Thin films and quasi-two-dimensional systems show a wide range of ordering effects and related pattern-formation phenomena. The origins of these phenomena can often be traced to competition between the atomic (or molecular) interactions in the system and the resulting inherent frustration of the system. In magnetic thin films, a wide range of magnetic structures are possible as a result of the competition between the long-ranged dipolar interactions and localized interactions. This article reviews recent experimental and theoretical work which has developed our understanding of ordering and pattern formation in these films and in related structures.

Dipolar effects in magnetic thin films and quasi-two-dimensional systems

Low-teperature specific-heat C measurements have been made on the high-T/sub c/ superconductors RBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ (R = Nd, Sm, Gd, Dy, and Er; deltaapprox. =0.1) for 0.5approx.

Long-range magnetic ordering in the high-Tc superconductors RBa

The first measurements of the 1/f-noise spectrum in copper oxide superconductors are presented. The key features of our results are (1) no noise is found in the superconducting state, (2) in the normal state the noise is large, comparable in magnitude to that in metal-insulator composites, and (3) the temperature dependencies of the noise and the resistivity are opposite, unlike either metals or semiconductors. Implications for possible conduction models are considered.

1/f-noise-power measurements of copper oxide superconductors in the normal and superconducting states

We have measured the noise spectrum near the transition temperature of the high-{ital T}{sub {ital c}} superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin film which shows a rather broad transition temperature width with the onset transition temperature {ital T}{sub {ital c},onset}=90 K and the zero-resistance temperature {ital T}{sub {ital c},zero}=30 K. We observed two noise peaks, one near the onset transition temperature (81 K) and another one near the zero-resistance temperature (45 K). These two noise peaks show distinctively different dependences on the magnetic field and driving current. Furthermore, the frequency dependence of the noise spectrum at 81 K shows 1/{ital f} behavior while that of the noise spectrum at 45 K shows 1/{ital f}{sup (1+{alpha})} behavior with {alpha} approaching 1, as the magnetic field is increased. The noise peak near the zero-resistance temperature region is believed to be due to the noise associated with the motion of free vortices.

Noise measurement near the transition region in YBa2Cu3O7- delta thin-film superconductor.

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearTc, on a number of YBa2Cu3O7 samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu2+ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.

Specific heat of YBa2Cu3O7

The specific heat $c$, resistance $R$, and magnetic susceptibility $\ensuremath{\chi}$ of the high-temperature superconductor $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ have been measured from 1.5 to 180 K and compared with those of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and ${\mathrm{La}}_{1.85}$${\mathrm{Ba}}_{0.15}$Cu${\mathrm{O}}_{4}$. For the Gd compound, an antiferromagnetic transition is seen in the specific-heat data at 2.24 K, and a Curie-Weiss dependence of $\ensuremath{\chi}$ is also seen, with $7.98{\ensuremath{\mu}}_{B}$ per Gd ion and $\frac{1}{\ensuremath{\chi}}$ extrapolating to zero at -5.89 K. The entropy associated with this peak in the specific heat is consistent with an assignment of $J=\frac{7}{2}$ to the total angular-momentum quantum number of the Gd ion, indicating that crystal-field effects are small. The specific heat of the Gd compound is larger than that of the Y compound at these temperatures; this is attributed to the larger mass of the Gd ion. A peak in $\frac{c}{{T}^{3}}$ near 20 K in the Gd compound indicates a sharply defined peak in the phonon density of states near 10 meV. The superconducting transition temperature ${T}_{c}$ of $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ indicated by the specific-heat data is 93.8 K, and the jump in the specific heat at ${T}_{c}$ is 301 mJ/K g-atom, in agreement with previous observations for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. Evidence is seen for a temperature-dependent phonon density of states in $\mathrm{Gd}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, and it is pointed out that such a temperature dependence is also seen in previously published data for ${\mathrm{La}}_{1.85}$${\mathrm{Ba}}_{0.15}$Cu${\mathrm{O}}_{4}$ and ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$Cu${\mathrm{O}}_{4}$.

Specific heat of Gd Ba 2 Cu 3 O 7 − δ in the normal and superconducting states

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Specific heat of Gd Ba 2 Cu 3 O 7 − δ in the normal and superconducting states