Hysteresikäyrä

Hysteresikäyrä (Finnish for hysteresis curve) is a graphical representation that shows the relationship between two interacting variables when the system exhibits a memory effect. The most common example is the magnetization (M) of a ferromagnetic material plotted against the applied magnetic field (H). When the field is increased from zero, the magnetization follows one path, and when the field is subsequently decreased, it follows a different path, creating a closed loop. This loop illustrates that the current magnetization depends not only on the present magnetic field but also on the history of how the field was applied.

The area enclosed by the hysteresis loop corresponds to the energy lost per cycle, primarily as heat due to internal friction or domain wall movement. In magnetic media such as transformers, motors, and recording tapes, a tightly bounded hysteresis curve (low coercivity and remanence) is desirable to minimize energy loss, whereas materials with larger loops are useful for magnetic memory storage where non‑volatile states are required.

Hysteresis is not limited to magnetism. Mechanical hysteresis curves plot stress versus strain in materials that show plastic deformation or viscoelastic behavior. In electronics, a Zener diode’s voltage‑current characteristic exhibits hysteresis at the breakdown threshold when reverse bias is cycled. Biological systems also display hysteresis, such as in the heart’s calcium handling or in neural firing thresholds.

Mathematically, hysteresis can be modeled with various phenomenological frameworks, including Preisach models and Rayleigh laws, which capture the cumulative effects of many elementary hysteretic units. These models are crucial for designing devices that either exploit or suppress hysteresis.