* Corresponding author. Fax: # 55-11-818-6984. E-mail address: cbecerra@if.usp.br (C.C. Becerra). Journal of Magnetism and Magnetic Materials 226}230 (2001) 610}611 Low-"eld remanent magnetization in the disordered antiferromagnet Cs  Fe  In  Cl  ) H  O C.H. Westphal, Z.V. Carvalho, A. Paduan-Filho, C.C. Becerra*, F. Palacio Instituto de Fn & sica, Universidade de Sa J o Paulo, CP 66318, 05315-970 Sa J o Paulo SP, Brazil Instituto de Ciencia de Materiales de Aragon, CSIC, Universidad de Zaragoza, E-50009 Zaragoza, Spain Abstract Magnetization measurements on single crystals of the site-diluted antiferromagnet A  Fe  In  Cl  ) H  O (A"Cs) were carried out at low magnetic "elds (H ) applied along the easy axis. The data revealed that a remanent magnetization M  develops below the Ne H el temperature ¹  . This M  (T) is parallel to the easy axis, saturates for H&1 Oe and it increases with decreasing T. It has also the same temperature dependence as other diluted systems of the same family (A"K,Rb). For all these systems the normalized M  (t)/M  (t"0.3), where t"T/T  is the reduced temperature, is independent of x and follows a universal curve.  2001 Elsevier Science B.V. All rights reserved. Keywords: Diluted antiferromagnet; Magnetic ordering; Cesium iron penta-chloride Recently, the appearance below T  of a remanent magnetization M  in some diluted low-anisotropy anti- ferromagnets [1}3] was reported. Such M  is observed parallel to the easy axis of antiferromagnetic alignment in "elds as low as few m-Oe. This behavior was found in compounds of the series A  Fe  In  Cl  ) H  O (A"K or Rb); in Mn  Zn  F  , and in Mn  Zn  TiO  [4]. A M  (T) was also observed in the diluted quasi one- dimensional antiferromagnet (CH  NH  )Mn  Cd  Cl  ) 2H  O [5]. In these works it is shown that if appropriate reduced variables are chosen it is possible to collapse all the low-"eld data for the M  (¹) of these compounds having di!erent degrees of dilution and crystallographic structures, on to a single-universal magnetization curve [6]. Of the material so far investigated the Rb}Fe/In solid solutions seems to be that which most e!ectively mani- fests the remanent magnetization phenomenon. In this case even the pure the compound Rb  FeCl  ) H  O ex- hibits a very small M  (¹) that follows the same universal behavior of the diluted systems. It has been suggested that lattice defects which develop in the crystals during the crystallization process, as well as the non-magnetic ions, act as domain wall pinning centers and are the cause of M  [3]. The fact that M  is quite signi"cant in pure Rb  FeCl  ) H  O seems to indicate that this system is certainly more prone to such imperfections. It is worth noting that the next compound in the A  FeCl  ) H  O series, the Cs  FeCl  ) H  O, has a di!erent crystal struc- ture due to the larger ionic radius of the Cs ion. It seems that the Rb  FeCl  ) H  O is close to the stability edge of its structure. This would favor the formation of defects in the crystallization process that would create additional contributions to the disorder of the lattice. In this work we perform an analogous investigation in the Cs system, that is on the `other sidea of the referred stability edge! We "nd that in this system (Cs}Fe/In), M  is also present and that the undiluted compound has also a non-negli- gible remanent component in the magnetization. The antiferromagnets A  Fe  In  Cl  ) H  O (A"Rb, K,Cs) (hereafter refered to, respectively, as Rb}Fe/In, K}Fe/In and Cs}Fe/In) can be described as 3D-Heisen- berg AF model with the a being the easy axis. The magnetization M(¹) of Cs}Fe/In samples was measured with a SQUID magnetometer. M(¹) is given by M(¹, H)"M  (¹)# , H, where M  (¹) is the remanent magnetization and  , is the parallel susceptibility of the 0304-8853/01/$- see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00698-3