C. F. Majkrzak

Magnetic rare-earth superlattices (invited)

The magnetic structures of several single‐crystal, magnetic rare‐earth superlattice systems grown by molecular‐beam epitaxy are reviewed. In particular, the results of recent neutron diffraction investigations of long‐range magnetic order in Gd‐Y, Dy‐Y, Gd‐Dy, and Ho‐Y periodic superlattices are presented. In the Gd‐Y system, an antiphase domain structure develops for certain Y layer spacings, whereas modified helical moment configurations are found to occur in the other systems, some of which are commensurate with the chemical superlattice wavelength. References are made to theoretical interaction mechanisms recently proposed to account for the magnetic states of these novel materials.

Interlayer exchange in magnetic superlattices (invited)

A simple model for calculating the Ruderman–Kittel–Kasuya–Yosida exchange interaction between layers of a magnetic rare‐earth (RE) metal separated by yttrium is presented. The calculation is semiphenomenological and makes use only of previously measured or calculated quantities. It is valid when the thickness of the magnetic RE layers is much smaller than that of the Y layers. The coupling is found to be of fairly long range and to be consistent in sign with the ordering observed in Gd‐Y superlattices. For the actual case of comparable layer thicknesses, a discussion of the superlattice wave functions is given. We show that our simple model is very plausible for constituent metals with simple band structures. For the actual band structure of Gd, eigenstates that are evanescent in the Gd region may provide a justification of our model and account for the agreement with experiment. Further theoretical and experimental work is necessary for understanding these systems.

Magnetic structure of multiple bilayers of thin films of Fe and Ge

A diffraction study of multiple bilayers consisting of thin films of Fe and Ge was performed using both x rays and polarized neutrons. A reduction in the moment of a fraction of the Fe atoms due primarily to interdiffusion at the interface is indicated.

Synthetic magnetic rare‐earth Dy‐Y superlattices

The long‐range helimagnetic order in synthetic rare‐earth Dy‐Y superlattices has been studied. The magnetic spiral of Dy maintains its coherence in a (Dy38 A‐Y38.6 A)80 superlattice, but not in a (Dy38A‐Y120.4 A)80 superlattice where the intervening Y layers are 42 atomic planes thick. Furthermore, the spiral periodicity shows a markedly different temperature dependence from that of bulk Dy, including a smaller range of variation and a ‘‘lock‐in’’ at low temperature.

Polarized neutron diffraction studies of Gd-Y synthetic superlattices

The microscopic magnetic structures of coherent, single‐crystal epitaxially grown superlattices consisting of successive bilayers of NGd basal planes of hexagonal‐close‐packed Gd followed by NY such atomic planes of nonmagnetic Y have been studied by polarized neutron diffraction. It has been found that for NGd=10, either a simple parallel alignment of the ferromagnetic Gd layers or an antiphase domain structure occurs, depending on the number of intervening Y planes in an oscillatory manner. These data are consistent with a RKKY coupling mechanism between Gd layers. In addition to the investigation of the interlayer interactions, measurements of the magnetization profile across the thickness of a Gd layer have been performed.

Measurement of the induced moment magnetic form factor of a heavy fermion superconductor

Polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced in a single crystal sample of CeCu2Si2 by a magnetic field of 50 kOe. We find that at 4.2 K (>Tc), the induced magnetization is predominantly of 4f electronic character. No evidence was found of any change in the magnetic form factor between 300 and 4.2 K.

Neutron-diffraction study of the magnetic ordering in superconducting NdRh/sub 4/B/sub 4/

The results of neutron-diffraction measurements are reported which confirm the development of long-range magnetic order in superconducting NdRh/sub 4/B/sub 4/. Two distinct antiferromagnetic transitions occur below the superconducting phase-transition temperature T/sub SC/ = 5.4 K, one at T/sub MH/approx. =1.5 K and the other at T/sub ML/approx. =1.0 K. In both phases, the body-centered tetragonal sublattice of Nd atoms orders antiferromagnetically with the Nd/sup 3 +/ moments aligned along the unique c axis. The magnetic moment is modulated sinusoidally along the (100) direction in the higher-temperature magnetic phase and along the (110) direction in the lower-temperature phase. The measured saturation moment is 3.4 +- 0.5..mu../sub B/. No ferromagnetic component could be detected in the higher-temperature magnetic phase within an experimental sensitivity of 0.3..mu../sub B/.

“Forbidden” magnon scattering in the weak ferromagnet MnSi

Abstract “Forbidden” magnon scattering (i.e. scattering in which magnons are created by increasing rather than decreasing the sample magnetization) has been observed in MnSi over the temperature range from 0.3 Tc to 2Tc in a magnetic field of 7.5 kOe. The existence of “forbidden” magnon scattering is in disagreement with theories of ferromagnetism formulated on the Random Phase Approximation. The linear behaviour of the inelastic neutron scattering intensity difference between the allowed magnon and its forbidden counterpart vs magnetization can be explained by the SKP model, which is based upon a slowly fluctuating spin density picture of itinerant ferromagnetism. Normally such behaviour is expected only for a relatively “strong” ferromagnet, but this picture seems also valid for a “weak” ferromagnet like MnSi.

Neutron scattering study of MnSi proving no ‘‘exchange hole’’

Neutron scattering experiments have been performed on MnSi below TC with the double‐axis powder scattering technique using unpolarized neutrons, and also with the polarization analysis technique. The magnetic scattering intensity has not shown any anomaly around q=0.5 A−1, in contrast to the previous results of Ziebeck et al. who found a large intensity peak at this momentum transfer. Thus, the hypothesis of Ziebeck et al. of observing an ‘‘exchange hole’’ is excluded.

Structural, electrical, and magnetic properties of Fe/Pd superlattices (abstract)

We have made Fe/Pd superlattices with periodic modulation wavelength Λ ranging from 33.2 to 188.4 A. Long‐range structural coherency is inferred from several x‐ray diffraction techniques. Fe and Pd were found to grow (110) and (111) parallel to the substrate, respectively. The in‐plane electrical resistivity was measured in the range 7.5–300 K by the four‐point probe method. The dependence of the resistivity on Λ will be explained by a model using interfacial scattering of electrons. Room‐temperature magnetic properties of the samples were measured using conversion electron Mossbauer spectroscopy and vibrating sample magnetometry. We found all the samples to be ferromagnetic. Both the intrinsic hyperfine magnetic field and the saturation magnetization show an increase as Λ decreases. We found that both of these physical quantities strongly depend on the number of atomic layers of (110) Fe in Λ. We did not find any induced magnetism in (111) Pd to within the experimental error. We determined the magnetic m...