The microscopic model for the magnetic subsystem in HoNi2B2C

We demonstrate that the system of localized magnetic moments in HoNi$_2$B$_2$C can be described by the 4-positional clock model. This model, at a proper choice of the coupling constants, yields several metamagnetic phases in magnetic field at zero temperature in full agreement with the experimental phase diagram. The model incorporates couplings between not nearest neighbors in the direction perpendicular to the ferromagnetic planes. The same model leads to a c-modulated magnetic phase near the Curie temperature. The theoretical value of the modulation wave-vector agrees surprisingly well with that observed by the neutron diffraction experiment without new adjustable parameters.Comment: 4 pages, 3 Postscript figures, RevTeX, uses EPSF.st

Spectra and magnetic properties of large spins in external fields

Journals published by the American Physical Society can be found at http://publish.aps.org/Spectra and magnetic properties of large spins J (e.g., spins possessed by ions or molecules), placed into a crystal electric field (CEF) of an arbitrary symmetry point group, are shown to change drastically when J changes by 1/2 or 1. At a fixed field symmetry and configuration of its N extrema situated at the p-fold symmetry axis, physical characteristics of the spin depend periodically on J with the period equal to p. The problem of the spectrum and eigenstates of the large spin J is equivalent to an analogous problem for a scalar charged particle confined to a sphere S-2 and placed into the magnetic field of the monopole with the charge J. This analogy, as well as the strong difference between close values of J, stems from the Berry phase occurring in the problem. For energies close to the extrema of the.CEF, the problem can be formulated as Harper's equation on the sphere. The (2J + 1)-dimensional space of states is split into smaller multiplets of classically degenerated states. These multiplets in turn are split into submultiplets of states transforming according to specific irreducible representations of the symmetry group determined by J and p. We classify possible configurations and corresponding spectra. Experimental realizations of large spins in a symmetric environment are proposed and physical effects observable in these systems are analyzed. [S1050-2947(99)00709-X]

Phase-Diagram of Ultrathin Ferromagnetic-Films with Perpendicular Anisotropy

Journals published by the American Physical Society can be found at http://journals.aps.org

Spin tunneling and topological selection rules for integer spins

We present topological interference effects for the tunneling of a single large spin, which are caused by the symmetry of a general class of magnetic anisotropies. The interference originates from spin Berry phases associated with different tunneling paths exposed to the same dynamics. Introducing a generalized path integral for coherent spin states, we evaluate transition amplitudes between ground as well as low-lying excited states. We show that these interference effects lead to topological selection rules and spin-parity effects for integer spins that agree with quantum selection rules and which thus provide a generalization of the Kramers degeneracy to integer spins. Our results apply to the molecular magnets Mn12 and Fe8.Comment: 4 pages, 3 EPS figures, REVTe

Angular dependence of metamagnetic transitions in DyAgSb2

Journals published by the American Physical Society can be found at http://journals.aps.org/Measurementsof the magnetization of DyAgSb2 reveal a complex system of up to 11 well-defined metamagnetic states for the field applied within the basal plane. Measurements of the magnetization vs the angle the applied field makes with respect to the [110] axis show the Dy3+ moments are constrained to lie along one of the four [110] directions within the basal plane. From the angular dependence of the critical fields and plateau magnetizations, the net distribution of the moments may be deduced for each state. Finally, the coupling constants are calculated within the framework of the "four-position clock model." [S0163-1829(99)04302-7]

Phenomenological Theory of Superconductivity and Magnetism in Ho1−x_{1-x}Dyx_xNi2_2B2_2C

The coexistence of the superconductivity and magnetism in the Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C is studied by using Ginzburg-Landau theory. This alloy shows the coexistence and complex interplay of superconducting and magnetic order. We propose a phenomenological model which includes two magnetic and two superconducting order parameters accounting for the multi-band structure of this material. We describe phenomenologically the magnetic fluctuations and order and demonstrate that they lead to anomalous behavior of the upper critical field. The doping dependence of $T_c$ in Ho$_{1-x}$Dy$_x$Ni$_2$B$_2$C showing a reentrance behavior are analyzed yielding a very good agreement with experimental data.Comment: 4 pages, 3 figures, REVTeX, submitted to PR

Berry's phase for large spins in external fields

It is shown that even for large spins $J$ the fundamental difference between integer and half-integer spins persists. In a quasi-classical description this difference enters via Berry's connection. This general phenomenon is derived and illustrated for large spins confined to a plane by crystalline electric fields. Physical realizations are rare-earth Nickel Borocarbides. Magnetic moments for half-integer spin (Dy$^{3+}$, $J=15/2$) and magnetic susceptibilities for integer spin (Ho$^{3+}$, $J=8$) are calculated. Experiments are proposed to furnish evidence for the predicted fundamental difference.Comment: 4 pages RevTe