Magnetic multilayer pigment flake and coating composition

1. A multilayer pigment flake comprising:

one or more layers of a ferromagnetic or ferrimagnetic alloy;

one or more dielectric layers; and

a central opaque reflecting layer of aluminum;

wherein the one or more layers of ferromagnetic or ferromagnetic alloy include first and second semi-transparent layers;

wherein the ferromagnetic or ferrimagnetic alloy is an iron-chromium alloy or an iron-chromium-aluminum alloy having a substantially nickel-free composition;

wherein the one or more dielectric layers include first and second transparent dielectric layers;

wherein the first transparent dielectric layer overlies the first semi-transparent magnetic layer;

wherein the central opaque reflecting layer overlies the first transparent dielectric layer;

wherein the second transparent dielectric layer overlies the central opaque reflecting layer; and

wherein the second semi-transparent absorbing magnetic layer overlies the second transparent dielectric layer.

2. The pigment flake of claim 1 , wherein the composition of the ferromagnetic or ferrimagnetic alloy consists essentially of about 5 wt % to about 30 wt % chromium, about 0 wt % to about 18 wt % aluminum, and a balance of iron.

3. The pigment flake of claim 1 , wherein the ferromagnetic or ferrimagnetic alloy is an iron-chromium alloy, and wherein the composition of the ferromagnetic or ferrimagnetic alloy consists essentially of about 5 wt % to about 15 wt % chromium, and a balance of iron.

4. The pigment flake of claim 1 , wherein the magnetic alloy is an iron-chromium-aluminum alloy, and wherein the composition of the ferromagnetic or ferrimagnetic alloy consists essentially of about 10 wt % to about 30 wt % chromium, about 1 wt % to about 18 wt % aluminum, and a balance of iron.

5. The pigment flake of claim 1 , wherein the one or more layers of ferromagnetic or ferrimagnetic alloy are amorphous thin-film layers.

6. The pigment flake of claim 1 , wherein the pigment flake has an interference layer structure, such that the pigment flake changes color with viewing angle or angle of incident light.

7. The pigment flake of claim 1 , wherein the pigment flake has a layer-thickness profile selected to optimize resonant microwave absorption, such that the pigment flake absorbs microwave radiation and can be heated with microwave radiation.

8. The pigment flake of claim 1 having a particle size of about 2 μm to about 20 μm.

9. The pigment flake of claim 1 , wherein the ferromagnetic or ferromagnetic alloy is an iron-chromium-aluminum alloy, and wherein the composition of the magnetic alloy consists essentially of about 18 wt % to about 25 wt % chromium, about 6 wt % to about 15 wt % aluminum, and a balance of iron.

10. The pigment flake of claim 1 , wherein the dielectric layer is selected from: SiO 2 , Al 2 O 3 , MgF 2 , SiO, and ZnS.

11. The pigment flake of claim 1 , wherein the dielectric layers have a thickness from 100-5,000 nm.

12. The pigment flake of claim 1 , wherein the layers of ferromagnetic or ferrimagnetic alloy have thickness about 20 nm to about 1,000 nm.

13. The pigment flake of claim 1 , disposed on a substrate.

14. A magnetic coating composition, comprising:

a binder medium; and

a plurality of multilayer pigment flakes according to claim 1 , disposed in the binder medium.

15. The coating composition of claim 14 , wherein the plurality of pigment flakes each have an interference layer structure, such that the plurality of pigment flakes change color with viewing angle or angle of incident light.

16. The coating composition of claim 14 , wherein the plurality of pigment flakes each have a layer-thickness profile selected to optimize resonant microwave absorption, such that the plurality of pigment flakes absorb microwave radiation and can be heated with microwave radiation.

17. The coating composition of claim 14 , wherein the binder medium is a high-viscosity binder medium, and wherein the coating composition is an intaglio ink.

18. The coating composition of claim 14 , wherein the binder medium includes a curable resin.