Spin Fluctuations and Anharmonicity in Itinerant Electron Magnetism

Abstract

The paper overviews recent investigations of thermodynamics and magnetic dynamics of itinerant electron magnets with strongly coupled (anharmonic) spin fluctuations (SF). A novel classification scheme for SF, dependent on their spatial dispersion and quantum effects is presented, including the generalized Fermi liquid (FL), soft-mode (SM), and localized moments (LM) regimes. It is shown that the conventional SCR theory of Moriya holds only in the weak coupling limit of the SM regime and cannot be applied to real magnets where spin anharmonicity induced by zero-point SF is expected to be strong. To account for the effects of strong spin anharmonicity, a variational SM theory of SF is presented. These effects essentially influence thermodynamics of itinerant magnets and affect the criterion for magnetic instabilities, equations of state, low temperature specific heat, non-FL and quantum critical behavior, etc. Nonlinear magnetic dynamics of itinerant magnets is analyzed based on the phenomenological spin-invariant dynamical equations both in the spin wave and relaxational regimes. Macroscopic magnetic dynamics and different mechanisms of magnon damping in the spin wave regime are shown to be essentially different from the results arising for the Heisenberg magnets. The relaxational regime may be strongly influenced by nonlinear effects of mode-mode couplings. The spectrum of overdamped longitudinal SF is shown to be radically changed due to their scattering by magnons, which may result in a quasielasic peak in the SF spectrum and inelastic SF near the magnon frequencies, depending on nonlinear coupling. The quasielastic SF are suppressed at low temperatures and develop at elevated temperatures, dominating near the Curie point. The results are applied to the magnetoresistive manganites.

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

“…In weak itinerant magnets SF were shown to have a paramagnon-like nature and arise in the electron-hole continuum due to the Landau damping mechanism [1,2]. This quasielastic type of SF was shown to describe well the properties of weak itinerant magnets with both weak [1] and strong [3] spin anharmonicity and was recognized as a driving force of their magnetic phase transitions.The linear Landau magnetic relaxation mechanism was shown to dominate in the low-temperature region [3][4][5]. With the rise in temperature non-linear relaxation processes due to coupling of SF may strongly affect their properties and change their linear nature to the non-linear one.…”

Phenomenological model of longitudinal spin fluctuations in itinerant antiferromagnets

“…Magnetic dynamics and SF in itinerant magnets play a decisive role in many technically important applications including Invar alloys [2], colossal magnetoresistive materials [3], high temperature superconductors [4], and newly discovered iron pnictide superconductors [5]. Whereas dynamics of transverse SF in itinerant magnets is well understood theoretically, the properties of longitudinal SF are still a puzzling point in the physics of magnetism (for a review see [6]). Contrary to dynamics of transverse fluctuations represented by the near-precession motion of magnetization density, linear dynamics of longitudinal SF in itinerant ferromagnets is thought to be dominated by Landau damping in the electron-hole continuum.…”

“…Non-linear effects of the coupling of transverse and longitudinal SF result in additional channels of magnetic relaxation for both transverse and longitudinal SF. In itinerant ferromagnets, this leads to a novel mechanism of magnon damping caused by three-mode scattering processes of emission (absorption) of a longitudinal SF by a magnon [7], and it dominates over all other mechanisms of magnon damping specific for itinerant magnets [6,8]. In both Heisenberg and itinerant ferromagnets, coupling of longitudinal and transverse modes essentially affects quasielasic longitudinal SF and gives rise to inelastic longitudinal SF near the magnon frequencies [9][10][11], which is still under debate both theoretically [10][11][12][13][14] and experimentally (see, e.g., [15]).…”

Magnetism and anomalous properties of plutonium

“…Spin fluctuations are customarily characterized by the amplitude of the fluctuating magnetization density m(k, ω) = χ(k, ω)B(k, ω), where ω, k are the frequency and the wave vector of the fluctuations arising as a response of the system to a random magnetic field B(k, ω). Their spectrum is characterized by the dynamic magnetic susceptibility χ(k, ω), whose form is determined by the equations of motion of the Fermi-liquid of strongly correlated electrons and the crystal lattice [17]. To illustrate the effects of the dynamic spin fluctuations, we shall assume that in plutonium, because of the comparatively narrow band of the 5ƒ electrons, the spatial dispersion of the dynamic magnetic susceptibility is comparatively weak.…”

“…According to the soft-mode theory [17], assuming spin anharmonicity to be weak we have (9) where F 0 (ω) = k B Tln[1 -exp(−˙ω/k B T)] + ˙ω/2 is the energy of a harmonic oscillator. From Eq.…”