NdFeB system sintered magnet

Abstract

The present invention aims to provide a NdFeB system sintered magnet capable of improving the magnetization characteristic. The NdFeB system sintered magnet is a NdFeB system sintered magnet with the c axis oriented in one direction, characterized in that: the median of the grain size of the crystal grains at a section perpendicular to the c axis is 4.5 m or less, and the area ratio of the crystal grains having grain sizes of 1.8 m or smaller on the aforementioned section is 5% or lower. The median of the grain size is decreased (to 4.5 m or less), whereby improve the coercive force is improved. Simultaneously, the area ratio of the crystal grains having grain sizes of 1.8 m or smaller is decreased (to 5% or lower) to reduce the number of crystal grains having no magnetic wall formed, whereby the magnetization characteristic is improved.

Claims

1. A NdFeB system sintered magnet with a c axis oriented in one direction comprising: a median of a grain size of crystal grains at a section perpendicular to the c axis is 4.5 m or smaller, the grain size being calculated as a diameter of a circle having an area equal to a sectional area of each crystal grain; and an area ratio defined by a ratio of an area of the crystal grains having grain sizes of 1.8 m or smaller on the section perpendicular to the c axis over an area of all crystal grains in the section perpendicular to the c axis is 5% or lower; wherein the crystal grains having grain sizes of 1.8 m or smaller are single-domain grains.

2. A NdFeB system sintered magnet with a c axis oriented in one direction, comprising: a median of a grain size of crystal grains at a section perpendicular to the c axis is 4.5 m or smaller, the grain size being calculated as a diameter of a circle having an area equal to a sectional area of each crystal grain; and an area ratio defined by a ratio of an area of the crystal grains having grain sizes of 1.6 m or smaller on the section perpendicular to the c axis over an area of all crystal grains in the section perpendicular to the c axis is 2% or lower; wherein the crystal grains having grain sizes of 1.6 m or smaller are single-domain grains.

3. The NdFeB system sintered magnet according to claim 1, wherein a content ratio of a rare-earth element is 31% by weight or higher.

4. The NdFeB system sintered magnet according to claim 1, contains one or more kinds of metal elements having a melting point of 700 C. or lower.

5. The NdFeB system sintered magnet according to claim 4, wherein the metal element or elements are one or more kinds selected from a group of Al, Mg, Zn, Ga, In, Sn, Sb, Te, Pb and Bi.

6. The NdFeB system sintered magnet according to claim 1, wherein an element of Dy and/or Tb is diffused only in regions near grain boundaries of the crystal grains.

7. The NdFeB system sintered magnet according to claim 2, wherein a content ratio of a rare-earth element is 31% by weight or higher.

8. The NdFeB system sintered magnet according to claim 2, contains one or more kinds of metal elements having a melting point of 700 C. or lower.

9. The NdFeB system sintered magnet according to claim 8, wherein the metal element or elements are one or more kinds selected from a group of Al, Mg, Zn, Ga, In, Sn, Sb, Te, Pb and Bi.

10. The NdFeB system sintered magnet according to claim 2, wherein an element of Dy and/or Tb is diffused only in regions near grain boundaries of the crystal grains.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram for explaining the reason why single-domain grains do not become magnetized in a comparatively weak magnetic field.

(2) FIG. 2 is a graph showing the magnetization characteristics in Examples 1 and 2 of the NdFeB system sintered magnet according to the present invention as well as in Comparative Example 1.

(3) FIG. 3 is a graph showing magnetization characteristics in Present Examples 1G-3G as well as in Comparative Examples 1G and 2G.

(4) FIG. 4 is a graph showing magnetization characteristics in Present Examples 2, 4 and 5 as well as in Comparative Example 3.

(5) FIG. 5 is a graph showing magnetization characteristics in Present Examples 2G, 4G and 5G as well as in Comparative Example 3G.

(6) FIG. 6 is a graph showing magnetization characteristics in Present Examples 2G and 6G.

(7) FIG. 7 is an optical micrograph at a c.sub. plane of the NdFeB system sintered magnet in Present Example 1.

(8) FIG. 8 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Present Example 1.

(9) FIG. 9 is a graph showing the grain size distribution on a c.sub.// plane of the NdFeB system sintered magnet in Present Example 1.

(10) FIG. 10 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Present Example 2.

(11) FIG. 11 is a graph showing the grain size distribution on a c.sub.// plane of the NdFeB system sintered magnet in Present Example 2.

(12) FIG. 12 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Present Example 3.

(13) FIG. 13 is a graph showing the grain size distribution on a c.sub.// plane of the NdFeB system sintered magnet in Present Example 3.

(14) FIG. 14 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Present Example 4.

(15) FIG. 15 is a graph showing the grain size distribution on a c.sub.// plane of the NdFeB system sintered magnet in Present Example 4.

(16) FIG. 16 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Present Example 5.

(17) FIG. 17 is a graph showing the grain size distribution on a c.sub.// plane of the NdFeB system sintered magnet in Present Example 5.

(18) FIG. 18 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Comparative Example 1.

(19) FIG. 19 is a graph showing the grain size distribution on a c.sub.// plane of the NdFeB system sintered magnet in Comparative Example 1.

(20) FIG. 20 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Comparative Example 2.

(21) FIG. 21 is a graph showing the grain size distribution on a c.sub. plane of the NdFeB system sintered magnet in Comparative Example 3.

(22) FIG. 22 is a graph showing the relationship between the median D.sub.50 of the grain size of the crystal grains and the area ratio of the crystal grains whose grain sizes on a c.sub. plane are 1.8 m or smaller.

DESCRIPTION OF EMBODIMENTS

(23) Examples of the NdFeB system sintered magnet according to the present invention are hereinafter described using FIGS. 2 through 22.

Examples

(24) In the present examples, NdFeB system sintered magnets with five different compositions shown as Composition 1 through Composition 5 in Table 1 were created by a press-less method, which will be described later.

(25) TABLE-US-00001 TABLE 1 Compositions of NdFeB System Sintered Magnets (before Grain Boundary Diffusion Process) (Unit: % by Weight) TRE Nd Pr Dy B Co Ga Al Cu Fe Composition 1 33.22 33.1 0.12 0 0.98 0 0 0.20 0.10 bal. Composition 2 31.28 23.83 5.04 2.41 0.98 0 0 0.17 0.13 bal. Composition 3 31.31 26.5 4.81 0 0.99 1.97 0 0.22 0.12 bal Composition 4 31.49 26.2 5.01 0.28 0.98 0 0.2 0.18 0.13 bal. Composition 5 29.15 21.9 4.87 2.38 0.92 1.08 0 0.26 0.13 bal.

(26) The numerical values shown in Table 1 represent the content ratios of the respective elements in percent by weight. The TRE in Table 1 means the total of the content ratios of the rare-earth elements. In the present case, it represents the total of the content ratios of Nd, Pr and Dy.

(27) Initially, a lump of alloy prepared as the starting material was coarsely pulverized by a hydrogen pulverization method and then finely pulverized using a jet mill to obtain alloy powder. For Compositions 1, 4 and 5, the alloy powder was prepared with the target value of the average particle size set at 3 m. For Compositions 2 and 3, multiple kinds of alloy powder with different target values of the average particle size were prepared. Next, the alloy powder was put in a container having a cavity whose inner space is shaped like a plate. Without compression-molding the alloy powder in the container, a magnetic field was applied to the alloy powder in the thickness direction of the cavity to magnetically orient the c axes in the direction parallel to the thickness direction. Then, the alloy powder in the container was heated as is, to sinter the powder. The obtained sintered body was removed from the container and worked into a piece measuring 7 mm7 mm in planer shape and 3 mm in thickness. In this manner, samples of the NdFeB system sintered magnet were created as Present Examples 1-6 and Comparative Examples 1-3. Table 2 shows the composition and the particle size of alloy powder of each of those samples. The conditions applied in grouping the samples into Present and Comparative Examples will be described later.

(28) TABLE-US-00002 TABLE 2 Composition and Particle Size of Alloy Powder of Each Sample Median D50 of Particle Size Sample Composition of Alloy Powder [m] Present Example 1 1 3.05 Present Example 2 2 2.88 Present Example 3 3 2.83 Present Example 4 2 3.28 Present Example 5 2 3.73 Present Example 6 4 2.91 Comparative Example 1 5 2.87 Comparative Example 2 3 4.31 Comparative Example 3 2 4.86

(29) Table 3 shows the result of a measurement of magnetic characteristics of Present Example 1, 2 and 4-6 as well as those of Comparative Examples 1 and 3. The measured magnetic characteristics were as follows: the residual magnetic flux density B.sub.r, the saturation magnetization J.sub.s, the coercive force H.sub.cB determined from the B-H (magnetic flux density-magnetic field) curve, the coercive force H.sub.cJ determined from the J-H (magnetization-magnetic field) curve, the maximum energy product BH.sub.Max, B.sub.r/J.sub.s, the magnetic field H.sub.k corresponding to 90% of B.sub.r, and the squareness ratio SQ (=H.sub.k/H.sub.cJ).

(30) TABLE-US-00003 TABLE 3 Magnetic Characteristics of Samples B.sub.r J.sub.s H.sub.cB H.sub.cJ BH.sub.Max B.sub.r/J.sub.s H.sub.k SQ Sample [kG] [kG] [kOe] [kOe] [MGOe] [%] [kOe] [%] Present 13.614 14.358 11.035 16.115 43.7 94.8 14.987 93.0 Example 1 Present 13.481 14.218 13.049 21.846 44.6 94.8 20.928 95.8 Example 2 Present 13.364 13.972 13.099 21.013 44.0 95.6 19.844 94.4 Example 4 Present 13.454 14.093 13.172 20.231 44.6 95.5 19.217 95.0 Example 5 Present 13.697 14.333 13.156 16.132 45.7 95.6 15.222 94.4 Example 6 Comparative 14.121 14.757 13.411 19.262 48.5 95.7 17.952 93.2 Example 1 Comparative 13.264 13.990 12.928 19.257 43.2 94.8 18.339 95.2 Example 3

(31) A comparison of Present Examples 2, 4 and 5 with Comparative Example 3, all of which have the same composition, shows that Present Examples 2, 4 and 5 have higher magnetic characteristics than Comparative Example 3; in particular, their coercive force H.sub.cJ is characteristically high. The reason for the low coercive forces H.sub.a of Present Examples 1 and 6 as compared to the other Present and Comparative Examples shown in Table 3 is because the material used for the samples of Present Examples 1 and 6 did not contain Dy. Accordingly, those two examples cannot be simply compared with the other ones.

(32) Table 4 shows the result of a measurement of the aforementioned magnetic characteristics performed on all the samples after a grain boundary diffusion process. The grain boundary diffusion process is a process including the steps of attaching a powder or similar material containing Dy and/or Tb to the surface of the sintered body of a NdFeB system magnet and heating it to a temperature of 750 to 950 C. to diffuse the element of Dy and/or Tb only in regions near the grain boundaries of the crystal grains in the sintered body. This process is known to be capable of improving the coercive force while reducing the decrease in the maximum energy product (for example, see Patent Literature 2). In both Present and Comparative Examples, the grain boundary diffusion process was performed by attaching powder of TbNiAl alloy (containing 92 atomic percent of Tb, 4 atomic percent of Ni and 4 atomic percent of Al) to the surface of each sample and heating the samples to 900 C. Each sample after the grain boundary process is hereinafter represented by the original sample name with suffix G, such as Present Example 1G or Comparative Example 1G. The result demonstrates that the effect of improving the coercive force while reducing the decrease in the maximum energy product was obtained with any sample, regardless of whether it was a Present or Comparative Example.

(33) TABLE-US-00004 TABLE 4 Magnetic Characteristics of Samples after Grain Boundary Diffusion Process B.sub.r J.sub.s H.sub.cB H.sub.cJ BH.sub.Max B.sub.r/J.sub.s H.sub.k SQ Sample [kG] [kG] [kOe] [kOe] [MGOe] [%] [kOe] [%] Present 13.192 13.811 12.859 27.897 42.6 95.5 26.651 95.5 Example 1G Present 13.157 13.842 12.867 34.467 42.4 95.1 32.961 95.6 Example 2G Present 13.709 14.414 13.153 24.994 44.9 95.1 24.095 96.4 Example 3G Present 13.212 13.867 12.901 32.900 42.8 95.3 31.783 96.6 Example 4G Present 13.209 13.799 12.913 33.050 42.7 95.7 32.126 97.2 Example 5G Present 13.490 14.101 13.068 26.532 44.5 95.7 25.331 95.5 Example 6G Comparative 13.836 14.383 13.041 29.631 44.3 96.2 26.836 90.6 Example 1G Comparative 13.733 14.483 13.127 22.521 44.9 94.8 21.675 96.2 Example 2G Comparative 13.055 13.739 12.701 31.174 41.5 95.0 30.086 96.5 Example 3G

(34) An experiment for measuring the magnetization characteristic of each sample was performed. The experimental method was as follows: Initially, the sample was placed in an air-core coil and magnetized in the direction of orientation of the crystal by a pulsed magnetic field generated by passing a pulsed electric current through the air-core coil. Then, the application of the magnetic field was discontinued (i.e. the external magnetic field was set to zero), whereupon a demagnetizing field H.sub.d associated with the magnetization occurred in the sample (the value of H.sub.d corresponds to that of the magnetic field H at the load point at which the B-H curve in the second quadrant intersects with a straight line having a slope proportional to the permeance coefficient p.sub.c), causing the magnetization to remain. The magnetic flux resulting from this magnetization (as measured in terms of the value B.sub.d of the magnetic flux density at the load point of the B-H curve) was detected using a search coil with the number of turns of 60 (this coil was different from the aforementioned air-core coil used for applying the pulsed magnetic field) and a flux meter (FM2000, manufactured by Denshijiki Industry Co., Ltd.). In the experiment, while the intensity of the applied magnetic field was increased in a stepwise manner, the operation of discontinuing the applied magnetic field and detecting the magnetic flux was performed at each step until the detected magnetic flux reached saturation. The magnetization ratio was calculated as a proportion of the magnetic flux in a weak magnetic field, with the largest value of the detected magnetic flux defined as 100%.

(35) FIG. 2 shows the result of the experiment of the magnetization characteristic measurement performed on Present Examples 1 and 2 as well as Comparative Example 1. The experimental result shows that the intensity of the magnetizing field at which the magnetization ratio reached 100% was 25 kOe or higher for Present Example 1, 30 kOe or higher for Present Example 2, and 35 kOe for Comparative Example 1. Thus, Present Examples 1 and 2 could be completely magnetized with weaker magnetic fields than Comparative Example 1. When the magnetizing field was 25 kOe or weaker, Present Example 1 had the highest magnetization ratio, followed by Present Example 2 and Comparative Example 1. When the magnetizing field was 20 kOe, the magnetization ratios of Present Examples 1 and 2 exceeded 90%, while that of Comparative Example 1 was 90% or lower.

(36) FIG. 3 shows the result of the experiment of the magnetization characteristic measurement performed on Present Examples 1G-3G as well as Comparative Examples 1G and 2G. In any of these cases, as compared to the samples before the grain boundary diffusion process shown in FIG. 2, the magnetization ratio is lower at any intensity of magnetic field, and a plateau area is present in the magnetization curve. These facts suggest that the magnetization characteristic has been deteriorated. Such a deterioration in magnetization characteristic is inevitable as long as the grain boundary diffusion process is performed, since it results from the fact that the grain boundary diffusion process increases the magnetization of individual crystal grains and makes the reversal of magnetization more difficult. However, the fact that the magnetization characteristics of Present Examples 1G-3G are higher than that of Comparative Example 1G also confirms that the present invention exhibits a certain effect when the comparison is made among the magnets which have undergone the grain boundary diffusion process. Comparative Example 2G is comparable to those of Present Examples 1G-3G in terms of magnetization characteristic. However, its coercive force H.sub.cJ is comparatively low, as shown in Table 4.

(37) FIG. 4 shows the result of the experiment of the magnetization characteristic measurement performed on Present Examples 2, 4 and 5 as well as Comparative Example 3, which all have the same composition. Regardless of the distinction of Present and Comparative Examples, these samples required a comparatively high magnetic field of 35 kOe in order to achieve a magnetization ratio of 100%. Meanwhile, these samples exceeded a magnetization ratio of 90% when the magnetic field was 20 kOe, regardless of the distinction of Present and Comparative Examples. Among Present Examples 2, 4 and 5, Present Example 2 having the highest magnetization ratio and the least noticeable plateau area can be said to have the highest magnetic characteristic. Comparative Example 3 has a high magnetization characteristic but a low coercive force, as noted earlier. Thus, it is not Comparative Example 3 but Present Examples 2, 4 and 5 that has achieved the objective of the present invention, i.e. to obtain a NdFeB system sintered magnet having both a high coercive force and a high magnetization ratio.

(38) FIG. 5 shows the result of the experiment of the magnetization characteristic measurement performed on Present Examples 2G, 4G and 5G as well as Comparative Example 3G which were all subjected to the grain boundary diffusion process. Similar to the case of FIG. 3, those samples have their magnetization characteristic deteriorated as compared to the samples before the grain boundary diffusion process. However, they show a tendency similar to Present Examples 2, 4 and 5 as well as Comparative Example 3 shown in FIG. 4.

(39) FIG. 6 shows the result of the experiment of the magnetization characteristic measurement performed on Present Example 6G, together with the magnetization characteristic of Present Example 2G. Present Example 6G is similar to Present Example 2G in respect of the composition and the particle size of the alloy powder, except it contains 0.2% by weight of Ga. Present Example 6G has a higher magnetization characteristic than Present Example 2G. It can be said that such a high magnetization characteristic results from the fact that Present Example 6G contains Ga.

(40) An experiment for determining the grain size distribution of the crystal grains in Present Examples 1-5 and Comparative Examples 1-3 was performed in order to clarify the reason why the previously described differences in the magnetic characteristics and the magnetization characteristic occurred among the samples.

(41) In this experiment, optical microphotographs of three randomly selected visual fields with an actual size of approximately 140 m110 m were taken at 1000-fold magnification on a plane perpendicular to the thickness (c axis) of the NdFeB system sintered magnet (c.sub. plane) and on a plane parallel to the thickness direction (which is hereinafter called the c.sub.// plane). As one example, FIG. 7 shows an optical microphotograph on a c.sub. plane in Present Example 1. Next, an image analysis of those optical microphotographs using an image analyzer (LUZEX AP, manufactured by Nireco Corporation) was performed as follows: Initially, an image processing for adjusting the brightness, contrast and other parameters was performed so as to make the grain boundaries of the crystal grains clearly visible. Subsequently, the sectional area of each crystal grain was calculated. Then, on the assumption that the section of each crystal grain was a circle whose area equals the calculated sectional area of that crystal grain, the circle's diameter was calculated as the grain size of the crystal grain. Such a calculation of the grain size was performed for all the crystal grains in the three visual fields, and the grain size distribution was computed.

(42) FIGS. 8-21 show the computed grain size distributions of the crystal grains in the NdFeB system sintered magnets of Present Examples 1-5 and Comparative Examples 1-3. In any of these graphs of grain size distribution, the crystal grains were divided into unit grain sizes defined at grain-size intervals of 0.2 m (0-0.2 m, 0.2-0.4 m, and so on). The number of grains was counted for each unit grain size, and the area ratio was calculated by n.sub.i.sub.i/S, where n.sub.i is the number of grains at each unit grain size, .sub.i is the average sectional area at each unit grain size, and S is the sectional area of the entire target of the measurement (see the insert in each figure). Furthermore, the sum of the area ratios obtained at the unit grain sizes equal to or less than a currently-focused unit grain size is defined as the accumulated area ratio at that unit grain size. Accordingly, the accumulated area ratio at a unit grain size of 1.8 m corresponds to the aforementioned area ratio of the crystal grains having grain sizes of 1.8 m or smaller. In each figure, the larger graph shows the accumulated area ratio within a grain-size range of 2.5 m or less, while the insert shows the area ratio and the accumulated area ratio over the entire grain-size range. The total number n of crystal grains within the entire area of the measurement target is also shown in some figures. For Comparative Examples 2 and 3, only the data of the c.sub. plane are shown.

(43) From these grain-size distribution graphs, the accumulated area ratios at grain sizes of 1.6 m and 1.8 m were calculated, the results of which were as shown in Table 5 (c.sub. plane) and Table 6 (c.sub.// plane).

(44) TABLE-US-00005 TABLE 5 (c.sub. plane) Accumulated Area Grain Size D50 of Ratio (%) Crystal Grains Grain Size Grain Size [m] 1.8 m 1.6 m Present Example 1 3.42 2.8 1.08 Present Example 2 3.55 2.8 1.58 Present Example 3 3.6 4.4 2.3 Present Example 4 3.8 4.2 2.4 Present Example 5 4.0 3.5 2.1 Comparative Example 1 3.2 7.5 3.99 Comparative Example 2 5.2 1.1 0.7 Comparative Example 3 6.3 1.2 0.9

(45) TABLE-US-00006 TABLE 6 (c.sub. plane) Accumulated Area Grain Size D50 Ratio (%) [m] of Grain Size Grain Size Crystal Grains 1.8 m 1.6 m Present Example 1 3.0 8.0 3.62 Present Example 2 2.9 6.6 3.73 Present Example 3 3.7 4.2 2.5 Present Example 4 3.4 4.5 2.9 Present Example 5 3.6 4.9 3.2 Comparative Example 1 3.2 7.5 3.99

(46) From the results shown in these tables, the following facts can be extracted: On the c.sub. plane, the area ratio of the crystal grains having grain sizes of 1.8 m or smaller was 5% or lower in any of Present Examples 1-5, while Comparative Example 1 had a high value of 7.5%. By contrast, on the c.sub.// plane, there was no significant difference in the area ratio of the crystal grains having grain sizes of 1.8 m or smaller between Present and Comparative Examples. The median D.sub.50 of the grain size of the crystal grains was less than 4.5 m in any of those examples and there was no noticeable difference between Present and Comparative Examples as well as between the c.sub. and c.sub.// planes. Comparative Examples 2 and 3, in which the area ratio of the grain size of 1.8 m or smaller on the c.sub. plane was lower than 5%, are not included in the present invention, since the median D.sub.50 of the grain size of the crystal grains, which is the index relating to the coercive force, is greater than 4.5 m.

(47) Thus, it has been demonstrated that a magnetization ratio of 90% or higher can be achieved using an external magnetic field of 20 kOe in the samples of Present Examples 1-5 in which the area ratio of the crystal grains having grain sizes of 1.8 m or smaller on a c.sub. plane is 5% or lower. This is probably due to the fact that the volume occupied by crystal grains having small grain sizes (or the area occupied on a section of the sintered magnet) is thereby reduced, so that single-domains are less likely to be formed.

(48) It should be noted that the area ratio of the crystal grains having grain sizes of 1.6 m or smaller on a c.sub. plane is 2% or lower in Present Examples 1 and 2, whereas the ratio is higher than 2% in Present Examples 3-5. This result corresponds to the fact that no plateau area is noticeable in Present Examples 1 and 2.

(49) FIG. 22 is a graph created based on the experimental results of Present Examples 1-5 and Comparative Examples 1-3, which shows the relationship between the median D.sub.50 of the grain size of crystal grains and the area ratio of the crystal grains having grain sizes of 1.8 m or smaller on a c.sub. plane (the accumulated area ratio at 1.8 m). This graph shows that there is a trade-off between the two indices. That is to say, reducing the median D.sub.50 of the grain size to improve the coercive force inevitably increases the accumulated area ratio at 1.8 m on the c.sub. plane and consequently deteriorates the magnetization characteristic. Conversely, decreasing the accumulated area ratio at 1.8 m on the c.sub. plane to improve the magnetization characteristic inevitably increases the median D.sub.50 of the grain size and consequently decreases the coercive force. Accordingly, when determining these two indices, it is necessary to strike a balance between the two so that the median D.sub.50 of the grain size will be 4.5 m or less while the accumulated area ratio at 1.8 m on the c.sub. plane will be 5% or less.

REFERENCE SIGNS LIST

(50) 10 . . . NdFeB System Sintered Magnet 11 . . . Multi-Domain Grain 12 . . . Single-Domain Grain 13 . . . Magnetic Domain Formed in Multi-Domain Grain 14 . . . Reverse Magnetic Domain Formed in Single-Domain Grain