ALBERT

Notes: We present heat capacity measurements of NiO/CoO superlattices grown by reactive sputtering. Neutron diffraction studies of similar superlattices have shown antiferromagnetic ordering through several bilayers despite the short-range nature of the spin interaction in the constituent materials. Specific heat measurements were made using a unique thin film microcalorimeter capable of measuring the heat capacity of thin films from 1.5 K to well above room temperature. We examine the effect of exchange coupling at the interfaces by varying the thickness of the bilayers. For thin bilayers (26 A(ring)), we observe a single broad heat capacity peak similar to a Ni0.5Co0.5O alloy. This peak is at a temperature which corresponds to the superlattice magnetic blocking temperature. For thicker bilayers ((approximately-greater-than)50 A(ring)), two broad maxima occur which approach the individual Néel temperatures of CoO and NiO with increasing bilayer thickness. The ordering temperature of the NiO layers is more suppressed than expected, indicating a more pronounced size effect compared to the CoO layers. The magnetic entropy S=R(ln 3+ln 2)/2 for the superlattices, within the uncertainties of the measurement, is conserved. We compare the temperature dependences of the specific heats to various models. © 1996 American Institute of Physics.

Notes: The large, growth-induced magnetic anisotropy in amorphous rare earth-transition metal alloys such as Tb-Fe are shown to depend strongly on the deposition temperature and only weakly on deposition rate or deposition technique (e.g., sputtering versus electron beam co-evaporation). These dependencies can be well fit with a thermally activated form involving minimization of surface energy during the growth by a re-orienting of adatom configurations over potential energy barriers. In this model, the growing film lowers its surface energy by a partial alignment of local clusters, presumably such as to maximize the number of in-plane bonds, although chemical effects undoubtedly also play an important role. These effects are somewhat analogous to a surface reconstruction which becomes trapped into the growing film by low bulk diffusion rates. In particular, a two-level model with a flat distribution of energy barriers is here shown to provide an excellent fit to the observations. Such a model leads to a ln(t) dependence on deposition rate and an exponential dependence on deposition temperature. We have also studied the subsequent irreversible relaxation of the anisotropy upon annealing. This relaxation is strongly influenced by the original growth temperature. In particular, the higher the original growth temperature, the more resistant the film is to subsequent relaxation. This result has important technological implications. As is commonly observed, the relaxation is well fit by a two-level model, again with a flat distribution of energy barriers over a range of energies, producing a ln(t) dependence on annealing time and a thermally activated dependence on annealing temperature. In annealing, of course, the lower energy state is isotropic, unlike the surface-induced anisotropic state produced during growth. The influence of the growth temperature on this relaxation implies that the actual process of creating the anisotropic state during the growth has the consequence of eliminating free volume in the sample, thereby raising the energy barriers to subsequent relaxation.

Notes: Thin films of amorphous rare earth–transition metal alloys are well known to possess perpendicular magnetic anisotropy. The vapor deposition process possesses factors that break the symmetry of an idealized, bulk, isotropic, amorphous material. These include the substrate, the incident atomic beams, incident energetic Ar atoms, and any applied field. Structural anisotropy such as columnar microstructure, nonperpendicular growth direction, stress and strain, and preferential growth of crystalline grains with low energy surfaces, are known consequences of the growth process. Evidence exists, however, that the anisotropy in these amorphous alloys is of a more subtle local nature. The perpendicular magnetic anisotropy in amorphous Tb-Fe in a broad range of compositions increases dramatically with increasing deposition temperature, including temperatures well above the Curie temperature. It is not strongly dependent on stress and is independent of film thickness. Upon annealing, the anisotropy vanishes. The nonrandomness in the amorphous structure thus does not appear to be related to nanocrystallites, which would presumably be enhanced by annealing, nor to kinetic effects of incident atomic beam directions which would be reduced by an increased deposition temperature. The dependence on deposition temperature can be fitted to a thermally activated form, implying an important role of surface diffusion and surface energy in the formation of the anisotropic local structure. We suggest that this process be generically thought of as a texturing of the amorphous phase, analogous to the preferred texturing of the low surface energy grains in polycrystalline growth. The dependence of the magnetic anisotropy of a-Tb-Fe on deposition temperature and rate will be presented and compared to a particular model involving reorientation of adatom configurations into lower energy orientations. Comparisons to other a-RE-TM alloys will be made, in particular to a-Gd-Fe and to a-Er-Fe (with the opposite sign Steven's coefficient). The results of an extensive computer simulation of a competing model involving chemically driven preferential site occupation will also be presented.

Notes: Using extended x-ray absorption fine structures (EXAFS) measurements we have investigated the atomic environment around the Fe atom in a series of amorphous Tb0.26Fe0.74 films having different magnetic anisotropy energies owing to different deposition temperatures. The polarization properties of synchrotron radiation allowed the separate study of structure parallel and perpendicular to the sample plane. An anisotropy between these two structures was observed. Modeling results indicate this anisotropy is due to anisotropic pair correlations where the Fe–Fe pairs are statistically preferred in-plane and the Fe–Tb pairs out-of-plane. The amplitude of this anisotropy scales with both the substrate temperature and the magnetic anisotropy energy. A ≈1% in-plane compression of the Fe–Fe distance was measured between the in-plane and out-of-plane structure of the sample grown at 77 K. This sample had no detectable local chemical anisotropy suggesting that intrinsic stress plays an important role in determining its magnetic anisotropy.

Notes: A model is proposed for a novel surface-to-surface segregation process which would be observed during growth by vapor deposition of thin polycrystalline films of alloys exhibiting classic surface segregation. The model depends on sufficient surface mobility to allow equilibration between surfaces of different grains and insufficient bulk mobility to allow equilibration between the surface and bulk of each grain before the present surfaces are covered by the next layer of material. This high ratio of surface to bulk mobility is easily found under standard deposition conditions. The model leads to an inhomogeneous film in which the composition of each grain is dependent on its crystallographic orientation.

Notes: We present heat capacity and magnetic measurements of antiferromagnetic (AFM) CoO/Ni0.5Co0.5O superlattices grown by reactive sputtering. X-ray data verify the structure and the high quality of the superlattice. Neutron-diffraction studies of similar superlattices have shown AFM ordering through several bilayers despite the short-range nature of the spin interaction in the constituent materials. We have recently developed a unique thin film microcalorimeter capable of measuring thin films from 1.5 K to well above room temperature, permitting specific heat measurements on these superlattices for the first time. Magnetic measurements were made by coupling the superlattices to a 30 nm Ni81Fe19 overlayer and measuring the temperature dependence of the exchange anisotropy field. We examine the effect of exchange coupling at the interfaces by varying the thickness of the bilayers and their constituents. When the layers of the CoO/Ni0.5Co0.5O superlattice are thin, we observe a single broad heat capacity peak at a temperature between the Néel temperatures of bulk CoO and Ni0.5Co0.5O. This peak is at a temperature that corresponds to the superlattice magnetic blocking temperature, the temperature at which the exchange field goes to zero. For thicker layers, we observe the disappearance of the superlattice peak, and the emergence of two broad peaks close to the individual Néel temperatures of CoO and Ni0.5Co0.5O. We compare the temperature dependence of the specific heat of the superlattices to various models.

Notes: We have deposited (100) and (111) oriented single-crystal Co1−xPtx films (x=0.65, 0.75, and 0.86) over a range of growth temperatures from −50 to 800 °C. The Curie temperature is increased by up to 200 °C over the value expected for the homogeneous, chemically disordered alloy in the as-deposited films (of both orientations) grown near 400 °C. Measurements of the onset of magnetic ordering below the Curie temperature indicate separation into Co-rich and Pt-rich regions. High resolution x-ray measurements show no shift in the lattice constant or broadening of the x-ray peaks, and no observable strain for x=0.75, suggesting that the separated regions are small and epitaxially coherent. We interpret this as evidence for a previously unobserved miscibility gap. The bulk phase diagram shows no phase separation, but magnetic energy tends to drive the system toward immiscibility as demonstrated by the calculations of several workers. We suggest that the observed miscibility gap is an equilibrium surface effect, trapped into the bulk film by low bulk mobility. Preliminary work by Rosengren and Kundrotas supports this idea. Large perpendicular magnetic anistropy is found in those films that exhibit an anomalously high Curie temperature (films grown near 400 °C). This anisotropy is likely related to the phase separation. After annealing at high temperatures, the Curie temperature approaches the homogeneous values and the anisotropy relaxes. © 1996 American Institute of Physics.

Notes: The microstructure of oxidized amorphous Tb-Fe alloy thin films prepared by magnetron cosputtering has been studied as a function of alloy composition and the substrate temperature at which the films were grown. Transmission electron microscopy and Auger electron spectroscopy with ion-sputter depth profiling were used to characterize the films. The composition of the alloy has no dramatic effect on the oxidation rate; it does, however, effect the microstructure of the α-Fe2O3 layer that forms on the alloy. Oxidized alloys originally containing more than 27 at. % Tb show an extremely fine-grained structure, with grain sizes on the order of 30 A(ring). Alloys containing less Tb developed large (1–2 μm) [001]-oriented single-crystal α-Fe2O3 grains amongst the fine-grained α-Fe2O3. For alloys with less then 18 at. % Tb, only the large grains remain. Samples grown at deposition temperatures ranging from 85 K to 400 °C show a similar structure after oxidation.

Notes: The magnetic anisotropy Ku of magnetron cosputtered amorphous Tb-Fe has been found to drop precipitously when the composition is within several at. % of 22 at. % Tb, a composition which is the room-temperature compensation point. The samples showing this low value for Ku are magnetically saturated (fields up to 100 kOe were used) and torque curves have the expected uniaxial behavior. The unexpected decrease is correlated with a previous finding of anomalous, nonuniaxial anisotropy for films prepared with less than 22 at. % Tb when nonperpendicular incident angles are used for the deposition. We hypothesized that an undetected phase separation was occurring in these Fe-rich samples. The present study examines films prepared on rotating substrates, essentially eliminating the effect of incident angles. As previously predicted, with no incident angles the phase separation is unobservable by magnetic anisotropy, but presumably is still present and may account for the observed drop in Ku. Analogous results have been seen in amorphous Ho-Fe; the relevant composition is far from magnetic compensation, supporting the conclusion that the coincidence of compensation with the anomalies in Tb-Fe is accidental. This behavior of the anisotropy is inconsistent with simple models for the atomic structure and the source of Ku. The effect of deposition temperature will also be discussed.