Andrew Scott Albrecht

Two-dimensional S = 1 2 Heisenberg antiferromagnets: Synthesis, structure, and magnetic properties

The magnetic susceptibility and magnetization of two layered S=½ Heisenberg antiferromagnets with moderate exchange are reported. The two isostructural compounds, (2-amino-5-chloropyridinium) 2 CuBr 4 [(5CAP) 2 CuBr 4 ] and (2-amino-5-methylpyridinium) 2 CuBr 4 [(5MAP) 2 CuBr 4 ], contain S=½, Cu(II) ions related by C centering, yielding four equivalent nearest neighbors The crystal structure of the synthesized compound, (5CAP) 2 CuBr 4 , shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the ab plane, separated by the organic cations along the c axis. Magnetic pathways are available through the bromide-bromide contacts within the layers and provide for moderate antiferromagnetic exchange. Susceptibility measurements indicate interaction strengths to he 8.5(2) K and 6.5(2) K and ordering temperatures of 5.1(2) K and 3.8(2) K for (5CAP) 2 CuBr 4 and (5MAP) 2 CuBr 4 , respectively. High-field magnetization experiments on both compounds show upward curvature of M(H,T). Magnetization measurements made at T = 1.3 K show saturation occurs in (5MAP) 2 CuBr 4 at 18.8 T and in (5CAP) 2 CuBr 4 at 24.1 T. The magnetization curves are consistent with recent theoretical predictions. Single-crystal magnetization measured at 2.0 K indicates a spin-flop transition at 0.38 T and 0.63 T for (5CAP) 2 CuBr 4 and (5MAP) 2 CuBr 4 , respectively.

Magnetic studies of the two-dimensional, S=1/2 Heisenberg antiferromagnets (5CAP)2CuCl4 and (5MAP)2CuCl4

The magnetic materials (5CAP)2CuCl4 and (5MAP)2CuCl4 form effective square planar lattices of CuCl42− anions. The Cu2+ ions have weak antiferromagnetic interactions in the plane through Cl–Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give J=0.57 K for (5CAP)2CuCl4 and J=0.38 K for (5MAP)2CuCl4. χ(T,H=0) for these materials has a rounded maximum at Tpeak=1.1 K for (5CAP)2CuCl4 and Tpeak=0.62 K for (5MAP)2CuCl4. A kink in χ(T,H=0) for the (5MAP)2CuCl4 suggests a phase transition to three-dimensional long range order at TC=0.44 K. Measurements of χ(T,H) show that the saturation field HC for the transition to a fieldinduced saturated state is 3.78 T for (5CAP)2CuCl4 and 2.17 T for (5MAP)2CuCl4. Thus these materials are excellent model systems in which to explore the full H–T phase diagram of the two-dimensional S=1/2 Heisenberg antiferromagnet.

New Square S=1/2 Heisenberg Antiferromagnetic Lattices: Pyridinium Tetrahalocuprates and Bispyrazinecopper(II) Tetrafluoroborate

Abstract As part of a program to create new low dimensional S=1/2 antiferromagnetic Heisenberg compounds with moderate exchange, we have prepared four new copper complexes with square magnetic lattices. The complexes L2[CuX4] have been prepared (L = 2-amino-5-methylpyridinium, X = Cl, Br; L = 2-amino-5-chloropyridinium, X = Br). The compounds are isomorphous, giving monoclinic crystals; space group C2/c. The tetrahalometallate ions lie in C-centered layers, forming a square magnetic lattice. The layers are separated by the organic cations. Preliminary analysis of the magnetic data shows that the exchange constant, J, increases in the order Br > Cl and 2-amino-5-chloropyridinium > 2-amino-5-methylpyridinium. A new member of the pyrazine bridged family M(pyrazine)2X2 has also been prepared and studied. Cu(pz)2 (BF4)2 also crystallizes in the space group C2/c. Analysis of the temperature dependence of the susceptibility yields and exchange value of −7.98 K, similar to that found for the known Cu(pz)2(ClO4)2 ...

High-Field Magnetization Studies of Two-Dimensional Copper Antiferromagnets

Abstract The magnetic susceptibilities and high-field magnetization data for members of two families of S = 1/2, two-dimensional, square antiferromagnetic lattices are reported. The known compounds bis-2-amino-5-methylpyridinium tetrabromocuprate, 1, and bis-2-amino-5-chloropy-ridinium tetrabromocuprate, 2, crystallize in the space group C2/c with the tetrabromocuprate ions related by the C-centering and unit cell translations, leading to a square magnetic lattice. Similarly bispyrazinecopper(II) perchlorate, 3, and bispyrazinenitratocopper(II) hex-afluoro-phosphate, 4, crystallize in the space groups C2/m and I4/mcm respectively and also generate square magnetic lattices. High-field magnetization studies, up to 60 Tesla, have determined their saturation fields to be 21 T (1), 27 T (2), 53 T (3) and 32 T (4). All show upward curvature in plots of magnetization as a function of field and fall on a universal curve in good agreement with recent theoretical predictions.

Magnetic properties of a new amorphous magnet

An amorphous phase of Co3BTCA2(H2O)6 has been prepared (BTCA=1,3,5‐benzenetri‐ carboxylate). The static properties (dc susceptibility, hysteresis) of Co3BTCA2(H2O)6 are similar to those of other ‘‘glassy’’ magnetic systems, but the dynamics of Co3BTCA2(H2O)6 (absence of long relaxation times, disappearance of frequency dependence at low temperatures) are unusual. The amorphous phase is characterized by a narrow hysteresis loop at low temperature. For the hysteresis curve at 4.2 K, the coercive field is several hundred Oersted and the remnant magnetization is on the order of 10−2 Msat. The temperature dependence of the dc magnetic susceptibility shows an unusual field dependence below 30 K, with the largest effective moments being obtained in the smallest applied fields. The freezing temperature, Tf, in zero field, as determined by both dc and ac susceptibilities, has been found to be Tf=23 K. The ac susceptibility shows the characteristic spin glass frequency dependence, yet loses all frequency dependence...

Properties of Molecular-Based Frustrated Magnets

Abstract The magnetic properties of cobalt complexes of BTCA (BTCA = 1,3,5 benzenetricarboxylic acid) have been investigated. The trianion of the BTCA ligand has the ability to create lattices with competing interactions leading to frustration. A dodecahydrate phase, Co3BTCA2(H2O)12, is obtained from room temperature aqueous solutions and consists of pendant chains with very small interactions. Varying the reaction conditions leads to an amorphous phase which displays many of the features of a spin glass: hysteresis, remanent magnetization, frequency dependent susceptibility, aging effects, and an inverse field dependence of the dc susceptibility. The dehydration state and corresponding magnetic properties are strongly dependent on the temperature of the original reaction. Reaction temperatures below 78 °C lead to the pendant chain while temperatures greater than 78 °C yield more interesting glassy phases. These phases exhibit many properties of reentrant spin glasses.

Oxamide Derived Bimetallic Ferrimagnets

Abstract New oxamide-based bimetallic ferrimagnets have been synthesized and studied. Reaction of oxamide with copper nitrate and Bu4OH in DMSO solution yields the soluble complex (Bu4N)2[Cu(C2H2N202)2]. Subsequent addition of nickel(II), or cobalt(II), perchlorate results in precipitation of the bimetallic materials. Magnetic susceptibility studies of both compounds show minima in the χT products, characteristic of ferrimagnetism. The minimum occurs at 45 K for the nickel-copper complex. The minimum for the cobalt-copper complex is too broad to locate accurately. The moments for both complexes reach constant values below 5 K. The cobalt-copper complex shows hysteresis at 4.2 K with a coercive field of 400 G, and exhibits long relaxation times at low temperatures.