Elementary spin excitations in ultrathin itinerant magnets

Elementary spin excitations (magnons) play a fundamental role in condensed matter physics, since many phenomena e.g. magnetic ordering, electrical (as well as heat) transport properties, ultrafast magnetization processes, and most importantly electron/spin dynamics can only be understood when these quasi-particles are taken into consideration. In addition to their fundamental importance, magnons may also be used for information processing in modern spintronics.     

Exchange interactions,spin waves,and transition temperatures in itinerant magnets

This contribution reviews an ab initio two-step procedure to determine exchange interactions, spin-wave spectra, and thermodynamic properties of itinerant magnets. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature. In the second step, parameters of an effective classical Heisenberg Hamiltonian are determined using the magnetic force theorem and the one-electron Green functions. The Heisenberg Hamiltonian and methods of statistical physics are employed in subsequent evaluation of magnon dispersion laws, spin-wave stiffness constants, and Curie/Néel temperatures. The applicability of the developed scheme is illustrated by selected properties of various systems such as transition and rare-earth metals, disordered alloys including diluted magnetic semiconductors, ultrathin films, and surfaces. A comparison to other ab initio approaches is presented as well.     

On the dynamics of itinerant electron magnets in the paramagnetic state

It is shown how disordered local-moment states, in a metal such as Fe, may be stabilized by constraining magnetic fields. Effective interaction parameters between moments are defined in terms of spin density functional theory. On removing the constraints the moments initially precess but then decay rapidly, the latter process leading to an energy width of S(q, ω) at large q of order $\text{(k}{}_{\mathrm{B}}\text{T}{}_{\mathrm{C}}\text{W)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, where T_C is the Curie temperature and W is the width of the d-band. T he consequences for neutron scattering experiments are discussed and the rather different case of Ni is considered.     

Orbital polarization in itinerant magnets

We propose a parameter-free scheme of calculation of the orbital polarization (OP) in metals, which starts with the strong-coupling limit for the screened Coulomb interactions in the random-phase approximation (RPA). For itinerant magnets, RPA can be further improved by restoring the spin polarization of the local-spin-density approximation through the local-field corrections. The OP is then computed as the self-energy correction in the static GW method, which systematically improves the orbital magnetization and the magnetic anisotropy energies in transition-metal and actinide compounds.     

Non-linear spin fluctuations and phase transitions in itinerant electron magnets: CMR manganites

We present a review analyzing the effects of coupling of transverse magnons with longitudinal spin fluctuations in isotropic itinerant ferro- and antiferromagnets. It is shown that this coupling essentially changes the spectrum of longitudinal fluctuations. At low-temperatures their spectrum is dominated by the linear Landau relaxation, is purely quasielastic and described by a broad central peak of a paramagnon type. On approaching the critical temperature non-linear magnetic relaxation due to mode–mode couplings can dominate and lead to a rapid increase of the central peak and to a new mechanism of magnetic phase transitions governed by non-linear spin fluctuations. The formalism is applied to the CMR manganites where the observed quasielastic fluctuations can be viewed as non-linear spin-lattice fluctuations strongly affected by magnons.     

Spin-charge fluctuations in itinerant helical magnets

Using a generalized random phase approach quantum fluctuations beyond the helical mean-field states of the Hubbard model off half filling are discussed. The collective mode spectrum is shown to exhibit spin-charge mixing and is found to be strongly renormalized by coupling to low-energy incoherent particle-hole excitations. Results for the collective mode dispersion which displays a significant anisotropy are presented for selected helical phases. Moreover phase space regions of ‘missing’ modes are identified with a finite wave length instability of the helical states suggesting novel inhomogeneous phases of the doped Hubbard model. P.A.C.S. : 71.27.+a, 75.30.Fv, 75.40.Gb     

Photoinduced change in the spin state of itinerant correlated electron systems

A photoinduced spin-state change in the itinerant correlated electron system is studied. A photon introduced in the low-spin band insulator induces a bound state of the high-spin state and a photoexcited hole. This bound state brings a characteristic peak in the pump-probe optical absorption spectra which are completely different from the spectra in thermal-excited states. The present results well explain the recent experiments of the ultrafast optical spectroscopy in perovskite cobaltites.     

稀土元素三角格子体系中的阻挫磁性与量子涨落

稀土元素三角格子阻挫体系在近几年受到了广泛的关注,稀土元素中存在较强的自旋轨道耦合作用,容易形成各向异性的磁相互作用,结合复杂的晶体场结构,可以产生很多新奇的物理性质,实现包括量子自旋液体、内禀量子伊辛磁体和隐藏序在内的一系列新奇量子态。文章以最近的中子散射研究为例,介绍稀土元素三角格子体系中的新奇磁关联与量子涨落现象。     

Theoretical study of spin waves in the itinerant ferromagnetic state of MnAs

The dynamic susceptibility is studied for the ferromagnetic state of MnAs on the basis of the itinerant electron theory. The spin waves exist throughout the entire Brillouin zone and the existence of an optical spin-wave branch is predicted.     

On structural instabilities in itinerant magnets and superconductors

The influence of a structural distortion on the itinerant antiferromagnetic (IAF) phase and on the superconducting (S) phase is investigated using a free electron band structure for the electron and hole pockets. For equal concentrations of electrons and holes a metastable phase is found in which the lattice distortion coexists with the IAF or S phase. For unequal concentrations of electrons and holes the critical temperature and the value of the order parameter for the IAF or S phase (in the coexistence region) will always be enhanced by the onset of the structural distortion. The theoretical predictions are compared in the IAF case with the occurrence of a spin flip transition in chromium accompanied by a tetragonal distortion, and in the S case with experimental results on A-15 compounds exhibiting a martensitic phase transition.     

Partially disordered antiferromagnetic state in two-dimensional triangular-lattice antiferromagnet

The partially disordered antiferromagnetic (PDA) state, as an exotic phase peculiar to the antiferromagnet with Ising spin in triangular lattice, is investigated by using Monte Carlo simulations of Wang-Landau algorithm and Glauber algorithm. It is revealed that PDA state, as the ground state of the triangular antiferromagnetic system, presents the complicated spin configuration due to geometrical frustration. The formation of multi-domain structure within the framework of honeycomb antiferromagnet results in the high degeneracy of PDA state. And this degeneracy of ground state can be lifted by a small magnetic field. Consequently the system shows the ferrimagnetic state, and the magnetization plateau of 1/3 saturate value (Ms) is observed in many experiments. Moreover, due to the multi-domain structure of PDA state and those spins on domain walls, the metastable steps may manifest themselves superposed on the 1/3Ms plateau in some special cases.     

Effects of spin fluctuations in itinerant electron ferromagnets

Effects of spin fluctuations on the paramagnetic spin susceptibility, magnetization and specific heat are calculated in gaussian statistics as a power series of k_BT. The results are applied to nickel and iron. It is shown that in effect spin fluctuations reduce the molecular field coefficient above the Curie temperature.     

Renormalization of spin polarised itinerant electron bands in the normal state of a model ferromagnetic superconductor

This paper studies the normal state properties of itinerant electrons in a toy model,which is constructed according to the model for coexisting ferromagnetism and superconductivity proposed by Suhl [Suhl H 2001 Phys.Rev.Lett.87 167007].In this theory with ferromagnetic ordering based on localized spins,the exchange interaction J between conduction electrons and localized spin is taken as the pairing glue for s-wave superconductivity.It shows that this J term will first renormalize the normal state single conduction electron structures substantially.It finds dramatically enhanced or suppressed magnetization of itinerant electrons for positive or negative J.Singlet Cooper pairing can be ruled out due to strong spin polarisation in the J > 0 case while a narrow window for s-wave superconductivity is opened around some ferromagnetic J.     

Localized spin excitations in magnets with triangular spin ordering

Some types of localized spin excitations in hexagonal antiferromagnets with triangular spin ordering are investigated. The character of relaxation of these spin excitations is studied.     

Chiral spin pairing in helical magnets

A concept of chiral spin pairing is introduced to describe a vector-chiral liquid-crystal order in frustrated spin systems. It is found that the chiral spin pairing is induced by the coupling to phonons through the Dzyaloshinskii-Moriya interaction and the four-spin exchange interaction of the Coulomb origin under the edge-sharing network of magnetic and ligand ions. This produces two successive second-order phase transitions upon cooling: an O(2) chiral spin nematic, i.e., spin cholesteric, order appears with an either parity, and then the O(2) symmetry is broken to yield a helical magnetic order. Possible candidate materials are also discussed as new multiferroic systems.     

Ultrafast spin dynamics in multisublattice magnets

We propose a general theoretical framework for ultrafast laser-induced spin dynamics in multisublattice magnets. We distinguish relaxation of relativistic and exchange origin and show that when the former dominates, nonequivalent sublattices have distinct dynamics despite their strong exchange coupling. Even more interesting, in the exchange dominated regime sublattices can show highly counterintuitive transitions between parallel and antiparallel alignment. This allows us to explain recent experiments with antiferromagnetically coupled sublattices, and predict that such transitions are possible with ferromagnetic coupling as well. In addition, we predict that exchange relaxation enhances the demagnetization speed of both sublattices only when they are antiferromagnetically coupled.