Stabilization of metastable expanded face-centered-tetragonal manganese

The structural and magnetic properties of Mn prepared on single crystalline face-centered-tetragonal (fct) Co(001) were investigated. Mn grows coherently up to at least 50 monolayers (ML) and adopts a metastable expanded fct(001) phase [c/a = 1.055(5)]. This new fct-Mn phase was recently predicted theoretically by Hafner and Spisák. Studies of magnetic Mn/Co interface exchange interactions prove the room temperature antiferromagnetic state for thicknesses above 2.5 ML. The magnetic anisotropy of the thin Mn is high enough to induce a significant exchange anisotropy for Mn thicknesses as low as 6 ML. The potential of fct-Mn to become a novel model system for systematic studies on the exchange interactions at antiferromagnet/ferromagnet interfaces is discussed.     

Magnetic properties of ErIG and approximation of isotropic exchange

A one-ion model calculation which considers the Er³⁺ ion subject to the crystal field, the isotropic exchange field and the applied magnetic field is compared with the following experimental data: the easy axis direction, the transition temperature, the temperature and field dependence of the magnetization, the compensation temperature, the magnetocrystalline anisotropy and the umbrella structure of Er³⁺ magnetic moments in ErIG. The crystal field parameters in Y(Er)GG and ErGG are discussed and determined first. The same parameters are used for ErIG, only those of second order are allowed to vary. In spite of the isotropic exchange approximation, the experiments (except the controversial umbrella structure and anisotropy data) are well reproduced within the framework of this model. Further improvements are expected by inclusion of the anisotropy of the exchange interaction.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.     

Magnetic studies in evaporated Ni/Pd multilayers

The magnetic properties of Ni/Pd multilayers, prepared by sequential evaporation in ultrahigh vacuum, have been studied. The Ni thickness dependence of the magnetization and magnetic anisotropy is discussed. The temperature dependence of the spontaneous magnetization is well described by a T^3/2 law in all multilayers. A spin-wave theory has been used to explain the temperature dependence of the spontaneous magnetization, and the approximate values for the exchange interactions for various Ni layer thicknesses have been obtained.     

Spin wave relaxation and magnetic properties in [M/Cu] super-lattices; M=Fe, Co and Ni

In this work, we study the elementary excitations and magnetic properties of the [M/Cu] super-lattices with: M=Fe, Co and Ni, represented by a Heisenberg ferromagnetic system with N atomic planes. The nearest neighbour (NN), next nearest neighbour (NNN) exchange, dipolar interactions and surface anisotropy effects are taken into account and the Hamiltonian is studied in the framework of the linear spin wave theory. In the presence of the exchange alone, the excitation spectrum E(k) and the magnetization 〈S^z〉/S analytical expressions are obtained using the Green's function formalism. The obtained relaxation time of the magnon populations is nearly the same in the Fe and Co-based super-lattices, while these magnetic excitations would last much longer in the Ni-based super lattice. A numerical study of the surface anisotropy and long-ranged dipolar interaction combined effects are also reported. The exchange integral values deduced from a comparison with experience for the three super-lattices are coherent.     

Effect of single ion anisotropy on the critical temperature of classical quasi-one-dimensional magnets

The effect of single ion anisotropy energy on the three-dimensional ordering temperature of a classical quasi-one-dimensional magnetic chain is estimated using a mean field approximation for the interchain coupling and a classical spin field model for a chain. Numerical results are presented for T_c as a function of the interchain and interchain exchange interactions and the single ion anisotropy.     

Magnetic interactions in RECu2Si2 compounds (RE = Tb-Tm)

The RKKY theory and the molecular field model including CEF effects (by Noakes and Shenoy) are applied for the explanation of magnetic interactions in the RECu₂Si₂ family (RE = Tb-Tm). The negative sign of the obtained exchange integral J_sf may point to the presence of interband mixing. The predicted magnetic anisotropy and enhancement of ordering temperatures T_N-5 above the de Gennes values are consistent with experimental ones only on the basis of the B⁰₂ model with parameter A⁰₂ as for CdCu₂Si₂. They are not consistent if one uses the full CEF Hamiltonian with 5 A^m₁ parameters as for TmCu₂Si₂. A short discussion about the role of magnetic interactions other than the simple RKKY exchange is also given.     

Magnetic domain structures and magnetotransport properties in Co-Ag granular thin films

The magnetic microstructures and magnetotransport properties in granular Co_xAg_1-x films with 17%≤x≤62% were studied. Magnetic force microscopy (MFM) observations showed the presence of magnetic stripe domains in as-deposited samples with x≥45% and the evolution of the magnetic domain patterns to in-plane domains with annealing. A perpendicular magnetic anisotropy as high as about 8×10⁵ ergs/cc for as-deposited Co₆₂Ag₃₈ and about 6×10⁵ ergs/cc for as-deposited Co₄₅Ag₅₅ was observed by magnetization and torque measurements. With increasing annealing temperature, the perpendicular magnetic anisotropy became negative. The origin of the perpendicular magnetic anisotropy may be attributed to a rhombohedral distortion of the cubic cell due to residual substrate-film stresses. The magnetic stripe domains are the consequence of the interplay of the indirect or direct exchange, perpendicular magnetic anisotropy and dipolar interactions. Finally, magnetoresistance (MR) curves displayed training behaviours and different shapes when measured with different configurations (parallel, transverse and perpendicular). It is proposed that the existence and the evolution of the magnetic domain structures strongly affect the magnetotransport properties due to the extra contribution of the electron scattering at the domain walls. Furthermore, an anisotropic MR also contributes to the overall MR curves. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 23 May 2001     

Patterned media with composite in-plane and perpendicular anisotropy recording layers

Magnetization reversal mechanism in nanostructures composed of exchange coupled bi-layers with in-plane and perpendicular anisotropy was investigated. Micromagnetic simulation was carried out for bit-patterned media with areal density of 5 Tb/in², as example. Magnetization of thermally stable recorded bit using a single layer may not switch under write field. However, a complete and fast switching is possible with an exchange coupling to a layer with in-plane anisotropy. By adjusting the thicknesses and intrinsic properties of the two layers, the composite recording layer still can retain perpendicular anisotropy. The exchange coupled structure with dual-anisotropy can be extended to magnetic memories.     

Magnetization plateaus and supersolid phases in an extended Shastry–Sutherland model

New magnetization plateaus and supersolid phases are identified in an extended Shastry–Sutherland model – with additional longer range interactions and exchange anisotropy – over a wide range of interaction parameters and an applied magnetic field. The model is appropriate for describing the low energy properties of some members of the rare earth tetraborides. Using a plaquette representation and exact mapping of ground state configurations in the Ising limit, supplemented by large scale quantum Monte Carlo simulations to include the effects of exchange interactions, we have identified several magnetization plateaus and associated spin supersolid phases. Our methods enable us to elucidate the underlying structure and mechanism of formation of the supersolid phases, thus gaining deeper understanding into their emergence from competing interactions. Additionally, in this regime we find evidence of a fractional plateau at 1∕9 saturation magnetization, which may hold the clue to understanding the fractional plateau in TmB₄ that exhibits magnetic hysteresis.     

The influence of anisotropy of exchange interactions on the magnetic properties of the Li2CuO2 cuprate

A quantum spin model with competing ferro- and antiferromagnetic exchange interactions was studied. The model described a special class of quasi-one-dimensional cuprates. The influence of anisotropy of exchange interactions on the properties of the model was analyzed. It was shown that, under certain conditions, the ferromagnetic state was the ground state, and the spectrum of excitations was characterized by the presence of bound magnon states. The results are used to analyze the magnetic properties of the Li₂CuO₂ quasi-one-dimensional cuprate.     

Magnetic anisotropy of Pm impurity in rare earth–nickel compound

Low-temperature nuclear orientation was applied to study hyperfine interactions of ¹⁴²Pr, ¹⁴⁷Nd and ^143,144Pm nuclei in Pr^0.5Nd^0.5Ni single crystal. Angular distributions, temperature dependence and external magnetic field effects on the γ-ray anisotropy are presented. A Nd-Pm exchange interaction seems to dominate the magnetic properties of Pm ions in this system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Domain state–dependent magnetic formation of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles analyzed via magnetic resonance

Magnetic properties, arising from surface exchange and interparticle interactions of the Fe₃O₄ (magnetite) nanoparticles, were investigated in the temperature range of 5–300 and 120–300 K using vibrating sample magnetometer technique and electron spin resonance spectroscopy, respectively. The research was based on to figure out the origin of intraparticle interactions and the change of interparticle interactions in wide size range Fe₃O₄ nanoparticles. The analyses were done for samples having almost same particle size distributions. The average particle sizes were changed in between 30 ± 2 and 34 ± 2 nm. The observed magnetization values were demonstrated the mixture of single-domain size particles, exhibiting both single-domain (SD) and superparamagnetic (SPM) states. The symmetry of resonance curves changed according to the ratio of SD and SPM-stated particles in mixture under located temperature. The changes of anisotropy up to domain state were understood by freezing magnetic moment in glycerol matrix from room temperature to 120 K under 5-kG field. The shift of H _R values to higher magnetic fields and the more symmetric resonance spectrum proved the effect of anisotropy and interparticle interactions fields on magnetic behave. In addition, the origin of intra-interaction was exposed from Fe³⁺ centers and exchange coupling in between Fe²⁺, Fe³⁺, and O⁻, and Fe³⁺ centers found from g factor (g).     

Multifrequency time-resolved EPR (9.5GHz and 95GHz) on covalently linked porphyrin-quinone model systems for photosynthetic electron transfer: effect of molecular dynamics on electron spin polarization

Covalently linked porphyrin–quinone model systems for photosynthetic electron transfer were examined by using time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (0.34T/9.5GHz, X-band) and high field and frequency (3.4T/95GHz, W-band). The paramagnetic transients studied were the light-induced spin-correlated radical pair states of the donor–acceptor complex in polar solvents below the melting point and in the soft glass phase of a liquid crystal. It is shown that the systems form strongly exchange-coupled radical pairs, whose TREPR lineshapes are determined mainly by fast electron recombination together with both spin–lattice relaxation and modulation of the exchange interaction. Below the melting point the spin–lattice relaxation rate naturally slows down, but that of the spin on the quinone site is still of the order of 10⁶ s^-1. Most probably this is due to contributions from spin–rotation interaction, and dependent on the molecular orientation with respect to the magnetic field. This relaxation anisotropy is related to anisotropic motion of the quinone site in the solvent cage. The results allow conclusions to be drawn concerning the molecular dynamics and flexibility of the systems. To yield long-lived radical pair states that would mimic photosynthetic electron transfer, the two mechanisms described, modulation of exchange and spin–rotation interactions, have to be suppressed by reducing the molecular flexibility of the complex.     

Energy contributions in magnetite nanoparticles: computation of magnetic phase diagram,theory, and simulation

In this study, we present a theoretical analysis of magnetization processes by considering energy contributions in magnetite fine particles. The focus is on the K _S-driven magnetic phase transition taking place between the low surface-anisotropy ferrimagnetic state and the hedgehog configuration obtained in the high surface-anisotropy limit. Analytical expressions of energy terms (exchange, magnetocrystalline anisotropy, surface-anisotropy) are presented and their magnitudes are computed for different particle sizes. Monte Carlo simulations were also carried out for comparison purposes. A core–shell model is implemented for simulating magnetite nanoparticles between 2 and 10 nm in diameter. Our simulation framework is based on a three-dimensional classical Heisenberg-like Hamiltonian with nearest magnetic neighbors interactions. It includes exchange coupling, cubic magnetocrystalline anisotropy for core ions, and single-ion site surface-anisotropy for those atoms belonging to the shell. The magnetic phase diagram and comparisons between analytical and numerical results are presented and discussed.     

Anisotropic exchange contributions to the magnetic susceptibility of transition metal ions in hexagonal close packed metals

When the effect of anisotropic exchange, together with crystal-field potentials is included in the Hamiltonian for magnetic impurity ions in a non-cubic metal, an additional contribution to the magnetic susceptibility is obtained. This produces a change in the temperature dependence of the susceptibility at low temperatures and gives an anisotropy varying as 1/T². The anisotropy in exchange, deduced from experiment, is 0.0086 per cent for Zn: Mn and between 2.4 and 7.2 per cent for Zn : Cr with J₆ > J_⊥ in both cases.     

Magnetic Properties of Spin-1/2 Quantum Heisenberg Ferromagnet with Spatially Anisotropy

The spin-1/2 quantum Heisenberg ferromagnet with anisotropic spin exchange interactions in three dimensions are investigated by means of the Green's function method. In the Tyablikov approximation, the correlation functions, the magnetization, and the susceptibilities are computed. The magnetic properties of this model are found to be dependent of anisotropy.     

Magnetization reversal in CoPt-based hard–soft homocomposites

CoPt-based hard–soft sputtered bilayers with (1 1 1) texture have been produced by appropriate heat treatment of the bottom layer. Two samples with different degrees of chemical ordering of the hard layer are compared. The anisotropy of the hard layer determines its robustness against destabilization from the soft one. Detailed measurements of the soft layer minor hysteresis loop features as a function of the magnetic state of the hard layer are proposed as a means to study the nature of interfacial exchange interactions and the mechanism of magnetization reversal. When hard layer anisotropy is not robust enough, the reversed soft layer can induce irreversible changes to the magnetic structure at the interface leading to a decoupling of exchange field from the magnetic state of the hard layer.     

Uniaxial magnetic anisotropy of rhombohedral CoCO3 crystals at T = 0 K

A method for calculating the contribution of exchange interaction to uniaxial anisotropy with the use of g’ factors has been worked out using CoCO₃ crystals as an example. The calculated contribution of dipole-dipole interactions to the anisotropy of CoCO₃ is 0.93 cm^?1. The sum of the contributions to the anisotropy constant of CoCO₃ with the inclusion of the dipole-dipole interactions is 36.1 cm^?1.