Magnetic "dead" layer at a complex oxide interface

Interface Effects on Magnetic Flux Pinning in La0.7Sr0.3MnO3/YBa 2Cu3O7-x Bilayers

The magnetic pinning properties of a ferromagnet/superconductor hybrid structure consisting of a La_0.7Sr_0.3MnO₃(LSMO) layer with various thicknesses on top of a fixed thickness YBa₂Cu₃O_7-x (YBCO) layer are investigated in this article. The existence of a weakly magnetic layer was identified at the interface between YBCO and LSMO by a ferromagnetic resonance (FMR) study. Magnetic moment and anisotropy of the interfacial layer were probed using the angular-dependent FMR study. This layer gives rise to an additional flux pinning contribution to the bulk magnetic pinning from the LSMO layer. Our study provides insight into the complex interface physics in the LSMO/YBCO bilayer system, promoting a new pathway for the development of novel flux pinning-related functionality.

Switchable electronic and enhanced magnetic properties of CrI3 edges

Owing to its novel electronic and magnetic properties, two-dimensional CrI₃ has great potential in the application of spintronic devices. However, as an inevitable line defect, the properties of the edges of CrI₃ remain elusive. Here, via first-principles calculations with spin-orbit coupling, we investigated the thermodynamic stabilities, electronic and magnetic properties of thirteen CrI₃ edges with different structures. We showed that zigzag edges are more stable than armchair edges, and a CrI₃ nanoribbon can be either metallic or insulating depending on its chemical growth conditions. The edge stability and associated electronic properties can be understood in terms of the octahedron ligand field and electron counting model. In most cases, both the magnetic moment and Curie temperature can be enhanced by edges, which are in startle contrast to the surfaces of three-dimensional ferromagnetic materials, where a magnetic dead layer is often observed.

Confinement-Induced Giant Spin-Orbit-Coupled Magnetic Moment of Co Nanoclusters in TiO2 Films

High magnetization materials are in great demand for the fabrication of advanced multifunctional magnetic devices. Notwithstanding this demand, the development of new materials with these attributes has been relatively slow. In this work, we propose a new strategy to achieve high magnetic moments above room temperature. Our material engineering approach invoked the embedding of magnetic nanoclusters in an oxide matrix. By precisely controlling pulsed laser deposition parameters, Co nanoclusters are formed in a 5 at % Co-TiO₂ film. The presence of these nanoclusters was confirmed using transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption fine structure. The film exhibits a very high saturation magnetization of 99 emu/cm³. Detailed studies using X-ray magnetic circular dichroism confirm that Co has an enhanced magnetic moment of 3.5 μ_B/atom, while the Ti and O also contribute to the magnetic moments. First-principles calculations supported our hypothesis that the metallic Co nanoclusters surrounded by a TiO₂ matrix can exhibit both large spin and orbital moments. Moreover, a quantum confinement effect results in a high Curie temperature for the embedded Co nanoclusters. These findings reveal that 1-2 nm nanoclusters that are quantum confined can exhibit very large magnetic moments above room temperature, representing a promising advance for the design of new high magnetization materials.

Field dependence of the ferromagnetic/superconducting proximity effect in a YBCO/STO/LCMO multilayer

The interaction between superconductivity and magnetism in spatially confined heterostructures of thin film multilayers is investigated in the ferromagnetic manganite La2/3Ca1/3MnO3 (LCMO) and the high-temperature superconductor YBa2Cu3O7-δ (YBCO) mediated by an intermediate insulating SrTiO3 (STO) layer. The STO layer is used to mediate and tune the range of interactions between the ferromagnet and superconductor. A magnetically depleted layer with zero-magnetisation within the LCMO layer is shown by polarised neutron reflectometry measurements. This zero-magnetisation layer is caused by the onset of superconductivity in YBCO despite being separated by an insulating layer with a thickness much larger than the superconducting coherence length. The magnetic field dependence of this interaction is also explored. We show that the magnetism of the depleted layer can be restored by applying a magnetic field that partially destroys the superconductivity in YBCO, restricting the electronic interaction between the materials.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

Modulated self-reversed magnetic hysteresis in iron oxides

The steadfast rule of a ferromagnetic hysteresis loop claims its saturation positioned within the first and third quadrants, whereas its saturation positioned in the second and fourth quadrants (named as self-reversed magnetic hysteresis) is usually taken as an experimental artifact and is always intentionally ignored. In this report, a new insight in this unique hysteresis phenomenon and its modulation were discussed in depth. Different iron oxides (magnetite, maghemite and hematite) with varying dimensions were soaked in FeCl₃ aqueous solution and absorbed Fe³⁺ cations due to their negative enough surface zeta potentials. These iron oxides@Fe³⁺ core-shell products exhibit well pronounced self-reversed magnetic hysteresis which concurrently have typical diamagnetic characteristics and essential ferromagnetic features. The presence of pre-magnetized Fe³⁺ shell and its negatively magnetic exchange coupling with post-magnetized iron-oxide core is the root cause for the observed phenomena. More strikingly, this self-reversed magnetic hysteresis can be readily modulated by changing the core size or by simply controlling Fe³⁺ concentration in aqueous solution. It is anticipated that this work will shed new light on the development of spintronics, magnetic recording and other magnetically-relevant fields.

Induced ferromagnetism at BiFeO3/YBa2Cu3O7 interfaces

Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from high-temperature superconductivity and giant magnetoresistance to magnetism and ferroelectricity. In addition, when TMOs are interfaced with each other, new functionalities can arise, which are absent in individual components. Here, we report results from first-principles calculations on the magnetism at the BiFeO₃/YBa₂Cu₃O₇ interfaces. By comparing the total energy for various magnetic spin configurations inside BiFeO₃, we are able to show that a metallic ferromagnetism is induced near the interface. We further develop an interface exchange-coupling model and place the extracted exchange coupling interaction strengths, from the first-principles calculations, into a resultant generic phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the presence of a hysteresis loop in field-dependent magnetization data. The emergence of interfacial ferromagnetism should have implications to electronic and transport properties.

The onset of ferromagnetism and superconductivity in [La0.7Sr0.3MnO3(n u.c.)/YBa2Cu3O7(2 u.c.)]20 superlattices

With the aim of studying the interface magnetism, the onset of ferromagnetism and the onset of the transition to the superconducting state a series of [La0.7Sr0.3MnO3(n u.c.)/YBa2Cu3O7(2 u.c.)]20 (LSMO/YBCO) superlattices with nominally varying layer thickness of the LSMO from one to four unit cells (u.c.) was prepared and characterized by x-ray diffraction, electronic transport, magnetization and ferromagnetic resonance measurements. Spontaneous magnetization was observed for a superlattice with four u.c. LSMO layer thickness in a multilayer structure. Superlattices with 3 u.c. of LSMO and lower layer thicknesses did not show a signature of ferromagnetism. The onset of superconductivity was observed for superlattices with one and two LSMO layer u.c. thickness.

Magnetic proximity effect in YBa2Cu3O7/La(2/3)Ca(1/3)MnO3 and YBa2Cu3O7/LaMnO(3+δ) superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu3O7 and ferromagnetic-metallic La0.67Ca0.33MnO3 or ferromagnetic-insulating LaMnO(3+δ). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO3 and almost absent for ferromagnetic-insulating LaMnO(3+δ). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

Control of octahedral tilts and magnetic properties of perovskite oxide heterostructures by substrate symmetry

Perovskite transition-metal oxides are networks of corner-sharing octahedra whose tilts and distortions are known to affect their electronic and magnetic properties. We report calculations on a model interfacial structure which avoids chemical influences and show that the symmetry mismatch imposes an interfacial layer with distortion modes that do not exist in either bulk material, creating new interface properties driven by symmetry alone. Depending on the resistance of the octahedra to deformation, the interface layer can be as small as one unit cell or extend deep into the thin film.

Interface ferromagnetism and orbital reconstruction in BiFeO3-La(0.7)Sr(0.3)MnO3 heterostructures

We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with ferromagnet La(0.7)Sr(0.3)MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L(2,3) edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge.

Low-temperature magnetic and transport anisotropy in manganite thin films

The stability of striped magnetic phases in films of La(1-x)A(x)MnO(3) perovskites is investigated. A variational analysis is developed for different film thicknesses at fixed hole density (x = 0.3) and the competition among magnetic phases as a function of the transfer integral and the temperature is analyzed. The stabilization of an in-plane striped magnetic phase is observed with reducing the film thickness at low temperatures below the metal-insulator transition temperature. Within the adopted variational scheme, treating perturbatively the residual electron-phonon interaction, the dependence of the in-plane resistivity on temperature for different thicknesses is calculated. At low temperatures, due to the striped magnetic phase, the resistivity shows an important in-plane anisotropy. The obtained results are found to be consistent with experiments.