A metal bar suddenly becomes stiff when heated, and at the same time loses or gains magnetic properties. In alloys based on nickel and manganese, such transformations occur not smoothly but abruptly — atoms rearrange into a new order, and the material changes its behavior. In this work, researchers studied four very similar compositions of Ni-Mn-Cu-Ga, where the copper fraction differed by only fractions of a percent. That was enough to obtain three different scenarios: in one case, magnetism disappeared first, then the lattice changed; in another, the order was reversed; and in the third, both events occurred simultaneously.

The main trick is a new thermodynamic analysis. Usually, the magnetic transition temperature (Curie point) and the lattice rearrangement temperature are measured separately, but here they influence each other. The authors took into account how changes in the crystal structure alter the strength of interaction between electron spins — the so-called exchange parameter that determines whether a material is magnetic. It turned out that for the same alloy, the Curie point in the high-temperature phase (austenite) and in the low-temperature phase (martensite) differs by more than 50 degrees. The same piece of metal has two different "magnetic personalities" depending on how its atoms are arranged.

The practical benefit of the method is that it allows extracting hidden transition temperatures from standard magnetization measurements. Previously, this required complex experiments, but now it is enough to track how the magnetic response changes upon heating and apply the new calculation. This is important for fine-tuning ferromagnetic shape-memory alloys — materials that can bend and return to their original shape under a magnetic field. They are used in microactuators, sensors, and even medical instruments where silent movements without motors are needed.