Disproportionate influence of site disorder on the evolution of magnetic phases in anti-Heusler alloy Al$_2$MnFe

TL;DR

The paper addresses how site-disorder types control magnetic phase evolution in the anti-Heusler Al$_2$MnFe. By combining synthesis, X-ray diffraction, magnetization, ac-susceptibility, Mössbauer spectroscopy, transport, heat capacity, and DFT/SPRKKR calculations with SQS modeling, it identifies a dominant B2-type disorder on octahedral sites and a modest ~12% Mn–Al inter-site swap as the key driver: ferromagnetic order sets in at $T_{ m C} \sim 113$ K while a reentrant spin-glass state emerges below $T_{ m f} \sim 20$ K due to competing AFM interactions involving Mn at 8$c$. Theoretical results show Mn on 4$a$/$4$b$ sites couple ferromagnetically, whereas Mn on 8$c$ couples antiparallel, explaining the observed glassiness; the energy preference for B2 over L2$_1$ supports the structural model. Overall, the work demonstrates that inter-site disorder in moment-carrying sublattices can disproportionately influence magnetic ground states, providing guidelines to tailor magnetic and transport properties for anti-Heusler spintronics.

Abstract

Anti-Heusler alloys, being a new addition to the Heusler alloys family, exhibit atomic disorders, and almost all of them are reported as a re-entrant spin-glass system. Although such spin-glass feature is generally attributed to the inherent atomic disorder, a comprehensive and extensive investigation on the individual roles of different types of disorders in magnetic interactions remains lacking for any of the reported anti-Heusler systems. As an illustrative case, we have carried out an in-depth experimental as well as theoretical investigation of structural, magnetic, and transport properties of a polycrystalline anti-Heusler alloy, Al$_2$MnFe. While the major atomic disorder is found to be among Fe and Mn atoms, which are randomly distributed among the two octahedral sites, 4$a$ and 4$b$ (B2-type disorder), a relatively small fraction ($\sim$12\%) of Mn atoms also replace Al atoms at the tetrahedral 8$c$ site. Magnetically, the system undergoes two transitions: a paramagnetic to a ferromagnetic transition at $T_{\rm C}\sim$113~K, followed by a spin-glass phase transition below $T_{\rm f}\sim$20~K. Here, the magnetic moment is primarily confined to Mn atoms. Very interestingly, our theoretical analysis reveals that the ferromagnetic spin arrangement remains rather robust in spite of the 50\% disorder of moment-carrying Mn atoms between the two octahedral sites, but a much smaller ($\sim$12\%) cross-distribution of Mn atoms between octahedral and tetrahedral sites are sufficient to impose a reentrant spin-glass state at low temperature. Our analysis brings forth the importance of understanding the role of individual types of swap-disorder on magnetic properties in the anti-Heusler family of materials.

Figures (7)

• Figure 1: Primitive cell of full-HA and anti-HA. Table describes reversing of atoms ($X$ and $Z$) from full-HA to anti-HA.
• Figure 2: Rietveld refinement of the X-ray diffraction pattern assuming (a) L2$_{\rm 1}$ ordered structure, (b) B2-type and (c) B2 + 12% Mn-Al disordered structure, and its associated atomic arrangements are shown in (d), (e), and (f), respectively. 4$a$ and 4$b$ octahedral symmetry sites are represented in (g) and (h), whereas 8$c$ tetrahedral symmetry site is depicted in (i).
• Figure 3: (a) Temperature dependence ZFC and FC magnetization curve under 100 Oe applied field (left), $H/M$ curve (right), d$M$/d$T$ curve (lower panel); (b) Field dependence magnetization curve at 2 K, upper (I) and lower (II) inset shows spontaneous magnetization fitting and coercivity respectively; (c) Wait time dependent magnetic relaxation measured in ZFC protocol at $T$= 5 K (d) $M(T)$ curve under various applied field, where closed symbols are FC curves whereas the open symbols represents ZFC curves. Left and right arrow shows the gradual change of $T_{\rm P}$ and $T_{\rm C}$ respectively. (e) $M(H)$ curve measured at different temperatures, inset represent temperature dependence spontaneous magnetization value, which follows the SW equation. (f) Temperature dependent relaxation with fixed wait time, $t_{\rm w}$= 60 s. (g) Shifting of hump in relaxation rate $S(t)$ due to different wait time. Memory effect measured in (h) intermediate cooling and (i) intermediate heating protocol. Inset (I) of (h) is the merging data of interval 1 and 3 measured in cooling protocol. (j) Magnetic phase diagram of Al$_2$MnFe, obtained from zero-field-cooled (ZFC) magnetization data. The low temperature dashed curve is drawn to show the spin glass (SG) region, where the disordered spins are frozen, depicted by arrows. The ferromagnetic (FM) region is in between the two dashed curve, where spins are arranged in parallel to each other, shown by parallel arrows. The high temperature region above the dashed line is the paramagnetic (PM) region, where the spins are randomly oriented and vibrates due to thermal energy.
• Figure 4: Frequency dependent (a) real ($\chi^{\prime}$) and (c) imaginary $\chi^{\prime\prime}$ part of ac-susceptibility measured at fixed excitation field $h$= 5 Oe.The real part curve get bifurcated below $T_{\rm f}$, which also increase to higher temperature (in imaginary part also) due to the applied higher frequency, shown in inset. (b) and (d) depicts excitation field dependence $\chi^{\prime}$ and $\chi^{\prime\prime}$ respectively, measured at fixed frequency $f$= 5329 Hz. Here the curve bifurcated below $T_{\rm C}$ and subsequently merges after $T_{\rm f}$. The arrow shows $T_{\rm f}$ does not changes due to the variation of excitation field. (e) Blown up of the frequency dependent real part of ac susceptibility curve. Inset (I) Arhenius law fitting, (II) critical scaling theory fitting and (III) Vogel Fultcher theory fitting of the freezing temperatures with respect to the applied frequencies.
• Figure 5: Mössbauer spectrum measured at (a) $T$=300 K, (b) $T$ = 5 K, (c) at six different temperatures in between 5-300 K,(d) The points shows the temperature evolution of full width half maxima (FWHM) and the line is the guide to eye.(e) Temperature dependent zero field longitudinal resistivity ($\rho_{xx}(T)$, measured in heating cycle. Inset shows the change of slope at $T_{\rm C}$. (f) $\rho_{xx}(T)$ at $H=$ 0 and 50 kOe measured in both heating and cooling cycle. (g) Field dependent longitudinal resistivity ($\rho_{xx}(H)$) at $T$= 100 and 120 K. (h) Magnetoresistance (%) data, $\rho_{xx}(H)$ of various temperatures in the range of 5-250 K . (i) Heat capacity curve of Al$_2$MnFe. Inset show the low temperature fitting with $\gamma T+ \beta T^3$.