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Correlations & Competition Between the Lattice, Electrons, & Magnetism

FWP/Project Description: 
Project Leader(s):
 
Principal Investigators:
 
Postdocs:
Pinaki Das, Benjamin Ueland, Qiang Zhang
 
Graduate Students:
Wageesha Jayasekara, Aashish Sapkota, Gregory Tucker

Research Project Overview

The properties of novel materials, such as high-temperature superconductors, charge/orbital ordering systems, and multiferroics, are all sensitively controlled by correlations and competition among the lattice, electronic, and magnetic degrees-of-freedom.  A complete understanding of the interrelations between these systems and the necessary conditions for enhancing or tailoring desirable physical properties have been identified as a Grand Challenge to the scientific community.  Neutron and x-ray scattering are powerful techniques that directly probe the structural, electronic, and magnetic aspects of complex ground states, phase transitions, and corresponding excitations. Within this FWP, the varied expertise of the PIs in different scattering methods is employed in a synergistic approach and systems are studied using a wide range of neutron and x-ray techniques.  The experimental program is supported by a closely coupled effort in ab initio band structure calculations, theoretical modeling, and scattering simulations. We also enjoy strong collaborations with many of the other FWPs in the Ames Laboratory, especially Complex States, Emergent Phenomena & Superconductivity in Intermetallic & Metal-like Compounds, Extraordinary Responsive Rare Earth Magnetic Materials, Innovative & Complex Metal-Rich Materials, and Magnetic Nanosystems: Making, Measuring, Modeling and Manipulation

Some Recent Projects

Iron Pnictides: Over the last two years, we have continued our investigations of the iron-based superconductors and our major emphasis has turned to understanding the nature of spin fluctuations and their connection with superconductivity.

  • In CaFe2As2, for example, the wide Q and high energy capabilities of ARCS at the SNS allowed us to conclusively show that the Fe moment is completely quenched in the non-superconducting collapsed tetragonal phase.
  • We have demonstrated that the onset of superconductivity in Ba(Fe1-xCox)2As2 coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe-like antiferromagnetic (AFM) order for samples in a compositional range from x = 0.04 to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. 
  • We have also investigated the dispersion of the spin resonance below Tc that appears at QAFM = (1/2 1/2 1) in the 122 iron arsenide compounds.  In the cuprates, the dispersion of the resonance is downwards towards the nodes in the d-wave superconducting gap forming the characteristic hourglass shape below Tc.  For Ba(Fe0.963Ni0.037)2As2, however, we found that the resonance disperses upwards and showed, with the assumption of an s± superconducting order parameter, that the details of the resonance’s dispersion are determined by the normal state spin fluctuations (e.g. the in-plane anisotropic magnetic correlation length). 
  • Our inelastic neutron scattering measurements on a set of co-aligned samples of antiferromagnetic LaFeAsO demonstrated that the magnetic interactions are essentially two-dimensional .  The spin-wave velocities, within the Fe layer, and the magnitude of the spin gap, are similar to the AFe2As2 based materials. However, the ratio of interlayer and intralayer exchange is found to be less than ∼10-4 in LaFeAsO, very similar to the cuprates, and ∼100 times smaller than that found in AFe2As2 compounds.

Manganese and cobalt arsenides: A closely related effort focuses on the manganese and cobalt arsenides. 

  • Low levels of K substitutions for Ba induce metallic behavior in BaMn2As2. However strong AFM ordering (TN > 500 K) remains, suggesting that charge conductivity and AFM order are independent of one another.  In recent work we have shown: (1) the local-moment AFM ordering is very robust up to at least 40% K substitution; and (2) using polarized neutron diffraction, we demonstrated that itinerant ferromagnetism coexists with the AFM order below 100 K.  These results are consistent the weak coupling described above.
  • Dilute substitutions of Co for Fe in the AFe2As2 compounds (A = Ca, Ba, Sr) destabilizes the stripe-like AFM ordering by shrinking (enlarging) the hole (electron) pockets and detuning the nesting condition. Ultimately, the suppression of stripe AFM ordering upon Co substitutions of a only few percent allows a superconducting ground state to appear in the presence of substantial spin fluctuations at QAFM. Further Co substitutions (> 12% Co) lead to a complete suppression of both stripe-like spin fluctuations and superconductivity.  We have found that, at the other end of the compositional range, SrCo2As2 is close to an instability toward stripe-like AFM order, exhibiting steeply dispersing and quasi-two-dimensional paramagnetic excitations near QAFM.  This is quite surprising for several reasons: (1) The sister compound, CaCo2As2, orders antiferromagnetically in the A-type AFM structure (ferromagnetic planes antiferromagnetically coupled along the c-axis; (2) Band-structure calculations find a large density of states at the Fermi energy that was proposed to drive a ferromagnetic instability or A-type AFM ordering; and (3) There is no clear nesting feature favoring stripe AFM order in SrCo2As2, raising the general issue of what drives the stripe-like magnetic ordering in the iron pnictides. The ACo2As2 compounds manifest other interesting behaviors as well.  For example, both BaCo2As2 and SrCo2As2 manifest negative c-axis thermal expansion coefficients, which is unusual for paramagnetic metals.

Magnetic oxides: We have grown a high quality single crystal FeV2O4 and conducted elastic and inelastic neutron scattering to determine the phase diagram of this unique spinel oxide.

  • FeV2O4 features two transition metal ions that both possess spin and orbital degrees of freedom that are strongly coupled, giving rise to unique properties that are manifested by three structural transitions of which two are accompanied by magnetic transitions.  Fe2+ occupies the diamond-like A-site in the cubic spinel structure whereas V3+ occupies the pyrochlore B-site. FeV2O4 is an excellent candidate to investigate the roles of orbital ordering at not only the B site, but also at the A site. The recent discovery of multiferroicity in FeV2O4 with a coexistence of ferroelectricity and non-collinear ferrimagnetism, in contrast to the antiferromagnetism in most of the multiferroics, further motivates us to focus on this system. FeV2O4 undergoes three transitions from the high temperature cubic phase to Tetragonal-I at TS = 140 K (due to Fe orbital ordering); Tetragonal-I to Orthorhombic at TN1 = 110 K accompanied by ferrimagnetic ordering (iron up-spin – vanadium down-spin); and Orthorhombic to Tetragonal-II accompanied by non-collinear ferrimagnetic order at TN2 = 70 K (vanadium spins canted) and the emergence of ferroelectricity.  Our neutron scattering studies elucidated the different roles of the two orbital-active Fe2+ and V3+ species in the magnetic excitations.   For example, the strong spin-orbit coupling for Fe2+ induces a significant energy gap below TN1 with little contribution from the V3+. The absence of a change in the energy gap below TN2 is evidence for either a very weak spin-orbit coupling or significantly quenched orbital moment of the V3+.

Magnetic quasicrystals and related compounds: We have started a new program to investigate magnetism in quasiperiodic crystals. 

  • In a close collaboration with the Complex States, Emergent Phenomena & Superconductivity in Intermetallic & Metal-like Compounds FWP in the Ames Laboratory we discovered a new family of local-moment bearing binary quasicrystals, i-R-Cd (R = Gd through Tm + Y).  The discovery is particularly exciting because these quasicrystals represent the compositionally simplest system for the study of the magnetic interactions in aperiodic systems. Furthermore, the existence of a corresponding set of cubic approximants, RCd6, to the icosahedral phase allows for direct comparison between the low-temperature magnetic states of crystalline and quasicrystalline phases with fundamentally similar local structures, since RCd6 may be described as a body-centered cubic packing of the same clusters of atoms as found in the newly discovered i-R-Cd icosahedral phase.
  • Using x-ray resonant magnetic scattering and neutron diffraction on 114Cd  enriched samples, we have demonstrated that the RCd6 approximants manifest long-range magnetic order at low temperatures, whereas the related icosahedral phase exhibits only spin-glass-like freezing at low temperatures .

Helpful Links

On-line Physics Publications                        Other Neutron & X-ray Sources                    
arXiv.org e-Print archvie Neutron Sciences at Oak Ridge
APS Journals NIST Center for Neutron Research
AIP Journals Lujan Neutron Scattering Center (Los Alamos)
Science Direct Journals Missouri University Research Reactor (MURR)
  National High Magnetic Field Laboratory
Useful information Advanced Photon Source
CCP14 ISIS Pulsed Neutron Source
Neutron Scattering Society of America (NSSA) Institut Laue-Langevin Neutron for Science
American Minerologist Crystal Structure Database Swiss Spallation Neutron Source (PSI)
X-ray Interactions with Matter Laboratoire Leon Brillouin (LLB)
Carlfriesite Mineral Data  
Crystallography Open Database  

Publications

2014

Kong T; Budko S L; Jesche A; McArthur J; Kreyssig A; Goldman A I; Canfield P C . 2014.Magnetic and transport properties of i-R-Cd icosahedral quasicrystals (R=Y, Gd-Tm). Physical Review B. 90:014424.

Pathak A K; Paudyal D; Jayasekara W T; Calder S; Kreyssig A; Goldman A I; Gschneidner K A; Pecharsky V K . 2014. Unexpected magnetism, Griffiths phase, and exchange bias in the mixed lanthanide Pr0.6Er0.4Al2. Physical Review B. 89:224411.

Taufour V; Foroozani N; Tanatar M A; Lim J; Kaluarachchi U; Kim S K; Liu Y; Lograsso T A; Kogan V G; Prozorov R; Bud'ko S L; Schilling J S; Canfield P C . 2014. Upper critical field of KFe2As2 under pressure: A test for the change in the superconducting gap structure. Physical Review B. 89:220509.

Tucker G S; Fernandes R M; Pratt D K; Thaler A; Ni N; Marty K; Christianson A D; Lumsden M D; Sales B C; Sefat A S; Bud'ko S L; Canfield P C; Kreyssig A; Goldman A I; McQueeney R J . 2014. Crossover from spin waves to diffusive spin excitations in underdoped Ba(Fe1-xCox)(2)As-2. Physical Review B. 89:180503.

Ueland B G; Kreyssig A; Prokes K; Lynn J W; Harriger L W; Pratt D K; Singh D K; Heitmann T W; Sauerbrei S; Saunders S M; Mun E D; Bud'ko S L; McQueeney R J; Canfield P C; Goldman A I . 2014. Fragile antiferromagnetism in the heavy-fermion compound YbBiPt. Physical Review B. 89:180403.

Weber F; Pintschovius L; Reichardt W; Heid R; Bohnen K P; Kreyssig A; Reznik D; Hradil K . 2014. Phonons and electron-phonon coupling in YNi2B2C. Physical Review B. 89:104503.

Jesche A; McCallum R W; Thimmaiah S; Jacobs J L; Taufour V; Kreyssig A; Houk R S; Bud'ko S L; Canfield P C . 2014. Giant magnetic anisotropy and tunnelling of the magnetization in Li-2(Li1-xFex)N. Nature Communications. 5:3333.

Siemons W; Beekman C; MacDougall G J; Zarestky J L; Nagler S E; Christen H M . 2014. A complete strain-temperature phase diagram for BiFeO3 films on SrTiO3 and LaAlO3 (001) substrates. Journal of Physics D-Applied Physics. 47:034011.

2013

Quirinale D G; Anand V K; Kim M G; Pandey A; Huq A; Stephens P W; Heitmann T W; Kreyssig A; McQueeney R J; Johnston D C; Goldman A I . 2013. Crystal and magnetic structure of CaCo1.86As2 studied by x-ray and neutron diffraction. Physical Review B. 88:174420.

Roy B; Pandey A; Zhang Q; Heitmann T W; Vaknin D; Johnston D C; Furukawa Y . 2013.Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4 spinel. Physical Review B. 88:174415.

Soh J H; Tucker G S; Pratt D K; Abernathy D L; Stone M B; Ran S; Bud'ko S L; Canfield P C; Kreyssig A; McQueeney R J; Goldman A I . 2013. Inelastic Neutron Scattering Study of a Nonmagnetic Collapsed Tetragonal Phase in Nonsuperconducting CaFe2As2: Evidence of the Impact of Spin Fluctuations on Superconductivity in the Iron-Arsenide Compounds. Physical Review Letters. 111:227002.

Zhang Q; Tian W; Li H F; Kim J W; Yan J Q; McCallum R W; Lograsso T A; Zarestky J L; Bud'ko S L; McQueeney R J; Vaknin D . 2013. Magnetic structures and interplay between rare-earth Ce and Fe magnetism in single-crystal CeFeAsO. Physical Review B. 88:174517.

Kogan V G . 2013. Elastic contribution to interaction of vortices in uniaxial superconductors. Physical Review B. 88:144514. 

Kreyssig A; Beutier G; Hiroto T; Kim M G; Tucker G S; De Boissieu M; Tamura R; Goldman A I . 2013. Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant. Philosophical Magazine Letters. 93:512-520.

Goldman A I; Kong T; Kreyssig A; Jesche A; Ramazanoglu M; Dennis K W; Bud'ko S L; Canfield P C . 2013. A family of binary magnetic icosahedral quasicrystals based on rare earths and cadmium. Nature Materials. 12:714-718. 

Zimmermann A S; Sondermann E; Li J Y; Vaknin D; Fiebig M . 2013. Antiferromagnetic order in Li(Ni1-xFex)PO4 (x=0.06, 0.20). Physical Review B. 88:014420.

Pandey A; Ueland B G; Yeninas S; Kreyssig A; Sapkota A; Zhao Y; Helton J S; Lynn J W; McQueeney R J; Furukawa Y; Goldman A I; Johnston D C . 2013. Coexistence of Half-Metallic Itinerant Ferromagnetism with Local-Moment Antiferromagnetism in Ba0.60K0.40Mn2As2. Physical Review Letters. 111:047001. 

Pandey A; Quirinale D G; Jayasekara W; Sapkota A; Kim M G; Dhaka R S; Lee Y; Heitmann T W; Stephens P W; Ogloblichev V; Kreyssig A; McQueeney R J; Goldman A I; Kaminski A; Harmon B N; Furukawa Y; Johnston D C . 2013. Crystallographic, electronic, thermal, and magnetic properties of single-crystal SrCo2As2. Physical Review B. 88:014526.

Kim M G; Soh J; Lang J; Dean M P M; Thaler A; Bud'ko S L; Canfield P C; Bourret-Courchesne E; Kreyssig A; Goldman A I; Birgeneau R J . 2013. Spin polarization of Ru in superconducting Ba(Fe0.795Ru0.205)(2)As-2 studied by x-ray resonant magnetic scattering. Physical Review B. 88:014424.

Hahn S E; Tucker G S; Yan J Q; Said A H; Leu B M; McCallum R W; Alp E E; Lograsso T A; McQueeney R J; Harmon B N . 2013. Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering. Journal of Applied Physics. 113:17e153.

Kim M G; Tucker G S; Pratt D K; Ran S; Thaler A; Christianson A D; Marty K; Calder S; Podlesnyak A; Bud'ko S L; Canfield P C; Kreyssig A; Goldman A I; McQueeney R J . 2013.Magnonlike Dispersion of Spin Resonance in Ni-doped BaFe2As2. Physical Review Letters. 110:177002.

Lamsal J; Tucker G S; Heitmann T W; Kreyssig A; Jesche A; Pandey A; Tian W; McQueeney R J; Johnston D C; Goldman A I . 2013. Persistence of local-moment antiferromagnetic order in Ba1-xKxMn2As2. Physical Review B. 87:144418.

Ramazanoglu M; Lamsal J; Tucker G S; Yan J Q; Calder S; Guidi T; Perring T; McCallum R W; Lograsso T A; Kreyssig A; Goldman A I; McQueeney R J . 2013. Two-dimensional magnetic interactions in LaFeAsO. Physical Review B. 87:140509.

Zhang Q; Wang W J; Kim J W; Hansen B; Ni N; Bud'ko S L; Canfield P C; McQueeney R J; Vaknin D . 2013. Magnetoelastic coupling and charge correlation lengths in a twin domain of Ba(Fe1-xCox)(2)As-2 (x=0.047): A high-resolution x-ray diffraction study. Physical Review B. 87:094510.

Hahn S E; Tucker G S; Yan J Q; Said A H; Leu B M; McCallum R W; Alp E E; Lograsso T A; McQueeney R J; Harmon B N . 2013. Magnetism-dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering. Physical Review B. 87:104518.

Zhang Q; Wang W J; Kim J W; Hansen B; Ni N; Bud'ko S L; Canfield P C; McQueeney R J; Vaknin D . 2013. Magnetoelastic coupling and charge correlation lengths in a twin domain of Ba(Fe1-xCox)(2)As-2 (x=0.047): A high-resolution x-ray diffraction study. Physical Review B. 87:094510.

Pratt D K; Chang S; Tian W; Taskin A A; Ando Y; Zarestky J L; Kreyssig A; Goldman A I; McQueeney R J . 2013. Checkerboard to stripe charge ordering transition in TbBaFe2O5. Physical Review B. 87:045127.

Previous Years

FWP Highlights: 
  • A new metallic material — based on the substitution of manganese for iron in an iron–arsenide superconductor — has been discovered in which the microscopic magnets of the electron current carriers provided by the potassium atoms all line up in the same direction at low temperatures whereas the neighboring microscopic magnets of the manganese atoms line up in opposite directions to each other.  This material, Ba0.6K0.4Mn2As2, thus exhibits a novel magnetic behavior with ferromagnetic and antiferromagnetic behavior coexisting.  Interestingly, the ordered moments in the two magnetic substructures are aligned perpendicular to each other.  Understanding what causes this unique magnetic structure may help researchers to understand the mechanism of high temperature superconductivity and to design materials for the new field of spintronics, where electron magnets are used in electronic devices for information processing rather than their charge.

  • The propagation of a novel magnetic excitation in the superconducting state, called a spin resonance, has been observed in iron arsenide superconductorsfor the first time. How the resonance disperses depends upon the direction probed within the single crystals studied. Propagation of the spin resonance reveals details about the superconducting state and highlights qualitative differences between iron arsenide and cuprate superconducting materials.  The magnetic excitation appears in the superconducting state with upwards dispersion in iron arsenide superconductors. By contrast, in cuprate superconductors the dispersion is downwards. The neutron scattering measurements designed to study the spin resonance were performed on a single crystal of a nickel-doped barium–iron–arsenide superconductor [Ba(Fe0.963Ni0.037)2As2] at the Spallation Neutron Source and the High Flux Isotope Reactor, U.S. Department of Energy user facilities.  Neutron spin resonance is considered to be a hallmark of unconventional superconductivity, thus a detailed understanding is important to future developments of superconducting materials. 

  • Researchers have discovered a new family of stable quasicrystals made from only two elements, a rare earth and cadmium.  The family includes the first magnetic binary quasicrystals.  Quasicrystals are metallic alloys that lack the periodic order seen in conventional crystals.  Instead, they exhibit aperiodic, long-range order and have “forbidden” rotational symmetries (for example, five-fold).  Most are made from three or four elements.  Binary quasicrystals are rare and sought by scientists because they offer a cleaner way to investigate the relationships between the unusual structures of quasicrystals and their properties.  For this family, there also are crystalline approximants — materials that are periodic but with atomic decorations similar to those of quasicrystals — that are close in composition allowing comparisons with related crystalline materials.  The Department of Energy’s Advanced Photon Source was used to confirm their structure.  The discovery of this family enables researchers to study the nature of magnetic interactions in quasicrystals.  Thus far, studies show the magnetic behavior of these quasicrystals and related approximants is vastly different.