Tuning nature and temperature of structural and magnetic phase transitions of (M=Ag, Ni)
Introduction
Refrigeration based on the magnetocaloric effect (MCE) is a proven [1], environmentally friendly and efficient alternative to conventional gas-compression technology. Although research and development focuses on room temperature household applications, wide temperature span applications such as gas liquefaction [2] may greatly benefit from the flexibility and extreme tunability presented by magnetocaloric materials.
Materials with first order magnetostructural transitions (FOMST), where the magneto-elastic coupling in a material is strong enough to make both crystal and magnetic lattices change discontinuously, show especially strong magnetocaloric effects with high magnetic entropy changes and adiabatic temperature [3], While some materials undergo order-order magnetostructural transitions leading to a giant inverse magnetocaloric effect [[4], [5], [6], [7], [8]], most materials exhibiting giant magnetocaloric effects, undergo disorder-order magnetostructural transitions between the high temperature paramagnetic phase and a low temperature ferri- or ferromagnetic phase [[9], [10], [11], [12]].
There are a few compounds among Mn-rich antiperovskites that show a large magnetocaloric effect. The most prominent example is [13]. Recently, a large magnetic entropy change of 13.52 J/Kkg (for a 5 T field change) was reported for at its phase transition around 145 K [14]. The high temperature phase of Mn-rich antiperovskites with the composition , where M is a metal and X a main group element like N or C, has a cubic structure with symmetry. This structure consists of a cubic primitive cell of the metal M with the faces occupied by Mn and X filling the octahedral hole in the center [15] (see Fig. 1 left).
is paramagnetic at room temperature and crystallizes in the cubic structure described above. At 143 K it undergoes a magneto-structural phase transition to a ferrimagnetic low-temperature phase with tetragonal structure. The low temperature phase has symmetry and the c/a of the tetragonal cell is estimated to be 0.9853 while the volume of the cell is nearly conserved through the transition [16] (see Fig. 1 right). The magnetic structure of the low temperature phase consists of two small induced moments located on the Cu atoms and six considerably larger moments located on the Mn atoms. In this structure two Mn1 atoms occupy the 1c (0.5,0.5,0) site and the Cu atoms occupy the 1a site (0.0.0), while the four remaining Mn2 atoms occupy the 2e site (0,0.5,0.5). The magnetic structure can be described as alternating Mn1 – Mn2 layers which stack along the c-axis. The moments in the Mn1 layer (including the small Cu induced moment) are aligned along the c axis and show a ferromagnetic coupling. However, the four Mn moments on the Mn2 layer are canted from the c axis towards the [111] direction with a canting angle of [17].
Transition metal substitutions in give rise to a number of interesting properties [18]. Ding et al. report a near zero coefficient of resistivity (NZ-TCR) in in a tunable temperature range. With increasing Cu content, the magnetic ordering temperature decreases as well as the ferromagnetic interactions of the low temperature state [19,20]. is also reported to show NZ-TCR with a tunable temperature range. The nature of the resistivity is found to be dependent on the magnetic structure of the material. For a silver content below x = 0.25 the materials are reported to be ferromagnetic with slightly increasing ordering temperatures with increasing Ag content [21]. We have recently reported a tunable, giant magnetocaloric effect with low hysteresis in [22], where the Curie temperature increases with increasing carbon content.
As relevant properties like the transition temperature and the magnetic or magnetocaloric properties can be tuned by shifting the transition temperature using, for example, compositional changes, it is of importance to understand the influence of such compositional changes. Therefore, in this paper, we investigate the influence of C and M (M = Ag, Ni) substitution on the magnetic transition and magnetocaloric properties of compounds.
Section snippets
Experimental
Polycrystalline samples of (with x = 0, 0.2 and y = 0, 0.1, 0.2, 0.3) and (with x = 0, 0.1 and y = 0, 0.1, 0.2, 0.3) were synthesized via a solid state reaction. powder, prepared by flowing ammonia over Mn (≥99.99%) powder at 1037 K for 4 h, was thoroughly mixed with additional Mn, Cu (≥99%), Ni (≥99.9%), Ag (≥99.99%) and C (p. a.) sealed in a quartz ampule under low pressure Ar and heated to 1033 K for 5 days. After that the as-prepared samples were
Results
At room temperature, XRD reveals that all samples crystallize in the cubic structure with space group (see Fig. 2a and Fig. S2). Small amounts of an antiferromagnetic secondary () phase as well as antiferromagnetic MnO () impurities were present in all samples. Compounds with high Ag content contain small amounts of pure Ag. SEM/EDX pictures also show the impurities (Fig. S1 and Table S1). Pictures from samples with Ni show some spots with significantly higher Ni
Discussion
The changing magnetic and structural transitions of with decreasing Cu content and simultaneous N substitution can be interpreted as follows: Ag substitution on the Cu position has two competing influences on the magnetism of . First, the larger radius of Ag leads to a bigger cell size and thus longer Mn-Mn distances (see Fig. 2, Table S4). By increasing the Mn-Mn distance the bandwidth at the Fermi energy gets narrower leading to a larger density of states at the Fermi
Summary
All in all, the synthesis of with antiperovskite structure and their magnetic and structural transitions were studied and results are summarized in Fig. 7. Cu substitution by Ag or Ni lead to a competition between ferri- and antiferromagnetic interactions in the magnetic low temperature phase and higher transition temperatures compared to . Also, the transition is accompanied by an isosymmetric cubic-cubic transition instead of the cubic-tetragonal transition that is
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