Frequency dependent ac magnetic susceptibility in YBa2Cu3O7−δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures

The field frequency dependence of the ac magnetic susceptibility in YBa2Cu3O7−δ/–La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 structures was studied. The trilayers were grown in situ on (0 0 1)-oriented SrTiO3 substrates using a dc sputtering technique. The ac magnetic susceptibility measurements were performed using an inductive technique with exciting field amplitude hac = 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 YBa2Cu3O7−δ/–La1/3Ca2/3MnO3/–La2/3Ca1/3MnO3 trilayers displayed a diamagnetic behavior below the superconducting critical temperature Tc (∼77 K) and a combined effect, probably stemming from a complex interplay between the ferromagnetic and antiferromagnetic character of the constituents La2/3Ca1/3MnO3 and La1/3Ca2/3MnO3, at T > Tc. The observed Curie temperature Tc of ∼260 K was associated with the F-layer. The imaginary 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, TI, which separates the creep and flux flow regimens. The TI was obtained using the Jönnson model, which was applied to the real branch of the measured magnetic susceptibility. According to this model, χ′(T,f,hac)=K(T,hac)f3S(T,hac)/2χ′(T,f,hac)=K(T,hac)f3S(T,hac)/2, where f stands for the measuring frequency and S = kBT/Ueff for the vortex relaxation rate with Ueff being the effective vortex pinning energy. Farther, K(T, hac) is a factor which depends on the critical current density Jc . Additionally, Ueff was obtained by measuring the resistance of the YBa2Cu3O7−δ layer in the YBa2Cu3O7−δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructure in the superconducting transition region. This value was found to be comparable to those determined for the flux creep and flux flow regimens.