Robust Interfacial Exchange Bias and Metal-Insulator Transition Influenced by the LaNiO3 Layer Thickness in La0.7Sr0.3MnO3/LaNiO3 Superlattices

Atomic scale interfacial magnetism and origin of metal-insulator transition in (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices: a first principles study

Interfacial magnetism and metal-insulator transition at LaNiO[Formula: see text]-based oxide interfaces have triggered intense research efforts, because of the possible implications in future heterostructure device design and engineering. Experimental observation lack in some points a support from an atomistic view. In an effort to fill such gap, we hereby investigate the structural, electronic, and magnetic properties of (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices with varying LaNiO[Formula: see text] thickness (n) using density functional theory including a Hubbard-type effective on-site Coulomb term. We successfully capture and explain the metal-insulator transition and interfacial magnetic properties, such as magnetic alignments and induced Ni magnetic moments which were recently observed experimentally in nickelate-based heterostructures. In the superlattices modeled in our study, an insulating state is found for n=1 and a metallic character for n=2, 4, with major contribution from Ni and Mn 3d states. The insulating character originates from the disorder effect induced by sudden environment change for the octahedra at the interface, and associated to localized electronic states; on the other hand, for larger n, less localized interfacial states and increased polarity of the LaNiO[Formula: see text] layers contribute to metallicity. We discuss how the interplay between double and super-exchange interaction via complex structural and charge redistributions results in interfacial magnetism. While (LaNiO[Formula: see text])[Formula: see text]/(CaMnO[Formula: see text])[Formula: see text] superlattices are chosen as prototype and for their experimental feasibility, our approach is generally applicable to understand the intricate roles of interfacial states and exchange mechanism between magnetic ions towards the overall response of a magnetic interface or superlattice.

Dimensionality control of magnetic coupling at interfaces of cuprate-manganite superlattices

The dimensionality of the crystal structure plays a vital role in artificial heterostructures composed of different transition metal oxides. Nonlinear layer-thickness dependence of the exchange bias effect was observed in high-quality SrCuO₂/La_0.7Sr_0.3MnO (LSMO) superlattices induced in the present work by dimensional evolution. In the SCO(n)/LSMO(8) superlattices with thickness below the critical value (5 u.c.), the exchange bias effect decreased and the saturated magnetization increased with increase in SCO thickness. By contrast, the exchange bias effect increased and the saturated magnetization decreased in S(n)L(8) superlattices with thickness above the critical value. This is because the lattice SCO material underwent a breathing-like structural transformation from the planar to a chain-like structure. The results indicate the interfacial superexchange coupling mainly present in the chain-like S(n)L(8) superlattices through X-ray absorption spectroscopy and first principles calculations. This superexchange coupling generated a weak localized magnetic moment to pin the adjacent ferromagnetic layer. However, in the thicker S(n)L(8) superlattices, evolution of magnetic properties was induced by the long-range antiferromagnetic order in the planar SCO layer. Our findings demonstrate that the dimensionality driven structural variation is an effective method to manipulate the electronic reconstruction and the associated physical properties, paving a pathway for the advancement of strongly correlated materials.

Strategy to Induce Multiferroic Property in (RTiO3 )n /(RVO3 )n Superlattices: A First-Principles Study

By first-principles calculations, lanthanide contraction is applied on a 1/1 (with symmetric center) and a 2/2 (with non-centrosymmetric polar structure) RTiO₃ /RVO₃ superlattices to realize quasi-continuous structural distortion modulation. The strong correlations of microscopic structural distortion, magnetic coupling and charge disproportionation accompanying metal-insulator transition (MIT) are clarified. It is found that the effect of lanthanide contraction on the 1/1 and 2/2 RTiO₃ /RVO₃ superlattices can induce ferromagnetic to antiferromagnetic transition within ab VO₂ plane and the MIT occurs within these superlattices. And the MIT phenomenon is attributed to the charge disproportionation on V sites caused by the magnetic coupling transition. More structural distortion in the 2/2 RTiO₃ /RVO₃ superlattice is necessary than that of the 1/1 RTiO₃ /RVO₃ superlattice to induce the similar magnetic and MIT transition originating from the smaller interface/volume ratio. Based on these results, combining lanthanide contraction and epitaxial strain effects, multiferroic property is realized on 2/2 YTiO₃ /YVO₃ superlattice. Among all the structural parameters, aspect ratio c/a and Ti-O-V bond angles along the [001] direction are found to play the vital roles in the relevant transition process. Therefore, our calculations provide a microscopic guidance to design and synthesize new multiferroic materials.

Defect-Induced Exchange Bias in a Single SrRuO3 Layer

Exchange bias stems from the interaction between different magnetic phases, and therefore, it generally occurs in magnetic multilayers. Here, we present a large exchange bias in a single SrRuO₃ layer induced by helium ion irradiation. When the fluence increases, the induced defects not only suppress the magnetization and the Curie temperature but also drive a metal-insulator transition at a low temperature. In particular, a large exchange bias field up to ∼0.36 T can be created by the irradiation. This large exchange bias is related to the coexistence of different magnetic and structural phases that are introduced by embedded defects. Our work demonstrates that spintronic properties in complex oxides can be created and enhanced by applying ion irradiation.

Observation of Superconductivity in the LaNiO3/La0.7Sr0.3MnO3 Superlattice

In the pursuit of high-temperature superconductivity like that in cuprates, artificial heterostructures or interfaces have attracted tremendous interest. It has been a long-sought goal to find similar unconventional superconductivity in nickelates. However, as far as we know, this has not yet been experimentally realized. To approach this objective, we synthesized a prototypical superlattice that consists of ultrathin LaNiO₃ and La_0.7Sr_0.3MnO₃ layers. Both zero resistance and the Meissner effect are observed using resistive and magnetic measurements of the superlattice. These are experimental indicators for superconductivity in new superconductors. X-ray linear dichroism causes the NiO₂ planes to develop electron-occupied x²-y² orbital order similar to that of cuprate-based superconductors. Our findings demonstrate that artificial interface engineering is suitable for investigating novel physical phenomena, such as superconductivity.

Interfacial Spin Glass State and Exchange Bias in the Epitaxial La0.7Sr0.3MnO3/LaNiO3 Bilayer

We study the magnetic properties of an epitaxial growth bilayer composed of ferromagnetic La0.7Sr0.3MnO3 (LSMO) and paramagnetic LaNiO3 (LNO) on SrTiO3 (STO) substrates. We find that the stack order of the bilayer heterostructure plays a key role in the interfacial coupling strength, and the coupling at the LSMO(top)/LNO(bottom) interface is much stronger than that at the LNO(top)/LSMO(bottom). Moreover, a strong spin glass state has been observed at the LSMO/LNO interface, which is further confirmed by two facts: first, that the dependence of the irreversible temperature on the cooling magnetic field follows the Almeida-Thouless line and, second, that the relaxation of the thermal remnant magnetization can be fitted by a stretched exponential function. Interestingly, we also find an exchange bias effect at the LSMO/LNO bilayer below the spin glass freezing temperature, indicating that the exchange bias is strongly correlated with the spin glass state at its interface.

Exchange Bias Effect and Orbital Reconstruction in (001)-Oriented LaMnO3/LaNiO3 Superlattices

Paramagnetic LaNiO₃ (LNO)-based heterostructures have been attracting the attention of researches, especially since the interesting exchange bias (EB) effect has been observed in (111)-oriented LaMnO₃ (LMO)/LNO superlattices (SLs). However, this effect is not expected to occur in the (001) direction SLs. In this paper, we report the observation of an unexpected EB effect in (001)-oriented (LMO)₃/(LNO)_t SLs. The orbits of interfacial Mn/Ni ions preferentially occupy the strain-stabilized x² - y² in ultrathin LNO layers [t ≤ 4 unit cells (u.c.)]. Conversely, as the LNO layer becomes thicker (t ≥ 6 u.c.), the EB effect is absent, and the orbits are reconstructed to form the 3z² - r² preferential occupancy. The absence of the EB in thicker LNO-based SLs is attributed to the interfacial charge transfer suppressed by orbital reconstruction as a consequence of the increasing LNO thickness. In the thinner LNO-based SLs, the larger charge transfer results in stronger localized magnetic moments for the cause of the EB effect. These results provide a useful interpretation of the relationship between macroscopic magnetic properties and the microscopic electronic structure in oxide-based heterostructures.

Electrically tunable tunneling rectification magnetoresistance in magnetic tunneling junctions with asymmetric barriers

The development of multifunctional spintronic devices requires simultaneous control of multiple degrees of freedom of electrons, such as charge, spin and orbit, and especially a new physical functionality can be realized by combining two or more different physical mechanisms in one specific device. Here, we report the realization of novel tunneling rectification magnetoresistance (TRMR), where the charge-related rectification and spin-dependent tunneling magnetoresistance are integrated in Co/CoO-ZnO/Co magnetic tunneling junctions with asymmetric tunneling barriers. Moreover, by simultaneously applying direct current and alternating current to the devices, the TRMR has been remarkably tuned in the range from -300% to 2200% at low temperature. This proof-of-concept investigation provides an unexplored avenue towards electrical and magnetic control of charge and spin, which may apply to other heterojunctions to give rise to more fascinating emergent functionalities for future spintronics applications.

The Exchange Bias of LaMnO3/LaNiO3 Superlattices Grown along Different Orientations

With the goal of observing and explaining the unexpected exchange bias effect in paramagnetic LaNiO₃-based superlattices, a wide range of theoretical and experimental research has been published. Within the scope of this work, we have grown high-quality epitaxial LaMnO₃(n)-LaNiO₃(n) (LMO/LNO) superlattices (SLs) along (001)-, (110)-, and (111)-oriented SrTiO₃ substrates. The exchange bias effect is observed in all cases, regardless of growth orientation of the LMO/LNO SLs. As a result of a combination of a number of synchrotron based x-ray spectroscopy measurements, this effect is attributed to the interfacial charge transfer from Mn to Ni ions that induces localized magnetic moments to pin the ferromagnetic LMO layer. The interaction per area between interfacial Mn and Ni ions is nearly consistent and has no effect on charge transfer for different orientations. The discrepant charge transfer and orbital occupancy can be responsible for the different magnetic properties in LMO/LNO superlattices. Our experimental results present a promising advancement in understanding the origin of magnetic properties along different directions in these materials.