Magnetoresistance and magnetic ordering fingerprints in hydrogenated graphene

Spin-dependent features in the conductivity of graphene, chemically modified by a random distribution of hydrogen adatoms, are explored theoretically. The spin effects are taken into account using a mean-field self-consistent Hubbard model derived from first-principles calculations. A Kubo transport methodology is used to compute the spin-dependent transport fingerprints of weakly hydrogenated graphene-based systems with realistic sizes. Conductivity responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic macroscopic states, constructed from the mean-field solutions obtained for small graphene supercells. Magnetoresistance signals up to ～7% are calculated for hydrogen densities around 0.25%. These theoretical results could serve as guidance for experimental observation of induced magnetism in graphene.     

Magnetoresistance of neodymium metal

The transverse and longitudinal magnetoresistance on the polycrystalline d.h.c.p. Nd have been measured at 1.38 K. 4.2 K, 13.96 K and 20.4 K with applied fields up to 20 kOe.     

Jahn-Teller antiferroelectric ordering in praseodymium hydroxide

Electric and magnetic susceptibility experiments on Pr(OH)₃ show one- and three-dimensional ordering of Pr electric dipoles similar to that observed in PrC1₃. The smaller unit cell of Pr(OH)₃ leads to substantially stronger Pr-Pr interactions and to a phase transition at 1.25 K compared with 0.4 K in PrC1₃.     

Double exchange model for magnetic hexaborides

A microscopic theory for rare-earth ferromagnetic hexaborides, such as Eu1-xCaxB6, is proposed on the basis of the double-exchange Hamiltonian. In these systems, the reduced carrier concentrations place the Fermi level near the mobility edge, introduced in the spectral density by the disordered spin background. We show that the transport properties such as the Hall effect, magnetoresistance, frequency dependent conductivity, and dc resistivity can be quantitatively described within the model. We also make specific predictions for the behavior of the Curie temperature T(C) as a function of the plasma frequency omega(p).     

Magnetoresistance in magnetic domain wall systems

The contribution to the magnetoresistance (MR) was calculated for a single magnetic domain wall present in a small 2D impure ferromagnetic wire. On assuming low-temperatures and spin-independent impurity scattering the mechanism responsible for a positive contribution to MR is quantum interference. It is found that the contribution increases with impurity concentration and reduces with the width of the wall. The influence on the MR of a homogeneous electric field is proved to be spin-dependent.     

Magnetoresistance of dilute magnetic alloys

The magnetoresistance of dilute magnetic alloys is calculated. The difficulties appearing in the calculation of scattering times within the density wave model are discussed and overcome semi-empirically.     

Magnetoresistance and spin-transfer torque in magnetic tunnel junctions

We comment on both recent progress and lingering puzzles related to research on magnetic tunnel junctions (MTJs). MTJs are already being used in applications such as magnetic-field sensors in the read heads of disk drives, and they may also be the first device geometry in which spin-torque effects are applied to manipulate magnetic dynamics, in order to make non-volatile magnetic random access memory. However, there remain many unanswered questions about such basic properties as the magnetoresistance of MTJs, how their properties change as a function of tunnel-barrier thickness and applied bias, and what are the magnitude and direction of the spin-transfer-torque vector induced by a tunnel current.     

Transverse magnetic ordering

There is an increasing number of ferromagnets and antiferromagnets which are observed to undergo either a further long range magnetic order or spin glass transition in components of the moment transverse to either the ferromagnetic or antiferromagnetic ordering direction. Necessary conditions include exchange frustration and some atomic disorder. We discuss the observation of transverse antiferromagnetic order in the ferromagnet (Fe, Mn)₃Si and the transverse spin glass phase observed in the ferromagnetic glassy metal a-(Fe, Zr) and the antiferromagnet γ-Mn–Cu.     

Spin-spiral ordering in magnetic superconductors

The spin-spiral ordering in magnetic superconductors is caused by the interplay among the photon-field, the persistent current, and localized spin moments. The dynamical spin-susceptibility of this system is derived and the dynamical properties are studied in connection with this spin-spiral ordering.     

强磁场下锗电阻温度传感器的磁致电阻效应研究

以锗温度传感器GR-1400-AA为研究对象,研究了其在0～16 T磁场下、2～100K温区内的磁致电阻效应。结果表明:在2～20K温区,GR-1400-AA磁效应随温度的升高急剧降低,且场强越高磁效应的变化率越大;在20～100K温区,磁效应随温度的升高趋于平缓;GR-1400-AA磁效应随场强的升高而升高,且温度越低,磁效应的变化率越高,2.1K、16T处的磁效应为2120.3%;GR-1400-AA由磁阻引起的测量温差随温度的升高而升高,随场强的增大而增大,且全为负温差,在6K、2T处和16T、100K处的测量温差分别为-0.24K、-60.4K。不推荐应用在磁场环境下。     

Magnetoresistance in silicon based ferrite magnetic tunnel junction

We report magnetoresistance for silicon based magnetic tunnel junction. We used cobalt ferrite & cobalt nickel ferrite as free layer and pinned layer. The magnetoresistance measured at room temperature through silicon by fabricating FM/Si/FM magnetic tunnel junction. Magnetoresistance shows a loop type behavior with 3.7%. We have successfully demonstrated spin tunneling through silicon with ferrite junction that opens the door for potential candidate for spintronics devices. The spin-filtering effect for this double spin-filter junction is also discussed.     

Structure and magnetic ordering in SrNpO3

In the course of studies of alkaline earth oxo-neptunates Mössbauer spectra of²³⁷Np in SrNpO₃ were taken. From the point of view of lattice symmetries two characteristically different sites for the Np ions were found. The transition temperature T_C=(30.5±0.5) K was confirmed. The phase transition turned out to be of second order. Below T_C the lattice orders antiferromagnetically.     

Spin-glass-like magnetic ordering in Zn substituted magnetite magnetic fluids

Electron spin resonance (ESR) spectra of magnetic fluids involving polydispersed Zn(0.5)Fe(0. 5)Fe(2)O(4) (FZ5) and Zn(0.7)Fe(0. 3)Fe(2)O(4) (FZ7) nanomagnetic particles are scanned from 4.2 to 300K. The FZ7 fluid exhibits certain distinct features below 40K which are different from FZ5 fluid. These include (i) an isotropic shift in resonance field in zero-field-cooled ESR study, (ii) deviation of resonance field from sin(2)theta behavior (where theta is the angle between axis of the particle and field) in field cooled (FC) sample and (iii) abrupt increase in anisotropy field for FC sample. The results are analyzed in light of the core-shell model for nanomagnetic particles.     

强磁场下氧化钌温度传感器的磁致电阻效应研究

以氧化钌温度传感器(RX-202A)为研究对象,研究了其在0¹6 T磁场下、216 T磁场下、2⁴0 K温区内的磁致电阻效应。结果表明:RX-202A的磁效应随场强的升高而升高,随温度呈波动变化,在低温区先由正效应转变为负效应,后又随温度的升高而逐渐降低,并逐渐趋于0,RX-202A在2.1 K、16T处的磁效应为0.83;RX-202A由磁效应引起的测量误差随温度呈波动变化,在2.240 K温区内的磁致电阻效应。结果表明:RX-202A的磁效应随场强的升高而升高,随温度呈波动变化,在低温区先由正效应转变为负效应,后又随温度的升高而逐渐降低,并逐渐趋于0,RX-202A在2.1 K、16T处的磁效应为0.83;RX-202A由磁效应引起的测量误差随温度呈波动变化,在2.2¹0K温区随场强的变化差别较小,在1010K温区随场强的变化差别较小,在10³8K温区随场强的增大而增大,RX-202A在2.2K、16T处,38K、16T处产生的温度测量误差分别为-0.099K和0.364K。     

Superexchange pairing and magnetic ordering in cuprate superconductors

The first analytic study of superexchange ion interaction in the cuprate layer of high-T _c superconductors is reported. It is shown that the superexchange nonadditive contributions are dominant in the onset of long-range magnetic order in a system, as well as are responsible for hole pairing to produce an energetically preferable, stable spin configuration. The Heisenberg parameter and the pair binding energy obtained ab initio are in a good quantitative agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 2127–2131 (December 1999)     

Local moments and magnetic ordering in transition metals

Stoner theory predicts a complete collapse of the local magnetic moment at the Curie temperature. In this paper, the effect of local moment formation in the non-magnetic state is discussed. The energy between the magnetic and non-magnetic state is found to be considerably reduced. This observation is consistent with the low observed magnetic ordering temperatures of the 3d metals when compared to the values derived from band theory.     

Anisotropy of the magnetic moment of neodymium in garnets

The theoretical and experimental study of the magnetic moments of Nd³⁺ ions in aluminium and gallium garnets respectively was performed. The calculation of the field dependence of the magnetic moment of Nd³⁺ in YAG, based on the complete set of 9 crystal field parameters determined from the analysis of the optical spectra, gives at 4·2 K the anisotropy of the magnetic moment of about 10%. The magnetization curves of Nd₃Ga₅O₁₂ were measured in static fields up to 46 kOe and found to be anisotropic and lying very near to the theoretical curves for Nd: YAG. The pulse field measurements up to 210 kOe provided no reliable results probably owing to the strong magnetocaloric effect. It is concluded that the crystal field effects are not sufficient to explain the whole magnitude of the anisotropy of magnetization in Nd-Sc-Fe garnet observed in our earlier work.The authors wish to thank Mr. P.Pechatý for the growing of the crystals and Mr. J.Vank for their careful orientation.