Magnetochemistry: advances in theory and experimentation

Zero-field splitting in metal complexes

Constructing magnetic molecular solids by employing three-atom ligands as bridges

Framework-structured weak ferromagnets are new rising stars in molecule-based magnetic materials. The framework structures are powerful carriers for long-range ordering of spins. And weak ferromagnetism due to spin canting is an effective approach for magnets because of its frequent occurrence and desired spontaneous magnetization as long as the canting angle γ is large enough. In this critical review, we provide an overview of the various framework-structured weak ferromagnets based on different grades of ligands (from mono-atom to three-atom-like ligands). Particular emphasis is given to the relationships between structural features and the properties, rational employment of the ligands, and weak ferromagnetic strategies for molecule-based magnets with exciting properties and applications (273 references).

Framework-structured weak ferromagnets

We present the crystal structures and magnetic properties of a series of magnetic compounds, MII{N(CN)2}2, where M=Cu (1), Ni (2), Co (3) and Fe (4), and [Mn{N(CN)2}2(C2H5OH)2]Z·(CH3)2CO (5). In the isostructural compounds 1–4, the dicyanamide anion is triply coordinating through its three nitrogen atoms. It bridges the metal ions to form infinite 3D metal-organic frameworks with a rutile-type structure. The framework contains doubly bridged M(–Nâ–·C–N–Câ–·N–)2 ribbons that link approximately orthogonally through the amide nitrogen atoms. The Jahn–Teller distortion in 1 has a strong influence on the packing arrangement (M–N bond lengths: 1.98 and 2.47 A for 1 and 2.10 and 2.15 A for 3). On lowering the temperature the bond distances in 1 remain unchanged except for a decrease of the M–Namide length to 2.45 A. Magnetic data for 1 obey the Curie–Weiss law (Θ=-2.1 K). 2 and 3 are ferromagnets with Curie temperatures (TC) of 9 and 21 K and are characterized by hysteresis loops of 710 and 7975 Oe at 2 K, remnant magnetization, magnetization approaching the expected saturation (gS) of 2 and 3 µB in high field, absorptive component (χ″) in the AC magnetization and λ peak in the heat capacity data. 4 is similarly characterized and shows behaviour that is characteristic of a canted antiferromagnet: the Weiss constant is temperature dependent (+3 K in the range 200–300 K), there is a sharper peak than for 1 or 2 in the AC magnetization and the isothermal magnetization at 3 K increases monotonically to ≈1.3 µB (expected to be 4 µB for ferromagnetic alignment of the spins) in a field of 8 T. Its coercive field (17800 Oe) is the largest observed for any metal-organic compound and exceeds those of alloys of SmCo5 and Nd2Fe14B. The maximum energy product (B · H) is the highest for 3 and is comparable to alloys of Sm–Co. We attribute the large coercive field to a combination of single ion and particle shape anisotropies. 5 is paramagnetic at high temperature with Θ=-3 K. Below 16 K it behaves as a canted antiferromagnet with a very weak resultant spontaneous magnetization.

Hard magnets based on transition metal complexes with the dicyanamide anion, {N(CN)2}-,

The crystal structure at room temperature and the heat-capacity and magnetic susceptibilities at low temperatures of single crystals of [${(\mathrm{C}{\mathrm{H}}_{3})}_{3}$NH]Co${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O are reported. The orthorhombic crystals belong to the space group $\mathrm{Pnma}$ with $a=16.671(3)$ \AA{}, $b=7.273(1)$ \AA{} $c=8.113(2)$ \AA{}, and $Z=4$. The structure consists of chains of edge-sharing trans- [Co${\mathrm{Cl}}_{4}$${(\mathrm{O}{\mathrm{H}}_{2})}_{2}$] octahedra running parallel to the $b$ axis. At 4.135 \ifmmode^\circ\else\textdegree\fi{}K, [${(\mathrm{C}{\mathrm{H}}_{3})}_{3}$NH]Co${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O undergoes a second-order phase transition. The heat-capacity data have been analyzed using Onsager's solution for the two-dimensional Ising model. The lattice contribution was taken into account by using the lattice of the isostructural [${(\mathrm{C}{\mathrm{H}}_{3})}_{3}$NH]Cu${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O and a corresponding states procedure. The three principal-axis single-crystal susceptibilities display an unusual amount of anisotropy. Two of the data sets (one parallel and one perpendicular to the chemical chain) have been fit to an Ising linear-chain model. A small molecular-field correction significantly improved one of those fits. The $a$-axis susceptibility displays an unusually sharp rise, with decreasing temperature, very close to ${T}_{N}$ along with the suggestion of a net moment persisting below the transition. Thus, a Dzyaloshinsky-Moriya antisymmetric exchange interaction was assumed to be operative, since it is symmetry alowed in this space group, and Moriya's molecular-field-susceptibility calculation was used to fit this data set. All four of the data sets are quite reasonably described, the specific heat and one of the susceptibility sets within experimental error, by the following set of parameters: rectangular lattice exchange parameters of $\frac{J}{k}=(7.7\ifmmode\pm\else\textpm\fi{}0.2)$ \ifmmode^\circ\else\textdegree\fi{}K and $\frac{{J}^{\ensuremath{'}}}{k}=(0.09\ifmmode\pm\else\textpm\fi{}0.01)$ \ifmmode^\circ\else\textdegree\fi{}K; spectroscopic splitting parameters of ${g}_{a}=2.95\ifmmode\pm\else\textpm\fi{}0.05$, ${g}_{b}=3.90\ifmmode\pm\else\textpm\fi{}0.01$, and ${g}_{c}=6.54\ifmmode\pm\else\textpm\fi{}0.01$; and a Dzyaloshinsky-Moriya antisymmetric exchange parameter of $|\frac{D}{k}|=(4.9\ifmmode\pm\else\textpm\fi{}0.2)$ \ifmmode^\circ\else\textdegree\fi{}K. Finally, a consistent spin structure is proposed in which the intrachain interaction is ferromagnetic with the compound ordering antiferromagnetically below 4.135\ifmmode^\circ\else\textdegree\fi{}K such that there is a small net moment in the $a$ direction.

An Anisotropic Low-Dimensional Ising System, [ ( C H 3 ) 3 NH]Co Cl 3 · 2 H 2 O: Its Structure and Canted Antiferromagnetic Behavior

Magnetic-susceptibility and heat-capacity measurements on single crystals of Ni${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O, which is isomorphic to Mn${\mathrm{Cl}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O and Mn${\mathrm{Br}}_{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} 4${\mathrm{H}}_{2}$O, indicate an antiferromagnetic-paramagnetic transition at (2.99\ifmmode\pm\else\textpm\fi{}0.01) \ifmmode^\circ\else\textdegree\fi{}K. The large zero-field splitting of the ${\mathrm{Ni}}^{2+}$ ion in this material results in an 11 \ifmmode^\circ\else\textdegree\fi{}K doublet-singlet splitting of its ground state with the doublet lying lower. A molecular-field model has been used to obtain an exchange-parameter value of $\frac{\mathrm{zJ}}{k}=\ensuremath{-}5.25$ \ifmmode^\circ\else\textdegree\fi{}K. The effect of the crystal field anisotropy on the transition temperature has been interpreted and the results seem to indicate that the molecular-field theory gives a better approximation than do the Green's-function techniques.

Antiferromagnetic Ordering and Crystal Field Behavior of Ni Cl 2 4 H 2 O

Heat-capacity and magnetic-susceptibility measurements at low temperatures on single crystals of RbCo${\mathrm{Cl}}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O are described here which indicate that this material is the first reported example of a magnet which can be formally described as having a primarily one-dimensional Dzyaloshinsky-Moriya-like character. Although the compound is not isomorphic to its Cs analog, there are some similarities in their magnetic behavior. Specifically, RbCo${\mathrm{Cl}}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O magnetically orders at ${T}_{N}=2.94\ifmmode\pm\else\textpm\fi{}0.01$ K and displays spin canting with an unusual amount of anisotropy observed in the single-crystal paramagnetic susceptibilities. The data have been fit using a linear-chain model in which the symmetric part of the intrachain spin-spin interaction is relatively small compared to the Dzyaloshinsky-Moriya antisymmetric contribution. Although the model does not result in unique predictions for the exchange parameters, the present data can be described with symmetric and antisymmetric exchange parameters of $\frac{J}{k}=0$ and $|\frac{D}{k}|=59$ K, respectively. The spectroscopic splitting parameters of ${g}_{a}=4.12$ and ${g}_{{c}^{\ensuremath{'}}}=5.2$ have also been determined from the fits. The results are consistent with the presence of only two sublattices which are severely canted with respect to each other. The spins apparently lie in the $b{c}^{\ensuremath{'}}$ plane and the angle between the sublattices approaches 90\ifmmode^\circ\else\textdegree\fi{}.

Spin canting in RbCo Cl 3 ·2 H 2 O, a Dzyaloshinsky-Moriya linear chain magnet

The room-temperature crystal structure as well as low-temperature magnetic studies which include heat-capacity, proton-resonance, and magnetic-susceptibility experiments are reported for Mn${(\mathrm{SCN})}_{2}$${({\mathrm{C}}_{2}{\mathrm{H}}_{5}\mathrm{O}\mathrm{H})}_{2}$. The compound belongs to the space group $\frac{P{2}_{1}}{c}$ with $a=8.535(3)$ \AA{}, $b=6.339(3)$ \AA{}, $c=10.919(4)$ \AA{}, $\ensuremath{\beta}=94.68\ifmmode^\circ\else\textdegree\fi{}$, and $z=2$. The structure consists of polymeric sheets of thiocynate-bridged manganese ions with the sheets being separated by the ethanol groups, which are coordinated to the manganese ions. The compound magnetically orders at 10.30\ifmmode\pm\else\textpm\fi{}0.01 K with the magnetic space group being $\frac{P{2}_{1}}{c}$. In the ordered state the Mn spins lie within the polymeric sheets and are canted such as to give a net magnetization along the $b$ axis. In zero-field domain effects result in essentially equal volumes of crystal with net magnetizations along the positive and negative $b$ axis. In the paramagnetic region the susceptibility and heat-capacity results are reasonably well described by the two-dimensional square-planar Heisenberg spin-$\frac{5}{2}$ model with $\frac{J}{k}=\ensuremath{-}0.88$ K.

J. N. McElearney

Papers

An Anisotropic Low-Dimensional Ising System, [ ( C H 3 ) 3 NH]Co Cl 3 · 2 H 2 O: Its Structure and Canted Antiferromagnetic Behavior

Antiferromagnetic Ordering and Crystal Field Behavior of Ni Cl 2 4 H 2 O

Spin canting in RbCo Cl 3 ·2 H 2 O, a Dzyaloshinsky-Moriya linear chain magnet

Structural and magnetic properties of Mn ( SCN ) 2 ( C 2 H 5 O H ) 2 , a new two-dimensional Heisenberg magnet

Two anisotropic Heisenberg linear chain manganese compounds