Magnetoelectric control of superparamagnetism
A magnetoelectric composite device having a free (i.e. switchable) layer of ferromagnetic nanocrystals mechanically coupled a ferroelectric single crystal substrate is presented, wherein application of an electrical field on the composite switches the magnetic state of the switchable layer from a superparamagnetic state having no overall net magnetization to a substantially single-domain ferromagnetic state.
1. A magnetoelectric device, comprising:
a superparamagnetic element; and
a dielectric element coupled to the superparamagnetic element;
wherein the superparamagnetic element is coupled to the dielectric element such that presence of an electric field switches the magnetic state of the superparamagnetic element between a superparamagnetic state and a substantially single-domain ferromagnetic state; and
wherein the superparamagnetic state comprises substantially no overall net magnetization.
2. A magnetoelectric device as recited in claim 1 , wherein the device is configured to switch the magnetic state of the superparamagnetic element at room temperature.
3. A magnetoelectric device as recited in claim 2 , wherein the electric field is used to turn on and off a permanent magnetic moment of the device.
4. A magnetoelectric device as recited in claim 2 , wherein the superparamagnetic element is mechanically coupled to the dielectric element such that the presence of the electric field induces a strain between the superparamagnetic element and the dielectric element to switch the magnetic state.
5. A magnetoelectric device as recited in claim 4 :
wherein the superparamagnetic element comprises a plurality of nanoparticles; and
wherein the dielectric element comprises a substrate comprising a ferroelectric material mechanically coupled to the nanoparticles.
6. A magnetoelectric device as recited in claim 5 , wherein the dielectric element comprises a piezoelectric substrate.
7. A magnetoelectric device as recited in claim 6 , wherein a free layer comprises Ni nanocrystals embedded within a PT layer.
8. A magnetoelectric device as recited in claim 7 , wherein the substrate comprises PMN-PT mechanically coupled to the nanocrystals.
9. A magnetoelectric device as recited in claim 4 , wherein the substrate comprises upper and lower electrodes disposed on both sides of the substrate.
10. A magnetoelectric device as recited in claim 9 :
wherein the upper electrode and the nanoparticles partially oxidize to promote adhesion; and
wherein said adhesion is configured to facilitate strain transfer between the substrate and the nanoparticles.
11. A magnetoelectric device as recited in claim 2 , wherein the superparamagnetic element comprises a material having a non-zero magnetostriction configured such that any induced magnetoelastic anisotropy causes magnetic dipoles in the superparamagnetic element to align either parallel or perpendicular to a dominant compressive strain direction.
12. A multiferroic composite, comprising:
a switchable superparamagnetic element having an electric-field-induced anisotropy; and
a ferroelectric element coupled to the superparamagnetic element;
wherein the superparamagnetic element is coupled to the ferroelectric element such that presence of an electric field switches the magnetic state of the superparamagnetic element between a superparamagnetic state and a substantially single-domain ferromagnetic state; and
wherein the superparamagnetic state comprises substantially no overall net magnetization.
13. A composite as recited in claim 12 , wherein the composite is configured to switch the magnetic state of the superparamagnetic element at room temperature.
14. A composite as recited in claim 13 , wherein the electric field is used to turn on and off a permanent magnetic moment of the composite.
15. A composite as recited in claim 13 :
wherein the superparamagnetic element comprises a first layer having a plurality of nanoparticles;
wherein the ferroelectric element comprises a piezoelectric substrate; and
wherein the superparamagnetic element is mechanically coupled to the piezoelectric substrate such that the presence of an electric field induces a strain between the superparamagnetic element and the dielectric element to switch the magnetic state.
16. A composite as recited in claim 15 , wherein the superparamagnetic element comprises Ni nanocrystals embedded within a PT layer.
17. A composite as recited in claim 16 , wherein the substrate comprises PMN-PT.
18. A composite as recited in claim 15 , wherein the substrate comprises upper and lower electrodes disposed on both sides of the substrate.
19. A composite as recited in claim 18 :
wherein the upper electrode and the nanoparticles partially oxidize to promote adhesion; and
wherein said adhesion is configured to facilitate strain transfer between the substrate and the nanoparticles.
20. A composite as recited in claim 13 , wherein the superparamagnetic element comprises a material having a non-zero magnetostriction configured such that any induced magnetoelastic anisotropy causes magnetic dipoles in the superparamagnetic element to align either parallel or perpendicular to a dominant compressive strain direction.
21. A composite as recited in claim 13 , wherein the composite is a component within a magnetic memory circuit.