Antiferromagnetic Coupling Strength Research Articles

Change of Interlayer Exchange Coupling in Fe/Y Multilayers by Hydrogenation

We have investigated the change of interlayer exchange coupling in Fe (3.0 nm)/Y (t nm) multilayers upon increasing the thickness of the spacer layer by hydrogenation. The coupling behavior changes from an antiferromagnetic (AFM) state to a non-coupled (NC) state, both before and after hydrogenation. The maximum value of coupling strength (J1) is found to become much lower after hydrogenation than before hydrogenation. The range of the spacer layer thickness for maintaining AFM coupling is found to be extended from 1.4 nm to 1.8 nm by hydrogenation. Both the difference in the AFM coupling strength and the extension of the spacer layer thickness of AFM coupling should be attributed to the change from a Y layer to a yttrium–hydride (Y–H) layer by hydrogenation.

Effects of Interface Roughness on Interlayer Coupling in Fe/Cr/Fe Structure

Effect of interface roughness on antiferromagnetic coupling between Fe layers in a Fe/Cr/Fe trilayer, with Cr layer having a wedge form has been studied. All the samples have been deposited simultaneously on substrates having different roughness, thus it is being considered that there is no variation in the morphological features like grain size and grain texture of the films. Measurements have been done as a function of Cr spacer layer thickness and the peak value of antiferromagnetic coupling strength is compared among different trilayers, thus any influence of spacer layer thickness fluctuation from sample to sample has also been avoided. The samples are characterized by X-ray reflectivity (XRR) and magneto-optic Kerr effect (MOKE). XRR results show that the roughness of the substrate is not replicated at the successive interfaces. Antiferromagnetic coupling between Fe layers decreases with the increase of roughness of Fe/Cr/Fe interfaces.

Oscillatory interlayer exchange coupling in[Pt∕Co]n∕NiO∕[Co∕Pt]nmultilayers with perpendicular anisotropy: Dependence onNiOandPtlayer thicknesses

Interlayer exchange coupling has been studied in a series of ${[\mathrm{Pt}({t}_{\mathrm{Pt}}\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{Co}(4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})]}_{n}∕\mathrm{NiO}({t}_{\mathrm{NiO}})∕{[\mathrm{Co}(4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{Pt}({t}_{\mathrm{Pt}}\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})]}_{n}$ multilayers with perpendicular anisotropy. The coupling oscillates between antiferromagnetic and ferromagnetic as a function of ${t}_{\mathrm{NiO}}$ with a period of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and the oscillatory behavior is related to the antiferromagnetic ordering of the $\mathrm{NiO}$ layer. This interlayer coupling between two $\mathrm{Co}∕\mathrm{Pt}$ multilayers is shown to occur domain by domain by magnetic force microscopy imaging. For the strongest antiferromagnetic coupling at ${t}_{\mathrm{NiO}}=11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, an oscillation with a period of $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is superposed onto the exponential decay of the coupling strength as a function of ${t}_{\mathrm{Pt}}$. The exponential decay with ${t}_{\mathrm{Pt}}$ is ascribed to the exponential decay of the coupling between the $\mathrm{Co}$ layers across the $\mathrm{Pt}$ layers in each $\mathrm{Co}∕\mathrm{Pt}$ multilayer, and the superposed oscillatory behavior can be attributed to multiple reflections of electron waves at the $\mathrm{Co}∕\mathrm{Pt}$ interfaces and their interference. A linear dependence of the antiferromagnetic coupling strength on $1∕n$, (where $n$ is the number of repeats), is suggestive of a surface interaction for this interlayer coupling.

Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation.

The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.

Effect of halogen displacements on the exchange coupling in methyloxo-bridged binuclear copper (II) complexes: a density functional study

Effect of halogen displacements on magnetic coupling for methyloxo-bridged copper(II) dimers is investigated by means of DFT-BS (broken-symmetry) approach. Both coupling constants J and bridging O atomic charges( Q O) increase with the increasing displacement degree n. The displacements lead to β electron transfer from Cl to Cu atoms, α electron from O atoms to methyl groups, which brings down spin densities on Cu 2O 2 core and further spread them toward the periphery of the models. The resulted spin delocalization reduces antiferromagnetic coupling strength for methyloxo-bridged copper (II) dimers. Furthermore, the decrease of spin densities on O p x , O p y and Cu d xy orbitals is the leading reason for the reduction of coupling strength as halogen displacements.

New unsymmetrical μ-phenoxo bridged binuclear copper(II) complexes

A series of binuclear Cu II complexes [Cu2XL] nþ having two copper(II) ions bridged by different motifs (X = OH � , MeCO � ,o r Cl � ) have been prepared using the ligands: H2L 1 ¼ 4-methyl-2-[N-(2-{dimethylamino}ethyl-N¢methyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol, H2L 2 ¼ 4-nitro-2-[N-(2-{dimethylamino}ethyl-N¢-methyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol, H2L 3 ¼ 4-methyl-2-[N-(2-{diethylamino}ethyl-N¢-ethyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol and H2L 4 ¼ 4-nitro-2-[N-(2-{diethylamino}ethyl-N¢-ethyl)aminomethyl]-6-[(prolin-1-yl)methyl]phenol. The complexes have been characterized by spectroscopic, analytical, magnetic and electrochemical measurements. Cryomagnetic investigations (80–300 K) revealed anti-ferromagnetic exchange between the Cu II ions (� 2J in the range )50 to )182 cm � 1 ). The strength of anti-ferromagnetic coupling lies in the order: OAc > OH > Cl. Cyclic voltammetry revealed the presence of two redox couples, assigned to Cu II /Cu II /Cu II /Cu I /Cu I /Cu I . The first reduction potential is sensitive to electronic effects from the aromatic ring substituents and steric effect on the donor nitrogens (side arm) of the ligand systems.

Coexistence of the uniaxial anisotropy and the antiferromagnetic coupling in Co/Ir(1 1 1) superlattices

We have investigated the structural and the magnetic properties of carefully grown Co/Ir(1 1 1) superlattices composed of a few atomic mono-layers (MLs) of each element. We found that the superlattice of [Co(3 ML)/Ir(3 ML)] has its magnetic easy-axis perpendicular to the surface and has quite large antiferromagnetic (AF) coupling between adjacent Co layers. As a result, the magnetic hysteresis curve shows a spin-flip at H∼53 kOe . Adopting a simple two sublattice coherent rotation model, we analyzed the MH curves and obtained the AF coupling strength J AF and the perpendicular anisotropy K as well. The estimated K of [Co(3 ML)/Ir(3 ML)](1 1 1) is even larger than that of Co/Pt multilayer which is known as a typical strong perpendicular anisotropy system.

Origin of peak-dip-hump structure in the in-plane optical conductivity of the high-TCcuprates: Role of antiferromagnetic spin fluctuations of short-range order

An improved U(1) slave-boson approach is applied to study the optical conductivity of the two-dimensional systems of antiferromagnetically correlated electrons over a wide range of hole doping and temperature. Interplay between the spin and charge degrees of freedom is discussed to explain the origin of the peak-dip-hump structure in the in-plane conductivity of high-${T}_{C}$ cuprates. The role of spin fluctuations of short-range order (spin singlet pair) is investigated. It is shown that the spin fluctuations of the short-range order can cause the midinfrared hump, by exhibiting a linear increase of the hump frequency with the antiferromagnetic Heisenberg coupling strength.

Interlayer exchange coupling and giant magnetoresistance inFe/V(001) superlattices

Magnetization and magnetoresistivity studies of Fe/V (001) superlattices are reported. The first giant magnetoresistance peak with respect to the vanadium and iron layer thicknesses is investigated. The interlayer antiferromagnetic coupling strength is found to show a peak at a vanadium layer thickness of 13 atomic monolayers ($\approx$ 20 \AA) with a full width at half maximum of about 2 monolayers. The antiferromagnetic coupling shows a maximum at an iron layer thickness of about 6 monolayers ($\approx$ 9 \AA) for series of superlattices with vanadium thicknesses around 13 monolayers. The magnitude of the giant magnetoresistance shows a similar variation as the antiferromagnetic coupling strength.

Thermal stability investigations on laminated antiferromagnetically coupled media

The effect of the antiferromagnetic coupling strength (J), on the thermal stability factors (SF =KuV/kBT) obtained by different techniques is studied. For this purpose, laminated antiferromagnetically coupled (LAC) media were prepared with different values of J. The results indicated that values of SF are almost the same, when the value of J is smaller than J=0.1 erg/cm2. However, for larger values of J (0.3 erg/cm2), it is very clear that the different methods give different values of SF. It was explained that the difference in the values of SF comes mainly from the pinning of the bottom layer and the different time scales of application of the reverse field. In general, the method that uses larger time scales gave larger values of stability factor. The studies also indicate a possibility that the stability factors obtained from Sharrock's equation may not be accurate for LAC media with very high values of J.

Improved thermal stability of synthetic ferrimagnetic media with enhanced exchange coupling strength

The effect of the antiferromagnetic interlayer exchange coupling strength J between the initial or stabilization magnetic layer L1 and the top magnetic layer L2 on the thermal stability of synthetic ferrimagnetic media (SFM) is investigated. Enhanced J is achieved by the insertion of a thin Co-rich hexagonal close-packed layer between the Ru layer and L1, and such a structure did not change the read/write properties significantly. The contribution of the anisotropy energy of L1 to the thermal stability of SFM is improved by increasing J. However, a significant increase in the contribution of L1 to the thermal stability is achieved at relatively low J values (0.14 erg/cm2) which translate into only a 3–4% increase in the switching field H0.

Contribution of the magnetic anisotropy of the stabilization layer to the thermal stability of synthetic ferrimagnetic media

A study is reported on synthetic ferrimagnetic media, which is made of an initial or stabilization magnetic layer (L1) closer to the substrate and a thicker top magnetic layer (L2) separated by a Ru spacer layer. The effects of the magnetic anisotropy (KU-L1) and the thickness (tL1) of the stabilization layer on the thermal stability of synthetic ferrimagnetic media are systematically studied. The magnetic anisotropy KU-L1 is varied by changing the Pt content of CoCrPtB alloy while a constant thickness and the same composition are retained for the L2. Media coercivity, stabilization factor KUV/kBT, and thermal stability increase as a function of tL1. For a particular tL1 value, these factors are improved for higher KU-L1, though the interlayer antiferromagnetic coupling strength is decreased due to compositional changes.

Exchange coupling strength in synthetic ferrimagnetic media

The antiferromagnetic exchange coupling strength (H/sub ex/) between two CoCrPtB magnetic layers in bilayer synthetic ferrimagnetic media (SFM) is investigated. H/sub ex/ is found to be inversely proportional to the thickness and magnetization of the initial magnetic layer (L1). H/sub ex/ also shows a decrease with increasing temperature. An exchange coupling strength of J/spl ap/0.05 erg/cm/sup 2/ was obtained from H/sub ex/. For further improvement of thermal stability at higher recording densities, the thickness or magnetic anisotropy of L1 may be increased. On the other hand, for an effective SFM, the coercivity of L1 must be lower than H/sub ex/. Therefore, a large H/sub ex/ is critical for better thermal stabilization.

Effect of structural and chemical parameters on magnetic coupling in hydroxo-bridged Cu(II) dimer

The influence of various structural and chemical parameters on magnetic coupling in model hydroxo-bridged Cu(II) dimers has been analyzed using the density functional theory and the broken symmetry approach. These parameters, such as the environment around copper atom, the electro negativity of the nonbridging ligands, the Cu–O–Cu angle, the Cu–O distance, the out-of-plane displacement of hydroxo group and the hinge distortion of the bridge, are all the factors that can affect magnetic coupling. The change of antiferromagnetic coupling strength does not depend on the Cu⋯Cu distance simply. In antiferromagnetic complexes, the spin delocalization from magnetic centers towards their neighbors is remarkable and weakens with ferromagnetic coupling strengthening. The effect of asymmetry at the Cu 2O 2 core has been studied also.

The symmetry-broken formalism applied to the electronic structure of an iminosemiquinone copper(II) catalyst: a key to the qualitative understanding of its reactivity.

A concise outline of the known derivation of the singlet-triplet energy-gap equations within the symmetry-broken wavefunction framework is given. They allow a computation of the singlet-triplet energy gap for molecules that exhibit a weak antiferromagnetic coupling of electrons. The accuracy of the equations is assessed by computation of the singlet-triplet gaps in model Na2 molecules. Various antiferromagnetic coupling strengths are simulated by the use of different Na-Na bond lengths in the computations. The singlet-triplet energy gaps obtained with the different equations are compared with the gaps computed with the more accurate coupled-cluster methods. Subsequently, the equations are applied to an iminosemiquinone copper(II) complex found previously to have remarkable catalytic properties. The application is performed by employing wave-function equations but with quantities computed within the density functional framework. The electronic ground state of this complex is computed to be a singlet state, which is also the experimental finding. Moreover, the experimental geometry and the singlet-triplet gap are reasonably reproduced by the computation. A straightforward method to determine the magnetic orbitals is suggested and applied. We illustrate that the form of the magnetic orbitals indicates in a qualitative manner that hydrogen-atom abstraction should be a major reaction pathway of the iminosemiquinone copper(II) complex. Hydrogen-atom abstraction has been suggested previously to be the rate-determining step in a catalytic process initiated by the iminosemiquinone copper(II) complex. The results support the notion that the form of the magnetic orbitals might be a qualitative indicator for the reactivity of molecules that exhibit weak antiferromagnetic coupling.

Electronic structure of Sr2FeMoO6

We have analyzed the unusual electronic structure of Sr2FeMoO6 combining ab initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength ( U(eff)) at the Mo site due to these renormalized interaction strengths.

Influence of the nature of the buffer on the coupling and the transport properties in Co/Ru/Co sandwiches

A series of Co30 Å/Ru5 Å/Co30 Å sandwiches has been prepared by ion beam sputtering on a glass substrate using different buffer layers in order to optimize the antiferromagnetic (AF) coupling strength and the giant magnetoresistance (GMR) signal. The best GMR of about 1.7% is obtained for a Co/Ru/Co sandwich deposited on the Fe50 Å/Co5 Å/Cu30 Å buffer. On the basis of the GMR and exchange coupling results, the two more interesting buffers have been chosen for a detailed analysis. Two additional series have been grown with varying Ru spacer layer thickness on these two buffers: 150 Å Ru and Fe50 Å/Co5 Å/Cu30 Å buffer layers. For the two series, oscillations in magnetoresistance and AF indirect exchange coupling have been observed as a function of the Ru thickness with the same period of about 10 Å. The first AF peak is observed at a thickness of 6 Å Ru for the series grown on Ru150 Å buffer and 4 Å for the series grown on a Fe50 Å/Co5 Å/Cu30 Å buffer layer. At this first peak, both GMR and AF coupling are largely enhanced with values JAF=−2.6 erg/cm2 and a GMR of 1.34% for the Fe50 Å/Co5 Å/Cu30 Å buffer compared to JAF=−0.75 erg/cm2 and a GMR of 0.08% for the Ru150 Å buffer. Atomic force microscopy and x-ray diffraction studies have also been performed in order to understand the origin of the observed increase in GMR and AF exchange coupling between the two series.

Study of the barrier height in exchange coupled Fe/Fe1−xSix (x>0.70) multilayers

Fe/Fe 1−x Si x multilayers show distinct antiferromagnetic (AF) coupling for a wide spacer composition range 0.50<x⩽1.00. As the Si content x increases, the spacer changes from metallic to insulating and the AF coupling strength (J) is significantly enhanced from 0.05 to 1.20 erg/cm2. We have explained the temperature dependence of the coupling constants J1 and J2 in terms of the quantum interference model by taking an unknown energy difference Δ(=U−εF) as a fitting parameter, where εF is the Fermi level of Fe and U is the potential of the Fe1−xSix. The aim of the present work is to determine the quantity Δ experimentally for the insulating composition range of x>0.70. The quantity Δ was evaluated both from I–V characteristics and the temperature dependence of the resistivity with the current perpendicular to the sample plane using a crossed electrode geometry junction. It is found that the barrier height increases from 0.15 to 0.70 eV with increasing the Si content x. These values almost agree with the parameter Δ deduced from the temperature dependence of J1 and J2. This agreement supports the validity of our previous calculations based on the quantum interference model.

Structure and Magnetic Properties of Fe/Si Multilayers

Fe/Si multilayers have been grown on MgO(100) and Si(100) substrates by dc magnetron sputtering. From x-ray diffraction analysis, the Fe(100) orientation was dominated in samples with MgO(100) as substrates, and the Fe(110) orientation was dominated in samples with Si(100) as substrates. Ferromagnetic coupling was observed for samples with Si spacer thickness greater than 17 Å or less than 11 Å, and antiferromagnetic coupling occurred for samples with Si spacer thickness between 11 Å and 17 Å. The strength of antiferromagnetic coupling was stronger in samples on MgO. This indicated that the magnetic coupling between Fe layers may be closely related to the structural orientation of the magnetic layers. For the Fe 60Å/Si 13Å/Fe 60 Å trilayer on the MgO(100) substrate, Mr/Ms was equal to 0.3, however, it increased to 0.95 on Si (100).

Spin Gap Characteristic of Y(Ba1−xGdx)2Cu3O7−δ

Y(Ba1−xGdx)2Cu3O7−δ compounds with x = 0 ∼ 0.15 are prepared using the solid reaction technique. With structure analysis by Rietveld refinement of x-ray diffraction, we find that Gd3+ ions prefer to occupy Y sites within lighter doping x ≤ 0.08 due to ion size effects, then begin partially to occupy Ba sites with doping content increasing, which gives vital influence on superconductivity and spin-gap properties. The magnetic doping effects of Gd3+ ions on spin-gap properties are investigated in detail by contrast of the distinguished behaviors between T* and Tc, indicating that spin-gap temperature is not completely determined by the carrier density, but strongly dependent on the strength of interplane antiferromagnetic coupling. Finally, we propose an expression of in-plane resistivity dependent on the maximal width of spin-gap Δ0 to derive their values for different samples, which almost keep constant with the increase of Gd doping contents.