Strong Random Anisotropy Research Articles

Structural and Hysteretic Properties of La0.7Ca0.3−xSrxMnO3 Manganites Using the Hydrothermal Route

Manganitesof La0.7Ca0.3−xSrxMnO3 have been produced using the hydrothermal method. The influence of the concentration of La, Ca, and Sr on the structural, morphological, and magnetic properties was studied and analyzed. The x-ray diffraction studies showed the occurrence of orthorombic and trigonal symmetries in the crystalline structure, depending on the stoichiometry. X-ray photoelectron spectroscopy studies were performed, showing the presence of the elements La3d, Ca2p, Sr3d, Mn2p, and O1s. In all samples, a deficiency of oxygen was observed and ascribed to hydroxide formation. The FTIR spectra shows the Mn–O–Mn bonds in the stretching vibration mode; on the other hand, a great relative amount of Mn4+ ions that are not included in the manganite phase was observed. Furthermore, morphology analyses developed using scanning electron microscopy showed cauliflower-type microstructures in all samples. The dependence of magnetization on magnetic field was also studied where a low coercivity and a high-saturation magnetization were evidenced. Values of the squareness ratio coefficient ranged between 0.0587 and 0.0867, indicating that the individual particles exhibit a strong random anisotropy.

Irreversibility lines in the T–H phase diagram of Cr70Fe30 thin films

Irreversibility in magnetization has been measured at fixed magnetic fields, H≤70kOe, on Cr70Fe30 thin films. The observed variations of the weak and strong irreversibility temperatures with the external magnetic field in the ferromagnetic (FM) and re-entrant (RE) regimes, respectively, reveal that both the FM and RE states are mixed phases in which longitudinal FM order coexists with a transverse spin glass order. However, the RE state distinguishes itself from the FM one in a strong mixing of the longitudinal and transverse spin components caused by strong random anisotropy. The pinning of domain walls at the magnetic inhomogeneities is the common cause of coercivity and the irreversibility in magnetization.

Microstructure and magnetic properties of nanocrystalline (Ni0·5Zn0·5)xCo1−xFe2O4 ferrite by spraying co-precipitation method

Nanocrystalline (Ni0·5Zn0·5)xCo1−xFe2O4 with 0⩽x⩽1·0 are prepared by spraying co-precipitation method. Their microstructure and magnetic properties are investigated. The results show that nanocrystalline (Ni0·5Zn0·5)xCo1−xFe2O4 ferrites are spinel phase. Coercivity and retentivity of specimens increase with cobalt content. The enhancement of the saturation magnetisation up to a maximum value of 102·1 A m2 kg−1 in nanocrystalline (Ni0·5Zn0·5)0·5Co0·5Fe2O4 ferrite is observed. This can be attributed to the change of the net magnetic moment between the sublattices of structure for the substitution of Ni2+ and Zn2+ ions with Co2+ ions. The coercivity of nanocrystalline (Ni0·5Zn0·5)0·5Co0·5Fe2O4 ferrite increases monotonously in the grain size regime d<69 nm, and then coercivity decreases with grain size. The initial increase in coercivity of the nanocrystalline (Ni0·5Zn0·5)0·5Co0·5Fe2O4 ferrite with increasing size is due to the enhanced role of the surface and its strong random anisotropy.

Neutron study of magnetic ordering of planar spins in a random anisotropy antiferromagnet: Evidence for quantum tunnelling?

Large As/V substitutions in DyVO 4 result in planar Dy spins ordering antiferromagnetically in the presence of strong random anisotropy disorder. Neutron measurements on DyAs 0.35 V 0.65 O 4 show the development of sharp but not resolution-limited magnetic diffraction peaks below 1.6 K. Magnetic ordering appears to equilibrate on a time scale of hours as the temperature is reduced in small steps, but, remarkably, the time scale is nearly independent of temperature.

Magnetic phase diagram, static properties and relaxation of the insulating spin glass Co 1− xMn x(SCN) 2(CH 3OH) 2

The magnetic behavior of Co 1− x Mn x (SCN) 2(CH 3OH) 2 has been studied by DC magnetization and susceptibility measurements on mixtures spanning the complete composition range. The pure components are a quasi-two-dimensional Heisenberg antiferromagnet (Mn system) and a three-dimensional Ising antiferromagnet (Co system). The crystal structure of the cobalt constituent is determined, and is closely related to that of the manganese constituent. Competing orthogonal spin anisotropies should occur in mixtures, and frustration effects arising from competing ferromagnetic and antiferromagnetic interactions may also arise. The Curie and Weiss constants, in χ M= C/( T− θ), vary regularly with composition. C versus x is essentially linear while θ versus x shows a definite curvature, analysis of which reveals that the unlike-ion exchange interaction is antiferromagnetic and stronger than the like-ion interactions. The magnetic susceptibility is field dependent, more markedly so with increasing x. Plots of M/ H versus T exhibit maxima at low temperatures only for mixtures substantially richer in cobalt than manganese. Magnetic transition temperatures are estimated from these data. Magnetization versus field isotherms evolve with composition and with temperature; those for x=0.24 5 and 0.16 9 exhibit S-shapes for temperatures at or below the identified transitions. The nonlinear susceptibility versus temperature for x=0.24 5 displays structure but does not diverge. The temperature dependence of the thermoremanent magnetization (TRM) for x=0.24 5 shows characteristic features but does not follow any simple form. The time-dependence of the TRM is fitted at a series of temperatures employing a stretched exponential decay form. The thermal variation of the fit parameters is systematic and suggests that temperatures just below 3 K and slightly above 6 K have special significance. Over a limited temperature range the TRM is found to scale approximately as T log 10(t/τ 0) , with τ 0≈10 −12 s. Strong and weak irreversibility lines are determined for x=0.24 5; both vary as τ g∝ h 0.56, with zero-field temperatures of T s(0)=5.5 5 K and T w(0)=9.8 5 K, respectively. The exponent is closer to that recently predicted (0.53) for a short-range three-dimensional Ising spin glass than to the value 2/3 of the DeAlmeida–Thouless line in the infinite range mean-field Ising model. The existence of strong random anisotropy may account for the presence of a weak irreversibility line with the observed exponent. The T– x magnetic phase diagram exhibits a crossing of paramagnetic-ordered state phase boundaries and an associated tetracritical point at x≈0.20 5 and T≈2.6 0 K. Spin glass properties are apparent for compositions close to the tetracritical point.

Static magnetic properties and relaxation of the insulating spin glassCo1−xMnxCl2⋅H2O

The magnetic properties of ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{Cl}}_{2}{\mathrm{\ensuremath{\cdot}}\mathrm{H}}_{2}\mathrm{O}$ are examined by dc magnetization and susceptibility measurements, for $x=0.05,$ 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 0.95 between 1.8 and 300 K. The pure components are a quasi-one-dimensional Heisenberg antiferromagnet (Mn) and an antiferromagnetic reentrant spin glass (Co) with some low-dimensional character. The Curie and Weiss constants, in ${\ensuremath{\chi}}_{M}=C/(T\ensuremath{-}\ensuremath{\theta}),$ show regular composition dependence, with $\ensuremath{\theta}(x)$ varying nonlinearly from positive to negative values as x increases. Antiferromagnetic maxima often occur, and transition temperatures are estimated for most mixtures. The $T\ensuremath{-}x$ diagram shows two descending boundaries from either composition extreme; any transition temperatures for $x=0.5--0.8$ are lower than we can measure. Magnetization isotherms evolve with composition, and associated hysteretic effects weaken with increasing x. The nonlinear susceptibility for $x=0.30$ shows structure, but does not diverge. The thermoremanent magnetization (TRM) is examined in detail for $x=0.30,$ 0.40, and 0.50. Its temperature dependence shows characteristic features, but does not follow any simple form. Systematic variation in the TRM with cooling field and composition is apparent. The time dependence of the TRM is fit using a stretched exponential decay form. Systematic variations in the fit parameters with temperature, cooling field, and composition emerge. For low to moderate temperatures, the TRM is found to scale according to $T{\mathrm{log}}_{10}(t/{\ensuremath{\tau}}_{0}),$ with ${\ensuremath{\tau}}_{0}\ensuremath{\approx}{10}^{\ensuremath{-}12}--{10}^{\ensuremath{-}13}\mathrm{s}.$ For $x=0.30$ and 0.50, strong and weak irreversibility lines are determined. The former conform better to a recent prediction for the short-range three-dimensional Ising spin glass, ${\ensuremath{\tau}}_{g}\ensuremath{\propto}{h}^{0.53},$ than to the DeAlmeida-Thouless mean-field form ${\ensuremath{\tau}}_{g}\ensuremath{\propto}{h}^{2/3};$ best-fit exponents are slightly less than 0.53. For the weak irreversibility lines, the dependence of ${\ensuremath{\tau}}_{g}$ on field is much weaker than the Gabay-Toulouse form ${\ensuremath{\tau}}_{g}\ensuremath{\propto}{h}^{2}.$ The presence of strong random anisotropy is a possible explanation. A monotonic decrease of ${T}_{s}$ with increasing x is found. A reentrant spin glass state still occurs for $x=0.30;$ for $x=0.40$ and 0.50, the spin glass transition is not reentrant. Irreversible effects are maximized in the $x=0.3--0.4$ region; they are absent for mixtures richer in manganese than cobalt.

High field magnetisation studies in amorphous CoDyB alloys

We have prepared amorphous Co80-xDyxB20 ribbons with 0 ≤ x ≤ 15 by melt spinning and studied their magnetic properties in the temperature range 1.5–300 K under applied fields of H ≤ 15 T. The Dy moment was found to be 8.6μB, which is lower than the theoretical value of 10μB, indicating the non-collinearity of Dy spins due to strong random anisotropy. For χ < 10, the local anisotropy K1 is 1.5 × 107 erg cm−3, and 3.8 × 107 erg cm−3 for χ > 10.

Cubic models with random anisotropy

A Monte Carlo algorithm has been used to study the cubic ferromagnet with a random uniaxial single-site anisotropy on simple cubic lattices, in the strong anisotropy limit. For both two-component and three-component spins the phase diagram contains a ``domain-type'' ferromagnetic phase for strong random anisotropy, in addition to the simple ferromagnetic phase which is seen for weak random anisotropy. For two-component spins both paramagnet-ferromagnet transitions seem to be second order. For three-component spins the magnetization probably disappears discontinuously, and there appears to be a thin layer of infinite susceptibility phase between the paramagnetic and ferromagnetic phases.

Low-temperature behavior of random-anisotropy magnets.

An improved computer annealing algorithm has been used to study the low-energy states of magnets with strong random anisotropy on simple-cubic lattices. For classical Heisenberg spins with isotropically random uniaxial anisotropy, the ferromagnetic correlations at T=0 can be described by a scaling exponent \ensuremath{\eta} of about 0.2 and a correlation length of about 10 lattice units. The ground-state energy ${\mathit{E}}_{0}$ is (-1.118\ifmmode\pm\else\textpm\fi{}0.003)J. For XY spins with random p-fold anisotropy, the ground states are ferromagnetic, with magnetizations of 0.45\ifmmode\pm\else\textpm\fi{}0.02, 0.715\ifmmode\pm\else\textpm\fi{}0.015, and 0.843\ifmmode\pm\else\textpm\fi{}0.006, for p=2, 3, and 4, respectively, and the values of ${\mathit{E}}_{0}$/J are -1.5075\ifmmode\pm\else\textpm\fi{}0.0015, -2.229\ifmmode\pm\else\textpm\fi{}0.003, and -2.543\ifmmode\pm\else\textpm\fi{}0.002.

Phase-transition behavior in a random-anisotropy system.

The character of the magnetic transition is studied for a magnetic glass with strong random-anisotropy and ferromagnetic interactions. Field-dependent ac susceptibility measurements are presented near the ordering temperature of ${\mathrm{Tb}}_{64}$${\mathrm{Fe}}_{20}$${\mathrm{Ga}}_{16}$. The data are analyzed with a scaling theory for the singular susceptibility. An excellent collapse of the $H$, $T$ data is obtained for reduced temperatures in the range 0.002-0.13. These results, which are compared with recent theoretical work, provide strong evidence for a true phase transition in a random-anisotropy system.

Ordering in ferromagnets with random anisotropy.

We summarize and extend our study (using real-space response and correlation functions) of the properties of a continuous-symmetry ferromagnet with random anisotropy, distinguishing between the cases of weak and strong random anisotropy. For the weak-anisotropy case we find three different magnetic regimes, according to the strength of the external magnetic field H. In zero H, the net magnetization is zero, although the ferromagnetic correlation length (FCL) is large. We call a ferromagnet in this first regime a correlated spin glass (CSG). It has a very large magnetic susceptibility, and hence a relatively small coherent anisotropy converts it into a nearly typical ferromagnetic domain structure. Also, a relatively small magnetic field nearly aligns the CSG, producing the second regime, which we call a ferromagnet with wandering axis (FWA). The FWA is a slightly noncollinear structure in which the tipping of the magnetization with respect to the field varies over the system. The tipping angle is correlated over a (field-dependent) correlation length which is smaller than the FCL of the CSG. As the field increases the correlation length in the FWA decreases, until the third regime is reached, wherein the tipping angles (which are smaller than in the FWA) are completely uncorrelated from site to site. We obtain the magnetization or susceptibility (as appropriate) for each of these three regimes. We also show that the temperature dependence of the (single-ion) random anisotropy strength can provide a plausible explanation for certain classes of reentrant phenomena and susceptibility cusps observed in magnetization studies. Neutron scattering studies appear to be consistent with the predicted ${H}^{\mathrm{\ensuremath{-}}1/2}$ dependence of the FCL in the FWA regime, and display the expected rise of the FCL in the CSG regime as the random anisotropy strength decreases with increasing temperature.

Random magnetism in amorphous rare-earth alloys (invited)

Several aspects of the magnetic transitions seen in rare-earth metallic glasses are discussed, particularly with reference to recent theoretical work. These include: (a) apparent double transitions observed in Gd glasses where exchange fluctuations are important, (b) evidence for a correlated speromagnetic state recently predicted by Chudnovsky and Serota, and (c) the analysis of a Tb glass with strong random anisotropy in terms of an Ising-type spin-glass transition.

Random anisotropy in amorphous ferromagnets

Random anisotropy is present in all amorphous magnetic materials, and depending on its strength, it can dramatically affect the magnetic behavior. We describe the effect of strong random anisotropy in materials such as amorphous Tb-Fe and Dy-Fe at low temperature and also examine the role which weak random anisotropy might play in even ideally homogeneous soft materials. Much of our analysis is based on the simple model proposed by Harris, Plischke and Zuckerman for a ferromagnet with random-axis uniaxial anisotropy. We describe computer simulation results for this model and then develop scale length arguments which allow us to describe fluctuations in the magnetization direction. For a perfectly isotropic distribution of anisotropy axis we find that the conventional ferromagnetic ground state is unstable. The new ground state has large frozen in fluctuations but probably has a considerable moment and is, therefore, not spin glass-like. This system does not support domain walls of the conventional type. For a system with both a macroscopic anisotropy axis and random anisotropy, we can have domain walls. We present a theory for the intrinsic coercivity which gives values of about 10−6 Oe for Fe-metalloids and 0.2 Oe for Gd-Co-Mo. This indicates that inhomogeneities of larger than atomic scale are limiting the behavior of present materials. A model illustrating aspects of magnetic resonance behavior is also described.