Effect of temperature-dependent shape anisotropy on coercivity with aligned Stoner-Wohlfarth soft ferromagnets

The temperature variation effect of shape anisotropy on the coercivity, HC(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni, fcc Co and bcc Fe, are investigated within the framework of Neel-Brown (N-B) analysis. An extended N-B equation is thus proposed,by introducing a single dimensionless correction function, the reduced magnetization, m(\tao) = MS(T)/MS(0), in which \tao = T/TC is the reduced temperature, MS(T) is the saturation magnetization, and TC is the Curie temperature. The factor, m(\tao), accounts for the temperature-dependent effect of the shape anisotropy. The constants, H0 and E0, are for the switching field at zero temperature and the potential barrier at zero field, respectively. According to this newly derived equation, the blocking temperature above which the properties of superparamagnetism show up is described by the expression, TB = E0m^2(\tao)/[kBln(t/t0)], with the extra correction factor m^2(\tao). The possible effect on HC(T) and the blocking temperature, TB, attributed to the downshift of TC resulting from the finite size effect has been discussed also.Comment: 22 pages, 2 figures, 1 table, Accepted by Phys. Rev.

Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCo2_2O4_4

Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase.Comment: 5 pages, 3 figure

Spin-polarized electronic structures and transport properties of Fe-Co alloys

The electrical resistivities of Fe-Co alloys owing to random alloy disorder are calculated using the Kubo-Greenwood formula. The obtained electrical esistivities agree well with experimental data quantitatively at low temperature. The spin-polarization of Fe50Co50 estimated from the conductivity (86%) has opposite sign to that from the densities of the states at the Fermi level (-73%). It is found that the conductivity is governed mainly by s-electrons, and the s-electrons in the minority spin states are less conductive due to strong scattering by the large densities of the states of d-electrons than the majority spin electrons.Comment: 3 pages, 4 figure

Tuning the Curie temperature of FeCo compounds by tetragonal distortion

Combining density-functional theory calculations with a classical Monte Carlo method, we show that for B2-type FeCo compounds tetragonal distortion gives rise to a strong reduction of the Curie temperature $T_{\mathrm{C}}$. The $T_{\mathrm{C}}$ monotonically decreases from 1575 K (for $c/a=1$) to 940 K (for c/a=\sqrtwo). We find that the nearest neighbor Fe-Co exchange interaction is sufficient to explain the $c/a$ behavior of the $T_{\mathrm{C}}$. Combination of high magnetocrystalline anisotropy energy with a moderate $T_{\mathrm{C}}$ value suggests tetragonal FeCo grown on the Rh substrate with $c/a=1.24$ to be a promising material for heat-assisted magnetic recording applications.Comment: 4 pages, 2 figure

Synthesis and characterization of core-shell structure silica-coated Fe29.5Ni70.5 nanoparticles

In view of potential applications of magnetic particles in biomedicine and electromagnetic devices, we made use of the classical Stober method base-catalysed hydrolysis and condensation of tetraethoxysilane (TEOS) to encapsulate FeNi nanoparticles within a silica shell. An original stirring system under high power ultrasounds made possible to disperse the otherwise agglomerated particles. Sonication guaranteed particles to remain dispersed during the Stober synthesis and also improved the efficiency of the method. The coated particles are characterized by electron microscopy (TEM) and spectroscopy (EDX) showing a core-shell structure with a uniform layer of silica. Silica-coating does not affect the core magnetic properties. Indeed, all samples are ferromagnetic at 77 K and room temperature and the Curie point remains unchanged. Only the coercive force shows an unexpected non-monotonous dependence on silica layer thickness.Comment: Regular paper submited to international peer-reveiwed journa

Quantized spin wave modes in magnetic tunnel junction nanopillars

We present an experimental and theoretical study of the magnetic field dependence of the mode frequency of thermally excited spin waves in rectangular shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned from MgO-based magnetic tunnel junctions. The spin wave frequencies were measured using spectrally resolved electrical noise measurements. In all spectra, several independent quantized spin wave modes have been observed and could be identified as eigenexcitations of the free layer and of the synthetic antiferromagnet of the junction. Using a theoretical approach based on the diagonalization of the dynamical matrix of a system of three coupled, spatially confined magnetic layers, we have modeled the spectra for the smallest pillar and have extracted its material parameters. The magnetization and exchange stiffness constant of the CoFeB free layer are thereby found to be substantially reduced compared to the corresponding thin film values. Moreover, we could infer that the pinning of the magnetization at the lateral boundaries must be weak. Finally, the interlayer dipolar coupling between the free layer and the synthetic antiferromagnet causes mode anticrossings with gap openings up to 2 GHz. At low fields and in the larger pillars, there is clear evidence for strong non-uniformities of the layer magnetizations. In particular, at zero field the lowest mode is not the fundamental mode, but a mode most likely localized near the layer edges.Comment: 16 pages, 4 figures, (re)submitted to PR

Dynamics of magnetization coupled to a thermal bath of elastic modes

We study the dynamics of magnetization coupled to a thermal bath of elastic modes using a system plus reservoir approach with realistic magnetoelastic coupling. After integrating out the elastic modes we obtain a self-contained equation for the dynamics of the magnetization. We find explicit expressions for the memory friction kernel and hence, {\em via} the Fluctuation-Dissipation Theorem, for the spectral density of the magnetization thermal fluctuations. For magnetic samples in which the single domain approximation is valid, we derive an equation for the dynamics of the uniform mode. Finally we apply this equation to study the dynamics of the uniform magnetization mode in insulating ferromagnetic thin films. As experimental consequences we find that the fluctuation correlation time is of the order of the ratio between the film thickness, $h$, and the speed of sound in the magnet and that the line-width of the ferromagnetic resonance peak should scale as $B_1^2h$ where $B_1$ is the magnetoelastic coupling constant.Comment: Revised version as appeared in print. 12 pages 9 figure

Magneto-optical Kerr Effect Studies of Square Artificial Spin Ice

We report a magneto-optical Kerr effect study of the collective magnetic response of artificial square spin ice, a lithographically-defined array of single-domain ferromagnetic islands. We find that the anisotropic inter-island interactions lead to a non-monotonic angular dependence of the array coercive field. Comparisons with micromagnetic simulations indicate that the two perpendicular sublattices exhibit distinct responses to island edge roughness, which clearly influence the magnetization reversal process. Furthermore, such comparisons demonstrate that disorder associated with roughness in the island edges plays a hitherto unrecognized but essential role in the collective behavior of these systems.Comment: Physical Review B, Rapid Communications (in press

Magnetic dynamics with spin transfer torques near the Curie temperature

We use atomistic stochastic Landau-Lifshitz-Slonczewski simulations to study the interaction between large thermal fluctuations and spin transfer torques in the magnetic layers of spin valves. At temperatures near the Curie temperature $T_{\rm C}$, spin currents measurably change the size of the magnetization (i.e. there is a {\it longitudinal} spin transfer effect). The change in magnetization of the free magnetic layer in a spin valve modifies the temperature dependence of the applied field-applied current phase diagram for temperatures near $T_{\rm C}$. These atomistic simulations can be accurately described by a Landau-Lifshitz-Bloch + Slonczewski equation, which is a thermally averaged mean field theory. Both the simulation and the mean field theory show that a longitudinal spin transfer effect can be a substantial fraction of the magnetization close to $T_{\rm C}$.Comment: 8 pages, 6 figure

170 Nanometer Nuclear Magnetic Resonance Imaging using Magnetic Resonance Force Microscopy

We demonstrate one-dimensional nuclear magnetic resonance imaging of the semiconductor GaAs with 170 nanometer slice separation and resolve two regions of reduced nuclear spin polarization density separated by only 500 nanometers. This is achieved by force detection of the magnetic resonance, Magnetic Resonance Force Microscopy (MRFM), in combination with optical pumping to increase the nuclear spin polarization. Optical pumping of the GaAs creates spin polarization up to 12 times larger than the thermal nuclear spin polarization at 5 K and 4 T. The experiment is sensitive to sample volumes containing $\sim 4 \times 10^{11}$ $^{71}$Ga$/\sqrt{Hz}$. These results demonstrate the ability of force-detected magnetic resonance to apply magnetic resonance imaging to semiconductor devices and other nanostructures.Comment: Submitted to J of Magnetic Resonanc