News

New Ph.D. Student Starts

On the 15th, Daniel Jetter officially joined our group as a Ph.D. student. Daniel has been working in our group since January 2020, when he started as a visiting Masters student, under the external supervision of Prof. Reinhold Kleiner at the University of Tübingen. He worked on the development of a nanometer-scale SQUID-on-cantilever probe for sensitive magnetic imaging. Daniel finished his Masters in earlier this month.

Physics Today Webinar

On February 11th at 17:00 CET, Prof. Poggio will give a webinar on the Physics Today platform on the physics underlying magnetic imaging techniques, including MFM, scanning SQUID microscopy, and scanning NV-center microscopy. He will discuss how to best apply these techniques to shed light on magnetization patterns, spin configurations, and current distributions in a number of exciting condensed matter systems. Dr. Jelena Trbovic of Zurich Instruments will also discuss how lock-in amplifiers can be used with such local probing techniques to enhance their sensitivity and contrast. Register here.

Paper on Magnetic Mesocrystals Out in PRB

On the 4th, Physical Review B published our paper entitled, Magnetic anisotropy of individual maghemite mesocrystals. Maghemite mesocrystals are magnetic superstructures of maghemite nanoparticles, which are arranged in highly ordered lattices of up to a few micrometers in size. Although measurements on disordered ensembles have been carried out, determining the magnetic properties of individual mesoscopic crystals is challenging due to their small total magnetic moment. In our paper, we describe how to overcome these challenges by utilizing sensitive dynamic cantilever magnetometry to study individual mesocrystals.

Our measurements reveal an unambiguous cubic anisotropy, resulting from the crystalline anisotropy of the constituent maghemite nanoparticles and their alignment within the mesoscopic lattice. The signatures of anisotropy and its origins come to light because we combine the self-assembly of highly ordered mesocrystals with the ability to resolve their individual magnetism. This combination is promising for future studies of the magnetic anisotropy of other nanoparticles, which are too small to investigate individually.

Post-doc Dr. Boris Groß and Ph.D. student Simon Philipp carried out all of the DCM experiments, numerical simulations, and data analysis. External collaborators synthesized the mesocrystals.

Simon gives talk at JEMS

On the 7th, Ph.D. student Simon Philipp gave a talk at the Joint European Magnetic Symposia (JEMS) 2020 held remotely in Lisbon, Portugal. His talk was entitled, “Cantilever torque magnetometry on individual spherical Janus particles”.

Paper on Soft X-ray Microscopy Published

On the 10th, Optica published a paper, on which we collaborated, entitled, Soft x-ray microscopy with 7 nm resolution. The manuscript discusses the combination of newly developed soft x-ray Fresnel zone plate lenses with advanced scanning control and careful optical design to achieve x-ray microscopy with a spatial resolution of 7 nm. Read more about this work in write-ups at Science Daily or Phys.org.

Our group’s contribution to this collaboration was made by post-doc Dr. Boris Groß, who did the micromagnetic simulations used to analyze and interpret the observations.

PhD Student Arrives

On the 1st, Andriani Vervelaki started as Ph.D. student in the group. Andriani comes to us from Crete University in Greece, where she earned her Bachelor of Science in 2018 and her Masters degree in 2020 in physics and photonics, respectively. Welcome Andriani!

Nicola Rossi Moves On

Dr. Nicola Rossi, who started as a Ph.D. student in June 2014, graduated in July 2019, and stayed on as a post-doctoral researcher, left the group at the end of the month. After a short and well-deserved break, he will start work at AMS in Rüschlikon, Switzerland. Nicola has been a key member of the group for many years, performing the group’s first experiments with nanowire scanning probes, leading these efforts, and later providing crucial support to our SQUID-on-tip experiments. He will be sorely missed in the group. We wish him the best of luck in his next adventure and hope he won’t be a stranger!

Post-doc Starts Work

On the 6th, Dr. Kousik Bagani started work in our group as a post-doctoral researcher. Kousik earned his Bachelor of Science in Physics in 2008 from Panskura Banamali College at Vidyasagar University and his Masters of Science in Physics in 2010 from Jadavpur University, both in India. In 2016, he received his Ph.D. in Physics from the Saha Institute of Nuclear Physics at Calcutta University. There he wrote a thesis on “Role of Defects in Anomalous Change of Physical Properties in Graphitic and Oxide Nano-Materials” under the supervision of Prof. Sangam Banerjee. From 2017 to 2020, he worked as a post-doctoral researcher at the Weizmann Institute in Israel in the group of Prof. Eli Zeldov on developing the a new generation of nanometer-scale SQUID-on-tip sensors. Welcome Kousik!

FIBsuperProbes Now Underway

On the 1st, our new FET Open collaborative project, entitled “Focused Ion Beam fabrication of superconducting scanning Probes” or FIBsuperProbes for short, had its official start. We are the lead and coordinating partner of a project, which includes with researchers from IBM Research in Zürich led by Dr. Armin Knoll, from the University of Tübingen led by Prof. Dieter Kölle, and from the Consejo Superior de Investigaciones Cientificas (CSIC) in Zaragoza led by Prof. Jose María De Teresa. We plan to develop a new method — based on forcused ion beam (FIB) technology — of fabricating nanometer-scale superconducting scanning probes. The FET Open provides nearly 3M EUR of research funds to the 4 partner groups over the course of 3.5 years. The collaboration’s website can be found here.

New Masters Student Starts

This month, Aurèle Kamber started work in our group as a Masters student working with Dr. Floris Braakman. Aurèle received his B.S. in Nanoscience with focus in Physics in 2019 from the University of Basel. Welcome!

New Post-doc Joins Group

On the 15th, Dr. Francesco Fogliano started work in our group as a post-doctoral researcher. Francesco earned his Bachelor of Science in Physics in 2012 and his Masters of Science in Physics in 2015, both from the University of Pisa. In 2019, he received his Ph.D. in Physics from the Néel Institute in Grenoble. There he worked on ultrasensitive nanowire force sensors at very low temperatures under the supervision of Dr. Olivier Arcizet, Dr. Benjamin Pigeau, and Dr. Jean-Philippe Poizat. Welcome Francesco!

Paper published on Stability of Néel-type Skyrmions

On the 3rd, Phys. Rev. B published our paper entitled, Stability of Néel-type skyrmion lattice against oblique magnetic fields in GaV₄S₈ and GaV₄Se₈.

The discovery of magnetic skyrmions has spurred renewed interest in non-centrosymmetric magnets. Their topologically protected spin-texture, which can be stable even at room temperature, their nanometer-scale size, and their easy manipulation via electric currents and fields make skyrmions a promising platform for information storage and processing applications.

Until recently, most investigations in bulk crystals have focused on Bloch-type skyrmions, in which the local magnetization rotates perpendicular to the skyrmion’s radial direction. Recently, however, Néel-type skyrmions, in which the local magnetization rotates parallel to its radial direction, have been observed in bulk GaV₄S₈, GaV₄Se₈, and GaMo₄S₈. These materials, which crystallize in the cubic lacunar spinel structure, are multiferroic and could enable nearly dissipation free manipulation of the skyrmions magnetic order by electric fields. In addition to having different spin textures and phase diagrams, the lack of a competing conical phase makes Néel-type skyrmions more robust than their Bloch-type counterparts.

The paper presents experiments, in which we use dynamic cantilever magnetometry (DCM) to map the magnetic phase boundaries in GaV₄S₈ and GaV₄Se₈ as a function of the strength and orientation of magnetic field. We determine the corresponding phase diagrams, which reproduce the major features predicted by a recent theoretical model. The measurements constitute a direct experimental confirmation of the robustness of Néel-type skyrmions to oblique magnetic fields in two materials with uniaxial magnetic anisotropy of opposite signs. In addition to magnetic transitions between the cycloidal, skyrmion lattice, and field-polarized ferromagnetic states, in GaV₄Se₈, we also observe sharp anomalies in the DCM, which we assign to field-driven transformations of magnetic states confined to polar domain walls (DWs).

Our work represents a new and original application of DCM to the study of magnetic skyrmions. It serves both as a template for future studies of skyrmion-hosting materials and provides new information about lacunar spinels and Néel-type skyrmions. In particular, the measured magnetic phase diagrams for GaV₄S₈ and GaV₄Se₈ are in good qualitative agreement with the theoretical predictions and provide insight for how to improve these initial models. Furthermore, the DCM measurements yield new evidence for distinct magnetic states confined to polar structural DWs and their transition from the Cyc to FM state.

The project represents a tight collaboration between our group, the group of István Kézsmárki from the University of Augsburg, Sándor  Bordács from Budapest Univeristy, and Andrey Leonov from Hiroshima University. Our group contributed by making sensitive magnetic torque measurements at low temperature as a function of magnetic field magnitude and direction. Combined with information gained by the other experimental techniques, we were able to construct a complete picture of the sample’s magnetic states. Former Ph.D. student Dr. Andrea Mehlin did much of the early experiments and analysis along with Dr. Boris Groß. Ph.D. student Simon Philipp contributed to experiments in the later part of the project. In the final months of the project, Boris worked extensively on the data analysis and on creating a model with our collaborators that was consistent with all of the measurements.