Speakers
Description
Since the discovery of the exchange bias phenomenon, which is a shift in the hysteresis loop when coupling a ferromagnet (FM) to an antiferromagnet (AFM), a lot of scientific efforts were made in order to explain this effect. Although exchange bias is widely used in spin valves, it’s origin is not yet completely understood and sometimes obscured by the disorder in the antiferromagnetic grains and the various mechanisms that contribute to this effect. This makes it difficult to study these systems, using only theoretical models. Here we show that this complex interaction can be modeled within the standard micromagnetic software MuMax3[1].
Apart from a shift of the hysteresis loop, also a training effect in several polycrystalline FM/AFM bilayers can be found, i.e. the bias field and coercivity of the system decrease for an increasing number of hysteresis cycles n. This effect can be subdivided into 2 types: a thermal and an athermal component. The latter contains the largest contribution and is only present for n=1.
A common way to model these 2 effects is by considering the presence of pinned and rotatable uncompensated AFM spins at the FM/AFM interface. We are able to simulate these 2 effects in a polycrystalline Co(30nm)/CoO(3nm) bilayer taking into account the local granular structure of the AFM. We can also proof that the athermal component of the training effect can be attributed to the reorientation of rotatable uncompensated AFM spins during the first hysteresis loop. This leads to a reduction of the coercivity and as well as the bias field, as has been confirmed by experimental data[2].
[1] Arne Vansteenkiste, Jonathan Leliaert, Mykola Dvornik, Mathias Helsen, Felipe Garcia-Sanchez, and Bartel Van Waeyenberge. The design and verification of mumax3. AIP Advances, 4(10), 2014.
[2] T. Dias, E. Menéndez, H. Liu, C. Van Haesendonck, A. Vantomme, K. Temst, J. E. Schmidt, R. Giulian, and J. Geshev. Rotatable anisotropy driven training effects in exchange biased co/coo films. Journal of Applied Physics, 115(24), 2014.
