Search

Rare-earth orthoferrite SmFeO3 is an outstanding single-phase multiferroic material, holding great potential in novel low-power electronic devices. Nevertheless, simultaneous magnetic and ferroelectric orders as well as magnetoelectric (ME) coupling effect at room temperature (RT) in this system have not been demonstrated yet. In this study, epitaxial SmFeO3 films were successfully prepared onto tensile-strain Nb-SrTiO3 (Nb-STO) substrates by a pulsed laser deposition (PLD) method. Measurement results show that the films exhibit obvious ferromagnetic and ferroelectric orders at RT. Meanwhile, the magnetic anisotropy gradually changes from out-of-plane (OP) to in-plane (IP) direction with increasing film thickness, which is attributed to the variations of O 2p-Fe 3d hybridization intensity and Fe 3d-orbit occupancy caused by the strain-relaxed effect. Moreover, electrically driven reversible magnetic switching further proves that the SmFeO3 films exhibit the RT ME coupling effect, suggesting promising applications in new-generation electric-write magnetic-read data storage devices.

Thin films with perpendicular magnetic anisotropy (PMA) play an essential role in the development of technologies due to their excellent thermal stability and potential application in devices with high density, high stability, and low energy consumption. Many studies have focused on the relationship between the resistivity of heavy metals and the PMA of the neighbouring magnetic metals in magnetic multi-layered films. However, reports on the effects of heavy metals non-adjacent to the magnetic metals on the PMA are rare. Herein, we demonstrate the influence of the heavy metal Ta underlayer non-adjacent to the magnetic Co layer on the PMA and thermal stability in the Ta/Pt/Co/Pt heterostructures. A type of amorphous Ta film having an ultra-high resistivity (ρmax = 3.9 × 105 μΩ cm) was optimized by DC sputtering at a high sputtering Ar pressure, low sputtering power, and large target-to-substrate distance. The value of resistivity is three orders of magnitude higher than that of the β-Ta underlayer. We found that this special Ta underlayer can effectively improve the PMA and thermal stability of the magnetic Co layer based on the anomalous Hall and planar Hall effect measurements. The maximum magnetic anisotropic field reaches 1.1 T at a low temperature. It is very likely that the ultra-high resistivity leads to the increase in the additional electron scattering in the Ta/Pt interface, while the latter results in the enhancement of the PMA and thermal stability in the structure. These results reveal the inherent relationship between the resistivity of the heavy metal underlayer and PMA, and provide a novel approach to improve the PMA and thermal stability of heavy metal/magnetic metal multi-layered films.

In this work, epitaxial growth of LaMnO3 thin films on different substrates using pulsed laser deposition under tensile and compressive strain was studied. The intrinsic exchange bias effect was observed in the single A-type antiferromagnetic LaMnO3 films no matter whether the tensile or compressive strain was supplied by the substrates. Due to the lattice mismatch between the film and different substrates, the intense strain can induce MnO6 octahedral rotation in the bottom region of the film neighboring the substrate, which leads to the distortion of MnO6 octahedron and the net magnetic behavior. However, the upper part maintains the original A-type antiferromagnetic order due to strain relaxation. The exchange bias effect in single films is attributed to the coupling between the bottom canted magnetic part and the upper antiferromagnetic region. The observation of exchange bias in single films on different substrates enables the emergence of a new class of biasing components in spintronics, which are based on strain-engineering.

${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ ($\mathrm{YIG}$) thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are reported. The films are deposited by magnetron sputtering at room temperature first and then annealed in ${\mathrm{O}}_{2}$ at high temperature. The substrates are ${\mathrm{Gd}}_{3}({\mathrm{Sc}}_{2}{\mathrm{Ga}}_{3}){\mathrm{O}}_{12}$, which share the same crystalline structure as $\mathrm{YIG}$, but have a lattice constant slightly larger than that of $\mathrm{YIG}$; the lattice mismatching gives rise to an out-of-plane compressive strain and PMA in the $\mathrm{YIG}$ films. The PMA is confirmed by vibrating sample magnetometer, magneto-optical Kerr effect, anomalous Hall effect, and angle-dependent ferromagnetic resonance (FMR) measurements. The damping of the films is analyzed through frequency-dependent FMR measurements. As an example, an 8-nm-thick $\mathrm{YIG}$ film shows an effective PMA field of about 2800 Oe, a nearly square hysteresis loop, and a damping constant of only $4.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. As an illustration of possible applications of such films in spintronic devices, current-induced switching of the magnetization of the PMA $\mathrm{YIG}$ films is demonstrated by the use of $\mathrm{YIG}$/$\mathrm{Pt}$ bilayered Hall bar devices.

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 La0.7Sr0.3MnO3 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.

Recent theoretical studies indicated that the Curie temperature of perovskite manganite thin films can be increased by more than an order of magnitude by applying appropriate interfacial strain to control orbital ordering. In this work, we demonstrate that the regular intercalation of BaTiO3 layers between La0.67Sr0.33MnO3 layers effectively enhances ferromagnetic order and increases the Curie temperature of La0.67Sr0.33MnO3/BaTiO3 superlattices. The preferential orbital occupancy of eg(x 2 –y 2 ) in La0.67Sr0.33MnO3 layers induced by the tensile strain of BaTiO3 layers is identified by X-ray linear dichroism measurements. Our results reveal that controlling orbital ordering can effectively improve the Curie temperature of La0.67Sr0.33MnO3 films and that in-plane orbital occupancy is beneficial to the double exchange ferromagnetic coupling of thin-film samples. These findings create new opportunities for the design and control of magnetism in artificial structures and pave the way to a variety of novel magnetoelectronic applications that operate far above room temperature.

Nanoscale manipulation of the transport and magnetic properties of a material offers a distinguished possibility for the development of novel multifunctional devices with ultra-small dimensions, high densities and low power consumption. This is highly desirable for promising applications in nonvolatile data storage. Here, a novel device with multilayered NiOx/Pt (x < 1) nanowire arrays as the switching matrix was constructed by a simple electrodeposition method combining a partial oxidization process. The device exhibits bipolar resistive switching behavior with free-forming, low switching voltage, and stable endurance properties. More importantly, its magnetoresistance and magnetic properties including saturated magnetization and coercivity can be simultaneously tuned during the resistive switching process. Both the Ni-rich region constructed through electric field-induced oxygen ion migration and the pre-existence of Ni nanoparticles in NiOx segments are responsible for the resistive switching and the corresponding modulation of the magnetoresistance and magnetic properties. This work provides a simple yet effective approach for the modulation of resistance and magnetization through electrical and magnetic control, which can provide a new way of designing a multifunctional magnetoelectric device.

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...

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.

A SiO 2 -Co film with a Pt bottom electrode and an Au top electrode was deposited by magnetron sputtering at room temperature. The sample exhibits bipolar resistive switching properties. Both oxygen vacancies and Co nanoparticles in the SiO 2 -Co film contribute to resistive switching properties and the formation of conducting filaments. The sample, at high and low resistance states, exhibits magnetoresistance at 300 K and 10 K, respectively; this may be due to spin-dependent tunneling between Co particles through the SiO 2 barriers. Thus, the combination of resistive switching and magnetoresistance in a simple Pt/SiO 2 -Co/Au structure leads to the formation of multiple resistance states, which is promising for future applications of multiple-state non-volatile storage.

Mg doped ZnO films with hexagonal wurtzite structure were deposited on c-cut sapphire Al2O3 substrates by pulsed laser deposition. Both room temperature ferromagnetism and band gap of the films simultaneously tuned by the concentration of oxygen vacancies were performed. Our results further reveal that the singly occupied oxygen vacancies should be responsible for the room temperature ferromagnetism and band gap narrowing. Singly occupied oxygen vacancies having the localized magnetic moments form bound magnetic polarons, which results in a long-range ferromagnetic ordering due to Mg doping. Moreover, band gap narrowing of the films is probably due to the formation of impurity band in the vicinity of valence band, originating from singly occupied oxygen vacancies. These results may build a bridge to understand the relationship between the magnetic and optical properties in oxide semiconductor, and are promising to integrate multiple functions in one system.

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...

We demonstrate the synthesis of Fe 1−x Cr x Te 2 (0≤x≤0.6) nanoparticles by high-temperature organic-solution-phase method. The microstructure and magnetic properties were investigated by X-ray diffraction, transmission electron microscope and vibrating sample magnetometer. Cr doping can effectively improve the magnetic properties and morphology of FeTe 2 . Compared with the undoped FeTe 2 , Fe 1−x Cr x Te 2 spherical nanoparticles exhibit fine grain size of about 5 nm and uniform distribution. An ultrahigh room-temperature saturation magnetization of 7.34 emu/g can be obtained for Fe 0.85 Cr 0.15 Te 2 . It is 20 times higher than that of pure FeTe 2 . The lattice expansion caused by Cr doping is favorable for the enhancement of Fe–Fe and Fe–Cr ferromagnetic interaction in Fe 1−x Cr x Te 2 . Cr doping may be an effective approach to improve the magnetic properties of the other transition-metal chalcogenides.

Co/ZnO:Al granular films were made on glass substrates by sequential magnetron sputter deposition of ultrathin Co layer and ZnO:Al layer at room temperature. The as-deposited films consist of superparamagnetic Co particles dispersed in ZnO:Al (~2% Al) semiconductor matrix. Distinguished magnetoresistance effect at room temperature was obtained in the as-deposited films, which obviously reduced after annealing due to the growth of Co particles. The size of important magnetic particles was analyzed by Langevin function for hysteresis loops and magnetoresistance curves at room temperature. It was found that small magnetic particle contribute to magnetoresistance behavior and large particles dominate the room temperature magnetism in Co/ZnO:Al granular films.

One-dimensional magnetic nanowires have attracted much attention in the last decades due to their unique physical properties and potential applications in magnetic recording and spintronics. In this work, ordered arrays of Co/Cu multilayered nanowires which can be exploited to develop magnetoresistive sensors were successfully prepared using porous anodic alumina (PAA) templates. The structure and morphology of the multilayered nanowire arrays were characterized by transmission electron microscopy and scanning electron microscopy. The nanowire arrays are highly ordered and the average diameter is about 50 nm, which is controlled by the pore diameter of the PAA templates. The influences of period number and Cu layer thickness on the magnetic and the giant magnetoresistance (GMR) properties were investigated. The coercivity and remanence ratio increase first and then gradually tend to be stable with the increase of period number and the Cu layer thickness, while the GMR ratio increases first and then decreases with the increase of the period number accompanied by an oscillatory behavior of GMR as the Cu layer thickness changes, which are ascribed to the spin dependence electron scattering in the multilayers. The optimum GMR of −13% appears at Co (50 nm)/Cu (5 nm) with 200 deposition cycles in our experimental conditions.

Ionic liquid gating has emerged as an attractive means of manipulating the magnetic properties of materials. Here, we report ionic liquid gating modulating the ferromagnetism of a Co film. A weak and strong saturation magnetization can be reversibly obtained by applying a negative voltage and a positive one, respectively. The modulation may be related with the oxidization and reduction of Co. Negative (or positive) gating voltages drive the oxygen ions to inject into (or extract from) Co layer of the sample, which oxidizes (or reduces) Co. These gating processes are reversible. Moreover, the magnetism of the gated states is considerably stable. It is a significant step towards designing low-power electric-controlling magnetic devices in future.

Tunneling anisotropic magnetoresistance (TAMR) in the magnetic tunnel junctions (MTJs) driven by spin–orbit coupling (SOC) has attracted much attention for potential spintronic applications based on magnetic manipulation of electric transport. However, it has been believed that heavy metals are indispensable for the large TAMR. Here we experimentally show a TAMR of up to 46% in a La0.7Sr0.3MnO3 (LSMO) based MTJ without heavy metals. This value represents the largest TAMR for MTJs with half-metallic electrodes. We demonstrate that the TAMR ratio is enhanced by over one order of magnitude by tuning interface termination layer, achieving quasilocalized states at the interface Fermi level on the basis of magnetization orientation. These results are also supported by our first-principles density functional calculations. This finding illustrates a crucial role of interface tuning in the TAMR effect, opening a route for engineering the large anisotropic magnetoresistance by interface termination control and manipulating high spin polarization without using heavy metals.

The ferroelectric HfO2 thin film has attracted a lot of research interest due to being Pb free and its excellent compatibility with the Si-based semiconductor process. However, methods to obtain thicker HfO2 thin films with strong ferroelectricity have yet to be explored. In this work, a 50 nm-thick La-doped HfO2 thin film was prepared using pulsed laser deposition, and significant room temperature ferroelectricity with a remnant polarization (Pr) of 27 µC/cm2 was achieved through annealing in N2 with a rapid-heating-temperature process. The ferroelectricity is mainly related to the increase in the content of the (002)-oriented orthogonal phase formed by the rapid-heating-temperature treatment. Furthermore, this special annealing process was verified in a 50 nm-thick Tm-doped HfO2 film, and the Pr of 48 µC/cm2 was observed. This value is the highest value reported so far in doped HfO2 films with a thickness of 50 nm or greater. These results provide a new approach to prepare thicker ferroelectric HfO2-based thin films.

The interfacial Dyzaloshinskii-Moriya interaction (DMI) plays a crucial role in stabilizing a chiral Neel-type magnetic domain wall in perpendicularly magnetized multilayers. In this work, we investigate the effect of oxide layers on interfacial DMI in ultrathin Pt/Co/SmOx and Pt/Co/AlOx stacks with the perpendicular magnetic anisotropy. The DMI effective field was characterized using the field driven domain wall creep and current-induced hysteresis loop shift methods. The experimental results show that an enhanced interfacial DMI is obtained in Pt/Co/SmOx stacks, which may be related to the oxidation degrees of the Co layer in view of the difference of Co/metallic oxide interfaces. Moreover, the X-ray photoelectron spectroscopy with the argon ion etching technique was used to further verify the weaker oxidation of the Co layer existed in Pt/Co/SmOx than that in Pt/Co/AlOx in the same annealing process due to the smaller electronegativity of the Sm compared to the Al element. Our results provide a promising route to effectively manipulate the interfacial DMI in spin-orbit-torque-based spintronic devices through the element electronegativity.

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.

In this work, we report the resistance switching behavior and its modulating magnetic properties and magnetoresistance of an Au/ZnO-Co/SiO2-Co/Pt structure. The ZnO-Co/SiO2-Co bilayers prepared by magnetron sputtering at room temperature contain Co nanoparticles embedded in the oxide matrices. Repeatable bipolar resistance switching is observed in the Au/ZnO-Co/SiO2-Co/Pt structure, accompanied with a change of magnetization and magnetoresistance. At high/low resistance state, the sample exhibits weak/strong saturation magnetization and large/small magnetoresistance ratio. The accumulation of oxygen vacancies and the appearance of oxygen-deficient ZnO1-x, more Co atoms and Zn atoms in the switching areas due to the motion of oxygen ions under electric stimuli may be counted for the occurrence of the resistive switching effect. During the processes, the variations of the number of oxygen vacancies and Co atoms offer the opportunity to modulate magnetoresistance and ferromagnetism of the sample. This work shows that the resistance, magnetoresistance and magnetic properties of the Au/ZnO-Co/SiO2-Co/Pt structure can be electrically tuned, which may supply clues for designing multifunctional devices.

Heavy metal/ferromagnetic metal bi-layered structures that exhibit both strong perpendicular magnetic anisotropy (PMA) and large spin-orbit torque (SOT) efficiency have high potential in high-density, low-power memory, and logic device applications. Here, we report the enhancement of PMA and SOT in Ta/Pt/Co/Ta multi-layered heterostructures through interfacial diffusion. The structures can exhibit PMA fields of 9100 Oe at 300 K and 14100 Oe at 10 K and an effective spin Hall angle (SHA) of 0.61 ± 0.03 at 300 K. These values are larger than the corresponding values reported previously for similar heterostructures. The current-induced magnetization switching was demonstrated. The critical switching current density is on the order of 106 A/cm2, and the corresponding switching efficiency is higher than that reported for similar structures. X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy analyses suggest a strong correlation between the observed PMA and SOT enhancement and the interfacial diffusion during the sputtering growth of the samples. It is very likely that the interfacial diffusion gives rise to enhanced spin-orbit coupling at the interface, while the latter results in enhancement in the PMA, SHA, and switching efficiency in the structure.

High field linear magnetoresistance is an important feature for magnetic sensors applied in magnetic levitating train and high field positioning measurements. Here, we investigate linear magnetoresistance in Pt/FePt/ZnO/Fe/Pt multilayer magnetic sensor, where FePt and Fe ferromagnetic layers exhibit out-of-plane and in-plane magnetic anisotropy, respectively. Perpendicular anisotropy L10-FePt reference layer with large coercivity and high squareness ratio was obtained by in situ substrate heating. Linear magnetoresistance is observed in this sensor in a large range between +5 kOe and −5 kOe with the current parallel to the film plane. This L10-FePt based sensor is significant for the expansion of linear range and the simplification of preparation for future high field magnetic sensors.

The achievement of high temperature ferromagnetism in perovskite manganites has proved both fundamentally and technologically important for spintronics devices. However, high operating temperatures have not been achieved due to the depression of the Curie temperature and the rapid spin filtering efficiency loss, which are the main obstacles for practical applications. Here, we report unexpected room temperature insulating ferromagnetism in ultrathin (110) oriented La0.7Sr0.3MnO3 (LSMO) films. The relationships between room temperature ferromagnetism, charge transfer, and orbital occupancy are investigated, with X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD) measurements. Our results suggest that the room temperature insulating ferromagnetism is originated from super-exchange interaction between Mn2+ and Mn3+. The formation of Mn2+ ions is related to the charge transfer induced by oxygen vacancies. Moreover, a preferential orbital occupancy of eg(3z2-r2) in Mn3+ ions is crucial to the...