Energy landscape of noncollinear exchange coupled magnetic multilayers
arxiv(2024)
摘要
We conduct an exploration of the energy landscape of two coupled
ferromagnetic layers with perpendicular-to-plane uniaxial anisotropy using
finite-element micromagnetic simulations. These multilayers can be used to
produce noncollinearity in spin-transfer torque magnetic random-access memory
cells, which has been shown to increase the performance of this class of
computer memory. We show that there exists a range of values of the interlayer
exchange coupling constants for which the magnetic state of these multilayers
can relax into two energy minima. The size of this region is determined by the
difference in the magnitude of the layer anisotropies and is minimized when
this difference is large. In this case, there is a wide range of experimentally
achievable coupling constants that can produce desirable and stable
noncollinear alignment. We investigate the energy barriers separating the local
and global minima using string method simulations, showing that the stabilities
of the minima increase with increasing difference in the anisotropy of the
ferromagnetic layers. We provide an analytical solution to the location of the
minima in the energy landscape of coupled macrospins, which has good agreement
with our micromagnetic results for a case involving ferromagnetic layers with
the same thickness and anisotropy, no demagnetization field, and large exchange
stiffness. These results are important to understand how best to employ
noncollinear coupling in the next generation of thin film magnetic devices.
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