Noelia Marcano

Epitaxial strain modifies the physical properties of thin films deposited on single-crystal substrates. In a previous work, we demonstrated that in the case of La_2/3Ca_1/3MnO₃ thin films the strain induced by the substrate can produce the segregation of a non-ferromagnetic layer (NFL) at the top surface of ferromagnetic epitaxial La_2/3Ca_1/3MnO₃ for a critical value of the tetragonality τ, defined as τ = |c − a|a, of τ_C ≈ 0.024. Although preliminary analysis suggested its antiferromagnetic nature, to date a complete characterization of the magnetic state of such an NFL has not been performed. Here, we present a comprehensive magnetic characterization of the strain-induced segregated NFL. The field-cooled magnetic hysteresis loops exhibit an exchange bias mechanism below T ≈ 80 K, which is well below the Curie temperature of the ferromagnetic La_2/3Ca_1/3MnO₃ layer. The exchange bias and coercive fields decay exponentially with temperature, which is commonly accepted to describe spin-glass (SG) behavior. The signatures of slow dynamics were confirmed by slow spin relaxation over a wide temperature regime. Low-energy muon spectroscopy experiments directly evidence the slowing down of the magnetic moments below ~100 K in the NFL. The experimental results indicate the SG nature of the NFL. This SG state can be understood within the context of the competing ferromagnetic and antiferromagnetic interactions of similar energies.

Chemical doping and epitaxy can be used to tailor the magnetoelectric properties of multiferroic thin films, such as SrMnO₃. Here, we study the dependence of the magnetic order temperatures of Sr_1−xBa_xMnO₃ thin films on epitaxial strain and Ba content. Combining low-energy muon spin spectroscopy and scanning transmission electron microscopy, the broadness of the magnetic transition is attributed to the presence of a Mn-O-Mn angle gradient along the out-of-plane direction. We also demonstrate that the unit cell volume is the key parameter to determine the Néel temperature in Sr_1−xBa_xMnO₃ thin films showing G-type antiferromagnetic order. The occurrence of a simultaneously ferroelectric and ferromagnetic ground state at high strain levels is suggested for the Sr_0.8Ba_0.2MnO₃ thin film deposited on TbScO₃.

The perovskite (Sr,Ba)MnO₃ system is an ideal candidate for tailoring electrical and magnetoelectric properties through the accurate control of Ba content and epitaxial strain due to the strong coupling between polar instability, spin order, and lattice. Here, first, the polar order is proved to be induced in Sr_1−xBa_xMnO₃ thin films through lattice expansion either by epitaxial strain or chemical pressure, which correlates with the evolution of the dielectric properties. Second, due to strong spin–phonon coupling, a large magnetoelectric response is found in the (Sr,Ba)MnO₃ system, in which the dielectric constant drops up to 50% when the antiferromagnetic order emerges, larger than most magnetoelectric oxides. More important, this coupling between magnetism and dielectric properties can be tuned from ≈18% to ≈50% by appropriately selecting Ba content and epitaxial strain. Third, a clear trend of increasing the band gap energy on increasing the unit cell volume either by epitaxial strain or chemical pressure is found, which opens the way for engineering the semiconducting properties of (Sr,Ba)MnO₃ system at will. Thus, this work proves the possibility to design the electrical response and the magnetoelectric coupling in (Sr,Ba)MnO₃ system.

Magnetization studies have been performed on a polycrystalline sample of Dy${}_{5}$Si${}_{3}$Ge as a function of an applied magnetic field (up to 50 kOe) and hydrostatic pressure (up to 10 kbar) in the 5\char21{}300 K temperature range. The anomalous behavior of the magnetic susceptibility indicates that a Griffiths-like phase exists at low magnetic fields and pressures up to 10 kbar. We present evidence that the high-temperature second-order ferromagnetic transition can be coupled with the low-temperature first-order crystallographic transformation into a single first-order magnetic-crystallographic transformation using a magnetic field and hydrostatic pressure as tuning parameters. The effect of pressure on the Griffiths-like phase is reported and analyzed in the framework of the complex competition between the interslab and intraslab magnetic interactions.

DOI: http://dx.doi.org/10.1103/PhysRevB.87.199904 ©2013 American Physical Society