Slow magnetic relaxations in the anisotropic Heisenberg chain compound Mn(III) tetra(ortho-fluorophenyl)porphyrin-tetracyanoethylene M. Balanda, 1 M. Rams, 2 S. K. Nayak, 3 Z. Tomkowicz, 2, * W. Haase, 4 K. Tomala, 2 and J. V. Yakhmi 5 1 H. Niewodniczański Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Kraków, Poland 2 Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland 3 Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India 4 Institut für Physikalische Chemie, Technische Universität Darmstadt, Petersenstrasse 20, 64287 Darmstadt, Germany 5 Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 25 June 2006; revised manuscript received 29 October 2006; published 18 December 2006 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 = -217 K. At low temperatures, slow magnetic relaxations below 13 K and irreversible magnetic behavior below 4.5 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 E =117 K. These relaxations are interpreted as a thermally activated reversal of magnetization through the energy barrier E, associated with creation and propagation of domain walls in one-dimensional anisotropic Heisenberg systems. DOI: 10.1103/PhysRevB.74.224421 PACS numbers: 75.50.Xx, 75.10.Pq, 75.40.Gb, 75.60.Ch I. INTRODUCTION Molecular magnetism is a dynamically developing field of science with prospects for applications. Recently, so-called single molecule magnets 1 SMM and single chain magnets 2–4 SCM have been objects of interest. SMM ma- terials are composed of molecules with a high spin and strong easy axis anisotropy. Each such molecule can function as a magnet at temperatures below its superparamagnetic blocking temperature. Due to this property, SMM can be used to build the ultimate high-density memory. Therefore, considerable efforts are devoted to the search for new mate- rials with similar properties but with higher blocking tem- peratures. The interest in SCM is a natural consequence in this field. The number of SCM reported to date is not large. These materials are composed of ferro- or ferrimagnetic chains, which are well isolated Ising systems. Only recently, one SCM was reported, chains of which are anisotropic Heisenberg systems. 5 In our previous studies 6,7 we synthe- sized manganese porphyrin-radical polymers with alcoxy substitutions having very high interchain distance up to 30 Å. Such materials are good candidates for SCM. Unex- pectedly, a phase transition to the state with spontaneous magnetization was observed. A theoretical model explaining this phase transition takes into account superspin interactions and a strong single ion anisotropy. 8 The family of metalloporphyrin-tetracyanoethylene based magnets provides an unusual opportunity for magnetic stud- ies, due to the relative ease of modifying the structure through, e.g., various substitutions at the porphyrin disc pe- riphery. In this way, magnetic dimensionality can be varied by increasing the interchain separation. Molecular structure of manganese tetraphenylporphyrin-tetracyanoethylene, MnTXPPTCNE, where X =R or R is a substituent, is shown in Fig. 1. In the center of the porphyrin disc the Mn III ion is located, which forms the chemical bond with the TCNE radical. The spin of the Mn III ion S = 2 is strongly antiferromag- netically coupled with a delocalized spin s = 1 2 of the TCNE radical. The constant of the exchange coupling for some of these compounds is of the order of 100 cm -1 . 9 Although the family of metalloporphyrin-radical magnets offers a wide range of magnetic behavior, there is some com- mon property of nearly all representatives—partially, or even fully spin glass—or superparamagneticlike character of these substances. 10–12 It is known 11 that a disorder may come from solvent molecules, which are present in free space between chains, as well as from some freedom in orientation of the bridging TCNE molecule. We reported the partially glassy character of MnTXPPTCNE·solvent, X = alcoxy. 7,13 As suggested, this glassiness was caused by the presence of frus- trated clusters of segments of chains coupled by dipolar interactions. 13 It was observed that with increasing magnetic field, the behavior gradually became superparamagnetic or SCM-like, as if the field decoupled the chains field symme- try incompatible with dipolar coupling. Gîrţu et al. 14,15 suggested that the cluster glasslike behav- ior observed for the two manganese porphyrin-TCNE com- pounds with different solvents: MnTPPTCNE ·yo-xylene and MnTPPTCNE·yo-dichlorobenzene is caused by disorder leading to the origin of one-dimensional clusters coupled through dipole-dipole interactions to form three-dimensional domains. Etzkorn et al. 16 studied spin glass transition for MnTPPTCNE·21,3-C 6 H 4 Cl 2 , which occurs near 4 K and proposed a quasi-1D fractal cluster glass model describing magnetic viscous behavior of this transi- tion. The dimension D of spin clusters was determined to be in the range 0.8–1.5. As temperature was lowered, D became greater than 1, meaning that clusters included portions of neighboring chains. The origin of glassiness in these compounds is intriguing because compounds are one-dimensional anisotropic systems with strong intrachain coupling. It is not apparent how the PHYSICAL REVIEW B 74, 224421 2006 1098-0121/2006/7422/2244219 ©2006 The American Physical Society 224421-1