Effective Exchange Coupling Research Articles

The microstructure and magnetization reversal behavior of melt-spun (Nd1−xCex)-Fe-B ribbons

In this study, the alloy ingots with nominal compositions of (Nd1−xCex)31FebalCo0.2Ga0.1B (x = 0, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%) were prepared and then melt-spun to form nanocrystalline ribbons at the wheel speed of 20 m/s. XRD results show that all melt-spun ribbons exhibit the tetragonal structure (Nd,Ce)2Fe14B phase with the space group P42/mmm. The Curie temperature and lattice constant decrease with the increase of Ce content. The Curie temperature decreases gradually from 306 to 247 °C with the increase of Ce content. Those results indicate that Ce element has been incorporated into Nd2Fe14B main crystalline phase and formed (Ce,Nd)-Fe-B hard magnetic phase. It is also found that the remanence ratio (Mr/Ms) decreases from 0.693 to 0.663 and the coercivity (Hc) decreases from 18.7 to 14.2 kOe with the increase of Ce content. However, a relatively high coercivity of 18.3 kOe for (Nd1−xCex)31FebalCo0.2Ga0.1B (x = 0.2) melt-spun ribbon is achieved. The coercivity is sensitive to microstructure. The AFM patterns show the sample (x = 0.2) has the most uniform and finest microstructure. The magnetization reversal behavior (δM plots) is discussed in detail. The positive δM value is observed in every sample, which confirms the existence of exchange coupling interaction. Evidently, the δM maximum value reaches 0.9 in the sample (x = 0.2). It is indicated that the intergranular exchange coupling effect is the strongest, which is consistent with coercivity enhancing.

Exchange-bias reversal in Mn2−xNi1+xGa films with antisite disorder

We report on the exchange-bias reversal in Mn2−xNi1+xGa (x = 0, 0.2, 0.4, and 0.6) thin films grown by molecular beam epitaxy on GaAs substrates. Exchange bias reversal is observed in these intrinsically ferrimagnetic films together with a reduction in the total moment even under increasing of Ni content which usually increases the total moment. These magnetic features indicate that antisite disorder is responsible for engender changes in the exchange-bias and total moment. Density functional theory calculations were performed to determine the magnetic moment dependence with the volume of cubic austenite and tetragonal martensite structures. A simple phenomenological approach is used to extract an effective exchange coupling constant from exchange bias fields.

On The Density Functional Theory Treatment of Lanthanide Coordination Compounds: A Comparative Study in a Series of Cu-Ln (Ln = Gd, Tb, Lu) Binuclear Complexes.

The nontrivial aspects of electron structure in lanthanide complexes, considering ligand field (LF) and exchange coupling effects, have been investigated by means of density functional theory (DFT) calculations, taking as a prototypic case study a series of binuclear complexes [LCu(O2COMe)Ln(thd)2], where L2- = N,N'-2,2-dimethyl-propylene-di(3-methoxy-salicylidene-iminato) and Ln = Tb, Lu, and Gd. Particular attention has been devoted to the Cu-Tb complex, which shows a quasi-degenerate nonrelativistic ground state. Challenging the limits of density functional theory (DFT), we devised a practical route to obtain different convergent solutions, permuting the starting guess orbitals in a manner resembling the run of the β electron formally originating from the f8 configuration of the Tb(III) over seven molecular orbitals (MOs) with predominant f-type character. Although the obtained states cannot be claimed as the DFT computed split of the 7F multiplet, the results are yet interesting numeric experiments, relevant for the ligand field effects. We also performed broken symmetry (BS) DFT estimation of exchange coupling in the Cu-Gd system, using different settings, with Gaussian-type and plane-wave bases, finding a good match with the coupling parameter from experimental data. We also caught BS-type states for each of the mentioned series of different states emulated for the Cu-Tb complex, finding almost equal exchange coupling parameters throughout the seven LF-like configurations, the magnitude of the J parameter being comparable with those of the Cu-Gd system.

The itinerant magnetism in a 3d-4d double perovskite Sr2CrMoO6

In this work, we use the exact diagonalization and Monte Carlo calculations to study magnetic behaviors of the 3d-4d double perovskite Sr2CrMoO6. The model is described by a quantum Hamiltonian induced by the hybridization mechanism in Sr2CrMoO6 via the double exchange, considering the transition metal Mo5+ (σ=1/2, 4d1) cation totally non-magnetic and classical core spins S=3/2 located at sites of Cr3+ (S=3/2, 3d3) cations. We have defined a Hamiltonian matrix and determined eigen-energies which are functions of core spins interactions. At ground state, we have found that the ferromagnetic phase of core spins stabilizes the system for the electronic density n=0.25. To study magnetic properties at finite temperature, we have defined an effective magnetic Hamiltonian for spins, approving the Monte Carlo simulations for systems of high sizes. Thus, the exchange coupling effect, the magnetization and the magnetic susceptibility are investigated for different sizes, and the critical temperature is determined.

Structural, magnetic and microwave properties of exchange coupled and non-exchange coupled BaFe12O19/NiFe2O4 nanocomposites

In the present work, exchange coupled and non-exchange coupled BaFe12O19 (BaM)/NiFe2O4 (NiF) nanocomposites with varying composition were prepared by adopting two different processing methods. Phase formation and microstructural changes were analyzed by X-ray diffraction (XRD) and Scanning electron microscopy (SEM) respectively. Effect of exchange coupling on magnetic and microwave properties (in Ku-band) were studied by vibrating sample magnetometer and vector network analyzer respectively. XRD patterns confirmed the co-existence of BaM and NiF phases in composites without any secondary phase. SEM micrographs showed the well distinguished BaM and NiF grains in exchange coupled composites. Smooth hysteresis loops were observed for exchange coupled composites in contrast to non-exchanged coupled, which showed stepped-loops for all studied compositions. Higher saturation magnetization (Ms) was also found in exchange coupled composites. Whereas, Ms showed a linear decrease with NiF content in non-coupled composites. Coercivity found to decrease with NiF content with distinct trend among the two systems. Frequency dependent complex permittivity and permeability showed large variation between the two systems. Reflection losses at multiple frequencies were observed in exchange coupled composites.

Effect of dysprosium on structural and physical properties of Ba2NiCoFe12O22 Y-type hexaferrites

A series of Ba2NiCoDy x Fe12-x O22 (x = 0.00, 0.05, 0.01, 0.15, 0.20, 0.25) nano-structured Y-type hexaferrites were synthesized by the sol-gel auto-combustion technique. XRD analysis revealed Y-type hexagonal phase is formed. The increase in Dy content leads to the slight increase in the lattice parameter c indicating the distortion in the cell due to difference in atomic radii. SEM micrograph confirms the increase in grain size with the substitution of Dy. The irregular platelet-like particles were also observed in SEM profiles. Electrical permittivity decreases with the increase of frequency due to interfacial polarization in accordance with Maxwell Wagner model. At high frequencies, grain boundaries were effective and at low frequencies, the permittivity decreases due to conducting grains. Magnetization and coercivity also decreases as the Dy doping increases. The decrease in magnetization may be attributed to segregation of Dy at grain boundaries. The decrease in coercivity is due to decrease in exchange coupling effect between neighboring domains.

Magnetic excitations from the two-dimensional interpenetrating Cu framework in Ba2Cu3O4Cl2

We report detailed neutron scattering studies on Ba$_2$Cu$_3$O$_4$Cl$_2$. The compound consists of two interpenetrating sublattices of Cu, labeled as Cu$_{\rm A}$ and Cu$_{\rm B}$, each of which forms a square-lattice Heisenberg antiferromagnet. The two sublattices order at different temperatures and effective exchange couplings within the sublattices differ by an order of magnitude. This yields an inelastic neutron spectrum of the Cu$_{\rm A}$ sublattice extending up to 300 meV and a much weaker dispersion of Cu$_{\rm B}$ going up to around 20 meV. Using a single-band Hubbard model we derive an effective spin Hamiltonian. From this, we find that linear spin-wave theory gives a good description to the magnetic spectrum. In addition, a magnetic field of 10 T is found to produce effects on the Cu$_{\rm B}$ dispersion that cannot be explained by conventional spin-wave theory.

Synthesis of magnetically exchange coupled SrFe12O19/FeCo composites through cryogenic ball milling

SrFe12O19/FeCo composite particles with different mass ratios of SrFe12O19 to FeCo were synthesized through a cryogenic ball milling process. The corresponding products were characterized with scanning electron microscopy (SED), transmission electron microscopy (TEM), x-ray diffraction (XRD) and vibrating sample magnetometer (VSM) for crystal morphology, elemental distribution, crystal phases, and magnetic properties. The results showed that when the mass percentage of FeCo was less than 15%, smooth magnetic hysteresis loops could be obtained from SrFe12O19/FeCo composite particles, indicating effective magnetic exchange coupling between the SrFe12O19 and FeCo particles. A further FeCo mass increase resulted in kinks in the magnetic hysteresis loop and destroyed the magnetic exchange coupling. As a comparison, room temperature ball milling of SrFe12O19/FeCo (95/5 wt%) cannot achieve magnetic exchange coupling between SrFe12O19 and FeCo due to FeCo nanoparticle agglomeration.

All-Silicon Switchable Magnetoelectric Effect through Interlayer Exchange Coupling.

The magnetoelectric (ME) effect originating from the effective coupling between electric field and magnetism is an exciting frontier in nanoscale science such as magnetic tunneling junction (MTJ), ferroelectric/piezoelectric heterojunctions etc. The realization of switchable ME effect under external electric field in d0 semiconducting materials of single composition is needed especially for all-silicon spintronics applications because of its natural compatibility with current industry. We employ density functional theory (DFT) to reveal that the pristine Si(111)-3×3 R30° (Si3 hereafter) reconstructed surfaces of thin films with a thickness smaller than eleven bilayers support a sizeable linear ME effect with switchable direction of magnetic moment under external electric field. This is achieved through the interlayer exchange coupling effect in the antiferromagnetic regime, where the spin-up and spin-down magnetized density is located on opposite surfaces of Si3 thin films. The obtained coefficient for the linear ME effect can be four times larger than that of ferromagnetic Fe films, which fail to have the reversal switching capabilities. The larger ME effect originates from the spin-dependent screening of the spin-polarized Dirac fermion. The prediction will promote the realization of well-controlled and switchable data storage in all-silicon electronics.

Exchange bias and strain effect co-modulated magnetic symmetry in La0.6Sr0.4MnO3/orthorhombic-YMnO3 multiferroic heterostructures

The exchange bias and strain effect co-modulated magnetic symmetry in all oxide La0.6Sr0.4MnO3 (LSMO) and orthorhombic YMnO3 (YMO) multiferroic heterostructures were studied. Because of the lattice mismatch between the LSMO and YMO layers, the LSMO layer exhibits a 90° rotation growth on the YMO layer. The strain induced growth not only leads to a 90° phase shift in the anisotropic magnetoresistance (AMR) curves, but also brings a two-fold symmetric magnetoelastic coupling energy along the LSMO direction. With the incorporation of magnetoelastic coupling energy and exchange coupling energy, the exchange bias induced torque shows a phase shift and causes the asymmetry of the peak position and value in the AMR curves. This work illustrates a modulated magnetic symmetry in ferromagnetic/multiferroic systems by interfacial exchange coupling and strain effect, which will benefit the design of magnetoelectric devices.

Magnetic Relaxation Dynamics of a Centrosymmetric Dy2 Single-Molecule Magnet Triggered by Magnetic-Site Dilution and External Magnetic Field.

A centrosymmetric Dy2 single-molecule magnet (SMM) and its doped diamagnetic yttrium analogues, Dy0.19Y1.81 and Dy0.10Y1.90, were solvothermally synthesized to investigate the effects of intramolecular exchange coupling and quantum tunneling of magnetization (QTM) on the magnetic relaxation dynamics. Constructed from two hula-hoop-like DyIII ions and a pair of phenoxido groups, the antiferromagnetically coupled Dy2 exhibits a thermal-activated zero-field effective energy barrier (Ueff) of 277.7 K and negligible hysteresis loop at 2.0 K. The doping of a diamagnetic YIII matrix with 90.5% and 95.0% molar ratios reveals the single-ion origin of the Orbach channel, increases the relaxation time by partially quenching the QTM process, and induces an open hysteresis loop until 5.0 K. In contrast, an optimal dc field of 1.0 kOe improves the barrier height up to 290.1 K through complete elimination of the QTM and delays the relaxation time of the direct relaxation pathway. More interestingly, the collaborative dual effects of magnetic-site dilution and external magnetic field make the effective energy barrier and relaxation time increase 8.1% and 49 times, respectively. Thus, the overall magnetization dynamics of the Dy2 system systematically elaborate the inherent interplay of the QTM and Orbach processes on the effective energy barrier, highlighting the vital role of the relaxation time on the coercive hysteresis loop.

The effects of interfacial exchange coupling in Fe/ErFeO3 heterostructures

Exploring exchange bias in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures is vital for both fundamental magnetism and practical application. However, in the case of conventional FM/AFM systems, the essential field cooling process above the Néel temperature of AFM materials hinders their application if the Néel temperature is far higher than room-temperature. Here, we report the effects of interfacial exchange coupling in Fe/ErFeO3 heterostructures. The magnetic-field-induced switchable exchange bias, originating from the AFM exchange coupling between Fe film and Dzyaloshinskii–Moriya-interaction-induced net moment of ErFeO3 along c axis, is successfully achieved without field cooling or in-field growth process of AFM. Different from the most previous pinning layer using a hard FM or traditional AFM, ErFeO3 pinning layer has the advantages of both the magnetic field sensitivity (~780 Oe) and ultrahigh dynamic frequency. In addition, although Fe film is polycrystalline, it exhibits a strong uniaxial magnetic anisotropy resulted from the so-called ‘spin-flop-coupling effect’, i.e. the magnetic coupling between Fe film and the compensated G-type AFM spins of EFO along a axis. Interestingly, the exchange bias field and asymmetric switching field offer entirely different information about the asymmetry of magnetization reversal near hard axis. The asymmetric switching field is further proved to be an effective measure to determine the weak unidirectional magnetic anisotropy for film with nearly 180° domain wall displacement. Our experimental results provide a practical method to establish room-temperature exchange bias in FM/G-type AFM without field cooling. Furthermore, the magnetic-field-induced switchable exchange-bias, the spin-flop coupling effect and the angular dependent asymmetry of magnetization reversal in the vicinity of hard axis in Fe/ErFeO3 heterostructures may provide new insights on the interfacial exchange coupling in FM/AFM systems.

Physical Mechanisms of Exchange Coupling Effects in Nanoparticulate Diluted Magnetic Oxides Obtained by Laser Pyrolysis

TiO2 nanoparticles, undoped and doped with Fe, have been prepared by laser pyrolysis and further investigated with respect to morphological, structural and magnetic aspects by transmission electron microscopy, diffractometry, Mossbauer spectroscopy, and magnetometry. The obtained nanoparticles, consisting of mainly anatase phase, agglomerate in clusters of tenths of units and present a large size distribution in the range from 5 to 40 nm. The anatase to rutile weight ratio (about 9) and the morphology of particles is similar in all analyzed samples (doped by up to 12 at. % Fe). Only Fe3+ ions in high spin configuration were observed mainly at the surface of TiO2 nanoparticles, either distributed or forming fine clusters of Fe oxide. Both a paramagnetic phase and a superparamagnetic one with blocking temperature lower than 50 K are superposed over a long-range ferromagnetic phase specific to diluted magnetic oxide systems. The influence of doping Fe ions on the magnetic behavior of each phase is discussed ...

Well-controlled exchange bias effect in MnO@Mn3O4 core-shell nanoparticles with an inverted coupling structures

The influence of the varied thickness of antiferromagnetic (AFM) and ferrimagnetic (FiM) nanostructure in MnO@Mn3O4 core-shell nanoparticles (NPs) on the exchange bias effect has been investigated. The MnO@Mn3O4 core-shell NPs with varied radius of MnO core (RMnO) and thickness of Mn3O4 shell (TMn3O4) have been synthesized by in-situ oxidation of the pure MnO NPs in organic solution containing a mild oxidant at different temperatures. The exchange bias effect can be tuned by changing the RMnO (TMn3O4) of the MnO@Mn3O4 core-shell NPs. The strongest exchange coupling effect of both 17.2 and 25.0 nm MnO@Mn3O4 core-shell NPs occur when RMnO=TMn3O4=R/2. It is expected that the finding of the control of exchange bias effect by varied RMnO and TMn3O4 in inverted core-shell NPs is helpful for the further understanding of the exchange bias mechanism.

The fabrication and characterization of MnAl/FeCo composite thin films with enhanced maximum energy products

This paper reported a method to increase the maximum energy products of MnAl thin films through magnetic exchange coupling. MnAl and MnAl/FeCo thin films were deposited through DC magnetron sputtering on Si wafer substrate. The morphology of the thin films was investigated through SEM, TEM and AFM, and the magnetic properties were investigated through VSM. Results show that there is an effective magnetic exchange coupling between MnAl and FeCo thin films when the FeCo layer is no larger than 8nm. The magnetic exchange coupling was demonstrated by smooth magnetic hysteresis loops, enhanced maximum energy products, and positive peaks in the Henkel Plots. The results reported in this paper could be used to further increase the magnetic performance of MnAl thin films.

Synthesis of magnetically exchange coupled CoFe2O4/CoFe2 core/shell composite particles through spray pyrolysis

Abstract Magnetically exchange coupled CoFe2O4/CoFe2 composite particles were synthesized through spray pyrolysis, followed by H2 reduction. The H2 concentration was controlled to be 5, 8 and 10 vol%, respectively, in three parallel experiments. The products show a core/shell structure, with CoFe2O4 as the core, and CoFe2 as the shell. The structure is confirmed by TEM-EDX and STEM analysis. The XRD patterns indicate the existence of both CoFe2O4 and CoFe2 phases in the composites. The magnetic hysteresis loop of CoFe2O4/CoFe2 composite particles reduced by 5 vol% H2 is smooth and kink-free, indicating an effective exchange coupling between CoFe2O4 and CoFe2. Henkel Plots are further used to investigate the magnetic exchange coupling in the composites. This paper presented a convenient, environmentally friendly and effective method to synthetize magnetically exchange coupled composite particles.

Magnetic properties of a doped graphene-like bilayer

A doped graphene-like bilayer is described using a four-sublattice Heisenberg model both ferromagnetic and antiferrimagnetic couplings. The magnetic properties of the bilayer system are studied using the Heisenberg model, retarded Green's function and the linear spin-wave approximation. The spin-wave spectra, energy gap, and the magnetization and quantum fluctuation of the system at the ground state are calculated with various intra- and interlayer couplings. The results indicate that the effect of antiferromagnetic exchange coupling on the magnetic properties of the system is significant. Magnetizations at low temperature show intersection points due to the quantum effects.

Exchange biased Co3O4 nanowires: A new insight into its magnetic core–shell nature

We investigated interfacial exchange coupling effect in nano casted Co3O4 nanowires. Magnetometry measurements indicated that the magnetic response of the wires has two contributions. First one from the core of the wire which has characteristics of a 2D-DAFF(two-dimensional diluted antiferromagnet in a field). The second one is from uncompensated surface spins which get magnetically ordered towards the field direction once field cooled below 25K. Below 25K, the net magnetization of the core of the wire gets exchange coupled with the uncompensated surface spins giving rise to exchange bias effect. The unique 2D-DAFF/spin-glass core/shell heterostructure showed a pronounced training effect in the first field cycling itself. The magnitude of exchange bias field showed a maximum at intermediate cooling fields and for the higher cooling field, exchange bias got reduced.

Long-range exchange interaction between magnetic impurities in graphene

The effective spin exchange coupling between impurities (adatoms) on graphene mediated by conduction electrons is studied as a function of the strength of the potential part of the on-site energy $U$ of the electron-adatom interaction. With increasing $U$, the exchange coupling becomes long-range, determined largely by the impurity levels with energies close to the Dirac points. When adatoms reside on opposite sublattices, their exchange coupling, normally antiferromagnetic, becomes ferromagnetic and resonantly enhanced at a specific distance where an impurity level crosses the Dirac point.

Tm(iii) complexes undergoing slow relaxation of magnetization: exchange coupling and aging effects.

The present study focuses on the dynamic magnetic behaviour of exchange coupled 3d-4f complexes containing the scarcely investigated non-Kramers Tm3+ center, the 3d metal ions being either the low-spin Fe3+ (1) or the diamagnetic Co3+ (2) ion. Both complexes display field-induced slow relaxation of magnetization. The field and temperature dependences of the relaxation rate provided indication of relevant contributions from quantum tunnelling, direct, Orbach and Raman processes, with only minor effects from exchange coupling interactions. Furthermore, the aged sample of 2 exhibited an additional relaxation process, possibly due to structural modifications accompanied by solvent loss, highlighting the importance of a careful consideration of this factor when analysing the magnetization dynamics in solvated systems.