arXiv subjects

L. B. Steren

Publications and source records attributed to L. B. Steren.

11 recordsLinked to original sources

arXiv2026

Open Nanoacoustic Resonators Based on SrTiO$_3$/YBa$_2$Cu$_3$O$_{7-x}$ Superlattices

We report the design and experimental demonstration of an open nanophononic cavity based on a hybrid oxide superlattice composed of SrTiO$_3$ (STO) and YBa$_2$Cu$_3$O$_{7-x}$ (YBCO), combined with a metallic Ni transducer for coherent phonon generation. The STO/YBCO periodic stack acts as an acoustic distributed Bragg reflector supporting confined longitudinal acoustic phonons in the sub-THz regime, while the Ni layer enables efficient ultrafast optical excitation and detection by time-domain Brillouin scattering. Transient reflectivity measurements reveal confined acoustic dynamics and a well-defined cavity resonance, in agreement with transfer-matrix calculations of acoustic reflectivity and mode profiles. These results demonstrate phonon confinement in multifunctional oxide heterostructures and establish complex oxide superlattices as a platform for hybrid nano-acoustic resonators and ultrafast phonon control of correlated electronic phases.

cond-mat.mes-hall↗

arXiv2024

Magnetic transition and spin-polarized two-dimensional electron gas controlled by polarization switching in strained CaMnO$_3$/BaTiO$_3$ slabs

$Ab$ $initio$ calculations show the presence of a strong magnetoelectric interfacial coupling in CaMnO$_3$ ultra-thin film grown on a strained BaTiO$_3$ ferroelectric film. This heterostructure presents a polarization driven magnetic transition from a G-type to an A-type antiferromagnetic structure. Together with this magnetic transition we find a metallic behaviour at the interface between these two insulators, where the charge character of the carriers can be tuned from electrons to holes by switching the electric polarization direction of the FE film. Besides, the electron gas is spin-polarized while the hole-gas is not.

cond-mat.mtrl-sci↗

arXiv2020

Strain induced magnetic transition in CaMnO$_3$ ultra thin films

The effect of high tensile strain and low dimensionality on the magnetic and electronic properties of CaMnO$_3$ ultrathin films, epitaxially grown on SrTiO$_3$ substrates, are experimentally studied and theoretically analyzed. By means of ab initio calculations, we find that, both, the high strain produced by the substrate and the presence of the free surface contribute to the stabilization of an in-plane ferromagnetic coupling, giving rise to a non-zero net magnetic moment in the ultrathin films. Coupled with this change in the magnetic order we find an insulator-metal transition triggered by the quantum confinement and the tensile epitaxial strain. Accordingly, our magnetic measurements in 3nm ultrathin films show a ferromagnetic hysteresis loop, absent in the bulk compound due to its G-type antiferromagnetic structure.

cond-mat.mtrl-sci↗

arXiv2013

R-M interactions in R2BaMO5 (R=Y or Gd and M=Cu or Zn)

R2BaMO5 (R = Gd, Y and M = Cu, Zn) oxides have been studied by specific heat, dc magnetic susceptibility, and electron paramagnetic resonance (EPR). For one member of the series without magnetic moment at M, namely Gd2BaZnO5, measurements reveal long range antiferromagnetic order at TN(Gd2BaZnO5) = 2.3(0.1)K, much lower than the Curie Weiss temperature of Θ=15.9(0.3)K. This indicates the existence of competing interactions that introduce a large degree of magnetic frustration in the system. For Y2BaCuO5 the Cu Cu interactions are responsible for the broad maximum in the magnetic contribution to the specific heat centered at 18.5(0.01)K that stretches beyond the instrumental limit of 25K. The strong Cu Cu interactions also present in Gd2BaCuO5, combined with the Gd-Cu interaction, polarize the Gd sublattice giving measurable contributions to the specific heat in the same temperature range. In addition, they broaden the Gd EPR line and saturate its integrated intensity. The ordering temperature of Gd ions is raised to TN(Gd2BaCuO5)=12.0(0.1)K.

cond-mat.mtrl-sci↗

arXiv2009

Direct observation of electronic inhomogeneities induced by point defect disorder in manganite films

We have investigated the influence of point defect disorder in the electronic properties of manganite films. Real-time mapping of ion irradiated samples conductivity was performed though conductive atomic force microscopy (CAFM). CAFM images show electronic inhomogeneities in the samples with different physical properties due to spatial fluctuations in the point defect distribution. As disorder increases, the distance between conducting regions increases and the metal-insulator transition shifts to lower temperatures. Transport properties in these systems can be interpreted in terms of a percolative model. The samples saturation magnetization decreases as the irradiation dose increases whereas the Curie temperature remains unchanged.

cond-mat.mtrl-sci↗

arXiv2009

Influence of ion implantation on the magnetic and transport properties of manganite films

We have used oxygen ions irradiation to generate controlled structural disorder in thin manganite films. Conductive atomic force microscopy CAFM), transport and magnetic measurements were performed to analyze the influence of the implantation process in the physical properties of the films. CAFM images show regions with different conductivity values, probably due to the random distribution of point defect or inhomogeneous changes of the local Mn3+/4+ ratio to reduce lattice strains of the irradiated areas. The transport and magnetic properties of these systems are interpreted in this context. Metal-insulator transition can be described in the frame of a percolative model. Disorder increases the distance between conducting regions, lowering the observed TMI. Point defect disorder increases localization of the carriers due to increased disorder and locally enhanced strain field. Remarkably, even with the inhomogeneous nature of the samples, no sign of low field magnetoresistance was found. Point defect disorder decreases the system magnetization but doesn t seem to change the magnetic transition temperature. As a consequence, an important decoupling between the magnetic and the metal-insulator transition is found for ion irradiated films as opposed to the classical double exchange model scenario.

cond-mat.mtrl-sci↗

arXiv2007

Detection of the magneto-structural phase coexistence in MnAs epilayers at a very early stage

We report on the appearance of magnetic stripes in MnAs/GaAs(100) epilayers at temperatures well below the ferromagnetic transition of the system. The study has been performed by ferromagnetic resonance experiments (FMR) on MnAs epilayers grown on (100) and (111) GaAs substrates. The FMR spectra of the MnAs/GaAs(100) samples at 180 K reveal the appearance of zones of different magnetic behavior with respect to the low-temperature homogeneous ferromagnetic phase. The angular and the temperature dependence of the spectra serve us to detect the inter-growth of the non-magnetic phase into the ferromagnetic phase at a very early stage of the process. The experimental data show that the new phase nucleates in a self-arranged array of stripes in MnAs/GaAs(100) thin films while it grows randomly in the same films grown on GaAs(111).

cond-mat.mtrl-sci↗

arXiv2006

Magnetization reversal and anomalous coercive field temperature dependence in MnAs epilayers grown on GaAs(100) and GaAs(111)B

The magnetic properties of MnAs epilayers have been investigated for two different substrate orientations: GaAs(100) and GaAs(111). We have analyzed the magnetization reversal under magnetic field at low temperatures, determining the anisotropy of the films. The results, based on the shape of the magnetization loops, suggest a domain movement mechanism for both types of samples. The temperature dependence of the coercivity of the films has been also examined, displaying a generic anomalous reentrant behavior at T$>$200 K. This feature is independent of the substrate orientation and films thickness and may be associated to the appearance of new pinning centers due to the nucleation of the $β$-phase at high temperatures.

cond-mat.mtrl-sci↗

arXiv2005

Thermal enhancement of the antiferromagnetic exchange coupling between Fe epilayers separated by a crystalline ZnSe spacer

We have put into evidence the existence of an antiferromagnetic coupling between iron epilayers separated by a ZnSe crystalline semiconductor. The effect has been observed for ZnSe spacers thinner than 4 nm at room-temperature. The coupling constant increases linearly with temperature with a constant slope of ~5.5x 10-9 J/m2K. The mechanisms that may explain such exchange interaction are discussed in the manuscript. It results that thermally-induced effective exchange coupling mediated by spin-dependent on and off resonant tunnelling of electrons via localized mid-gap defect states in the ZnSe spacer layer appears to be the most plausible mechanism to induce the antiferromagnetic coupling.

cond-mat.mtrl-sci↗

arXiv2004

Antiferromagnetism at the YBa2Cu3O7 / La2/3Ca1/3MnO3 interface

The magnetic properties of a series of YBa2Cu3O7-x/La2/3Ca1/3MnO3 (YBCO/LC1/3MO) superlattices grown by dc sputtering at high oxygen pressures (3.5 mbar) show the expected ferromagnetic behaviour. However, field cooled hysteresis loops at low temperature show the unexpected existence of exchange bias, effect associated with the existence of ferromagnetic/antiferromagnetic (F/AF) interfaces. The blocking temperature (TB) is found thickness dependent and the exchange bias field (HEB) is found inversely proportional to the FM layer thickness, as expected. The presence of an AF material is probably associated to interface disorder and Mn valence shift towards Mn4+.

cond-mat.mtrl-sci↗

arXiv2002

Impurity and band effects competition on the appearence of Inverse Giant Magnetoresistence in Cu/Fe multilayers with Cr

We have studied the dependence of impurity vs. band effects in the appearance of inverse giant magnetoresistance (IGMR) in Cu/Fe superlattices whit Cr. Current in plane (CIP) and current perpendicular to the plane (CPP) geometries are considered. For the calculation of the conductivities we have used the linearized Boltzmann equation in the relaxation time approximation. Cr impurity effects are taken into account through the spin dependent relaxation times and the band effects through the semiclassical velocities obtained from the LDA calculated electronic structure. The larger the Cr/Fe hybridization strength, the bigger is the tendency towards IGMR. In particular, in CIP geometry roughness at these interfaces increases the IGMR range. The results are compared with experiments and we conclude that the experimental GMR curves can only be explained if Cr bands are present.

cond-mat.mtrl-sci↗