Magnetization switching in high-density magnetic nanodots by a fine-tune sputtering process on a large-area diblock copolymer mask

partially_open10Ordered magnetic nanodot arrays with extremely high density provide unique properties to the growing field of nanotechnology. To overcome the size limitations of conventional lithography, a fine-tuned sputtering deposition process on mesoporous polymeric template fabricated by diblock copolymer self-assembly is herein proposed to fabricate uniform and densely spaced nanometer-scale magnetic dot arrays. This process was successfully exploited to pattern, over a large area, sputtered Ni80Fe20 and Co thin films with thicknesses of 10 and 13 nm, respectively. Carefully tuned sputter-etching at a suitable glancing angle was performed to selectively remove the magnetic material deposited on top of the polymeric template, producing nanodot arrays (dot diameter about 17 nm). A detailed study of magnetization reversal at room temperature as a function of sputter-etching time, together with morphology investigations, was performed to confirm the synthesis of long-range ordered arrays displaying functional magnetic properties. Magnetic hysteresis loops of the obtained nanodot arrays were measured at different temperatures and interpreted via micromagnetic simulations to explore the role of dipole-dipole magnetostatic interactions between dots and the effect of magnetocrystalline anisotropy. The agreement between measurements and numerical modelling results indicates the use of the proposed synthesis technique as an innovative process in the design of large-area nanoscale arrays of functional magnetic elements.openBarrera, G; Celegato, F; Coïsson, M; Manzin, A; Ferrarese Lupi, F; Seguini, G; Boarino, L; Aprile, G; Perego, M; Tiberto, PBarrera, Gabriele; Celegato, F; Coïsson, M; Manzin, A; Ferrarese Lupi, F; Seguini, G; Boarino, L; Aprile, G; Perego, M; Tiberto,

MICROMAGNETIC STUDIES OF THE TRANSITION BETWEEN VORTEX AND SINGLE-DOMAIN STATES IN SUB-100 NM NANODOTS

Understanding energy barriers involved in nucleating and annihilating magnetic vortices in nanodots is important for magnetic memories and nano-oscillators. We used a “rigid-vortex approximation'' and micromagnetic approach to calculate the total magnetic energy of a nanodot for various magnetic configurations. This was done for 20 nm-thick iron nanodots with different diameters (30, 40, 65, and 80 nm) as a function of applied magnetic field. By analyzing the energy landscape for different magnetic configurations, we calculated the energy barrier for switching from the vortex to the single-domain state (vortex annihilation) and the converse (vortex nucleation). The applied fields required to overcome these two barriers are compared to those obtained from the simulations of the magnetic reversal and to the experimental values.1 The role of the thermal fluctuations in the temperature dependence of these critical fields is analyzed by comparison of the energy barriers with the thermal energy, kBT. Work is supported by Texas A&M University, TAMU-CONACYT Collaborative Research Program. 1. R. K. Dumas, et. al., Appl. Phys. Lett. 91, 202501 (2007)

Atomistic spin model simulations of magnetic nanomaterials

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and microstructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelization strategies used which enable the routine simulation of extended systems with full atomistic resolution

Thermally nucleated magnetic reversal in CoFeB/MgO nanodots

Power consumption is the main limitation in the development of new high performance random access memory for portable electronic devices. Magnetic RAM (MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising candidate for reducing the power consumption given its non-volatile nature while achieving high performance. The dynamic properties and switching mechanisms of MTJs are critical to understanding device operation and to enable scaling of devices below 30 nm in diameter. Here we show that the magnetic reversal mechanism is incoherent and that the switching is thermally nucleated at device operating temperatures. Moreover, we find an intrinsic thermal switching field distribution arising on the sub-nanosecond time-scale even in the absence of size and anisotropy distributions or material defects. These features represent the characteristic signature of the dynamic properties in MTJs and give an intrinsic limit to reversal reliability in small magnetic nanodevices

Effect of dipolar interactions on the magnetization of a cubic array of nanomagnets

We investigated the effect of intermolecular dipolar interactions on a cubic 3D ensemble of 5X5X4=100 nanomagnets, each with spin $S = 5$. We employed the Landau-Lifshitz-Gilbert equation to solve for the magnetization $M(B)$ curves for several values of the damping constant $\alpha$, the induction sweep rate, the lattice constant $a$, the temperature $T$, and the magnetic anisotropy field $H_A$. We find that the smaller the $\alpha$, the stronger the maximum induction required to produce hysteresis. The shape of the hysteresis loops also depends on the damping constant. We find further that the system magnetizes and demagnetizes at decreasing magnetic field strengths with decreasing sweep rates, resulting in smaller hysteresis loops. Variations of $a$ within realistic values (1.5 nm - 2.5 nm) show that the dipolar interaction plays an important role in the magnetic hysteresis by controlling the relaxation process. The $T$ dependencies of $\alpha$ and of $M$ are presented and discussed with regard to recent experimental data on nanomagnets. $H_A$ enhances the size of the hysteresis loops for external fields parallel to the anisotropy axis, but decreases it for perpendicular external fields. Finally, we reproduce and test an $M(B)$ curve for a 2D-system [M. Kayali and W. Saslow, Phys. Rev. B {\bf 70}, 174404 (2004)]. We show that its hysteretic behavior is only weakly dependent on the shape anisotropy field and the sweep rate, but depends sensitively upon the dipolar interactions. Although in 3D systems, dipole-dipole interactions generally diminish the hysteresis, in 2D systems, they strongly enhance it. For both square 2D and rectangular 3D lattices with ${\bm B}||(\hat{\bm x}+\hat{\bm y})$, dipole-dipole interactions can cause large jumps in the magnetization.Comment: 15 pages 14 figures, submitted to Phys. Rev.

Recent Advances in Nanomagnetism

The Special Issue on Recent Advances in Nanomagnetism is a compilation of articles, addressing various aspects of magnetic properties and behaviour in low dimensional magnetic materials. One contribution addresses the novel magnetic properties in a nanohybrid of iron oxide and carbide nanoparticles grown in diamond. Magnetic textures, such as skyrmion structures, form an important area of research in nanomagnetism, this forms the topic of another contribution. Several aspects of magnetisation dynamics are addressed in other contributions and relate to the developments of microresonators and microantennas applied to the study of magnetic nanostructures; the ferromagnetic resonance behaviour in nanodot systems are also considered. Materials development forms an important area of study in nanomagnetism, and, as such, the preparation conditions, such as annealing under an applied field, can have important effects on the magnetic properties of thin films and low dimensional structures. Such considerations form the study of one of the contributions. Perpendicular magnetic anisotropy has a number of important applications in magnetic storage materials; this is the subject of two further contributions