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15 Sep 2010

Volume 108, Issue 6, Articles (06xxxx)

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back to top Magnetism and Superconductivity

Three-dimensional analysis of BaZrO3 pinning centers gives isotropic superconductivity in GdBa2Cu3O7−δ

K. Kaneko, K. Furuya, K. Yamada, S. Sadayama, J. S. Barnard, P. A. Midgley, T. Kato, T. Hirayama, M. Kiuchi, T. Matsushita, A. Ibi, Y. Yamada, T. Izumi, and Y. Shiohara

J. Appl. Phys. 108, 063901 (2010); http://dx.doi.org/10.1063/1.3486213 (4 pages) | Cited 5 times

Online Publication Date: 17 September 2010

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BaZrO3 doped GdBa2Cu3O7−δ superconductor was prepared by pulsed layer deposition using a KrF excimer laser, which showed isotropic characteristics of critical current density, JC, for magnetic-field orientations, ∼ 4.0×109 A m−2 at 1.0 T. Microstructural and morphological characterization of resulting artificial pinning centers were conducted by transmission electron microscopy and electron tomography, which confirmed the presence of one-dimensional artificial pinning centers, BaZrO3 nanorods. Since there is noticeable anisotropy from GdBa2Cu3O7−δ superconductor without doping BaZrO3, the pinning observed for all magnetic-field orientations are due to the high density and high dispersion of these BaZrO3 nanorods with very wide angular distributions.
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74.25.Wx Vortex pinning (includes mechanisms and flux creep)
74.25.Sv Critical currents
74.72.-h Cuprate superconductors
74.25.F- Transport properties

Epitaxial growth and interfacial magnetism of spin aligner for remanent spin injection: [Fe/Tb]n/Fe/MgO/GaAs-light emitting diode as a prototype system

E. Schuster, R. A. Brand, F. Stromberg, F.-Y. Lo (駱芳鈺), A. Ludwig, D. Reuter, A. D. Wieck, S. Hövel, N. C. Gerhardt, M. R. Hofmann, H. Wende, and W. Keune

J. Appl. Phys. 108, 063902 (2010); http://dx.doi.org/10.1063/1.3476265 (4 pages) | Cited 6 times

Online Publication Date: 17 September 2010

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We have successfully grown and characterized [Fe/Tb]10/Fe(001)/57Fe(001)/MgO(001) multilayer contacts on a GaAs-based light emitting diode. Using 57Fe conversion-electron Mössbauer spectroscopy at room temperature (RT) and at 4.2 K, we provide atomistic proof of large perpendicular Fe spin components in zero external field at and below RT at the 57Fe(001)/MgO(001) interface. Further, indirect evidence of large interfacial Fe atomic moments is provided. Our contacts serve as a prototype spin aligner for remanent electrical spin injection at RT.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
76.80.+y Mössbauer effect; other γ-ray spectroscopy
68.65.Ac Multilayers
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
72.25.-b Spin polarized transport
68.55.A- Nucleation and growth

Improved coupling of Josephson junction arrays to the open space

F. Song (宋凤斌), M. He (何明), M. I. Faley, L. Fang (方兰), and A. M. Klushin

J. Appl. Phys. 108, 063903 (2010); http://dx.doi.org/10.1063/1.3482023 (5 pages) | Cited 2 times

Online Publication Date: 17 September 2010

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The substrates of Josephson junctions have recently been considered as dielectric resonator antennas in order to increase self-emission power from the junctions. Further investigations and interesting phenomena demonstrating this mechanism are presented in this paper. Junction arrays of the same layout were located on various substrates, and Shapiro steps and self-emission were studied in the course of the experiments and simulations. It is demonstrated that suitable substrates can be utilized to improve the coupling of the junctions to the open space for both irradiation and self-emission at the same time.
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74.81.Fa Josephson junction arrays and wire networks
74.50.+r Tunneling phenomena; Josephson effects

Manipulating magnetic moment in a magnetic domain wall under transverse magnetic fields near Walker threshold

Youngman Jang, Seungha Yoon, Seungkyo Lee, Kisu Lee, and B. K. Cho

J. Appl. Phys. 108, 063904 (2010); http://dx.doi.org/10.1063/1.3488011 (4 pages) | Cited 1 time

Online Publication Date: 17 September 2010

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Current-induced domain wall (DW) motion under transverse magnetic fields was investigated through micromagnetic simulation using the Landau–Lifshitz–Gilbert equation containing adiabatic and nonadiabatic spin torque terms. It was found that the transverse field aligned antiparallel to the magnetic moment of the DW promotes a nucleation of an antivortex core, which causes a temporal Walker breakdown and then causes the magnetic moment of the DW to be aligned parallel to the transverse field. On the other hand, the transverse field aligned parallel to the magnetic moment of the DW induces the nucleation of an antivortex core at an edge of a nanowire to be delayed, resulting in the increase in Walker threshold current. The effect of transverse field on current-induced DW motion should be considered carefully for the spintronic applications that utilize DW motion.
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75.60.Ch Domain walls and domain structure
75.75.-c Magnetic properties of nanostructures
75.78.Cd Micromagnetic simulations

Influences of cracking of coated superconducting layer on voltage-current curve, critical current, and n-value in DyBCO-coated conductor pulled in tension

S. Ochiai, T. Arai, A. Toda, H. Okuda, M. Sugano, K. Osamura, and W. Prusseit

J. Appl. Phys. 108, 063905 (2010); http://dx.doi.org/10.1063/1.3488014 (8 pages) | Cited 5 times

Online Publication Date: 17 September 2010

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Influences of cracking of coating layer under applied tensile strain on V(voltage)-I(current) curve, critical current, and n-value of DyBa2Cu3O7−δ coated conductor were studied experimentally and analytically. The experimentally measured variations in V-I curve, critical current, and n-value with increasing applied strain and the correlation of n-value to critical current were described well by the partial crack-current shunting model of Fang et al. Also, the variations in the ratio of shunting current to overall critical current and the ratio of voltage developed in the cracked region to overall voltage with extension of crack, and the variation in critical current with the ratio of noncracked area to overall cross-sectional area of superconducting layer were revealed.
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74.78.-w Superconducting films and low-dimensional structures
74.25.Sv Critical currents
74.25.Ld Mechanical and acoustical properties, elasticity, and ultrasonic attenuation
74.72.-h Cuprate superconductors

Pinning analyses on epitaxial YBa2Cu3O7−δ films with BaZrO3 inclusions

A. Augieri, G. Celentano, V. Galluzzi, A. Mancini, A. Rufoloni, A. Vannozzi, A. Angrisani Armenio, T. Petrisor, L. Ciontea, S. Rubanov, E. Silva, and N. Pompeo

J. Appl. Phys. 108, 063906 (2010); http://dx.doi.org/10.1063/1.3477451 (5 pages) | Cited 1 time

Online Publication Date: 17 September 2010

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The introduction of artificial pinning sites in YBa2Cu3O7−δ (YBCO) epitaxial thin films has been obtained by pulsed laser deposition (PLD) technique from YBCO-BaZrO3 (BZO) composite targets with BZO concentration ranging from 2.5 to 7 mol %. The typical critical temperature, Tc, drop observed in YBCO-secondary phase films grown by PLD has been successfully recovered by increasing the deposition temperature. Transport properties analyses revealed improved pinning force values for YBCO-BZO films for BZO concentration above 5 mol %. These films exhibited an anomalous dual peak structure which has been ascribed to the presence of two pinning contributions. The crossover field, Bm, separating these two pinning regimes has been observed to be temperature independent suggesting the occurrence of the matching field effect. The measured value of Bm agreed with the matching field value, BΦ = KndΦ0, as evaluated from transmission electron microscopy investigations. The mark of this phenomenon is retained down to 10 K, making YBCO-BZO more performing than pristine YBCO films at applied magnetic fields below Bm in all the temperature range inspected. On the contrary, in the high magnetic field range (above Bm), the effectiveness of correlated pinning is progressively reduced on lowering the temperature suggesting that at low temperatures the dominant pinning contribution arises from isotropic pinning centers.
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74.25.Wx Vortex pinning (includes mechanisms and flux creep)
74.78.-w Superconducting films and low-dimensional structures
74.25.F- Transport properties
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
74.72.-h Cuprate superconductors

Size-dependent spin state and ferromagnetism in La0.8Ca0.2CoO3 nanoparticles

I. Fita, V. Markovich, A. Wisniewski, D. Mogilyansky, R. Puzniak, P. Iwanowski, L. Meshi, L. Titelman, V. N. Varyukhin, and G. Gorodetsky

J. Appl. Phys. 108, 063907 (2010); http://dx.doi.org/10.1063/1.3468463 (9 pages) | Cited 3 times

Online Publication Date: 17 September 2010

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Magnetic and structural properties of nanocrystalline low-doped La0.8Ca0.2CoO3 cobaltites with particle size of 8, 13, 23, and 50 nm, prepared by the glycine-nitrate method, were investigated in temperature range 5–320 K, magnetic field up to 50 kOe and under hydrostatic pressure up to 10 kbar. With particle downsizing, a noticeable expansion of unit cell, with concomitant changes in the rhombohedral structure toward the cubic one was observed. It was found that the increased surface-disorder effect strongly suppresses the ferromagnetic state in La0.8Ca0.2CoO3 nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, MS, and Curie temperature, TC, when particle’s size decreases from 23 to 8 nm. The effective magnetic moment μeff was found also to decrease distinctly due to the strong interdependence between Co–O–Co interactions and Co spin state. The size-induced magnetic disorder drives the La0.8Ca0.2CoO3 nanoparticles to a dominant glassy behavior for 8 nm particles. This is evidenced by the fact that the freezing temperature varies with magnetic field in a strict conformity with the de Almeida–Thouless law for spin glasses and also by the observation of characteristic slowing down in the spin dynamics. The applied pressure suppresses TC, MS, and coercive field HC, like it is observed for bulk La0.8Ca0.2CoO3. Nevertheless, in nanoparticles the pressure effect on TC is noticeably stronger, while HC diminishes with pressure much slower then in bulk material.
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75.75.Jn Dynamics of magnetic nanoparticles
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.50.Dd Nonmetallic ferromagnetic materials

The stress-induced anisotropy in the (110) plane of the magnetostrictive TbDyFe [110] oriented crystal

Zhibin Wang, Jinghua Liu, Chengbao Jiang, and Huibin Xu

J. Appl. Phys. 108, 063908 (2010); http://dx.doi.org/10.1063/1.3480814 (4 pages) | Cited 4 times

Online Publication Date: 20 September 2010

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The magnetostriction curve of [110]-oriented Tb0.3Dy0.7Fe1.9 crystal is measured under different compressive prestress. It is observed that the saturation magnetostriction keeps on increasing with the increase in compressive prestress from 0 to 75 MPa. The distribution of the free energy within the (110) plane normal to the stress axis is calculated to clarify the enhancement of saturation magnetostriction under large compressive prestress. It is found that with the increase in the prestress, the lowest energy positions deviate from the initial four 〈111〉 easy axes in the (110) plane, approaching the [1math0] and [math10] orientations. The stress-induced anisotropy results in the change in initial domain configuration and further the enhancement of saturation magnetostriction, which well explains the experimental results.
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75.80.+q Magnetomechanical effects, magnetostriction
75.30.Gw Magnetic anisotropy
61.50.-f Structure of bulk crystals
65.40.G- Other thermodynamical quantities

Spin-wave resonances in exchange-coupled Permalloy/garnet bilayers

J. Ben Youssef, V. Castel, N. Vukadinovic, and M. Labrune

J. Appl. Phys. 108, 063909 (2010); http://dx.doi.org/10.1063/1.3475646 (5 pages)

Online Publication Date: 20 September 2010

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Exchange coupling at the interface between a single-crystal garnet film with a perpendicular anisotropy and a Permalloy film with an in-plane anisotropy was investigated both experimentally and theoretically. Wide-band (2–20 GHz) ferromagnetic resonance measurements in parallel configuration reveal spectra with multiple resonance lines (up to four). These data were analyzed by means of a standing spin-wave model extended to account for in-plane/out-of-plane anisotropy bilayers and to include a finite value of the bilinear interfacial exchange coupling J. The field and frequency evolutions of the observed multiple resonance peaks were satisfactorily reproduced using the value J = 0.18±0.05 erg/cm2.
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75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
75.50.Gg Ferrimagnetics
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Influence of uniaxial anisotropy on the domain pinning fields of ferromagnetic Ga1−xMnxAs films

Sangyeop Lee, Hakjoon Lee, Taehee Yoo, Sanghoon Lee, X. Liu, and J. K. Furdyna

J. Appl. Phys. 108, 063910 (2010); http://dx.doi.org/10.1063/1.3486210 (5 pages) | Cited 4 times

Online Publication Date: 21 September 2010

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The domain pinning fields of ferromagnetic Ga1−xMnxAs films were investigated using the planar Hall effect (PHE). Two in-plane components of GaMnAs films’ anisotropy fields (H4∥ and HU∥), which determine the direction of magnetic easy axes in the (001) plane, were obtained from the PHE’s angular dependence fitted with magnetic free energy within the scheme of the Stoner–Wohlfarth model. The domain pinning fields were obtained both with and without consideration of the deviation angle, δ, of magnetic easy axes from the 〈100〉 crystallographic direction of each sample. The values of domain pinning fields are clearly different between the two methods of analysis and the discrepancy increases with δ. This indicates that the correct direction of the magnetic easy axis (i.e., the influence of uniaxial anisotropy) must be considered to obtain precise values of pinning fields in GaMnAs films.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
75.30.Gw Magnetic anisotropy
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

Broadband electromagnetic wave absorbers prepared by grading magnetic powder density

Masahiro Itoh, Masao Terada, Fumiyoshi Shogano, and Ken-ichi Machida

J. Appl. Phys. 108, 063911 (2010); http://dx.doi.org/10.1063/1.3487477 (5 pages) | Cited 3 times

Online Publication Date: 21 September 2010

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Resin compacts including iron-based magnetic powders were prepared using a centrifugal molding technique. Energy dispersive x-ray analyses demonstrated the formation of a concentration gradient of the magnetic powder in the resin compacts. The resultant concentration-graded resin compacts exhibited better broadband electromagnetic wave absorption than the homogeneous resin compacts prepared as a reference. This absorption ability was further enhanced by attaching a urethane foam plate to the absorber surface.
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75.50.Tt Fine-particle systems; nanocrystalline materials
81.10.Fq Growth from melts; zone melting and refining
41.20.Jb Electromagnetic wave propagation; radiowave propagation

Angular dependence of tunnel magnetoresistance in magnetic tunnel junctions and specific aspects in spin-filtering devices

F. Montaigne, C. Tiusan, and M. Hehn

J. Appl. Phys. 108, 063912 (2010); http://dx.doi.org/10.1063/1.3466778 (8 pages) | Cited 1 time

Online Publication Date: 22 September 2010

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We propose a general formalism to describe accurately the angular dependence of the magnetoresistance. A parabolic band model is used to determine without approximation the conductance of arbitrary complex heterostructures. Simple analytical expressions are obtained in some limit cases. Particularly, we show that significant deviation from the cosine dependence is expected for ferromagnetic barriers. Numerical computations are used to quantify the deviation from the cosine dependence for normal and ferromagnetic barriers and support the precedent conclusion. Finally, the influence of the applied voltage on the angular dependence of magnetoresistance is discussed.
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85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields

Spatially resolved measurements of the magnetocaloric effect and the local magnetic field using thermography

D. V. Christensen, R. Bjørk, K. K. Nielsen, C. R. H. Bahl, A. Smith, and S. Clausen

J. Appl. Phys. 108, 063913 (2010); http://dx.doi.org/10.1063/1.3487943 (4 pages) | Cited 6 times

Online Publication Date: 22 September 2010

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The magnetocaloric effect causes a magnetic material to change temperature upon application of a magnetic field. Here, spatially resolved measurements of the adiabatic temperature change are performed on a plate of gadolinium using thermography. The adiabatic temperature change is used to extract the corresponding change in the local magnetic field strength. The measured temperature change and local magnetic field strength are compared to results obtained with a numerical model, which takes demagnetization into account and employs experimental data.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.60.-d Domain effects, magnetization curves, and hysteresis

Magnetic characterization of micropatterned Nd–Fe–B hard magnetic films using scanning Hall probe microscopy

M. Kustov, P. Laczkowski, D. Hykel, K. Hasselbach, F. Dumas-Bouchiat, D. O’Brien, P. Kauffmann, R. Grechishkin, D. Givord, G. Reyne, O. Cugat, and N. M. Dempsey

J. Appl. Phys. 108, 063914 (2010); http://dx.doi.org/10.1063/1.3486513 (7 pages) | Cited 8 times

Online Publication Date: 23 September 2010

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Scanning Hall probe microscopy has been used for the quantitative measurement of the z-component (out-of-plane) of the stray magnetic fields produced by Nd–Fe–B hard magnetic films patterned at the micron scale using both topographic and thermomagnetic methods. Peak-to-peak field values in the range 20–120 mT have been measured at scan heights of 25–30 μm above the samples. Quantitative comparison between calculated and measured field profiles gives nondestructive access to the micromagnets’ internal magnetic structure. In the case of topographically patterned films the average value of remanent magnetization is extracted; in the case of thermomagnetically patterned films the depth of magnetization reversal is estimated. The measured field profiles are used to derive the spatial variation in the field and field gradient values at distances in the range 0.1–10 μm above the micromagnet arrays. These length-scales are relevant to the application of the micromagnet arrays for lab-on-chip applications (trapping and confinement of magnetic particles). Very large field and field gradient values as high as 1.1 T and 4.1×106 T/m, respectively, are estimated.
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75.70.Ak Magnetic properties of monolayers and thin films
72.20.Pa Thermoelectric and thermomagnetic effects
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.60.Jk Magnetization reversal mechanisms
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Phase coexistence induced by cooling across the first order transition in Ni–Co–Mn–Sb shape memory alloy

Ajaya K. Nayak, K. G. Suresh, and A. K. Nigam

J. Appl. Phys. 108, 063915 (2010); http://dx.doi.org/10.1063/1.3483951 (7 pages) | Cited 2 times

Online Publication Date: 23 September 2010

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The first order austenite to martensitic transition in the off-stoichiometric Heusler alloy Ni–Co–Mn–Sb has been studied using magnetization, electrical resistivity, and heat capacity measurements with different field cooling protocols and thermal cyclings. The coexistence of high temperature, high magnetic austenite phase along with the low temperature and low magnetic martensitic phase after field cooling has been confirmed by all these measurements. The time dependence of these data quite vividly illustrates the metastability of the supercooled/superheated phase in the disorder-broadened first order transition.
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81.30.Kf Martensitic transformations
64.70.kd Metals and alloys
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.40.Gh Other heat and thermomechanical treatments
65.40.Ba Heat capacity

Fluctuation conductivity of single-crystalline BaFe1.8Co0.2As2 in the critical region

Soo Hyun Kim, Chang Ho Choi, Myung-Hwa Jung, Jung-Bum Yoon, Young-Hun Jo, X. F. Wang, X. H. Chen, X. L. Wang, Sung-Ik Lee, and Ki-Young Choi

J. Appl. Phys. 108, 063916 (2010); http://dx.doi.org/10.1063/1.3478716 (4 pages) | Cited 4 times

Online Publication Date: 23 September 2010

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The magnetofluctuation conductivity, called excess conductivity, originated from the forming of the superconducting droplet near to the mean-field transition temperature, was measured for the optimally doped BaFe1.8Co0.2As2 single crystals with a critical temperature, Tc, of 24.6 K. This measurement of the excess conductivity for magnetic fields up to 9 T was compared with the thermodynamic scaling theory in the critical region, in which not only the Gaussian fluctuation but also fourth order terms of the order parameter are included. An analysis of the excess conductivity showed that the superconductivity followed three-dimensional scaling rather than two-dimensional scaling even though the sample had a layered structure.
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74.40.-n Fluctuation phenomena
74.25.fc Electric and thermal conductivity
74.70.Xa Pnictides and chalcogenides
74.25.Bt Thermodynamic properties
74.25.Ha Magnetic properties including vortex structures and related phenomena

A robust approach for the growth of epitaxial spinel ferrite films

J. X. Ma, D. Mazumdar, G. Kim, H. Sato, N. Z. Bao, and A. Gupta

J. Appl. Phys. 108, 063917 (2010); http://dx.doi.org/10.1063/1.3488638 (5 pages) | Cited 8 times

Online Publication Date: 23 September 2010

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Heteroepitaxial spinel ferrites NiFe2O4 and CoFe2O4 films have been prepared by pulsed laser deposition at various temperatures (175–690 °C) under ozone/oxygen pressure of 10 mTorr. Due to enhanced kinetic energy of ablated species at low pressure and enhanced oxidation power of ozone, epitaxy has been achieved at significantly lower temperatures than previously reported. Films grown at temperature below 550 °C show a novel growth mode, which we term “vertical step-flow” growth mode. Epitaxial spinel ferrite films with atomically flat surface over large areas and enhanced magnetic moment can be routinely obtained. Interestingly, the growth mode is independent of the nature of substrate (spinel MgAl2O4, perovskite SrTiO3, and rock-salt MgO) and film thickness. The underlying growth mechanism is discussed.
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68.55.aj Insulators
81.15.Fg Pulsed laser ablation deposition

Size-driven magnetic transitions in La1/3Ca2/3MnO3 nanoparticles

V. Markovich, I. Fita, A. Wisniewski, D. Mogilyansky, R. Puzniak, L. Titelman, and G. Gorodetsky

J. Appl. Phys. 108, 063918 (2010); http://dx.doi.org/10.1063/1.3488619 (9 pages) | Cited 3 times

Online Publication Date: 24 September 2010

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Magnetic properties of electron-doped La1/3Ca2/3MnO3 manganite nanoparticles with average particle size ranging from 12 to 42 nm, prepared by the glycine-nitrate method, have been investigated in temperature range 5–300 K and in magnetic fields up to 90 kOe. Reduction in the particle size suppresses antiferromagnetism and decreases the Néel temperature. In contrast to bulk crystals, the charge ordering does not occur in all studied nanoparticles, while a weak ferromagnetism appears above 200 K. Low temperature magnetic hysteresis loops indicate upon exchange bias effect displayed by horizontal and vertical shifts in field cooled processes. The spontaneous and remanent magnetization at low temperature shows a relatively complex variation with particle size. The size-induced structural/magnetic disorder drives the La1/3Ca2/3MnO3 nanoparticles to a pronounced glassy behavior for the smallest 12 nm particles, as evidenced by large difference between zero field cooled and field cooled magnetization, frequency dependent ac-susceptibility, as well as characteristic slowing down in the spin dynamics. Time evolution of magnetization recorded in magnetic fields after field cooling to low temperatures exhibits pronounced relaxation and a very noisy behavior that may be caused by formation of some collective states. Magnetic properties of the nanoparticle samples are compared with those of La0.2Ca0.8MnO3 nanoparticles. These results shed some light on the coupling between charges and spin degrees of freedom in antiferromagnetic manganite nanoparticles.
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75.75.Jn Dynamics of magnetic nanoparticles
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Et Exchange and superexchange interactions
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Three-dimensional magnetization evolution and the role of anisotropies in thin Fe/MgO films: Static and dynamic measurements

E. Carpene, E. Mancini, C. Dallera, E. Puppin, and S. De Silvestri

J. Appl. Phys. 108, 063919 (2010); http://dx.doi.org/10.1063/1.3488639 (10 pages) | Cited 4 times

Online Publication Date: 24 September 2010

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We have quantitatively characterized the real-space components of the magnetization vector M in thin epitaxial Fe(001)/MgO(001) films through an experimental set-up based on the magneto-optical Kerr effect. The capabilities of the method permit to investigate the magnetization reversal under the effect of an applied field directly on the real-space trajectories of M, providing a straightforward interpretation of the magnetization switching mechanisms in terms of magnetic anisotropies and domains formation. Using the pump-probe technique we also studied the three-dimensional precession dynamics of the magnetization vector triggered by a femtosecond laser pulse, revealing how the anisotropy fields (magnetocrystalline and shape) affect the observed features of the precessional dynamics, i.e., the frequency and the amplitude of motion. Our quantitative approach permits a deeper understanding of the basic mechanisms underlying spin dynamics and it can be successfully applied to a large class of magnetic thin layers.
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75.70.-i Magnetic properties of thin films, surfaces, and interfaces
78.20.Ls Magneto-optical effects
75.60.Jk Magnetization reversal mechanisms
75.30.Gw Magnetic anisotropy
75.30.Ds Spin waves

Composition dependence of magnetic and magnetotransport properties in C60–Co granular thin films

Isamu Sugai, Seiji Sakai, Yoshihiro Matsumoto, Hiroshi Naramoto, Seiji Mitani, Koki Takanashi, and Yoshihito Maeda

J. Appl. Phys. 108, 063920 (2010); http://dx.doi.org/10.1063/1.3471800 (7 pages) | Cited 3 times

Online Publication Date: 24 September 2010

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Composition dependence of magnetic and magnetotransport properties in C60Cox thin films exhibiting large magnetoresistance (MR) effect was investigated in the Co composition range of x = 8–20, where x denotes the number of Co atoms per C60 molecule. From the superparamagnetic magnetization curves observed, the average diameter (dave) of Co nanoparticles dispersed in the matrix phase was evaluated to be approximately 1 nm for the sample of x = 8, and increased with the Co composition, x. By analyzing the temperature (T) dependence of resistivity based on the model by Abeles et al. [Adv. Phys. 24, 407 (1975)] , the average charging energies (〈mathC〉) of Co nanoparticles were evaluated to be 2–9 meV for the samples of x = 8–17 while the considerably weak temperature dependence suggested much smaller values of 〈mathC〉 for the samples of x>17. The composition dependence of dave and 〈mathC〉 revealed a structural transition from well-defined granular structures in the range of x = 8–17 to magnetically and electronically coupled states of Co nanoparticles over x ∼ 17. As a result of the structural change, the MR behavior became different between the two composition regions separated at x ∼ 17. In particular, for the samples of x = 8–17, the bias-voltage (V) dependence of the MR ratio in the low-V region fits well with an unusual exponential form of MR = MR0 exp(−V/Vc) at T<20 K, and it is most remarkable that the zero-bias MR ratio (MR0) is in proportion to 〈mathC〉 and also that the fitting parameter Vc is closely correlated with 〈mathC〉. These results indicate that the charging effect of Co nanoparticles plays an important role in the anomalously large MR effect of C60–Co granular films. In addition, the power-law dependence of MR on T (MR∝T−α,α ∼ 2) was also observed at relatively high temperature range T ≥ 10 K in the wide range of the composition.
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75.70.Ak Magnetic properties of monolayers and thin films
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
73.61.Ng Insulators

Stabilization of surface spin glass behavior in core-shell Fe67Co33–CoFe2O4 nanoparticles

G. Hassnain Jaffari, S. Rizwan Ali, S. K. Hasanain, Gernot Güntherodt, and S. Ismat Shah

J. Appl. Phys. 108, 063921 (2010); http://dx.doi.org/10.1063/1.3478738 (5 pages) | Cited 4 times

Online Publication Date: 24 September 2010

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Magnetic properties of Co33Fe67–CoFe2O4 (core-shell) nanoparticles are presented. Both dc magnetization and ac susceptibility measurements indicate a spin glass (SG) like transition occurring at TF ∼ 175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature (TF(H)) following the well known Almeida–Thouless line, δTF ∼ H2/3. Additionally, the particles exhibit a large exchange bias (HEB ∼ 1357 Oe) arising from the core-shell (ferromagnetic-SG) coupling. The unusually high SG transition temperature and large exchange bias effects are attributed to a combination of several factors including the thickness of the amorphous oxide shell and large values of the exchange and anisotropy constants associated with the CoFe2O4 shell.
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75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Gw Magnetic anisotropy
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Lk Spin glasses and other random magnets
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

The effect of annealing on the junction profile of CoFeB/MgO tunnel junctions

Hui He, Kirill Zhernenkov, Miriana Vadalá, Numan Akdogan, Dmitry Gorkov, Radu M. Abrudan, Boris P. Toperverg, Hartmut Zabel, Hitoshi Kubota, and Shinji Yuasa

J. Appl. Phys. 108, 063922 (2010); http://dx.doi.org/10.1063/1.3483956 (5 pages)

Online Publication Date: 24 September 2010

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The tunnelling magnetoresistance of CoFeB/MgO tunnel junctions is exceptionally high, although the electrodes and the barrier are grown at room temperature in the amorphous state. For their functionality annealing steps up to high temperatures are required. We have analyzed in detail the changes in the chemical and magnetization profile upon annealing up to 360°. The multilayers used for this study are similar to those which are used in magnetic tunnel junctions, however with five repeats. In particular, we have used hard non-resonant and soft resonant magnetic x-ray scattering in order to unravel any changes upon annealing. The multilayers exhibit superior structural quality, which hardly changes with annealing. Surprisingly, only little recrystallization of the CoFeB and the MgO layers can be discerned by x-ray diffraction.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.-m Magnetotransport phenomena; materials for magnetotransport
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.40.Gh Other heat and thermomechanical treatments
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
81.40.Rs Electrical and magnetic properties related to treatment conditions

Microwave magnetoelectric effects in bilayers of piezoelectrics and ferrites with cubic magnetocrystalline anisotropy

A. S. Tatarenko, A. B. Ustinov, G. Srinivasan, V. M. Petrov, and M. I. Bichurin

J. Appl. Phys. 108, 063923 (2010); http://dx.doi.org/10.1063/1.3483957 (4 pages) | Cited 3 times

Online Publication Date: 24 September 2010

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The strain mediated microwave magnetoelectric coupling is studied in bilayers of ferrites with cubic anisotropy and piezoelectrics. The strength of magnetoelectric coupling is determined from data on ferromagnetic resonance frequency shift Δf versus applied electric field E. Studies at 6–10 GHz on samples of nickel zinc ferrite and polycrystalline lead zirconium titanate, lead magnesium niobate–lead titanate or lead zinc niobate-lead titanate (PZN–PT) show shifts Δf on the order of 30–150 MHz for E = 10 kV/cm. Bilayers of nickel zinc ferrite and PZN-PT demonstrated the highest magnetoelectric coefficient of 5.3 Oe cm/kV. Theoretical estimates of the frequency shifts are in very good agreement with the data.
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75.85.+t Magnetoelectric effects, multiferroics
75.70.-i Magnetic properties of thin films, surfaces, and interfaces
75.30.Gw Magnetic anisotropy
77.55.Nv Multiferroic/magnetoelectric films
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.50.Gg Ferrimagnetics

A physical model of exchange bias in [Pd/Co]5/FeMn thin films with perpendicular anisotropy

Lin Lin, Naganivetha Thiyagarajah, Ho Wan Joo, Jang Heo, Ky Am Lee, and Seongtae Bae

J. Appl. Phys. 108, 063924 (2010); http://dx.doi.org/10.1063/1.3471803 (9 pages) | Cited 5 times

Online Publication Date: 24 September 2010

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A physical model of perpendicular exchange bias (PEB) has been established based on the total energy equation per unit area of an exchange bias system by assuming coherent rotation of the magnetization. The anisotropy energy of antiferromagnetic (AFM) layer, KAFM×tAFM, as well as ferromagnetic (FM) multilayers, KFM,eff×tFM, and the interfacial exchange coupling energy, Jex were considered as primary physical parameters in building up the physical model of PEB phenomenon. It was proposed that the PEB is a result of the energy competition between KAFM×tAFM, KFM,eff×tFM, and Jex; where KAFM×tAFM ≥ Jex, is a critical condition to observe exchange bias in the system. In particular, it was revealed that Jex is directly relevant to the net magnetization of FM and AFM spin structure, Jex∝cos αAFM×cos βFM, in the perpendicular direction rather than the magnetization angle difference observed in an in-plane system. The physical role of perpendicular anisotropy energy, KFM,eff×tFM was also found to be significant to enhance the PEB. These physical characteristics are completely different from those are observed from an exchange bias system with in-plane anisotropy. The physical validity of the proposed PEB model was confirmed using different structures of exchange biased [Pd/Co]5/FeMn thin films with perpendicular anisotropy. The experimentally analyzed results demonstrated that the physical model of PEB proposed in this work is agreed well with the experimentally observed PEB phenomenon. Furthermore, the proposed model was found to be effective to design and to predict a new PEB system for the advanced spintronics applications.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Gw Magnetic anisotropy
75.30.Et Exchange and superexchange interactions
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Investigation of Fe–Si–N films as magnetic overcoat for high density recording disk drives

M. Gauvin, E. E. Fullerton, and F. E. Talke

J. Appl. Phys. 108, 063925 (2010); http://dx.doi.org/10.1063/1.3484035 (5 pages) | Cited 2 times

Online Publication Date: 24 September 2010

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A 50-nm-thick Fe–Si–N films were deposited via reactive magnetron cosputtering of independent Fe and Si targets, in Ar/N2 gas mixture, under different dc Fe target power conditions. Magnetic properties, mechanical hardness and tribological properties were characterized as a function of the Fe target power by magnetometry, nanoindentation, and nanoscratch testing, respectively. Deposited samples were found to be ferromagnetic with a coercivity of approximately 20 Oe and a saturation magnetization increasing from 200 to 1100 emu/cm3 as a function of Fe sputter power, i.e., values typical of soft magnetic materials. The mechanical hardness was found to be between 50% and 70% of the hardness of a pure SiNx film. Nanotribological properties of films deposited with a Fe target power ≥ 80 W degraded rapidly.
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75.50.Ss Magnetic recording materials
81.15.Cd Deposition by sputtering
75.70.-i Magnetic properties of thin films, surfaces, and interfaces
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