Glassy Magnets Composed of Metals Coordinated to 7,7,8,8-tetracyanoquinodimethane:  M(TCNQ)₂ (M = Mn, Fe, Co, Ni)

Abstract

The homologous series M(TCNQ)2 (M = Mn(II), Fe(II), Co(II), and Ni(II); TCNQ = 7,7,8,8-tetracyanoquinodimethane) prepared from reactions of [M(CH3CN)6][BF4]2 and [n-Bu4N][BF4] in CH3CN has been carefully analyzed from the perspective of synthetic issues and physical characterization, including complete magnetic analyses by the tools of dc and ac magnetometry. The preparative method was optimized to definitively establish the reproducibility of the chemistry as judged by infrared spectroscopy, thermal gravimetric analysis, powder X-ray crystallography, and elemental analysis. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) studies results are also in accord with the conclusion that these materials are pure, isostructural phases. The dc magnetic measurements reveal a spontaneous magnetization for the four materials at low temperatures with a weak field coercivity of 20, 750, 190, and 270 G at 2 K for Mn(TCNQ)2, Fe(TCNQ)2, Co(TCNQ)2, and Ni(TCNQ)2, respectively. At low temperatures, ac susceptibility measurements confirm the presence of a magnetic phase at 44, 28, 7, and 24 K for Mn(TCNQ)2, Fe(TCNQ)2, Co(TCNQ)2, and Ni(TCNQ)2, respectively, but do not support the description of this system as a typical magnet. In the absence of the ac magnetic data, the behavior is indicative of ferri- or ferromagnetic ordering (depending on the metal), but in fact a complete investigation of their physical properties revealed their true nature to be a glassy magnet. The glassiness, which is a high magnetic viscosity known to originate from randomness and frustration, is revealed by a frequency dependence of the ac susceptibility data and is further supported by a lack of a lambda peak in the heat capacity data. These results clearly demonstrate that molecule-based materials with a presumed magnetic ordering may not always be exhibiting truly cooperative behavior.

Recent Advances on Anilato-Based Molecular Materials with Magnetic and/or Conducting Properties

“…Variable-temperature dc magnetic measurements under an applied magnetic field of 0.1 T revealed strong metal-radical magnetic interactions, leading to magnetic ordering less than 8 K (Figure 32) due to the strong magnetic coupling that has previously been observed in Fe III H 2 An 3− complexes [198]. Below 250 K the observed strong deviations from the Curie-Weiss behavior, observed in systems with strong π-d interactions [199], have been attributed to the competition between ferromagnetic and antiferromagnetic interactions, both maybe present in 43, leading to magnetic glassiness [200]. A Curie-Weiss fit of the inverse magnetic susceptibility data from 250 to 300 K results in a Curie temperature of θ = 134 K and a Curie constant of C = 6.1 emu•K/mol.…”

A muon spin relaxation study of the metal-organic magnet Ni(TCNQ)2

“…15 lSR in Ni(TCNQ-D4) 2 was shown to be sensitive to the ferromagnetic phase transition close to 20 K but there was no evidence to suggest that this transition is accompanied by an onset to a magnetically glassy state. However, below 6 K, the ZF lSR data suggest the onset of a second magnetic state, which may be a glassy magnetic behaviour reported elsewhere, 5 but further measurements are needed below 2 K to confirm this conclusion. LF studies indicate the presence of nuclear magnetic fields, which can be decoupled in 5 mT.…”

“…The ZF relaxation parameter, k 1 , is shown in Figure 3(b) and the peak in the temperature response below 20 K indicating the magnetic transition. At lower temperatures (T 6 K), the value of k 1 begins to rise and the asymmetry of component 2 also appears to exhibit a peak at around 4 K. From the magnetization data in Figure 1, at such low temperatures the ZFC curve changes slope and it may be that this deviation is associated with the glassy magnetic state reported by Clerac et al 5 The relaxation data shown in Figure 2(a) at T > T C (50 K), which is described by the sum of two gaussian terms, support the lower temperature results in suggesting that there are at least two muon stopping sites. In this higher temperature range, it is likely that the relaxation is due to nuclear dipolar fields associated with nitrogen atoms of the TCNQ molecule rather than the deuterium atoms.…”

“…1,2 One of the first examples of charge transfer magnetic materials was the metallocene-TCNQ (7,7,8,8- 3,4 A further advancement in metal-organic magnetism was with the synthesis of the M(TCNQ) 2 series, where M ¼ Fe, Mn, Co, and Ni. 5 These new compounds were unsolvated and there was clear evidence of crystallinity from x-ray diffraction data where the unit cell was predicted to be tetragonal. For the Ni(TCNQ) 2 compound, this showed a bulk ferromagnet transition with a T C ¼ 20:8 K (h ¼ 37 K); however, it is believed that there is a glassy component associated with the magnetic transition.…”

“…Ni(TCNQ-D 4 ) 2 was synthesized using a similar method to Clerac et al, but using TCNQ-D 4 which was produced using a method reported by Dolphin et al 7 The temperature dependent magnetization of Ni(TCNQ-D 4 ) 2 was measured with a Quantum Design MPMS using an applied field of 2.5 mT; the results are shown in Figure 1. Also included in the figure are data obtained from a Ni(TCNQ) 2 synthesized in our laboratory following the method of Clerac et al 5 The magnetic measurements showed a difference in T C of $3 K between the protio (T C A selection of ZF lSR spectra from Ni(TCNQ-D 4 ) 2 , recorded at temperatures above and below T C , are shown in Figure 2(a). At temperatures well above the T C , the best fit to the data is using the sum of two gaussian relaxations 10 while at temperatures close to and below T C , the data can be described well using a summation of two single exponential relaxations, suggesting two muon implantation sites…”