Publications by Thorsten Hesjedal
Journal of Applied Physics 115 (2014) 2
Nanoscale research letters 9 (2014) 127-
: High-density growth of single-crystalline Bi2Se2Te nanowires was achieved via the vapour-liquid-solid process. The stoichiometry of samples grown at various substrate temperatures is precisely determined based on energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy on individual nanowires. We discuss the growth mechanism and present insights into the catalyst-precursor interaction.
Modelling ferromagnetic resonance in magnetic multilayers: Exchange coupling and demagnetisation-driven effects
JOURNAL OF APPLIED PHYSICS 115 (2014) ARTN 17D140
APPLIED PHYSICS LETTERS 104 (2014) ARTN 253103
Scientific Reports Nature Publishing Group 4 (2014)
Stacking nonvolatile memory cells into a three-dimensional matrix represents a powerful solution for the future of magnetic memory. However, it is technologically challenging to access the data in the storage medium if large numbers of bits are stacked on top of each other. Here we introduce a new type of multilevel, nonvolatile magnetic memory concept, the magnetic abacus. Instead of storing information in individual magnetic layers, thereby having to read out each magnetic layer separately, the magnetic abacus adopts a new encoding scheme. It is inspired by the idea of second quantisation, dealing with the memory state of the entire stack simultaneously. Direct read operations are implemented by measuring the artificially engineered ‘quantised’ Hall voltage, each representing a count of the spin-up and spin-down layers in the stack. This new memory system further allows for both flexible scaling of the system and fast communication among cells. The magnetic abacus provides a promising approach for future nonvolatile 3D magnetic random access memory.
EUROPEAN PHYSICAL JOURNAL-APPLIED PHYSICS 66 (2014) ARTN 10401
New Journal of Physics 15 (2013) 10
Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi 2 Te 3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi 2 Te 3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with T C varying between 9 and 13 K (bulk values) and a saturation moment that reaches4.4(5) μ B per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab -plane. The electrical transport data show an anomaly around T C that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n -doped with carrier concentrations of ∼ 0.5–3.0 × 10 20 cm −3 . X-ray magnetic circular dichroism (XMCD) at the Mn L 2,3 edge at 1.8 K reveals a large spin magnetic moment of4.3(3) μ B /Mn, and a small orbital magnetic moment of0.18(2) μ B /Mn. The results also indicate a ground state of mixed d 4 –d 5 –d 6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga 1− x Mn x As. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c -axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.
Applied Physics Letters 102 (2013) 062403
Perpendicular magnetic tunnel junctions (p-MTJs) with MgO barriers are interesting for high-density information-storage devices. Chemically ordered L10-FePt is a potential electrode due to its large perpendicular magnetocrystalline anisotropy. To-date, a single theoretical study on L10-FePt/MgO p-MTJ based on an idealized structure reported significant dependence of spin-dependent tunneling on interface structure. [Y. Taniguchi et al., IEEE Trans. Magn. 44, 2585 (2008).] We report a structural study of epitaxial L10-FePt(001)//MgO(001)//L10-FePt(001) p-MTJs, focusing on the interfaces using aberration-corrected scanning transmission electron microscopy. Interfaces are semi-coherent, with oxygen atomic-columns of MgO located opposite to iron atomic-columns in L10-FePt. Up to three lattice planes show atomic-column steps, the origin of which is attributed to antiphase boundaries in L10-FePt.
Applied Physics Letters 102 (2013) 242412
Thin film GdBiTe3 has been proposed as a candidate material in which to observe the quantum anomalous Hall effect. As a thermal non-equilibrium deposition method, molecular beam epitaxy (MBE) has the ability to incorporate large amounts of Gd into Bi2Te3 crystal structures. High-quality rhombohedral (GdxBi1−x)2Te3 films with substitutional Gd concentrations of x ≤ 0.4 were grown by MBE. Angle-resolved photoemission spectroscopy shows that the topological surface state remains intact up to the highest Gd concentration. Magnetoresistance measurements show weak antilocalization, indicating strong spin orbit interaction. Magnetometry reveals that the films are paramagnetic with a magnetic moment of 6.93 μB per Gd3+ ion.
SPIN World Scientific Publishing 3 (2013) 1350008
Multivalued logic devices are promising candidates for achieving high-density, low-power memory and logic functionalities. We present a ferromagnetic multilayer Hall junction device with four distinct resistance - and thus logic - states. The states can be encoded as a quaternary bit and decoded into two binary bits. We demonstrate a nonvolatile full adder that is based on a single Hall junction, the extraordinary Hall balance. The device can be easily integrated into complex logic circuits for logic-in-memory architectures.
Scientific Reports Nature Publishing Group 3 (2013) 2087
Magnetoresistance (MR) effects are at the heart of modern information technology. However, future progress of giant and tunnelling MR based storage and logic devices is limited by the usable MR ratios of currently about 200% at room-temperature. Colossal MR structures, on the other hand, achieve their high MR ratios of up to 106% only at low temperatures and high magnetic fields. We introduce the extraordinary Hall balance (EHB) and demonstrate room-temperature MR ratios in excess of 31,000%. The new device concept exploits the extraordinary Hall effect in two separated ferromagnetic layers with perpendicular anisotropy in which the Hall voltages can be configured to be carefully balanced or tipped out of balance. Reprogrammable logic and memory is realised using a single EHB element.
Applied Physics Letters 101 (2012) 072412
Methane chemical vapor deposition on transition metal/GaAs samples - A possible route to Haeckelite carbon nanotubes?
Surface and Interface Analysis 44 (2012) 456-465
We present a systematic study of atmospheric chemical vapor deposition growth of carbon nanotubes (CNTs) on patterned, transition metal/GaAs samples employing methane as the carbon feedstock. Controlled CNT growth was found to occur from the exposed metal-semiconductor interface, rather than from the metal or semiconductor surfaces themselves. A fast sample loading system allowed for a minimization of the exposure to high temperatures, thereby preventing excessive sample damage. The optimum growth temperature for CrNi/GaAs interfaces is 700 °C (at a methane flow rate of 700 sccm). Possible growth scenarios involving the Ni-As-Ga system and its interaction with C is discussed. Raman spectroscopy of the CNTs revealed the presence of pentagon-heptagon defects. Closer analysis of the spectra points towards a mixture of so-called Haeckelite CNTs. © 2011 John Wiley & Sons, Ltd.
Journal of Nanoscience and Nanotechnology 12 (2012) 1006-1023
Methane chemical vapor deposition on transition metal/GaAs samples - a possible route to Haeckelite carbon nanotubes?
Surface and Interface Analysis (2011)
IEEE Transactions on Magnetics (2012)
Physical Review B: Condensed Matter and Materials Physics 86 (2012) 174420
Using a history-dependent method, it is possible to prepare magnetic superlattices, consisting of alternating hard and soft layers, in transverse exchange spring states. The procedure, which involves both physical rotation and magnetization routines, is illustrated using a (110)-oriented [DyFe2(60 Å)/YFe2(240 Å)]15 multilayer film. In small applied fields, it is shown that the magnetic response of a transverse magnetic exchange spring is reversible. However, in fields of up to 14 T, the Dy moments are pulled up out of their local in-plane [00¯1] minimum into an out-of-plane  (or equivalent ) axis. The reversible transverse exchange spring state is then lost. Thereafter, the magnetic loop is characterized by an irreversible out-of-plane magnetic exchange spring state.
Physical Review B - Condensed Matter and Materials Physics 86 (2012)
Magnetic doping of topological insulators (TIs) is a prerequisite for their application as spin-based devices. Using x-ray magnetic circular dichroism (XMCD) we investigate the influence of an ultralow coverage (∼0.5% of a monolayer) of magnetic atoms on a TI substrate. For Fe and Co adatoms on Bi 2Te 3 at ∼1.5 K we find an orbital-to-spin magnetic moment ratio of ∼0.45. The magnetization curve of the Fe atoms recorded by XMCD is in quantitative agreement with a paramagnetic behavior with no indication of long-range magnetic order. The spectral shape of the XMCD indicates that the adatoms are weakly hybridized with the substrate and form narrowband states. The results show that the adatoms are not capable of breaking time-reversal symmetry. © 2012 American Physical Society.
Are carbon nanotubes a naturally occurring material? Hints from methane CVD using lava as a catalyst
Current Nanoscience 7 (2011) 294-296
Single-walled carbon nanotubes (SWNTs) were grown using methane CVD with lava as a catalyst and substrate. Metal-oxide phases embedded in the lava are reduced in the presence of hydrogen, thereby promoting catalytic growth. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show a correlation between the growth of carbonaceous nanomaterials and the presence of iron in the alumina matrix. Raman spectroscopy of the carbon deposits proves the occurrence of SWNTs. Although this growth route lacks ef-ficiency, it provides evidence for the claim that SWNTs are a natural allotrope of carbon and that volcanoes may provide an environment for their synthesis. © 2011 Bentham Science Publishers Ltd.