Giant Magnetoelastic Coupling in a Metallic Helical Metamagnet

Abstract

Using high resolution neutron diffraction and capacitance dilatometry we show that the thermal evolution of the helimagnetic state in CoMnSi is accompanied by a change in inter-atomic distances of up to 2%, the largest ever found in a metallic magnet. Our results and the picture of competing exchange and strongly anisotropic thermal expansion that we use to understand them sheds light on a new mechanism for large magnetoelastic effects that does not require large spin-orbit coupling.PACS numbers: 65.40. De, 61.05.fm, 75.80.+q, 64.60.Kw Most materials change shape in a magnetic field. Usually the effect, and the so-called magneto-elastic interaction from which it derives are small, especially away from a phase transition. However, large magneto-elastic interactions are crucial to a range of new, technological materials in which multiple order parameters exist simultaneously and are coupled. These include ferromagnetic shape memory materials [1], multiferroics [2], and magnetic refrigerant materials [3]. Despite their relevance, atomically-resolved observations of large magneto-elastic effects within a given crystal structure are very rare.Giant magneto-elastic coupling was recently observed in multiferroic hexagonal manganites [2], where it is two orders of magnitude larger than in any other magnetic material. In those compounds it is believed to be central to magneto-electric coupling and multi-ferroicity and is thought to arise, unusually, from strongly varying exchange interactions where the co-ordination of Mn atoms in MnO 5 trigonal bipyramids removes the orbital degeneracy and Jahn-Teller mechanism typically found in MnO 6 -derived structures. Here we show that exchangederived giant magneto-elastic interactions are not limited to multiferroic oxides, but may be much more general, if one examines materials that possess competing exchange interactions relieved by temperature or applied field. The system we study is CoMnSi, a metallic antiferromagnet.We previously observed a large MCE in CoMnSi, a metamagnet that has a field-and temperature-induced transition from a low temperature, non-collinear incommensurate helical antiferromagnetic (AFM) state to a high magnetisation state [4]. CoMnSi is structurally similar to MnP, a system in which the field-dependence of non-collinear magnetic states has been well studied [5] and in which a rare Lifshitz tricritical point is seen [6]. However the similarity of the magnetic structures in MnP and in CoMnSi is not so well known, due to a lack of single crystals and of temperature-dependent neutron diffraction data. Here we focus on the iso-structural evolution of the crystal lattice within the helical groundstate of CoMnSi. We show that, within the AFM state and well below the zero field Néel temperature there is a giant, and opposing, change in the two shortest Mn-Mn distances, of around 2%. This change has two consequences. Firstly it brings about an Invar-like effect in sample volume in zero magnetic field. Secondly, it couples strongly to the suppresion of helimagn...

Wide temperature span of entropy change in first-order metamagnetic MnCo1−xFexSi

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Strong magnetoelastic coupling and unconventional electric polarization in the triangular-lattice multiferroic Li0.99Cu0.01CrODey1, Karmakar2, Majumdar3 et al. 2013Phys. Rev. B194181View full textAdd to dashboardCite

“…In such a backdrop of versatile abundance of scientific studies, 2-5 the evidence of magnetoelasticity with isostructural transition is quite rare, while those inheriting large atomic displacement are even rarer. 7,8 In the following study we observe a fairly large oxygen displacement at an isostructural transition, giving rise to ∼ 0.02 A step-like increase in Cr−O bond length as obtained from the x-ray diffraction studies using high-flux synchrotron source in a two-dimensional (2D) triangular lattice antiferromagnet (AFM), Li 0.99 Cu 0.01 CrO 2 (LCCO). Most interestingly, this displacement enhances remarkably (∼ 10 times) due to minimal Cu doping in LiCrO 2 (LCO).…”

Structural and magnetic properties of MnCo 1−x Fe x Si alloys

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