Magnetoresistive stacks and methods therefor

1. A magnetoresistive device, comprising:

an intermediate region;

a magnetically fixed region on one side of the intermediate region; and

a magnetically free region on an opposite side of the intermediate region, wherein the magnetically free region includes at least a first ferromagnetic region and a second ferromagnetic region separated by a non-magnetic insertion region, wherein one of the first ferromagnetic region and the second ferromagnetic region includes at least a boron-rich ferromagnetic layer positioned between two boron-free ferromagnetic layers, and wherein another of the first ferromagnetic region and the second ferromagnetic region includes a boron-containing ferromagnetic layer and a ferromagnetic layer including iron or cobalt contacting each other,

the boron-rich ferromagnetic layer including an alloy consisting of (i) boron and (ii) only one of iron or cobalt,

the boron-containing ferromagnetic layer including an alloy consisting of (i) boron and (ii) at least one of iron or cobalt, wherein a boron composition in the boron-rich ferromagnetic layer is higher than a boron composition in the boron-containing ferromagnetic layer, and

the two boron-free ferromagnetic layers including an alloy consisting of cobalt and iron.

2. The magnetoresistive device of claim 1 , wherein at least one of the two boron-free ferromagnetic layers is formed contacting the boron-rich ferromagnetic layer.

3. The magnetoresistive device of claim 1 , wherein the alloy consisting of cobalt and iron has a composition between approximately 4-80% atomic percent of cobalt.

4. The magnetoresistive device of claim 1 , wherein the alloy consisting of (i) boron and (ii) only one of iron or cobalt has a composition between approximately 45-55% atomic percent of boron.

5. The magnetoresistive device of claim 1 , wherein the alloy consisting of (i) boron and (ii) only one of iron or cobalt has a composition between approximately 40-60% atomic percent of boron.

6. The magnetoresistive device of claim 1 , wherein the alloy consisting of cobalt and iron has a composition between approximately 14-75% atomic percent of cobalt.

7. The magnetoresistive device of claim 1 , wherein the alloy consisting of cobalt and iron has a crystalline microstructure, and the alloy consisting of (i) boron and (ii) only one of iron or cobalt has an amorphous microstructure.

8. The magnetoresistive device of claim 1 , wherein the alloy consisting of iron and cobalt has a composition Co X Fe 100-X , where X is between 4 and 96 in atomic percent.

9. A magnetoresistive device, comprising:

an intermediate region;

a magnetically fixed region on one side of the intermediate region; and

a magnetically free region on an opposite side of the intermediate region, wherein the magnetically free region includes at least a first ferromagnetic region and a second ferromagnetic region separated by a non-magnetic insertion region, wherein one of the first ferromagnetic region and the second ferromagnetic region includes at least a boron-rich ferromagnetic layer positioned between two boron-free ferromagnetic layers, wherein another of the first ferromagnetic region and the second ferromagnetic region includes a boron-containing ferromagnetic layer and a ferromagnetic layer including iron or cobalt contacting each other, and wherein a boron composition in the boron-rich ferromagnetic layer is higher than a boron composition in the boron-containing ferromagnetic layer,

the boron-rich ferromagnetic layer including

(a) an alloy consisting of (i) boron and (ii) only one of iron or cobalt, or (b) an alloy of cobalt, iron, and an element M, where element M is one of tantalum, hafnium, zirconium, or chromium, and boron having a composition (CoFeM) X B 100-X , wherein X is between 40 and 80 in atomic percent, and

the two boron-free ferromagnetic layers including an alloy consisting of iron and cobalt.

10. The magnetoresistive device of claim 9 , wherein the two boron-free ferromagnetic layers include between approximately 4-80% atomic percent of cobalt.

11. The magnetoresistive device of claim 9 , wherein the boron-rich ferromagnetic layer includes between approximately 40-60% atomic percent of boron.

12. The magnetoresistive device of claim 9 , wherein the two boron-free ferromagnetic layers include between approximately 14-75% atomic percent of cobalt, and the boron-rich ferromagnetic layer includes between approximately 45-55% atomic percent of boron.

13. The magnetoresistive device of claim 9 , wherein the non-magnetic insertion region is approximately 3 angstroms (Å) thick.

14. The magnetoresistive device of claim 9 , wherein the boron-rich ferromagnetic layer is approximately 2-9 angstroms (Å) thick.

15. The magnetoresistive device of claim 9 , wherein at least one of the two boron-free ferromagnetic layers is approximately 1.5 angstroms (Å) thick.

16. The magnetoresistive device of claim 9 , wherein at least one of the boron-containing ferromagnetic layer or the ferromagnetic layer containing iron or cobalt is approximately 1-3 angstroms (Å) thick.

17. The magnetoresistive device of claim 9 , wherein each of the two boron-free ferromagnetic layers is approximately 3-30 angstroms (Å) thick.

18. The magnetoresistive device of claim 9 , wherein the other of the first ferromagnetic region and the second ferromagnetic regions does not include a boron-rich ferromagnetic layer.

19. The magnetoresistive device of claim 9 , further comprising another intermediate region on an opposite side of the magnetically free region from the intermediate region.