Probabilistic neuron circuits
Neuron circuit structures are presented which employ magnetic tunnel junction (MTJ) elements that change state probabilistically in response to application of electrical source currents that emulate synaptic activity. Some implementations form probabilistic neuron circuits using homogeneous perpendicular spin-transfer torque (STT) MTJ elements. These neuron circuits include a perpendicular STT reference MTJ element coupled via an electrical node with a perpendicular STT neuron MTJ element that can change state. The electrical node for each neuron circuit couples a neuron MTJ element or “perturbation” element to a reference element, and also to an electrical current employed to influence probabilistic magnetization state changes in the perturbation MTJ element. A read current can be applied to the perturbation element to produce an output voltage at the electrical node indicative of a magnetization state of the perturbation element.
1. A circuit comprising:
a perturbation magnetic tunnel junction (MTJ) element;
a reference element;
an output node coupling a free layer of the perturbation MTJ element and a first terminal of the reference element to a synaptic current source, wherein the synaptic current source comprises a plurality of MTJ elements connected in parallel and a selection element to selectively couple a synaptic current from the plurality of MTJ elements to the output node;
a read current source coupled to a pinned layer of the perturbation MTJ element; and
a reference potential coupled to a second terminal of the reference element.
2. The circuit of claim 1 , wherein the perturbation MTJ element comprises a first perpendicular spin transfer torque (STT) MTJ element having an alterable magnetization state, and wherein the reference element comprises a second perpendicular STT MTJ with a magnetization state held in a predetermined state.
3. The circuit of claim 1 , wherein the synaptic current source comprises at least a first selection element to selectively couple a synaptic current to the output node, wherein the read current source comprises at least a second selection element to selectively couple one of a read current and a reset current to the pinned layer of the perturbation MTJ element, and wherein circuitry supplying the reference potential comprises a third selection element to selectively couple an electrical reference to the second terminal of the reference element.
4. The circuit of claim 1 , wherein the perturbation MTJ element and the reference element form a voltage divider having an output voltage at the output node.
5. An apparatus comprising:
a perturbation magnetic tunnel junction (MTJ) element coupled to a control line through a node; and
a reference MTJ element coupled to the control line through the node;
wherein during a synaptic phase, the perturbation MTJ element changes state probabilistically in response to a synaptic current provided by the control line through the node; and
wherein during a read phase, a read current established between the perturbation MTJ element and the reference MTJ element results in an output at the node representative of the state of the perturbation MTJ element
wherein a synaptic current source that feeds the control line comprises a plurality of MTJ elements coupled in parallel, and a selection element to selectively couple a synaptic current from the plurality of MTJ elements to the node.
6. The apparatus of claim 5 , wherein the output comprises a voltage spike at the node that results from passing the read current through at least the perturbation MTJ element when the perturbation MTJ element is in an opposite magnetization state relative to the reference MTJ element.
7. The apparatus of claim 5 , wherein the read current is introduced to a terminal of the perturbation MTJ element opposite the node, wherein a terminal of the reference MTJ element opposite the node is coupled to a ground potential to form a voltage divider between the perturbation MTJ element and the reference MTJ element, and wherein the output comprises a voltage spike resultant from the read current based on a magnetization state of at least the perturbation MTJ element.
8. The apparatus of claim 5 , wherein a free layer of the perturbation MTJ element is coupled to the node, and a free layer of the reference MTJ is coupled to the node.
9. The apparatus of claim 5 , further comprising:
responsive to a reset current established through at least the perturbation MTJ element, the perturbation MTJ element is reset into an antiparallel magnetization state, and wherein the reference MTJ element comprises a magnetization state held in an antiparallel state.
10. The apparatus of claim 5 , wherein an alternating application of the synaptic current and the read current is repeated until a voltage spike at the node is produced as the output responsive to the read current.
11. The apparatus of claim 5 , wherein varying properties of the synaptic current provided by the plurality of MTJ elements influences a probabilistic behavior of the perturbation MTJ element.
12. A method comprising:
during a write phase, sending a synaptic current from a source circuit through a perturbation magnetic tunnel junction (MTJ) element coupled to a node to stochastically influence a magnetization state of the perturbation MTJ element, wherein the source circuit comprises a plurality of MTJ elements connected in parallel and a selection element to selectively couple the synaptic current from the plurality of MTJ elements to the node; and
during a read phase, isolating the source circuit from the node, sending a read current through the perturbation MTJ element and a reference MTJ element coupled by the node, and producing an output voltage at the node representative of the magnetization state of the perturbation MTJ element.
13. The method of claim 12 , further comprising:
repeating the write phase and the read phase until a voltage spike occurs at the node during the read phase indicating a change in the magnetization state of the perturbation MTJ element from an initial magnetization state to an opposite magnetization state relative to the reference MTJ element.
14. The method of claim 13 , further comprising:
during a reset phase, sending a reset current through the perturbation MTJ element and the reference MTJ element coupled by the node to place the perturbation MTJ element into the initial state comprising an antiparallel magnetization state.
15. The method of claim 12 , further comprising:
during the read phase, introducing the read current to a terminal of the perturbation MTJ element opposite the node, and coupling a terminal of the reference MTJ element opposite the node to a ground potential.
16. The method of claim 12 , further comprising:
during the write phase, sending the synaptic current from the plurality of MTJ elements coupled in parallel, wherein the plurality of MTJ elements each output a corresponding portion of the synaptic current that stochastically influences a change in the magnetization state of the perturbation MTJ element.
17. A system to produce a probabilistic output, comprising:
a synaptic current source comprising:
a plurality of magnetic tunnel junction (MTJ) elements that together provide a synaptic current;
a first switching element configured to selectively provide the synaptic current to a node of a neuron circuit during a write phase;
the neuron circuit comprising:
a perturbation MTJ element;
a reference MTJ element;
the node coupling a free layer of the perturbation MTJ element and a free layer terminal of the reference MTJ element, wherein an output produced at the node during a read phase comprises the probabilistic output;
a second switching element configured to couple a read potential to a pinned layer of the perturbation MTJ element during the read phase and couple a reference potential to the pinned layer of the perturbation MTJ element during the write phase; and
a third switching element configured to couple the reference potential to a pinned layer of the reference MTJ element during the read phase and decouple the reference potential from the pinned layer of the reference MTJ during the write phase.
18. The system of claim 17 , wherein:
the second switching element is further configured to couple a reset potential to the pinned layer of the perturbation MTJ element during a reset phase; and
the third switching element is further configured to couple the pinned layer of the reference MTJ element to the reference potential during the reset phase.
19. The system of claim 17 , further comprising:
an output element configured to present an output voltage indicating the probabilistic output, wherein a first output voltage level indicates at least the perturbation MTJ element was probabilistically placed in an opposite magnetization state relative to the reference MTJ element responsive to the synaptic current.
20. The system of claim 17 , wherein magnitude and duration properties of the synaptic current provided by the synaptic current source influences a probabilistic magnetization state change in the perturbation MTJ element.
21. The system of claim 17 , wherein the perturbation MTJ element undergoes a probabilistic magnetization state change in response to the synaptic current and at least thermal noise present in the perturbation MTJ element.
22. The system of claim 17 , further comprising:
interconnect elements configured to interconnect an output representing the probabilistic output produced at the node as an input to one or more other synaptic current sources associated with one or more further neuron circuits.
23. A stochastic device comprising:
means for selectively producing a synaptic current through a perturbation element coupled to a node to stochastically influence a state change of the perturbation element from an antiparallel magnetization state to a parallel magnetization state; and
means for selectively producing a read current through the perturbation element and a reference element coupled with the perturbation element by the node; and
means for producing an output at the node representative of a present magnetization state of the perturbation element.
24. The stochastic device of claim 23 , further comprising:
means for selectively producing a reset current through the perturbation element and the reference element coupled by the node to place the perturbation element into an initial state comprising the antiparallel magnetization state.