1) IMRAM, Tohoku Univ., Sendai, Japan, 2) ESICMM, Tsukuba, Japan
Abstract:
During the last decade, so-called multi-scale analyses have been rapidly developed in the fields of microstructure analysis and theoretical analysis on magnetism and magnetization reversal. These multi-scale approaches have been adopted to the structural and magnetic studies of Nd-Fe-B magnets. On the other hand, experimental studies on magnetization reversal processes have never been carried out from the view point of multi-scale analysis. In this study, we propose the macroscopic and microscopic experimental approaches on the magnetization reversal processes of Nd-Fe-B magnets.
As a macroscopic approach, thermal activation analyses, such as magnetic viscosity measurements and time dependent coercivity measurements, reveal the detail of energy barrier for magnetization reversal [1]. Generally, the energy barrier is assumed to be a form of Eb = E0(1-H/H0)n, where E0 is the barrier height at zero magnetic field, H the magnetic field, H0 the intrinsic coercivity without thermal fluctuation, n the constant of 1~2. These parameters strongly depend on the magnetization reversal process, but they also depend on the field direction with respect to the magnetic easy axis. Thus using a highly oriented sample is indispensable for the thermal activation analyses. A newly developed hot-deformed Nd-Fe-B magnet with Nd–Cu eutectic diffusion under expansion constraint is a suitable sample for these analyses, because it exhibits very high coercivity of about 20 kOe and excellent c-axis orientation [2]. Thus the analyzed results for the as-hot deformed (AHD) and Nd–Cu gran boundary diffused (GBD) magnets are summarized in Table 1. These two magnets exhibit quite different Hc, but the values of n for both magnet are almost the same to be about 1, suggesting that the wall pinning process is dominant for both hot-deformed magnets [3]. The larger value of E0 for GBD magnet compared with that of AHD one may reflect the well isolation of Nd2Fe14B grains due to Nd-Cu grain boundary diffusion process [4]. Temperature dependent these parameters will make us more deeply understand the magnetization reversal process of Nd-Fe-B magnets. In addition to this macroscopic approach, we use a sample of micropatterned NdFe-B dot with a microcoil as the microscopic approach. The Nd-Fe-B dot is fabricated from a well oriented NdFe-B thin film. Combining with this sample and a large amplitude nanosecond pulse generator, the microscopic observation of nucleation and wall pinning processes and evaluation of distributions of reversal field and energy barrier can be expected. Figure 1 shows an optical microscopy image of the sample and an example of MFM image after application of a pulse field of 8.7 kOe.
The authors would like to thank T. Akiya, H. Sepehri-Amin, T. Ohkubo, and K. Hono of NIMS and K. Hioki and A. Hattori of Daido Steel Co. Ltd. for provision of hot-deformed magnets and fruitful discussions. This work is partly supported by the KAKENHI (24360261), ESICMM funded by MEXT, and the Management Expense Grants for National Universities Corporation from MEXT.
[1] R. Goto et al., J. Appl. Phys.(in press), [2] T. Akiya et al., Scripta Mater., 81, 48 (2014), [3] P. Gaunt, J. Appl. Phys. 59, 4129 (1986), [4] H. Sepehri-Amin et al., Acta Mater., 61, 6622 (2013). [5] N. Kikuchi et al., J. Appl. Phys. 105, 07D506 (2009).
As a macroscopic approach, thermal activation analyses, such as magnetic viscosity measurements and time dependent coercivity measurements, reveal the detail of energy barrier for magnetization reversal [1]. Generally, the energy barrier is assumed to be a form of Eb = E0(1-H/H0)n, where E0 is the barrier height at zero magnetic field, H the magnetic field, H0 the intrinsic coercivity without thermal fluctuation, n the constant of 1~2. These parameters strongly depend on the magnetization reversal process, but they also depend on the field direction with respect to the magnetic easy axis. Thus using a highly oriented sample is indispensable for the thermal activation analyses. A newly developed hot-deformed Nd-Fe-B magnet with Nd–Cu eutectic diffusion under expansion constraint is a suitable sample for these analyses, because it exhibits very high coercivity of about 20 kOe and excellent c-axis orientation [2]. Thus the analyzed results for the as-hot deformed (AHD) and Nd–Cu gran boundary diffused (GBD) magnets are summarized in Table 1. These two magnets exhibit quite different Hc, but the values of n for both magnet are almost the same to be about 1, suggesting that the wall pinning process is dominant for both hot-deformed magnets [3]. The larger value of E0 for GBD magnet compared with that of AHD one may reflect the well isolation of Nd2Fe14B grains due to Nd-Cu grain boundary diffusion process [4]. Temperature dependent these parameters will make us more deeply understand the magnetization reversal process of Nd-Fe-B magnets. In addition to this macroscopic approach, we use a sample of micropatterned NdFe-B dot with a microcoil as the microscopic approach. The Nd-Fe-B dot is fabricated from a well oriented NdFe-B thin film. Combining with this sample and a large amplitude nanosecond pulse generator, the microscopic observation of nucleation and wall pinning processes and evaluation of distributions of reversal field and energy barrier can be expected. Figure 1 shows an optical microscopy image of the sample and an example of MFM image after application of a pulse field of 8.7 kOe.
The authors would like to thank T. Akiya, H. Sepehri-Amin, T. Ohkubo, and K. Hono of NIMS and K. Hioki and A. Hattori of Daido Steel Co. Ltd. for provision of hot-deformed magnets and fruitful discussions. This work is partly supported by the KAKENHI (24360261), ESICMM funded by MEXT, and the Management Expense Grants for National Universities Corporation from MEXT.
[1] R. Goto et al., J. Appl. Phys.(in press), [2] T. Akiya et al., Scripta Mater., 81, 48 (2014), [3] P. Gaunt, J. Appl. Phys. 59, 4129 (1986), [4] H. Sepehri-Amin et al., Acta Mater., 61, 6622 (2013). [5] N. Kikuchi et al., J. Appl. Phys. 105, 07D506 (2009).
Table 1 Energy barrier parameters evaluated at 150
oC for as-hot-deformed (AHD) and Nd-Cu grain boundary diffused (GBD) magnets, respectively
Fig. 1 (Left) Optical microscopy image of Nd-Fe-B dot with microcoil and (right) an example of MFM image of the dot after a pulse field application.