Nanometers-thick self-organized Fe stripes: bridging the gap between surfaces and magnetic materials

Abstract

We have fabricated 5 nm-high Fe(110) stripes by self-organized (SO) growth on a slightly vicinal R(110)/Al203(1120) surface, with R=Mo, W. Remanence, coercivity and domain patterns were observed at room temperature (RT). This contrasts with conventional SO epitaxial systems, that are superparamagnetic or even non-magnetic at RT due to their flatness. Our process should help to overcome superparamagnetism without compromise on the lateral size if SO systems are ever to be used in applications.Arrays of epitaxial nanometer-sized (1-50 nm) magnetic structures can be grown by self-organization (SO). However such structures are superparamagnetic or even non-magnetic at room temperature (RT) [1,2,3]. Indeed the energy barrier opposing spontaneous magnetization flipping roughly scales with KV , with K the magnetic anisotropy per unit volume, and V the system's volume. 3D clusters of similar lateral size can overcome superparamagnetism at RT by increasing K[4]. This seems not sufficient in epitaxial SO [3,5,6] because SO deposits are generally very flat, implying a very small V . Therefore, beating superparamagnetism in SO deposits without compromising on the lateral density seems to imply increasing their thickness t.One way to force SO deposits to grow vertically and overcome superparamagnetism at RT is sequential deposition [7,8]. We proposed a second route, that consists in annealing a thin continuous film deposited on a vicinal surface to form an array of several atomic layers (AL)-thick stripes [8,9]. In the early reports, concerning Fe/Mo(110) stripes, a stable thickness t = 6 AL (∼ 1.2 nm) was observed above 1-2 ALs of wetting. Yet this was not thick enough to observe static coercivity at RT, which could be obtained only for multidisperse assemblies of islands and stripes, thicker on the average. In this Letter we report the growth of thicker stripes, in the case of Fe/W(110): t ∼ 25 AL (∼ 5 nm). Such stripes display at RT functional features of magnetic materials: coercivity, remanence and domains, unlike conventional SO systems. The microscopic origin of the self-organization process is also unravelled.The samples are epitaxially grown by pulsed laser deposition in a multi-chamber ultra-high vacuum setup (base pressure 7 × 10 −9 Pa), with in situ STM, RHEED and Auger spectroscopy [10]. Commercial (1120) sapphire wafers with a residual miscut angle ǫ < 0.1 • are buffered with refractory metal films (Mo or W, < ∼ 10 nm-thick), whose surface consists of an array * Olivier.Fruchart@grenoble.cnrs.fr of atomically-flat terraces of width ∼ 200 nm, separated by mono-atomic steps [9]. Fe is then deposited at 150 • C and annealed at 400 − 450 • C, covered with 1 nm Mo for controlling the magnetic anisotropy, and finally capped by 4 nm Al as a protection against oxidation. AFM (PSI Autoprobe CP) and hysteresis loops (QD MPMS-XL) were performed ex situ. Samples were then dc-demagnetized ex situ with the field applied perpendicular to the stripes. Magnetic and chemical imaging was performed under zero external field using ...

Magnetic imaging with spin-polarized low-energy electron microscopy

“…These results show how the relative importance of different magnetic anisotropy energies contributing to the SRT observed in Fe/W(110) films can be influenced by carefully tuned annealing schedules and by the choice of the substrate atomic step structure. Complementing information on island morphologies and magnetic properties of annealed Fe/W(110) films available from prior studies [119,120], the capability of SPLEEM to directly correlate, on the basis of real time in situ observations, surface morphology and local distribution of magnetization, has provided crucial insights into these SRT mechanisms.…”

Self-organization and magnetic domain microstructure of Fe nanowire arrays

“…[6][7][8] [11][12][13] Different factors have been proposed as the driving mechanism for the rotation of the magnetic easy axis after annealing: reduction of the magnetoelastic anisotropy and contribution of the shape anisotropy, 13 or reduction of the surface anisotropy in the coalesced 3D islands. 11 In this work, we use spin-polarized low-energy electron microscopy (SPLEEM) to obtain spatially resolved magnetic information during the morphological transition.…”

Kinetic self-organization of trenched templates for the fabrication of versatile ferromagnetic nanowires

“…2), for which layer-by-layer growth occurs with PLD on a smooth surface 12,15 . For all deposits we have used W templates, which remain perfectly stable up to 300…”

“…A prerequisite is that some versatility of geometrical as well as physical properties is achieved, like tuning the magnitude of the MAE. We focus here on the fabrication and properties of wires, which lie at the background of the fast-developing field of spin electronics making use of the propagation of domain walls in wires for storage or logics devices 7,8,9 .SO magnetic wires are often achieved by stepdecoration of vicinal surfaces 10,11,12 . This approach is not versatile as a new crystal has to be prepared with a specific miscut whenever the period needs to be changed.…”

“…SO magnetic wires are often achieved by stepdecoration of vicinal surfaces 10,11,12 . This approach is not versatile as a new crystal has to be prepared with a specific miscut whenever the period needs to be changed.…”