High SNR CPP reader using high frequency standing wave interference detection
An apparatus includes a current perpendicular to the plane sensing element, a DC current source connected to the sensing element, a microwave AC current source connected to supply AC current to the sensing element, and a detector for measuring a voltage representative of phase difference between the AC current and AC voltage across the multilayer structure when the sensing element is subjected to a magnetic field. A method for sensing a magnetic field is also provided.
1. An apparatus comprising:
a current perpendicular to the plane sensing element;
a DC current source connected to the sensing element;
a microwave AC current source connected to supply AC current to the sensing element; and
a detector for measuring a voltage representative of phase difference between the AC current and AC voltage across the multilayer structure when the sensing element is subjected to a magnetic field.
2. The apparatus of claim 1 , wherein the detector comprises:
transmission line coupled to the sensing element;
a bias tee for coupling the DC current source to the transmission line;
a microwave source;
a three port component for coupling the microwave source to the transmission line; and
a microwave detector coupled to the transmission line.
3. The apparatus of claim 2 , wherein the transmission line has a length chosen such that the microwave detector measures standing wave power in a range from a minimum to a maximum power level.
4. The apparatus of claim 1 , wherein current from the DC current source causes precession of a direction of magnetization of a free layer in the sensing element and the precession of the direction of magnetization is phase locked to current from the AC current source.
5. The apparatus of claim 4 , wherein the detector comprises an amplitude modulation detection circuit.
6. The apparatus of claim 1 , further comprising:
a magnetic field source providing a bias magnetic field for the sensing element.
7. The apparatus of claim 1 , further comprising:
a magnetic storage medium, wherein the phase difference is induced by magnetic fields from a magnetic storage medium.
8. The apparatus of claim 1 , wherein the sensing element comprises:
a point contact sensing element.
9. The apparatus of claim 1 , wherein the sensing element comprises:
a device driven by spin momentum transfer.
10. The apparatus of claim 1 , wherein the sensing element comprises:
spin valve nano-pillars.
11. The apparatus of claim 1 , wherein the sensing element includes an electromagnet, and magnetic field from the electromagnet is substantially perpendicular to the plane of the sensing element.
12. A method comprising:
applying an AC current and a DC current to a current perpendicular to the plane sensing element; and
detecting a phase difference between the AC current and AC voltage across the sensing element when the sensing element is subjected to a magnetic field.
13. The method of claim 12 , further comprising:
coupling a transmission line to the sensing element, wherein a standing wave is formed on the transmission line due to impedance mismatch between the transmission line and the sensing element.
14. The method of claim 13 , wherein the detecting step measures standing wave power and a change in the standing wave power is representative of a phase shift between resistance precession of the sensing element and the AC current.
15. The method of claim 12 , wherein the sensing element comprises:
a magnetically biased multilayer structure including a reference layer and a free layer separated by a spacer layer, wherein the reference layer has magnetic anisotropy perpendicular to the plane of the reference layer and the free layer has magnetic anisotropy in the plane of the free layer.
16. The method of claim 15 , wherein the DC current causes precession of a direction of magnetization of the free layer, and wherein the precession of the direction of magnetization is phase locked to the AC current.
17. The method of claim 15 , wherein the multilayer structure is subjected to a bias magnetic field substantially out of the plane of the free layer.
18. The method of claim 15 , wherein the multilayer structure is subjected to a bias magnetic field substantially in the plane of the free layer.
19. The method of claim 12 , wherein the phase difference is induced by magnetic fields from a magnetic storage medium.
20. The method of claim 12 , wherein the sensing element comprises:
a point contact sensing element.
21. The method of claim 12 , wherein the sensing element comprises:
a device driven by spin momentum transfer.