Showing papers on "Ferromagnetism published in 2003"

Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO

Abstract: The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute ( 700 °C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.

Wide band gap ferromagnetic semiconductors and oxides

Abstract: Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. T...

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta).

Abstract: The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

Extraordinary magnetoelasticity and lattice softening in bcc Fe-Ga alloys

Abstract: Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. λ100 exhibits two peaks as a function of Ga content. At room temperature, λ100 reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, λ100 drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling |b1| of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c11−c12) which approaches zero near 27% Ga. λ111 is significantly smaller in magnitude than λ100 over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%.

Glassy ferromagnetism and magnetic phase separation in La 1 − x Sr x CoO 3

Abstract: We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and magnetotransport properties of the glassy ferromagnet ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}.$ The compositions studied span the range from the end-member ${\mathrm{LaCoO}}_{3}$ $(x=0.0)$ through to $x=0.7.$ These materials have attracted attention recently, primarily due to the spin-state transition phenomena in ${\mathrm{LaCoO}}_{3}$ and the unusual nature of the magnetic ground state for finite x. In this paper we present a consistent picture of the magnetic behavior of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ in terms of short-range ferromagnetic ordering and intrinsic phase separation. At high Sr doping $(xg0.2)$ the system exhibits unconventional ferromagnetism (with a Curie temperature up to 250 K), which is interpreted in terms of the coalescence of short-range-ordered ferromagnetic clusters. Brillouin function fits to the temperature dependence of the magnetization as well as high-temperature Curie-Weiss behavior suggest that the ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are both in the intermediate spin state. At lower Sr doping $(xl0.18)$ the system enters a mixed phase that displays the characteristics of both a spin glass and a ferromagnet. A cusp in the zero-field-cooled dc magnetization, a frequency-dependent peak in the ac susceptibility and time-dependent effects in both dc and ac magnetic properties all point towards glassy behavior. On the other hand, field cooling results in a relatively large ferromagneticlike moment, with zero-field-cooled and field-cooled magnetizations bifurcating at an irreversibility point. Even in the region above $x=0.2$ the out-of-phase component of the ac susceptibility shows frequency-dependent peaks below the Curie temperature (indicative of glassy behavior) which have previously been interpreted in terms of the freezing of clusters. All of the results are consistent with the existence of a strong tendency towards magnetic phase separation in this material, a conclusion which is further reinforced by consideration of the electronic properties. The metal-insulator transition is observed to be coincident with the onset of ferromagnetic ordering $(x=0.18)$ and has a behavior in the doping dependence of the low-temperature conductivity which is strongly suggestive of percolation. This can be interpreted as a percolation transition within the simple ferromagnetic cluster model. On the metallic side of the transition the system exhibits colossal magnetoresistance-type behavior with a peak in the negative magnetoresistance (\ensuremath{\sim}10% in 90 kOe) in the vicinity of the Curie temperature. As the transition is approached from the metallic side we observe the onset of a negative magnetoresistance that increases in magnitude with decreasing temperature, reaching values as large as 90% in a 90-kOe field. This magnetoresistance is enhanced at the metal-insulator transition, where it persists even to room temperature.

FeRh/FePt exchange spring films for thermally assisted magnetic recording media

Abstract: The temperature-dependent magnetic response of exchange-coupled FePt/FeRh thin films is described. The FePt forms a high magnetocrystalline anisotropy, high-coercivity ferromagnetic layer. The FeRh layer is antiferromagnetic at room temperature but, upon heating above a transition temperature, becomes ferromagnetic with a large magnetic moment and low magnetocrystalline anisotropy, forming an exchange–spring system and significantly lowering the coercive field of the composite system. This feature opens intriguing possibilities for media applications for thermally assisted magnetic recording where the ferromagnetic phase of FeRh is exploited to help write the media while the antiferromagnetic phase supports the long-time stability.

High-temperature ferromagnetism in Ni2+-doped ZnO aggregates prepared from colloidal diluted magnetic semiconductor quantum dots.

Abstract: Ferromagnetism with T c > 350 K is observed in the diluted magnetic semiconductor Ni 2 + :ZnO synthesized from solution. Whereas colloidal Ni 2 + :ZnO nanocrystals are paramagnetic, their aggregation gives rise to robust ferromagnetism. The appearance of ferromagnetism is attributed to the increase in domain volumes and the generation of lattice defects upon aggregation. The unusual temperature dependence of the magnetization coercivity is discussed in terms of a temperature-dependent exchange interaction involving paramagnetic Ni 2 + ions.

Ferromagnetism in Mn-implanted ZnO:Sn single crystals

Abstract: We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segregated secondary phases, and strongly suggests that a near-room-temperature dilute magnetic semiconducting oxide has been realized. Based on these results, ZnO doped with Mn and Sn may prove promising as a ferromagnetic semiconductor for spintronics.

Resonance magnetoelectric effects in layered magnetostrictive-piezoelectric composites

Abstract: Magnetoelectric interactions in bilayers of magnetostrictive and piezoelectric phases are mediated by mechanical deformation. This work is concerned with the theory and companion data for magnetoelectric (ME) coupling at electromechanical resonance (EMR) in the layered samples. Estimated ME voltage coefficient versus frequency profiles for nickel, cobalt, or lithium ferrite and lead zirconate titanate (PZT) predict a giant ME effect at EMR with the highest coupling expected for cobalt ferrite-PZT. There is excellent agreement between the theory and data for layered nickel ferrite-PZT; the ME voltage coefficient at resonance increases by a factor of 40 compared to low frequency values. Similar measurements on layered ferromagnetic alloy-PZT and bulk ferrite-PZT reveal even a stronger EMR assisted enhancement in ME coupling.

Synthesis and characterization of CoFe2O4 magnetic nanoparticles prepared by temperature-controlled coprecipitation method

Abstract: Magnetic nanoparticles of cobalt ferrite (CoFe2O4) have been synthesized in a homogeneous aqueous solution without any template and subsequent heat treatment. The average particle size could be varied in the range of 2–14 nm by controlling coprecipitation temperature of Co2+ and Fe3+ ions in alkaline solution although the size distribution is pretty wide. As the precipitation temperature increased in the range of 20–80°C, the average particle size also increased. However, there is a considerable change in XRD crystallinity and the average size of the nanoparticles at the precipitation temperature between 40°C and 60°C. While the nanoparticles prepared at the temperature below 40°C show superparamagnetic relaxation at room temperature with blocking temperatures between 75 and 200 K, the samples prepared at the temperature higher than 60°C consist of both superparamagnetic and ferrimagnetic nanoparticles that result in magnetic coercivity at room temperature. Mossbauer spectra of the samples also confirmed their magnetic properties and wide size distribution in each sample. The analysis of the spectra gave a rough estimation of the ratio of superparamagnetic and ferromagnetic nanoparticles in each sample at various temperatures.

Room-temperature ferromagnetism in a II-VI diluted magnetic semiconductor Zn(1-x)Cr(x)Te.

Abstract: The magnetic and magneto-optical properties of a Cr-doped II-VI semiconductor ZnTe were investigated. Magnetic circular dichroism measurements showed a strong interaction between the sp carriers and localized d spins, indicating that Zn(1-x)Cr(x)Te is a diluted magnetic semiconductor. The Curie temperature of the film with x=0.20 was estimated to be 300+/-10 K, which is the highest value ever reported for a diluted magnetic semiconductor in which sp-d interactions were confirmed. In spite of its high Curie temperature, Zn(1-x)Cr(x)Te film shows semiconducting electrical transport properties.

Giant Planar Hall Effect in Epitaxial (Ga,Mn)As Devices

Abstract: Large Hall resistance jumps are observed in microdevices patterned from epitaxial (Ga,Mn)As layers when subjected to a swept, in-plane magnetic field. This giant planar Hall effect is 4 orders of magnitude greater than previously observed in metallic ferromagnets. This enables extremely sensitive measurements of the angle-dependent magnetic properties of (Ga,Mn)As. The magnetic anisotropy fields deduced from these measurements are compared with theoretical predictions.

Ferromagnetism induced by clustered Co in Co-doped anatase TiO2 thin films.

Abstract: We investigated ferromagnetism of a newly discovered ferromagnetic semiconductor Co-doped anatase ${\mathrm{T}\mathrm{i}\mathrm{O}}_{2}$ thin film, using the magnetic circular dichroism (MCD) at the Co ${L}_{2,3}$ absorption edges. The magnetic moment was observed to be $\ensuremath{\sim}0.1{\ensuremath{\mu}}_{B}/\mathrm{C}\mathrm{o}$ in the measurements, but the MCD spectral line shape is nearly identical to that of Co metal, showing that the ferromagnetism is induced by a small amount of clustered Co. With thermal treatments at $\ensuremath{\sim}400\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the MCD signal increases, and the moment reaches up to $\ensuremath{\sim}1.55{\ensuremath{\mu}}_{B}/\mathrm{C}\mathrm{o}$, which is $\ensuremath{\sim}90%$ of the moment in Co metal. In the latter case, the cluster size was observed to be 20--60 nm.

Dynamic exchange coupling in magnetic bilayers.

Abstract: A long-range dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance and explained by an adiabatic spin-pump theory. In such a resonance the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the resonant linewidth when the resonance fields are well apart, and results in a dramatic linewidth narrowing when the resonant fields approach each other.

Half-Metallic Ferromagnetism and Structural Stability of Zincblende Phases of the Transition-Metal Chalcogenides

Abstract: An accurate density-functional method is used to study systematically half-metallic ferromagnetism and stability of zincblende phases of 3d-transition-metal chalcogenides. The zincblende CrTe, CrSe, and VTe phases are found to be excellent half-metallic ferromagnets with large half-metallic gaps (up to 0.88 eV). They are mechanically stable and approximately 0.31-0.53 eV per formula unit higher in total energy than the corresponding nickel-arsenide ground-state phases, and therefore would be grown epitaxially in the form of films and layers thick enough for spintronic applications.

Curie temperatures of III–V diluted magnetic semiconductors calculated from first principles

Abstract: Curie temperatures of the diluted magnetic semiconductors (Ga, Mn)As, (Ga, Mn)N, (Ga, Cr)As and (Ga, Cr)N are evaluated from first principles. The electronic structure is calculated in the local spin density approximation by using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation to describe the substitutional and spin disorder. From the total energy differences between the ferromagnetic state and the spin-glass state, realistic estimations of Curie temperatures are achieved by using a mapping on the Heisenberg model in the mean-field approximation. Effects of additional carrier doping treatments are also investigated. Very large Curie temperatures are obtained, lying above room temperature for (Ga, Mn)N, (Ga, Cr)As and (Ga, Cr)N. Upon hole doping the Curie temperature of (Ga, Mn)N further increases, while (Ga, Mn)As shows a plateau behavior.

Synthesis of cubic zirconium and hafnium nitride having Th3P4 structure.

Abstract: High-pressure synthesis is a powerful method for the preparation of novel materials with high elastic moduli and hardness. Additionally, such materials may exhibit interesting thermal, optoelectronic, semiconductuing, magnetic or superconducting properties. Here, we report on the high-pressure synthesis of zirconium and hafnium nitrides with the stoichiometry M3N4, where M = Zr, Hf. Synthesis experiments were performed in a laser-heated diamond anvil cell at pressures up to 18 GPa and temperatures up to 3,000 K. We observed formation of cubic Zr3N4 and Hf3N4 (c-M3N4) with a Th3P4-structure, where M-cations are eightfold coordinated by N anions. The c-M3N4 phases are the first binary nitrides with such a high coordination number. Both compounds exhibit high bulk moduli around 250 GPa, which indicates high hardness. Moreover, the new nitrides, c-Zr3N4 and c-Hf3N4, may be the first members of a larger group of transition metal and/or lanthanide nitrides with interesting ferromagnetic or superconducting behaviour.

Structural and doping effects in the half-metallic double perovskite A 2 CrWO 6 (A=Sr, Ba, and Ca)

Abstract: The structural, transport, magnetic, and optical properties of the double perovskite A2CrWO6 with A = Sr,Ba,Ca have been studied. By varying the alkaline earth ion on the A site, the influence of steric effects on the Curie temperature TC and the saturation magnetization has been determined. A maximum TC = 458 K was found for Sr2CrWO6 having an almost undistorted perovskite structure with a tolerance factor f~1. For Ca2CrWO6 and Ba2CrWO6 structural changes result in a strong reduction of TC. Our study strongly suggests that for the double perovskites in general an optimum TC is achieved only for f~1, that is, for an undistorted perovskite structure. Electron doping in Sr2CrWO6 by a partial substitution of Sr2+ by La3+ was found to reduce both TC and the saturation magnetization Ms. The reduction of Ms could be attributed both to band structure effects and the Cr/W antisites induced by doping. Band structure calculations for Sr2CrWO6 predict an energy gap in the spin-up band, but a finite density of states for the spin-down band. The predictions of the band structure calculation are consistent with our optical measurements. Our experimental results support the presence of a kinetic energy driven mechanism in A2CrWO6, where ferromagnetism is stabilized by a hybridization of states of the nonmagnetic W site positioned in between the high spin Cr sites.

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

Abstract: We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

The giant magnetocaloric effect of optimally prepared Gd5Si2Ge2

Abstract: The appropriate heat treatment of the Gd5Si2Ge2 alloy prepared from high-purity Gd results in a considerable enhancement of its magnetocaloric effect. The maximum magnetic entropy change increases by ∼80% (from ∼20 to 36 J/kg K) and the adiabatic temperature change increases by ∼55% (from ∼11 to 17 K) for a magnetic field change from 0 to 50 kOe when compared to the as arc-melted material. The magnetic ordering temperature is slightly reduced from ∼277 K in the as arc-melted material to ∼272 K in the fully homogenized and annealed Gd5Si2Ge2. The behavior of the isothermal magnetization as a function of magnetic field changes to some extent, while the heat capacity anomaly at the phase transition temperature is considerably sharper. The 1570 K heat treatment results in the partial ordering and redistribution of the Si and Ge atoms between their corresponding crystallographic sites in the monoclinic Gd5Si2Ge2-type structure and to the removal of the polymorphic orthorhombic Gd5Si2Ge2 phase with the Gd5Si4-t...

Shape memory ferromagnets

Abstract: In ferromagnetic alloys with shape memory large reversible strains can be obtained by rearranging the martensitic domain structure by a magnetic field. Magnetization through displacement of domain walls is possible in the presence of high magnetocrystalline anisotropy, when martensitic structure rearrangement is energetically favorable compared to the reorientation of magnetic moments. In ferromagnetic Heusler alloys Ni2+xMn1–xGa the Curie temperature exceeds the martensitic transformation temperature. The fact that these two temperatures are close to room temperature offers the possibility of magnetically controlling the shape and size of ferromagnets in the martensitic state. In Ni2+xMn1–xGa single crystals, a reversible strain of ~6% is obtained in fields of ~1 T.

Ferromagnetism in cobalt-implanted ZnO

Abstract: The magnetic and structural properties of cobalt-implanted ZnO single crystals are reported. High-quality, (110)-oriented single-crystal Sn-doped ZnO substrates were implanted at ∼350 °C with Co to yield transition metal concentrations of 3–5 at. % in the near-surface (∼2000 A) region. After implantation, the samples were subject to a 5 min rapid thermal annealing at 700 °C. Magnetization measurements indicate ferromagnetic behavior, with hysteresis observed in the M vs H behavior at T=5 K. Coercive fields were ⩽100 Oe at this measurement temperature. Temperature-dependent magnetization measurements showed evidence for ordering temperatures of >300 K, although hysteresis in the M vs H behavior was not observed at room temperature. Four-circle x-ray diffraction results indicate the presence of (110)-oriented hexagonal phase Co in the ZnO matrix. From the 2θ full width at half maximum (FWHM) of the Co (110) peak, the nanocrystal size is estimated to be ∼3.5 nm, which is below the superparamagnetic limit at ...

Temperature-dependent magnetization in diluted magnetic semiconductors

Abstract: We calculate magnetization in magnetically doped semiconductors assuming a local exchange model of carrier-mediated ferromagnetic mechanism and using a number of complementary theoretical approaches. In general, we find that the results of our mean-field calculations, particularly the dynamical mean-field theory results, give excellent qualitative agreement with the experimentally observed magnetization in systems with itinerant charge carriers, such as ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ with $0.03lxl0.07,$ whereas our percolation-theory-based calculations agree well with the existing data in strongly insulating materials, such as ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}.$ We comment on the issue of non-mean-field like magnetization curves and on the observed incomplete saturation magnetization values in diluted magnetic semiconductors from our theoretical perspective. In agreement with experimental observations, we find the carrier density to be the crucial parameter determining the magnetization behavior. Our calculated dependence of magnetization on external magnetic field is also in excellent agreement with the existing experimental data.

Kondo effect in quantum dots coupled to ferromagnetic leads.

Abstract: We study the Kondo effect in a quantum dot coupled to ferromagnetic leads and analyze its properties as a function of the spin polarization of the leads. Based on a scaling approach, we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation of motion technique, we study nonlinear transport through the dot. For parallel alignment, the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.

Large Tunneling Magnetoresistance at Room Temperature Using a Heusler Alloy with the B2 Structure

Abstract: A Co2Cr0.6Fe0.4Al Heusler alloy film exhibited a B2 structure, which was deposited using a magnetron sputtering system on a thermally oxidized Si substrate at room temperature without any buffer layers. The film exhibited the magnetic moment of 2.04µB per formula unit, nearly the integer number of Bohr magnetons, suggesting a localized nature of ferromagnetism similar to that of many Heusler compounds, which is a necessary condition for half metallicity. A spin- valve-type tunneling junction with a Co2(Cr, Fe)Al Heusler alloy film was fabricated using metal masks, which consists of Co2Cr0.6Fe0.4Al(10 nm)/AlOx (1.8 nm)/CoFe (3 nm)/NiFe (5 nm)/IrMn (15 nm)/Ta (5 nm), deposited on a thermally oxidized Si substrate without a buffer layer. The junction demonstrated large tunneling magnetoresistances of 16% at room temperature and 26.5% at 5 K.

Identification of novel compositions of ferromagnetic shape-memory alloys using composition spreads

Abstract: Exploration of new ferroic (ferroelectric, ferromagnetic or ferroelastic) materials continues to be a central theme in condensed matter physics and to drive advances in key areas of technology. Here, using thin-film composition spreads, we have mapped the functional phase diagram of the Ni–Mn–Ga system whose Heusler composition Ni2MnGa is a well known ferromagnetic shape-memory alloy. A characterization technique that allows detection of martensitic transitions by visual inspection was combined with quantitative magnetization mapping using scanning SQUID (superconducting quantum interference device) microscopy. We find that a large, previously unexplored region outside the Heusler composition contains reversible martensites that are also ferromagnetic. A clear relationship between magnetization and the martensitic transition temperature is observed, revealing a strong thermodynamical coupling between magnetism and martensitic instability across a large fraction of the phase diagram.

Martensitic transformation and shape memory effect in ferromagnetic Heusler alloy Ni2FeGa

Abstract: We have synthesized ferromagnetic Heusler alloy Ni2FeGa using the melt-spinning technique. The Ni2FeGa ribbon, having a high chemical ordering L21 structure, exhibits a thermoelastic martensitic transformation from cubic to orthorhombic structure at 142 K and a premartensitic transformation. The alloy has a relatively high Curie temperature of 430 K, a magnetization of 73 Am2/kg, and a low saturated field of 0.6 T. The textured samples with preferentially oriented grains show a completely recoverable two-way shape memory effect with a strain of 0.3% upon the thermoelastic martensitic transformation.

Vortex formation in narrow ferromagnetic rings

Abstract: The high-symmetry ring geometry is shown to exhibit a wide range of intriguing magnetostatic and magnetodynamic properties, which we survey in this topical review. We consider first the patterning and deposition techniques, which are used to fabricate ring structures (diameters between 0.1 and 2??m) and discuss their respective advantages and disadvantages. The results of direct nanoscale imaging of the novel magnetization configurations present in rings with different geometrical parameters (including discs) are discussed. These results give valuable insight into the influence of the magnetic anisotropies governing the magnetic states. The different types of domain walls that arise are compared quantitatively to micromagnetic simulations. The magnetodynamic switching between the different magnetic states is described in detail. In particular we elaborate on the different geometry-dependent magnetic switchings, since the different transitions occurring allow us to determine which energy terms govern the reversal process. We discuss a process by which fast (sub-ns) and controlled switching can be achieved, therefore making rings an attractive geometry for applications, in addition to studying fundamental issues of nanomagnetism.

Magnetic semiconductors based on cobalt substituted ZnO

Abstract: We have investigated the microstructure and the magnetic properties of cobalt substituted ZnO thin films deposited on sapphire (0001) substrates by pulsed laser deposition. We have optimized the growth condition using in situ monitoring by reflection high-energy electron diffraction. We found that ferromagnetic films need to be grown at low oxygen partial pressure (<10−6 Torr). Films with 25% of Co are ferromagnetic at room temperature with clear out-of-plane anisotropy. We have looked for spurious origins of the ferromagnetic signal and found none.

Ferromagnetism in laser deposited anatase Ti 1-x Co x O 2-δ films

Abstract: Pulsed laser deposited films of Co-doped anatase TiO 2 are examined for Co substitutionality, ferromagnetism, transport, magnetotransport, and optical properties. Our results show limited solubility (up to ∼2%) of Co in the as-grown films and formation of Co clusters thereafter. For the Ti 0 . 9 3 Co 0 . 0 7 O 2 - δ sample, which exhibits a Curie temperature (T C ) over 1180 K, we find the presence of 20-50 nm Co clusters as well as a small concentration of Co incorporated into the remaining matrix. After being subjected to the high-temperature anneal during the first magnetization measurement, the very same sample shows a T C ∼650 K and almost full matrix incorporation of Co. This T C is close to that of as-grown Ti 0 . 9 9 Co 0 . 0 1 O 2 - δ sample (∼700 K). The transport, magnetotransport, and optical studies also reveal interesting effects of the matrix incorporation of Co. These results are indicative of an intrinsic Ti 1 - x Co x O 2 - δ diluted magnetic semiconductor with T C of about 650-700 K.