Structurally driven metamagnetism in MnP and related Pnma compounds

Abstract:

We investigate the structural conditions for metamagnetism in MnP and related materials using Density Functional Theory. A magnetic stability plot is constructed taking into account the two shortest Mn-Mn distances. We find that a particular Mn-Mn separation plays the dominant role in determining the change from antiferromagnetic to ferromagnetic order in such systems. We establish a good correlation between our calculations and structural and magnetic data from the literature. Based on our approach it should be possible to find new Mn-containing alloys that possess fieldinduced metamagnetism and associated magnetocaloric effects. 

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https://arxiv.org/pdf/1003.5193.pdf


I. INTRODUCTION 

The magnetocaloric effect (MCE) is the change of temperature of a material in a changing magnetic field, and was first observed in iron by Emil Warburg.

...... In this paper, we focus on MnP-type (Pnma space group) binary and ternary orthorhombic materials and in particular on what structural factors support antiferromagnetic order and field-induced metamagnetism. Our motivitation is to allow prediction of new materials that may have large MCEs.

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The paper is organised as follows: in sec. II, the underlying Pnma crystal structure is introduced together with the details of our computational method. Sec. III describes the results of ab-initio electronic calculations  based on DFT theory. We will show that the ferromagnetic structure of the prototype MnP binary alloy would undergo a change in its magnetic state as a result of isodirectional lattice expansion. This change, however can also be realised by chemical pressure through alloying as is demonstrated in Sec. IV. Examples from the literature for both 3d transition metal addition as well as p-block partial replacement in MnP will be discussed. In Sec. V, we extend this analysis to other Mn-based ternary (TiNiSi structure) compositions that crystallize in the Pnma space group. Finally, we summarise the practical factors that can scale the crystal structure into the regime where a change of magnetic state can be expected.

II. CRYSTAL STRUCTURE AND COMPUTATIONAL DETAILS 

A. Crystal Structure 

The orthorhombic crystal structure of MnP (Pnma, 62) is shown in Fig.1 with unit cell dimensions a=5.268 Å, b=3.172 Å and c=5.918 Å at room temperature. Both the Mn and the P element occupy the 4c (x, 1 4 , z) crystallographic positions with xMn = 0.0049(2), zMn = 0.1965(2) and xP = 0.1878(5), zP = 0.5686(5)20. The orthorhombic structure can be regarded as a distortion from the higher symmetry hexagonal NiAs (P63/mmc, 194) type structure with the atomic positions of Ni (4c) shifted from xMn = 0, zN i = 1 4 to the xMn, zMn positions and the As (4c) atoms moved from xN i = 1 4 , zN i = 7 12 to xP and zP . Furthermore the two structures can be related as follows: , bortho = ahex and cortho = √ 3 × ahex.

III. RESULTS: MNP

In this article, we fix the lattice type as Pnma as found experimentally and concentrate on the electronic and magnetic properties of the prototype MnP binary alloy. We calculate the effect of isotropic lattice expansion and compression on non-magnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) solutions in order to find the critical lattice parameters where the crossover from one magnetic state into another can occur. For this study a single unit cell consisting of 8 atoms (4 Mn and 4 P) is used, which allows three different collinear antiferromagnetic configurations (AFM1, AFM2 and AFM3) and a collinear ferromagnetic (FM) one to be constructed, as show in Table I.