Free layer in magnetoresistive random-access memory

1 . A magnetoresistive random-access-memory (MRAM) comprising:

a reference layer;

a tunnel barrier layer of magnesium-oxide (MgO); and

a free layer, wherein the free layer comprises:

a first cobalt-iron-boron (CoFeB) layer on top of the tunnel barrier layer;

a spacer layer on top of the first CoFeB layer;

a second CoFeB layer on top of the spacer layer; and

a capping layer of MgO on top of the second CoFeB layer,

wherein the first and the second CoFeB layer include respectively a first region adjacent to the tunnel barrier layer and the capping layer respectively and a second region adjacent to the spacer layer, wherein the first regions of the first and the second CoFeB layer include crystallized cobalt-iron (CoFe) and the second regions of the first and the second CoFeB layer include amorphous CoFe alloy, and

wherein the spacer layer includes two layers of iron and three layers of a refractory metal stacked alternately with the two layers of iron, wherein each of the two layers of iron has a thickness between 0.5 nm and 1.0 nm to have magnetic power to couple with the first and the second CoFeB layer; a middle layer of the three layers of the refractory metal has a thickness between 0.05 nm and 0.3 nm such that it does not magnetically decouple the two layers of iron from each other; and each of the two outer layers of the three layers of the refractory metal has a thickness between 0.1 nm and 0.4 nm to provide absorption of boron but do not decouple the two layers of iron from the first and the second CoFeB layer, which directly contact the two outer layers of the three layers of the refractory metal with the bottom outer layer of the three layers of the refractory metal in direct contact with the first CoFeB layer and the top outer layer of the three layers of the refractory metal in direct contact with the second CoFeB layer.

2 . The MRAM of claim 1 , wherein the spacer layer includes a concentration level of iron ranging from 20 at. % to 80 at. %.

3 . The MRAM of claim 1 , wherein the first regions of the first and the second CoFeB layer are depleted of boron.

4 . The MRAM of claim 1 , wherein the first region of the first CoFeB layer has a thickness that is between 20% to 40% of a thickness of the first CoFeB layer.

5 . The MRAM of claim 1 , wherein a thickness of the first region of the first CoFeB layer is between 0.2 nm and 0.5 nm, and a thickness of the second region of the first CoFeB layer is between 0.2 nm and 0.7 nm.

6 . A magnetoresistive random-access-memory (MRAM) comprising:

a reference layer;

a tunnel barrier layer above the reference layer; and

a free layer above the tunnel barrier layer, wherein the free layer includes:

a first cobalt-iron-boron (CoFeB) layer on top of the tunnel barrier layer;

a spacer layer on top of the first CoFeB layer;

a second CoFeB layer on top of the spacer layer; and

a capping layer on top of the second CoFeB layer;

wherein the spacer layer is an alloy of a refractory metal and iron including two layers of iron with each having a thickness between 0.5 nm and 1.0 nm and three layers of the refractory metal stacked alternately with the two layers of iron and a thickness of the layer of the refractory metal between the two layers of iron is between 0.05 nm and 0.3 nm such that it does not magnetically decouple the two layers of iron, with the iron having a concentration level ranging from 20 at. % to 80 at. % of the alloy.

7 . The MRAM of claim 6 , wherein the refractory metal is selected from a group consisting of zirconium (Zr), niobium (Nb), hafnium (Hf), and rhenium (Re).

8 . The MRAM of claim 6 , wherein the tunnel barrier layer is a layer of magnesium-oxide (MgO) and the first CoFeB layer is directly on top of the tunnel barrier layer, the first CoFeB layer is depleted of boron (B) to include a first region adjacent to the tunnel barrier layer and a second region adjacent to the spacer layer; wherein the first region includes crystallized cobalt-iron (CoFe) and the second region includes amorphous CoFe alloy.

9 . The MRAM of claim 8 , wherein the first region of the first CoFeB layer has a thickness that is between 20% to 40% of a thickness of the first CoFeB layer.

10 . The MRAM of claim 8 , wherein a thickness of the first region of the first CoFeB layer is between 0.2 nm and 0.5 nm, and a thickness of the second region of the first CoFeB layer is between 0.2 nm and 0.7 nm.

11 . The MRAM of claim 6 , wherein the capping layer is a layer of magnesium-oxide (MgO) and is directly on top of the second CoFeB layer, the second CoFeB layer is depleted of boron (B) to include a first region adjacent to the capping layer and a second region adjacent to the spacer layer; wherein the first region includes crystallized cobalt-iron (CoFe) and the second region includes amorphous CoFe alloy.

12 . A magnetic tunnel junction (MTJ) stack comprising:

a reference layer;

a tunnel barrier layer above the reference layer; and

a free layer above the tunnel barrier layer,

wherein the free layer includes a first cobalt-iron-boron (CoFeB) layer; a spacer layer of an alloy comprising a refractory metal and iron; a second CoFeB layer; and a capping layer, wherein the refractory metal is zirconium (Zr), niobium (Nb), hafnium (Hf), or rhenium (Re), and the spacer layer being sandwiched between the first and the second CoFeB layer, and

wherein the spacer layer includes two layers of iron and three layers of the refractory metal stacked alternately with the two layers of iron, wherein each of the two outer layers of the refractory metal has a thickness between 0.1 nm and 0.4 nm to provide absorption of boron; each of the two layers of iron has a thickness between 0.5 nm and 1.0 nm to have magnetic power to couple with the first and the second CoFeB layer; and the layer of the refractory metal between the two layers of iron has a thickness between 0.05 nm and 0.3 nm to not magnetically decouple the two layers of iron.

13 . The MTJ stack of claim 12 , wherein the tunnel barrier layer is a layer of magnesium-oxide (MgO), and the first CoFeB layer is directly on top of the tunnel barrier layer to include a first region adjacent to the tunnel barrier layer and a second region adjacent to the spacer layer; wherein the first region includes crystallized cobalt-iron (CoFe) and the second region includes amorphous CoFe alloy.

14 . The MTJ stack of claim 13 , wherein both the first region and the second region of the first CoFeB layer include less than 5 at. % of boron.

15 . The MTJ stack of claim 13 , wherein a thickness of the first region of the first CoFeB layer is between 0.2 nm and 0.5 nm, and a thickness of the second region of the first CoFeB layer is between 0.2 nm and 0.7 nm.

16 . The MTJ stack of claim 12 , wherein the spacer layer includes a concentration level of iron ranging from 20 at. % to 80 at. %.