Large magnetoresistance in carbon-coated Ni/NiO nanoparticles

We report here a large magnetoresistance (MR) observed in carbon-coated Ni/NiO nanostructures synthesized by a chemical method. The crystalline nature and particle size of the graphitic-carbon-coated Ni/NiO nanostructure was investigated by X-ray diffraction study and field emission scanning electron microscope images. The Raman spectroscopy confirms the presence of graphite layer over the Ni/NiO nanostructure. The field-cooled (FC) magnetic hysteresis curves show exchange bias effect suggesting possible Ni/NiO core–shell structure. The temperature-dependent magnetization data show bifurcation in FC–zero-field-cooled curves, indicating the superparamagnetic behaviour and competing ferromagnetic (FM) and antiferromagnetic interactions in the nanocomposite. MR studies show a large negative MR of \(\sim \)20% at 18 K and \(\sim \)4.2% at room temperature, revealing significant enhancement of FM interactions at low temperatures and spin-dependent tunnelling of current through the nanocomposite.     

自旋阀结构中的力致磁阻效应

采用变分法研究了外应力场下铁磁单层膜、铁磁/反铁磁双层膜系统的磁化性质,进而研究了由铁磁单层膜和铁磁/反铁磁双层膜所构建的自旋阀结构中的磁电阻与外应力场之间的关系.结果表明,铁磁膜中的磁化性质与膜面内所加应力场的大小,方向密切相关,而反铁磁层的嵌入将明显地改变着铁磁层的磁矩向应力场方向磁化的行为.特别地,在应力场方向垂直于铁磁易轴情况下,当应力场日H_λ=2(K_1+K_(up)/3M)时,将发生磁化从易轴方向到应力方向的突变.为此,可采用自旋阀结构,通过其膜面内的应力场所调控的磁电阻效应,构建纳米尺度下的力磁传感器.     

Enhanced magnetic properties of cobalt nanoparticles on FeMn films

Monodisperse cobalt nanoparticles are synthesized by the thermal decomposition of Co₂(CO)₈ in phenyl ether and subsequently deposited on antiferromagnetic (AFM) FeMn films and glass substrates, respectively. Magnetic measurement shows that the as-prepared Co nanoparticles are superparamagnetic and can be transformed into ferromagnetic (FM) through thermal treatment. While keeping monodisperse, the annealed FM Co nanoparticles on AFM FeMn films show a much larger coercivity than the ones on glass substrates due to FM/AFM exchange coupling. Accordingly, we propose a convenient method to enhance magnetic properties of nanoparticles.     

Exchange Bias Effect and Ferromagnetic Resonance Study of NiO/NiFe/NiO Trilayers with Different Thicknesses of NiO Layers

Antiferromagnetic (AFM₁)-ferromagnetic (FM)-antiferromagnetic (AFM₂) multilayer films are fabricated using a magnetron sputtering technique. The AFM₁, FM, and AFM₂ layers are NiO, NiFe, and NiO films, respectively, where AFM₁ and AFM₂ are of the same thickness ranging from 10 to 60 nm. The exchange bias effect and ferromagnetic resonance of NiO/NiFe (50 nm)/NiO have been investigated using the vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) meter. Both saturation magnetization (M _s) and coercivity (H _c) increase first and then decrease with the increasing thickness of NiO layers. The maximum values (M _s = 457 kA/m, H _c = 1.86 kA/m) are achieved when the NiO thickness is 30 nm. The in-plane resonance field (H _r∥) and ferromagnetic resonance linewidth (ΔH _∥) show the lowest values (H _r = 77.4 kA/m, ΔH _∥ = 7.2 kA/m) which occur when the thickness of NiO layers is 30 nm. The dependence between exchange bias and NiO film thickness in NiO/NiFe/NiO trilayers has also been discussed.     

Study of Electronic,Mechanical, Magnetic,and Optical Properties of Mg<Subscript>0.75</Subscript>TM<Subscript>0.25</Subscript>S/Se (TM = Fe,Co, Ni): A First Principle Approach

A systematic study of the magnetism, mechanical, and optical behaviors of Mg_0.75TM_0.25S/Se (TM = Fe, Co, Ni) ordered alloys was conducted using the full-potential linearized augmented plane wave plus local orbital method. The ferromagnetic (FM) and antiferromagnetic (AFM) ground state energy differences and enthalpy of formation scrutinized their dynamical stability in the FM state. The elastic parameters decided their mechanical stability, ductile behavior, and partial ionic and covalent characters. Moreover, the calculated band structures and density of states revealed the type and origin of magnetism in terms of exchange splitting energies and exchange constants. The Fe- and Ni-doped MgS/Se alloys exhibited half metallic ferromagnetic (HMF) characteristics, while the Co-doped MgS/Se alloy depicted ferromagnetic semiconducting characters. A reduction in the magnetic moments of the transition metals was observed compared with their free space value, which is due to strong p-d hybridization. Furthermore, the optical spectrum showed the maximum light absorbed in the visible and in the UV region of the electromagnetic spectrum, which makes them potential candidates for optoelectronic and spintronic device applications.     

Magnetic properties and first-order magnetic phase transition in single crystal FeRh thin film

To understand the mechanism of first-order magnetic phase transition in ordered FeRh thin films, the magnetic properties and first-order antiferromagnetic (AFM)–ferromagnetic (FM) phase transition behavior of single-crystal FeRh thin film are investigated in detail. The first-order magnetic phase transition is seen at a temperature of around 120 °C during heating and 145 °C cooling processes in perpendicular direction. The M–H loops measured isothermally amidst the AFM–FM transition regime show an opening at high magnetic field, which indicate a reversible AFM–FM transition induced by magnetic field. The clusters of the FM phase nucleate in the AFM matrix heterogeneously and vice versa during the first-order phase transition and the mechanism of nucleation and growth kinetics of the first-order magnetic phase transition in ordered FeRh thin film is quite similar to that of the crystallization of solids described by the Avrami model.     

The cytotoxicity of NiO nanoparticle with borate capping

The impact of surface capping on cytotoxicity of NiO nanoparticle was investigated with Escherichia coil (E.coli) in this work. The NiO nanoparticle and NiO nanoparticle capped by borate (denoted as NiO-borate) were synthesized by hydrothermal method. The average size of both nanoparticles is about 4.0 nm. The plate experiments demonstrated that NiO-borate nanoparticles show lower cytotoxicity than NiO nanopaticles. Further spectrophotometric analysis revealed that the concentration of both extracellular and intercellular Ni2+ in NiO-borate system were lower than that of uncapped one. Intracellular ICP-AES analysis also showed the concentration of Ni element was higher than Ni2+, suggesting the NiO nanoparticles might penetrate into the cellular interior. Comprehensive AFM, SEM and TEM observation illustrated both NiO-borate and NiO nanoparticles lead to the collapse of cellular body, the convex on the cell wall and the damage of cell wall ultimately. In summary, the surface capping with borate on NiO nanopaticles will suppress the release of the Ni2+ ions and impede the contact between the NiO nanoparticle and cell wall, which ultimately decreased the cytotoxicity of NiO nanoparticles.     

Fabrication of ferromagnetic Ni epitaxial thin film by way of hydrogen reduction of NiO

The ferromagnetic epitaxial Ni (111) thin film on the oxide substrate could be obtained by an epitaxy method, employing pulsed laser deposition (PLD) of epitaxial NiO (111) film on the sapphire (α-Al₂O₃) substrate and successive hydrogen reduction. The epitaxial NiO (111) film was deposited on the sapphire (0001) substrate at room temperature by PLD, and was reduced into the Ni epitaxial film by annealing (300 °C to 700 °C) in the hydrogen atmosphere, suggesting the possible formation of epitaxial [Ni metal/α-Al₂O₃] multilayer. The epitaxy of Ni film was proved by ex situ X-ray diffraction. The ferromagnetic anisotropy of the epitaxial Ni film was examined by superconducting quantum interference magnetometry.     

Magnetic Properties of Zn1?x Ni x O (0.25≤x≤ 0.50) Prepared by Solid-State Reactions

In this study the origin of ferromagnetism in ZnO-based bulk systems has been investigated using Ni-doped ZnO samples, Zn_1−x Ni _x O with 0.25≤x≤0.50, prepared by solid-state reactions. The structural characterizations indicated that the Ni²⁺ ions almost uniformly distributed in all the samples, and the samples have hexagonal wurtzite structure; however, when x is increased toward 0.50, a new NiO phase is formed. A ferromagnetism (FM) has been observed for all the samples at and below the room temperature. In other words, the room temperature results of (M–H) curves show that the FM observed is intrinsic for all the Ni-doped ZnO samples. However, the saturated magnetizations decrease gradually with increasing Ni concentration. This indicates that, in addition to FM, the excessive doping of Ni in ZnO also causes an antiferromagnetic (AFM) contribution which increases with increasing Ni amount. This result is also supported by the magnetization against temperature measurements. Furthermore, the trend of the ac-susceptibility (χ) versus temperature curves, measured under an ac-magnetic field of 100 Oe, also support our conclusion about the antiferromagnetic contribution to ferromagnetism in our samples.     

An investigation of flower shaped NiO nanostructures by microwave and hydrothermal route

The face centered cubic phase of NiO nanostructures were successfully synthesized from microwave and hydrothermal methods. The structural properties of the synthesized material were analyzed by X-ray diffraction (XRD) studies. The thermal analysis revealed the transformation of Ni(OH)₂ into NiO at about 380 °C. The obtained scanning electron microscopy images exhibited less agglomerated flowers for hydrothermal reacted NiO than the microwave processed samples. The sheet like morphologies of NiO nanostructures were confirmed by transmission electron microscope and the obtained particles sizes were comparable to the calculated values from XRD data. The UV–Vis and photoluminescence spectra results showed that the absorption edges of the NiO nanoflowers have a blue-shift due to quantum confinement effect. The Raman spectrum exhibited the transformation of antiferromagnetic to superparamagnetic transition confirmed from the absence of magnon peak. The XPS spectrum presented the observation of Ni 2p and O 1s levels with higher intense peak nature for hydrothermal treated NiO than microwave. The hysteresis loops of the NiO samples prepared by both hydrothermal and microwave heating methods revealed the weak ferromagnetic behaviors at room temperature. Based on the experimental observations and analysis, a possible hydrothermal reaction mechanism is proposed to synthesize flower shaped NiO nanostructured materials with improved structural, optical, morphological and magnetic properties compared to microwave synthesis.     

Magnetic and Structural Properties of Ni/NiO Nanoparticles Prepared Using Nickel Acetate and Polyvinyl Acetate Precursor

Ni/NiO nanoparticles were synthesized using a mixture of nickel acetate and polyvinyl acetate as precursors, followed by heating between 350–450 °C. The prepared samples were investigated using XRD, TEM and SQUID magnetometry. Depending on the heating temperature, the average sizes of Ni and NiO crystallites were found to be in the ranges of 25–37 nm and 10–26 nm, respectively. The analysis of XRD patterns shows that the volume percentage of Ni can be easily controlled by the heating temperature. Magnetization measurements show that the samples consist of ferromagnetic moments which are blocked at low temperatures and a disordered NiO component which causes spin-glass like behavior.     

Magnetic Perovskite Superlattices Grown by Pulsed Laser Deposition

Magnetic multilayers of (SrRuO₃) _m (SrMnO₃) _n were grown artificially using the pulsed laser deposition technique on (001)-oriented SrTiO₃ substrates. The temperature and magnetic field dependent reisistivity of the superlattices consisting of ferromagnetic (FM) SrRuO₃ and antiferromagnetic (AFM) SrMnO₃ are studied as a function of the SrRuO₃ unit cells. We observed a pinned/biased moment instead of the biased field in the superlattices and we redefine the structure as AFM/FM(Pin)/FM(Free)/FM(Pin) units below a critical field.     

Ionic Gel Modulation of RKKY Interactions in Synthetic Anti‐Ferromagnetic Nanostructures for Low Power Wearable Spintronic Devices

To meet the demand of developing compatible and energy‐efficient flexible spintronics, voltage manipulation of magnetism on soft substrates is in demand. Here, a voltage tunable flexible field‐effect transistor structure by ionic gel (IG) gating in perpendicular synthetic anti‐ferromagnetic nanostructure is demonstrated. As a result, the interlayer Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction can be tuned electrically at room temperature. With a circuit gating voltage, anti‐ferromagnetic (AFM) ordering is enhanced or converted into an AFM–ferromagnetic (FM) intermediate state, accompanying with the dynamic domain switching. This IG gating process can be repeated stably at different curvatures, confirming an excellent mechanical property. The IG‐induced modification of interlayer exchange coupling is related to the change of Fermi level aroused by the disturbance of itinerant electrons. The voltage modulation of RKKY interaction with excellent flexibility proposes an application potential for wearable spintronic devices with energy efficiency and ultralow operation voltage.     

Oxygen implanted Ni/NiO exchange bias system: fabrication, structure and magnetic property

Ion implantation is used to build a nanometer scale anti-ferromagnetic (AFM) cluster embedded exchange bias (EB) system. Ni film with a thickness of 100 nm is deposited on the Si (100) substrate using magnetron sputtering, 140 keV O+ is chosen to implant into the Ni film to form NiO AFM clusters, of which the size is estimated by X-ray diffraction based on synchrotron radiation (SR-XRD). By measuring hysteresis loop after field-cooling, significant shifts of loop along the applied field are observed. By increasing the implantation dose up to 3 x 10(17)/cm2 and annealing samples in N2 atmosphere, we discuss the origin of EB effect observed and the size confinement effect which lowers down the Néel temperature (T(N)) of NiO cluster to below room temperature.     

SiO2 / Ni 核壳结构纳米粒子的制备及其磁性能

以硅酸钠为主要原料, 通过液相沉淀法在纳米镍粉表面包覆了一层SiO₂ 。应用XRD、FTIR、TEM、TGA、DSC 和VSM 对复合粉体的结构、形貌和磁性能进行了研究。结果表明, SiO₂ 以非晶态的形式包覆在纳米镍粒子表面, 形成了核壳结构, 降低了纳米粉体的团聚现象。TGA 和DSC 结果表明, SiO₂ 的包覆提高了纳米粉体的抗氧化性。磁性分析结果表明, 粉体包覆前由于表面氧化层(NiO) 的存在, 粉体的磁滞回线偏移; 包覆后的粉体由于SiO₂的存在, 饱和磁化强度降低, 矫顽力升高。      

层状CoPS3中磁性的压力与载流子掺杂可控调节

最近发现的层状铁磁材料吸引了研究者的广泛兴趣,但是其有限的数量严重阻碍了进一步的发展.作为一个更大的家族,层状反铁磁材料为加深对基本磁性物理的理解和推动高性能自旋电子学的应用提供了一个出色的平台.本文中,我们选取了过渡金属磷三硫族化合物中代表性的反铁磁CoPS3材料,并通过系统的第一性原理计算证实压力或载流子掺杂可有效...     

First-Principles Investigation of Electronic Structure Features of TbOFeAs Compound

We report a theoretical investigation on the electronic and magnetic properties of rare-earth pnictide parent compound, such as TbOFeAs. Employing first-principles method supplemented by the local spin density approximation (LSDA), we discuss the electronic structure with the incorporation of the role of Coulomb on-site repulsion (U) of Tb 4f states as well as the spin-orbit (SO) coupling on the magnetic and nonmagnetic phases. For ferromagnetic (FM) and antiferromagnetic (AFM) phases, we have determined the spin and orbital magnetic moments of Tb ions and confer the significance of the spin-orbit interaction of Tb 4f states in this parent compound. In the FM state, the reduction of Fe moment is about a factor of 3.5 with respect to AFM configuration. The most energetically favorable state is AFM configuration. Our theoretical findings surmise that the magnetic moments on Fe sites carry an AFM order. Based on LSDA + U + SO approximation, we infer that the Tb magnetic moments also carry an AFM order, albeit the spin Tb sites in TbO layer possess the same orientation as the Fe spins in FeAs layer. With the incorporation of on-site Coulomb repulsion and spin-orbit interaction in AFM state, the Fe 3d states are large near the Fermi level and this phase is illustrating a metallic behavior. Moreover, the Fermi surface topology and nesting features are presented.     

Effect of a NiO capping layer on the temperature dependence of the interlayer coupling in Co/Pt multilayer with perpendicular anisotropy

The effect of an ultra thin NiO(11 Å) capping layer on the temperature dependence of the interlayer coupling of a [Pt(26.5 Å)/Co(4 Å)]₃ multilayer with perpendicular anisotropy has been investigated. In the presence of the NiO(11 Å) capping layer, the interlayer coupling between the Co layers has been observed to demonstrate interesting temperature-dependent transitions. Only the antiferromagnetic (AF) interlayer coupling exists at T ≥ 270 K. With the decrease of temperature, the coexistence of ferromagnetic (FM) and antiferromagnetic interlayer coupling is observed in the temperature range of 220 K ≤ T≤ 270 K. When the temperature is lower than 220 K, only the ferromagnetic interlayer coupling exists. The temperature-dependent polarization of the Pt layers can be one of the origins for the observed AF-to-FM transitions.     

Exchange-biased NiFe2O4/NiO nanocomposites derived from NiFe-layered double hydroxides as a single precursor

NiFe₂O₄ nanoparticles (<10 nm) embedded in a NiO matrix have been fabricated by calcining the corresponding Ni^IIFe^III-layered double hydroxide (LDH) precursors at high temperature (500 °C). Compared with the NiFe₂O₄/NiO nanocomposite obtained by calcination of a precursor prepared by a traditional chemical coprecipitation method, those derived from NiFe-LDH precursors show much higher blocking temperatures (T _B) (?380 K). The enhanced magnetic stability can be ascribed to the much stronger interfacial interaction between NiFe₂O₄ and NiO phases due to the topotactic nature of the transformation of the LDH precursor to the NiFe₂O₄/NiO composite material. Through tuning the Ni^II/Fe^III molar ratio of the NiFe-LDH precursor, the NiFe₂O₄ concentration can be precisely controlled, and the T _B value as well as the magnetic properties of the final material can also be regulated. This work represents a successful example of the fabrication of ferro(ferri)magnetic (FM)/antiferrimagnetic (AFM) systems with high magnetic stability from LDH precursors. This method is general and may be readily extended to other FM/AFM systems due to the wide range of available LDH precursors.      

E‐field Control of the RKKY Interaction in FeCoB/Ru/FeCoB/PMN‐PT (011) Multiferroic Heterostructures

E‐field control of antiferromagnetic (AFM) orders is promising for the realization of fast, compact, and energy‐efficient AFM applications. However, as the AFM spins are strongly pinned, the E‐field control process is mainly based on the exchange bias regulation that usually confines at a low temperature. Here, a new magnetoelectric (ME) coupling mechanism for the modulation of AFM orders at room temperature is explored. Based on the FeCoB/Ru/FeCoB/(011) Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ (PMN‐PT) synthetic antiferromagnetic (SAF) heterostructures, the external E‐field generates relative magnetization switching in the two ferromagnetic (FM) layers, leading the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction tuning. This voltage‐induced switching behavior can be repeated in a stable and reversible manner for various SAFs, which is a key challenge in the E‐field control of AFM coupling and is not resolved yet. The voltage‐induced RKKY interaction changes by analyzing the dynamic optical and acoustic modes is quantified, and with first‐principles calculations, it is found that the distortion of the Fermi surface by the lattice reconstruction is the key of the relative magnetization switching and RKKY interaction modulation. This voltage control of the RKKY interaction in ME heterostructures provides an easy way to achieve the next generation of AFM/FM spintronic applications.