Magnetovolume instabilities and ferromagnetism versus antiferromagnetism in bulk fcc iron and manganese

Moruzzi, V. L.; Marcus, P. M.; Kübler, J.

doi:10.1103/physrevb.39.6957

Cited by 400 publications

(145 citation statements)

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“…Fe nanoparticles, allowing optimisation to values higher than those in bulk b.c.c. Fe, in agreement with various calculations [18,[23][24][25] that predict a strong dependence of f.c.c. Fe atomic moments on lattice parameter.…”

Section: Introductionsupporting

confidence: 77%

Structure and magnetism in Cr-embedded Co nanoparticles

Baker¹,

Kurt²,

Roy³

et al. 2016

J. Phys.: Condens. Matter

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We present the results of an investigation into the atomic structure and magnetism of 2 nm diameter Co nanoparticles embedded in an antiferromagnetic Cr matrix. The nanocomposite films used in this study were prepared by co-deposition directly from the gas phase, using a gas aggregation source for the Co nanoparticles and a molecular beam epitaxy (MBE) source for the Cr matrix material. Co K and Cr K edge extended x-ray absorption fine structure (EXAFS) experiments were performed in order to investigate atomic structure in the embedded nanoparticles and matrix respectively, while magnetism was investigated by means of a vibrating sample magnetometer. The atomic structure type of the Co nanoparticles is the same as that of the Cr matrix (b.c.c.) although with a degree of disorder. The net Co moment per atom in the Co/Cr nanocomposite films is significantly reduced from the value for bulk Co, and decreases as the proportion of Co nanoparticles in the film is decreased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.25   /atom. After field cooling to below 30 K all samples showed an exchange bias, which was largest for the most dilute sample. Both the structural and magnetic results point towards a degree of alloying at the nanoparticle/matrix interface, leading to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core.

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Section: Introductionsupporting

confidence: 77%

Structure and magnetism in Cr-embedded Co nanoparticles

Baker¹,

Kurt²,

Roy³

et al. 2016

J. Phys.: Condens. Matter

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“…In the case of Fe, the traditional local order parameter, µ, gave conflicting results computationally, with the transition appearing as first [3] or second [2] order. It was not until the transition was interpreted as a topological change of the elctronic charge density, ρ( r), that the underlying nature of the phase change became apparent [4].…”

Section: Pacs Numbersmentioning

confidence: 99%

Topological Catastrophe and Isostructural Phase Transition in Calcium

2010

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We predict a quantum phase transition in fcc Ca under hydrostatic pressure. Using density functional theory, we find at pressures below 80 kbar, the topology of the electron charge density is characterized by nearest neighbor atoms connected through bifurcated bond paths and deep minima in the octahedral holes. At pressures above 80 kbar, the atoms bond through non-nuclear maxima that form in the octahedral holes. This topological change in the charge density softens the C elastic modulus of fcc Ca, while C44 remains unchanged. We propose an order parameter based on applying Morse theory to the charge density, and we show that near the critical point it follows the expected mean-field scaling law with reduced pressure. PACS numbers:The theory and characterization of solid-solid isostructural phase transitions has been an area of intense experimental and theoretical research since they were described in 1949 [1]. What sets these transitions apart from others is that while crystalline properties change-sometimes discontinuously-crystallographic structure does not, making these transitions purely electronic in nature. Both first order and continuous transitions are known to exist. A well known example of the former is the γ → α transition in fcc Ce [1], while a magneto-volume instability in fcc Fe is an example of the later [2].In the case of Fe, the traditional local order parameter, µ, gave conflicting results computationally, with the transition appearing as first [3] or second [2] order. It was not until the transition was interpreted as a topological change of the elctronic charge density, ρ( r), that the underlying nature of the phase change became apparent [4]. While this approach is promising, no general correlations have been drawn between isostructural phase transitions and ρ( r). We use this approach to predict an isostructural phase transition in calcium under hydrostatic pressure. We then apply Morse theory to the charge density and propose an order parameter for the transition. We show that this order parameter scales in the appropriate way with reduced pressure near the quantum critical point.One of the advantages of casting theories of isostructural phase transition in terms of ρ( r) is that it is a quantum observable. Though ρ( r) is often calculated by way of first principles methods (e.g. density functional theory (DFT)), the total charge density can also be measured via X-ray diffraction techniques, and the spin-polarized charge density can be determined using spin-polarized neutron diffraction. As it is also known that all ground state properties depend on the charge density [5], it seems appropriate to seek relationships between the structure * Electronic address: trjones@mines.edu of ρ( r), changes to that structure, and corresponding changes to properties. We note that previous work has proposed that DFT could be used as a general framework for analyzing quantum phase transitions in electronic systems [6].The electron charge density is a 3 dimensional scalar field. Morse theory tells us the ...

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“…Less clear is the PM/LS phase change. The situation yields ambiguous results if µ is used as an order parameter, with the transition appearing as first [27,29] or second [31] order. Small changes in total energy associated with this phase change, 0.001 eV/atom [31], make the correct assignment of moment problematic.…”

mentioning

confidence: 99%

“…However, as with magnetic bcc Fe, the spin densities give rise to distinct topologies. In the case of fcc (γ) Fe, three magnetic phases are observed, high-spin, lowspin, and paramagnetic (HS, LS, and PM respectively) phases, each characterized by a different range of lattice constants [27][28][29][30][31][32]. From the VASP calculations we find the HS phase corresponds to a ≥ 3.564Å, while the LS, low volume phase, occurs for a ≤ 3.563Å, with the PM phases equilibrating at a = 3.45Å.…”

mentioning

confidence: 99%

Topology of the Spin-Polarized Charge Density in bcc and fcc Iron

2008

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We investigate the topology of the spin-polarized charge density in bcc and fcc iron. While the total spin-density is found to possess the topology of the non-magnetic prototypical structures, in some cases the spin-polarized densities are characterized by unique topologies; for example, the spin-polarized charge densities of bcc and high-spin fcc iron are atypical of any known for nonmagnetic materials. In these cases, the two spin-densities are correlated: the spin-minority electrons have directional bond paths with deep minima in the minority density, while the spin-majority electrons fill these holes, reducing bond directionality. The presence of two distinct spin topologies suggests that a well-known magnetic phase transition in iron can be fruitfully reexamined in light of these topological changes. We show that the two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to high-spin) are different. The former follows the Landau symmetrybreaking paradigm and proceeds without a topological transformation, while the latter also involves a topological catastrophe. Bader's topological theory of molecular structure, Atoms in Molecules (AIM), has been successfully applied to a variety of crystalline systems [1][2][3][4][5][6][7][8][9][10][11]. It has been employed to investigate the nature of bonding in materials ranging from high temperature alloys to biological systems. These have yielded surprising results, such as second neighbor bond paths in B2 ionic crystals [3] and transition metal aluminides [4], with the magnitude of the latter correlating to failure properties [5]. Other studies have used bond path properties to offer first principles explanations of stress-induced failure in brittle and ductile alloys [7], as well as shear elastic constants in a variety of pure metals and alloys [8][9][10]. Building on these ideas and using the rigorous definitions of bond paths afforded by the theory, the anomalous behavior of iridium under shear was also explained [11].One of the attractive features of AIM is its reliance on the charge density, a quantum mechanical observable, that is most often calculated but can, in principle, be measured via X-ray diffraction techniques [12][13][14][15]. In a similar fashion, the spin-polarized charge density is an observable that can be calculated or measured using spinpolarized neutron diffraction. Despite the information and insights that have come from topological investigations of the total charge density, the same analysis has yet to be performed on spin-polarized densities. Here, we report the results from the first such studies, exploring the spin-minority and spin-majority topologies of bodycentered-cubic (bcc) and face-centered-cubic (fcc) iron.This first application of AIM to spin-density sheds light on the origins of the magnetic phase transitions of fcc iron. It is argued that this system undergoes two distinct phase transitions during volume expansion: a second- * Electronic address: trjones@mines.edu order phase change, from a paramagnetic to a l...

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Magnetovolume instabilities and ferromagnetism versus antiferromagnetism in bulk fcc iron and manganese

Cited by 400 publications

References 20 publications

Structure and magnetism in Cr-embedded Co nanoparticles

Structure and magnetism in Cr-embedded Co nanoparticles

Topological Catastrophe and Isostructural Phase Transition in Calcium

Topology of the Spin-Polarized Charge Density in bcc and fcc Iron

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