Material Systems

Superconductors

Struktur des Hochtemperatursupraleiters YBCO

In 1911, the Dutch physicist Heike Kamerlingh Onnes discovered at the University of Leiden that mercury looses its electrical resistance below a temperature of 4 K, the so-called critical temperature. Since then, many different superconducting materials were found, to some extent with considerably higher critical temperatures. Yttrium-barium-copper-oxide (YBCO), which is known since 1987, is fabricated and investigated it the emg. This material exhibits a critical temperature of around 90 K. Hence, liquid nitrogen (77K) can be employed for cooling instead of using the much more expensive helium. A variety of methods is available for the characterization of the films – some of them during the fabrication process - which are fabricated by pulsed laser deposition. These superconductors can further be used as basis for superconducting device which are also produced and used for measurement tasks in the institute.

Grafik: Widerstandsverlauf einer YBCO-Probe — Resistance curve of YBCO sample

Foto einer YBCO-Schicht auf einem 10 x 10 mm² Strontiumtitanat-Substrat. — Photograph of YBCO-film on a 10 x 10 mm² strontium titanate substrate

YBa₂Cu₃O₇ was discovered in 1987 by M. K. Wu et al. as the first superconductor with a transition temperature above the boiling point of liquid nitrogen (77 K).
M.K. Wu et al. , Phys. Rev. Lett. 58 , 908 (1987)

Transition temperature: T_c = 92 K
Structure: rhombic
Lattice constants: a = 3.8231 Å, b = 3.8864 Å, c = 11.6807 Å

Semiconductors

Magnetic Materials

La_1-xCa_xMnO₃ is a complex magnetic material. Its crystal structure is cubic perovskite with a lattice constant of approx. 0,386 nm.

Its magnetic properties change with temperature. They can be controlled by changing the Ca-concentration.

Our main interest is in the ferromagnetic region of Ca-content between 20% and 50%. With increasing temperature the material changes its phase from a ferromagnetic metal to a paramagnetic insulator. In external magnetic fields the material exhibits a very large change in the electric resistance (up to 25000%), called the CMR (colossal magnetoresistance) effect.

One possible application is a sensors for magnetic fields. Additionally, La_1-xCa_xMnO₃ can be combined with ferroelectric materials for novel multiferroic devices.

Grafik Lanthankalziummanganat Gitter — Conventional unit cell of La1-xCaxMnO3

Phasendiagramm von LCMO (angelehnt an: J. Klein, Dissertation, Köln (2001) — Phasediagram of La1-xCaxMnO3 (from: J. Klein, Dissertation, Köln (2001)

(Fe₃O₄)

Magnetite has a cubic crystal structure with the space group of Fd3m (No. 227).
Theoretically, the unit cell is made up of eight cubic units with a lattice d-spacing of 8.396 Å. It contains 56 atoms, including 32 oxygen atoms, 16 Fe³⁺ and 8 Fe²⁺
and may be denoted as (Fe³⁺)^tetr₈ [Fe³⁺Fe²⁺]^oct₈ O32. In its unit cell as shown in Fig. 1, the oxygen onions form a closed-packed FCC lattice. Also, there is 32
octahedral (B site) and 64 tetrahedral (A site) sites in the unit cell. The Fe²⁺ cations occupy 1/4 of the octahedral interstitial sites (i.e. 8 Fe²⁺) and Fe³⁺ ones
evenly ll 1/4 of the octahedral (i.e. 8 Fe³⁺) and 1/8 of the tetrahedral (i.e. 8 Fe³⁺) sites. This crystallographic conguration is denoted inverse spinel.

From the magnetic moment conguration point of view, magnetite is categorizedas ferrimagnetic materials. Its magnetic properties is reected by the splitting of the 5d orbitals as visualized in Fig. 2. The 5d orbitals are split into two subsets due to the inuence of oxide ligands, implying that all Fe³⁺+ and Fe²⁺ ions have ve and four unpaired electrons, respectively.
As can be seen, in the octahedral coordination, Fe³⁺ and Fe²⁺ ions are coupled ferromagnetically through a so called double exchange mechanism. The electron
whose spin is directed in the opposite direction of the others and colored red, can be exchanged between two octahedral coordination. On the other hand, the Fe³⁺ ions in tetrahedral and octahedral sites are coupled antiferromagnetically via the oxygen atom, implying that the Fe³⁺ spins cancel out each other and thus merely unpaired spins of Fe²⁺ in octahedral coordination contribute to the magnetization.
This magnetic moment conguration accounts for the ferrimagnetism seen in magnetite.

Fig. 1: Magnetite crystal structure with the space group of Fd3m cubic lattice Fig. 2: Schematic depiction of the splitting of the 5d orbitals in octahedral and tetrahedral coordination

Fig. 1: Maghemite crystal structure with the space group of Fd3m cubic lattice.

(Fe₃O₃)

Maghemite likewise to magnetite has a cubic crystal structure with the lattice d-spacing of 8.33 Å. As it can be deduced from its chemical formula γ-Fe₂O₃, there only exists Fe³⁺ cations which are arbitrarily distributed in 16 octahedral and 8 tetrahedral interstitial sites in the FCC packing of oxygen anions as shown in Fig.1. The Fe²⁺ cation vacancies () are located in the octahedral sites and their arrangement in the maghemite structure plays a preponderant role in magnetic response of maghemite. When the vacancies are randomly distributed its space group is (No. 227) and its formula unit may be written as O₃₂. The described crystal arrangement is known as normal spinel. Since the spins in the octrahedral and tetrahedral sites are oriented anti-parallel, maghemite is a ferrimagnetic.

Wüstite has a defected salt-like crystallographic structure (, space group No. 225 and the normal lattice d-spacing of 4.303 Å). In its unit cell, oxygen atoms occupy the main face-centred cubuc (FCC) places and Fe²⁺ cations fill all octahedral interstitial sites as visualized in Figure below. The presence of many vacancies in its structure causes high mobility Fe²⁺ cations towards the surface and their consequence oxidation to Fe³⁺. This means that wüstite is a thermodynamically unstable phase and tends to fully oxidize to magnetite by being exposed to air. Regarding the magnetic properties, wüstite showas antiferromagnetic (AFM) behaviour, revealing a negligible magnetic response even in the presence of a magnetic field owning to its anti-parallel spin configuration.

2012

Synthesis of Single-Core Iron Oxide Nanoparticles as a Potential Tracer for Magnetic Particle Imaging
Aidin Lak, Thilo Wawrzik, Frank Ludwig, Meinhard Schilling
Magnetic Particle Imaging, Springer Proceedings in Physics, Volume 140, 91–95, 2012

2002

Thickness dependent phase separation in La_0.7Ca_0.3MnO₃ films
R. Rauer, J. Bäckström, D. Budelmann, M. Kurfiss, M. Schilling, M. Rübhausen, K. Dörr, S.L. Cooper
Appl. Phys. Lett., Volume 81, 3777–3779, 2002

Material Systems

Superconductors

Semiconductors

Magnetic Materials

For All Visitors

For Students

Internal Tools

Contact