Magnetostructural coupling from competing magnetic and chemical bonding effects

An understanding of the origins of giant magnetostructural coupling is developed for the compound MnAs, a magnetic material that has served as a prototype for many promising technologies including caloric refrigeration, magnetic actuation, and spintronics. We demonstrate that strong coupling between magnetism and crystal structure arises from an orbital-specific competition between exchange energies and kinetic (bonding) energies and that thermally activated spin fluctuations drive the unusual first-order phase transition from high to low symmetry upon heating. The underlying mechanism raises the prospect of an exotic paramagnetic state featuring local fluctuations in atomic positions and bonding on the timescale of the moment fluctuations. The results should inform the design of new materials with enhanced magnetostructural coupling, found at the border between structural and magnetic stability.

Figure 1

Structure of MnAs in its (a) hexagonal and (b) orthorhombic forms. In (a), the orthohexagonal cell is shown with dashed lines and, at the bottom, the arrows indicate the directions of the atomic displacements that give rise to the orthorhombic structure. (c) The previously proposed $C$-type antiferromagnetic structure of the orthorhombic phase. All correlations out of the shown plane are ferromagnetic.

Figure 2

(a) Two-dimensional (2D) energy surface of ferromagnetic MnAs as a function of Mn distortion and As distortion. (b) One-dimensional (1D) energy surfaces of FM, AFM, and PM MnAs as a function of Mn distortion. (c) Corresponding Mn local moment magnitudes.

Figure 3

(a)–(d) Projected Mn $d$-orbital partial densities of states for the ferromagnetic hexagonal structure, using a projection scheme oriented around the Mn-Mn contacts. Majority spin states are shown as positive density of states (DOS) values, and minority are shown as negative DOS values. (a) and (b) $d_{x^2-y^2},d_{xy},d_{yz}$, and $d_{xz}$ orbitals are all fully spin polarized, stabilized by Hund's coupling. (c) The $d_{z^2}$ orbital is more dispersive and is stabilized by Mn-Mn bonding as shown in (d) the partial charge density of the minority spin channel within 1 eV below the Fermi level.

Figure 4

Single-orbital properties of MnAs in the FM (a)–(e) and $C$-type AFM (f)–(i) structures. (a) Mn $d_{x^2-y^2}$ orbitals oriented along Mn-Mn contacts. (b) and (d) Projected partial $d_{x^2-y^2}$ DOS for the hexagonal and orthorhombic structures, respectively. (c) Projected $d_{x^2-y^2}$ moment and negative integrated crystal orbital Hamilton populations ($-\mathrm{iCOHP}$) of the Mn-Mn zigzag contact as a function of Mn distortion. (e) Partial charge density of the minority spin channel within 1 eV below the Fermi level for low-moment ferromagnetic orthorhombic MnAs. (f)–(i) show the same data as (a)–(e) except for the AFM case.

Figure 5

(a) Structurally relaxed SQS used to simulate the paramagnetic state. (b) Projection of the supercell atomic positions into a single orthorhombic cell, showing Mn and As occupying a distribution of positions along the structural distortion modes. (c) Distributions of bond distance and (d) negative integrated crystal orbital Hamilton population for the 48 unique Mn-Mn zigzag contacts.