J.M. Broto

Magnetic hysteresis and superantiferromagnetism in ferritin nanoparticles

Abstract The magnetic behaviour of nanoparticles of antiferromagnetic ferritin has been investigated by means of magnetic measurements. First, using a combination of low-field susceptibility and magnetisation measurements (up to 5.5×10 4 xa0Oe) in the superparamagnetic regime (30–250xa0K), we propose a method to estimate the Neel temperature of ferritin, by which we obtained T N ≃500xa0K. The hysteresis loop at 2.5xa0K has been measured, and interpreted with a simple model, where the irreversibilities are due to the switching of the uncompensated moments. The magnetisation curve at 2.5xa0K up to 30×10 4 xa0Oe has also been measured, and we show that it can be interpreted in terms of superantiferromagnetism, which is a finite size effect predicted by Neel in 1961, but which has not been previously experimentally evidenced. Superantiferromagnetism occurs in small antiferromagnetic particles with an even number of ferromagnetic reticular planes, and results in an enhancement of the antiferromagnetic susceptibility and in a non-linear field dependence of the magnetisation at higher fields.

Tuning the magnetic ground state of a tetranuclear nickel(II) molecular complex by high magnetic fields

Electron spin resonance and magnetization data in magnetic fields up to 55 T of a novel multicenter paramagnetic molecular complex [L_2Ni_4(N_3)(O_2C Ada)_4](Cl O_4) are reported. In this compound, four Ni centers each having a spin S = 1 are coupled in a single molecule via bridging ligands (including a mu_4-azide) which provide paths for magnetic exchange. Analysis of the frequency and temperature dependence of the ESR signals yields the relevant parameters of the spin Hamiltonian, in particular the single ion anisotropy gap and the g factor, which enables the calculation of the complex energy spectrum of the spin states in a magnetic field. The experimental results give compelling evidence for tuning the ground state of the molecule by magnetic field from a nonmagnetic state at small fields to a magnetic one in strong fields owing to the spin level crossing at a field of ~25 T.

Charge and orbital order in rare-earth and Bi manganites: a comparison

A comparative investigation of the charge modulation tendencies in R–M–MnO3 and Bi–Sr–MnO3 manganites reveals remarkable differences in these series of materials. By means of magnetic, magnetotransport, synchrotron X-ray and neutron diffraction, as well as pulsed high magnetic fields data, we compare the systematic electronic and magnetic properties of several rareearth-based manganites and bismuth-based Sr manganites. The differences between these types of systems are rationalized in terms of the role that the different electronic structure of bismuth and rare earth plays in these materials.

Unveiling the magnetic structure of graphene nanoribbons.

We perform magnetotransport measurements in lithographically patterned graphene nanoribbons down to a 70 nm width. The electronic spectrum fragments into an unusual Landau levels pattern, characteristic of Dirac fermion confinement. The two-terminal magnetoresistance reveals the onset of magnetoelectronic subbands, edge currents and quantized Hall conductance. We bring evidence that the magnetic confinement at the edges unveils the valley degeneracy lifting originating from the electronic confinement. Quantum simulations suggest some disorder threshold at the origin of mixing between chiral magnetic edge states and disappearance of quantum Hall effect.

Magnetic behaviour of Cu2FeGeSe4

Measurements of magnetic susceptibility s as a function of temperature „ and of magnetisation M as a function of applied magnetic eld H at a number of xed temperatures were made on polycrystalline samples of Cu 2 FeGeSe 4 . The s versus „ data show that an antiferromagnetic transition occurs at 20 K and that a second transition occurs at & 8K , indicating a transition to weak ferromagnetic form. The M versus H curves indicated that at all temperatures below &70 K bound magnetic polarons (BMP) occur, in the paramagnetic, antiferromagnetic and weak ferromagnetic ranges. Below 8 K, the M versus H curves exhibited magnetic hysteresis, and this is attributed to the interaction of the BMPs with tetragonally anisotropic matrix. The B versus H curves were well tted by a Langevin-type of equation, and the variation of the tting parameters determined as a function of temperature. These showed that above 20 K the total BMP magnetisation fell almost linearly with increasing temperature and e!ectively disappeared at 70 K. The number of BMPs remained practically constant with temperature having a mean value of 6.55]1018/cm3. The analysis gave a value of 213 l B for the average magnetic moment of a BMP, corresponding to 42.4 Fe atoms. Using a simple spherical model, this gives the radius of a BMP as 12.0 As . ( 2000 Elsevier Science B.V. All rights reserved.

Antiferromagnetic dimers of ni(II) in the S= 1 spin-ladder Na2Ni2(C2O4)3(H2O)2

We report the synthesis, crystal structure, and magnetic properties of the

Probing the electronic properties of individual carbon nanotube in 35 T pulsed magnetic field

S=1

Spin state transition: the origin of structural, magnetic and metal–insulator transitions in GdBaCo2O5+δ (δ≈0.5)

2-leg spin-ladder compound

Crystallographic and magnetic properties of Cu2FeGeSe4 and Cu2FeGeTe4 compounds

{mathrm{Na}}_{2}{mathrm{Ni}}_{2}{({mathrm{C}}_{2}{mathrm{O}}_{4})}_{3}{({mathrm{H}}_{2}mathrm{O})}_{2}

Magnetization measurements on fine cobalt particles

. The magnetic properties were examined by magnetic susceptibility and pulsed high field magnetization measurements. The magnetic excitations have been measured in high field high frequency ESR. Although the Ni(II) ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. The analysis of the temperature dependent magnetization data leads to a magnetic exchange constant of