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We report a detailed single-crystal and powder neutron diffraction study of Co2TiO4 and Co2SnO4 between the temperatures 1.6 K and 80 K to probe their spin structures in the ground state. For both compounds the strongest magnetic intensity was observed for the (111)M reflection due to ferrimagnetic ordering, which sets in below TN = 48.6 K and 41 K for Co2TiO4 and Co2SnO4, respectively. An additional low intensity magnetic reflection (200)M was noticed in Co2TiO4 due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and the single-crystal neutron data of Co2TiO4 we noticed a significant broadening of the magnetic (111)M reflection, possibly results from the disordered character of the Ti and Co atoms on the B site. Practically, the same peak broadening was found for the neutron powder data of Co2SnO4. On the other hand, from our single-crystal neutron diffraction data of Co2TiO4 we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case it can be expected that competing Jahn-Teller effects act along different crystallographic axes can induce anisotropic local strain. In fact, for both ions Ti 3+ and Co 3+ the 2tg levels split into a lower dxy level and yields a higher two-fold degenerate dxz/dyz level. As a consequence, one can expect a tetragonal distortion in Co2TiO4 with c/a < 1, which could not significantly detected in the present work.

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We report magnetic small-angle neutron scattering (SANS) data for the nanocrystalline rare earth metal Terbium in its paramagnetic state. Whereas critical scattering dominates at large momentum transfer, q, the (magnetic-) field response of the scattering at small q arises from the spatial nonuniformity of the paramagnetic susceptibility tensor. The finding of an interrelation between SANS and the susceptibility suggests a way for characterizing the nonuniform magnetic interactions in hard magnets by neutron scattering.

Temperature dependent transport measurements on ultrathin antiferromagnetic Mn films reveal a heretofore unknown non-universal weak localization correction to the conductivity which extends to disorder strengths greater than 100 kΩ per square. The inelastic scattering of electrons off of gapped antiferromagnetic spin waves gives rise to an inelastic scattering length which is short enough to place the system in the 3d regime. The extracted fitting parameters provide estimates of the energy gap (∆ ≈ 16 K) and exchange energy (J ≈ 320 K). PACS numbers: 75.45.+j, 72.15.Rn, 75.50.Ee, 75.30.Ds Thin-film transition metal ferromagnets (Fe, Co, Ni, Gd) and antiferromagnets (Mn, Cr) and their alloys are not only ubiquitous in present day technologies but are also expected to play an important role in future developments [1]. Understanding magnetism in these materials, especially when the films are thin enough so that disorder plays an important role, is complicated by the long standing controversy about the relative importance of itinerant and local moments [2][3][4]. For the itinerant transition metal magnets, a related fundamental issue centers on the question of how itinerancy is compromised by disorder. Clearly with sufficient disorder the charge carriers become localized, but questions arise as to what happens to the spins and associated spin waves and whether the outcome depends on the ferro/antiferro alignment of spins in the itinerant parent. Ferromagnets which have magnetization as the order parameter are fundamentally different than antiferromagnets which have staggered magnetization (i.e., difference between the magnetization on each sublattice) as the order parameter [5]. Ferromagnetism thus distinguishes itself by having soft modes at zero wave number whereas antiferromagnets have soft modes at finite wave number [6]. Accordingly, the respective spin wave spectrums are radically different. These distinctions are particularly important when comparing quantum corrections to the conductivity near quantum critical points for ferromagnets [7] and antiferromagnets [8].Surprisingly, although there have been systematic studies of the effect of disorder on the longitudinal σ xx and transverse σ xy conductivity of ferromagnetic films [9][10][11][12][13], there have been few if any such studies on antiferromagnetic films. In this paper we remedy this situation by presenting transport data on systematically disordered Mn films that are sputter deposited in a custom designed vacuum chamber and then transferred without exposure to air into an adjacent cryostat for transport studies to low temperature. The experimental procedures are similar to those reported previously: disorder, characterized by the sheet resistance R 0 measured at T = 5 K, can be changed either by growing separate samples or by gentle annealing of a given sample through incremental stages of disorder [14]. Using these same procedures our results for antiferromagnets however are decidedly different. The data are well described over a large range of dis...

We investigate the effects of Fe and Co substitutions on the phase stability of the martensitic phase, mechanical, electronic and magnetic properties of magnetic shape memory system Mn2NiGa by first-principles Density functional theory(DFT) calculations. The evolution of these aspects upon substitution of Fe and Co at different crystallographic sites are investigated by computing the electronic structure, mechanical properties (tetragonal shear constant, Pugh ratio and Cauchy pressure) and magnetic exchange parameters. We find that the martensitic phase of Mn2NiGa gradually de-stabilises with increase in concentration of Fe/Co due to the weakening of the minority spin hybridisation of Ni and Mn atoms occupying crystallographically equivalent sites. The interplay between relative structural stability and the compositional changes are understood from the variations in the elastic modulii and electronic structures. We find that like in the Ni2MnGa-based systems, the elastic shear modulus C can be considered as a predictor of composition dependence of martensitic transformation temperature Tm in substituted Mn2NiGa, thus singling it out as the universally acceptable predictor for martensitic transformation in Ni-Mn-Ga compounds over a wide composition range. The magnetic properties of Mn2NiGa are found to be greatly improved by the substitutions due to stronger ferromagnetic interactions in the compounds. The gradually weaker(stronger) Jahn-Teller distortion (covalent bonding) in the minority spin densities of states due to substitutions lead to a half-metallic like gap in these compounds resulting in materials with high spin-polarisation when the substitutions are complete. The substitutions at the Ga site result in two new compounds Mn2NiFe and Mn2NiCo with very high magnetic moments and Curie temperatures. Thus, our work indicates that although the substitutions de-stabilise the martensitic phase in Mn2NiGa, new magnetic materials with very good magnetic parameters and potentially useful for novel magnetic applications can be obtained. This can trigger interests in the experimental community for further research on substituted Mn2NiGa.