Daniel Haskel - Academia.edu (original) (raw)

Papers by Daniel Haskel

Physical Review Letters, 2006

A Comment on the Letter by D. Haskel et al., Phys. Rev. Lett. 95, 217207 (2005)PRLTAO0031-900710.... more A Comment on the Letter by D. Haskel et al., Phys. Rev. Lett. 95, 217207 (2005)PRLTAO0031-900710.1103/PhysRevLett.95.217207. The authors of the Letter offer a Reply.

Physical Review Letters, Nov 1, 2010

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIr... more The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ˜1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L2,3 absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

The strong local-moment magnetism in divalent Eu metal (4f^7) suppresses superconductivity. At ex... more The strong local-moment magnetism in divalent Eu metal (4f^7) suppresses superconductivity. At extreme pressure Eu is expected to become trivalent and nonmagnetic (4f^6). Recently, superconductivity in Eu was discovered by Debessai et al. for pressures above 80 GPa [1]. However, Eu's transition temperature lies near 2 K, nearly an order of magnitude lower than for comparable trivalent d-electron metals, possibly because Eu is not fully trivalent but rather mixed valent. Here we report recent experimental results on Eu's valence and magnetic state to Mbar pressures through synchrotron x-ray absorption near edge structure (XANES), synchrotron x-ray magnetic circular dichroism (XMCD), and synchrotron M"ossbauer spectroscopy (SMS).[4pt] [1] M. Debessai, T. Matsuoka, J. J. Hamlin, J. S. Schilling, and K. Shimizu, Phys. Rev. Lett. 102, 197002 (2009).

Iron in concentrations as small as 4 at. % (where the distance between Fe atoms is more than thre... more Iron in concentrations as small as 4 at. % (where the distance between Fe atoms is more than three lattice spacings) produces ferromagnetic behavior at temperatures up to 50 K above the Gd Curie temperature TC in Gd100-xFex nanostructures. X-ray diffraction and XAFS show that Gd100-xFex nanostructures made by inert-gas-condensation and melt-spinning have nanoscale hcp Gd grains with Gd-Fe grain

Using element-specific x-ray resonance techniques, we have investigated the interfacial magnetic ... more Using element-specific x-ray resonance techniques, we have investigated the interfacial magnetic coupling in SrRuO3/SrMnO3 superlattices. A strong out-of-plane SRO anisotropy coupled with AFM Ru-Mn interactions result in a canted Mn structure with a significant induced net Mn moment that reduces to zero under a strong magnetic field. At T > TC SRO, the SRO anisotropy is removed and the planar

ABSTRACT The appearance of magnetism in otherwise non-magnetic materials has recently been report... more ABSTRACT The appearance of magnetism in otherwise non-magnetic materials has recently been reported for a number of nanoscale materials. Coupled with the size-dependent optical and electronic properties of the nanocrystalline materials, this magnetic behavior opens the possibility for an extended range of technological applications. As such, identifying the origin of the magnetism is an extremely important goal, yet this remains the subject of some controversy in the literature. We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the particle size and surface ligands. Using SQUID magnetometry and x-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand endgroup functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed.

Physical Review B, 2014

A combination of x-ray absorption, x-ray-diffraction, and transport measurements at high pressure... more A combination of x-ray absorption, x-ray-diffraction, and transport measurements at high pressure is used to investigate the interplay between the electronic properties of Ir 5d states and lattice degrees of freedom in the weakly ferromagnetic insulator BaIrO 3 . Although the Ir 5d local magnetic moment is highly stable against lattice compression, remaining nearly unperturbed to at least 30 GPa, the weak ferromagnetism (net ordered moment) is quickly quenched by 4.5 GPa (3% volume reduction). Under chemical pressure, where Sr is substituted for the larger Ba in BaIrO 3 , the local magnetic moment on Ir remains stable, but the weak ferromagnetism is quenched after only 1.7% volume reduction. The magnetic ordering temperature T m is also more strongly suppressed by chemical pressure compared to physical pressure. In addition, under ∼23-at. % Sr doping, BaIrO 3 undergoes a transition to a paramagnetic metallic state. Resistivity measurements indicate that BaIrO 3 remains an electrical insulator to at least 9 GPa, a much higher pressure than required to quench the weak ferromagnetism (∼4.5 GPa). Such a disparate response of transport and magnetic properties to chemical and physical pressure is likely rooted in the different compression rates of the (a,c) lattice parameters with Sr doping and applied pressure and the effect of related lattice distortions on electronic bandwidth and exchange interactions in this strongly spin-orbit-coupled system.

Advanced Functional Materials, 2014

Physical Review Letters - PHYS REV LETT, 2007

We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5... more We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5} edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu{sub 0.95}FeSb is a ferrimagnet in which the Fe 3d moment and the Eu{sup 2+} 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L edge XAS, we find that about 13% of the Eu have a formal valence of 3{sup +}. We ascribe the origin of ferrimagnetism at a relatively high transition temperature T{sub c} of 85 K in Eu{sub 0.95}FeSb to f-electron interaction with the nearly ferromagnetic [FeSb]{sup 2.2-} host lattice.

Codoping with Co has been shown to promote successful dilution of magnetic dopants of Mn and Fe i... more Codoping with Co has been shown to promote successful dilution of magnetic dopants of Mn and Fe into Ge up to a combined doping level of 15 at. %. Local structure, magnetic moment, and spin-dependant states of these diluted magnetic semiconductors (DMS) have been probed as a function of composition using X-ray and tunneling spectroscopy techniques. Extended X-ray Absorption Fine Structure (EXAFS) measurements reveal an increased level of substitution for each dopant species with respect to traditional single dopant DMS materials. A codoped environment results in 70% of Mn and 90% of Co successfully incorporating substitutionally into the Ge lattice. The remaining dopants are seen to form interstitial dimmers and trimers rather than metallic precipitates. Tunneling spectroscopy measurements on these systems through Al2O3 barriers using both Nb and Fe as spin detectors will be discussed in their correlation with the structural results.

Physical Review Letters, 2007

We combine x-ray magnetic circular dichroism spectroscopy at Fe L 2;3 edges, at Eu M 4;5 edges, x... more We combine x-ray magnetic circular dichroism spectroscopy at Fe L 2;3 edges, at Eu M 4;5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu 0:95 Fe 4 Sb 12 is a ferrimagnet in which the Fe 3d moment and the Eu 2 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L 3 edge XAS, we find that about 13% of the Eu have a formal valence of 3 . We ascribe the origin of ferrimagnetism at a relatively high transition temperature T C of 85 K in Eu 0:95 Fe 4 Sb 12 to f-electron interaction with the nearly ferromagnetic Fe 4 Sb 12 2:2ÿ host lattice.

Physical Review Letters, 2010

The electronic structure and magnetism of Ir 5d 5 states in nonmetallic, weakly ferromagnetic BaI... more The electronic structure and magnetism of Ir 5d 5 states in nonmetallic, weakly ferromagnetic BaIrO 3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be $1:5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L 2;3 absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO 3 . The local moment behavior persists even as the metallicparamagnetic phase boundary is approached with Sr doping or applied pressure.

Physical Review Letters, 2008

Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that a... more Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that are element specific, we have elucidated the role of Ho 3+ in multiferroic HoMnO3. In zero field, Ho 3+ orders antiferromagnetically with moments aligned along the hexagonal c direction below 40 K, and undergoes a transition to another magnetic structure below 4.5 K. In applied electric fields of up to 1 × 10 7 V/m, the magnetic structure of Ho 3+ remains unchanged.

Physical Review Letters, 2012

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr 2 IrO 4 at am... more X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr 2 IrO 4 at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit J eff ¼ 1 2 state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split J eff ¼ 1 2 , 3 2 states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of 17GPa,wherethe′′weak′′ferromagneticmomentisquencheddespitetransportmeasurementsshowinginsulatingbehaviortoatleast40GPa.ThemagnetictransitionhasimplicationsfortheoriginoftheinsulatinggapandthenatureofexchangeinteractionsinthisSOcoupledsystem.TheexpectationvalueoftheangularpartoftheSOinteraction,hLAˊSi,extrapolatestozeroat17 GPa, where the ''weak'' ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, hL Á Si, extrapolates to zero at 17GPa,wherethe′′weak′′ferromagneticmomentisquencheddespitetransportmeasurementsshowinginsulatingbehaviortoatleast40GPa.ThemagnetictransitionhasimplicationsfortheoriginoftheinsulatinggapandthenatureofexchangeinteractionsinthisSOcoupledsystem.TheexpectationvalueoftheangularpartoftheSOinteraction,hLAˊSi,extrapolatestozeroat80-90 GPa where an increased bandwidth strongly mixes J eff ¼ 1 2 , 3 2 states and SO interactions no longer dominate the electronic ground state of Sr 2 IrO 4 .

Physical Review Letters, 2008

Fe K-edge x-ray magnetic circular dichroism of magnetite (Fe 3 O 4 ) powders was measured with sy... more Fe K-edge x-ray magnetic circular dichroism of magnetite (Fe 3 O 4 ) powders was measured with synchrotron radiation under variable pressure and temperature conditions in diamond anvil cell. The magnetic dichroism was observed to decrease discontinuously by 50% between 12 and 16 GPa, independent of temperature. The magnetic transition is attributed to a high-spin to intermediate-spin transition of Fe 2 ions in the octahedral sites and could account for previously observed structural and electrical anomalies in magnetite at this pressure range. The interpretation of x-ray magnetic circular dichroism data is supported by x-ray emission spectroscopy and theoretical cluster calculations.

Physical Review Letters, 2006

A Comment on the Letter by D. Haskel et al., Phys. Rev. Lett. 95, 217207 (2005)PRLTAO0031-900710.... more A Comment on the Letter by D. Haskel et al., Phys. Rev. Lett. 95, 217207 (2005)PRLTAO0031-900710.1103/PhysRevLett.95.217207. The authors of the Letter offer a Reply.

Physical Review Letters, Nov 1, 2010

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIr... more The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ˜1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L2,3 absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

The strong local-moment magnetism in divalent Eu metal (4f^7) suppresses superconductivity. At ex... more The strong local-moment magnetism in divalent Eu metal (4f^7) suppresses superconductivity. At extreme pressure Eu is expected to become trivalent and nonmagnetic (4f^6). Recently, superconductivity in Eu was discovered by Debessai et al. for pressures above 80 GPa [1]. However, Eu's transition temperature lies near 2 K, nearly an order of magnitude lower than for comparable trivalent d-electron metals, possibly because Eu is not fully trivalent but rather mixed valent. Here we report recent experimental results on Eu's valence and magnetic state to Mbar pressures through synchrotron x-ray absorption near edge structure (XANES), synchrotron x-ray magnetic circular dichroism (XMCD), and synchrotron M"ossbauer spectroscopy (SMS).[4pt] [1] M. Debessai, T. Matsuoka, J. J. Hamlin, J. S. Schilling, and K. Shimizu, Phys. Rev. Lett. 102, 197002 (2009).

Iron in concentrations as small as 4 at. % (where the distance between Fe atoms is more than thre... more Iron in concentrations as small as 4 at. % (where the distance between Fe atoms is more than three lattice spacings) produces ferromagnetic behavior at temperatures up to 50 K above the Gd Curie temperature TC in Gd100-xFex nanostructures. X-ray diffraction and XAFS show that Gd100-xFex nanostructures made by inert-gas-condensation and melt-spinning have nanoscale hcp Gd grains with Gd-Fe grain

Using element-specific x-ray resonance techniques, we have investigated the interfacial magnetic ... more Using element-specific x-ray resonance techniques, we have investigated the interfacial magnetic coupling in SrRuO3/SrMnO3 superlattices. A strong out-of-plane SRO anisotropy coupled with AFM Ru-Mn interactions result in a canted Mn structure with a significant induced net Mn moment that reduces to zero under a strong magnetic field. At T > TC SRO, the SRO anisotropy is removed and the planar

ABSTRACT The appearance of magnetism in otherwise non-magnetic materials has recently been report... more ABSTRACT The appearance of magnetism in otherwise non-magnetic materials has recently been reported for a number of nanoscale materials. Coupled with the size-dependent optical and electronic properties of the nanocrystalline materials, this magnetic behavior opens the possibility for an extended range of technological applications. As such, identifying the origin of the magnetism is an extremely important goal, yet this remains the subject of some controversy in the literature. We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the particle size and surface ligands. Using SQUID magnetometry and x-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand endgroup functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed.

Physical Review B, 2014

A combination of x-ray absorption, x-ray-diffraction, and transport measurements at high pressure... more A combination of x-ray absorption, x-ray-diffraction, and transport measurements at high pressure is used to investigate the interplay between the electronic properties of Ir 5d states and lattice degrees of freedom in the weakly ferromagnetic insulator BaIrO 3 . Although the Ir 5d local magnetic moment is highly stable against lattice compression, remaining nearly unperturbed to at least 30 GPa, the weak ferromagnetism (net ordered moment) is quickly quenched by 4.5 GPa (3% volume reduction). Under chemical pressure, where Sr is substituted for the larger Ba in BaIrO 3 , the local magnetic moment on Ir remains stable, but the weak ferromagnetism is quenched after only 1.7% volume reduction. The magnetic ordering temperature T m is also more strongly suppressed by chemical pressure compared to physical pressure. In addition, under ∼23-at. % Sr doping, BaIrO 3 undergoes a transition to a paramagnetic metallic state. Resistivity measurements indicate that BaIrO 3 remains an electrical insulator to at least 9 GPa, a much higher pressure than required to quench the weak ferromagnetism (∼4.5 GPa). Such a disparate response of transport and magnetic properties to chemical and physical pressure is likely rooted in the different compression rates of the (a,c) lattice parameters with Sr doping and applied pressure and the effect of related lattice distortions on electronic bandwidth and exchange interactions in this strongly spin-orbit-coupled system.

Advanced Functional Materials, 2014

Physical Review Letters - PHYS REV LETT, 2007

We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5... more We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5} edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu{sub 0.95}FeSb is a ferrimagnet in which the Fe 3d moment and the Eu{sup 2+} 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L edge XAS, we find that about 13% of the Eu have a formal valence of 3{sup +}. We ascribe the origin of ferrimagnetism at a relatively high transition temperature T{sub c} of 85 K in Eu{sub 0.95}FeSb to f-electron interaction with the nearly ferromagnetic [FeSb]{sup 2.2-} host lattice.

Codoping with Co has been shown to promote successful dilution of magnetic dopants of Mn and Fe i... more Codoping with Co has been shown to promote successful dilution of magnetic dopants of Mn and Fe into Ge up to a combined doping level of 15 at. %. Local structure, magnetic moment, and spin-dependant states of these diluted magnetic semiconductors (DMS) have been probed as a function of composition using X-ray and tunneling spectroscopy techniques. Extended X-ray Absorption Fine Structure (EXAFS) measurements reveal an increased level of substitution for each dopant species with respect to traditional single dopant DMS materials. A codoped environment results in 70% of Mn and 90% of Co successfully incorporating substitutionally into the Ge lattice. The remaining dopants are seen to form interstitial dimmers and trimers rather than metallic precipitates. Tunneling spectroscopy measurements on these systems through Al2O3 barriers using both Nb and Fe as spin detectors will be discussed in their correlation with the structural results.

Physical Review Letters, 2007

We combine x-ray magnetic circular dichroism spectroscopy at Fe L 2;3 edges, at Eu M 4;5 edges, x... more We combine x-ray magnetic circular dichroism spectroscopy at Fe L 2;3 edges, at Eu M 4;5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu 0:95 Fe 4 Sb 12 is a ferrimagnet in which the Fe 3d moment and the Eu 2 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L 3 edge XAS, we find that about 13% of the Eu have a formal valence of 3 . We ascribe the origin of ferrimagnetism at a relatively high transition temperature T C of 85 K in Eu 0:95 Fe 4 Sb 12 to f-electron interaction with the nearly ferromagnetic Fe 4 Sb 12 2:2ÿ host lattice.

Physical Review Letters, 2010

The electronic structure and magnetism of Ir 5d 5 states in nonmetallic, weakly ferromagnetic BaI... more The electronic structure and magnetism of Ir 5d 5 states in nonmetallic, weakly ferromagnetic BaIrO 3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be $1:5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L 2;3 absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO 3 . The local moment behavior persists even as the metallicparamagnetic phase boundary is approached with Sr doping or applied pressure.

Physical Review Letters, 2008

Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that a... more Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that are element specific, we have elucidated the role of Ho 3+ in multiferroic HoMnO3. In zero field, Ho 3+ orders antiferromagnetically with moments aligned along the hexagonal c direction below 40 K, and undergoes a transition to another magnetic structure below 4.5 K. In applied electric fields of up to 1 × 10 7 V/m, the magnetic structure of Ho 3+ remains unchanged.

Physical Review Letters, 2012

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr 2 IrO 4 at am... more X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr 2 IrO 4 at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit J eff ¼ 1 2 state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split J eff ¼ 1 2 , 3 2 states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of 17GPa,wherethe′′weak′′ferromagneticmomentisquencheddespitetransportmeasurementsshowinginsulatingbehaviortoatleast40GPa.ThemagnetictransitionhasimplicationsfortheoriginoftheinsulatinggapandthenatureofexchangeinteractionsinthisSOcoupledsystem.TheexpectationvalueoftheangularpartoftheSOinteraction,hLAˊSi,extrapolatestozeroat17 GPa, where the ''weak'' ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, hL Á Si, extrapolates to zero at 17GPa,wherethe′′weak′′ferromagneticmomentisquencheddespitetransportmeasurementsshowinginsulatingbehaviortoatleast40GPa.ThemagnetictransitionhasimplicationsfortheoriginoftheinsulatinggapandthenatureofexchangeinteractionsinthisSOcoupledsystem.TheexpectationvalueoftheangularpartoftheSOinteraction,hLAˊSi,extrapolatestozeroat80-90 GPa where an increased bandwidth strongly mixes J eff ¼ 1 2 , 3 2 states and SO interactions no longer dominate the electronic ground state of Sr 2 IrO 4 .

Physical Review Letters, 2008

Fe K-edge x-ray magnetic circular dichroism of magnetite (Fe 3 O 4 ) powders was measured with sy... more Fe K-edge x-ray magnetic circular dichroism of magnetite (Fe 3 O 4 ) powders was measured with synchrotron radiation under variable pressure and temperature conditions in diamond anvil cell. The magnetic dichroism was observed to decrease discontinuously by 50% between 12 and 16 GPa, independent of temperature. The magnetic transition is attributed to a high-spin to intermediate-spin transition of Fe 2 ions in the octahedral sites and could account for previously observed structural and electrical anomalies in magnetite at this pressure range. The interpretation of x-ray magnetic circular dichroism data is supported by x-ray emission spectroscopy and theoretical cluster calculations.