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

A review is presented of the current research and development of shape-memory materials, including shape-memory alloys, shape-memory ceramics and shape-memory polymers. The shape-memory materials exhibit some novel performances, such as sensoring (thermal, stress or field), large-stroke actuation, high damping, adaptive responses, shape memory and superelasticity capability, which can be utilized in various engineering approaches to smart systems. Based on an extensive literature survey, the various shape-memory materials are outlined, with special attention to the recently developed or emerged materials. The basic phenomena in the materials, that is, the stimulus-induced phase transformations which result in the unique performance and govern the remarkable changes in properties of the materials, are systematically lineated. The remaining technical barriers, and the challenges to improve the present materials system and develop a new shape memory materials are discussed.

Shape-memory materials and hybrid composites for smart systems: Part I Shape-memory materials

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

Spin exchange interactions and magnetic structures of extended magnetic solids with localized spins: theoretical descriptions on formal, quantitative and qualitative levels

A review is given on the basic properties of 2:14:1 hard magnetic phases as well as technological procedures used for manufacturing R-Fe-B and R-Fe-C permanent magnets. First, we analyse the phase diagrams, crystal structure of hard magnetic phases and composition ranges in which these phases form solid solutions, as a result of various types of substitution. Then, the magnetic properties of and -based compounds are described. The routes used for manufacturing R-Fe-B and R-Fe-C permanent magnets are detailed. The magnets' microstructure as well as their coercivities are described in correlation with composition and manufacturing routes. The properties of nanostructure magnets are then presented. Finally, the stability of Nd-Fe-B and Nd-Fe-C magnets in various working conditions is analysed.

Permanent magnets based on R-Fe-B and R-Fe-C alloys

High field magnetization of CuO powder is measured in pulsed magnetic fields up to 360 kOe. The spin-flop field H c and the perpendicular magnetic susceptibility χ ⊥ are found to be 104 kOe and 2.7×10 -6 emu/g at 4.2 K, respectively. The exchange field H E of 6000±600 kOe and the anisotropy field H A of 0.9±0.1 kOe are estimated. CuO can substantially be regarded as a one-dimensional magnet with an intrachain exchange interaction J 0 / k of 400K and frustration effects are expected between the chains.

High Field Magnetism of CuO

The temperature dependence of the magnetic susceptibility, electrical resistivity, and specific heat, as well as the magnetization in fields up to 35 T, of polycrystalline samples of UTGe (T=transition metal) compounds has been studied. The development from nonmagnetic UCoGe and URuGe through the weak ferromagnet URhGe, which displays the highest γ value, to complicated types of the local 5f‐moment ordering for T=Pd, Ni, and Pt is in agreement with the expected trends of gradually reduced 5f‐d hybridization.

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Specific heat and magnetic behavior of UTGe compounds

Magnetization measurements have been performed on the single crystal samples of Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ), NENP, and Ni(C 3 H 10 N 2 ) 2 NO 2 (ClO 4 ), NINO in pulsed high magnetic fields. No magnetization appears up to about 10 T reflecting the presence of the Haldane gap and the linear magnetization appears in both compounds above this field. The observed anisotropy of the transition field is explained by the Haldane gap energy E g and the crystalline field constants D and E in the lowest excited triplet. The parameters are estimated as E g =16.8 K, D =-16.1 K, and E =-1.3 K and E g =14.2 K, D =-11.5 K and E =2.1 K for NENP and NINO, respectively.

Magnetization Measurement of NENP and NINO in High Magnetic Field

Results of magnetic studies of pseudoternary U(Ru, Rh)Al and U(Ru, Rh)Ga systems are presented. Reduction of the 5f-4d hybridization with increasing Rh content is reflected in a gradual transition from paramagnetic (spin fluctuation) behaviour of URuX to ferromagnetism in URhX. The huge uniaxial magnetic anisotropy can be attributed to the anisotropic 5f-ligand hybridization.

Hybridization and magnetism in U(Ru, Rh)X, X=Al, Ga

Magnetic, specific heat and resistivity studies of UNiAl single-crystals are reported. The results suggest the antiferromagnetic ordering below TN = 19 K. The high value of γ = 164 mJ⧸mol K2 and the low magnetic moment ≌0.6μB indicate the itinerant nature of 5f magnetism. Almost all the magnetic response is concentrated along the c-axis of the hexagonal structure of UNiAl. The strong uniaxial anisotropy is reflected in the specific heat behaviour in magnetic fields applied in different crystallographic directions. An anisotropic resistivity has also been observed.

M. Ono

Papers

High Field Magnetism of CuO

Specific heat and magnetic behavior of UTGe compounds

Magnetization Measurement of NENP and NINO in High Magnetic Field

Hybridization and magnetism in U(Ru, Rh)X, X=Al, Ga

Antiferromagnetic Correlations in UNiAl