Library

Notes: Abstract Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK us = 0.96, 0.63, and 0.3 ergs/cm2 respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K 1∥S=0.012 ergs/cm2. Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Notes: A complex domain configuration found in an iron whisker can be generated by a function with three parameters. The configuration is generated by passing a current along the axis of a {100} whisker with a square cross section. It has four domains in which the magnetization circulates about a central domain magnetized along the z axis. There are four {110} 90° walls and four {100} 90° walls separating these domains. Two of the parameters are known a priori from micromagnetics: a1 describes type I 90° walls between two domains with in-plane magnetization; and a2 describes type II 90° walls between the domains with in-plane magnetization and the central domain with the magnetization out of the plane. The third parameter h, which determines the width of the central domain, is adjustable to minimize the total micromagnetic energy in response to magnetic fields. The magnetization pattern is generated from a vector potential A(x,y)zˆ. Calculating Ms curl A(x,y)zˆ produces Mx and My. Then Mz is found from M2x+M2y+M2z=M2s. The potential has four parts: A(x,y)=Ax+Az+Ah+Ac, where Ax and Ay are the same function (with x and y interchanged) used to create the in-plane domains and the type I walls, Ah generates the type II walls, and Ac describes the outer corners. The resulting magnetization is divergence free. The analytic functions are convenient for calculation of the magnetic response to applied fields. © 1996 American Institute of Physics.

Notes: Monomolecular layers of exfoliated MoS2 are incorporated in the process of the formation of Co ferrite from aqueous solutions. The presence of MoS2 is found to limit the coalescence of particles during reduction to Fe(Co) metal. That MoS2 inhibits the full conversion to metal is confirmed by Mössbauer effect spectroscopy. Magnetization measurements are interpreted on the basis of noncollinear magnetization patterns resulting from competing surface anisotropies.

Notes: Cobalt ferrite, CoxFe3−xO4 with 0.55≤x≤0.8, is produced by precipitation from aqueous solutions. Drying and annealing at temperatures below 300 °C results in a coercivity that increases with increasing temperature. Mössbauer effect spectroscopy and magnetic measurements are consistent with the interpretation that these Co ferrites have a (mostly) normal spinel structure as made. These convert to a (mostly) inverse spinel structure on thermal aging. No significant morphological changes occur during this transformation. Coercivities as high as μ0Hc=0.2 T are observed.

Notes: The 1994 3M-Intermag meeting featured an invited session on Fe16N2 in which many of the research workers in this exciting field put forth apparently conflicting views on the nature and possibilities for the use of, perhaps, the most promising transition metal based alloy for magnetic applications. The debate centers on the promise of 3μB per atom at transition metal densities if pure Fe16N2 could be made. A particularly compelling resolution of the conflicts was not mentioned at the meeting, even though it has been in the past. The conflict arises primarily from the original classification of Fe16N2 as a phase denoted by α″ to distinguish it from Fe8N classified as a phase denoted by α′ and the substitutional alloy α-Fe(N). The conflicts arise because data are interpreted as a mixture of two phases α′ and α″. For any measured property, it is then sufficient to know the ratio of the two component phases for two different compositions to deduce the apparent properties of the individual phases. But this is not the case. The distinction between α″ and α′ is not that of two separate phases but, rather, the end points of a continuum characterized by the degree of order. This makes it necessary to produce the fully ordered state in order to determine its properties, unless it is possible to have enough data about those properties for various degrees of order to justify extrapolation to the fully ordered state. The most ordered Fe16N2 is produced using molecular beam epitaxy. It also shows the highest magnetic moments. The high magnetic moments are supported by Mössbauer measurements of large hyperfine fields in bulk materials (not the molecular beam epitaxy material) and by some of the theoretical calculations for the ordered state.One of the criticisms of the molecular beam epitaxy work is that those thin films do not decompose into Fe4N and Fe(N) on heating as do bulk alloys of the same composition. (The implication is that the thin films contain something else.) But this criticism is not valid, simply because one cannot predict the stability of a highly ordered state from observations of a weakly ordered state. This is particularly the case where the ordering is accompanied by large lattice distortions. The existing literature is analyzed to extract the dependence of magnetic moment on the degree of order. These considerations lend support to those who are pursuing Fe16N2 for its potential for producing a magnetic material with 3μB per atom at transition metal densities. How to increase the degree of order in bulk materials becomes a central concern. © 1996 American Institute of Physics.

Notes: For ferromagnetic sandwiches where two iron layers are separated by a middle layer, the strength of the coupling sharply varies with each change in the thickness of the middle layer. If the interfaces were sharp on the atomic scale, one could observe this directly, or else one can use micromagnetics to take into account the actual morphology of less than ideal interfaces as shown by Slonczewski. As an approximation the average magnetization in each layer is found from a phenomenological energy that contains the competition between bilinear and biquadratic exchange constants that depend on surface roughness. More detailed micromagnetic calculations using relaxation methods to treat spatial variations of the magnetization in each layer are given for Fe–Cr–Fe sandwiches. The approximate calculations give jumps in the magnetization at critical fields, but these are smeared out in the more detailed calculations. For the special case of equivalent layers, the phenomenological expression for the energy can be used to obtain analytic expressions for the field dependence of the magnetization. © 1996 American Institute of Physics.

Notes: Collective magnetic dipoles are written into sections of commercial magnetic recording tapes by application of high fields at angles θset with respect to the tape axis. The media is rotated 2000 times about the tape normal in a lower magnetic field Hm in the plane of the tape. The magnetization along the direction of Hm is measured in steps of Δθ=π/5. Harmonic analysis of the angular dependence of the magnetization is used to discover how the dipole term depends on cycle number n, Hm, the direction of rotation, and θset. The data are analyzed using μd(n)=μd(∞)+[μd(1)−μd(∞)]n−γ. This dipole disappears on rotation for an infinite number of cycles in fields Hm(approximately-greater-than)Hcrit. For Hm〈Hcrit, μd(∞) depends on Hm much as Ms(T) depends on temperature. For Hm close to Hcrit, γ becomes as small as 0.1 for which 1010 cycles would be required to produce 90% of the change. © 1996 American Institute of Physics.

Notes: An iron whisker with a current along its axis has four domains in a frame about a central core. The domains in the frame are magnetized perpendicular to the axis. The central core is magnetized along the axis. The magnetoelastic energy and its corresponding effective field, found analytically using the calculus of variations, account for the experimental observation that for each current, below a critical current, there is a narrow range of fields for which there are two stable sizes for the central core. © 1996 American Institute of Physics.

Notes: Bismuth doped yttrium iron garnet thin films formed on the surfaces of a gadolinium gallium garnet substrate are used as magneto-optical indicators for quantitative studies of the micromagnetics of an iron whisker. The method does not require image processing. A field applied perpendicular to the whisker axis splits a 180° domain wall into two 90° sections slightly separated by a section magnetized in the direction of the perpendicular field. A field along the axis of the whisker has a similar effect. Where the region between the two split 90° sections intersect the surface, there is a magnetic charge that strongly affects the domain pattern of the indicator film. More generally, domain walls become visible because the local internal susceptibility is greater at the walls than in the domains. The magneto-optical indicator can be used to measure both susceptibilities. © 1996 American Institute of Physics.

Notes: A collective dipole is written into a section of commercial magnetic recording tape containing metal particles by applying a saturating field in the plane of the tape at some particular angle θset with respect to the tape axis. The section is then rotated for many cycles n in a lower magnetic field. The field is reduced to zero at the end of the rotation to the angle θ in the nth cycle. Then the components of the remanent dipole μx(n,θ) and μy(n,θ) are measured. The experiment is then started over beginning with saturation. By plotting the components on a μx,μy diagram, one discovers a remarkable pattern of almost closed ellipses moving with cycle number n such that μcx(n) and μcy(n), the coordinates of the apparent centers of the ellipses, migrate toward the origin. The decay of the two components follows μx,y(n,θ)=μx,y(∞,θ)−Δμx,y(θ)n−p, with small values of p〈0.2 in the critical field near in magnitude to the coercive field. The ellipses become centered about the origin after many cycles if the field is near or above the critical field, but not for lower fields. For θset≠mπ/2, the results depend on the sense of rotation (clockwise or counterclockwise) in the field. There are so many more particles in the tape than can be treated in a computer simulation that simulations only begin to suggest the effects observed. © 1997 American Institute of Physics.