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Posted: Nov 22, 2011

A new practical strategy for magnetic-force-microscope cantilevers with high isotropic coercivity

(Nanowerk News) A magnetic force microscope (MFM) can determine the distribution of stray fields at a level of tens of nanometers near the surface of magnetic films, and therefore is an effective tool for observing the domain structures in magnetic grains of submicrometer size.
At present, the coercivity of normal MFM cantilevers is about 0.3 kOe. Being affected by the magnetism of the measured material, the stability of these cantilevers is unsatisfactory. By applying a FePt layer, the coercivity can reach ~10 kOe. However, the accompanying high-temperature (over 750°C) annealing spoils the resolution owing to the growth of the grains and the subsequent increase in tip radius. It is essential for the fabrication of a high-performance MFM cantilever to obtain larger coercivity at lower annealing temperature.
This image shows the coercivity of FexPt100-x films
Fig 1: This image shows the coercivity of FexPt100-x films (x = [10, 85]).
The group of Professor LI Guoqing at Southwest University of China proposed a novel method to tackle this problem. With inadequate annealing at 500°C, a kind of hard/soft composite was formed in Fe60Pt40 films. The strong interaction of the exchange spring between the hard phase and soft phase contributes to larger coercivity exceeding 5 kOe as the sample is magnetized along any direction (see Fig. 1).
The annealing temperature is lower, and the magnetic properties are isotropic. Cantilevers coated with this kind of material have improved stability. The research results were published in Scientia Sinica Phys, Mech & Astron (Jiang D M, Chen Y C, Xiang H, et al. Structure and magnetic properties of FexPt100-x films (in Chinese). Scientia Sinica Phys, Mech & Astron (Chinese Ver), 2011, 41(10): 1156—1165).
By researching the properties of (001) textured Fe-Pt hard/soft composite on MgO(001) substrate, the group found the coercivity is favorable even if the sample is magnetized along the hard axis of the hard phase. A so-called tri-domain model was used to schematize the mechanism in view of the slight coherent strain near the hard–soft interface (see Fig. 2).
These are models for the magnetization of Fe60Pt40 hard/soft composite magnetized along the hard axis of the magnetic hard phase.
Fig 2: These are models for the magnetization of Fe60Pt40 hard/soft composite magnetized along the hard axis of the magnetic hard phase.
With a coating of this kind of hard/soft composite, the MFM resolution is better than 13 nm. This research has initiated the design of the coating layer for the MFM cantilever by choosing an off-stoichiometric composition and a matching lower annealing temperature to generate the exchange spring. The adequate saturation magnetization of the Fe-enriched alloy satisfies the sensitivity of the MFM cantilever. These efforts will benefit the fabrication of an excellent MFM cantilever. This research project was partially supported by a grant from the National Natural Science Foundation of China.
Source: Science in China Press
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