Bulk nanomaterials can be engineered by independently creating and then layering subunits of different functional components, according to research published in a recent issue of the journal Small. In a demonstration of their method, the international research team constructed a three-component, technologically important magnetic nanocomposite material with a record high energy density for magnetic materials in its class.  

As a result of intracomponent interactions, multicomponent nanomaterials could outperform some key single component materials used in electronics, energy-conservations devices, and biotechnologies. Reaching optimal performance in such materials requires precisely controlling the structure of each component during synthesis. However, as the number of the components grows, this control becomes increasingly more difficult. Components often require competing physical conditions to achieve their ideal grain size, shape, alignment along specific crystallographic orientations, or other structural properties. Chemical approaches for overcoming this difficulty are currently limited to one- and two-dimensional nanomaterials. New approaches such as multi-field coupling deformation work in three-dimensions but are limited to two functional components.

Inspired by the complex structures that can be built with simple building blocks, a team of researchers from Yanshan University in China, Beijing University of Technology, and The Pennsylvania State University considered a different approach, building complex nanomaterials with subunits of functional components. “This strategy enables us to separately manipulate conflicting constituents in different subunits,” says Xiangyi Zhang, a team leader from Yanshan University. “As such, it becomes very easy to generate a bulk multicomponent nanomaterial with the desired structures,” he says. 

As a test case, the research team focused on the bulk nanocomposite material (SmCo + FeCo)/NdFeB. Both SmCo and NdFeB are rare-earth hard magnetic compounds. FeCo is a soft magnetic material. Theoretical work has shown if the components have the ideal structure, this hard-soft combination could produce a permanent magnetic material with an unmatched energy density. The soft magnetic component is key to designing low-cost, high-performing nanocomposite magnets.

The three components of (SmCo + FeCo)/NdFeB bring complementary functionalities to the bulk material but have competing needs in terms of structural control. For example, the ideal grain size of FeCo favors synthesis at lower temperatures (e.g., below 600°C), but the magnetic alignment of NdFeB occurs in the temperature range 750-950°C. 

Applying the building block approach, Zhang and his team created one subunit with the functional components SmCo and FeCo. Using mechanical milling to control grain size, distribution, and fraction, the researchers made an amorphous matrix of SmCo with FeCo nanocrystals evenly distributed throughout. They mixed this with a subunit of NbFeB nanocrystalline powder. A bulk sample of the mixture was then deformed under high-pressure thermal compression (HPTC).

When the researchers characterized a cylindrical piece of the deformed sample using x-ray diffraction, transmission electron microscopy, and elemental analysis, the results revealed alternating layers of (FeCo + SmCo) and NdFeB nanostructures that were micrometers thick.

Further analysis revealed that the resulting layered architecture maintained the desired structures of the components. In addition, strain-energy anisotropies in the SmCo matrix and NdFeB crystal caused the hard magnetic grains to align along their easy magnetization axes under HPTC despite temperatures below 750°C. As a result of these factors, the (SmCo + FeCo)/NdFeB had an energy density of 31 MGOe (247 kJ/m³), a record for this class of materials with high soft-magnetic fractions. The material also displayed good long-term thermal stability.

Challenges remain in creating bulk nanocomposite magnets that can match the energy density and ease with which rare-earth magnets can be synthesized, but according to David Sellmyer, director of the Nebraska Center for Materials and Nanoscience at the University of Nebraska-Lincoln, “This work is a valuable step on the path toward creating bulk exchange-coupled magnets with important technological applications.” Sellmyer was not involved in this study.

“The authors have performed a useful study of hard-soft exchange-coupled magnets that addresses one of the persistent limitations of the field. Namely, several groups have been able to synthesize hard-soft nanoscale magnets with high energy products (above about 40 MGOe), but only in thin-film form,” Sellmyer says.

Although the test case was a multicomponent bulk nano-magnet, the researchers anticipate that this method of simultaneously controlling all of the structural properties of the constituents in a bulk multicomponent nanostructure can be applied much more broadly, for example to thermoelectric materials, multiferroics, and more complex bulk hybrid nanomaterials.

Read the abstract in Small.