Crystal and magnetic structure of antiferromagnetic Mn$_{2}$PtPd

We have investigated the crystal and magnetic structure of Mn${}_{2}$PtPd alloy using powder x-ray and neutron diffraction experiments. This compound is believed to belong to the Heusler family having crystal symmetry $\mathit{I}$4/$\mathit{mmm}$ (TiAl${}_{3}$-type). However, in this work we found that the Pd and Pt atoms are disordered and thus Mn${}_{2}$PtPd crystallizes in the $\mathit{L}$1${}_{0}$ structure having $\mathit{P}$4/$\mathit{mmm}$ symmetry (CuAu-I type) like MnPt and MnPd binary alloys. The lattice constants are $\mathit{a}$ = 2.86 \r{A} and $\mathit{c}$ = 3.62 \r{A} at room temperature. Mn${}_{2}$PtPd has a collinear antiferromagnetic spin structure below the N\'{e}el temperature $\mathit{T}$${}_{N}$ = 866 K, where Mn moments of $\mathrm{\sim}$4 $\mu$${}_{B}$ lie in the $\mathit{ab}$-plane. We observed a strong change in the lattice parameters near $\mathit{T}$${}_{N}$. The sample exhibits metallic behaviour, where electrical resistivity and carrier concentration are of the order of 10${}^{-5}$ $\Omega$ cm and 10${}^{21}$ cm${}^{-3}$, respectively.


Introduction
Antiferromagnets (AFMs) are special among magnetic materials as they display magnetic ordering but with zero magnetic moment. Their practical use has been established in many fields especially in spintronics, for instance as pinning layers in giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) devices [1][2][3][4][5][6][7][8][9][10][11][12][13]. Here, the AFM acts as a passive component. The absence of stray fields is a great advantage of AFMs over ferromagnets and thus AFMs may even replace ferromagnets as an active component in spintronic devices.
However, it is difficult to manipulate the AFMs due to their vanishing magnetic moment.
Recent advances in controlling the antiferromagnetic configurations by electrical switching bear a good prospect for a new era of applications [14]. In addition, high ordering temperatures and large magneto-crystalline anisotropy are desired to enhance the scope for more universal use.
Heusler alloys X2YZ are a particularly promising class of materials for the search of new AFMs for applications due to their great variability in chemical composition and properties [15][16][17][18][19][20].
In this regard the recently reported phase Mn2PtPd is of interest, which was identified in a high throughput computational study by Sanvito et al. [21] as a potential new ferromagnetic cubic Heusler alloy. Its subsequent experimental realization, however, rather suggested this material to be a tetragonal antiferromagnetic Heusler compound having space group I4/mmm (TiAl3type) with lattice constants a = 4.03 Å and c = 7.24 Å [21]. A TiAl3-type structure would imply ordering of Pt and Pd atoms. A magnetic transition near 320 K was reported [21], but details on the magnetic behaviour are unknown yet. On the other hand, several Mn based binary alloys MnTM with various transition metals TM like Ni, Pt, Pd, Rh, Ir have been reported which are AFMs having high Néel temperatures and the crystal symmetry P4/mmm (L10, CuAu-I type) [22][23][24][25][26][27][28][29][30][31][32]. Binary alloys having cubic (B2, CsCl-type) and tetragonal (L10, CuAu-I type) structures are the building blocks to design Heusler materials [33]. Combining two binary alloys XY and XZ belonging to the B2 structure results in a regular Heusler (L21, Cu2MnAl-type) structure, given the condition that Y and Z atoms order, whereas a tetragonal Heusler (TiAl3type) compound is expected in the case of combining two L10 structures. However, ternary alloys X2YZ adapt the same structure as their binary precursor if there is a complete disorder of Y and Z atoms. Hence, taking into account the chemical similarity between Pd and Pt atoms, Mn2PtPd could also be considered as a CuAu-I type phase MnPt0.5Pd0.5 with a disordered arrangement of Pt and Pd atoms. In this paper we report our powder X-ray and neutron diffraction studies on Mn2PtPd as well as magnetization and electrical transport measurements.
We will show that Mn2PtPd is a CuAu-I type antiferromagnet with a Néel temperature TN = 866(5) K.

Experimental details
A polycrystalline ingot of Mn2PtPd was prepared by arc melting stoichiometric amounts of constituent elements in the presence of high purity Ar atmosphere. About 2.5 wt % of extra Mn was used to compensate the weight loss due to the evaporation of Mn during the melting. The sample was ground and characterized at room temperature by powder X-ray diffraction (XRD) using a Huber G670 camera [Guinier technique, λ = 1.54056 Å (Cu−Kα1 radiation)]. Field and temperature dependent magnetization measurements were performed using a vibrating sample magnetometer (MPMS3, Quantum Design). Temperature dependent magnetization M(T) measurements were carried out from 2 to 400 K in zero field cooled (ZFC) and field cooled (FC) modes. High temperature magnetization measurements were performed during heating and cooling from 300 to 1000 K using the oven option of the MPMS3. The electrical transport properties were investigated using a physical property measurement system (PPMS, Quantum Design). The Hall measurements were performed on a rectangular bar using a five probe geometry at different temperatures from 2 K to 300 K in fields up to 5 T. For the neutron diffraction study, the sample was powdered by grinding followed by annealing at 773 K in an evacuated quartz tube for 24 hours. Neutron powder diffraction experiments were carried out on the instrument E6 at the BER II reactor of the Helmholtz-Zentrum Berlin. This instrument uses a pyrolytic graphite (PG) monochromator selecting the neutron wavelength λ = 2.42 Å. In order to investigate in detail the crystal structure of Mn2PtPd a powder pattern was collected at 1012 K, well above the magnetic ordering temperature of TN = 866 K. The sample was heated up in a quartz ampoule using a high-temperature furnace (AS Scientific Products Ltd., Abingdon, GB). The temperature dependence of the crystal and magnetic structure was investigated between 296 and 1012 K, where 18 and 6 patterns were collected below and above TN, respectively. Neutron powder patterns were recorded between the diffraction angles (2θ) 5.5 and 136.5 °. Rietveld refinements of the powder diffraction data were carried out with the program FullProf [34], using the nuclear scattering lengths b(Mn) = 3.73 fm, b(Pd) = 5.91 fm, and b(Pt) = 9.63 fm [35]. The magnetic form factor of the Mn atoms was taken from Ref. [36]. Intensity (arb. unit) 2 (degree)

Results and discussion
al. [21] using space group I4/mmm (TiAl3-type, a = 4.03 Å and c = 7.24 Å). However, the absence of some additional peaks in the experimental pattern which are present in the calculated I4/mmm pattern suggests that Pt and Pd are atomically disordered and thus Mn2PtPd rather crystallizes in P4/mmm symmetry. Below we will show the detailed structural analysis using the powder neutron diffraction data. To determine the magnetic behaviour and the ordering temperature we have performed magnetization measurements as a function of temperature and field.  [25,26,[29][30][31]. This is further support that Mn2PtPd rather is a CuAu-I type than a TiAl3-type AFM material. For obtaining low temperature M(T) data, the sample was cooled down to 2 K and measurements were carried out in the zero field cooled (ZFC) and field cooled (FC) modes from 2 to 400 K at 0.1 T. Both ZFC and FC magnetization curves followed the same path. In contrast to Ref. [21] we did not observe any indication for a magnetic transition near 320 K. At temperatures below 100 K an up-turn in the magnetization is apparent. A similar behaviour has been reported for MnPt alloys due to a very small deviation from the equiatomic composition [26]. Therefore, a small amount of off-stoichiometry in the sample could be the reason for this kind of behaviour and also for the irreversibility in the magnetization being observed at 470 K on cooling ( figure   2(a)). In figure 2  The crystal structure of Mn2PtPd was refined from a powder neutron diffraction data set collected at 1012 K ( figure 3(a)), which is above the magnetic ordering temperature of TN = 866 K. The refinements were carried out in the tetragonal space groups P4/mmm (No. 123) and    . For these two models we practically obtained the same residuals, which shows that we are not able to determine the moment direction within the tetragonal ab plane. It has been reported that MnPt exhibits two different magnetic structures [30]. At high temperatures above 750 K, the Mn moment lies in the basal plane whereas it undergoes a spin-flip transition in a broad temperature region ranging from 750 to 570 K, leading to a magnetic structure with Mn moments aligned parallel to the tetragonal c axis. By contrast, MnPd exhibits only the magnetic structure with the moments lying in the ab plane.
The same spin structure is also adopted by Mn2PtPd throughout the whole temperature range.
There are no indications for a spin-flip transition. It is noted that we did not observe any significant change in the magnetic configuration below 470 K which indicates that the irreversibility in the M(T) curve (figure 2a) is rather related to off-stoichiometry or defects than being an intrinsic bulk property. 8 The determined moment value at 296 K is μexp(Mn) = 4.09 (10) μB, which is similar as for many other magnetic intermetallic alloys containing Mn [24][25][26]29,31,37]. The temperature dependence of the magnetic moment of the Mn atom was investigated in the temperature range from 296 to 1012 K. In figure 4(a) it is seen that the saturation of the magnetic moments is almost reached at 296 K. The magnetic order disappears at the Néel temperature TN = 866 (5) K. The lattice parameters, the c/a ratio, and the volume of the unit cell are displayed in figure   4 (b) and 4 (c) respectively. The lattice parameters at 296 K are a = 2.86 Å and c = 3.62 Å.
Interestingly, the Rietveld refinements of the powder patterns revealed a strong change of the lattice parameters near the Néel temperature, which results in a pronounced increase of the tetragonal distortion as reflected in the c/a ratio. This is ascribed to the fact that the apical bond length da(Mn-Mn) is strongly elongated below TN, whereas the equatorial bond length de(Mn-Mn) is shortened (see table 1). A similar strong magnetoelastic coupling on going from the paramagnetic to the AFM phase was observed for MnPd and the lattice anomaly was attributed to deformation sensitive nearest neighbour Mn-Mn interactions [29]. On the other hand the cell volume is continuously decreasing to lower temperatures without pronounced anomalies near the Néel temperature.    [26] and MnPd alloys [25]. We measured the Hall resistivity ρH at different temperatures between 2 and 300 K as shown in figure 5 (b). The straight line behaviour of the Hall resistivity with the applied field is expected for the collinear AFM structure which shows a normal Hall effect. Anomalous Hall effects scaling with the magnetization are only found for ferromagnets or peculiar types of noncollinear antiferromagnets [38]. The calculated carrier concentration from Hall measurements is shown in the inset of figure 5 (b). The carrier concentration is given by the equation = 0 / , where μ0 is the vacuum permeability, H is the applied field and e is the elementary charge. The carrier concentration is of the order of 10 21 cm -3 . The Hall resistivity decreases and carrier concentration increases with increase in temperature.

Conclusions
We found that Mn2PtPd adopts the L10 (CuAu-I)-type crystal structure similar to MnPt and MnPd binary alloys. As for MnPd the magnetic moments of the Mn atoms (~4 μB) are antiferromagnetically coupled in the ab-plane without evidence for a spin-flip transition.
Mn2PtPd has an ordering temperature TN = 866 K which is the average of the ordering