Mary Beth Stearns

Simple explanation of tunneling spin-polarization of Fe, Co, Ni and its alloys

Abstract Values of the electron spin-polarization and their proportionality to bulk moments are easily explained by realizing that they are due to the itinerant d-like electrons. The proportionality of the tunneling current of superconductors and ferromagnets to the three-dimensional density-of-states is seen to occur naturally in the independent particle model. In contrast to tunneling, photoelectrons are dominated by localized electrons; these are also seen to be in agreement with calculated band structure.

Variation of the Internal Fields and Isomer Shifts at the Fe Sites in the FeAl Series

The Mossbauer technique was used to measure the internal magnetic fields and isomer shifts of Fe57 atoms residing at the various sites in the FeAl series. Alloys of Al content varying from 0 to 50 at. % were studied. In the disordered region (<21 at. % Al) five different type sites were observed, corresponding to Fe atoms having 8 through 4 nearest‐neighbor (nn) Fe atoms. The internal field decreased by about 0.07 HFe for each nn Al atom. In the ordered region sites having 8 through 2 nn Fe atoms were observed. For A type sites (4 nn Fe atoms and 4 nn Al atoms at 25 at. % Al) the internal field decreased by 0.11–0.12 HFe for each nn Al atom. In the region above about 30 at. % Al the alloys are not ferromagnetic but information on the type ordering could be obtained from the isomer shifts. Over the whole series the isomer shift indicated that there is progressively less electronic charge at the nucleus as the number of nn Al atoms increased.

Spin Density Measurements and the Spin Susceptibility of the 4s Conduction Electrons in Fe

It has been observed that the Mossbauer spectra of many ferromagnetic alloys can be analyzed to give the effect on the internal field at an 57Fe nucleus of a solute atom in any of the first six neighbor shells surrounding an Fe atom. The success of such an analysis indicates that the effects of many elements (e.g., Al, Si, Mn, V, Cr) are quite localized and, in particular, the results of the Al and Si alloys indicate that these atoms can be interpreted as acting like holes in the Fe lattice. From considerations of the various contributions to the hyperfine field in Fe, it appears that for the FeAl and FeSi alloy systems the observed changes can be attributed to changes in the spin polarization of the s conduction electrons as a function of distance from an Fe atom. The main features of the observed variation are: (a) an oscillating polarization whose amplitude of oscillation is much greater than that corresponding to a Ruderman‐Kittel‐Kasuya‐Yosida type calculation and whose sign is such as to show that t...

Hyperfine field and magnetic behavior of Heusler alloys

The hyperfine fields and Curie temperatures of the Heusler alloys (X2MnZ) are analyzed and show that: (1) The s‐conduction electron polarization (sCEP) is essentially independent of the X or Z element. It is also independent of the difference in the number of valence electrons between a Cd impurity, at a Z site and the Z atom it replaces. This behavior is in contradiction to the basic assumptions made in the charge perturbation models for hyperfine fields but in agreement with the assumptions of the volume overlap model. (2) The behavior of the Curie temperature shows that it is the d‐conduction electron polarization not the sCEP that dominates the coupling between the magnetic Mn atoms in the ferromagnetic Heusler alloys.

On the Origin of Ferromagnetism and the Hyperfine Fields in Fe, Co AND Ni

A specific model for the origin of ferromagnetism in Fe is assumed which attributes the origin of the ferromagnetism to the indirect coupling of localized d‐like electrons by a small number of itinerant d electrons. We find that about 5% of the d‐electrons are in itinerant d bands; the other 95% being in d‐bands which are sufficiently narrow that they can be considered localized. Band calculations and Fermi surface measurements strongly support such a model. Using this model we give the scaling rules for the three contributions to the hyperfine field at any solute atom in an Fe, Co or Ni matrix and evaluate some of these contributions. The analysis indicates that the CEP curves for 4s electrons of Co and Ni are similar to that measured for Fe. Thus the origin of ferromagnetism in Co and Ni is also assumed to be due to a small fraction of itinerant d electrons in these metals.

Studies of Hyperfine Fields in Iron Alloys

The 57Fe hyperfine field spectra of several bcc iron‐rich binary alloys have been studied by the Mossbauer effect, the spin‐echo technique, and by frequency‐swept nuclear magnetic resonance. The alloys contained low concentrations of Al, Si, Mn, Cr, V, Co, or Ni. By computer analysis, the experimental spectra have been decomposed into component lines, assuming a model where the effects of the individual neighbor shells are additive. The resulting shifts in hyperfine fields due to the three nearest‐neighbor shells obtained by the three techniques are compared and discussed.

Magnetization and structure of Mn‐Ni and Mn‐Co layered magnetic thin films

The large number of d‐like conduction electrons in Mn causes the local moments of Mn to be aligned antiferromagnetically, whereas the smaller number of itinerant d electrons in Fe, Co, and Ni causes the d‐conduction electron polarization (dCEP) to align these materials ferromagnetically. With the aim of altering the dCEP in a controlled manner, a series of thin films of total thickness of about 1.2 μ has been prepared by evaporating alternating layers of Ni and Mn or Co and Mn on highly polished oriented sapphire substrates. The layer thicknesses were varied between about 5 and 100 A. Interference patterns from x‐ray measurements indicate that the Ni‐Mn films are layered with Ni(111) and predominantly β‐Mn (221) layers. The Co‐Mn films show predominantly hcp‐Co (002) alignment and a Mn alignment which is similar to that of the Ni‐Mn films. A wide variety of magnetic behavior is seen in these films, as expected due to the systematic variation of the dCEP across the films.

Origin of magnetic alignment in heusler alloys

Abstract It is shown that the magnetic alignment in ferromagnetic Heusler alloys is caused by the positive polarization of the d-like conduction electrons at the Mn-Mn distance. The two other polarizing interactions, s-d exchange and super-exchange, are found to give negative polarization contributions at the Mn-Mn distance.

Theory of spin wave spectra in Heusler alloys

The spin wave spectra of three Heusler alloys (X2MnZ) measured by Ishikawa et al have been analysed by means of a model containing the three magnetic interactions which we believe are present in these materials: Coulomb exchange between localized dl and itinerant di electrons, s‐dl exchange, and superexchange via the Z atoms. Reasonable fits to the data were obtained in which the main variation between alloys was a differing di‐dl interaction representing the change in di electron density as X is varied. This di‐dl interaction dominates the first three near‐neighbor exchange parameters and appears to be responsible for ferromagnetism in these alloys.

Interpretation of the hyperfine field values measured at positive muons in Fe

Abstract The measured hyperfine field at a μ + stopped in Fe is interpreted as being due to the 4s conduction electron polarization (CEP) in Fe and is used to extend the CEP curve to closer distances than nearest neighbors.