Edinburgh Research Explorer Competing antiferromagnetic orders in the double perovskite Mn2MnReO6 (Mn3ReO6)

The new double perovskite Mn2MnReO6 has been synthesised at high pressure. Mn(2+) and Re(6+) spins order antiferromagnetically through two successive transitions that are coupled by magnetoelastic effects, as order of the Mn spins at 109 K leads to lattice distortions that reduce frustration prompting Re order at 99 K.

Double perovskites A2BB'O6 with ordering of B/B' transition metal cations on the ABO3 perovskite-type lattice are an important group of oxide materials. 1 Some examples such as Sr2FeMoO6 and Sr2FeReO6 are ferrimagnetic, spin-polarised conductors with large low-field tunnelling magnetoresistances. 2 Many other double perovskites have antiferromagnetic ground states that may be frustrated due to the tetrahedral geometry of the B and B' sublattices, 3,4 leading to a spin liquid ground state in Ba2YMoO6. 5 The A 2+ (= Ca, Sr, Ba, Pb) cations in double perovskites synthesised at ambient pressure are relatively large and nonmagnetic. However, perovskites with only the smaller high spin Mn 2+ ion at the A sites have recently been synthesised at high pressure and temperature conditions, introducing additional magnetic functionality. MnVO3 perovskite is metallic due to itinerancy of the V 4+ 3d 1 states, as found in CaVO3 and SrVO3, but also has coexisting helimagnetic order of localised S = 5/2 Mn 2+ spins. 6 The double perovskite Mn2FeSbO6 also has low temperature incommensurate antiferromagnetic Mn spin order. 7 Mn2FeReO6, also recently discovered through high pressure synthesis, 8,9 is particularly notable as it has a high Curie temperature (520 K), ferrimagnetic Fe 3+ /Re 5+ spin order, and negative tunnelling magnetoresistance like other A2FeReO6 double perovskites. The Mn 2+ spins enhance the bulk magnetisation giving a record value for transition-metal double perovskites, 8  10-20 mg samples of Mn2MnReO6 were synthesised from a stoichiometric mixture of Mn3O4 and ReO2 in a Pt capsule at 8 GPa and 1400 °C in a Walker-type multi-anvil press. The best polycrystalline product was highly phase pure with traces of MnO (<3 wt%) also observed. Small single crystals were separated from the walls of the capsule from one run and used for structure determination. Results are summarised in Table I and further details of the single crystal analysis and powder xray and neutron studies are in ESI.
Mn2MnReO6 has a monoclinic double perovskite structure like those of A2MnReO6 (A = Ca and Sr) analogues. 10,11 A small antisite disorder (3.7 %) for Mn/Re at B/B' sites was observed in the single crystal, showing that the degree of cation ordering is very high. Bond valence sum (BVS) calculations in Table I show that the charge distribution is Mn2 2+ Mn 2+ Re 6+ O6, in contrast to that of Mn2 2+ Fe 3+ Re 5+ O6. The crystal structure is substantially distorted due to the small Mn 2+ cations at the A sites. The ideal 12-coordination of the MnA site is split into four short (2.10 -2.16 Å), four medium (2.61 -2.79 Å) and four long (3.33 -3.56 Å) MnA-O distances. The four short bonds create a distorted tetrahedral environment around MnA. The MnBO6 and ReO6 octahedra are also distorted, with ReO6 showing a small tetragonal compression (two 1.88 and four 1.93-1.97 Å bonds) consistent with Jahn-Teller distortion from orbital order of the 5d 1 Re 6+ ions. Large tilts of the octahedra are observed, with MnB-O-Re angles of 135 -140ᵒ deviating far from the ideal 180ᵒ value.
Magnetic susceptibility measurements in Figure 1 show that Mn2MnReO6 is Curie-Weiss paramagnetic at high temperatures. A fit to the inverse susceptibility gives a Weiss temperature of θ = -147(1) K, showing that antiferromagnetic exchange interactions are dominant, and a paramagnetic moment of µeff = 4.9(1) µB per transition metal ion, close to the predicted spin-only value of 5.20 µB for Mn2 2+ Mn 2+ Re 6+ O6. The susceptibility maximum at 109 K evidences an antiferromagnetic spin ordering, and a change of slope at 99 K is consistent with the second magnetic transition revealed by neutron diffraction below. Divergence of zero field cooled (ZFC) and field cooled (FC) susceptibilities below ~40 K may evidence a trace of the ferrimagnetic impurity Mn3O4, although this was not seen in the powder diffraction patterns. No magnetic or structural anomalies from Mn2MnReO6 are apparent in the neutron data near 40 K. Four high pressure products were combined to give a ~70 mg sample of Mn2MnReO6 for neutron powder diffraction to investigate the low temperature properties further. Figure 2 displays the profiles at 10 and 200 K. Rietveld refinements showed that the monoclinic P21/n structure is retained down to the lowest measured temperature of 10 K. A greater degree (12%) of MnB/Re antisite disorder was observed than in the single crystal, this probably reflects slight compositional variations between the combined polycrystalline samples. The onset of antiferromagnetic order in Mn2MnReO6 is marked by the appearance of several magnetic diffraction peaks below 109 K. However, the temperature variation of the magnetic intensities (see inset to Figure 3) shows that a further spin transition occurs at 99 K. All of the Mn2MnReO6 magnetic peaks are indexed by the k = (½ ½ 0) propagation vector. Analysis of the 10 K magnetic neutron data reveals that all of the moments lie in the ab-plane (details are in ESI). The MnA, MnB and Re spins form three independent antiferromagnetic sublattices. Initial refinements showed that MnA and MnB spins are approximately perpendicular, and best fits were obtained when they were constrained to be parallel to [110] and [1͞ 10] directions respectively. Re moments are small and were constrained to be collinear with MnB spins; other directions did not improve the fit. The thermal evolution of the refined model shows that the MnA and MnB spins order at the upper transition at TMn = 109 K, whereas Re moments order separately at TRe = 99 K. Thermal variations of the ordered moments and lattice strains are shown in Figure 3, and magnetic structures in the two regimes are in Figure 4.
The (upper) magnetic ordering temperature of 109 K for Mn2MnReO6 is comparable to those of 110 and 120 K for Ca2MnReO6 and Sr2MnReO6. 10 However the latter materials are both ferrimagnets, with simultaneous k = (0 0 0) order of Mn and Re spins observed in a neutron diffraction study of Sr2MnReO6. 11 Hence the k = (½ ½ 0) antiferromagnetism of Mn2MnReO6 with separate ordering transitions for the two transition metal B-site sublattices, which is very unusual in double perovskites, shows that interactions of B-site spins with Please do not adjust margins Please do not adjust margins MnA moments are significant and suppress the Re spin order. Antiferromagnetic order within B/B' tetrahedral networks is frustrated in double perovskites, but Mn2MnReO6 also has frustrated interactions between A and B/B' cations, as each MnA spin has 2 up and 2 down spins from the surrounding MnB and Re spin tetrahedra, and each B/B'-site spin has 4 up and 4 down MnA spins as neighbours. This results in perpendicular alignment of A and B/B' spins to maximise antisymmetric Dzyaloshinskii-Moriya interactions. The relative strengths of the competing antiferromagnetic orders are revealed by the moment variations in Fig. 3. Below TMn = 109 K the ordered MnB moment rises rapidly up to 1.9µB at 99 K whereas MnA increases more slowly to 1.1 µB, showing that dominant MnB order partially frustrates MnA spins and fully frustrates Re order. However, below TNA = 99 K further MnB order is frustrated as the moment saturates at 2.0 µB while lessfrustrated MnA and Re spins rise to saturated moments of 4.5 and 1.0 µB, close to ideal values of 5 and 1 µB respectively.  Although the magnetic structure of Mn2MnReO6 appears frustrated as discussed above, this is not reflected by the ratio -θ/TNB = 1.3 which is close to unity. This demonstrates that monoclinic lattice distortion relieves much of the frustration by breaking the degeneracy of MnB-O-Re superexchange interactions. Further evidence comes from observed anomalies in lattice parameters at the two transitions as shown in Figure 3 and ESI. The change in thermal expansion of c from positive to negative on cooling through TMn increases the monoclinic distortion as a,b < c/√2. This reduces frustration further and so is the likely factor that drives the long range order of Re spins at 99 K. The proximity of the two antiferromagnetic transitions thus arises from magnetoelastic effects. Re spin order further changes the magnetoelastic coupling as evidenced by the anomaly in β at TRe.
In conclusion, Mn2MnReO6 is the first example of a A2BB'O6 double perovskite with antiferromagnetically ordered transition metal spins at all cation sites. Frustration between the three antiferromagnetic sublattices results in perpendicular orientations of the A and B/B' spins and long range magnetic ordering through two successive antiferromagnetic transitions. These show an unusual coupling through magnetoelastic effects, as order of the Mn spins at TMn = 109 K leads to lattice distortions that reduce frustration leading to Re spin order at TRe = 99 K. Around 650 A2BB′O6 double perovskite oxides are previously reported 1 but Mn2FeReO6 and Mn2MnReO6 are the only two with magnetic transition metal ions at all sites, and both have novel magnetic properties due to the presence of A-site Mn 2+ , so it is likely that many more interesting 'all transition metal' double perovskites will be accessible through high pressure synthesis.
We thank EPSRC, STFC and the Royal Society for support and provision of ILL beamtime, and Dr. C. Ritter for assistance with data collection.