INTEC   05402
INSTITUTO DE DESARROLLO TECNOLOGICO PARA LA INDUSTRIA QUIMICA
Unidad Ejecutora - UE
artículos
Título:
Magnetic Properties of Ferromagnetic Quasi-1D Copper-Peptide Compounds: Exchange Interactions and Very Low Temperature Phase Transitions
Autor/es:
CHAGAS, EF; RAPP, RE; RODRIGUES, DE; CASADO, NMC; RAFAEL CALVO
Revista:
JOURNAL OF PHYSICAL CHEMISTRY B
Editorial:
American Chemical Society, USA
Referencias:
Año: 2006 vol. 110 p. 8052 - 8063
ISSN:
1089-5647
Resumen:
The magnetic properties of the Cu(II)-peptide compounds (L-tyrosyl-L-leucinato)Cu(II)
and (L-tryptophyl-glycinato)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 = moH
£ 9 T), and ac magnetic
susceptibility (with B0 = 0 for 0.03 < T < 3 K and B0
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 ½ and used the method of Bonner and Fisher. A global fit of the model to
the specific heat and magnetization data gives 2J0/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 2J0 are discussed in terms of
structural properties of the carboxylate bridges in the two compounds.
Using the parameters obtained from the global fit we
calculated 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 1T 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 three dimensional magnetic phase transitions. The interchain exchange
couplings 2J1
producing the 3D magnetic order are ferromagnetic and have magnitudes 2J1/kB
~ 0.015 K and 0.073 K for Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively.