MRAM with means of controlling magnetic anisotropy
View Patent ↗A CPP MTJ MRAM unit cell utilizes transfer of spin angular momentum as a mechanism for changing the magnetic moment direction of a free layer. The strength of the switching field, H s of the cell is controlled by the magnetic anisotropy of the cell which, in turn, is controlled by a combination of the shape anisotropy and the stress and magnetostriction of the cell free layer. The coefficient of magnetostriction of the free layer can be adjusted by methods such as adding Nb or Hf to alloys of Ni, Fe, Co and B or by forming the free layer as a lamination of layers having different values of their coefficients of magnetostriction. Thus, by tuning the coefficient of magnetostriction of the cell free layer it is possible to produce a switching field of sufficient magnitude to render the cell thermally stable while maintaining a desirable switching current.
1. A spin transfer MTJ MRAM cell comprising:
an MTJ cell including a non-magnetic tunneling barrier layer formed between a magnetically pinned layer having a fixed magnetization and a magnetically free layer having a switchable magnetization;
wherein said free layer has a shape anisotropy in a plane of said free layer and;
wherein said free layer is surrounded by material;
wherein said free layer is under a tensile or compressive stress in the plane of said free layer produced substantially by residual tensile or compressive stresses caused by differential expansion and contraction of said surrounding material and;
wherein said free layer is formed having a material composition and structure that can be varied during said formation and wherein said free layer has a coefficient of magnetostriction, λ that depends upon said free layer material composition and structure, whereby, upon the exertion of said stresses
said free layer acquires a total in-plane magnetic anisotropy, H ktotal , that may include a shape induced magnetic anisotropy, but is most predominantly an anisotropy due to stress and magnetostriction and whereby said MTJ cell acquires, thereby, an in-plane switching field, H s , that has a component resulting from said shape induced anisotropy and a component resulting from said stress induced anisotropy whereby
a perpendicularly directed critical current is able to switch the direction of magnetization of said free layer, but
the magnitude of the switching field maintains the resulting magnetization of said free layer constant and free from variations caused by thermal agitation in the absence of said critical current.
2. The cell of claim 1 wherein said free layer is a ferromagnetic alloy comprising the elements Fe, Co, Ni, B, wherein the atomic proportions of the alloyed elements determine the coefficient of magnetostriction, λ, of the layer.
3. The cell of claim 1 wherein said free layer is a ferromagnetic alloy comprising the elements Fe, Co, Ni, B with the addition of either Cu, Hf or Nb or combinations of Cu, Hf and Nb, wherein the atomic proportions of the alloyed elements determine the coefficient of magnetostriction, λ, of the layer.
4. The cell of claim 1 wherein said free layer is a composite ferromagnetic structure formed as a lamination of ferromagnetic layers wherein each layer has a different coefficient of magnetostriction from the other layers, whereby the overall coefficient of magnetostriction of the free layer is determined by adjusting the coefficients of magnetostriction of each layer forming the laminate and by adjusting the relative thickness of each said layer.
5. The cell of claim 1 wherein the free layer is formed as a layer of Ni x Fe 1-x with x between approximately 0.25 and 1.
6. The cell of claim 1 wherein the free layer is formed as a layer of Co x Fe 1-x B y with x between approximately 0.25 and 1 and y between approximately 0 and 0.5.
7. The cell of claim 1 wherein the free layer is formed as bilayer of Co x Fe 1-x B y /Ni z Fe 1-zx with x between approximately 0.25 and 1, y between approximately 0 and 0.5 and z between approximately 0.7 and 1.
8. The cell of claim 1 wherein the free layer is formed as bilayer of Co x Fe 1-x B y /(Ni 70 Fe 30 ) 1-z Cu z with x between approximately 0.25 and 1, y between approximately 0 and 0.5 and z between approximately 0.15 and 0.25.
9. The cell of claim 1 wherein the free layer is formed as bilayer of Co x Fe 1-x B y /U z V 1-z , where U is Co, Ni, Fe, B oar their alloys and V is Nb, Hf or their alloys, with x between approximately 0.25 and 1, y between approximately 0 and 0.5 and z between approximately 0.7 and 1.
10. The cell of claim 1 wherein said surrounding material is high-k dielectric material.