Oxide pyrochlores — A review

Within the past 20 years or so, there has occurred an explosion of interest in the magnetic behavior of pyrochlore oxides of the type $A_{2}^{3+}$$B_{2}^{4+}$O$_{7}$ where $A$ is a rare-earth ion and $B$ is usually a transition metal. Both the $A$ and $B$ sites form a network of corner-sharing tetrahedra which is the quintessential framework for a geometrically frustrated magnet. In these systems the natural tendency to form long range ordered ground states in accord with the Third Law is frustrated, resulting in some novel short range ordered alternatives such as spin glasses, spin ices and spin liquids and much new physics. This article attempts to review the myriad of properties found in pyrochlore oxides, mainly from a materials perspective, but with an appropriate theoretical context.

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Magnetic Pyrochlore Oxides

Heterometallic 3d−4f complexes are of high interest in molecular magnetism: the lanthanide ions bring large and, in most cases, anisotropic magnetic moments. The combination of 3d and 4f metal ions, which differ through their chemistries and stereochemical preferences, leads to a rich variety of heterometal complexes, ranging from discrete entities to 3-D coordination polymers. This paper reviews recent achievements in this field: (i) oligonuclear complexes for studying the nature of the 3d−4f exchange interaction; (ii) construction of single-molecule magnets; (iii) magnetic properties of 3d−4f coordination polymers.

3d-4f combined chemistry: synthetic strategies and magnetic properties.

The study of paramagnetic compounds based on 4d and 5d transition metals is an emerging research topic in the field of molecular magnetism. An essential driving force for the interest in this area is the fact that heavier metal ions introduce important attributes to the physical properties of paramagnetic compounds. Among the attractive characteristics of heavier elements vis-a-vis magnetism are the diffuse nature of their d orbitals, their strong magnetic anisotropy owing to enhanced spin–orbit coupling, and their diverse structural and redox properties. This critical review is intended to introduce readers to the topic and to report recent progress in this area. It is not fully comprehensive in scope although we strived to include all relevant topics and a large subset of references in the area. Herein we provide a survey of the history and current status of research that has been conducted on the topic of second and third row transition metal molecular magnetism. The article is organized according to the nature of the precursor building blocks with special topics being highlighted as illustrations of the special role of heavier transition metal ions in the field. This paper is addressed to readers who are interested in molecular magnetism and the application of coordination chemistry principles to materials synthesis (231 references).

Molecular magnetic materials based on 4d and 5d transition metals.

Weyl fermions have recently been observed in several time-reversal-invariant semimetals and photonics materials with broken inversion symmetry. These systems are expected to have exotic transport properties such as the chiral anomaly. However, most discovered Weyl materials possess a substantial number of Weyl nodes close to the Fermi level that give rise to complicated transport properties. Here we predict, for the first time, a new family of Weyl systems defined by broken time-reversal symmetry, namely, Co-based magnetic Heusler materials XCo_{2}Z (X=IVB or VB; Z=IVA or IIIA). To search for Weyl fermions in the centrosymmetric magnetic systems, we recall an easy and practical inversion invariant, which has been calculated to be -1, guaranteeing the existence of an odd number of pairs of Weyl fermions. These materials exhibit, when alloyed, only two Weyl nodes at the Fermi level-the minimum number possible in a condensed matter system. The Weyl nodes are protected by the rotational symmetry along the magnetic axis and separated by a large distance (of order 2π) in the Brillouin zone. The corresponding Fermi arcs have been calculated as well. This discovery provides a realistic and promising platform for manipulating and studying the magnetic Weyl physics in experiments.

Time-Reversal-Breaking Weyl Fermions in Magnetic Heusler Alloys.

A study of compounds GdT2Si2 by Mössbauer spectroscopy and by bulk magnetization measurements

The self-assembly reaction between trivalent lanthanide ions, 2,2':6',2' '-terpyridine (terpy) ligand, and octacyanotungstate(V) leads to the formation of two series of isomorphous cyano-bridged compounds: (i) one-dimensional (1-D) chains [Ln(terpy)(DMF)(4)][W(CN)(8)].6H(2)O.C(2)H(5)OH (Ln = Ce-Dy) and (ii) dinuclear molecules [Ln(terpy)(DMF)(2)(H(2)O)(2)][W(CN)(8)].3H(2)O (Ln = Ho, Er, Yb) and the ionic [Tm(III)(terpy)(DMF)(2)(H(2)O)(3)][WV(CN)(8)].4H(2)O.DMF (DMF = N,N-dimethylformamide) system. The crystal structures of 1-D chains consist of alternating {[W(CN)(8)]} and {[Ln(terpy)]} building blocks. The neighboring chains are weakly linked through the pi-pi stacking interactions of the aromatic rings, leading to two-dimensional supramolecular layers. The dinuclear species are weakly linked through the hydrogen bonds between H2O molecules and terminal cyano ligands resulting in a columnlike arrangement of dimers. Taking into account the ligand-field splitting and the exchange interaction, we have estimated the magnetic couplings between the Ln(III) and WV centers in a series of polycrystalline 1-D chains and in dimeric systems. The corresponding exchange constants have been shown to change the sign along the series of chains. The coupling is antiferromagnetic for 1 (J = -0.24 cm(-1)) and 2 (J = -0.07 cm(-1)), whereas 3 (J = +0.47 cm(-1)), 7 (J = +0.28 cm(-1)), and 8 (J = +0.23 cm(-1)) have ferromagnetic character. In the case of dimeric systems, the coupling constants seem to be independent of the lanthanide center. The splitting structures of the ground-state multiplets of the Ln(III) centers have been shown to explain the temperature dependences of the magnetic susceptibilities.

Tuning of Magnetic Properties of Polynuclear Lanthanide(III)−Octacyanotungstate(V) Systems: Determination of Ligand-Field Parameters and Exchange Interaction

Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

The reaction between Ln(NO3)3·xH2O, Cs3[WV(CN)8]·H2O and 2,2′:6′,2″-terpyridine (terpy) leads to the original isomorphous cyano-bridged [LnIII(terpy)(DMF)4][WV(CN)8]·6H2O [Ln = Gd (1), Sm (2)] 1-D chains. The crystal structures of {LnIIIWV} chains 1 and 2 consist of alternating {[W(CN)8]} and {[Ln(terpy)]} building blocks. The neighbouring 1-D chains are weakly linked through π–π stacking interactions of the aromatic rings leading to 2-D supramolecular layers. The layers are linked through hydrogen bonds between H2O molecules and terminal cyano ligands. Magnetic studies revealed a weak antiferromagnetic coupling (J = −2.3(2) K) within the {GdIIIWV} chains in 1. The positive effective coupling constant J = +2.0(5) K between the total angular momentum of the Sm(III) centre and the spin of the W(V) ion is equivalent to an antiferromagnetic character of the spin coupling between both centres in the {SmIIIWV} chains of 2. The magnetic measurements suggest that they display an isolated magnetic chain behaviour.

[Ln(terpy)]3+ (Ln = Sm, Gd) entity forms isolated magnetic chains with [W(CN)8]3−

Magnetic properties of manganese(III) tetra(ortho-fluorophenyl)porphyrin-tetracyanoethylene were studied by ac and dc measurements on polycrystalline samples. This compound consists of ferrimagnetic chains, in which magnetic moments of Mn ion $(S=2)$ in the center of a porphyrin disc and of a tetracyanoethylene radical $(s=1∕2)$ are antiferromagnetically coupled with the exchange integral of $J=\ensuremath{-}217\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At low temperatures, slow magnetic relaxations (below $13\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) and irreversible magnetic behavior (below $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) were observed, which strongly resemble behavior of a single chain magnet, reported for Ising systems. Temperature dependence of the relaxation time is well described by the Arrhenius equation with the activation energy $\ensuremath{\Delta}E=117\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These relaxations are interpreted as a thermally activated reversal of magnetization through the energy barrier $\ensuremath{\Delta}E$, associated with creation and propagation of domain walls in one-dimensional anisotropic Heisenberg systems.

K. Tomala

Papers

A study of compounds GdT2Si2 by Mössbauer spectroscopy and by bulk magnetization measurements

Tuning of Magnetic Properties of Polynuclear Lanthanide(III)−Octacyanotungstate(V) Systems: Determination of Ligand-Field Parameters and Exchange Interaction

Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

[Ln(terpy)]3+ (Ln = Sm, Gd) entity forms isolated magnetic chains with [W(CN)8]3−

Slow magnetic relaxations in the anisotropic Heisenberg chain compound Mn(III) tetra( ortho -fluorophenyl)porphyrin-tetracyanoethylene