Temperature-dependent interlayer exchange coupling strength in synthetic antiferromagnetic [Pt/Co]_2/Ru/[Co/Pt]_4 multilayers

In this work, we experimentally investigated the thermal stability of the interlayer exchange coupling field(Hex) and strength(-Jiec) in synthetic antiferromagnetic(SAF) structure of [Pt(0.6)/Co(0.6)]2/Ru(tRu)/[Co(0.6)/Pt(0.6)]4multilayers with perpendicular anisotropy. Depending on the thickness of the spacing ruthenium(Ru) layer, the observed interlayer exchange coupling can be either ferromagnetic or antiferromagnetic. The Hexwere studied by measuring the magnetization hysteresis loops in the temperature range from 100 K to 700 K as well as the theoretical calculation of the-Jiec. It is found that the interlayer coupling in the multilayers is very sensitive to the thickness of Ru and temperature. The Hexexhibits either a linear or a non-linear dependence on the temperature for different thickness of Ru. Furthermore, our SAF multilayers show a high thermal stability even up to 600 K(Hex= 3.19 kOe,-Jiec= 1.97 erg/cm~2 for tRu=0.6 nm, the unit 1 Oe = 79.5775 A·m-1), which was higher than the previous studies.

Effect of annealing time on the exchange coupling interactions and microstruc-ture of nanocomposite Pr_(7.5)Dy_1Fe_(71)Co_(15)Nb_1B_(4.5) ribbons

The influence of annealing time on the magnetic properties and microstructure of nanocomposite Pr7.5Dy1Fe71Co15Nb1B4.5 ribbons was systematically investigated by the methods of vibrating sample magnetometer （VSM）, X-ray diffraction （XRD） and high resolution transmission electron microscopy （HRTEM）. Interaction domains derived from strong exchange coupling interactions between hard and soft magnetic grains were imaged using magnetic force microscopy （MFM）. Maximum remanence, intrinsic coercivity, and maximum energy product values were obtained in the ribbons annealed at 700℃ for 15 min, which were composed of Pr2（Fe, Co）14B, α-（Fe, Co）, and slight Pr2（Fe, CO）17 phases. Although Jr, Hci, and （Bn）max decreased gradually with further increase of annealing time, it is emphasized that comparatively high Jr and Hci and （BH）max were obtained in a wide annealing time period of 15 to 360 min. The shape of initial magnetization curves and hysteresis loops change as a function of annealing time, indicating different magnetization reversal routes, which can be fully explained by the corresponding microstructure.

Effect of chromium interlayer on magnetic exchange coupling of SmCo/Cr/TbFeCo multilayer thin films

A series of SmCo/Cr/TbFeCo multilayer thin films with perpendicular anisotropy were prepared by RF- magnetron sputtering system, and the effects of Cr interlayer thickness on magnetic properties and interlayer exchange coupling were investigated. It was found that the magnetic properties varied with the thickness of Cr interlayer, especially the values of saturation magnetization Ms and the coercivity Hc fluctuated periodically with the thickness of Cr interlayer. STM images revealed that the variation of coercivity Hc was attributed to the microstructure change of SmCo layer influenced by Cr interlayer, and the variation of Ms was related to interlayer exchange coupling.

Effect of exchange coupling on magnetic property in Sm–Co/α-Fe layered system

The hysteresis loops as well as the spin distributions of Sm-Co/a-Fe bilayers have been investigated by both three- dimensional （3D） and one-dimensional （1D） micromagnetic calculations, focusing on the effect of the interface exchange coupling under various soft layer thicknesses ts. The exchange coupling coefficient Alas between the hard and soft ,layers varies from 1.8 x10-6 erg/cm to 0.45 x 10-6 erg/cm, while the soft layer thickness increases from 2 nm to 10 nm. As the exchange coupling decreases, the squareness of the loop gradually deteriorates, both pinning and coercive fields rise up monotonically, and the nucleation field goes down. On the other hand, an increment of the soft layer thickness leads to a significant drop of the nucleation field, the deterioration of the hysteresis loop squareness, and an increase of the remanence. The simulated loops based on the 3D and 1D methods are consistent with each other and in good agreement with the measured loops for Sm-Co/a-Fe multilayers.

Large coercivity and unconventional exchange coupling in manganese-oxide-coated manganese gallium nanoparticles

The microstructures and magnetic properties of nanoparticles, each composed of an antiferromagnetic （AFM） manganese-oxide shell and a ferromagnetic-like core of manganese-gallium （MnGa） compounds, are studied. The coreshell structure is confirmed by transmission electron microscope （TEM）. The ferromagnetic-like core contains three kinds of MnGa binary compounds, i.e., ferrimagnetic （FI） DO22-type MnaGa, ferromagnetic （FM） Mn8Gas, and AFM DO19-type Mn3Ga, of which the first two correspond respectively to a hard magnetic phase and to a soft one. Decoupling effect between these two phases is found at low temperature, which weakens gradually with increasing temperature and disappears above 200 K. The exchange bias （EB） effect is observed simultaneously, which is caused by the exchange coupling between the AFM shell and FM-like core. A large coercivity of 6.96 kOe （1Oe = 79.5775 A·m^-1） and a maximum EB value of 0.45 kOe are achieved at 300 K and 200 K respectively.

Magnetic anisotropy,exchange coupling and Dzyaloshinskii–Moriya interaction of two-dimensional magnets

The two-dimensional(2D)magnets provide novel opportunities for understanding magnetism and investigating spin related phenomena in several atomic thickness.Multiple features of 2D magnets,such as critical temperatures,magnetoelectric/magneto-optic responses,and spin configurations,depend on the basic magnetic terms that describe various spins interactions and cooperatively determine the spin Hamiltonian of studied systems.In this review,we present a comprehensive survey of three types of basic terms,including magnetic anisotropy that is intimately related with longrange magnetic order,exchange coupling that normally dominates the spin interactions,and Dzyaloshinskii–Moriya interaction(DMI)that favors the noncollinear spin configurations,from the theoretical aspect.We introduce not only the physical features and origin of these crucial terms in 2D magnets but also many correlated phenomena,which may lead to the advance of 2D spintronics.

Anisotropic exchange coupling interaction between hard-hard magnetic grains in sintered SrFe_(12)O_(19)ferrites

Exchange coupling interaction in sintered magnetic materials is generally isotropic.In this study,the anisotropic exchange coupling interaction was found in sintered oblate cylindrical SrFe_(12)O_(19)(SrM)specimens obtained by the SrM nanopowders synthesized via a hydrothermal method.According to Henkel plots,the exchange coupling interaction between hard-hard magnetic grains was found in both as-pressed and sintered specimens.However,the exchange coupling interaction can only be found in the in-plane direction but not in the out-of-plane direction for thel sintered specimens.By building a model of a grain configuration,this anisotropy of the exchange coupling interaction was ascribed to the vertically arranged plate-like SrM grains with micrometers in width but nanometers in thickness,which was confirmed by morphologies of cross sections in fractured specimens.

Effect of zirconium content on exchange coupling and magnetization reversal of nanocrystalline Nd_(12.3)Fe_(81.7-x)Zr_xB_6 alloy

The effect of Zr content on exchange coupling and magnetization reversal of the Ndl2.3Fe81.7_xZrxB6 （x=0-3.0） ribbons was systematically investigated. Interaction domains were imaged by magnetic force microscopy （MFM）. The strength of interactions determined by Wohlfarth＇s analysis increased first with Zr content x increasing, reached the maximum value at x=l.5, and then decreased with x further increasing. Initial magnetization curves and dependence of coercivity and remanence on applied magnetic field showed that the mechanism of coercivity in all samples was mainly of exchange coupling pinning type, which was enhanced with x increasing. It was found by three-dimensional atom probe （3DAP） that Zr atoms did not partition into the Nd2Fe14B hard magnetic phase, but significantly enriched at the interfacial region.

The influence of interface exchange coupling on the demagnetization process for perpendicularly oriented FePt/α-Fe/FePt trilayers

The influence of the interface exchange coupling on the magnetization reversal process for a FePt/α-Fe/FePt tri-layer structure has been studied through a micromagnetic approach.The analytical formula of the nucleation field has been derived.It is found that the nucleation field increases as the interface coupling constant rises.Especially when the thickness of the soft layer is small,the influence of the exchange coupling on the nucleation field is significant.The angular distributions of the magnetization for various exchange coupling constants have been obtained by numerical calculation.It is found that the angular distribution of the magnetization is discontinuous at the interface of the hard and soft layers.In the meantime,the pinning field decreases with the increase of the thickness of the soft layer and the exchange coupling constant.

Dense skyrmion crystal stabilized through interfacial exchange coupling:Role fo in-plane anisotropy

A Monte Carlo simulated-annealing algorithm was used to study the magnetic state in an in-plane helimagnet layer on triangular lattice that exchange couples to an underlayer with strong out-of-plane anisotropy.In the single helimagnet layer with in-plane anisotropy(K),the formation of labyrinth-like domains with local spin spirals,instead of parallel stripes,is favored,and these domains rapidly transform into dense skyrmion crystals with increasing interfacial exchange coupling(J'),equivalent to a virtual magnetic field,and finally evolve to an out-of-plane uniform state at large enough J'.Moreover,with increasing K,the skyrmion crystal state can vary from regular 6-nearest-neighboring circular skyrmion arrangement to irregular squeezed skyrmions with less than 6 nearest neighbors when the in-plane anisotropy energy is higher than the interfacial exchange energy as the skyrmion number is maximized.Finally,we demonstrated that the antiferromagnetic underlayer cannot induce skyrmions while the chirality inversion can be achieved on top of an out-of-plane magnetization underlayer with 180°domain walls,supporting the experimental findings in FeGe thin film.This compelling advantage offers a fertile playground for exploring emergent phenomena that arise from interfacing magnetic skyrmions with additional functionalities.

Performance of switch between exchange bias and coercivity:Influences of antiferromagnetic anisotropy and exchange coupling

The study on temperature dependence of exchange bias field and coercivity is crucial to solving the writing/reading dilemma in magnetic recording.Motivated by recent experimental findings,a complete switch between exchange bias field and coercivity with temperature is proposed,and the performance,characterized by average switching temperature(T_(S))and switching temperature width(T_(W)),controlled by antiferromagnetic anisotropy(KAF)and exchange coupling(J_(AF))constants is studied based on a MonteCarlo simulation.The results show that a linear relationship between T_(S)and KAFis established when KAFis above a critical value,while T_(S)is weakly influenced by J_(AF).On the contrary,T_(W)is insensitive to KAF,while strongly depends on J_(AF).Besides overcoming thermal energy,the increase of KAFfor a small J_(AF)guarantees the completely frozen states in the antiferromagnetic layers during magnetizing at higher temperature,below which the exchange bias field exists with a negligible coercivity.Otherwise,for a large J_(AF),the uncompensated antiferromagnetic magnetization behavior during the ferromagnetic magnetization reversal becomes complicated,and the switching process in the low temperature range depends on the irreversibility of uncompensated antiferromagnetic magnetization reversal during magnetizing,while in the high temperature range mainly influenced by the field-cooling process,resulting in a large T_(W).This work provides an opportunity to control/optimize the performance of the temperatureinduced switch between unidirectional and uniaxial symmetries through precisely tuning KAFand/or J_(AF)to meet different application demands in the next generation information technology.

Effect of Phosphor Addition on Intergranular Exchange Coupling of Co-Pt Thin Films

In this paper, ternary Co-Pt-P thin films were prepared by Co-P chips pasted on the cobalt target. The structure magnetron sputtering with platinum and home-made and magnetic properties were investigated by X-ray diffraction （XRD） and vibrating sample magnetometer （VSM）, respectively. With increasing phosphor content, the coercivities of Co-12 at.% Pt-P films increase from 1034 to 1525 Oe owing to the exchange decoupling among magnetic grains. The decrease of inter-granular exchange coupling was confirmed by delta-M curve measurement and magnetic force microscopy （MFM）. Transmission electronic microscopy （TEM） with nano- beam composition analysis shows that phosphor segregated at cobalt-based grain boundaries is responsible for the exchange decoupling. It is thus suggested that the phosphor addition is effective to tune the exchange coupling of magnetic grains, particularly for perpendicular recording media.

Inter-granular exchange coupling and magnetic anisotropy of Ta/Ru/Co-23 at%Pt perpendicular thin films with different Ru underlayer thicknesses

In the present work, a series of Ta/Ru/Co- 23 at%Pt thin films with varied Ru underlayer thicknesses were fabricated by magnetron sputtering. All of the films show c-axis preferred orientation perpendicular to the film surface. The drop of c/a ratio and lattice expansion of Co- Pt layer with the increase in Ru underlayer thickness was revealed by X-ray diffraction （XRD）. The coercivity of the Ta/Ru/Co-Pt thin films increases drastically with Ru underlayer thickness increasing, due to the enhancement of effective magneto-crystalline anisotropy constant and exchange decoupling of magnetic nano-grains. The enhancement of effective magneto-crystalline anisotropy constant is ascribed to the lattice deformation of Co-Pt layer by mismatching the Ru layer and Co-Pt surface. Moreover, the exchange decoupling of magnetic nanograins is attributed to the further isolation of magnetic nano-grains.

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO_(3)(001)

Exchange coupling across the interface between a ferromagnetic(FM)layer and an antiferromagnetic(AFM)or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy,which has been extensively studied due to the important application in magnetic materials and devices.In this work,we observed a fourfold magnetic anisotropy in amorphous Co Fe B layer when exchange coupling to an adjacent Fe Rh layer which is epitaxially grown on an SrTiO_(3)(001)substrate.As the temperature rises from 300 K to 400 K,Fe Rh film undergoes a phase transition from AFM to FM phase,the induced fourfold magnetic anisotropy in the Co Fe B layer switches the orientation from the Fe Rh<110>to Fe Rh<100>directions and the strength is obviously reduced.In addition,the effective magnetic damping as well as the two-magnon scattering of the Co Fe B/Fe Rh bilayer also remarkably increase with the occurrence of magnetic phase transition of Fe Rh.No exchange bias is observed in the bilayer even when Fe Rh is in the nominal AFM state,which is probably because the residual FM Fe Rh moments located at the interface can well separate the exchange coupling between the below pinned Fe Rh moments and the Co Fe B moments.

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic bilayers

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic （FM/AF） bilayers has been investigated in detail by extending Slonczewski＇s ＇proximity magnetism＇ idea. Here three important parameters are discussed for FM/AF bilayers, i.e. interracial bilinear exchange coupling J1, interracial biquadratic （spin-flop） exchange coupling J2 and antiferromagnetic layer thickness tAF. The results show that both the occurrence and the variety of the exchange bias strongly depend on the above parameters. More importantly, the small spin-flop exchange coupling may result in an exchange bias without the interracial bilinear exchange coupling. However, in general, the spin-flop exchange coupling cannot result in the exchange bias. The corresponding critical parameters in which the exchange bias will occur or approach saturation are also presented.

The rotational anisotropies in the ferromagnetism/antiferromagnetism 1/ antiferromagnetism 2 exchange bias structures

The rotational anisotropies in the exchange bias structures of ferromagnetism/antiferromagnetism 1/antiferro- magnetism 2 are studied in this paper. Based on the model, in which the antiferromagnetism is treated with an Ising mean field theory and the rotational anisotropy is assumed to be related to the field created by the moment induced on the antiferromagnetic layer next to the ferromagnetic layer, we can explain why in experiments for ferromag- netism （FM）/antiferromagntism 1 （AFM1）/antiferromagnetism 2 （AFM2） systems the thickness-dependent rotational anisotropy value is non-monotonic, i.e. it reaches a minimum for this system at a specific thickness of the first anti- ferromagnetic layer and exhibits oscillatory behaviour. In addition, we find that the temperature-dependent rotational anisotropy value is in good agreement with the experimental result.

Ru thickness-dependent interlayer coupling and ultrahigh FMR frequency in FeCoB/Ru/FeCoB sandwich trilayers

The antiferromagnetic(AFM) interlayer coupling effective field in a ferromagnetic/non-magnetic/ferromagnetic(FM/NM/FM) sandwich structure, as a driving force, can dramatically enhance the ferromagnetic resonance(FMR) frequency. Changing the non-magnetic spacer thickness is an effective way to control the interlayer coupling type and intensity, as well as the FMR frequency. In this study, Fe Co B/Ru/Fe Co B sandwich trilayers with Ru thickness(tRu) ranging from 1 A to 16 A are prepared by a compositional gradient sputtering(CGS) method. It is revealed that a stress-induced anisotropy is present in the Fe Co B films due to the B composition gradient in the samples. A tRu-dependent oscillation of interlayer coupling from FM to AFM with two periods is observed. An AFM coupling occurs in a range of 2 A ≤ tRu≤ 8 A and over 16 A, while an FM coupling is present in a range of tRu< 2 A and 9 A ≤ tRu≤ 14.5 A. It is interesting that an ultrahigh optical mode(OM) FMR frequency in excess of 20 GHz is obtained in the sample with tRu= 2.5 A under an AFM coupling. The dynamic coupling mechanism in trilayers is simulated, and the corresponding coupling types at different values of tRuare verified by Layadi’s rigid model. This study provides a controllable way to prepare and investigate the ultrahigh FMR films.

Theoretical Studies on the Spin Exchange Interaction in Copper(II) Complexes Coordinated with Nitronyl Nitroxide

Nitronyl nitroxide radical 1, NIT (4, 4, 5, 5-tetramethyl-4, 5-dihydro-1H-imidazolyl-1- oxyl-3-oxide) and copper(II) chloride complexes with nitronyl nitroxide 2, [Cu(NITPh)2Cl2] (NITPh = 2-phenyl-4, 4, 5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide) were studied with density functional theory (DFT). The magnetic orbital analysis reveals that the antiferromagnetic coupling for complex 2 is due to the antibonding s*-orbital overlap between 22x-yd(Cu) and p* (NO) orbitals. Also, spin population and atomic charge distribution analysis suggest that for AFS of complex 2 the antiferromagnetic coupling between the radical ligands and the copper(II) ion originates from the spin delocalization induced by the a electron transfer from p*(NO) to 22x-yd(Cu) orbital.

Surface states modulated exchange interaction in Bi2Se3/thulium iron garnet heterostructures

We investigate the modulation of magnetic anisotropy of thulium iron garnet(TmIG)films by interfaced Bi2Se3 thin films.High quality epitaxial growth of Bi2Se3 films has been achieved by molecular beam epitaxy on TmIG films.By the method of ferromagnetic resonance,we find that the perpendicular magnetic anisotropy(PMA)of TmIG can be greatly strengthened by the adjacent Bi2Se3 layer.Moreover,the competition between topological surface states and thickness dependent bulk states of Bi2Se3 gives rise to the modulation of PMA of the Bi2Se3/TmIG heterostructures.The interfacial interaction can be attributed to the enhanced exchange coupling between Fe^3+ions of TmIG mediated by topological surface electrons of Bi2Se3.

Nanocrystalline exchange-coupled Pr-Fe-B permanent magnets

Nanocomposite Pr2Fe14B/α-Fe permanent magnets were prepared by melt spinning and subsequent crystallizahon of Pr8Fe86B6, amorphous Precursnors. The microstructure is a two-phase nanocomposite of Pr2 Fe14 B and softmagnetic α-Fe with an average size of 30nm. X-ray diffration, Thermomagnetic analysis and TEM analy0sis indicatetha amorphous Pr8Fe86B6, alloy crystallizes through the process of Am→Am→Am'+α-Fe→Pr2Fe23B3+α-Fe-Pr2Fe14B+α-FeThe highest value of remanence (Br), cocreivity (Hci) and maximum energy Product ((BH)max) of the nanocrystallinealloys are 1.10T, 340 kA/m and 110 kJ/m3 respechvely, exhibihng remarkable remanence enhancement. The effect ofannaling temperature and time on the microstructure and magnetic properties was also studied. The resultS show thatappropriate annealing temperature and time are important for obtaining the optimal microstructure and the bestmagnetic properties.