Complex antiferromagnetic order in the garnet Co3Al2Si3O12 (original) (raw)
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Complex magnetic order in the kagomé staircase compound Co3V2O8
Physical Review B, 2006
Co 3 V 2 O 8 ͑CVO͒ has a different type of geometrically frustrated magnetic lattice, a kagomé staircase, where the full frustration of a conventional kagomé lattice is partially relieved. The crystal structure consists of two inequivalent ͑magnetic͒ Co sites, one-dimensional chains of Co͑2͒ spine sites, linked by Co͑1͒ cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a rich variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k = ͑0,␦ ,0͒ with ␦ = 0.55 and the spin direction along the a axis. ␦ is found to decrease monotonically with decreasing temperature and then locks into a commensurate antiferromagnetic structure with ␦ = 1 2 for 6.9Ͻ T Ͻ 8.6 K. In this phase, there is a ferromagnetic layer where the spine site and cross-tie sites have ordered moments of 1.39 B and 1.17 B , respectively, and an antiferromagnetic layer where the spine-site has an ordered moment of 2.55 B , while the cross-tie sites are fully frustrated and have no observable ordered moment. Below 6.9 K, the magnetic structure becomes incommensurate again, and the presence of higherorder satellite peaks indicates that the magnetic structure deviates from a simple sinusoid. ␦ continues to decrease with decreasing temperature and locks in again at ␦ = 1 3 over a narrow temperature range ͑6.2Ͻ T Ͻ 6.5 K͒. The system then undergoes a strongly first-order transition to the ferromagnetic ground state ͑␦ =0͒ at T c = 6.2 K. The ferromagnetism partially relieves the cross-tie site frustration, with ordered moments on the spine-site and cross-tie sites of 2.73 B and 1.54 B , respectively. The spin direction for all spins is along the a axis ͑Ising-like behavior͒. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating that there is significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.
Temperature-dependent multi-k magnetic structure in multiferroic Co3TeO6
Materials Research Bulletin, 2012
A complex magnetic order of the multiferroic compound Co 3 TeO 6 has been revealed by neutron powder diffraction studies on ceramics and crushed single crystals. The compound adopts a monoclinic structure (s.g. C2/c) in the studied temperature range 2 K -300 K but exhibits successive antiferromagnetic transitions at low temperature. Incommensurate antiferromagnetic order with the propagation vector k 1 = (0, 0.485, 0.055) sets in at 26 K. A transition to a second antiferromagnetic structure with k 2 = (0, 0, 0) takes place at 21.1 K. Moreover, a transition to a commensurate antiferromagnetic structure with k 3 = (0, 0.5, 0.25) occurs at 17.4 K. The magnetic structures have been determined by neutron powder diffraction using group theory analysis as a preliminary tool. Different coordinations of the Co 2+ ions involved in the low-symmetry C2/c structure of Co 3 TeO 6 render the exchange-interaction network very complex by itself. The observed magnetic phase transformations are interpreted as an evidence of competing magnetic interactions. The temperature dependent changes in the magnetic structure, derived from refinements of highresolution neutron data, are discussed and possible mechanisms connected with the spin reorientations are described.
Magnetic Ordering and Spin Waves in Na0.82CoO2
Physical Review Letters, 2005
NaxCoO2, the parent compound of the recently synthesized superconductor NaxCoO2:yH2O, exhibits bulk antiferromagnetic order below ∼ 20 K for 0.75 ≤ x ≤ 0.9. We have performed neutron scattering experiments in which we observed Bragg reflections corresponding to A-type antiferromagnetic order in a Na0.82CoO2 single crystal and characterized the corresponding spinwave dispersions. The spin waves exhibit a strongly energy-dependent linewidth. The in-plane and out-of-plane exchange constants resulting from a fit to a nearest-neighbor Heisenberg model are similar in magnitude, which is unexpected in view of the layered crystal structure of NaxCoO2. Possible implications of these observations are discussed. 76.75.+i, 72.80.Ga, 71.30.+h The cobaltate Na x CoO 2 : yH 2 O has recently enjoyed intense attention. The composition with x ∼ 0.30, y ∼ 1.4 has been shown to be superconducting over a narrow range of x, with maximum transition temperature T c ∼ 5 K . This compound is particularly interesting because its structure is similar to that of the high-T c copper oxide superconductors. In both materials, superconducting sheets containing oxygen and a spin-1/2 transition metal are separated by layers of lower conductivity in an anisotropic crystal structure. However, a number of characteristics suggest that the superconductivity in this compound may be unusual in different ways from that found in the cuprates. For example, some experiments indicate that the symmetry of the Cooper pair wavefunction may be p-wave .
Physical Review B, 2017
In this contribution, we report that CaY2Co2Ge3O12 exhibits an unusual anisotropic and chainlike antiferromagnetic arrangement of spins despite crystallizing in the highly symmetric garnet structure. Using low-temperature powder neutron di↵raction and symmetry analysis, we identify a magnetic structure consisting of chain-like motifs oriented along the body diagonals of the cubic unit cell with moments pointing parallel to the chain direction due to the strong Ising character of the Co ions. Antiferromagnetic order sets in below 6 K and exhibits both temperature-and fieldinduced magnetic transitions at high fields. Combining the results, we present a magnetic phase diagram that suggests CaY2Co2Ge3O12 undergoes a quantum phase transition at low temperatures and moderate fields.
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Physical Review B, 2006
Co 3 V 2 O 8 ͑CVO͒ has a different type of geometrically frustrated magnetic lattice, a kagomé staircase, where the full frustration of a conventional kagomé lattice is partially relieved. The crystal structure consists of two inequivalent ͑magnetic͒ Co sites, one-dimensional chains of Co͑2͒ spine sites, linked by Co͑1͒ cross-tie sites. Neutron powder diffraction has been used to solve the basic magnetic and crystal structures of this system, while polarized and unpolarized single crystal diffraction measurements have been used to reveal a rich variety of incommensurate phases, interspersed with lock-in transitions to commensurate phases. CVO initially orders magnetically at 11.3 K into an incommensurate, transversely polarized, spin density wave state, with wave vector k = ͑0,␦ ,0͒ with ␦ = 0.55 and the spin direction along the a axis. ␦ is found to decrease monotonically with decreasing temperature and then locks into a commensurate antiferromagnetic structure with ␦ = 1 2 for 6.9Ͻ T Ͻ 8.6 K. In this phase, there is a ferromagnetic layer where the spine site and cross-tie sites have ordered moments of 1.39 B and 1.17 B , respectively, and an antiferromagnetic layer where the spine-site has an ordered moment of 2.55 B , while the cross-tie sites are fully frustrated and have no observable ordered moment. Below 6.9 K, the magnetic structure becomes incommensurate again, and the presence of higherorder satellite peaks indicates that the magnetic structure deviates from a simple sinusoid. ␦ continues to decrease with decreasing temperature and locks in again at ␦ = 1 3 over a narrow temperature range ͑6.2Ͻ T Ͻ 6.5 K͒. The system then undergoes a strongly first-order transition to the ferromagnetic ground state ͑␦ =0͒ at T c = 6.2 K. The ferromagnetism partially relieves the cross-tie site frustration, with ordered moments on the spine-site and cross-tie sites of 2.73 B and 1.54 B , respectively. The spin direction for all spins is along the a axis ͑Ising-like behavior͒. A dielectric anomaly is observed around the ferromagnetic transition temperature of 6.2 K, demonstrating that there is significant spin-charge coupling present in CVO. A theory based on group theory analysis and a minimal Ising model with competing exchange interactions can explain the basic features of the magnetic ordering.
Physical Review B, 2009
We report the results of the neutron diffraction experiments on the spin-chain compounds Ca 3 Co 2−x Fe x O 6 ͑x = 0.2 and 0.4͒ crystallizing in the rhombohedral structure ͑space group R3c͒ in the temperature range of 1.5-100 K. Additional magnetic Bragg peaks in the low-temperature neutron powder diffraction patterns mainly indicate an antiferromagnetic ordering for these iron-substituted compounds. The observed magnetic reflections can be indexed in the nuclear lattice with a propagation vector K = ͑0,0,1͒ which indicates that the centering translations have been lost in the magnetic structure. The Rietveld refinement of the neutron diffraction patterns for both the compounds confirms that the ⌫ 2 irreducible representation of the little group G k classifies the magnetic structure which corresponds with ferromagnetic planes perpendicular to the b axis, where the magnetic moment is aligned along the c axis, coupled antiferromagnetically along the b direction. We demonstrate that two different magnetic structures, ͑i͒ amplitude modulated structure with a propagation vector K = ͑0,0,1͒ and ͑ii͒ partially disordered antiferromagnet structure, are able to fit the same neutron diffraction pattern because the Fourier coefficients for each solution only differs in a global phase factor that cannot be determined by the experiment. The temperature-independent intensity of the ͑110͒ Bragg peak and the fact that the determined Fourier coefficients assure that the magnetic structure has net zero magnetic moment in the lattice confirm the absence of any phase transition to a ferrimagnetic state.
Crystal Chemistry and Competing Magnetic Exchange Interactions in Oxide Garnets and Spinels
Journal of Solid State Chemistry, 2019
In this contribution, we review the crystal chemistry and magnetic exchange pathways in atomically well-ordered oxide garnets and spinels. While superficially the topologies of these two structures appear quite different, both are capable of accommodating a large number of magnetic elements from across the periodic table within multiple coordination environments, making them ideal for identifying new design principles. We present a comparison of the crystal chemistry for the two structures and an overview of the various magnetic properties that have been observed when one or more magnetic ions occupy the individual sublattices. We identify that the large number of degenerate superexchange pathways between each sublattice in either structure plays a critical role in producing novel magnetic behavior.
Evolution of magnetic states in frustrated diamond lattice antiferromagneticCo(Al1−xCox)2O4spinels
Physical Review B, 2010
Using neutron powder diffraction and Monte Carlo simulations we show that a spin-liquid regime emerges at all compositions in the diamond-lattice antiferromagnets Co͑Al 1−x Co x ͒ 2 O 4. This spin-liquid regime induced by frustration due to the second-neighbor exchange coupling J 2 is gradually superseded by antiferromagnetic collinear long-range order ͑k =0͒ at low temperatures. Upon substitution of Al 3+ by Co 3+ in the octahedral B site the temperature range occupied by the spin-liquid regime narrows and T N increases. To explain the experimental observations we considered magnetic anisotropy D or third-neighbor exchange coupling J 3 as degeneracy-breaking perturbations. We conclude that Co͑Al 1−x Co x ͒ 2 O 4 is below the theoretical critical point J 2 / J 1 =1/ 8, and that magnetic anisotropy assists in selecting a collinear long-range ordered ground state, which becomes more stable with increasing x due to a higher efficiency of O-Co 3+-O as an interaction path compared to O-Al 3+-O.