Volume 35, Issue 6, June 2009
Index of content:
- LOW-TEMPERATURE MAGNETISM
35(2009); http://dx.doi.org/10.1063/1.3151991View Description Hide Description
Thermodynamic characteristics of the perovskite-like compound , exhibiting a structural phase transformation of the martensitic type with a characteristic temperature , are studied in the temperature range . Step-like hysteretic temperature behavior of the effective heat capacity is revealed at and attributed to the discrete kinetics and a latent heat of the martensitic transformation. The magnetic subsystem is found to exhibit a magnetic glass state below and temperature hysteresis of the magnetic susceptibility clearly pronounced in the and regions. The Debye and Einstein temperatures, and , respectively, derived from the experimental Debye–Waller factors for the , Mn, and O sublattices, are used to refine contributions from the structural and magnetic transformations to the heat capacity and to reveal thermodynamically nonequilibrium states.
Elementary excitations and thermodynamics of zig-zag spin ladders with alternating nearest-neighbor exchange interactions35(2009); http://dx.doi.org/10.1063/1.3151992View Description Hide Description
A one-dimensional spin-model in which the alternation of the exchange interactions between neighboring spins is accompanied by the next-nearest-neighbor (NNN) spin exchange (zig-zag spin ladder with alternation) is studied. The thermodynamic characteristics of the model quantum spin chain are obtained in the mean-field-like approximation. Depending on the strength of the NNN interactions, the model manifests either the spin-gapped behavior of low-lying excitations at low magnetic fields, or ferrimagnetic ordering in the ground state with gapless low-lying excitations. The system undergoes second-order or first-order quantum phase transitions, governed by the external magnetic field, NNN coupling strength, and the degree of the alternation. Hence, NNN spin–spin interactions in a dimerized quantum spin chain can produce a spontaneous magnetization. On the other hand, for quantum spin chains with a spontaneous magnetization, caused by NNN spin–spin couplings, the alternation of nearest-neighbor (NN) exchange interactions can cause destruction of that magnetization and the onset of a spin gap for low-lying excitations. Alternating NN interactions produce a spin gap between two branches of low-energy excitations, and the NNN interactions yield asymmetry of the dispersion laws of those excitations, with possible minima corresponding to incommensurate structures in the spin chain.
35(2009); http://dx.doi.org/10.1063/1.3151993View Description Hide Description
The microstructure and the magnetic and transport properties of as-deposited films prepared by pulsed laser deposition are investigated in a wide region of temperature and magnetic field. The microstructure analysis reveals that all films have a high -oriented texture, an orthorhombic crystal lattice, and a negligible quantity of inclusions. The observed strip-domain phase with a periodic spacing of about , the crystal lattice of which is the same as for the basic film phase, exhibits magnetic behavior typical for the Griffiths phase. Regions of the double-period modulated phase are found at room temperature in the film, which is interpreted as ordering with a partial transition at . At the same time, the investigation reveals that the magnetic and transport properties of the electron–doped films, driven by cation doping, are similar to those for the hole-doped manganites. Therefore, one can conclude that there is no fundamental difference between the mechanisms of spin ordering and charge transport in the hole-doped and electron–doped manganites.
Magnetization field-dependences and the “exchange bias” in ferro/antiferromagnetic systems. I. Model of a bilayer ferromagnetic35(2009); http://dx.doi.org/10.1063/1.3151994View Description Hide Description
A qualitative model explanation of the experimentally obtained field dependences of the magnetization in ferro- and antiferromagnetic media in contact with one another is proposed. In this model a thin ferromagnetic(FM) film on an antiferromagnetic(AFM) substrate consists of only two ferromagnetic layers. This is the simplest model which admits a spatially nonuniform FM state. In this exactly solvable model it shown that a range of fields exists where a stable collinear (canted) structure of the FM subsystem obtains. This structure corresponds to inclined sections of the field dependence of the magnetization which are not associated with the kinetics of the magnetization reversal process. In the model proposed, for systems with large easy-plane anisotropy the magnetization reversal process with “exchange bias” taken into account is strictly symmetric as a function of the field provided that the additional weak FManisotropy in the easy plane is neglected. When this anisotropy in the easy plane is taken into account hysteresis appears in the magnetization curve and the field dependence becomes asymmetric.