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15 Dec 2011

Volume 110, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 110, 121301 (2011); http://dx.doi.org/10.1063/1.3665219 (29 pages)

T. Fujita, M. B. A. Jalil, S. G. Tan, and S. Murakami
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back to top Magnetism and Superconductivity

Multi-domain resonance in textured Z-type hexagonal ferrite

Tomotsugu Kato, Hideto Mikami, and Shin Noguchi

J. Appl. Phys. 110, 123901 (2011); http://dx.doi.org/10.1063/1.3669369 (6 pages) | Cited 1 time

Online Publication Date: 16 December 2011

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The resonance frequency of a non-saturated Z-type hexagonal ferrite under a demagnetizing field effect was formulated. The permeabilities along the c-axis and the c-plane were measured by a specimen cut from the c-plane-textured Z-type ferrite, and the resonance frequency was measured as a μ″ peak. The resonance frequency measured from the permeability in the c-plane increases dramatically as the specimen’s thickness increases. On the other hand, the resonance frequency measured from the permeability along the c-axis remains fairly constant. We tried to analyze the reason for the large difference in the resonance frequency depending on the crystal orientation using multi-domain resonance theory. As a result, the resonance frequency of a textured Z-type ferrite has been expressed when the high frequency magnetic field is parallel and normal to the c-plane. Therefore, it is confirmed that a demagnetizing field increases the resonance frequency, especially when the high frequency magnetic field is parallel to the c-plane. This result would contribute to the development a high performance inductance element.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Gg Ferrimagnetics
75.60.-d Domain effects, magnetization curves, and hysteresis

Fluence dependence of ion implantation-induced exchange bias in face centered cubic Co thin films

J. Demeter, E. Menéndez, K. Temst, and A. Vantomme

J. Appl. Phys. 110, 123902 (2011); http://dx.doi.org/10.1063/1.3669445 (7 pages)

Online Publication Date: 19 December 2011

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The fluence dependence of exchange bias induced by oxygen ion implantation has been studied in highly textured face centered cubic Co films. These films exhibit a strong magnetocrystalline anisotropy prior to implantation. Upon implantation, the crystalline order is strongly reduced, even for the lowest implantation fluence, as shown by an isotropic magnetic behavior. Detailed analysis of the structural changes shows that the crystallite size remains basically unaltered upon implantation, suggesting that CoxOy is formed at the Co grain boundaries. A large suppression of the magnetocrystalline anisotropy is observed after implantation. This anisotropy has no influence on the unidirectional anisotropy associated to the exchange bias effect. Our study identifies a narrow implantation fluence window in which exchange bias by oxygen ion implantation is established. With increasing oxygen fluence, an increase in the magnitude of the exchange bias effect for higher fluences and, finally, a saturation of the exchange bias effect is observed in the studied fluence window. Moreover, the particular shape of the measured hysteresis loop is ascribed to a distribution of switching fields, which results from the implantation depth profile of oxygen throughout the Co film.
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75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
61.72.up Other materials
68.55.J- Morphology of films
61.72.Mm Grain and twin boundaries

The morphology of Al-based submicron Josephson junction

V. V. Roddatis, U. Hübner, B. I. Ivanov, E. Il’ichev, H.-G. Meyer, M. V. Koval’chuk, and A. L. Vasiliev

J. Appl. Phys. 110, 123903 (2011); http://dx.doi.org/10.1063/1.3670003 (4 pages)

Online Publication Date: 19 December 2011

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We present a detailed study of the microstructure of submicron Al/Al−O/Al Josephson junctions fabricated by the conventional shadow evaporation technique. The morphology of the dielectric Al−O layer, which plays the key role for junction transport properties, has been investigated by making use of high resolution electron microscopy. We demonstrate, that the flatness and thickness of the aluminum oxide layer strongly depends on its grain structure. The most pronounced thickness deviations are observed in the vicinity of so-called “triple points,” where the grain boundary crosses the interlayer, forming a two-grain contact. Additionally we show that even for the single-grain contact, the Al/Al−O interface is not atomically flat, which can cause additional flicker noise at subkelvin temperatures.
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74.50.+r Tunneling phenomena; Josephson effects
72.70.+m Noise processes and phenomena
61.72.Mm Grain and twin boundaries
61.72.-y Defects and impurities in crystals; microstructure

Nucleation of ReBa2Cu3Ox (Re = rare-earth) during high-rate metal-organic chemical vapor deposition growth

Vyacheslav F. Solovyov, Qiang Li, Y. Chen, A. Guevara, T. Shi, and V. Selvamanickam

J. Appl. Phys. 110, 123904 (2011); http://dx.doi.org/10.1063/1.3670030 (6 pages)

Online Publication Date: 19 December 2011

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Large-scale, high-rate epitaxial growth technology for the second-generation superconducting wire brings unique technological challenges for the thin-film coating industry. One of the most difficult steps of the process is controlling nucleation of a complex compound over a km-long low-cost oxide template. Here, we analyze early stages of industrial-scale epitaxial metal organic chemical vapor deposition (MOCVD) growth of ReBa2Cu3Ox (REBCO, Re = rare-earth) on buffered metal substrates. The nucleation event is detected by high-flux synchrotron X-ray diffraction and confirmed by atomic force microscopy. REBCO nuclei exhibit a strong preference for edges of the buffer grain, indicating that (001) steps of the buffer grains are preferred nucleation sites. It is concluded that random nucleation of REBCO is caused by agglomerates of small buffer grains.
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74.78.-w Superconducting films and low-dimensional structures
74.72.-h Cuprate superconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Variable angle magnetometry for exchange-coupled multilayers with in-plane and perpendicular anisotropy

N. L. Yakovlev, R. Sbiaa, and S. N. Piramanayagam

J. Appl. Phys. 110, 123905 (2011); http://dx.doi.org/10.1063/1.3665191 (6 pages)

Online Publication Date: 20 December 2011

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Co/Pd multilayers, including exchange coupled structures with thin layers of Co with different exchange coupling strengths, were investigated using variable angle magnetometry. Hysteresis loops of perpendicular and in-plane components of magnetization were measured using two-axis vibrating sample magnetometer and variable angle magneto-optical Kerr effect system. Co/Pd multilayer structures have high perpendicular remanence and the magnetization reversal mechanism is mainly domain wall motion. For the systems with Co and Co/Pd multilayers, the magnetization reversals of high coercivity (Co/Pd) multilayer and low coercivity Co single layer were found to be independent, if the soft layer (Co) is thicker than 2 nm. In the samples with a high degree of exchange coupling, the magnetization of the multilayer has two components, normal and in-plane. The proportion between them varies with field strength and orientation of the sample. Taking this into account, it was possible to determine the anisotropy energy of the (Co/Pd) multilayer, which is in the range of 400–420 kJ/m3.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
78.20.Ls Magneto-optical effects
75.60.Jk Magnetization reversal mechanisms
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Enhancement of the upper critical field in codoped iron-arsenic high-temperature superconductors

F. Weickert, M. Nicklas, W. Schnelle, J. Wosnitza, A. Leithe-Jasper, and H. Rosner

J. Appl. Phys. 110, 123906 (2011); http://dx.doi.org/10.1063/1.3665700 (6 pages)

Online Publication Date: 21 December 2011

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We present the first study of codoped iron-arsenide superconductors of the 122 family (Sr/Ba)1-x Kx Fe2-y Coy As2 with the purpose to increase the upper critical field Hc2 compared to single doped Sr/BaFe2 As2 materials. Hc2 was investigated by measuring the magnetoresistance in high pulsed magnetic fields up to 64 T. We find, that Hc2 extrapolated to T = 0 is indeed enhanced significantly to ≈ 90 T for polycrystalline samples of Ba0.55 K0.45 Fe1.95 Co0.05 As2 compared to ≈ 75 T for Ba0.55 K0.45 Fe2 As2 and BaFe1.8 Co0.2 As2 single crystals. Codoping thus is a promising way for the systematic optimization of iron-arsenic based superconductors for magnetic-field and high-current applications.
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74.25.Op Mixed states, critical fields, and surface sheaths
74.62.Dh Effects of crystal defects, doping and substitution
74.81.-g Inhomogeneous superconductors and superconducting systems, including electronic inhomogeneities
61.72.up Other materials
74.25.F- Transport properties
72.20.My Galvanomagnetic and other magnetotransport effects

Exchange bias and enhanced coercivity in phase separated La0.45Sr0.55MnO3 and Pr0.55(Ca0.65S0.35)0.45MnO3 films

Byeong-geon Kim, Ashvani Kumar, Sanghoon Ki, Sangwoo Kim, and Joonghoe Dho

J. Appl. Phys. 110, 123907 (2011); http://dx.doi.org/10.1063/1.3666023 (5 pages)

Online Publication Date: 21 December 2011

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An epitaxial La0.45Sr0.55MnO3 (LSMO) film, which has a paramagnetic-to-ferromagnetic transition at ∼260 K and a partial ferromagnetic-to-antiferromagnetic transition at ∼150 K upon cooling, was synthesized on (001) (La0.18Sr0.82)(Al0.59Ta0.41)O3 with pulsed laser deposition. The x-ray diffraction pattern showed interference fringes around the (002) LSMO peak, indicating a successful growth of a high quality and smooth film. When the LSMO film was cooled down in a magnetic field of 5 kOe, it exhibited a large exchange bias (HEX) of ∼490 Oe and an enhanced coercivity (HC) of ∼800 at 10 K, respectively. The observed exchange bias effect, which is associated with phase coexistence of the ferromagnetic (FM) and antiferromagnetic (AF) phases, displayed a distinctive training effect, which is probably due to a competing spin order at the boundary between the FM and AF phases. Similarly, we also observed a large exchange bias and enhanced coercivity in a phase separated Pr0.55(Ca0.65S0.35)0.45MnO3 film, which had simultaneous FM and AF transitions at ∼150 K. Our results show that the exchange bias effect can be induced even in single layer films with a magnetic phase coexistence state of FM and AF phases, which may be exploited as a very simple structure for spin-valve devices.
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75.50.Vv High coercivity materials
75.70.Ak Magnetic properties of monolayers and thin films
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
81.15.Fg Pulsed laser ablation deposition
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.-s Critical-point effects, specific heats, short-range order

Modulating magnetism of ZnO:C with vacancy and substitution

W. Q. Li, J. X. Cao, J. W. Ding, and Xuedong Hu

J. Appl. Phys. 110, 123908 (2011); http://dx.doi.org/10.1063/1.3666051 (7 pages)

Online Publication Date: 21 December 2011

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We investigate magnetic properties of C-doped ZnO (ZnO:C) systems in the presence of defects such as O and Zn vacancies and substitution of Li and Al for Zn. Specifically, we use density functional theory to calculate electronic structures, local magnetic moment, polarization energy, etc., for these material systems. We find that the magnetic moment at a C atom in ZnO:C can be suppressed by the presence of an oxygen vacancy and enhanced by the presence of a Zn vacancy, and depends sensitively on the distance between the C atom and such a vacancy. Our results provide an explanation of the experimental observation that the C-induced magnetic moment in ZnO:C varies widely, from 1.3 μB to 3.0 μB. We also show that Lithium and Aluminum implantations in ZnO:C can effectively tailor the magnetic moment and ferromagnetism, regardless of the distance between the implanted atoms and the C atoms. In particular, our results indicate that Curie temperature in a ZnO:C system can potentially be increased by Al implantation.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
61.72.jd Vacancies
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Nr Semiconductor compounds

Finite-size scaling relation of the Curie temperature in barium hexaferrite platelets

Jun Wang, Fan Zhao, Wei Wu, and Guo-meng Zhao

J. Appl. Phys. 110, 123909 (2011); http://dx.doi.org/10.1063/1.3670964 (5 pages) | Cited 1 time

Online Publication Date: 22 December 2011

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High-temperature magnetic measurements were carried out on barium ferrite BaFe12 O19 nanoparticles coated with amorphous silica. We find that the Curie temperature of this material decreases with decreasing particle size, in agreement with the finite-size scaling theory. In contrast to what one expects, the observed particle-size dependence of the Curie temperature does not follow a finite-size scaling relation for a zero-dimensional magnetic system. Instead, the data follow a finite-size scaling relation for a two-dimensional magnetic system with the scaling exponent ν = 0.78±0.06. The validity of the two-dimensional scaling relation in this material is due to the fact that the nanoparticles have a platelet-like shape.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Gg Ferrimagnetics
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.Fk Domain structures in nanoparticles

Anomalies of inverse direct current susceptibility in spin-chain compounds Ca3Co2-xMnxO6

S. S. Sheng, Z. W. Ouyang, N. M. Xia, J. Chen, Y. Y. Wu, Z. C. Xia, and L. Li

J. Appl. Phys. 110, 123910 (2011); http://dx.doi.org/10.1063/1.3671025 (5 pages) | Cited 1 time

Online Publication Date: 22 December 2011

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Various types of anomalies of inverse dc susceptibility (H/M) have been observed in spin-chain compounds Ca3Co2-xMnxO6. For x = 1.0, the H/M curve exhibits a field-dependent downturn from the paramagnetic Curie-Weiss behavior, signaling the presence of a Griffiths-like cluster phase. A slight decrease of x leads to a rapid suppression of the Griffiths-like singularity, followed by an appearance of field-independent, non-Griffiths-like behavior at x = 0.92. This feature persists until x = 0.33, at and below which the H/M curve presents an unambiguous, field-independent upturn from the Curie-Weiss law. These Mn-doping-dependent anomalies of H/M curves are associated with complex competing magnetic interactions.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.20.Ck Nonmetals
61.72.up Other materials
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)

Characterizing longitudinal and transverse relaxation rates of ferrofluids in microtesla magnetic fields

Ming-Jye Chen, Shu-Hsien Liao, Hong-Chang Yang, Hsin-Yi Lee, Yi-Jia Liu, Hsin-Hsien Chen, Herng-Er Horng, and Shieh-Yueh Yang

J. Appl. Phys. 110, 123911 (2011); http://dx.doi.org/10.1063/1.3671420 (6 pages) | Cited 4 times

Online Publication Date: 23 December 2011

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Shortening spin-lattice relaxation rates (1/T1) or spin-spin relaxation rates (1/T2) is the purpose of magnetic resonance imaging contrast agents. In this work, an ultralow field nuclear magnetic resonance spectrometer and imager are set up to characterize the spin relaxation rates of Fe3O4 superparamagnetic iron oxide (SPIO) for image contrast. It was found that both 1/T1 and 1/T2 increase linearly when the magnetic susceptibility χ of SPIO increases by increasing the concentration of SPIO dispersed in water. In an applied field, magnetic moments of SPIO generate microscopic field gradients that weaken the field homogeneity, in turn de-phasing the proton’s nuclear spin and enhancing the relaxation rates. A T1-contrast image is demonstrated, using SPIO as the contrast agent and high-Tc superconducting quantum interference devices as the detector. T1-contrast imaging in microtesla fields might provide a potential modality for discriminating cancer.
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76.60.Es Relaxation effects
82.56.Na Relaxation
75.30.Cr Saturation moments and magnetic susceptibilities
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.50.Mm Magnetic liquids
75.50.Tt Fine-particle systems; nanocrystalline materials

Remote domain wall chirality measurement via stray field detection

M. A. Bashir, M. T. Bryan, D. A. Allwood, T. Schrefl, J. S. Claydon, G. Burnell, and C. H. Marrows

J. Appl. Phys. 110, 123912 (2011); http://dx.doi.org/10.1063/1.3671615 (5 pages) | Cited 1 time

Online Publication Date: 23 December 2011

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We have calculated magnetostatic fields from transverse and vortex magnetic domain walls in a planar Ni81Fe19 nanowire using micromagnetic simulations. Magnetic fields were calculated either side of the wire in the wire plane. Asymmetries in the stray field from the two types of domain walls can be understood in terms of monopole, dipole and quadrupole contributions. These calculations indicate the required sensitivity of a nearby sensor for domain wall detection or being able to distinguish domain wall character. We demonstrate this by modeling the electrical response of a magnetoresistive multilayer element to a domain wall in a nearby magnetic nanowire.
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75.60.Ch Domain walls and domain structure
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.75.Fk Domain structures in nanoparticles
68.65.Ac Multilayers

Hysteretic spin-wave excitation in spin-torque oscillators as a function of the in-plane field angle: A micromagnetic description

G. Finocchio, A. Prattella, G. Consolo, E. Martinez, A. Giordano, and B. Azzerboni

J. Appl. Phys. 110, 123913 (2011); http://dx.doi.org/10.1063/1.3671632 (6 pages)

Online Publication Date: 23 December 2011

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This paper describes a full micromagnetic characterization of the magnetization dynamics driven by spin-polarized current in anisotropic spin-torque oscillators (STOs). For field angles approaching the hard in-plane axis, the excited mode is uniform and a super-critical Hopf-bifurcation takes place at the critical current density JC. For field angles close to the easy axis of the free layer, the excited mode is localized (non-uniform) and a sub-critical Hopf-bifurcation occurs at JC. In this latter region, a hysteretic behaviour is, therefore, found. We demonstrate numerically that the non-linearities of the STO are strongly reduced when the oscillation frequency at the critical current is near the ferromagnetic resonance (FMR) frequency computed at zero bias current, and in particular, this condition corresponds to the field orientation at which a minimum in the FMR-frequency is achieved.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.78.-n Magnetization dynamics
72.25.-b Spin polarized transport
75.30.Gw Magnetic anisotropy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Bi-quadratic interlayer exchange coupling in Co2MnSi/Ag/Co2MnSi pseudo spin-valve

Hari S. Goripati, Masamitsu Hayashi, T. Furubayashi, T. Taniguchi, H. Sukegawa, Y. K. Takahashi, and K. Hono

J. Appl. Phys. 110, 123914 (2011); http://dx.doi.org/10.1063/1.3671634 (7 pages) | Cited 1 time

Online Publication Date: 23 December 2011

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Bi-quadratic interlayer exchange coupling is found below 100 K in a Co2MnSi/Ag/Co2MnSi current-perpendicular-to-plane pseudo spin valves. The bi-quadratic coupling constant J2 was estimated to be ∼−0.30 erg/cm2 at 5 K and the strong temperature dependence of the coupling strength points its likely origin to the “loose spin” model. Application of current of ∼2 × 107 A/cm2 below 100 K leads to an increase in the magnetoresistance (MR), indicating current induced antiparallel alignment of the two magnetic layers. These results strongly suggest that the presence of the bi-quadratic interlayer exchange coupling causes the reduction of the magnetoresistance at low temperature and illustrates the importance of understanding the influence of interlayer exchange coupling on magnetization configuration in magnetic nanostructures.
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81.05.Bx Metals, semimetals, and alloys
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
75.30.Et Exchange and superexchange interactions
75.47.Np Metals and alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Magnetic properties of Fe chains on Cu2N/Cu(100): A density functional theory study

Jeremy W. Nicklas, Amita Wadehra, and John W. Wilkins

J. Appl. Phys. 110, 123915 (2011); http://dx.doi.org/10.1063/1.3672444 (4 pages)

Online Publication Date: 27 December 2011

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We present a density functional study of the magnetic properties of Fe adatoms on Cu2N/Cu(100) surface. The magnetic anisotropy energies of a single Fe atom are in excellent agreement with the available experiments. Our results for the spin densities and exchange coupling strengths for Fe dimer and trimer establish antiferromagnetic configuration to be the ground state due to predominant superexchange interaction mediated by nitrogen atoms in this system.
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75.30.Et Exchange and superexchange interactions
75.30.Ds Spin waves
75.30.Gw Magnetic anisotropy
75.50.Ee Antiferromagnetics
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
68.43.Fg Adsorbate structure (binding sites, geometry)

Tunable fringe magnetic fields induced by converse magnetoelectric coupling in a FeGa/PMN-PT multiferroic heterostructure

Trifon Fitchorov, Yajie Chen, Bolin Hu, Scott M. Gillette, Anton Geiler, Carmine Vittoria, and Vincent G. Harris

J. Appl. Phys. 110, 123916 (2011); http://dx.doi.org/10.1063/1.3672822 (4 pages)

Online Publication Date: 27 December 2011

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The fringe magnetic field, induced by magnetoelectric coupling in a bilayer Fe-Ga/Pb(Mg1/3Nb2/3)O3_PbTiO3 (PMN-PT) multifunctional composite, was investigated. The induced external field is characterized as having a butterfly hysteresis loop when tuned by an applied electric field. A tuning coefficient of the electrically induced fringe magnetic field is derived from the piezoelectric and magnetostrictive properties of the composite. A measured maximum tuning coefficient, 4.5 Oe/(kV cm−1), is found to agree well with theoretical prediction. This work establishes a foundation in the design of transducers based on the magnetoelectric effect.
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75.85.+t Magnetoelectric effects, multiferroics
77.80.Dj Domain structure; hysteresis
77.65.-j Piezoelectricity and electromechanical effects
75.80.+q Magnetomechanical effects, magnetostriction

Weak d0 magnetism in C and N doped ZnO

Anh Pham, M. H. N. Assadi, Y. B. Zhang, A. B. Yu, and S. Li

J. Appl. Phys. 110, 123917 (2011); http://dx.doi.org/10.1063/1.3669491 (7 pages)

Online Publication Date: 27 December 2011

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We present an ab initio study of carbon and nitrogen substituting oxygen in zinc oxide structure. Detailed spin-polarized total-energy calculations of the various defect and dopant at different charge states and geometries indicate a non-zero spin magnetic moment only found from the CO-2 while NO shows no sign of localized magnetic moment. It is also revealed that CO has a tendency towards forming C2 complexes inside the ZnO structure with very weak antiferromagnetic spin arrangement. Furthermore, it was found that oxygen vacancy and hydrogen interstitial could not induce ferromagnetism in C doped ZnO.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Ee Antiferromagnetics
75.50.Pp Magnetic semiconductors
61.72.jd Vacancies
61.72.jj Interstitials
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Nanomagnetic engineering of the properties of domain wall atom traps

T. J. Hayward, A. D. West, K. J. Weatherill, T. Schrefl, I. G. Hughes, and D. A. Allwood

J. Appl. Phys. 110, 123918 (2011); http://dx.doi.org/10.1063/1.3671631 (8 pages) | Cited 1 time

Online Publication Date: 29 December 2011

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We have used the results of micromagnetic simulations to investigate the effects of nanowire geometry and domain wall magnetization structure on the characteristic parameters of magnetic atom traps formed by domain walls in planar ferromagnetic nanowires. It is found that when traps are formed in the near-field of a domain wall both nanowire geometry and wall structure have a substantial effect on trap frequency (how tightly atoms are spatially confined) and adiabaticity (how closely the atoms’ magnetic moments track the applied field direction within the trap). We also show that in certain regimes a trap’s depth depends only on the amplitude of an externally applied rotating magnetic field, thus allowing it to be tuned independently of the trap’s other critical parameters.
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75.60.Ch Domain walls and domain structure
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
73.21.Hb Quantum wires
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.50.Tt Fine-particle systems; nanocrystalline materials

Griffiths phase-like behavior and spin-phonon coupling in double perovskite Tb2NiMnO6

Harikrishnan S. Nair, Diptikanta Swain, Hariharan N., Shilpa Adiga, Chandrabhas Narayana, and Suja Elzabeth

J. Appl. Phys. 110, 123919 (2011); http://dx.doi.org/10.1063/1.3671674 (6 pages) | Cited 1 time

Online Publication Date: 29 December 2011

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The Griffiths phase-like features and the spin-phonon coupling effects observed in Tb2NiMnO6 are reported. The double perovskite compound crystallizes in monoclinic P21/n space group and exhibits a magnetic phase transition at Tc ∼ 111 K as an abrupt change in magnetization. A negative deviation from ideal Curie--Weiss law exhibited by 1/χ(T) curves and less-than-unity susceptibility exponents from the power-law analysis of inverse susceptibility are reminiscent of Griffiths phase-like features. Arrott plots derived from magnetization isotherms support the inhomogeneous nature of magnetism in this material. The observed effects originate from antiferromagnetic interactions that arise from inherent disorder in the system. Raman scattering experiments display no magnetic-order-induced phonon renormalization below Tc in Tb2NiMnO6, which is different from the results observed in other double perovskites and is correlated to the smaller size of the rare earth. The temperature evolution of full-width-at-half-maximum for the stretching mode at 645 cm−1 presents an anomaly that coincides with the magnetic transition temperature and signals a close connection between magnetism and lattice in this material.
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63.20.K- Phonon interactions
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.30.Hv Other nonmetallic inorganics

Uncompensated antiferromagnetic moments in Mn-Ir/FM (FM = Ni-Co, Co-Fe, Fe-Ni) bilayers: Compositional dependence and its origin

Hirokazu Takahashi, Yohei Kota, Masakiyo Tsunoda, Tetsuya Nakamura, Kenji Kodama, Akimasa Sakuma, and Migaku Takahashi

J. Appl. Phys. 110, 123920 (2011); http://dx.doi.org/10.1063/1.3672450 (9 pages)

Online Publication Date: 30 December 2011

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Ferromagnetic (FM) material dependence of the uncompensated (UC) antiferromagnetic (AF) moments in AF/FM exchange biased bilayers has been studied using the x-ray magnetic circular dichroism technique in the AF/FM (AF = γ-Mn-Ir, FM = Ni-Co, Co-Fe, Fe-Ni) bilayers. The direction and magnitude of the UC-Mn moment change significantly when the composition of the FM layer changes. The crystal structure of the FM layer affects the magnitude of the UC-Mn moments. The UC-Mn moments and the FM moments of Fe-rich alloys prefer the anti-parallel alignment. Conversely, the UC-Mn moments align parallel to the FM moments in Co-rich or Ni-rich regions. A first-principles calculation pertaining to the L12-Mn3Ir/FM (FM = Ni4-nCon, Co4-nFen, Fe4-nNin; n = 0, 1, 2, 3) bilayer system was carried out to characterize the UC-Mn moments near the interface. It was found that the UC-Mn moments originate from the reorientation of the magnetic moments of Mn and other ferromagnetic atoms near the AF/FM interface. The calculated result for the compositional dependence of the UC-Mn moment is in good agreement with the obtained experimental data. As a result, the dependence of the UC-Mn moment on the composition of the FM layer can be explained qualitatively based on the model that the band filling fraction modifies the direction and the magnitude of exchange coupling between AF and FM atoms, depending on the crystal structure and the composition of the FM layer.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Ee Antiferromagnetics
78.20.Ls Magneto-optical effects
75.30.Et Exchange and superexchange interactions
75.50.Bb Fe and its alloys
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Transport properties of superconducting MgB2 composites with carbon-encapsulated Fe nanospheres

Viorel Sandu, Gheorghe Aldica, Stelian Popa, Petre Badica, Elena Cimpoiasu, Florian Dumitrache, and Elena Sandu

J. Appl. Phys. 110, 123921 (2011); http://dx.doi.org/10.1063/1.3672820 (7 pages)

Online Publication Date: 30 December 2011

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We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm2 for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain boundaries and/or on point defects.
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74.25.F- Transport properties
74.25.Ha Magnetic properties including vortex structures and related phenomena
75.75.-c Magnetic properties of nanostructures
74.25.Sv Critical currents
74.81.Bd Granular, melt-textured, amorphous, and composite superconductors
74.10.+v Occurrence, potential candidates
74.62.Dh Effects of crystal defects, doping and substitution

Low field anisotropic colossal magnetoresistance in Sm0.53Sr0.47MnO3 thin films

Manoj K. Srivastava, M. P. Singh, Amarjeet Kaur, F. S. Razavi, and H. K. Singh

J. Appl. Phys. 110, 123922 (2011); http://dx.doi.org/10.1063/1.3672838 (5 pages)

Online Publication Date: 30 December 2011

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Sm0.53Sr0.47MnO3 (SSMO) thin films (thicknesses ∼200 nm) were deposited by on-axis dc magnetron sputtering on the single crystal LSAT (001) substrates. These films are oriented along the out of plane c-direction. The ferromagnetic and insulator-metal transition occurs at TC ∼ 96 and TIM ∼ 91 K, respectively. The magnetization easy axis is observed to lie in the plane of the film while the magnetic hard axis is found to be along the normal to this. The magnetotransport of the SSMO films, which was measured as a function of angle (θ) between the magnetic field (H) and plane of the film, shows colossal anisotropy. Magnetoresistance (MR) decreases drastically as θ increases from 0° (H//easy axis) to 90° (H//hard axis). The out-of-plane anisotropic MR is as high as 88% at H = 3.6 kOe and 78 K. The colossal anisotropy has been explained in terms of the magnetic anisotropies at play and the magnetic domain motion in applied magnetic field.
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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)
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.30.Gw Magnetic anisotropy
75.60.Ch Domain walls and domain structure
75.47.Gk Colossal magnetoresistance

Thermomagnetic conversion efficiencies for ferromagnetic materials

Chin-Jui Hsu (許晉睿), Samuel M. Sandoval, Kyle P. Wetzlar, and Gregory P. Carman

J. Appl. Phys. 110, 123923 (2011); http://dx.doi.org/10.1063/1.3672844 (7 pages) | Cited 1 time

Online Publication Date: 30 December 2011

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The theory of thermomagnetic generation is reviewed and an efficiency analysis using experimentally measured magneto-thermal properties of 3d transitional and 4f rare earth ferromagnetic elements is presented in this study. While theoretical results suggest that 55% of Carnot efficiency is possible, experimental data indicate values smaller than 25% of Carnot efficiency unless large magnetic field (e.g., Ha ∼ 80 kOe) is applied. For smaller magnetic fields representative of NdFeB permanent magnets (e.g., Ha = 3 kOe), the largest efficiencies are obtained for operating ferromagnetic materials over a smaller temperature difference (ΔT = 5 K). Furthermore, single crystal materials are found to have superior efficiencies, as do elements that undergo an order-to-order phase transition. Both of these later results relate to increased magnetization changes over a given ΔT. These results are subsequently used to postulate that a single domain structure will produce larger efficiencies due to the higher magnetization present over a wide range of magnetic fields when compared to multi-domain materials. Calculations for a Gd single domain suggest efficiencies on the order of 30% are possible, representing a threefold increase from multi-domain Gd at relatively small magnetic fields.
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72.15.Jf Thermoelectric and thermomagnetic effects
75.30.Sg Magnetocaloric effect, magnetic cooling
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Ww Permanent magnets
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Dipolar interactions in magnetic nanowire aggregates

Thomas Maurer, Fatih Zighem, Weiqing Fang, Frédéric Ott, Grégory Chaboussant, Yaghoub Soumare, Kahina Ait Atmane, Jean-Yves Piquemal, and Guillaume Viau

J. Appl. Phys. 110, 123924 (2011); http://dx.doi.org/10.1063/1.3671540 (6 pages) | Cited 1 time

Online Publication Date: 30 December 2011

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We investigate the effect of dipolar interactions on the magnetic properties of nanowire aggregates. Micromagnetic simulations show that dipolar interactions between wires are not detrimental to the high coercivity properties of magnetic nanowires, even in very dense aggregates. This is confirmed by experimental magnetization measurements and Henkel plots, which show that the dipolar interactions are small. Indeed, we show that misalignment of the nanowires in aggregates leads to a coercivity reduction of only 30%. Direct dipolar interactions between nanowires, even as close as 2 nm, have small effects (maximum coercivity reduction of ∼15%) and are very sensitive to the detailed geometrical arrangement of the wires. These results strengthen the potential of elongated single domain particles for applications requiring high coercivity properties.
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75.75.-c Magnetic properties of nanostructures
81.07.Gf Nanowires
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
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