Theory explains ferromagnetic superconductor behavior

Credit: @tsarcyanide/MIPT Press Workplace

Scientists from France and Russia have actually provided a theoretical description for the behavior of a just recently found product integrating superconducting and ferromagnetic residential or commercial properties. The brand-new theoretical design likewise anticipates up until now unnoticed results in products of this kind. The research study was released in Physical Evaluation Letters.

Ferromagnetism and superconductivity are, in a manner, 2 opposed propensities that apparently cannot exist side-by-side in one crystal. Undoubtedly, a superconductor accommodates an electrical present with no resistance. When put in an electromagnetic field, such a product expels that field from its bulk in what is referred to as the Meissner result. By contrast, a ferromagnet is allured and therefore brings an electromagnetic field in its bulk. It would appear, for that reason, that a product cannot concurrently display superconductivity and ferromagnetism.

Nevertheless, europium-based substances have actually just recently become the focus of research study attention, when observations revealed they might concurrently display ferromagnetism and superconductivity. Besides its significance for essential science, the coexistence of these 2 phenomena in one product provides interesting possibilities for gadget style. It holds the pledge of superconducting spintronics, that is, gadgets dealing with details encoded by spins, without any dissipation.

A normal refrigerator magnet is an example of a ferromagnet whose so-called Curie point lies above space temperature level. Listed below that crucial temperature level, a ferromagnetic product is allured due to the parallel positioning of the intrinsic magnetic momenta, or spins, of outer-shell electrons. It might appear counterproductive, however down at the tiny scale, the nature of this spontaneous purchasing is electrical instead of magnetic: The Coulomb interaction energy of the electrons in a ferromagnet is lower for the parallel spin setup. As an outcome, each spin might be considered residing in an average, or exchange, field produced by the other spins.

Why ferromagnetism ruins superconductivity

There are 2 systems moderating the interaction of superconducting electrons and magnetic minutes. Specifically, the electro-magnetic and the exchange one.

Anticipated in 1956 by Vitaly Ginzburg, the electro-magnetic system includes evaluating Meissner currents. As mentioned above, an external electromagnetic field does not permeate into the bulk of a superconductor. To compensate the external field in the bulk, evaluating currents run along the surface area of the superconductor. The generation of such currents triggers the energy to increase. If the external field is more powerful than a particular crucial worth, the included energy due to the screening currents surpasses the energy of condensation. It ends up being more beneficial for the superconductor to shift into the regular state and enable the field into the bulk. Considering that normal magnetizations in ferromagnets are much greater than the crucial fields of superconductors, uniform ferromagnetism damages superconductivity.

The exchange system includes an interaction in between a ferromagnet’s exchange field and the electrons making it possible for superconductivity. These are in fact bound states of 2 electrons with opposite momenta and spins, called Cooper sets. The exchange field tends to line up the electron spins in parallel to each other, ruining Cooper sets and for that reason superconductivity. This is referred to as the paramagnetic result.

How ferromagnetism can exist side-by-side with superconductivity

It ends up that a product can concurrently display the ferromagnetic and superconducting residential or commercial properties, supplied that a person of the bought states is nonuniform. Undoubtedly, a nonuniform field is evaluated to a lower level. This implies that a nonuniform magnetic structure will not damage superconductivity by means of the electro-magnetic system. Taking just the exchange interaction into account, the introduction of nonuniform magnetic structure in the superconducting state was anticipated as early as 1959. The duration of this structure is far smaller sized than the particular size of a Cooper set. As an outcome, at the scale of a Cooper set, the typical exchange field reduces, and when ferromagnetism emerges, it does not destroy superconductivity. As temperature level decreases, at some time the exchange field reaches the paramagnetic limitation, and after that superconductivity is gone. Regrettably, for all formerly understood ferromagnetic superconductors, the temperature level window accommodating synchronised ferromagnetism and superconductivity was just about 0.1 kelvins.

“The early research on nonuniform magnetism in ferromagnetic superconductors only considered the electromagnetic interaction. However, it soon turned out that this was not applicable to any material known back then: The exchange interaction was always dominant. This led to a temporary suspension of the research focusing on the electromagnetic mechanism,” research study co-author Zhanna Devizorova from the MIPT Lab of Optoelectronics for 2-D Products stated.

Brand-new chances opened when europium-based ferromagnetic superconductors appeared. A phosphorus-doped substance of europium, iron, and arsenic with the formula EuFe2As2 is an example. What makes this product exceptional is that the paramagnetic result ruining superconductivity is highly reduced in it, and the electro-magnetic interaction controls. The factor for this is that ferromagnetism in P-doped EuFe2As2 is supplied by the localized electrons from the 4f shells of europium atoms, while superconductivity is moderated by iron’s 5d conduction electrons. In this substance, the europium atoms are placed in such a method that the electrons accountable for superconductivity are fairly independent from those accountable for ferromagnetism. The 2 subsystems are practically self-governing. This leads to a really weak exchange field acting upon the conduction electrons.

The paramagnetic result suppression in EuFe2As2 implies that ferromagnetism and superconductivity exist side-by-side in a relatively large range of temperature levels. It is therefore an exceptional product for speculative research study into the unique stages that emerge due to the supremacy of the electro-magnetic system and display these 2 unique buyings at the exact same time. For instance, in 2015 a group of speculative physicists from MIPT and somewhere else utilized that product to imagine the magnetic structure of such stages utilizing magnetic force microscopy.

Now, these speculative information have actually been qualitatively described by a theory advanced in the research study reported here. Its authors show how the nonuniform magnetic structure with a sinusoidal magnetization profile slowly changes into a domain-type structure as the temperature level decreases. This so-called Meissner-domain structure was experimentally observed in EuFe2As2 in between 17.8-18.25 kelvins. The duration of the structure showed considerably smaller sized than that in a routine ferromagnet. This comes from the effect of superconductivity.

More cooling activates a first-order shift into the ferromagnetic vortex state identified by existing side-by-side Abrikosov vortices and ferromagnetic domains. The group determined the specifications of this shift. In a superconductor, a vortex is an entity with an electromagnetic field at its core. It is evaluated from the outdoors by Meissner currents. The scientists revealed that the size of the domains in the vortex state is practically the like in a routine ferromagnetic product. The theory proposed in the research study likewise anticipates a brand-new result: the domain walls accommodating Abrikosov vortices perpendicular to the vortices in the domains.

“We developed a theory of nonuniform magnetic states in ferromagnetic superconductors, in which the electromagnetic interaction between superconductivity and ferromagnetism dominates,” Devizorova included. “Besides qualitatively explaining the current speculative information on such states in EuFe2As2, we forecast a brand-new result, which can now be evaluated experimentally.”

At this moment, the research study falls under the world of essential science. Nevertheless, by comprehending the interaction in between ferromagnetism and superconductivity, hybrid gadgets might be created later, which would utilize both superconducting and ferromagnetic products and come in handy for spintronics.

Superconductivity and ferromagnetism battle an even match

More details:
Zh. Devizorova et al. Theory of Magnetic Domain Stages in Ferromagnetic Superconductors, Physical Evaluation Letters (2019). DOI: 10.1103/PhysRevLett.122.117002

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Moscow Institute of Physics and Technology

Theory explains ferromagnetic superconductor behavior (2019, July 10)
obtained 11 July 2019

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