Journal of Magnetism and Magnetic Materials 221 (2000) 99 }102 Secondary magnetic relaxations in Mn  complexes Marco Evangelisti, Juan Bartolome H * Facultad de Ciencias, Instituto de Ciencia de Materiales de Arago & n, C.S.I.C., Universidad de Zaragoza, E-50009 Zaragoza, Spain Abstract We present new experiments on the zero-"eld magnetic relaxation in Mn  acetate and Mn  2-Cl benzoate. By dynamical magnetic measurements, this study shows that a small portion of few percent of the clusters does not undergo the main relaxation process and that they relax faster in the temperature region of 2 K. The e!ective relaxation times of these secondary processes follow an Arrhenius law with energy barrier of approximately 23 and 30 K for the acetate and the 2-Cl benzoate, respectively.  2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic relaxation; AC-susceptibility 1. Introduction Experiments have shown evidence at the me- soscopic scale of quantum tunneling of the magnet- ization through an anisotropy energy barrier for the molecular nano-magnetic complexes Mn  acetate [1}3] and Mn  2-Cl benzoate [4]. Both can be thought of as an ensemble of identical mag- netic clusters of net spin S"10 at low temper- atures, with a strong uniaxial anisotropy. However, several authors have pointed out, using AC-suscep- tibility technique, that there is a magnetic minority component in almost all Mn  complexes, di!erent from the well-studied main component, which undergoes fast relaxation in the 2 K region [5}8]. Since any relaxation observation is dominated at each temperature by the fastest process, it is of paramount importance to ascertain which is the * Corresponding author. Tel.: #34-976-761218; fax: #34- 976-761229. E-mail address: barto@posta.unizar.es (J. Bartolome H ). magnetic species that is observed in a particular relaxation experiment. Our goal is to clarify this point in the frequency domain 0.1}990 Hz and in the temperature region of 2 K, where the minority species relaxes most e!ectively. 2. Experimental results We have performed dynamical magnetic measurements on oriented crystallites of Mn  acetate and on a single crystal of Mn  2-Cl ben- zoate. The magnetic core and the local symmetry of each molecule are the same in both compounds [9]. However, in contrast to the acetate where the clus- ters are iso-oriented, the easy axes of the 2-Cl ben- zoate lie alternatively on the directions [0 1 1] and [0 l  1], being nearly perpendicular to their nearest neighbours. The experiments were performed in a commercial SQUID magnetometer with an AC- susceptibility measurement option. The AC-"eld was applied along the easy axes of the molecules of Mn  acetate and along one of the [0 1 1] and 0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 3 8 1 - 4