Thermal spin transport phenomena and magnetic proximity effect in Pt/ferromagnet bilayer systems. Spintronics and spin caloritronics

Bougiatioti P. Thermal spin transport phenomena and magnetic proximity effect in Pt/ferromagnet bilayer systems. Spintronics and spin caloritronics. Bielefeld: Universität Bielefeld; 2019

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

Longitudinal spin Seebeck coefficient: heat flux vs. temperature difference method

Sola A, Bougiatioti P, Kuepferling M, et al. Longitudinal spin Seebeck coefficient: heat flux vs. temperature difference method. Scientific Reports. 2017;7(1): 46752.The determination of the longitudinal spin Seebeck effect (LSSE) coefficient is currently plagued by a large uncertainty due to the poor reproducibility of the experimental conditions used in its measurement. In this work we present a detailed analysis of two different methods used for the determination of the LSSE coefficient. We have performed LSSE experiments in different laboratories, by using different setups and employing both the temperature difference method and the heat flux method. We found that the lack of reproducibility can be mainly attributed to the thermal contact resistance between the sample and the thermal baths which generate the temperature gradient. Due to the variation of the thermal resistance, we found that the scaling of the LSSE voltage to the heat flux through the sample rather than to the temperature difference across the sample greatly reduces the uncertainty. The characteristics of a single YIG/Pt LSSE device obtained with two different setups was (1.143 ± 0.007) 10^−7 Vm/W and (1.101 ± 0.015) 10^−7 Vm/W with the heat flux method and (2.313 ± 0.017) 10^−7 V/K and (4.956 ± 0.005) 10^−7 V/K with the temperature difference method. This shows that systematic errors can be considerably reduced with the heat flux method

Quantitative Disentanglement of the Spin Seebeck, Proximity-Induced, and Ferromagnetic-Induced Anomalous Nernst Effect in Normal-Metal-Ferromagnet Bilayers

We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{\textrm{4-x}}$ ($4\geq x \geq 0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe$_2$O$_4$ to metallic Ni$_{33}$Fe$_{67}$. A proximity-induced ANE could only be identified in the metallic Pt/Ni$_{33}$Fe$_{67}$ bilayer in contrast to Pt/NiFe$_2$O$_{\rm x}$ ($x>0$) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity

Nonlocal magnon spin transport in NiFe2O4 thin films

Shan J, Bougiatioti P, Liang L, Reiss G, Kuschel T, van Wees BJ. Nonlocal magnon spin transport in NiFe2O4 thin films. Applied Physics Letters. 2017;110(13): 132406.We report magnon spin transport in nickel ferrite platinum (Pt)/(NiFe2O4, NFO) bilayer systems at room temperature. A nonlocal geometry is employed, where the magnons are excited by the spin Hall effect or by the Joule heating induced spin Seebeck effect at the Pt injector and detected at a certain distance away by the inverse spin Hall effect at the Pt detector. The dependence of the nonlocal magnon spin signals as a function of the magnetic field is closely related to the NFO magnetization behavior. In contrast, we observe that the magnetoresistance measured locally at the Pt injector does not show a clear relationship with the average NFO magnetization. We obtain a magnon spin relaxation length of 3.1 ± 0.2 μm in the investigated NFO samples

Correlation of tunnel magnetoresistance with the magnetic properties in perpendicular CoFeB-based junctions with exchange bias

Manos O, Bougiatioti P, Dyck D, et al. Correlation of tunnel magnetoresistance with the magnetic properties in perpendicular CoFeB-based junctions with exchange bias. JOURNAL OF APPLIED PHYSICS. 2019;125(2): 023905.We investigate the dependence of magnetic properties on the post-annealing temperature/time, the thickness of the soft ferromagnetic electrode, and the Ta dusting layer in the pinned electrode as well as their correlation with the tunnel magnetoresistance ratio, in a series of perpendicular magnetic tunnel junctions of materials sequence Ta/Pd/IrMn/CoFe/Ta(x)/CoFeB/MgO(y)/CoFeB(z)/Ta/Pd. We obtain a large perpendicular exchange bias of 79.6 kA/m for x = 0.3 nm. For stacks with z = 1.05 nm, the magnetic properties of the soft electrode resemble the characteristics of superparamagnetism. For stacks with x = 0.4 nm, y = 2 nm, and z = 1.20 nm, the exchange bias presents a significant decrease at post-annealing temperature T-ann = 330 degrees C for 60 min, while the interlayer exchange coupling and the saturation magnetization per unit area sharply decay at T-ann = 340 degrees C for 60 min. Simultaneously, the tunnel magnetoresistance ratio shows a peak of 65.5% after being annealed at T-ann = 300 degrees C for 60 min, with a significant reduction down to 10% for higher annealing temperatures (T-ann >= 330 degrees C) and down to 14% for longer annealing times (T-ann = 300 degrees C for 90 min). We attribute the large decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the pinned electrode. Published under license by AIP Publishing