This application relates to magnetic materials and structures having at least one free ferromagnetic layer.
Various magnetic materials use multilayer structures which have at least one ferromagnetic layer configured as a “free” layer whose magnetic direction can be changed by an external magnetic field or a control current. Magnetic memory devices may be constructed using such multilayer structures where information is stored based on the magnetic direction of the free layer.
One example for such a multilayer structure is a spin valve (SV) which includes at least three layers: two ferromagnetic layers and a conducting layer between the two ferromagnetic layers. Another example for such a multilayer structure is a magnetic or magnetoresistive tunnel junction (MTJ) which includes at least three layers: two ferromagnetic layers and a thin layer of a non-magnetic insulator as a barrier layer between the two ferromagnetic layers. The insulator for the middle barrier layer is not electrically conducting and hence functions as a barrier between the two ferromagnetic layers. However, when the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer.
Notably, the resistance to the electrical current across the MTJ or SV structures varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers are parallel to each other, the resistance across the MTJ or SV structures is at a minimum value RP. When the magnetizations of the two ferromagnetic layers are anti-parallel with each other, the resistance across the MTJ or SV is at a maximum value RAP. The magnitude of this effect is commonly characterized by the tunneling magnetoresistance (TMR) in MTJs or magnetoresistance (MR) in SVs defined as (RAP-RP)/RP.