Entanglement spectrum and magnetization plateaus in an S = 1 bond-alternative Heisenberg chain with single-ion anisotropy and magnetic field

Entanglement spectrum and quantum phase transitions (QPTs) in S = 1 bond-alternative antiferromagnetic Heisenberg chain with single-ion anisotropy and magnetic field are investigated by the infinite time-evolving block decimation (iTEBD) method. The combined effects of the single-ion anisotropy and magnetic field have been discussed, and a rich ground-state phase diagram is obtained. We find that the single-ion anisotropy is advantageous to the stability of the 1/2 magnetization plateau. Both entanglement entropy and entanglement spectrum, as two model-independent measures, are capable of describing all the QPTs. Especially, doubly degenerate entanglement spectrum on even bond is observed in the 1/2 plateau phase. Besides constant spontaneous magnetization, three magnetization plateaus (M ^z = 0, 1/2, and 1) are found to have constant entanglement entropy, entanglement spectrum, and nearest-neighbor correlation. In addition, all the QPTs in such a model have been determined to belong to the second-order category.     

4f-electron-phonon coupling and lattice dynamics in the ferromagnet LiTbF4

The polarized Raman spectra of LiTbF₄ have been investigated at several temperaturesT150 K, and the temperature dependence of the selfenergies of optical phonons due to the magnetoelastic interaction have been determined. The sizes of these temperature shifts, of the magnetic phonon splittings and of the level repellings between resonant optical phonons and crystal field states have been calculated using the theory of magnetoelastic interaction by Fulde and Thalmeier without any adjustable parameters, considering the modulation of the crystalline electric field by the optical phonons. The phonon modes of LiTbF₄ have been determined in a rigid-ion approximation. Reasonable to good agreement with the experimental results has been obtained for most of the phonon modes.     

Magnetic-field-induced quantum criticality in a spin-<Emphasis Type="Italic">S</Emphasis> planar ferromagnet with single-ion anisotropy

The effects of single-ion anisotropy on quantum criticality in a d-dimensional spin-S planar ferromagnet is explored by means of the two-time Green’s function method. We work at the Tyablikov decoupling level for exchange interactions and the Anderson-Callen decoupling level for single-ion anisotropy. In our analysis a longitudinal external magnetic field is used as the non-thermal control parameter and the phase diagram and the quantum critical properties are established for suitable values of the single-ion anisotropy parameter D. We find that the single-ion anisotropy has sensible effects on the structure of the phase diagram close to the quantum critical point. However, for values of the uniaxial crystal-field parameter below a positive threshold, the conventional magnetic-field-induced quantum critical scenario remains unchanged.     

Peculiarities of the Giant Magnetoimpedance in Permalloy-Based Film Structures in the Important Temperature Range for Practical Applications

Multilayer structures based on Fe19Ni81 films have been obtained by ion-plasma sputtering and investigated on an impedance spectroscopy apparatus equipped with a temperature unit. An increase in the magnetoimpedance ratios for the total impedance and its real part has been found for the multilayer structure (Cu/FeNi)₅/Cu/(Cu/FeNi)₅ upon heating from 25 to 50°C. The maximum of the giant magnetoimpedance ratio of the total impedance ΔZ/Z = 56% has been observed at a frequency of 80 MHz with a sensitivity of 18%/Oe, while the maximum of the real part ΔR/R = 170% was observed at the frequency of 10 MHz with the sensitivity of 46%/Oe. Magnetization and resistivity at the direct current have been found to depend insignificantly on the temperature and, hence, the relaxation mechanism due to the magnetoelastic anisotropy was proposed as the most probable mechanism of increasing the value and sensitivity of the magnetoimpedance effect.     

Structural and magnetic characterization of rhombohedral Ga1.2Fe0.8O3 ceramics prepared by high-pressure synthesis

The rhombohedral α- Ga_1.2Fe_0.8O₃ ceramics have been synthesized by using a high pressure technique at a pressure of 5 GPa and a temperature of 800 °C from orthorhombic ε- Ga_1.2Fe_0.8O₃ ceramics, which were identified to be isostructural with α- Fe₂O₃ and α- Ga₂O₃. The low temperature magnetism has been studied for α- Ga_1.2Fe_0.8O₃, the saturation magnetization is at 5 K, and the Morin temperature has not been found. Moreover, it is most probable that the spin reorientation of α- Ga_1.2Fe_0.8O₃ has been found at 50 K resulted from the change of magnetic dipole anisotropy and single-ion anisotropy with temperature.     

Quantum analogue of the spin-flop transition for a spin pair

Quantum (step) magnetization curves have been analyzed for a spin pair with antiferromagnetic interaction in the presence of a magnetic field that is parallel to the easy magnetization axis. Both semiclassical and numerical analyses have been performed for a wide range of the anisotropy parameter and spins up to S ? 100. In the dependence of the anisotropy character (single-ion or exchange), a spin jump larger than unity can appear in the magnetization curve or jumps can be concentrated in a narrow range of the field. In addition, regions of the problem parameters have been revealed where behavior is semiclassical for low spins on the order of S = 5 and where behavior is substantially quantum even for S → ∞.     

On $\mathsf{c = 1}$ critical phases in anisotropic spin-1 chains

Quantum spin-1 chains may develop massless phases in presence of Ising-like and single-ion anisotropies. We have studied c = 1 critical phases by means of both analytical techniques, including a mapping of the lattice Hamiltonian onto an O(2) NL M, and a multi-target DMRG algorithm which allows for accurate calculation of excited states. We find excellent quantitative agreement with the theoretical predictions and conclude that a pure Gaussian model, without any orbifold construction, describes correctly the low-energy physics of these critical phases. This combined analysis indicates that the multicritical point at large single-ion anisotropy does not belong to the same universality class as the Takhtajan-Babujian Hamiltonian as claimed in the past. A link between string-order correlation functions and twisting vertex operators, along the c = 1 line that ends at this point, is also suggested.Received: 16 July 2003, Published online: 24 October 2003PACS: 75.40.-s Critical-point effects, specific heats, short-range order - 75.10.Jm Quantized spin models - 02.70.-c Computational techniques     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Temperature dependence of the afr frequencies of a uniaxial antiferromagnet

The method of two-time Green's functions is used to calculate the temperature dependences of the antiferromagnetic-resonance (AFR) frequencies for a uniaxial antiferromagnet in the field range below sublattice-reversal fields. The AFR frequency has a [K(T)/M(T)]^1/2 dependence on the anisotropy constant K(T). For the simplest case of single-ion anisotropy, the normalized AFR frequency is proportional to the relative sublattice magnetization: (T) /(0) (T).Translated from Izvestiya VUZ. Fizika, No. 12, pp. 39–44, December, 1969.     

The single-ion anisotropy effects in the mixed-spin ternary-alloy

The effect of single-ion anisotropy on the thermal properties of the ternary-alloy in the form of $A{B}_p{C}_{1?p}$ is investigated on the Bethe lattice (BL) in terms of exact recursion relations. The simulation on the BL consists of placing A atoms (spin-1/2) on the odd shells and randomly placing B (spin-3/2) or C (spin-5/2) atoms with concentrations p and $1?p$, respectively, on the even shells. The phase diagrams are calculated in possible planes spanned by the system parameters: temperature, single-ion anisotropy, concentration and ratio of the bilinear interaction parameters for $z=3$ corresponding to the honeycomb lattice. It is found that the crystal field drives the system to the lowest possible state therefore reducing the temperatures of the critical lines in agreement with the literature.     

Thermodynamics of a quasi-two-dimensional heisenberg ferromagnet with single-ion anisotropy

A quasi-two-dimensional ferromagnet (S = 1) with easy axis type anisotropy and interplanar exchange J and intraplanar exchange K is examined within the framework of the spin-wave approximation. An expression for the Curie temperature that is dependent on both the anisotropy parameter D (D/J 1) and on the exchange parameter ratio =K/J is obtained from the self-consistent equations. The important role of the single-ion anisotropy in the formation of the long-range magnetic order in two-dimensional systems is shown.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 94–101, March, 1988.The authors are grateful to S. S. Aplesnin for performing all the numerical computations and to I. S. Sandalov and A. F. Sadreev for useful discussions.     

Magnetic phase transition in Heisenberg antiferromagnetic films with easy-axis single-ion anisotropy

The staggered susceptibility of spin-1 and spin-3/2 Heisenberg antiferromagnet with easy-axis single-ion anisotropy on the cubic lattice films consisting of n=2, 3, 4, 5 and 6 interacting square lattice layers is studied by high-temperature series expansions. Sixth order series in J/k_BT have been obtained for free-surface boundary conditions. The dependence of the Néel temperature on film thickness n and easy-axis anisotropy D has been investigated. The shifts of the Néel temperature from the bulk value can be described by a power law n^−λ with a shift exponent λ, where λ is the inverse of the bulk correlation length exponent. The effect of easy-axis single-ion anisotropy on shift exponent of antiferromagnetic films has been studied. A comparison is made with related works. The results obtained are qualitatively consistent with the predictions of finite-size scaling theory.     

Thermal expansion and magnetostriction of GdAg2, and relations to the magnetoelastic paradox

The antiferromagnet GdAg₂ has been shown to be a good model system for the magnetoelastic paradox (MEP), because it exhibits large symmetry conserving magnetoelastic strains and the antiferromagnetic propagation vector breaks the tetragonal lattice symmetry (therefore a large symmetry breaking magnetoelastic strain can be expected in a single magnetic structure). As in many similar Gd based compounds no symmetry breaking strain has been found in the experiment. In order to investigate this MEP further, we have measured magnetostriction and magnetization on a textured polycrystal. The behaviour closely resembles that of GdNi₂B₂C, the prototype system for the magnetoelastic paradox (MEP). Our forced magnetostriction data indicate that the crystal distorts in applied magnetic field and gives further evidence that the MEP is a low field effect. The observed phase transitions are in agreement with available specific heat and neutron diffraction data. Moreover, the saturation magnetic field was measured in high pulsed magnetic fields and agrees well with the value calculated from the Standard Model of Rare Earth Magnetism (SMREM).     

Phase transitions in random uniaxial systems with dipolar interactions

The critical behaviour of random uniaxial ferromagnetic (ferroelectric) systems with both short range and long range dipolar interactions is investigated, using the field theoretic renormalization method of Brézin et al. for the free energy above and below the transition pointT _c. The randomness is due to externally introduced fluctuations in the short range interactions (quenched case) or (and) magneto-elastic coupling to the lattice (annealed case). Strong deviations in the critical behaviour with respect to the pure systems are found. In the quenched case e.g. the specific heatC and the coefficientf ₂ (ofM ³ in the equation of state, whereM is the magnetization) change fromC |ln|t^1/3,f ₂ |ln|t^–1 in the pure system toC is the reduced temperature andA _±,C _± are constants) in the random situation. This change should e.g. be experimentally observable by deuterization of the ferroelectric tri-glycine sulfate where the logarithmic behaviour off ₂ has already been detected in the pure case. For nonvanishing magnetoelastic coupling a complex critical behaviour is obtained and discussed. We find the interesting result that if both quenched randomness and a weak magnetoelastic coupling are present the quenched random critical behaviour dominates in the close vicinity ofT _c. Finally the influence of the magnetoelastic coupling on the longitudinal phonons in investigated and it is found that the relative changes in the corresponding elastic constant and structure factor are proportional to the specific heat and the wavevector dependent energy-energy correlation function respectively, suggesting new experiments.     

Elastic constants and quadrupolar interactions in the (LaCe)B6 series

Elastic constants of (La_1–xCe_x)B₆(x=0.01, 0.03, 0.1) dilute alloys have been determined by ultrasonic measurements at temperatures between 0.5 K and 4.2 K. The temperature dependence ofc ₄₄ and (c ₁₁–c ₁₂)/2 is in excellent agreement with calculations including magnetoelastic and interionic quadrupolar interactions. The single ion coupling constantsg (=₃, ₃) are independent of concentration. The systematic concentration dependence of quadrupolar coupling constants g'g(x) from the dilute (x=0.01) alloy to the compound CeB₆ (x=1) has been studied. Evidence of Grüneisenparameter coupling in the bulk modulus but no indication for a ₈ ground state splitting or a quadrupolar Kondo effect has been found. In addition the magnetic field dependence of elastic constants up to 6.5 Tesla was investigated.     

Formation of quantum magnetization plateaux in mixed-spin Ising chains with single-ion anisotropy

We investigate the physical processes which give rise to a multi-plateau ground-state magnetization curve in ferrimagnetic Ising chains with alternating spins (S,s) and different single-ion anisotropies on each sublattice of the system under an applied magnetic field, by using an elaboration of the molecular-field theory. Our analysis is started with the system () for which we use the transfer-matrix technique for comparison. In this system, we find a double-plateau structure (initial and saturation) in the magnetization curve for all values of , independent of anisotropies. Then we study two more elaborate systems, comparing the results with density-matrix renormalization group calculations, and finally generalize our argument to the general case. We find that for a specified range of the anisotropy parameters, the system exhibits 2s+1 plateaux, including the two classical and all those allowed for general quantum spin chains. This follows a similar rule as that known for spin-S(S≥1) Ising chains with single-ion anisotropy, for which 2S+1 plateaux appear in the ground-state magnetization curve, surviving even at low temperatures.     

Optimization of multiferroic properties in the absence of single-ion magnetic anisotropy

The dependence of the average magnetization per site of perovskite B sublattice has been calculated on the assumption of weak single-ion magnetic anisotropy for a quasy-binary solution of Fe³⁺ and nonmagnetic ions. The technique of application of the obtained results to optimize the magnetoelectric characteristics of solid solutions is qualitatively described.