Dirk Menzel

Technische Universität Braunschweig · Institute of Condensed Matter Physics

We present a detailed study of the electronic and magnetic ground-state properties of Fe1−xCoxSi using a combination of macroscopic and microscopic experimental techniques. From these experiments we quantitatively characterize the metal-insulator transition and magnetic/nonmagnetic quantum phase transition occurring at low-doping levels in Fe1−xCox...

We present a detailed study of the electronic and magnetic ground state properties of Fe$_{1-x}$Co$_{x}$Si using a combination of macroscopic and microscopic experimental techniques. From these experiments we quantitatively characterize the metal-insulator transition and magnetic/non-magnetic quantum phase transition occurring at low doping levels...

Multiferroic BiFeO3 (BFO) ceramics are synthesized by non-thermodynamically stable methods, such as fast firing, cryomilling and Spark Plasma Sintering (SPS) with applied pressure of 60 MPa and high heating rate of 150 K/min. We obtained BFO ceramics and powders that present enhanced magnetic and electrical properties. The observed enhancements are...

Ferromagnetic inverse opal-like structures can be considered as ordered three-dimensional network of nanoislands connected to each other by elongated links. These systems can serve as a playground for studying 3D nanomagnetism. At the same time the structure period (700 nm) and sample size are large enough to apply convenient integral and surface s...

Samples of (Mn1−xCox)Si with x=0.15 and x=0.17 were grown, and their physical properties: magnetization, magnetic susceptibility, resistivity, and heat capacity were studied. The data analysis included previous results at x=0.057,0.063,and0.09. The doping of MnSi with Co completely destroys the helical phase transition, but it saves the helical flu...

The samples of (Mn$_{1-x}$Co$_x$)Si with $x=0.15$ and $x=0.17$ were grown and their physical properties: magnetization and magnetic susceptibility, resistivity and heat capacity were studied. The data analysis included also the previous results at $x=0.057, 0.063, 0.09$. The indicated doping MnSi with Co completely destroys the helical phase transi...

Strontium ferromolybdate, Sr2FeMoO6, is an important member of the family of double perovskites with the possible technological applications in the field of spintronics and solid oxide fuel cells. Its preparation via a multi-step ceramic route or various wet chemistry-based routes is notoriously difficult. The present work demonstrates that Sr2FeMo...

We compare the low-temperature photoluminescence (PL) intensities of a range of GaInN/GaN quantum well (QW) structures under identical excitation conditions, mounting the samples side by side. Normalizing the measured intensity to the absorbed power density in the QWs, we find that low-temperature PL efficiencies of several samples, which show clos...

Bismuth ferrite (BiFeO 3) particles are prepared by a combined mechanochemical−thermal processing of a Bi 2 O 3 + α-Fe 2 O 3 mixture. Structural, magnetic, hyperfine, morphological and chemical properties of the as-prepared BiFeO 3 are studied using X-ray diffraction (Rietveld refinement), 57 Fe Mössbauer spectroscopy, SQUID magnetometry, electron...

We propose a superconducting spin valve based on a Josephson junction with B20-family magnetic metal as barrier material. Our analysis shows that the states of this element can be switched by reorienting the intrinsic non-collinear magnetization of the spiral magnet. This reorientation modifies long-range spin-triplet correlations and thereby influen...

We present a combined experimental and theoretical study of the mineral atacamite Cu2Cl(OH)3. Density-functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field,...

We have grown and characterized three samples of Co-doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity, and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results it follows that impurities (Co and Fe) eliminate the first-o...

The present study is focused on the investigation of the magnetic properties of hexagonally ordered iron nanowire arrays electrodeposited into anodic alumina templates. A series of 9 arrays of nanowires with a diameter of 52 nm, an array periodicity of 101 nm, and a length varying from 3.6 to 21.2 μm is analyzed. Scanning electron microscopy, X-ray...

We present a detailed study of the magnetic and electronic properties of U2Rh3Si5, a material that has been demonstrated to exhibit a first-order antiferromagnetic phase transition. From a high-magnetic-field study, together with extensive experiments in moderate fields, we establish the magnetic phase diagrams for all crystallographic directions....

Ordered ferromagnetic nanowire arrays are widely studied due to the diversity of possible applications. However, there is still no complete understanding of the relation between the array’s parameters and its magnetic behavior. The effect of vortex states on the magnetization reversal of large-diameter nanowires is of particular interest. Here, we...

de Die Kombination von Materialien mit unterschiedlichen magnetischen Eigenschaften auf der Nanoskala führt durch Kopplungseffekte zu Phänomenen wie der sogenannten unidirektionalen Austauschanisotropie, die z. B. für Speichermedien oder magnetische Sensoren großes Potenzial besitzt. Während solche Strukturen bislang nur über komplexe Prozesse der...

We present a detailed study of the magnetic and electronic properties of $\mathrm{U}_2\mathrm{Rh}_3\mathrm{Si}_5$, a material that has been demonstrated to exhibit a first order antiferromagnetic phase transition. From a high magnetic field study, together with extensive experiments in moderate fields, we establish the magnetic phase diagrams for a...

Arrays of ferromagnetic nanowires are promising for diverse areas of practical application ranging from data storage to drug delivery. This makes it essential to study their magnetic behavior and magnetization reversal mechanisms. Here, we report on the fabrication of ordered hexagonal arrays of iron nanowires by templated electrodeposition with th...

Ordered ferromagnetic nanowire arrays are widely studied due to the diversity of possible applications. However, there is still no complete understanding of the relation between the array’s parameters and its magnetic behavior. The effect of vortex states on the magnetization reversal of large-diameter nanowires is of particular interest. Here, we...

Electromagnetic properties of complex oxide solid solutions containing Ce and Y attract increasing interests due to their high application potential. Their properties are known to be dependent on many factors including grain size and crystal defects. Here we focus on unique features of nanocrystalline Ce1-xYxO2-δ (x ≤ 0.3) solid solutions prepared...

We present a study of the magnetoresistivity of thin film MnSi in high magnetic fields. We establish that the magnetoresistivity can be understood in terms of spin fluctuation theory, allowing us to compare our data to studies of bulk material. Despite of a close qualitative resemblance of bulk and thin film data, there are clear quantitative diffe...

Nanostructured composite thin films showing magnetic exchange coupling at the material interface have attracted great interest for the development of electronic components such as spin-valves. Besides the commonly performed fabrication of multilayer systems, the utilization of nanoparticle building blocks holds great potential for thin films with t...

We report that Cu 9 O 2 (SeO 3) 4 Cl 6 is a new multiferroic compound. Comprehensive studies of this compound have been carried out on single crystals as well as polycrystalline samples. Magnetic susceptibility χ and specific heat C with H ∥ c axis both reveal an anomaly at T N = 37 K associated with the long-range antiferromagnetic order. However,...

The spin wave stiffness was measured by small-angle neutron scattering method in the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi with x=0.25,0.30,0.50. It has been shown that the spin wave dispersion in the fully polarized state is anisotropic due to Dzyaloshinskii-Moriya interaction. It is reflected in the neutron scattering pattern as t...

We report results of studying the magnetization, specific heat, and thermal expansion of a single crystal with nominal composition (Mn1−xFex)Si with x=0.15. We found no thermodynamic evidence in favor of a second-order phase transition in this material. The trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e., app...

We report results of studying the magnetization, specific heat and thermal expansion of a single crystal with nominal composition Mn 0.85 Fe 0.15 Si . We found no thermodynamic evidences in favor a second order phase transition in the 15 % Fe substituted MnSi. The trajectory corresponding to the present composition of (MnFe)Si is a critical one, i....

We present a combined experimental and theoretical study of the mineral atacamite Cu$_2$Cl(OH)$_3$. Based on {\it ab initio} band structure calculations, we derive a magnetic coupling scheme of essentially a $S = 1/2$ sawtooth chain. Experimentally, we fully characterize the long-range antiferromagnetically ordered state and field-induced behavior,...

SpineMan is designed as a prototype of a soft robotic manipulator that is constructed of alternating hard and soft segments similar to the human spine. Implementing such soft segments allows to surpass the rigidity of conventional robots and ensures safer workspaces where humans and machines can work side by side with less stringent safety restrict...

Fe3+ S=5/2 ions form saw-tooth-like chains along the a axis of the oxoselenite Fe2O(SeO3)2 and an onset of long-range magnetic order is observed for temperatures below TC=105 K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations with small linewidths emerge...

Fe$^{3+}$ $S = 5/2$ ions form saw-tooth like chains along the $a$ axis of the oxo-selenite Fe$_2$O(SeO$_3$)$_2$ and an onset of long-range magnetic order is observed for temperatures below $T_C = 105$ K. This order leads to distinct fingerprints in phonon mode linewidths and energies as resolved by Raman scattering. In addition, new excitations wit...

We have carried out a structural and magnetic characterization of Co8Zn8+y Mn4−y−x Fex by means of bulk techniques, powder neutron scattering and Mössbauer spectroscopy. From our data, we derive the main magnetic parameters and carry out a detailed structural characterization. We observe that disorder is present in our samples in various forms and...

Thin film MnSi has been epitaxially grown on Si(111) substrate and investigated in terms of magnetoresistance and Hall measurements under applied hydrostatic pressure up to 2.58 GPa. Although the behavior of the thin films is qualitatively similar to bulk material, at a high pressure above 2 GPa, an additional magnetic ordering occurs, resulting in...

We present a study of the magnetoresistivity of thin film MnSi in high magnetic fields. We establish that the magnetoresistivity can be understood in terms of spin fluctuation theory, allowing us to compare our data to studies of bulk material. Despite of a close qualitative resemblance of bulk and thin film data, there are clear quantitative diffe...

We present a comparative study of the (magneto)transport properties, including Hall effect, of bulk, thin film and nanostructured MnSi. In order to set our results in relation to published data we extensively characterize our materials, this way establishing a comparatively good sample quality. Our analysis reveals that in particular for thin film...

The synthesis of FePt nanocrystals is typically performed in an organic solvent at rather high temperatures, demanding the addition of the in situ stabilizers oleic acid and oleylamine to produce monomodal particles with well-defined morphologies. Replacing frequently-used solvents with organic media bearing functional moieties, the use of the stab...

A novel metastable phase of DyGe2.85 synthesised in the AuCu3-structure under high pressure, has been studied by means of the magnetic susceptibility and electrical resistivity measurements (under the pressure P≤3.1 GPa), neutron powder diffraction and time-differential γ-γ perturbed angular correlations (TDPAC) using ¹¹¹Cd nuclear probes. Two dist...

The stiffness of spin waves in the Fe0.75Co0.25Si helimagnet with the Dzyaloshinskii–Moriya interaction in a state fully magnetized by an external field has been measured by the small-angle neutron scattering method. It has been shown that the dispersion of magnons in this state is anisotropic because the neutron scattering pattern consists of two...

The magnetic system of the pseudobinary compound Mn1−xCoxGe has been studied using small-angle neutron scattering and susceptibility measurements. It is found that Mn1−xCoxGe orders magnetically at low temperatures in the whole concentration range of x∈[0/0.9]. Four different states of the magnetic structure have been found at low temperatures: the...

The small-angle neutron scattering is used to measure the spin-wave stiffness in the field-polarized state of the Dzyaloshinskii-Moriya helimagnets Mn1−xFexSi with x=0.03, 0.06, 0.09, and 0.10. The Mn1−xFexSi compounds are helically ordered below Tc and show a helical fluctuation regime above Tc in a wide range up to TDM. The critical temperatures...

An experimental method was developed to set the magnetization direction in ferromagnetic layers of Si(100)/Cu/NiFe/Cu/Co/IrMn/Cu morphologically modified spin valves. Large uniaxial anisotropies emerged in the NiFe and Co layers due to modifications of their morphologies induced by the shadowing effect that arises from oblique depositions. Therefor...

MnSi, which crystallizes in the cubic B20 structure, shows intriguing magnetic properties involving the existence of skyrmions in the magnetic phase diagram. Bulk MnSi has been intensively investigated and thoroughly characterized, in contrast to MnSi thin film, which exhibits widely varying properties in particular with respect to electronic trans...

In this study, magnetic nanocomposites were developed and used as adsorbents for lead and copper from aqueous media. Structural, surface, magnetic and textural properties of functionalized maghemite nanoparticles synthesized by alkaline co-precipitation were studied. The surfaces of the iron oxide nanoparticles (Nps) were modified with different ch...

The magnetic entropy change [ΔSM(T, H)] around the phase transition temperature TC is investigated by the scaling method for Fe0.5Co0.5Si, which exhibits a skyrmion phase below TC. The parameters of ΔSM(T, H) exhibit field dependent behaviors. The ΔSM(T, H) curves under high field can be well scaled into a single universal curve independent of exte...

The magnetic system of the pseudobinary compound Mn$_{1-x}$Co$_{x}$Ge has been studied using small-angle neutron scattering and SQUID-measurements. It is found that Mn$_{1-x}$Co$_{x}$Ge orders magnetically at low temperatures in the whole concentration range of $x \in [0 \div 0.9]$. Three different states of the magnetic structure have been found:...

Recent studies have dealt with the electronic and magnetic ground state properties of the tetraboride material MnB4. So far, however, the ground state properties could not be established unambiguously. Therefore, here we present an experimental study on single-crystalline MnB4 by means of resistivity and magnetization measurements. For this, we hav...

Dimensionality is one of the most important parameters in the determination of the physical properties. Therefore, tuning of effective dimensionality is of significant importance for modulating the functionality of materials. In this work, we find that the spin-dimensionality can be changed by Co-doping in the Fe1−x Cox Si system. Investigation of...

Magnetization hysteresis loops of a three-dimensional nanoscale analog of spin ice based on the nickel inverse opal-like structure (IOLS) have been studied at room temperature. The samples are produced by filling nickel into the voids of artificial opal-like films. The spin ice behavior is induced by tetrahedral elements within the IOLS, which have...

Nickel, palladium and copper complexes of the tripyrrolic ligand hexaethyltripyrrindione (H3Et6tpd) have been prepared and characterized by UV/Vis and EPR spectroscopy, as well as by single crystal X-ray structure determination. In all cases the metal ion is coordinated in a distorted square-planar geometry carrying a water ligand at the fourth coo...

The magnetic system of the Mn$_{1-x}$Fe$_{x}$Ge solid solution is ordered in a spiral spin structure in the whole concentration range of $x \in [0.0 \div 1.0]$. The close inspection of the small-angle neutron scattering data reveals the quantum phase transition from the long-range ordered (LRO) to short range ordered (SRO) helical structure upon in...

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide...

Recent studies have dealt with the electronic and magnetic ground state properties of the tetraboride material MnB$_4$. So far, however, the ground state properties could not be established unambiguously. Therefore, here we present an experimental study on single-crystalline MnB$_4$ by means of resistivity and magnetization measurements. For this,...

The growth process of hydrophilic iron oxide nanoparticles synthesized via the non-aqueous sol–gel method using TEG as high-boiling solvent has been characterized. A model of the growth process is presented, based on a combination of analytical methods, especially XRD, SAXS and TGA. In contrast to previously studied particles synthesized in benzyl...

Temperature dependencies of the electrical resistivity, magnetic susceptibility, and heat capacity have been obtained in the cubic phase of TbGe2.85 (the AuCu3 structure), synthesized at high pressure. The macroscopic measurements indicate that a charge density wave is formed below 145 K and an antiferromagnetic ordering is realized below 19 K. Hyp...

Homoleptic iron complexes of six bis(pyridylimino)isoindoline (bpi) ligands with different substituents (H, Me, Et, tBu, OMe, NMe2 ) at the 4-positions of the pyridine moieties have been prepared and studied with regard to temperature-dependent spin and redox states by a combination of (57) Fe Mössbauer spectroscopy, SQUID magnetometry, single-crys...

Abstract The B20 compound MnSi exhibits a skyrmion phase in a certain range of temperature (T) and external magnetic field (H) in addition to the helimagnetic ordering. In this work, the magnetic entropy change (ΔSM) in the single crystal helimagnet MnSi has been investigated by the scaling analysis. The ΔSM(T,H) curves in higher field range (2 kOe...

Monosilicides of $3d$ metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ series and their...

Monosilicides of 3d-metals frequently show a chiral magnetic ordering with the absolute configuration defined by the chirality of the crystal structure and the sign of the Dzyaloshinskii-Moriya interaction (DMI). Structural and magnetic chiralities are probed here for Fe$_{1-x}$Co$_x$Si series and their mutual relationship is found to be dependent...

The crystal structure of a disordered form of Cr$_{1/3}$NbS$_2$ has been characterized using diffraction and inelastic scattering of synchrotron radiation. In contrast to the previously reported symmetry (P6$_3$22), the crystal can be described by a regular twinning of an average P6$_3$ structure with three disordered positions of the Cr ions. Shor...

The critical behavior of the single-crystal helimagnet MnSi is investigated by means of bulk dc magnetization at the boundary between the conical state and paramagnetic phase. We obtain the critical exponents (beta = 0.242 +/- 0.006, gamma = 0.915 +/- 0.003, and delta = 4.734 +/- 0.006), where the self-consistency and reliability are verified by th...

The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the 57Fe Mössbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale i...

In this paper, we critically review the usual explanation of the blue shift in the temperature dependence of the light emission, which is commonly observed for polar GaInN/GaN quantum wells at intermediate temperatures. We demonstrate that this blue shift is not necessarily caused by a thermally induced change of occupation in an inhomogeneously br...

We have synthesized the solid solutions of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ge}$ compounds with $x$ running from 0.0 to 0.9. Small-angle neutron scattering and magnetization measurements have shown that these compounds are ordered into the spin helix structure below the critical temperature ${T}_{c}$. The helix is transform...

The magnetic structure of the cubic helimagnet MnGe was studied by small-angle neutron scattering in a wide temperature range from 10 to 300 K. We show that MnGe undergoes a complex order-disorder phase transition covering more than 100 K above the critical temperature ${T}_{N}=(130\ifmmode\pm\else\textpm\fi{}2)$ K. Moreover, the helical structure...

(Co1-xNix) 3Sb4O6F6 (x = 0, 0.25, 0.5, 0.75, 1) is prepared by hydrothermal treatment of CoF2 and/or NiF2, and Sb2O3 (autoclave, 230 °C, 4 d).

The enhanced thermopower of the correlated semiconductor FeSi is found to be robust against the sign of the relevant charge carriers. At $T$\,$\approx$\,70 K, the position of both the high-temperature shoulder of the thermopower peak and the nonmagnetic-enhanced paramagnetic crossover, the Nernst coefficient $\nu$ assumes a large maximum and the Ha...

Using high pressure method polycrystalline powder samples of Mn1-xFexGe and Fe1-yCoyGe have been synthesized with x/y running from 0.0 to 1.0. The crystallite size for these compounds is in the order of 10 microns. SQUID magnetization and small angle neutron scattering (using SANS-1 at the MLZ, Garching) have revealed the helical magnetic ordering...

The growth process of iron oxide nanoparticles during synthesis via the non-aqueous sol–gel method has been analyzed. Samples obtained after reaching reaction temperature T R = 200 °C and during the following 23 h of synthesis were characterized in detail by X-ray diffraction measurements, transmission electron microscopy, X-ray photoelectron spect...

Compounds within the solid solution (Co1–xNix)3Sb4O6F6 were prepared by the hydrothermal method. The compounds crystallize in the noncentrosymmetric cubic space group I 4̅3m with unit cell parameters a = 8.176(1) Å for M = Co and a = 8.0778(1) Å for M = Ni. The crystal structure is made up by corner sharing [MO2F4] octahedra via the fluorine atoms....