arXiv subjects

J. Langer

Publications and source records attributed to J. Langer.

11 recordsLinked to original sources

arXiv2023

Absorption and birefringence study for reduced optical losses in diamond with high NV concentration

The use of diamond color centers such as the nitrogen-vacancy (NV) center is increasingly enabling quantum sensing and computing applications. Novel concepts like cavity coupling and readout, laser threshold magnetometry and multi-pass geometries allow significantly improved sensitivity and performance via increased signals and strong light fields. Enabling material properties for these techniques and their further improvements are low optical material losses via optical absorption of signal light and low birefringence. Here we study systematically the behavior of absorption around 700 nm and birefringence with increasing nitrogen- and NV-doping, as well as their behavior during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen-doping yet substitutional nitrogen does not seem to be the direct absorber. Birefringence reduces with increasing nitrogen doping. We identify multiple crystal defect concentrations via absorption spectroscopy and their changes during the material processing steps and thus identify potential causes of absorption and birefringence as well as strategies to fabricate CVD diamonds with high NV density yet low absorption and low birefringence.

quant-ph↗

arXiv2022

Rapid determination of single substitutional nitrogen N$_s^0$ concentration in diamond from UV-Vis spectroscopy

Single substitutional nitrogen atoms N$_s^0$ are the prerequisite to create nitrogen-vacancy (NV) centers in diamonds. They serve as the electron donors to create the desired NV$^-$ center, provide charge stability against photo-ionisation, but also are the main source of decoherence. Therefore, precise and quick determination of N$_s^0$ concentration is a key advantage to a multitude of NV-related research in terms of material improvement as well as applications. Here we present a method to determine the N$_s^0$ concentration based on absorption spectroscopy in the UV-Visible range and fitting the 270 nm absorption band. UV-Visible spectroscopy has experimental simplicity and widespread availability that bear advantages over established methods. It allows a rapid determination of N$_s^0$ densities, even for large numbers of samples. Our method shows further advantages in determining low concentrations as well as the ability to measure locally, which is highly relevant for diamonds with largely varying N$_s^0$ concentrations in a single crystal. A cross-check with electron paramagnetic resonance (EPR) shows high reliability of our method and yields the absorption cross section of the 270~nm absorption band, $\sigma=1.96\pm0.15$ cm$^{-1}\cdot$ppm$^{-1}$ (in common logarithm) or $\sigma_e=4.51\pm0.35$ cm$^{-1}\cdot$ppm$^{-1}$ (in natural logarithm), which serves as a reference to determine N$_s^0$ concentrations, and makes our method applicable for others without the need for a known N$_s^0$-reference sample and calibration. We provide a rapid, practical and replicable pathway that is independent of the machine used and can be widely implemented as a standard characterization method for the determination of N$_s^0$ concentrations.

quant-ph↗

arXiv2022

Key points in the determination of the interfacial Dzyaloshinskii-Moriya interaction from asymmetric bubble domain expansion

Different models have been used to evaluate the interfacial Dzyaloshinskii-Moriya interaction (DMI) from the asymmetric bubble expansion method using magneto-optics. Here we investigate the most promising candidates over a range of different magnetic multilayers with perpendicular anisotropy. Models based on the standard creep hypothesis are not able to reproduce the domain wall (DW) velocity profile when the DW roughness is high. Our results demonstrate that the DW roughness and the interface roughness of the sample layers are correlated. Furthermore, we give guidance on how to obtain reliable results for the DMI value with this popular method. A comparison of the results with Brillouin light scattering (BLS) measurements on the same samples shows that the BLS approach often results in higher measured values of DMI.

cond-mat.mes-hall↗

arXiv2021

Creation of nitrogen-vacancy centers in chemical vapor deposition diamond for sensing applications

The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV$^-$ concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over 4 orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2-20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin-spin coherence times T$_2$ ranging from 45.5 to 549 $\mu$s for CVD diamonds containing 168 to 1 parts per billion NV$^-$ centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.

quant-ph↗

arXiv2021

Size-dependent enhancement of passive microwave rectification in magnetic tunnel junctions with perpendicular magnetic anisotropy

Spintronic rf detectors were demonstrated, recently, for energy harvesting and wireless communication at low input power. Here we report on the optimization of the rectified output dc voltage using magnetic tunnel junctions (MTJ) with strong perpendicular anisotropy (PMA) of both the polarizing and the free layer. The magnetization of the polarizing layer is fixed out of plane, while the free layer thickness is adjusted so that its magnetization orientation changes from in plane to out of plane. The rectification dc output voltage lies in the mV range for moderate rf powers, with a signal to noise ratio of 10 to 100 for Prf = -25dBm. It shows a strong dependence on the dimensions of the MTJ: it increases by a factor of 5 to 6 when reducing the diameter from 150nm to 20nm. This enhancement can be doubled when reducing the FL thickness from 1.8nm to 1.6nm. This dimensional enhancement is attributed to the change of the effective anisotropy of the excited free layer, and the MTJ resistance. The results are of interest for the design of spintronic based rf detectors with optimized sensitivity.

cond-mat.mes-hall↗

arXiv2021

Multiple magneto-ionic regimes in Ta/Co$_{20}$Fe$_{60}$B$_{20}$/HfO$_{2}$

In Ta/CoFeB/HfO2 stacks a gate voltage drives, in a nonvolatile way, the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state with IPA. The IPA$\,\to\,$PMA regime is found to be significantly faster than the PMA$\,\to\,$IPA regime, while only the latter shows full reversibility under the same gate voltages. The effective damping parameter also shows a marked dependence with gate voltage in the IPA$\,\to\,$PMA regime, going from 0.029 to 0.012, and only a modest increase to 0.014 in the PMA$\,\to\,$IPA regime. The existence of two magneto-ionic regimes has been linked to a difference in the chemical environment of the anchoring points of oxygen species added to underoxidized or overoxidized layers. Our results show that multiple magneto-ionic regimes can exist in a single device and that their characterization is of great importance for the design of high performance spintronics devices.

physics.app-ph↗

arXiv2021

Quality assessment of in situ plasma etched diamond surfaces for CVD overgrowth

In situ plasma etching is a common method to prepare diamond substrates for epitaxial overgrowth to effectuate higher quality. However, there is no practical, direct, qualitative method established so far to assess the performance of the etching pretreatment. We propose an optimization of the pretreatment process on grounds of high-resolution X-ray diffraction measurements to judge the structural quality gain of the diamond substrates and the effectiveness of the polishing-induced subsurface damage removal. The obtained data shows, that parameters like thickness nor misorientation angle of the diamond substrates seem to influence the gain of the structural quality. The process duration, however, is an important key factor, when the amount of material removal and the arising roughness are discussed. Furthermore, the impact of the oxygen-to-hydrogen ratio is examined. And with rising oxygen percentage, the structural quality gain remains similar, only the overall as well as local mean roughness increases exponentially. Within the utilized reactor setup, the best results are obtained after a 20-minute in situ hydrogen plasma-etching step. The optimal pretreatment process, however, will always embody a tradeoff and needs to be optimized for each reactor type. Due to the introduced method a better evaluation and comparison of the achievements is accomplishable.

cond-mat.mtrl-sci↗

arXiv2017

Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires

Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quantify the DMI for a given material system. Here we extract the DMI at the Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary measurement schemes namely asymmetric domain wall motion and the magnetic stripe annihilation. By using the two different measurement schemes, we find for W(5 nm)/Co20Fe60B20(0.6 nm)/MgO(2 nm) the DMI to be 0.68 +/- 0.05 mJ/m2 and 0.73 +/- 0.5 mJ/m2, respectively. Furthermore, we show that this DMI stabilizes skyrmions at room temperature and that there is a strong dependence of the DMI on the relative composition of the CoFeB alloy. Finally we optimize the layers and the interfaces using different growth conditions and demonstrate that a higher deposition rate leads to a more uniform film with reduced pinning and skyrmions that can be manipulated by Spin-Orbit Torques.

cond-mat.mtrl-sci↗

arXiv2011

Determination of spin-dependent Seebeck coefficients of CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars

We investigate the spin-dependent Seebeck coefficient and the tunneling magneto thermopower of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) in the presence of thermal gradients across the MTJ. Thermal gradients are generated by an electric heater on top of the nanopillars. The thermo power voltage across the MTJ is found to scale linearly with the heating power and reveals similar field dependence as the tunnel magnetoresistance. The amplitude of the thermal gradient is derived from calibration measurements in combination with finite element simulations of the heat flux. Based on this, large spin-dependent Seebeck coefficients of the order of (240 \pm 110) \muV/K are derived. From additional measurements on MTJs after dielectric breakdown, a tunneling magneto thermopower up to 90% can be derived for 1.5 nm MgO based MTJ nanopillars.

cond-mat.mes-hall↗

arXiv2011

Tunneling magneto thermo power in magnetic tunnel junction nanopillars

We study the tunneling magneto thermo power (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by a micropatterned electric heater line. Thermo power voltages up to a few tens of \muV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence with the applied temperature gradient. The thermo power signal varies by up to 10 \muV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100 \muV/K and a large TMTP change of the tunnel junction of up to 90%.

cond-mat.mtrl-sci↗

arXiv2008

Biased quasi ballistic spin torque magnetization reversal

We explore the fundamental time limit of ultra fast spin torque induced magnetization reversal of a magnetic memory cell. Spin torque precession during a spin torque current pulse and free precessional magnetization ringing after spin torque pulse excitation is detected by time resolved magneto transport. Adapting the duration of the spin torque excitation pulse to the spin torque precession period allows suppression of the magnetization ringing and thus coherent control of the final orientation of the magnetization. In the presence of a hard axis bias field such coherent control enables an optimum ultra fast, quasi ballistic spin torque magnetization reversal by a single precessional turn directly from the initial to the reversed equilibrium state.

cond-mat.mtrl-sci↗