arXiv subjects

C. Frontera

Publications and source records attributed to C. Frontera.

16 recordsLinked to original sources

arXiv2020

Self-assembled line network in BiFeO3 thin films

In this work we report on the controlled fabrication of a self-assembled line network in highly epitaxial BiFeO3 thin films on top of LaAlO3 in the kinetically limited grown region by RF sputtering. As previously shown in the case of manganite thin films, the remarkable degree of ordering is achieved using vicinal substrates with well-defined step-terrace morphology. Nanostructured BiFeO3 thin films show mixed-phase morphology. Besides typical formation following (100) and (010) axes, some mixed phase nanodomains are detected also in-between the regular line network. These particular microstructures open a playground for future applications in multiferroic nanomaterials.

cond-mat.mtrl-sci↗

arXiv2017

Band structure of CuMnAs probed by optical and photoemission spectroscopy

Tetragonal phase of CuMnAs progressively appears as one of the key materials for antiferromagnetic spintronics due to efficient current-induced spin-orbit torques whose existence can be directly inferred from crystal symmetry. Theoretical understanding of spintronic phenomena in this material, however, relies on the detailed knowledge of electronic structure (band structure and corresponding wave functions) which has so far been tested only to a limited extent. We show that AC permittivity (obtained from ellipsometry) and UV photoelectron spectra agree with density functional calculations. Together with the x-ray diffraction and precession electron diffraction tomography, our analysis confirms recent theoretical claim [Phys.Rev.B 96, 094406 (2017)] that copper atoms occupy lattice positions in the basal plane of the tetragonal unit cell.

cond-mat.mtrl-sci↗

arXiv2016

Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn

Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field. Moreover, a large list of aniferromagnetic semiconductors and metals with N\'eel temperatures above room temperature exists. In the present manuscript, we persevere in the quest for the limits of how large can anisotropic magnetoresistance be in antiferromagnetic materials with very large spin-orbit coupling. We selected IrMn as a prime example of first-class moment (Mn) and spin-orbit (Ir) combination. Isothermal magnetotransport measurements in an antiferromagnetic-metal(IrMn)/ferromagnetic-insulator thin film bilayer have been performed. The metal/insulator structure with magnetic coupling between both layers allows the measurement of the modulation of the transport properties exclusively in the antiferromagnetic layer. Anisotropic magnetoresistance as large as 0.15 % has been found, which is much larger than that for a bare IrMn layer. Interestingly, it has been observed that anisotropic magnetoresistance is strongly influenced by the field cooling conditions, signaling the dependence of the found response on the formation of domains at the magnetic ordering temperature.

cond-mat.mtrl-sci↗

arXiv2016

An invisible non-volatile solid-state memory

Information technologies require entangling data stability with encryption for a next generation of secure data storage. Current magnetic memories, ranging from low-density stripes up to high-density hard drives, can ultimately be detected using routinely available probes or manipulated by external magnetic perturbations. Antiferromagnetic resistors feature unrivalled robustness but the stable resistive states reported scarcely differ by more than a fraction of a percent at room temperature. Here we show that the metamagnetic (ferromagnetic to antiferromagnetic) transition in intermetallic Fe0.50Rh0.50 can be electrically controlled in a magnetoelectric heterostructure to reveal or cloak a given ferromagnetic state. From an aligned ferromagnetic phase, magnetic states are frozen into the antiferromagnetic phase by the application of an electric field, thus eliminating the stray field and likewise making it insensitive to external magnetic field. Application of a reverse electric field reverts the antiferromagnetic state to the original ferromagnetic state. Our work demonstrates the building blocks of a feasible, extremely stable, non-volatile, electrically addressable, low-energy dissipation, magnetoelectric multiferroic memory.

cond-mat.mtrl-sci↗

arXiv2015

Strain-induced nonsymmorphic symmetry breaking and removal of Dirac semimetallic nodal line in an orthoperovskite iridate

By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the Pbnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.

cond-mat.str-el↗

arXiv2015

Room-temperature antiferromagnetic memory resistor

The bistability of ordered spin states in ferromagnets (FMs) provides the magnetic memory functionality. Traditionally, the macroscopic moment of ordered spins in FMs is utilized to write information on magnetic media by a weak external magnetic field, and the FM stray field is used for reading. However, the latest generation of magnetic random access memories demonstrates a new efficient approach in which magnetic fields are replaced by electrical means for reading and writing. This concept may eventually leave the sensitivity of FMs to magnetic fields as a mere weakness for retention and the FM stray fields as a mere obstacle for high-density memory integration. In this paper we report a room-temperature bistable antiferromagnetic (AFM) memory which produces negligible stray fields and is inert in strong magnetic fields. We use a resistor made of an FeRh AFM whose transition to a FM order 100 degrees above room-temperature, allows us to magnetically set different collective directions of Fe moments. Upon cooling to room-temperature, the AFM order sets in with the direction the AFM moments pre-determined by the field and moment direction in the high temperature FM state. For electrical reading, we use an antiferromagnetic analogue of the anisotropic magnetoresistance (AMR). We report microscopic theory modeling which confirms that this archetypical spintronic effect discovered more than 150 years ago in FMs, can be equally present in AFMs. Our work demonstrates the feasibility to realize room-temperature spintronic memories with AFMs which greatly expands the magnetic materials base for these devices and offers properties which are unparalleled in FMs.

cond-mat.mtrl-sci↗

arXiv2014

Engineering the microstructure and magnetism of La$_2$CoMnO$_6$ thin films by tailoring oxygen stoichiometry

We report on the magnetic and structural properties of ferromagnetic-insulating La$_2$CoMnO$_6$ thin films grown on top of (001) STO substrates by means of RF sputtering technique. Careful structural analysis, by using synchrotron X-ray diffraction, allows identifying two different crystallographic orientations that are closely related to oxygen stoichiometry and to the features (coercive fields and remanence) of the hysteresis loops. Both Curie temperature and magnetic hysteresis turn out to be dependent on the oxygen stoichiometry. In situ annealing conditions allow tailoring the oxygen content of the films, therefore controlling their microstructure and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2013

Possible Scale Invariant Linear Magnetoresistance in Pyrochlore Iridates Bi$_2$Ir$_2$O$_7$

We report the observation of a linear magnetoresistance in single crystals and epitaxial thin films of the pyrochlore iridate Bi$_2$Ir$_2$O$_7$. The linear magnetoresistance is positive and isotropic at low temperatures, without any sign of saturation up to 35 Tesla. As temperature increases, the linear field dependence gradually evolves to a quadratic field dependence. The temperature and field dependence of magnetoresistance of Bi$_2$Ir$_2$O$_7$ bears strikingly resemblance to the scale invariant magnetoresistance observed in the strange metal phase in high Tc cuprates. However, the residual resistivity of Bi$_2$Ir$_2$O$_7$ is more than two orders of magnitude higher than the curpates. Our results suggest that the correlation between linear magnetoresistance and quantum fluctuations may exist beyond high temperature superconductors.

cond-mat.str-el↗

arXiv2013

Tuning the electronic properties of J_eff=1/2 correlated semimetal in epitaxial perovskite SrIrO3

We investigated the electronic properties of epitaxially stabilized perovskite SrIrO3 and demonstrated the effective strain-control on its electronic structure. Comprehensive transport measurements showed that the strong spin-orbit coupling renders a novel semimetallic phase for the J_eff=1/2 electrons rather than an ordinary correlated metal, elucidating the nontrivial mechanism underlying the dimensionality-controlled metal-insulator transition in iridates. The electron-hole symmetry of this correlated semimetal was found to exhibit drastic variation when subject to bi-axial strain. Under compressive strain, substantial electron-hole asymmetry is observed in contrast to the tensile side, where the electron and hole effective masses are comparable, illustrating the susceptivity of the J_eff=1/2 to structural distortion. Tensile strain also shrinks the Fermi surface, indicative of an increasing degree of correlation which is consistent with optical measurements. These results pave a pathway to investigate and manipulate the electronic states in spin-orbit-coupled correlated oxides, and lay the foundation for constructing 5d transition metal heterostructures.

cond-mat.str-el↗

arXiv2013

Solving and refining novel thin film phases using Cu X-ray radiation: the epitaxy-induced CuMnAs tetragonal phase

We present a combined experimental and computational method which enables the precise determination of the atomic positions in a thin film using CuKα radiation, only. The capabilities of this technique surpass simple structure refinement and allow solving unknown phases stabilized by substrate-induced stress. We derive the appropriate corrections to transform the measured integrated intensities into structure factors. Data collection was performed entirely on routinely available laboratory diffractometers (CuKα radiation); the subsequent analysis was carried out by single-crystal direct methods (δ recycling procedure) followed by the least-squares refinement of the structural parameters of the unit cell content. We selected an epitaxial thin film of CuMnAs grown on top of a GaAs substrate, which formed a crystal structure with tetragonal symmetry, differing from the bulk material which is orthorhombic. Here we demonstrate the new tetragonal form of epitaxial CuMnAs grown on GaAs substrate and present consistent high-resolution scanning transmission electron microscopy and stoichiometry analyses.

cond-mat.mtrl-sci↗

arXiv2012

Epitaxy-distorted spin-orbit Mott insulator in Sr2IrO4 thin films

High quality epitaxial thin films of Jeff=1/2 Mott insulator Sr2IrO4 with increasing in-plane tensile strain have been grown on top of SrTiO3(001) substrates. Increasing the in-plane tensile strain up to ~0.3% was observed to drop the c/a tetragonality by 1.2 %. X-ray absorption spectroscopy detected a strong reduction of the linear dichroism upon increasing in-plane tensile strain towards a reduced anisotropy in the local electronic structure. While the most relaxed thin film shows a consistent dependence with previously reported single crystal bulk measurements, electrical transport reveals a charge gap reduction from 200 meV down to 50 meV for the thinnest and most epitaxy-distorted film. We argue that the reduced tetragonality plays a major role in the change of the electronic structure, which is reflected in the change of the transport properties. Our work opens the possibility for exploiting epitaxial strain as a tool for both structural and functional manipulation of spin-orbit Mott systems.

cond-mat.mtrl-sci↗

arXiv2011

Valence change of praseodymium in Pr0.5Ca0.5CoO3 investigated by x-ray absorption spectroscopy

X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(+5)%] of Pr3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomic-like 4f1 states (Pr4+) and f- symmetry states on the O 2p valence band (Pr3+L states) indicative of a strong 4f- 2p interaction.

cond-mat.str-el↗

arXiv2006

High and low-temperature crystal and magnetic structures of epsilon-Fe2O3 and their correlation to its magnetic properties

The crystal and magnetic structures of the orthorhombic e-Fe2O3 have been studied by simultaneous Rietveld refinement of X-ray and neutron powder diffraction data in combination with Mossbauer spectroscopy, as well as magnetisation and heat capacity measurements. It has been found that above 150 K the e-Fe2O3 polymorph is a collinear ferrimagnet with the magnetic moments directed along the a axis, while the magnetic ordering below 80 K is characterised by a square-wave incommensurate structure. The transformation between these two states is a second order phase transition and involves subtle structural changes mostly affecting the coordination of the tetrahedral and one of the octahedral Fe sites. The temperature dependence of the e-Fe2O3 magnetic properties is discussed in the light of these results.

cond-mat.mtrl-sci↗

arXiv2005

Magnetoelectric Coupling in epsilon-Fe2O3

Nanoparticles of the ferrimagnetic epsilon-Fe2O3 oxide have been synthesized by sol-gel method. Here, we report on the measurements of the dielectric permittivity as a function of temperature, frequency and magnetic field. It is found that, coinciding with the transition from collinear ferrimagnetic ordering to an incommensurate magnetic state occurring at about 100 K, there is an abrupt change (about 30 %) of permittivity suggesting the existence of a magnetoelectric coupling in this material. Indeed, magnetic field dependent measurements at 100 K have revealed an increase of the permittivity by about 0.3 % in 6 T. Prospective advantages of epsilon-Fe2O3 as multiferroic material are discussed.

cond-mat.mtrl-sci↗

arXiv2004

Electronic self-doping of Mo-states in A2FeMoO6 (A=Ca, Sr and Ba) half-metallic ferromagnets - a Nuclear Magnetic Resonance study

A systematic study of (A,A')2FeMoO6 (A,A'=Ca, Sr, Ba) ferromagnetic oxides with double perovskite structure has been performed using 95,97Mo and 57Fe NMR spectroscopy. These oxides are isoelectronic but have substantially different Curie temperatures. The NMR analysis provides clear evidence that the magnetic moment at Mo sites is not constant but varies sensitively with the ionic size of the alkaline ions. The 95,97Mo frequency, and thus the electronic charge at Mo ions, is found to be smaller in Ba and Ca than in Sr-based oxides. The charge release from Mo sites is accompanied by an uptake at Fe sites, and thus a self-doping Fe-Mo process is observed. This process is controlled by relevant structural parameters: the Fe-O-Mo bond length and bending. A clear relationship between the Curie temperature and the magnetic moment and thus electron density at Mo sites has been disclosed. The relevance of these findings for the understanding of ferromagnetic coupling in double perovskites is discussed.

cond-mat.mtrl-sci↗

arXiv2002

Selective Spin-State Switch and Metal-Insulator Transition in \boldmath $\rm GdBaCo_2O_{5.5}$

Ultra-high resolution synchrotron diffraction data for $\rm GdBaCo_2O_{5.5}$ throw new light on the metal-insulator transition of Co$^{3+}$ Ba-cobaltites. An anomalous expansion of CoO$_6$ octahedra is observed at the phase transition on heating, while CoO$_5$ pyramids show the normal shrinking at the closing of the gap. The insulator-to-metal transition is attributed to a sudden excitation of some electrons in the octahedra ($t_{2g}^6$ state) into the Co $e_g$ band (final $t_{2g}^4e_g^2$ state). The $t_{2g}^5e_g^1$ state in the pyramids does not change and the structural study also rules out a $d_{3x^2-r^2}/d_{3y^2-r^2}$ orbital ordering at $T_{MI}$.

cond-mat.mtrl-sci↗