Haile Ambaye

Polarization screening-induced magnetic phase gradients at complex oxide interfaces

Thin-film oxide heterostructures show great potential for use in spintronic memories, where electronic charge and spin are coupled to transport information. Here we use a La0.7Sr0.3MnO3 (LSMO)/PbZr0.2Ti0.8O3 (PZT) model system to explore how local variations in electronic and magnetic phases mediate this coupling. We present direct, local measurements of valence, ferroelectric polarization and magnetization, from which we map the phases at the LSMO/PZT interface. We combine these experimental results with electronic structure calculations to elucidate the microscopic interactions governing the interfacial response of this system. We observe a magnetic asymmetry at the LSMO/PZT interface that depends on the local PZT polarization and gives rise to gradients in local magnetic moments; this is associated with a metal-insulator transition at the interface, which results in significantly different charge-transfer screening lengths. This study establishes a framework to understand the fundamental asymmetries of magnetoelectric coupling in oxide heterostructures.

Kondo physics in non-local metallic spin transport devices

The non-local spin-valve is pivotal in spintronics, enabling separation of charge and spin currents, disruptive potential applications and the study of pressing problems in the physics of spin injection and relaxation. Primary among these problems is the perplexing non-monotonicity in the temperature-dependent spin accumulation in non-local ferromagnetic/non-magnetic metal structures, where the spin signal decreases at low temperatures. Here we show that this effect is strongly correlated with the ability of the ferromagnetic to form dilute local magnetic moments in the NM. This we achieve by studying a significantly expanded range of ferromagnetic/non-magnetic combinations. We argue that local moments, formed by ferromagnetic/non-magnetic interdiffusion, suppress the injected spin polarization and diffusion length via a manifestation of the Kondo effect, thus explaining all observations. We further show that this suppression can be completely quenched, even at interfaces that are highly susceptible to the effect, by insertion of a thin non-moment-supporting interlayer.

Spatial arrangement of organic compounds on a model mineral surface: implications for soil organic matter stabilization.

The complexity of the mineral-organic carbon interface may influence the extent of stabilization of organic carbon compounds in soils, which is important for global climate futures. The nanoscale structure of a model interface was examined here by depositing films of organic carbon compounds of contrasting chemical character, hydrophilic glucose and amphiphilic stearic acid, onto a soil mineral analogue (Al2O3). Neutron reflectometry, a technique which provides depth-sensitive insight into the organization of the thin films, indicates that glucose molecules reside in a layer between Al2O3 and stearic acid, a result that was verified by water contact angle measurements. Molecular dynamics simulations reveal the thermodynamic driving force behind glucose partitioning on the mineral interface: The entropic penalty of confining the less mobile glucose on the mineral surface is lower than for stearic acid. The fundamental information obtained here helps rationalize how complex arrangements of organic carbon on soil mineral surfaces may arise.

From X-Ray Telescopes to Neutron Focusing

In the case of neutrons the refractive index is slightly less than unity for most elements and their isotopes [1]. Consequently, thermal and cold neutrons can be reflected from smooth surfaces at grazing-incidence angles. Hence, the optical technologies developed for x-ray astronomy can be applied for neutron focusing. The focusing capabilities of grazing incidence neutron imaging optics have been successfully demonstrated using nickel mirrors. The mirrors were fabricated using an electroformed nickel replication process at Marshall Space Flight Center. Results of the neutron optics experiments and current status of the multilayer coating replication technique development are presented.

Polarized neutron diffraction at a spallation source for magnetic studies

The availability of high-power spallation neutron sources, along with advances in the development of coupled moderators and neutron polarizers, has made it possible to use polarized neutrons on time-of-flight diffractometers for in situ studies of phenomena contributing to field-induced magnetization of a material. Different electronic and structural phenomena that contribute to the overall magnetization of a material can be studied and clearly identified with polarized neutron diffraction measurements. This article reports the first results from polarized neutron diffraction experiments on a time-of-flight instrument at a spallation source. Magnetic field-induced rotation of electron spins in an Ni–Mn–Ga single crystal was measured with polarized neutron diffraction at the MAGICS reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. The difference in intensities measured with spin-up and spin-down polarized neutrons is proportional to the field-induced magnetization of the crystal. The polarized neutron measurements indicate that the magnetic form factor for the 3d electrons of Mn in Ni–Mn–Ga is lower than the value reported earlier for an ideal spherical symmetry of electronic distribution. Future experiments for studying field-induced magnetization in materials following the current methodology are outlined.

Probing Giant Magnetism on Fe-N Thin Film by Polarized Neutron Reflectivity

With the development in hard drive and permanent magnet industry, higher satu‐ ration magnetization (Ms) or magnetic induction (Bs) material is on high demand. According to the Slater-Pauling curve, the highest Bs value is ~2.45 T, which be‐ longs to FeCo alloy. However, in 1972, Kim and Takahashi [1] announced that the new material Fe-N thin film exhibited an increase in the Bs value of 18%. From then on, tons of research works [2, 3, 4] had been dedicated in this area with conclusions on both sides, achieving either high Bs or low Bs. Among those works, the Bs values were obtained from the measurement of the thin film magnetic moment and its vol‐ ume, which might cause a considerable amount of error depending on the accuracy of the film thickness and area measurement. Other concerns also include the uncer‐ tainty of Ms value due to the subtraction of Fe underlayer. Indeed, a direct meas‐ urement of Bs is the key to clarify the discrepancies between these results. Here, we are presenting the method of polarized neutron reflectometry (PNR) to measure the Bs of the partially ordered Fe16N2 thin film. PNR allows the interface magnetism study, [5] the absolute magnetization determination and magnetic depth profile in single thin films, [6, 7, 8, 9] and complicated superlattice structures. [10, 11] In the following scenarios, different partially ordered Fe16N2 thin films are fabricated and are measured using PNR subsequently. Their Bs values are obtained using the fit‐ ting results of nuclear scattering length density (NSLD) and magnetic scattering length density (MSLD). Also, a PNR application on FeN thin film will also help us to understand the switching picture of the FeN thin film with external applied field.

User s Guide for REFoffSpec Version 1.5.4

This document is a user s guide for the IDL software REFoffSpec version 1.5.4 whose purpose is to aggregate for analysis NeXus data files from the magnetism and liquids reflectometer experiments at the Oak Ridge National Laboratory Spallation Neutron Source. The software is used to scale and align multiple data files that constitute a continuous set for an experimental run. The User s Guide for REFoffSepc explains step by step the process using a specific example run. Output screens are provided to orient the user at each step. The guide documents in detail changes made to the original REFoffSpec code during the period November 2009 and January 2011. At the time of the completion of this version of the code it was accessible from the sns_tools interface as a beta version.