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Zhao et al. Microstructures 2023;3:2023022 https://dx.doi.org/10.20517/microstructures.2022.46 Page 3 of 9

determined using a ThermoFisher iCAP6300 Inductive Coupled Plasma Emission Spectrometer (ICP). The
ingots were remelted three times to ensure homogeneity. 2wt.% excessive Mn elements were added to
compensate for losses during the melting process. The as-prepared ingots were annealed in encapsulated
quartz tubes at 973 K for 120 h and followed by furnace cooling. The nature of the single-phase was checked
using a Rigaku MiniFlex 600 X-ray diffractometer. The temperature-dependent X-ray diffraction (XRD)
patterns were collected using Bruker D8 Advance X-ray diffractometer. The diffraction patterns were fitted
to a cubic unit cell (space group Pm m) in Jana2006 . The calorimetric data were collected as a function of
[30]
temperature and pressure using a high-pressure differential scanning calorimeter (µDSC7, Setaram). The
samples were enclosed in a high-pressure vessel made of Hastelloy. Constant pressure scans were performed
at 0.1, 30, 60, and 90 MPa in the temperature region from 290 to 390 K, respectively. After subtracting the
baseline background, the heat flow data can be converted to entropy changes. The magnetic properties are
characterized using a Magnetic Properties Measurement System (MPMS-XL, Quantum Design) and a
Physical Property Measurement System (PPMS-14T, Quantum Design).

RESULTS AND DISCUSSIONS
Mn Pt crystallizes in the ordered Cu Au-type structure [31,32] , where Mn atoms are located on the face
1+x
3-x
3
centers of the cubic lattice formed by Pt atoms, as shown in Figure 1A. The compound with the
stoichiometric composition magnetically orders into a colinear AFM state at about T ~365 K (magnetic
t
[32]
transition temperature of Mn Pt in the literature ), where a negligibly small tetragonal distortion is
3
observed. As depicted, magnetic moments carried by Mn atoms are aligned along the c-axis, and the
magnetic unit cell is constructed along the c-axis with doubled chemical unit cell. Note that the Mn atoms
located on the ab-plane carry no ordered magnetic moment. The four Mn atoms form a square lattice,
and the diagonal magnetic moments are parallel while the adjacent ones are anti-parallel. Spaced by the
non-magnetic Mn atom, the magnetic moments of the two layers of the square lattices are
antiferromagnetically coupled. As the temperature decreases below T, such a colinear AFM state transforms
t
into a triangle-lattice AFM state, where magnetic moments are located on the (111) plane and point in the
<211> direction, leading to a two-dimensional geometric spin frustration. This arrangement of magnetic
moments ensures that the magnetic unit cell is identical to the chemical unit cell. This phase transition is a
typical first-order magnetic phase transition, even if the lattice symmetry is maintained . Based on the
[33]
triangular AFM structure, anomalous Hall effects have been predicted and observed in films as well as bulk
single crystals [34,35] .

In the Mn Pt system, the magnetic properties, in particular, T , are strongly dependent on the
3-x
t
1+x
composition x. Shown in Figure 1B is the heat flow data of the x = 0.1 alloy under ambient pressure, where
an endothermic peak is found at 360 K while an exothermic peak at 340 K with a thermal hysteresis of about
20 K, which is a signature of the first-order phase transition. The temperature corresponding to the peak in
the heat flow curve is defined as the phase transition temperature. In this paper, we uniformly regard the
transition temperature of the cooling process as the phase transition temperature of the sample. The
-1
entropy change at this transition is derived from being about 12.31 J kg K , which is kind of small
-1
compared to other systems that exhibit strong first-order transitions. The reason will be clarified afterward.
[28]
-1
-1
[29]
-1
-1
For example, the entropy change is 22.3 J kg K in Mn GaN while 43 J kg K in Mn NiN . The
3
3
temperature-dependent XRD was used to monitor the lattice distortion during the phase transition. The
contour plots of the XRD patterns are shown in Figure 1C as a function of temperature (T) and scattering
vector (Q). The patterns can be indexed based on the reported cubic structure. Within our resolution, there
is no distinguishable symmetry change in the temperature from 300 to 410 K. Four strong Bragg peaks are
observed, and the (210) peak obviously shifts towards the lower Q around 360 K. The determined lattice
constant displays a sudden jump at 360 K [Figure 1D], corresponding to relative changes in lattice constant