Spin-State Transition in the Fe Pnictides

Abstract

We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca1−xRExFe2As2 (RE=La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ∼0.9µB at T = 300 K to ∼0.45µB at T = 70 K. In the collapsed tetragonal (cT) phase of Nd-and Pr-doped samples (T<70K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca1−xRExFe2As2 (RE= Pr and Nd) exhibits a spin-state transition and provide direct evidence for a non-magnetic Fe 2+ ion in the cT-phase, as predicted by Yildirim. We argue that the gradual change of the the spin-state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.PACS numbers: 74.70. Xa, 75.20.Hr, 78.70.En, 75.30.Wx The interesting orbital physics found in many 3d and 4d transition metal compounds, such as manganites [1,2] and ruthenates [2], seems to play an important role in the iron based superconductors as well [3][4][5][6][7][8][9]. In the iron pnictides, many low-energy probes such as transport [10], scanning tunnelling microscopy [11], inelastic neutron scattering [12], angle-resolved photoemission spectroscopy [13,14], and most recently magnetic torque measurements [15] have reported a strong in-plane anisotropy of electronic properties. These results have spurred a great deal of interest in the orbital physics of the iron pnictides, in particular the possibility of orbital order [3][4][5][6][7][8][9].An important aspect of the orbital physics is the competition between the Hund's rule coupling constant J H and the crystal field splitting, ∆ CF . In the case of LaCoO 3 , the energy scales of ∆ CF and J H are similar, resulting in spin-state transition; Co 3+ ions take on a low-spin state (S=0) at low temperature, but go into thermally excited high/intermediate-spin (S=2 or S=1) states at elevated temperature [16,17] Among the iron based superconductors, CaFe 2 As 2 offers perhaps the best system to investigate the competition between ∆ CF [20] and J H , and its effect on the spin-state. Like many iron pnictides, CaFe 2 As 2 goes from a high temperature tetragonal phase (T-phase) to an orthorhombic and antiferromagnetically (AFM) ordered phase, below T N ≈ 170 K [21]. More importantly, CaFe 2 As 2 takes on yet another structural phase at low temperatures through application of a modest pressure of 0.35 GPa [22] or chemical doping, with rare-earths [23] or phosphorus [24]. Upon entering this phase, known as the collapsed tetragonal phase (cT-phase), the lattice undergoes a ∼ 10% reduction along the c-axis and an ∼2% increase along the a-axis. This is accompanied by a disappearance of the AFM order [22], supression of spin fluctuations [25], and recovery of Fermi liquid behavior [24]. It is thus clear that an unusually dramatic lattice i...

Magnetic Field Effects in a Correlated Electron System with Spin-State Degree of Freedom — Implications for an Excitonic Insulator —

“…The rich magnetic field effects are owing to the combinations of the LS and HS states in the wave function and in the real space configuration. The present results provide a possible interpretation for the recently observed H-induced phases in LaCoO 3 . This study suggests that the correlated electron systems with SSDF is a quarry for novel quantum phenomena induced by the strong magnetic field, similar to the frustrated magnets.…”

“…There are multiple spin states in a single magnetic ion due to the different local electron configurations. Transitions between the multiple spin states are often seen in the iron and cobalt ions which are included not only in correlated electron materials, [1][2][3] but also in biomaterials, 4) and earth innercore materials. [5][6][7] A driving force of the spin-state transition is attributed to a competition between the crystalline-field effect and the Hund's coupling, which stabilize the low-spin (LS) and high-spin (HS) states, respectively.…”

Evidence of Mott physics in iron pnictides from x-ray spectroscopy

“…In both cases a clear increase of the IAD value with hole-doping is observed, although the trend is somewhat smaller for the K-doped samples. If the FeCrAs spectrum is used as the reference for the IAD analysis as done in [22,23], the lineshape of the difference spectra for both the K-and Cr-doped compounds has the typical reported [22][23][24][25][26] shape showing a bump in the energy position of the Kβ' satellite and the downward-upward feature around the Kβ 1,3 peak. Interestingly, when using the parent compound BaFe 2 As 2 as the reference instead, more structure is revealed.…”

Optical spectroscopy study of the collapsed tetragonal phase ofCaFe2(As0.935P0.065)2singl

“…Magnetic and electronic structures are found to be strongly influenced by the transition. In the cT phase, the Fe local moment is quenched 27,28 , the AFM order or magnetic fluctuation disappears [29][30][31] and the standard Fermi liquid behavior recovers. 16,23 In addition, in sharp contrast to the paramagnetism in the T phase, intrinsic superconductivity is absent in the cT phase.…”