In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

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Electronic structure calculations with dynamical mean-field theory

Simple rules for the understanding of Heusler compounds

The discovery1 of superconductivity at high pressure (albeit over a restricted range) in the ferromagnetic material UGe2 raised the possibility that bulk superconductivity might be found in other ferromagnets. The exact symmetry of the paired state and the dominant mechanism responsible for the pairing, however, remain unidentified. Meanwhile, the conjecture that superconductivity could occur more generally in ferromagnets has been fuelled by the recent observation of a low-temperature transition that suggests an onset of superconductivity in high-quality crystals of the itinerant-ferromagnet ZrZn2 (ref. 2), although the thermodynamic signature of this transition could not be detected. Here we show that the ferromagnet URhGe is superconducting at ambient pressure. In this case, we find the thermodynamic signature of the transition—its form is consistent with a superconducting pairing of a spin-triplet type, although further testing with cleaner samples is needed to confirm this. The combination of superconductivity and ferromagnetism may thus be more common and consequently more important than hitherto realized.

Coexistence of superconductivity and ferromagnetism in URhGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.

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Superconductivity on the Border of Weak Itinerant Ferromagnetism in UCoGe

Results are presented of magnetic and specific-heat studies performed on some equiatomic compounds of the series U TX with a special emphasis on non-ferromagnetic UNiAl, UCoAl, URuA1 and URuGa. The occurrence of magnetism in these compounds is attributed mainly to the 5f electrons of uranium. The systematics of the ground-state properties across the series is discussed with respect to delocalization mechanisms of the 5f electrons. The possible role of spin fluctuations in the low-temperature behaviour of UCoAl, URuAl and URuGa is indicated by magnetic measurements. The compound with the highest γ value, UNiAl (γ = 167 mJ/mol K 2 ), behaves as an itinerant antiferromagnet.

Systematics across the UTX series (T = Ru, Co, Ni; X = Al,Ga, Sn) of high-field and low-temperature properties of non-ferromagnetic compounds

We produced magnesium ferrite (MgFe 2 O 4 ) nanoparticles by hydrothermal synthesis in supercritical water. Suspensions containing varying ratios of Mg(OH) 2 and Fe(OH) 3 at room temperature were pressurized to 30 MPa, fed into a tubular reactor by high-pressure pump, and rapidly heated to reaction temperature by mixing with supercritical water. The MgFe 2 O 4 phase forms at 460 °C. The Mg/Fe molar ratio was varied from 0.5 to 1.5 with the goal of obtaining single-phase MgFe 2 O 4 . At the stoichiometric ratio for MgFe 2 O 4 , Mg/Fe = 0.5, the product contains both MgFe 2 O 4 and α-Fe 2 O 3 . At Mg/Fe = 1.0 and 1.5, the product is the desired single-phase MgFe 2 O 4 . The synthesized MgFe 2 O 4 nanoparticles, with particle size of about 20 nm, exhibit superparamagnetic behavior.

Vladimir Sechovsky

Papers

Systematics across the UTX series (T = Ru, Co, Ni; X = Al,Ga, Sn) of high-field and low-temperature properties of non-ferromagnetic compounds

Continuous synthesis of fine MgFe2O4 nanoparticles by supercritical hydrothermal reaction

Magnetic properties of the 5f itinerant electron metamagnet UCoAl under high pressure

Thermal properties of metamagnetic transition in heavy-fermion systems

Chapter 1 Magnetism of ternary intermetallic compounds of uranium