JP2013089805A - Method for inspecting permanent magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inspecting a permanent magnet, capable of detecting the presence or absence of a low coercive force portion in a crystal grain unit in order to evaluate coercive force stability of the permanent magnet.SOLUTION: A method for inspecting a permanent magnet includes simultaneously observing a crystal grain boundary in an AFM image and a magnetic domain in an MFM image for the same visual field by using a magnetic force microscope to determine the presence or absence of a low coercive force portion in a crystal grain unit on the basis of a magnetic domain shielding rate=[A/(A+B)]×100 (%), as a rate of the grain boundary shielding the magnetic domain, measured from a length A of the grain boundary shielding the magnetic domain, and a length B of the grain boundary not shielding the magnetic domain.

Description

  The present invention relates to an inspection method for evaluating the coercive force stability of a permanent magnet.

The permanent magnet needs to have a large magnetic flux density and a coercive force. In particular, rare earth magnets typified by neodymium magnets (Nd ₂ Fe ₁₄ B) are used for various applications as extremely strong permanent magnets with high magnetic flux density. In order to increase the coercive force, dysprosium (Dy) is added. Dy replaces part of Nd in the vicinity of the crystal grain boundary to increase the coercive force. However, since Dy is a rare rare earth element and expensive, it is necessary to reduce the amount of use as much as possible. Therefore, a Dy diffusion magnet is known in which a small amount of dysprosium (Dy) is added and Dy is concentrated at the grain boundaries by diffusion treatment.

  Even if this Dy diffusion magnet has a high coercive force as a whole, if there is a portion having a low coercive force locally, the magnetization changes from the low coercive force portion during long-term use, and the coercive force as a whole. There was a problem that decreased.

  Even if the entire magnet is magnetically measured as small pieces of about 1 mm square, and all the pieces have a predetermined coercive force, the low-maintenance that causes a change in magnetization during long-term use in the pieces. There may be a magnetic site.

  A divided piece of about 1 mm square is composed of a large number of crystal grains (about several μm). The added Dy is distributed in the crystal grains near the crystal grain boundary. Therefore, the coercive force expression unit in the diffusion magnet is a crystal grain. Therefore, it is necessary to check the presence or absence of a low coercive force site that causes a decrease in coercive force during long-term use on a crystal grain basis.

  In Patent Document 1, as a method and apparatus for determining the magnetization state of a permanent magnet, a non-magnetized region image projected on a magnetic viewer is photographed, and the photographed image is processed to image a permanent magnet for a voice coil motor. It is disclosed that the quality of the magnetic state is determined. With this method, it is not possible to check for the presence or absence of a low coercive force site on a crystal grain basis.

  In Patent Documents 2 and 3, as a magnetic device inspection apparatus and inspection method, there is a method of measuring a magnetic distribution generated by a cantilever having a magnetic probe or a probe coated with a magnetic substance and lever tip displacement detection means. It is disclosed.

  In Patent Document 4, as a rare earth sintered magnet and a method for producing the same, a main phase composed of a compound having an R2Fe14B type crystal structure and a grain boundary phase located at a grain boundary portion of the main phase are formed, and the grain boundary phase is amorphous. It is disclosed to suppress the occurrence of reverse magnetic domains composed of a magnetic layer portion and a nonmagnetic crystal layer portion.

  However, the techniques disclosed in Patent Documents 2 and 3 cannot check the presence or absence of a low coercive force site in units of crystal grains.

  Therefore, there has been a demand for a method for detecting the presence or absence of a low coercive force site in units of crystal grains.

JP 2008-058054 A JP 2004-347435 A JP 2010-175534 A JP 2004-111481 A

  An object of this invention is to provide the inspection method of the permanent magnet which can detect the presence or absence of a low coercive force site | part in a crystal grain unit.

  In order to achieve the above object, according to the present invention, a crystal grain boundary and a magnetic domain are observed at the same time for the same field of view with a magnetic force microscope, and a low level is maintained for each crystal grain based on the ratio of the grain boundary blocking the magnetic domain. A method for inspecting a permanent magnet is provided, wherein the presence or absence of a magnetic part is determined.

  According to the present invention, the grain boundary and the magnetic domain are observed simultaneously for the same field of view by the magnetic force microscope, and based on the ratio of the grain boundary blocking the magnetic domain, it is determined whether or not there is a low coercive force site on a crystal grain basis. It is possible to inspect whether or not a magnetization change occurs during long-term use on a crystal grain basis.

FIG. 1 shows an atomic force microscope image (AFM image) and a magnetic force microscope image (MFM image) that are simultaneously observed by a magnetic force microscope (MFM), and regions where crystal grain boundaries obtained from these observed images block magnetic domains. It is a schematic diagram shown. FIG. 2 is a graph showing the correlation between the coercive force and the grain boundary magnetic domain cutoff rate obtained in the examples. FIG. 3 shows observation images of various samples having different coercive forces plotted in FIG. 2 using a magnetic force microscope.

  The present invention utilizes the fact that an atomic force microscope image (AFM image) and a magnetic force microscope image (MFM image) can be simultaneously observed by a magnetic force microscope (MFM). A description will be given with reference to FIG.

  By using a magnetic force microscope (MFM), the AFM image shown in FIG. 1 (1) and the MFM image shown in FIG. 1 (2) can be observed simultaneously for the same observation field. From the superposition of the AFM image and the MFM image, a region where the crystal grain boundary blocks the magnetic domain is obtained as shown in FIG. The grain boundary magnetic domain cutoff rate (or magnetic domain cutoff rate) is calculated by the following equation for a plurality (N: N = about 5 to 10) of observation regions.

Magnetic domain cutoff rate (%) = [grain boundary length for magnetic domain cutoff (A) / total grain boundary length (A + B)] × 100
As described in detail in the examples, the relationship between the coercive force and the magnetic domain cutoff rate is obtained in advance for the production lot to be inspected, and the lower limit of the magnetic domain cutoff rate at which a predetermined coercive force is obtained for all of the plurality of observation regions. If the value is equal to or greater than the value, it is regarded as a non-defective product.

  For a rare earth magnet (crystal grain size: 2 to 10 μm) subjected to diffusion treatment by adding Dy 4.5% to the Nd—Fe—B base composition, the coercive force and magnetic domain cutoff rate were measured according to the following procedure and conditions.

  In order to obtain various coercive forces and magnetic domain block rates in the same lot, durability treatment up to 1000H was performed at 250 ° C. after magnetization.

<Coercivity measurement>
The coercive force was measured with a VSM for samples that had been endured for various times.

<Magnetic force microscope (MFM) observation>
After the coercive force measurement, the surface of the sample to be observed was exposed to a magnetic force microscope (MFM) after revealing crystal grain boundaries by mechanical polishing and, if necessary, by night etching. The observation visual field was 10 μm square.

  The grain boundary is confirmed by the AFM image, and the length (A) of the grain boundary that blocks the magnetic domain and the length (B) of the grain boundary that is not blocked are measured while confirming the magnetic domain by the MFM image. The target magnetic domain was a linear magnetic domain having a large aspect ratio. The grain boundary length was measured manually on the photographed image (photograph). However, image analysis can also be performed mechanically.

  From the measured values of A and B, the magnetic domain cutoff rate [A / (A + B)] × 100 (%) was calculated.

  The results are shown in FIG. Two samples were measured for each durability time.

  In the range from the endurance time zero (no endurance treatment) to the endurance time 1000H, the coercive force 26.0 kOe to 23.0 kOe, the magnetic domain cutoff rate 97% and 98% to 81% and 84% range change Obtained. As shown in FIG. 2, a clear correlation is recognized between the coercive force and the magnetic domain cutoff rate.

  FIG. 3 shows (1) initial state (magnetic domain cutoff rate: 97%), (2) after durability 250 ° C. × 214H (magnetic domain cutoff rate: 82%), and (3) after durability 250 ° C. × 1000 H (magnetic domain cutoff rate: 81%) AFM-MAM image. In each figure, the region where the grain boundary blocks the magnetic domain is surrounded by a circle.

  According to the method of the present invention, it can be estimated that a coercive force of 26.9 kOe or more can be secured by using the correlation shown in FIG.

  In addition, although the Example was described about the Nd-Fe-B type Dy diffusion magnet, the application object of this invention does not need to be limited to this. The present invention can be applied to any magnetic material capable of observing crystal grain boundaries and magnetic domains with a magnetic force microscope (MFM).

  ADVANTAGE OF THE INVENTION According to this invention, the inspection method of the permanent magnet which can detect the presence or absence of a low coercive force site | part per crystal grain is provided.

Claims (5)

  It is characterized by observing crystal grain boundaries and magnetic domains simultaneously for the same field of view with a magnetic force microscope, and determining the presence or absence of a low coercive force site for each crystal grain based on the magnetic domain blocking rate, which is the rate at which the grain boundaries block magnetic domains. Inspection method for permanent magnets. In claim 1, the grain boundary is confirmed by an AFM image by a magnetic force microscope, the magnetic domain is confirmed by an MFM image, and the length A of the grain boundary that blocks the magnetic domain and the length B that does not block the magnetic domain are measured and measured. From the values of A and B, the following formula:
Magnetic domain cutoff rate = [A / (A + B)] × 100 (%)
A method for inspecting a permanent magnet, characterized in that a magnetic domain cutoff rate is obtained by:   3. The method for inspecting a permanent magnet according to claim 1, wherein a magnetic domain cutoff rate is obtained for a linear magnetic domain having a large aspect ratio.   By the method described in any one of Claims 1-3, the relationship between a coercive force and a magnetic domain interruption | blocking rate is previously calculated | required about the manufacturing lot made into a test object, and predetermined coercive force is given about all the several observation area | regions. A method for producing a permanent magnet, comprising a step of determining whether the product is a non-defective product if it is equal to or higher than the lower limit value of the magnetic domain cutoff rate and is a defective product if it is less than the lower limit value.   5. The method of manufacturing a permanent magnet according to claim 4, wherein the plurality is 5 to 10.