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12 May 2008

Volume 92, Issue 19, Articles (19xxxx)

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Appl. Phys. Lett. 92, 193301 (2008); http://dx.doi.org/10.1063/1.2920199 (3 pages)

Wei Chen, Hong Liang Zhang, Han Huang, Lan Chen, and Andrew Thye Shen Wee
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GaMnAs-based hybrid multiferroic memory device

M. Overby, A. Chernyshov, L. P. Rokhinson, X. Liu, and J. K. Furdyna

Appl. Phys. Lett. 92, 192501 (2008); http://dx.doi.org/10.1063/1.2917481 (3 pages) | Cited 50 times

Online Publication Date: 12 May 2008

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We report a nonvolatile hybrid multiferroic memory cell with electrostatic control of magnetization based on strain-coupled GaMnAs ferromagnetic semiconductor and a piezoelectric material. We use the crystalline anisotropy of GaMnAs to store information in the orientation of the magnetization along one of the two easy axes, which is monitored via transverse anisotropic magnetoresistance. The magnetization orientation is switched by applying voltage to the piezoelectric material and tuning magnetic anisotropy of GaMnAs via the resulting stress field.
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85.50.Gk Non-volatile ferroelectric memories
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Stoichiometric growth of high Curie temperature heavily alloyed GaMnAs

S. Mack, R. C. Myers, J. T. Heron, A. C. Gossard, and D. D. Awschalom

Appl. Phys. Lett. 92, 192502 (2008); http://dx.doi.org/10.1063/1.2927481 (3 pages) | Cited 21 times

Online Publication Date: 13 May 2008

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Heavily alloyed, 100 nm Ga1−xMnxAs (x>0.1) films are obtained via low-temperature molecular beam epitaxy by utilizing a combinatorial technique which allows systematic control of excess arsenic during growth. Reproducible electronic, magnetic, and structural properties are optimized in a narrow range of stoichiometric growth conditions. In contrast to a prediction of the Zener model of hole-mediated ferromagnetism, the Curie temperature of the stoichiometric material is independent of x (for x>0.1), while substitutional Mn content is proportional to x within a large window of growth conditions.
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75.70.-i Magnetic properties of thin films, surfaces, and interfaces
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
68.55.ag Semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors

Temperature dependence of the interface moments in Co2MnSi thin films

N. D. Telling, P. S. Keatley, L. R. Shelford, E. Arenholz, G. van der Laan, R. J. Hicken, Y. Sakuraba, S. Tsunegi, M. Oogane, Y. Ando, K. Takanashi, and T. Miyazaki

Appl. Phys. Lett. 92, 192503 (2008); http://dx.doi.org/10.1063/1.2927482 (3 pages) | Cited 3 times

Online Publication Date: 13 May 2008

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X-ray magnetic circular dichroism (XMCD) is utilized to explore the temperature dependence of the interface moments in Co2MnSi (CMS) thin films capped with aluminum. By increasing the thickness of the capping layer, we demonstrate enhanced interface sensitivity of the measurements. L21-ordered CMS films show no significant temperature dependence of either the Co or Mn interface moments. However, disordered CMS films show a decreased moment at low temperature possibly caused by increased Mn–Mn antiferromagnetic coupling. It is suggested that for ordered L21 CMS films the temperature dependence of the tunneling magnetoresistance is not related to changes in the interface moments.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.70.Ak Magnetic properties of monolayers and thin films
78.20.Ls Magneto-optical effects
78.70.Dm X-ray absorption spectra
75.47.-m Magnetotransport phenomena; materials for magnetotransport
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)

In-field magnetic force microscope study of dipolar interaction in an ideally ordered Co nanorod array fabricated using nanoimprint lithography

T. Wang, Y. Wang, Y. Fu, T. Hasegawa, T. Washiya, H. Saito, S. Ishio, F. S. Li, H. Oshima, K. Itoh, K. Nishio, and H. Masuda

Appl. Phys. Lett. 92, 192504 (2008); http://dx.doi.org/10.1063/1.2928202 (3 pages) | Cited 2 times

Online Publication Date: 13 May 2008

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We have performed experimental studies and numerical calculations on the dipolar interaction in an ideally ordered hexagonal nanorod array. The dipolar field in an ordered Co nanorod array was measured using an in-field magnetic force microscope (MFM) with a controlled applied field. The average dipolar fields of neighboring nanorods acting on a center nanorod were calculated by approximating the monopole with a magnetic charge m = MSS on the end of each nanorod. The largest dipolar field obtained from numerical calculation is agreement with that from in-field MFM measurements.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
81.16.Nd Micro- and nanolithography
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Gw Magnetic anisotropy

Giant magnetocaloric effect in the Ising antiferromagnet DySb

W. J. Hu, J. Du, B. Li, Q. Zhang, and Z. D. Zhang

Appl. Phys. Lett. 92, 192505 (2008); http://dx.doi.org/10.1063/1.2928233 (3 pages) | Cited 43 times

Online Publication Date: 13 May 2008

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The magnetic phase transitions and the magnetocaloric effect in the Ising antiferromagnet DySb have been studied. A field-induced sign change of the magnetocaloric effect has been observed which is related to a first-order field-induced metamagnetic transition from the antiferromagnetic to the ferromagnetic states at/below the Néel temperature TN, while the negative field-induced entropy change is found to be associated with the first-order magnetic transition from the paramagnetic to the ferromagnetic states above TN. The large magnetic-entropy change (−20.6 J/kg K at 11 K for a field change of 7 T), together with small hysteresis, suggests that DySb could be a potential material for magnetic refrigeration in the low-temperature range.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.10.Hk Classical spin models
75.50.Ee Antiferromagnetics
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Room temperature soft ferromagnetism in the nanocrystalline form of YCo2, A well-known bulk Pauli paramagnet

S. Narayana Jammalamadaka, E. V. Sampathkumaran, V. Satya Narayana Murthy, and G. Markandeyulu

Appl. Phys. Lett. 92, 192506 (2008); http://dx.doi.org/10.1063/1.2929380 (3 pages) | Cited 5 times

Online Publication Date: 13 May 2008

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The Laves phase compound YCo2 is a well-known exchange-enhanced Pauli paramagnet. We report here that, in the nanocrystalline form, this compound interestingly is an itinerant ferromagnet at room temperature with a low coercive field. The magnitude of the saturation moment (about 1μB per formula unit) is large enough to infer that the ferromagnetism is not a surface phenomenon in these nanocrystallites. Since these ferromagnetic nanocrystallites are easy to synthesize with a stable form in air, one can explore applications, particularly where the hysteresis is a disadvantage.
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75.20.En Metals and alloys
75.50.Cc Other ferromagnetic metals and alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Measurement and noise performance of nano-superconducting-quantum-interference devices fabricated by focused ion beam

L. Hao, J. C. Macfarlane, J. C. Gallop, D. Cox, J. Beyer, D. Drung, and T. Schurig

Appl. Phys. Lett. 92, 192507 (2008); http://dx.doi.org/10.1063/1.2917580 (3 pages) | Cited 41 times

Online Publication Date: 14 May 2008

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Science and industry demand ever more sensitive measurements on ever smaller systems, as exemplified by spintronics, nanoelectromechanical system, and spin-based quantum information processing, where single electronic spin detection poses a grand challenge. Superconducting quantum interference devices (SQUIDs) have yet to be effectively applied to nanoscale measurements. Here, we show that a simple bilayer deposition route, combining photolithography with focused ion beam patterning, produces high performance nanoscale SQUIDs. We present results of noise measurements on these nanoSQUIDs which correspond to a magnetic flux sensitivity of around 0.2 μΦ0/Hz1/2. This represents one of the lowest noise values achieved for a SQUID device operating above 1 K.
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07.50.Hp Electrical noise and shielding equipment
85.25.Dq Superconducting quantum interference devices (SQUIDs)

Magnetic flux noise in MgB2 superconductor

Neeraj Khare, D. P. Singh, and Ajai K. Gupta

Appl. Phys. Lett. 92, 192508 (2008); http://dx.doi.org/10.1063/1.2924309 (3 pages) | Cited 1 time

Online Publication Date: 15 May 2008

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Magnetic flux noise in MgB2 polycrystalline sample is measured using a high-TC rf-superconducting quantum interference device in the temperature range of 6–40 K. A small magnetic field ( ∼ 200 mG) was applied while cooling the sample. The flux noise exhibits 1/fα type of behavior with α ∼ 1.0–1.3 and shows enhanced noise around 24 and 37 K. The flux noise seems to originate from thermally activated vortex hopping. The large magnetic noise at 24 K indicates the presence of larger density of pinning sites with energies ∼ 0.061 eV leading to enhanced magnetic fluctuations at temperatures much below TC.
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74.25.Ha Magnetic properties including vortex structures and related phenomena
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)

Joule heating in ferromagnetic nanowires: Prediction and observation

Kab-Jin Kim, Jae-Chul Lee, Sug-Bong Choe, and Kyung-Ho Shin

Appl. Phys. Lett. 92, 192509 (2008); http://dx.doi.org/10.1063/1.2926374 (3 pages) | Cited 12 times

Online Publication Date: 15 May 2008

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We present an analytic theory of the Joule heating in metallic nanowires. The steady state is calculated for heat conduction through the insulation layer and then the transient state is considered from the thermodynamics law. The temperature is predicted to exhibit a quick exponential decay to a steady state within a few tens of nanoseconds. The decay time is linearly dependent on the temperature coefficient and both increase to saturation values with the increasing wire width. The validity of the theory is experimentally confirmed by the in situ measurement of the temperature-dependent electric resistance.
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75.75.-c Magnetic properties of nanostructures
75.50.Cc Other ferromagnetic metals and alloys
75.30.Sg Magnetocaloric effect, magnetic cooling
81.07.Bc Nanocrystalline materials

Investigations of half and full select disturb rates in a toggle magnetic random access memory

R. P. Robertazzi, D. C. Worledge, and J. Nowak

Appl. Phys. Lett. 92, 192510 (2008); http://dx.doi.org/10.1063/1.2931030 (3 pages)

Online Publication Date: 15 May 2008

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Immunity to half select disturbs and self-activated toggle errors were measured for toggle magnetic memory cells at high switching speeds (200 ps rise time pulses, 100 ns in duration). Measurements of the immunity of cells to word line or bit line half selects indicate lifetimes for magnetic reversal of at least 1037 disturbs can be expected for properly designed cells. The lifetime for error free toggling has also been measured. The data indicate that error free toggle switching of a cell can be expected to be greater than 1018 toggles for optimized write fields.
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84.30.Sk Pulse and digital circuits

Observation of spin-glass behavior in antiperovskite Mn3GaN

Bo Song, Jikang Jian, Huiqiang Bao, Ming Lei, Hui Li, Gang Wang, Yanping Xu, and Xiaolong Chen

Appl. Phys. Lett. 92, 192511 (2008); http://dx.doi.org/10.1063/1.2931058 (3 pages) | Cited 13 times

Online Publication Date: 15 May 2008

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The dc magnetization, ac susceptibility, and isothermal remanent magnetization measurements are carried out for antiperovskite Mn3GaN. Strong indication for a frozen state with freeing temperature of Tf at ∼ 133 K is found, determined from the difference between field-cooled and zero-field-cooled magnetizations. The ac susceptibility measurements show a peak around Tf, with the peak position shifting as a function of the driving frequency f and magnetic field H, respectively. Relaxation effects are observed after switching the external magnetic field below Tf. These findings consistently demonstrate that Mn3GaN exhibits a canonical spin-glass state at low temperature.
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75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Sg Magnetocaloric effect, magnetic cooling
75.50.Lk Spin glasses and other random magnets

Spin-dependent density of states in Ga1−xMnxAs probed by tunneling spectroscopy

H. Saito, A. Yamamoto, S. Yuasa, and K. Ando

Appl. Phys. Lett. 92, 192512 (2008); http://dx.doi.org/10.1063/1.2920207 (3 pages) | Cited 3 times

Online Publication Date: 15 May 2008

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We conducted spin-dependent tunneling spectroscopy measurements in Fe/ZnSe/Ga1−xMnxAs magnetic tunnel diodes that had a metal-insulator-semiconductor structure. The tunneling magnetoresistance signal in differential conductance spectra (dI/dV−V) was found to disappear for bias voltages (V) higher than 350 meV for a bias direction where electrons tunnel into Ga1−xMnxAs. This indicates that there is exchange splitting in the density of states of Ga1−xMnxAs up to 350 meV above the Fermi energy.
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73.21.Ac Multilayers
73.20.At Surface states, band structure, electron density of states
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
72.20.My Galvanomagnetic and other magnetotransport effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.Pq Other materials

Oppositely rotating eigenmodes of spin-polarized current-driven vortex gyrotropic motions in elliptical nanodots

Ki-Suk Lee, Young-Sang Yu, Youn-Seok Choi, Dae-Eun Jeong, and Sang-Koog Kim

Appl. Phys. Lett. 92, 192513 (2008); http://dx.doi.org/10.1063/1.2926666 (3 pages) | Cited 5 times

Online Publication Date: 16 May 2008

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The authors found that there exist two different rotational eigenmodes of oppositely rotating sense in spin-polarized current-driven vortex gyrotropic motions in soft magnetic elliptical nanodots. Simple mathematical expressions were analytically calculated by adopting vortex-core (VC)-rotation-sense-dependent dynamic susceptibility tensors based on the linearized Thiele equation [ Phys. Rev. Lett. 30, 230 (1973) ]. The numerical calculations of those analytical expressions were confirmed by micromagnetic simulations, revealing that linear-regime steady-state VC motions driven by any polarized oscillating currents can be interpreted simply by the superposition of the clockwise and counterclockwise rotational eigenmodes. The shape of the orbital trajectories of the two eigenmodes is determined only by the lateral dimension of elliptical dots. Additionally, the orbital radii and phases of the two eigenmodes’ VC motions were found to markedly vary with the frequency of applied currents, particularly across the vortex eigenfrequency and according to the vortex polarization, which results in overall VC motions driven by any polarized oscillating currents.
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73.63.-b Electronic transport in nanoscale materials and structures
72.25.-b Spin polarized transport
75.60.-d Domain effects, magnetization curves, and hysteresis

Equation of motion for a domain wall movement under a nonuniform transverse magnetic field

Chun-Yeol You

Appl. Phys. Lett. 92, 192514 (2008); http://dx.doi.org/10.1063/1.2931069 (3 pages) | Cited 6 times

Online Publication Date: 16 May 2008

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We investigate the dynamics of the domain wall in a nonuniform field driven domain wall motion memory concept. The equation of motion for a transverse Néel-type domain wall under a nonuniform transverse magnetic field is obtained with a collective coordinate approach. The validity of the equation of the motion is confirmed with micromagnetic simulations. We find that the domain wall velocity depends on the domain wall width, Gilbert damping parameter α, saturation magnetization, and the gradient of the field. The domain wall velocity of ∼ 100 m/s is obtained with typical material as Permalloy with a moderate field gradient (100 Oe/μm). It has promising results for memory applications.
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75.60.Ch Domain walls and domain structure
75.40.Mg Numerical simulation studies

Stress induced martensite in epitaxial Ni–Mn–Ga films deposited on MgO(001)

M. Thomas, O. Heczko, J. Buschbeck, L. Schultz, and S. Fähler

Appl. Phys. Lett. 92, 192515 (2008); http://dx.doi.org/10.1063/1.2931082 (3 pages) | Cited 15 times

Online Publication Date: 16 May 2008

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Biaxial tensile stress in epitaxial Ni52Mn23Ga25 films on MgO(001) was measured at different temperatures using an adapted x-ray stress analysis. A stress of up to 105 MPa originates from different thermal expansions of substrate and film and partially from the substrate-film misfit. The film transforms to twinned orthorhombic martensite at 319 K. The stress increases the martensitic transformation temperature of about 63 K. This is in good agreement with respect to the transformation temperature expected from the composition of the film. The presence of biaxial tensile stress leads to twin boundary selection, hence, there are no twin planes perpendicular to the substrate.
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81.30.Kf Martensitic transformations
65.40.De Thermal expansion; thermomechanical effects
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