Pulsed Field-Induced Magnetization Switching in Antiferromagnetic Ferrihydrite Nanoparticles

The dynamic magnetization switching of ferrihydrite nanoparticles has been investigated by a pulsed magnetometer technique in maximum fields H_max of up to 130 kOe with pulse lengths of 4, 8, and 16 ms. Ferrihydrite exhibits antiferromagnetic ordering and defects cause the uncompensated magnetic moment in nanoparticles; therefore, the behavior typical of magnetic nanoparticles is observed. The dynamic hysteresis loops measured under the above-mentioned conditions show that the use of pulsed fields significantly broadens the temperature region of existence of the magnetic hysteresis and the coercivity can be governed by varying the maximum field and pulse length. This behavior is resulted from the relaxation effects typical of conventional ferro- and ferrimagnetic nanoparticles and the features typical of antiferromagnetic nanoparticles.     

Non-equilibrium effects in the magnetic behavior of Co3O4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co₃O₄ nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co₃O₄ nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.     

Exchange bias in a generalized Meiklejohn–Bean approach

A generalized Meiklejohn–Bean model is considered in order to derive an analytic expression for the dependence of the exchange bias field on the layer thickness involved in ferromagnetic/antiferromagnetic heterosystems, on the orientation of the applied magnetic field with respect to the magnetic easy axes and on the quenched magnetization M_AF of the antiferromagnetic pinning layer. While M_AF is a well-known feature of field-cooled dilute antiferromagnets, it seems to occur quite generally also in pure AF pinning substrates. The new analytic expressions are successfully compared with recent experimental results and Monte Carlo investigations.     

Magnetic phase transition in ɛ-In x Fe2 − x O3 nanowires

This paper reports on a study of magnetic properties of ordered arrays of ?-In _x Fe_{2 ? x} O₃ (x = 0.24) nanowires possessing a high room-temperature coercive force of 6 kOe. Lowering the temperature below 190 K brings about a sharp decrease of the coercive force and magnetization of nanowires driven by the magnetic phase transition from the ferrimagnetic into antiferromagnetic phase. The transition is accompanied by a decrease of the magnetic anisotropy constant, which accounts for the anomalous frequency dependence of the position of the maximum in the temperature dependence of dynamic magnetic susceptibility. In the low-temperature phase, a spin-flop transition in the magnetic field of 28 kOe has been observed at T = 2 K. Lines related to the high-temperature hard-magnetic and low-temperature phases have been identified in electron spin resonance spectra of the nanowires. A line lying near zero magnetic field and evolving from the nonresonant signal related to the microwave magnetoresistance of the sample has also been detected.     

Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH₂O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe³⁺ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT ².     

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Magnetic properties of electron-doped La_0.23Ca_0.77MnO₃ manganite nanoparticles, with average size of 12 and 60?nm, prepared by the glycine?Cnitrate method, have been investigated in the temperature range 5?C300?K and magnetic fields up to 90?kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T ^3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T?<?100?K.     

Studying the magnetic properties of CoSi single crystals

The magnetic properties of CoSi single crystals have been measured in a range of temperatures T = 5.5–450 K and magnetic field strengths H ≤ 11 kOe. A comparison of the results for crystals grown in various laboratories allowed the temperature dependence of magnetic susceptibility χ(T) = M(T)/H to be determined for a hypothetical “ideal” (free of magnetic impurities and defects) CoSi crystal. The susceptibility of this ideal crystal in the entire temperature range exhibits a diamagnetic character. The χ(T) value significantly increases in absolute value with decreasing temperature and exhibits saturation at the lowest temperatures studied. For real CoSi crystals of four types, paramagnetic contributions to the susceptibility have been evaluated and nonlinear (with respect to the field) contributions to the magnetization have been separated and taken into account in the calculations of χ(T).     

Magnetization process and principal magnetic susceptibilities in RbMnF3

The temperature dependence of χ_⊥, initialχ_p (powder), and evaluated χ_∥ for RbMnF₃ are reported for the temperature range of 4.2–106 K. A shallow minimum at $\text{T}\text{T}{}_{\mathrm{N}}\text{? 0.72}$ is observed in χ_⊥. The field dependence of the magnetization shows a smooth rotation of the spins to a normal direction above H_c ? 2.43 kOe at 4.2 K.     

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms N _Fe ～ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.     

Enhanced temperature-independent magnetoresistance below the metal-insulator transition temperature of epitaxial La0.2Nd0.4Ca0.4MnO3 thin films

Epitaxial La_0.2Nd_0.4Ca_0.4MnO₃ thin films have been deposited at 800°C on LaAlO₃ substrate using pulsed laser deposition technique. The structural and magnetotransport properties of the films have been studied. The sharp peak in the temperature dependence of the resistance corresponding to metal-to-insulator transition (T _p) has been observed at a temperature of T _p=82 K, 97 K and 110 K for 0 Oe, 20 kOe and 40 kOe magnetic fields, respectively. The film exhibits a large nearly temperature-independent magnetoresistance around 99% in the temperature regime below T _p. The zero field-cooled (ZFC) and field-cooled (FC) magnetization data at 50 Oe shows irreversibility between the ZFC and FC close to the ferromagnetic transition temperature T _c=250 K. The ZFC temperature data of the film displays ferromagnetic behavior for higher temperature regime T _c=250 K>T>T _p=82 K, and a decrease in magnetization with decreasing temperature up to 5 K below 82 K exhibiting a sort of antiferromagnetic behavior in the low temperature regime (T<82 K=T _p=T _N).     

Magnetic and electronic properties of the antiferromagnetic YbFe4Al8 compound

The magnetic, electrical and electronic properties of the tetragonal ternary YbFe₄Al₈ compound have been investigated. This compound was supposed to be an antiferromagnetic superconductor due to the negative magnetization signal appearing at a low field of the field cooling mode, however, based on the measurements of the temperature dependence of magnetization and resistivity we do not confirm the presence of superconductivity in this material and we ascribe the negative magnetization to the complicated non-collinear magnetic structure. A switch to the antiferromagnetic order at about 150 K has been visible both on the M(T) and ρ(T) curves. The valence state of the Yb ions has been studied by X-ray photoemission spectroscopy. The valence band spectrum at the Fermi level exhibits the domination of the hybridized Yb(4f) and Fe(3d) states.     

Magnetic properties of polycrystalline films of CoCr2O4 and CoFe0.5Cr1.5O4 multiferroics

This paper reports on the first study of the magnetic properties of polycrystalline films of CoCr₂O₄ and CoFe_0.5Cr_1.5O₄ multiferroics. The study covered, in particular, magnetization reversal curves and temperature dependences of the magnetization at temperatures ranging from 4.2 to 300 K in magnetic fields of up to 10 kOe. It has been shown that the Curie temperature and the pattern of the temperature dependence of the magnetization depend on the cation composition of the multiferroic. The temperature dependence of the magnetization of polycrystalline CoCr₂O₄ films has revealed an anomaly in the temperature range 10–70 K.     

Magnetic properties of biomineral particles produced by bacteria <Emphasis Type="Italic">Klebsiella oxytoca</Emphasis>

Ferrihydrite nanoparticles (2–5 nm in size) produced by bacteria Klebsiella oxytoca in the course of biomineralization of iron salt solutions from a natural medium exhibit unique magnetic properties: they are characterized by both the antiferromagnetic order inherent in a bulk ferrihydrite and the spontaneous magnetic moment due to the decompensation of spins in sublattices of a nanoparticle. The magnetic susceptibility enhanced by the superantiferromagnetism effect and the magnetic moment independent of the magnetic field provide the possibility of magnetically controlling these natural objects. This has opened up the possibilities for their use in nanomedicine and bioengineering. The results obtained from measurements of the magnetic properties of the ferrihydrite produced by Klebsiella oxytoca in its two main crystalline modifications are reported, and the data obtained are analyzed theoretically. This has made it possible to determine numerical values of the magnetic parameters of real biomineral nanoparticles.     

Modeling of the blocking temperature of a system of core/shell nanoparticles

A theoretical study was conducted on the size dependence of the blocking temperature T_b of a system of interacting core/shell nanoparticles. A method for estimating the blocking temperature of interacting core/shell nanoparticles is presented, which allows T_b to be calculated more precisely than using the “Neel relation”. It was shown that with an increase in the intensity of the magnetostatic interaction (concentration of nanoparticles), the blocking temperature increases, while the growth of the external magnetic field leads to the opposite effect. Moreover, the T_b of large nanoparticles changes more significantly. Comparing different approaches, we identify a precise method for determining the blocking temperature from ZFC and FC magnetization curves.     

Ferrimagnetism and hyperfine interactions in RFe5Al7(R = rare earth)

Magnetization and Mössbauer (⁵⁷Fe, ¹⁵⁵Gd) studies of RFe₅Al₇(R = Y, Sm to Lu, ThMn₁₂ crystal structure) in magnetic fields up to 50 kOe and temperatures 4.1 to 500 K have been performed. The Mössbauer studies yield the distribution of the iron ions among the various inequivalent crystallographic and magnetic sites, the hyperfine fields and their temperature dependence. The magnetization curves display a great variety of unusual magnetic phenomena. Among those; strong anisotropy, magnetic and thermal hysteresis (H_c = 24 kOe for DyFe₅Al₇ at 4.1 K), negative magnetization at low temperatures when the sample is cooled in a magnetic field (even in 1 Oe), compensation points, maxima points and time-dependent magnetization. Most of the phenomena observed can be explained in terms of a spin structure previously suggested for the RFe₆Al₆ compounds, composed of 4 magnetic sublattices. The rare earth moments lie antiparallel to the iron moments in the (j) site and to the ferromagnetic component of a canted antiferromagnetic structure of iron in the (f) site. Iron in the (i) site is nonmagnetic.     

Magnetization anisotropy in the AFM and SDW phases of CeB6

The angular dependences of the magnetization and Hall resistance have been investigated by the method of the sample rotation in the magnetic field in the high-quality single-crystal samples in the paramagnetic and magnetically ordered phases of CeB₆ in the magnetic field up to 60 kOe. It has been shown that, as CeB₆ undergoes the transition from the antiferromagnetic modulated phase to the so-called antiferroquadrupolar phase, the easy-magnetization axis in the [110] plane changes from 〈100〉 to 〈110〉. The magnetic field dependences of the anisotropic component of the magnetization differ radically in these magnetically ordered phases. The analysis provides evidence in favor of the formation of a state with the spin density wave (SDW phase) in the temperature range T _N ≈ 2.3 K < T < T _Q ≈ 3.3 K in CeB₆.     

Mössbauer and magnetic studies of orthorhombic FeSiO3

Magnetic properties of orthoferrosilite FeSiO₃ have been examined using susceptibility, magnetization measurements and Mössbauer spectroscopy. From magnetic and Mössbauer measurements, one obtains close values of the magnetic ordering temperature, T_N=39±1 K and T_N=41±1 K, respectively. The magnetic order is characterized by strong ferromagnetic coupling of Fe²⁺ moments within the ribbons and a weak antiferromagnetic coupling of the moments between adjacent ribbons. The 4.2 K Mössbauer spectra can be fitted with two different hyperfine magnetic fields H_hf=68 kOe and H_hf=314 kOe which can be assigned to Fe²⁺ in the octahedrally coordinated M1 and M2 sites, respectively, of the FeSiO₃ structure.     

Transition De Phase Metamagnetique Du Bromure Ferreux

In a magnetic field parallel to the magnetization axis of an antiferromagnetic Fe Br₂ single crystal, a caracteristic metamagnetic behaviour is observed. The transition from an antiferromagnetic phase to a paramagnetic phase is studied by help of magnetization measurements in a steady field (H < 60 kOe). The measurement precision has allowed a detailed study of the magnetization isotherms, caracteristic of a first order magnetization phase transition (T < T_c = 4, 7 K) and of a second order phase transition (T_c < T < T_N = 14, 2 K).We have observed an original phase diagram. In a certain temperature and field range, the ordered phase is stable on the high temperature side of the transition point. Some theoretical studies in an Ising model, or in the hypothesis of a strong magnetoelastic coupling forecast the existence of such a magnetic phase diagram.At present, we proceed to a theoretical study, in a molecular field approximation, of the magnetic phase diagram of compounds similar to Fe Br₂ where we take into account the relative values of parameters J₁, J₂ and D associated with ferromagnetic and antiferromagnetic interactions and crystalline anisotropy.     

Biquadratic exchange interactions in the Europium monochalcogenides

We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetization data of the paramagnetic phase and elaborate the consequences this additional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility χ₃ obeys a Curie-Weiss law at suffciently high temperatures and that the associated Curie-Weiss temperature θ₃ is a measure for the biquadratic interaction strength. For all these materials the biquadratic interaction is ferromagnetic (θ₃ > 0). This leads to a conflicting situation in the case of EuTe for which θ₁ < 0. We attribute the peculiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that moment expected for perfect magnetic order, to the presence of biquadratic interactions. The critical field B_c follows a T² law in the spin-wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility χ₃ diverges at a temperature T* which is 2.5 K and 1.2 K above the ordering temperature, respectively. In the temperature range T_c < T < T* the magnetization curves exhibit some weak but definite anomaly which might be interpreted as a field-induced transition into the ferromagnetic state. A new multicritical point has been identified along the critical field curve B_c of EuSe.     

Temperature dependence of the lattice parameters and magnetic properties of Er3Ir single crystals

Er₃Ir single crystals were grown by the Czochralski method from a levitated melt. The electrical resistivity thermal dependence exhibits ordering temperature of the erbium sublattice at 40 K and a spin reorientation process at 22 K. The DC and AC magnetic susceptibility show antiferromagnetic ordering in the form of an asymmetric peak. The magnetization in strong magnetic fields up to 140 kOe exhibits anisotropy. The lattice parameters’ thermal dependence of Er₃Ir and Er₃Ni show anisotropy and anomalous behaviour.