Transition metal ferromagnetic films with perpendicular magnetic anisotropy (PMA) have ferromagnetic resonance (FMR) linewidths that are one order of magnitude larger than soft magnetic materials, such as pure iron (Fe) and permalloy (NiFe) thin films. A broadband FMR setup has been used to investigate the origin of the enhanced linewidth in Ni|Co multilayer films with PMA. The FMR linewidth depends linearly on frequency for perpendicular applied fields and increases significantly when the magnetization is rotated into the film plane. Irradiation of the film with Helium ions decreases the PMA and the distribution of PMA parameters. This leads to a great reduction of the FMR linewidth for in-plane magnetization. These results suggest that fluctuations in PMA lead to a large two magnon scattering contribution to the linewidth for in-plane magnetization and establish that the Gilbert damping is enhanced in such materials (α ≈ 0.04, compared to α ≈ 0.002 for pure Fe).PACS numbers: 75.47.-m,85.75.-d,75.70.-i,76.50.+g Magnetic materials with perpendicular magnetic anisotropy (PMA) are of great interest in information storage technology, offering the possibility of smaller magnetic bits [1] and more efficient magnetic random access memories based on the spin-transfer effect [2]. They typically are multilayers of transition metals (e.g., Co|Pt, Co|Pd, Ni|Co) with strong interface contributions to the magnetic anisotropy [3], that render them magnetically hard. In contrast to soft magnetic materials which have been widely studied and modeled [4,5,6,7], such films are poorly understood. Experiments indicate that there are large distributions in their magnetic characteristics, such as their switching fields [1]. An understanding of magnetization relaxation in such materials is of particular importance, since magnetization damping determines the performance of magnetic devices, such as the timescale for magnetization reversal and the current required for spin-transfer induced switching [2,8].Ferromagnetic resonance (FMR) spectroscopy provides information on the magnetic damping through study of the linewidth of the microwave absorption peak, ∆H, when the applied field is swept at a fixed microwave frequency. FMR studies of thin films with PMA show very broad linewidths, several 10's of mT at low frequencies ( 10 GHz) for polycrystalline alloy [9], multilayer [10] and even epitaxial thin films [11]. This is at least one order of magnitude larger than the FMR linewidth found for soft magnetic materials, such as pure iron (Fe) and permalloy (FeNi) thin films [5]. Further, it has recently been suggested that the FMR linewidth of perpendicularly magnetized CoCrPt alloys cannot be explained in terms of Landau-Lifshitz equation with Gilbert damping [12], the basis for understanding magnetization dynamics in ferromagnets:Here M is the magnetization and γ= |gµ B / | is the gyromagnetic ratio. The second term on the right is the damping term, where α is the Gilbert damping constant. This equation describes precessional motion of th...

Bit Patterned Media (BPM) for magnetic recording provides a route to thermally stable data recording at >1 Tb/in 2 and circumvents many of the challenges associated with extending conventional granular media technology. Instead of recording a bit on an ensemble of random grains, BPM is comprised of a well ordered array of lithographically patterned isolated magnetic islands, each of which stores one bit. Fabrication of BPM is viewed as the greatest challenge for its commercialization. In this article we describe a BPM fabrication method which combines rotary-stage e-beam lithography, directed self-assembly of block copolymers, self-aligned double patterning, nanoimprint lithography, and ion milling to generate BPM based on CoCrPt alloy materials at densities up to 1.6 Td/in 2 (teradot/inch 2 ). This combination of novel fabrication technologies achieves feature sizes of <10 nm, which is significantly smaller than what conventional nanofabrication methods used in semiconductor manufacturing can achieve. In contrast to earlier work which used hexagonal closepacked arrays of round islands, our latest approach creates BPM with rectangular bitcells, which are advantageous for integration of BPM with existing hard disk drive technology. The advantages of rectangular bits are analyzed from a theoretical and modeling point of view, and system integration requirements such as provision of servo patterns, implementation of write synchronization, and providing for a stable head-disk interface are addressed in the context of experimental results. Optimization of magnetic alloy materials for thermal stability, writeability, and tight switching field distribution is discussed, and a new method for growing BPM islands from a specially patterned underlayer -referred to as "templated growth" -is presented. New recording results at 1.6 Td/in 2 (roughly equivalent to 1.3 Tb/in 2 ) demonstrate a raw error rate <10 -2 , which is consistent with the recording system requirements of modern hard drives. Extendibility of BPM to higher densities, and its eventual combination with energy assisted recording are explored.Index Terms-Bit patterned media, hard disk drive, block copolymer, self-assembly, double patterning, e-beam lithography, sequential infiltration synthesis, nanoimprint lithography, templated growth, thermal annealing, Co alloys, magnetic multilayers, interface anisotropy, magnetic recording, write synchronization, prepatterned servo, areal density.

Single crystal Ni/Co͑111͒ superlattices have been grown by molecular beam epitaxy. The Ni thickness is 3 ML whereas the Co thickness varies from 0.2 to 4 ML. The superlattices were studied using magnetometry and ferromagnetic resonance spectroscopy and they all exhibit strong perpendicular to the plane magnetic anisotropy. The maximum magnetocrystalline anisotropy is obtained for one cobalt monolayer. Kerr microscopy measurements show the variation of domain pattern as the Co layer thickness changes.

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study thin Co/Ni synthetic layers with perpendicular anisotropy confined in spin-valve based nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude rf excitation, from 1 to 20 GHz. These results are compared with those obtained using conventional rf field driven FMR on extended films with the same Co/Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth and approximately the same linewidth vs frequency slope, implying the same damping parameter. The critical current for magnetic excitations is determined from measurements of the resonance linewidth vs dc current and is in accord with the one determined from I-V measurements.Spin-transfer torque has been theoretically predicted and experimentally demonstrated to drive magnetic excitations in nanostructured spin valves and magnetic tunnel junctions [1,2,3,4,5]. With an rf current, spin transfer can be used to study ferromagnetic resonance [6,7]. This technique, known as spin-torque driven ferromagnetic resonance (ST-FMR), enables quantitative studies of the magnetic properties of thin layers in a spin-transfer device. Specifically, the layer magnetic anisotropy and damping can be determined [8], which are important parameters that need to be optimized in spin-torque-based memory and rf oscillator applications.Spin-transfer memory devices will likely include magnetic layers with perpendicular magnetic anisotropy that counteracts their shape-induced easy-plane anisotropy. This will allow efficient use of spin current for magnetic reversal with a reduced switching threshold [9] and a faster switching process [10]. Recent work by Mangin et al. [11] has demonstrated improvements of spin-torque efficiency in a spin valve that has perpendicularly magnetized Co/Ni synthetic layers. For further optimization of perpendicular anisotropy materials, it is important to have quantitative measurements of their anisotropy field and damping in a nanostructured device, as both of these parameters directly affect the threshold current for spintransfer induced switching.In this Letter, we present ST-FMR studies of bilayer nanopillars, where the thin (free) layer is composed of a Co/Ni synthetic layer and the thick (fixed) layer is pure Co. The magnetic anisotropy and damping of the Co/Ni have been determined by ST-FMR. We compare these results with those obtained from extended films with the same Co/Ni layer stack measured using traditional rf field driven FMR.Pillar junctions with submicron lateral dimensions ( Fig. 1(a)) were patterned on a silicon wafer using a nanostencil process [12]. Junctions were deposited using metal evaporation with the layer structure 1.5 nm (a)

The magnetic properties and magnetization dynamics of polycrystalline ultra-thin Co layers were investigated using a broadband ferromagnetic resonance (FMR) technique at room temperature. A variable thickness (1 nm ≤ t ≤ 10 nm) Co layer is sandwiched between 10 nm thick Cu layers (10 nm Cu|t Co|10 nm Cu), while materials in contact with the Cu outer interfaces are varied to determine their influence on the magnetization damping. The resonance field and the linewidth were studied for in-plane magnetic fields in field swept experiments at a fixed frequency, from 4 to 25 GHz.The Co layers have a lower magnetization density than the bulk, and an interface contribution to the magnetic anisotropy normal to the film plane. The Gilbert damping, as determined from the frequency dependence of the linewidth, increases with decreasing Co layer thickness for films with outer Pt layers. This enhancement is not observed in structures without Pt layers. The result can be understood in terms of a non-local contribution to the damping due to spin pumping from Co through the Cu layer and spin relaxation in Pt layers. Pt layers just 1.5 nm thick are found to be sufficient to enhance the damping and thus act as efficient "spin-sinks." In structures with Pt outer layers, this non-local contribution to the damping becomes predominant when the Co layer is thinner than 4 nm. PACS numbers: