Nonmagnetic Hosts Research Articles

Defect-induced magnetism in TiO2: An example of quasi 2D magnetic order with perpendicular anisotropy

Magnetic order at room temperature induced by atomic lattice defects, like vacancies, interstitials, or their pairs, has been observed in a large number of different non-magnetic hosts, such as pure graphite, oxides, and silicon-based materials. High Curie temperatures and time-independent magnetic response at room temperature indicate the extraordinary robustness of this new phenomenon in solid-state magnetism. In this work, we review experimental and theoretical results of pure TiO2 (anatase), whose magnetic order can be triggered by low-energy ion irradiation. In particular, we discuss the systematic observation of an ultrathin magnetic layer with perpendicular magnetic anisotropy at the surface of this oxide.

Intrinsically Magnetic Cells: A Review on Their Natural Occurrence and Synthetic Generation.

The magnetization of non-magnetic cells has great potential to aid various processes in medicine, but also in bioprocess engineering. Current approaches to magnetize cells with magnetic nanoparticles (MNPs) require cellular uptake or adsorption through in vitro manipulation of cells. A relatively new field of research is “magnetogenetics” which focuses on in vivo production and accumulation of magnetic material. Natural intrinsically magnetic cells (IMCs) produce intracellular, MNPs, and are called magnetotactic bacteria (MTB). In recent years, researchers have unraveled function and structure of numerous proteins from MTB. Furthermore, protein engineering studies on such MTB proteins and other potentially magnetic proteins, like ferritins, highlight that in vivo magnetization of non-magnetic hosts is a thriving field of research. This review summarizes current knowledge on recombinant IMC generation and highlights future steps that can be taken to succeed in transforming non-magnetic cells to IMCs.

Laser‐modulated RKKY interaction in two‐dimensional metals

The conduction electrons can deliver an indirect exchange interaction, namely the RKKY interaction, between impurities dissolved in nonmagnetic metal hosts by the interaction between the conduction electron spins and localized impurity spins. The effect of the laser field on the RKKY interaction between two localized spins in two‐dimensional metals is studied by a quantum theory in this paper. The calculation results indicate an intense laser field can effectively manipulate this kind of indirect exchange interaction. The threshold of field strength for achieving this effect is given, and the exchange integrals at different impurity separations and laser field strengths are exhibited. It is discussed that both the finite spatial extension of the interaction potential and thermal energy of irradiation with a laser can influence the RKKY interaction.

Size effects on the local magnetism and Kondo behavior of isolated Fe impurities in nanocrystalline metallic hosts

Local magnetism of 3d impurities in nonmagnetic metallic hosts is a subject of intense experimental and theoretical investigations. A key problem is the study of Kondo effect arising from antiferromagnetic exchange interaction between the impurity moment and host-conduction electrons leading to moment instability below a characteristic temperature TK. While extensive studies have been carried out for many d and f impurities in bulk metallic hosts 1,2 little information is available on the Kondo behavior of single magnetic impurities in nanocrystalline materials. Recently resistivity studies in thin films and nanowires of AusFed ,C usCrd, and CusFed alloys have revealed the suppression of Kondo interaction below a critical thickness and width. 3 The results, initially interpreted as being due to confinement of the Kondo screened electron cloud, were later shown to arise from size

Giant thermopower in carbon nanotubes: A one-dimensional Kondo system

The electrical transport properties of single-wall carbon nanotubes are shown to be strongly influenced by the presence of transition-metal impurities derived from the catalyst introduced to stimulate their growth. Data on thermoelectric power and electrical resistance in the temperature range 10‐400 K were obtained on a series of samples prepared using M-Y catalysts ~M5Cr, Mn, Co, Fe, Ni!. The unusual transport behavior observed is tentatively assigned to an interaction between the magnetic moment of the M atom and the spin of the conduction p electrons of the nanotubes, i.e., the Kondo effect. @S0163-1829~99!50240-3# Small diameter (d,2 nm) single-wall carbon nanotubes ~SWNT! have now been prepared in sufficient yield from catalyzed carbon plasmas to allow the widespread study of the chemical and physical properties of these molecular filaments. 1 In this work, we present the results of a systematic study of the transport properties of SWNT mats produced from a series of transition-metal‐Y catalysts which show that the identity of the catalyst has a pronounced effect on the electrical transport. It is known from numerous studies on nonmagnetic metallic hosts ~X! that only ;10‐100 ppm magnetic impurity ~M! can have a dramatic effect on the electrical transport properties of the host. 2 A giant peak in the thermopower vs temperature and an associated resistivity minimum were reported in the systems X: M, where X5Au, Ag, Cu, and M 5Ti, V, Cr, Mn, Fe, Co, Ni. 2,3 These observations are explained theoretically as the result of an interaction between the magnetic moments of the impurity atoms and the conduction electron spins of the host. 2,3 This interaction was shown to lead to a new spin-dependent scattering mechanism and a narrow hybridization peak or ‘‘Kondo resonance’’ in the electronic density of states positioned near the Fermi energy E F . 3,4 The Kondo temperature can be written approxi

Magnetic contribution to the segregation energies in magnetic-nonmagnetic systems

The segregation energy of magnetic materials (Fe, Co, Ni) embedded in some nonmagnetic hosts (Pd, Ag, Cu) is calculated using an approximate procedure based on ab initio calculations of three-dimensional periodic systems, performed in the spin polarized and paramagnetic states. Slabs with full layers of magnetic impurities or with ordered alloy layers are considered, separated by sufficient empty space so as to simulate free surfaces. We find that the considered magnetic atoms always prefer to lie embedded in the host metal and not to be located at the surfaces (positive segregation energy). The magnetic calculation gives a lower segregation energy than the nonmagnetic one, the difference being larger for AgCo and CuCo than for PdCo and FeCu. It is zero for CuNi. We have found, however, that this difference is independent of the local magnetization at the impurities. For example, it is 0.06 eV for Fe in Cu, where the magnetic moment is 2.4 ${\ensuremath{\mu}}_{B},$ and 0.12 eV for Co in Cu, where the magnetic moment is 1.6 ${\ensuremath{\mu}}_{B}.$

Low-temperature nuclear-spin relaxation times and line shapes for Hsub2 in solid nonmagnetic hosts.

We calculate the nuclear-spin relaxation time ${T}_{1}$ and line-shape function for isolated ortho- (J=1) ${\mathrm{H}}_{2}$ molecules in solid nonmagnetic hosts located at sites with various crystal-field symmetries. This work includes modifications of earlier work so that the formalism can be used in regimes where ${\ensuremath{\Gamma}}_{m}$, the molecular angular momentum relaxation rate, is not large compared to the molecular-nuclear spin coupling constants ${\ensuremath{\omega}}_{c}$ and ${\ensuremath{\omega}}_{d}$. The formalism also allows for a nonzero value of the quadrupolarization 〈3${J}_{z}^{2}$-${J}^{2}$〉. With these changes the theory can be used at temperatures at and below about 4 K where line shapes evolve from Lorentzians into Pake doublets.

Crystalline electric fields in metals

A method is described for calculating crystal fields for rare-earth ions in non-magnetic hosts. Screening is included by performing a Bohm-Pines-like unitary transformation on the starting Hamiltonian which contains kinetic-energy and Coulombic energies of electrons in the presence of fixed nuclei. Contributions from core orbitals are included as well as components from a 5d virtual bound state assumed to contain no more than one electron. The coupling between the 4f impurity and the majority s-like conduction band is assumed to be weak. Degenerate perturbation theory has been used and in zeroth order it has been assumed that the impurity potential is sufficiently strong to remove one state from the s-band. The many-body nature of the problem has been incorporated using a second quantised representation. Further unitary transformations are performed to weaken the 4f-conduction band coupling before averaging over the conduction states to obtain an effective 4f Hamiltonian.

Very diluted RE's in (Sc, Y, La)Al 2: Crystals field systematics and spin dynamics

Inelastic neutron scattering results are presented which extend earlier work especially to RE's in ScAl 2 and furthermore to some RE's in LaAl 2 (Pr, Nd, Tb, Dy, Ho in ScAl 2; Pr, Ho in LaAl 2). For different RE's we report about the systematics of CF-parameters within and between the non-magnetic hosts (Sc, Y, La)Al 2. A detailed study of the magnetic single ion behavior and the description with the dynamical single ion susceptibility as calculated by Becker, Fulde and Keller was done on 3000 ppm of Tm, Tb and Er in YAl 2 and LaAl 2 over a temperange between 1.5 K and room temperature.

Effects of higher-Jstates on nuclear-spin relaxation times forH2in solid nonmagnetic hosts

Effects of higher-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>J</mml:mi></mml:math>states on nuclear-spin relaxation times for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>in solid nonmagnetic hosts

Magnetic polarization clouds, percolation, and the critical concentration for ferromagnetism

The magnetic properties of many alloys are described in terms of magnetic polarization clouds. This is true in particular of dilute alloys of magnetic atoms in strongly exchange-enhanced hosts and of concentrated but magnetically dilute alloys of Ni in nonmagnetic hosts. In such alloys the locally averaged magnetization density is observed by diffuse neutron scattering to fall off as a Yukawa function away from the center of a polarization cloud. We present numerical calculations which confirm this Yukawa form and present an approximate theoretical derivation which gives a formula for the inverse range κ of a polarization cloud in an alloy in terms of the uniform exchange-enhancement factor α and the band susceptibility Γ(q) for the alloy. Assuming ferromagnetism in such alloys to arise from percolation amongst the polarization clouds, we also derive a relationship between the critical concentration c*cr, of polarization clouds for the onset of ferromagnetism and the average magnetization μ*cr and inverse range κcr of those clouds. This relationship is found to be in good agreement with experiment. Approximate calculated values of c*cr, μ*, and κ are presented for two cases. The effects of chemical ordering and clustering are considered briefly.

Symmetry analysis of states of ion pairs in crystals

The symmetry classification of the states of a pair of interacting ions in a crystal is obtained by a formal symmetry reduction not dependent upon any assumptions about the details of the interaction. The theory is given in versions valid with or without spin-orbit coupling, and for pairs in magnetic or non-magnetic hosts. Some discrepancies with previous work are discussed.

Nuclear-spin-lattice relaxation times forH2in solid nonmagnetic hosts

We calculate the nuclear-spin relaxation times ${T}_{1}$ and ${T}_{2}$ for isolated ortho ($J=1$) ${\mathrm{H}}_{2}$ molecules in solid nonmagnetic hosts located at sites with various crystal fields. The mechanism considered for the relaxation of a nuclear spin to lattice equilibrium proceeds via the molecular angular momentum and is valid in the limit where molecular spin-lattice decay rates are much greater than molecular-nuclear-spin coupling frequencies. We find that many relaxation properties, including the value of ${T}_{1}$ at the ${T}_{1}$ minimum and the product of ${T}_{1}{T}_{2}$ at temperatures well below the temperature of the ${T}_{1}$ minimum, depend crucially on the crystal field at the ${\mathrm{H}}_{2}$ molecular site. Thus measurements of ${T}_{1}$ and ${T}_{2}$ can yield definite information about the environment of ${\mathrm{H}}_{2}$ molecules in a host lattice.

?+SR in imperfect solids

The use of μ+SR as a probe of point defects in metals is discussed. The importance of separating the effects due to the presence of the muon from those intrinsic to the defect-host system is stressed. It is suggested that studies of magnetic impurities in non-magnetic hosts may yield data which are relatively probe-independent. A theoretical analysis of the depolarization of muons coupled to magnetic impurities is outlined. It is pointed out that measurements of the field dependence of the depolarization rate can give information about impurity spin correlation times.

Crystal fields in Sc, Y, and the heavy-rare-earth metals Tb, Dy, Ho, Er, Tm, and Lu

Experimental investigations of the magnetic properties of dilute alloys of the rare-earth solutes Tb, Dy, Ho, Er, and Tm in the nonmagnetic hosts Lu, Y, and Sc have been performed. These measurements, which include and supplement earlier published results, have been analyzed and crystal-field parameters for all these 15 alloy systems deduced. The consistency of the parameters was confirmed by a variety of magnetic measurements, including neutron spectroscopy. Crystal-field parameters have also been derived for the ions in pure magnetic rare-earth metals and their alloys using the results for the dilute alloys supplemented with paramagnetic measurements up to high temperatures on the concentrated systems. Mean values and standard deviations of the higher-order crystal-field parameters for all Y and Lu alloys are $\frac{{B}_{40}}{\ensuremath{\beta}}=6.8\ifmmode\pm\else\textpm\fi{}0.9$ K, $\frac{{B}_{60}}{\ensuremath{\gamma}}=13.6\ifmmode\pm\else\textpm\fi{}0.7$ K, and $\frac{{B}_{66}}{\ensuremath{\gamma}}=(9.7\ifmmode\pm\else\textpm\fi{}1.1)\frac{{\ensuremath{\beta}}_{60}}{\ensuremath{\gamma}}$. These values---with the inaccuracies somewhat increased---are expected to be representative also for the magnetic rare-earth metals. For rare-earth ions in the Sc host the values $\frac{{B}_{40}}{\ensuremath{\beta}}=9.9\ifmmode\pm\else\textpm\fi{}1.9$ K, $\frac{{B}_{60}}{\ensuremath{\gamma}}=19.8\ifmmode\pm\else\textpm\fi{}1.5$ K, and $\frac{{B}_{66}}{\ensuremath{\gamma}}=(9.4\ifmmode\pm\else\textpm\fi{}0.9)\frac{{\ensuremath{\beta}}_{60}}{\ensuremath{\gamma}}$ were deduced. $\frac{{B}_{20}}{\ensuremath{\alpha}}$ is a host-sensitive parameter which has the average values of -102.7, -53.4, and 29.5 K for rare-earth ions in Y, Lu, and Sc, respectively. There is also evidence that this parameter varies with the solute. $\frac{{B}_{20}}{\ensuremath{\alpha}}$ for ions in the pure magnetic rare-earth metals and their alloys shows a linear variation with $\frac{c}{a}$ ratio characteristic of each ion. The results indicate a contribution from anisotropic exchange to the high-temperature paramagnetic anisotropy of approximately 20% for Tb, Dy, Ho, and Er, and approximately 10% for Tm.

Temperature Dependence of the Ni^2+ Ion Absorption Transition in Magnetic Host Materials

The temperature dependence of the Ni2 + ion absorption-transition maxima has been studied in the magnetic host materials of KNi1–xMgxF3, RbFeF3, RbCoF3, RbMnF3, MnF2, and KMnF3. The behavior of the absorption transitions in these materials differs from the behavior due to purely thermal effects observed in the nonmagnetic hosts. Additional shifts, due to the exchange interactions responsible for the magnetic properties, are superimposed on the thermal shifts. In the pure-nickel salts, the departures from purely thermal behavior occur in the vicinity of the ordering temperature of the crystals. In KNi1–xMgxF3, crystals, the departures are observed in materials of sufficient Ni2+ concentration, below the Néel temperature of the pure material. In magnetic materials where the Ni2+ ion is an impurity, the magnetically induced shifts in the nickel absorption bands are observed near the ordering temperature of the host crystal.

Spin polarization around magnetic impurities

Numerical results are given for the spin polarization around 3d series magnetic impurities in non-magnetic hosts. The wave-number dependence of the conduction electron scattering and the magnetic ion structure are taken into account.

Hyperfine fields at impurities in metals

We give a very brief survey of the present ideas on hyperfine fields for dilute impurities in ferromagnetic hosts and for dilute magnetic impurities in non-magnetic hosts. There are three main groups of impurities corresponding to different partially filled atomic shells and so to different dominant hyperfine field mechanisms. These are the rare earths, the transition elements, and the non-magnetic elements having only s-p shells partially filled.

Properties of the Hyperfine Structure of Dilute and very Dilute Fe in Nonmagnetic Metallic Hosts

The Mössbauer effect has proven fruitful for investigating the magnetic properties of dilute concentrations of Fe in metals. Measurements of the magnetic hyperfine field Hhf at Fe impurities in nonmagnetic metal hosts yield information relating to the formation of localized moments and to the onset of impurity-impurity interactions. These studies fall into two classes: (A) Systems in which the impurity concentration is sufficiently low (or the measurement temperature sufficiently high) that impurity-impurity interactions play a negligible role and the Hhf depends only upon the nature of the impurity and of the host. (B) Systems in which interactions play a significant role and magnetic transitions can occur. In the first category, measurements on a number of cubic transition metals and alloys will be compared with other experimental data and with theories of the formation of localized magnetic moments. In the second category, dilute alloys of Fe with Au, and Pd will be compared with the results of other experiments and with theoretical concepts of direct and indirect exchange interactions.