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Magnetic read/write processes with vacuum SPM
(Nanowerk NanoBusiness Article) The use of vacuum SPM improves sensitivity of magnetic and electrostatic interactions non-contact measurements significantly. The enhanced sensitivity is achieved due to increasing of cantilever quality factor (Q-factor) in vacuum environment.
Q-factor increases in more than 10 times at pressure below 10-1 torr, which is achievable even by forvacuum pump means. But after following vacuum level groth cantilever Q-factor changes slowly.
NT-MDT’s equipment Solver HV and NTEGRA Aura allow to carry out measurements in vacuum under the pressure below 10-1 torr.
The sample used in the following experiments is ferromagnetic particles ordered array of the following parameters: ∼35-40 nm diameter, 120 nm period, height 7 nm. Such an array was made by electron beam lithography on the CoPt film of 7 nm height with perpendicular magnetic anisotropy. .
Picture 1: SEM images of sample
Picture 2 shows sample MFM image obtained by one-pass technique, that allows to gain MSM image right after the first pass. For this purpose the magnetic measurements are performed at certain Z-scanner position without feed-back control. (There is standard two-pass method, which includes topography measurements during first pass and long-range interaction during second pass). The advantage of one-pass technique is absence of tip-sample contact, that reduce the probability of unwilling reversal magnetization during scanning. Thus the preliminary adjustment of sample slope is necessary for such technique, in order to reduce the difference in tip-sample separation at different X,Y-position. This can be easily done by measuring head legs adjustment.
Picture 2: MFM images of sample
In picture 3 you can see the MFM image gained at the different distance between tip and sample.
The bright spots in pic. 2 correspond to repulsion force, when tip magnetization direction is opposite to the one of magnetic particle. The dark spots correspond to attraction force near particles with magnetization aligned in the same direction as tip magnetic moment.
Picture 3: MFM pictures obtained at different tip-sample distance
The most important parameters influencing MFM writing and reading processes are tip-sample separation and thickness of magnetic layer on the tip. Too thick magnetic layer on the tip or too small tip-sample distance lead to uncontrolled magnetic reversal. On the other hand, too thin tip layer or too large tip-sample distance make system unsuitable for writing.
Picture 3 demonstrates this situation clearly. The cantilever covered by 50-nm CoCr-alloy film easily switches magnetic state of particles: during scanning repulsion becomes attraction on some particles (see pic. 3a). (Slow scanning was carried out bottom-up) The increased tip-sample distance leads to scanning without switching, however, in this case the resolution of the final image is poor (pic. 3b).
In order to perform bit-by-bit writing, the sample was preliminarily magnetized in direction opposite to tip magnetization. Then the MFM image shows only repulsion.
Picture 4: Scheme of magnetic writing
The scheme of controllable switching of particle magnetization by tip is shown in Pic. 4. The local changes of the particle magnetic moment are carried out by magnetic tip approaching to the sample. Magnetic reversal occurs when local magnetic field of the tip exceeds the particle coercitivity. The result is visible on MFM image as dark spots (attraction) at light background (repulsion). Thus data writing is carried out by certain particles magnetic reversal. Data reading is performed by one-pass scanning.
After careful fitting of tip magnetic layer thickness the 30 nm layer of CoCr-alloy was found as most suitable for controllable local magnetic switching.
For the result testing purpose the four individual particles located in determined positions, were switched by such tip (pic. 5). This experiment shows high reliability and sensitivity of nano-scaled particles read/write processes performed in vacuum.
Picture 5: Controllable switching in ordered magnetic particles array
 A.M.Alekseev, B.A.Gribkov, D.S.Nikitushkin, V.L.Mironov, S.N.Vdovichev and A.A.Fraerman. Investigations of MFM tip induced remagnetization effects in CoPt ferromagnetic nanoparticles, Proceedings of MISM-2008 (Moscow, Russia), p. 260.