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

The one-dimensional chain complex [Cu2(μ2-1,1-N3)2(μ2-1,3-NO3)2(μ2-1,3-Me3NCH2CO2)2]n (1) contains three different bridge groups, μ2-1,1-azide (end-on), μ2-nitrate, and μ2-syn,syn-carboxylate, arra...

An Azide-Bridged Copper(II) Ferromagnetic Chain Compound Exhibiting Metamagnetic Behavior.

The paper presents the review of AC susceptibility measurements in study of magnetic phase transitions and spin dynamics. Examples of several types of phase transitions in conventional and molecular magnets are shown. The phenomenon of magnetic relaxation is brie y ascribed and illustrated with results on relaxation in low-dimensional molecular magnets, such as single molecule magnets, single chain magnets and quasi 1-dimensional systems showing magnetic phase transition to the long-range ordered phase. The following experimental possibilities of the AC technique, which allow to get insight into properties of novel magnetic materials are mentioned: (i) change of the AC eld amplitude (in order to distinguish ferro-(ferri) and antiferromagnets); (ii) use of a static eld HDC (to conclude on the type of magnetic order and to study the phase diagram); (iii) measurement of nonlinear susceptibilities (to conclude on the type of phase transition); (iv) probing the dependence of both components of susceptibility on frequency of the AC eld (to study magnetic relaxation); (v) tests of the DC eld e ect on the frequency dependence of χAC (to analyze the possibility of blocking or reentrant spin-glass transition).

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AC Susceptibility Studies of Phase Transitions and Magnetic Relaxation: Conventional, Molecular and Low-Dimensional Magnets

This review critically analyzes current practice in the design, calibration, sensitivity determination, and operation of alternating-field susceptometers, and examines applications in magnetic susceptibility measurements of superconductors. Critical parameters of the intrinsic and coupling components of granular superconductors may be deduced from magnetic susceptibility measurements. The onset of intrinsic diamagnetism corresponds to the initial decrease in electrical resistivity upon cooling, but the onset of intergranular coupling coincides with the temperature for zero resistivity. The lower critical field may be determined by the field at which the imaginary part of susceptibility increases from zero. Unusual features in the susceptibility of superconductor films, such as a magnetic moment that is independent of film thickness and the variation of susceptibility with angle, are related to demagnetization. Demagnetizing factors of superconductor cylinders are significantly different from those commonly tabulated for materials with small susceptibilities. Rules for the susceptibility of mixtures with specific demagnetizing factors are used to estimate the volume fraction of superconducting grains in sintered materials. Common misunderstandings of the Meissner effect, magnetic units, and formula conversions are discussed. There is a comprehensive summary of critical-state formulas for slabs and cylinders, including new equations for complex susceptibility in large alternating fields. Limitations on the use of the critical-state model for deducing critical current density are listed and the meaning of the imaginary part of susceptibility is considered.

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Alternating-Field Susceptometry and Magnetic Susceptibility of Superconductors

Low-dimensional magnetic compounds, especially zero-dimensional magnetic clusters and 1D chain compounds that show single-molecule and single-chain magnetic properties, are currently of ongoing interest for the design and synthesis of new molecule-based magnets. Well-documented examples include Mn12 [1] and Fe8 [2] for single-molecule magnets (SMMs), and [Co(hfacac)2(NITPhOMe)] (hfacac = hexafluoroacetylacetonate, NITPhOMe = 4′-methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) and [Mn2(saltmen)2Ni(pao)2L2](A)2 (saltmen = N,N′-1,1,2,2-tetramethylethylenebis(salicylideneiminate), pao = pyridine-2-aldoximate, and A= anion) for single-chain magnets (SCMs). Although they are not associated with real 3D magnetic order, the slow magnetic relaxation and a hysteresis loop below the blocking temperature make them behave like classical magnets, and good candidates for potential high-density information storage. The slow relaxation phenomenon, which is one of the main characters for both SMMs and SCMs, is quite common for many systems, including magnetic systems, liquid crystals, polymers, high-critical-temperature (Tc) superconductors, and metallic glasses. Typical examples of magnetic systems, besides SMMs and SCMs, are spin-glass magnets, cluster spin-glass magnets, and classical superparamagnets. The origin of the relaxation is quite complex and varies in different systems. Randomness (defects, disorder, etc.) and frustration (geometrical or competitive) are thought to be responsible for the spin-glass system. For SMMs and SCMs, the strong uniaxial magnetic anisotropy (where the zero-field splitting parameter D < 0) and the large spin values of the ground states (for SCMs, one can say that the total spin number ST =∞) are two main reasons for the very slow relaxation in these systems. In long-range ordered magnets, such as ferromagnets or ferrimagnets, the relaxation can also be induced by the dynamics of domain walls, such as domain-wall displacement and the domain magnetization rotation and so on. In order to obtain a magnetic ground-state for a chain, individual magnetic moments cannot cancel out. This condition can be satisfied if the chain is ferromagnetic (e.g., [[FeL(CN)4]4Co(H2O)x] (L= 2,2′-bipyridine or 1,10-phenanthroline, x = 1 or 2), and [Co(bt)(N3)2] (bt = 2,2′-bithiazoline)) or ferrimagnetic, such as [Co(hfacac)2(NITPhOMe)]. [3a] There is another way to achieve this goal in an antiferromagnetic chain: the spin-canting that arises from the non-colinear spin arrangements because of the existence of an antisymmetrical component of the superexchange interaction (Dzyaloshinsky– Moriya interaction, DMI) or the appropriate anisotropy. Such is the case in the reported SCM of Co(H2L)(H2O) (H4L= 4-Me-C6H4-CH2N(CH2PO3H2)2). [3g]

Weak Ferromagnetism and Dynamic Magnetic Behavior in a Single End‐to‐End Azide‐Bridged Nickel(II) Chain

The crystal structure of [C6H11NH3] CuCl3, cyclohexylammonium trichlorocuprate(II) (CHAC), is orthorhombic, space group P212121 with a = 19.441(5), b = 8.549(2) and c = 6.190(1) A. The salt contains chains of CuCl−3 ions along the c axis. From magnetization and susceptibility measurements it is found that the compound behaves as a one-dimensional S = 1 2 Heisenberg ferromagnet with J1/k = 70(2) K. Antiferromagnetic ordering with a weak ferromagnetic moment along the a axis occurs below Tc = 2.18(2) K. From the metamagnetic phase diagram the interchain interactions are derived using mean field theory: z2J2/z1J1 = 1.1 × 10−3 and z3J3/z1J1 = −1.0 × 10−4. Also a small anisotropy (J|/J⊥ ≈ 0.01) is found in the intrachain interaction. The measurements indicate that CHAC is one of the best approximations to the 1d Heisenberg ferromagnet known to date.

Crystal structure and magnetic properties of cyclohexylammonium trichlorocuprate(II): A quasi 1d Heisenberg S = 12 ferromagnet

The compound (C3H7NH3)2MnCl4 is a quasi 2-d canted antiferromagnet. Along the c-axis a weak ferromagnetic moment (30 G cm3/mole for T = 4.2 K) was found below Tc = 39.2 K. The susceptibility of a single crystal, with h⌈c and T

Experimental study of the effect of domains on the A.C. susceptibility of the weak ferromagnet (C3H7NH3)2MnCl4

The compounds (C 3 H 7 NH 3 ) 2 MnCl 4 and (C 3 H 7 NH 3 ) 2 MnBr 4 are good approximations of the two-dimensional quadratic Heisenberg antiferromagnet. From measurements of the magnetic susceptibility the exchange constants at 80 K have been obtained, J / k = −4.45(5) K and −4.50(5) K, respectively. These exchange constants are compared with theoretical results for the MnXMn superexchange path. A slight temperature dependence of J is observed, resulting in values which are about 5% lower at 300 K. The chloride is found to show weak ferromagnetism below T c = 39.2(2) K, which is ascribed to an antisymmetric exchange term. The canting angle of the spins is only 0.05°. The bromide compound orders at 47(3) K but shows no sign of weak ferromagnetism.

Spin canting and exchange in the two-dimensional antiferromagnets (C3H7NH3)2 and (C3H7NH3)2MnBr4

A.c. susceptibility measurements on the 1d Ising ferromagnet Co[(CH 3 ) 3 NH]Cl 3 ·2H 2 O (CoTAC) are reported. At zero field these measurements were performed at temperatures between 1.2 and 150 K, with the a.c. field h parallel to the three orthogonal crystal axes. The data show the 3d AFM ordering along c to occur at T c = 4.179(5) K, but the moments are canted over 22° towards the a axis. The results were satisfactorily described with an intrachain exchange interaction J b / k = 13.8(5) K and interchain interactions z c J c / k = 0.28(5) K and z a J a / k = -0.0320(5) K. From the field-dependent susceptibilities the metamagnetic phase diagram was obtained. The tricritical point occurred at an internal field of 29(1) Oe and T c = 4.125(5) K. Along the crystal a axis CoTAC behaves as a weak ferromagnet. From the frequency dependence of χ it was possible to determine the domain-wall relaxation times. These time constants were found to vary exponentially with T . from which it was suggested that the spin reorientation arises due to impurity relaxation. The normalized susceptibilities as a function of reduced temperature demonstrated the occurrence of two lattice dimensionality crossovers. The spontaneous (weak) ferromagnetic moment parallel to a varies with T in a way similar as calculted for a 3d Ising ferromagnet.

H.A. Groenendijk

Papers

Crystal structure and magnetic properties of cyclohexylammonium trichlorocuprate(II): A quasi 1d Heisenberg S = 12 ferromagnet

Experimental study of the effect of domains on the A.C. susceptibility of the weak ferromagnet (C3H7NH3)2MnCl4

Spin canting and exchange in the two-dimensional antiferromagnets (C3H7NH3)2 and (C3H7NH3)2MnBr4

Magnetic properties of the quasi 1d Ising ferromagnet Co[(CH3)3NH]Cl3·2H2O

AC susceptibilities of the metamagnet [(CH3)3NH]CoCl3 · 2H2O as a function of frequency, field and temperature