J.X. Shen

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.

MAGNETIC-PROPERTIES OF EPITAXIAL SINGLE-CRYSTAL ULTRATHIN FE3SI FILMS ON GAAS (001)

Magnetic properties of Fe3Si films with thickness from 2 to 210 monolayers (ML) epitaxially grown on GaAs (001) were studied using a superconducting quantum interference device and alternating gradient force magnetometers. Growth of these single‐crystal intermetallic compound films were carried out in a multichamber molecular beam epitaxy (MBE) system. The samples were covered in situ with Au 50 A thick to prevent oxidation when the samples were removed from the MBE chamber. All the films are ferromagnetic even for samples as thin as 2 ML. The easy magnetization direction of the films is parallel to the film surface. The magnetic coercivity forces (Hc) of the samples increase as the film thickness decreases to 10 ML, and then decrease when the film thickness decreases further to 2 ML.

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.

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.

Temperature‐dependent interface magnetism and magnetization reversal in Co/Pt multilayers

We report on the temperature dependence of the magnetic properties and interface magnetism of Co/Pt multilayers. The magnetic properties including magnetization and anisotropy change substantially as the temperature varies from 300 to 10 K for samples with Co layer thickness in the range from 3 to 7 A. The interface anisotropy of about 0.38 erg/cm2 is nearly independent of temperature. The magnetization reversal is dominated by domain wall motion for the thinner Co layers and dominated by nucleation for the thicker Co layers.

Magnetic and magneto‐optic properties of sputtered Co/Ni multilayers

We have investigated the magnetic and magneto‐optic properties of Co/Ni multilayers deposited on Ag and Au buffer layers. The samples with Au buffer layers show perpendicular magnetic anisotropy, but those with Ag buffer layers do not. The structure and degree of crystalline alignment of the buffer layer are evidently crucial to development of perpendicular magnetic anisotropy. We also present the results of polar Kerr rotation measurements as a function of wavelength and layer thickness of the multilayers.

Electronic structure and magneto-optical properties of MnBi and MnBiAl

MnBiAl films are of considerable current interest for possible magneto‐optical applications because of their perpendicular anisotropy, large polar Kerr rotation, reduced grain size, and structural stability. We report here experimental and theoretical studies of the effect of Al alloying on the electronic structure and magneto‐optical properties of MnBi. Our measured spectral dependencies of the polar Kerr rotation in the two systems are similar. We carried out relativistic self‐consistent spin‐polarized electronic structure calculations on MnBi and MnBiAl, and the calculated densities of states are in good agreement with available x‐ray photoemission spectroscopy data for both materials. The spin magnetic moment of Mn increases and the overall orbital moment decreases with the addition of Al to MnBi. The latter effect implies the lowering of spin‐orbit interactions upon alloying MnBi with Al, and the two effects together suggest that the spectral dependencies of the Kerr rotation should be similar, as is...

Studies of domain dynamics in amorphous Dy/Fe multilayers

Compositionally‐modulated Dy/Fe films were prepared by dc magnetron sputtering onto glass substrates. For this series of samples, the Dy thickness was fixed at 5 A, while the Fe thickness was varied from 4 to 6 A. To study the magnetization reversal, the samples were first saturated. The magnetic field was then reversed to a value near the coercive field, and the Kerr rotation was measured as a function of time after reversal. We interpret our results in terms of the model of domain reversal first described by Fatuzzo for ferroelectrics. For some Fe thicknesses, we found that the reversal process was dominated by domain nucleation, while for other thicknesses it was dominated by domain wall motion. The observed behavior is discussed in terms of the magnetic and structural properties of the films.

Low‐temperature characterization of the magnetic properties of MnBiAl thin films

The magnetic properties of thin‐film samples of MnBi0.8Alx with aluminum concentrations of x=0.0, 0.4, 0.6, and 0.8 were systematically studied over a temperature range of 20 to 300 K. The as‐deposited films are amorphous and nonmagnetic, but highly textured polycrystalline films that are ferromagnetic are formed by annealing at 350 °C. Our measurements show that the coercivity of such films rapidly decreases, then approaches a constant value (4 kOe for x=0.4) with increasing annealing time. Magnetic measurements show that both anisotropy and coercivity decrease with decreasing temperature. Unlike bulk MnBi, our MnBi0.8Alx thin films do not have a spin reorientation transition at low temperatures. This may be due to impediment of the lattice contraction by the Al atoms doped into the interstitial sites of the MnBi lattice.