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

Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-sigma-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.

New Concepts in Tetrathiafulvalene Chemistry.

Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of eight particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by using strong transverse magnetic fields to tune the quasi-one-dimensional Ising ferromagnet CoNb2O6 (cobalt niobate) through its critical point. Spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviors.

http://science.sciencemag.org/content/sci/327/5962/177.full.pdf

Quantum Criticality in an Ising Chain: Experimental Evidence for Emergent E8 Symmetry

Bulk ferromagnetism in the β-phase crystal of the p-nitrophenyl nitronyl nitroxide radical

The reaction between the [Mn(BS)(H2O)]+ monomeric and [Mn2(μ-BS)2(H2O)2]2+ dimeric cations and [Fe(CN)6]3- gave rise to cation−anion interaction via the formation of [FeC⋮NMn] bridges. Depending on the nature of the Schiff base and regardless of the stoichiometry used, either the trimeric anion [{Mn(BS)}2{Fe(CN)6}]- (BS = 3-MeOsalen, 6; 5-Clsalen, 7; 5-Brsalen, 8; salcy, 10) or the pentameric cation [{Mn(BS)}4{Fe(CN)6}]+ (BS = saltmen, 9) is formed, which has been assembled by the K+ cation or the ClO4- anion, respectively. The X-ray analysis of 6 revealed a two-dimensional network layer structure. The magnetic measurements showed its metamagnetic behavior, where the ferromagnetic interaction operates within each layer and the antiferromagnetic interaction operates between the layers. The Neel temperature, TN, is 9.2 K, and the critical field at 2 K is 300 Oe. The temperature dependent magnetic susceptibilities of 7 and 8 are in agreement with a discrete, symmetrical, trinuclear structure Mn(III)Fe(III)Mn...

Complexes derived from the reaction of manganese(III) Schiff base complexes and hexacyanoferrate(III): Syntheses, multidimensional network structures, and magnetic properties

Single crystals of the perovskite-type ${3d}^{1}$ metallic alloy system ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{VO}}_{3}$ were synthesized in order to investigate metallic properties near the Mott transition. The substitution of a ${\mathrm{Ca}}^{2+}$ ion for a ${\mathrm{Sr}}^{2+}$ ion reduces the bandwidth $W$ due to a buckling of the V-O-V bond angle from $\ensuremath{\sim}180\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{SrVO}}_{3}$ to $\ensuremath{\sim}160\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{CaVO}}_{3}.$ Thus, the value of $W$ can be systematically controlled without changing the number of electrons making ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{VO}}_{3}:$ one of the most ideal systems for studying bandwidth effects. The Sommerfeld-Wilson ratio $(\ensuremath{\simeq}2),$ the Kadowaki-Woods ratio (in the same region as heavy fermion systems), and a large ${T}^{2}$ term in the electric resistivity, even at 300 K, substantiate a large electron correlation in this system, though the effective mass, obtained by thermodynamic and magnetic measurements, shows only a systematic but moderate increase in going from ${\mathrm{SrVO}}_{3}$ to ${\mathrm{CaVO}}_{3},$ in contrast to the critical enhancement expected from the Brinkmann-Rice picture. It is proposed that the metallic properties observed in this system near the Mott transition can be explained by considering the effect of a nonlocal electron correlation.

Bandwidth control in a perovskite-type 3 d 1 -correlated metal Ca 1 − x Sr x VO 3 . I. Evolution of the electronic properties and effective mass

A possible mechanism of ferromagnetic coupling of a series of organic radical ferromagnets is presented, based on their crystal structures and molecular-orbital calculations. The investigated radicals are 4-(arylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyls (abbreviated as Ar–CH=N–TEMPO, where Ar = phenyl, 4-methylthiophenyl, 4-chlorophenyl, 4-biphenylyl, and 4-phenoxyphenyl), and 4-hydroxyimino-TEMPO. All of these radical crystals have characteristic features that oxygen atoms of N–O radical sites of TEMPO moieties always locate near methyl- and/or methylene-hydrogens at β-positions of N–O in the TEMPO moieties of adjacent molecules. A positive spin on the N–O radical induces negative spins on the β-hydrogen atoms due to an intramolecular spin-polarization, ON(↑)–C(↓)–C(↑)–H(↓), which in turn induces a positive spin on the N–O sites of the adjacent molecules. This spin-alternation mechanism of ferromagnetic coupling among N–O sites via hydrogen bridges is also applicable to the case of an organic ...

Proposed Mechanism of Ferromagnetic Interaction of Organic Ferromagnets : 4-(Arylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyls and Related Compounds

A series of 4-arylmethyleneamino-2,2,6,6-tetramethylpiperidin-1-yloxyls (4-arylmethyleneamino-TEMPO) and related compounds were synthesized, and their magnetic susceptibilities were measured by a SQUID magnetometer in the temperature range of 1.8—100 K. Of 165 radicals investigated, 52 kinds of radicals exhibited intermolecular ferromagnetic interactions. These were confirmed by the increase of effective magnetic moments in low-temperature regions. Positive Weiss temperatures (θ), ranging from +0.03 to +0.75 K, were found for these materials. Over 100 kinds of radicals exhibited antiferromagnetic interactions with θ ranging from −0.01 to −24 K. The surprisingly high probability of finding organic radicals with intermolecular ferromagnetic interaction may be understood by the characteristic molecular arrangements in the crystals. An oxygen atom of an NO radical site of a piperidin-1-yloxyl moiety is apt to locate near methyl- and/or methylene-hydrogens of β-positions of the adjacent molecules, and the resu...

Organic Radicals Exhibiting Intermolecular Ferromagnetic Interactions with High Probability: 4-Arylmethyleneamino-2,2,6,6-tetramethylpiperidin-1-yloxyls and Related Compounds

We present the temperature dependence of the magnetic susceptibility χ(T), electrical resistivity ρ(T) and low-temperature specific heat C(T) of CeRu4Sb12. χ(T) shows a broad peak around 100 K characteristic to intermediate-valence compounds but the small Weiss temperature of −26 K and ρ(T) suggest that the compound is classified as a Kondo compound. The ground state is very anomalous and non-Fermi liquid behaviors were observed in C/T and ρ(T) at low temperatures.

Masayasu Ishikawa

Papers

Bandwidth control in a perovskite-type 3 d 1 -correlated metal Ca 1 − x Sr x VO 3 . I. Evolution of the electronic properties and effective mass

Proposed Mechanism of Ferromagnetic Interaction of Organic Ferromagnets : 4-(Arylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyls and Related Compounds

Organic Radicals Exhibiting Intermolecular Ferromagnetic Interactions with High Probability: 4-Arylmethyleneamino-2,2,6,6-tetramethylpiperidin-1-yloxyls and Related Compounds

Anomalous magnetic properties of CeRu4Sb12

Searching for spontaneous magnetic order in an organic ferromagnet. μSR studies of β-phase p-NPNN