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Topic Name: Researchers Successfully Unveiled Three-Dimensional Visualisation of Magnetic Fields Inside Solid, Non-Transparent Materials

Category: Mechanical

Research persons: Nikolay Kardjilov

Location: Hahn-Meitner-Institute (HMI) in Berlin, Germany

Details

3-D images are not only useful in medicine; the observation of internal structures is also invaluable in many other fields of scientific investigation. Recently, researchers from the Hahn-Meitner-Institute (HMI) in Berlin in cooperation with University of Applied Sciences in Berlin have succeeded, for the first time, in a direct, three-dimensional visualisation of magnetic fields inside solid, non-transparent materials. This is announced by Nikolay Kardjilov and colleagues in the current issue of the journal Nature Physics, the on-line contributions to which will be released in advance on the 30th of March at 13.00 US Eastern Time.

The researchers in the imaging group used neutrons, subatomic particles that have zero net charge, but do have a magnetic moment, making them ideal for investigating magnetic phenomena in magnetic materials. When in an external magnetic field, the neutrons behave like compass needles, all aligning to point on the direction of the field. Neutrons also have an internal angular momentum, often referred to by physicists as spin, a property that causes the needle to rotate around the magnetic field, similar to the way in which the Earth rotates on its axis. When all of the magnetic moments point in the same direction then the neutrons are said to be spin-polarised. If a magnetic sample is irradiated with such neutrons, the magnetic moments of the neutrons will begin to rotate around the magnetic fields they encounter in the sample and the direction of their spin changes.

Kardjilov's group used this phenomenon as a measurement parameter for tomography experiments using two spin polarisers (which only allow the passage of neutrons whose spin points in a specific direction) to polarise and then analyse the neutrons. By detecting changes in the spins, it is possible to “see” the magnetic fields within the sample.

Kardjilov explains this by comparison with a medical CT scan; when a specimen is irradiated with x rays the density of the materials present alters the intensity of the light. "It's the same with our magnetic specimen, which changes the spin rotation of the neutrons", says Nikolay Kardjilov. "The equipment only allows passage of neutrons with a specific spin rotation, and this generates the contrast according to how the magnetic properties are distributed within the specimen. By rotating the specimen we can reconstruct a three-dimensional image."

Since 2005, Nikolay Kardjilov has built up the neutron tomography section at HMI and now his group is the first to use spin rotation as a measurement signal for three-dimensional imaging. Normally, neutron imaging relies on the different levels of absorption of radiation by different materials to produce contrast. The measurement of magnetic signals is a novel concept and its success lies partly in the polarisers and analysers, and the detector system, which have been developed and built by the HMI researchers.

Magnetism is one of the central research fields at HMI. To understand high temperature superconductivity, for example, it is vital to understand how magnetic flux lines are distributed and how these flux lines can be established in the material. With Kardjilov's experimental setup, it is now possible, among other things, to visualise magnetic domains in magnetic crystals three-dimensionally.

Note for Magnetic Field
In physics, a magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. Magnetic fields surround electric currents, magnetic dipoles, and changing electric fields.

When placed in a magnetic field, magnetic dipoles tend to align their axes to be parallel with the magnetic field, as can be seen when iron filings are in the presence of a magnet. Magnetic fields also have their own energy, with an energy density proportional to the square of the field intensity. The magnetic field is typically measured in either teslas (SI units) or gauss (cgs units).

There are some notable specific incarnations of the magnetic field. For the physics of magnetic materials, see magnetism and magnet, and more specifically ferromagnetism, paramagnetism, and diamagnetism. For constant magnetic fields, such as are generated by stationary dipoles and steady currents, see magnetostatics. For magnetic fields created by changing electric fields, see electromagnetism.

The electric field and the magnetic field are tightly interlinked, in two senses. First, changes in either of these fields can cause ("induce") changes in the other, according to Maxwell's equations. Second according to Einstein's theory of special relativity, a magnetic force in one inertial frame of reference may be an electric force in another, or vice-versa. Together, these two fields make up the electromagnetic field, which is best known for underlying light and other electromagnetic waves.

The rotating magnetic field is a key principle in the operation of alternating-current motors. A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was conceptualized by Nikola Tesla, and later utilised in his, and others, early AC (alternating-current) electric motors. A rotating magnetic field can be constructed using two orthogonal coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and so, in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case. The ability of the three-phase system to create a rotating field, utilized in electric motors, is one of the main reasons why three-phase systems dominate the world's electrical power supply systems.

Because magnets degrade with time, synchronous motors and induction motors use short-circuited rotors (instead of a magnet) following the rotating magnetic field of a multicoiled stator. The short-circuited turns of the rotor develop eddy currents in the rotating field of the stator, and these currents in turn move the rotor by the Lorentz force.

Note for Subatomic Particle
A subatomic particle is an elementary or composite particle smaller than an atom. Particle physics and nuclear physics are concerned with the study of these particles, their interactions, and non-atomic matter.

Subatomic particles include the atomic constituents electrons, protons, and neutrons. Protons and neutrons are composite particles, consisting of quarks. A proton contains two up quarks and one down quark, while a neutron consists of one up quark and two down quarks; the quarks are held together in the nucleus by gluons. There are six different types of quark in all ('up', 'down', 'bottom', 'top', 'strange', and 'charm'), as well as other particles including photons and neutrinos which are produced copiously in the sun. Most of the particles that have been discovered are encountered in cosmic rays interacting with matter and are produced by scattering processes in particle accelerators. There are dozens of subatomic particles.

In particle physics, the conceptual idea of a particle is one of several concepts inherited from classical physics, the world we experience, that are used to describe how matter and energy behave at the molecular scales of quantum mechanics. As physicists use the term, the meaning of the word "particle" is one which understands how particles are radically different at the quantum-level, and rather different from the common understanding of the term.

The idea of a particle is one which had to undergo serious rethinking in light of experiments which showed that the smallest particles (of light) could behave just like waves. The difference is indeed vast, and required the new concept of wave-particle duality to state that quantum-scale "particles" are understood to behave in a way which resembles both particles and waves. Another new concept, the uncertainty principle, meant that analyzing particles at these scales required a statistical approach. All of these factors combined such that the very notion of a discrete "particle" has been ultimately replaced by the concept of something like wave-packet of an uncertain boundary, whose properties are only known as probabilities, and whose interactions with other "particles" remain largely a mystery, even 80 years after quantum mechanics was established.

Energy and matter we have studied from Einstein's hypotheses are analogous: matter can be austerely denoted in terms of energy. Thus, we have only discovered two mechanisms in which energy can be transferred. These are particles and waves. For example, light can be expressed as both particles and waves. This paradox is known as the Duality Paradox.

Particles are discrete, their energy is centralized into what appears to be a finite space, which possesses absolute boundaries and its contents we contemplate to be homogenous i.e. the same at any point within the particle. Particles subsist at a particular location. If they are demonstrated on a 3D graph, they have x, y, and z coordinates. They can never exist in more than one location at once, and to travel to a different place in space, a particle must move to it under the laws of kinematics, acceleration, velocity and so forth.

Note for Magnetic Moment
In physics, astronomy, chemistry, and electrical engineering, the term magnetic moment of a system (such as a loop of electric current, a bar magnet, an electron, a molecule, or a planet) usually refers to its magnetic dipole moment, and is a measure of the strength of the system's net magnetic source. Specifically, magnetic dipole moment quantifies the contribution of the system's internal magnetism to the external dipolar magnetic field produced by the system (i.e. the component of the external magnetic field that drops off with distance as the inverse cube). Any dipolar magnetic field pattern is symmetric with respect to rotations around a particular axis, therefore it is customary to describe the magnetic dipole moment that creates such a field as a vector with a direction along that axis. For quadrupolar, octupolar, and higher-order multipole magnetic moments, see Multipole expansion.

Fundamentally, contributions to any system's magnetic moment may come from sources of two kinds: (1) motion of electric charges, such as electric currents and (2) the intrinsic magnetism of elementary particles, such as the electron.

Contributions due to the sources of the first kind can be calculated from knowing the distribution of all the electric currents (or, alternatively, of all the electric charges and their velocities) inside the system, by using the formulas below. On the other hand, the magnitude of each elementary particle's intrinsic magnetic moment is a fixed number, often measured experimentally to a great precision. For example, any electron's magnetic moment is measured to be −9.284764×10-24 J/T. The direction of the magnetic moment of any elementary particle is entirely determined by the direction of its spin (the minus in front of the value above indicates that any electron's magnetic moment is antiparallel to its spin).

Finally, the net magnetic moment of any system is a vector sum of contributions from one or both types of sources. For example, a hydrogen atom's magnetic moment is a vector sum of the following contributions: the intrinsic moment of the electron, the orbital motion of the electron around the proton, and the intrinsic moment of the proton.

Any molecule has a well-defined magnitude of magnetic moment, which may depend on the molecule's energy state. Typically, the overall magnetic moment of a molecule is a combination of the following contributions, in the order of their typical strength:
magnetic moments due to its unpaired electron spins (paramagnetic contribution), if any
orbital motion of its electrons, which in the ground state is often proportional to the external magnetic field (diamagnetic contribution)
the combined magnetic moment of its nuclear spins, which depends on the nuclear spin configuration.

At Hahn-Meitner-Institut Berlin work about 800 employees, among them about 300 scientists. They research properties of materials, develop solar cells of new generation and practice a user facility at research reactor BER II open to the national and international community.

In figure, The magnetic field of a dipol magnet visualized by spinpolarized neutrons