Research and Development / Development of world’s largest Halbach-type magnetic circuit

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Development of world’s largest Halbach-type magnetic circuit

The Magnetic Materials Research Center (Director: Masakatsu Honshima) of Shin-Etsu Chemical Co., Ltd. (President and CEO: Chihiro Kanagawa) has successfully developed the world’s largest large-scale magnetic circuit for a Halbach type permanent magnet. With a total weight of approximately 9.5 tons, it will be used mainly on the production processes for MR sensors (magnetoresistance elements) for use in MRAMs (magnetoresistance random access memories) and encoders for position detection.


Strong magnetic field Halbach-type magnetic circuit


FIG 1: Strong magnetic field Halbach-type magnetic circuit


Click the thumbnail for the large figure.



In order to realize the MRAM, which is a so-called spin RAM on a gigabit level, a heat treatment process in a magnetic field is indispensable. Nevertheless, the required generational magnetic field is extremely large at more than 1 Tesla, and the skew angle and homogeneity require numerical values that are extremely close to zero. Up to now, electromagnets and superconductive magnets have been used. Recently, however, a large number of semiconductor makers are adopting the permanent magnet method thanks to the good magnetic field performance and magnetic field stability, in addition to their being maintenance-free. Conventionally, introduction stopped at 8-inch processes. Recently, however, with the trend toward 12-inch processes, needs have increased for larger Halbach-type magnetic circuits.


  Magnetic circuit
  internal diameter
  500 mm
  Circuit external
  1.4 meters
  Circuit height
  1.0 meters
  Circuit weight
  Approx. 10 tons
  Process space
  φ300 X 300H mm
  Avg. magnetic
  field strength
  1.05 Tesla
  Process space
  Skew angle
  ±0.87°or less


Table: Magnetic Circuit Characteristics


Shin-Etsu Chemical had already been producing a variety of large-scale Halbach circuits up to 1 or 1.5 Tesla. The most-recently developed magnetic circuit was a world-first in terms of introduction of equipment for 12-inch dimensions. The new development is a cylinder with a diameter of 1.4 meters and a height of 1 meter, with a total weight of about 10 tons, making it probably the world’s largest permanent magnetic circuit generating a strong magnetic field. This magnetic circuit was completed at the end of last year, after which it was introduced to equipment makers where it is presently undergoing a variety of evaluation tests.


The Halbach-type array (otherwise known as dipole ring) is a structure considered the best in terms of magnetic efficiency in a magnetic circuit composed solely of permanent magnets. The array of the magnets is designed in a way to be the same as the attitude of the external magnetic field in a donut configuration. That means it generates a magnetic field in the internal diameter space (inner side of the donut configuration) with extremely high homogeneity. On the other hand, because the magnetic array is complex, it was believed that construction of large circuits that can generate a ferromagnetic field of 1T or higher would be extremely difficult. In the future, such a magnetic circuit is expected to find use in various fields requiring a stable ferromagnetic field source, such as semiconductor memories, high-performance HDD drive heads, medical equipment and basic research.


History of Development


Generally speaking, large magnetic circuits allow creation of static magnetic stability and complex magnetic fields. On the other hand, compared to electromagnets and superconducting magnets, they are high-priced, so that use has been limited up to now basically to such areas as medical treatment or basic research fields.


Nevertheless, thanks to recent technological advances and new trends in energy-saving, we witness a trend toward switching to permanent magnets for the magnetic fields that were generated up to now with electromagnets. Especially in such wide domains as heat processing in magnetic fields, when generating a magnetic field of 1 Tesla or higher, compared to electromagnets requiring huge amounts of electric power and water for cooling, permanent magnets with their stable magnetic fields and their maintenance-free construction, offer extremely good advantages on the whole, even in terms of the cost of introducing such equipment.


In order to generate a ferromagnetic field with a permanent magnetic circuit, the most efficient approach is to array magnets in a Halbach array. If this array is used, it becomes possible to generate ferromagnetic fields over a wide range, something that was difficult with conventional permanent magnet type magnetic circuits.
Because the Halbach-type magnetic circuit has a complex magnet layout, it is difficult to produce magnetic circuits. Up to now, production was limited to small items with an internal diameter of about 100 mm. In order to realize a large version of such a Halbach-type magnetic circuit, Shin-Etsu Chemical carried out optimization of the magnet configuration with three-dimensional magnetic field structure compound analysis while also reassessing production processes.


Magnetic circuit analysis


FIG 2: Magnetic circuit analysis


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Description of Development


The size of the magnetic field generated in the inner-diameter side of the Halbach-type magnetic circuit is controlled by the volume of the permanent magnet comprising the magnetic circuit. Meanwhile, the magnetic field homogeneity and skew angle increase in proportion to the section count of the magnets composed in a circular configuration and the ratio (H/R) of the length of the central axis of the cylinder (H) and the radius (R). As a result, it is possible to increase the magnetic field strength by simply increasing the external diameter of the magnetic circuit, although this will worsen the magnetic field homogeneity and skew angle. The higher the section count in a circular direction, the closer one comes to an ideal Halbach array, so that it is possible to make effective use of the internal diameter side process space. Nevertheless, it is necessary to prepare magnetic segments in a variety of magnetization directions, which is not beneficial in terms of costs.


Circumferential segmentation


FIG 3: Circumferential segmentation


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The magnetic field specifications of heat treatment for general MRAMs require an average magnetic field of 1 Tesla or higher in the process space, magnetic field homogeneity within +/-3%, and a skew angle no more than +/-1 degree. For example, if the process space is a cylindrical shape with a diameter or 300 millimeters and a height of 300 millimeters, the size of the magnetic circuit required to satisfy the above magnetic field specifications must have a diameter of about 1.5 meters and a height of 1 meter and an overall weight of 10 tons or more, including the auxiliary devices.
Furthermore, the section count in a circular direction must take magnetic field analysis and limitations on the production line into consideration, so that 24 divisions were considered to be the most efficient.


The rare earth permanent magnet used in the Halbach-type magnetic circuit has an energy product that is about 10 times that of a ferrite magnetic, which means the generational magnetic field is high. On the other hand, in attempting to create it with a complex array, there is the additional influence of the repulsive force of the segments and their suction force, which makes production extremely difficult. Shin-Etsu Chemical used three-dimensional magnetic field structure compound analysis to measure the internal stress of the magnetic circuit generated during assembly and thus minimize the influence of suction and propulsive force during assembly. This made it possible to develop large-size magnetic segments in which the load during production could be lowered.


Magnet segment


FIG 4: Magnet segment


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With permanent magnets, the magnetic field is not generated on the outside with the materials themselves. Instead it passes through the magnetization process before it can finally become a magnet. For example, in the case of a neodymium magnet, a strong magnetic field of 2 Tesla or higher must be impressed on the magnet. Acting as the means for generation of the strong magnetic field in this case is a pulse coil or electromagnetic. Because it is difficult to make large versions of this equipment, they are not suited for magnetization of large magnets for which the magnetization direction is 10 centimeters or higher. For this reason, production itself of large magnet systems was considered meaningless in terms of common sense up to now.


Magnetization of large magnets was made possible with superconducting coils. However, because there were no makers with experience in producing superconducting coils that could generate magnetic fields of 3 Tesla or greater with a large caliber, Shin-Etsu Chemical developed a superconducting magnet with an internal diameter of 800 millimeters together with a superconducting coil maker. By using this device, it was possible to magnetize magnets with a maximum size of 50 square centimeters.


Superconducting  magnetizer


FIG 5: Superconducting magnetizer



In order to carry out high-precision magnetic field evaluation of the completed Halbach-type magnetic circuit, a special magnetic field measuring device and Gaussmeter Hall probe were developed. By using this equipment, it was easily possible to measure magnetic fields in the micro-Tesla range, which was difficult with conventional Gauss meters.




MRAM (Magnetoresistive Random Access Memory)
  Short for Magnetoresistive Random Access Memory. The structure is similar to a DRAM and has a structure in which the capacitor section of the DRAM is replaced by a TMR element (tunnel magnetism resistor). If a condition is chosen in which the relative attitude of magnetization of the two ferromagnetic electrodes is either parallel or reverse-parallel, the tunnel magnetic resistance change, making it possible to store 1 bit of information with a single TMR element. If the electrical resistance of the element is measured, the information stored in the TMR element can be read without any damage. This is possible at high speeds and with high integration, making it one of the most likely candidates for a next-generation memory device.


Measurement values on internal diameter space of magnetic circuit (top) and skew angle (bottom)


FIG 6: Measurement values on internal diameter space of magnetic circuit (top) and skew angle (bottom)


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Skew Angle
  Generally speaking, this refers to the angle created by the axial center of propeller blades and the surface of the blades. In this case, however, it refers to the source magnetic field in relation to the main magnetic field (principal component magnetic field which is uniaxial). In other words, this refers to Bx and By if the main magnetic field is Bz, which is expressed as the inverse function of the tangent. In the case of heat treatment in a magnetic field, it is desirable that the skew angle within the process space be within 1 degree.

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