Study On The Exchange Bias Behavior Of Magnetic Bilayers

Recently, nanomagnetic materials have gained high interest from both fundamental and application research, stemming partially from the applications in high-density magnetic storage media and biomedicine. For the former, the most challenge is to beat the superparamagnetism at finite temperatures, which causes thermal destabilization of the nano-scale recording units. It is found that exchange bias in composite nanostructures may be the core technology to solve this issue. Exchange bias is one of essential physical mechanisms in the basic structures of hard disk driver head and magnetic random access memory and it has been extensively studied during the last several decades. Motivated by recent experimental findings, exchange bias in the nanostructures with various morphologies, including the ferromagnetic-amorphous magnetic bilayers, ferromagnetic-antiferromagnetic bilayers and a certain shift of the ferromagnetic-antiferromagnetic bilayers, is studied by performing a modified Monte Carlo method, based on the classical Heisenberg model.In the present paper, the standard Monte Carlo simulation is modified. We not only consider the thermal fluctuations of all spins along the arbitrary directions, but also calculate the energy of each spin at any time in order to clarify whether there are the energy barriers during the reversal of spin. The acceptation probability in the standard Monte Carlo simulation is dependent on the energy difference between the new and initial states. Whereas in this paper the acceptation probability is determined by the energy difference between the saddle point and the initial state, as long as the rotation of spin needs to straddle the energy barriers. After the modification, the Monte Carlo simulation has proved useful for the systems with finite anisotropies.For the ferromagnetic-amorphous magnetic bilayers, when the spins anisotropy orientation in the domains is random, no matter how much of the system parameters, the hysteresis loop of system is symmetrical about the origin of coordinates. There is no exchange bias phenomenon. The coercive field and magnetic energy of the system increases with enhancing spins anisotropy in the domains, however there is a little change in saturation magnetization. When the spins anisotropy orientation obey normal distribution around the z axis or the orientation is constant, there is no exchange bias effect in case the spins anisotropy within the amorphous layer is smaller. While the spins anisotropy within magnetic domains increases to3.5, the exchange bias behavior of the system begins to appear. We found that the exchange bias field increases with increasing the spins anisotropy within domains, however coercive field shows a non-monotonic change with the enhancing the spins anisotropy. The value of the exchange bias and coercive field is almost no change in changing the cooling field, but the spin coupling within the amorphous layer domains play an important role in the exchange bias effect. At the same time the ferromagnetic or antiferromagnetic interface coupling of the bilayers and the coupling strength obviously impact the exchange bias and coercive field. This is rooted in the spins anisotropy in the domains pinning effect on the ferromagnetic spins and the net remanence at the interface.Then we investigated the exchange bias effects of ferromagnetic-antiferromagnetic bilayers system. The results show that when the antiferromagnetic spins anisotropy is small, there is no obvious exchange bias whatever we change the system parameters. But the coercive field of the system is greatly impacted. Even more interesting, we observed a small antiferromagnetic hysteresis effect. The reason is that the anti-parallel antiferromagnetic spins arrangement and the effects of spins frustration make the system stay in metastable state. When we set a larger antiferromagnetic spins anisotropy value (greater than5), the system shows significant exchange bias effect and the exchange bias field enhances with increasing the antiferromagnetic spins anisotropy. Antiferromagnetic spins anisotropy, interface coupling and cooling field have a great impact on the exchange bias behavior. The results show that the antiferromagnetic plays a key role in the exchange bias system.Finally, we calculate the exchange behavior of ferromagnetic-antiferromagnetic bilayers with a certain displacement between two layers. We find that, under the same parameters, the exchange bias effect in such system is weaker than that in the standard ferromagnetic-antiferromagnetic system. At the same time, we do not observe the small hysteresis which appears in the standard FM-AFM system. This is because in such model, the number of nearest neighbor spins between different layers is4that is different with the standard FM-AFM system. In this case, the antiferromagnetic spins anisotropy is not only pinning the ferromagnetic spins but also be coupled strongly by the ferromagnetic spins. As a result, the metastable state of antiferromagnetic spins distribution becomes unstable.In summary, a modified Monte Carlo method is used to study the exchange bias in the bilayers composite nanostructures with various morphologies. By means of calculating magnetization, microscopic spin configurations and energy distributions, hysteresis loops, and others, the microscopic origin of exchange bias in the nanostructures with various morphologies is shown.