Aktuelles 2018

Aktuelles 2018

PHD Graduation in FM-Group

We congratulate our colleague Tim Oliver Helbig to the successful defense of his doctoral thesis „Demagnetizing and hardening mechanisms in Nd-Fe-B and Sr-hexaferrite permanent magnets“ on Friday the 26th of October 2018. After the exam the group celebrated together the after-defense ceremony.

We thank him for the years we could work together and wish him good luck for his future career. Let´s stay in touch

Dr. Fabian Rhein mit Doktorvater Oliver Gutfleisch
Dr. Fabian Rhein mit Doktorvater Oliver Gutfleisch

Wir gratulieren Dr. Fabian Rhein

Wir gratulieren unserem Kollegen und RESPONSE assoziiertem (ehemaligen) Doktoranden Dr. Fabian Rhein zu einer erfolgreich bestandenen Verteidigung seiner Doktorarbeit mit dem Titel Nanoskalige Magnete und Magnetkomposite auf Ferritbasis. Obwohl die Arbeit selbst extern bei der SIEMENS AG angefertigt wurde, fand auch ein großer Teil der Arbeit an der TU Darmstadt statt was sich auch in gemeinsamen Publikationen widerspiegelt. Nach der Überreichung des Doktorhutes durch den Doktorvater Oliver Gutfleisch, erfolgte die weitere Feier bei Speis und Trank in der nahegelegenen Darmstädter Brauerei. Hier musste Fabian zudem in einer inoffiziellen Nachprüfung seine Kenntnisse über die Geschichte der SIEMENS AG unter Beweis stellen, sowie seine Kenntnisse regionaler Küche.

X-ray diffractometer set-up with magnet and cold finger
X-ray diffractometer set-up with magnet and cold finger

X-ray diffractometer for the investigation of temperature- and magnetic field-induced structural phase transitions

Tom Faske and Wolfgang Donner

J. Appl. Cryst. (2018). 51

https://doi.org/10.1107/S1600576718004892

This article reports the development and characterization of a laboratory-based high-resolution X-ray powder diffractometer equipped with a 5.5 T magnet and closed-cycle helium cryostat that is primarily designed for the investigation of magneto-structural phase transitions. Unique features of the diffractometer include the position-sensitive detector, allowing the collection of an entire diffraction pattern at once, and the high energy resolution with Mo Kα1 radiation. The ability to utilize a lower energy resolution but higher photon flux by switching to an X-ray mirror monochromator makes it a versatile setup for a variety of compounds. In this contribution, details of the design and performance of the instrument are presented along with its specifications.

Magnetic domain structure and microstructure of Sr-hexaferrite sintered magnets
Magnetic domain structure and microstructure of Sr-hexaferrite sintered magnets

In-situ magnetic force microscopy analysis of magnetization and demagnetization behavior in Al3+ substituted Sr-hexaferrite

F. Rhein, T. Helbig, V.Neu, M. Krispin, O. Gutfleisch

Acta Materialia 146 (2018) 85-96

https://doi.org/10.1016/j.actamat.2017.12.010

The sintering temperature of an Al3+substituted Sr-hexaferrite compositewas systematically varied from 1180°C to 1280°C resulting in different microstructures. The grain size was found to range from a few hundred nanometers to several hundred micrometers depending on Al content and sintering temperature. Adding an Al substituted powder to a commercial powder increased the coercivity from 360 mT to 470 mT, at the same time, decreasing remanence from 350 mT to 305 mT. Magnetization and demagnetization processes from the thermally demagnetized state (TDS) and DC-demagnetized state (DCD) have been investigated systematically by in-situ magnetic force microscopy (MFM) under magnetic field.

From the surface domain contrast a polarizationwas derived which quantitatively matches the global i.e. bulk polarization obtained by superconducting quantum interface device (SQUID) magnetometry. The shape of the initial polarization curve and the polarization from the DCD state were correlated with the in-situ MFM data revealing a distinctly different magnetization behavior depending on grain size. The presented results enable a better understanding of local nucleation mechanisms, global influences of pinning centers and further opportunities to improve rare earth (RE) free permanent magnets based on ferrites.

Doktor Dominik Gölden mit Doktorhut
Dr. Dominik Gölden mit Doktorhut

Wir gratulieren Dr. Dominik Gölden

Wir gratulieren unserem Kollegen Dr. Dominik Gölden zur erfolgreichen Verteidigung seiner Doktorarbeit mit dem Titel Magnetocrystalline anisotropy of iron thin films with interstitial nitrogen and boron am 24ten Januar 2018. Nach bestandener Prüfung und dem Überreichen des traditionellen Doktorhutes, wurde angestoßen und noch bis spät in den Abend gefeiert. Bedingt durch den unmittelbaren Wechsel in die Industrie und den neuen Job, war dies auch gleichzeitig die inoffizielle Abschiedsfeier von Dominik. Das verbleibende RESPONSE Team wünscht Dominik viel Glück und Erfolg bei seinem weiteren beruflichen Werdegang.

Origin of field-induced discontinuous phase transitions in Nd2Fe17

L. V. B. Diop, M. D. Kuz’min, K. P. Skokov, Y. Skourski, and O. Gutfleisch

Phys. Rev. B 97, 054406 (2018)

https://doi.org/10.1103/PhysRevB.97.054406

Magnetic properties of a trigonal ferromagnet Nd2Fe17 have been studied on single crystals in steady (14 T) and pulsed (32 T) magnetic fields. The easy-magnetization direction lies close to the [120] axis, deviating from the basal plane by 2.9° (at 5 K). Of particular interest is the low-temperature magnetization process along the high-symmetry axis [001], which is the hard direction. This process is discontinuous and involves two first-order phase transitions (FOMPs). One of them (at 20 T) is a symmetry FOMP similar to that observed in Sm2Fe17. The second transition (at 10.4 T) is without precedent: as the magnetization turns abruptly towards the applied field, it also changes its azimuthal orientation (the angle φ) by 60°. Both transitions can be reasonably accounted for by the presence of a significant sixth-order trigonal anisotropy term.