Interaction Effects in Dilute Cluster-Assembled Magnetic Nanostructures

We have prepared dilute nanostructured magnetic samples by co-deposition of preformed Cobalt clusters with a narrow size distribution around 40 atoms/cluster in Silver matrices. Magnetoresistance measurements are used to derive information about the magnetic structure of the samples. Effects of cluster size distribution or anisotropy can be neglected in our samples. Deviations from simple Langevin-type magnetization are observed as a function of temperature and identified as due to inter-cluster interactions. Pairwise magnetostatic and indirect exchange interactions as well as the model of interacting superparamagnets are found not to be adequate to explain the observed temperature dependences. We propose an interpretation as correlated spin glass, which shows that for small clusters spin glass behavior can be observed even at high dilutions.

Observations of Pauli paramagnetic effects on the flux line lattice in CeCoIn5

Pratyush Das, Sandrine Gerber, Joël Mesot, Jonathan White

From small-angle neutron scattering studies of the flux line lattice (FLL) in CeCoIn5, with magnetic field applied parallel to the crystal c-axis, we obtain the field and temperature dependence of the FLL form factor (FF), which is a measure of the spatial variation of the field in the mixed state. We extend our earlier work (Bianchi et al 2008 Science 319 177) to temperatures up to 1250 mK. Over the entire temperature range, paramagnetism in the flux line cores results in an increase of the FF with field. Near H-c2 the FF decreases again, and our results indicate that this fall-off extends outside the proposed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) region. Instead, we attribute the decrease to a paramagnetic suppression of Cooper pairing. At higher temperatures, a gradual crossover toward more conventional mixed state behavior is observed.

2010

Magnetic states and spin-wave modes in single ferromagnetic nanotubes

Daniel Rüffer

In this thesis the electrical properties, magnetic states and spin wave resonances of individual magnetically hollow ferromagnetic nanotubes have been studied. They were prepared from the different materials Nickel (Ni), Permalloy (Py) and Cobalt-Iron-Boron (CoFeB), deposited as shells onto non-magnetic Gallium-Arsenide (GaAs) semiconductor nanowires via Atomic Layer Deposition (ALD), thermal evaporation and magnetron sputtering, respectively. The resulting nanotubes had lengths between 10 to 20 μm, diameters of 150 to 400 nm and tube walls (shells) which were 20 to 40nm thick. Structural analysis of the tubes by Transmission Electron Microscopy revealed a poly(nano)crystalline (Ni, Py) and amorphous (CoFeB) structure. Electrical transport experiments as a function of temperature revealed different transport mechanisms for each of the materials. Electron-phonon scattering dominated the temperature dependence of the resistivity in Ni, while a clear evidence for electron magnon scattering was observed in Py. Electron-electron interaction in granular and amorphous media was identified as the major contribution to the temperature dependence in CoFeB. The Anisotropic Magnetoresistance (AMR) ratios have been determined for all tubes and different temperatures. Ni nanotubes exhibited a large relative AMR effect of 1.4% at room temperature. The AMR measurements provided information about the magnetic configurations as well as the magnetization reversal mechanism. Indications for the formation of vortex segments in Ni tubes were found for the magnetization reversal when the magnetic field was perpendicular to the nanotube axis. In cooperation with the Poggio group in Basel, cantilever magnetometry has been used for the further characterization of the nanotube magnetization. The magnetization curves were compared to the AMR measurements and finite element method (FEM) micromagnetic simulations. The comparison between the experimental results and the simulations suggested that the roughness of Ni tubes gave rise to segmented magnetic switching. An almost perfect axial alignment of the remanent magnetization has been observed in Py and CoFeB nanotubes. The influence of the inhomogeneous internal field in transverse magnetic fields was investigated by simulation. The segment-wise alignment of spins with the field direction is argued to provoke characteristic kinks in the hysteresis curve and measured AMR effect. Magnetothermal spatial mapping experiments using the anomalous Nernst effect (ANE) complemented the magnetotransport experiments in cooperation with the group of Prof. Grundler in Munich. Here, first evidence of end-vortices entering the nanotube before reversal could be found. Electrically detected spin wave resonance experiments have been performed in cooperation with the group of Prof. Grundler on individual nanotubes. The detected voltage, generated by the spin rectification effect, revealed multiple resonances in the GHz frequency. The experimentally observed resonances were compared to calculated ones extracted from dynamic simulations. With this comparison, the signatures could be attributed to azimuthally confined spin-wave modes. The deduced dispersion relation suggested the quantization of exchange dominated spin waves in that resonance frequencies follow roughly a quadratic dependence on the wave vector.

EPFL2014

Temperature and field dependence of magnetic domains in La1.2Sr1.8Mn2O7

Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic magnetic force microscopy, domain structures for the layered manganite La1.2Sr1.8Mn2O7 as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature T-C, and appear either via the application of a c-axis magnetic field, or by decreasing the temperature further. At temperatures well below T-C, new domain walls, stable at zero field, can be formed by the application of a c-axis field. Magnetic structures are seen also at temperatures above T-C : these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.

Amer Physical Soc2015

Magnetic states and spin-wave modes in single ferromagnetic nanotubes

Daniel Rüffer

EPFL2014