Magnetic Imaging on the Nanometer Scale
prime supplier of scanning probe microscope instruments and corresponding cooling systems. Tis company is spearheading the development of ultra-low vibration, cryogen-free cooling equipment, enabling application of MFM and SHPM at temperatures as low as 50 mK without the need for costly liquid helium or nitrogen.
Acknowledgments We would like to thank our colleagues H. Qian,
C. Debuschewitz, and R. Pohlner for assistance with some of the measurements, and Simon Bending for useful discussions. A special thanks goes also to A. Erb, H.H. Wen, R. Kramer, and E. Fullerton for providing samples.
References [1] VV Volkov and Y Zhu, Ultramicroscopy 98 (2004) 271–81. [2] P Weinberger, Phil Mag Lett 88 (2008) 897. [3] Y Martin and HK Wickramasinghe, Appl Phys Lett 50 (1987) 1455; and JJ Saenz et al., J Appl Phys 62 (1987) 4293.
Figure 5: Upper part: SHPM measurements on a degraded Bi2Sr2CaCu2O8+x substrate showing strong surface pinning effects at 4.2 K and 2.5 gauss magnetic field. The figure to the right shows a linecut
through one of
the vortices, displaying the field distribution approximately 100 nm above the surface (attocube applications lab, 2011; sample courtesy of A. Erb, TU Munich). Lower part: SHPM measurement of a BaFeO ferromagnet, measured at 300 K. The figure to the right shows a linecut through one of the magnetic domains,
indicating the strong magnetization of the ferromagnet (attocube applications labs, 2011; sample courtesy of R. Kramer, Grenoble).
and coherence length ξ, respectively [13]. Another example for quantitative field distribution measurements is given in the lower part of Figure 5, depicting ferromagnetic domains in the compound barium hexaferrite: it shows extremely strong magnetization (see the right hand side of Figure 5) and is currently investigated in the course of the search for replacements for rare earth materials [24]. Improved cooling technology. From a different perspec-
tive, low-temperature research on magnetic nanostructures faces yet another challenge—liquid helium shortage and the corresponding increase in operation costs [25]. Tis leads to the demand for cryogen-free cooling systems. Sensitive techniques such as MFM and SHPM, however, require specially designed products optimized for ultra-low vibration levels. Combining the latter with high magnetic fields has become possible only very recently because of a proprietary (top-loading) design by attocube systems: mechanical vibrations created by a pulse-tube coldhead are decoupled from the measurement platform, resulting in peak-to-peak vibration amplitudes of less than 4.2 nm at the sample location while retaining probe cooldown times as fast as 1 hour to 4 K [26]. Tis technology can be extended to temperatures as low as 50 mK, as recently demonstrated [27].
Summary It has been shown that low temperature MFM and SHPM
play a crucial role in many fields of fundamental research. With a large number of high-impact customer publications and a company history of more than a decade, attocube systems is a
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[4] AM Chang et al ., Appl Phys Lett 61 (1992) 1974. [5] J Paglione and RL Greene, Nature Physics 6 (2010) 645. [6] I Žutić et al., Rev Mod Phys 76 (2004) 323–410. [7] R New, “Te Future of Magnetic Recording Technology,” Hitachi Global Storage Technologies, 2008.
[8] G Binnig, CF Quate, and C Gerber, Phys Rev Lett 56 (1986) 930.
[9] SJ Bending, Adv Phys 48 (1999) 449, and private communication with SJ Bending (2011).
[10] D Rugar, HJ Mamin, and P Guethner, Appl Phys Lett 55 (1989) 2588.
[11] L Abelmann et al., “Magnetic Force Microscopy— Towards Higher Resolution” in Magnetic Microscopy of Nanostructures, eds. H Hopster, HP Oepen, Springer, Berlin, 2005; Y Zhu, “Chapter 11: Magnetic force microscopy”, 411–451, in Modern Techniques for Characterizing Magnetic Materials, ed. Y Zhu, Springer, New York, 2005.
[12] K Babcock et al., Mat Res Soc Symp 355 (1995) 311. [13] VV Khotkevych, MV Milošević, and SJ Bending, Rev Sci Instrum 79 (2008) 123708.
[14] RS Popović, Hall Effect Devices, ed. A Hilger, Bristol, UK, 1990.
[15] K Vervaeke, E Simon, G Borghs, and VV Moshchalkov, Rev Sci Instrum 80 (2009) 074701.
[16] P Kejik et al., Sens Actuators, A 129 (2006) 212. [17] JG Bednorz and KA Mueller, Z Phys B—Condensed Matter 64 (1986) 189.
[18] AA Abrikosov, Zh Eksp i Teor Fiz 32, (1957) 1442; Soviet Phys JETP 5 (1957) 1174.
[19] Takahashi et al., Nature 453 (2008) 376. [20] Y Kamihara, T Watanabe, M Hirano, and H Hosono, J Am Chem Soc 130 (2008) 3296.
[21] Y Yin et al., Phys Rev Lett 102 (2009) 097002. [22] O Auslaender et al., Nature Physics 5 (2009) 35; Reichhardt, Nature Physics 5 (2009) 15.
[23] H Schmid, Ferroelectrics 162 (1994) 317. [24] F Mazaleyrat et al., arXiv:cond-mat/1103.5840. [25] KH Kaplan, Physics Today (June 2007) 31. [26]
http://www.attocube.com/attoCRYO/attoDRY1000.htm [27]
http://www.attocube.com/attoCRYO/attoDRY5000.htm
www.microscopy-today.com • 2011 November
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