Influence of Crystalline Fields on the Susceptibilities of Salts of Paramagnetic Ions. II. The Iron Group, Especially Ni, Cr and Co

Schlapp, Robert; Penney, W. G.

doi:10.1103/physrev.42.666

Cited by 204 publications

(34 citation statements)

References 10 publications

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“…As reported previously, XRD refinements suggest a cation inversion x = 0.02 ± 0.04, among the lowest in the literature. Our previous fits of magnetization to a Curie-Weiss temperature dependence also yielded an effective moment µ ef f = 4.89 ± 0.03µ B and Weiss constant Θ = −109 ± 1 K. The large effective moment for Co 2+ in a tetrahedral crystal field environment can be understood as an effect of spin-orbit coupling 27,28 , but it has recently been pointed out that this value is also larger than many other reports in the literature 13 . On this last issue, we note that these other publications typically extract moments and Weiss constants from fits to inverse susceptibility, rather than magnetization itself with a background term and used a different temperature range.…”

Section: Crystal Growth and Characterizationmentioning

confidence: 61%

Revisiting the ground state of CoAl2O4 : Comparison to the conventional antiferromagnet MnAl2O

MacDougall¹,

Aczel²,

Su³

et al. 2016

Phys. Rev. B

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The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which unique incommensurate or 'spin-spiral liquid' ground states are predicted. Our previous single-crystal neutron scattering study instead classified it as a 'kineticallyinhibited' antiferromagnet, where the long ranged correlations of a collinear Néel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. The current paper provides new data sets from a number of experiments, which support and expand this work in several important ways. We show that the phenomenology leading to the kinetically-inhibited order is unaffected by sample measured and instrument resolution, while new low temperature measurements reveal spin correlations are unchanging between T = 2 K and 250 mK, consistent with a frozen state. Polarized diffuse neutron measurements show several interesting magnetic features, which can be entirely explained by the existence of short-ranged Néel order. Finally, and crucially, this paper presents some of the first neutron scattering studies of single crystalline MnAl2O4, which acts as an unfrustrated analogue to CoAl2O4 and shows all the hallmarks of a classical antiferromagnet with a continuous phase transition to Néel order at TN = 39 K. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T * . The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange.PACS numbers: 75.30.Ds, 75.50.Lk, 78.70.Nx The A-site spinels, AB 2 X 4 with A magnetic, have seen a surge of interest in the past decade, due to a series of interesting experimental observations 1-15 and theoretical predictions of novel spin-liquid ground states [16][17][18][19][20] . Magnetic cations in these materials comprise a bi-partite diamond lattice, and novel behavior is argued to be the result of a competition between nearest (J 1 ) and nextnearest (J 2 ) neighbor superexchange interactions 2,21 . This is demonstrated explicitly by the calculations of Bergman et al. 16 , who have shown for spin-only materials that the collinear Néel structure favored by J 1 is progressively destabilized with increasing J 2 , until a Lifshitz point is encountered at J2 J1 = 1 8 . For greater J 2 , the ground state is predicted to be a novel 'spiral spin liquid' (SSL) state characterized by fluctuations between an infinitely degenerate set of incommensurate spin spirals, whose propagation wavevectors form a series of calculable manifolds in reciprocal space ('spiral surfaces'). Further calculations [16][17][18] predict that these mass degeneracies are lifted by low-lying thermal or quantum fluctuations, driving first-order phase transitions via the order-by-disorder mechanism 22 to either unique spin spiral or Néel ordered states, depending o...

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Section: Crystal Growth and Characterizationmentioning

confidence: 61%

Revisiting the ground state of CoAl2O4 : Comparison to the conventional antiferromagnet MnAl2O

MacDougall¹,

Aczel²,

Su³

et al. 2016

Phys. Rev. B

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“…The presence of a gap may be significant, because recent Monte Carlo calculations have suggested that a single-ion anisotropy is necessary to stabilize Néel order in CoAl 2 O 4 (35). The gap likely arises from the effect of spinorbit coupling, which is known to be important for the Co 2þ ion in a tetrahedral environment (28), and incidentally provides an explanation for the enhanced local moment inferred from bulk magnetization measurements (44,45). Finite domain size effects may also play a role.…”

Section: Resultsmentioning

confidence: 99%

Kinetically inhibited order in a diamond-lattice antiferromagnet

MacDougall

Gout

Zarestky

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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Frustrated magnetic systems exhibit highly degenerate ground states and strong fluctuations, often leading to new physics. An intriguing example of current interest is the antiferromagnet on a diamond lattice, realized physically in A-site spinel materials. This is a prototypical system in three dimensions where frustration arises from competing interactions rather than purely geometric constraints, and theory suggests the possibility of unusual magnetic order at low temperature. Here, we present a comprehensive single-crystal neutron scattering study of CoAl 2 O 4 , a highly frustrated A-site spinel. We observe strong diffuse scattering that peaks at wavevectors associated with Néel ordering. Below the temperature T Ã ¼ 6.5 K, there is a dramatic change in the elastic scattering lineshape accompanied by the emergence of well-defined spin-wave excitations. T Ã had previously been associated with the onset of glassy behavior. Our new results suggest instead that T Ã signifies a first-order phase transition, but with true long-range order inhibited by the kinetic freezing of domain walls. This scenario might be expected to occur widely in frustrated systems containing firstorder phase transitions and is a natural explanation for existing reports of anomalous glassy behavior in other materials. F rustration occurs in spin systems when constraints prevent the formation of a ground state satisfying all of the pairwise interactions. The defining characteristics of frustration are massive ground-state degeneracy and concomitant strong fluctuations. The latter suppress magnetic order and lead to spin-liquid regimes extending to low temperature. There has been great interest in such spin liquids because of the plethora of emergent phenomena (1-3) resulting from an extreme sensitivity to small, often neglected degeneracy-breaking effects. Noteworthy examples include unusual short-range correlations (4, 5), apparent spinglass behavior in the absence of disorder (6-8), collective ring excitations in pyrochlore spinels (9-11), spontaneously reduced dimensionality in rare-earth titanates (12), and topological excitations like magnetic monopoles in spin-ice materials (13-15).The degeneracy-breaking effects can be small terms in the interaction Hamiltonian that enter the problem in a nonperturbative way. Examples include dipolar interactions in the pyrochlores (16) or spin-lattice interactions in multiferroics (17). Interestingly, degeneracy can also be broken in the complete absence of such interaction terms, through a mechanism known as "order-by-disorder" (18). Order-by-disorder refers to the scenario where the entropy associated with thermal or quantum fluctuations lowers the free energy of one ordered state compared to others of equal energy, thereby stabilizing order. Order-by-disorder was first predicted in studies of the diluted Ising model on a rectangular lattice (18). It has since emerged as a central feature in theories of various frustrated antiferromagnets, including the face-centered cubic (FCC) (19,20) as well a...

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“…In order for V to have this C3h symmetry, ·it has been shown (43) that the L, M terms to consider are those given in Table 1. The crystal field potential V is then treated as a perturbation on Hion· Solutions to, (8) will be taken as,…”

Section: {6)mentioning

confidence: 99%

“…In the following years many workers in the field contributed greatly to establishing that this model had some validity. Work on spectra and paramag.-netic susceptibilities in the 1930's was carried out by many scientists (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Some of the conclusions that were arrived at during this time for rare earth salts were (a) Russell It is the purpose of this investigation-to obtain the ~plitting factors for the magnetic field parallel to the caxis of some of the levels that Johnsen (35) did not measure and_ to measure and interpret the spectra for the magnetic .…”

Section: Introductionmentioning

confidence: 99%

Nature of the Energy Levels in Hydrated Thulium Ethylsulphate

Bartel

1964

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Influence of Crystalline Fields on the Susceptibilities of Salts of Paramagnetic Ions. II. The Iron Group, Especially Ni, Cr and Co

Cited by 204 publications

References 10 publications

Revisiting the ground state of CoAl2O4 : Comparison to the conventional antiferromagnet MnAl2O

Revisiting the ground state of CoAl2O4 : Comparison to the conventional antiferromagnet MnAl2O

Kinetically inhibited order in a diamond-lattice antiferromagnet

Nature of the Energy Levels in Hydrated Thulium Ethylsulphate

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