C. D. Hu

Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu2O2

A complete phase diagram of Zn substituted quantum quasi-two-dimensional helimagnet LiCu2O2 has been presented. Helical ordering transition temperature (T_h) of the original LiCu2O2 follows finite size scaling for less than ~ 5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution > 5.5% quenches the long-range helical ordering and introduces an intriguing Zn level dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for T_c (Zn > 0.055,H). The magnetic coupling constants of nearest-neighbor (nn) J1 and next-nearest-neighbor (nnn) J2 (alpha=J2/J1) are extracted from high temperature series expansion (HTSE) fitting and N=16 finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin driven multiferroic material.

The electron-paramagnetic resonance linewidth of double-exchange interaction systems

Westudied the electron-paramagnetic resonance linewidth of colossal magnetoresistance manganites. Starting from the quantum Langevin equation, we derived the transverse susceptibility of the double-exchange interaction systems and identified the damping function to be the inverse of the relaxation time T2. We argued that the anisotropic energy caused by the crystal field was the most probable relaxation mechanism. The spin correlation caused by the double-exchange interaction was examined with the Schwinger boson approach. Taking into account the constraints of Schwinger bosons, we found that the linewidth is approximately proportional to the temperature. At the low-temperature end (near the Curie temperature TC), there is a minimum, below which the linewidth increases sharply. At the high-temperature limit, the linewidth is proportional to . Most importantly, the linewidth is a universal function of T/TC.

Dynamic susceptibility study on the skyrmion phase stability of Fe0.7Co0.3Si

The AC susceptibilities of Fe1−xCoxSi (0.1 ≤ x ≤ 0.7) alloys were measured and compared with that of MnSi. The range of skyrmion phase in the H-T phase space was enlarged and moved to higher temperatures when approximately 1/3 of the Fe was substituted with Co. Comparing with MnSi, the skyrmion phase of Fe0.7Co0.3Si was found thermodynamically more stable under a much lower and narrower critical field near 280 ± 50 Oe. The stability range in the H-T phase space and the free energy reduction ΔE (as a function of the applied magnetic field) for both the Fe0.7Co0.3Si and MnSi were compared.

The Dzyaloshinskii?Moriya interaction in metals

The Dzyaloshinskii-Moriya (DM) interaction in metals is an important mechanism for many magnetic properties. We start with the s-d exchange model and spin-orbit interaction for weak itinerant ferromagnetic systems to establish the form of DM interaction for metallic magnetic systems. The s-d exchange interaction is treated accurately and the conduction electron-mediated magnetism gives a form of DM interaction which is different from that in insulators. The implications of our result to spiral spin states and skyrmion lattices are also discussed.

Mn vacancy defects, grain boundaries, and A-phase stability of helimagnet MnSi

Mn vacancy defect and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of A-phase (skyrmion lattice) formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors creates both grain boundaries and intrinsic Mn vacancy defect of various levels in MnSi. The results of combined transport, specific heat, and AC spin susceptibility measurements are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. The finite-size effect and Mn vacancy level dependent helical phase transition temperature T(c) have been identified and verified. The stability of A-phase in H-T phase space has been examined through AC spin susceptibility data analysis.

Manganese Deficiency in MnSi Single Crystal and Skyrmion Pinning

The dc magnetization and ac spin susceptibility measurement results of MnxSi single crystals with x ≈ 0.96-1 are reported. Magnetic phase diagrams for MnxSi of different Mn deficiency levels are mapped and compared. We provide strong evidence that the existence of manganese deficiency is common in the MnSi single crystals due to the high Mn vapor loss during growth, and the Mn deficiencies could be used as pinning centers to stabilize the Skyrmion phase within the magnetic field-temperature phase space.

Spiral spin state in a zigzag spin-chain system

We considered a spin chain with nearest neighbor and next nearest neighbor exchange interactions, anisotropic exchange interaction and Dzyaloshinskii-Moriya interaction. The conditions of the spiral spin state as the ground state were analyzed. Our method was to build the connection between the spiral state and the fully polarized state with a unitary transformation. Under this transformation, anisotropic exchange interaction and Dzyaloshinskii-Moriya interaction can be transformed to each other. Then we used positive semi-definite matrix theorem to identify the region of fully polarized state as the ground state for the transformed Hamiltonian, and it is the region of spiral spin state as the ground state of the original Hamiltonian. We also found that the effect of Dzyaloshinskii-Moriya interaction is important. Its strength is related to the pitch angle of spiral spins. Our method can be applied to coupled spin chains and two dimensional triangular lattice systems. The result can be compared with experiment data.

Spin Pump on a Finite Antiferromagnetic Chain through Bulk States

We studied the possibility of the spin pump in a S=1/2 antiferromagnetic chain. The spin chain is mapped into a fermion system and bosonization is utilized to transform the equation of motion to a sine-Gordon equation. The sine-Gordon equation on a finite chain with different boundary conditions is solved. Among numerous solutions, the static soliton is compatible with the original physical system. By varying adiabatically a angle in the phase space composed of applied electric and magnetic fields, the spin states change between the Neel state and dimer state and a quantized spin is transported by the bulk state from one end of the system to the other.

A study of the solutions of the combined sine-cosine-Gordon equation

We have studied the solutions of the combined sine-cosine-Gordon Equation found by Wazwaz [A.M. Wazwaz, Travelling wave solutions for combined and double combined sine-cosine-Gordon equations by the variable separated ODE method, Appl. Math. Comput. 177 (2006) 755] using the variable separated ODE method. These solutions can be transformed into a new form. We have derived the relation between the phase of the combined sine-cosine-Gordon equation and the parameter in these solutions. Its applications in physical systems are also discussed.

Quantum spin pumping at a fractionally quantized magnetization state for a system with competing exchange interactions

We study the quantum spin pumping of an antiferromagnetic spin-1/2 chain with competing exchange interactions. We show that spatially periodic potential modulated in space and time acts as a quantum spin pump. In our model system, an applied electric field causes a spin gap to its critical ground state by introducing bond-alternation exchange interactions. We study quantum spin pumping at different quantized magnetization states and also explain physically the presence and absence of quantum spin pumping at different fractionally quantized magnetization states.