Design and fabrication of non-superparamagnetic high moment magnetic nanoparticles for bioapplications

We present a new type of magnetic nanoparticles for bioapplications. Multilayered nanodisks consisting of two magnetic layers separated by a non-magnetic layer with two capping layers were designed and fabricated. Two key magnetic requirements for bioapplications, a high saturation magnetic moment and a near-zero remanence, were achieved through the magnetostatic interlayer coupling between two magnetic layers. Capping layers provide functionalization sites for biomolecule attachment. A pillar-template-based synthesis method was employed for fabrication. Nanodisks with a diameter of 70 nm and a thickness of 60 nm were produced in large quantity. The magnetic characterization shows that each nanodisk possesses a magnetic moment equivalent to 100 10-nm Co nanoparticles and a near-zero remanent moment.     

Tunnel magnetoresistance of magnetic junctions with cubic symmetry of the layers

A tunnel magnetic junction is considered with magnetic hard and magnetic soft layers of cubic symmetry. The magnetic switching of the layers is analyzed for a magnetic field perpendicular to the initial magnetizations. In such a situation, an additional peak in the tunnel magnetoresistance ratio appears at the magnetic field value that is substantially lower than the anisotropy field of the soft layer.     

Layer-resolved microscopy of magnetic domains in multi-layered systems

Photoelectron emission microscopy in connection with magnetic circular dichroism in soft X-ray absorption can be used for the microscopic imaging of magnetic domains in layered thin film structures consisting of several magnetic layers. Due to the element-selectivity of the method, the different magnetic layers in such a structure can be imaged separately, provided that they contain different elements. This has been applied for the investigation of Co/Cu/Ni trilayers, epitaxially grown on Cu (001). The magnetic coupling between the Co and Ni layers can be directly visualized from comparing layer-resolved magnetic domain images of both layers. As a consequence of the competition between the anisotropy energies of the two magnetic layers and the magnetic coupling energy, spin-reorientation transitions between collinear and non-collinear magnetic configurations are observed. Apart from this globally observable magnetic interlayer coupling a micromagnetic coupling mechanism is also evident from the layer-resolved domain images. It is caused by magnetostatic interaction of local stray fields from domain walls. Received: 22 August 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-345/5511-223, E-mail: kuch@mpi-halle.de     

Ab initio theory of perpendicular transport in layered magnetic systems

The spin-polarized electron conductance perpendicular to layers of a random magnetic multilayer is evaluated from first principles. We employ the Landauer formulation in the framework of the tight-binding linear muffin-tin orbital approach and the surface Green-function technique. The disorder in the bulk and at interfaces is included in terms of lateral supercells confined to individual atomic layers. The application is made to interleave and separate multilayers with a different order of magnetic and non-magnetic layers with generally non-collinear alignments of layer magnetizations.     

Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO(R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe_2As_2(R =Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magneticfield-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.     

Magnetic structure of the spin-1/2 layer compound NaNiO2

We have carried out high resolution neutron powder diffraction experiments aiming at a determination of the magnetic structure of the S=1/2 layer compound NaNiO2. The magnetic moments are ferromagnetically aligned in the NiO2 layers and antiparallel between layers. The direction of the magnetic moment has a small component along the a-direction.     

Stray field interaction of stacked amorphous tapes

In this study, magnetic cores made of amorphous rectangular tape layers are investigated. The quality factor Q of the tape material decreases rapidly, however, when stacking at least two tape layers. The hysteresis loop becomes non-linear, and the coercivity increases. These effects are principally independent of the frequency and occur whether tape layers are insulated or not. The Kerr-microscopy was used to monitor local hysteresis loops by varying the distance of two tape layers. The magnetization direction of each magnetic domain is influenced by the anisotropy axis, the external magnetic field and the stray field of magnetic domains of the neighboring tape layers. We found that crossed easy axes (as the extreme case for inclined axes) of congruent domains retain the remagnetization and induce a plateau of the local loop. Summarizing local loops leads to the observed increase of coercivity and non-linearity of the inductively measured loop. A high Q-factor can be preserved if the easy axes of stacked tape layers are identical within the interaction range in the order of mm.     

Magnetic-state tuning of the rhombohedral graphite film by interlayer spacing and thickness

Based on first-principles total-energy calculations, we systematically investigate how the electronic and magnetic properties of rhombohedral graphite thin films depend on the interlayer spacing and number of layers. Our calculations show that the magnetic ordering of the thin films depends on the interlayer spacing. Thin films under compression normal to the layers possess finite magnetic moments, indicating parallel spin coupling between the two surfaces. We also find that thin graphite films with seven or more atomic layers exhibit magnetic ordering while films with six or less atomic layers are metallic with no magnetic ordering.     

Co-Zr/Pd多层膜的磁性与结构

Co-Zr/Pd多层膜由高频溅射方法制得．磁性合金Co-Zr层厚度固定为1.8nm，改变Pd层厚度0.5—6nm．由振动样品磁强计测量，发现随Pd层厚度增加，磁化强度发生周期性振荡变化，周期约为1nm，这是由Pd层的极化振荡引起的．经X射线衍射测得Pd层厚度超过1.3nm时，磁性合金Co-Zr层发生晶化，而厚的Co-Zr单层膜是非晶结构．X射线大角衍射图中的超晶格峰表明，在Co-Zr层和Pd层之间存在相关生长．而且还发现，随Pd层厚度增加，样品在垂直膜面方向的晶粒尺寸及fcc（111）面的面间距发生周期性 关键词：     

Optical measurements of field-induced phenomena of the magnetic phase transition in quasi 2D MnS layers grown by MBE

We used a sensitive optical method to study the magnetic phase transition of antiferromagnetic MnS layers. The method is applicable for very small numbers of spins, e.g., thin single layers. We studied the optical and magnetic properties of MnS layers using the internal optical transition of the manganese 3d-shell. The temperature dependence of the Mn-emission exhibits a pronounced minimum revealing the para- to anti-ferromagnetic phase transition. The MnS layers were grown by molecular beam epitaxy, embedded between diamagnetic ZnSe cladding layers on a (100)-GaAs substrate. It was found that the Néel-temperature itself is influenced by the biaxial strain and can be changed in an external magnetic field in case of quasi 2D MnS-layers. The phase diagram reveals a weak Ising like anisotropic contribution in case of a 1.8 nm thin layer, whereas a 8.6 nm thick layer behaves still like an ideal isotropic Heisenberg system.     

Microwave and static magnetic properties of multi-layered iron-based films

Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability. To produce a bulk massive sample from thin films, multi-layer films made of a number of many thin films may be used. The paper reports on an experimental study of microwave and static magnetic properties of multi-layer iron-based films and composite samples made of these. The multi-layer films under study are deposited onto a mylar substrate by a magnetron-sputtering process performed in Ar atmosphere with controlled N₂ admixture. The sputtered iron layers are alternated with SiO₂ layers. The measured static and microwave magnetic properties of the bulk sample are found to differ from the properties of constituent films. This is an evidence for strong interactions between the magnetic layers in the sample, which interact at distances exceeding greatly the distance between adjacent magnetic layers. A theoretic model is developed to account for magneto-dipole interactions between iron films in a multi-layer system. The model explains the anomalously high demagnetization field of the sample observed in the measurements.     

Hyperfine structure of highly charged ions

¹⁹⁷Au Mössbauer spectra from Au/TM (TM = Fe, Co, Ni) multilayers consist mainly of two components. One component shows a large hyperfine magnetic field due to the hybridization at the interface between Au and ferromagnetic layers. The other component is nonmagnetic arising from the middle part of the Au layers. From the fractional area of the magnetic components in each spectrum, the Au atoms in 0.4 nm Au layers are perturbed by the Fe and Ni layers, and Co layers perturb 0.3 nm Au layers at the interface.     

Calculation of critical states of superconducting multilayers based on numerical solution of the Ginzburg-Landau equations for superconducting plates

Numerical methods are used to analyze the Ginzburg-Landau equations for a superconducting plate carrying transport current in a magnetic field. Critical current is calculated as a function of the applied magnetic field strength for superconducting plates with different thicknesses. The relations between the field dependence of critical current and the distributions of order parameter, magnetic field, and supercurrent in a plate are analyzed. The field-dependent critical currents computed for plates are used to determine the critical current as a function of the applied magnetic field strength and local magnetic field and current distributions for multilayers in parallel magnetic fields. The constituent superconducting layers are assumed to interact only via magnetic field. A simple method is proposed for analyzing the critical states of multilayers in magnetic fields of arbitrary strength, based on elementary transformations of the critical current-density distribution over individual layers in zero applied magnetic field. The method can be used to analyze experimental results.     

Pinning of domain walls in two-layer ferromagnetic nanowire with scattering fields of nanoparticles

The results of a micromagnetic simulation of the pinning-depinning processes of a domain wall (DW) in a rectangular ferromagnetic nanowire (NW) consisting of two magnetic layers with scattering fields of two rectangular two-layer nanoparticles (NPs) located on NW opposite sides and oriented perpendicular to its axis are presented. The features of magnetization reversal of this system in the external magnetic field are studied depending on direction of the magnetic moments of the nanoparticle layers. The value of the depinning field in such a system depends essentially on mutual orientation of NP magnetic moments and NW magnetization. The possibility to realize a magnetic logic cell performing the “conjunction” operation of ternary logic is discussed.     

Roughness of layer interfaces and phase diagram of magnetic multilayer structures

The dependence of the type of magnetic ordering in multilayer structures on the magnitude of the exchange interaction between the magnetic layers and on the degree of roughness of the interfaces between the layers is investigated. In the case of a three-layer system the regions of existence of the domain structure, of noncollinear ordering of the homogeneous magnetic layers, and of the collinear ferromagnetic phase and collinear antiferromagnetic phase are found. Fiz. Tverd. Tela (St. Petersburg) 39, 1244–1247 (July 1997)     

Slow Oscillations of the Perpendicular Magnetization of the Pt/Co/Ir/Co/Pt Spin Valve

The time dependences of the transfer of the magnetic moment between stable magnetic states of heterostructures with two Pt/Co/Ir/Co/Pt ferromagnetic layers with the perpendicular magnetization have been studied. Spontaneous oscillations of the macroscopic magnetization with a period of several hours are observed after switching the magnetic field to a new value. The phase portrait of the magnetic relaxation corresponds to damped oscillations. The macrospin oscillation may be due to the high nucleation rate of the reverse magnetization phase induced by the exchange and magnetic dipole interaction between the phase nucleation centers, which arise in different layers. The changes in the Zeeman energy of the system under magnetic oscillations are considered.     

Coherent Fan Magnetic Structure in Dy/Gd Superlattices

The magnetic structure of pseudo-single-crystal (0001) Dy/Gd superlattices, as well as its modification with the temperature, is determined by the combined application of SQUID magnetometry, circular magnetic dichroism, and polarized neutron reflectometry. It is established that a fan magnetic order is formed in the Dy layers in relatively low magnetic fields at temperatures below 170 K. Then, this order propagates coherently throughout the superlattice. The magnetic moments in the Dy layers lie in the basal plane, and in the Gd layers, they are oriented mainly along the hexagonal c axis.     

Waveguide modes of 1D photonic crystals in a transverse magnetic field

We analyze waveguide modes in 1D photonic crystals containing layers magnetized in the plane. It is shown that the magnetooptical nonreciprocity effect emerges in such structures during the propagation of waveguide modes along the layers and perpendicularly to the magnetization. This effect involves a change in the phase velocity of the mode upon reversal of the direction of magnetization. Comparison of the effects in a nonmagnetic photonic crystal with an additional magnetic layer and in a photonic crystal with magnetic layers shows that the magnitude of this effect is several times larger in the former case in spite of the fact that the electromagnetic field of the modes in the latter case is localized in magnetic regions more strongly. This is associated with asymmetry of the dielectric layers contacting with the magnetic layer in the former case. This effect is important for controlling waveguide structure modes with the help of an external magnetic field.     

Selection of magnetic screens by numerical calculations

To choose the parameters of magnetic screens, a technique for numerically calculating the 3D magnetic field distribution is developed for the case when field sources are locates in open regions. The problem is solved in vector magnetic potential by the finite integration method. A modification of the method of perfectly matched absorbing boundary layers for the case of magnetic field allows calculation of the field in real 3D screens. The numerical solutions are tested using absorbing layers by comparing with the known analytical solutions. The parameters of finite-size screens effectively decreasing the magnetic field intensity are found.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.