On the possible coexistence of itinerant-electron antiferromagnetism with the itinerant-electron ferromagnetism

It is shown that in an electron gas with coexisting charge density wave and spin density wave a spontaneous magnetization can occur. In the first paramagnetic region (T,T_t;Δ_s = Δ_t = 0) the spin susceptibility is independent of temperature (Pauli paramagnetism). In the second paramagnetic region (T_K<T<T_t,Δ_t≠0;Δ_s=0) the total susceptibility of the spin density wave is enhanced by a temperature dependent paramagnetic susceptibility with a Curie type law. This reflects the instability of the system to the formation of a charge density wave or to ferromagnetism. At sufficiently low temperatures (T～T_K) the spin susceptibility is anomalous great a fact which cannot be explained with the standard Fedders-Martin theory. Experimental results for pure chromium are in good agreement with our predictions.     

Coexistence of the itinerant-electron antiferromagnetism and the spin-glasses

The Born approximation has been used to calculate the phase diagram of the itinerant-electron antiferromagnet containing magnetic impurities in the spin-glass state.     

The influence of the short-range ferromagnetic order on the itinerant-electron antiferromagnetism

The short-range spin—spin impurities interaction in the itinerant-electron antiferromagnet is considered in a model with quenched impurities. In the mean field approximation the phase diagram is calculated.     

Spin glass and antiferromagnetism in Kondo-lattice disordered system

The competition between spin glass (SG), antiferromagnetism (AF) and Kondo effect is studied here in a model which consists of two Kondo sublattices with a Gaussian random interaction between spins in different sublattices with an antiferromagnetic mean J ₀ and standard deviation J. In the present approach there is no hopping of the conduction electrons between the sublattices and only spins in different sublattices can interact. The problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields which can be solved at mean field level within the static approximation and the replica symmetry ansatz. The obtained phase diagram shows the sequence of phases SG, AF and Kondo state for increasing Kondo coupling. This sequence agrees qualitatively with experimental data of the Ce₂Au_1-x Co _x Si₃ compound.Received: 9 April 2003, Published online: 9 September 2003PACS: 05.50.+q Lattice theory and statistics; Ising problems - 64.60.Cn Order disorder transformations; statistical mechanics of model systems     

Spin susceptibilities of metals in the liquid state

Conduction electron spin susceptibility values are deduced for the 18 liquid metals for which total magnetic susceptibility data are available in the literature. A “best” set of ionic susceptibility values is chosen using the criterion that the electronic susceptibilities should not show a marked systematic dependence upon atomic number for a given valency. The results strongly suggest that the spin susceptibility is quite generally 10–15% larger than that predicted by current theoretical calculations of the electron-electron enhancement in a Fermi liquid.     

A pair approximation of the spin fluctuation theory of itinerant-electron ferromagnets

We discuss a pair approximation of the spin fluctuation theory, which is an extension of the single-site theory proposed by the present author and can include the effect of the short-range magnetic order (SRMO). Numerical calculations of b.c.c. iron show that SRMO for neighbouring sites j and l, $\text{Γ≡}\text{〈m}{}_{\mathrm{j}}\text{m}{}_{\mathrm{l}}\text{〉}\text{〈m}{}^2{}_{\mathrm{j}}\text{〉}$, is about 0.21–0.14 at $\text{T}\text{T}{}_{\mathrm{c}}\text{=1.0?2.0}$. This is considerably smaller than the value (Γ～0.8) proposed by Korenman, Murray and Prange for an analysis of sloppy spin waves of iron, while it coincides with the value (Γ≌0.12–0.18) given by Shastry, Edwards and Young with the use of the spherical Heisenberg model with exchange interactions extending to far nearest neighbours.     

Spin susceptibilities for the Hubbard model in a band approach

The properties of spin excitations superposed on a uniform ground state with antiferromagnetic (or spiral) spin structure are studied in a 2D Hubbard model. Expressions are derived for the spin susceptibility in the random phase approximation (RPA) using split Hubbard bands as a zeroth approximation. The calculated collective modes with dispersion ω(Q)=c|Q−(π, π)| near Q∼(π, π) reproduce well the characteristics of the spin excitations observed in undoped cuprates. For doped systems with an antiferromagnetic structure of the ground state, calculating X″(Q,ω→0) gives the same mode with a peak at Q∼(π, π), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure, X″(Q,ω→0) peaks occur with incommensurate quasimomenta Q that are coupled to the spirality vector. Zh. éksp. Teor. Fiz. 116, 1058–1080 (September 1999)     

Coexistence of ferro- and antiferromagnetism — A spin fluctuation theory

The coexistence of and transition between ferro- and antiferromagnetism in itinerant electron system are investigated by extending the spin fluctuation theory of ferromagnetic metals by Usami and Moriya. Calculation is made on a model density of states, which simulates the one for d metals with a body centered cubic crystal structure. The result shows that the wavevector-dependent susceptibility χ_q has a two peaks at q = 0 and q = Q for the suitable choice of parameters and the coexistence is realized when the amplitude of spin fluctuation takes a proper value. Paramagnetic susceptibility of this system is also discussed.     

Spin and spin-valley Hall effects in a honeycomb lattice with antiferromagnetism and spin-orbit couplings

We study linear response to a longitudinal electric field on an antiferromagnetic honeycomb lattice with intrinsic and Rashba spin-orbit couplings (SOCs). It is found that the spin-valley Hall effect could emerge alone or coexist with the spin Hall effect. The spin and spin-valley Hall conductivities exhibit some peculiarities that depend on the distinct topological states of the graphene lattice. Furthermore, the spin and spin-valley Hall conductivities could be remarkably modulated by changing the Fermi level. Our findings suggest that the antiferromagnetic honeycomb lattice with SOCs is an excellent platform for potential applications of spintronics and valleytronics.     

Atomic diamagnetic susceptibilities from the statistical Thomas-Fermi-Amaldi theory

We report the calculation of atomic diamagnetic susceptibilities from the statistical Thomas-Fermi-Amaldi theory. The importance of the Fermi-Amaldi correction for the Thomas-Fermi theory is examined. The calculated values are compared with those obtained from self-consistent-field wave functions as well as with respect to available experimental data. Several general trends are pointed out and the advantages of the statistical procedure are stressed.     

Mode-mode coupling theory of itinerant electron antiferromagnetism in superconducting state

It has been considered since the first discovery of a high-T(c) cuprate that an antiferromagnetic (AF) state and a superconducting (SC) state are separated in it. However, it is very intriguing that the coexistence of the AF and SC states has recently been observed in HgBa(2)Ca(4)Cu(5)O(12+) (Hg-1245). Moreover, it is very novel that this coexistence of these two states appears if the SC-transition temperature T(c) is higher than the AF-transition temperature T(N). The mode-mode coupling theory can provide a clear elucidation of this novel phenomenon. A key point of this theory is that the AF susceptibility consists of the random-phase-approximation (RPA) term and the mode-mode coupling one. The RPA term works to make a positive contribution to the emergence of the antiferromagnetic critical point (AF-CP). In contrast, the mode-mode coupling term works to make a negative contribution to the emergence of the AF-CP. However, the growth of the SC-gap function in the d(x(2)-y(2))-wave SC state works to suppress the negative contribution of the mode-mode coupling term to the emergence of the AF-CP. Moreover, the effect of SC fluctuations near the SC-transition temperature T(c) suppresses the mode-mode coupling term of the AF susceptibility that works to hinder the AF ordering. For these two reasons, there is a possibility that the d(x(2)-y(2))-wave SC state is likely to promote the emergence of the AF-CP. Namely, the appearance of the above-mentioned novel coexistence of the AF and SC states observed in Hg-1245 can be explained qualitatively on the basis of this idea.     

SO(4) theory of antiferromagnetism and superconductivity in Bechgaard salts

Motivated by recent experiments with Bechgaard salts, we investigate the competition between antiferromagnetism and triplet superconductivity in quasi-one-dimensional electron systems. We unify the two orders in an SO(4) symmetric framework, demonstrating the existence of such symmetry in one-dimensional Luttinger liquids. SO(4) symmetry strongly constrains the phase diagram, leading to coexistence regions of antiferromagnetic, superconducting, and normal phases, as observed in (TMTSF)(2)PF(6). We predict a sharp neutron scattering resonance in superconducting samples.     

Edge and surface antiferromagnetism in ABO3 perovskite-type nanoparticle within the effective field theory

ABSTRACT

The edge and surface antiferromagnetic properties of the ABO₃ perovskite-type nanoparticle and their components (A, B and O) are investigated through the Effective Field Theory developed by Kaneyoshi. It is found that the Edge AntiFerroMagnetism (EAFM) causes a decrease in the Curie temperature and separation in the magnetizations. However, the Surface AntiFerroMagnetism (SAFM) causes a compensation behaviour (T_Comp?=?1.23) in the total magnetisation of the ABO₃ perovskite-type nanoparticle. The magnetisation of the central B component has been affected by the edge antiferromagnetism more strongly than by the surface antiferromagnetism. Moreover, coercivity (Hc) and remanence magnetizations (Mr) decrease for the EAFM, but they increase for the SAFM. Triple hysteresis behaviour is obtained for the SAFM. Therefore, it is suggested that there is a strong relationship between the compensation behaviour and the triple hysteresis behaviour.     

Spin fluctuation theory of ferromagnetic metals

Ferromagnetic metals at finite temperatures are discussed using the functional integral method along the lines of the unifield spin fluctuation theory of Moriya and Takahashi. We develop a simple approach which makes use of only the density of states but still takes account of the nonlocal nature of the spin fluctuations with the aid of a single-site coherent potential approximation. The effect of the charge density fluctuation is also taken into account within the saddle point approximation. The results of numerical calculations for various model densities of states including those for bcc and fcc d-metals are presented. Particularly, the Curie temperatures and the Curie-Weiss magnetic susceptibilities for Fe, Co and Ni seem to be fairly well reproduced.     

QCD susceptibilities and nuclear-matter saturation in a relativistic chiral theory

We investigate the evolutions with density of the QCD scalar susceptibility and of the sigma mass in a chiral relativistic theory of nuclear matter, in the mean-field approximation. In order to reach saturation we need to introduce the scalar response of the nucleons. The consequences are a quite mild density dependence of the sigma mass and the progressive decoupling of the quark density fluctuations from the nucleonic ones at large densities.     

Spin dynamics from time-dependent spin-density-functional theory

We derive the spin-wave dynamics of a magnetic material from the time-dependent spin-density-functional theory in the linear response regime. The equation of motion for the magnetization includes, besides the static spin stiffness, a "Berry curvature" correction and a damping term. A gradient expansion scheme based on the homogeneous spin-polarized electron gas is proposed for the latter two quantities, and the first few coefficients of the expansion are calculated to second order in the Coulomb interaction.     

Theory of magnetoelectrical effect for the itinerant-electron antiferromagnet

The microscopic theory of the magnetoelectrical effect in the itinerant-electron antiferromagnet at T ≠ 0 is presented. The magnetic moment, magnetic susceptibility and the electric polarizability have been calculated as functions of temperature.