How to advance permanent ferrite magnet coercivity

Magnetic Coercivity

Permanent ferrite magnet can still keep magnetic filed after demagnetization, which makes it widely used for amplifier, DC motor and all kinds of magnetrons. The power and stability of magnetic fields mainly depends on four parameters of magnetic material, ie. Br, Hcb, Hcj and (BH)max, the higher the better.

Br is inductance of magnet on behave of permanent magnet ferrite after demagnetization, (BH)max stands for the energy of magnetic field that permanent magnet ferrite furnishes for outer magnetic route. Permanent magnetic ferrite is always working under opened-circuit state, which working point spots on demagnetization curve. Value of (BH)max depends on the separation coefficient of Br and Hcb and demagnetization cure. If the working point occurs around the (BH)max, we hope the (BH)max is as large as possible. The energy of magnet will be higher that (BH)max of permanent magnetic material supply under a same volume as possible as the material of permanent magnet waste less. So what do Hcb and Hcj stand for respectively?

At first, let us come back to the technical index—Hcb, the value of counter magnetic field to be required when Br comes to zero in hysteresis graph. That is to say, the intersection cross between demagnetization curve and abscissa is named Hcb.

Hcj is the value of magnetic field when Mr falls to zero on hysteresis curve, which is the intersection across inner demagnetization curve and abscissa.

Based on the definition of these parameters, we can see that the coercivity stands for the level of anti-magnetic interference. Permanent magnets of higher coercivity can work under higher temperature.

Using critical coercivity parameter Hk has more practical meaning in project, which can properly reflect the stability of magnet when working in dynamic environment. Hk is on behalf of the value of magnetic filed when the value of magnetization Mr comes down to 0.9Mr. On the sinuosity of inner demagnetization curve, the strength of magnetization and energy of magnetic fields will decrease speedily if strengthen the outer magnetic fields further. So it is more practical to pay attention to the value Hk.

Any point on the hysteresis curve stands for the corresponding energy value provided to outer magnetic route by unit volume magnet. When the outer magnetic field value reaches to Hcb, the induction strength of magnet comes down to zero, which means no energy supplied to outer magnetic route and the permanent magnet posses energy itself. When the point is below inner hysteresis curve, it stands for the internal magnetic energy of storage per unit volume, which ensures permanent magnet supply energy for outer magnetic circuit. When the magnetic strength come to Hcj with Mr upto zero, which will lead to a full demagnetization without storage of magnetic energy. Accordingly, Hcj stands for the energy of magnet and anti-interference level to outer magnetic field.

Hcj and Hcb stand for the anti-interference value to outer magnetic field, so they are the higher the better. From application aspect, when Hcj comes to a certain high value, which is close to Hcb, the closer the better.

Ways to advance coercivity

Magnetic experts said that the difference of plumbum shape of magnet and spinel shape of magnet are “R block” where is unavailable for less-symmetrical hexahedron 2b crystal to spinel ferrite. Fe3+ has such a great interplay of anion around triangle particle fields that yields a anisotropic higher than cubic crystal. That is to say, Fe3+ supply a higher anisotropic in the 2b. Besides 2b, 12k and Fe3+ on other positions make a good contribution to anisotropic. To advance the coercivity by mechanism, we have gained a certain progress. Magnetic remanence value Br has been very near to the theoretic value in industry production, while the value of coercivity are still 1/3 less than the theoretic value-616ka/m(Sr ferrite) at least.

Following are common ways to advance coercivity in industrial manufacturing process

  1. make the size of particle distributed on average at state of single domain, the particle size of single domain for permanent magnetic ferrite are around 0.9u normally. Under the condition of single domain, the magnet vector is enabled to rotate and Hcj is higher. In facts, the single domain and multi domain are co-existence under mass production. The core of anti-magnetization is still coming up and the magnet vector is also rotated. Analyzer is important for researching the manufacture machinery and the particle of single domain. Now people are using average particle size analyzer step by step to test particle size distribution, balling mill and grinder are used for making particle of single domain. Avoid using the vibrating mill as possible can prevent forming sheet shape particle.
  2. it is an important method to advance coercivity by adjusting the formula and manufacture technology. The formula value “n” has some effects on coercivity. When n<6, in the same manufacturing technology, the coercivity advances with the increasing of the proportion of Fe2O3; when n>6, change the formula additive and manufacturing technology, the coercivity can reach 400ka/m or above, and Br will also reach above 400mt. Under special condition, when rate of mol comes to 8 for Fe2O3 and SrCO3, which can lead both Br and Hcj to theoretic value.
  3. Herein comes a discussion on the change of coercivity value under n=6.1 when altering the additive:

    A. n=6.1, both first additive and second additive are: 0.4% CaCO3 and 0.4% kaolin

    Magnetic parameters: Br:390mT, HCB:180kA/m, HCJ:190ka/m, (BH)max:27.5kJ/m3

    In A formula, when adding 0.7% Cr2O3 and 0.6% Al2O3 the second time, the magnetic performance becomes to: Br≥380Mt, HCB≥250ka/m, HCJ≥270ka/m, Al and Cr will replace Fe ion of crystal, coercivity is advanced, but Br will lower a little.

    In A formula, if second additive becomes to: 0.6% CaCO3, 0.8% SrSO4, 1.0% Cr2O3, 0.4% Al2O3, the magnetic parameter becomes to: 397mT Br, HCB≥276kA/m, HCJ≥288kA/m, (BH)max:29kJ/m3; adding SrSO4 can increase the activity of magnetic material and advance the value of Br and HC.

    In A Formula, if the second additive changes to: 0.6% CaCO3, 0.8% SrSO4, 1.0% Cr2O3, 0.4% Al2O3, the magnetic performance parameter will change to: Br≥390mT, HCJ≥310kA/m.

    B. if adding some SrCO3 in the second additive, it will help to generate M phase and complete growth of broken crystal particle, If SiO2 is added into the second additive, it will assemble on the ferrite crystal particles to strengthen the surface resistance and prevent the crystal from growing. When the adding rate exceeds 0.2%, the force of coervicity will increase. Surely, it is also favorable to advance coervicity by lowing the temperature of sintering or shortening the attemperation time on the artwork.

The way to make high performance permanent ferrite

Just as mentioned in many files, the efficiency of molding will be low and make it more complicated through milling the particle of permanent magnetic ferrite. It will decrease Br to improve coercivity through add oxid such as Al or Cr. If want to get coercivity up to 320kA/m(over 4000Oe), meanwhile to keep the value of Br below 400MT or less, it can be hit through ion replacement which is certificated feasible in results of research and practice by many experts. The property of mass production is as follow:

  1. Br:426mT, HCB:297.3kA/m, HCJ:371.7kA/m, (BH)max:34.3kJ/m3
  2. Br:4260Gs, HCB:3735Oe, HCJ:4670Oe, (BH)max:4.31MGOe

It is feasible to choose oxide of rare earth La and oxide of Co to replace hexangular crystal Sr ion and Fe ion of permanent magnetic ferrite, whose ionic radius is nearly the same. If keeping the number of ion the same in the replacing process, and balancing the compensate charge, it can advance the coervicity and Br greatly. By proving of testing, its property is best when the adding weight rate is within 0.5—5%,

TDK have released FB9 series, some labs of our country have also produced the same grade of permanent magnetic ferrite with FB9, which is near the theoretic value of permanent magnetic ferrite. Innovation for old products should include heighten produce efficiency, lowing cost and creating new technical artwork besides improving parameters.