Superparamagnetism and magnetic defects in Fe₂VAl and Fe₂VGa

Abstract

We have investigated the magnetic properties of Heusler-type Fe2VAl and Fe2VGa by means of susceptibility, magnetization, and nuclear magnetic resonance (NMR) measurements. As evidenced by the static and dynamic magnetic susceptibilities, both compounds exhibit the characteristic features of a superparamagnetic glass. The freezing temperatures of our samples are 23 K for Fe2VAl and 9 K for Fe2VGa. A detailed analysis indicates the coexistence of individual magnetic defects with large-moment superparamagnetic clusters. The temperature-dependent magnetization and NMR linewidth are dominated by isolated magnetic defects, identified as Fe antisite defects. The concentration of these defects, determined from NMR linewidths, is 4.5×10-3 in Fe2VAl, in good agreement with the value deduced from previous field-dependent specific heat measurements.

The zero-magnetization Heusler ferrimagnet

“…Even a small concentration of antisite defects in the L2 1 structure, for example, can modify the magnetic moment and Fermi-level spin polarization of the phase. [4][5][6] A Slater-Pauling plot for a series of L2 1 Heusler compounds shows that the moment per unit cell is linear in electron count, with slope À1. 1 The moment disappears when there are 24 electrons per formula, or an average of six per atom.…”

NMR and Mössbauer study of spin dynamics and electronic structure ofFe2+xV1−xLue1, Li2, Ross3 et al. 2003Phys. Rev. B200110View full textAdd to dashboardCite

“…20 In Fe 2+x V 1−x Al T c goes monotonically to zero as x goes to zero, 9 although Fe 2 VAl exhibits a sample-dependent low-temperature transition which may be residual ferromagnetism 10,21 or a superparamagnetic freezing temperature. 7,19 Furthermore in Fe 2 VAl and Fe 2 VGa localized magnetic defects have been observed and associated with Fe antisites, 6,12,15 while there is also evidence for larger magnetic clusters in the materials. 7,19 No magnetic splitting was observed in Mössbauer studies near the Fe 2 VAl composition, 10,22,23 while the dominance of defects and clusters makes a percolative behavior seem likely.…”

“…7,19 Furthermore in Fe 2 VAl and Fe 2 VGa localized magnetic defects have been observed and associated with Fe antisites, 6,12,15 while there is also evidence for larger magnetic clusters in the materials. 7,19 No magnetic splitting was observed in Mössbauer studies near the Fe 2 VAl composition, 10,22,23 while the dominance of defects and clusters makes a percolative behavior seem likely.…”

“…Samples used for all measurements were a portion of the same ingots used in previous NMR 7,16,24 and specific heat 6 measurements. These are polycrystalline samples prepared by arc melting.…”

“…3 The Heusler alloys Fe 2+x V 1−x Al and Fe 2+x V 1−x Ga are ordered intermetallics which exhibit GMR near T c , 4,5 although in addition to thermal disorder, localized magnetic clusters play a prominent role in these materials, even for the nominally nonmagnetic composition x = 0. 6,7 There has been considerable interest in understanding the role of these clusters for the magnetic transitions, and the unusual electronic behavior associated with these materials. 8,9,10 Both Fe 2 VAl and Fe 2 VGa have been shown to be semimetallic and intrinsically non-magnetic, via electronic structure calculations 11,12,13,14,15 NMR measurements, 7,16,17 and optical measurements.…”