Stacking-dependent exchange bias in two-dimensional ferromagnetic/antiferromagnetic bilayers

A clear microscopic understanding of exchange bias is crucial for its application in magnetic recording, and further progress in this area is desired. Based on the results of our first-principles calculations and Monte Carlo simulations,we present a theoretical proposal for a stacking-dependent exchange bias in two-dimensional compensated van der Waals ferromagnetic/antiferromagnetic bilayer heterostructures. The exchange bias effect emerges in stacking registries that accommodate inhomogeneous interlayer magnetic interactions between the ferromagnetic layer and different spin sublattices of the antiferromagnetic layer. Moreover, the on/off switching and polarity reversal of the exchange bias can be achieved by interlayer sliding, and the strength can be modulated using an external electric field. Our findings push the limits of exchange bias systems to extreme bilayer thickness in two-dimensional van der Waals heterostructures, potentially stimulating new experimental investigations and applications.

Mean-field and high temperature series expansion calculations of some magnetic properties of Ising and XY antiferromagnetic thin-films

In this work, the magnetic properties of Ising and XY antiferromagnetic thin-films are investigated each as a function of Neel temperature and thickness for layers （n = 2, 3, 4, 5, 6, and bulk （∞） by means of a mean-field and high temperature series expansion （HTSE） combined with Pade approximant calculations. The scaling law of magnetic susceptibility and magnetization is used to determine the critical exponent γ, veff （mean）, ratio of the critical exponents γ/v, and magnetic properties of Ising and XY antiferromagnetic thin-films for different thickness layers n = 2, 3, 4, 5, 6, and bulk （∞）.

Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet(AFM)/ferromagnet(FM)heterostructure is a popular system for studying the spin–orbit torque(SOT)of AFMs.However,the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field,namely the magnetic moment drag effect,which further influences the characteristic of SOT efficiency.In this work,we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance(ST-FMR)method.A full analysis on the AFM/FM systems with exchange bias is performed,and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy.The ST-FMR results can be well fitted by this model.We obtained the relative accurate SOT efficiencyξ_(DL)=0.058 for the IrMn film.This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.

Theoretical Investigation of Influence of Mechanical Stress on Magnetic Properties of Ferromagnetic/Antiferromagnetic Bilayers Deposited on Flexible Substrates

Effect of mechanical stress on magnetic properties of an exchange-biased ferromagnetic/antiferromagnetic bilayer deposited on a flexible substrate is investigated. The hysteresis loops with different magnitudes and orientations of the stress can be classified into three types. The corresponding physical conditions for each type of the loop are deduced based on the principle of minimal energy. The equation of the critical stress is derived, which can judge whether the loops show hysteresis or not. Numerical calculations suggest that except for the magnitude of the mechanical stress, the relative orientation of the stress is also an important factor to tune the exchange bias effect.

Model of hybrid interfacial domain wall in ferromagnetic/antiferromagnetic bilayers

A general model of a hybrid inteffacial domain wall （HIDW） in ferromagnetic/antiferromagnetic exchange biased bilayers is proposed, where an inteffacial domain wall is allowed to extend into either the ferromagnetic or antiferromagnetic layer or across both. The proposition is based on our theoretical investigation on thickness and field dependences of ferromagnetic domain wall （FMDW） and antiferromagnetic domain wall （AFDW）, respectively. Good match of the simulation to the hysteresis loops of a series of NiFe/FeMn exchange-biased bilayers confirms the existence of the HIDW, where the AFDW part is found to preferentially occupy the entire antiferromagnetic layer while the FMDW shrinks with the increased magnetic field as expected. The observed asymmetry between the ascending and descending branches of the hysteresis loop is explained naturally as a consequence of different partition ratios between AFDW and FMDW.

Coexistence of antiferromagnetic and ferromagnetic in Mn-doped anatase TiO_2 nanowires

A series of Mn-doped TiO2 nanowires(NWs) were prepared by hydrothermal method at the mole fraction of Mn changing from 0 to 12.0%.X-ray powder diffraction(XRD) analysis shows that all the samples have pure anatase structure.SEM and TEM studies show that the diameter and the length of the Mn-doped TiO2 NWs are larger than those of the undoped TiO2 NWs.Energy dispersive X-ray spectroscopy(EDX) reveals that the samples are composed of Ti,Mn and O.According to magnetization measurements,all samples show ferromagnetic behavior,but only the undoped TiO2 NWs are completely ferromagnetic with a saturated magnetization about 1.0 mA·m2/kg.Mn-doped TiO2 samples exhibit antiferromagnetic and ferromagnetic(AF-FM) behaviors simultaneously.Photoluminescence(PL) spectra demonstrate the existence of MnO2 sublattice.These observations indicate that an AF-WF crossover is induced by the coexistence of TiO2 sublattice and MnO2 sublattice.

Magnetization of Coupled Ultrathin Ferromagnetic Films

The magnetization of coupled ferromagnetic films is calculated by Green's function method. The coupling can either be ferromagnetic or antiferromagnetic. For the latter case, a concept of pseudo-spin is suggested to make calculation possible. A pseudo-spin is actually an anti-spin with its properties being analogue to other known anti particles such as a hole. The decreasing of Curie point as the coupling strength decays is computed. It is noted that with the same strength, antiferromagnetic coupling has higher Curie point than ferromagnetic coupling.

Crystallization behavior of Ni_(54)Mn_(25.1)Ga_(20.9) ferromagnetic shape memory thin film

NiMnGaferromagnetic shape memory thin film was deposited onto Al foil using r.f.magnetron sputtering technique.The crystallization behavior of the film was investigated by XRD and DSC.The activation energy of crystallization of the film was calculated by Kissinger’s method.The results show that the crystallization temperature of NiMnGafree-standing thin film in martensite state is 372 ℃,and the activation energy of crystallization is about 191.9 kJ·mol-1.

Preparation,structure and ferromagnetic properties of the nanocrystalline Ti_(1-x)Mn_xO_2 thin films grown by radio frequency magnetron co-sputtering

This paper reported that the Mn-doped TiO2 films were prepared by radio frequency （RF） magnetron cosputtering. X-ray diffraction measurements indicate that the samples are easy to form the futile structure, and the sizes of the crystal grains grow big and big as the Mn concentration increases. X-ray photoemlssion spectroscopy measurements and high resolution transmission electron microscope photographs confirm that the manganese ions have been effectively doped into the TiO2 crystal when the Mn concentration is lower than 21%. The magnetic property measurements show that the Ti1-xMnxO2 （x = 0.21） films are ferromagnetic at room temperature, and the saturation magnetization, coercivity, and saturation field are 16.0 emu/cm^3, 167.5 × 80 A/m and 3740 × 80 A/m at room temperature, respectively. The room-temperature ferromagnetism of the films can be attributed to the new futile Ti1-xMnxO2 structure formed by the substitution of Mn^4＋ for Ti^4＋ into the TiO2 crystal .lattice, and could be explained by O vacancy （Vo）-enhanced ferromagnetism model.

Simultaneous control of ferromagnetism and ferroelasticity by oxygen octahedral backbone stretching

Coexistence of ferromagnetism and ferroelasticity in a single material is an intriguing phenomenon,but has been rarely found.Here we studied both the ferromagnetism and ferroelasticity in a group of LaCoO3 films with systematically tuned atomic structures.We found that all films exhibit ferroelastic domains with four-fold symmetry and the larger domain size(higher elasticity)is always accompanied by stronger ferromagnetism.We performed synchrotron x-ray diffraction studies to investigate the backbone structure of the CoO6 octahedra,and found that both the ferromagnetism and the elasticity are simultaneously enhanced when the in-plane Co–O–Co bond angles are straightened.Therefore the study demonstrates the inextricable correlation between the ferromagnetism and ferroelasticity mediated through the octahedral backbone structure,which may open up new possibilities to develop multifunctional materials.

Peculiarity of magnetoresistance of discontinuous ferromagnetic thin films

The magnetoresistive properties of discontinuous ferromagnetic Fe and Co thin films deposited by electron-beam sputtering onto glass substrates at room temperature were investigated. Tunnel magnetoresistance(MR) was observed for all of the as-deposited samples. The maximum MR was observed for Fe thin films with an effective thickness of 17 nm. In the case of the Co thin films, the annealing process led to a change of the type of MR to anisotropic at Co film thicknesses(dCo) of 15 ≤ d_(Co) ≤ 25 nm and to positive isotropic at thicknesses of d_(Co) < 15 nm. By contrast, the MR type of Fe thin films did not change.

AN ANALYSIS ON CHARACTERISTIC OF MAGNETOELASTICITY OF A MAGNETIC FIELD MICROSENSOR MADE OF CANTILEVERED PLATES WITH FERROMAGNETIC FILMS

The purpose of this paper is to study the magnetomechanicalcharacteristic of a microsensor which is composed of a cantileveredbeam-plate with ferromagnetic films in order to measure the magneticfield from the deformation of plate when the microsensor is locatedin the magnetic field. To this end, a nu- merical approach made up ofthe finite element method for magnetic field and the finitedifference method for deflection of the microsensor is proposed toperform the numerical analysis of deflection under magnetoelasticinteraction. Some quantitative results of a case study for themagnetoelastic characteristic between the mag- netic field anddeflection of the microsensor in the magnetic field are given. Theresults show that this mi- crosensor can be used not only to measurethe magnitude of magnetic intensity, but also to possibly monitor thedirection of the vector of the magnetic field.

关于Planar Ferromagnets and Antiferromagnets泛函的径向极小元的注记

就Bethuel,Brezis和Helein提出的问题讨论了Planar Ferromagnets and Antiferromagnets泛函在H={u(x)=(sinf(r)|xx|,cosf(r))∈H1(B1,S2);f(0)=0,f(1)=2π,r=|x|}中的径向极小元的一些性质,其中包括此泛函的径向极小元的零点的分布及若干个上界估计,并给出了这一问题的肯定回答.

Ferromagnetic Resonance Study on the Permalloy Submicron Rectangular Arrays Prepared by Electron Beam Lithography

In this paper, we report a ferromagnetic resonance study on the permalloy film of submicron sized rectangular arrays prepared by electron beam lithography and the theoretical simulation to the non uniform demagnetizing effect and ferromagnetic resonance data. By theoretical simulation, the magnetization, gyromagnetic ratio and g value of the sample are determined. The theoretical curves of the dependence of the resonance field on the field orientation φ H fit well with the experimental data. When the steady magnetic field is applied near the film normal, a series of additional regular peaks (up to eight ) appeared in the FMR spectrum on the low field side of the main FMR peak. The resonance field of these side peaks decreases linearly with the peak number. The possible physical mechanism of these multiple peaks was discussed.

High sum-frequency generation in dielectric/antiferromagnet/Ag sandwich structures

We present a method to increase the sum-frequency （SF） outputs in dielectric/antiferromagnet（AF）/Ag sandwich structures for a fixed input power. Two incident waves simultaneously illuminate the upper surface, one is oblique and the other is normal to it. Numerical calculations based on the SiO2/MnF2/Ag and ZnF2/MnF2/Ag structures show that the SF outputs on the upper film increase a few times as compared to those of a single AF film when the thickness of the AF film is one-quarter of the vacuum wavelength. Moreover, the SF outputs generated near the higher resonant frequency will be higher than those obtained near the lower resonant frequency. An optimum AF film thickness is achieved through investigating its effect on the SF outputs in the two different dielectric sandwich structures.

A Chemical Synthesis of Ferromagnetic Zn0.99Co0.01O Nano-Needles

Zn0.99Co0.01O nano-needles are synthesized by using pure ZnO powder as the starting material via chemical reactions in ammonia aqueous solution. The nano-needles show the room-temperature ferromagnetism （RTFM） characterized by using a superconducting quantum interference device. Non-reductive chemical synthesis steps ensure to prevent forming Co-metal nanoclusters within the doped sample. All the results of thermal gravimetric analysis, Fourier transform infrared spectroscopy, x-ray diffraction and ultraviolet spectroscopy demonstrate that Co ions have doped into ZnO lattices and occupied some Zn sites without changing the wurtzite structure of ZnO lattices, and no potential second phase except for the doped Co ions substituting the Zn sites in ZnO lattice can account for the observed RTFM behaviour. Moreover, the synthesis process is of high reproducibility over 80% which is higher than that of commonly-used sol-gel method.

Exchange bias in ferromagnet/antiferromagnet bilayers

Since the exchange bias （EB） effect was discovered in the Co/CoO core-shell nanoparticles, it has been extensively studied in various ferromagnet （FM）/antiferromagnet （AFM） bilayers due to its crucial role in spintronics devices. In this article, we review the investigation of the EB in our research group. First, we outline basic features of the EB, including the effects of the constituent layer thickness, the microstructure and magnetization of the FM layers, and we also discuss asymmetric magnetization reversal process in wedged-FM/AFM bilayers. Secondly, we discuss the mechanisms of the positive EB and the perpendicular EB. Thirdly, we demonstrate the hysteretic behavior of the angular dependence of the EB and analyze the EB training effect. Finally, we discuss the roles of the rotatable anisotropy in the two phenomena.

Interlayer exchange coupling in (Ga,Mn) As ferromagnetic semiconductor multilayer systems

This paper describes interlayer exchange coupling (IEC) phenomena in ferromagnetic multilayer structures, focusing on the unique IEC features observed in ferromagnetic semiconductor (Ga,Mn)As-based systems. The dependence of IEC on the structural parameters, such as non-magnetic spacer thickness, number of magnetic layers, and carrier density in the systems has been investigated by using magnetotransport measurements. The samples in the series show both a typical anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR)-like effects indicating realization of both ferromagnetic (FM) and antiferromagnetic (AFM) IEC in (Ga,Mn)As-based multilayer structures. The results revealed that the presence of carriers in the nonmagnetic spacer is an important factor to realize AFM IEC in this system. The studies further reveal that the IEC occurs over a much longer distance than predicted by current theories, strongly suggesting that the IEC in (Ga,Mn)As-based multilayers is a long-range interaction. Due to the long-range nature of IEC in the (Ga,Mn)As-based systems, the next nearest neighbor (NNN) IEC cannot be ignored and results in multi-step transitions during magnetization reversal that correspond to diverse spin configurations in the system. The strength of NNN IEC was experimentally determined by measuring minor loops that correspond to magnetization flips in specific (Ga,Mn)As layer in the multilayer system.

Flexible tuning microwave permeability spectrum in [ferromagnet/antiferromagnet]_n exchange-biased multilayer stack structure

NiFe/[IrMn/NiFe/IrMn] 5 /[NiFe/IrMn] 4 /NiFe structured exchange-biased multilayer films are designed and prepared by magnetron sputtering. The static and the microwave magnetic properties are systematically investigated. The results reveal that adding a partially pinned ferromagnetic layer can effectively broaden the ferromagnetic resonance linewidth toward the low frequency domain. Moreover, a wideband multi-peak permeability spectrum with a 3.1-GHz linewidth is obtained by overlapping the spectra of different partially pinned ferromagnetic layers and [antiferromagnet/ferromagnet/antiferromagnet] n stacks. Our results show that the linewidth of the sample can be feasibly tuned through controlling the proper exchange bias fields of different stacks. The designed multilayered thin films have potential application for a tunable wideband high frequency noise filter.

Ferromagnetism in Eu-doped ZnO films deposited by radio-frequency magnetic sputtering

This paper reports that Eu-doped ZnO films were successfully deposited on silicon （100） by radio-frequency magnetic sputtering. The x-ray diffraction patterns indicate that Eu substitutes for Zn in the lattice. Ferromagnetic loops were obtained by using superconducting quantum interference device at 10 K and room temperature. No discontinuous change was found in both of the zero-field-cooled and field-cooled curves. The observed ferromagnetism in Eu-doped ZnO can be attributed to a single magnetic phase. The saturation magnetisation decreased remarkably for the Eu-doped ZnO prepared by introducing 5% of oxygen in the sputtering gas or by the post annealing in O2, suggesting that the defects play key roles in the development of ferromagnetism in Eu-doped ZnO films.