Revisiting the magnetic structure of Holmium at high pressure by using neutron diffraction.

Low-temperature neutron diffraction experiments at [Formula: see text] GPa have been conducted to investigate the magnetic structures of metallic Holmium at high pressures by employing a long d-spacing high-flux diffractometer and a Paris-Edinburgh press cell inside a cryostat. We find that at [Formula: see text] GPa and [Formula: see text] K, no nuclear symmetry change is observed, keeping therefore the hexagonal closed packed (hcp) symmetry at high pressure. Our neutron diffraction data confirm that the ferromagnetic state does not exist. The magnetic structure corresponding to the helimagnetic order, which survives down to 5 K, is fully described by the magnetic superspace group formalism. These results are consistent with those previously published using magnetization experiments.

Isoreticular two-dimensional magnetic coordination polymers prepared through pre-synthetic ligand functionalization.

Chemical functionalization is a powerful approach to tailor the physical and chemical properties of two-dimensional (2D) materials, increase their processability and stability, tune their functionalities and, even, create new 2D materials. This is typically achieved through post-synthetic functionalization by anchoring molecules on the surface of an exfoliated 2D crystal, but it inevitably alters the long-range structural order of the material. Here we present a pre-synthetic approach that allows the isolation of crystalline, robust and magnetic functionalized monolayers of coordination polymers. A series of five isostructural layered magnetic coordination polymers based on Fe(II) centres and different benzimidazole derivatives (bearing a Cl, H, CH3, Br or NH2 side group) were first prepared. On mechanical exfoliation, 2D materials are obtained that retain their long-range structural order and exhibit good mechanical and magnetic properties. This combination, together with the possibility to functionalize their surface at will, makes them good candidates to explore magnetism in the 2D limit and to fabricate mechanical resonators for selective gas sensing.

Giant direct and inverse magnetocaloric effect linked to the same forward martensitic transformation.

Metamagnetic shape memory alloys have aroused considerable attraction as potential magnetic refrigerants due to the large inverse magnetocaloric effect associated to the magnetic-field-induction of a reverse martensitic transformation (martensite to austenite). In some of these alloys, the austenite phase can be retained on cooling under high magnetic fields, being the retained phase metastable after field removing. Here, we report a giant direct magnetocaloric effect linked to the anomalous forward martensitic transformation (austenite to martensite) that the retained austenite undergoes on heating. Under moderate fields of 10 kOe, an estimated adiabatic temperature change of 9 K has been obtained, which is (in absolute value) almost twice that obtained in the conventional transformation under higher applied fields. The observation of a different sign on the temperature change associated to the same austenite to martensite transformation depending on whether it occurs on heating (retained) or on cooling is attributed to the predominance of the magnetic or the vibrational entropy terms, respectively.

Effects of A-site disorder in the properties of A2CoMnO6 (A = La, Tb).

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn(4+)-O-Co(2+) superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb(3+) moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.

Evolution of magnetoelectric properties of Sc-diluted TbMnO3.

The magnetoelectric properties of the TbMn(1-x)Sc(x)O3 series have been studied at low temperatures by means of heat capacity, magnetic measurements and impedance spectroscopy. TbMnO3 exhibits as expected three transitions upon lowering the temperature corresponding to the magnetic ordering of the two sublattices (Mn and Tb) and the ferroelectric transition. Ferroelectricity disappears with Sc dilution for x > 0.1 because the non-collinear magnetic arrangement is destroyed. The dilution of Mn with a non-magnetic ion is also detrimental to the magnetic ordering of both Mn and Tb sublattices. The system evolves to a magnetic glassy state for the intermediate compositions. Formal TbScO3 shows Sc-deficiency and long range magnetic ordering of Tb(3+) moments in the ab-plane brought by the direct interaction between Tb(3+) ions. This ordering is different from the one found in TbMnO3 due to the lack of magnetic coupling between Tb- and Mn-sublattices. A small substitution of Sc by Mn in TbScO3 destroys the Tb ordering giving rise to a magnetic glass behaviour. This effect is ascribed to the partial polarization of Tb sublattice by the paramagnetic Mn which competes with the direct Tb-Tb exchange.

Enhancement of ferromagnetic correlations on multiferroic TbMnO3 by replacing Mn with Co.

The structural, electronic and magnetic properties of TbMn(1-x)Co(x)O(3) (0.1 ≤ x ≤ 0.9) compounds are reported. The samples are isostructural to TbMnO(3) adopting the orthorhombic distorted perovskite structure (Pbnm), except for x = 0.4, 0.5 and 0.6, where an ordered double perovskite structure (P2(1)/n) is found. X-ray absorption spectra at the Mn and Co K edges show an incomplete charge transfer between Mn and Co atoms yielding a mixed valence state Mn(3+)/Mn(4+) and Co(3+)/Co(2+) for the whole series. Neutron powder diffraction measurements show the development of a ferromagnetic ground state for the intermediate compositions (0.3 ≤ x ≤ 0.6) indicating that the ferromagnetic superexchange Mn(4+)-O-Co(2+) interaction is the strongest among a wide set of competitive interactions. The ferromagnetic ordering is, however, not fully achieved and coexists with glassy magnetic properties. With increasing concentration of Co (x ≥ 0.7) the long range ordering vanishes and only a glassy magnetic behavior with slow dynamics is found. These properties could be related to the existence of magnetically inhomogeneous small clusters arising from competitive magnetic interactions.

The transition from ferromagnet to cluster-glass in La(1-x)Tb(x)Mn1/2Sc1/2O3.

We report the structural and magnetic properties of the La(1-x)Tb(x)Mn(1/2)Sc(1/2)O(3) series. LaMn(1/2)Sc(1/2)O(3) shows a long range ferromagnetic ordering in agreement with a fully polarized Mn-sublattice. The substitution of La with Tb produces structural changes which affect the magnetic properties. This substitution leads to a contraction in the unit cell volume that mainly reduces the M-O-M bond angle (M = Mn, Sc). The bending of this angle decreases the Mn-O-Mn superexchange interaction and enhances the competition between nearest-neighbour and next-nearest-neighbour interactions. Accordingly, the magnetic ground state changes from ferromagnetic to a glassy magnetic state. Large thermal irreversibility between zero-field-cooled and field-cooled conditions is observed for all samples. The study of the dynamic magnetic properties has been performed using the frequency dependent real part of the ac magnetic susceptibility. The use of both the Vogel-Fulcher law and the conventional critical slowing down law yields similarly good accuracies in the fits. The relaxation times obtained from both laws concur with the existence of a cluster-glass for x ≥ 0.5 samples.

Effect of atomic order on the martensitic and magnetic transformations in Ni-Mn-Ga ferromagnetic shape memory alloys.

The influence of long-range L2(1) atomic order on the martensitic and magnetic transformations of Ni-Mn-Ga shape memory alloys has been investigated. In order to correlate the structural and magnetic transformation temperatures with the atomic order, calorimetric, magnetic and neutron diffraction measurements have been performed on polycrystalline and single-crystalline alloys subjected to different thermal treatments. It is found that both transformation temperatures increase with increasing atomic order, showing exactly the same linear dependence on the degree of L2(1) atomic order. A quantitative correlation between atomic order and transformation temperatures has been established, from which the effect of atomic order on the relative stability between the structural phases has been quantified. On the other hand, the kinetics of the post-quench ordering process taking place in these alloys has been studied. It is shown that the activation energy of the ordering process agrees quite well with the activation energy of the Mn self-diffusion process.

Magnetic structure of Gd5Si2Ge2 and Gd5Si2Ge1.9M0.1 (M = Ga, Cu).

Powder x-ray diffraction patterns of the doped compounds Gd(5)Si(2)Ge(1.9)M(0.1) (M = Ga, Cu) show the same crystal structure, orthorhombic Gd(5)Si(4)-type, in the ferromagnetic and paramagnetic phases. This is different from Gd(5)Si(2)Ge(2), whose paramagnetic phase is monoclinic. The magnetic structure at low temperature, solved from diffraction experiments with hot neutrons, is the same in all the three compounds, collinear ferromagnetic with moments along the crystal b-axis, or F(y)F(By) according to Bertaut's notation. These results, combined with those of heat capacity and magnetocaloric effect, indicate, similarly to Gd(5)Si(4), a second-order, purely magnetic, transition in the doped compounds explaining the absence of hysteresis.

Probing and comparing electron doping and miss-site effects in Re-based double perovskites.

We have studied the Sr(2-x)La(x)Fe(1+x/2)Re(1-x/2)O(6) series in order to check how the increase in the number of Fe-O-Fe superexchange interactions affects the magnetic properties of this family of double perovskites. In these compounds the addition of La(3+) can be compensated by an increase of the Fe(3+) ratio leading to a non-electron-doped system. The unit cell of these samples expands on replacing Sr by La and the cationic ordering disappears for x>1 samples. Spontaneous magnetization at 5 K is observed in a wide range of concentrations but the magnetization at 5 T decreases as the La content increases. This result may be explained in terms of a ferrimagnetic ground state of the double perovskite. The magnetic ordering temperature increases with increasing the La and Fe content in agreement with an increase in the number of Fe-O-Fe interactions. The comparison to the electron-doped samples suggests that this effect is not enough to explain on its own the stronger T(C)-rise in Sr(2-x)La(x)FeReO(6) compounds.