Composition and size dependence of magnetic properties of FePt/Fe exchange-spring films

• Exchange-spring behavior of FePt/Fe multi-layers was studied via micro-magnetic simulation.
• As total thickness is 20 nm, optimal magnetic property is shown when the thickness ratio of Fe is 10%.
• As total thickness is 20 nm, nucleation fields are positive when the thickness ratio of Fe is 25% and larger.
• As total thickness is 20 nm, coercivity is greatly reduced when the thickness ratio of Fe is 25% and larger.
• Under fixed volume and the thickness ratio of Fe (10%), switching field can be reduced by reducing the bottom size.

The composition and size dependence of the magnetic properties of FePt/Fe exchange-spring bilayer films was studied using micromagnetic simulation. Based on the simulated hysteresis loops for composite layers with an identical thickness of 20 nm and different composition ratios, it can be observed that when the thickness ratio of Fe is 10%, an exchange-spring effect with a negative nucleation field appears; the switching field is greatly reduced compared to the rigid magnetic FePt, and the squareness ratio reaches its maximum value. When the thickness ratio of Fe is 25% and more, the nucleation fields become positive; meanwhile, the coercivity is smaller than the switching field, and the squareness ratio decreases because of the increase in the thickness of the Fe film. In addition, at a fixed thickness ratio and total volume, the switching field of the FePt/Fe bilayer films is further reduced, accompanied by a decrease in the squareness ratio due to an increase in the thickness of the Fe layer.