Spontaneous Antiferromagnetic Order Research Articles

The temperature dependence of coercivity for Ni nanowires: Possible effect of NiO antiferromagnetic clusters

A study about coercivity as a function of the temperature is presented for nickel nanowires in porous alumina membranes. Changes in coercivity values in temperatures between 50 K and 300 K and results by X-ray diffraction (XRD) suggest that, the effects are motivated by the antiferromagnetic phase transition of small clusters of nickel oxide on the nanowires surface. This is supported by the temperature range at which the antiferromagnetic transition of NiO occurs and the possibility of interaction Ni/NiO by exchange coupling. The temperature value for spontaneous antiferromagnetic ordering of NiO, decreases with the diminution of the clusters size. The lower Néel temperature reported for these clusters is 50 K, which is in agreement with the coercivity diminution presented in the present paper. The effects of magnetostriction, magnetocrystalline anisotropy and dipolar interactions coexist with our proposal (local pinning due to exchange), giving a high complexity to the coercivity temperature behavior of these nanowires. Finally, our hypothesis was corroborated by the XRD analysis, confirming the existence of peaks belonging to the Ni and NiO phases.

Enhanced magnetoelectric effect of the exactly solved spin-electron model on a doubly decorated square lattice in the vicinity of a continuous phase transition

Magnetoelectric properties of a coupled spin-electron model on a doubly decorated square lattice in an external electric field applied along the crystallographic axis [11] are rigorously examined with the help of generalized decoration-iteration transformation. The phase diagram, spontaneous magnetization and electric polarization are exactly calculated and their dependencies are comprehensively investigated under a concurrent influence of temperature and electric field. It is found that the electric field mostly stabilizes at zero temperature the spontaneous antiferromagnetic order with respect to the ferromagnetic one. At finite temperatures the external electric field gradually suppresses a spontaneous ferromagnetic (antiferromagnetic) order emergent close to a quarter (half) filling. An enhanced magnetoelectric response is detectable in vicinity of a continuous phase transition at which the spontaneous magnetization vanishes and the electric polarization displays a weak-type singularity. It is demonstrated that reentrant phase transitions of the ferromagnetic or antiferromagnetic phase may be induced at moderate values of the electric field, which simultaneously produces a sharp kink in a critical line of the ferromagnetic phase nearby a quarter filling.

Spontaneous antiferromagnetic order and strain effect on electronic properties of α-graphyne

Using hybrid exchange-correlation functional in ab initio density functional theory calculations, we study magnetic properties and strain effect on the electronic properties of α-graphyne monolayer. We find that a spontaneous antiferromagnetic (AF) ordering occurs with energy band gap (∼0.5 eV) in the equilibrated α-graphyne. Bi-axial tensile strain enhances the stability of AF state as well as the staggered spin moment and value of the energy gap. The antiferromagnetic semiconductor phase is quite robust against moderate carrier filling with threshold carrier density up to 1.7 × 1014 electrons/cm2 to destabilize the phase. The spontaneous AF ordering and strain effect in α-graphyne can be well described by the framework of the Hubbard model. Our study shows that it is essential to consider the electronic correlation effect properly in α-graphyne and may pave an avenue for exploring magnetic ordering in other carbon allotropes with mixed hybridization of s and p orbitals.

Nuclear cooling and spin properties of rhodium down to picokelvin temperatures

The lowest temperatures ever measured have been produced by cascade adiabatic nuclear demagnetization of copper and rhodium. Ultimately, spontaneous antiferromagnetic ordering of rhodium nuclei is anticipated. High-sensitivity SQUID-NMR measurements at high nuclear polarization in low magnetic fields have been used to study the mutual interactions between the nuclear spins of rhodium in the metal.

Two-Dimensional Coordination Polymers with One-Dimensional Magnetic Chains: Hydrothermal Synthesis, Crystal Structure, and Magnetic and Thermal Properties of [MCl2(4,4‘-bipyridine)] (M = Fe, Co, Ni, Co/Ni)

Two-dimensional transition-metal coordination polymers [MCl2(bpy)] (M = Fe, Co, Ni; Co/Ni, bpy = 4, 4‘-bipyridine) have been synthesized in superheated water solutions. The hydrothermal routes resulted in four compounds [FeCl2(bpy)](I), [CoCl2(bpy)](II), [NiCl2(bpy)](III), and [(Co/Ni)Cl2(bpy)](IV). Compounds I−IV are isostructural and belong to orthorhombic crystal system, space group Cmmm (No. 65). Crystal data for I: a = 11.929(2) Å, b = 11.447(2) Å, c = 3.638(1) Å, V = 496.77(18) Å3, Z = 2. Crystal data for II: a = 11.993(2) Å, b = 11.374(2) Å, c = 3.611(1) Å, V = 492.57(18) Å3, Z = 2. The structure is a noninterpenetrating two-dimensional network containing transition-metal centers octahedrally coordinated by four bridging chlorine and two bpy ligands at trans positions. The adjacent bpy ligands within a single layer are parallel to each other at a distance of 3.6 Å. Spontaneous antiferromagnetic ordering was found in all compounds from the magnetic susceptibility χ(T) measurements under low fields. The transition temperatures are 10.0, 5.0, 8.5, and 7.5 K, for I, II, III, and IV, respectively. The μeff values yielded from fitting the high temperature χ(T) indicate the high spin states of metal ions. In addition, a metamagnetic transition was observed in the field-dependent magnetization measurement for all four compounds. The magnetic properties observed in this system are attributed to the ferromagnetic intrachain M−M exchange interaction through the Cl2 bridges along the c axis and the antiferromagnetic interchain interaction between the M ions. TGA studies indicate that all four compounds are thermally stable with an on-set temperature of weight loss greater than 400 °C.

Antiferromagnetism in solid oxygen

Solid oxygen is known to have spontaneous antiferromagnetic order in its lowest temperature phase. Muons implanted in \alpha\mbox- O2 in zero applied magnetic field take up positions where the local field is non‐zero and precess at a frequency proportional to the sublattice magnetisation. The reduction of the magnetisation as the temperature increases toward TN is similar to that expected for the case of an anisotropic AFM coupling among spins of O_2 molecules. A fit to a simple non‐interacting, harmonic spin wave model yields an anisotropy‐induced gap in the spin wave density of states Eg/kB=80.5(6) K, implying a large anisotropy constant K\approx 16J.

Evidence for Fe4+ in YBa2 (Cu1-xMx)3O7-y (M=57Fe, 57Co) by absorption and emission Mössbauer spectroscopy.

$^{57}\mathrm{Fe}$ absorption and $^{57}\mathrm{Co}$ emission M\ossbauer spectroscopy was applied to iron- and cobalt-doped ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${0}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ high-temperature superconductors (M${=}^{57}$Fe, $^{57}\mathrm{Co}$; 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$\ensuremath{\le}x\ensuremath{\le}0.1, y=0 and y=0.8) between 4.2 and 295 K with and without external magnetic field. four iron species A, B, C, and D, are observed with different dominance at different x and y. Aftereffects of the electron capture are ruled out. At 4.2 K spontaneous antiferromagnetic order is shown by contributions A, B, and C in spectra of samples with ${x}_{\mathrm{Fe}\ensuremath{\ge}0.06}$. A model-independent separation of the magnetically split subspectra shows an asymmetry parameter of the electric field gradient above 0.8 for A and B. Temperature and external magnetic field dependence of the magnetic splittings is indicative of superparamagnetic relaxation of small clusters. The species A, B, and C are magnetically interrelated. We suggest a preferential Cu(1) substitution by Fe and Co. Three of the four iron species, viz. A, B, and D have been assigned to high-spin ${\mathrm{Fe}}^{4+}$; C is high spin ${\mathrm{Fe}}^{3+}$. We provide evidence that Co is mostly in the four-valent state, too. Four-valent Fe and Co substitution in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{y}}$ is in reasonable agreement with the dopant concentration dependence of ${T}_{c}$. Possible crystallographic assignments of all four iron (cobalt) sites are discussed.