A crystal the size of a fingernail sits between the plates of a capacitor. A magnetic field is switched on, and the crystal stretches like a ribbon pulled at both ends: its length increases by 60, or even 120, percent. This is not a typo. Ordinary iron in a magnetic field changes its length by millionths of a percent, whereas here the change is large enough to see with the naked eye.
The materials in question are rare-earth orthoferrites, compounds of iron, oxygen, and a rare-earth element. The researchers grew single crystals of HoFeO₃ and DyFeO₃, cooled them to a few kelvins, and applied a field of 90 kilo-oersteds. Along one of the axes, the crystal began to grow like a plant stem in time-lapse footage. Along the other two axes, it barely changed. This anisotropy is the key to understanding the mechanism.
Why does this happen? The crystals contain two magnetic sublattices: an iron one and a rare-earth one. Holmium and dysprosium ions have very strong spin-orbit coupling, the interaction between an electron's intrinsic spin and its motion around the nucleus. Like a spring, this coupling ties the ion's magnetic moment to its position in the lattice. When an external field flips the magnetic moments, the ions shift, and the entire crystal abruptly changes shape. This is a first-order phase transition, a jump from one state to another, with hysteresis: if the field is removed, the crystal does not immediately return to its original length.
To confirm that the effect is tied specifically to the rare-earth ion, the researchers measured a third crystal, GdFeO₃. Gadolinium has weak spin-orbit coupling. And what did they find? The magnetostriction was a hundred times smaller, and there was no structural transition at all. This is direct proof: the giant deformation is triggered by rare-earth anisotropy, not by iron.
Along with the elongation, the electrical response changes too: the dielectric permittivity along the c axis drops by more than 65%. A crystal that breathes under a magnetic field also changes its electrical properties. This combination opens the way to magnetically controlled actuators, strain sensors, and adaptive structures that operate at low temperatures. For now it is cryogenic exotica, but the principle has been found: rare-earth orthoferrites are a platform for extremely large reversible deformations controlled by a field.
Magnetostriction was discovered by James Joule in 1842: an iron rod lengthened by millionths of a percent in a magnetic field. In the new work, the deformation is a million times larger.
In Arthur C. Clarke's novel Rendezvous with Rama, an alien ship changes its geometry under the action of external fields. Rare-earth orthoferrites with their 120% elongation resemble such a "smart" material, although they work only at cryogenic temperatures.