arXiv subjects

Niclas Teichert

Publications and source records attributed to Niclas Teichert.

8 recordsLinked to original sources

arXiv2021

Single-pulse all-optical switching in amorphous Dy$_x$Co$_{1-x}\text{ }$ and Tb$_x$Co$_{1-x}$

Repeated uniform switching of the magnetization of thin films of ferrimagnetic amorphous Gd$_{x}$(FeCo)$_{1-x}$ in response to single fast laser pulses is well established. Here we report unusual toggle switching in thin films of sperimagnetic amorphous Dy$_x$Co$_{1-x}$ and Tb$_x$Co$_{1-x}$ with $\it{x} \simeq$ 0.25 irradiated with single 200 fs pulses of 800 nm laser light. The samples have strong local random anisotropy due to the non-S state rare earth. The compensation temperature of the films is $\le$ 180 K and their Curie temperature is $\simeq$ 500 K. They are mostly switched by the first pulse, and subsequent pulses lead to partial re-switching of a decreasing amount of the irradiated area, with a granular structure of submicron regions of switched and unswitched material. Individual switched domains about 700 nm in size are observed around the edge of the irradiated spots where the fluence is at the threshold for switching. Results are discussed in terms of a random anisotropy model where the ratio of local anisotropy to exchange is temperature dependent and close to the threshold for strong pinning.

cond-mat.mtrl-sci↗

arXiv2020

Magnetic reversal and pinning in a perpendicular zero moment half-metal

Compensated ferrimagnets are promising materials for fast spintronic applications based on domain wall motion as they combine the favourable properties of ferromagnets and antiferromagnets. They inherit from antiferromagnets immunity to external fields, fast spin dynamics and rapid domain wall motion. From ferromagnets they inherit straightforward ways to read out the magnetic state, especially in compensated half metals, where electrons flow in only one spin channel. Here, we investigate domain structure in compensated half-metallic Mn2Ru0.5Ga films and assess their potential in domain wall motion-based spin-electronic devices. Our focus is on understanding and reducing domain wall pinning in unpatterned epitaxial thin films. Two modes of magnetic reversal, driven by nucleation or domain wall motion, are identified for different thin film deposition temperatures $(T_{dep})$. The magnetic aftereffect is analysed to extract activation volumes $(V^*)$, activation energies $(E_A)$, and their variation $(ΔE_A)$. The latter is decisive for the magnetic reversal regime, where domain wall motion dominated reversal (weak pinning) is found for $ΔE_A<0.2$ eV and nucleation dominated reversal (strong pinning) for $ΔE_A>0.5$ eV. A minimum $ΔE_A=28$ meV is found for $T_{dep}=290°$C. Prominent pinning sites are visualized by analysing virgin domain patterns after thermal demagnetization. In the sample investigated they have spacings of order 300 nm, which gives an upper limit of the track-width of spin-torque domain-wall motion-based devices.

cond-mat.mtrl-sci↗

arXiv2020

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents

Spin orbit torque driven switching is a favourable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in-situ characterisation using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, and using this, multi-level switching is demonstrated. The result is that spin-switching current densities, down to 800 kA cm$^{-2}$, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements, but also neuromorphic and probabilistic computing.

cond-mat.mes-hall↗

arXiv2017

Electrical transport and optical band gap of NiFe$_\textrm{2}$O$_\textrm{x}$ thin films

We fabricated NiFe$_\textrm{2}$O$_\textrm{x}$ thin films on MgAl$_2$O$_4$(001) substrates by reactive dc magnetron co-sputtering varying the oxygen partial pressure during deposition. The fabrication of a variable material with oxygen deficiency leads to controllable electrical and optical properties which would be beneficial for the investigations of the transport phenomena and would, therefore, promote the use of such materials in spintronic and spin caloritronic applications. We used several characterization techniques in order to investigate the film properties, focusing on their structural, magnetic, electrical, and optical properties. From the electrical resistivity measurements we obtained the conduction mechanisms that govern the systems in high and low temperature regimes, extracting low thermal activation energies which unveil extrinsic transport mechanisms. The thermal activation energy decreases in the less oxidized samples revealing the pronounced contribution of a large amount of electronic states localized in the band gap to the electrical conductivity. Hall effect measurements showed the mixed-type semiconducting character of our films. The optical band gaps were determined via ultraviolet-visible spectroscopy. They follow a similar trend as the thermal activation energy, with lower band gap values in the less oxidized samples.

physics.app-ph↗

arXiv2016

How to enable bulk-like martensitic transformation in epitaxial films

The present study is dedicated to the influence of different substrate and buffer layer materials on the martensitic transformation in sputter deposited epitaxial shape memory Heusler alloys. For this, the magnetocaloric Heusler alloy Ni-Co-Mn-Al \cite{Teichert2015b} is grown on MgO(001), MgAl$_{2}$O$_{4}$(001), and MgO(001)/V substrates, which exhibit a lattice misfit to the Ni-Co-Mn-Al between $-1.2\%$ and $3.6\%$. By temperature dependent X-ray diffraction measurements it is shown that the optimum buffer layer for shape memory Heusler films is not one with minimum lattice misfit, but one with minimum Young's modulus and moderate misfit because an elastic buffer layer can deform during the martensitic transformation of the Heusler layer. Furthermore, epitaxial strain caused by a moderate lattice misfit does not significantly change the martensitic transformation temperatures.

cond-mat.mtrl-sci↗

arXiv2015

Exchange bias effect in martensitic epitaxial Ni-Mn-Sn thin films applied to pin CoFeB/MgO/CoFeB magnetic tunnel junctions

The exchange bias effect is commonly used to shift the coercive field of a ferromagnet. This technique is crucial for the use of magnetic tunnel junctions as logic or memory devices. Therefore, an independent switching of the two ferromagnetic electrodes is necessary to guarantee a reliable readout. Here, we demonstrate that the intrinsic exchange bias effect of Ni-Mn-Sn can be used to apply a unidirectional anisotropy to magnetic tunnel junctions. For this, we use epitaxial Ni-Mn-Sn films as pinning layers for microfabricated CoFeB/MgO/CoFeB magnetic tunnel junctions. We compare the exchange bias field ($H_{\text{EB}}$) measured after field cooling in $-10$\,kOe external field by magnetization measurements with $H_{\text{EB}}$ obtained from tunnel magnetoresistance measurements. Consistent for both methods we find an exchange bias of about $H_{\text{EB}}=130$\,Oe at 10\,K, which decreases with increasing temperature and vanishes above 70\,K.

cond-mat.mtrl-sci↗

arXiv2015

Structure and Giant Inverse Magnetocaloric Effect of Epitaxial Ni-Co-Mn-Al Films

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be 14M. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent X-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that Ni$_{41}$Co$_{10.4}$Mn$_{34.8}$Al$_{13.8}$ films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5\,J\,kg$^{-1}$K$^{-1}$ for $μ_0 ΔH=5\,\text{T}$.

cond-mat.mtrl-sci↗

arXiv2015

Vanadium sacrificial layers as a novel approach for the fabrication of freestanding Heusler Shape Memory Alloys

In this study we report a method for the preparation of freestanding magnetocaloric thin films. Non-stoichiometric Heusler alloys Ni-Mn-Sn, Ni-Co-Mn-Sn and Ni-Co-Mn-Al are prepared via sputter deposition. A sacrificial vanadium layer is added between the substrate and the Heusler film. By means of selective wet-chemical etching the vanadium layer can be removed. Conditions for the crystallization of Vanadium layers and epitaxial growth of the Heusler films are indicated. Magnetic and structural properties of freestanding and as-prepared films are compared in detail. The main focus of this study is on the influence of substrate constraints on the Martensitic transistion.

cond-mat.mtrl-sci↗