Magnetic Imaging on the Nanometer Scale
Tese quantum objects are referred to as vortices and can be considered single entities. Te density of vortices can be tuned and is directly proportional to the applied external magnetic field—as long as the applied external magnetic field remains below the critical field Hc2, where the sample becomes normal conducting. Trough the mutual repulsion of neighboring circular currents, a vortex lattice forms, which in the easiest case is hexagonal [18]. In addition to the mutual repulsion between vortices, pinning forces are present to a variable extent in every superconducting material. Tese forces cause vortices to stick to certain locations on the surface or in the bulk of the superconductor. Tis behavior has important implications for some applications like the construction of superconducting magnets: upon application of an electrical current, vortices experience a Lorentz force, which causes them to move. Vortex motion, however, induces a voltage and thus electrical resistance, which is not desired in such coils. Hence, (artificial) vortex pinning plays an important role in minimizing electrical losses due to moving vortices. Te vortex lattice can easily be observed on a freshly
cleaved Bi2Sr2CaCu2O8+x (Bi-2212) cuprate superconductor, as imaged using an attoMFM in Figure 3 (leſt). Te observed lattice is almost perfectly hexagonal, indicating that hardly any pinning is present in this compound. With an inter-vortex spacing d of 700 ± 34 nm, the vortex separation is in good agreement with the expected value of (4/3)1/4(φ0/H)1/2 = 729 nm for an external magnetic field of 45 gauss. Vortices and vortex lattices have been intensively studied
high-Tc superconductors
in Bi-2212, YBCO, and many other cuprate compounds [19]. Tis effort has significantly improved the understanding of the different vortex types, phases, and pinning mechanism existing, but it has not solved the mystery of the origin of high-Tc superconductivity. Te recently discovered new class of iron-arsenic based (pnictides) [20] gave new spirit
to solving the puzzle of high-Tc superconductivity. One of the first MFM images recorded on the pnictide compound Ba1-xKxFe2As2 was demonstrated by attocube systems in 2009 and is shown in Figure 3 (right). In contrast to the Bi-2212, the pnictide sample shows strong pinning leading to a significantly disordered vortex lattice. Independent investigations on the related compound BaFe1.8Co0.2As2 confirm these results
and relate the observed pinning to bulk pinning rather than surface pinning [21], which may enable further tailoring of the properties of these new materials to even higher critical currents as desired for applications such as wiring for superconducting ultra-high-field magnets or power grids. In contrast to many other magnetic imaging techniques,
MFM can also be used for local manipulation of samples: in 2009 O. Auslaender et al. managed to individually drag single vortices, thus being able to directly probe the interaction with the local disorder potential [22]. Whereas previous experiments usually yielded information on the properties of bulk pinning as experienced by large ensembles of vortices, this approach allowed control of the properties of vortex matter on a local scale. Data storage. Another field of research with high appli-
cation potential for MFM is data storage and the accompanying material science. Although market demands for high disc capacity have already been addressed by altering hard disc magnetization orientation from longitudinal to perpendicular, further means of increasing storage density are under consideration. One of the most promising candidates, with storage densities of 1 Tbit/in² and beyond, is bit-patterned media (BPM), where single domain particles are defined by lithography or self-assembly. BPM eliminates the random noise associated with multi-grain bits, defines sharper transitions between bits, and overcomes the problem of poor thermal stability compared to conventional recording media [7]. MFM proves to be an ideal tool for the characterization of such materials in cryogenic conditions, where high magnetic fields are readily available and magnetic switching and hysteresis of BPM can be investigated free of thermal effects (see Figure 4). In contrast to MFM, SHPM is most frequently used in
applications where quantitative information on local magnetic properties is required. Typical applications are local hysteresis and domain structure measurements in ferromagnetic and multiferroic materials [23], as well as flux penetration studies [13] in superconductors. Te latter experiment is depicted in the upper part of Figure 5, where the flux distribution on a degraded Bi2Sr2CaCu2O8+x surface is measured. Te emergent field of the vortex can be modeled within the framework of the London approximation, yielding penetration depth λ
Figure 3: Vortex images recorded using low-temperature MFM on the cuprate Bi2Sr2CaCu2O8+x (left, attocube applications labs, 2009; sample courtesy of A. Erb, TU Munich) and the pnicitide Ba1-xKxFe2As2 (right, attocube applications labs, 2009; sample courtesy of H.H. Wen, Beijing), respectively. Measurements were conducted at 4.1 K, 45 gauss, and 70 nm tip-sample distance in both cases. Dark-bright contrast is approximately 200 mHz in both cases.
2011 November •
www.microscopy-today.com
Figure 4: Bit patterned media sample: Magnetic domain switching upon externally applied magnetic field, measured at 4.2 K. Dark-bright contrast is approximately 6.2 Hz for all measurements (sample courtesy of E. Fullerton, Hitachi [USA]).
37
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76