H. Kępa

Probing hole-induced ferromagnetic exchange in magnetic semiconductors by inelastic neutron scattering.

The effect of hole doping on the exchange coupling of the nearest neighbor (NN) Mn pairs in Zn(1-x)MnxTe is probed by inelastic neutron scattering. The difference in the NN exchange energy DeltaJ1 in the presence and in the absence of the holes is determined. The obtained value of DeltaJ1 is in good agreement with the predictions of the Zener/RKKY model, even on the insulator side of the metal-insulator transition.

Antiferromagnetic phase transition and interlayer spin correlations in short-period EuTe/PbTe superlattices

Abstract Neutron diffraction and magnetization studies of short-period antiferromagnetic (AF) semiconducting (1xa01xa01) EuTe/PbTe superlattices are presented. Due to the symmetry-breaking lattice mismatch strain, only a single AF configuration forms in the EuTe layers, instead of four allowed by symmetry in bulk EuTe. The strain and the finite layer thickness also lead to a strong shift in the Neel temperature. Neutron-diffraction spectra exhibit pronounced patterns of satellites, indicating spin correlations between successive EuTe layers separated by PbTe layers, and the transfer of magnetic interactions across the diamagnetic spaces. Experiments on samples with doped PbTe layers and in external magnetic fields rule out that the coupling is caused by any of the mechanisms known to occur in metallic superlattices. Hence, our data strongly suggest the existence of a new interlayer coupling mechanism.

Determination of antiferromagnetic interactions in Zn(Mn)O, Zn(Co)O, and Zn(Mn)Te by inelastic neutron scattering

The nearest-neighbor magnetic exchange interactions in Zn0.95Mn0.05O, Zn0.95Co0.05O, and Zn0.98Mn0.02Te were investigated by measuring inelastic neutron scattering from isolated exchange-coupled spin-spin pairs. The experiments on Zn0.98Mn0.02Te were carried out at atmospheric pressure and then at 4kbar pressure in order to determine the dependence of the exchange parameter on the spin-spin distance.

Polarized neutron reflectivity studies of magnetic semiconductor superlattices

Abstract Polarized neutron reflectivity studies of EuS/PbS, EuS/YbSe and GaMnAs/GaAs superlattices performed at the NIST Center for Neutron Research are presented. Pronounced antiferromagnetic (AFM) interlayer coupling has been found in EuS/PbS superlattices for a very broad range of PbS spacer thicknesses. Similar, but weaker, AFM coupling is also present in EuS/YbSe, although only for relatively thin YbSe layers. Neutron polarization analysis shows distinct in-plane asymmetry of the magnetization directions of EuS layers in both systems under investigation. For GaMnAs/GaAs superlattices, ferromagnetic (FM) interlayer correlations have been observed. Polarized neutron reflectometry investigations of several GaMnAs/GaAs superlattices have revealed that the manganese magnetic moments in individual GaMnAs layers, in spite of low Mn concentration, form a truly long range, that is in certain cases a single domain, ferromagnetic state.

INTERLAYER CORRELATIONS IN ANTIFERROMAGNETIC SEMICONDUCTOR SUPERLATTICES EUTE/PBTE

Abstract We report neutron diffraction studies of short-period (111) [(EuTe)ξ|(PbTe)η]N superlattices (SLs), with a special emphasis on the interlayer spin coherence phenomena seen in these systems. We summarize the results of experiments on over thirty samples, in which we have investigated the dependence of interlayer coupling effects on the EuTe and PbTe layer thicknesses, on doping of the PbTe layers, and on external magnetic fields.

Neutron and X-ray reflectometry studies of rough interfaces in a Langmuir-Blodgett film

Abstract Neutron and X-ray reflectometry are used to study interlayer roughness and islanding in a 20-bilayer barium stearate Langmuir-Blodgett film with alternating hydrogenated and deuterated bilayers. The interlayer roughness is highly conformal, and analysis of the diffuse X-ray peak widths suggests it is approximately self-affine with a roughness exponent of h ≈ 0.82. This roughness exponent describes the film in all directions, even though steps on the substrate interrupt the correlation of the film across the steps. The neutron and X-ray specular reflectivities contain modulated Kiessig fringes, indicating the presence of islands on the top of the film. Odd Bragg peaks in the neutron specular reflectivity are broadened, implying long-range disorder in the H-D bilayer structure. These data suggest that the islands arose from incomplete coverage during the film preparation.

Studies of intrinsic exchange interactions in Zn(Mn)O, Zn(Mn)S and Zn(Mn)Te at 4 kbar by inelastic neutron scattering

The intrinsic antiferromagnetic (AFM) interactions between nearest‐neighbor Mn ions in Zn(Mn)O, Zn(Mn)S, Zn(Mn)Te have been studied by inelastic neutron scattering at ambient pressure and at 4 kbar. To find how the J values in a single alloy change when the Mn‐Mn distance R decreases we have launched systematic measurements of the nearest‐neighbor exchange constants J1 in these alloys using inelastic neutron scattering spectrometry. We present the J1 data so far obtained at 4 kbar for the three aforementioned systems, as well as the result of measurements of the pressure dependence of the Mn‐Mn distance. We discuss our findings in the context of the analysis presented in the paper by Szuszkiewicz et al.

Numerical simulation of neutron diffraction from correlated/uncorrelated magnetic multilayers

Abstract We report stochastic diffraction simulations of magnetic multilayered systems. Results show that spectrum fringes from a system of many magnetic domains is a non-random effect - i.e., such fringes cannnot occur in the total absence of interlayer spin correlations. Simulation of diffraction spectra from superlattices with interlayer coupling probability 0

Polarized neutron diffraction studies of antiferromagnetic EuTe/PbTe superlattices (abstract)

PbTe is a nonmagnetic semiconductor, and EuTe, also a semiconductor, is a type II fcc antiferromagnet below TN=10 K. Our earlier diffraction studies of [(EuTe)m|PbTe)3m]N samples (with m=2–7, N=200–600) showed that the type II ordering is preserved in the EuTe layers,1 but the lattice strain always selects an arrangement with the (111) ferromagnetic (FM) sheets (characteristic for the type II structure) parallel to the layers. In addition, samples with n⩽5 scans through the antiferromagnetic (AFM) maxima reveal pronounced patterns of satellite peaks—a clear signature of interlayer spin coherence, indicating that Eu–Eu interactions are transferred through PbTe barriers even as thick as 55 A! The origin of this surprisingly long-range interaction cannot be understood on the grounds of the existing theories of interlayer coupling, in which high concentrations of conduction electrons always play a crucial role,2 because EuTe is insulating at low T and in PbTe n is as low as ∼1017u2002cm−3. In order to identify th...

Computer simulation of neutron diffraction from coupled and uncoupled magnetic superlattices (abstract)

Neutron diffraction is a powerful tool for investigating interlayer coupling phenomena in magnetic superlattices. In the absence of interlayer spin coherence the diffraction spectrum has the form of a broad smooth maximum, i.e., it essentially reproduces the shape of the single-layer structure factor. In contrast, spectra from correlated superlattices exhibit characteristic fringe patterns. However, doubts are sometimes expressed as to whether the presence of fringes indeed provides meaningful evidence for interlayer coherence; such opinions are usually based on results of computer simulations that produce “fringelike” features even though the magnetization in consecutive layers changes in a random fashion. Clarity in this issue is certainly of importance, especially in studies of systems with antiferromagnetic layers, in which neutron diffraction is practically the only experimental tool capable of detecting interlayer spin coherence. We report the results of thorough computer modeling studies that show that the “pseudofringes” obtained from simulations on uncoupled superlattice models disappear after averaging the data from a sufficiently large number of random configurations. It is a reasonable expectation that in a real superlattice each magnetic layer breaks into a large number of domains, so neutrons passing through the sample “see” different situations. Hence, realistic modeling does require averaging over many random configurations, and our results point out that in following such a procedure, the “fringelike” features eventually wash out, and the simulated spectra become fully consistent with the analytical single-layer structure factor. However, there is still the possibility that in special situations the spectra from uncoupled systems may exhibit fringes. Suppose that the system symmetry allows discrete domain states (e.g., corresponding to in-plane easy axes), and that the average domain size and the superlattice period are both small in comparison to the coherence length of the incident neutron wave (since the coherence length in a typical experiment is of the order of 1000 A, this is not an unusual situation). Then, arguably, there is always a finite probability that equivalent domains located in different layers will fall into the “coherence volume,” and give rise to constructive interference, even though the domains are not coupled by any interactions. In view of that, our work included simulations on several different model systems that might, in principle, exhibit such an effect. Again, the averaged data from many thousands of runs showed no anomalies at the expected fringe positions, providing more evidence that neutron diffraction allows a fully reliable distinction of coupled and uncoupled superlattice systems.Neutron diffraction is a powerful tool for investigating interlayer coupling phenomena in magnetic superlattices. In the absence of interlayer spin coherence the diffraction spectrum has the form of a broad smooth maximum, i.e., it essentially reproduces the shape of the single-layer structure factor. In contrast, spectra from correlated superlattices exhibit characteristic fringe patterns. However, doubts are sometimes expressed as to whether the presence of fringes indeed provides meaningful evidence for interlayer coherence; such opinions are usually based on results of computer simulations that produce “fringelike” features even though the magnetization in consecutive layers changes in a random fashion. Clarity in this issue is certainly of importance, especially in studies of systems with antiferromagnetic layers, in which neutron diffraction is practically the only experimental tool capable of detecting interlayer spin coherence. We report the results of thorough computer modeling studies that show ...