Multiferroics: Magnetic puzzle solved

The spin and orbital configurations of LuFe₂O₄ are solved by X-ray spectroscopy and theoretical modeling.

Multiferroic materials exhibit simultaneous ferromagnetic and ferroelectric properties, and their coupling — controlling the magnetic properties by electric fields and vice versa — could provide the basis for wide ranging applications.

Although it is accepted that the properties of multiferroics stem from the spatial arrangements of spins and charges, a detailed understanding of the physics underlying these characteristics remains lacking. A classical example is lutetium-iron oxide (LuFe₂O₄). Determining the spin structure of this material is particularly important in order to understand the origin of its exceptional magnetic properties including high magnetic anisotropy and huge magnetic coercivity.

Now, J.-H. Park and colleagues in Korea and Japan¹ have taken up this challenge and solved the magnetic puzzle governing the properties of this material through X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) measurements and theoretical calculations.

Fig. 1: Schematic illustration of the spin structure of iron ions in FeO double layers.

The structure of LuFe₂O₄ consists of hexagonal double layers of FeO intercalated with oxygen and lutetium planes. Below 340 K, the FeO planes exhibit charge ordering, that is, Fe³⁺ and Fe²⁺ are ordered spatially such that in one layer an Fe³⁺ ion is placed amongst Fe²⁺ ions to form a hexagon, and in the other layer the opposite occurs. By analyzing XMCD data, the team concluded that all Fe²⁺ spins are aligned with the magnetization, while 1/3 of the Fe³⁺ spins are parallel and 2/3 are antiparallel. The puzzling aspect of this finding is that the total magnetization possible with this configuration is lower than the saturation magnetization measured for the material.

The team suspected that such a discrepancy could be due to the influence of the orbital magnetic moment, and this was confirmed by XAS and XMCD. An additional electron in the d shell of the Fe²⁺ ion fills the lowest orbital state, but due to spin–orbit interaction, this state becomes non-degenerate, resulting in a large unquenched orbital momentum. In addition to explaining the saturation magnetization, this large orbital moment is also the origin of the magnetic anisotropy and coercivity of LuFe₂O₄.

Apart from their fundamental relevance, the results may be important for other applications. “The high magnetic anisotropy in LuFe₂O₄ is solely due to iron and our results are important to develop new permanent magnets without rare earths,” says Park.