Remanent magnetic induction

Figure 2 shows the two hysteresis loops for a medium and a head material. The coercivity, FT, the saturation magnetization, M or induction, Bs, remanent magnetization, M or induction, B and the permeability, JJ, differ for the two materials. 

Mr = remanent magnetization, 6 = layer thickness, d = head-to-medium spacing, Q = a value related to the field gradient of an inductive head ( -0.75),... 

Magnetic field Magnetic induction Magnetization Spontaneous magnetization Saturation magnetization Magnetic flux Magnetic moment Coercive field Remanence... 

Figure 1. Typical curve representing the dependence of magnetic induction B on magnetic field H for a ferromagnetic material. When His first applied, B follows curve a as the favorably oriented magnetic domains grow. This curve flattens as saturation is approached. When His then reduced, B follows curve b, but retains a finite value (the remanence B,) M H = 0. In order to demagnetize the material, a negative field -H (where H is called the coercive field or coercivity) must be applied. As H is further decreased and then increased to complete the cycle (curve c), a hysteresis loop is obtained. The area within this loop is a measure of the energy loss per cycle for a unit volume of the material.

Energetic analysis shows the dependence of using magnetie materials from relation of coercive force on magnetization to remanent induction. The permanent magnets... 

If, after the material has been magnetically saturated to the value Bs, the field is reduced to zero, the magnetization vectors rotate out of line with the field towards the nearest preferred direction which is determined in part by magnetocrystalline anisotropy. The magnetization is thus prevented from complete relaxation to the virgin curve and hence, for zero field, there is a remanent induction Bx. In order to reduce the induction to zero a reverse field II,. has to be applied. The coercive field or coercivity II,. depends in part on crystalline anisotropy, as might be expected. 

It must have a high spontaneous magnetization, Ms, in the temperature range of practical interest (typically around room temperature). The saturation intrinsic induction, Bis = AttMs (in gauss) sets the upper limits for the remanent flux density, Br = Bls, and for the energy product, BH)mzx = ( B1%)2 (in MGOe). 

Both methods are, in principle, suited to the production of any REPM alloy, but the adaptation to Sm(Co, Fe, Cu, Zr)x and now Nd-Fe-B alloys required extensive development efforts. The R/D or KOR powders have much higher oxygen contents than induction melted alloys, about 1000-2500 ppm by weight. This requires compensation by adding extra RE (Sm), and there are more oxide particles in the final magnet, slightly reducing its remanence. 

As discussed in Section 4.5, the desired properties in materials for permanent magnet applications are essentially a high remanent induction, Bf, a large coercive field (either gH or jifc) and a large energy product A brief discussion of the various mechanisms leading to increased coercivity is presented first. Since most of the new hard magnets are intermetallic compounds, a short account of their anisotropies is included. 

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