Large uniaxial magnetostriction with sign inversion at the first order phase transition in the nanolaminated Mn2GaC MAX phase

Abstract

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 Mn 2 GaC 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 Mn 2 GaC, 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.Inherently nanolaminated M n+1 AX n (n = 1, 2, 3) compounds attract tremendous interest, since these materials provide the unique anisotropic structural and physical properties important for diverse applications 1,2 . These compounds, collectively known as MAX phases, are composed of an early transition metal (M), a p-element from the A-group elements (A) and X being either C or N. MAX phases have a hexagonal structure and belong to the space group P6 3 /mmc with the primitive unit cell given by 8 atoms: 4 M, 2 A and 2 X (for n = 1). These systems exhibit an atomically laminated structure composed of M-X-M (M 2 X) slabs interleaved by A-element atomic layers. The atomic layers are stacked along the c-axis. The layered, highly anisotropic crystal structure results in mechanical properties usually associated with ceramics, such as high stiffness, damage tolerance and resistance to corrosion and thermal shock 1,2 . The chemical bonding of the M, A and X elements is anisotropic and comprises metallic, covalent and ionic character 2 . The strong hybridization between d orbitals of the M-element and 2p states of the X-element results in directed covalent bonds along the M-X-M chains in basal planes 2,3 . The M-A bonding is generally weaker and accompanied by partial charge transfer from the M-element to the A-element, giving rise to the ionic contribution 2-4 . Metallic-like bonding between d states of the M-element occurs in the

Impact of strain, pressure, and electron correlation on magnetism and crystal structure of Mn2GaC from first-principles

“…This is indicative of a parameter forcing the magnetic moment to be localized, raising a question of the proper value of U eff to describe Mn 2 GaC in particular and magnetic MAX phases in general. These results indicate the PBE or PBE + U (with U eff ≤ 0.25 eV) is the most appropriate functionals to use until proven otherwise for describing Mn 2 GaC when compared to experimental results [26][27][28][29] .…”

“…The first experimental evidence of magnetic MAX phases were quaternary Mn-doped Cr 2 GeC 10 – 12 , Cr 2 GaC 13 – 17 and Cr 2 AlC 14 , 18 , 19 followed by (Mo,Mn) 2 GaC 20 , 21 and (V,Mn) 3 GaC 2 22 . Examples of ternary MAX phases with demonstrated magnetic properties are Cr 2 AlC 23 , 24 , Cr 2 GeC 11 , 24 , Cr 2 GaC 13 , 25 , Cr 2 GaN 25 , and Mn 2 GaC 26 – 29 . The Cr-based phases have all been made in bulk form and as thin films, whereas Mn 2 GaC have been synthesized in thin film form only.…”

“…Based on supercell calculations, the magnetic critical order-disorder temperature T c has been predicted to be 660 ± 133K 31 . Subsequent measurement found a Néel temperature of 507 K, at which Mn 2 GaC changes from a (suggested) collinear AFM state to the paramagnetic (PM) state 29 .…”

“…c-axis magnetostriction of 450 ppm 29 . Moreover, Mn 2 GaC exhibits neutron-diffraction peaks consistent with long-range AFM order with a periodicity of two structural unit cells 28 .…”

Long-term stability and thickness dependence of magnetism in thin (Cr0.5Mn0.5)2GaC MAX phase films

“…[12][13][14]. Recent studies revealed a high magnetic ordering temperature (above 200 K) which is strongly influenced by the Mn concentration at the M-site and by the choice of the A-element [15][16][17][18][19][20][21][22]. The highest phase purity and structural quality has been achieved in (Cr 0.5 Mn 0.5 ) 2 GaC films epitaxially grown on MgO (111) substrates [23].…”