This invention relates to a structure with switchable magnetic properties.
In certain applications it is advantageous if the magnetic permeability of a material can be tailored for that application at least within a specified frequency range and more especially if its magnetic permeability could be switched between selected values. In our co-pending UK Patent Application No. 2346485 (International Patent Application No. WO 00/41270) and in a publication entitled Magnetism from Conductors and Enhanced Non-Linear Phenomena, IEEE Transaction on Microwave Theory and Techniques, 1999, 47, 2075-2084, J B Pendry, A J Holden, D J Robbins and W J Stewart, a structured material is disclosed which exhibits a magnetic permeability at a selected frequency, typically a microwave frequency (GHz). The content of these documents is hereby incorporated by way of reference thereto.
The structured material described in these documents comprises an array of capacitive elements which include a low resistance electrically conducting path and in which the elements are arranged such that a magnetic component of electromagnetic radiation within a selected frequency band induces an electrical current to flow around the path and through the associated element. The size of the elements and their spacing are selected such as to provide a selected magnetic permeability in response to the electromagnetic radiation. Such a structure allows a material to be fabricated which is designed to have a selected fixed magnetic permeability for a selected frequency of electromagnetic radiation.
As shown in FIGS. 1(a) and (b) one such structured material 2 comprises an array of capacitive elements 4 each of which consists of two concentric metallic electrically conducting cylindrical tubes: an outer cylindrical tube 6 and an inner cylindrical tube 8. Both tubes 6, 8 have a longitudinal (i.e running in an axial direction) gap 10 and the two gaps 10 are offset from each other by 180xc2x0. The elements 4 are arranged in a regular square array and are positioned on centres at a distance a apart. The outer tube 6 has a radius r and the inner 8 and outer 6 cylindrical tubes are separated by a distance d. The gap 10 prevents the flow of dc electrical current around either of the cylinders 6, 8. However the self capacitance between the two cylindrical tubes 6, 8 allows an ac current, j, to flow when the material is subjected to electromagnetic radiation 12 having a magnetic field component H which is parallel to the axis of the tubes 6, 8. It is shown that such a structure has an effective magnetic permeability xcexceff (xcfx89) which is given by:                                           μ            eff                    ⁡                      (            ω            )                          =                  1          -                      [                                                            π                  ⁢                                      xe2x80x83                                    ⁢                                      r                    2                                                                    a                  2                                                            1                +                                                      2                    ⁢                    σ                    ⁢                                          xe2x80x83                                        ⁢                    i                                                        ω                    ⁢                                          xe2x80x83                                        ⁢                                                                  r                        ⁢                        μ                                            0                                                                      -                                                      3                    ⁢                                          dc                      0                      2                                                                                                  π                      2                                        ⁢                                          ω                      2                                        ⁢                                          r                      3                                                                                            ]                                              Eq        .                  xe2x80x83                ⁢        1            
in which xcfx89 is the angular frequency, "sgr" the resistivity of the cylindrical tubes, i the {square root over (xe2x88x921)} and c0 the velocity of light. From Eq. 1 it can be seen that by appropriate selection of the size r and spacing a of the cylindrical tubes a structure having a selected magnetic permeability at a given frequency xcfx89 can be obtained.
For ease of fabrication it proposed in UK Patent Application No. 2346485 (International Patent Application No. WO 00/41270) to construct each capacitive element 4 in the form of a stack of concentric split rings 26, 28 as shown in FIGS. 2(a) and 2(b). A stack of such rings is shown to be equivalent to the concentric cylindrical tubes described above and has a magnetic permeability given by:                                           μ            eff                    ⁡                      (            ω            )                          =                  1          -                      [                                                            π                  ⁢                                      xe2x80x83                                    ⁢                                      r                    1                    2                                                                    a                  2                                                            1                +                                                                            2                      ⁢                                                                        l                          ⁢                          σ                                                1                                                                                    ω                      ⁢                                              xe2x80x83                                            ⁢                                              r                        1                                            ⁢                                              μ                        0                                                                              ⁢                  i                                -                                                      3                    ⁢                                          lc                      0                      2                                                                                                  πω                      2                                        ⁢                                          r                      1                      3                                        ⁢                                          ln                      [                                              xe2x80x83                                            ⁢                                                                        2                          ⁢                                                      c                            1                                                                                                    d                          1                                                                    ]                                                                                            ]                                              Eq        .                  xe2x80x83                ⁢        2            
where r1 is the inside radius of the inner ring 28, a the lattice spacing of the rings, l the separation between the rings in a given column in an axial direction, d1 the separation between the rings in a radial direction, c1 the width of each ring in a radial direction and "sgr"1 the resistance per unit length of each ring.
A further microstructured material described in United Kingdom Patent Application No. 2346485 (International Patent Application No. WO 00/41270) is constructed using a stack of conducting elements which comprise a single spiral shaped conductor 34 as illustrated in FIGS. 3(a) and 3(b).
It is also suggested that in United Kingdom Patent Application No. 2346485 (International Patent Application No. WO 00/41270) that the magnetic permeability of the structured material could he made to be switchable by incorporating an non-linear dielectric medium, such as Barium Strontium Titanate (BST) or other ferroelectric material, into the structure. The magnetic permeability of the structure is switched by changing the permittivity of the ferroelectric material by applying an electric field across the ferroelectric material. It is suggested that the ferroelectric material could be incorporated between the cylindrical tubes of each capacitive element (FIG. 1(b)) or between each of the concentric rings in a radial direction (FIG. 2(a)). The inclusion however of a ferroelectric material, such as BST, decreases the resonant frequency of the structure by a factor of more than 30 times. To increase the resonant frequency to a selected value to obtain the desired magnetic permeability at a given frequency requires the self capacitance of each capacitive element to be reduced by the same factor. When it is intended that the structured magnetic material is to operate at microwave frequency, that is in the GHz region, this would require a structure composed of capacitive elements which were impractical to fabricate. To overcome this problem it is proposed in United Kingdom Patent Application No. 2346485 (International Patent Application No. WO 00/41270) that the structure comprises an array of single, rather than concentric, cylindrical tubes each of which has two gaps running in an axial direction. A ferroelectric is provided in the gaps and the magnetic permeability switched by changing the permeability of the ferroelectric material using an electrical static switchable electric field. Although such a structured material is capable of operation at microwave frequencies it is impractical to fabricate capacitive elements sufficiently small for operation at radio frequencies in the MHz region. Furthermore even for microwave operation the construction of such a structured material is difficult and expensive.
The present invention has arisen in an endeavour to provide a structured material having a magnetic permeability which can be switched between selected values at a selected wavelength of operation, which can be readily fabricated and which is suitable for operation at radio frequencies (MHz).
According to the present invention there is provided a structure with switchable magnetic properties comprising an array of capacitive elements in which each capacitive element includes a low resistance conducting path and is such that a magnetic component of electromagnetic radiation lying within a predetermined frequency band induces an electrical current to flow around said path and through said associated element and wherein the size of the elements and their spacing apart are selected such as to provide a predetermined permeability in response to said received electromagnetic radiation, characterised in that each capacitive element comprises a plurality of stacked planar sections each of which comprises at least two concentric spiral conducting members which are electrically insulated from each other and which have a switchable permittivity material therebetween.
The magnetic permeability of the structure can be readily switched to a selected value by applying a static electric field across the switchable permittivity material. This is conveniently achieved by applying a dc voltage between the conducting spiral members of each capacitive element. In the context of this patent application the term spiral is to be construed broadly and is not restricted to a plane curve which is traced about a fixed point from which it continuously recedes. The term includes any unclosed loop of more than one turn which recedes away from a centre point. As such the term encompasses spirals which are square, rectangular, triangular, hexagonal or have other geometric forms.
Preferably the spirals are substantially circular in form. Alternatively they are square or rectangular in form.
Advantageously the switchable permittivity material comprises a ferroelectric material, preferably Barium Strontium Titanate. Alternatively it can comprise a liquid crystal.
Preferably the capacitive elements are arranged on a square array. Advantageously alternate spiral conducting members in a given row unwind in an opposite sense. With such an arrangement the structure advantageously further comprises electrically conducting connecting tracks connecting respective spiral members in a given column.
Preferably the structure is configured for operation at radio frequencies (MHz).
The structures of the invention are non-magnetic in a steady magnetic field.