This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since the early 1950s but only recently discovered to be superconductive at a remarkably high critical temperature Tc = 40 K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB2 will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and wires. The largest critical current densities, greater than 10 MA cm−2, and critical fields, 40 T, are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is yet to be resolved as a wide range of values have been reported, γ = 1.2–9. Also, there is no consensus on the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB2 holds the record for the highest Tc among simple binary compounds. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materialss; several compounds have since been announced to be superconductive: TaB2, BeB2.75, C–S composites, and the elemental B under pressure.

https://iopscience.iop.org/article/10.1088/0953-2048/14/11/201/pdf

Review of the superconducting properties of MgB2

This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since early 1950's, but recently discovered to be superconductive at a remarkably high critical temperature Tc=40K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, transparency of grain boundaries to current promises that MgB2 will be a good material for both large scale applications and electronic devices. During the last seven month, MgB2 has been fabricated in various forms, bulk, single crystals, thin films, tapes and wires. The largest critical current densities >10MA/cm2 and critical fields 40T are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is still to be resolved, a wide range of values being reported, between 1.2 and 9. Also there is no consensus about the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, being the tip of an iceberg who awaits to be discovered. Up to date MgB2 holds the record of the highest Tc in its class. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materials, several compounds being already announced to become superconductive: TaB2, BeB2.75, C-S composites, and the elemental B under pressure.

Review of superconducting properties of MgB2

Superconductivity and ferromagnetic ordering are two antagonistic types of ordering, and their mutual influence leads to many interesting phenomena which have been studied recently in ternary compounds. Theoretical analysis of ferromagnetic materials which are type II superconductors near the superconducting transition point T cl shows that they become type I near the magnetic transition point T M. The proposed theory constructed for the case T M « T cl predicts the formation of a transverse domain-like (DS phase) magnetic structure below T M. The electronic spectrum appears to be gapless in the DS phase of clean compounds with a re-entrant transition. The change from type II to type I behaviour as the sample is cooled to T M has been observed in ErRh4B4. Experimental data for HoMo6S8, HoMo6Se8 and ErRh4B4 give evidence for the coexistence of super-conductivity and non-uniform magnetic ordering below T M. Mutual influence of superconducting and magnetic orderings is also studied.

Coexistence of superconductivity and magnetism theoretical predictions and experimental results

Gold nanoparticles currently elicit an intense and very broad research activity because of their peculiar properties Be it in catalysis, optics, electronics, sensing or theranostics, new applications are found daily for these materials Approximately a decade ago a report was published with magnetometry data showing that gold nanoparticles, most surprisingly, could also be magnetic, with features that the usual rules of magnetism were unable to explain Many ensuing experimental papers confirmed this observation, although the reported magnetic behaviours showed a great variability, for unclear reasons In this review, most of the experimental facts pertaining to “magnetic gold” are summarized The various theories put forth for explaining this unexpected magnetism are presented and discussed We show that despite much effort, a satisfying explanation is still lacking and that the field of hypotheses should perhaps be widened

Magnetism in gold nanoparticles

Rigorous conditions for the metamagnetic transitions between the paramagnetic and ferromagnetic states and between the weak ferromagnetic and normal ferromagnetic states are derived. The expressions for the conditions and the critical field are derived in the simple model for an itinerant electron system and for a combined system of localized magnetic moments and itinerant electrons. The results are applied to TiBe2, YCo2, Pd, compounds of rare earth metals and transition metals, Co(S, Se)2, etc. It is explained that TiBe2 and perhaps YCo2 and Pd will not show the metamagnetic transition in consistent with the experimental results.

https://hal.archives-ouvertes.fr/jpa-00209372/document

Itinerant electron metamagnetism

Superconducting and magnetic properties of new ternary borides with the CeCo3B2-type structure

https://escholarship.org/content/qt6cs4v77g/qt6cs4v77g.pdf?t=oaz8cq

Enhanced paramagnetism of TiBe2 and ferromagnetic transitions in TiBe2-xCux

Superconducting Dy(IrxRh1−x)4B4 system

Magnetic susceptibility on semimagnetic semiconductors

Detailed measurements of the magnetization of HoRh4B4 and GdRh4B4 near the magnetic transition temperature are presented. In contrast to the ferromagnetic behavior of GdRh4B4, no spontaneous magnetization is found to develop below T
min HoRh4B4. This raises doubts about the nature of the magnetic order in that material as well as in DyRh4B4 and TbRh4B4. Anisotropy and time dependence of the magnetization are found in these last three compounds. The behavior of the very small remanent magnetization and of the paramagnetic susceptibility of HoRh4B4 is shown. These results point to a complex magnetic order, possibly helical or sinusoidal antiferromagnetism with a long wavelength.

F. Acker

Papers

Superconducting and magnetic properties of new ternary borides with the CeCo3B2-type structure

Enhanced paramagnetism of TiBe2 and ferromagnetic transitions in TiBe2-xCux

Superconducting Dy(IrxRh1−x)4B4 system

Magnetic susceptibility on semimagnetic semiconductors

Low-temperature magnetic behavior of HoRh4B4