bluuebunny/crossref_metadata_2025 huggingface dataset:
Volume dependence of the exchange interaction and Curie temperature in Co2MGa (M = Ti and Fe): A first-principles study
Authors: Liu X. B. , Altounian Z.
Published: March 2011

Magnetic moment, exchange interaction and Curie temperature (TC) have been calculated for Co2TiGa and Co2FeGa by a first-principles density functional calculation combined with a linear response method. The exchange interaction is dominated by Co-Co pairs in Co2TiGa while that of Co2FeGa is mainly contributed by Fe-Co pairs. Based on the mean field multiple-sublattices model, the estimated TC is about 114 K for M = Ti and 1270 K for M = Fe, calculated with the experimental lattice constant, in good agreement with the experimental values (128 K and 1093 K for M = Ti and Fe, respectively). With increasing lattice constant, a, from 95% to 105% of the experimental value (aexp.), the moment per formula unit mf.u. changes from 0.43 μB to 1.0 μB and TC increases from 27 K to 142 K in Co2TiGa. However, mf.u. increases slightly from 4.98 μB to 5.40 μB while TC decreases from 1330 K to 1190 K with increasing a from 95% to 105% of aexp. in Co2FeGa. These different volume dependences of TC are ascribed to the weak ferromagnetism in Co2TiGa and the strong ferromagnetism in Co2FeGa.
Study of magnetization state transition in closely spaced nanomagnet two-dimensional array for computation
Authors: Kumari Anita , Sarkar Sudeep , Pulecio Javier F. , Karunaratne D. K. , Bhanja Sanjukta
Published: March 2011

The present study investigated the dipole–dipole interaction for finite 2D arrays of ferromagnetic circular nanomagnets. Starting with two basic arrangements of coupled nanomagnets namely, longitudinal and transverse, different diameters, and thicknesses are studied. The phase plot results exhibit for longitudinal arrangements that the single domain state is pervasive over a large range of thickness values as compared to the transverse arrangement or isolated nanomagnet cases. The study is further extended to finite arrays (3 × 3 and 5 × 5) of circular nanomagnets. The magnetic force microscopy results show that arrays of nanomagnets favors antiferromagnetic ordering at remanence. We have correlated our experimental results with micromagnetic simulations. Based on our study, we can conclude that nanomagnets with 100 nm in diameter, 15 nm in thickness, and 20 nm in spacing have single domain states in an array configuration with one-step switching, which results in fast operation, a property ideal for computing.
Electronic structure analysis of the quasi-one-dimensional oxide Sr6Co5O15 within the LDA+U method
Authors: Botana A. S. , Botta P. M. , de la Calle C. , Piñeiro A. , Pardo V. , Botana J. , Pereiro M. , Baldomir D. , Alonso J. A. , Arias J. E.
Published: March 2011

The quasi-one-dimensional cobalt oxide Sr6Co5O15 is studied using first-principles electronic-structure calculations and Boltzmann transport theory. We have been able to describe the electronic structure, characterized by the structural one-dimensionality and a particular type of charge ordering, with unexpected electronic structure of the different Co atoms. The origin of the large unquenched misaligned orbital angular momenta comes out naturally from a correct description of the different crystal-field environments. The evolution with the on-site Coulomb repulsion (U) of the electronic structure and the transport properties is discussed, with a best agreement with experiment found for the smallest value of U that allows to converge the correct in-chain ferrimagnetic ground state.
Nanoscale confined mode ferromagnetic resonance imaging of an individual Ni81Fe19 disk using magnetic resonance force microscopy (invited)
Authors: Lee Inhee , Obukhov Yuri , Hauser A. J. , Yang F. Y. , Pelekhov Denis V. , Hammel P. Chris
Published: March 2011

We demonstrate and characterize the localized Ferromagnetic Resonance (FMR) modes created in an individual micron-sized Ni81Fe19 disk by means of a strong inhomogeneous probe field applied anti-parallel to the saturated magnetization of the sample. Our variational calculation accurately predicts the frequencies of the localized modes in FMR spectra, and characterizes the sizes and all related internal magnetic fields of the Bessel function modes in a simple model analogous to particle wavefunctions in a quantum well. The localized modes enable FMR imaging of the nonuniform demagnetizing field inside a Py disk demonstrating a novel magnetic resonance imaging technique able to map internal fields in ferromagnets with spectroscopic accuracy.
Scalability of spin field programmable gate array: A reconfigurable architecture based on spin metal-oxide-semiconductor field effect transistor
Authors: Tanamoto Tetsufumi , Sugiyama Hideyuki , Inokuchi Tomoaki , Marukame Takao , Ishikawa Mizue , Ikegami Kazutaka , Saito Yoshiaki
Published: March 2011

The scalability of a field programmable gate array (FPGA) using a spin metal-oxide-semiconductor field effect transistor (MOSFET) (spin FPGA) with a magnetocurrent (MC) ratio in the range of 100–1000% is discussed for the first time. The area and speed of million-gate spin FPGAs are numerically benchmarked with CMOS FPGA for 22, 32, and 45 nm technologies including a 20% transistor size variation. We show that the area is reduced and the speed is increased in spin FPGA due to the nonvolatile memory function of spin MOSFET.
Nucleation of vortex pairs in exchange biased nanoelements
Authors: Silva A. S. M. , Dantas Ana L. , Rebouças G. O. G. , Carriço A. S.
Published: March 2011

We report a theoretical investigation of interface effects in the magnetic order of interface biased iron and Permalloy™ elliptical nano-elements. Contrary to intuition, there is a partial pinning of the interface layer, favoring double vortex states along the hysteresis loop. Interface biasing affects the relative chirality and the distance of the vortices. Unbiased nanoelements may nucleate vortex pairs with the same chirality separated by an antivortex. For interface biased nanoelements the vortex pair forms with opposite chirality separated by a magnetic domain.
Detection of Rashba field using a rotational applied field
Authors: Jang Hyun Cheol , Park Youn Ho , Koo Hyun Cheol , Kim Hyung-jun , Chang Joonyeon , Kim Hijung
Published: March 2011

The intrinsic Rashba field in an InAs quantum well is observed using a potentiometric geometry in which a circular shaped permalloy electrode is utilized with a rotational applied field. From the ratio of the curve shift to the applied field and the amplitude of potentiometric signal, a Rashba field of 8.84 T can be estimated. This method is a very simple and accurate alternative to quantitatively observe the Rashba field.
Total power loss density in a soft magnetic 49% Co–49% Fe–2% V-alloy
Authors: Pieper W. , Gerster J.
Published: March 2011

The total power loss density of a soft magnetic alloy with wt. % 49 Co–49 Fe–2 V was investigated. Measured loss data of strip material with thickness 0.1 mm were in good agreement with the statistical loss theory that is based on separation of losses into hysteresis, eddy current, and excess contributions. As a standard reference material a low-loss 2.7 wt. % Si–Fe nonoriented electrical steel (with P = 2.25 W/kg at 1.5 T and 50 Hz at thickness 0.35 mm) was also investigated. The results indicate that the amount of excess loss is distinctly higher for the SiFe electrical steel while hysteresis loss is slightly lower. As a consequence the total loss density is lower for the CoFe alloy above a limit value of frequency that is identified for material thicknesses 0.1 mm, 0.2 mm, and 0.35 mm at several induction values. This result makes the material attractive for an increasing number of applications where energy efficiency is important. Kerr-microscopic images of the magnetic domain structure of the two materials show smaller domains of the CoFe alloy as the potential reason for the difference in excess losses.
Set-based corral control in stochastic dynamical systems: Making almost invariant sets more invariant
Authors: Forgoston Eric , Billings Lora , Yecko Philip , Schwartz Ira B.
Published: March 2011

We consider the problem of stochastic prediction and control in a time-dependent stochastic environment, such as the ocean, where escape from an almost invariant region occurs due to random fluctuations. We determine high-probability control-actuation sets by computing regions of uncertainty, almost invariant sets, and Lagrangian coherent structures. The combination of geometric and probabilistic methods allows us to design regions of control, which provide an increase in loitering time while minimizing the amount of control actuation. We show how the loitering time in almost invariant sets scales exponentially with respect to the control actuation, causing an exponential increase in loitering times with only small changes in actuation force. The result is that the control actuation makes almost invariant sets more invariant.
Synthesis of air stable FeCo nanoparticles
Authors: Tzitzios V. , Basina G. , Niarchos D. , Li Wanfeng , Hadjipanayis G.
Published: March 2011

Nanoparticulate FeCo alloys have been synthesized by Fe(CO)5 and Co2(CO)8 thermal decomposition in paraffin oil in the presence of oleic acid and oleyl amine. The crystal structure and morphology of the nanoparticles were confirmed by powder x-ray diffraction and transmission electron microscopy. The size and crystallinity of the particles was found to depend on the reaction conditions such as the precursors concentration, reaction time, and carbonyls injection temperature. Also the Fe/Co ratio can be easily controlled by controlling the Fe and Co carbonyls ratio. The as-made nanoparticles were annealed at 500 °C under CH4 stream in order to be protected from future oxidation. This treatment leads to the formation of a graphitic shell around the particles which also protects them from sintering. Additionally, these particles can be functionalized with 1-pyrenebutiric acid in order to be soluble in various solvents.
Microscopic theory of spin torque induced by spin dynamics in magnetic tunnel junctions
Authors: Miura Daisuke , Sakuma Akimasa
Published: March 2011

We studied the charge and spin currents in magnetic tunnel junctions in the presence of spin dynamics on the basis of a tight-binding scheme; the spin dynamics is assumed to be present only in one of the two ferromagnetic electrodes. The charge current is pumped by the dynamical spins having the form ṁ1⋅m2, where mα (α=1, 2) denotes the direction of magnetization in the electrodes and m1 represents the dynamic spin. In addition, three types of spin currents are induced by the dynamical spins. One of these spin currents has the form ṁ1×m2, whose coefficient is proportional to the product of the spin polarizations of both the electrodes. This term can possibly prevent magnetization switching, which is an effect that differs from both the Gilbert damping and spin transfer torque effects. Even in the absence of spin dynamics, the spin current exists in the form m1×m2. We have confirmed that the coefficient of this static term is equal to the effective exchange interaction between the two ferromagnetic electrodes.
Formation of magnetic domains and domain walls in epitaxial Fe3O4(100) elements (invited)
Authors: Fonin M. , Hartung C. , Rüdiger U. , Backes D. , Heyderman L. , Nolting F. , Rodríguez A. Fraile , Kläui M.
Published: March 2011

Magnetic domains and domain walls in epitaxial Fe3O4(100) elements (rings and wires) are imaged using magnetic force microscopy and photoemission electron microscopy. We show that the interplay between the four-fold magnetocrystalline anisotropy and the shape determines the equilibrium domain structure. Domain walls with a characteristic zig-zag structure are observed in Fe3O4(100) elements initially magnetized along one of the magnetocrystalline hard axes. We attribute the formation of zig-zag domain walls to the competition of the four-fold magnetocrystalline anisotropy, the exchange and dipolar coupling. A direct correlation between the wire width and the spin structure of zig-zag domain walls is found.
Nonlocal ultrafast magnetization dynamics in the high fluence limit
Authors: Kuiper K. C. , Malinowski G. , Longa F. Dalla , Koopmans B.
Published: March 2011

In order to explain a number of recent experimental observations of laser-induced femtosecond demagnetization in the large fluence limit, we discuss the consequences of a recently proposed nonlocal approach. A microscopic description of spin flip scattering is implemented in an effective three temperature model, including electronic heat diffusion. Effects of finite film thickness on the demagnetization transients are discussed. Our results show a clear saturation of the ultrafast demagnetization, in excellent agreement with experimental observations.
Microstructures and magnetostriction of two-phase Fe66-Pd30-Ni4 high-temperature ferromagnetic shape memory alloys
Authors: Lin Yin-Chih , Lin Chien-Feng , Yang Jin-Bin , Lee Hwa-Teng
Published: March 2011

The microstructures and magnetostrictive strains of ferromagnetic shape memory Fe-Pd30 alloy systems with additions of Ni elements (4 at.%) have been investigated by a magnetostriction meter, scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The research results show that the magnetostrictive strains of the Fe66-Pd30-Ni4 alloys after homogenization treatment (λ‖s=79×10-6) are higher than those of the as received materials (λ‖s=55×10-6). The lower magnetostriction of the as received metal is due to segregation-impeded parts of the L10 twin boundary motion in realistic magnetic fields. In addition, an important discovery in this study is that doping the Fe-Pd30 alloy system with Ni substitution for Fe seems to prevent the decomposition of L10 + L1m twin phase into stoichiometric L10 + L1m + αbct structures when the strain-forged alloys are solution treated and recrystallization annealed, and then aged at 400 °C for 100 hours. The magnetostrictive strains of the 400 °C/100 h aged sample are maintained with λ||s=62×10-6; λ⊥s=-11×10-6, and the XRD analysis revealed a complete absence of αbct phasein the aged sample. This magnetic property of the alloys is suitable for application in a high temperature and high frequency (T<400 °C) environment. The strain-forged samples were solution treated (ST) and recrystallization annealed, and then aged at 500∼700 °C for 100 hours. XRD analysis revealed the formation of an αbct phase when the specimen was aged at 500 °C for 100 hours. This αbct phase occurrence causes large reductions in the magnetostrictive strains. When the samples were aged at 600∼700 °C for 100 hours, overaging occurred, the martensitic stoichiometric L10 + L1m + αbct lamellar structures dissolved into the matrix simultaneously, and the magnetostrictive strains gradually recovered.
Magnetostriction in high-pressure synthesized PrxNd1−xFe1.9 (0 ≤ x ≤ 1) alloys
Authors: Shi Y. G. , Zhai L. , Tang S. L. , Chen R. J.
Published: March 2011

Polycrystalline PrxNd1−xFe1.9 (0 ≤ x ≤ 1) magnetostrictive alloys were synthesized by high-pressure annealing. The crystal structure, Curie temperature, easy magnetic direction, and magnetostriction of the alloys were investigated. Due to the effects of high pressure, PrxNd1−xFe1.9 alloys are stabilized in a C15 cubic Laves phase with MgCu2-type structure. The Curie temperature decreases linearly with the increasing concentration of Pr. The easy magnetic direction at room-temperature of thealloys rotates continuously from 〈100〉 for x = 0 to 〈111〉 for x = 1. The magnetostriction ofPr0.5Nd0.5Fe1.9 shows a peak at low magnetic field, which indicates the possible anisotropy compensation between Pr and Nd sublattices.
Effect of partial substitution of Ni by Co on the magnetic and magnetocaloric properties of Ni50Mn35In15 Heusler alloy
Authors: Pathak Arjun K. , Dubenko Igor , Xiong Yimin , Adams Philip W. , Stadler Shane , Ali Naushad
Published: March 2011

The magnetic and magnetocaloric properties of Ni48Co2Mn35In15 were studied using magnetization and heat capacity measurements. The magnetic entropy change (ΔSM) was evaluated from both magnetizing and demagnetizing fields. An inverse ΔSM for the magnetizing and demagnetizing processes were found to be 20.5 and 18.5 J kg−1 K−1, respectively, for ΔH = 5 T at the martensitic transition (T = TM). The normal ΔSM was found to be −5.4 J kg−1 K−1 for both fields at the paramagnetic/ferromagnetic transition (T = TC). The effective refrigeration capacity at TM and TC for magnetizing field was found to be 268 and 243 J/kg (285 and 243 J/kg for the demagnetizing field), respectively. We have also estimated the density of states, the Debye temperature, and the inverse adiabatic temperature change to be 4.93 states/eV f.u., 314 K, and −3.7 K, respectively, from the measured heat capacity data.
Magnetism and transport properties of α-Mn structure Mn3Ge thin film
Authors: Dung Dang Duc , Feng Wuwei , Shin Yooleemi , Cho Sunglae
Published: March 2011

We have newly synthesized the α-Mn structured Mn3Ge thin films on GaAs(001) substrate. The single phase Mn3Ge film was grown at a growth temperature of 150 °C, while the secondary phase was observed above a growth temperature of 250 °C. The Mn3Ge films exhibited ferrimagnetism with a Curie temperature of 334 K. The saturation magnetizations were 255.9 and 313.8 emu/cm3 and the corresponding coercive fields were 453 and 1166 Oe at 10 K for the samples grown at 150 and 300 °C, respectively. The ferrimagnetism was confirmed by the anomalous Hall effect data.
Ce-doped EuO: Magnetic properties and the indirect band gap
Authors: Liu Pan , Tang Jinke , Colón Santana Juan A. , Belashchenko Kirill D. , Dowben Peter A.
Published: March 2011

We have prepared and investigated thin films of EuO doped with the rare-earth element cerium. X-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy were used to determine the quality of these films prepared by pulsed laser deposition. Ce doping leads to an enhanced Curie temperature near 150 K, close to that seen for oxygen-deficient EuO1−x. However, the magnetization of Ce-doped EuO exhibits differences from that observed for Gd-doped and oxygen-deficient samples. The high-resolution angular-resolved photoemission from Ce-doped EuO reveals filling of conduction-band states near the X point. This indicates that the band gap in EuO is indirect, and that at 2% doping Ce-doped EuO1−x is at least semimetallic.
Comparison of electrical techniques for magnetization dynamics measurements in micro/nanoscale structures
Authors: Lim Sang-Hyun , Wallis T. M. , Imtiaz Atif , Gu Dazhen , Krivosik Pavol , Kabos Pavel
Published: March 2011

Three all-electrical detection techniques—differential resistance (dV/dI), rf/microwave transmission, and spin rectification measurements—were used to measure the magnetic response of individual patterned Permalloy structures. The investigated techniques enable the identification of distinctive magnetization states, such as vortex nucleation and excitation, domain wall motion, ferromagnetic resonance, and spin wave excitations both in the quasistatic and dynamic regimes. The techniques differ in signal-to-noise ratio and sensitivity to the different types of magnetization responses studied here, suggesting that the use of multiple techniques can significantly improve the understanding of magnetically complex structures.
Magnetic properties of Aucore-Coshell nanoparticles
Authors: Wen Tianlong , Krishnan Kannan M.
Published: March 2011

Aucore-Coshell nanoparticles were synthesized by a wet-chemical method, and their magnetic properties were characterized. By comparing the blocking temperature of Aucore-Coshell nanoparticles to pure cobalt nanoparticles of the same size, it is proposed that cobalt spins in the shell are all aligned by exchange interaction to form single domain nano shells. At the same time, a demagnetizing field in the gold core develops and is aligned along the magnetization direction of the cobalt shell. A greatly enhanced coercivity of Aucore-Coshell nanoparticles at 10 K, compared to the pure cobalt sample, was observed. We propose that the enhanced coercivity of Aucore-Coshell nanoparticles is due to the pinning effect of cobalt spins at the Au/Co interface. The pinning mechanisms include strain pinning and demagnetizing field pinning.
Effect of Cr deficiency on physical properties of triangular-lattice antiferromagnets CuCr1−xO2 (0 ≦ x ≦ 0.10)
Authors: Ling D. C. , Chiang C. W. , Wang Y. F. , Lee Y. J. , Yeh P. H.
Published: March 2011

Structural, transport, and magnetoelectric (ME) properties of delafossite oxides CuCr1−xO2 with 0 ≦ x ≦ 0.10 were extensively investigated. The Rietveld refinement shows that the Cu-O bond length decreases with increasing Cr deficiency, indicative of the presence of a mixed valence state of Cu+/Cu2+ and an enhancement of the hybridization between Cu 3d and O 2p orbitals. As a result, it leads to a decrease of room-temperature resistivity by two orders of magnitude. The deduced effective moment for the Cr-deficient samples is larger than the one only taking into account the contribution from Cr3+ with S = 3/2. This demonstrates that Cu2+ is present in the Cr-deficient samples, giving rise to excess holes at the Cu site. Below TN(Cr) ∼ 24 K, the magnetocapacitance [ε(H)−ε(0)]/ε(0) exhibits a distinct field dependence and deviates from the square of magnetization M2. These findings suggest that the ME coupling in CuCr1−xO2 with higher x is modulated by an increase of the spin fluctuations in the CrO2 triangular lattice through the interplay between charge and spin degrees of freedom.
Exchange bias effect in NiO/NiFe2O4 nanocomposites
Authors: Gong W. J. , Liu W. , Li D. , Guo S. , Liu X. H. , Feng J. N. , Li B. , Zhao X. G. , Zhang Z. D.
Published: March 2011

A series of (100−x)NiO/(x)NiFe2O4 nanocomposites (x = 0, 2.5, 5, 8.3, 12.5, 25) synthesized by a chemical coprecipitation method have been investigated. The exchange bias field HE of the nanocomposites reaches a maximum at x = 2.5, and then decreases with increasing x. The decrease of HE is attributed to the formation of isolated ferrimagnetic NiFe2O4 clusters, which is confirmed by observation with the use of high resolution transmission electron microscopy. The temperature dependence of HE and the coercivity HC for pure NiO is different from those with other samples, which is due to the exchange coupling between the uncompensated antiferromagnetic core and disordered surface shell of NiO nanoparticles.
Magnetic and magnetocaloric properties of Gd6X2Si3 (X = Ni, Co) and Ln6Co2Si3 (Ln = Pr, La)
Authors: Pathak Arjun K. , Dubenko Igor , Stadler Shane , Ali Naushad
Published: March 2011

Phase compositions and crystal structures of Gd6X2Si3 (X = Ni, Co,) and Ln6Co2Si3 (Ln = Pr, La) have been studied. The magnetic properties of Gd6X2Si3 (X = Ni, Co) and Ln6Co2Si3 (Ln = Pr, La) have been evaluated from magnetization measurements performed by a superconducting quantum interference device magnetometer in a temperature interval of 5–400 K, and at magnetic fields up to 5T. The crystal structures of Gd6X2Si3 (X = Co, Ni) and Ln6Co2Si3 (Ln = Pr, La) were found to be hexagonal at 300 K. The Curie temperature (TC), effective and magnetic moment of the compounds (at5 K) have been determined for Gd6X2Si3 (X = Ni, Co), and Ln6Co2Si3 (Ln = Pr, La). TC was found to depend on composition, and reaches maximum value of about 300 K for the Gd6X2Si3 system. Themagnetocaloric effect (magnetic entropy changes and relative cooling power (RCP) was found to depend on X and Ln, and the maximum RCP values were found to be larger than 500 J/kg near room temperature for the Gd6Ni2Si3 and Gd6Co2Si3 compounds. This value of RCP is comparable to the prototype magnetic refrigeration material, Gd.
Magnetic domain structure of La0.7Sr0.3MnO3 nanoislands: Experiment and simulation
Authors: Kim Eun Ji , Watts John L. R. , Harteneck Bruce , Scholl Andreas , Young Anthony , Doran Andrew , Suzuki Yuri
Published: March 2011

We have studied the magnetic domain structure of La0.7Sr0.3MnO3 (LSMO) nanoislands with dimensions on the order of 160 ~ 720 nm. Domain structures of (001)-oriented LSMO rectangular nanoislands exhibit single and multiple flux-closed domain states depending on aspect ratio. (110)-oriented rectangular nanoislands, with a strain-induced magnetically easy axis parallel to the long axis of the rectangle, exhibit single and two domain states depending on aspect ratio. (110) elongated hexagonal islands, with similar aspect ratios, uniformly exhibit single domain states due to the apparent suppression of domain nucleation at tapered edges. These results are consistent with object-oriented micromagnetic framework simulations of LSMO islands.
Static and dynamic strain effects in La0.7Ce0.3MnO3 thin films
Authors: Chen L. , Hu F. X. , Wang J. , Shen J. , Sun J. R. , Shen B. G. , Yin J. H. , Pan L. Q.
Published: March 2011

Both static and dynamic strain effects of La0.7Ce0.3MnO3 (LCeMO) thin films that were grown on (001) oriented SrTiO3 (STO) and 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) substrates were investigated. A charge-orbital ordering (COO) transition was found in the region of low temperature for strained (001)-LCeMO/STO films. The gradual vanishing of the COO transition with the relaxation of tensile epitaxial strain indicates the key role of tensile strain in the formation of COO. Furthermore, the results of dynamic strains on (001)-LCeMO/PMN-PT film show that even a small strain modification of ∼ −0.1% could evidently change the magnetotransport properties around TCOO, verifying the dominant role of the tensile strain.
Structural and magnetic characterizations of Mn2CrO4 and MnCr2O4 films on MgO(001) and SrTiO3(001) substrates by molecular beam epitaxy
Authors: Jhuang Yu-Cing , Kuo K. M. , Chern G.
Published: March 2011

In this report, we grow MnCr2O4 (124) films, and Mn2CrO4 (214) films, for comparison, on MgO and SrTiO3 (STO) substrates by oxygen-plasma-assisted molecular beam epitaxy and also carry out the measurements of the magnetization as a function of temperature (5–200 K) and external field (0–5 T) of these films. The results of magnetization versus temperature further show that ferrimagnetic transition (Tc) occurs at 44 K for 214 films grown on both MgO and STO. The Tc of 124 films raises to 45 K but is only observed for the film grown on STO. No or very weak magnetization is observed for the 124 films grown on MgO(001). The vanishing of the magnetization of the 124 films grown on MgO is attributed to the instability of spinel structure for Cr-rich films. Magnetic hysteresis curves of the films further show the magnetization saturation (Ms) of 124 is <1/2 bohr magneton, which is noticeably smaller than the bulk Ms ~ 1 bohr magneton. The reduction of Ms in these chromites films may result from the cation distribution in that Cr cations distribute in both A and B sites, which is different from the bulk materials. The distribution of Cr cations in both A and B is attributed to the low temperature growth and similar results have been found in previous growth of various ferrite films.
The investigation of reversible strain and polarization effect in (011)-La0.9Ba0.1MnO3 film using field effect configuration
Authors: Wang J. , Hu F. X. , Chen L. , Sun J. R. , Shen B. G.
Published: March 2011

We have investigated the influence of the electric bias field on the magnetic and transport properties of (011)-oriented La0.9Ba0.1MnO3 (LBMO) thin film epitaxially grown on (011)-0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) single crystal substrate. It was found that strain and polarization effects induced by electric bias coexist in the whole temperature range and both of them can modulate the transport properties of (011)-LBMO on PMN–PT. The relative change of resistance ΔR/R exhibits peak values of −22% and −32% for bias fields of +12 and −12 kV/cm, respectively, around metal-insulator transition temperature, TMI. However, the sign of ΔR/R shows polarity dependence at temperature far below TMI. Further careful analysis demonstrates that these two opposite behaviors can be ascribed to the different influence of strain and polarization effects on transport properties.
Jahn–Teller effect and magnetic property in anisotropic triangular compound NaxMnO2
Authors: Ouyang Sheng-De , Liu Da-Yong , Zou Liang-Jian , Lin Hai-Qing
Published: March 2011

In this paper we investigate the Jahn–Teller distortion and magnetic property in parent and doped anisotropic triangular compound NaxMnO2. We first determine the Jahn–Teller distortion modes of the MnO6 octahedra by minimizing the potential energy. Then we demonstrate by the cluster self-consistent field method that the magnetic ground state of NaxMnO2 is striped antiferromagnetic at x = 0. The spin arrangement agrees with the experimental data. With the increase of hole doping concentration, the manganese spins gradually transform from canted striped antiferromagnetic phase to ferromagnetic one.
Ferromagnetism enhanced by structural relaxation of biaxially compressed LaCoO3 films
Authors: Mehta Virat , Suzuki Yuri
Published: March 2011

Epitaxial LaCoO3 films were synthesized on LaAlO3 substrates to explore the role of epitaxial strain and structure on the ferromagnetism observed in these biaxially compressed films. Coherent strain and tetragonal structure were only achieved in thin film samples grown using higher energy densities. The strain relaxed with increasing thickness and was accompanied by increasing mosaic spread. Higher magnetization values were consistently seen in fully relaxed films grown using lower laser energy density. These results suggest that epitaxial strain is not the only factor determining the ferromagnetism and that the microstructure and defects may play a significant role.
Interfacial effects in La2/3Sr1/3MnO3 thin films with different complex oxide capping layers
Authors: Valencia S. , Konstantinovic Z. , Gaupp A. , Schmitz D. , Balcells Ll. , Martínez B.
Published: March 2011

Interfacial effects in sputtered La2/3Sr1/3MnO3 thin films with different capping layers (MgO, LaAlO3, SrTiO3, NdGaO3, and Au) have been locally investigated by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn L3,2-edge. Data were acquired by using the total electron yield detection mode thus guaranteeing maximum sensitivity to the interface. The data show that LaAlO3 capping almost does not modify the bulklike Mn valence at the interface. In case of SrTiO3 and Au, the presence of divalent Mn is detected, whereas MgO and NdGaO3 capping lead to an increase of the Mn valence oxidation state. The modification of the nominal Mn valence state leads to depressed surface magnetization.
Fabrication of L1-ordered FePt granular media using an in-line sputtering system
Authors: Takekuma I. , Sayama J. , Nakamura K. , Nemoto H. , Tanahashi K.
Published: March 2011

FePtAg-SiO2 granular films were fabricated using an in-line sputtering system with a composite alloy target. Two heating methods were investigated in consideration of practical manufacturing processes. Structural analyses showed that the FePtAg-SiO2 film had better L10 ordered structure and clearer grain boundaries when deposited on the preheated substrate than when annealed after deposition. Adding Ag improved both the L10 ordering and the face centered tetragonal (001) texture. Torque magnetometry indicated that the FePtAg-SiO2 film had a uniaxial perpendicular anisotropy of 1.4 × 107 (erg/cc) at an ordering parameter S of 0.72.
Demonstration and analysis of high resolution differential read-head
Authors: Shiimoto Masato , Katada Hiroyuki , Yasui Kan , Nakagawa Takeshi , Meguro Kenichi , Nagasaka Keiichi , Ito Naoto , Hoshiya Hiroyuki , Nishida Yasutaka , Nakamoto Kazuhiro
Published: March 2011

We investigated properties of a differential read-head focusing on resolution, output signal, and bit error rate (BER) through experiments and calculations. We successfully observed particular waveforms of the differential head. The fabricated differential heads showed much higher resolution and better BER than conventional heads. Through measurements and calculations, we clarified that as gap-length (GL) between two spin-valves decreases, the resolution for the differential head increases, but the output signal decreases. Accordingly, the differential head has optimal GL to achieve the best BER. The optimal GL is almost the same as the shortest bit length. We also clarified that a calculated differential head with optimized GL has better BER than a conventional head with a shield-gap length of 20 nm, especially at higher linear density. Therefore, the differential head is one of the candidates for reader structures for high-areal-density hard disk drives.
Engineering synchronization of chaotic oscillators using controller based coupling design
Authors: Padmanaban E. , Hens Chittaranjan , Dana Syamal K.
Published: March 2011

We propose a general formulation of coupling for engineering synchronization in chaotic oscillators for unidirectional as well as bidirectional mode. In the synchronization regimes, it is possible to amplify or to attenuate a chaotic attractor with respect to other chaotic attractors. Numerical examples are presented for a Lorenz system, Rössler oscillator, and a Sprott system. We physically realized the controller based coupling design in electronic circuits to verify the theory. We extended the theory to a network of coupled oscillators and provided a numerical example with four Sprott oscillators.
Chemical stability of highly (0001) textured Sm(CoCu)5 thin films with a thin Ta capping layer
Authors: Zhao Haibao , Wang Hao , Liu Xiaoqi , Zhang Tao , Wang Jian-Ping
Published: March 2011

With the highest magnetocrystalline anisotropy constant (Ku) among practical magnetic materials, SmCo5 could be a very attractive candidate for future high areal density magnetic recording. However, its corrosion resistance is always a concern in recording media applications. In this paper, the chemical stability and microstructures of highly (0001) textured Sm(CoCu)5 thin films with and without a 3 nm Ta capping layer were reported. For Sm(CoCu)5 thin films without a capping layer, the coercivity decreases significantly (from 8kOe to 1kOe) within one month. Sm(CoCu)5 thin films capped with a thin Ta layer (3 nm) behave differently. Even exposed to a laboratory environment (25 °C) over 3 years, the Ta-capped Sm(CoCu)5 thin films are stable in terms of structural and magnetic properties, i.e., there were no changes in X-ray diffraction peaks and vibrating sample magnetometer hysteresis loops. Microstructure of Ta-capped Sm(CoCu)5 thin films showed that Sm(CoCu)5 formed a domelike particle assembly structure on a smooth Ru underlayer and were well covered by partially oxidized Ta capping layer, as shown by TEM cross-section micrographs. Accelerated corrosion treatment (130 °C, 95% relative humidity, 6 h) was performed on Ta-capped Sm(CoCu)5 thin films. X-ray photoelectron spectroscopy (XPS) results showed that no Co was detected on the sample surface before the corrosion treatment, but strong XPS signals of CoOx and Co(OH)x were observed after treatment. Therefore, none of our Sm(CoCu)5 thin films can pass the accelerated corrosion test. Hcp-phased CoPt-alloys are proposed as better capping materials for Sm(CoCu)5 thin films in future high-density magnetic recording applications.
Interface-dependent magnetic anisotropy of Fe/BaTiO3: A first principles study
Authors: Choi Heechae , Hwang Yubin , Lee Eung-Kwan , Chung Yong-Chae
Published: March 2011

Using first principles calculations, we investigated interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the (1×1) surface unit and a Fe monolayer (ML) is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2–terminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy.
Optimum design consideration for interferometric spin wave logic operations
Authors: Nakashima Y. , Nagai K. , Tanaka T. , Matsuyama K.
Published: March 2011

In the present study, the operational modes and the structural design are optimized to realize potential performance in the interferometric spin wave logic circuits. Successive functional operations, such as generation, propagation, and inductive detection of spin wave packets are numerically studied by using micromagnetic simulations. The logic input is coded with the phase of pulsed microwave currents applied through the generators. Among the various kinds of the investigated spin wave (SW) modes, the backward volume mode exhibits superior performance. Various structural and operational parameters, including the pulsed microwave frequency and the film thickness of the magnetic strip, were optimized by taking the inductive output voltage (Vout) as a quantitative criterion. The several orders of difference obtained in the Vout for the different logic inputs demonstrates the successful exclusive-not-OR operation.
Quantitative analysis of interaction between domain walls and magnetic nanoparticles
Authors: Klein Todd , Dorroh Daniel , Li Yuanpeng , Wang Jian-Ping
Published: March 2011

We have studied the potential energy and effective field induced by the presence of a single superparamagnetic particle above a magnetic domain wall in a 5 nm ferromagnetic film (Ms = 800 emu/cm3) with uniaxial crystalline anisotropy (Ku < 107 erg/cm3). The wall width, wall type (head-to-head, Néel, and perpendicular Bloch), film dimensions, particle height, and external applied field are found to affect the performance of particle sensing systems. Results and optimization strategies derived from this model are presented. The calculated change in depinning field (ΔHdp) is compared against experimental data and micromagnetic simulation. This comparison provides justification for further development in terms of integration with micromagnetic simulations.
Giant magnetocaloric effect of thin Ho films
Authors: Filho F. C. Medeiros , Mello V. D. , Dantas A. L. , Sales F. H. S. , Carriço A. S.
Published: March 2011

We report a theoretical study of the impact of finite size on the magnetocaloric effect (MCE) of thin Ho films. For strong external field strengths, the adiabatic temperature change ΔTad is comparable to the value found in bulk Ho, reaching about 12 K for an external field strength change ΔH = 50 kOe. For thicknesses below the helix period, there is a large enhancement. In this thickness range, the helical state does not form, leading to a giant MCE reaching ΔT/ΔH = 6.5 K/T for ΔH = 2 kOe.
Spatial light phase modulators with one-dimensional magnetophotonic crystals driven by piezoelectric films
Authors: Mito S. , Takagi H. , Lim P. B. , Baryshev A. V. , Inoue M.
Published: March 2011

We demonstrate a novel spatial light phase modulator composed of a one-dimensional magnetophotonic crystal driven by a piezoelectric film. With the application of a constant magnetic bias field perpendicular to the film plane, the magneto-optic Kerr rotation angle of the modulator was subject to continuous change from 3.8–2.4° with the voltage up to 50 V and hence a continuous phase modulation for rays with circular polarization. Surprisingly, the magneto-optical response of the modulator showed nonvolatile behavior and the light phase was maintained at a designated value without applying any stress. Based upon the above experimental results, magneto-optic spatial light phase modulators with a 64 × 64 array of magnetic pixels were constructed. The fabricated device had a dimension of 15 μm2 and demonstrated two-dimensional optical modulation of the phase of reflected rays.
Serious discrepancies between nuclear magnetic resonance and neutron scattering measurements in the critical region of classic two-dimensional magnets and anomalous behaviors at low temperatures
Authors: Bykovetz N. , Klein J. , Lin C. L.
Published: March 2011

A characterization of 2D critical behavior in the classic 2D antiferromagnets K2NiF4, Rb2MnF4, and K2MnF4 made originally by Birgeneau et al. [Phys. Rev. B 1, 2211 (1970)], using neutron scattering (NS), is used to review NMR data in these compounds and in some classic 2D ferromagnets. On analyzing the NMR data, very serious discrepancies were found between the NMR and NS results for the critical exponent β, with the values of the NMR βs being some 50% higher. Some novel results were also found, including 2D cases where β=0.33 (but characterized by a non-3D parameter, D∼1). The discrepancies surely need to be resolved, and if the present analysis of the NMR data is further validated, these results could present a clear and nontrivial hurdle for current critical-region theories. Additionally, from the NMR data it was found that the low-temperature behaviors of these classical 2D systems are at odds with the predictions of conventional spin-wave theory analyses.
High temperature annealing stability of magnetic properties in MgO-based perpendicular magnetic tunnel junction stacks with CoFeB polarizing layer
Authors: Rahman M. Tofizur , Lyle Andrew , Hu Guohan , Gallagher William J. , Wang Jian-ping
Published: March 2011

We studied Co/Pd multilayers (MLs) and CoFeB spin polarizing layer for MgO-based perpendicular magnetic tunnel junctions (MTJs) by engineering the exchange coupling between CoFeB and Co/Pd MLs and the interface morphology of Co and Pd in Co/Pd MLs. One of the key challenges of maintaining perpendicular anisotropy in both the fixed and free layers was achieved by tuning the energy balance between the Co/Pd MLs and the CoFeB layers. A perpendicular squareness ratio of near unity in M–H loops of full stack structures clearly indicated excellent perpendicular anisotropy even after annealing at 350 °C for 1 h in vacuum. Very low intermixing with sharp Co/Pd interfaces confirmed by the small angle x-ray reflectivity measurements was believed to be the key to high temperature annealing stability of magnetic properties. Our results shed light on the mechanisms resulting in low TMR (tunneling magnetoresistance) for Co/M (M=Pd, Pt, Ni) multilayer-based MTJs in this study as well as in previously published reports.
High power density vibration energy harvester with high permeability magnetic material
Authors: Xing X. , Yang G. M. , Liu M. , Lou J. , Obi O. , Sun N. X.
Published: March 2011

An alternative design of vibration energy harvester was demonstrated to significantly increase the output power density. This design had two magnetic solenoids fixed at two sides of a spring supported hard magnet pair with anti-parallel magnetization, which produced a spatially inhomogeneous bias magnetic field for switching the flux inside the solenoids during vibration. Experimental results showed an output voltage of 2.52 V and a power density 20.84 mW/cm3 at 42 Hz, with a half peak working bandwidth of 6 Hz. This vibration energy harvester design leads to high output voltage, power and power density that are critical for real applications.
Magnetic properties of EuS spin filter tunnel contacts to silicon
Authors: Müller Martina , Schreiber Reinert , Schneider Claus M.
Published: March 2011

We investigate the magnetic properties of the ferromagnetic insulator EuS in view of its potential in spin-filter tunnel contacts to silicon. We prepared thin polycrystalline EuS films directly on (001) oriented Si substrates that show well-defined magnetic properties down to the monolayer regime. Addressing the question of magnetic coupling between a EuS magnetic tunnel barrier and a CoO/Co magnetic electrode, we succeeded in realizing an independent magnetic switching behavior in this spin-valve-type system. These results are important prerequisites for future spin-dependent transport experiments.
Extrinsic damping contribution in soft magnetic thin films detected by permeability spectra
Authors: Xu Feng , Li Shandong , Ong C. K.
Published: March 2011

This work focuses on the identification of extrinsic damping contribution in FeCoN and FeCoSiN soft magnetic thin films by microwave permeability spectra measurements. The comparison of hysteresis loops shows that FeCoSiN thin film has a stronger in-plane uniaxial anisotropy. Through the analysis on the field-dependent frequency linewidth, the extrinsic damping contribution from two-magnon-scattering in FeCoN thin films is identified. The two-magnon-scattering damping contribution is greatly enhanced in FeCoSiN film due to the granular microstructure which brings about much more interface area between the two phases.
Metal-insulator transition in Nd1−x Eux NiO3 probed by specific heat and anelastic measurements
Authors: Barbeta V. B. , Jardim R. F. , Torikachvili M. S. , Escote M. T. , Cordero F. , Pontes F. M. , Trequattrini F.
Published: March 2011

Oxides RNiO3 (R = rare-earth, R ≠ La) exhibit a metal-insulator (MI) transition at a temperature TMI and an antiferromagnetic (AF) transition at TN. Specific heat (CP) and anelastic spectroscopy measurements were performed in samples of Nd1−xEuxNiO3, 0 ≤ x ≤ 0.35. For x = 0, a peak in CP is observed upon cooling and warming at essentially the same temperature TMI = TN ∼ 195 K, although the cooling peak is much smaller. For x ≥ 0.25, differences between the cooling and warming curves are negligible, and two well defined peaks are clearly observed: one at lower temperatures that define TN, and the other one at TMI. An external magnetic field of 9 T had no significant effect on these results. The elastic compliance (s) and the reciprocal of the mechanical quality factor (Q−1) of NdNiO3, measured upon warming, showed a very sharp peak at essentially the same temperature obtained from CP, and no peak is observed upon cooling. The elastic modulus hardens below TMI much more sharply upon warming, while the cooling and warming curves are reproducible above TMI. Conversely, for the sample with x = 0.35, s and Q−1 curves are very similar upon warming and cooling. The results presented here give credence to the proposition that the MI phase transition changes from first to second order with increasing Eu doping.
Magnetic behavior of 10 nm-magnetite particles diluted in lyotropic liquid crystals
Authors: Arantes F. R. , Figueiredo Neto A. M. , Cornejo D. R.
Published: March 2011

A magnetic study of 10nm magnetite nanoparticles diluted in lyotropic liquid crystal and common liquids was carried out. In the liquid crystal the ZFC-FC curves showed a clear irreversible behavior, and it was possible to distinguish the nematic from the isotropic phase since the magnetization followed the dependence of the nematic order parameter with the temperature. This behavior could be mimicked by Monte Carlo simulation.
The influence of quantum spin fluctuations on magnetic instability
Authors: Antropov V. P. , Solontsov A.
Published: March 2011

We analyze the magnetic stability of the ground state of magnetic systems, taking into account strongly coupled zero-point spin fluctuations. The formalism is applied to the elemental 3d-metals Fe and Ni and to two phases of the 5f-metal Pu. Strong suppression of local magnetism due to spin fluctuations is obtained for xα- Pu. Such inclusion of spin fluctuations changes the character of the magnetism in α-Pu from localized to itinerant.