Quantum critical point and spin fluctuations in lower-mantle ferropericlase

Abstract

Ferropericlase [(Mg,Fe)O] is one of the most abundant minerals of the earth's lower mantle. The high-spin (HS) to low-spin (LS) transition in the Fe 2+ ions may dramatically alter the physical and chemical properties of (Mg,Fe)O in the deep mantle. To understand the effects of compression on the ground electronic state of iron, electronic and magnetic states of Fe 2+ in (Mg 0.75 Fe 0.25 )O have been investigated using transmission and synchrotron Mössbauer spectroscopy at high pressures and low temperatures (down to 5 K). Our results show that the ground electronic state of Fe 2+ at the critical pressure P c of the spin transition close to T = 0 is governed by a quantum critical point (T = 0, P = P c ) at which the energy required for the fluctuation between HS and LS states is zero. Analysis of the data gives P c = 55 GPa. Thermal excitation within the HS or LS states (T > 0 K) is expected to strongly influence the magnetic as well as physical properties of ferropericlase. Multielectron theoretical calculations show that the existence of the quantum critical point at temperatures approaching zero affects not only physical properties of ferropericlase at low temperatures but also its properties at P-T of the earth's lower mantle. , which has a 79% abundance (1-6). Great interest recently has been focused on the pressure-induced electronic transition in (Mg,Fe)O, in which the Fe 2+ ions transform from the high-spin (HS) state [total spin momentum (S) = 2] to the low-spin (LS) state (S = 0) (7-14). A series of observed physical and chemical properties of (Mg,Fe)O have been altered dramatically by the spin transition in the relevant range of pressure-temperature (P-T) conditions of the deep earth's mantle, including thermal (15, 16) and electrical (17) conductivity, density (18,19), incompressibility (18), and sound velocity (20). Most previous investigations on the spin transition of iron in ferropericlase were performed at either room temperature or high temperatures. At pressures near the spin cross-over, the Fe ions may exist in a mixed condition, with HS and LS states coexisting as the result of thermal fluctuations. This mixed state results in a wide cross-over with a smooth, continuous transition from the HS to the LS state with a pressure interval (ΔP) of ∼20 GPa (9, 14). Studies on the electronic and magnetic states at low temperatures may provide crucial information on the ground state of the Fe ions in ferropericlase at high pressures (21).We have investigated electronic and magnetic properties of Fe 2+ in two representative compositions of ferropericlase (Mg 1-x Fe x )O (x = 0.25 and 0.2) at high pressures and low temperatures using transmission Mössbauer spectroscopy (TMS) and synchrotron Mössbauer spectroscopy (also called nuclear forward scattering, NFS) in diamond anvil cells (DACs) up to 90 GPa. Hyperfine parameters of iron ions derived from the Mössbauer spectra are used to construct the magnetic phase diagram. We find quantum critical point in the phase diagram at 55 GPa and low temperatures wher...

Spin crossovers in Mott–Hubbard insulators at high pressures

“…From the experimental data for Mg 1 À x Fe x O, T N ¼37 K at P ¼0 and T N ¼ 55 K at P¼55 GPa [12]. Thus, 2a t ¼t O /0.0122 1/GPa is very close to the value 2a t t 0 ¼0.0121 1/GPa found for FeBO 3 from the high pressure experiments [14].…”

“…The baric dependence of a spin gap is a D ¼7.8 meV/GPa [12]. Numerical solution of this equation results in a first order quantum phase transition shown in the Fig.…”

“…An interesting example of the HS-LS transition was recently observed in ferropericlase (Mg,Fe)O [12] which is one of the most abundant minerals of the Earth's lower mantle. The high pressure and low temperature investigations by TMS and SMS Mössbauer spectroscopy allowed to establish a fundamental understanding of the ground electronic and magnetic states in (Mg 0.75 ,Fe 0.25 )O [12].…”

“…Pressure dependence of the Neel temperature in the antiferromagnet (Mg 0.75 Fe 0.25 )O with spin-crossover at P c ¼ 55 GPa. Non-self consistent (red crosses) and self consistent (empty blue circles) calculated data are compared with experimental data (black triangles) [12]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.…”

Reduced radiative conductivity of low spin FeO6-octahedra in FeCO3 at high pressure and temperature

“…Theoretical computations indicate that the fraction of HS states is small (<0.5) at the base of the lower mantle (Tsuchiya et al, 2006;Holmstrom and Stixrude, 2015;Lyubutin et al, 2013;Sturhahn et al, 2005). If such, the temperature-induced broadening and intensification of the 1 A 1g → 1 T 1g band as well as the red-shift of the AE (∼1 cm −1 /K) observed in this study will diminish the ability of LS FP to conduct heat by radiation at CMB conditions.…”