Hybrid oxide/metal cap layer for boron-free free layer

1. A magnetic tunnel junction stack comprising:

a pinned layer;

a main oxide barrier layer on the pinned layer;

a free layer on the main oxide barrier layer; and

a hybrid oxide/metal cap layer on the free layer,

wherein the hybrid oxide/metal cap layer comprises:

a first oxide layer on the free layer;

a second oxide layer on the first oxide layer; and

a metallic cap layer on the second oxide layer,

wherein the free layer is free of boron (B), and

wherein a thickness of the first oxide layer is about 1 to 6 Å and a total thickness of the first oxide layer and the second oxide layer is about 4 to 14 Å.

2. The magnetic tunnel junction stack of claim 1 , wherein

the first oxide layer comprises a first oxide, and

the second oxide layer comprises a second oxide, the second oxide having a band gap equal to or greater than a band gap of the first oxide.

3. The magnetic tunnel junction stack of claim 1 , wherein the first oxide layer comprises one or more oxides selected from AlO x , ZnO x , TiO x , VO x , GaO x , YO x , ZrO x , NbO x , HfO x , TaO x , SiO x , MgGaO x , Hf—Zr—O x , Hf—Si—O x , Zr—Si—O x , Hf—Al—O x , Zr—Al—O x , and In—Ga—Zn—O.

4. The magnetic tunnel junction stack of claim 1 , wherein the second oxide layer comprises one or more oxides selected from MgO, MgAlO, MgTiO, and AlO x .

5. The magnetic tunnel junction stack of claim 1 , wherein the metallic cap layer comprises a metal element having a higher Gibbs free energy of oxide formation than a metal element in the first oxide layer and a metal element in the second oxide layer.

6. The magnetic tunnel junction stack of claim 1 , wherein the metallic cap layer comprises one or more metals selected from Ru, W, Mo, Co, Fe, Ni, CoFe, FeNi, CoNi, CoFeB, CoFeBMo, and CoFeBW.

7. The magnetic tunnel junction stack of claim 1 , wherein the hybrid oxide/metal cap layer is free of boron (B).

8. The magnetic tunnel junction stack of claim 1 , wherein the first oxide layer is an amorphous layer or a semi-crystalline layer.

9. The magnetic tunnel junction stack of claim 1 , wherein the second oxide layer is an amorphous layer or a crystalline layer or a semi-crystalline layer.

10. The magnetic tunnel junction stack of claim 1 , wherein the free layer comprises Co x Fe 1-x wherein 0<x<1, CoFeNi, Co 2 FeAl, Co 2 MnSi, Co 2 FeMnSi, Co 2 FeSi, MnGa, and/or MnGe.

11. The magnetic tunnel junction stack of claim 1 , wherein a damping factor (α) of the free layer is about 0.006 or lower.

12. The magnetic tunnel junction stack of claim 1 , wherein the free layer has a M s t of greater than about 80 μemu/cm 2 , and an H κ of greater than +1 kOe.

13. A method of manufacturing a magnetic tunnel junction stack, the method comprising:

forming a main oxide barrier layer on a pinned layer;

forming a free layer on the main oxide barrier layer; and

forming a hybrid oxide/metal cap layer on the free layer,

wherein the forming of the hybrid oxide/metal cap layer comprises:

forming a first oxide layer on the free layer;

forming a second oxide layer on the first oxide layer; and

forming a metallic layer on the second oxide layer, and

wherein the free layer is free of boron (B), and

wherein a thickness of the first oxide layer is about 1 to 6 Å and a total thickness of the first oxide layer and the second oxide layer is about 4 to 14 Å.

14. The method of claim 13 , wherein the forming of the first oxide layer or the forming of the second oxide layer is by direct sputtering of an oxide target.

15. The method of claim 13 , wherein the forming of the first oxide layer or the forming of the second oxide layer comprises:

depositing a metallic layer, and

oxidizing the metallic layer to provide the first or second oxide layer.

16. The method of claim 13 , wherein

the first oxide layer comprises a first oxide, and

the second oxide layer comprises a second oxide, the second oxide having a band gap equal to or greater than a band gap of the first oxide.

17. The method of claim 13 , wherein the first oxide layer comprises one or more oxides selected from AlO x , ZnO x , TiO x , VO x , GaO x , YO x , ZrO x , NbO x , HfO x , TaO x , SiO x , MgGaO x , Hf—Zr—O x , Hf—Si—O x , Zr—Si—O x , Hf—Al—O x , Zr—Al—O x , and In—Ga—Zn—O.

18. The method of claim 13 , wherein the second oxide layer comprises one or more oxides selected from MgO, MgAlO, MgTiO, and AlO x .

19. The method of claim 13 , wherein the metallic layer comprises one or more metals selected from Ru, W, Mo, Co, Fe, Ni, CoFe, FeNi, CoNi, CoFeB, CoFeBMo, and CoFeBW.

20. The method of claim 13 , wherein a damping factor (α) of the free layer is about 0.006 or lower.