Yuhao Bai

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

Artificial heterostructures based on LaNiO3 (LNO) have been widely investigated with the aim to realize the insulating antiferromagnetic state of LNO. In this work, we grew [(La0.7Sr0.3MnO3)5-(LaNiO3)n]12 superlattices on (001)-oriented SrTiO3 substrates by pulsed laser deposition and observed an unexpected exchange bias effect in field-cooled hysteresis loops. Through X-ray absorption spectroscopy and magnetic circular dichroism experiments, we found that the charge transfer at the interfacial Mn and Ni ions can induce a localized magnetic moment. A remarkable increase of exchange bias field and a transition from metal to insulator were simultaneously observed upon decreasing the thickness of the LNO layer, indicating the antiferromagnetic insulator state in 2 unit cells LNO ultrathin layers. The robust exchange bias of 745 Oe in the superlattice is caused by an interfacial localized magnetic moment and an antiferromagnetic state in the ultrathin LNO layer, pinning the ferromagnetic La0.7Sr0.3MnO3 layers together. Our results demonstrate that artificial interface engineering is a useful method to realize novel magnetic and transport properties.

Realization of resistive switching and magnetoresistance in ZnO/ZnO-Co composite materials

Combining resistive switching and magnetoresistance in a system exhibits great potential for application in multibit nonvolatile data storage. It is in significance and difficulty to seek a material with resistances that can be stably switched at different resistance states modulated by an electrical field and a magnetic field. In this paper, we propose a novel electrode/ZnO/ZnO-Co/electrode device in which the storage layer combines a nanostructured ZnO-Co layer and a ZnO layer. The device exhibits bipolar resistive switching characteristics, which can be explained by the accumulation of oxygen vacancies due to the migration of oxygen ions by external electrical stimuli and the contribution of Co particles in the ZnO-Co layer. Moreover, the magnetoresistance effect at room temperature can be observed in the device at high and low resistance states. Therefore, through electrical and magnetic control, four resistance states are achieved in this system, presenting a new possibility towards enhancing data densities by many folds.

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.

The Exchange Bias of LaMnO 3 /LaNiO 3 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.

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.

Magnetic properties and magnetic reversal process of exchange-coupled Nd2Fe14B/α″-Fe16N2 bilayers

The hysteresis loops and the magnetic reversal process of exchange-coupled Nd2Fe14B/α″-Fe16N2 bilayers, with a deviation angle of easy axis β from the applied magnetic field direction and different thicknesses of soft phase were investigated using the object-oriented micromagnetic framework. Results show that a deviation of easy axis in Nd2Fe14B/α″-Fe16N2 bilayers can affect the magnetic properties and significantly decrease the energy product and coercivity. The remanence increases, whereas the coercivity of the bilayers drops monotonically, with the increase in thickness of the soft magnetic layer (Ls). A well-oriented bilayer with Ls = 4 nm exhibits the largest maximum energy product (i.e., 881.67 kJ/m3), which is larger than that of the other common exchange spring materials that we calculated. Therefore, the proposed nanocomposite magnet is one of the most promising exchange spring systems to achieve maximum energy product.

The antiferromagnetic state in ultrathin LaNiO3 layer supported by long-range exchange bias in LaNiO3/SrTiO3/La0.7Sr0.3MnO3 superlattices

Research on the novel antiferromagnetic (AFM) state in ultrathin LaNiO3 (LNO) layers is very significant because bulk LNO is always exhibits paramagnetic state. Recently, an unexpected exchange bias effect has been observed in LNO-based heterostructures, but the reason for this effect is considered as the interfacial localized magnetic moment rather than the AFM state in ultrathin LNO layers. To this end, we have grown a prototypical superlattice composed of non-magnetic spacer layers as a LNO/spacer/La0.7Sr0.3MnO3 (LSMO) system to support the AFM state in ultrathin LNO layers. Verified through X-ray absorption spectroscopy measurements, the interfacial localized magnetic moment induced by charge transfer can be effectively eliminated by inserting a spacer layer. Moreover, the long-range exchange bias effect can still be observed in these LNO/spacer/LSMO superlattices, and the strength of the coupling decreases with increasing non-magnetic spacer layer thickness. Therefore, this phenomenon suggests that the antiferromagnetic state in an ultrathin LNO layer plays a key role in the long-range exchange bias. Our findings will provide important clues for simplifying the behavior of confusing systems.