Giant Magnetic Moment Research Articles

Ferromagnetism in Co doped CeO2: Observation of a giant magnetic moment with a high Curie temperature

We report room temperature ferromagnetism in single crystal Ce1−xCoxO2−δ (x⩽0.05) films deposited on a LaAlO3(001) substrate. Films were grown by a pulsed laser deposition technique and were thoroughly characterized using x-ray diffraction, high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy and scanning transmission electron microscopy-Z contrast, x-ray photoelectron spectroscopy, optical transmission spectroscopy, and magnetic measurements. These films are transparent in the visible regime and exhibit a very high Curie temperature ∼740–875K with a giant magnetic moment. Our results indicate that the ferromagnetic property is intrinsic to the CeO2 system and is not a result of any secondary magnetic phase or cluster formation.

Large magnetic moments and anomalous exchange coupling in As-doped Mn clusters

We report electronic and magnetic properties of pure and arsenic-doped manganese clusters from density functional theory using spin-polarized generalized gradient approximation for the exchange-correlation energy. We see for both the cases, noncollinear treatment of spins are indeed necessary. The arsenic atom stabilizes manganese clusters, which is further accompanied by the ferromagnetic $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ coupling for ${\mathrm{Mn}}_{2}\mathrm{As}$ and ${\mathrm{Mn}}_{4}\mathrm{As}$ clusters, resulting in large cluster magnetic moments, 9 and $17\phantom{\rule{0.3em}{0ex}}{\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$, respectively. We show in doped clusters, exchange coupling are anomalous and behave quite differently from the Ruderman-Kittel-Kasuya-Yosida-type predictions. Finally, it is suggested that, if grown in the low temperature MBE, the giant magnetic moments due to ferromagnetic Mn ordering and their large enough exchange coupling parameter in ${\mathrm{Mn}}_{2}\mathrm{As}$ and ${\mathrm{Mn}}_{4}\mathrm{As}$ could play an important role on the ferromagnetism and on the observed large variation in the Curie temperature of the Mn-doped III-V semiconductors.

Giant magnetic moment in an anomalous ferromagnetic insulator: Co-dopedZnO

Ferromagnetic insulators that exhibit strong ferromagnetism at the atomic level are believed to be suitable for magnetic dielectric barriers in spintronic devices and solid-state qubits in quantum computing. Here a giant magnetic moment of $6.1{\ensuremath{\mu}}_{B}∕\mathrm{Co}$ and a high Curie temperature ${T}_{C}$ of $790\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are observed in $(4\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%)$ Co-doped $\mathrm{ZnO}$ films, which is not carrier mediated, but co-exists with the dielectric state. Direct current reactive magnetron co-sputtering is used to grow ${\mathrm{Zn}}_{0.96}{\mathrm{Co}}_{0.04}\mathrm{O}$ dilute magnetic insulator on ${\mathrm{LiNbO}}_{3}$ (104) substrates at considerably low growth temperature $(\ensuremath{\sim}200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, which is significant for complementary metal oxide semiconductor technology. X-ray photoelectron spectroscopy and x-ray-absorption spectroscopy reveal a solid solution of cobalt in $\mathrm{ZnO}$, where Co is in the $2+$ state substituting for Zn. A supercoupling mechanism in terms of bound magnetic polarons is proposed to discuss the ferromagnetism in the dielectric ground state of $\mathrm{Co}:\mathrm{ZnO}$, which would lead to different consideration for the origin of giant magnetic moment and high-temperature ferromagnetism in transition-metal doped oxides.

Electronic and Magnetic Properties of Ti1-xMxO2-δ(M=Co and Fe) Thin Films Grown by Sol-gel Method

Electronic and magnetic properties of <TEX>$Ti_{1-x}M_xO_{2-\delta}$</TEX> (M=Co and Fe) thin films grown by sol-gel method have been investigated. Anatase and rutile <TEX>$Ti_{1-x}Co_xO_{2-\delta}$</TEX> films were successfully grown on <TEX>$Al_2O_3$</TEX> (0001) substrates and exhibited p-type electrical conductivity while the undoped films n-type conductivity. Room temperature vibrating sample magnetometry measurements on the anatase and rutile <TEX>$Ti_{1-x}Co_xO_{2-\delta}$</TEX> films with same x (<TEX>$=4.8 at.{\%}$</TEX>) showed quite similar magnetic hysteresis curves with the saturation magnetic moment of <TEX>$\~4 {\mu}_B$</TEX> per Co ion despite their differences in structural and electronic properties. Such giant magnetic moment is attributable to the unquenched orbital moment of the <TEX>$Co^{2+}$</TEX> ions substituting the octahedral <TEX>$Ti^{4+}$</TEX> sites. Similar ferromagnetic behavior was observed for <TEX>$Ti_{1-x}Fe_xO_{2-\delta}$</TEX> films that are highly resistive compared to the Co doped samples. Saturation magnetic moment was found to decrease for higher x, i.e., <TEX>$\~2$</TEX> and <TEX>$\~1.5 {\mu}_B$</TEX> per Fe ion for x=2.4 and 5.8 at. <TEX>$\%$</TEX>, respectively. Conversion electron <TEX>$M\ddot{o}ssbauer$</TEX> spectroscopy measurements predicted the coexistence of <TEX>$Fe^{2+}$</TEX> and <TEX>$Fe^{3+}$</TEX> ions at the octahedral sites of <TEX>$Ti_{1-x}Fe_xO_{2-\delta}$</TEX>.

Transparent Cr-doped SnO2 thin films: ferromagnetism beyond room temperature with a giant magnetic moment

Laser ablated Cr-doped SnO2 thin films grown on various kinds of substrates all showferromagnetism well beyond room temperature. Surprisingly, films ofSn0.95Cr0.05O2 grown onLaAlO3 substrates have a giantmagnetic moment of 6 μB/Cr, which is 20–30 times larger than that of films grown under the same conditions onSrTiO3 and R-cut sapphire substrates. All films are highly transparent.

Effect of the Mn clustering in (GaMn)N on the magnetic transition temperature

(GaMn)N is a topic of much current controversy. Rao and Jena suggested [Phys. Rev. Lett. 89, 185504 (2002)] that a high Curie temperature detected in some of the (GaMn)N samples is a result of the formation of small Mn clusters carrying giant magnetic moments. We report a detailed density-functional-theory study of the effect of the Mn clustering on the electronic structure, exchange interactions, and Curie temperature in (GaMn)N. In contrast to the prediction, the Curie temperature decreases with the formation of the clusters. The main reason for this decrease is the splitting of the impurity band because of the intracluster $\mathrm{Mn}\mathrm{Mn}$ interaction that results in a decreased double-exchange effect. Additionally, the clustering increases the sensitivity of the magnetic properties with respect to the compensating defects.

Ferromagnetism in transition-metal-dopedTiO2thin films

V, Cr, Fe, Co, and Ni had been chosen to substitute partially for Ti in $\mathrm{Ti}{\mathrm{O}}_{2}$. By applying appropriate conditions, transition-metal-doped $\mathrm{Ti}{\mathrm{O}}_{2}$ thin films deposited on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ substrates by laser ablation which show a good crystallinity and ferromagnetism above room temperature, could be obtained. The tendency of the dependence of magnetization versus element seems to be in accord with what theories have predicted. Among all the dopants, V appears to be the most promising candidate since its doping results in semiconducting films with a giant magnetic moment. Films are free of dopant particles or clusters. Structural and magnetic measurements have revealed that the room temperature ferromagnetism in $\mathrm{V}∕\mathrm{Cr}∕\mathrm{Fe}∕\mathrm{Co}∕\mathrm{Ni}$-doped thin films must originate from the transition-metal-doped $\mathrm{Ti}{\mathrm{O}}_{2}$ matrices.

Comparative x-ray absorption spectroscopy study of Co-doped SnO2 and TiO2

We performed x-ray absorption spectroscopy measurements at the cobalt L2,3 edge and the oxygen K edge of Co-doped SnO2 and Co-doped TiO2. Our measurements confirm that doped cobalt atoms are in the same local environment in both compounds. Furthermore, the results support the idea that cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in SnO2, but not in TiO2.

Ferromagnetism at room temperature with a large magnetic moment in anatase V-doped TiO2 thin films

V-doped TiO2 thin films were grown by laser ablation on LaAlO3 substrates. In the chosen range of the growth conditions, all V:TiO2 films have an anatase structure and exhibit semiconducting and ferromagnetic behaviors at room temperature. V:TiO2 films have a giant magnetic moment and they seem to be far better ferromagnetic than Co/Fe/Ni-doped TiO2 films. This study has proved that a few percent of V substituting for Ti in TiO2 can result in a potential diluted magnetic semiconductor.

Nitrogen-induced magnetic transition in small chromium clusters

Using density functional theory with generalized gradient approximation for exchange and correlation, we show that otherwise antiferromagnetically coupled chromium atoms in very small chromium clusters couple ferromagnetically when doped with a nitrogen atom, thus leading to giant magnetic moments. For example, the magnetic moment of Cr2N is found to be 9μB while that of Cr2 is 0μB. Strong bonding between Cr and N atoms brings about this magnetic transition. The Cr atoms nearest neighbor to N couple ferromagnetically with each other and antiferromagnetically with nitrogen. The significance of these results in understanding the ferromagnetic order in Cr-doped GaN is discussed.

Hybridization reduction of the magnetization for N-doped FeCo superlattices

In search of a soft magnetic material exhibiting high magnetization, we investigated the magnetic and structural properties of an (Fe3Co)4Nm superlattice (SL) with (m=2) and without (m=0) N doping via first principles full-potential linearized augmented plane wave calculations in order to examine the possible magnetic enhancement by (i) N addition (which has drawn considerable attention since the first report (in 1972) of a giant magnetic moment for quasistable α″-Fe16N2) and (ii) the lowered dimensionality of the SL. The structural optimization was fully accomplished by total energy and atomic force calculations within the generalized-gradient approximation. Despite the lattice expansion by 9% due to the N insertion, the magnetization of (Fe3Co)4N2 was found to be reduced from the value of the Fe3Co superlattice by the strong hybridization of N with Fe and Co.

Interaction between Cs and Fe

Following the fact that giant magnetic moments of Fe observed as impurities either in bulk or thin film of cesium, the component as well as the distance dependence of Fe–Cs interactions were studied by density functional theory with local spin density approximation. Modeled ordered alloys of FexCs1−x (x=1.0, 0.75, 0.5, 0.25, 0) are studied by placing Fe and Cs at various lattice sites with different interatomic distances on a body-centered-cubic crystal structure. The total spin moments per Fe atom decrease as the Cs concentration increases in modeled FexCs1−x, which indicates that stronger hybridization between Fe and Cs exists when there are more Cs atoms in the system. The spin moments per Fe atom become larger with an increase in the Fe–Cs distance in Fe0.5Cs0.5. Polarization to the cesium atom is small even for shorter Fe–Cs distances. The interaction between Cs and Fe is analyzed by the change of Fe 3d states in these systems, and is significantly different from that in either single iron or bulk iron.

Giant magnetic moments of nitrogen-doped Mn clusters and their relevance to ferromagnetism in Mn-doped GaN.

Calculations based on density-functional theory show that the stability and magnetic properties of small Mn clusters can be fundamentally altered by the presence of nitrogen. Not only are their binding energies substantially enhanced, but also the coupling between the magnetic moments at Mn sites remains ferromagnetic irrespective of their size or shape. In addition, these nitrogen-doped Mn clusters carry giant magnetic moments ranging from 4mu(B) in MnN to 22mu(B) in Mn5N. It is suggested that the giant magnetic moments of MnxN clusters may play a key role in the ferromagnetism of Mn-doped GaN which exhibit a wide range (10-940 K) of Curie temperatures.

Electromagnetic pulse from final gravitational stellar collapse

We employ an effective gravitational stellar final collapse model which contains the relevant physics involved in this complex phenomena: spherical radical infall in the Schwarzschild metric of the homogeneous core of an advanced star, giant magnetic dipole moment, magnetohydrodynamic material response and realistic equations of state (EOS). The electromagnetic pulse is computed both for medium size cores undergoing hydrodynamic bounce and large size cores undergoing black hole formation. We clearly show that there must exist two classes of neutron stars, separated by maximum allowable masses: those that collapsed as solitary stars (dynamical mass limit) and those that collapsed in binary systems allowing mass accretion (static neutron star mass). Our results show that the electromagnetic pulse spectrum associated with black hole formation is a universal signature, independent of the nuclear EOS. Our results also predict that there must exist black holes whose masses are less than the static neutron star stability limit.

Spin polarization of a 2D-electron gas and the magnetization problem of nano-particles

ESR signal from the surface of Pd/Ni nano-particle with Ni mono-layer are investigated. Because of the giant magnetic moment effect in the Pd/Ni particle, the ESR signal is quite strong. The characteristics of ESR can be clearly observed from the data of g-shift and the temperature dependency of the satellite signal. Similar characteristics of non-magnetic Pd and Au nano-particles Pd and Au nano-particles are also observed, but their intensities are not so strong. The mechanism, which results in the spin polarization in the film of electron gas, cannot be understood by the computer simulation method in case of the nano-particle.

On the origin of the giant magnetic moments of Fe and Co in Cs Films

To unravel the mystery of the recently observed giant magnetic moments of Fe and Co in Cs films, orbital-polarization corrected relativistic spin density functional calculations have been performed. Unlike other transition–metal systems where the orbital magnetic moments are quenched, Fe and Co in Cs as well as in other alkali metals are found to possess a giant orbital moment of 2–3 μ B along with a large spin moment. Also, these free atom-like spin and orbital magnetic moments in Cs would not be squashed under large lattice contractions up to 23% around the impurity atoms. The induced moments on the host atoms are small. The results offer an explanation for the origin of the giant magnetic moments of Fe and Co in Cs films.

Spin Modulation Wave in Giant Magnetic Moment Systems PdCo and PtCo Alloys

Susceptibility and neutron scattering measurements were performed for the giant magnetic moment systems PdCo and PtCo alloys. Short range order of spin modulation wave was observed on the ferromagnetic spin alignment. The results were compared with those observed for other giant magnetic moment systems PdFe and PtFe alloys. A big difference exists for a response to an applied magnetic field probably due to a difference of magnetization easy axes in these systems. A hardness of the saturation magnetization of PdCo alloy would be ascribed to a complex magnetic structure of this system.

ChemInform Abstract: α-Fe16N2: Giant Magnetic Moment or Not?

We reviewed the present situation of the α-Fe 16 N 2 problem for α-Fe 16 N 2 films with and without a giant magnetic moment in connection with structure and magnetic moment. The results of X-ray diffraction and transmission electron microscopy analysis did not show any difference between α-Fe 16 N 2 films with and without a giant magnetic moment. However, the Mossbauer spectra were quite different from each other. The hyperfine field of Fe 16 N 2 film synthesized by the present authors splits into three hyperfine field interactions. However, for the Fe 16 N 2 with a giant magnetic moment, the hyperfine field interaction did not split and is almost the same as that of pure α-Fe. Therefore, the interpretation of Mossbauer spectra is the key item to solving the α-Fe 16 N 2 problem.

Origin of the giant magnetic moments of Fe impurities on and in Cs films

We have explored the origin of the observed giant magnetic moments ( approximately 7&mgr;(B)) of Fe impurities on the surface and in the bulk of Cs films, using the relativistic local-spin-density-approximation method. We have found that Fe impurities in Cs behave differently from those in noble metals or in Pd. Whereas the induced spin polarization of Cs atoms is negligible, the Fe ion itself is a source of the giant magnetic moment. The 3d electrons of Fe in Cs are localized as the 4f electrons in rare-earth ions so that the orbital magnetic moment becomes as large as the spin magnetic moment. The calculated total magnetic moment M = 6.43&mgr;(B) is close to the experimentally observed value.

α-Fe16N2: giant magnetic moment or not?

Abstract We reviewed the present situation of the α“-Fe16N2 problem for α”-Fe16N2 films with and without a giant magnetic moment in connection with structure and magnetic moment. The results of X-ray diffraction and transmission electron microscopy analysis did not show any difference between α“-Fe16N2 films with and without a giant magnetic moment. However, the Mössbauer spectra were auite different from each other. The hyperfine field of α”-Fe16N2 film synthesized by the present authors splits into three hyperfine field interactions. However, for the α“-Fe16N2 with a giant magnetic moment, the hyperfine field interaction did not split and is almost the same as that of pure α-Fe. Therefore, the interpretation of Mössbauer spectra is the key item to solving the α”-Fe16N2 problem.