Institut Néel Grenoble

Micro-magnetic devices are developed for application in bio & automobile sensors, etc. The materials in these sensors must be characterized after integration in the device. Scanning Hall probe microscopy (SHPM) is a tool of choice for detecting the stray-field of the packaged devices. In this project, I have developed a SHPM setup at Institut Néel with Dr. Klaus Hasselbach [1]. The setup is being currently utilized for the study of micro-magnet arrays being developed for applications in micro-systems [2-4].

I have also been involved in the micro-magnetic imprinting (µMI) process [2] of fabrication of hard magnetic powder based micro-flux sources (at Institut Néel with Dr. Nora Dempsey). Micro-patterned hard magnetic films serve as templates or master structures in this fabrication process.

I also undertook some modelling of micro-magnetic structures.

In SHPM the Hall sensor measures the z-component of the stray magnetic field emanating from the sample. I have built and used a microscope (development and installation of digital regulation for the microscope and programming user-friendly and reliable LabVIEW software).

Some of the salient features of the SHPM setup are:

• The probe is mounted on a quartz tuning fork allowing topographic feedback and constant probe - sample separation. (Scanning at a fixed height without such feedback (flyover mode) is also possible).

• Hall crosses of sizes varying between 1-5 µm have been used (Hall probes developed by Prof. Roman Kramer at Institut Néel).

• Minimum possible scan resolution given by the stepper motor driven linear stages is 0.1 µm.

• Ability to perform scans at various heights from the plane of the sample, from 1 µm up to 35 mm.

• Detection of large fields ~ 1 T, such as from strong permanent magnets.

• Magnetic field sensitivity ~ 0.1 mT (taking into account the system noise during scanning, but primarily limited by Johnson noise).

Using this microscope, we have studied thin film samples with micro-magnet arrays. The images below show the z-component of the stray field pattern (B) produced at heights of (a) 5 μm, (b) 20 μm, and (c) 50 μm above μMI structures made with crushed melt spun ribbon (average particle size = 5 μm). For each image, scan resolution = 2.5 μm and scan area = 160 × 400 pixels = 400 μm × 1000 μm. The adjoining graph shows B(x) profiles across the dashed lines shown on each image (cf. Ref. [1] for further details).