Specific heat and magnetic ordering of NdNi2B2C

In this work, the specific heat of NdNi₂B₂C was computed with the three sets of crystal-field parameters proposed by previous authors. All curves of the heat capacity plotted with the calculated results exhibit sharp peaks around the magnetic transition temperature TN as experimentally observed. To understand the mechanism of its magnetic ordering, we also calculated the magnetization of the material in low temperature region with the ground crystal-field (CF) level, the two lowest CF levels, and the full CF levels of J=9/2 multiplet respectively for comparison. Using the two eigenstates of the ground CF level, we derived a formula for 〈Jx〉 with mean-field approach for theoretical analysis. Both our numeric and theoretical results suggest that the two lowest CF levels play dominant roles in the magnetic process of the material below TN. It is also very interesting to notice that the ground CF level itself results in a larger TN, but the inclusion of the first excited CF doublet in calculations instead hinders the magnetic ordering, leading to a weakly reduced transition temperature.     

Reduced Magneto-Resistivity of a Rare-Earth Crystalline and the Degeneracy Removals of Its Crystal-Field Levels

A theoretical approach is generalized and employed to calculate the magneto-resistivity of a rare-earth crystalline （CeAh） with degenerate ground crystal-field （CF） level in the presence of external fields. The calculated results show that when a magnetic field is applied in the c-direction, the magneto-resistivity may be reduced by more than 90% in certain cases in comparison with the pure CF contribution at the same temperature, demonstrating the strong effects of the degeneracy removals of the CF levels on the magnetic resistivity.     

Infrared study in high magnetic fields of the crystal-field excitations in PrMnO3

Pr³⁺ ion crystal field (CF) excitations in PrMnO₃ single crystals have been studied by infrared transmission, in the 1800–8000 cm⁻¹ range, as a function of temperature and applied magnetic field up to 13 T. No noticeable frequency shifts which might occur below T_N∼100 K, as a result of the antiferromagnetic transition, are observed in the Pr³⁺ CF levels. A set of CF parameters that fit the experimental levels as well as the low temperature Pr³⁺ magnetic moment in PrMnO₃ has been determined.     

Magnetic field induced ordering in quasi-one-dimensional quantum magnets

Three-dimensional magnetic ordering transitions are studied theoretically in strongly anisotropic quantum magnets. An external magnetic field can drive quasi-one-dimensional subsystems with a spin gap into a gapless regime, thus inducing long-range three-dimensional magnetic ordering due to weak residual magnetic coupling between the subsystems. Compounds with higher spin degrees of freedom, such as N-leg spin-1/2 ladders, are shown to have cascades of ordering transitions. At high magnetic fields, zero-point fluctuations within the quasi-1D subsystems are suppressed, causing quantum corrections to the ordering temperature to be reduced. Received 24 March 2000     

Ordered phase of nuclear spins in uniaxial ferromagnets at ultralow temperatures

Paper investigates the onset of nuclear magnetic ordering caused by the indirect Suhl-Nakamura interaction in ferromagnets. The necessary condition for nuclear spin ordering with definite ordering vector is obtained. Particularly, it is shown that ferromagnetically ordered phase of nuclear spins could be observed only in case of disk shaped samples. The spectrum of the nuclear spin excitations is also found. Received 25 January 1999 and Received in final form 5 May 1999     

Orbital Order and Orbital Excitations in Degenerate Itinerant Electron Systems

We present a theory of orbital ordering in orbital-degenerate itinerant electron systems. The orbital instability in a two-orbital degenerate Hubbard model is investigated in the random phase approximation （RPA）. After demonstrating the criteria for the formation of orbital ordering or the orbital density wave ordering, we find that the orbital and the spin-orbital collective excitation spectra in the ferro-orbital ordered phase exhibit finite gaps. The possible application of the present theory in orbital-ordered 4d compounds is also discussed.     

A crystal field investigation of the properties of RCuAs2 (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb)

A crystal field (CF) investigation of the magnetic properties and heat capacities of RCuAs₂ (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb) has been carried out using the observed average magnetic susceptibilities (1.8-300 K) of the title compounds. The CF parameters proposed for the systems show a systematic variation throughout the rare-earth series. Other physical properties dependent on the CF are also computed and compared with available experimental data. The experimental heat capacity data reported for a limited range of temperature agree well with computed heat capacity for all the compounds (except SmCuAs₂ and YbCuAs₂). CF J mixing was found to be appreciable for all the samples except YbCuAs₂.     

Truncated forms of the second-rank orthorhombic Hamiltonians used in magnetism and electron magnetic resonance (EMR) studies are invalid—Why it went unnoticed for so long?

This paper deals with the truncated forms of the second-rank orthorhombic Hamiltonians employed in magnetism and electron magnetic resonance (EMR) studies. Consideration of the intrinsic features of orthorhombic Hamiltonians reveals that the truncations, which consist in omission of one of three interdependent orthorhombic terms, are fundamentally invalid. Implications of the invalid truncations are: loss of generality of quantized spin models, misinterpretation of physical properties of systems studied (e.g. maximum rhombicity ratio and relative parameter values), and inconsistent notations for Hamiltonian parameters that hamper direct comparison of data from various sources. Truncated Hamiltonian forms identified in our survey are categorized and systematically reviewed. Examples are taken from studies of various magnetic systems, especially those involving transition ions, as well as model magnetic systems. The pertinent studies include magnetic ordering in three- and lower dimensions, e.g. [(CH₃)₄N]MnCl₃ (TMMC), canted ferromagnets, Haldane gap antiferromagnets, single molecule magnets exhibiting macroscopic quantum tunneling, e.g. Mn₁₂ complexes with spin S=10. Our study provides better insight into magnetic and spectroscopic properties of pertinent magnetic systems, which calls for reconsideration of the experimental and theoretical results based on invalid truncated Hamiltonians. The physical nature of Hamiltonians used in magnetism and EMR studies and other types of inappropriate terminology occurring, especially in model magnetism studies, require separate discussion.     

Paradoxical magnetic cooling in a structural transition model

In contrast to the experimentally widely used isentropic demagnetization process for cooling to ultra-low temperatures we examine a particular classical model system that does not cool, but rather heats up with isentropic demagnetization. This system consists of several magnetite particles in a colloidal suspension, and shows the uncommon behavior of disordering structurally while ordering magnetically in an increasing magnetic field. For a six-particle system, we report an uncommon structural transition from a ring to a chain as a function of magnetic field and temperature. Received 5 September 2000     

A microscopic mechanism for rejuvenation and memory effects in spin glasses

Aging in spin glasses (and in some other systems) reveals astonishing effects of `rejuvenation and memory' upon temperature changes. In this paper, we propose microscopic mechanisms (at the scale of spin-spin interactions) which can be at the origin of such phenomena. Firstly, we recall that, in a frustrated system, the effective average interaction between two spins may take different values (possibly with opposite signs) at different temperatures. We give simple examples of such situations, which we compute exactly. Such mechanisms can explain why new ordering processes (rejuvenation) seem to take place in spin glasses when the temperature is lowered. Secondly, we emphasize the fact that inhomogeneous interactions do naturally lead to a wide distribution of relaxation times for thermally activated flips. `Memory spots' spontaneously appear, in the sense that the flipping time of some spin clusters becomes extremely long when the temperature is decreased. Such memory spots are capable of keeping the memory of previous ordering at a higher temperature while new ordering processes occur at a lower temperature. After a qualitative discussion of these mechanisms, we show in the numerical simulation of a simplified example that this may indeed work. Our conclusion is that certain chaos-like phenomena may show up spontaneously in any frustrated and inhomogeneous magnetic system, without impeding the occurrence of memory effects. Received 5 February 2001 and Received in final form 27 April 2001     

Ground-state long-range order in quasi-one-dimensional Heisenberg quantum antiferromagnets: high-order coupled-cluster calculations

We investigate the ground-state magnetic long-range order of quasi-one-dimensional quantum Heisenberg antiferromagnets for spin quantum numbers s = 1/2 and s = 1. We use the coupled cluster method to calculate the sublattice magnetization and its dependence on the inter-chain coupling J_⊥. We find that for the unfrustrated spin-1/2 system, an infinitesimal inter-chain coupling is sufficient to stabilize magnetic long-range order, in agreement with results obtained by other methods. For s = 1, we find that a finite inter-chain coupling is necessary to stabilize magnetic long-range order. Furthermore, we consider a quasi one-dimensional spin-1/2 system, where a frustrating next-nearest neighbor in-chain coupling is included. We find that for stronger frustration as well, a finite inter-chain coupling is necessary to have magnetic long-range order in the ground state, and that the strength of the inter-chain coupling necessary to establish magnetic long-range order is related to the size of the spin gap of the isolated chain.     

Theoretical estimates for proton-NMR spin–lattice relaxation rates of heterometallic spin rings

Heterometallic molecular chromium wheels are fascinating new magnetic materials. We reexamine the available experimental susceptibility data on MCr₇ wheels in terms of a simple isotropic Heisenberg Hamiltonian for M=Fe, Ni, Cu, and Zn and find in that FeCr₇ needs to be described with an iron–chromium exchange that is different from all other cases. In a second step we model the behavior of the proton spin lattice relaxation rate as a function of applied magnetic field for low temperatures as it is measured in nuclear magnetic resonance (NMR) experiments. It appears that CuCr₇ and NiCr₇ show an unexpectedly reduced relaxation rate at certain level crossings.     

Maximizing Thermal Entanglement of XY Model Using Site-Dependent Magnetic Fields in Specific Directions

We study the thermal entanglement by means of concurrence in a two-qubit isotropic XY model in the presence of site-dependent external magnetic fields in arbitrary directions. We find that at a given temperature and magnetic field strength, the mirror symmetry of the two fields about the x-y plane is a necessary condition for maximum entanglement. However, if there is no constraint on the field strengths, then the necessary condition for maximum entanglement reduces to the configuration that the two fields are vertical, anti-parallel and with the same strength. We also investigate the anisotropic XY model and find that the above conclusion more or less holds.     

Magnetism in systems with reduced dimensionality and chemical disorder: The local environment effects

We study influence of the local chemical environment, the so-called local environment effects, on the electronic structure and properties of magnetic systems with reduced dimensionality and chemical disorder, and show that they play a crucial role in a vicinity of magnetic instability. As a model, we consider Fe–Ni Invar. We present results obtained from ab initio calculations of the electronic structure, magnetic moments, and exchange interactions in random fcc Fe–Ni alloy, for a single monolayer alloy film on a Cu (0 0 1) substrate as well as in the bulk. We analyze the difference between the film and the bulk magnetization, which is found to be most pronounced for dilute alloys. We also analyze a sensitivity of the individual magnetic moments and effective exchange parameters to the local chemical environment of the atoms.     

Spin frozen in the Kondo-lattice compound: CeCuSi2

In this paper, we present the experimental results of X-ray powder diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements as well as Ce-L_III-edge X-ray absorption spectrum of the Ce-based intermetallic compound CeCuSi₂. The results revealed that CeCuSi₂ is a Kondo-lattice compound with no superconducting or magnetic-phase transition above 0.4 K. In addition, we found spin-glass behavior in the DC susceptibility measurements. The AC susceptibility measurements and the magnetic entropy calculation also confirm the presence of the spin-glass phase. The possible formation mechanism for the spin-frozen state is also discussed in this paper.     

Magnetoelastic effect in an exchange model

The effect of the interplay between magnetism, charge ordering and lattice distortion within a like double and super-exchange model is studied in low-dimensional systems. An important magnetoelastic effect that leads to a lattice contraction is presented in conjunction with an analytical minimization for a three-site one-dimensional model. The model is discussed in connection with the magnetism, charge ordering and the contraction of the rungs experimentally observed within the three-leg ladders (3LL) present in the oxyborate Fe₃O₂BO₃.     

Negative Magnetoresistivity of ErBi and Its Crystal-Field Levels

A theoretical approach is generalized and employed in this work to evaluate the magnetoresistivity of ErBi in external magnetic fields. The calculated results and theoretical analyses show that when an external magnetic field is applied in the z-direction, the magnetoresistivity can be reduced considerably due to the degeneracy lifting of the crystal-field levels. However, when the magnetic field is exerted along the x-axis, the magnetoresistivity will be increased because of the formations of new magnetic states of the Er ion and its transitions within and between these new states.     

Influence of Randomness and Localization on RKKY Interactions in Diluted Magnetic Semiconductors

We show that the spatially random distribution of magnetic moments of dopants in diluted magnetic semiconductors can partially localize the itinerant carriers and change the carrier-mediated indirect RKKY interaction. From numerical calculations of the electron states taking into account the interaction with magnetic impurities which are random both in spatial positions and in orientations of magnetic moments, we obtain the electron states and the RKKY interaction as a function of the distance between magnetic dopants L and of the sp - d exchange integral J. With the increase of disorder, the localization of itinerant electrons become stronger and the long-range regular oscillatory behaviour of the RKKY interaction gradually disappears and is replaced by severe fluctuations. The randomness and localization may enhance the RKKY interaction between dopants with short and middle distances and in favour of the ferromagnetism.     

Effect of impurity clustering on ferromagnetism in dilute magnetic semiconductors

We establish a model to investigate the effect of clustering of impurities on the ferromagnetism in dilute magnetic semiconductors (DMS). The Curie temperature Tc is calculated by the mean-field theory on a lattice with randomly distributed clusters of magnetic impurities which are interacting with each other by carrier mediated RKKY exchange coupling together with the nearest-neighbor (NN) direct exchange interaction. We consider different types and sizes of the clusters and find that the clustering of impurities can either enhance or reduce Tc, depending on the type and strength of the NN exchange interaction. If the NN interaction is antiferromagnetic and strong compared with the RKKY interaction, the clustering will reduce Tc. On the other hand, if it is ferromagnetic interaction or weak antiferromagnetic one, the clustering can enhance Tc. The trend of enhancing Tc is magnified if the average size of clusters increases. The clustering also changes the distribution of polarizations of impurities. The obtained results provide natural explanations on the fact that the ferromagnetism of DMS samples depends on the preparing and annealing processes even though the density of the magnetic impurities is kept the same.