* Corresponding author. Fax: #54-2944-445299. E-mail address: sirenam@ib.cnea.gov.ar (M. Sirena). Journal of Magnetism and Magnetic Materials 226 } 230 (2001) 847}848 Magnetic after-e!ect in manganite "lms M. Sirena*, L.B. Steren, J. Guimpel Centro Ato & mico Bariloche, Av. Ezequiel Bustillo, C.P. 8400, 9500 S.C. de Bariloche, Argentina Abstract The time dependence of the magnetic and transport properties on La  Sr  MnO  "lms and bulk samples has been studied through magnetization and resistivity measurements. A magnetic after-e!ect has been observed in all samples. At low temperatures, the low-"eld magnetization, can be described by the function M(t)"M  #M  exp(!t/)# S(H, ¹)ln(t). The resistivity increases logarithmically in the same temperature range, indicating the evolution of the sample to a more disordered state. Above a characteristic temperature, this behaviour is reversed and an increase of the magnetization with time is observed. The relaxation parameters depend on the bulk or "lms character of the samples. In the latter case, a dependence on the "lm thickness was found. A direct correlation between the time dependence of the resistivity and magnetization curves in manganite compounds was found.  2001 Elsevier Science B.V. All rights reserved. Keywords: Manganite thin "lms; Magnetic after-e!ect; Magnetic structure; Magnetoresistance The study of manganites with perovskite structure, A  A  MnO  , has been intensi"ed in the last years, due to the discovery of the colossal magnetoresistance e!ect (CMR) [1] in these compounds. In fact, a large magnetoresistance is observed near the ferromagnetic ordering of the Mn moments, where the system also presents an insulator}metal transition. Time hysteresis e!ects have been observed in magnetoresistance measurements, dependent on the "eld sweeping time. The knowledge of this phenomenon is critical for applications of these materials as magnetoresistance sensors. In order to clarify these results we have measured the magnetic after-e!ect (MAE) in La  Sr  MnO  bulk and sput- tered "lms of di!erent thicknesses. The "lms were grown by DC magnetron sputtering on (1 0 0) MgO and (1 0 0) SrTiO  substrates. Bulk samples have also been measured for comparison. The time de- pendence of the magnetization has been measured in a SQUID magnetometer at several temperatures, be- tween 4 and 200 K. For each M vs. t measurement, the magnetization of the samples is "rst saturated with a magnetic "eld of 1 T, parallel to the "lm surface. After 5 min, the "eld is turned o! and the measurement started (t  "0). A time window of 8h has been studied. The resistivity of the samples is measured using a standard four-probe method and following the same procedure described above for the magnetization. In thin "lms, the Curie temperature is shifted to lower temperatures (200 K(T  (250 K) with respect to the bulk value (¹  +370 K) due to the substrate in#uence on the Mn}O}Mn coupling [2]. Typical magnetization vs. time curves, measured at di!erent temperatures, are shown in Fig. 1. An important MAE is noticed in all the curves. No systematic temperature dependence of the absolute magnetization change, M  !M  has been found . The magnetization still relaxes after several hours of measurement, varying as much as 10% from its initial value M  . The main change observed is in the "rst hour. Below a characteristic temperature ¹  , the mag- netization presents a general behaviour described by the addition of an exponential decay function, noticed in the short time window, and a slower !ln(t) law. As the temperature is increased, the magnetization curves pro- gressively lose their logarithmic character, approaching a single exponential decay function. The whole set of magnetization data can be described in terms of a simple phenomenological model consisting in the addition of 0304-8853/01/$- see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)01027-1