Stable polarization of permanent magnets over the lifetime of the application is an important aspect in electrical machine design. Specification of the long-term stability of magnet material is difficult, since knowledge of the phenomenon is incomplete. To be able to optimize magnet material selection, the long-term magnetic behavior of the material must also be understood. This study shows that material with a very square JH curve is stable until a certain critical operating temperature is reached. Major losses are detected as the critical temperature is exceeded. Material with a rounder JH curve does not show a well-defined critical temperature, but increasing losses over a large temperature range. The critical temperature of a material is also dependent on the field conditions. Results differ whether the tests are performed in an open or closed magnetic circuit. In open-circuit tests, the opposing field is not homogeneously distributed throughout the volume of the magnet and thus the long-term behavior is different than that in closed-circuit conditions. Open-circuit tests seem to give bigger losses than closed-circuit tests in cases where the permeance coefficient of the open-circuit sample is considered to be the average permeance coefficient, calculated according to the dimensions of the magnet.

During the last decade, the utilization of sintered NdFeB magnets in large motor and generator applications has become more common. The remanence of NdFeB material is superior in comparison to other types of magnet materials. The challenge is coercivity, which decreases rapidly as the temperature rises. Coercivity at elevated temperatures can be increased by partial substitution of Nd with Dy. Increasing Dy content, however, decreases the remanence and is therefore an unfavorable way of improving stability at elevated temperatures. In addition, dysprosium is not as abundant as neodymium in ores and thus it is much more expensive than Nd. A lot of effort has recently been put into development work for lower Dy content magnets with higher coercivities.

One way to avoid excess Dy additions in magnets is the optimization of the application by means of appropriate material selection. This requires, however, precise knowledge of the magnetic behavior of the materials concerned. FE-modeling is an efficient way of designing the application. The optimization of the magnetic circuit is thus much easier today than a few decades ago. Irreversible polarization losses occurring in magnets during the operation of machines are difficult to estimate, detect, and compensate accurately. Consequently, it is more preferable to avoid any demagnetization.

Many studies have recently been published on the demagnetization risks of permanent magnets in different types of machines. the behavior of the magnet material is represented by an idealized BH curve, meaning that the magnetic field density of the magnet is considered to decrease linearly with an increasing opposing external magnetic field, until a certain knee point is reached. If the opposing field exceeds the knee point, some irreversible losses will occur. The same principle, but utilizing the nominal BH curves given by the magnet producers, is used in.

In addition to FEM calculations, it is important to understand how to convert the knee point value that is used into the real material properties and especially into specifications when ordering magnets. The magnetic behavior of commercial permanent magnet material is not necessarily close to ideal. Usually BH curves are measured only for one sample in the production lot and there is always some variation in properties from sample to sample. There might also be some inhomogeneities inside the magnet, and the measured BH curve only shows the average behavior of the material. Locally, the response to the opposing magnetic field can differ from the measured response.

These effects can be taken into account by setting tolerances for the remanence and coercivity values and for the homogeneity of these properties. However, these parameters do not define the shape of the BH curve. To ensure similar magnetic behavior to that applied in the FE model, the JH curve of the applied material should be very square-like. A common way to quantify the shape of the curve is to use a parameter termed squareness factor (SF) (or in some papers squareness ratio SR):
where  refers to the field at which 10% of the remanence is lost (=field at 90% of ). A squareness factor of 1 describes an ideal magnet material in which the magnetization of all domains is reversed in the same opposing field, the coercive field. In real materials this is almost impossible to achieve, but the closer the SF approaches unity, the better the homogeneity of the material. Consequently, SF is considered as the quality measure of a magnet material.

The shape of the JH curve depends on the microstructure of the magnet material, and the microstructure depends on the production process. Factors affecting the squareness include the mean grain size and its standard deviation and grain shape homogeneity. In addition, all kinds of defects in the microstructure, especially that soft magnetic phases can easily deteriorate the squareness. If there is roundness in the material’s JH and BH curves, it can be treated by a partial demagnetization (also termed stabilization or preageing) by heat treatment or a reversal field pulse. If a slight demagnetization is performed in a closed circuit, the recoil curve will be close to linear one.

The most difficult property to take into account in the optimization of materials specifications is time-dependent demagnetization. Standard demagnetization curves are measured in a timescale of seconds and the so-called thermal aftereffect or ageing is not visible in the curves. This ageing is due to a phenomenon called magnetic viscosity. The phenomenon in NdFeB magnets has been studied since the 1980’s, but mostly from a theoretical point of view. Those measurements were performed in a timescale of seconds. The results are not necessarily applicable to a timescale of years. The magnitude of the thermal aftereffect in permanent magnets depends on the microstructure of the material but also on the external conditions like temperature and magnetic field.

The magnetic behavior of a magnetized permanent magnet is expected to be defined by the measured demagnetization curve. In our previous work, we have studied the time-dependent demagnetization of different types of NdFeB-based magnets to find some practical information and a clear connection between the demagnetization curve of a magnet material and the long-term magnetic behavior of a magnet produced from that material. The shape of the demagnetization curve was found to have an effect on time-dependent demagnetization. In this paper we present a more detailed analysis of the effects of the JH curve squareness on the long-term magnetic behavior of two different sintered NdFeB materials. We also compare the measurement results obtained in open magnetic circuit and closed magnetic circuit conditions. Open-circuit measurement results always contain information not only about the behavior of the material but also about the effect of the geometry of the sample. The influence of the geometrical factors is also discussed.