Multiple-bits-per-cell voltage-controlled magnetic memory
View Patent ↗Voltage controlled magneto-electric tunnel junctions and memory devices are described which provide efficient high speed voltage switching of non-volatile magnetic devices (MeRAM) at high cell densities. A multi-bit-per-cell (MBPC) MeRAM is described which requires only a single transistor to write and read two data bits from the one MBPC MeRAM cell.
1. A voltage-controlled magnetic anisotropy (VCMA) switch apparatus, comprising:
at least three ferromagnetic (FM) layers having at least one FM fixed layer and at least a first FM free layer, and a second FM free layer; and
a dielectric (DE) layer interposed between each of said FM layers separating them from one another;
wherein material, shape and thickness of said FM free layers and said FM fixed layer are selected to have in-plane (IP) and out-of-plane (OOP) anisotropies;
wherein OOP anisotropy of said FM free layers is affected by interface properties between said FM layers and said DE layers, and is controlled by voltages applied across said DE layers as an applied voltage and polarity;
wherein a magnetization orientation of said first FM free layer is switched in response to voltages of a first polarity, while magnetization orientation of said second FM free layer is switched in response to voltages of a second polarity by utilizing a voltage-controlled magnetic anisotropy (VCMA) effect;
wherein said apparatus is configured for precessional motion of magnetization;
wherein said magnetization orientation of said first FM free layer is changed in response to width of a pulse of a voltage level of a first polarity and said second FM free layer is changed in response to width of a pulse of a voltage level of a second polarity; and
wherein width of each said pulse is timed at approximately one-half of the precessional period of each said FM free layer.
2. The apparatus recited in claim 1 , wherein said magnetization of each of said FM free layers is switchable between two magnetization orientation states that are either in-plane (IP) or out-of-plane (OOP).
3. The apparatus recited in claim 1 , wherein said magnetization orientation states are parallel (P) and anti-parallel (AP) with respect to magnetization orientation of said FM fixed layer.
4. The apparatus recited in claim 1 , wherein said apparatus switches magnetic states in response to voltage application of given polarities, and does not require a flow of electric charge currents through said apparatus or through a conductor disposed proximal said apparatus to perform said switching.
5. The apparatus recited in claim 1 , wherein said switching of magnetization orientation of each of said FM free layers is determined in response to the applied voltage which induces magnetization rotation and allows full switching to occur in response to presence of a stray magnetic field to switch magnetization to its final switched state in said first FM free layer and said second FM free layer.
6. The apparatus recited in claim 5 , wherein said at least one stray field arises from said ferromagnetic (FM) fixed layer.
7. The apparatus recited in claim 1 , wherein each said ferromagnetic (FM) free layer comprises one layer or a combination of layers.
8. The apparatus recited in claim 1 , wherein said ferromagnetic (FM) fixed layer comprises one layer or a combination of layers.
9. The apparatus recited in claim 1 , wherein said apparatus comprises an in-plane (IP) voltage-controlled magnetic anisotropy (VCMA) switch apparatus having two bits per cell.
10. The apparatus recited in claim 1 , wherein said apparatus comprises an out-of-plane (OOP) voltage-controlled magnetic anisotropy (VCMA) switch apparatus having two bits per cell.
11. The apparatus recited in claim 1 , wherein magnetization orientation of said apparatus can be read by measuring electrical resistance through the FM free layers, DE layers, and FM fixed layer in response to tunneling magnetoresistance (TMR) properties.
12. The apparatus recited in claim 1 , wherein DE layer thickness differs between said first FM free layer and said FM fixed layer, and between said second FM free layer and said FM fixed layer, so that different tunneling resistances are created across each of the tunnel barriers leading to four different resistance states.
13. The apparatus recited in claim 1 , wherein said ferromagnetic material is selected from the group of ferromagnetic materials consisting of Fe, Co, Pt, Pd, FePd, CoFe, and CoFeB.
14. The apparatus recited in claim 1 , wherein said dielectric material comprises MgO.
15. The apparatus recited in claim 1 , wherein said dielectric material comprises a material with a large dielectric constant which is equal to or larger than approximately 10.
16. The apparatus recited in claim 1 , wherein said OOP anisotropy is tuned by placing an additional non-magnetic metal layer adjacent to an FM layer, where said metal layer increases said OOP anisotropy or said VCMA effect.
17. The apparatus recited in claim 16 , where said metal layer comprises Tantalum (Ta) or Hafnium (Hf).
18. The apparatus recited in claim 1 , wherein said OOP anisotropy is tuned by placing an additional dielectric layer adjacent to an FM layer, where said dielectric layer increases said OOP anisotropy or said VCMA effect.
19. The apparatus recited in claim 18 , where said dielectric layer comprises Magnesium Oxide (MgO), Hafnium Oxide (HfOx), or Tantalum Oxide (TaOx).
20. The apparatus recited in claim 1 , wherein said apparatus is incorporated as multiple bits within a multi-bit-per-cell (MBPC) magneto-electric random access memory (MeRAM) in which bit state is written to different FM free layers in response to application of a first and second voltage levels, at a first and second polarity.