Roger D. Kirby

Nanostructured magnetic films for extremely high density recording

Abstract This paper presents recent results on high coercivity, nanocrystalline magnetic films. Elementary models for magnetization decay, thermal stability, and noise are discussed along with requirements on grain size and anisotropy for recording at extremely high areal densities (approaching 100 gigabits per square inch). Characterization of nanocrystalline films by activation-volume measurements and its relationship to nanostructure are emphasized. A number of recently discovered films with high potential for future extremely high density recording are discussed.

Enhancement of coercivity in nanometer-size CoPt crystallites

In this study, we showed that a magnetic coercivity (Hc) as high as 37 kOe was obtained in a CoPt thin film that contains separated nanometer-size CoPt crystallites. We prepared CoPt thin films with thicknesses of 5 and 175 nm by magnetron sputtering. After annealing in an Ar/H2 atmosphere at temperatures from 650 to 750 °C for 3–12 h, we measured the magnetic properties and found that magnetic Hc relates to annealing temperature, annealing time and film thickness. From atomic force microscopy and magnetic force microscopy studies, the magnetic single domain size of CoPt is in the range of 100–200 nm. The high Hc is likely due to the well-separated nanometer-size crystallites and the well-ordered fct phase of CoPt alloy.

Magnetization reversal and defects in Co/Pt multilayers

(Co 3 A/Pt 10 A)×N (N=8, 16, and 30 bilayers) multilayer thin films were prepared by sputtering onto an 850‐A‐thick SiN layer that had been deposited on a silicon (111) substrate. We used the polar Kerr effect to measure the time dependence of magnetization reversal over the temperature range 90–300 K. Direct domain observations were also carried out. The results show that the domain expansion process depends strongly on the number of bilayers. Uniform domain expansion was found only in the thinner samples. From the time dependence of the magnetization reversal measurements over the temperature range 90–300 K, the activation energy and volume associated with domain wall motion were found to be 1.2 eV and 2.3×10−18 cm3 for N=8 sample. The N=16 and N=30 samples seem to have a broad distribution of activation energies. Our observations suggest that both the coercivity and magnetization reversal are controlled by the defects that come from the interface between the Co and Pt.

Effects of surface morphology on magnetic properties of Ni nanowire arrays in self-ordered porous alumina

Ni nanowire arrays were produced by electrodeposition of Ni into self-assembled porous anodic alumina templates. With different anodization voltages, the wires show different surface morphologies. The temperature dependence of the coercivity and activation volume of wires with regular shape can be explained by thermal activation over an energy barrier with a 3/2 power dependence on the field. The wires with irregular surface morphologies show abnormal temperature dependences of the coercivity and activation volume. Possible mechanisms for these behaviours are discussed.

Formation of an anisotropy lattice in Co∕Pt multilayers by direct laser interference patterning

We report on the use of direct laser interference patterning to form an “anisotropy” lattice in Co∕Pt thin film multilayers. Co∕Pt multilayers have been extensively studied and, for the compositions studied here, are characterized by strong perpendicular magnetic anisotropy in which the magnetic moment is perpendicular to the film plane. In direct laser interference patterning, two-to-four coherent laser beams from a pulsed Nd:YAG laser strike the sample surface simultaneously, and for sufficiently intense beams the sample properties are modified locally where interference maxima occur. Kerr rotation, magnetic force microscopy, and atomic force microscopy measurements after patterning by one pulse from the laser show that the films have a regular array of “dots” with in-plane magnetization in a background matrix of perpendicular magnetization. Such patterning holds promise for the study of model nanoscale magnetic systems.

Magneto-optic properties of evaporated Mn-Bi-Al films

Abstract Thin films of Mn 50 Bi x Al y were made using the co-evaporation method. X-ray diffraction measurements confirmed the presence of MnBi, and weak diffraction lines suggested the presence of a second phase, possibly the MnAl τ phase. Hysteresis loops obtained both from polar Kerr rotation measurements and magnetization measurements were found to be anomalous. These measurements showed that the magnetizations of the two phases are parallel to each other and perpendicular to the film surface, and that the signs of the Kerr rotation are opposite for the two phases. Further, the magnetizations of the two phases are not strongly coupled to one another. A simple model calculation which includes the anisotropy energy of each phase is able to reproduce the salient features of the experimental hysteresis loops.

Magneto‐optical properties of MnBiAl thin films

Mn‐Bi‐Al thin films were produced by sequential evaporation of the constituents, followed by an anneal at 300 °C. The temperature and composition dependencies of the Kerr rotation angle, absolute reflectivity, and magnetic anisotropy were measured. The results show that, up to 30 at. % Al concentration, the thin films retain the pure MnBi hexagonal structure. Further, for suitable Al content, the films have the same large Kerr rotation as MnBi. Pure MnBi films exhibit perpendicular anisotropy at room temperature and in‐plane anisotropy for temperatures below 142 K. In contrast, the Al‐doped films prepared here have perpendicular anisotropy down to at least 85 K. The increased coercivities of the Al‐doped films are attributed to the occupation of grain‐boundary and interstitial sites of the NiAs‐type hexagonal structure by the Al atoms.

Raman scattering from 1T-TaS2

Abstract Raman measurements on the 1 T -polytype of TaS 2 are reported. In the commensurate charge density wave state, a large number of Raman-active peaks are observed below 400 cm -1 . Most of these peaks are attributed to k = 0 optic phonons resulting from superlattice formation.

Magneto-optical and structural properties of nanocrystalline MnBi-based films

Abstract In this paper we discuss recent research on MnBi and MnBiX films where X denotes alloying elements whose purpose is to modify the properties of the parent compound in beneficial ways. While MnBi has a large Kerr response, large uniaxial anisotropy and can be grown in thin-film form with its easy axis normal to the film plane, it also has large grain sizes and a high-temperature structural instability near the Curie point, both of which are detrimental to magneto-optic recording applications. We report systematic studies of Al doping which show that it does not enhance the Kerr response nor eliminate the high temperature structural instability. Al does, however, promote small grain sizes which are required for a low noise magneto-optical recording medium. Preliminary results are reported on sputtered samples with a number of other dopants, and some of these have promising properties.

Origin of the structural transition in TiSe2

Abstract Thermoelectric power and electron diffraction measurements have been made on well characterized crystals of TiSe 2 , Zr x Ti 1− x Se 2 and V x Ti 1− x Se 2 in an effort to determine the importance of charge carriers to the periodic lattice distortion. When the energy bands are uncrossed by alloying with Zr, the superlattice is critically suppressed. This result and the effect of vanadium doping, as well as earlier investigations, suggest that both electrons and holes are crucial. Superlattice formation in these systems appears to be due to interaction between carriers and a particular zone boundary phonon. Fermi surface nesting alone is not sufficient to account for the instability.