Magnetic read/write processes with vacuum SPM

Magnetic read/write processes with vacuum SPM
A. Alekseev¹ and B. Gribkov^2
1NT-MDT Co, Russia           ²IPM RAS, Russia

The use of vacuum SPM significantly improves sensitivity of non-contact measurements of magnetic and electrostatic interactions. The enhanced sensitivity is the result of increase of cantilever quality factor (Q-factor) in vacuum. At pressure below 10-1 torr, which is achievable even with forvacuum pump, Q-factor is increased in more than 10 times and changes slowly after following increase of vacuum level. Measurements in vacuum below 10-1 torr are available with SPMs SOLVER HV-MFM and NTEGRA Aura.
The sample used in our experiments is based on ~7 nm CoPt film with perpendicular magnetic anisotropy deposited on silicon. The ordered array of particles with diameter of ~35-40 nm and period of 120 nm was made by electron beam lithography on such film (Fig.1) [1].

Fig. 1. SEM image of sample.		Fig. 2. MFM image of sample.

Fig. 2. shows MFM image of sample, which was obtained by one-pass technique in contrast to standard two-pass method, which includes measurements of topography during first pass and long-range interaction during second pass. The advantage of one-pass technique is absence of topography measurements when tip and sample are close to each other and then probability of magnetization reversal is reduced. With this method magnetic measurements are performed at certain Z-scanner position without feed-back control. The preliminary adjustment of sample slope is necessary with such technique in order to reduce difference in tip-sample separation at different X,Y-position. It can be easily done by adjustment of measuring head legs.
The bright spots in Fig. 2 correspond to repulsion force, when tip magnetization direction is opposite to the one of magnetic particle. The dark spots correspond to attraction near particles with magnetization aligned in the same direction as tip magnetic moment. The most important parameters influencing MFM writing and reading 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 magnetic reversal. Too thin layer on the tip or too large tip-sample distance are not suitable for writing. The cantilever covered by 50-nm CoCr-alloy film easily switches magnetic state of particles: repulsion becomes attraction on some particles during scanning (Fig. 3a). The increased tip-sample distance leads to scanning without switching, however, resolution is poor (Fig. 3b).

a)		b)
Fig. 3. MFM pictures obtained at different tip-sample distance.

In order to perform writing the sample was preliminarily magnetized in direction opposite to tip magnetization. Then only repulsion exists in the MFM image. The scheme of controllable switching of particle magnetization by tip is shown in Fig. 4. The local changes of the sample magnetic moment are performed by approaching of the magnetic tip to the sample. If local magnetic field of the tip exceeds coercitivity of particle then magnetic reversal occurs. The result is visible on MFM image as a dark spot (attraction) against a bright background (repulsion). 

 
Fig. 4. Scheme of magnetic writing.

After careful adjustment of magnetic layer thickness on the tip it was found that 30 nm layer of CoCr-alloy is most suitable for controllable local magnetic switching in the sample used here. The four individual particles were switched by such tip in preset positions (Fig. 5). This experiment shows high reliability and sensitivity of read/write processes performed in vacuum.

 
Fig. 5. Controllable switching in ordered magnetic particles array.

1. 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.