Study of Magnetic Properties of Fe-Doped CuO: Monte Carlo Simulations

Using Monte Carlo simulations, we are studying the magnetic properties of Fe-doped CuO thin films. The total magnetizations and the susceptibilities are studied as a function of the effect doping, external magnetic field, and exchange coupling. The critical temperature is discussed as a function of the effect of iron concentration. On the other hand, we investigate the effect of increasing temperatures on the coercive field for a constant value of exchange coupling and a fixed concentration. The coercive magnetic field is found to decrease with increasing temperature values until reaching its null value. The effect of increasing the exchange coupling amount on the saturation magnetic field H _s is illustrated. A linear growth of the saturation magnetic field is found as a function of the exchange coupling interaction. To complete this study, we presented and discussed the magnetic hysteresis cycle loops.     

Multiferroic Heterostructures Integrating Ferroelectric and Magnetic Materials

Multiferroic heterostructures can be synthesized by integrating monolithic ferroelectric and magnetic materials, with interfacial coupling between electric polarization and magnetization, through the exchange of elastic, electric, and magnetic energy. Although the nature of the interfaces remains to be unraveled, such cross coupling can be utilized to manipulate the magnetization (or polarization) with an electric (or magnetic) field, known as a converse (or direct) magnetoelectric effect. It can be exploited to significantly improve the performance of or/and add new functionalities to many existing or emerging devices such as memory devices, tunable microwave devices, sensors, etc. The exciting technological potential, along with the rich physical phenomena at the interface, has sparked intensive research on multiferroic heterostructures for more than a decade. Here, we summarize the most recent progresses in the fundamental principles and potential applications of the interface‐based magnetoelectric effect in multiferroic heterostructures, and present our perspectives on some key issues that require further study in order to realize their practical device applications.     

单相多铁性磁电体磁电起源及耦合机理分析

单相多铁性磁电体是指在同一相下既有铁电性又有铁磁性的材料,其丰富的物理机制和极大的应用潜力近来受到国内外学术界和工程应用领域的广泛关注.电极化和磁化通过内禀的磁电耦合作用为下一代多功能电子器件的设计提供了额外的自由度.简单回顾了单相磁电材料研究的历史以及最新进展,从其导电性、对称性、化学d0性、结构畸变等方面详细讨论了单相多铁性磁电体磁电性的起源及耦合,并简单分析了磁电材料产业化进程中存在的问题,展望了多铁性磁电体的研究及应用前景.     

Multiferroic and magnetoelectric materials--novel developments and perspectives

Magnetoelectric (ME) materials are of utmost interest in view of both fundamental understanding and novel desirable applications. Despite its smallness, the linear ME effect has been shown to control spintronic devices very efficiently, e.g., by using the classic ME antiferromagnet Cr?O?. Similar nano-engineering concepts exist also for type-I multiferroic single phase materials like BiFeO? and BiMnO?. Record high ME response has been realized in stress-strain coupled multiphase magnetoelectrics like PZT/FeBSiC composites, enabling applications in sensors. In type-II multiferroics, whose ferroelectricity is due to modulated magnetic ordering, the ME coupling is of fundamental interest. Higher-order ME response characterizes disordered systems, which extend the conventional multiferroic scenario toward ME multiglass (e.g., Sr(1-x)MnxTiO?).     

AFM‐Based Spin‐Exchange Microscopy Using Chiral Molecules

Local magnetic imaging at nanoscale resolution is desirable for basic studies of magnetic materials and for magnetic logic and memories. However, such local imaging is hard to achieve by means of standard magnetic force microscopy. Other techniques require low temperatures, high vacuum, or strict limitations on the sample conditions. A simple and robust method is presented for locally resolved magnetic imaging based on short‐range spin‐exchange interactions that can be scaled down to atomic resolution. The presented method requires a conventional AFM tip functionalized with a chiral molecule. In proximity to the measured magnetic sample, charge redistribution in the chiral molecule leads to a transient spin state, caused by the chiral‐induced spin‐selectivity effect, followed by the exchange interaction with the imaged sample. While magnetic force microscopy imaging strongly depends on a large working distance, an accurate image is achieved using the molecular tip in proximity to the sample. The chiral molecules' spin‐exchange interaction is found to be 150 meV. Using the tip with the adsorbed chiral molecules, two oppositely magnetized samples are characterized, and a magnetic imaging is performed. This method is simple to perform at room temperature and does not require high‐vacuum conditions.     

Room temperature magnetic exchange coupling in multiferroic BaTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> magnetoelectric superlattice

Multiferroic BaTiO₃/CoFe₂O₄ superlattice films are deposited by laser molecular beam epitaxy. The film growth modes are studied by in situ reflection high energy electron diffraction and the film structures are revealed by high resolution transmission electron microscopy study. Ferroelectric switching behavior was studied by piezoresponse force microscopy, and it shows that good ferroelectricity was retained in the superlattice. Such a multiferroic superlattice also shows a magnetic exchange coupling under room temperature. Detailed analysis reveals that different growth modes and the substrate strain effect may be responsible for the magnetic exchange coupling.     

Effect of Variable Biquadratic Exchange Coupling on the Magnetic Hysteresis of Uniaxial Antiferromagnetic Co/Ru/Co Films

We investigated biquadratic exchange coupling strength in sputtered uniaxial antiferromagnetic Co/Ru/Co trilayers, which can be tuned by applying a magnetic field perpendicular to the sample surface during deposition. The perpendicular field induces a columnar grain structure that results in a perpendicular magnetic anisotropy for single Co layers. The perpendicular magnetic anisotropy is detected by combined magnetization and Brillouin light scattering measurements. In trilayers, increasing the perpendicular field during deposition results in a monotonic increase of the biquadratic exchange coupling strength, so these samples provide an experimental realization of the effect of variable biquadratic coupling strength on magnetic hysteresis behavior. The hard-axis remanence is a direct measure of biquadratic exchange coupling strength, which we demonstrated both by experiment and by a Stoner-Wohlfarth model that includes the relevant magnetic parameters.     

NiO／NiFeCo／Cu／NiFeCo自旋阀结构的巨磁电阻效应及影响因素的研究

用电子蒸发的方法制备ＮｉＯ／ＮｉＦｅＣ／Ｃｕ／ＮｉＦｅＣｏ自旋阀多层膜,通过磁场中退火得到好的偏置型自旋阀ＧＭＲ效应。通过对制备态以及磁场退火后样品的ＭＲ曲线的研究,讨论了交换耦合作用,单层磁性能以及层间耦合作用对材料ＧＭＲ效应的大小和磁场灵敏度的影响,得出提高交换耦合作用,改善单层磁性能和尽可能减小层间耦合将有得于得到高性能的偏置型自旋阀ＧＭＲ材料的结论。     

Properties,Benefits, and Application of Nanocrystalline Structures in Magnetic Materials

The magnetic properties of nanocrystalline hard magnetic and soft magnetic are summarized. When the grain size becomes of the order f the magnetic exchange length exchange coupling occurs. The different concepts of exchange coupling in these materials are discussed. Exchange coupling leads in isotropic hard magnetic materials to a remanence enhancement. Soft magnetic materials exhibit due to exchange coupling a lower coercivity, lower losses and consequently also improved properties.     

Zero-Temperature Study of a Tetrameric Spin-1/2 Chain in a Transverse Magnetic Field

We consider an alternating Heisenberg spin-1/2 antiferromagnetic–ferromagnetic chain with the space-modulated dominant antiferromagnetic exchange and anisotropic ferromagnetic coupling (tetrameric spin-1/2 chain). The zero-temperature effect of a symmetry breaking transverse magnetic field on the model is studied numerically. It is found that the anisotropy effect on the ferromagnetic coupling induces two new gapped phases. We identified their orderings as a kind of the stripe antiferromagnetic phase. As a result, the magnetic phase diagram of the tetrameric chain shows five gapped quantum phases, and the system is characterized by four critical fields which mark quantum phase transitions in the ground state of the system with the changing transverse magnetic field. We have also exploited the well-known bipartite entanglement (name as concurrence) and global entanglement tools to verify the occurrence of quantum phase transitions and the corresponding critical points.     

A New Mechanism for Ferromagnetism in C60-Tdae1

Based on the topology of C⁶⁰ and the resulting non-disjoint nature of the lowest unoccupied molecular orbitals, we propose a new model for ferromagnetic exchange in C⁶⁰-TDAE. Within the Hubbard model, we find that the ferromagnetic exchange integral is stabilized to first order in the inter-ball transfer integral, while the antiferromagnetic coupling is stabilized only to second order. This difference is adequate to counter the larger phase space available for stabilizing the antiferromagnetic state. Thus, the ground state is found to be ferromagnetic for reasonable inter-ball transfer integrals.     

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.     

Color Patterning of Luminescent Perovskites via Light‐Mediated Halide Exchange with Haloalkanes

Lead halide perovskite possesses a semiconductor bandgap that is readily tunable by a variation in its halide composition. Here, a photo‐activated halide exchange process between perovskite nanocrystals and molecular haloalkanes is reported, which enables the perovskite luminescence to be controllably shifted across the entire visible spectrum. Mechanistic investigations reveal a mutual exchange of halogens between the perovskite crystal surface and a chemisorbed haloalkane, yielding nanocrystals and haloalkanes with mixed halide contents. Exchange kinetics studies involving primary, secondary, and tertiary haloalkanes show that the rate of reaction is governed by the activation barrier in the breakage of the covalent carbon–halogen (C? X) bond, which is a function of the C? X bond energy and carbon radical stability. Employing this halide exchange approach, a micrometer‐scale trichromatic patterning of perovskites is demonstrated using a light‐source‐integrated inkjet printer and tertiary haloalkanes as color‐conversion inks. The haloalkanes volatilize after halide exchange and leave no residues, thereby offering significant processing advantage over conventional salt‐based exchange techniques. Beyond the possible applications in new‐generation micro‐LED and electroluminescent quantum dot displays, this work exemplifies the rich surface and photochemistry of perovskite nanocrystals, and could lead to further opportunities in perovskite‐based photocatalysis and photochemical sensing.     

Giant Rashba-type spin splitting in bulk BiTeI

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.     

Magneto‐Ionic Switching of Superparamagnetism

Electrochemical reactions represent a promising approach to control magnetization via electric fields. Favorable reaction kinetics have made nanoporous materials particularly interesting for magnetic tuning experiments. A fully reversible ON and OFF switching of magnetism in nanoporous Pd(Co) at room temperature is demonstrated, triggered by electrochemical hydrogen sorption. Comprehensive magnetic characterization in combination with high‐resolution scanning transmission electron microscopy reveals the presence of Co‐rich, nanometer‐sized clusters in the nanoporous Pd matrix with distinct superparamagnetic behavior. The strong magneto‐ionic effect arises from coupling of the magnetic clusters via a Ruderman–Kittel–Kasuya–Yoshida‐type interaction in the Pd matrix which is strengthened upon hydrogen sorption. This approach offers a new pathway for the voltage control of magnetism, for application in spintronic or microelectromagnetic devices.     

The effect of cluster size on thin film media noise

A dynamic micromagnetic model of thin film magnetic recording media with generalised structure has been used to study the effect of cluster size on thin film media noise. Clusters formed by common crystallography and by intergranular exchange coupling have been simulated, and the effects of clustering on hysteresis and noise are presented. For small clusters of a few grains, crystallographic correlations are shown to have the same effect on noise as intergranular exchange coupling, giving an increase in magnetic feature size and noise     

Radical Polymers for Organic Electronic Devices: A Radical Departure from Conjugated Polymers?

Radical polymers are aliphatic or nonconjugated polymers bearing organic robust radicals as pendant groups per repeating unit. A large population of the radical redox sites allows the efficient redox gradient‐driven electron transport through the polymer layer by outer‐sphere self‐exchange reactions in electrolyte solutions. The radical polymers are emerging as a new class of electroactive materials useful for various kinds of wet‐type energy storage, transport, and conversion devices. Electric‐field‐driven charge transport by hopping between the densely populated radical sites is also a remarkable aspect of the radical polymers in the solid state, which leads to many dry‐type devices such as organic memories, diodes, and switches.     

Exchange coupling and its applications in magnetic data storage

The continuing scaling of magnetic recording is facing more and more scientific and technological challenges because both the read sensor and recording bit are approaching sub-50 nm regime with the ever increasing areal density in hard disk drives. One of the key and indispensable elements for both high-sensitivity sensors and high-density media is the exchange bias between a ferromagnetic and an antiferromagnetic layer or the exchange coupling between two ferromagnets via a non-magnetic spacer. In the nanometer regime, the exchange coupling between ferromagnet and antiferromagnet or two ferromagnets through a conductive spacer is governed by the intergrain exchange interaction which has its origin in electron spins. Interlayer exchange coupling in multilayer or trilayer essentially originates from the quantum confinement effect. In this paper, we first review the physical origin and various theoretical models of the two types of exchange couplings, followed by a review of the applications of the exchange bias and interlayer exchange coupling in data storage with emphasis on the advanced read sensor and advanced media including perpendicular media and patterned media.     

Thermomagnetic data writing and erasing in DyCo/NiFe (TbFe/NiFe) film structures

A new principle of data writing and erasing is proposed based on the exchange coupling between magnetic layers with mutually orthogonal orientations of the effective magnetization. The new method provides for a significant decrease in energy consumption. The possibility of using bilayer magnetic films of the (rare earth metal-transition metal)/NiFe type possessing unidirectional anisotropy as the media for thermomagnetic writing and erasing of magnetooptical data is studied.     

Multiferroics with Spiral Spin Orders

Cross correlation between magnetism and electricity in a solid can host magnetoelectric effects, such as magnetic (electric) induction of polarization (magnetization). A key to attain the gigantic magnetoelectric response is to find the efficient magnetism–electricity coupling mechanisms. Among those, recently the emergence of spontaneous (ferroelectric) polarization in the insulating helimagnet or spiral‐spin structure was unraveled, as mediated by the spin‐exchange and spin–orbit interactions. The sign of the polarization depends on the helicity (spin rotation sense), while the polarization direction itself depends on further details of the mechanism and the underlying lattice symmetry. Here, we describe some prototypical examples of the spiral‐spin multiferroics, which enable some unconventional magnetoelectric control such as the magnetic‐field‐induced change of the polarization direction and magnitude as well as the electric‐field‐induced change of the spin helicity and magnetic domain.