Ferromagnetic Bulk Research Articles

Magnetism in transition metal substituted ZnO

Structural and magnetic properties have been studied for polycrystalline Zn1−xTMxO, where TM (transition metal ions) = Mn, Fe and Co. Phase pure samples demonstrate Curie-Weiss behavior with antiferromagnetic interactions. Different synthesis conditions lead to formation of impurities that are responsible for spin-glass behavior (ZnMnO3 impurity in Zn1−xMnxO) or high-temperature ferromagnetic ordering (Co metal in Zn1−xCoxO or (Zn,Fe)3O4 in Zn1−xFexO). No bulk ferromagnetism was observed for singlephase materials, contrary to the existing theories. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and magnetic properties of transition metal substituted ZnO

Structural and magnetic properties have been studied for polycrystalline Zn1−xTMxO, where TM (transition metal ions)=Mn, Fe, and Co. No bulk ferromagnetism was observed for single-phase materials, contrary to the existing theories. Single-phase samples demonstrate paramagnetic Curie–Weiss behavior with antiferromagnetic interactions, similar to other diluted magnetic semiconductors. Nonoptimal synthesis conditions lead to formation of second phases that are responsible for spin-glass behavior {ZnMnO3 impurity for Zn1−xMnxO [S. Kolesnik et al., J. Supercond. 15, 251 (2002)]} or high-temperature ferromagnetic ordering [Co metal for Zn1−xCoxO with the Curie temperature TC>800 K or (Zn,Fe)3O4 for Zn1−xFexO with TC=440 K].

Searching for a magnetic proximity effect in magnetite–carbon structures

In order to study a possible magnetic proximity effect in magnetite–carbon structures, we have performed magnetization measurements of graphite–magnetite composites with different mass ratios as well as the measurement of the magnetoresistance of one of them and of the magnetization of a magnetite–carbon bilayer. The overall results do not indicate the induction of bulk ferromagnetism in graphite and disordered carbon structures through their contact with magnetite.

Interactions between demagnetizing field and minor-loop development in bulk ferromagnets

A phenomenon of mutual influence of the demagnetizing field and a gradual increase of width of minor loops in bulk ferromagnets during magnetization processes is scrutinized by computational modeling. Classical static micromagnetics is here enriched by a suitable activated dissipation mechanism allowing for rate-independent hysteresis. The activation threshold is allowed to depend, again by a rate-independent manner, on local history of a magnetization process, which reflects (only on a mesoscopical level) gradual development of a domain structure. Influence of the shape of the specimen is thus automatically involved via nonlocal interactions through the demagnetizing field.

Resonance magnetoelectric effects in layered magnetostrictive-piezoelectric composites

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.

Flat-band ferromagnetism in undoped and doped polyaminotriazole crystal

Motivated from the flat-band ferromagnetism conceived theoretically for a single chain of five-membered rings (polyaminotriazole) by Arita, Suwa, Kuroki, and Aoki [Phys. Rev. Lett. 88, 127202 (2002)], we have studied whether the magnetism can indeed occur as a bulk, i.e., in a three-dimensional crystal of the polymer, by means of the spin-density-functional calculation. We find that the intrachain ferromagnetism is robust against crystallization as far as the flat band is made half filled. We have further investigated the actual crystal doped with various compounds, where ${\mathrm{HF}}_{2}$ is shown to put the system close to the bulk ferromagnetism while stronger anions such as ${\mathrm{BF}}_{4}$ or ${\mathrm{PF}}_{6}$ should be promising.

Ferromagnetic Monolayers. A Fresh Look at Fundamentals

AbstractFor Abstract see ChemInform Abstract in Full Text.

Extrinsic Origin of High-Temperature Ferromagnetism in CaB6

The discovery of ferromagnetism in CaB 6 doped with La was surprising, because there is no magnetic element in the compound nevertheless the system involved a ferromagnetic Curie temperature as high as 600 K. In order to resolve the origin of the ferromagnetism, we have made magnetization measurements and chemical analysis of CaB 6 and LaB 6 samples synthesized under various conditions. We have found that the ferromagnetism disappears almost completely by the acid etching, and that there is strong correlation between the magnitude of reduced magnetization and the iron concentration involved in the acid. Therefore we conclude that the ferromagnetism is of extrinsic origin and can be ascribed to iron impurity phases, probably FeB and Fe 2 B that have nearly the same Curie temperature as CaB 6 . We suggest that these iron impurities originate from the crucible used in the synthesis. Furthermore we have measured the thermoelectric power at high temperatures including the Curie temperature. There seems to be no relation between the appearance of the ferromagnetism and the transport properties, and no evidence for the bulk ferromagnetism has been observed.

Influence of Co doping on properties of Pr 0.8Na 0.2Mn (1− y) Co yO 3 perovskites

The influence of the cobalt substitution for manganese ions in the series of the perovskites Pr 0.8Na 0.2Mn (1− x) Co x O 3 (0⩽ x⩽0.1) was investigated. The study of electric and magnetic properties was carried out on sintered polycrystalline samples. The composition of x=0.04 exhibits an insulator to metal-like (I–M) transition at ∼106 K, connected with a ferromagnetic arrangement. For x=0.1, however, an insulating behavior persists down to low temperatures in spite of the transition to the bulk ferromagnetism. The observed properties are related to an acting of the cobalt ions as point defects. They disturb the tendency to charge ordering and instead of the antiferromagnetic arrangement typical for x=0 ferromagnetic double-exchange interactions Mn 3+–O 2−–Mn 4+ and Mn 3.5+ δ –O 2−–Co 2+, decisive for the resulting behavior, arise.

Magnetic Properties and Structures of the α- and δ-Phases ofp-NPNN

We have investigated the crystal structures and magnetic properties of the α- and δ-phases of an organic radical p -NPNN ( p -nitrophenyl α-nitronyl nitroxide), which yielded the first organic bulk ferromagnet, β- p -NPNN (the β-phase). The results are compared with those of the β- and γ-phases, which show distinct magnetic ordering due to intermolecular ferromagnetic couplings. Common structural features are found in the four phases. Unlike the other phases, the α-phase exhibits a weak antiferromagnetic interaction. This is attributable to phenyl–phenyl overlapping found only in the α-phase. The crystal structure of δ- p -NPNN contains a packing motif similar to that of the γ-phase. Though the δ-phase exhibits ferromagnetic interactions, it shows no magnetic ordering down to 0.46 K. It is suggested that ferromagnetic two-leg spin ladders are formed in the δ-phase, instead of a two-dimensional network as in γ- p -NPNN. This low dimensionality explains the absence of a magnetic transition.

Possible flat-band ferromagnetism in an organic polymer

Motivated from the flat-band form magnetism conceived in the physics of correlated electrons; we predict that a polymer of five-membered rings can exhibit band (i.e. itinerant) ferromagnetism. We have first identified that the right material having a flat one-electron dispersion should be polyaminotriazole, for which we have performed a generalized gradient approximation (GGA) calculation based on the spin density functional. The result shows that the ground state is indeed ferromagnetic when the flat band is made half-filled. From the structural optimization, we also show that the magnetism overcomes the Peierls instability, unlike conjugated polymers such as polyacetylene. We have also confirmed that the mechanism for the ferromagnetism is the flat-band ferromagnetism by comparing the first-principles wavefunction with those in the Hubbard model. Possibility of the bulk ferromagnetism for the three dimensional polyaminotriazole crystal with chemical carrier doping is also discussed.

Bulk superconductivity inRu0.9Sr2YCu2.1O7.9

An Ru 0 . 9 Sr 2 YCu 2 . 1 O 7 . 9 sample synthesized by high-pressure high-temperature solid-state reaction underwent (weak) ferromagnetic transition at ∼150 K followed by a bulk superconducting transition at ∼30 K. It showed a clear Meissner signal in the field-cooled process up to external magnetic field of a few hundred Oe (∼300 Oe). These results appear to indicate coexistence of bulk superconductivity and ferromagnetism. At the same time, both magnetic-susceptibility and hysteresis data can be interpreted as mere superimposition of superconducting diamagnetic and (weak) ferromagnetic responses without any anomalous behavior. The genuine coexistence of bulk superconductivity and ferromagnetism in this compound might not yet be conclusive.

Ferromagnetic monolayers: a fresh look at fundamentals

AbstractThe preparation of single ferromagnetic layers in ultrahigh vacuum provides interesting new phenomena in magnetism. Three examples will be given: (i) Pseudomorphic growth of Ni, Co, Fe on fcc‐Cu single crystals produces ferromagnets with tetragonal and trigonal symmetry. This enlarges the magnetic anisotropy energy (hard magnets) by several orders of magnitude. (ii) In two‐dimensional ferromagnetic films the spin‐fluctuations are much more dominant than in bulk ferromagnets. As a result the magnetic ordering temperature of coupled ferromagnets can be shifted by 50–100 K. (iii) By varying the film thickness the Curie‐temperature can be shifted to any convenient value. This allows to study in a detailed manner critical phenomena, e.g. the critical exponent γ for Fe/V‐multilayers.

Insulator–metal transition in Nd 0.8Na 0.2Mn (1− x) Co xO 3 perovskites

The electric and magnetic properties of the perovskites Nd 0.8Na 0.2Mn (1− x) Co x O 3 (0⩽ x⩽0.2) prepared by the usual ceramic procedure were investigated. The insulator-to-metal-like (IM) transition, closely related to a ferromagnetic arrangement, was revealed for the composition of x=0.04 and a similar tendency was detected for x=0. The insulating behavior persists down to low temperatures for higher contents of cobalt ions in spite of the transition to the bulk ferromagnetism. The properties are interpreted in terms of the steric distortion, tilting of the Mn(Co)O 6 octahedra and the double-exchange interactions of the type Mn 3+–O 2−–Mn 4+and Mn 3.5+ δ –O 2−–Co 2+, respectively. Presence of antiferromagnetic domains in the ferromagnetic matrix for the most of cobalt-substituted samples is supposed.

Magnetic Properties of La1-xSrxMnO3 Nanocrystals Embedded in A Mesoporous Silicate

AbstractWe have prepared the diluted assemblies of La1-xSrxMnO3 (x = 0, 0.05, 0.15 and 0.175) nanocrystals (ø ∼ 2∼3 nm) embedded in amorphous SiO2 to clarify the nano-magnetism of both the antiferromagnetic and ferromagnetic La1-xSrxMnO3. All the samples of x = 0, 0.05, 0.15, and 0.175 exhibited superparamagnetism with the blocking temperature of ∼40 K. Except the hump at ∼40 K, the antiferromagnetic x = 0 and 0.05 samples showed simply paramagnetic-like characteristics in both the temperature dependent dc and ac susceptibilities. The ferromagnetic x = 0.15 and 0.175 samples demonstrated complicated characteristics in the temperature dependent susceptibilities. The samples indicated antiferromagnetic and ferromagnetic behavior above ∼50 K and ∼100 K, respectively. The field dependent magnetization is consistent with the predictions of systems having randomly oriented and noninteracting fine particles (ø ∼2∼3 nm). For the ferromagnetic La1-xSrxMnO3 nanocrystals, the antiferromagnetism at crystal surfaces suppressed largely the ferromagnetism in bulk.

Temperature dependence of the extraordinary Hall effect in magnetic granular alloys

We present the results of theoretical investigation of the temperature dependence of the extraordinary Hall effect (EHE) in granular metal–metal and metal–insulator alloys in the case of electron–phonon scattering at high temperatures. Skew scattering is assumed to be the dominant mechanism of the EHE. The calculations were carried out using Zhang–Levy model and the effective-medium approximation. The single-site electron–phonon interaction model was considered by analogy to that one in the theory of disordered alloys. In the case of strong spin-dependent scattering there is an additional term in the temperature dependence of the EHE coefficient of magnetic granular alloys in comparison with that for bulk ferromagnets. This term is linear with T 3. The similar temperature dependence for the EHE conductivity in granular metal–metal and metal–insulator alloys takes place in spite of the different origin of giant magnetoresistance in these systems. The strong temperature dependence of the EHE coefficient can be viewed as an evidence of enhanced spin–orbit interaction at interfaces between granules and the matrix. We show a linear correlation between the interface contribution to the EHE coefficient and the interface contribution to alloy resistivity. The obtained results are in a qualitative agreement with the recent experimental data for nanocomposites.

Specimen shape influence on hysteretic response of bulk ferromagnets

Influence of shape of a specimen is questioned by computational modeling. A new mesoscopical-level model modifying the conventional micromagnetics in a suitable way is used. Numerical simulations made for uniaxial ferromagnets as, e.g., CoZrDy demonstrate explicit dependence of the hysteretic response on the geometry of the specimen. In particular, it is shown how non-ellipsoidal shape of the specimen causes the main hysteresis loop to be round. Minor loops occur only if there is more than one activation threshold (i.e. one coercive force). One option is to include spatial fluctuations in the coercive force, which is demonstrated computationally, as well.

Disappearance of Bulk Ferromagnetism of Iron at High Pressures above 9 GPa

At high pressures exceeding 9 GPa, the disappearance of the ferromagnetism of iron has been detected by magnetization measurements. The value of this critical pressure is consistent with the theories in which the Stoner condition for ferromagnetism has been examined along with the martensitic transition path from the bcc to the hcp structure of iron. The sluggish or hysteresis behavior of the compressed iron is discussed.

Exchange interactions and Curie temperatures of 3D- and 2D-ferromagnets

Effective exchange interactions in bulk ferromagnets as well as in magnetic overlayers on Cu(001) covered by a Cu-cap layer of varying thickness were determined from first principles by mapping of corresponding total energies onto the effective Heisenberg model in the framework of the adiabatic approximation and magnetic force theorem. The effective Heisenberg model is then used to determine spin-wave stiffness constants and Curie temperatures evaluated in the framework of the random-phase approximation. Calculations are in a fair agreement with available experimental data for bulk ferromagnets and reproduce an oscillatory Curie temperature of magnetic overlayers as a function of Cu-cap thickness in a qualitative agreement with recent experiments.

Formation and Soft Magnetic Properties of Co-Fe-Si-B-Nb Bulk Glassy Alloys

Soft ferromagnetic bulk glassy alloys in Co-Fe-Si-B base system were formed in the diameter range up to 1 mm at the composition of (Co 0.705 Fe 0.045 Si 0.1 B 0.15 ) 96 Nb 4 by copper mold casting. Since no bulk glass formation has been obtained in the Co-Fe-Si-B system, the addition of 4%Nb is very effective for the increase in the glass-forming ability. The effectiveness was interpreted by satisfaction of the three component rules for formation of bulk glassy alloys. The bulk glassy alloys exhibit the glass transition before crystallization. The glass transition temperature (T g ), the supercooled liquid region defined by the difference between Tg and crystallization temperature (T x ), ΔT x (= T x - T g ) and the reduced glass transition temperature (T g /T i ) are 823K, 37 K and 0.61, respectively. The bulk glassy alloys also exhibit soft magnetic properties with saturation magnetization (I s ) of about 0.60T and low coercive force (H c ) below 3A/m. The synthesis of the Co-based bulk glassy alloy rods with glass transition and good soft magnetic properties is important for future development as a new type of soft magnetic bulk material.