Magnetization reversal of (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets as per soft x-ray magnetic circular dichroism microscopy

We investigated magnetization reversal of (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets as per x-ray magnetic circular dichroism microscopy. Magnetization reversal initially occurs at the ferromagnetic grain boundaries or in the vicinity of nonmagnetic Sm oxides. As the demagnetization field increases after magnetization reversal, the reversal region extends into the grain from these areas by the magnetic domain wall motion. Energy-dispersive x-ray analysis using an electron probe micro-analyzer shows that, at the grain boundaries, the Fe concentration is higher and the Cu concentration is lower compared to that inside of the grains; and concentrations of Sm, Co, Fe, and Cu vary in the vicinity of Sm oxides. By measuring the Co L3 absorption intensity, we verified that local coercivities in these areas are very low compared to those inside of the grain. These results imply that the magnetization reversal that occurs in these areas is induced by the variation in the composition. The results obtained in our research will be useful for improving the magnetic properties of Sm–Co magnets. VC 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0005635 Sm–Co magnets including RCo5 and R2Co17 type magnets, where R is a rare earth element, are still very important in various applications. These include automotive sensors such as anti-lock braking, high-temperature motors such as in trains and aircraft, and quadrupole field magnets used in high-energy accelerators. Sm–Co magnets were developed in the late 1960s and 1970s and reached more than 240 kJ/m of maximum energy product [(BH)max] in Sm2Co17 type magnets. 6 These magnets show excellent magnetic performance of magnetic properties with high remanence and coercivity, where the remanence (or the residual magnetization) and coercivity are denoted by Br and HcJ, respectively. They also have high Curie temperatures and exhibit excellent heat resistance over 150 C. The magnetic properties of Sm2Co17 magnets continue to improve. Recently, researchers reported that the magnetic properties of a Sm2Co17-type magnet, Sm(Co0.572Fe0.35Cu0.06Zr0.018)7.8, achieved a very high (BH)max (more than 280 kJ/m ) by improving solution heat treatment, aging heat treatment, and optimizing the Fe content. In Sm2Co17-type magnets, the cellular structure (in which there is a Sm2Co17 phase and a plate-like SmCo5 phase enriched in Cu) forms as a result of aging heat treatment, and the magnetic domain walls are constrained by the difference in the magnetic wall energy between the aforementioned two phases. The coercivity mechanism of these magnets has mainly been studied by investigations of the cellular structure using transmission electron microscopy. Additionally, the squareness of the demagnetization curve defined by Hk/HcJ, where Hk is the magnetic field at which magnetization is 90% of Br, is less than that of Nd–Fe–B sintered magnets. Recently, Kerr effect microscopy indicated that the origin of the grain boundaries of Sm2Co17 magnets that have high squareness in the demagnetization curve pertains to the initial stage of magnetization reversal. Knowledge of where the initial magnetization reversal Appl. Phys. Lett. 117, 022409 (2020); doi: 10.1063/5.0005635 117, 022409-1 VC Author(s) 2020 Applied Physics Letters ARTICLE scitation.org/journal/apl

Sm-Co magnets including RCo 5 and R 2 Co 17 type magnets, where R is a rare earth element, are still very important in various applications. These include automotive sensors such as anti-lock braking, high-temperature motors such as in trains and aircraft, and quadrupole field magnets used in high-energy accelerators. Sm-Co magnets were developed in the late 1960s and 1970s [1][2][3][4][5] and reached more than 240 kJ/m 3 of maximum energy product [(BH) max ] in Sm 2 Co 17 type magnets. 6 These magnets show excellent magnetic performance of magnetic properties with high remanence and coercivity, where the remanence (or the residual magnetization) and coercivity are denoted by B r and H cJ , respectively. They also have high Curie temperatures and exhibit excellent heat resistance over 150 C.
The magnetic properties of Sm 2 Co 17 magnets continue to improve. Recently, researchers reported that the magnetic properties of a Sm 2 Co 17 -type magnet, Sm(Co 0.572 Fe 0.35 Cu 0.06 Zr 0.018 ) 7.8 , achieved a very high (BH) max (more than 280 kJ/m 3 ) by improving solution heat treatment, aging heat treatment, and optimizing the Fe content. 7,8 In Sm 2 Co 17 -type magnets, the cellular structure (in which there is a Sm 2 Co 17 phase and a plate-like SmCo 5 phase enriched in Cu) forms as a result of aging heat treatment, and the magnetic domain walls are constrained by the difference in the magnetic wall energy between the aforementioned two phases. 9,10 The coercivity mechanism of these magnets has mainly been studied by investigations of the cellular structure using transmission electron microscopy. Additionally, the squareness of the demagnetization curve defined by H k /H cJ , where H k is the magnetic field at which magnetization is 90% of Br, is less than that of Nd-Fe-B sintered magnets.
Recently, Kerr effect microscopy indicated that the origin of the grain boundaries of Sm 2 Co 17 magnets that have high squareness in the demagnetization curve pertains to the initial stage of magnetization reversal. [11][12][13] Knowledge of where the initial magnetization reversal occurs and how the magnetization reversal domains develop is important to further improve magnetic properties, especially the squareness of the demagnetization curve. Toward this goal, we investigated the magnetization reversal of (Sm, Ce) 2 (Co, Fe, Cu, Zr) 17 magnets using soft x-ray magnetic circular dichroism (XMCD) microscopy, which gives higher spatial resolution and a larger magnetic effect compared to Kerr microscopy.
A highly oriented Sm 2 Co 17 -type magnet with the composition of Sm 0.67 Ce 0.33 (Co 0.73 Fe 0.2 Cu 0.05 Zr 0.02 ) 7.2 was provided by Shin-Etsu Chemical Co., Ltd. The samples used for magnetic property measurements were of dimensions 7 mm Â 7 mm Â 7 mm. The magnetic properties were measured using a pulsed field magnetometer (TPM-2-08S25VT-C, Toei Industry Co., Ltd, Tokyo Japan). A maximum magnetic field of 8 T was applied for magnetic saturation. The hysteresis curve measured using the pulsed field magnetometer was subjected to a demagnetization correction.
XMCD microscopy was performed at the soft X-ray beamline, BL25SU, of SPring-8. 14 For these experiments, magnets were cut from a block magnet into a pillar shape with dimensions of 0.6 mm Â 0.6 mm Â L mm, where the easy magnetization direction is along the L-axis. This magnet was fractured perpendicular to the L-axis under high vacuum ($4.89 Â 10 À6 Pa) to prevent oxidation of the cracked surface in the sub-chamber and was then transferred into the main chamber ($4.6 Â 10 À7 Pa). The XMCD measurements were conducted at room temperature with an external magnetic field (H) as follows: À5.0 T H þ5.0 T. The measurements were performed on a surface perpendicular to the easy magnetization direction. The incident x-rays and external magnetic field were parallel to the easy magnetization direction. The measurement area was 55.95 lm Â 60.00 lm, and the scanning step was 150 nm.
X-ray absorption intensity was measured using the total electron yield at the Co L 3 pre-edge (774.0 eV), Co L 3 edge (778.5 eV), Sm M 5 pre-edge (1066.0 eV), and Sm M 5 edge (1078.6 eV). In these energy regions, the x-ray beam was focused to approximately 100 nm Â 100 nm, and the focal depth was approximately 65 lm. The probing depth was 1.5 nm. The XMCD ratio was determined by the ratio of the difference to the sum (referring to the absorption intensities attributable to positive and negative photon helicity). The XMCD ratio and its map were used to observe local magnetic properties and magnetization reversal, respectively. David Billington et al. describe the XMCD microscopy measurement method in more detail. 15 Composition mapping and line scan measurements of Sm, Co, Fe, Cu, and oxygen were obtained from the intensities of the characteristic x-rays of Sm L a , Co L, Fe K a , Cu L, and oxygen K in the energydispersive x-ray (EDX) spectrum using a field-emission scanning electron microscope (FE-SEM SU8000, Hitachi High-Technologies Corporation, Tokyo) equipped with an EDX spectrometer (XFlash FlatQUAD 5060F, Bruker Corporation) at an accelerating voltage of 10 keV.  Figure  2(a), þ5.0 T, shows that the magnetization of every grain (red area) is saturated. White regions correspond to nonmagnetic inclusions or voids. In Fig. 2(b), À0.4 T, the magnetization reversal region (blue) was initiated from the grain boundaries, previously reported using the Kerr effect, 11 and also from the vicinity of the nonmagnetic phases or inclusions. In Fig. 2(c), À0.5 T, the magnetization reversal regions extend to inside the grains from the grain boundaries or nonmagnetic inclusions. When the demagnetization field reached À1.1 T [ Fig.  2(d)], the total area of the red region approached that of the blue region and magnetization declined to zero.
From the demagnetization curve shown in Fig. 1 and the XMCD maps of Fig. 2, we can see that magnetization reversal occurs only at grain boundaries until near the inflection point of the demagnetization curve or H k . After the magnetization reversal region extended into the grains, magnetization decreased quickly, eventually reaching coercivity.

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scitation.org/journal/apl Figure 3 shows an enlarged XMCD map at a magnetic field of À0.5 T. Here, the magnetization reversal region extended into the grains from the grain boundaries and the boundaries of nonmagnetic phases or inclusions. Every magnetization reversal area appearing inside grains comes into contact with the grain boundary and nonmagnetic inclusions as the demagnetization increased. In contrast, Fig. 4 shows some magnetization reversal regions [encircled in Figs. 4(b) and 4(c)] that emerged without coming into contact with the grain boundaries and nonmagnetic inclusions, also noted in previous research. 12,13 Our initial hypothesis is that magnetization reversal might have originated from nucleation or coherent rotation. However, these reversal regions might have originated from grain boundaries or

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scitation.org/journal/apl nonmagnetic phases underneath the surface and reached the specimen surface via magnetic domain wall motion induced by increasing the applied demagnetization field. Figure 5(a) shows a Co L 3 XMCD map of H ¼ À0.5 T, Fig. 5(b) shows a corresponding SEM image, Fig. 5(c) shows EDX results for Sm, and Fig. 5(d) shows EDX results for oxygen. The EDX maps indicate that the nonmagnetic inclusions inside the grains were Sm oxide.
It would be expected that the composition at the grain boundaries and in the vicinity of the Sm oxide phase inside the grains deviates from that of the grains. To verify this expectation, we obtained SEM images of particular regions within Fig. 6(a), where the first includes two grains and their grain boundary [ Fig. 6(b)] and the corresponding EDX line scan for Sm, Co, Fe, and Cu [ Fig. 6(b)], and the second shows inside the grain in the vicinity of Sm-oxide [ Fig. 6(c)] and the corresponding EDX line scan for Sm, Co, Fe, and Cu. The Fe concentration at the grain boundary was higher than that inside the grain. In contrast, it was found that the Cu concentration at the grain boundary was lower than that inside the grain from the line scan in Fig. 6(b). In the vicinity of Sm oxide, the Co, Fe, and Cu concentration decreased sharply. We hypothesize that these composition variations are responsible for the coercivity drop in these areas. Figure 7 shows the local magnetic properties evaluated from the Co L 3 edge XMCD signal intensity. The magnetic properties of the grain boundary and the triplet point deteriorated in their coercivities compared to those inside of the grain. The triplet point, which is the junction of the grain boundaries, is expected to have a composition similar to the grain boundaries. It is easy to conjecture that the reverse magnetization area initiated from the grain boundary or from the triplet points.
In conclusion, we investigated magnetization reversal of Sm 0.67 Ce 0.33 (Co 0.73 Fe 0.2 Cu 0.05 Zr 0.02 ) 7.2 via XMCD microscopy. The magnetization reversal initially occurs at the grain boundaries or in the vicinity of the Sm oxide inside the grains. When the magnetic field is within the range of À0.4 to À0.5 T, the magnetization reversal domains extend into the grains, where the starting point of magnetization reversal into the grain is shown in Fig. 7. Upon extension into the grains, the reversal domains propagate further into the grains as the reverse magnetic field increases. The Co concentration in the grain boundaries is nearly identical to that inside the grains, whereas the Fe concentration is higher and the Cu concentration is lower at the grain boundaries. We verified that Sm, Co, Fe, and Cu concentrations also differ in the vicinity of Sm oxide phases, indicating that these compositional variations are responsible for the deterioration of coercivity in these regions (where initial magnetization reversal occurs). The local magnetic properties measured by the Co L 3 edge XMCD ratio show that, in these regions, magnetic properties deteriorated in coercivity compared to those inside of the grains.
See the supplementary material for the Sm M 5 XMCD map of the specimen demagnetized at À1.28 T (near the bulk coercivity) after full magnetization at 5 T and measured at zero magnetic field and for the propagation of magnetization reversal with the changing external magnetic field.