Tunnel magnetoresistance (TMR) structures with MGO barrier and methods of making same

1. A system, comprising:

a free layer; and

a tunnel barrier layer having microstructure and composition characteristic of in situ natural oxidation of magnesium,

wherein the tunnel barrier layer is a multilayer structure of individual layers of magnesium oxide stacked directly upon one another,

a first plurality of the layers of the tunnel barrier layer having microstructure and composition characteristic of in situ naturally oxidized magnesium,

a second plurality of the layers of the tunnel barrier layer having microstructure and composition characteristic of reactive deposition of magnesium in the presence of oxygen,

wherein the first plurality of the layers is interleaved with the second plurality of the layers.

2. A system as recited in claim 1 , wherein each of the layers of the tunnel barrier layer is less than about 4 angstroms thick and greater than 0 angstroms thick.

3. A system as recited in claim 1 , wherein the tunnel barrier layer is less than about 9 angstroms thick and greater than 0 angstroms thick.

4. A system as recited in claim 1 , further comprising:

magnetic media;

at least one head for reading from and writing to the magnetic media, each head having:

a sensor comprising the free layer and the tunnel barrier layer;

a writer coupled to the sensor;

a slider for supporting the head; and

a control unit coupled to the head for controlling operation of the head.

5. A system as recited in claim 1 , further comprising a third layer of the tunnel barrier layer, the third layer being a discretely formed layer of magnesium oxide having microstructure and composition characteristic of in situ naturally oxidized magnesium, the third layer being directly adjacent one of the layers in the first plurality of layers of the tunnel barrier layer.

6. A system as recited in claim 1 , further comprising a third layer of the tunnel barrier layer, the third layer being a discretely formed layer of magnesium oxide having microstructure and composition characteristic of reactive deposition of magnesium in the presence of oxygen, the third layer being directly adjacent one of the layers in the second plurality of layers of the tunnel barrier layer.

7. A method for forming a system as recited in claim 1 , the method comprising:

forming a first layer of magnesium above at least one of the free layer and a reference layer;

exposing the first layer of magnesium to an oxidizing environment for causing oxidation of the first layer of magnesium;

forming a second layer of magnesium above the oxidized first layer of magnesium; and

exposing the second layer of magnesium to the oxidizing environment for causing oxidation of the second layer of magnesium.

8. A method as recited in claim 7 , further comprising removing the oxidizing environment, from a vicinity of the first layer prior to forming the second layer of magnesium.

9. A method as recited in claim 7 , further comprising forming a layer of magnesium oxide by reactive deposition above the oxidized first layer of magnesium.

10. A method as recited in claim 9 , further comprising forming the second layer of magnesium above the oxidized first layer of magnesium and the layer of magnesium oxide formed by reactive deposition, and exposing the second layer of magnesium to the oxidizing environment for causing oxidation of the second layer of magnesium.

11. A method as recited in claim 10 , further comprising removing the oxidizing environment from a vicinity of the first layer prior to forming the layer of magnesium oxide by reactive deposition.

12. A method as recited in claim 7 , further comprising forming a layer of magnesium oxide by reactive deposition above the oxidized second layer of magnesium.

13. A method as recited in claim 7 , wherein at least one of the layers of magnesium is formed by sputtering.

14. A method as recited in claim 7 , wherein the oxidizing environment includes gaseous oxygen.

15. A method as recited in claim 7 , wherein the first layer of magnesium is less than about 3 angstroms thick and greater than 0 angstroms thick.

16. A method for forming a system as recited in claim 1 , the method comprising:

forming a first layer of magnesium above at least one of a free layer and a reference layer;

exposing the first layer of magnesium to an oxidizing environment for causing oxidation of the first layer of magnesium; and

forming a layer of magnesium oxide above the at least one of the free layer and the reference layer by reactive deposition.

17. A method as recited in claim 16 , further comprising forming a second layer of magnesium oxide by reactive deposition above the oxidized first layer of magnesium.

18. A method as recited in claim 16 , further comprising removing the oxidizing environment from a vicinity of the first layer of magnesium prior to forming a subsequent layer.

19. A method as recited in claim 18 , further comprising forming a second layer of magnesium above the layer of magnesium oxide formed by reactive deposition, and exposing the second layer of magnesium to the oxidizing environment for causing oxidation of the second layer of magnesium.

20. A method as recited in claim 16 , wherein the layer of magnesium oxide formed by reactive deposition is formed above the oxidized first layer of magnesium.

21. A method as recited in claim 16 , wherein the oxidizing environment: includes gaseous oxygen.

22. A method as recited in claim 16 , wherein the first layer of magnesium is less than about 3 angstroms thick and greater than 0 angstroms thick.