Magnetically nanostructured state in a Ni-Mn-Sn shape-memory alloy

Abstract

For certain compositions Ni-Mn-Sn and related magnetic shape memory alloys undergo a martensitic transition at temperatures in the range 300 to 400 K, with the emergence of novel magnetic properties below the transition. While Ni 50 Mn 50 is an antiferromagnet, substitution of Sn on some fraction of the Mn sites in Ni 50 Mn 50-x Sn x leads to competing ferromagnetic (F) and antiferromagnetic (AF) phases at low temperatures. Details of this magnetic phase coexistence are, however, significantly lacking, particularly with respect to the AF phase. The present investigations use zero applied magnetic field 55 Mn NMR as a local probe of the magnetic properties of the alloy Ni 50 Mn 50-x Sn x with x = 10. Rich multi-peak spectra are observed, and the various components are definitively assigned to nanoscale F or AF regions. Measurements of the static nuclear hyperfine field distributions as a function of temperature, and in small applied fields, together with nuclear relaxation rates, provide detailed information on the size distributions, relative concentrations, and physical natures of these F and AF regions. The results show that the nanoscale magnetic features of the x = 10 system are substantially more complex than previous studies have suggested. We argue that the general approach used in these experiments is applicable to other such complex metal alloys, and could yield many additional insights.

Phase separation and superparamagnetism in the martensitic phase ofNi50−xCoxMn

“…As discussed in our prior work, 22 further confirmation of the F / AF assignments is provided by measurements of spectral frequency shifts in low (~ 1T) applied magnetic fields; lines due to the F component downshifts with H, in contrast to the behavior of spectral lines from AF regions.…”

“…22 μ , up to 8 T using a spin echo spectrometer with automated frequency sweep capability over the range f = 150 -450 MHz. The polycrystalline metallic grains were embedded in paraffin wax to minimize skin depth effects.…”

“…The approximately linear T-dependence (solid black fit line) suggests a mechanism involving dynamics of the F clusters, as discussed below, and established in our prior work. 22 In order to also analyze the evolution of the spectral weight with temperature, the peaks for the x = 7 and x = 14 samples were fit with multiple Gaussian curves (not shown). For x = 7, the behavior of the Curielaw-corrected area of the F components in Region H is plotted (in arbitrary units) as a function of T in Fig.…”

“…22 If there is a distribution of cluster sizes it follows that there is a corresponding distribution of Finally, the transverse or spin-spin relaxation time T 2 , obtained from stretched exponential fits to the spin echo decay curves at x = 7, is found to be weakly T dependent, and, for the F components in Region H, decreases from 110 s μ to <10 s μ as T increases from 1.6 K to 120 K. Fig. 3 shows that the spectral area decreases steadily over this same temperature range, as the regions in which T 2 < 10 s μ no longer contribute significantly to spin echoes.…”

“…The procedure is described in Ref. 22 and involves the use of the following expression for the nuclear spin-spin relaxation rate for a globular cluster of volume V …”