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Large uniaxial magnetostriction with sign inversion at the first order phase transition in the nanolaminated Mn2GaC MAX phase
Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, 47057, Duisburg, Germany..
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, 47057, Duisburg, Germany.; National University of Science and Technology «MISIS», 119049, Moscow, Russian Federation..
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. Grein Research ehf. Dunhaga 5, Reykjavik, Iceland..
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, no 1, article id 2637Article in journal (Refereed) Published
Abstract [en]

In 2013, a new class of inherently nanolaminated magnetic materials, the so called magnetic MAX phases, was discovered. Following predictive material stability calculations, the hexagonal Mn2GaC compound was synthesized as hetero-epitaxial films containing Mn as the exclusive M-element. Recent theoretical and experimental studies suggested a high magnetic ordering temperature and non-collinear antiferromagnetic (AFM) spin states as a result of competitive ferromagnetic and antiferromagnetic exchange interactions. In order to assess the potential for practical applications of Mn2GaC, we have studied the temperature-dependent magnetization, and the magnetoresistive, magnetostrictive as well as magnetocaloric properties of the compound. The material exhibits two magnetic phase transitions. The Néel temperature is T N  ~ 507 K, at which the system changes from a collinear AFM state to the paramagnetic state. At T t  = 214 K the material undergoes a first order magnetic phase transition from AFM at higher temperature to a non-collinear AFM spin structure. Both states show large uniaxial c-axis magnetostriction of 450 ppm. Remarkably, the magnetostriction changes sign, being compressive (negative) above T t and tensile (positive) below the T t . The sign change of the magnetostriction is accompanied by a sign change in the magnetoresistance indicating a coupling among the spin, lattice and electrical transport properties.

Place, publisher, year, edition, pages
London: Nature Publishing Group, 2018. Vol. 8, no 1, article id 2637
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-145680DOI: 10.1038/s41598-018-20903-2OAI: oai:DiVA.org:liu-145680DiVA, id: diva2:1190598
Available from: 2018-03-15 Created: 2018-03-15 Last updated: 2018-03-15Bibliographically approved
In thesis
1. Synthesis and characterization of magnetic nanolaminated carbides
Open this publication in new window or tab >>Synthesis and characterization of magnetic nanolaminated carbides
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

MAX phases are a group of nanolaminated ternary carbides and nitrides, with a composition expressed by the general formula Mn+1AXn (𝑛 = 1 − 3), where M is a transition metal, A is an A-group element, and X is carbon and/or nitrogen. MAX phases have attracted interest due to their unique combination of metallic and ceramic properties, related to their inherently laminated structure of a transition metal carbide (Mn+1Xn) layer interleaved by an A-group metal layer.

This Thesis explores synthesis and characterization of magnetic MAX phases, where the A-group element is gallium (Ga). Due to the low melting point of Ga (T = 30 °C), conventional thin film synthesis methods become challenging, as the material is in liquid form at typical process temperatures. Development of existing methods has therefore been investigated, for reliable/reproducible synthesis routes, including sputtering from a liquid target, and resulting high quality material. Routes for minimizing trial-and-error procedures during optimization of thin film synthesis have also been studied, allowing faster identification of optimal deposition conditions and a simplified transfer of essential deposition parameters between different deposition systems.

A large part of this Thesis is devoted towards synthesis of MAX phase thin films in the Cr-Mn-Ga-C system. First, through process development, thin films of Cr2GaC were deposited by magnetron sputtering. The films were epitaxial, however with small amount of impurity phase Cr3Ga, as confirmed by X-ray diffraction (XRD) measurements. The film structure was confirmed by scanning transmission electron microscopy (STEM) and the composition by energy dispersive X-ray spectroscopy (EDX) inside the TEM.

Inspired by predictive ab initio calculations, the new MAX phase Mn2GaC was successfully synthesized in thin film form by magnetron sputtering. Structural parameters and magnetic properties were analysed. The material was found to have two magnetic transitions in the temperature range 3 K to 750 K, with a first order transition at around 214 K, going from non-collinear antiferromagnetic state at lower temperature to an antiferromagnetic state at higher temperature. The Neél temperature was determined to be 507 K, changing from an antiferromagnetic to a paramagnetic state. Above 800 K, Mn2GaC decomposes. Furthermore, magnetostrictive, magnetoresistive and magnetocaloric properties of the material were iv determined, among which a drastic change in lattice parameters upon the first magnetic transition was observed. This may be of interest for magnetocaloric applications.

Synthesis of both Cr2GaC and Mn2GaC in thin film form opens the possibility to tune the magnetic properties through a solid solution on the transition metal site, by alloying the aforementioned Cr2GaC with Mn, realizing (Cr1-xMnx)2GaC. From a compound target with a Cr:Mn ratio of 1:1, thin films of (Cr0.5Mn0.5)2GaC were synthesized, confirmed by TEM-EDX. Optimized structure was obtained by deposition on MgO substrates at a deposition temperature of 600 ºC. The thin films were phase pure and of high structural quality, allowing magnetic measurements. Using vibrating sample magnetometry (VSM), it was found that (Cr0.5Mn0.5)2GaC has a ferromagnetic component in the temperature range from 30 K to 300 K, with the measured magnetic moment at high field decreasing by increasing temperature. The remanent moment and coercive field is small, 0.036 μB, and 12 mT at 30 K, respectively. Using ferromagnetic resonance spectroscopy, it was also found that the material has pure spin magnetism, as indicated by the determined spectroscopic splitting factor g = 2.00 and a negligible magnetocrystalline anisotropy energy.

Fuelled by the recent discoveries of in-plane chemically ordered quaternary MAX phases, so called i-MAX phases, and guided by ab initio calculations, new members within this family, based on Cr and Mn, were synthesized by pressureless sintering methods, realizing (Cr2/3Sc1/3)2GaC and (Mn2/3Sc1/3)2GaC. Their structural properties were determined. Through these phases, the Mn content is the highest obtained in a bulk MAX phase to date.

This work has further developed synthesis processes for sputtering from liquid material, for an optimized route to achieve thin films of controlled composition and a high structural quality. Furthermore, through this work, Mn has been added as a new element in the family of MAX phase elements. It has also been shown, that alloying with different content of Mn gives rise to varying magnetic properties in MAX phases. As a result of this Thesis, it is expected that the MAX phase family can be further expanded, with more members of new compositions and new properties.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. p. 58
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1918
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-145679 (URN)10.3384/diss.diva-145679 (DOI)9789176853429 (ISBN)
Public defence
2018-04-16, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
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Supervisors
Available from: 2018-03-15 Created: 2018-03-15 Last updated: 2019-09-30Bibliographically approved

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