Magnetic Ground State Structure Research Articles

CALPHAD: Method for calculation of finite temperature thermodynamic properties for magnetic allotropes—Case study on Fe, Co and Ni

A method for calculation of finite temperature thermodynamic properties for magnetic allotropes has been developed. The developed method is based on quasi-harmonic approximations using the Debye–Grüneisen model. Electronic contributions are included and magnetic disordering contributions are added with the empiric model often used in CALPHAD (CALculation of PHAse Diagrams) databases. The developed method also includes two new ways to estimate the Debye temperature (θD(V0)) which enables calculation of finite temperature thermodynamic properties also for allotropes that are dynamically unstable at 0 K. Also a new CALPHAD approach for estimation of Curie- (TC) or Néel- (TN) temperatures is presented. The developed method was used to calculate Gibbs energy as a function of temperature (G(T)) and isobaric heat capacity as a function of temperature (CP(T)) for the body-centred-cubic (bcc), the face-centred-cubic (fcc) and the hexagonal-closed-packed (hcp) allotropes of Fe, Co and Ni. The results were compared with the SGTE Unary database. By using the developed method the phase transitions in Fe and Co were predicted. The calculations of the metastable allotropes fcc-Fe, hcp-Fe and bcc-Ni in this work indicates that the descriptions of these allotropes in the SGTE Unary database need to be updated, especially regarding the magnetic parameters. Furthermore, contradictory to previously reported results in literature, the calculations in this work show that the magnetic ground state structure of fcc-Fe is the double layer antiferromagnetic structure (AFM-D) and not a spin-spiral. The final conclusion is that by using the developed method it is possible to calculate finite temperature thermodynamic properties, not only, for stable and metastable magnetic allotropes, but also for magnetic allotropes that are dynamically unstable at 0 K, which is very important for the development of thermodynamic databases.

Magnetic switching in Crx (x = 2–8) and its oxide cluster series

First principle studies on the magnetic ground state structure, noncollinearity, binding energy and various electronic properties of a series of Crx (x = 2–8) clusters are performed. In order to investigate the effect of ionization and oxidation on the clusters, the anionic (Crx−) and oxidized (CrxO2) analogues of those clusters are also studied in detail. To calculate adiabatic electron affinity of CrxO2 clusters, additionally CrxO2− analogues are also included in the present work. An interesting even (non-magnetic) – odd (magnetic) feature in the considered cluster series has been noticed. The similar behavior is also reflected from their electronic properties as even (less reactive) – odd (more reactive). The most of the neutral and ionized chromium clusters, viz., Crx and Crx− are found to be noncollinear in their ground states, whereas oxidation stabilized those clusters into the collinear spin alignments. The bond distances of Cr clusters are found to be close with available experimental studies.

Density-Functional Theory Study of Ce@C<sub>82</sub>

The magnetic ground state structure of the metallofullerene Ce@C82 is confirmed by the density functional calculations. The results show that the Ce atom is located inside the C82 cage with site at the C2 symmetry axis. The effective magnetic moment of Ce@C82 is increased relative to the value of a free Ce3+ ion. The reason is that there is hybridization between unoccupied Ce-4f states and carbon-π states, which result in a general ferromagnetic coupling of the Ce-4f spin with the remaining unpaired spin in the hybridized molecular orbital.

Magnetic excitations in the geometric frustrated multiferroic CuCrO2

In this paper detailed neutron scattering measurements of the magnetic excitation spectrum of CuCrO${}_{2}$ in the ordered state below ${T}_{\mathrm{N}1}=24.2$ K are presented. The spectra are analyzed using a model Hamiltonian which includes intralayer exchange up to the next-next-nearest neighbor and interlayer exchange. We obtain a definite parameter set and show that exchange interaction terms beyond the next-nearest neighbor are important to describe the inelastic excitation spectrum. The magnetic ground state structure generated with our parameter set is in agreement with the structure proposed for CuCrO${}_{2}$ from the results of single crystal diffraction experiments previously published. We argue that the role of the interlayer exchange is crucial to understand the incommensurability of the magnetic structure as well as the spin-charge coupling mechanism.

Synthesis, structure, and properties of tetragonalSr2M3As2O2(M3=Mn3,Mn2Cu, andMnZn2) compounds containing alternatingCuO2-type and FeAs-type layers

Polycrystalline samples of Sr2Mn2CuAs2O2, Sr2Mn3As2O2, and Sr2Zn2MnAs2O2 were synthesized. Their temperature- and applied magnetic field-dependent structural, transport, thermal, and magnetic properties were characterized by means of x-ray and neutron diffraction, electrical resistivity rho, heat capacity, magnetization and magnetic susceptibility measurements. These compounds have a body-centered-tetragonal crystal structure (space group I4/mmm) that consists of MO2 (M = Zn and/or Mn) oxide layers similar to the CuO2 layers in high superconducting transition temperature Tc cuprate superconductors, and intermetallic MAs (M = Cu and/or Mn) layers similar to the FeAs layers in high-Tc pnictides. These two types of layers alternate along the crystallographic c-axis and are separated by Sr atoms. The site occupancies of Mn, Cu and Zn were studied using Rietveld refinements of x-ray and neutron powder diffraction data. The temperature dependences of rho suggest metallic character for Sr2Mn2CuAs2O2 and semiconducting character for Sr2Mn3As2O2 and Sr2Zn2MnAs2O2. Sr2Mn2CuAs2O2 is inferred to be a ferrimagnet with a Curie temperature TC = 95(1) K. Remarkably, we find that the magnetic ground state structure changes from a G-type antiferromagnetic structure in Sr2Mn3As2O2 to an A-type ferrimagnetic structure in Sr2Mn2CuAs2O2 in which the Mn ions in each layer are ferromagnetically aligned, but are antiferromagnetically aligned between layers.

Noncollinear magnetism in Permalloy

Permalloy is an important material in a wide variety of magnetic systems, most notably in giant-magnetoresistive read heads. However, despite this great interest, its properties are not fully understood. For an in depth analysis of important physical properties as, e.g., electric transport or magnetic anisotropy, a detailed understanding of the distribution of magnetic moments on an atomic level is necessary. Using our first principles locally self-consistent multiple scattering method, we calculate the magnetic ground state structure for a large supercell model of Permalloy. Our code allows us to solve both the usual nonrelativistic Schrödinger equation as well as the fully relativistic Dirac equation and to find the magnitude and direction of the magnetic moments at each atomic site. While the nonrelativistic calculation yields a collinear ground state in accordance with previous calculations, we find the ground state for the fully relativistic calculation to be slightly noncollinear. We also investigate the influence of variations in the iron concentration on the distribution of magnetic moments.

Ising-axis conversion in [formula omitted

The so-called Ising-axis conversion is an irreversible metamagnetic transition observed in the orthorhombic antiferromagnets RCu 2 ( R = lanthanide ) in high magnetic fields applied along the hard axis of magnetization. In fields parallel to the crystallographic c -axis it converts the magnetic Ising axis from the a - to the c -direction for R = Ce , Pr, Tb and Dy. This transition is accompanied by a giant magnetostriction effect ( Δ L / L ≈ 1 % ) and the new state remains stable even after switching off the field. The axis conversion process is not limited to the ordered region only and occurs also in the paramagnetic range. To study the competition between the magnetic and the quadrupolar interactions an Y-diluted system is studied. High-quality single crystals of the ( Tb 0.5 Y 0.5 ) Cu 2 compound were grown by the Czochralski pulling technique and subjected to the specific heat, magnetization, neutron diffraction and axis-conversion experiments. The dilution of the Tb sublattice leads to weakening of the magnetic exchange, manifested by lowering of the Néel temperature from T N = 54 K in pure TbCu 2 to T N = 30 K in ( Tb 0.5 Y 0.5 ) Cu 2 , while the magnetic ground state structure remains unchanged. The Ising-axis conversion has been studied by magnetization measurements. The c -axis-magnetization curve exhibits several metamagnetic transitions corresponding to the conversion of the easy-axis of magnetization. The temperature dependence of the critical fields is well comparable to that of the pure TbCu 2 . From the experiments follows that the quadrupolar interaction is the driving force of the conversion process.

Magnetic phase transition and magnetic structure of ground state in CsDyW O

Magnetic phase transition in the CsDyW2O8 magnet has been studied by means of low temperature specific heat C(T) measurements. The magnetic ordering temperature of the Dy3+ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy3+ ions. The behavior of the C(T) dependencies above and below TN is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state.

Complex magnetism in ultra-thin films: atomic-scale spin structures and resolution by the spin-polarized scanning tunneling microscope

In this paper we present a density functional theory investigation of complex magnetic structures in ultra-thin films. The focus is on magnetically frustrated antiferromagnetic Cr and Mn monolayers deposited on a triangular lattice provided by a Ag (111) substrate. This involves non-collinear magnetic structures, which we treat by first-principles calculations on the basis of the vector spin-density formulation of the density functional theory. We find for Cr/Ag (111) a coplanar non-collinear periodic 120° Neel structure, for Mn/Ag (111) a row-wise antiferromagnetic structure, and for Fe/Ag (111) a ferromagnetic structure as magnetic ground states. The spin-polarized scanning tunneling microscope (SP–STM) operated in the constant-current mode is proposed as a powerful tool to investigate complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. We discuss a recent application of this operation mode of the SP–STM on Mn/W (110), which led to the first observation of a two-dimensional antiferromagnet on a non-magnetic metal. The future potential of this approach is demonstrated by calculating SP–STM images for different magnetic structures of Cr/Ag (111). The results show that the predicted non-collinear magnetic ground state structure can clearly be discriminated from competing magnetic structures. A general discussion of the application of different operation modes of the SP–STM is presented on the basis of the model of Tersoff and Hamann.

Magnetic structure of ground state of the KDy(WO 4) 2 single crystal

The data of low-temperature magnetic measurements of the monoclinic KDy(WO 4) 2 crystal are used to calculate the exchange and dipole–dipole interactions energy. The last are used for the determination of the magnetic structure of the ground state.