At Spintec, in collaboration with the Laboratory of Advanced Microscopy of SP2M, we have developed a sample preparation technique for electron holography imaging. With this technique, we can draw a map of the magnetic field in a sample consisting of magnetic bilayers developed for multilevel magnetic storage.
The sample is a pre-patterned substrate, i.e. a matrix of pads prepared on a silicon substrate by conventional microelectronic techniques, and on which magnetic materials for storage of information are deposited. In our approach, each pad contains two pieces of information: the average value of the magnetic field above the pad and the field gradient between the two edges of the pad. Thus, we obtain four distinct magnetic configurations (2 bits).
The goal here is to measure by electron holography (inset) the spatial distribution of the magnetic field around these pads, like the one that is captured by a read-head. The sample must be very thin, about 100 nm typically. The pads are 180 × 80 nm spaced by 80 nm. It is therefore necessary to reduce the sample to a thin slice consisting of a single row of dots.
These slices are obtained by FIB (inset). The sample is first thinned from the rear face down to a thickness of about 5 microns. At this thickness and with a voltage of 30 kV, it is then possible to observe the dot array by electronic transparency. The substrate can then be milled by a beam of gallium from the rear face. This avoids having to use protective layers that could affect the observation of the magnetic field around the dots. Once the sample is reduced to a single row, it is glued onto a sample holder and observed by electron holography perpendicular to the plane of the strip (Figs. 1 and 2.)
The hologram that is obtained is then used to calculate the spatial distribution of the magnetic field within the sample. The measurements obtained are fully consistent with theoretical predictions (Fig. 3.)
Fig. 2: Sample preparation: slices of (a) a line of dots, (b) six lines and (c) ten lines. (D) Sample slice glued to holder.
Electron holography is an imaging method that uses a transmission electron microscope and is based on the phase difference between two halves of the electron beam. One passes through the sample to form the wave image, while the other part forms the reference wave. The interference pattern between these two waves gives the hologram, i.e. the spatial phase shift induced by the magnetic field of the sample. It is then possible to calculate through integration the spatial distribution of the magnetic field.
Fig. 3: Interference Images (a) measured and (b) obtained by simulation. The magnetic configuration of the dots is presented in (c), which also shows the distribution of the magnetic field around the dots.
FIB (Focused Ion Beam)
An FIB is an instrument that resembles a scanning electron microscope that uses a focused ion beam (typically gallium) instead of an electron beam. The ions etch away the target atoms, therefore the FIB is a tool for microfabrication and not observation. It is used for preparing samples for transmission electron microscopy which requires very thin samples. The equipment acquired jointly by the PTA, the PFNC and the CMTC, also includes an electron column for in situ imaging.
Further reading: Moritz J et al., IEEE Magnetics Letters 2 (2011) 4500104
Maj : 17/02/2014 (919)