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2D Magnetic Particles

Selected Article

Nat. Comm 7, 13611 (2016)
Critical exponents and scaling invariance in the absence of a critical point

Proc. R. Soc. A 472, 2195 (2016)
Thirty per cent contrast in secondary-electron imaging by scanning field-emission microscopy
Phys. Rev. B 89, 014429 (2014)
Domain-wall free energy in Heisenberg ferromagnets

Phys. Rev. B 87, 115436 (2013)
Scale invariance of a diodelike tunnel junction
Two Dimensional Magnetic Particles
Ch. Stamm, F. Marty, A. Vaterlaus, V. Weich, S. Egger, U. Maier, U. Ramsperger, H. Fuhrmann, and D. Pescia
SCIENCE 282, 449 (1998)
The local orientation of the magnetization in a magnet is determined by the balance of various energy terms. Generally, the state of lowest energy is one with domains: The magnetization is homogeneous within a domain and varies its orientation from domain to domain [1]. This general statement turns out to be wrong for atomically thin particles. Flat magnetic structures of cobalt and iron with variable size and shape were fabricated by putting a mask in front of the sample surface during the growth of a thin layer. The mask is produced with a focused ion beam system [2]. This technique allows the growth of structures as small as 100 nm. Their magnetic properties are probed with a Scanning Kerr Microscope (SKEM: lateral resolution 1 mm) or a Scanning Electron Microscope with Polarization Analysis of the secondary electrons (SEMPA: lateral resolution 10 nm). The structural characterization is done with a scanning tunneling microscope (STM).
Figure 1: SEMPA and SKEM images of Co particles with various size. The magnetization is measured in the remanent state. The gray scale range is proportional to the electron spin polarization or to the Kerr asymmetry. The arrows indicate the direction of the magnetization, which is parallel to a [110] in-plane axis. a, b: extended Co layer (d = 4.5 AL, Hrev = 3.52 kA/m). The layer is shown as grown (a) and after applying a field amounting to -0.995 Hrev . SKEM image (c) and corresponding histeresis loop (d) of a 6 AL Co dot (Hrev = 0.76 kA/m). e: SEMPA image of square dots with a size from 0.5 to 4.5 mm (d = 10 AL, Hrev = 8 kA/m). f: SEMPA image of dots with diameter 130 nm (d = 2 AL) and 300 nm (d = 2.5 AL). g: like f but after applying a switching field of 27.2 kA/m (Hrev = 11.2 kA/m). The magnetization changed sign (dark corresponds to a negative electron spin polarization).
Reducing the lateral size of in-plane magnetized two dimensional cobalt films down to 100 nanometers (nm) did not essentially modify their magnetic properties; neither domains penetrated the particle, nor was any sizable shape anisotropy observed. Fig.1 illustrates this behavior. The observed properties can be explained by evaluating the balance of the same energies which determine the behavior of three dimensional magnets. A small or vanishing interaction between magnetic particles is essential for possible storage application. Three dimensional particles do not fulfill this requirement. The mutual interaction between two dimensional particles is found to be negligible, the small test particle shown in Fig. 2 feels no measurable stray field from its neighbors. A stray field would result in different external fields needed to switch the test particle from white to black and from black to white, respectively.
Figure 2: SEMPA images showing a small Co element (Hrev= 2.64 kA/m, d = 4 AL) sandwiched between two elliptical particles (Hrev = 3.2 kA/m and 2.9 kA/m, d = 4 AL). Before a was taken, all particles were magnetized up by a large positive magnetic field and then exposed to a negative magnetic field of -2.48 kA/m. This negative fields did not switch the state of the particles. : A magnetic field of Hrev- = -2.64 kA/m has reversed M of the smaller Co dot, which now appears dark in the image. c: A magnetic field of Hrev+ = 2.64 kA/m has switched M of the smaller Co dot in the positive direction.
These results suggest that only a few atoms forming a 2D in-plane magnetized dot may provide a stable, elementary bit for nano-recording [3].
[1] E. Gu, E. Ahmad, S.J. Gray, C. Daboo, J.A.C. Bland, L.M. Brown, M. Rührig, A.J. McGibbon, and J.M. Chapman, Phys. Rev. Lett.78, 1158 (1997).
[2] FIB sources at the Paul Scherrer Institute in Villingen, Switzerland and at the National Research Institute for Metals in Tsukuba, Japan are used to produce the masks.
[3] J. Harris and D. Awschalom, Physics World, 19 (1999).
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