We have studied the size effect on the L1 0 ordering of FePt ͑001͒ nanoparticles epitaxially grown on MgO ͑001͒. From the dark field images using 110 superlattice spots excited by incident electron beam along ͓11l͔ ͑l =4-6͒, the critical size for L1 0 ordering has been evaluated to be d = 1.5-2 nm below which no ordering occurs. Further, we have taken an electron diffraction pattern of each FePt nanoparticle using a nanometersized electron beam and determined the respective long-range order parameter S by analyzing the superlattice/ fundamental diffraction intensity ratio based on the multislice method. It is found that the order parameter S sharply drops below d ϳ 3 nm and decreases to zero for d Ͻ 2 nm, the result is almost consistent with thermodynamic calculations previously reported. The present work unambiguously shows that the ordering of L1 0 FePt is entirely inhibited when its size is less than d ϳ 2 nm. Such size effect is not so serious for practical applications of FePt to permanent magnets or magnetic recording media because the effect is significant only for d Ͻ 2 nm where the L1 0 FePt would be magnetically unstable due to severe thermal agitation.Equiatomic FePt forms a chemically ordered L1 0 structure below 1300°C, where there are alternating atomic planes of Fe and Pt along the c axis, 1 resulting in a tetragonal distortion in the unit cell and extremely large magnetic anisotropy energy of 7 ϫ 10 7 erg/ cc. 2 L1 0 FePt is expected to exhibit excellent hard magnetic properties even when its size is as small as 3 to 4 nm, due to its large magnetic anisotropy. Therefore much attention has been placed on fabrication and magnetic properties of nanostructured L1 0 FePt both from scientific and technological interests. Especially selfassembled monodisperse FePt nanoparticles are of particular interest for future ultrahigh density heat-assisted magnetic recording. 3,4 Recently a few studies have reported that equiatomic FePt is not transformed into the equilibrium L1 0 order phase when its size is less than several nanometers. 5-12 For example, Takahashi et al. 5 evaluated the long-range order parameter S of FePt granular films and found that the critical size for ordering was d ϳ 3 nm, being somewhat larger than the thermodynamic calculation using optimized nearest-neighbor Lennard-Jones potentials. A very similar critical size has been reported in thermodynamic calculations 9 and Monte Carlo simulations 11 based on the regular lattice Ising model. 13 Such instability of the order phase is mainly due to loss of bonding energy at the surface sites, in addition to other surface effects. 11,14 In the previous experiment on FePt-Al 2 O 3 granular films, 5,6 the order parameter S was determined from the electron diffraction intensity ratio I 110 / I 111 under the assumption of the kinematical diffraction theory, where I 110 and I 111 are the intensity for superlattice 110 and fundamental 111 diffractions, respectively. Furthermore, the size effect was discussed based on the mean particle size d m in spite ...

The electronic structure of the one-dimensional Ni complexes, ͓Ni͑chxn͒ 2 X]X 2 "XϭCl, Br; ͑chxn͒ϭ1R,2R-cyclohexanediamine…, is studied together with the discrete Ni complexes, ͓NiX 2 (͓14͔aneN 4 ͔͒ClO 4 "X ϭCl; Br;͓͑14͔aneN 4 ͒ϭ1,4,8,11-tetraazacyclotetradecane…,using optical spectroscopy, x-ray photoelectron spectroscopy ͑XPS͒ and Auger electron spectroscopy. The optical spectra show that the Br compounds have a smaller gap as compared with the Cl compounds. An analysis using a simple X-Ni-X trimer model on the optical spectra and the Ni 2p XP spectra yields quantitative estimates for the charge transfer ͑CT͒ energy ⌬ and the transfer energy T for discrete and one-dimensional Ni complexes. The analysis on the Ni LVV Auger spectra in conjunction with the valence XP spectra indicates that the average on-side d-d Coulomb energy U in the one-dimensional and discrete Ni complexes is about 5 eV, quite similar to the case of the Ni dihalides. The obtained results demonstrate that the one-dimensional Ni complexes are CT insulators. We discuss the differences in the electronic structures of the one-dimensional Ni complexes compared with the Ni dihalides and the one-dimensional Pt complexes on the basis of the estimated parameter values of ⌬, T, and U.

A stress-operated memory device consisting of an ellipsoidal magnetic particle array and an electrostrictive grid is proposed. In the device, the magnetic state of the particle can be controlled only by the magnetostriction effect. Each particle is located at the intersection of the grid and has an in-plane uniaxial anisotropy. A pair of electric contacts is connected to the end of each wire. In the writing process, the driving voltages are simultaneously applied to two pairs of the selected contacts. This allows to apply a local electric field whose direction and amplitude can be regulated by varying the voltage intensity and polarity. The exerting stress on the magnetic particle results in the linear magnetostriction and hence an additional anisotropy energy in the particle. The in-plane total energy minimum, corresponding to the magnetization direction, follows the local electric field. Consequently the magnetization of the single magnetic particle located at the intersection can therefore be selectively switched.

We have investigated switching behaviors of a spin-vortex core in circular magnetic dots to evaluate them as a new candidate for magnetic memory elements. The spin-vortex core with the lateral dimension of exchange length can take two possible states with spins around the dot center directing either up- or downward. These two spin configurations are very stable and their polarities can be switched by a large vertical field. The mean switching field reaches about 2.5 kOe for Ni80Fe20 dots with 1 μm in diameter and 80 nm in height. This result could open the way for developing a new bistable magnetic memory element utilizing the spin vortex.

Effect of fast neutron irradiation induced defects on the metamagnetic transition In Ce(Fe0.96Ru0.04)2 J. Appl. Phys. 112, 063922 (2012) Electronic structure, optical and magnetic properties of Co2FeGe Heusler alloy films J. Appl. Phys. 112, 063909 (2012) Giant induced magnetic anisotropy In strain annealed Co-based nanocomposite alloys Appl. Phys. Lett. 101, 102408 (2012) Highly zero-biased magnetoelectric response in magnetostrictive/piezoelectric composite J. Appl. Phys. 112, 024504 (2012) Resonance magnetoelectric effects in a layered composite under magnetic and electrical excitations Temperature dependent magnetic properties of vortices trapped in a lithographically patterned Permalloy disk were examined. A large residual magnetization at 5 K was observed in hysteresis curves unlike theoretical prediction. The residual magnetization, coercive field, and initial susceptibility were found to be dependent on temperature. Escaping from the pinning potential was facilitated by the increase of temperature, and the pinning temperature T pin was 9.6 K. The vortex is effectively pinned at the pinning potential when TϽT pin . This physical picture is well supported by the temperature variations of ac susceptibility for the biased dc field. The energy barrier is probably originated from defects such as the edge and surface roughnesses, and irregular grain boundaries in the disk.