C. Fermon

Propagating spin wave spectroscopy in a permalloy film: A quantitative analysis

We report on the microwave response of micrometer-wide antennas inductively coupled to a 30 nm thick permalloy film. We show that the self-inductance of a single antenna measures the coupling between the exciting current and the spin wave modes of the film. On the other hand, the signal transmitted to a second, distant antenna is used to observe the free relaxation law of a magnetostatic wave packet. All measurements, performed over a frequency range of 1–20 GHz, are quantitatively consistent with magnetostatic wave theory combined with Gilbert formulation of the damping.

Permalloy cylindrical submicron size dot arrays

Abstract Arrays of cylindrical permalloy dots of diameter varying from 88 to 200 nm were fabricated using high-resolution X-ray and electron beam lithography techniques. Reduction of the diameter induces a drastic modification of the magnetisation reversal process. For single domain dots, interdot dipolar coupling plays an important role for the global stable state. Magnetic force microscopy and alternating gradient magnetometry results are presented and discussed.

Magnetocardiography with sensors based on giant magnetoresistance

Biomagnetic signals, mostly due to the electrical activity in the body, are very weak and they can only be detected by the most sensitive magnetometers, such as Superconducting Quantum Interference Devices (SQUIDs). We report here biomagnetic recordings with hybrid sensors based on Giant MagnetoResistance (GMR). We recorded magnetic signatures of the electric activity of the human heart (magnetocardiography) in healthy volunteers. The P-wave and QRS complex, known from the corresponding electric recordings, are clearly visible in the recordings after an averaging time of about 1 min. Multiple recordings at different locations over the chest yielded a dipolar magnetic field map and allowed localizing the underlying current sources. The sensitivity of the GMR-based sensors is now approaching that of SQUIDs and paves way for spin electronics devices for functional imaging of the body.

Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes: Influence of annealing temperature

Low frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. The authors investigated the noise for different degrees of crystallization and CoFeB∕MgO interface quality depending on the annealing temperature. The authors report an extremely low 1∕f noise, compared to magnetic junctions with Al2O3 barriers. The origin of the low frequency noise is discussed and it is attributed to localized charge traps with the MgO barriers.

Numerical and theoretical analysis of multiquantum magic-angle spinning experiments

Using a recent investigation of the Floquet’s theorem for magic-angle spinning nuclear magnetic resonance simulations (NMR), a procedure for computing multiquantum magic-angle spinning spectra is derived. The general formalism which is introduced here can be applied more generally to any solid-state NMR two-dimensional experiments. All interactions and their time dependency are considered during the pulses. Furthermore, for powder patterns, a formal average is possible on γ (the third component of the Euler angle describing the orientation of the crystallite) which leads to great simplifications and to an improved computing efficiency. As an application, the intensity of the spinning sidebands in the two-dimensional multiquantum magic-angle spinning spectrum is investigated. The recently reported appearance of numerous spinning sidebands in the multiquantum dimension is discussed. Such effects appear naturally in the present formalism which provides a theoretical framework for further investigations. Simu...

Low frequency noise in arrays of magnetic tunnel junctions connected in series and parallel

Low frequency noise and small output voltage are the strongest limitations to the use of magnetic tunnel junctions (MTJs) for magnetic sensor applications, replacing giant magnetoresistance (GMR) and anisotropic magnetoresistance sensors. In this paper, we explore the possibility of using arrays with a large number of MTJs connected in parallel/series to overcome these limitations. MTJ’s sensor arrays of more than 3×103 junction elements in different configurations have been fabricated. Low frequency noise and detectivity have been measured and compared to an analytical model, with detectivities of about 16 nT/Hz1/2 achieved in a series/parallel architecture. This detectivity levels are competitive with single devices based on GMR, with the advantage of not requiring additional flux concentrators.

Toward a magnetoresistive chip cytometer: Integrated detection of magnetic beads flowing at cm/s velocities in microfluidic channels

This work describes an integrated device comprising microfluidic channels and incorporated spin-valve sensors sensitive enough to count, determine the magnetic orientation, flowing height, and speed of single micron-sized magnetic beads moving with velocities of 8–35 mm/s. Sensor signals of 3–100 μVp-p correspond to bead moments at different directions indicating a physical rotation of the beads and a slow response (seconds) of the bead moment to magnetizing field changes.

Spin wave quantization in laterally confined magnetic structures (invited)

An overview of the current status of the study of spin wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the most important issues; the quantization of localized spin waves due to the in-plane confinement of spin waves in elements, dipolar coupling between the quantized modes, and the localization of the modes within rectangular elements due to an inhomogeneous demagnetizing field.

Magnetoresistive Detection of Magnetic Beads Flowing at High Speed in Microfluidic Channels

An integrated device including microfluidic channels and incorporated magnetoresistive spin valve sensors has been developed and used to detect single magnetic bead motion at cm/s velocities. The sensitivity of the system is high enough to determine the magnetic orientation, the flowing height and the speed of each particle for velocities between 10-23 mm/s and the counting capability of the device is fully working for this range of velocities and below this limit. Sensor signals (between 3-100 muVp-p) correspond to bead moments at different directions indicating a physical rotation of the beads, and a slow response (seconds) of the bead moment to magnetizing field changes. Subsequent magnetic characterization of the micron-sized beads indicates that they are composed of strongly interacting magnetic nanoparticles.

RF Response of Superconducting-GMR Mixed Sensors, Application to NQR

When coupled to a Giant Magneto-Resistive (GMR) sensor, a superconducting loop containing a constriction can be a very sensitive device for magnetic field detection . The bandwidth of a GMR sensor is wide with only a limitation in the GHz regime mainly due to the ferromagnetic resonances of the free layer. We have studied the radiofrequency response of this mixed sensor and we present femtotesla-range sensitivity obtained up to 10 MHz. An application of this kind of sensor is Nuclear Quadrupolar Resonance (NQR) and low field Nuclear Magnetic Resonance (NMR). We present results showing that mixed sensors can be better than optimized resonant coils for frequencies below 10 MHz opening new possibilities not only for magnetic imaging and low field Nuclear Magnetic Resonance but also for a lot of applications of magnetic sensing in the RF domain.