The creation of large magnetic fields is a necessary component in many technologies, ranging from magnetic resonance imaging, electric motors and generators, and magnetic hard disk drives in information storage. This is typically done by inserting a ferromagnetic pole piece with a large magnetisation density M S in a solenoid. In addition to large M S , it is usually required or desired that the ferromagnet is magnetically soft and has a Curie temperature well above the operating temperature of the device. A variety of ferromagnetic materials are currently in use, ranging from FeCo alloys in, for example, hard disk drives, to rare earth metals operating at cryogenic temperatures in superconducting solenoids. These latter can exceed the limit on M S for transition metal alloys given by the Slater-Pauling curve. This article reviews different materials and concepts in use or proposed for technological applications that require a large M S , with an emphasis on nanoscale material systems, such as thin and ultra-thin films. Attention is also paid to other requirements or properties, such as the Curie temperature and magnetic softness. In a final summary, we evaluate the actual applicability of the discussed materials for use as pole tips in electromagnets, in particular, in nanoscale magnetic hard disk drive read-write heads; the technological advancement of the latter has been a very strong driving force in the development of the field of nanomagnetism.
The microwave emission of magnetic tunnel junction nanopillars with symmetric CoFeB/MgO/CoFeB interfaces and a 9-nm-thick free layer was studied as a magnetic field was applied in plane at different angles H with respect to the easy axis. As H was increased, a more complicated mode spectrum containing modes of larger amplitude was observed. The character of the different modes was deduced by screening many devices fabricated from the same wafer, observing the variation of frequency both between different junctions and with H, and through comparison with micromagnetic simulation. Within selected junctions at certain H values, the dependence of the frequency, amplitude, and linewidth of the lowest-frequency edge-localized mode upon the applied field and current can be interpreted in terms of combined thermal and spin-transfer-torque (STT) excitation. Fitting to a simple analytical model yielded values for the in-plane and out-of-plane spin STT. The out-of-plane STT was fo und to contain terms with both a linear and a quadratic dependence on the bias current. The linear term was often found to dominate, as expected for a tunnel junction with asymmetric rather than symmetric interfaces, suggesting that the fuller structure of the free and reference layers should be taken into account. The amplitude of the in-plane STT was somewhat larger than expected, suggesting that the dominant edge mode occupies only part of the area of the free layer but is subject to a proportionately larger fraction of the torque exerted upon the free layer as a whole. This might be expected when the resonant modes of the free layer interact and the lowest-frequency mode dominates
The search for materials or systems exhibiting a high magnetic saturation has been of longstanding importance. It has been suggested that increased saturation could be achieved by coupling a transition metal via a spacer to a rare earth. We report Gd/Cr/Fe 70 Co 30 multilayer stacks and find reduced yet modulating magnetic moment as a function of Cr thickness. Through a micro structural analysis the lowered moment is indicated by the nucleation of the ultrathin Gd films into an fcc phase. We discuss the possible solution in terms of quasi-perfect lattice match seed material to promote growth of hcp Gd.Corresponding authors: * cward19@qub.ac.uk # r.m.bowman@qub.ac.uk
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