BIPOLAR SPIN-TRANSFER SWITCHING

• US 20120294078A1
• Filed: 11/16/2011
• Published: 11/22/2012
• Est. Priority Date: 08/19/2003
• Status: Active Grant

• Current Assignee: New York University
• Original Assignee: New York University
• Inventors: Kent, Andrew, Bedau, Daniel, Liu, Huanlong
• Granted Patent: US 8,755,222 B2
• Time in Patent Office: Days
• US Class Current: 365/171
• CPC Class Codes: G11C 11/16 using elements in which the storage effect is based on magnetic spin effect
• CPC Class Codes: G11C 11/161 details concerning the memory cell structure, e.g. the layers of the ferromagnetic memory cell
• CPC Class Codes: G11C 11/1659 Cell access
• CPC Class Codes: G11C 11/1675 Writing or programming circuits or methods
• CPC Class Codes: G11C 11/5607 using magnetic storage elements
• CPC Class Codes: H01L 29/66984 Devices using spin polarized carriers
• CPC Class Codes: H10B 61/00 Magnetic memory devices, e.g. magnetoresistive RAM [MRAM] devices
• CPC Class Codes: H10N 50/10 Magnetoresistive devices

Abstract

Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to the free layer to achieve large spin-transfer torques and ultra-fast energy efficient switching. OST-MRAM devices that incorporate a perpendicularly magnetized spin-polarizing layer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer, exhibit improved performance over prior art devices. The switching is bipolar, occurring for positive and negative polarity pulses, consistent with a precessional reversal mechanism, and requires an energy less than 450 fJ and may be reliably observed at room temperature with 0.7 V amplitude pulses of 500 ps duration.

Claims

1. A magnetic device comprising:

a perpendicularly 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 at least 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;

wherein application of a current pulse, having either positive or negative polarity and a selected amplitude and duration, through a magnetic device switches the magnetization vector.

2. The magnetic device as defined in claim 1 wherein spin transfer torques associated with the perpendicularly magnetized polarized layer and an in-plane magnetized form of the reference layer can be described by,

3. The magnetic device as defined in claim 2 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.

4. The magnetic device as defined in claim 2 wherein the device switching is precessional and bipolar for both polarities for selected values of β

.

5. The magnetic device as defined in claim 2 wherein beta less than or about equal to 1 provides at least one of reduced direct current switching errors and precessional and fast switching.

6. The magnetic device as defined in claim 2 wherein the magnetization direction becomes precessional for the beta less than or about equal to 1, thereby providing higher precession frequencies for higher current amplitudes.

7. The magnetic device as defined in claim 2 wherein β

is selected from the group of about 1 or greater than 1 and pulse polarity controls a final magnetization state of the free magnetic layer and independent of the current pulse duration.

8. The magnetic device as defined in claim 7 wherein spin polarization of the reference layer is increased by selecting mating materials for the reference layer and a magnetic tunnel junction layer adjacent thereto.

9. The magnetic device as defined in claim 8 wherein the mating materials are selected from the group of (a) CoFeB and MgO, (b) NiFe and MgO, and (c) CoFe and MgO.

10. The magnetic device as defined in claim 7 wherein spin polarization from the polarizing layer is reduced by selecting a particular composition therefore.

11. The magnetic device as defined in claim 10 wherein the particular composition comprises a Co/Ni multilayer.

12. The magnetic device as defined in claim 11 wherein the multilayer is selected from the group of Co/Ni on Co/Pd, CoNi on Co/Pt.

13. The magnetic device as defined in claim 12 wherein thickness of the Co/Ni can be varied, thereby controlling the spin polarization.

14. The magnetic device as defined in claim 7 wherein spin polarization from the polarizing layer is reduced by further including a nonmagnetic layer disposed between the polarizing layer and the free magnetic layer, thereby controlling the spin polarization of carriers from the polarizing layer.

15. The magnetic device as defined in claim 14 wherein the nonmagnetic layer comprises Cu with controlled defects, thereby reducing the spin polarization.

16. The magnetic device as defined in claim 14 wherein the nonmagnetic layer can have varying layer thickness, thereby reducing spin polarization from the polarizing layer incident on the free magnetic layer.

17. A method of controlling a memory array having a plurality of cells each comprising:

determining an initial state of a cell within the memory array

determining if the initial state is the same as a write state corresponding to information to be written to the cell;

if the initial state is different from the write state, applying a current pulse of either positive or negative polarity and of a selected amplitude and duration through the magnetic device switching the magnetization vector.

18. A memory array comprising:

at least one bit cell including;

a magnetic device having;

a magnetic layer having a fixed magnetization vector;

a free magnetic layer having a variable magnetization vector having at least a first stable state and a second stable state;

a non-magnetic layer separating the magnetic layer with fixed magnetization vector and the free magnetic layer;

wherein application of a current pulse having either positive or negative polarity and a sufficient amplitude and duration through the magnetic device switches the magnetization vector from either the first stable state to the second stable state or from the second stable state to the first stable state; and

at least one transistor for current control and readout wherein application of a voltage to the at least one bit cell activates the bit cell.