Ferrimagnetism and reentrant phenomena in a tetragonal Ising nanoparticle

ABSTRACT

The phase diagrams and magnetizations in a tetragonal Ising nanoparticle have been investigated by using the effective-field theory with correlations and the core–shell model. For the spin-1/2 atoms in the shell, a transverse field is included to cant their spin direction. The results show that they are heavily dependent on the given physical parameters. The ferrimagnetic behaviours, as well as novel type behaviours, have been found in the thermal variation of magnetization. The reentrant phenomena are also obtained.     

A quadrangular transverse Ising nanowire with an antiferromagnetic spin configuration

The phase diagrams and the temperature dependences of magnetizations in a transverse Ising nanowire with an antiferromagnetic spin configuration are investigated by the use of the effective-field theory with correlations (EFT) and the core–shell concept. Many characteristic and unexpected behaviors are found for them, especially for thermal variation of total magnetization m_T. The reentrant phenomenon induced by a transverse field in the core, the appearance of a compensation point, the non-monotonic variation with a compensation point, the reentrant phenomena with a compensation point and the existence of both a broad maximum and a compensation point have been found in the thermal variations of m_T.     

Change in the magnetic moment of a ferromagnetic nanoparticle under polarized current

The magnetization reversal of a ferromagnetic Fe₃O₄ nanoparticle with a volume of the order of several thousands of cubic nanometers under the influence of spin-polarized current has been investigated on a high-vacuum scanning tunneling microscope, where one of the electrodes is a magnetized iron wire needle and the second electrode is a ferromagnetic nanoparticle on a graphite substrate. The measured threshold current of magnetization reversal, i.e., the lowest value of the current corresponding to the magnetization reversal, is found to be I_thresh ≈ 9 nA. A change in the magnetization of a nanoparticle is revealed using the giant magnetoresistance effect, i.e., the dependence of the weak polarized current (I < I_thresh) on the relative orientation of the magnetizations of the electrodes.     

Magnetic anisotropy and anisotropic tunneling magnetoresistance of compacted half-metal CrO<Subscript>2</Subscript> nanopowders

Low-temperature magnetic and magnetoresistive properties of compacted ferromagnetic halfmetal CrO₂ powders with nanoparticle shape anisotropy are studied. Magnetic anisotropy induced by the formation of magnetic texture during compaction is revealed. It is shown that tunneling magnetoresistance anisotropy is associated with the difference between a sample’s rates of magnetization in the longitudinal and transverse fields.     

Ferrimagnetic magnetizations of transverse Ising thin films with diluted surfaces

The temperature dependences of longitudinal and transverse magnetizations in transverse Ising thin films with diluted surfaces which are coupled antiferromagnetically to the bulk are studied by the use of the effective-field theory (EFT) with correlations. Novel features are obtained for the thermal variations of longitudinal magnetization, being different from those of the bulk ferrimagnetic materials, such as the possibility of two compensation points. They are depending on the thickness of a film and the surface dilution. These characteristic phenomena come from the competition between the surfaces and the bulk when the ratios of the physical parameters (transverse field and exchange interaction) between the surfaces and the bulk are selected as some large values.     

Non-collinear magnetoelectronics

The electron transport properties of hybrid ferromagnetic|$|$normal metal structures such as multilayers and spin valves depend on the relative orientation of the magnetization direction of the ferromagnetic elements. Whereas the contrast in the resistance for parallel and antiparallel magnetizations, the so-called giant magnetoresistance, is relatively well understood for quite some time, a coherent picture for non-collinear magnetoelectronic circuits and devices has evolved only recently. We review here such a theory for electron charge and spin transport with general magnetization directions that is based on the semiclassical concept of a vector spin accumulation. In conjunction with first-principles calculations of scattering matrices many phenomena, e.g. the current-induced spin-transfer torque, can be understood and predicted quantitatively for different material combinations.     

Magnetic properties of half-metallic semi Heusler Co1−xCuxMnSb compounds

A study of the half-metallic character of the semi Heusler alloys Co_1−xCu_xMnSb (0?x?0.9) is presented. We investigated the saturation magnetization M_S at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature T_C. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μ_B for Mn³⁺ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with M_S (5 K)=3.78 μ_B/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above T_C, the χ⁻¹ vs T curves follow very well the Curie–Weiss law. The effective moment μ_eff and paramagnetic Curie temperature θ are derived. A comparison between the values of M_S at 5 K and μ_eff shows a transition from localized to itinerant spin system in these compounds.     

Some characteristic phenomena in a transverse Ising nanotube

The phase diagram and magnetizations of a cylindrical nanotube described by the transverse Ising model are investigated by the use of the effective field theory with correlations. Some comparisons between the nanotube and the nanowire have been given for the phase diagrams. In particular, the temperature dependences of longitudinal magnetization in the system with a negative shell–core interaction are investigated. Some characteristic phenomena (new types in ferrimagnetism) which have not been observed in the nanowire as well as similar phenomena are found in the thermal variations, depending on the ratio of the physical parameters in the surface shell and the core. The possibilities of two compensation points and a field induced compensation point in the nanotube are also discussed.     

Ultrasmall iron oxide nanoparticles: Magnetic and NMR relaxometric properties

Ultrasmall iron oxide (USPIO) nanoparticles, with diameter mostly less than 3 nm dispersed in an organic carrier fluid were synthesized by polyol route. The evolution of ZFC-FC magnetization curves with temperature, as well as the shift of the ac susceptibility peaks upon changing the frequency, reveal that the nanoparticles in the fluid are non-interacting and superparamagnetic with the blocking temperature T_B ∼10 K. The Mössbauer spectra analysis proposed the core/shell structure of the nanoparticles consisting of stoichiometric γ-Fe₂O₃ core and non-stoichiometric shell. The nanoparticle surface layer has a great influence on their properties which is principally manifested in significant reduction of the magnetization and in a large increase in magnetic anisotropy. Magnetic moments do not saturate in fields up to 5 T, even at the lowest measured temperature, T = 5 K. The average magnetic particle diameter is changed from 1.3 to 1.8 nm with increasing magnetic field from 0 to 5 T which is noticeably smaller than the particle sizes measured by TEM. The estimated effective magnetic anisotropy constant value, K_eff = 2 × 10⁵ J/m³, is two orders of magnitude higher than in the bulk maghemite. Measurements of the longitudinal and transverse NMR relaxivity parameters on water diluted nanoparticle dispersions at 1.5 T gave the values r₁ = 0.028 mmol⁻¹ s⁻¹, r₂ = 0.050 mmol⁻¹ s⁻¹ and their ratio r₂/r₁ = 1.8. Continuous increase of the T₁-weighted MRI signal intensity with increasing Fe concentration in the nanoparticle dispersions was observed which makes this ferrofluid to behave as a positive T₁ contrast agent.     

Dynamic phase transitions in a cylindrical Ising nanowire under a time-dependent oscillating magnetic field

The dynamic phase transitions in a cylindrical Ising nanowire system under a time-dependent oscillating external magnetic field for both ferromagnetic and antiferromagnetic interactions are investigated within the effective-field theory with correlations and the Glauber-type stochastic dynamics approach. The effective-field dynamic equations for the average longitudinal magnetizations on the surface shell and core are derived by employing the Glauber transition rates. Temperature dependence of the dynamic magnetizations, the dynamic total magnetization, the hysteresis loop areas and the dynamic correlations are investigated in order to characterize the nature (first- or second-order) of the dynamic transitions as well as the dynamic phase transition temperatures and the compensation behaviors. The system strongly affected by the surface situations. Some characteristic phenomena are found depending on the ratio of the physical parameters in the surface shell and the core. According to the values of Hamiltonian parameters, five different types of compensation behaviors in the Néel classification nomenclature exist in the system. The system also exhibits a reentrant behavior.     

Resonant inelastic X-ray scattering and its magnetic circular dichroism

Magnetic circular dichroism in resonant inelastic X-ray scattering (MCD-RIXS) of ferromagnetic systems is discussed for the longitudinal geometry (LG), where the directions of incident X-ray and magnetization are parallel, and the transverse geometry (TG), where they are perpendicular. MCD-RIXS in LG is represented by two successive real processes, X-ray absorption and X-ray emission, while MCD-RIXS in TG is given by the interference process. We describe an application of MCD-RIXS in LG to the detection of Ce 4f² contribution in Ce, L₃ absorption edge of CeFe₂, and theoretical and experimental studies of MCD-RIXS in TG for Gd and Sm systems.     

Magnetizations of a transverse Ising nanowire

Magnetizations of a cylindrical nanowire described by the transverse Ising model are investigated by the use of the effective field theory with correlations (EFT), since the phase diagrams of the system have been examined in the previous work (J. Magn. Magn. Mater. (2010), in press) by using the two theoretical frameworks of the mean field theory and the EFT. The temperature dependences of longitudinal and transverse magnetizations in the system are strongly affected by the surface situations. Many characteristic phenomena are found in the thermal variations, depending on the ratio of the physical parameters in the surface shell and the core. In particular, the effects of the two transverse fields at the surface shell and in the core to these magnetizations have been firstly clarified.     

Synthesis, structure and magnetic properties of the new double perovskite Ca2CrSbO6

The title double perovskite has been synthesized by solid-state reaction in air. The crystal structure has been studied from powder X-ray diffraction data. Rietveld fits to the pattern show that this compound has a monoclinic symmetry [a=5.4932(3) Å, b=5.4081(3) Å, c=7.6901(5) Å, β=90.0022(1)°, at 300 K] defined in the space group P2₁/n, where the Cr and Sb cations are almost completely ordered in the B-sublattice of the perovskite structure. Magnetic susceptibility and magnetization measurements show that this compound behaves as a Curie-Weiss paramagnet at high temperatures with μ_eff=3.53(1) μ_B and θ_P=8 K, and exhibits a robust ferromagnetic component below the ordering temperature of T_C=13 K, with a saturation magnetization of 2.36 μ_B/f.u. at 5 K. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element, such as Sb, occupying one half of the B positions of the perovskite structure.     

One-dimensional bulk ferromagnets: NdAl2 and hcp cobalt

It is shown experimentally that NdAl₂ and hcp cobalt are one-dimensional (1D) bulk ferromagnets. For hcp cobalt this is only under the condition that the sample is magnetically saturated, i.e. that all moments are aligned parallel to the hexagonal c-axis. In 1D magnets the transverse interactions need not to be zero but must be sufficiently weak such that the transverse correlation length does not diverge at the critical temperature. The transverse interactions are then not relevant and the phase transition is driven by the longitudinal interactions. On the other hand, magnetic Bragg scattering relies on finite transverse correlations. For NdAl₂ no conventional magnetic Bragg scattering is observed if all moments are aligned vertical to the scattering plane by a magnetic field. For hcp cobalt the scattering intensity is considerably reduced in this geometry instead of having its maximum. From this observation it can be concluded that the transverse correlation length is practically zero in NdAl₂ but has a finite value in hcp cobalt. The macroscopic magnetization shows normal ferromagnetic saturation.     

Critical properties of the paramagnetic-to-ferromagnetic transition in nanocrystalline Gd diluted with Fe

Nanocrystalline Gd_0.946Fe_0.054 of average grain size 68 nm was prepared by melt-spinning. The magnetic behavior in the vicinity of the paramagnetic to ferromagnetic transition was investigated via dc magnetization and ac susceptibility measurements. The transition temperature and effective critical exponents for the order parameter and zero-field susceptibility were determined using Arrott-Noakes and Kouvel-Fisher analyses. The values obtained were T_C=291.71±0.07 K, β_eff=0.385±0.009, and γ_eff=1.24±0.03, respectively. Correction to scaling analysis indicated that the asymptotic exponents were both smaller than the effective ones within the reduced-temperature range investigated, contrary to the behavior seen in monocrystalline Gd. This behavior can be explained in terms of a crossover from 3D short-range Heisenberg universality class to the 3D Ising universality class due to increased anisotropy induced by the high magnetic fields used in the measurements and also possibly due to strain.     

Effect of nanoparticles on the magnetic properties of Mn–Zn soft ferrite

In this paper, the effect of nanostructures on the magnetic properties like the specific saturation magnetization (σ_S) and the coercivity (H_C) for Mn_0.4Zn_0.6Fe₂O₄ ferrite prepared by the co-precipitation method has been presented. We have shown by means of X-ray diffraction that the resulting ferrite is made up of nanoparticles, and that the average size of these nanoparticles calculated with the Scherrer formula depends upon the sintering temperature. When the sintering temperature is increased from 500 to 900 °C, the average nanoparticle diameter varies from 19.3 to 36.4 nm. The nanoparticle phase is further confirmed by scanning electron microscopy (SEM). Both results are found to be in good agreement. The magnetic properties are explained on the basis of the single-domain and multi-domain theory.     

Anisotropy and dynamic properties of Co2MnGe Heusler thin films

Co₂MnGe films of 30 and 50 nm in thickness were grown by RF-sputtering. Their magnetic anisotropies, dynamic properties and the different excited spin wave modes have been studied using conventional ferromagnetic resonance (FMR) and Microstrip line FMR (MS-FMR). From the in-plane and the out-of-plane resonance field values, the effective magnetization (4πM_eff) and the g-factor are deduced. These values are then used to fit the in-plane angular-dependence of the uniform precession mode and the field-dependence of the resonance frequency of the uniform mode and the first perpendicular standing spin wave to determine the in-plane uniaxial, the four-fold anisotropy fields, the exchange stiffness constant and the magnetization at saturation. The samples exhibit a clear predominant four-fold magnetic anisotropy besides a smaller uniaxial anisotropy. This uniaxial anisotropy is most probably induced by the growth conditions.     

The critical properties of magnetic films

Within the framework of the transverse Ising model and by using the effective field theory with a probability distribution technique that accounts for the self spin correlations, we have studied the critical properties of an L-layer film of simple cubic symmetry in which the exchanges strength are assumed to be different from the bulk values in N_S surface layers. We derive and illustrate the expressions for the phase diagrams, order parameter profiles and susceptibility. In such films, the critical temperature can shift to either lower or higher temperature compared with the corresponding bulk value. We calculate also some magnetic properties of the film, such as the layer magnetizations, their averages and their profiles and the longitudinal susceptibility of the film. The film longitudinal susceptibility still diverges at the film critical temperature as does the bulk longitudinal susceptibility.     

Ground state phase diagrams and the tricritical behaviors of Ising metamagnet in both external longitudinal and transverse field

The ground state magnetic properties of a two-sublattice Ising metamegnet in both external longitudinal and transverse fields are studied within the mean-field approach. A parameter α=(Z₁J₁+Z₂J₂)/(Z₁J₁−Z₂J₂) which reflects the strength ratio of spin coupling in the plane and in adjacent planes is introduced. The ground state energy, the longitudinal staggered magnetization, the longitudinal total magnetization and the transverse total magnetization are calculated. The ground state phase diagrams in Ω−h and Ω−α plane are presented. The results show that when Ω is given, the longitudinal critical magnetic field increases when α decreases; the phase transition changes always from first order to second order with increase in the longitudinal magnetic field h or decrease in α. The reentrant phenomenon occurs in the range α?−0.66, Ω?0.21, h?0.78.     

Surface effects on a multilayer and multisublattice cubic nanowire with core/shell

A multilayer and multisublattice cubic nanowire is studied based on a shell/core and the effective-field theory. The formulas of the longitudinal and transverse magnetization for each sublattice of the nanowire are given. The surface parameters have intense effects on the magnetizations and phase diagrams (phase transition temperatures and compensation temperatures) of the system. Two compensation points do exist for certain values of the surface transverse field, the surface single-ion anisotropy and the surface exchange coupling in the system. This interesting phenomenon may be potential applications in information storage technology.