Frequency dependent ac magnetic susceptibility in YBa 2 Cu 3 O 7d /La 1/3 Ca 2/3 MnO 3 /La 2/3 Ca 1/3 MnO 3 heterostructures K. Gross a , O. Mora ´n b, * , E. Baca a a Grupo de Ingenieria de Nuevos Materiales, Departamento de Fı ´sica, Universidad del Valle, A.A. 25360 Cali, Colombia b Laboratorio de Materiales Cera ´ micos y Vı ´treos, Departamento de Fı ´sica, Universidad Nacional de Colombia sede Medellı ´n, A.A. 568 Medellı ´n, Colombia Received 9 June 2006; received in revised form 30 June 2007; accepted 10 July 2007 Available online 25 September 2007 Abstract The field frequency dependence of the ac magnetic susceptibility in YBa 2 Cu 3 O 7d /–La 1/3 Ca 2/3 MnO 3 /La 2/3 Ca 1/3 MnO 3 structures was studied. The trilayers were grown in situ on (0 0 1)-oriented SrTiO 3 substrates using a dc sputtering technique. The ac magnetic suscep- tibility measurements were performed using an inductive technique with exciting field amplitude h ac = 0.1 Oe, applied parallel to (0 0 1)-direction of the substrate. The signal frequency was varied between 1 and 51.1 kHz. The YBa 2 Cu 3 O 7d /–La 1/3 Ca 2/3 MnO 3 /– La 2/3 Ca 1/3 MnO 3 trilayers displayed a diamagnetic behavior below the superconducting critical temperature T c (77 K) and a combined effect, probably stemming from a complex interplay between the ferromagnetic and antiferromagnetic character of the constituents La 2/3 - Ca 1/3 MnO 3 and La 1/3 Ca 2/3 MnO 3 , at T > T c . The observed Curie temperature T c of 260 K was associated with the F-layer. The imag- inary branch of the magnetic susceptibility showed loss peaks pinned at 72 K, which was independent of frequency. This temperature was found to be very close to the irreversibility temperature, T I , which separates the creep and flux flow regimens. The T I was obtained using the Jo ¨ nnson model, which was applied to the real branch of the measured magnetic susceptibility. According to this model, v 0 ðT ; f ; h ac Þ¼ KðT ; h ac Þf 3SðT ;hac Þ=2 , where f stands for the measuring frequency and S=k B T/U eff for the vortex relaxation rate with U eff being the effective vortex pinning energy. Farther, K(T, h ac ) is a factor which depends on the critical current density J c . Additionally, U eff was obtained by measuring the resistance of the YBa 2 Cu 3 O 7d layer in the YBa 2 Cu 3 O 7d /La 1/3 Ca 2/3 MnO 3 /La 2/3 Ca 1/3 MnO 3 hetero- structure in the superconducting transition region. This value was found to be comparable to those determined for the flux creep and flux flow regimens. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.78.Fk; 75.47.Lx Keywords: Heterostructures; Superconductivity; Manganites 1. Introduction The possibility of controlling the superconducting order parameters of conventional or high temperature supercon- ductors (HTS) through ferromagnetic materials is based on two main effects, namely the spin-polarized current injec- tion (SPCI), and the proximity effect (PE) [1–3]. The SPCI refers to the injection of spin-polarized carriers from a fer- romagnetic gate electrode into a superconductor material. It is expected that the injected spin-polarized quasiparticles provokes a non-equilibrium state in the superconductor, which in turn leads to a diminished concentration of Coo- per pairs and hence to the depression of the superconduc- tivity. This phenomenon has clearly been observed in superconducting/ferromagnetic bilayers (S/F) and super- conducting/barrier/ferromagnetic (S/barrier/F) trilayers, where the barrier can be a normal or an insulator material [1–7]. The proximity effect has mainly been observed in S/F 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.07.011 * Corresponding author. Tel.: + 57 44309887; fax: +57 44309332. E-mail address: omoranc@unalmed.edu.co (O. Mora ´n). www.elsevier.com/locate/physc Available online at www.sciencedirect.com Physica C 467 (2007) 136–140