The uniaxial perpendicular and magnetocrystalline anisotropies in strained 〈100〉-oriented Ni-Cu superlattices have been determined by in-plane and out-of-plane ferromagnetic resonance and magnetization measurements for varying Ni and Cu thicknesses. X-ray-diffraction patterns of the superlattice structure for these films indicate that the Ni lattice contracts while the Cu lattice dilates along the growth direction. Utilizing the measured strain with known elastic and magnetostriction constants for bulk Ni, the calculated strain-induced magnetoelastic anisotropies due to an inverse magnetostriction are in good quantitative agreement with the experimentally determined uniaxial perpendicular anisotropies.

We describe a state of enhanced dc conductance, terminated by an abrupt transition as a function of bias current, that exists at low temperature in metal point contacts that also display a signal near zero bias attributable to Kondo scattering from a class of fast-tunneling atomic two-level systems. We argue that the regime of enhanced conductance is a consequence of the presence of a highly correlated non-Fermi-liquid electronic screening state about these two-level tunneling systems. The critical-current transition marks a sudden change in the electronic state during which this non-Fermi-liquid state is disrupted, and replaced by a lower-conductance normal state at high bias. We describe in detail the temperature and magnetic-field dependence of the critical-current transitions, as well as their hysteresis as a function of bias.

${\mathrm{NdNi}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${\mathrm{O}}_3$ perovskites with 0.3≤x≤0.8 show a peak anomaly at low temperatures (≊28 K) in their ${\mathrm{\chi }}_{\mathrm{ac}}$(T) curve. The frequency dependence of the peak anomaly and magnetization experiments after different thermal cycles, i.e., cooling the samples in the presence of a magnetic field or zero-field cooling, shows that the anomaly is associated with magnetic relaxation processes indicating a new spin-glass transition. X-ray diffraction, transmission electron microscopy, and analysis of the magnetic data agree with the absence of small magnetic impurities which could give smaller relaxation phenomena. A discussion of the microscopic origin of such behavior has also been given.

The effect of fractons on the exciton dynamics in dilute magnets is investigated theoretically. It is predicted that the total intensity of the hot exciton-fracton absorption band varies with temperature as ${\mathit{T}}^{\mathit{d}\mathrm{¯}+2\mathrm{}\mathit{d}\mathrm{¯}/\mathit{D}}$, where d¯ is the fracton dimension and D the fractal dimension. Also, the fracton-exciton interaction results in a quadratic temperature dependence of the exciton intersublattice relaxation rate in dilute antiferromagnets, substantially different from that (cubic temperature dependence) of magnon-exciton scattering. Implication of the results for experimental study of the magnon-fracton crossover in dilute magnets is demonstrated.

The influence of the magnetic properties of the amorphous matrix on the macroscopic behavior of soft nanocrystalline materials is analyzed. The random-anisotropy model is generalized to two magnetic phase systems. It is shown that recent observations dealing with a magnetic hardening at the first stage of nanocrystallization and a magnetic hardening at temperatures close to the matrix Curie temperature can be well understood with this generalization.

Magnetic properties of ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Pt}}_{\mathit{x}}$ alloys were studied in both the disordered and ordered states for x=0.25, 0.35, 0.44, and 0.50. Different magnetic fields were applied in the range from 0 to 800 Oe for the disordered state and from 0 to 10 kOe for the ordered state under the zero-field cooling condition with the temperature range from 5 to 400 K. For x=0.25 and 0.35 alloys, a ferromagnetic moment was observed in both the disordered and ordered states for the whole magnetic-field range. In the ordered state, an unusual behavior of the magnetic moment was observed, which is independent of magnetic field. For the x=0.44 alloy, a ferromagnetic moment was observed in the disordered state, but peculiar results were observed in the ordered state. A weak ferromagnetic moment appeared at 0 and 10 Oe magnetic field but from 50 up to 1000 Oe magnetic field, an antiferromagnetic-type feature is seen, where the antiferromagnetic-paramagnetic transition temperature ${\mathit{T}}_{\mathit{N}}$ decreases monotonically with increasing magnetic field. Further, from 1200 Oe up to 10 kOe, the paramagnetism is evident. Hysteresis-loop data and the curve of the inverse of the magnetic susceptibility confirmed antiferromagnetism in the alloy. This is the first observation to our knowledge for such a change under an applied magnetic field in the binary alloys. For the x=0.50 alloy, a ferromagnetic moment in the disordered state and a clear paramagnetic moment in the ordered state were observed for the whole magnetic-field range.

We report the results of Lorentz electron microscopy studies of the microscopic magnetic reversal behavior of epitaxial Fe/GaAs(001) thin films in the thickness range 150–450 Å in order to explain how the microscopic processes give rise to the observed macroscopic reversal behavior. The results are described in the context of scanning tunneling microscopy and transmission electron microscopy studies of the film microstructure and of magneto-optical Kerr effect magnetometry studies of the macroscopic magnetic switching behavior. We find that striking differences in the domain structure occur according to the crystallographic direction along which the field is applied. For applied fields aligned close to the in-plane 〈110〉 directions (hard axes), magnetization reversal proceeds via a combination of coherent rotation and displacements of 90° domain walls aligned along the 〈110〉 directions. For magnetization reversal along the in-plane 〈100〉 directions (easy axes), an irregular checkerboard domain structure develops at the transition field: well-defined 90° and 180° domain walls coexist, aligned along the 〈110〉 and 〈100〉 directions, respectively. The magnetization reversal along the 〈100〉 directions takes place via a combination of domain nucleation and wall displacements. The magnetic reversal of unfavorably oriented domains is found to occur by a combination of two 90° reorientations.

Phase transitions of Ising ferromagnets with nearest-neighbor ferromagnetic exchange interactions and competing biquadratic exchange interactions on a square lattice are investigated using a Monte Carlo method for spin values S=1, 3/2, 2, and 5/2. We find that for all the spin values studied in this paper, there always exists a transition line for each spin value in the high-temperature region. However, in the lower-temperature region, different transition behavior may occur for different spin values. In the T-K plane, a line running within the ordered phase, along the nonzero temperature region from T=0 and K/J=-1/3 to T=0 and K/J=-1.0 for spin S=3/2 and from T=0 and K/J=-1/6 to T=0 and K/J=-1/3 for spin S=2, separates the region of equal sublattice magnetizations from that of unequal sublattice magnetizations. For spin value S=5/2, two low-temperature lines in the T-K plane separate the region of equal sublattice magnetizations from that of unequal sublattice magnetizations with the one line running in the nonzero temperature region from T=0 and K/J=-0.1 to T=0 and K/J=-1/6 and the other line running from T=0 and K/J=-1/3 to T=0 and K/J=-1.0. A comparison with other approximate techniques is also made.

The ground-state properties of a spin-1 Heisenberg ferromagnet with an arbitrary crystal-field potential are studied using the linked-cluster series expansion. In the linked-cluster expansion method quantum and spin-fluctuation correlations have been systematically included. We have obtained the magnetization and susceptibility series at zero temperature for a general lattice. The quantum fluctuations play an essential role in determining the magnetic ordering at T=0. The dependence of magnetic ordering on the crystal-field potentials and the critical values of the crystal-field potential for which magnetic ordering exists are discussed. The observed values of magnetization from the linked-cluster series expansion are appreciably depressed from its mean-field approximation values due to the quantum fluctuations.

The square-lattice spin-1/2 antiferromagnet containing a dilute concentration δ of randomly placed ferromagnetic nearest-neighbor bonds is studied at low-temperature via nonlinear σ-model techniques and by exact diagonalization. We generally find that long-range Néel order is destroyed above a critical strength in the defective ferromagnetic exchange-coupling constant given by ‖${\mathit{K}}_{\mathit{c}}$‖/J∼${\mathrm{\delta }}^{\mathrm{-}1/2}$. We also observe large statistical fluctuations both in the spin stiffness and in the antiferromagnetic structure factor near this critical point, suggesting the onset of a spin-glass phase.

Chubukov’s proposal concerning the possibility of a nondimerized quantum nematic phase in the ground-state phase diagram of the bilinear-biquadratic spin-1 chain is studied numerically. Our results do not support the existence of this phase, but instead they indicate a direct transition from the ferromagnetic into the dimerized phase.

The ground-state phase diagram of the Kondo chain with an anisotropic exchange is investigated. Four different phases are identified, two types of Néel phases, the Haldane phase, and the Kondo singlet phase. The boundaries between these phases in the strong coupling region are determined by mapping to effective models: the spin-1 Heisenberg model with single-ion anisotropy and the Ising model in a transverse magnetic field. It is shown that these transition lines extend to the weak-coupling limit and they are estimated by using the exact numerical diagonalization for a finite system.

The extraordinary Hall resistivity of magnetic multilayers is derived by using the Kubo formalism. It is found that the commonly used scaling relation between the extraordinary Hall resistivity and the ordinary resistivity is not valid. Hall voltages in the plane of the layers and perpendicular to the plane of the layers are quite different, just as the ordinary resistivities are different for these two geometries. Recent experimental results are discussed within this formulation.

We study the critical behavior of the surface on a semi-infinite simple-cubic-lattice Ising model with a bimodal random surface field by a large-cell mean-field renormalization-group method. Our results show that a surface ferromagnetic phase exists at weak random fields above the bulk critical temperature, and no signs of the tricritical point are found until the surface critical temperature decreases to that of the bulk.

The general formula of exchange coupling between transition-metal ferromagnetic layers spaced by a nonferromagnetic transition-metal layer with flat interfaces is given within a one-band tight-binding model for a simple cubic lattice, which determines self-consistently the Fermi level so as to take account of the dependence of the Fermi level on the magnetic configuration and thicknesses of layers, for thin magnetic trilayers. For large exchange splitting and thickness of ferromagnetic layers, assuming the minority-spin energy bands in the ferromagnets matches that of the spacer, we made detailed calculations to obtain both the analytic expression of interlayer exchange coupling energies and its simple asymptotic expression for large spacer thickness, which is accurate enough even for spacer thickness N=6. In some reasonable range of parameter, for smaller spacer thickness, intrinsic biquadratic coupling given by our theory is strong enough to be a dominant mechanism of noncolinear aligning of magnetization direction of two ferromagnetic layers, which plays a similar role as that of the magnetostatic interaction induced by interfacial roughness. The situation still holds even for large exchange splitting, different from existing theoretical works based on the free-electron model. Our results imply that intrinsic biquadratic coupling is more sensitive to the models chosen than bilinear coupling.

Full magnetization curves of S=1 antiferromagnetic Heisenberg chains with periodic and open boundaries are investigated by a quantum Monte Carlo method. The boundary condition has a significant effect on the magnetization process especially in the field range where the Zeeman energy is less than the Haldane gap. The magnetization of the periodic chain in the range is strongly suppressed at low temperatures because the ground state is nonmagnetic. On the other hand, the magnetic ground of the open chain bring about finite magnetization. The difference between both the magnetization curves is also investigated in detail by subtracting the magnetization of the periodic chain with L spins from one of the open chains with L+1 spins. It agrees well with the double of the Brillouin function for S=1/2, rather than the Brillouun function for S=1.

The temperature dependence of the giant magnetoresistance (GMR) in ${\mathit{M}}_1$/N/${\mathit{M}}_2$ multilayers consisting of magnetic ${\mathit{M}}_{\mathit{i}}$ (i=1,2) and nonmagnetic N layers, is discussed with the use of the finite-temperature band theory in which the effect of spin fluctuations is taken into account by means of the static functional-integral method combined with the coherent potential approximation. It is shown that the temperature dependence of the MR ratio, ΔR/R, shows a variety of behaviors depending on the values of ${\mathit{a}}_1$ and ${\mathit{a}}_2$ where ${\mathit{a}}_{\mathit{i}}$=${\mathrm{\Delta }}_{\mathit{i}\mathrm{\uparrow }}$/${\mathrm{\Delta }}_{\mathit{i}\mathrm{\downarrow }}$ and ${\mathrm{\Delta }}_{\mathit{i}\mathit{s}}$ is the imaginary part of the coherent potential of an s-spin electron at the interface of the ${\mathit{M}}_{\mathit{i}}$ layer. In the normal MR where ${\mathit{a}}_1$>1 and ${\mathit{a}}_2$>1 (or ${\mathit{a}}_1$<1 and ${\mathit{a}}_2$<1), ΔR/R decreases as the temperature is raised. On the contrary, in the inverse MR where ${\mathit{a}}_1$>1,${\mathit{a}}_2$<1 (or vice versa) and ΔR/R<0, it increases although its absolute magnitude decreases. In a multilayer where normal and inverse MR’s coexist, ΔR/R may show a maximum or almost constant behavior in a fairly wide temperature range.

By high-current injection from a point contact into bulk superconducting Ta a nonequilibrium state is produced that may be described as a normal bubble within the superconductor half-space. In driving the diameter of this bubble by the external bias we observe two different types of oscillatory behavior of the conductance. At zero magnetic field the quasiparticle current across the droplet is modulated due to geometrical resonances by multiple Andreev and normal reflections. At magnetic fields somewhat below the critical field, the superconducting surrounding of the normal region forms a symmetric dc SQUID. The magnetic flux through the normal region thus modulates a small supercurrent contribution. Equal numerical values of the bubble diameter are obtained by quantitative evaluation of both effects.

The Hall coefficient, dc conductivity, and hence the mobility, as well as the ac conductivity and the dielectric constant in the ${\mathrm{CuO}}_2$ planes are reported for single crystals of ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ containing ∼0.2 mole % holes introduced by doping with oxygen or strontium. In general, the transport properties are quite similar for the two kinds of acceptors. A smaller value of the binding energy is found for the Sr acceptor than the oxygen acceptor. In addition, the in-plane dielectric constant is found to be larger in the Sr-doped sample, and the characteristic temperature for variable-range hopping is smaller. The latter observations indicate that the radius of the hole’s orbit when bound to the acceptor is larger for Sr than for O, consistent with the smaller binding energy. Despite these differences in the bound states of the acceptors, the holes introduced by O and Sr appear to affect the antiferromagnetic order in the same way since the two crystals have the same Néel temperature. The temperature dependencies of the in-plane mobility for the two systems are very similar, and are well-described by a simple model combining polar optical phonon scattering and the scattering of charged impurities. The quantitative agreement with theory for the optical phonon scattering confirms the polaronic nature of the charge carriers.

We have measured optical reflection spectra for the alkali-metal fullerides ${\mathrm{K}}_3$${\mathrm{C}}_{60}$, ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$, ${\mathrm{K}}_4$${\mathrm{C}}_{60}$, ${\mathrm{Rb}}_4$${\mathrm{C}}_{60}$, and ${\mathrm{Rb}}_6$${\mathrm{C}}_{60}$ grown by a cosublimation method in an energy region of 0.08–6 eV. A gap structure peaked at 0.5 eV was observed in ${\mathrm{K}}_4$${\mathrm{C}}_{60}$, indicating that this compound is an insulator. This result is in good agreement with other kinds of experiments, but contrary to a simple band theory. The metallic conductivity spectra of ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$${\mathrm{C}}_{60}$ were formed of a Drude part and midinfrared absorption. The energy of the latter contribution was close to the optical gap of ${\mathrm{K}}_4$${\mathrm{C}}_{60}$. The infrared-active molecular vibrational ${\mathit{T}}_{1\mathit{u}}$(4) mode shows an anomalous dependence on the alkali concentration x, and a large splitting (about 30 ${\mathrm{cm}}^{\mathrm{-}1}$) in the x=4 phase. These results suggest that the interaction between the doped electrons and the ${\mathit{T}}_{1\mathit{u}}$(4) vibration mode is extremely strong. The relation between the anomalous phonons and the midinfrared absorption in ${\mathrm{K}}_4$${\mathrm{C}}_{60}$ and ${\mathrm{K}}_3$${\mathrm{C}}_{60}$ is discussed.