Magnetic properties of <italic>L</italic>1<sub>0</sub>-FePt(001)/<italic>A</italic>1-FePt bilayer exchange springs

Abstract

Soft A 1-FePt (30 nm) films were magnetron-sputtered on MgO(001) single-crystal substrates at an elevated temperature of 400°C, and subsequently annealed at diffrent temperatures in range of T a=[400°C, 700°C] for 6 h in order to turn them into hard magnets with different degrees of A 1→ L 10 transformation. Then second layers of A 1-FePt with different thicknesses were covered at 100°C to obtain L 10-FePt/ A 1-FePt bilayer magnetic exchange springs with (001) textures. At T a≤600°C, the coverage of hard layer was ~100%, though the grain size increased with T a. The surface roughness reduced from 2.45 to 1.02 nm if the film was annealed at T a=500°C. The surface of hard layer flattened by annealing should benefit to improve the strength of interlayer exchange interaction. At T a=500°C and 600°C, A 1-FePt and L 10-FePt coexisted in the hard layer with an ordering degree of 0.61 and 0.84, respectively. The direction of magnetic easy axis began to switch from in plane to out of plane at T a=500°C, indicating a vertical magnetocrystalline anisotropy was generated. The out-of-plane magnetization curves showed rectangle-like shapes at 500°C≤ T a≤600°C, in spite of the soft phase left in hard layer, and a moderate coercive force of ~5 kOe was obtained at T a=500°C. This indicates the exchange interaction between coexistent two phases in hard layer was strong enough to pin the soft magnetic moments with the hard ones. At T a=700°C, the coercive force exceeded 20 kOe, which is too huge to be affected by exchange springs. By using the film of T a=500°C as hard layer, L 10/ A 1-FePt bilayer film with 20 nm thick soft layer kept rigid behaviors with almost the same coercive force of single hard layer, and respective reversal steps for soft layer and hard layer appeared in the magnetization curve if the thickness of soft layer was 30 nm, with a reduced coercive force of ~4 kOe. As double shoulders should appear in the hysteresis loop if the thickness of soft layer exceeds twice of domain wall thickness of hard layer, i.e., the so-called interlayer exchange length, this indicates that the hard/soft interlayer exchange length was in the range of 20 nm l ex L 10/ A 1-FePt exchange spring. The reason is due to that the coexistence of A 1-FePt and L 10-FePt in hard layers lowered the efficient uniaxial magnetocrystalline anisotropy. This lengthened distance of interlayer exchange could be maintained at T a=600°C. This is benefit to design the properties of magnetic springs for different applications.