An almost never stated but very important assumption in any B-H curve is that B is parallel to the applied field H, and H is the only field that exists. In anisotropic materials or an anisotropic sample, when the field H is not applied parallel or perpendicular to the anisotropy axis, the vectors B and H are usually not parallel. A B-H curve measured in this way is almost meaningless. For example, measuring the B-H loop of a nail with the field H applied at an angle to the length of the nail will give meaningless results. The field H must be applied parallel or perpendicular to the anisotropy axis so that B, M and H are parallel.
Another factor which is often ignored is that the maximum field Hm must be large enough to saturate the sample. This is conveniently determined when the upper parts of the loop are “closed”.
The demagnetizing field H necessary to cause a magnet which was previously saturated to have zero magnetization M. The units of M are emu/cm3 in cgs units and A/m in MKS-SI units. 1000 A/m = 1 emu/cm3. In general Hc and Hci are quite different with Hci usually being much larger than Hc. The relation between B, M and H in MKS-SI units is B = µ0(H + M), where µ0 is “the permeability of free space” µ0 = 4∏*10-7, and B = H + 4∏iM in cgs units. Strictly speaking these are vector equations but generally H and M, and hence B, are parallel and can be considered to be scalars.
A graph of the magnetization M plotted vs. H. The differences between B-H and M-H loops have generated a great deal of confusion. This is not surprising because many text books contain important errors! The two loops will be discussed in some detail below. The M-H loop tells one more about the basic properties of the magnetic material than does the B-H loop.