Magnetic stray fields in nanoscale magnetic tunnel junctions

Abstract

The magnetic stray field is an unavoidable consequence of ferromagnetic devices and sensors leading to a natural asymmetry in magnetic properties. Such asymmetry is particularly undesirable for magnetic random access memory applications where the free layer can exhibit bias. Using atomistic dipole-dipole calculations we numerically simulate the stray magnetic field emanating from the magnetic layers of an magnetic memory device with different geometries. We find that edge effects dominate the overall stray magnetic field in patterned devices and that a conventional synthetic antiferromagnet structure is only partially able to compensate the field at the free layer position. A granular reference layer is seen to provide near-field flux closure while additional patterning defects add significant complexity to the stray field in nanoscale devices. Finally we find that the stray field from a nanoscale antiferromagnet is surprisingly non-zero arising from the imperfect cancellation of magnetic sublattices due to edge defects. Our findings provide an outline of the role of different layer structures and defects in the effective stray magnetic field in nanoscale magnetic random access memory devices and atomistic calculations provide a useful tools to study the stray field effects arising from a wide range of defects.

Atomistic investigation of the temperature and size dependence of the energy barrier of CoFeB/MgO nanodots

“…fields due to the reference layer of the MTJ 31,32 that could reduce E b , as also discussed in the analysis of the effect of an external field. We remark that the thermal stability is one the main parameters for MRAM technology since it affects both the data retention of a device and the writing performances via the threshold current.…”

“…An applied field acting on the reference layer of a MTJ alters the energy landscape of the layer, and affects the reversal mechanism 22 . For simple MTJ geometries such as a single free layer MTJ 3,7,19 , the recording layer is subjected to the stray field coming from the reference layer which can affect the stability of the system 31,32 . We perform simulations at 300 K applying an external field B a =0.0 T, 0.1 T and 0.5 T along the positive z-direction perpendicular to the dot.…”

Spin transfer torque switching dynamics in CoFeB/MgO magnetic tunnel junctions

“…J ij represents the exchange coupling constant between spin i and j, k i u is the uniaxial energy constant on site i along the easy-axisê, µ i s is the atomic spin moment on the atomic site i, and H app is the external applied field. The magnetostatic contributions H dmg are calculated using a modified macrocell approach [19]. This method accounts for the contribution within each cell explicitly computing the interaction tensor from the atomistic coordinates, following the work of Bowden [20].…”

Field‐Free Spin–Orbit Torque Switching in Perpendicularly Magnetized Synthetic Antiferromagnets

“…[ 1–4 ] Analogous to the antiferromagnets, synthetic antiferromagnets (SAFs), formed through the exchange coupling due to Ruderman–Kittel–Kasuya–Yoshida (RKKY) interaction [ 5 ] between two ferromagnets mediated by a sandwiched metallic layer, are expectedly popular in the field of spintronics owning to several attractive properties. For instance, the SAFs are usually used as the reference layer or free layer in magnetic tunnel junctions (MTJs) so as to reduce the stray fields [ 6,7 ] and improve the thermal stability. [ 8–10 ] Besides, the SAFs exhibit a variety of domain structures, such as stripe and bubble domains, [ 11 ] implying that they are also useful for magnetic texture‐based devices.…”