Density-functional theory calculation of the intermolecular exchange interaction in the magnetic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Mn</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>dimer

Park, Kyungwha; Pederson, Mark R.; Richardson, Steven L.; Aliaga‐Alcalde, Núria; Christou, George

doi:10.1103/physrevb.68.020405

Cited by 64 publications

(35 citation statements)

References 18 publications

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“…However, more scrutinized studies are required to conclude that this, in fact, attributes to the antiferromagnetic coupling in the calculated lowest-energy state. Another intriguing point is that as shown in Table I, collinear spin excitations for Ni 4 have an order of magnitude lower energies than those for the Mn 12 -acetate 14 and Mn 4 monomer 15,16 . A majorityminority spin-flip gap ranges from 1.4 eV to 1.6 eV and a minority-majority spin-flip gap varies between 1.8 eV and 2.1 eV depending on ligands as shown in Table III.…”

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confidence: 97%

Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Park

Yang

Hendrickson

2005

Journal of Applied Physics

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Recent magnetic measurements on tetra-nickel molecular magnets [Ni(hmp)(ROH)Cl]4, where R=CH3, CH2CH3, or (CH2)2C(CH3)3 and hmp − is the monoanion of 2-hydroxymethylpyridine, revealed a strong exchange bias prior to the external magnetic field reversal as well as anomalies in electron paramagnetic resonance peaks at low temperatures. To understand the exchange bias and observed anomalies, we calculate the electronic structure and magnetic properties for the Ni4 molecules with the three different ligands, employing density-functional theory. Considering the optimized structure with possible collinear spin configurations, we determine a total spin of the lowest-energy state to be S = 0, which does not agree with experiment. We also calculate magnetic anisotropy barriers for all three types of Ni4 molecules to be in the range of 4-6 K. which has a total ground-state spin of S = 10 and magnetic anisotropy barrier of 65 K. 2,3 Magnetic hysteresis loop measurements on Mn 12 -acetate showed quantum tunneling of magnetic moment through the magnetic anisotropy barrier when the external magnetic field was applied along the easy axis. 3

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confidence: 97%

Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Park

Yang

Hendrickson

2005

Journal of Applied Physics

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“…Once the MEI extracted, the investigation of magnetism of several type of systems can be performed going from molecules, 3 transition metals alloys, 4,5 and surfaces, 6,7 diluted magnetic semiconductors, 8,9 to clusters, [10][11][12][13] and even for strongly correlated systems.…”

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Mapping the magnetic exchange interactions from first principles: Anisotropy anomaly and application to Fe, Ni, and Co

Lounis

Dederichs

2010

Phys. Rev. B

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Mapping the magnetic exchange interactions from model Hamiltonian to density-functional theory is a crucial step in multiscale modeling calculations. Considering the usual magnetic force theorem but with arbitrary rotational angles of the spin moments, a spurious anisotropy in the standard mapping procedure is shown to occur provided by bilinear like contributions of high-order spin interactions. The evaluation of this anisotropy gives a hint on the strength of nonbilinear terms characterizing the system under investigation. DOI: 10.1103/PhysRevB.82.180404 PACS number͑s͒: 75.30.Ϫm, 73.22.Ϫf, 75.70.Ϫi Multiscale modeling approaches are extremely important for describing huge magnetic systems, e.g., at the micrometer scale which would be impossible with only densityfunctional theory ͑DFT͒. In magnetism, usually the multiscale approach is performed after mapping the magnetic exchange interactions ͑MEI͒ of a classical Heisenberg model to the DFT counterparts. This is a crucial task which can lead to wrong results if not done carefully. The simple model is described bywhere J ij describes the pairwise ͑two-spin͒ MEI between spins at lattice sites i and j while e ជ i ͑1, , ͒ defines the direction of the local moment M ជ i . Sometimes, higher-order terms such as the four-spin or the biquadratic MEI are introduced in the previous Hamiltonian for a better mapping of the DFT results. 1,2Once the MEI extracted, the investigation of magnetism of several type of systems can be performed going from molecules, 3 transition metals alloys, 4,5 and surfaces, 6,7 diluted magnetic semiconductors, 8,9 to clusters, 10-13 and even for strongly correlated systems.14 Thermodynamical properties are then easily accessible such as Curie temperatures, specific heat or magnetic excitation spectra and spin waves stiffness in multidimensional systems.An elegant method to extract the MEI is based on a Greens-function technique which has been derived 20 years ago by Liechtenstein et al. 15 ͑noted in the text LKAG͒. Instead of calculating several magnetic configurations, this method, based on the magnetic force theorem ͑MFT͒, 16,17 allows the evaluation of the MEI from one collinear configuration which is usually ferromagnetic. Computationally, this method is thus very attractive.Assuming infinitesimal rotation angles of the magnetic moments ͑limit of infinite magnon wavelength͒ is necessary to get the final LKAG formula for the MEI. However, one should note that this formalism is used for arbitrary big rotation angles ͑finite magnon wavelength͒ as well. Thus, many improvements of the formalism have been proposed recently: Bruno 18 proposed a renormalized MFT using the constrained DFT ͑Ref. 19͒ leading to unrealistic high local density approximation ͑LDA͒ Curie temperature ͑T c ͒ for fcc Ni. The same effect has been observed using the proposal of Antropov. 20 Katsnelson and Lichtenstein 21 proposed in their recent publication a reconciliation between the old formalism 15 and the new renormalized theories. 18,20 They have shown that the impr...

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“…Xx, 71.15.Mb, 75.30.Gw, 75.30.Et The observation of resonant tunneling of magnetization in the single molecule magnet Mn 12 -acetate 1 (hereafter Mn 12 ) with a ground-state spin of S = 10 has led to many experimental and theoretical investigations. 2,3,4,5,6 A simple anisotropy Hamiltonian for S = 10 provides an excellent approximation to the physics occurring at low temperatures. Deriving the Hamiltonian from density-functional (DF) calculations is challenging but possible.…”

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“…Deriving the Hamiltonian from density-functional (DF) calculations is challenging but possible. 4,5,6 For a particular molecular geometry, a magnetic anisotropy tensor may be calculated considering spin-orbit coupling within a DF framework. In the principal-axes coordinates, the lowest-order spin Hamiltonian can be simplified to the following form:…”

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Incommensurate transverse anisotropy induced by disorder and spin-orbit-vibron coupling in Mn12 acetate

Park

Pederson

Baruah

et al. 2005

Journal of Applied Physics

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It has been shown within density-functional theory that in Mn12-acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field.PACS numbers: 75.50. Xx, 71.15.Mb, 75.30.Gw, 75.30.Et The observation of resonant tunneling of magnetization in the single molecule magnet Mn 12 -acetate 1 (hereafter Mn 12 ) with a ground-state spin of S = 10 has led to many experimental and theoretical investigations. 2,3,4,5,6 A simple anisotropy Hamiltonian for S = 10 provides an excellent approximation to the physics occurring at low temperatures. Deriving the Hamiltonian from density-functional (DF) calculations is challenging but possible. 4,5,6 For a particular molecular geometry, a magnetic anisotropy tensor may be calculated considering spin-orbit coupling within a DF framework. In the principal-axes coordinates, the lowest-order spin Hamiltonian can be simplified to the following form:where D and E are the uniaxial and second-order transverse anisotropy parameters and S z is the easy-axis component of the spin operator S. The S 4 symmetry of the ideal Mn 12 molecule causes the value of E to vanish and the lowest-order transverse terms to be fourth order. Only transverse anisotropy terms are responsible for the resonant tunneling between energy levels that are almost degenerate. Magnetic tunneling measurements, however, showed that some of the resonant tunneling occurred at a level lower than fourth order. For the Mn 12 , the calculated D value is -0.556 K, 4 which agrees well with experiment. The calculated (measured) value of D, however, does not account for all the measured anisotropy barrier of 65 K. 3,8 One needs a longitudinal anisotropy of order higher than second-order. In this spirit a fourth-order spin-orbit-vibron (SOV) interaction was proposed by Pederson et al. 11 Ideally, this SOV interaction can only contribute to modifying the second-order barrier and to activating two fourth-order transverse terms in the spin Hamiltonian. At this level a strong SOV interaction can complicate the tunneling experiments in several different ways. The simplest complication is that there are two different longitudinal 4th-order terms which scale as S 2 S 2 z and S 4 z respectively. If the S 4 z term is dominant, then at any resonant field only a single pair of states are involved with tunneling. The fourth-order transverse terms result in departures of the period of the tunnel-splitting oscillation from ∆H x = 2 2E(|D| + E)/gµ B that has been derived by Garg. 12 Such departures were first identified by Wernsdorfer et al. 13 and later modified to include the 4th-order transverse...

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Density-functional theory calculation of the intermolecular exchange interaction in the magneticMn4dimer

Cited by 64 publications

References 18 publications

Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Mapping the magnetic exchange interactions from first principles: Anisotropy anomaly and application to Fe, Ni, and Co

Incommensurate transverse anisotropy induced by disorder and spin-orbit-vibron coupling in Mn12 acetate

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