C16-Borides: High magnetic strength without rare earths and platinum

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Strong magnets are the basis for electric motors, generators, data storage, sensors, and energy-efficient drive systems, thus directly determining the performance, efficiency, and miniaturization of many technical applications. At the end of December, researchers at Georgetown University presented a new class of magnets free of rare earths in the journal Advanced Materials, based on so-called C16 high-entropy borides. The thin films consist exclusively of transition metals and boron – and, according to the researchers, still achieve magnetic anisotropies that approach those of established rare-earth permanent magnets. The work explicitly aims to relieve critical raw material supply chains, as well as applications in storage technology, spintronics, and energy-efficient magnets.

Previously, high-performance permanent magnets have been based either on rare earths such as neodymium – for example, in the widely used NdFeB magnets – or, in the thin-film sector, on alloys with a high platinum content. Both classes of materials are considered expensive, ecologically problematic in their extraction, and geopolitically risky. The researchers therefore explicitly refer to the search for alternatives made from more readily available elements. The presented C16 high-entropy borides are intended to address exactly this: high magnetic anisotropy without rare earths and without precious metals.

High-Entropy Chemistry Instead of New Crystal Structures

The starting point of the study is the tetragonal C16 boride structure – a lattice type known from classical compounds of transition metals with boron. The transition metals used are technically important elements such as iron, cobalt, nickel, and manganese. These so-called 3d metals, named after the electrons in their outer atomic shells (3d orbitals), are particularly relevant for magnetism. Instead of using only one or two of these metals, the researchers around Kai Liu and Gen Yin pursue a chemical approach: the metal sites in the lattice are occupied by a mixture of five or more different 3d metals. “High entropy” here means that the high chemical disorder – many different types of atoms on equivalent lattice sites – stabilizes the material and simultaneously changes its electronic properties.

Experimentally, the authors rely on so-called combinatorial co-sputtering: several metallic targets are deposited simultaneously onto a heated substrate, so that dozens of different compositions are created on a single wafer. This approach is intended to significantly accelerate the search for optimal material combinations.

Results: Thin films with pronounced preferred orientation

Structurally, the researchers' investigations showed that despite the high chemical diversity, an ordered, textured C16 phase forms. Magnetically, the targeted control of anisotropy is crucial: by varying the metal mixtures, the magnetization can be rotated from a flat orientation parallel to the layer to a pronounced “easy axis” behavior perpendicular to the layer. This perpendicular preferred orientation is essential for applications such as storage media or permanent magnets.

The result: the new material mixtures are significantly superior to simple boride compounds made of only two elements. According to the paper, they produce magnets that retain their magnetic strength much better (high coercivity) and have a stronger internal orientation (high anisotropy). Their quantum mechanical simulations link these effects to changes in spin-orbit coupling, i.e., the interaction between electron spin and orbital motion. The researchers mention future high-density storage media, spintronic components, and rare-earth-free permanent magnets as possible applications.

However, the new magnets are currently only thin-film laboratory samples in the micrometer range. Whether scalable permanent magnets with sufficient energy product and temperature stability can be produced from C16 high-entropy borides remains to be seen.

Without rare earths – but not independent

As promising as the approach is, although the C16 borides do without rare earths and platinum, boron and central transition metals are also not available everywhere.

The supply situation for boron is actually extremely tight. The EU Critical Raw Materials Act, which came into force in May 2024, classifies boron as a strategic raw material. The EU is almost 100 percent dependent on imports for boron, which come almost exclusively from Turkey. Economically relevant deposits are lacking in Europe. Elemental boron is also not harmless; in higher doses, it is toxic to reproduction and development, which is why the EU has classified boric acid and borates as toxic to reproduction.

This import dependency fits into the overall picture of European raw material supply, as the debate about rare earths and platinum metals also shows. Even with intensified recycling and re-industrialization, Europe remains dependent on global supply chains for numerous critical raw materials. New magnetic materials can shift dependencies, but they cannot solve the fundamental problem.

(vza)