Vl. E. Sinitsyn

Low-energy excitations and thermodynamical properties of the quantum (5/2, 1/2, 1/2) ferrimagnetic chain

The low-energy structures of the quantum ferrimagnetic Heisenberg chain consisting of (5/2, 1/2, 1/2) trimers are investigated theoretically. The results of the linear spin-wave theory are compared with those from the numerical exact diagonalization calculation and the matrix product method for the lowest optical mode. The temperature behaviour of thermodynamical properties such as magnetic susceptibility, specific heat and entropy are analysed in the framework of the modified spin-wave theory. The results of the calculations are used to explain the experimental data obtained for the molecule-based heterospin magnets [Mn(hfac)2BNOR ] (R = H, F, Cl, Br) with one-dimensional chain structure.

Coherent sliding dynamics and spin motive force driven by crossed magnetic fields in a chiral helimagnet

We demonstrated that the chiral soliton lattice formed out of a chiral helimagnet exhibits coherent sliding motion by applying a time-dependent magnetic field parallel to the helical axis, in addition to a static field perpendicular to the helical axis. To describe the coherent sliding, we use the collective coordinate method and numerical analysis. We also show that the time-dependent sliding velocity causes a time-varying Berry cap which causes the spin-motive-force. A salient feature of the chiral soliton lattice is appearance of the strongly amplified spin motive force which is directly proportional to the macroscopic number of solitons (magnetic kinks).

Symmetry adapted finite-cluster solver for quantum Heisenberg model in two dimensions: a real-space renormalization approach

We present a quantum cluster solver for the spin-S Heisenberg model on a two-dimensional lattice. The formalism is based on the real-space renormalization procedure and uses the lattice point group-theoretical analysis and non-Abelian SU(2) spin symmetry technique. The exact diagonalization procedure is used twice at each renormalization group step. The method is applied to the spin-half antiferromagnet on a square lattice, and a calculation of local observables is demonstrated. A symmetry-based truncation procedure is suggested and verified numerically.

Quantum dissipation theory of slow magnetic relaxation mediated by domain-wall motion in the one-dimensional chain compound [Mn(hfac)2BNOH]

Based on a quantum dissipation theory of open systems, we present a theoretical study of slow dynamics of magnetization for the ordered state of the molecule-based magnetic complex [Mn(hfac){sub 2}BNO{sub H}] composed from antiferromagnetically coupled ferrimagnetic (5/2,1) spin chains. Experimental investigations of the magnetization process in pulsed fields have shown that this compound exhibits a metamagnetic AF-FI transition at a critical field in the order of the interchain coupling. A strong frequency dependence for the ac susceptibility has been revealed in the vicinity of the AF-FI transition and was associated with an AF-FI interface kink motion. We model these processes by a field-driven domain-wall motion along the field-unfavorable chains correlated with a dissipation effect due to a magnetic system-bath coupling. The calculated longitudinal magnetization has a two-step relaxation after the field is switched off and are found in good agreement with the experiment. The relaxation time determined from the imaginary part of the model ac susceptibility agrees qualitatively with that found from the remanent magnetization data.

On a quantum plateau of magnetization in metal-organic quasi-one-dimensional ferrimagnets

The possibility of realizing a quantum plateau of magnetization in [Mn(hfac)2BNOR] metal-organic compounds is investigated theoretically. A model of a one-dimensional ferrimagnetic chain (5/2, 1) is used for calculating the magnetization as a function of an external field by the method of discrete path integral representation (DPIR). Within this model, the coexistence of classical and quantum plateaus of magnetization is revealed. It is shown that the critical field Hc1 that destroys the classical plateau (ground-state magnetization) is determined by the optical gap in zero field, which is estimated by the matrix-product method and a numerical method of exact diagonalization (recursion method).

The ground-state properties of the one-dimensional heterospin chain (5/2, 1/2, 1/2) with alternating exchange

The ground-state properties of the ferrimagnetic spin chain consisting of (5/2,1/2,1/2) trimers with antiferromagnetic exchange between the 5/2 and 1/2 spins and ferromagnetic exchange between the 1/2 and 1/2 spins are analysed in the framework of the quantum renormalization group in real space and the matrix product method. The results are compared with a spin-wave calculation for a spin chain with two types of spin, (5/2,1). The ground-state energy, the average magnetizations and the correlation length are found as functions of the ratio between the antiferromagnetic and ferromagnetic exchange. The analysis shows that the trimer spin chain is characterized by strong quantum fluctuations reducing the average magnetizations and possesses an extremely short correlation length.

Critical behavior of a monoaxial chiral helimagnet

We analyze the critical behavior of magnetically ordered phases appearing in a monoaxial chiral helimagnet in a weak external magnetic field. Using the formalism of the equations of state in the critical region, we determine the temperature dependence of the order parameters for the conical phase and the soliton-lattice phase. We calculated the critical exponents and show that they coincide with those in the three-dimensional Heisenberg model.

Generation of spin motive force in a soliton lattice

The generation of a spin motive force in a chiral helimagnet due to the action of two crossed magnetic fields is considered. The cases of pulsed and periodic magnetic fields directed along the helical axis under a perpendicular dc field are analyzed. It is shown that, in the case of a pulsed field, the spin motive force is related to dissipation, whereas in a periodic field, there is a reactive component that is not related to damping processes.

Magnetization and spin gap in two-dimensional organic ferrimagnet BIPNNBNO

A magnetization process in the two-dimensional ferrimagnet BIPNNBNO is analyzed. The compound consists of ferrimagnetic (1,1/2) chains coupled by two sorts of antiferromagnetic interaction. Whereas the behavior of the magnetization curve in higher magnetic fields can be understood within a process for the separate ferrimagnetic chain, the appearance of the singlet plateau at lower fields is an example of non-Lieb-Mattis type ferrimagnetism. By using the exact diagonalization technique for finite clusters of size 4 × 6, 4 × 8 and 4 × 10 we show that the interchain frustration coupling plays an essential role in stabilization of the singlet phase. These results are complemented by an analysis of four cylindrically coupled ferrimagnetic (1,1/2) chains via an Abelian bosonization technique and an effective theory based on the XXZ spin-1/2 Heisenberg model when the interchain interactions are sufficiently weak/strong, respectively.

Chapter 10 Magnetic properties of low-dimensional organic crystals of the PNNNO family

We investigate the “local structure” of the ground state of F2PNNNO (two-dimensional organic antiferromagnet of a spin S = 1) by means of real-space renormalization group (RSRG) and density-matrix renormalization group (DMRG) techniques. The results show that the system is close to a spin liquid with a singlet ground state.