Magnetic illusion : transforming a magnetic object into another object by negative permeability

We theoretically predict and experimentally verify the illusion of transforming the magnetic signature of a 3D object into that of another arbitrary object. This is done by employing negative-permeability materials, which we demonstrate can be emulated by tailored sets of currents. The experimental transformation of the magnetic response of a ferromagnet into that of its antagonistic material, a superconductor, is presented to confirm the theory. The emulation of negative-permeability materials by currents provides a pathway for designing devices for controlling magnetic fields

Tailoring magnetic fields in inaccessible regions

Controlling magnetism, essential for a wide range of technologies, is impaired by the impossibility of generating a maximum of magnetic field in free space. Here, we propose a strategy based on negative permeability to overcome this stringent limitation. We experimentally demonstrate that an active magnetic metamaterial can emulate the field of a straight current wire at a distance. Our strategy leads to an unprecedented focusing of magnetic fields in empty space and enables the remote cancellation of magnetic sources, opening an avenue for manipulating magnetic fields in inaccessible regions

Negative permeability in magnetostatics and its experimental demonstration

The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negative-permeability values. The understanding of these new solutions allow us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negative-permeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical applications

Controlling static magnetic fields with positive and negative permeability metamaterials

Magnetism is present in our daily life – it is found at the basis of technologies such as electric generators and transformers, data storage systems, sensors, and biomedical equipment – and it is usually controlled by ferromagnetic materials. Recently, the introduction of magnetic metamaterials and the transformation optics technique has enabled the development of a wide range of devices for controlling magnetic fields, offering possibilities beyond those of conventional magnetic materials. In this thesis, we apply these concepts to propose different strategies for controlling static magnetic fields in novel ways. First, we review and study in detail the interaction of static magnetic fields with conventional materials with extremely large (ferromagnets) and extremely low (perfect diamagnets) magnetic permeability. Results show that there are some properties of materials with permeability close to zero that remain yet to be exploited, such as the possibility of overlapping the field created by wires located in different positions as if they had all been placed at the same point. Then, we explore the properties of different metamaterials with positive permeability, which consist of arrangements of ferromagnetic or perfect diamagnetic materials or combinations of both. We demonstrate that extremely anisotropic two-dimensional and three-dimensional shells with extremely large radial permeability and extremely low angular permeability can achieve strong magnetic field concentrations inside their holes without distorting externally applied magnetic fields. This could be used to enhance the sensitivity of magnetic sensors. For the case of a spherical concentrator, results are confirmed experimentally. These shells are also shown to expel towards their exterior the magnetic field generated inside their hole. Interestingly, these properties can be applied to effectively enlarge magnetic materials; a magnetic material surrounded by a concentrating shell responds to the applied magnetic field as if it was a larger material. Finally, we present a different metamaterial device which can make a magnetic sensor undetectable. In this way, the sensor would detect the applied field without being detected, which is interesting for applications requiring non-invasive sensing. In the last part of the thesis, we introduce the concept of negative permeability for static magnetic fields. We theoretically and experimentally demonstrate that, even though materials with negative permeability do not naturally occur, they can be emulated in practice by suitably tailored arrangements of currents, which constitute an active metamaterial. We propose two applications of negative permeability in magnetostatics: magnetic illusion and perfect magnetic lensing. On the one hand, we show that negative permeability enables the transformation of the magnetic signature of an object into that of another one. The experimental transformation of a ferromagnetic sphere into a perfect diamagnetic sphere confirms the theoretical ideas. On the other hand, we demonstrate that a cylindrical shell with permeability -1 behaves as the analogous of a perfect lens for static magnetic fields and can be used to create images of magnetic sources. Since the images may appear in empty space, this shell could enable the creation and cancellation of magnetic sources remotely, something unachievable with positive permeability. To sum up, in this thesis we propose different strategies for shaping, controlling and even for creating static magnetic fields based on positive and negative permeability metamaterials

Controlling static magnetic fields with positive and negative permeability metamaterials

Magnetism is present in our daily life - it is found at the basis of technologies such as electric generators and transformers, data storage systems, sensors, and biomedical equipment - and it is usually controlled by ferromagnetic materials. Recently, the introduction of magnetic metamaterials and the transformation optics technique has enabled the development of a wide range of devices for controlling magnetic fields, offering possibilities beyond those of conventional magnetic materials. In this thesis, we apply these concepts to propose different strategies for controlling static magnetic fields in novel ways. First, we review and study in detail the interaction of static magnetic fields with conventional materials with extremely large (ferromagnets) and extremely low (perfect diamagnets) magnetic permeability. Results show that there are some properties of materials with permeability close to zero that remain yet to be exploited, such as the possibility of overlapping the field created by wires located in different positions as if they had all been placed at the same point. Then, we explore the properties of different metamaterials with positive permeability, which consist of arrangements of ferromagnetic or perfect diamagnetic materials or combinations of both. We demonstrate that extremely anisotropic two-dimensional and three-dimensional shells with extremely large radial permeability and extremely low angular permeability can achieve strong magnetic field concentrations inside their holes without distorting externally applied magnetic fields. This could be used to enhance the sensitivity of magnetic sensors. For the case of a spherical concentrator, results are confirmed experimentally. These shells are also shown to expel towards their exterior the magnetic field generated inside their hole. Interestingly, these properties can be applied to effectively enlarge magnetic materials; a magnetic material surrounded by a concentrating shell responds to the applied magnetic field as if it was a larger material. Finally, we present a different metamaterial device which can make a magnetic sensor undetectable. In this way, the sensor would detect the applied field without being detected, which is interesting for applications requiring non-invasive sensing. In the last part of the thesis, we introduce the concept of negative permeability for static magnetic fields. We theoretically and experimentally demonstrate that, even though materials with negative permeability do not naturally occur, they can be emulated in practice by suitably tailored arrangements of currents, which constitute an active metamaterial. We propose two applications of negative permeability in magnetostatics: magnetic illusion and perfect magnetic lensing. On the one hand, we show that negative permeability enables the transformation of the magnetic signature of an object into that of another one. The experimental transformation of a ferromagnetic sphere into a perfect diamagnetic sphere confirms the theoretical ideas. On the other hand, we demonstrate that a cylindrical shell with permeability -1 behaves as the analogous of a perfect lens for static magnetic fields and can be used to create images of magnetic sources. Since the images may appear in empty space, this shell could enable the creation and cancellation of magnetic sources remotely, something unachievable with positive permeability. To sum up, in this thesis we propose different strategies for shaping, controlling and even for creating static magnetic fields based on positive and negative permeability metamaterials

Negative permeability in magnetostatics and its experimental demonstration

The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negative-permeability values. The understanding of these new solutions allow us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negative-permeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical application

Negative permeability in magnetostatics and its experimental demonstration

The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negative-permeability values. The understanding of these new solutions allow us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negative-permeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical application

Enhancing the sensitivity of magnetic sensors by 3D metamaterial shells

Magnetic sensors are key elements in our interconnected smart society. Their sensitivity becomes essential for many applications in fields such as biomedicine, computer memories, geophysics, or space exploration. Here we present a universal way of increasing the sensitivity of magnetic sensors by surrounding them with a spherical metamaterial shell with specially designed anisotropic magnetic properties. We analytically demonstrate that the magnetic field in the sensing area is enhanced by our metamaterial shell by a known factor that depends on the shell radii ratio. When the applied field is non-uniform, as for dipolar magnetic field sources, field gradient is increased as well. A proof-of-concept experimental realization confirms the theoretical predictions. The metamaterial shell is also shown to concentrate time-dependent magnetic fields upto frequencies of 100 kHz