Effect of Temperature Variation on Magnetic Properties of Neodymium Magnets

The change in ambient temperature has the common and serious impact on the magnetic performance of Neodymium magnets. Demagnetization curves and B-H curves indicate that remanence of N40UH magnet falling slowly from 20 deg C to 180 deg C, and intrinsic coercivity sharply decreased with increasing temperature. B-H curve keep a straight line status below 120 deg C, and thus appear knee point under 150 deg C and 180 deg C because of the numerical values of intrinsic coercivity has already lower than remanance’s. In order to relieve the effect of temperature variation on magnetic properties, manufacturer will place magnetized Neodymium magnets to expect working temperature for a period of time, and hence eliminate the influence of high temperature. Even so, its magnetic properties are not able to maintain constant after such aging treatment.

Researchers have introduced some temperature stability parameters to characterize the effects of temperature on permanent magnetic properties, include Temperature Coefficient of remanance α_Br, Temperature Coefficient of intrinsic coercivity α_Hcj, reversible flux losses of open circuit L_rev, irreversible flux losses of open circuit L_irr, Reversible Temperature Coefficient of open circuit flux density and Max. Operating Temperature T_w.

Permanent magnets typically work in open-circuit condition and temperature-dependence of open-circuit flux density has more practical meaning among above temperature stability parameters. The open-circuit flux density decreased to B(T₁) from B(T₀) when ambient temperature increased to the certain T₁ from room temperature T₀, and open-circuit flux density will return to smaller B’(T₀) after ambient temperature back to room temperature. The open-circuit flux density show reversible change between B’(T₀) and B(T₁) when magnets are again between T₀ and T₁. The total flux losses L = B(T₀) – B(T₁) can be classified into two parts: reversible flux losses L_rev = B’(T₀) – B(T₁) and irreversible flux loss L_irr = B(T₀) – B’(T₀). The Reversible Temperature Coefficient of open-circuit flux density α_B(T) can be calculated by:

In fact, the α_B(T) is equal to Temperature Coefficient of remanance α_Br(T). Both α_B(T) and α_Br(T) is only determined by the intrinsic magnetic properties of material, or relationship M_s(T) between saturation magnetization M_s of the main phase and temperature. In other words, M_s(T) and Temperature Coefficient can be adjusted by addition of other elements. Irreversible flux losses are influenced by many factors, such as temeperature-dependence of intrinsic coercivity, magnetic viscosity, load line, service temperature and relevant duration.

Irreversible flux losses L_irr are refers to the short-time irreversible flux losses, therefore, it also can be called initial irreversible flux losses. The total aging flux losses also includes long-term irreversible flux losses which originate from metastability of magnetization state. Both of them are caused by the changing of the magnetic domain structure, but not structural change, and all can be recovered by re-magnetizing.