The work stands apart among studies of the magnetic and structural properties of rare-earth orthoferrites. Neighboring papers in the archive deal with superconductivity, graphene, quantum-like models and collective behavior, as well as magnetic textures and frustrations. Against this backdrop, the present study stands out for its direct measurement of giant magnetostriction in bulk single crystals at low temperatures. Until now, the structural properties of RFeO3 under a magnetic field had not been studied systematically, although electromagnetic responses along the c axis had already attracted attention thanks to strong spin-orbit coupling and spin-reorientation transitions.
Single crystals of HoFeO3, DyFeO3, and GdFeO3 were grown in a four-mirror furnace with halogen lamps in an oxygen atmosphere. Phase purity and structure were confirmed by X-ray diffraction on a Rigaku TTRAX II and by the Laue method. Magnetic measurements were performed on a PPMS and a SQUID-VSM. Thermal expansion and magnetostriction were measured with a capacitance dilatometer from the Vienna University of Technology using an Andeen-Hagerling AH2700A bridge. Samples about 1×1×1 mm³ in size were placed between the plates of a capacitor, and the field was applied along the axis being measured. The dielectric permittivity was determined with the same bridge under an electric field along the chosen direction.
In HoFeO3 at 90 kOe and 2 K, the relative elongation along the c axis exceeds 60%, while in DyFeO3 it reaches 115-120% at 5 K. Along the a and b axes, the magnetostriction is only 0.1-3% in HoFeO3 and about 10% in DyFeO3, that is, two orders of magnitude smaller. The transitions are accompanied by strong thermal hysteresis, which points to a first-order transition. The magnetodielectric effect is also anisotropic: the dielectric permittivity along c drops by more than 65% at 90 kOe and 35 K. In GdFeO3, where spin-orbit coupling is weak, the magnetostriction is two orders of magnitude smaller and the structural transition is absent. This confirms the key role of rare-earth anisotropy and the interaction between the Fe and R sublattices.
The work differs from neighboring studies in that it demonstrates for the first time a magnetically induced first-order structural transition with giant deformation in bulk single crystals of orthoferrites. Neighboring papers in the archive do not address magnetostriction on this scale: they study superconductivity, quantum models, or magnetic textures. Here, by contrast, it is shown that rare-earth orthoferrites can serve as a platform for extremely large reversible field-controlled deformations. This extends the class of materials with colossal magnetostriction beyond shape-memory alloys.
The topic could develop toward the search for room-temperature analogues with a similar response. It may be possible to study thin films and heterostructures based on RFeO3 for integration into devices. It would be interesting to examine the influence of pressure, electric field, and light on magnetostriction. Theoretical models linking spin-orbit interaction to lattice instability will help predict new compositions with an even larger effect.
The results could influence the development of magnetic actuators, sensors, transducers, and adaptive structures. They are also important for the physics of multiferroics and spintronics.
Next steps include measuring magnetostriction in other rare-earth orthoferrites (for example, ErFeO3, TmFeO3) and searching for compositions with a transition at higher temperatures. A detailed in situ structural study under a field is also needed to clarify the microscopic mechanism.
The work is connected to the unsolved problem of the coupling between magnetic order and the lattice in strongly correlated oxides. The question of the microscopic mechanism of the giant magnetostriction remains open: exactly how the spin-orbit interaction of rare-earth ions triggers the structural transition. It is also unclear why the first-order transition occurs only along the c axis and whether it is possible at room temperature.
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.