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Measuring in the magnetic field
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(Nanowerk NanoBusiness Article) Nowadays, some of the most promising fields of nanotechnology investigations are in nano-scaled objects local magnetization measuring. Investigation of ultra thin magnetic films will make it possible to increase storage devices capacity tenfold; spintronics elements creation will lead to the development of fundamentally new computes with “read/write/save” processes carried out on one single chip, magnetostriction could be useful for nanoelectronic devices construction.
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Magnetic-force microscopy allows visualizing and manipulating the magnetization with tens of nanometers resolution.
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There are six essentials of high-quality MFM:
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Increased sensitivity due to vacuum environment
Proper choice of the probe
Scanner with no magnetic parts (external field does not obstruct the imaging)
Accurate external field application
Many-pass compensation of electrostatic and other influences
Precise temperature changing during MFM measurements
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Vacuum environment
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Picture 1. MFM images of hard disk surface obtained in ambient air and in vacuum. Both images are of 1x1 µm.
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There are several ways to increase sensitivity and resolution of magnetic-force microscopy. The easiest one is placing the measuring system (sample, scanner and registration system) in the low vacuum environment. For example, 10-2 torr vacuum is enough for tenfold growth of the phase contrast in the two-pass dynamic MFM. But in this case, the “signal/noise” ratio gains fivefold. The high vacuum (up to 10-6 torr) allows to increase sensitivity greater, but comparing to the low vacuum the difference is insignificant.
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| Picture 2. Magnetic domain structure of ultra thin cobalt film (1.6 µm) 4,5 x 4,5 µm. The samples provided by Dr. A. Maziewski, Uniwersytet w Bialymstoku, Poland
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Proper choice of the probe
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Probe quality is another important factor that affects the resolution and sensitivity of MFM. The tip magnetic coating should be of suitable thickness for tip could “feel” the sample’s magnetic attraction. But at the same time the tip should be sharp enough to provide high spatial resolution. NT-MDT offers AFM silicon probes with CoCr magnetic coating of the tip for magnetic measuring. Cr protects the magnetic layer from the oxidation. The thickness of the coating is 30-40 nm.
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Scanner with no magnetic parts
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For the investigation of some magnetic effects it is necessary to apply external magnetic field to the sample. Usually, it causes certain difficulties as the regular SPM integrates some details that could be magnetized. As the result, any external field measurements lead to the distortion of AFM image.
This problem was solved by NT-MDT Co. Its’ first device for the magnetic measurements (1998) had scanner of special design with no magnetic parts.
But today the Company offers brand new equipment - NTEGRA nanolaboratory platform - with measuring head and base unit made of non-magnetic materials. That allows to avoid the probe shift while switching on/off the magnetic field. The scanner is equipped with close loop control sensors that carry out piezoceramics shift correction and provide exclusively precise probe positioning.
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External field application
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The external magnetic field could by applied in parallel and perpendicular way to scan surface. The NTEGRA nanolaboratory’s functionality allows to apply the external magnetic field up to +/-0.2 T in-plain the surface and +/-0.02 T in perpendicular way (vertical field).
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SPM system for measurements in the external magnetic field on the NTEGRA platform basis a) with the longitudinal magnetic field generator (left) and b) with transverse magnetic field generator (right):
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The longitudinal magnetic field generator is intended for the creation of magnetic field orientated in-plain of the sample. The generator consists of exciting coil with magnetic wires. The Hall detector with scale range up to 2 kgauss is installed at one of the wires poles in order for measuring the magnetic field value.
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The vertical magnetic field generator is intended for the creation of magnetic field normal to the flat of the sample. It consists of exciting coil with build-in Hall detector with scale range of 500 gauss, and a sample holder.
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| Picture 3. Film of yttrium-ferrous garnet in the presence of vertical magnetic field. The images of the same part of the surface 90 x 90 µm. The samples are provided by Prof. F.V.Lisovskiy, Radioelectronic Institute, Russia.
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Many-pass techniques
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There are several ways to carry out the compensation of electrostatic and topography influence, which are presented in picture 4.
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For samples possessing any electrostatic potential several passes should be performed in one session. On the scheme is an experiment with magnetization of nanoelectronic element: 1st pass shows topography; 2nd pass shows surface potential with topography influence compensated; 3rd pass shows magnetization with both electrostatic potential and topography compensated.
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| Picture 4. The scheme of three-pass magnetic measurement of nanoelectronic element
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MFM of high temperature samples
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Sample temperature can be changed during the MFM.
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| Picture 5. MFM images of the cobalt monocrystal with uniaxial anisotropy. Phase transition occurs when temperature increases. Images obtained from the same area, 14 x 40 µm. Sample courtesy of Prof. A.G. Pastushenkov, Tver University, Russia.
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Source: NT-MDT
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