Coexistence of cluster ferromagnetism and cluster spin-glass like behaviour in melt-quenched Cu2Mn0.5Fe0.5Al Heusler alloy
Introduction
Heusler alloys have been fascinating magnetic systems since their discovery because of their diverse magnetic properties such as itinerant magnetism, local magnetism, half-metallicity, spin-glass phase etc [[1], [2], [3], [4], [5]]. They are represented with a stoichiometric formula X2YZ, where X and Y represent transition elements, and Z is a sp valent element. The nature of magnetic interactions in these alloys depends on the occupancy of transition metal elements in the X and Y sites. Spin-glass behaviour was observed in Cu2-based Heusler alloys such as Cu2MnZ (Z = Al, In and Sn) amorphous films [6] and crystalline Cu2VAl [5]. The cause of glassiness in the former series of alloys was attributed to competition between ferromagnetic and antiferromagnetic interactions, while in the latter it was attributed to structural disorder. A re-entrant spin-glass (RSG) transition was seen in single crystal Cu2Mn0.7Ti0.3Al. Motoya et al. reported that the glassy nature in the alloy was from dynamic magnetic clusters and showed that the spin-glass and itinerant characters of magnetic electrons are closely related [7].
Cu2MnAl has been investigated by many researchers, as it showed ferromagnetism though none of its constituent atoms is ferromagnetic [[6], [7], [8], [9]]. Kübler et al. theoretically studied the coupling mechanisms in full Heusler alloys and showed that Cu2MnAl was a localized ferromagnetic system with moment localized at Mn site (Y site) [1]. As Cu and Al are nonmagnetic elements, the magnetic interactions of any transition element at Y position could be easily investigated by substitution. The substitution of Fe at the Mn site in Cu2MnAl converts it from a localized ferromagnetic system to an itinerant type [10]. Hence the nature of coupling mechanisms which defines the magnetic order in the system could be investigated by partial replacement of Mn by Fe. But the Partial substitution of Fe for Mn in Cu2MnAl is difficult to achieve by conventional arc melting method, as it results in the formation of secondary phases beyond 25% Fe [11]. The melt spinning technique can be employed to stabilize the L21 structure in these alloys.
During the process of going from a localized magnetic system to an itinerant system, we observed a magnetic CG like phase in the low-temperature regime for 50% substitution of Fe at Mn site. In the present paper, the possibility of glassy nature in the alloy was investigated by dc and ac magnetic measurements. In particular, we used the frequency dependence of fundamental and higher harmonic components of complex ac susceptibility, specific heat and electrical resistivity studies as tools for probing the possible presence of magnetic glassiness in the sample.
Section snippets
Experimental procedure
The Cu2Mn0.5Fe0.5Al alloy ribbon was prepared by melt-spinning method using an induction furnace. The copper wheel speed in the melt-spinning process was 2000 rpm. The thickness of the ribbon was around ∼20 μm. The crystal structure of the ribbon was analysed using XRD technique by PANalytical X-ray diffractometer (X'pertPRO: Cu-Kα radiation). The DC and AC magnetization measurements were carried out on the ribbon sample using SQUID VSM (Quantum design) in the temperature range 2–350 K. The
Structural properties
Fig. 1 displays the XRD pattern of Cu2Mn0.5Fe0.5Al ribbon. A full Heusler alloy crystallizes in ordered L21 structure and is characterized by order dependent reflections (111), (200) and other principal reflections [e.g. (220), (400), (422)] in the XRD pattern. The absence of order-dependent reflections (111), (200) may indicate the presence of atomic antisite disorder. In order to know the crystal structure and phase purity in the sample, Rietveld refinement was carried out on the XRD data
Summary and conclusions
In summary, the L21 structure was stabilized by employing rapid solidification process in the pseudo-ternary Heusler alloy Cu2Mn0.5Fe0.5Al. We studied the dynamic properties of the alloy by measuring the ac susceptibility response for different applied frequencies. The Tf values obtained from χ″ (T) vs. T data were fitted with VF and power law. From the two laws, it was found that the cluster freezing temperature was around 33 K with high τ0 values. The existence of non-linear susceptibility
Acknowledgements
This work was supported by the Indian Institute of Technology Madras, India. One of the authors B. V. thanks Hanuma Kumar Dara for his help in heat capacity and resistivity measurements. The authors acknowledge the Department of Science and Technology (DST), India for the financial support for providing the PPMS facility used in this study (Grant No. SR/FST/PSII-038/2016).
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