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Abstract: We report a comprehensive study on the magnetic ground state of La 1.5 Ca 0.5 Co O 4 combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the Co L 2 , 3 edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at T N ≈ 52 K . Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic A -centered lattice but with a and b axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group A 2 m m where two nonequivalent compressed and expanded Co O 6 octahedra are ordered showing a checkerboard pattern in the a b plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as ( 1 / 4 , 1 / 4 , 0 ) t , ( 1 / 4 , 1 / 4 , 1 ) t , and ( 1 / 4 , 1 / 4 , 1 / 2 ) t in the parent tetragonal cell were observed at low temperature at the Co L 2 , 3 -edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the Co L 2 edge, indicates that only Co 2 + -like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the a b plane of the parent tetragonal cell with a slight tilt in the c axis. Despite the onset temperatures for the three resonant magnetic reflections being the same, ≈ 55 K , different thermal behavior is observed between ( 1 / 4 , 1 / 4 , 1 / 2 ) t and ( 1 / 4 , 1 / 4 , L ) t ( L = integer ) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the ( 1 / 4 , 1 / 4 , 1 / 2 ) t magnetic reflection is at least ten times larger than that of the ( 1 / 4 , 1 / 4 , L ) t ( L = integer ) ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector k = ( 1 / 4 , 1 / 4 , 1 / 2 ) t that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in La 1.5 Ca 0.5 Co O 4 corresponds then to this propagation vector k = ( 1 / 4 , 1 / 4 , 1 / 2 ) t while ( 1 / 4 , 1 / 4 , 0 ) t and ( 1 / 4 , 1 / 4 , 1 ) t magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.

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Abstract: The development of high‐density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat‐assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2CoSb is introduced as a new hard magnet with potential for thin‐film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m−3 is combined with a saturation magnetization of μ0Ms = 0.52 T at 2 K (2.2 MJ m−3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare‐earth‐free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m−1 K−1, make Rh2CoSb a candidate for the development of heat‐assisted writing with a recording density in excess of 10 Tb in.−2.

Open Access

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Abstract: Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure. Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films. X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.

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Abstract: Doping Mn1−xCoxWO4 with Co produces a rich magnetic phase diagram with a large number of ferroelectric phases. We present resonant x-ray magnetic scattering experiments in the known collinear AF4 phase of multiferroic Mn0.85Co0.15WO4 showing that Mn and Co spins point to different directions despite they randomly occupy the same crystallographic site. The resultant noncollinear exotic spin configuration is determined by a competition of Mn2+ and Co2+ magnetic anisotropies and demands a reexamination of the ferroelectric phase diagram of this model family beyond the previous average description of its magnetic orders.