Patent Publication Number: US-2023149729-A1

Title: Spintronic nanodevice for low-power, cellular-level, magnetic neurostimulation

Description:
CROSS-REFERENCE OF RELATED APPLICATION 
     This application is a Section 371 National Stage Application of International Application No. PCT/US2021/025313, filed Apr. 1, 2021, which is incorporated by reference in its entirety and published as WO 2021/202834A1 on Oct. 7, 2021 and which claims priority of U.S. Provisional Application No. 63/004,857, filed Apr. 3, 2020. 
    
    
     BACKGROUND 
     By applying a voltage or a changing magnetic field to a nerve cell, it is possible to cause the nerve cell to “fire” during which the nerve cell depolarizes and then repolarizes. 
     In external magnetic stimulation, a strong alternating magnetic field is generated external to the body and is directed into the body. Within the body, the time-varying magnetic field induces an electric field that creates a current along the nerve cells that cause the cells to fire. 
     Such external systems require strong magnetic fields in order to penetrate into the body. However, as the magnetic fields increase in strength, the area affected by the magnetic fields also increases resulting in low resolution stimulus of the nerve cells. As a result, it is difficult to direct the external magnetic field to only a select number of nerve cells. 
     In implantable magnetic stimulation, a probe is placed in the vicinity of the nerve cells within the body and a magnetic field is generated at the end of the probe to stimulate the nerve so that it fires. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     SUMMARY 
     A neuro-stimulation system includes a stimulator controller, a probe, and a magneto-ionic stimulator positioned on the probe and electrically connected to the stimulator controller. The stimulator controller can apply a voltage to the magneto-ionic stimulator, wherein a change in the voltage causes a change in a magnetic field produced by the magneto-ionic stimulator. 
     In accordance with a further embodiment, a method of stimulating a neuron includes placing a magneto-ionic stimulator near the neuron. A voltage applied to the magneto-ionic stimulator is changed to change the strength of a magnetic field generated by the magneto-ionic stimulator such that an electric field is generated along the neuron. 
     In accordance with a still further embodiment, a neuro-stimulation system includes a stimulator controller, a probe, and a magneto-ionic stimulator positioned on the probe and electrically connected to the stimulator controller. The magneto-ionic stimulator produces a magnetic field that oscillates at a frequency less than 10 Hz. 
     In accordance with some embodiments, the neuro-stimulation system includes a layer of GdO x  in contact with a layer of Co. The stimulator controller applies a positive voltage across the GdO x  layer to cause hydrogen to appear at the boundary between the GdO x  layer and the Co layer. The magneto-ionic stimulator produces a weaker out-of-plane magnetic field when the hydrogen appears at the boundary. The stimulator controller removes the positive voltage across the GdO x  layer to cause the hydrogen to move away from the boundary between the GdO x  layer and the Co layer. The magneto-ionic stimulator produces a stronger out-of-plane magnetic field when the hydrogen moves away from the boundary. 
     In accordance with a further embodiment, the stimulator controller applies a negative voltage across the GdO x  layer to drive oxygen into the Co layer. The magneto-ionic stimulator produces a weaker out-of-plane magnetic field when the oxygen is driven into the Co layer. The stimulator controller applies a positive voltage across the GdO x  layer to drive oxygen out of the Co layer. The magneto-ionic stimulator produces a stronger out-of-plane magnetic field when the oxygen is driven out of the Co layer. 
     In accordance with a further embodiment, the magneto-ionic stimulator includes a layer of GdO x  in contact with a layer of Pd, which is in contact with a layer of Co. The stimulator controller applies a positive voltage across the GdO x  layer to drive hydrogen into the Pd layer. The magneto-ionic stimulator produces a weaker out-of-plane magnetic field when the hydrogen is driven into the Pd layer. The stimulator controller applies a negative voltage across the GdO x  layer to cause the hydrogen to move out of the Pd layer. The magneto-ionic stimulator produces a stronger out-of-plane magnetic field when the hydrogen moves out of the Pd layer. 
     In accordance with one embodiment, the magneto-ionic stimulator includes a layer of oxide in contact with a layer of CoFe x  alloy. In accordance with another embodiment, the magneto-ionic stimulator includes a layer of GdO x  in contact with a layer of Pd, which is in contact with a layer of Co. In accordance with another embodiment, the magneto-ionic stimulator includes a layer of CoFeB that is in contact with a layer of MgO. In a still further embodiment, the layer of MgO is further in contact with an oxide layer such as SiO x . 
     In accordance with one embodiment, the strength of the magnetic field produced by the magneto-ionic stimulator is controlled by controlling an amount of hydrogen at a boundary between two materials. 
     In accordance with a further embodiment, a neuro-stimulation system includes a stimulator controller, a support surface, and a spin orbit torque vortex stimulator positioned on the support surface and electrically connected to the stimulator controller such that the stimulator controller can apply a current to the spin orbit torque vortex stimulator. A change in the current causes a core of a magnetic field vortex produced by the spin orbit torque vortex stimulator to precess. 
     In accordance with a further embodiment, a neuro-stimulation system includes a stimulator controller, a support surface, and a magneto-ionic stimulator positioned on the support surface and electrically connected to the stimulator controller such that the stimulator controller can apply a voltage to the magneto-ionic stimulator. The magneto-ionic stimulator is an electrolytic gel and a change in the voltage causes a change in a magnetic field produced by the gel. 
     In accordance with a further embodiment, a neuro-stimulation system includes an array of magneto-ionic stimulators positioned proximate neurologic tissue and a controller changing a voltage applied to the magneto-ionic stimulators so as to cause a change in a magnetic field produced by the array of magneto-ionic stimulators. 
     In accordance with a further embodiment, the neuro-stimulation system further includes a wireless power receiver that receives power from a wireless power transmitter. 
     In accordance with a further embodiment, a method of stimulating a portion of an interoception system in a living body includes placing a magneto-ionic stimulator near the portion of the interoception system and changing a voltage applied to the magneto-ionic stimulator to change the strength of a magnetic field generated by the magneto-ionic stimulator such that an electric field is generated along the portion of the interoception system. 
     In accordance with a further embodiment, a method of stimulating tissue proximate the spine of a living body includes placing a magneto-ionic stimulator near the tissue and changing a voltage applied to the magneto-ionic stimulator to change the strength of a magnetic field generated by the magneto-ionic stimulator such that an electric field is generated along the tissue. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a neuro-stimulation system. 
         FIG.  2    is a perspective view of a probe and magneto-ionic stimulator in accordance with one embodiment. 
         FIG.  3    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  2    with the magneto-ionic stimulator in a first state. 
         FIG.  4    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  2    with the magneto-ionic stimulator in a second state. 
         FIG.  5    is a perspective view of a probe and magneto-ionic stimulator in accordance with a second embodiment. 
         FIG.  6    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  5    with the magneto-ionic stimulator in a first state. 
         FIG.  7    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  5    with the magneto-ionic stimulator in a second state. 
         FIG.  8    is a perspective view of a probe and magneto-ionic stimulator in accordance with a third embodiment. 
         FIG.  9    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  8    with the magneto-ionic stimulator in a first state. 
         FIG.  10    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  8    with the magneto-ionic stimulator in a second state. 
         FIG.  11    is a perspective view of a probe and magneto-ionic stimulator in accordance with a fourth embodiment. 
         FIG.  12    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  11    with the magneto-ionic stimulator in a first state. 
         FIG.  13    is a side sectional view of the probe and magneto-ionic stimulator of  FIG.  12    with the magneto-ionic stimulator in a second state. 
         FIG.  14    provides a side sectional view of a neurostimulator at the end of a probe in accordance with a fifth embodiment. 
         FIG.  15    provides a side sectional view of a neurostimulator at the end of a probe in accordance with a sixth embodiment. 
         FIG.  16    provides schematic view of neurostimulator with an on-chip wireless power transfer system in accordance with a seventh embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Below, embodiments of a magnetic field tissue stimulator are described. Each of these embodiments relies on a change in the out-of-plane magnetization of a thin layer of material as ions move within the stimulator. By applying a voltage, the ions are moved causing a resulting change in the magnetic field. As a result, a voltage control signal can be used to modulate the magnetic flux density of the magnetic field above the tissue stimulator. This fluctuating magnetic field creates an electric field in the target tissue that can, for example, cause neurons to fire. 
     A significant advantage of the embodiments is the frequency at which the magnetic field oscillates. Many prior art spintronic nano-sized stimulators are limited to operating in the MHz to GHz range. However, neurons do not react to such high frequency oscillations. Instead, the optimum frequency for neurostimulation is on the order of 1 Hz. The embodiments described below modulate the magnetic field at between 0.5-7.0 Hz thereby making them more effective spintronic neurostimulators. 
       FIG.  1    provides a neurostimulation system  100  that includes a stimulator controller  102 , a probe  104  having a support surface  106  and a magneto-ionic stimulator  108  located on support surface  106 . Support surface  106  has a diameter on the order of 1 mm while magneto-ionic stimulator  108  has a width on the order of 10 micrometers. Conductors  110  and  112  provide an electrical connection between stimulator controller  102  and magneto-ionic stimulator  108  that allow stimulator controller  102  to apply different voltages to magneto-ionic stimulator so as to change the out-of-plane magnetic field produced by magneto-ionic stimulator  108 . 
       FIG.  2    provides a perspective view of support surface  106  and stimulator  108 .  FIGS.  3  and  4    show side sectional views of support surface  106  and stimulator  108 . 
     Stimulator  108  has a top surface pointing away from support surface  106  and a bottom surface facing support surface  106 . The bottom surface of stimulator  108  is mounted on support surface  106  with an adhesive bead layer  114  between stimulator  108  and support surface  106 . Adhesive bead layer  114  follows the perimeter of the bottom surface of stimulator  108  such that portions of the bottom surface remain exposed to support surface  106 . 
     A layer  123  of Tantalum (Ta) is deposited on the top of a Si/SiO 2  substrate  122 . A layer  124  of platinum (Pt) is deposited on layer  123 . In accordance with one embodiment, Pt layer  124  has a height of 3 nm. Pt layer  124  is connected to conductor  112 . A layer  126  of cobalt (Co) is deposited on top of Pt layer  124 . In accordance with one embodiment, Co layer  126  has a height of 0.9 nm. A layer  128  of gadolinium oxide (GdO x ) is deposited on top of Co layer  126 . In accordance with one embodiment, GdO x  layer  128  has a height of 30 nm. A layer  130  of gold (Au) is deposited on top of GdO x  layer  128 . In accordance with one embodiment, Au layer  130  has a height of 3 nm. Au layer  130  is connected to conductor  110 . 
     In  FIG.  3   , stimulator controller  102  has placed conductors  110  and  112  at the same voltage so that there is no voltage between Au layer  130  and Pt layer  124 . In this state, Co layer  126  produces a magnetic field  300  (shown in dotted lines in  FIG.  3   ) that extends perpendicularly out of the top surface of Co layer  126  and interacts with neurons, such as neuron  302  in tissue  304  that Au layer  130  is pressed against. 
     In  FIG.  4   , stimulator controller  102  has applied a positive voltage between conductors  110  and  112  creating a corresponding positive voltage between Au layer  130  and Pt layer  124 . This positive voltage causes a layer of hydrogen to form at the interface between GdO x  layer  128  and Co layer  126 . This layer of hydrogen causes the magnetic field produced by Co layer  126  to rotate in plane thereby causing the magnetic field perpendicular to the top surface of Co layer  126  to disappear. As a result, there is no magnetic field passing through neuron  302 . 
     In order to stimulate neuron  302 , stimulator controller  102  applies positive voltage pulses on conductor  110  and connects conductor  112  to ground. Each voltage pulse creates a changing magnetic field that produces a corresponding electric field in neuron  302 . A rising edge in the voltage creates a corresponding falling edge in the magnetic field. In accordance with one embodiment, the switching time between the rising edge in voltage and the falling edge in the magnetic field is 100 ms. Similarly, a falling edge in the voltage creates a corresponding rising edge in the magnetic field. In accordance with one embodiment, the switching time between falling edge in the voltage and the rising edge in the magnetic field is 400 ms. The difference between the switching times is due to the fact that when a positive voltage is applied to conductor  110 , the voltage drives electrons into Pt layer  124  to facilitate the formation of the hydrogen layer. However, when conductor  110  is returned to ground, there is only a small electromotive force to draw electrons away from Pt layer  124  that will allow the hydrogen to move away from the interface. 
     In order to reduce the switching time for increasing the magnetic field, a negative voltage can be applied to conductor  110 . However, applying a negative voltage on conductor  110  will cause oxygen to move into Co layer  126 , which reduces the magnetic field instead of increasing the magnetic field. To avoid such oxidation of Co layer  126 , a second embodiment of the stimulator, magneto-ionic stimulator  508 , is provided as shown in  FIGS.  5 - 7   . 
       FIG.  5    provides a perspective view of stimulator  508  mounted on support surface  106 .  FIGS.  6  and  7    show side sectional views of support surface  106  and stimulator  508 . 
     Stimulator  508  has a top surface pointing away from support surface  106  and a bottom surface facing support surface  106 . The bottom surface of stimulator  508  is mounted on support surface  106  with an adhesive bead layer  514  between stimulator  508  and support surface  106 . Adhesive bead layer  514  follows the perimeter of the bottom surface of stimulator  508  such that portions of the bottom surface remain exposed to support surface  106 . 
     A layer  523  of Tantalum (Ta) is deposited on the top of a Si/SiO 2  substrate  522 . A layer  524  of platinum (Pt) is deposited on layer  523 . In accordance with one embodiment, Pt layer  524  has a height of 3 nm. Pt layer  524  is connected to conductor  112 . A layer  526  of cobalt (Co) is deposited on top of Pt layer  524 . In accordance with one embodiment, Co layer  526  has a height of 0.9 nm. A layer  527  of palladium (Pd) is deposited on top of Co layer  526 . In accordance with one embodiment, Pd layer  527  has a height of 4.5 nm. A layer  528  of gadolinium oxide (GdO x ) is deposited on top of Pd layer  527 . In accordance with one embodiment, GdO x  layer  528  has a height of 30 nm. A layer  530  of gold (Au) is deposited on top of GdO x  layer  528 . In accordance with one embodiment, Au layer  530  has a height of 3 nm. Au layer  530  is connected to conductor  110 . 
     In  FIG.  6   , stimulator controller  102  has placed a negative voltage on conductor  110  while keeping conductor  112  at ground so that there is negative voltage between Au layer  530  and Pt layer  524 . In this state, Co layer  526  produces a magnetic field  600  (shown in dotted lines in  FIG.  6   ) that extends perpendicularly out of the top surface of Co layer  526  and interacts with neurons, such as neuron  602  in tissue  604  that Au layer  530  is pressed against. 
     In  FIG.  7   , stimulator controller  102  has applied a positive voltage on conductor  110  creating a corresponding positive voltage between Au layer  530  and Pt layer  524 . This positive voltage causes a layer of hydrogen to form at the interface between GdO x  layer  528  and Co layer  526 . This layer of hydrogen causes the magnetic field produced by Co layer  526  to rotate in plane thereby causing the magnetic field perpendicular to the top surface of Co layer  526  to disappear. As a result, there is no magnetic field passing through neuron  602 . 
     In order to stimulate neuron  602 , stimulator controller  102  alternates between providing positive and negative voltage pulses on conductor  110  and connects conductor  112  to ground. The alternating voltage pulses create a changing magnetic field that produces a corresponding electric field in neuron  602 . 
     The addition of Pd layer  527  in the embodiment of  FIGS.  5 - 7   , protects Co layer  526  from oxidizing when a negative voltage is applied. During a positive voltage, hydrogen moves into Pd Layer  527  so that it still accumulates at the surface of Co layer  526 . During a negative voltage pulse, electrons are pulled away from Pt layer  524  causing the hydrogen atoms to become H +  protons that are then transported through GdO x  layer  528  while Pd layer  527  prevents oxygen from reaching Co layer  526 . 
       FIG.  8    provides a perspective view of support surface  106  and a magneto-ionic stimulator  808  of a third embodiment.  FIGS.  9  and  10    show side sectional views of support surface  106  and stimulator  808 . 
     Stimulator  808  has a top surface pointing away from support surface  106  and a bottom surface facing support surface  106 . The bottom surface of stimulator  808  is mounted on support surface  106  with an adhesive bead layer  814  between stimulator  808  and support surface  106 . Adhesive bead layer  814  follows the perimeter of the bottom surface of stimulator  808  such that portions of the bottom surface remain exposed to support surface  106 . 
     A layer  823  of Tantalum (Ta) is deposited on the top of a Si/SiO 2  substrate  822 . A layer  824  of platinum (Pt) is deposited on layer  823 . In accordance with one embodiment, Pt layer  824  has a height of 3 nm. Pt layer  824  is connected to conductor  112 . A layer of cobalt (Co) is deposited on top of Pt layer  824 . In accordance with one embodiment, the Co layer has a height of 0.9 nm. A layer  828  of Gadolinium oxide (GdO x ) is deposited on top of the Co layer. In accordance with one embodiment, GdO x  layer  828  has a height of 30 nm. A layer  830  of gold (Au) is deposited on top of GdO x  layer  828 . In accordance with one embodiment, Au layer  830  has a height of 3 nm. Au layer  830  is connected to conductor  110 . 
     Once constructed, a negative voltage is applied to conductor  110  to create a negative voltage between Au layer  830  and Pt layer  824 . This negative voltage causes oxygen to be forced into the surface of the Co layer thereby forming a CoO layer  826 . The negative voltage is then removed, leaving the oxygen in CoO layer  826 . 
     In  FIG.  9   , stimulator controller  102  has applied a positive voltage to conductor  110  to create a positive voltage between Au layer  830  and Pt layer  824 . In this state, the oxygen in CoO layer  826  is driven out of the layer producing a Co layer that generates a magnetic field  900  (shown in dotted lines in  FIG.  9   ) that extends perpendicularly out of the top surface of the Co layer and interacts with neurons, such as neuron  902  in tissue  904  that Au layer  830  is pressed against. 
     In  FIG.  10   , stimulator controller  102  has applied a negative voltage between conductors  110  and  112  creating a corresponding negative voltage between Au layer  830  and Pt layer  824 . This negative voltage drives oxygen back into the Co layer to reform CoO layer  826 . The oxygen causes the magnetic field to rotate into the plane of the CoO layer  826  so that the magnetic field perpendicular to the top surface of CoO layer  826  disappears. As a result, there is no magnetic field passing through neuron  902 . 
     In order to stimulate neuron  902 , stimulator controller  102  alternates between applying a positive voltage and a negative voltage on conductor  110  and connects conductor  112  to ground. Each voltage pulse creates a changing magnetic field that produces a corresponding electric field in neuron  902 . 
       FIG.  11    provides a perspective view of support surface  106  and a magneto-ionic stimulator  1108  of a fourth embodiment.  FIGS.  12  and  13    show side sectional views of support surface  106  and stimulator  1108 . 
     Stimulator  1108  has a top surface pointing away from support surface  106  and a bottom surface facing support surface  106 . The bottom surface of stimulator  1108  is mounted on support surface  106  with an adhesive bead layer  1114  between stimulator  1108  and support surface  106 . Adhesive bead layer  1114  follows the perimeter of the bottom surface of stimulator  1108  such that portions of the bottom surface remain exposed to support surface  106 . 
     A layer  1123  of tantalum (Ta) is deposited on the top of a Si/SiO 2  substrate  1122 . A layer  1124  of palladium (Pd) is deposited on layer  1123 . In accordance with one embodiment, Pd layer  1124  has a height of 10 nm. Pd layer  1124  is connected to conductor  112 . A layer  1126  consisting of multiple alternating layers of cobalt (Co) and palladium (Pd) is deposited on top of Pd layer  1124 . In accordance with one embodiment, Co/Pd multilayer  1126  has a height of 3 nm. A layer  1128  of tantalum is deposited on top of Co/Pd multilayer  1126 . In accordance with one embodiment, Ta layer  1128  has a height of 1 nm. A layer  1130  of CoFeB is deposited on top of tantalum layer  1128 . In accordance with one embodiment, CoFeB layer  1130  has a height of 1.3 nm. A layer  1132  of MgO is deposited on CoFeB layer  1130 . In accordance with one embodiment, MgO layer  1132  has a height of 2 nm. A layer  1134  of SiO x  is deposited on MgO layer  1132 . A layer  1136  of gold (Au) is deposited on top of SiO x  layer  1134 . In accordance with one embodiment, Au layer  1136  has a height of 3 nm. Au layer  1130  is connected to conductor  110 . 
     In  FIG.  12   , stimulator controller  102  has applied a positive voltage to conductor  110  to create a positive voltage between Au layer  1136  and Pd layer  1124 . In this state, ionic oxygen in SiO x  layer  1134  is driven away from MgO layer  1132  thereby allowing CoFeB layer  1130  to generate a magnetic field  1200  (shown in dotted lines in  FIG.  12   ) that extends perpendicularly out of the top surface of CoFeB layer  1130  and interacts with neurons, such as neuron  1202  in tissue  1204  that Au layer  1136  is pressed against. 
     In  FIG.  13   , stimulator controller  102  has applied a negative voltage between conductors  110  and  112  creating a corresponding negative voltage between Au layer  1136  and Pd layer  1124 . This negative voltage drives ionic oxygen toward MgO layer  1132  thereby causing the magnetic field to rotate into the plane of the CoFeB layer  1130  so that the magnetic field perpendicular to the top surface of CoFeB layer  1130  disappears. As a result, there is no magnetic field passing through neuron  1202 . 
     In order to stimulate neuron  1202 , stimulator controller  102  alternates between applying a positive voltage and a negative voltage on conductor  110  and connects conductor  112  to ground. Each voltage pulse creates a changing magnetic field that produces a corresponding electric field in neuron  1102 . 
     Each of the embodiments described above create a time-varying magnetic field has a frequency of oscillation that is limited by ionic transport speed. As a result, the switching provided by the embodiments is in the range of 0.5 Hz-100 kHz. This aligns well with the optimum frequency for stimulating neurons of ˜100 Hz. As a result, the embodiments are well-suited for neuron stimulation. 
       FIG.  14    provides a side sectional view of a neurostimulator  1408  at the end of probe  106 . Neurostimulator  1408  is a spin orbit torque vortex device, A bottom contact layer  1420  of neurostimulator  1408  is made of PtMn and has a height of 15 nm. A layer  1422  of CoFe is deposited on top of layer  1420  and has a height of 2.5 nm. A layer  1424  of Ru is deposited on layer  1422  and has a height of 0.85 nm. A layer  1426  of CoFeB is deposited on top of layer  1424  and has a height of 3 nm. Layer  1426  provides a fixed magnetic layer. A layer  1428  of MgO is deposited on top of layer  1426  and has a height of 1.075 nm. Layer  1428  acts as a barrier layer. A layer  1430  of NiFe is deposited on top of layer  1428  and has a height of 15 nm. Layer  1430  acts as a free layer. A cap layer  1432  of Ru is deposited on layer  1430  and has a height of 10 nm. 
     Neurostimulator  1408  uses a current I dc  in the plane of layer  1426  and a magnetic field H dc  that is perpendicular to the plane of layer  1426  to cause the core  1434  of a magnetic vortex within neurostimulator  1408  to precess  1436 . This results in an oscillating magnetic field external to neurostimulator  1408  that induces an oscillating electric field that can cause neuron  1402  to fire. The oscillations have a frequency on the order of GHz with the actual frequency being set by the size of the current in layer  1426 . This is a significant improvement over nanowire stimulation, which has a frequency between MHz and GHz. Neurostimulator  1408  also only requires 5 nW of power, which implies low thermal effects on tissue. 
       FIG.  15    provides a side sectional view of a neurostimulator  1508  at the end of probe  106 . Neurostimulator  1508  is a spin orbit torque vortex device. A bottom conductor layer  1520  of Cu has a layer  1522  of NiFe deposited on it. Layer  1522  provides a fixed magnetic layer. A thin layer  1524  of Cu is deposited on top of layer  1522  and acts as a barrier layer. A layer  1526  of NiFe is deposited on top of layer  1524  and acts as a free layer. A top conductor layer  1528  of Cu is deposited on layer  1526 . 
     Neurostimulator  1508  uses a current I dc  between top conductor layer  1528  and bottom conductor layer  1520  and a magnetic field H dc  that is perpendicular to the plane of layer  1522  to cause the core  1534  of a magnetic vortex within neurostimulator  1508  to precess  1536 . This results in an oscillating magnetic field external to neurostimulator  1508  that induces an oscillating electric field that can cause neuron  1502  to fire. The frequency of oscillation is on the order of 1.0 GHz. 
       FIG.  16    provides a schematic diagram of an alternative neurostimulation system  1600  that includes an implantable structure  1602  that can be surgically implanted within a living body  1604 . Implantable structure  1602  includes an array  1606  of neurostimulators that are mounted on a support surface  1605  of structure  1602  and that are in contact with tissue in living body  1604  after implantation. The neurostimulators can be any of the magneto-ionic neurostimulators discussed above. Implantable structure  1602  also supports a controller  1608  and a wireless receiver  1610 . Controller  1608  controls the application of voltage and/or current to the neurostimulators in array  1606  to thereby control the magnetic fields generated by the neurostimulators in array  1606 . Wireless receiver  1610  receives a wireless signal  1612  generated by a wireless transmitter  1614  outside of living body  1604 . Wireless signal  1612  generates a voltage in receiver  1610  that is then used to provide power to controller  1608 . Controller  1608  uses this power to apply the voltage and/or current to the neurostimulators in array  1606 . In accordance with one embodiment, wireless transmitter  1614  is contained within a mobile container  1616  that can be carried by the person implanted with structure  1602 . Mobile container  1616  also includes a battery  1618 , which provides power to wireless transmitter  1614 . 
     Although the neurostimulators have been discussed above in connection with neurons in the brain, the neurostimulators may be used in other parts of the neurologic system, such as neurologic tissue in the spine. In accordance with some embodiments, the neurostimulators are used on tissue of the interoception system of the body. 
     Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms for implementing the claims.