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1. (WO2010129390) GAS SENSOR
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WE CLAIM:

1. A gas sensor slack including a first ferromagnetic film layer; a spacer layer; and, a second ferromagnetic film layer, wherein the easy axis of magnetization of the second ferromagnetic layer is fixed, and orientation of the easy axis of magnetization of the first ferromagnetic layer relative to die second layer is dependent upon film thickness of said first film layer,

2. The gas sensor stack of claim 1 wherein die orientation of the easy axis of the first ferromagnetic layer at its ground or null state at ambient temperature and ambient atmosphere is in-plane.

3. The gas sensor stack of claim 2 wherein the orientation of the easy axis of the first ferromagnetic layer in a gas-clean state is out-of plane.

4. The gas sensor stack of claim 2 further including a heater.

5. The gas sensor stack of claim 4 wherein the heater is programmable,

6. The gas sensor stack of claim 5 further including an anti-ferromagnetic layer interposed between the second ferromagnetic layer and the heater,

7. A gas sensor including the gas sensor slack of claim 4, and further including a programmable heater power source.

8. The gas sensor of Claim 6 further including a resistance measurement circuit connected to the gas sensor stack of Claim 1 to measure the resistance of the sensor stack.

9. The gas sensor of Claim 7 further including a data processor, and at least one of the following: a display unit, an alarm unit or a process-control unit.

10. The gas sensor of claim 4 wherein the first ferromagnetic film layer is cobalt.

1 1. The gas sensor of claim 10 wherein the first cobalt film layer comprises at least two mono layers of cobalt.

12. The gas sensor of claim 10 wherein the second ferromagnetic layer is cobalt,

13. The gas sensor of claim 12 wherein the spacer layer is ruthenium.

14. The gas sensor of claim 10 further including a capping layer disposed over the top of the first ferromagnetic layer,

15. The gas sensor of claim 14 wherein the capping layer comprises one or more mono layers of gold.

16. A method for detecting the presence of a gas including the steps of: a. exposing a gas sensor including a sensor stack and healer according to claim 7 to an ambient atmosphere; b. heating the top ferromagnetic sensor stack of the gas sensor so as to dcsorb any gases that may have adsorbed to the exposed top surface of the sensor; c. allowing the exposed top surface of the gas sensor to cool; d. continuously measuring the resistance of the sensor stack during the cooling step; e. determining the point during cooling when the resistance of the sensor stack changes.

17. The method for detecting the presence of a gas according Io Claim 16, wherein the concentration of the gas may also be determined, including the further steps of: a. cycling the heater temperature between an upper and a lower valise; and, b. measuring the resistance of the gas sensor stack in response to said cycling of heater temperature.

18. The method of detecting the presence of a gases according to Claim 16, where more than one gas is to be detected, the method including the further steps of : a. cycling the heater temperature in a first heating cycle between an upper and lower value; b. measuring the resistance of the gas sensor slack in response Io said the changes in temperature during the first heating cycle; c. cycling the heater temperature between an upper and a lower value in a second heating cycle, wherein the upper and lower temperature values of the second heating cycle are different from the upper and lower temperature values of the first heating cycle; and thereafter, d. measuring the resistance of the gas sensor stack in response to the changes in temperature during the second heating cycle.

19. A method for preparing the gas sensor of claim 4 for use in detecting a gas, wherein the gas sensor stack is first heated to desorbed gas from the top surface of the said sensor stack such that the easy axis of the first ferromagnetic layer is flipped 90 degrees.

20. A wireless phone incorporating the gas sensor of Claim 15.