Presence Of Random Anisotropy Research Articles

Effect of random anisotropy on magnetization reversal in dipolarly coupled layered thin films

Magnetic thin films and multilayers offer many novel physical phenomena with tremendous potential for applications. Various growth processes and methods are in place to make homogeneous films. However, dispersion in local easy axes is an inherent issue which may give rise to an effective random anisotropy in thin film. It is important to investigate the effect of random anisotropy on the magnetic properties of the thin films. In this report we address the aspect of local dispersion induced random anisotropy on magnetization reversal in ferromagnetic (FM)/non-magnetic(NM)/ferromagnetic(FM) stacks. We show that the magnetization reversal is not only governed by the inter-layer interaction but the intrinsic anisotropies also play a crucial role. In this context we have studied the magnetization reversal of Co/AlOX/Co trilayers with various thicknesses of AlOX which acts as a spacer between the two Co layers. Presence of random anisotropy in addition to growth induced uniaxial anisotropy was observed in all the samples. Magneto-optic Kerr effect (MOKE) based microscopy revealed dipolar-coupled layer-by-layer magnetization reversal of the Co layers which was corroborated by polarized neutron reflectometry (PNR) experiments. Micromagnetic simulations confirmed that the presence of random anisotropy in addition to uniaxial anisotropy leads to layer-by-layer magnetization reversal.

Chirality-driven intrinsic spin-glass ordering and field-induced ferromagnetism in Ni3Al nanoparticle aggregates

Weak itinerant-electron ferromagnet Ni3Al is driven to magnetic instability (quantum critical point, QCP, where the long-range ferromagnetic order of the bulk ceases to exist) by reducing the average crystallite size to d=50nm. ‘Zero-field’ (H=0) linear and nonlinear ac-susceptibilities, measured on Ni3Al nanoparticle aggregates, with d=50nm (S1) and d=5nm (S2), provide strong evidence for two spin glass (SG)-like thermodynamic phase transitions: one at Ti(H=0)≃30K (Ti†(H=0)≃230K) and the other at a lower temperature Tp(H=0)≃8K (Th(H=0)≃52K) in S1 (S2). ‘In-field’ (H≠0) linear ac-susceptibility and dc magnetization demonstrate that the thermodynamic nature of these transitions is preserved in finite fields. The presently determined H–T phase diagrams for the samples S1 and S2 are compared with those predicted by the Kotliar–Sompolinsky and Gabay–Toulouse mean-field models and Monte Carlo simulations, based on the chirality-driven spin glass (SG) ordering scenario, for a three-dimensional nearest-neighbor Heisenberg SG system with or without weak random anisotropy. Such a detailed comparison permits us to unambiguously identify various ‘zero-field’ and ‘in-field’ SG phase transitions as: (i) the simultaneous paramagnetic (PM)-chiral glass (CG) and PM-SG phase transitions at Ti(H), (ii) the PM-CG transition at Ti†(H), (iii) the replica symmetry-breaking SG transition at Tp(H), and (iv) the continuous spin-rotation symmetry-breaking SG transition at Th(H). In the presence of random anisotropy, magnetization fails to saturate even at 90kOe in S1 whereas negligibly small anisotropy allows even fields as weak as 1kOe to saturate magnetization and induce ferromagnetism in S2. Due to the proximity to CG/SG-QCP, magnetization and susceptibility both exhibit non-Fermi liquid behavior over a wide range at low temperatures.

Absence of phase stiffness in the quantum rotor phase glass

We analyze here the consequence of local rotational-symmetry breaking in the quantum spin (or phase) glass state of the quantum random rotor model. By coupling the spin glass order parameter directly to a vector potential, we are able to compute whether the system is resilient (that is, possesses a phase stiffness) to a uniform rotation in the presence of random anisotropy. We show explicitly that the O(2) vector spin glass has no electromagnetic response indicative of a superconductor at mean-field and beyond, suggesting the absence of phase stiffness. This result confirms our earlier finding [Phys. Rev. Lett. 89, 027001 (2002)] that the phase glass is metallic, due to the main contribution to the conductivity arising from fluctuations of the superconducting order parameter. In addition, our finding that the spin stiffness vanishes in the quantum rotor glass is consistent with the absence of a transverse stiffness in the Heisenberg spin glass found by Feigelman and Tsvelik [Sov. Phys. JETP 50, 1222 (1979)].

Weak disorder in a two-dimensional dipole magnet

The effect of “random-field” disorder and “random-anisotropy-axis” disorder on a two-dimensional dipole ferromagnet is investigated. It is shown that disorder results in instability of the ferromagnetic phase. The correlation function of the magnetization is calculated with the aid of a self-consistent harmonic approximation. It is found that in the presence of a random field the correlation function is a power-law function of the distance. In the presence of random anisotropy the correlation function decreases logarithmically slowly as a function of distance.

Monte Carlo studies of two-dimensional random-anisotropy magnets.

We have carried out a systematic set of Monte Carlo simulations of the Harris-Plischke-Zuckermann lattice model of random magnetic anisotropy on a two-dimensional square lattice, using the classical Metropolis algorithm. We have considered varying temperature T, external magnetic field H (both in the reproducible and irreproducible limits), time scale of the simulation \ensuremath{\tau} in Monte Carlo steps and anisotropy ratio D/J. In the absence of randomness this model reduces to the XY model in two dimensions, which possesses the familiar Kosterlitz-Thouless low-temperature phase with algebraic but no long-range order. In the presence of random anisotropy we find evidence of a low-temperature phase with some disordered features, which might be identified with a spin-glass phase. The low-temperature Kosterlitz-Thouless phase survives at intermediate temperatures for low randomness, but is no longer present for large D/J. We have also studied the high-H approach to perfect order, for which there are theoretical predictions due to Chudnovsky.

Mössbauer study of disordered, quenched Pd80Co20.

We report M\"ossbauer source experiments on disordered $^{57}\mathrm{CoPd}_{80}$${\mathrm{Co}}_{20}$, and ask if there is any evidence for weak ferromagnetic random anisotropy (RA) near the critical point. In making this search we are encouraged by the fact that Co has an unquenched orbital angular momentum, and that there is evidence of local-moment disalignment in small-angle neutron scattering. Spectra taken over the temperature range 89<T<507 K were well fitted by a continuous magnetic hyperfine field distribution which can be described by a modal field, ${H}_{\mathrm{hf}{}^{m}(\mathrm{T}}$), and a full width at half maximum (FWHM) \ensuremath{\Delta}${H}_{\mathrm{hf}(\mathrm{T}}$). ${H}_{\mathrm{hf}{}^{m}(\mathrm{T}}$) was used to deduce \ensuremath{\beta}=0.38(3) for the range ${10}^{\mathrm{\ensuremath{-}}2}$1-T/${T}_{C}$ 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}1}$. Though insufficiently asymptotic for a strict test of theory, this value is in agreement with predictions for the random-exchange Heisenberg model. \ensuremath{\Delta}${H}_{\mathrm{hf}(\mathrm{T}}$) is found to increase above 320 K, a behavior that can be described via a local-moment mean-field theory which assumes the $^{57}\mathrm{Fe}$ M\"ossbauer site has a 17% FWHM distribution in the exchange constant and a 10% FWHM distribution in ${H}_{\mathrm{hf}(0}$). Neither the deduced critical behavior nor the observed hyperfine-field broadening requires the attribution of RA effects. The only evidence of the possible presence of RA in our experiments is the fact that cold working substantially broadens both ${T}_{C}$ and the hyperfine-field distribution near ${T}_{C}$.