Magnetic order in the double-layer manganites (La1-zPrz)1.2Sr1.8Mn2O7:: Intrinsic properties and role of intergrowth

We report on an investigation of the double-layer manganite series (La(1-z)Pr(z))(1.2)Sr(1.8)Mn(2)O(7) (0 <= z <= 1), carried out on single crystals by means of both macroscopic magnetometry and local probes of magnetism [muon-spin rotation (mu SR), (55)Mn NMR]. Muons and NMR demonstrate an antiferromagnetically ordered ground state at nonferromagnetic compositions (z >= 0.6), while more moderate Pr substitutions (0.2 <= z <= 0.4) induce a spin reorientation transition within the ferromagnetic phase. A large magnetic susceptibility is detected at T(C,N) < T < 250 K at all compositions. From (55) Mn NMR spectroscopy, such a response is unambiguously assigned to the intergrowth of a ferromagnetic pseudocubic phase (La(1-z)Pr(z))(1-x)Sr(x)MnO(3), with an overall volume fraction estimated as 0.5%-0.7% from magnetometry. Evidence is provided for the coupling of the magnetic moments of these inclusions with the magnetic moments of the surrounding (La(1-z)Pr(z))(1.2)Sr(1.8)Mn(2)O(7) phase, as in the case of finely dispersed impurities. We argue that the ubiquitous intergrowth phase may play a role in the marked first-order character of the magnetic transition and the metamagnetic properties above T(c) reported for double-layer manganites.