TY  - CONF
AU  - Pütter, Sabine
AU  - Syed Mohd, Amir
AU  - Mattauch, Stefan
AU  - Koutsioumpas, Alexandros
AU  - Brückel, Thomas
TI  - Polarized neutron investigation of ultrathin Co films on polycrystalline Pt under UHV
M1  - FZJ-2018-01934
PY  - 2017
AB  - Ultrathin magnetic films have been studied for a long time [1,2] but they are still in focus since in spintronics the electron spin has emerged as an additional degree of freedom, besides the electron charge [3]. Crucial for spintronics is the quality of the interface between the substrate and the film or within heterostructures, e.g.[4]. Apparently, ultra high vacuum (UHV) conditions are required for the study of their properties, as e.g. the magnetic interface anisotropy changes on thin film contamination with adsorbats [5] or on covering the thin film with protecting cap layers [6,7]. For this reason, in-situ experiments are preferable. A straightforward way to determine the magnetic moment of thin films and to study them with depth resolution is polarized neutron reflectometry (PNR) [8]. However,  PNR measurements of thin magnetic films under UHV conditions have not yet been performed, to our knowledge. The reason is that a UHV growth chamber occupies a lot of space which is generally limited at beam lines.In order to solve that problem, we have designed a small UHV chamber which is on one hand capable of transferring a sample from thin film fabrication at an MBE system to a neutron reflectometer and on the other hand can be used as measurement chamber for PNR [9]. It consists of a DN CF-40 cube with two opposing sapphire windows for the neutron beam, a combined non evaporable getter and ion pump for keeping the vacuum, a wobble stick, which is needed for in-situ sample transfer and also serves as sample holder for samples of up to 1 cm^2 and a valve for mounting the chamber to the MBE system and exchanging samples. The pressure in the transfer chamber is kept in the 10e−10 mbar range during transport and PNR measurement. The length of chamber is about 30 cm plus 50 cm translation length of the wobble stick and weighs about 10 kg.The magnetic neutron reflectometer with high incident angle (MARIA) at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany is a dedicated instrument for polarized neutron reflectometry of thin films [10]. In an adjacent building thin films can be fabricated with an MBE setup.Our UHV transport chamber was specially designed to fit into the magnet of MARIA and to be attached to the MBE setup. We present PNR measurements on polycrystalline Co thin films of different thickness which were grown in the MBE setup on 20 nm Pt/MgO(100) and measured afterwards at MARIA in a magnetic field of 300 mT at room temperature, under UHV conditions and at ambient air. Our experiments evidence that the UHV chamber works properly and proof that measurements in UHV are crucial.First of all, PNR investigations of a 30 Å Co film were performed in UHV. Afterwards the sample was taken out of the UHV chamber and was exposed to ambient air for 6 hours before measuring it with PNR again. Data treatment was performed with the program GenX [11].The measurements in UHV do not give any hint on oxidation of the Co film. In contrast, the corresponding PNR curves of the ex-situ measured Co film differ clearly from the ones measured in UHV.  As a result, the 30 Å Co film has transformed to 9 Å CoO_x film with 26 Å Co beneath in the six hours. To summarize, we evidence that the UHV chamber works properly which means samples which are delicate to handle in ambient conditions can be measured using our small UHV chamber. Second, we will discuss the issue of reduced magnetic moments at interfaces, which is also called „magnetic dead layers“. It is well known, that the magnetic moment of thin films is reduced at the interface between thin film and the substrate [12,13]. With our measurements we proof that the magnetic moment of Co films is reduced at each interface, also at a Pt cap layer. In literature, however, cap layers are only discussed in terms of the change of the perpendicular magnetic anisotropy [6,7], which is obviously only part of the story.As third point, we study by PNR how the magnetic moment changes for thin films with thickness. We performed PNR measurements under UHV conditions for Co film of thickness 1.5 nm, 2 nm, 3 nm and 6 nm. We observe that there is a magnetic gradient in the Co film up to 3 nm. Nevertheless, the average magnetic moment in Co film is increasing with its thickness and tends to be bulk value at 6 nm.  Access to PNR measurements at MARIA, sample preparation with the MBE system and the usage of the transport chamber for measuring is provided to all people interested via the MLZ user office system (www.mlz-garching) and the nanoscience foundry and fine analysis project (NFFA, www.nffa.eu) where you can also apply for funding.This project has received funding from the EU’s H2020 research and innovation programme under grant agreement n. 
T2  - Intermag Europe 2017
CY  - 24 Apr 2017 - 28 Apr 2017, Dublin (Ireland)
Y2  - 24 Apr 2017 - 28 Apr 2017
M2  - Dublin, Ireland
LB  - PUB:(DE-HGF)24
UR  - https://juser.fz-juelich.de/record/844524
ER  -