C.F. Majkrzak

Antiferromagnetic interlayer coupling in ferromagnetic semiconductor EuS/PbS(001) superlattices

Antiferromagnetic coupling between ferromagnetic layers has been observed for the first time in an all-semiconductor superlattice structure EuS/PbS(001), by neutron scattering and magnetization measurements. Spin-dependent superlattice band structure effects are invoked to explain the possible origin and the strength of the observed coupling.

Phase determination and inversion in specular neutron reflectometry

Abstract We present results testing the experimental feasibility of recently discovered solutions of the dynamical phase problem for specular reflection. Using layers of Cu, Ni, and Mo as references, the real and imaginary parts of the complex reflection amplitude were measured from neutron reflectivities for an asymmetric composite film consisting of deuterated polystyrene and Si. The reflection amplitude was also measured from neutron reflectivity without references for a symmetric deuterated polystyrene film. These amplitudes were inverted using the Gel’fand–Levitan–Marchenko equation to produce scattering length density profiles for the films studied. The inverted profiles compared reasonably well to the expected potentials. We conclude that such methods are practical with current instrumentation.

Neutron scattering on magnetic thin films: Pushing the limits (invited)

Neutron scattering has been the scattering technique of choice for the analysis of magnetic structures and their dynamics for many decades. The advent of magnetic thin film systems has posed new challenges since such samples have inherently small scattering volumes. By way of examples, recent progress in the application of neutron scattering for the study of both magnetic structure and dynamics in magnetic thin film systems will be presented. First, a combined high angle neutron scattering and polarized neutron reflectivity investigation of the magnetic order of Cr and its influence on the exchange coupling between the Fe layers in Fe/Cr superlattices is discussed. It is shown that in the whole thickness range up to 3000 A, the magnetic structure is governed by frustration effects at the Fe/Cr interfaces. Second, it is demonstrated that it is now possible to investigate the dynamic properties of magnetic thin films with neutron scattering. Unlike, e.g., Brillouin light scattering, inelastic neutron scatte...

Experimental demonstration of phase determination in neutron reflectometry by variation of the surrounding media

Abstract We have recently shown that it is possible to unambiguously determine the real part of the complex reflection coefficient for a thin film structure by measuring the neutron specular reflectivities for the film in contact with two different backing or two different fronting media, thus simplifying the reference methodology of phase determination and making it more practical. Here we demonstrate the technique in two different experiments. In the first, one backing medium is air, the other, heavy water. In the second, sapphire and silicon serve as two different fronting (incident) media. In addition, we show how the two possible branches of the imaginary part of the reflection coefficient inferred by the real part, while not required for inverting the data to find the film structure, can be a sensitive diagnostic for indicating whether the film of interest is inhomogeneous on a transverse scale comparable to the neutron coherence length. This is especially important in avoiding misinterpretation of data which result from the incoherent average of reflectivities from large-scale heterogeneities.

Polarized neutron reflectometry of a patterned magnetic film with a 3He analyzer and a position-sensitive detector

We have employed a polarized 3He spin filter in conjunction with a position-sensitive detector (PSD) to perform efficient polarization analysis of neutron diffuse reflectivity. This work was carried out on the NG-1 polarized neutron reflectometer at the National Institute of Standards and Technology Center for Neutron Research. We measured the specular and diffuse reflectivity of a patterned magnetic array that has periodic square holes in a Co film. Analysis of the data yielded spin-analyzed two-dimensional Qx−Qz reciprocal space maps for the sample in magnetized and demagnetized states. We compared the measurements obtained with a 3He analyzer and a PSD with those obtained using a conventional supermirror analyzer and a 3He proportional counter. The results are in good agreement. For this experiment, 3He gas was polarized by the spin-exchange optical pumping method and stored in a uniform magnetic field provided by a shielded solenoid. Improved optical pumping using a spectrally narrowed diode laser arr...

Inverting neutron reflectivity from layered film structures using polarized beams

Abstract It has recently been discovered that the phase of neutrons specularly reflected from a film structure can be determined exactly, even in the dynamical scattering region at small wave vector transfer, by using polarized beams and a buried magnetic reference layer. This is possible for both magnetic and nonmagnetic material films of interest. Given the reflection amplitude as a function of wave vector transfer, the scattering potential can be uniquely obtained by direct inversion, without resort to fitting. A review of some of the theory is presented, and new experimental results are reported.

Phase sensitive reflectometry and the unambiguous determination of scattering length density profiles

Exact methods for determining the complex neutron reflection amplitude for a thin film, which make use of multiple measurements of the specularly reflected intensities of composite systems, composed of the film adjacent to a reference layer and/or surrounding media, have been developed over the past several years. These techniques are valid even where the Born or distorted wave Born approximations break down. Thus, given both the modulus and phase of the specular reflection, a first-principles inversion can be performed which yields the scattering length density (SLD) depth profile of the film directly. Ideally, if the reflection amplitude is known for all wave vector transfers Q, the associated SLD profile is unique. Applying the aforementioned methods to a purely real SLD profile, which, effectively, is almost always that encountered in neutron reflection, at least two distinct reflectivity curves, corresponding to two different composite film systems, are required to determine the phase by direct algebraic computation, independently at each value of Q. Each of the composite systems consists of the common unknown part of the film plus a different reference layer segment and/or surrounding medium (e.g., the backing). Recently, investigations of certain classes of SLD profiles have been reported in the literature which examine whether a single X-ray reflectivity curve, given certain a priori knowledge about the system, i.e., about known parts of the film SLD and/or substrate, suffices to reconstruct the phase. Employing the exact formulation of phase sensitive reflectometry, we consider several illustrative and realistic cases in which a minimum of two reflectivity curves are required to distinguish the true SLD profile.

Polarized 3He analyzers for neutron reflectometry

Abstract At the National Institute of Standards and Technology (NIST) and Indiana University, we are developing polarized 3 He analyzers for neutron reflectometry. We have employed 3 He analyzers at two polarized neutron reflectometers, NG1 at the NIST Center for Neutron Research (NCNR) and POSY I at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory. The long-term goal for both efforts is to perform efficient studies of magnetic diffuse scattering. At the NCNR, we tested a 3 He analyzer by comparing measurements of specular scattering obtained with a supermirror analyzer to those obtained with a 3 He analyzer. For this test, we measured the spin-flip and nonspin-flip scattering from a Mn0.52Pd0.48/Fe thin film. The results with the 3 He analyzer show very good agreement with those obtained with the supermirror analyzer. We have also carried out tests of the 3 He analyzer for application to magnetic diffuse scattering experiments. We discuss the development of a 3 He analyzer for IPNS that will be employed for studies of patterned magnetic arrays.

Spin-flop tendencies in exchange-biased Co/CoO thin films

In order to study the antiferromagnetic (AFM) spin structure near the interface of exchange-biased bilayers, polarized neutron diffraction measurements were performed on a series of (111)u2009Cou2009(7.5 nm)/CoO (X nm) and CoO (X nm) thin films where X=20, 40, and 100 nm. In these samples, field cooling through the Neel temperature of the AFM increases the component of the CoO moment perpendicular to the applied field, relative to the parallel component. The subsequent application of a 500 Oe field perpendicular to the cooling direction rotates both the Co and CoO moments. Experiments on CoO films without Co showed a smaller difference between the parallel and perpendicular CoO moments in response to cooling and applied fields. Exchange coupling between the Co and CoO layers is apparently responsible for the increased projection of the AFM moments perpendicular to the cooling field.

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.