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Resonant magnetic exciton mode in the heavy-fermion antiferromagnet CeB6  [PDF]
G. Friemel,Yuan Li,A. V. Dukhnenko,N. Yu. Shitsevalova,N. E. Sluchanko,A. Ivanov,V. B. Filipov,B. Keimer,D. S. Inosov
Physics , 2011, DOI: 10.1038/ncomms1821
Abstract: Resonant magnetic excitations are widely recognized as hallmarks of unconventional superconductivity in copper oxides, iron pnictides, and heavy-fermion compounds. Numerous model calculations have related these modes to the microscopic properties of the pair wave function, but the mechanisms underlying their formation are still debated. Here we report the discovery of a similar resonant mode in the non-superconducting, antiferromagnetically ordered heavy-fermion metal CeB6. Unlike conventional magnons, the mode is non-dispersive, and its intensity is sharply concentrated around a wave vector separate from those characterizing the antiferromagnetic order. The magnetic intensity distribution rather suggests that the mode is associated with a coexisting order parameter of the unusual antiferro-quadrupolar phase of CeB6, which has long remained "hidden" to the neutron-scattering probes. The mode energy increases continuously below the onset temperature for antiferromagnetism, in parallel to the opening of a nearly isotropic spin gap throughout the Brillouin zone. These attributes bear strong similarity to those of the resonant modes observed in unconventional superconductors below their critical temperatures. This unexpected commonality between the two disparate ground states indicates the dominance of itinerant spin dynamics in the ordered low-temperature phases of CeB6 and throws new light on the interplay between antiferromagnetism, superconductivity, and "hidden" order parameters in correlated-electron materials.
Competition of ferromagnetic and antiferromagnetic spin ordering in nuclear matter  [PDF]
A. A. Isayev
Physics , 2003, DOI: 10.1134/1.1577751
Abstract: In the framework of a Fermi liquid theory it is considered the possibility of ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear matter with Skyrme effective interaction. The zero temperature dependence of ferromagnetic and antiferromagnetic spin polarization parameters as functions of density is found for SkM$^*$, SGII effective forces. It is shown that in the density domain, where both type of solutions of self--consistent equations exist, ferromagnetic spin state is more preferable than antiferromagnetic one.
Plateau of the Magnetization Curve of the S=1/2 Ferromagnetic-Ferromagnetic-Antiferromagnetic Spin Chain  [PDF]
Kiyomi Okamoto
Physics , 1995, DOI: 10.1016/0038-1098(96)00007-5
Abstract: I analytically study the plateau of the magnetization curve at $M/M_{\rm S} = 1/3$ (where $M_{\rm S}$ is the saturation magnetization) of the one-dimensional $S=1/2$ trimerized Heisenberg spin system with ferromagnetic ($J_{\rm F}$)-ferromagnetic ($J_{\rm F}$)-antiferromagnetic ($J_{\rm A}$) interactions at $T=0$. I use the bosonization technique for the fermion representation of the spin Hamiltonian through the Jordan-Wigner transformation. The plateau appears when $\gamma \equiv J_{\rm F}/J_{\rm A} \allowbreak < \gamma_{\rm C}$, and vanishes when $\gamma > \gamma_{\rm C}$, where the critical value $\gamma_{\rm C}$ is estimated as $\gamma_{\rm C} = 5 \sim 6$. The behavior of the width of the plateau near $\gamma_{\rm C}$ is of the Kosterlitz-Thouless type. The present theory well explains the numerical result by Hida.
Spin Exciton Formation inside the Hidden Order Phase of CeB6  [PDF]
Alireza Akbari,Peter Thalmeier
Physics , 2012, DOI: 10.1103/PhysRevLett.108.146403
Abstract: The heavy fermion metal CeB6 exhibits hidden order of antiferroquadrupolar (AFQ) type below T_Q=3.2K and subsequent antiferromagnetic (AFM) order at T_N=2.3K. It was interpreted as ordering of the quadrupole and dipole moments of a $\Gamma_8$ quartet of localised Ce $4f^1$ electrons. This established picture has been profoundly shaken by recent inelastic neutron scattering (G. Friemel et al., arXiv:1111.4151) that found the evolution of a feedback spin exciton resonance within the hidden order phase at the AFQ wave vector which is stabilized by the AFM order. We develop an alternative theory based on a fourfold degenerate Anderson lattice model, including both order parameters as particle-hole condensates of itinerant heavy quasiparticles. This explains in a natural way the appearance of the spin exciton resonance and the momentum dependence of its spectral weight, in particular around the AFQ vector and its rapid disappearance in the disordered phase. Analogies to the feedback effect in unconventional heavy fermion superconductors are pointed out.
Inelastic scattering in ferromagnetic and antiferromagnetic metal spintronics  [PDF]
R. A. Duine,P. M. Haney,A. S. Nunez,A. H. MacDonald
Physics , 2006, DOI: 10.1103/PhysRevB.75.014433
Abstract: We use a ferromagnetic voltage probe model to study the influence of inelastic scattering on giant magnetoresistance and current-induced torques in ferromagnetic and antiferromagnetic metal spin valves. The model is based on the Green's function formulation of transport theory and represents spin-dependent and spin-independent inelastic scatterers by interior voltage probes that are constrained to carry respectively no charge current and no spin or charge current. We find that giant magnetoresistance and spin transfer torques in ferromagnetic metal spin valve structures survive arbitrarily strong spin-independent inelastic scattering, while the recently predicted analogous phenomena in antiferromagnetic metal spin valves are partially suppressed. We use toy-model numerical calculations to estimate spacer layer thickness requirements for room temperature operation of antiferromagnetic metal spin valves.
High frequency study of the orbital ordering resonance in the strongly correlated heavy fermion metal CeB6  [PDF]
S. V. Demishev,A. V. Semeno,H. Ohta,S. Okubo,Yu. B. Paderno,N. Yu. Shitsevalova,N. E. Sluchanko
Physics , 2006,
Abstract: We report results of the study of the recently discovered magnetic resonance in the orbitally ordered phase of CeB6 (the orbital ordering resonance) in a wide frequency range 44-360 GHz. It is found that the g-factor for this resonance increases with frequency from g(44 GHz)~1.55 to g(>250 GHz)~1.7. In addition to the orbital ordering resonance for the frequencies exceeding 200 GHz a new magnetic resonance with the g-factor 1.2-1.3 is detected.
Exchange Bias in Ferromagnetic/Compensated Antiferromagnetic Bilayers  [PDF]
D. S. Deng,X. F. Jin,Ruibao Tao
Physics , 2001, DOI: 10.1103/PhysRevB.65.172402
Abstract: By means of micromagnetic spin dynamics calculations, a quantitative calculation is carried out to explore the mechanism of exchange bias (EB) in ferromagnetic (FM)/compensated antiferromagnetic (AFM) bilayers. The antiferromagnets with low and high Neel temperatures have been both considered, and the crossover from negative to positive EB is found only in the case with low Neel temperature. We propose that the mechanism of EB in FM/compensated AFM bilayers is due to the symmetry broken of AFM that yields some net ferromagnetic components.
Delta chain with ferromagnetic and antiferromagnetic interactions at the critical point  [PDF]
V. Ya. Krivnov,D. V. Dmitriev,S. Nishimoto,S. -L. Drechsler,J. Richter
Physics , 2014, DOI: 10.1103/PhysRevB.90.014441
Abstract: We investigate the spin-1/2 Heisenberg model on the delta chain (sawtooth chain) with ferromagnetic nearest-neighbor and antiferromagnetic next-neighbor interactions. For a special ratio between these interactions there is a class of exact ground states formed by localized magnons and the ground state is macroscopically degenerate with a large residual entropy per spin $s_0=\frac{1}{2}\ln 2$. An important feature of this model is a sharp decrease of the gaps for excited states with an increase of the number of magnons. These excitations give an essential contribution to the low-temperature thermodynamics. The behavior of the considered model is compared with that of the delta chain with both antiferromagnetic interactions.
On the probability of ferromagnetic strings in antiferromagnetic spin chains  [PDF]
Alexander G. Abanov,Vladimir E. Korepin
Physics , 2002, DOI: 10.1016/S0550-3213(02)00899-4
Abstract: We study the probability of formation of ferromagnetic string in the antiferromagnetic spin-1/2 XXZ chain. We show that in the limit of long strings with weak magnetization per site the bosonization technique can be used to address the problem. At zero temperature the obtained probability is Gaussian as a function of the length of the string. At finite but low temperature there is a crossover from the Gaussian behavior at intermediate lengths of strings to the exponential decay for very long strings. Although the weak magnetization per site is a necessary small parameter justifying our results, the extrapolation of obtained results to the case of maximally ferromagnetic strings is in qualitative agreement with known numerics and exact results. The effect of an external magnetic field on the probability of formation of ferromagnetic strings is also studied.
Local Density of States in the Antiferromagnetic and Ferromagnetic Kondo Models  [PDF]
P. Sinjukow,D. Meyer,W. Nolting
Statistics , 2002, DOI: 10.1002/1521-3951(200210)233:3<536::AID-PSSB536>3.0.CO;2-F
Abstract: Based on a simple approximation scheme we have computed the local density of states (LDOS) of the antiferromagnetic and ferromagnetic Kondo models for the full range of band occupations and coupling strengths. For both models the LDOS with its full energy dependence has not been calculated before. Arguments are given for the results to be qualitatively trustworthy despite the simplicity of the approximation scheme.
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