The effect of soft phase on magnetic properties and magnetization reversal process of Nd-Fe-B based multilayers

Permanent magnets are the silent workhorses of modern technology, found in everything from the motors that drive electric cars to the tiny speakers in our headphones. For decades, the strongest magnets available have been made from a specific combination of neodymium, iron, and boron. These materials are prized because they hold a powerful magnetic field, but scientists have hit a wall: they cannot easily make them any stronger without losing other essential qualities. To break through this limit, researchers have been exploring a different strategy: building magnets out of two distinct materials layered together. One layer is a "hard" magnetic material that resists losing its magnetism, acting as the anchor. The other is a "soft" magnetic material that is easy to magnetize but also easy to lose that magnetism. The hope is that if these two layers are placed close enough together, they will lock their magnetic directions together, creating a single material that is both incredibly strong and very stable.

A team of researchers at Guizhou University of Commerce in China has taken a deep dive into how these layers interact, using powerful computer simulations to model the behavior of these materials at a microscopic level. Instead of physically building these magnets, which can be difficult to control at such tiny scales, they created a digital laboratory. They simulated thin films made of neodymium-iron-boron as the hard layer and tested four different soft materials: iron, a specific iron-nitrogen compound, an iron-cobalt alloy, and an iron-boron compound. By stacking these layers in different arrangements—such as a soft layer sandwiched between two hard layers, or a hard layer sandwiched between two soft layers—they could watch exactly how the magnetic fields behaved as they changed the thickness of the soft layers.

The simulations revealed that the thickness of the soft layer is a critical dial that scientists can turn to tune the magnet's performance. As the soft layer gets thicker, the magnet becomes better at holding a strong magnetic field, but it becomes weaker at resisting outside forces that try to flip its magnetism. The researchers found that there is a sweet spot where these two competing effects balance out perfectly. For the most common combination they tested, with iron as the soft layer, the best performance occurred when the soft layer was about 4 nanometers thick. At this specific thickness, the material achieved a maximum energy product of 84.54 MGOe, a measure of how much magnetic energy the magnet can store. This was higher than other arrangements they tested, such as a single soft layer next to a hard one, or a hard layer sandwiched between two soft ones.

However, the most surprising discovery came when they swapped the soft material itself. When they used a compound made of iron and nitrogen, known as Fe16N2, as the soft layer, the results were even more dramatic. In this configuration, the magnet reached a maximum energy product of 109.38 MGOe at a soft layer thickness of 4 nanometers. This value is significantly higher than what was achieved with the other soft materials. The simulations showed that this superior performance comes from the unique way the iron-nitrogen compound interacts with the hard layer. The magnetic moments, which are the tiny internal arrows that point in the direction of magnetism, align very strongly across the boundary between the two materials. This strong connection allows the magnet to store more energy without losing its stability.

The researchers also looked closely at how the magnetization reverses, which is what happens when an external force tries to flip the magnet's poles. In the best-performing iron-nitrogen setup, the reversal process happens in a controlled, step-by-step manner. The magnetic arrows at the edges of the material begin to tilt first, and this tilt slowly spreads inward, like a wave moving through a crowd. This process is held in check by a mechanism called pinning, where the boundary between the hard and soft layers acts like a gate, preventing the reversal from happening too quickly or chaotically. This controlled behavior is exactly what is needed for a high-performance magnet. The study suggests that by carefully choosing the soft material and its thickness, scientists can design magnets that are far more powerful than what is currently possible, offering a clear path forward for the next generation of magnetic technology.