arXiv subjects

Markus Münzenberg

Publications and source records attributed to Markus Münzenberg.

At least 37 records · Page 2Linked to original sources

arXiv2019

Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures

Spin currents offer a way to control static and dynamic magnetic properties, and therefore they are crucial for next-generation MRAM devices or spin-torque oscillators. Manipulating the dynamics is especially interesting within the context of photo-magnonics. In typical $3d$ transition metal ferromagnets like CoFeB, the lifetime of light-induced magnetization dynamics is restricted to about 1 ns, which e.g. strongly limits the opportunities to exploit the wave nature in a magnonic crystal filtering device. Here, we investigate the potential of spin-currents to increase the spin wave lifetime in a functional bilayer system, consisting of a heavy metal (8 nm of $β$-Tantalum (Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer generates a transverse spin current when a lateral charge current passes through the strip. Using time-resolved all-optical pump-probe spectroscopy, we investigate how this spin current affects the magnetization dynamics in the adjacent CoFeB layer. We observed a linear spin current manipulation of the effective Gilbert damping parameter for the Kittel mode from which we were able to determine the system's spin Hall angles. Furthermore, we measured a strong influence of the spin current on a high-frequency mode. We interpret this mode an an exchange dominated higher order spin-wave resonance. Thus we infer a strong dependence of the exchange constant on the spin current.

cond-mat.mes-hall↗

arXiv2018

Tunnel magneto-Seebeck effect

The interplay of charge, spin and heat transport is investigated in the fascinating research field of spin caloritronics, the marriage of spintronics and thermoelectrics. Here, many new spin-dependent thermal transport phenomena in magnetic nanostructures have been explored in the recent years. One of them is the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions (MTJs) that has large potential for future nanoelectronic devices, such as nanostructured sensors for three-dimentional thermal gradients, or scanning tunneling microscopes driven by temperature differences. The TMS describes the dependence of the MTJ's thermopower on its magnetic cofiguration when a thermal gradient is applied. In this review, we highlight the successful way from first observation of the TMS in 2011 to current ongoing developments in this research area. We emphasize on different heating techniques, material designs, applications, and additional physical aspects such as the role of the thermal conductivity of the barrier material. We further demonstrate the efficient interplay between ab initio calculations and experiments within this field, as this has led, e.g., to the detection of large TMS ratios in MTJs with half-metallic Heusler electrodes.

cond-mat.mes-hall↗

arXiv2018

Thermal conductivity of thin insulating films determined by tunnel magneto-Seebeck effect measurements and finite-element modeling

In general, it is difficult to access the thermal conductivity of thin insulating films experimentally just by electrical means. Here, we present a new approach utilizing the tunnel magneto-Seebeck effect (TMS) in combination with finite-element modeling (FEM). We detect the laser-induced TMS and the absolute thermovoltage of laser-heated magnetic tunnel junctions with 2.6 nm thin barriers of MgAl$_2$O$_4$ (MAO) and MgO, respectively. A second measurement of the absolute thermovoltage after a dielectric breakdown of the barrier grants insight into the remaining thermovoltage of the stack. Thus, the pure TMS without any parasitic Nernst contributions from the leads can be identified. In combination with FEM via COMSOL, we are able to extract values for the thermal conductivity of MAO ($0.7$ W/(K$\cdot$m)) and MgO ($5.8$ W/(K$\cdot$m)), which are in very good agreement with theoretical predictions. Our method provides a new promising way to extract the experimentally challenging parameter of the thermal conductivity of thin insulating films.

cond-mat.mtrl-sci↗

arXiv2018

Anomalous Nernst effect on the nanometer scale: Exploring three-dimensional temperature gradients in magnetic tunnel junctions

Localized laser heating creates temperature gradients in all directions and thus leads to three-dimensional electron flux in metallic materials. Temperature gradients in combination with material magnetization generate thermomagnetic voltages. The interplay between these direction-dependent temperature gradients and the magnetization along with their control enable to manipulate the generated voltages, e.g. in magnetic nanodevices. We identify the anomalous Nernst effect (ANE) generated on a nanometer length scale by micrometer sized temperature gradients in magnetic tunnel junctions (MTJs). In a systematic study, we extract the ANE by analyzing the influence of in-plane temperature gradients on the tunnel magneto-Seebeck effect (TMS) in three dimensional devices. To investigate these effects, we utilize in-plane magnetized MTJs based on CoFeB electrodes with an MgO tunnel barrier. Due to our measurement configuration, there is no necessity to disentangle the ANE from the spin Seebeck effect in inverse spin-Hall measurements. The temperature gradients are created by a tightly focused laser spot. The spatial extent of the measured effects is defined by the MTJ size, while the spatial resolution is given by the laser spot size and the step size of its lateral translation. This method is highly sensitive to low voltages and yields an ANE coefficient of $K_N\approx 1.6\cdot 10^{-8}\,\mathrm{V/TK}$ for CoFeB. In general, TMS investigations in MTJs are motivated by the usage of otherwise wasted heat in magnetic memory devices for read/write operations. Here, the additionally generated ANE effect allows to expand the MTJs' functionality from simple memory storage to nonvolatile logic devices and opens new application fields such as direction dependent temperature sensing with the potential for further downscaling.

cond-mat.mes-hall↗

arXiv2017

Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes

Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some μV, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co$_2$FeAl and Co$_2$FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B based junctions.

cond-mat.mtrl-sci↗

arXiv2017

Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB based magnetic tunnel junctions by variation of the MgAl$_2$O$_4$ and MgO barrier thickness

We investigate the influence of the barrier thickness of Co$_{40}$Fe$_{40}$B$_{20}$ based magnetic tunnel junctions on the laser-induced tunnel magneto-Seebeck effect. Varying the barrier thickness from 1nm to 3nm, we find a distinct maximum in the tunnel magneto-Seebeck effect for 2.6nm barrier thickness. This maximum is independently measured for two barrier materials, namely MgAl$_2$O$_4$ and MgO. Additionally, samples with an MgAl$_2$O$_4$ barrier exhibit a high thermovoltage of more than 350$μ$V in comparison to 90$μ$V for the MTJs with MgO barrier when heated with the maximum laser power of 150mW. Our results allow for the fabrication of improved stacks when dealing with temperature differences across magnetic tunnel junctions for future applications in spin caloritronics, the emerging research field that combines spintronics and themoelectrics.

cond-mat.mtrl-sci↗

arXiv2017

Pumping laser excited spins through MgO barriers

We present a study of the tunnel magneto-Seebeck (TMS) effect in MgO based magnetic tunnel junctions (MTJs). The electrodes consist of CoFeB with in-plane magnetic anisotropy. The temperature gradients which generate a voltage across the MTJs layer stack are created using laser heating. Using this method, the temperature can be controlled on the micrometer length scale: here, we investigate, how both, the TMS voltage and the TMS effect, depend on the size, position and intensity of the applied laser spot. For this study, a large variety of different temperature distributions was created across the junction. We recorded two-dimensional maps of voltages generated by heating in dependence of the laser spot position and the corresponding calculated TMS values. The voltages change in value and sign, from large positive values when heating the MTJ directly in the centre to small values when heating the junction on the edges and even small negative values when heating the sample away from the junction. Those zero crossings lead to very high calculated TMS ratios. Our systematic analysis shows, that the distribution of the temperature gradient is essential, to achieve high voltage signals and reasonable resulting TMS ratios. Furthermore, artefacts on the edges produce misleading results, but also open up further possibilities of more complex heating scenarios for spincaloritronics in spintronic devices.

cond-mat.mes-hall↗

arXiv2016

A light induced metastable magnetic texture uncovered by in-situ Lorentz microscopy

Magnetic topological defects, including vortices and Skyrmions, can be stabilized as equilibrium structures by tuning intrinsic magnetic interactions and stray field geometries. Here, employing rapid quench conditions, we report the observation of a light-induced metastable magnetic texture, which consists of a dense nanoscale network of vortices and antivortices and exhibits glass-like properties. Our results highlight the emergence of complex ordering regimes in optically driven magnetic systems, opening up new pathways to harness ultrafast far-from-equilibrium relaxation processes.

cond-mat.mes-hall↗

arXiv2016

Comparison of laser induced and intrinsic tunnel magneto-Seebeck effect in CoFeB/MgAl$_2$O$_4$ and CoFeB/MgO magnetic tunnel junctions

We present a comparison of the tunnel magneto-Seebeck effect for laser induced and intrinsic heating. Therefore, Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23\,\% of the MgO MTJ emphasizes the influence of the CoFe composition. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alternatively be explained by the Brinkman model and the given junction properties. Especially, we are able to demonstrate that the symmetric contribution is solely influenced by the barrier asymmetry. Thus, we conclude that the symmetry analysis used for the intrinsic method is not suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect.

cond-mat.mtrl-sci↗

arXiv2015

Ultrafast photocurrents at the surface of the three-dimensional topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$

Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents.

cond-mat.mtrl-sci↗

arXiv2014

Generation of magnonic spin wave traps

Spatially resolved measurements of the magnetization dynamics induced by an intense laser pump-pulse reveal that the frequencies of resulting spin wave modes depend strongly on the distance to the pump center. This can be attributed to a laser generated temperature profile. On a CoFeB thin film magnonic crystal, Damon-Eshbach modes are expected to propagate away from the point of excitation. The experiments show that this propagation is frustrated by the strong temperature gradient

cond-mat.mtrl-sci↗

arXiv2014

Controlling boron redistribution in CoFeB/MgO magnetic tunnel junctions during annealing by variation of cap layer materials and MgO deposition methods

Magnetic tunnel junctions with crystalline MgO tunnel barrier and amorphous CoFeB electrodes received much attention due to their high tunnel magneto resistance ratio at room temperature. One important parameter for achieving high tunnel magneto resistance ratios is to control the boron diffusion from the electrodes especially during post growth annealing. By high resolution transmission electron microscopy and electron energy loss spectroscopy techniques we show that the cap layer material adjacent to the electrodes and the MgO deposition method are crucial to control boron redistribution. It is pointed out, that Ta cap layers acts as sinks for boron during annealing in contrast to Ru layers. Furthermore, radio frequency sputtered MgO tunneling barriers contain a rather high concentraion of boron in trigonal [BO$_3$]$^{3-}$ - environment after annealing in contrast to electron beam evaporated MgO which is virtually free from any boron. Our data further indicate that neither boron nor oxygen-vacancy-related gap states in the bulk of MgO barriers affect spin polarized transport for tunnel magneto resistance ratios at the level of 200%.

cond-mat.mtrl-sci↗

arXiv2014

On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect

Thermoelectric effects in magnetic tunnel junctions are currently an attractive research topic. Here, we demonstrate that the tunnel magneto-Seebeck effect (TMS) in CoFeB/MgO/CoFeB tunnel junctions can be switched on to a logic 1 state and off to 0 by simply changing the magnetic state of the CoFeB electrodes. We enable this new functionality of magnetic tunnel junctions by combining a thermal gradient and an electric field. This new technique unveils the bias-enhanced tunnel magneto-Seebeck effect, which can serve as the basis for logic devices or memories in a green information technology with a pure thermal write and read process. Furthermore, the thermally generated voltages that are referred to as the Seebeck effect are well known to sensitively depend on the electronic structure and therefore have been valued in solid-state physics for nearly one hundred years. Here, we lift Seebeck's historic discovery from 1821 to a new level of current spintronics. Our results show that the signal crosses zero and can be adjusted by tuning a bias voltage that is applied between the electrodes of the junction; hence, the name of the effect is bias-enhanced tunnel magneto-Seebeck effect (bTMS). Via the spin- and energy-dependent transmission of electrons in the junction, the bTMS effect can be configured using the bias voltage with much higher control than the tunnel magnetoresistance (TMR) and even completely suppressed for only one magnetic configuration, which is either parallel (P) or anti-parallel (AP). This option allows a readout contrast for the magnetic information of -3000% at room temperature while maintaining a large signal for one magnetic orientation. This contrast is much larger than the value that can be obtained using the TMR effect. Moreover, our measurements are a step towards the experimental realization of high TMS ratios, which are predicted for specific Co-Fe compositions.

cond-mat.mes-hall↗

arXiv2013

Spin-transfer torque switching below 20 kA/cm$^2$ in perpendicular magnetic tunnel junctions

We demonstrate the reduction of critical spin-transfer torque (STT) switching currents in Co-Fe-B/MgO based magnetic tunnel junctions (MTJ) with perpendicular magnetization anisotropy (PMA). The junctions yield tunnel magnetoresistance (TMR) ratios of up to 64% at 4 monolayer (ML) tunnel barrier thickness. In this paper, the reduction of the critical switching current density is studied. By optimizing the applied bias field during DC-STT measurements, ultra low critical switching current densities of less than 20 kA/cm$^2$, even down to 9 kA/cm$^2$, are found. With the reduced switching currents, our samples are ideal candidates for further experimental studies such as the theoretical predicted thermally driven spin-transfer torque effect.

cond-mat.mes-hall↗

arXiv2013

Time-resolved measurement of the tunnel magneto-Seebeck effect in a single magnetic tunnel junction

Recently, several groups have reported spin-dependent thermoelectric effects in magnetic tunnel junctions. In this paper, we present a setup for time-resolved measurements of thermovoltages and thermocurrents of a single micro- to nanometer-scaled tunnel junction. An electrically modulated diode laser is used to create a temperature gradient across the tunnel junction layer stack. This laser modulation technique enables the recording of time-dependent thermovoltage signals with a temporal resolution only limited by the preamplifier for the thermovoltage. So far, time-dependent thermovoltage could not be interpreted. Now, with the setup presented in this paper, it is possible to distinguish different Seebeck voltage contributions to the overall measured voltage signal in the μs time regime. A model circuit is developed that explains those voltage contributions on different sample types. Further, it will be shown that a voltage signal arising from the magnetic tunnel junction can only be observed when the laser spot is directly centered on top of the magnetic tunnel junction, which allows a lateral separation of the effects.

cond-mat.mes-hall↗

arXiv2013

Parameter space for thermal spin-transfer torque

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with 3 monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magneto resistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm$^2$. The thermally generated torque is calculated ab initio using the Korringa-Kohn-Rostoker and non-equilibrium Green's function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

cond-mat.mes-hall↗

arXiv2012

Photo-magnonics

In the framework of magnonics all-optical femtosecond laser experiments are used to study spin waves and their relaxation paths. Magnonic crystal structures based on antidots allow the control over the spin-wave modes. In these two-dimensional magnetic metamaterials with periodicities in the wave-length range of dipolar spin waves the spin-wave bands and dispersion are modified. Hence, a specific selection of spin-wave modes excited by laser pulses is possible. Different to photonics, the modes depend strongly on the strength of the magneto-static potential at around each antidot site - the dipolar field. While this may lead to a mode localization, also for filling fractions around or below 10%, Bloch states are found in low damping ferromagnetic metals. In this chapter, an overview of these mechanisms is given and the connection to spin-wave band spectra calculated from an analytical model is established. Namely, the plane-wave method yields flattened bands as well as band gaps at the antidot lattice Brillouin zone boundary.

cond-mat.mes-hall↗

arXiv2012

Spin-wave modes and band structure of rectangular CoFeB antidot lattices

We present an investigation of rectangular antidot lattices in a CoFeB film. Magnonic band structures are numerically calculated, and band gaps are predicted which shift in frequency by 0.9 GHz when rotating the external field from the long to the short axis of the unit cell. We demonstrate by time-resolved experiments that magnonic dipolar surface modes are split in frequency by 0.6 GHz which agrees well with the theoretical prediction. These findings provide the basis for directional spin-wave filtering with magnonic devices.

cond-mat.mtrl-sci↗