BHkäyrä

BHkäyrä, or B-H curve, is a graphical representation of the relationship between magnetic flux density (B) and magnetic-field strength (H) in magnetic materials. It is most commonly depicted as a hysteresis loop when the applied field H is cycled. The major loop shows the full magnetization cycle, while minor loops illustrate partial reversals. The loop demonstrates hysteresis: B does not exactly retrace its path when H is increased and then decreased; energy is dissipated as heat with each cycle, proportional to the loop area.

Key parameters extracted from the curve include saturation flux density B_s, remanent flux density B_r (the residual B when H is zero), and coercive field H_c (the H required to bring B to zero). The shape and size of the loop depend on the material, its microstructure, temperature, mechanical stress, excitation frequency, and the history of prior magnetization.

Equations and interpretation: B = μ0(H + M). In ferromagnets the magnetization M is nonlinear and exhibits hysteresis, so the B-H relationship is typically presented as data rather than a simple linear form.

Measurement methods: B-H curves are obtained by applying a controlled magnetic field to a sample and measuring both B and H, using devices such as magnetometers, B-H tracers, or vibrating-sample magnetometers. The data may be collected for the major loop or for various minor loops.

Applications: The BH-käyrä is essential for designing magnetic circuits in transformers, motors, and inductors, as well as for magnetic storage devices, material characterization, and assessing energy losses, permeability, and coercivity of materials.