Micromagnetic Simulation Of The Demagnetization Process In Antiferromagnetically Exchange-coupled Trilayers

Magnetic materials play a more and more prominent role in our contemporary society, and applications for permanent magnets have already been extended to many aspects of our lives. Following the development of magnetic materials, the scope of permanent magnets is still widening because of the properties of nanocomposite magnetic materials which combine the high remanence of a soft layer and the high coercivity of a hard layer, resulting in a rather high energy product. In addition, this new kind of material demonstrates magnetoresistance and perpendicular exchange-coupling, among other effects. For the past few years, magnetic composites having an exchange-coupling due to their hard/soft layers gradually became a hot topic for the preparation of high performance permanent magnetic materials, giving rise to numerous studies around the world. This paper utilizes traditional analytical methods as well as 3D simulations to investigate the magnetic reversal process in antiferromagnetically exchange-coupled hard/soft/hard trilayers. Basing our research on the micromagnetic theory, we systematically analyzed the hysteresis loops, remanences, and coercivities, among other properties of trilayer systems, according to layer thickness and magnetic moment distribution under all kinds of field. Our main research work is as follows:1. Two independent processes take part in the demagnetization process in antiferromagnetically exchange-coupled trilayers, the first one being the process from parallel to antiparallel state, the other one being the process from antiparallel to reversal parallel state. We used analytical simulation methods to carry out the derivation and calculation of the corresponding formulae, which allowed us to get the equations of the nucleation fields of these two processes. Through calculation we obtained the first nucleation field HN1 of the parallel to antiparallel state process, and discovered that the antiferromagnetic interface coupling(J) has a strong influence on it. When the absolute value of J tends to infinity, HN1 is tending to minus infinity, so that complete parallel state is unattainable in antiferromagnetic trilayers with strong coupling. The hard layer thickness Lh does not have much of an impact on HN1, while the soft layer thickness Ls has a rather obvious effect on HN1 only when it is small enough. About the process from antiparallel to reversal parallel state, Lh and J all have a marked impact on the second nucleation field HN2. Simultaneously, because of the inhibition inducted by J on HN2, the latter is bigger than the magnetocrystalline field of the hard layer Hk h.2. Micromagnetic methods have been applied to obtain angular distributions and hysteresis loops of hard/soft/hard trilayers, characterized by antiferromagnetic exchange coupling at interfaces. It is observed that compared to ferromagnetically exchange-coupled trilayers, the hysteresis loops of antiferromagnetically exchange-coupled trilayers differ widely. Hysteresis loops of the former look like a single square, while those of the latter look like two squares. It is worth noting that Ls has a very marked impact on the shape of the hysteresis loops. Another difference between the ferromagnetically and antiferromagnetically exchange-coupled trilayers is in the process from parallel to antiparallel state; the deflection angle of the magnetization in the hard layer of the latter gets bigger as H increases, and when close to the antiparallel state, the magnetic moments in the hard layer turn back.