Bipolar spin-transfer switching
A magnetic device includes a magnetized polarizing layer, a free magnetic layer, and a reference layer. The free magnetic layer forms a first electrode and is separated from the magnetized polarizing layer by a first non-magnetic metal layer. The free magnetic layer has a magnetization vector having a first and second stable state. The reference layer forms a second electrode and is separated from the free-magnetic layer by a second non-magnetic layer. Unipolar current is sourced through the polarizing, free magnetic and reference layers. Switching of the magnetization vector of the free magnetic layer from the first stable state to the second state is initiated by application of a first unipolar current pulse, and switching of the magnetization vector of the free magnetic layer from the second stable state to the first stable state is initiated by application of a second unipolar current pulse.
1. A magnetic device comprising:
a magnetized polarizing layer;
a free magnetic layer, the free magnetic layer forming a first electrode and separated from the magnetized polarizing layer by a first non-magnetic metal layer, the free magnetic layer having a magnetization vector having a first stable state and a second stable state;
a reference layer forming a second electrode and separated from the free-magnetic layer by a second non-magnetic layer, the reference layer having a fixed magnetization vector;
a current source that sources unipolar current through the magnetized polarizing layer, the free magnetic layer and the reference layer, the current source is capable of outputting a first unipolar current pulse of a first duration and having a selected amplitude, the first unipolar current pulse flowing orthogonally through the polarizing layer and having a positive polarity, the current source also capable of outputting a second unipolar current pulse of a second duration and having a selected amplitude, the second unipolar current pulse flowing orthogonally through the polarizing layer and having a negative polarity, and
wherein switching of the magnetization vector of the free magnetic layer from the first stable state to the second state is initiated by application of the first unipolar current pulse, and wherein switching of the magnetization vector of the free magnetic layer from the second stable state to the first stable state is initiated by application of the second unipolar current pulse, wherein switching to the second stable state is independent of the first duration of the first unipolar current pulse, and wherein switching to the first stable state is independent of the second duration of the second unipolar current pulse.
2. The magnetic device as in claim 1 wherein the polarizing layer is magnetized perpendicular to the free magnetic layer.
3. The magnetic device as in claim 1 , wherein the first duration and the second duration are the same.
4. The magnetic device as defined in claim 1 wherein spin transfer torques associated with the magnetized polarizing layer and an in-plane magnetized form of the reference layer can be described by,
ⅆ
m
^
ⅆ
t
=
-
γμ
0
m
^
×
H
→
eff
+
α
m
^
×
ⅆ
m
^
ⅆ
t
+
γ
a
J
m
^
×
(
m
^
×
m
^
P
)
-
βγ
a
J
m
^
×
(
m
^
×
m
^
R
)
where m represents the magnetization vector of the free layer magnetization, aJ is a term proportional to current of the unipolar current pulse and spin-polarization of the unipolar current pulse, the third term on the right hand side of this equation being spin transfer torque from the polarizing layer (mP) and the fourth term on the right hand side of the equation being a spin transfer torque from an in-plane magnetized form of the reference layer (mR), and β represents a ratio of magnitude of these spin transfer torques.
5. The magnetic device as defined in claim 4 wherein β>1 provides a range of current pulse amplitudes wherein switching of the magnetic device is directly from parallel to anti-parallel for a first current polarity and anti-parallel to parallel for a second current polarity.
6. The magnetic device as defined in claim 4 wherein the magnetic device switching is precessional and bipolar for both polarities for selected values of β.
7. The magnetic device as defined in claim 4 wherein β less than or about equal to 1provides precessional switching.
8. The magnetic device as defined in claim 4 wherein the magnetization direction becomes precessional for the β less than or about equal to 1, thereby providing higher precession frequencies for higher current amplitudes.
9. The magnetic device as defined in claim 4 wherein β is selected from the group of about 1or greater than 1 and pulse polarity and amplitude of the first unipolar current pulse controls a final magnetization state of the free magnetic layer.
10. The magnetic device as defined in claim 4 wherein β is selected from the group of about 1or greater than 1 and pulse polarity and amplitude of the second unipolar current pulse controls the second stable magnetization state of the free magnetic layer.
11. The magnetic device as defined in claim 9 wherein spin polarization of the reference layer is increased based upon materials of the reference layer and a magnetic tunnel junction layer adjacent thereto.
12. The magnetic device as defined in claim 11 wherein the materials are selected from the group of (a) CoFeB and MgO, (b) NiFe and MgO, and (c) CoFe and MgO.
13. The magnetic device as defined in claim 9 wherein spin polarization from the polarizing layer is reduced based upon a particular composition therefore.
14. The magnetic device as defined in claim 13 wherein the particular composition comprises a Co/Ni multilayer.
15. The magnetic device as defined in claim 14 wherein the multilayer is selected from the group of Co/Ni on Co/Pd, CoNi on Co/Pt.
16. The magnetic device according to claim 1 , wherein said first nonmagnetic metal layer is comprised of Cu.
17. The magnetic device according to claim 1 , wherein said second non-magnetic layer is comprised of MgO.