K. Hasselbach

DC-SQUID magnetization measurements of single magnetic particles

We present the first magnetization measurements of single submicronic particles at very low temperature made of either Ni, Co, CoZrMoNi or TbFe. As detector we use a micro-bridge-DC-SQUID deposited onto or next to the sample. Sample and detector are patterned by electron-beam lithography. The dynamics and the temperature dependence of the magnetization reversal is studied in view of Macroscopic Quantum Tunneling (MQT).

Persistent Currents in Mesoscopic Connected Rings

We report measurements of the low temperature magnetic response of a line of 16 GaAs/GaAlAs connected mesoscopic rings whose total length is much larger than l(straight phi). Using an on-chip micro-SQUID technology, we have measured a periodic response, with period h/e, corresponding to persistent currents in the rings of a typical amplitude of 0.40+/-0.08 nA per ring. Direct comparison with measurements on the same rings but isolated is presented.

Thermomagnetically patterned micromagnets

Thermomagnetic patterning (TMP) of 4 μm thick high performance NdFeB hard magnetic films deposited on Si substrates has been achieved using single pulsed laser irradiation. Uniaxially magnetised chessboard and stripe patterns with lateral feature sizes in the range 50–100 μm were produced. The depth of reversal was estimated, using both global (vibrating sample magnetometry) and localized (scanning Hall probe) measurements, to be in the range of 1.1–1.2 μm. A simple model provides semiquantitative agreement with the experimental results. A linear Halbach array was fabricated to demonstrate the potential of TMP for the realization of complex multidirectional microflux sources.

High sensitivity magnetization measurements of nanoscale cobalt clusters

Presented is a novel high sensitivity magnetometer allowing us to measure the magnetization reversal of about IO4 pB corresponding to a sensitivity of about IO-r6 emu. The detector is a niobium micro-bridge DC superconducting quantum interference device (SQUID), fabricated using electron-beam lithography. It is operational in the temperature range of O-7 K. Furthermore, we present a method to deposit on the SQUID loop a small number of Co clusters of about 2-5 nm in diameter. The first results obtained on these samples show that there is still a ferromagnetic coupling between the clusters and the magnetization reversal takes place by small avalanches

Superconducting proximity effect in a mesoscopic ferromagnetic wire

We present an experimental study of the transport properties of a ferromagnetic metallic wire (Co) in metallic contact with a superconductor (Al). As the temperature is decreased below the Al superconducting transition, the Co resistance exhibits a significant dependence on both temperature and voltage. The differential resistance data show that the decay length for the proximity effect is much larger than we would simply expect from the exchange field of the ferromagnet.

Mesoscopic quantum tunneling of the magnetization

Abstract Magnetic relaxation plateaux observed at low temperature in complex systems, are generally attributed to Quantum Tunneling of the Magnetization (QTM). If the experiments are not performed down to low enough temperatures, alternative interpretations can be given in terms of powerlaw distributions resulting from either switching field distributions or/and coupling between switching blocks leading to self-organized criticality. Besides, the first low-temperature/time-dependent magnetization experiments, performed on single sub-micronic particles and on arrays nanoparticles in molecular crystals, are described.

Micro-superconducting quantum interference device characteristics

We report on the dependence on field and temperature of the critical current of micro superconducting quantum interference devices (SQUIDs): SQUIDs with diameters as small as 1 μm using Dayem bridges as weak links. We model these SQUIDs by solving the Ginzburg–Landau equations with appropriate boundary conditions to obtain the supercurrent–phase relationships. These solutions show that the phase drops and depression of the order parameter produced by supercurrent flow are often distributed throughout the micro-SQUID structure, rather than being localized in the bridge area, for typical micro-SQUID geometries and coherence lengths. The resultant highly nonsinusoidal current–phase relationships Ic(φ) lead to reduced modulation depths and triangular dependences of the micro-SQUID critical currents on applied magnetic flux Ic(Φ). Our modeling agrees well with our measurements on both Al and Nb micro-SQUIDs.

Magnetic characterization of micropatterned Nd–Fe–B hard magnetic films using scanning Hall probe microscopy

Scanning Hall probe microscopy has been used for the quantitative measurement of the z-component (out-of-plane) of the stray magnetic fields produced by Nd–Fe–B hard magnetic films patterned at the micron scale using both topographic and thermomagnetic methods. Peak-to-peak field values in the range 20–120 mT have been measured at scan heights of 25–30 μm above the samples. Quantitative comparison between calculated and measured field profiles gives nondestructive access to the micromagnets’ internal magnetic structure. In the case of topographically patterned films the average value of remanent magnetization is extracted; in the case of thermomagnetically patterned films the depth of magnetization reversal is estimated. The measured field profiles are used to derive the spatial variation in the field and field gradient values at distances in the range 0.1–10 μm above the micromagnet arrays. These length-scales are relevant to the application of the micromagnet arrays for lab-on-chip applications (trappin...

Superconducting phase diagram of UPt3 studied by thermal expansion and specific heat

The specific heat and thermal expansion of UPt3 were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonalc axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges forH∥c is therefore qualitatively similar to that previously found forH⊥c, this time with a critical point (or region) aroundH*=8 kOe,T*=0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.

Scanning μ-superconduction quantum interference device force microscope

A scanning probe technique is presented: Scanning μ-superconducting quantum interference device (SQUID) force microscopy (SSFM). The instrument features independent topographic and magnetic imaging. The force microscope uses a piezoelectric quartz tuning fork as the detector and magnetic imaging is obtained by scanning μ-SQUID microscopy. The μ-SQUID is placed at the edge of a silicon chip attached to the tuning fork. A topographic vertical resolution of 0.02 μm is demonstrated and magnetic flux as weak as 10−3 Φ0 is resolved with a 1 μm diameter μ-SQUID loop. The SSFM operates in a dilution refrigerator in a cryogenic vacuum. Sample and probe can be cooled to 0.45 K.