WORKING OF MFM

2. Magnetization reversal and coercivity of magnetic-force microscopy tipsby A. Carl, J. Lohau, S.Krisch and S. WessermanIRFAN ELAHI Roll#01

3. MAGNETIC FORCE MICROSCOPY 1. Magnetic force microscopy (MFM) is a well established experimental technique used for imaging magnetization patterns of magnetic films with high resolution i.e. some tens of nanometers.2. This technique is a special mode of operation of atomic force microscopy (AFM) and uses a very sharp magnetic tip attached to a flexible cantilever. 3. Usually magnetic tip is mounted at the end of oscillating cantilever which is scanned at small distances up to 20nm across the surface. 4.Normally CoCr thin film coatings on Si cantilevers are used as magnetic tips.

4. WORKING OF MFM Generally, imaging is done using a two pass technique. During the first pass, the tip is scanned in tapping mode to measure the surface topography. In the second pass, the tip is lifted to a selected height (10-500 nm) and follows the same topography path, assuming that Van der Waals forces are neglected. The tip interacts with the stray field arising from the magnetic sample. The effect of this interaction determines the vertical motion of the tip as a function of the sample position as it scans across the sample. Repulsive interaction is showed by light while attractive by dark contrast.

6. APPLICATIONSNumerous experimental investigations have proven the versatility of magnetic force microscopy, thereby addressing/solving various questions in the field of ferromagnetism.1.Investigations of the characteristic formation of magnetic domains.2. Studies of the substructure of domain walls3. Quantitative measurements of the sample magnetization or stray fields.4. Of particular interest are quantitative investigations of the magnetization reversal of thin films or the switching characteristics of single-domain magnetic elements.

7. WHERE IS PROBLEMDuring the quantitative investigation of the magnetization reversal/ switching of thin films magnetic force microscope has to be operated in the presence of a variable external magnetic field. Thereby, the magnetization reversal of a given sample may be detected with MFM via changes in the image contrast (light, dark) resulting from the reorientation of the direction of magnetization of the sample with respect to the direction of magnetization of the tip. However, the interpretation of the image contrast may be ambiguous, since the applied magnetic field may also effect the magnetization direction of the MFM tip to some unknown extent.

8. WHAT WE NEED • Thus, in order to be able to differentiate between the contributions arising from the reorientation of the magnetization of both the sample and the MFM tip, A detailed knowledge of the magnetization reversal / switching of the MFM tip alone is needed.

9. SOLUTIONS TO THE PROBLEM In the past, a number of theoretical as well as experimental investigations have been performed in order to determine the magnetization reversal properties of MFM tips. Here we will discuss the current- carrying wires which have been used to meaure the switching of magnetic force microscopy tips. • ADVANTAGES The advantage of stray field produced by current carrying loop is that • This will definitely not be changed upon the application of an external magnetic field , as may be in case of ferromagnetic sample. • Image analysis then gives the exclusive measurement of the reversal of the magnetic tip. • To avoid the electrostatic interaction between current carrying wire and tip, an insulating layer is added to the wires.

10. CURRENT CARRYING TECHNIQUE • Definitive stray fields are produced by current carrying wires (circular) rather than ferromagnetic materials. In the very center of the current ring (x=0, y=0) the magnetic field is exactly directed into the z direction. The magnetic field in the center of the ring at different heights z along the z direction is given by the Biot–Savart law, where Ris the ring radius and I is the current running through the ring:

11. CURRENT PAPER STUDY • In this study a current carrying ring with radius = 2400nm has been used to produce a constant magnetic stray field which could be imaged with MFM. The MFM image contrast is analyzed only within the centre of current ring. • Thin 30nm single turn metal rings of various radii between 600 and 4750nm have been fabricated by standard electron beam lithography to provide well- defined magnetic fields (as external source) in z-direction. • Before use tips has been magnetized either in +z or –z direction as shown in figure.

12. Magnetization for CoCr coating as a function of external magnetic field orientated parallel or perpendicular to the plane of silicon substrate is within the plane of coating material. • Saturation magnetization Ms = 870 emu/cm3 and coercivity Hc = 360 Oe are same for both cases • Remanence Mr = 384 emu/cm3 for parallel but 86emu/cm3 for perpendicular This is characteristic for all MFM tips having CoCr coating which shows that easy axis is oriented within the plane, therefore • Once the tip has been magnetized in z direction (tip axis) the resultant remanent magnetization of the tip should point into the positive or negative z-direction as shown in previous figure.

13. For this reason only the component of stray field which is parallel to tip magnetization means parallel to plane of substrate has been affected. That is why during imaging an external field is applied in the z- direction in order to reverse the tip magnetization , from +z direction to –z direction. This will affect the image contrast that will change from light to dark contrast. • If the measured phase shift (which represents the image contrast) is plotted versus external magnetic field then, this will be direct measurement of the magnetization of the tip.

14. WAFERS EFFECT one thing more about the tip is that nevertheless all tips are coated with same magnetic material CoCr, their coercivities are different if these are coated on different Si wafers (not known why) but experimentally is verified by given table, same group wafers have same coercivity as in shown for A, B and C. Such a variation is remarkable, since all the tips are covered nominally with the same CoCr coating. Clearly, the apparent lack of reproducibility of the magnetic properties of MFM tips taken from different batches is still an unsatisfactory fact.

15. For all MFM tip/substrates we find that the easy direction of the magnetization is predominantly oriented within the plane of the CoCr coating as shown in Fig. • Therefore, once the tip itself has been magnetized into the z direction the long tip axis, the resultant remanent magnetization of the tip should predominantly point into the positive or negative z direction, as is indicated in Fig. • Figure schematically shows the tip apex of a pyramida MFM tip covered with a CoCr film. The arrows indicate the direction of the magnetization within the plane, resulting in an effective z component of the remanent tip magnetization.

16. For this, during imaging an external magnetic field is applied in the z direction in order to reverse the tip magnetization from, e.g., the +z direction to the -z direction. This will affect the image contrast in that it will be converted from light to dark contrast, or vice versa. If the measured phase shift (which represents the image contrast) is plotted versus external magnetic field, this then is a direct measurement of the hysteresis loop of the tip. • As stated in earlier, such a measurement is unambiguous only, if the measurement is performed on a sample, the magnetization (and, therefore, the stray field) of which will not be affected by the external magnetic field. This is the case for current-carrying wires or rings, since the stray field is produced by an electrical current.

17. Figure (a) shows AFM image of a current ring with radius of R =2400 nm, thickness t=30 nm, and wire width of w=900 nm. (b) Magnetic stray field produced by a currentthrough the ring leading to a definitive z component in the center of thecurrent ring. The origin of the coordinate system is located in the center ofthe ring.1. Using Biot-Savart law equation, a magnetic field of the value 27Oe is produced by providing 10mA current. This field is much smaller than the coercivity of CoCr coated tips which is approximately equal to 400 Oe to influence.2. Also the remanence of the tip is also not to be modified by this. 3. Now we apply the external magnetic field from 0 to 577 Oe to reverse the magnetization of the tip as shown in next figure.

18. FIG. Schematic representation of the current ring with stray-field geometry and the MFM tip for two different orientations of the tip magnetization, while a current of I=10 mA is conducted through the current ring, and an external magnetic field of H = ±1 kOe previous to both of the MFM measurements shown in figure.

19. ExplanationIn the beginning of the experiment (lower part of Fig. ,) the tip magnetization points to the opposite direction as compared to the magnetic stray field. Which leads to the light image contrast in the center of the ring. With increasing external magnetic field H, which is directed opposite to the tip magnetization, the z component of the tip magnetization gradually shrinks and changes sign at around H=415 Oe. At an external magnetic field of H=577 Oe the tip magnetization is completely reversed, pointing to the direction of the external magnetic field.

20. Plotting ∆ɸ versus external magnetic field values also allows us to characterize the magnetization reversal of the MFM tip. • The image contrast is now analyzed representing the phase shift measured in the centre of the ring. The phase shift varies from roughly ∆ɸ= 0.2° to ∆ɸ = - 0.2°, as indicated in previous contrast figure. • Note that the hysteresis loop is symmetric with respect to both H=0 and ∆ɸ=0, not shifted along ∆ɸ which in turn shows that • The tip magnetization was not modified by the small stray field of the current ring during the experiment.

21. Figure (a) shows, again, the tip hysteresis loop as measured with MFM (•), already given in as compared to a hysteresis loop measured for the same tip/substrate with a SQUID magnetometer (∆). The tip magnetization here is in arbitrary units. • Both the hysteresis loops agree well with respect to their shape which in turn confirms our above assumptions that the tip magnetization is oriented within the plane of the tip leading to effective z- component.

22. In conclusion, • an experimental study of the magnetization reversal and the coercivity of commercially available magnetic-force microscopy tips has been measured using a single-turn current-carrying ring, which is a necessary precondition for quantitative investigations of the hysteresis loop and coercivity of small magnetic samples.