Magnetic Layered Structures Research Articles

Smart structure for tailoring the magneto-optical Kerr effect

In the NaCl-type cerium, neodymium, and samarium compounds, the Kerr rotation at low temperatures is very large, compared to small values of common magnetic materials, such as Fe, Co, FeCo, and so on. A Kerr rotation of 14° in the infrared range was observed in CeSb compounds at a temperature of 2 K under a large applied field of 5 T. It is puzzling that the reflectivity in the region has a minimum of 25%, instead of zero. Although a 90° Kerr rotation was observed in the same materials, it was argued to be due to a surface state on the peculiar sample, instead of its intrinsic properties. The criteria for the 90° Kerr rotation were studied theoretically that the absorption part of the dielectric tensor is equal to zero, i.e., «xx 9 5«xy 8 50. In addition, the component «xx 8 must be confined between max$u«xy 9 u,12u«xy 9 u% and 1. The simultaneous fulfillment of these requirements imposes several limitations on the elements of the dielectric tensor, and then seriously reduces the number of magnetic compounds that can exhibit the Kerr angle of 90°. In fact, the limitations hold only for single thick homogeneous media. Magnetic layered structures are generally free from these limitations and much easier to obtain 90° Kerr rotation, since in this case reflection from the interfaces is of importance. In this consideration, Uspenskii and Harmon made numerical analysis on magnetic semiconductor/metal structure and found the 90° Kerr rotation in some structures, such as EuS ~EuO, CoCr2S4, and so on!/Ag~Cu and Au!. However, above single-crystalline homogeneous materials or layered structures are hard to make and do not have prerequisite for magneto-optical ~MO! storage materials. So it is simple to wonder whether there is any practical MO structure to realize large Kerr rotation in the visible range. The magneto-optical Kerr effect ~MOKE! has been studied in more general MO/ insulator ~IS! multilayers and layered structures, such as Fe/ alumina ~amorphous!, TbFe/SiO, and so on. For example, a Kerr rotation of as large as 20° was attained in Fe/alumina ~amorphous!. The MOKE enhancement is attributed to the optical interference. Although the sandwich structure of IS/ MO/IS was commonly used, up to date, no one has figured out the huge Kerr rotation of 90°. In this work we have made numerical calculations on the MOKE of the IS/MO/IS sandwiches on substrates of single crystal Si, employing a combination of SiN~5Si3N4! and TbFeCo~5Tb27Fe62Co11!. In

Evidence of anisotropic magnetic polarons inLa0.94Sr0.06MnO3by neutron scattering and comparison with Ca-doped manganites

Elastic and inelastic neutron-scattering experiments have been performed in a ${\mathrm{La}}_{0.94}{\mathrm{Sr}}_{0.06}{\mathrm{MnO}}_{3}$ untwinned crystal, which exhibits an antiferromagnetic canted magnetic structure with ferromagnetic layers. The elastic small q scattering exhibits a modulation with an anisotropic q dependence. It can be pictured by ferromagnetic inhomogeneities or large magnetic polarons with a platelike shape, the largest size $(\ensuremath{\approx}17 \AA{})$ and largest interpolaron distance $(\ensuremath{\approx}38 \AA{})$ being within the ferromagnetic layers. Comparison with observations performed on Ca-doped samples, which show the growth of the magnetic polarons with doping, suggests that this growth is faster for the Sr than for the Ca substitution. Below the gap of the spin-wave branch typical of the antiferromagnetic layered magnetic structure, an additional spin-wave branch reveals a ferromagnetic and isotropic coupling, already found in Ca-doped samples. Its q-dependent intensity, very anisotropic, closely reflects the ferromagnetic correlations found for the static clusters. All these results agree with a two-phase electronic segregation occurring on a very small scale, although some characteristics of a canted state are also observed suggesting a weakly inhomogeneous state.

Nanomagnets for sensors and data storage

Miniaturized magnetic devices are ubiquitous in the hard disks of computers and in tape storage systems. Giant magnetoresistance was discovered as recently as 1988 but already sensors using the effect are being incorporated into read heads for the highest density hard disk systems. As a result of intensive research and development, storage density on hard disks has increased dramatically at a rate greater than 60% per year. At sub-micron and nano-scale dimensions the properties of magnetic devices are strongly affected by their size and shape in a complex way resulting from the interplay between different types of magnetic energy. In some cases this causes a deterioration in the performance of existing devices, however it has also enabled entirely new devices to be proposed. Arrays of nanomagnets could be used for ultra-high density storage on hard disks or for fast and dense, non-volatile, solid state memory. Storage applications are possible because hysteresis in the nanomagnets creates two oppositely magnetized states which are stable in zero applied field and can store binary data. Magnetic sensors based on giant magnetoresistance in layered magnetic structures are among the most sensitive available for operation at room temperature and above. This paper describes the physical properties of nanomagnets and their role in present and future applications.

Layered magnetic structures in research and application

An overview is given on the status of research and applications in the field of layered magnetic structures and the historical development is indicated. Currently the research on interlayer exchange coupling, giant magnetoresistance and tunnel magnetoresistance is particularly active, therefore the basic understanding of these phenomena, the size of the effects, important issues and applications are discussed in some more detail.

Domain structure of magnetic layers deposited on patterned silicon

Arrays of Si square dots down to 200 nm in size were patterned on silicon substrates, and thin films of different magnetic materials were sputter deposited on these patterned substrates. The magnetic film covers the top of the dots, the bottom of the grooves and to much less extent the sidewalls of the dots. Single domain magnetic dots were obtained for Co/NiO bilayers and Co/Pt multilayers, without significant direct coupling mediated by the magnetic deposit on the sidewalls of the dots. Our results indicate that, in these arrays, the magnetic pinning forces are stronger than the estimated value of the largest demagnetizing magnetostatic field on each individual dot. As a result, any magnetic configuration could in principle be stored in such arrays. This approach seems therefore very promising for the preparation of magnetic storage media with ultrahigh density.

Polarized neutron reflectometry with phase analysis

A novel technique, polarized neutron reflectometry with phase analysis (PNRPA), is suggested, in which not only the moduli of reflection matrix elements but also up to three phase differences are measured. It is realized in the scheme with two flippers and an analyzer, by reflection of neutrons with the spin, in succession, parallel and antiparallel to two inclined axes fixed to the sample. More detailed information about magnetic layered structures can be thus obtained. An adequate mathematical formalism is given.

Magnetic polarons in layered antiferromagnetic systems

The energy and stability conditions are found for free ferromagnetic polarons (ferrons) in the antiferromagnetic semiconductors with a layered magnetic structure. Unlike a similar problem for the staggered antiferromagnets, solved long ago, this geometrically different problem was not considered yet. In particular, the difference between these two problems consists in the fact that the ferromagnetic region, which traps the charge carrier, is well defined for the staggered structure, and is not for the layered structure. The latter is due to ferromagnetic planes inherent in this structure. It is established that, nevertheless, the ferromagnetic region is almost spherical for large ferrons. The De Gennes single-spin magnetic polaron is unstable at realistic values of parameters. The energy and magnetic moment of the ferron bound to a donor or acceptor are calculated for the case of the double exchange.

Double Exchange via Degenerate Orbitals

We consider the double-exchange for systems in which doped electrons occupy degenerate orbitals, treating the realistic situation with double degenerate $e_g$ orbitals. We show that the orbital degeneracy leads in general to formation of anisotropic magnetic structures and that in particular, depending on the doping concentration, the layered magnetic structures of the A-type and chain-like structures of the C-type are stabilized. The phase-diagram that we obtain provides an explanation for the experimentally observed magnetic structures of some over-doped (electron-doped) manganites of the type Nd$_{1-x}$Sr$_x$MnO$_3$, Pr$_{1-x}$Sr$_x$MnO$_3$ and Sm$_{1-x}$Ca$_x$MnO$_3$ with $x > 0.5$.

Magnetic properties of Ce xLa 1− xSb and Ce xY 1− xP in high magnetic fields

Magnetic properties of single crystals of Ce X La 1− X Sb and Ce X Y 1− X P have been investigated by magnetization measurements in static or pulsed magnetic fields up to 30 T. The magnetic phase diagram was obtained, and the medium state region, FP-phase, is broad compared to the case of CeSb and it appears even at very low temperatures. We have also performed magnetization measurements of Ce 0.95Y 0.05P and made a magnetic phase diagram, which is similar to that of CeP. Therefore it is suggested that there are some magnetic layer structures in magnetic fields like in the case of CeP.

Nonlinear transport in tunnel magnetoresistance systems

The nonlinear electronic tunneling through ferromagnet (FM)/insulator (I)/FM single-barrier and FM/I/FM/I/FM double-barrier magnetic layered structures is studied using the Landauer-B\uttiker scattering approach. For the single-barrier junctions, the resulting bias dependence of tunnel magnetoresistance is qualitatively in agreement with experimental observation. The absolute value of magnetoresistance is shown to be enhanced in the double-barrier systems. Under suitable conditions, large negative and positive magnetoresistance effects occur alternately with increasing bias voltage. These features make double-barrier resonant-tunnel structures more attractive magnetoresistance devices.

Boltzmann equation for spin-dependent transport in magnetic inhomogeneous systems

From the Dyson equation of the nonequilibrium Green's functions, we derive a Boltzmann equation for the spin-dependent electronic transport, in which the electron distribution function is in general a $2\ifmmode\times\else\texttimes\fi{}2$ off-diagonal matrix in spin space. This equation is applicable to the transport in magnetic inhomogeneous systems, such as the magnetic layered structures and granular solids, with arbitrary magnetization configurations. For the parallel transport in the magnetic multilayers, it recovers the phenomenological Boltzmann equation proposed previously. It is shown that, as a consequence of continuity of the currents, the current density distribution depends only on the voltage drop between the ends of the sample. This allows us to determine the measurable physical quantities without calculating the actual internal electric field.

Differential scanning calorimetry evidence of compositional modulation and giant magnetoelastic wave amplitude in electrodeposited Co-P ribbons

Ribbons of Co-P with a transverse structural modulation in phosphorus content were produced by electrolytic deposition at pulsed current density. The composition modulation was evidenced by relating electron diffraction spectroscopy, X-ray diffraction, and differential scanning calorimetry. In particular it was shown that the latter technique can resolve the presence of regions containing a high P atomic fraction from regions of low P content. A very high amplitude of magneto elastic waves was obtained in this type of Co-P ribbons. This result was explained in terms of the mixed non-magnetic and magnetic layer structure which was produced by compositional variations. The significant magnetoelastic properties show that the investigated material could be useful as piezomagnetic core in ultrasonic technology and radioelectronics.

First-principles based semi-classical model for transport in magnetic layered structures

We present a first principles based semiclassical model for transport in an inhomogeneous magnetic layered structure. The approach solves the Boltzmann transport equation using the band structure, Bloch wave velocities and scattering matrices, describing the reflection and transmission of Bloch waves from interfaces, derived from ab-initio local spin density electronic structure calculations. The model has been tested for thick Co and Cu films as well as for a set of Co/Cu/Co spin valves.

Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mossbauer reflectometry and CEMS results on a [57Fe(2.55 nm)/FeSi\break(1.57 nm)]10 multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the 57Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < Bext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]10 varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.

Interface roughness effects in the giant magnetoresistance in magnetic multilayers

In-plane electronic transport in thin layered magnetic structures composed of two ferromagnetic films separated by a nonmagnetic spacer is analyzed theoretically in the Born approximation. Particular attention is paid to the role of interface roughness in the giant magnetoresistance (GMR) effect. The analysis applies to self-affine interfaces described by the k-correlation model. Our results show that GMR is sensitive to the roughness exponent H (0⩽H⩽1) in a manner that depends on spin asymmetries for bulk and interfacial scattering. The limit of low electron concentration is also considered.

Temperature dependence of noncollinear interlayer coupling

Based on the Anderson s-d mixing model in the presence of spin-orbit coupling, the noncollinear interlayer coupling in metallic magnetic layered structures is studied theoretically. After transforming the Anderson s-d mixing model into the s-d exchange model, both the isotropic and anisotropic Dzyaloshinsky-Moriya (DM) scattering potentials between the conduction electrons and localized moments are obtained. This anisotropic DM scattering potential can lead to a noncollinear interlayer coupling in magnetic metallic layered structures. At finite temperature, the anisotropic interlayer exchange coupling has the same spacer layer thickness dependence as the usual isotropic Ruderman-Kittel-Kasuya-Yosida (RKKY) interlayer coupling and consists of two terms. One term has the same temperature dependence as the isotropic coupling, but its oscillation with spacer layer thickness has a phase shift of π 2 . The other term has a different temperature dependence and no phase shift. Their relevance to the experimental results is also discussed.

High-density recording performance of media with CoCrPtTa/Cr/NiP on glass ceramic substrate

The comparison of magnetic recording performance at high density between two kinds of CoCrPtTa/Cr/NiP thin films, one with Co(112̄0)//Cr(200) and another with Co(101̄1)//Cr(110) texture, on glass ceramic substrate is reported. The testing results have shown that the former films have lower transition noise, smaller partial erasure effect, and better offtrack capability in partial response maximum likelihood channel which also enhances the estimated areal density. The better performance of the films with Co(112̄0)//Cr(200) texture may be attributed to microstructure of Cr underlayer which results in bicrystal structure of magnetic layer and smaller Cr grain size.

Exchange Enhancement of Pd Coating on Ni Fine Particles

Magnetic and structural properties of Ni fine particles (0.7±0.2 µm in diameter) coated with Pd (5 nm∼37 nm in thickness) were investigated in a hydrogen atmosphere by means of magnetometry and X-ray diffractometry. The spontaneous magnetic moment is newly induced on the Pd atoms even at room temperature by the magnetic proximity effect between Ni core and Pd layer. Relatively large coercive force indicates the presence of the interface anisotropy. The magnetic polarization and the coercive force are suppressed by the hydrogenation. The temperature dependence of the magnetization shows a broad maximum around 80 K and an increase below 50 K. The maximum in magnetization is due to the enhanced paramagnetic contribution of Pd, and the gradual increase below 50 K is due to the Ni moment in contact with ferromagnetic Pd. A model for layered magnetic structure of the Ni particles coated with Pd is proposed.

Theory of electromagnetic modes of magnetic effective-medium films

We study the surface and guided wave polaritons for a magnetic layered structure on a substrate. Using the effective-medium approximation we obtain analytic equations for implicit dispersion relations. These dispersion relations are solved numerically for a variety of layering patterns, and profiles showing the localization of both guided waves and surface waves are obtained. Explicit results are presented for an - superlattice on a substrate.

Detecting two components of magnetization in magnetic layer structures by use of a photoelastic modulator

A magneto-optic Kerr effect system for the measurement of magnetization in thin ferromagnetic layers based on a photoelastic modulator is described. The use of a quarter wave plate allows light with a variable polarization incident on the sample to be used. The polarization of the light as it passes through the system is treated algebraically using a matrix approach. A procedure for determining both the magneto-optic (MO) rotation presented by the sample and the traditional ellipsometric parameters is described, the consistency of the solutions being demonstrated by experiment. Typical MO rotation versus thickness curves for thin films of Permalloy (Ni79Fe21) deposited on glass for both the longitudinal and transverse field configurations are presented. These results demonstrate that the magnitude and sign (in the transverse case) of the MO rotation is strongly dependent on thickness for films thinner than 200 Å.