Effect of annealing on magnetic exchange coupling in CoPt/Co bilayer thin films

Abstract

Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase nanocomposite permanent magnets. The bilayers were prepared by magnetron sputter deposition of near-equiatomic CoPt with a thickness of 25 nm onto oxidized Si wafers. In the as-deposited state, CoPt had the A1 (fcc) structure and was magnetically soft. Before reinsertion into the sputtering chamber for the deposition of 2.8–16.7 nm thick Co layers, the CoPt films were annealed at 700 °C for 120 min to produce the magnetically hard, fully ordered L10 phase. The presence of exchange coupling in the bilayers was verified by magnetic hysteresis and recoil measurements and showed that only for Co thicknesses below 6.3 nm was this layer (in its as-deposited state) coupled through its full thickness to the CoPt layer. Annealing the bilayer samples at 300 and 550 °C for 20 min resulted in improvement of the interlayer magnetic coupling and produced clear differences in the magnetic reversal coherency and the recoil curves. However, for some samples, the improved coupling resulted in a decrease in coercivity, indicating that there is an optimum in the coupling strength for the attainment of high coercivity. Transmission electron microscopy studies of the bilayers in plan view showed that the increased interlayer coupling with annealing was a result of improved granular epitaxy of Co to CoPt.

Formation of L10-ordered CoPt during interdiffusion of electron-beam-deposited Pt/Co bilayer thin films on Si/SiO2 substrates by rapid thermal annealing

“…Based on the current state of Si technology, the preparation of ordered CoPt on Si substrates with a 10 nm scale would lead to expand the device applications. Some groups have reported ordered CoPt films on Si substrates, which were fabricated by a commonly used deposition method of sputtering [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. To fabricate nanoscale ordered CoPt, etching process such as Ar ion milling is performed after the preparation of ordered CoPt films by sputtering [37][38][39].…”

Role of diffused Co atoms in improving effective exchange coupling inSm−Co∕Fespring magnets

“…In layered structures, the magnetic anisotropy configuration and structural parameters can be readily controlled, thus making them an ideal model system for studying the fundamental properties of exchangespring magnets. [3][4][5][6] In a Sm-Co/ Fe magnet, the maximum energy product achievable is limited by a low nucleation field of the soft Fe phase. The effectiveness of the interphase exchange coupling at the Sm-Co/ Fe interface plays a crucial role in the overall magnetic properties, since the exchange coupling is strongly related to the Fe nucleation field.…”

Magnetization reversal in CoPt-based hard–soft homocomposites

“…Furthermore, since CoPt can be produced in either a high-anisotropy tetragonal or a low-anisotropy cubic phase, it is an ideal homocomposite system to study exchange coupling between hard and soft phases. The degree of chemical ordering and consequently the anisotropy of the hard layer can be varied by using different annealing temperatures in order to study the effect of hard [13]. In the following text, we report on the exchange coupling properties of homocomposites consisting of the hard-magnetic ordered tetragonal CoPt phase and soft-magnetic disordered cubic CoPt.…”