Static magnetic proximity effect in Pt/Ni1−x_{1-x}Fex_x bilayers investigated by x-ray resonant magnetic reflectivity

We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/Fe, Pt/Ni$_{33}$Fe$_{67}$, Pt/Ni$_{81}$Fe$_{19}$ (permalloy), and Pt/Ni bilayers. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight in the spatial distribution of the spin polarization of a non-magnetic metal across the interface to a ferromagnetic layer. The evaluation of the experimental results with simulations based on optical data from ab initio calculations provides the induced magnetic moment per Pt atom in the spin polarized volume adjacent to the ferromagnet. We find the largest spin polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni. Additional XRMR experiments with varying photon energy are in good agreement with the theoretical predictions for the energy dependence of the magnetooptic parameters and allow identifying the optical dispersion $\delta$ and absorption $\beta$ across the Pt L3-absorption edge

Static and non-equilibrium magnetic proximity effects in Pt/NiFe2O4 and Pt/Ni1 – xFex heterostructures

Klewe C. Static and non-equilibrium magnetic proximity effects in Pt/NiFe2O4 and Pt/Ni1 – xFex heterostructures. Bielefeld: Universität Bielefeld; 2016.This thesis adresses investigations of the static and the non-equilibrium magnetic proximity effect (MPE) in different bilayer structures consisting of Pt in contact to a ferromagnetic insulator (FMI) or a ferromagnetic metal (FMM). In particular, the ferromagnetic insulator NiFe2O4 (NFO) is studied in this regard. The first experimental chapter of the thesis focusses on the preparation of NFO thin films with different techniques for the application in spintronic and spin caloric devices. The film properties are studied in detail to obtain the optimum fabrication conditions and gain high quality samples. The fabricated films have already been successfully utilized in various investigations on the longitudinal spin Seebeck effect and the spin Hall magnetoresistance (SMR). In order to rule out static MPE induced parasitic effects in these experiments, the interfacial magnetic properties of the Pt/NFO double layers are adressed in the next chapter. In particular, x-ray resonant magnetic reflectivity (XRMR) is introduced as a powerful alternative to the commonly used x-ray magnetic circular dichroism when investigating interfacial spin polarizations, as it is highly interface sensitive and independent of the film thickness. Therefore, XRMR is well suited for studies on the static MPE. In addition to the XRMR measurements on Pt/NFO bilayers, auxiliary studies on YIG based heterostructures are out for a more comprehensive analysis of the static MPE in Pt/FMI systems. Furthermore, the technique of XRMR is explored in more detail and tested for its suitability for the observation of MPEs in different Pt/FMM reference samples. Especially, thickness independence as well as challenges in the quantitative analysis of experimental data are evaluated. The influence of the ferromagnet (FM) thickness and composition on the magnitude of the induced spin polarization is also monitored in different Pt/FM bilayer systems. In the last chapter the Pt/NFO heterostructures are tested for their magnetoresistive properties thus, their suitability for spintronic devices. In particular, the layers are checked for the occurence of an SMR

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe2O4 and on Fe of Various Thicknesses Investigated by XRMR

The longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4 films using Pt as a spin detector and compared to previously investigated NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects induced by the magnetic proximity effect in Pt can be excluded for the sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant magnetic reflectivity measurements show no magnetic response down to a limit of 0.04 {\mu}B per Pt atom comparable to the case of the chemicallydeposited NiFe2O4 films. These differently prepared films have various thicknesses. Therefore, we further studied Pt/Fe reference samples with various Fe thicknesses and could confirm that the magnetic proximity effect is only induced by the interface properties of the magnetic material.Comment: 4 pages, 4 figure

A bioluminescent mouse model of pancreatic β-cell carcinogenesis

The Rip1Tag2 transgenic mouse model of pancreatic β-cell carcinogenesis has been instrumental in identifying several hallmarks of cancer, including tumor cell evasion from apoptosis, tumor angiogenesis and tumor invasion. Moreover, Rip1Tag2 mice have been helpful in the development and testing of innovative cancer therapies and tumor imaging protocols. However, based on tumor localization in the mouse, primary tumor growth and metastatic dissemination cannot be easily monitored in a longitudinal axis by non-invasive and low-technology approaches. Here, we report the generation of a new transgenic mouse line as a versatile tool to study β-cell carcinogenesis. Transgenic expression of a bicistronic messenger RNA encoding simian virus large T antigen and firefly luciferase in pancreatic β-cells recapitulates insulinoma development in a reproducible multistage process. In the mouse line called RipTag-IRES-Luciferase line (RTL) 1, the β-cell-specific expression of luciferase allows the non-invasive monitoring of primary tumor growth over time in vivo and the detection and quantification of disseminated tumor cells and micrometastases in distant organs ex vivo. When crossed to mouse lines in which the expression of cancer ‘modifier' genes has been manipulated, tumor initiation and tumor progression are similarly affected as previously reported for Rip1Tag2 mice, indicating a robust tumor progression pathway shared between the two different transgenic mouse lines. Together, the data indicate that the RTL1 mouse line will be of great value to study anti-tumoral therapeutic approaches as well as to define the functional roles of cancer- and metastasis-related genes when crossed to appropriate transgenic or gene-targeted mouse line

Absorption of Transverse Spin Current in Ferromagnetic NiCu: Dominance of Bulk Dephasing over Spin-Flip Scattering

In ferromagnetic metals, transverse spin currents are thought to be absorbed via dephasing -- i.e., destructive interference of spins precessing about the strong exchange field. Yet, due to the ultrashort coherence length of $\approx$1 nm in typical ferromagnetic thin films, it is difficult to distinguish dephasing in the bulk from spin-flip scattering at the interface. Here, to assess which mechanism dominates, we examine transverse spin-current absorption in ferromagnetic NiCu alloy films with reduced exchange fields. We observe that the coherence length increases with decreasing Curie temperature, as weaker dephasing in the film bulk slows down spin absorption. Moreover, nonmagnetic Cu impurities do not diminish the efficiency of spin-transfer torque from the absorbed spin current. Our findings affirm that transverse spin current is predominantly absorbed by dephasing inside the nanometer-thick ferromagnetic metals, even with high impurity contents