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

With the development in hard drive and permanent magnet industry, higher saturation 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 belongs 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 volume, which might cause a considerable amount of error depending on the accuracy of the film thickness and area measurement. Other concerns also include the uncertainty of Ms value due to the subtraction of Fe underlayer. Indeed, a direct measurement 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 fitting 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

Superconducting triplet pairing in Ni--Ga-bilayer junctions

Ni--Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al$_2$O$_3$(/EuS)/Ni--Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)--Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)--Ga (30 nm) bilayers entail completely different physics, giving rise to novel conductance-peak subseries that we term conductance shoulders. Detecting the paramagnetic Meissner response in Ga from polarized neutron reflectometry provides the essential experimental hint that these conductance shoulders are caused by superconducting triplet pairings that Ni's ferromagnetic exchange interaction induces near thin Ni--Ga bilayers' interfaces -- most likely owing to inhomogeneously magnetized interface domains. We further substantiate our findings by means of a phenomenological theoretical model, clarifying that induced superconducting triplet pairings around the interface of Ni--Ga bilayers can indeed manifest themselves in the observed conductance shoulders. Arranging our work in a broader context demonstrates that Ni--Ga-bilayer junctions have a strong potential for efficient triplet-pairing engineering in superconducting-spintronics applications.Comment: 24 pages, 10 figures, 1 tabl

Highly Tunable Intrinsic Exchange Bias from Interfacial Reconstruction in Epitaxial NixCoyFe3-x-yO4(111)/{\alpha}-Al2O3(0001) Thin Films

Intrinsic exchange bias up to 12.6 kOe is observed in NixCoyFe3-x-yO4(111)/{\alpha}-Al2O3(0001) (0<=x+y<=3) epitaxial thin films where 0.15<=y<=2. An interfacial layer of rock-salt structure emerges between NixCoyFe3-x-yO4 thin films and {\alpha}-Al2O3 substrates and is proposed as the antiferromagnetic layer unidirectionally coupled with ferrimagnetic NixCoyFe3-x-yO4. In NiCo2O4(111)/{\alpha}-Al2O3(0001) films, results of reflection high energy electron diffraction, X-ray photoelectron spectroscopy, X-ray reflectometry, and polarized neutron reflectometry support that the interfacial layer is antiferromagnetic NixCo1-xO (0.32<=x<=0.49) of rock-salt structure; the interfacial layer and exchange bias can be controlled by growth oxygen pressure revealing the key role of oxygen in the mechanism of the interfacial reconstruction. This work establishes a family of intrinsic exchange bias materials with great tunability by stoichiometry and growth parameters and emphasizes the strategy of interface engineering in controlling material functionalities.Comment: Main Text: 14 pages, 5 figures; Supplemental Materials: 12 pages, 11 figure

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

Magnetic proximity-induced energy gap of topological surface states

Topological crystalline insulator surface states can acquire an energy gap when time reversal symmetry is broken by interfacing with a magnetic insulator. Such hybrid topological-magnetic insulator structures can be used to generate novel anomalous Hall effects and to control the magnetic state of the insulator in a spintronic device. In this work, the energy gap of topological surface states in proximity with a magnetic insulator is measured using Landau level spectroscopy. The measurements are carried out on Pb1-xSnxSe/EuSe heterostructures grown by molecular beam epitaxy exhibiting record mobility and a low Fermi energy enabling this measurement. We find an energy gap that does not exceed 20meV and we show that is due to the combined effect of quantum confinement and magnetic proximity. The presence of magnetism at the interface is confirmed by magnetometry and neutron reflectivity. The recovered energy gap sets an upper limit for the Fermi level needed to observe the quantized anomalous Hall effect using magnetic proximity heterostructures