Formation And Properties Of Exchange Springs In L1₀-FePt/A1-FePt Bilayer Films

Due to large uniaxial magnetic anisotropy(Ku=～7×107 erg/cc), high saturated magnetization(Ms=～1200 emu/cc), and stable chemical properties, the L10-FePt(FCT) alloy with nanograins can use to make storage media, magnetic tunnel junction or spin valve. However, the extremely high coercivity will bring about difficulty of magnetization reversal. The exchange springs in hard/soft multilayer films can efficiently reduce the switch fields of hard layers. Using Fe, Co, Ni and their alloys as the soft layers, however, it is not easy to obtain good L10-FePt-based exchange springs due to poor corrosion resistence and/or lattice mismatch. Soft Al-FePt(FCC) Soft Al-FePt(FCC) can epitaxially grow on L10-FePt. In this research, FePt films were deposited on heated MgO(110) substrates by using magnetron sputtering, and annealed to transit from Al phase into L10 phase growth of L10-FePt formed [110] texture by magnetron sputtering. Both the [001] direction (easy axis) and [110] direction (hard axis) of Llo-FePt are in plane with small demagnetization factors(N=-0). This can avoid the influence of the shape anisotropy. Then A 1-FePt layers were grown to obtain L10-FePt/Al-FePt bilayer films. The coervicity was adjusted by changing Al-â†'Llo transition degree of first layer and thickness of second layer. Focusing on effective exchange length of hard layer, the properties of exchange spring were discussed. The main results are shown as follows:1. On MgO(110) substrates, FePt (10 nm) films were deposited at 400℃, and subsequently annealed for 6 h at different temperatures in the range of Ta= [400-700] ℃. The structures, surface morphologies, and magnetic properties were analyzed. (110) textures were formed on MgO(110) substrates heated to 400℃. The as-deposited FePt films were in disordered Al phase. At Ta= 500℃, Al phase and L10phase coexisted with strong exchange coupling between them. The [001] direction (easy axis) of L10-FePt paralleled to one edge of the square substrate, and the [110] direction (diagonal of two hard axis, i. e., in hard plane) paralleled to the other edge of substrate. The magnetization curve showed rigid behaviors. Magnetized along [001] direction, the coercivity was about 12 kOe. At Ta≥ 600 ℃, the FePt films were composed of larger separated islands due to lattice misfit between FePt and MgO, though more Al-FePt transited into Llo-FePt. Damages of film continuity were suffered. The (110) supperlattice peak disappeared at Ta= 700℃, showing that the orientation also started to change.2. FePt(10 nm,Ta)/Al-FePt(20 nm) bilayer films were fabricated by depositing second FePt(20 nm) layers at 100℃ on the annealed FePt(10 nm) films. At Ta= 400℃(as deposited), the bilayer film showed soft behaviors. At Ta= 500℃, two jumps appeared in magnetization curve in field applied along [001] direction of L10-FePt. This implys the existence of interlayer exchange coupling. But reversal domains could generate in soft layer. The effective exchange length of hard layer might be less than 10 nm. The nucleated domain walls were pressed onto hard/soft interface by external magnetic field. When field reached 7 kOe, the walls broke into hard layer, and resultantly the magnetization reversed quickly. At Ta= 600℃, the jumps in magnetization curve disappeared with a maximal magnetization lower than saturation magnetization. The curve shifted toward negative direction of originally allpied field. This suggests that the switch filed of hard layer increased significantly due to grain disperse. The intergrain exchange coupling in hard layer was fairly weak. When the filed reached negative maximum for the first time, only partial magnetization in hard layer aligned in negative direction and reversed hardly in positivefield. At Ta= 700℃, the continuity of hard layer destroyed thoroughly. Magnetized along Llo-FePt [001] direction, the curve shifted toward positive direction of originally allpied field.3. at last, FePt(30 nm,Ta)/A 1-FePt(ts) bilayer films were fabricated at Ta= 500℃ and 600℃, ts= 0-40 nm to further analyse the interlayer exchange coupling. The results indicate:(1) At Ta= 500C, the magnetization curve of single hard layer (at ts= 0) showed a good rectangle in filed applied along Llo-FePt [001] direction. The coervicity was as high as 11 kOe. At ts = 20 nm, no obvious change was observed, including the curve shape and the coervicity. Different from FePt(10 nm, Ta= 500℃)/Al-FePt(20 nm), the jumps in magnetization curve did not appear. This indicates that the exchange length of hard layer was different. Annealing hard layer at the same temperature (7a= 5008℃), the degree of A1â†'L10 transition in it would be almost the same, but the width of Al regions (also the width of L10 regions) might increase with the thickness. As a result, the pinning effect of L10-FePt on A 1-FePt was weaker in a thicker harlayer. This whould lead to a longer effective exchange length (Lex= Ï€A/Km where A is the stiffness constant) of hard layer due to a lower effective magnetocrystalline anisotropy (Ku). Therefore, the two jumps in magnetization curve of FePt(30 nm, Ta= 500℃)/Al-FePt(20 nm) vanished. At ts=30 nm, the two jumps appeared again. This means that the effective exchange length of FePt(30 nm, Ta= 500℃) was longer than 10 nm but shorter than 15 nm.(2) At 7a= 600 ℃, the coervicity of single hard layer (at ts= 0) was less than 7 kOe, though more FePt transited into L10 phase. This might be the result of grain growth during annealing. At ts= 20 nm, the magnetization jumped two times. This means that the effective exchange length of hard layer became shorter than 10 nm due to a larger effective magnetocrystalline anisotropy.