Elsevier

Intermetallics

Volume 78, November 2016, Pages 50-54
Intermetallics

Magnetocaloric effect in HoMn2Si2 compound with multiple magnetic phase transitions

https://doi.org/10.1016/j.intermet.2016.09.002Get rights and content

Highlights

  • Below room temperature the compound under goes multiple magnetic transitions.

  • Magnetic transitions are connected with an iso-structural volume change.

  • Small thermal hysteresis is observed in HoMn2Si2 compound.

  • Thermomagnetic irreversibility is due to the narrow domain wall pinning effect.

  • Large magnetocaloric effect suggest that it is suitable for the low temperature MR.

Abstract

The structural, magnetic and magnetocaloric properties of the polycrystalline compound HoMn2Si2 have been studied. The temperature variation of magnetization and heat capacity show that below room temperature the compound under goes multiple magnetic transitions, at about 80, 15 and 3.5 K respectively. Moreover, the presence of the interesting thermomagnetic irreversibility in HoMn2Si2 is detected in the magnetization versus temperature plot, which can be ascribed to the narrow domain wall pinning effect. A broad and asymmetric peak is observed for the MCE response which might suggest the underlying first-order nature of the transition and/or the spin fluctuations of the Mn subsystem. The density of states N(EF) at the Fermi level and the Debye temperature have been determined and analyzed. Large magnetocaloric effect (ΔSM = −9 J/kg-K for a field change of 5 T around 9 K) suggest that this material is suitable for the low temperature magnetic refrigeration.

Introduction

In recent years, the magnetic refrigeration, a green cooling technology based on magnetocaloric effect (MCE), an intrinsic phenomenon of a magnetic material has attracted much attention due to its higher energy efficiency and eco-friendliness than the commonly used conventional gas compression refrigeration which uses a magnetic field to change the magnetic entropy of a material [1], [2], [3], [4], [5], [6]. The MCE is an intrinsic phenomenon of magnetic materials, which manifests as an isothermal magnetic entropy change (ΔSM) or an adiabatic temperature change (ΔTad) when the magnetic material is exposed to a varying magnetic field. Large values of ΔSM and ΔTad under a varying magnetic field are considered to be the most important indicator of magnetic cooling materials for the practical application, and therefore it is important to find new materials with large MCE. So to search for magnetic materials with large values ΔSM and ΔTad is the main mission in this field of research.

Typically, the large/giant MCE is observed in the material that undergoes first order magnetic phase transition. Large number of magnetic materials with large entropy change accompanied by a first order phase transition have been reported [1], [2], [3], [7]. Apart from MCE, the first order magnetic transition (FOMT) gives rise to various other functional properties of technological importance, such as magnetostrictive, magnetoresistance, shape memory effects etc. [8]. However, the first-order phase transition is usually accompanied by considerable thermal and/or magnetic hysteresis along with irreversibility, which is disadvantage for a magnetic refrigeration cycle [7], [9], [10], [11]. But some of them, especially rare-earth-based intermetallic compounds, have reversible large MCE along with a small hysteresis loss [12], [13], [14], [15]. Therefore it is desirable to search for rare-earth-based intermetallic materials that not only exhibit large reversible MCE but also have none or negligible thermal and field hysteresis loss.

The RT2X2 compounds, where R is rare earth intermetallic compounds (Gd, Tb, Dy, Ho etc.), T is transition metal, and X is Si/Ge, is an important class of materials, which exhibits a wide variety of interesting physical properties [16], [17], [18], [19]. Depending on the constituent element or composition, various properties like superconductivity, magnetic ordering, heavy-fermion properties, valence fluctuation, large MCE, etc. are observed [16], [17], [18], [19]. As Mn carries magnetic moment, (unlike many other T elements such as Co, Ni and Fe) and its magnetic states strongly depends on intraplanar Mn-Mn distances, the compounds with T = Mn attract particular interests [16], [20]. It is also known that, only in the case where T = Mn do they display long-range magnetic order, which, incidentally can exist even above 300 K. However, the rare earth lattice orders magnetically at lower temperatures. The RMn2X2 compounds crystallize in the ThCr2Si2-type body centered tetragonal structure (with space group I4/mmm) [21], [22] with repeated layers of atoms stacked in the sequence R-X-Mn-X-R [20], [23] along the c-axis (similar to R5(Si, Ge)4, a family of well-known large MCE materials) [24]. These characteristics of RT2X2 compounds offer scopes for selection of the magnetic state—and therefore the MCE—by controlling the interlayer and intralayer distances between magnetic atoms.

Recently, some RT2X2 compounds have been observed to possess not only large MCE but also a small hysteresis loss around their ordering temperature [18], [19]. The magnetic structure of HoMn2Si2 material has been studied using neutron diffraction dates from the 1990s [25]. This compound exhibits the crystal structure of ThCr2Si2 type with the Z parameter of Si in 4(e) site (space group 14/mmm). It exhibits antiferromagnetic ordering in the form of a flat spiral of Ho moments. Mn magnetic moments located in planes normal to the c-axis are coupled ferromagnetically, but moments in adjacent planes can be coupled either ferromagnetically or antiferromagnetically. They are aligned along the c axis.

In order to improve the application of magnetocaloric cooling, efforts have been paid to search for magnetic refrigerants in view of domestic or industrial applications near room temperature [1], [2], [3], [4], [5], [6], [7]. On the other hand, the research on systems exhibiting large MCEs at low temperature is also important due to their prospective application in the refrigeration for gas liquefaction of hydrogen and helium [26], [27], [28]. Magnetic refrigeration at liquid helium temperatures is indeed of interest especially in space, medical and other cryogenic applications [29]. Significant capital cost reduction can potentially be achieved with a magnetic hydrogen liquefier compared to conventional liquefiers. Magnetic refrigeration may allow hydrogen to be competitive as an alternative fuel source [30].

In this paper, for achieving a complete understanding of HoMn2Si2 material, a detailed investigation of the magnetic structure and magnetic phase transitions was presented. A large reversible MCE was observed, accompanied by a first order magnetic phase transition which is applicable for low temperature magnetic refrigeration. Characteristics of the magnetic transition, the origin of large MCE as well as its potential application in HoMn2Si2 were discussed.

Section snippets

Experimental details

Polycrystalline HoMn2Si2 sample was synthesised by an arc melting method under an argon atmosphere. The stoichiometric amounts of high purity Ho (99.9%), Mn (99.9%) and Si (99.99%) were melted four times for homogeneity on a water-cooled copper hearth. Around 3% excess Mn was added to compensate the Mn evaporation during the melting. The as-cast samples were sealed in an evacuated quartz tube and annealed for 1 week at 800 °C followed by furnace cooling. The sample was proved to be single phase

Results and discussion

A schematic of the crystal structure of HoMn2Si2 is provided in Fig. 1 along with Rietveld refinement of X ray data. The sample is confirmed to be single phase with the body centered tetragonal ThCr2Si2 type structure (space group I4/mmm). The calculated lattice parameters at room temperature [a (Å) = 3.922, c (Å) = 10.444 and V (Å3) = 16.044] derived from the refinement are in good agreement with the reported values [25], [31]. There is small amount of second phase present in the sample.

Conclusions

To summarize, three magnetic phase transitions have been detected in HoMn2Si2 below room temperature. An obvious magneto-history effect below 15 K is present which can be attributed to narrow domain wall pinning effect. First order nature of the transitions is revealed by Arrott plots. Large magnetic entropy change ΔSM = −9 J/kg-K is observed at 9 K with the application of 5 T magnetic field which is applicable for the low temperature magnetic refrigeration. The low temperature heat capacity

Acknowledgement

J. C. Debnath acknowledges the Alfred Deakin Postdoctoral Research Fellowship at Deakin University (Project ID: RM0000029546), Australia.

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