Anisotropic magnetic coupling of permalloy micron dots forming a square lattice

Abstract

Static magnetic and spin wave properties of square lattices of permalloy micron dots with thicknesses of 500 Å and 1000 Å and with varying dot separations have been investigated. A magnetic fourfold anisotropy was found for the lattice with dot diameters of 1 µm and a dot separation of 0.1 µm. The anisotropy is attributed to an anisotropic dipole-dipole interaction between magnetically unsaturated parts of the dots. The anisotropy strength (order of 10 5 erg/cm 3 ) decreases with increasing in-plane applied magnetic field.The physical properties of magnetic particles on the micro-and nanometer scale have attracted much interest in the last few years especially because of the promising prospects in future data storage and sensor applications [1][2][3][4]. One of the many unsolved problems is the origin and strength of magnetic interaction between the particles as a function of parameters like the dot separation and the aspect ratio (diameter to height ratio). In this Letter we report on first observations of an anisotropic in-plane coupling between magnetic dots arranged in a square lattice, which is correlated to inherent inhomogeneities of the dot magnetizations.Well separated dots of circular shape made of polycrystalline permalloy (Ni 80 Fe 20 ) will not exhibit any in-plane anisotropy.For dots with small enough separation any observed in-plane anisotropy is therefore a fingerprint for a magnetic coupling between the dots. Measurements of coupling induced anisotropies allow for a quantitative study of magnetic interdot coupling.The method of choice is Brillouin light scattering (BLS) from dipolar dominated spin wave modes (DamonEshbach modes) propagating within the dots [5]. The spin wave frequencies depend on the demagnetization factor of each dot, on the value and distribution of the magnetization vector, on anisotropies, and to a weak degree on the volume exchange constant. A study of the mode dispersion and of the dependence on the in-plane direction of the external field allows to determine the interdot coupling strength.We have studied regular square lattices of circular shaped dots of permalloy of 1 µm and 2 µm diameter and 500 Å and 1000 Å thickness. The films were prepared in ultrahigh vacuum onto Si/SiO 2 10x10 mm 2 substrates using an e --beam evaporator. The samples were patterned by synchrotron radiation based X-ray lithography performed at the L2M synchrotron station at the super ACO storage at LURE, Orsay, using ion beam etching to transfer the patterns into the permalloy films.A subtractive process was used as described elsewhere [7]. Each sample consists of circular dots arranged in a 1x1 mm 2 square lattice with a diameter/periodicity of 1/1.1, 1/2, 2/2.2 and 2/4 µm, respectively. For comparison, and in order to check for modifications of the magnetic bulk properties during the patterning process, the remaining parts of the samples were not patterned , i.e. consist of a continuous film. Depth profile measurements show that the etching process cuts 200 Å deep into the substrate between the do...

Inelastic light scattering in magnetic dots and wires

“…Last but not least, arrays of isolated magnetic dots are a good model system to study fundamental physical aspects of the next generation magnetic random access memory. Up to now there exist very few studies of magnetic dot arrays by BLS [6,7,8,28]. This is definitely connected to the fact that, on one hand, it is much more difficult to prepare dots with a well defined shape; and on the other hand, that the BLS cross section from dots is lower than that from wires simply due to the lower coverage of the surface by the magnetic material.…”

“…No inter-dot coupling was observed. Hillebrands et al [7,28] investigated spin wave properties of square lattices of micron-sized dots of permalloy with varying dot separations. Eight different samples comprising circular dots arranged in a 1x1 mm 2 square lattice with a diameter/periodicity of 1/1.1, 1/2, 2/2.2 and 2/4 µm, respectively, patterned into 500 and 1000 Å thick films were prepared.…”

Magnetic domain and domain-wall imaging of submicron Co dots by probing the magnetostrictive response using atomic force microscopy

“…On the other hand, Mathieu et al 26 found evidence for magnetostatic interaction between circular permalloy (Ni 80 Fe 20 ) dots with a diameter of 1 m on a square lattice with a 1.1 m period, while for the same type of dots with a 2 m period they found no magnetostatic interaction. Such studies are of current interest, since the magnetostatic interaction seems to strongly depend on the detailed structure of the system ͑dot material, shape, thickness, or type of array and center-tocenter or edge-to-edge distances͒.…”

Lateral Quantization of Spin Waves in Micron Size Magnetic Wires

“…Although static properties and coupling phenomena in magnetic films patterned on the micron scale have been studied extensively [1,2,3,4], high-frequency dynamic properties of such films have been rarely investigated. On the other hand, the study of spin wave properties in conventional finite size systems is well established, such as the investigation of so-called Walker-modes in magnetic spheroids [5], and of dipolar-dominated surface modes (Damon-Eshbach modes) in finite-thickness slabs with infinite lateral dimensions [6].…”