Photo-induced magnetic effects in

[MnR₄TPP][TCNE] molecular magnet

P.M. Zieliński¹, M. Bałanda¹, Robert Pełka¹, Tadeusz Wasiutyński¹

¹ The Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences, Radzikowskiego 152, 31-342 Cracow, Poland, tel.: +48(12)662823, fax.: +48(12)6628458, Pm.Zielinski@ifj.edu.pl

During the investigation into magnetic properties of charge transfer salt molecular magnet [Mn(OC₁₂H₂₅)₄TPP][TCNE]*2PhMe (TPP– tetraphenylporphyrin, TCNE– tetracyanoethylene) the photo-induced magnetic effects have been observed. The compound forms a pseudo 1D alternating acceptor-donor chains. The Mn-porphyrin based donors have spin S=2 while TCNE acceptors have spin s=1/2[1]. Transition from local order of ferrimagnetically coupled donor and acceptor spins to ferromagnetic long-range was observed at T_C = 22 K. Between T_C and ~ 8 K the substance shows properties characteristic of mixed spin-glass and ferromagnetic phase. At T_g ~ 8 K re-entrance to spin-glass is observed. Below T_g, at the virgin magnetisation curve the field induced magnetisation jump was reported and identified as the spin-flop transition[2]. Our research into magnetic properties of [MnR₄TPP][TCNE]*2PhMe and PME were performed in both static (DC) and dynamic (AC) modes with Lake Shore 7225 equipment using a specially designed sample probe with optical fibre. The optical system consisted of a Xe lamp with optical cut-off filters or He-Ne laser was used. Measurements of magnetic quantities were performed before and during illumination. Results of DC relaxation measurements obtained for different magnetic fields in He bath temperature (4,2 K) during illumination differ significantly from those obtained before illumination. Both the value and the time dependence of magnetisation changed during illumination (Fig. 1). Hysteresis loop measured before and during illumination was also found to undergo considerable change. On the other hand the AC measurements revealed only slight alteration in the real and imaginary components of the susceptibility. Simple theoretical description of the magnetisation process registered below 8 K, based on the models proposed by Palmer et al.[3] and Walton[4] was performed and the relaxation data were found to be consistent with the model.

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[1] J. S. Miller, J. C. Calabrese, R. S. McLean, A. J. Epstein,  ,,meso-(Tetraphenylporphinato)manganese(III)-tetracyanoethenide, [MnIIITPP]::[TCNE]. A New Structure-Type Linear-Chain Magnet with a Tc of 18K’’ : Advanced Materials, 1992, 4 490.

[2] M. Bałanda, K. Falk, K. Griesar, Z.Tomkowicz, T.Wasiutyński, W. Haase, ,,Spin Flop Transition in the Substituted Mn-Porphyrin Magnet [Mn(OC₁₂H₂₅)₄TPP][TCNE]*2PhMe’’, Moleqular Crystal And Liquid Crystal, 1999, 335, 133-141.

[3] R. G. Palmer, D. L. Stein, E. Abrahms, P. W. Anderson, ,,Models of Hierarchically Constrained Dynamics for glassy Relaxation’’ Physical Review Letters, 1984, 53, 958-961.

[4] D. Walton, ,,Relaxation and aging in Ising systems’’, Physical Review B, 1996, 53, 14980-14986.