B-site ordered A2B'B''O6 double perovskites have a variety of applications as magnetic materials.Here we show that diamagnetic d 10 and d 0 B'' cations have a significant effect on the magnetic interactions in these materials. We present a neutron scattering and theoretical study of the Mn 2+ double perovskite Ba2MnTeO6 with a 4d 10 Te 6+ cation on the B''-site. It is found to be a Type I antiferromagnet with a dominant nearest-neighbor J1 interaction. In contrast, the 5d 0 W 6+ analogue Ba2MnWO6 is a Type II antiferromagnet with a significant next-nearest-neighbor J2 interaction. This is due to a d 10 /d 0 effect, where the different orbital hybridization with oxygen 2p results in different superexchange pathways. We show that d 10 B'' cations promote nearest neighbor and d 0 cations promote next-nearest-neighbor interactions. The d 10 /d 0 effect could be used to tune magnetic interactions in double perovskites.

This is a repository copy of Site-selective d10/d0 substitution in a S=1/2 spin ladder Ba2CuTe1−xWxO6 (0≤x≤0.3).

The search for cheap, corrosion-resistant, thermally-mechanically stable functional magnetic materials, including soft magnetic and magneto-caloric materials has led to research focused on high entropy alloys (HEAs). Previous research shows that alloying elements with negative enthalpies of mixing can facilitate a second-order phase transition. On the other side of the spectrum, compositional segregation cause by positive enthalpy of mixing alloying additions (such as Cu) may also be used to tune magnetic properties. This paper studies the structural, magnetic and magneto-caloric effect of the FCC alloys CoFeNiCr y Cu x (x = 0.0, 0.5, 1.0 and 1.5, y = 0.0, 0.8 and 1.0) to tune these properties with Cu and Cr alloying. Scanning electron microscopy of the compositions show nanoparticles forming within the grains as the Cu concentration increases. Cr addition to CoFeNiCu 1.0 has a larger effect on the magnetic and magneto-caloric properties compared to the Cu addition to CoFeNiCr 1.0 . The addition of Cu (x = 0.5) to CoFeNiCr 1.0 improved both the saturation magnetisation and Curie temperature; addition of Cr (y = 1.0) to CoFeNiCu 1.0 decreased the Curie temperature by 900 K. All alloys were determined to have a second-order phase transition around their Curie temperature. The refrigerant capacity at 2 T was found to be similar to existing HEAs, although the Curie temperatures were lower than room temperature. Based on this data the CoFeNiCr 0.8 Cu composition was fabricated to increase the Curie temperature towards 300 K to explore these HEAs as new candidates for room temperature magneto-caloric applications. The fabricated composition showed Curie temperature, saturation magnetisation, and refrigerant capacity increasing with the small reduction in Cr content.

B-site ordered 4d1 and 5d1 double perovskites have a number of potential exotic ground states including multipolar order, quantum spin liquids and valence bond glass states. These arise from the complex interactions of spin-orbital entangled Jeff = 3/2 pseudospins on the geometrically frustrated fcc lattice. The 4d1 Mo5+ perovskite Ba2YMoO6 has been suggested to have a valence bond glass ground state. Here we report on the low temperature properties of powder samples of isostructural Ba2LuMoO6: the only other known cubic 4d1 perovskite with one magnetic cation. Our muon spectroscopy experiments show that magnetism in this material remains dynamic down to 60 mK without any spin freezing or magnetic order. A singlet-triplet excitation with a gap of Δ = 28 meV is observed in inelastic neutron scattering. These results are interpreted as a disordered valence bond glass ground state similar to Ba2YMoO6. Our results highlight the differences of the 4d1 double perovskites in comparison to cubic 5d1 analogues, which have both magnetic and multipolar order.

New and exotic ground states of magnetic materials are highly sought after and are extensively studied for the insights they provide into the thermodynamics of disorder and fundamental magnetic interactions. By controlling the crystal structure of an appropriate magnetic lattice, it is possible to cause the strong magnetic exchange interactions to sum to zero and so be frustrated. Due to the presence of this frustration, the lowest energy configuration that results may be crucially dependent on the tiniest of energy differences between a multitude of states that have (almost) the same energy. The keen interest in these materials arises from the fact that these finely balanced systems offer a way of probing classical or quantum mechanical interactions that are of fundamental importance but are too weak to be observed in non-frustrated systems. Here, we combine local and crystallographic probes of the cation-ordered double perovskite Ba2MnMoO6 that contains a face-centered cubic lattice of S = 5/2 Mn2+ cations. Neutron diffraction measurements below 9.27(7) K indicate that a fourfold degenerate non-collinear antiferromagnetic state exists with almost complete ordering of the Mn2+ spins. Muon spin relaxation measurements provide a local probe of the magnetic fields inside this material over the t1/2 = 2.2 µs lifetime of a muon, indicating a slightly lower Néel transition temperature of 7.9(1) K. The dc susceptibility data do not show the loss of magnetization that should accompany the onset of the antiferromagnetic order; they indicate that a strongly antiferromagnetically coupled paramagnetic state [θ = −73(3) K] persists down to 4 K, at which temperature a weak transition occurs. The behavior of this material differs considerably from the closely related compositions Ba2MnMO6 (M = W, Te), which show collinear ordering arrangements and well defined antiferromagnetic transitions in the bulk susceptibility. This suggests that the Mo6+ cation leads to a fine balance between the nearest and next-nearest neighbor superexchange in these frustrated double perovskite structures.