Magnetic Properties of Ferromagnetic Quasi-1D Copper-Peptide Compounds: ExchangeInteractions and Very Low Temperature Phase Transitions

EDSON F. CHAGAS; RAUL E. RAPP; DANIEL E. RODRIGUES; NIEVES M. C. CASADO; RAFAEL CALVORAFAEL CALVO

JOURNAL OF PHYSICAL CHEMISTRY B

American Chemical Society

Año: 2006 vol. 110 p. 8052 - 8066

1089-5647

The magnetic properties of the Cu(II)-peptide compounds (L-tyrosyl-L-leucinato)Cu(II) and (L-tryptophylglycinato) Cu(II), to be identified as Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively, have been investigated by specific heat (0.08 < T < 28 K), dc magnetization (2 < T < 80 K, with B0 = uo.Ho < 9 T), and ac magnetic susceptibility (with Bo = 0 for 0.03 < T < 3 K and Bo up to 9 T for 2 < T < 80 K) measurements. Above ~1 K, the specific heat and magnetization of both compounds display a ferromagnetic (FM) spin chain behavior sustained by syn-anti carboxylate bridges connecting equatorially Cu(II) ions at about 5 Å.To model this behavior, we calculated the eigenvalues of Heisenberg chains with up to 20 spins 1/2 and used the method of Bonner and Fisher. A global fit of the model to the specific heat and magnetization data gives 2Jo/kB = 3.60(5) K and 2.59(5) K for the intrachain exchange interactions in Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively (Hex(i,j) = -2J0 Si.Sj). These values of 2Jo are discussed in terms of structural properties of the carboxylate bridges in the two compounds. Using the parameters obtained from the global fit, wecalculated isothermal susceptibilities in agreement with the ac susceptibilities measured with small applied dc magnetic fields. However, the ac susceptibility measured with applied dc fields larger than 1 T lie between the values calculated for the isothermal and adiabatic susceptibilities. At 0.16 K for Cu(II)Tyr-Leu and 0.53 K for Cu(II)Trp-Gly, the observed specific heat and magnetic susceptibility display peaks associated to threedimensionalmagnetic phase transitions. The interchain exchange couplings 2J1 producing the 3D magnetic order are ferromagnetic and have magnitudes 2J1/kB ~ 0.015 and 0.073 K for Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively.