Elsevier

Journal of Alloys and Compounds

Volume 777, 10 March 2019, Pages 373-381
Journal of Alloys and Compounds

Coexistence of cluster ferromagnetism and cluster spin-glass like behaviour in melt-quenched Cu2Mn0.5Fe0.5Al Heusler alloy

https://doi.org/10.1016/j.jallcom.2018.10.327Get rights and content

Highlights

  • Stabilization of L21 structure by melt quenching.

  • Cluster glass phase revealed by spin dynamics and memory effect.

  • Coexistence of ferromagnetic interactions and glassy nature.

  • Lack of anomaly in specific heat at freezing temperature.

  • Electrical resistivity exhibits T3/2 dependence.

Abstract

We investigated the structural stability and magnetic properties of the melt-quenched Heusler alloy-Cu2Mn0.5Fe0.5Al using X-ray diffraction (XRD), scanning electron microscopy (SEM), dc magnetization and ac susceptibility measurements respectively. The structural analysis by Rietveld refinement showed that the alloy crystallised in L21 structure with no trace of secondary phase in the XRD pattern. The dc magnetic measurements exhibited a thermal irreversibility between field-cooled (FC) and zero field cooled (ZFC) magnetization curves at an irreversible temperature Tirr. A deviation from de Almeida –Thouless like line for Tirr vs. H data was observed, suggesting a possibility of cluster glass (CG) like phase in the sample. To confirm the CG behaviour, frequency dependent ac susceptibility (χac) measurements were carried out on the sample and was analysed by Vogel-Fulcher (VF) and Critical slowing down models. We obtained large relaxation time constants (τ0) from the fit to both the phenomenological models which indicate that the origin of glassy nature may not be from randomly oriented atomic spins like in spin-glasses (SG) but of magnetic clusters. The analysis of nonlinear susceptibilities of the sample confirmed the coexistence of ferromagnetic interactions and glassy nature in the sample. Further confirmation for the glassy nature in the sample was provided by ZFC memory experiments. No anomaly was found corresponding to long-range order in the heat capacity (CP(T)) measurement and a T3/2 dependence was also seen in the electrical resistivity which indicates the magnetic glassiness in the sample.

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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