page_title: Exchange spring magnet
section_title: Exchange spring magnet
hierarchical_section_title: Exchange spring magnet / Magnetic energy / Exchange spring magnet
caption_attribution_description: English: Illustration of magnetic moments of exchange spring magnet
context_page_description: An Exchange Spring Magnet is a magnetic material with high coercivity and high saturation properties derived from the exchange interaction between a hard magnetic material and a soft magnetic material, respectively. Coehoorn et al. were the first able to observe an actual exchange spring magnet. Exchange spring magnets are cheaper than many magnets containing rare earth/transition metals, as the hard phase of the magnet can be less than 15% of the overall magnet by volume.
context_section_description: In the exchange spring magnet, the hard phase has high coercivity and the soft phase has high saturation. The hard phase and the soft phase interact through their interface by exchange coupling.
  From left to right in Figure 3, an external field is first applied in an upward direction in order to saturate the magnet. Then, the external field is reversed and starts to demagnetize the magnet. Since the coercivity of the hard phase is relatively high, the moments remain unchanged so as to minimize the anisotropy and exchange energy. The magnetic moments in the soft phase start rotating to align with the applied field. Because of the exchange coupling at the soft/hard interface, the magnetic moments at the soft phase boundary have to align with the adjacent moment in hard phase. At the regions close to the interface, because of exchange coupling, the chain of magnetic moments acts like a spring. If the external field is increased, more moments in soft phase rotate downward, and the width of the transition region becomes smaller as the exchange energy density increases. The magnetic moments in the hard phase do not rotate until the external field is high enough that the exchange energy density in the transition region is comparable to the anisotropy energy density in the hard phase. At this point, the rotation of magnetic moments in the soft phase starts to affect the hard phase. As the external field surpasses the hard material's coercivity, the hard magnet gets fully demagnetized.
  In the previous process, when the magnetic moments in the hard magnet start to rotate, the intensity of external field is already much higher than the coercivity of the soft phase, but there is still a transition region in soft phase. If the thickness of the soft phase is less than twice as thick as the transition region, the soft phase should have a large effective coercivity, smaller than but comparable to the coercivity of the hard phase.
  In a thin soft phase, it is hard for the external field to rotate the magnetic moments, similar to a hard magnet with high saturation magnetization. After applying a high external field to partially demagnetize the magnetic moments in the hard phase and after subsequently removing the external field, the rotated moments in the soft phase can be rotated back by exchange coupling with the hard phase (Figure 5). This phenomenon is shown in the hysteresis loop of an exchange spring magnet (Figure 6).
  Comparing the exchange spring magnet's hysteresis loop with that of a conventional hard magnet demonstrates that the exchange spring magnet is more likely to recover from the opposing external field. When the external field is removed, the remanent magnetization can recover to a value close to its original. The name "exchange spring magnet" is derived from the reversibility of magnetization.
  The dimension of the soft phase inside the exchange spring magnet should be kept small enough so as to retain reversible magnetization. Additionally, the volume fraction of the soft phase needs to be as large as possible in order to achieve a high magnetization saturation. One viable material geometry is to fabricate a magnet by embedding hard particles inside a soft matrix. That way, the soft matrix material occupies the largest volume fraction while being close to the hard particles. The size and spacing of the hard particles is on the scale of nanometers. If the hard magnets are spheres on an fcc space lattice in the soft magnetic phase, the volume fraction of the hard phase can be 9%. Since the total magnetization saturation is summed up by volume fraction, it is close to the value of a pure soft phase.
caption: Figure 5: Magnetic moments of an exchange spring magnet upon introducing and removing a relatively large external magnetic field.
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