “negative” effect of twin structure, and the “positive” effect of dislocations on the coercivity is clarified in a systematic experimental study of $L{1}_0$-MnAl alloys. We find that the nucleation of magnetization reversal is preferentially activated along the twin boundaries and grows into the twin stripes. This suggests that twin structure reduces the domain wall nucleation field, so that the coercivity decreases by approximately 50% according to the direct comparison of twin-free and twinned magnets. In contrast, dislocations dramatically enhance the coercivity by acting as the pinning center, and a high density of dislocations can modify the dominant coercivity mechanism from nucleation to pinning in severely deformed MnAl magnets. With a decreasing dislocation density, the pinning field remains as a constant while the coercivity reduces monotonously, indicative of the “strong pinning” effect of dislocations on the magnetic domain wall, generating a positive correlation between coercivity and density of dislocation. High coercivity from 424 to 328 kA/m is obtained in deformed and annealed magnets with different densities of dislocations. Thus, the combination of eliminating twin structure and introducing high-density dislocations could overcome the present bottleneck in magnetic performance. This work may inspire avenues for the development of $L{1}_0$ rare-earth-free permanent magnetic alloys, and set up a pathway for accelerating the application process.

Figure 1

(a) TEM image showing the nanotwin structure in $L{1}_0$-MnAl alloy. (b) Optical morphologies showing the macrotwin structure in $L{1}_0$-MnAl alloy.

Figure 2

(a,b) Morphologies of the bonded magnets using the (a) twin-free particles and (b) twinned particles by optical microscopy; the inset figures are the enlargement of a corresponding particle. (c,d) Distribution of the particle sizes of (c) twin-free and (d) twinned magnets; the inset in each figure is the schematic figure of the microstructures of the corresponding magnet.

Figure 3

(a,b) Magnetic hysteresis loops of (a) twin-free magnet and (b) twinned magnet, measured along and perpendicular to the direction of texture at 300 K. (c,d) Initial magnetization curves of (c) twin-free magnet and (d) twinned magnet, measured along and perpendicular to the texture at 300 K. (e) The room-temperature XRD patterns of the cross-section surface of twin-free and twinned magnets; the XRD patterns of the isotropic as-annealed bulks and standard pattern of the $L{1}_0$ phase are also added for comparison. (f) The comparison of crystallographic orientation and remanence ratio of the MnAl magnets obtained in this work with literature data [31, 32, 35, 37, 38, 39, 40, 41].

Figure 4

(a) TEM image showing the morphology of twin structures of MnAl alloys. (b)The SAED pattern of the twin structure. (c) EBSD map of as-annealed $L{1}_0$-MnAl. (d) Pole figure of as-annealed $L{1}_0$-MnAl from three different crystallographic axes. (e) Histogram of misorientation angle. (f) Crystallographic phase relations between high-temperature $A3$ phase and low-temperature $L{1}_0$ phase [19]. (g) The twinning plane in $L{1}_0$ MnAl alloys. (h) A schematic figure showing the atomic distribution nearby a twin boundary. It should be mentioned that we only show the condition that the atomic ratio Mn:Al=1:1 for convenience.

Figure 5

(a–i) MOKE images showing the interaction of domain structures and twin boundaries during magnetization and demagnetization processes in twinned MnAl alloys. (j) Magnetization and demagnetization curve under the MOKE observation condition. (k) Schematic figures of magnetic domain evolution selected from the yellow rectangle in the MOKE images, corresponding to the data points 1–9 on the curve. (l) Schematic figures explaining the color contrast in the MOKE images and the evolution of domain structure during magnetization process.

Figure 6

(a–h) Lorentz TEM figures showing the magnetic domains and microstructures during magnetization and demagnetization processes in twinned MnAl alloys. (i) Magnetization and demagnetization curve under the LTEM observation condition. (j,k) Schematic figures of magnetic domain evolution of the region of yellow rectangles in LTEM figures during the (j) magnetization and (k) demagnetization process selected from and corresponding to the data points 1–8 on the hysteresis loop.

Figure 7

(a) Initial magnetization curves of melt-spun ribbon, directionally solidified bulk, hot-deformed sample [46], as-annealed bulk, ball-milled powders, and HPT specimens along random directions measured at 300 K. (b) $dM/dH$ curves of samples in (a), showing the nucleation or pinning mechanism of coercivity.

Figure 8

(a) The room-temperature XRD patterns of $L{1}_0$-MnAl before and after HPT deformation. (b) The lattice strain analysis based on the method of Williamson and Hall [49]. (c) TEM image showing the dislocation morphology of $L{1}_0$-MnAl after HPT deformation. (d) High-resolution image of $L{1}_0$-MnAl after HPT deformation, and the FFT pattern of the area in the yellow square is shown in the inset. (e) Image after inverse FFT operation, indicating high density of dislocations. (f) Schematic graph showing the lattice structure of dislocation in $L{1}_0$-MnAl alloys.

Figure 9

(a) The room-temperature XRD patterns of HPT-MnAl after a series of annealing treatment for 10 min under 200 °C, 250 °C, 300 °C, 350 °C, and 400 °C. (b) The enlargement of the (101) peak of (a) in the dashed rectangle. (${\mathrm{c}}_1-{\mathrm{f}}_1$): TEM images of twin structure evolution in $L{1}_0$-MnAl before HPT, after HPT, annealed for 10 min under 250 °C, and annealed for 10 min under 400 °C. (${\mathrm{c}}_2-{\mathrm{f}}_2$): TEM images showing the dislocation morphology in $L{1}_0$-MnAl before HPT, after HPT, annealed for 10 min under 250 °C, and annealed for 10 min under 400 °C. (${\mathrm{c}}_3-{\mathrm{f}}_3$): Schematic figures of the dislocation morphology corresponding to (${\mathrm{c}}_2-{\mathrm{f}}_2$), respectively.

Figure 10

(${\mathrm{a}}_1-{\mathrm{g}}_1$) Kikuchi patterns of the samples before HPT (${\mathrm{a}}_1$), after HPT (${\mathrm{b}}_1$), and annealed for 10 min under 200 °C (${\mathrm{c}}_1$), 250 °C (${\mathrm{d}}_1$), 300 °C (${\mathrm{e}}_1$), 350 °C (${\mathrm{f}}_1$), and 400 °C (${\mathrm{g}}_1$). (${\mathrm{a}}_2-{\mathrm{g}}_2$) Images after FFT of Kikuchi patterns corresponding to (${\mathrm{a}}_1-{\mathrm{g}}_1$), respectively. (h) Radius of the center of bright spots in the FFT images for different sample states. (i) Reciprocal of radius of the center spots in the FFT images and the dislocation density of samples.

Figure 11

(a–c) Initial magnetization curves (a), $dM/dH$ curves (b), and hysteresis loops (c) of $L{1}_0$-MnAl after HPT and series of annealing treatment. (d) Evolution of dislocation density, $H_{\mathrm{c}}$ and $M_{4\mathrm{MA}/m}$ with the different annealing treatment temperature.

Figure 12

The comparison of saturated magnetization and coercivity between $L{1}_0$-MnAl magnets obtained by a common preparation method ( [28, 31, 32, 33, 34, 55, 56, 57, 58, 59, 60] and this work).