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 with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Low frequency magnetic response in antiferromagnetically coupled Fe/Cr multilayers

The magnetic field and temperature dependence of the low frequency magnetic response of antiferromagnetically coupled Fe/Cr(100) multilayers has been studied between +/-500 Oe, from 2 to 300 K. At T = 2 K the losses exhibit an unusually strong frequency dependence which can be described within a single relaxation time scheme. This relaxation time proves to be strongly field dependent. These phenomena are specific for epitaxial multilayers with large magnetoresistance. The behavior of the relaxation time at low temperatures might be related to some quantum tunneling processes.     

Monte Carlo simulation of the reorientation transition in Heisenberg model with dipole interactions

The change of magnetic states in ultrathin films with temperature have been simulated by Monte Carlo method. A Heisenberg model with long-range dipole interactions was adopted in our calculations. The results were qualitatively in good agreement with the experimental phenomena. That is at low temperatures the magnetization is perpendicular to the plane, and at higher temperatures but below the Curie point, the magnetization is mostly within the plane. In between these two regions, the magnetization seems to be suppressed. The simulations show that the loss of magnetization is a consequence of the special magnetic states in which the local domains orientations are reverse with the neighbor ones.     

Thermally activated magnetization reversal process of self-assembled Fe55Co45 nanowire arrays

We have investigated the temperature dependence of the magnetic properties and the magnetic relaxation of the Fe₅₅Co₄₅ nanowire arrays electrodeposited into self-assembled porous alumina templates with the diameter about 10 nm. X-ray diffraction (XRD) pattern indicates that the nanowire arrays are BCC structure with [1 1 0] orientation along the nanowire axes. Owing to the strong shape anisotropy, the nanowire arrays exhibit uniaxial magnetic anisotropy with the easy magnetization direction along the nanowire axes. The coercivity at 5 K can be explained by the sphere chains of the symmetric fanning mechanism. The temperature dependence of coercivity can be interpreted by thermally activated reversal mechanism as being the localized nucleation reversal mechanism with the activation volume much smaller than the wire volume. Strong field and temperature-dependent magnetic viscosity effects were also observed.     

Free precession and transverse relaxation of hyperpolarized <Superscript>129</Superscript>Xe gas detected by SQUIDs in ultra-low magnetic fields

We studied the free precession of the nuclear magnetization of hyperpolarized ¹²⁹Xe gas in external magnetic fields as low as B₀ = 4.5 nT, using SQUIDs as magnetic flux detectors. The transverse relaxation was mainly caused by the restricted diffusion of ¹²⁹Xe in the presence of ambient magnetic field gradients. Its pressure dependence was measured in the range from 30 mbar to 850 mbar and compared quantitatively to theory. Motional narrowing was observed at low pressure, yielding transverse relaxation times of up to 8000 s.     

Temperature dependence of the magnetization of canted spin structures

Numerous studies of the low-temperature saturation magnetization of ferrimagnetic nanoparticles and diamagnetically substituted ferrites have shown an anomalous temperature dependence. It has been suggested that this is related to freezing of canted magnetic structures. We present models for the temperature dependence of the magnetization of a simple canted spin structure in which relaxation can take place at finite temperatures between spin configurations with different canting angles. We show that the saturation magnetization may either decrease or increase with decreasing temperature, depending on the ratio of the exchange coupling constants. This is in agreement with experimental observations.     

On the correlation between microstructure and magnetic losses in electrical steel

The magnetic properties of electrical steel such as magnetization curves, magnetization behavior and specific magnetic losses are related to the microstructure and the texture of the steel. A quantitative model, which describes the effect of microstructure and texture and their interplay on the magnetic losses P, is still missing. Based on experimental data for nonoriented electrical steels and FeSi-samples with high (Si, Al)-content, a more general formula is proposed for the dependence of P, at a given value of magnetic induction B, as a function of the mean value of the grain size d of the material and of the intensities of the relevant magnetic texture components.     

Magnetic relaxation in superfluid<Superscript>3</Superscript>He-B at very low temperatures

Main properties of the spin supercurrents and coherent precession of magnetization in the superfluid³He-B in hydrodynamic regime seem to be very well understood now. But recently surprisingly new unpredicted phenomena such as, for example, “catastrophic” relaxation, persistent spin precession, very strong magnetic relaxation, etc., have been observed in³He-B at ultralow temperatures in so-called non-hydrodynamic regime using both pulse and cw-NMR techniques. This paper deals with some of these new phenomena (a “linear term” in magnetic relaxation and a reduction of magnetization of coherent precession with magnetic field gradient) observed by cw-NMR technique, compares these results with new effects found by pulse NMR and speculates about the nature of these new phenomena.     

Local field dynamics in a resonant quantum tunneling system of magnetic molecules

We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual correlation effects in these systems. Received: 15 May 1998 / Revised: 10 July 1998 / Accepted: 11 July 1998     

Magnetic anisotropy and low-frequency dielectric response of weak ferromagnetic phase in κ-(BEDT-TTF)2Cu[N(CN)2]Cl, where BEDT-TTF is Bis(ethylenedithio)tetrathiafulvalene

We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor -(BEDT-TTF)₂Cu[N(CN)₂]Cl by means of magnetic anisotropy measurements and low-frequency dielectric spectroscopy. Magnetic anisotropy obeys Curie-Weiss law with negative Curie-Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal of the weak ferromagnetic magnetization are documented for the first time. A broad dielectric relaxation mode of moderate strength () emerges at 32 K, and weakens with temperature. The mean relaxation time, much larger than that expected for single-particle excitations, is thermally activated in a manner similar to the DC conductivity and saturates below 22 K. These features suggest the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain wall in a random ferromagnetic domain structure as the relaxation entity. We argue that the observed features might be well described if Dzyaloshinsky-Moriya interaction is taken into account. A Debye relaxation with similar temperature dependence was also observed and seems to be related to an additional ferromagnetic-like, most probably, field-induced phase. We tentatively associate this phase, whose tiny contribution was sample dependent, with a Cu²⁺ magnetic subsystem. Received 15 June 1998 and Received in final form 1 February 1999     

Variation of permeability of Nb-poor Finemet under different field amplitudes

The effect of magnetic field amplitude on permeability spectra was studied by using permeability spectroscopy. Results show that the permeability spectra can be greatly affected by magnetic field amplitude—they become complicated under a higher field, which means that the initial permeability increases with an increase of the applied field at first and then decreases with further increase. Meanwhile, another relaxation peak appears and the relaxation frequency increases during the same process. In addition, the permeability is improved significantly over the basic Finemet composition with appropriate substitution of Al for Nb. Moreover, the effect of annealing condition on the permeability spectra as well as the influence of eddy current was also discussed. All these phenomena were explained by using the dynamic magnetization theory and random anisotropy model.     

Slow Vortex Creep Induced by Strong Grain Boundary Pinning in Advanced Ba122 Superconducting Tapes

We report the temperature, magnetic field and time dependences of magnetization in advanced Ba122 superconducting tapes. The sample exhibits peculiar vortex creep behavior. Below 10 K, the normalized magnetization relaxation rate S = d ln(-M)/d ln(t) shows a temperature-insensitive plateau with a value comparable to that of low-temperature superconductors, which can be explained within the framework of collective creep theory. It then enters into a second collective creep regime when the temperature increases. Interestingly, the relaxation rate below 20 K tends to reach saturation with increasing the field. However, it changes to a power law dependence on the field at a higher temperature. A vortex phase diagram composed of the collective and the plastic creep regions is shown. Benefiting from the strong grain boundary pinning, the advanced Bal22 superconducting tape has potential to be applied not only in liquid helium but also in liquid hydrogen or at temperatures accessible with cryocoolers.     

Relaxation phenomena in ensembles of CoFe2O4 nanoparticles

Collective magnetic behavior of CoFe₂O₄ nanoparticles with diameters of 76, 16, 15 and 8 nm, respectively, prepared by different chemical methods has been investigated. Particle composition, size and structure have been characterized by inductive coupled plasma (ICP), transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Basic magnetic properties have been determined from the temperature dependence of magnetization and magnetization isotherms measurements. The three samples exhibit characteristic of a superparamagnetic system with the presence of strong interparticle interactions. Magnetic relaxation phenomena have been examined via frequency-dependent ac susceptibility measurements and aging and memory effect experiments. For the particles coated with oleic acid, it has been demonstrated that the sample reveals all attributes of a super-spin glass (SSG) system with strong interparticle interactions.     

Magnetic relaxation in pulse-magnetized high-temperature superconductors

A magnetic filed relaxation at the center of a pulse-magnetized single-domain Y–Ba–Cu–O superconductor at 78 K has been studied. In case of a weak magnetization, the magnetic flux density increases logarithmically and normalized relaxation rate defined as S = −d(lnB)/d(lnt) is negative (S = −0.037). When an external magnetic field magnitude increases, the relaxation rate first decreases in absolute value, then changes sign (becomes positive, S > 0) and after reaching some maximum finally reduces to a very small value. Non-monotonous dependence of S vs. H_a is explained by a non-homogeneous local temperature distribution during a pulse magnetization.     

Effects of R-site compositions on the meta-magnetic behavior of Tb_(1-x)Pr_x(Fe_(0.4)Co_(0.6))_(1.88)C_(0.05)(x=0, 0.8, and 1)

We investigate the low-temperature magnetic properties of intermetallic compounds Tb1-xPrx(Fe0.4Co0.6)1.88C0.05(x = 0, 0.8, and 1) by detailed magnetization measurements. Obvious temperature- and field-induced irreversibilities suggest the coexistence of multiple magnetic phases. Sharp magnetization jumps across the antiferromagnetic to ferromagnetic transition are observed only in the Pr-containing samples, indicating that the behavior of the avalanche-like growth of ferromagnetic clusters is mainly related to the light lanthanide Pr ions. In addition, the time relaxation, field sweep rate, and cooling field dependence of magnetization jumps in the sample with x = 1 are consistent with those in the martensitic scenario.     

Magnetic anomalies in nanocrystalline , a geometrically frustrated spin-chain compound

We have investigated the magnetic behavior of nano crystals, synthesized by high-energy ball-milling, for a well-known geometrically frustrated spin-chain system, Ca₃CoRhO₆, and compared its magnetic characteristics with those of the bulk form by measuring ac and dc magnetization. The features attributable to the onset of ‘partially disordered antiferromagnetism’ (characterizing the bulk form) are not seen in the magnetization data of the nano particles; the magnetic moment at high fields in the very low temperature range in the magnetically ordered state gets relatively enhanced in the nano particles. It appears that the ferromagnetic intrachain interaction, judged by the sign of the paramagnetic Curie temperature, is preserved in the nano particles. These trends are opposite to those seen in the nano particles of Ca₃Co₂O₆. However, the complex spin-dynamics as evidenced by large frequency dependence of ac susceptibility is retained in the nano particles as well. Thus, there are some similarities and dissimilarities between the properties of the nano particles and those of the bulk. We believe that these findings will be useful to understand correlation lengths deciding various properties of geometrical frustration and/or spin-chain phenomena.     

Natural ferromagnetic resonance in the disordered alloy Pd2AuFe

A contribution to electromagnetic power losses, additional to the losses due to eddy currents and exhibiting a resonance frequency dependence with the main maximum near 1 GHz, has been observed for the ferromagnetic alloy Pd₂AuFe in the frequency range 0.9 MHz–10 GHz in the absence of an external constant magnetic field. Investigations performed in a dc magnetic field show that this effect is a natural ferromagnetic resonance due to intradomain magnetization precession in the effective magnetic-anisotropy field. Fiz. Tverd. Tela (St. Petersburg) 40, 1900–1904 (October 1998)     

Polarization-dependent fractional loss improvement of laser scribed GO FeSi steels

Magnetization curves of untreated and laser scribed GO FeSi steels were measured for 19 different frequencies from 0.05 to 500 Hz and for four polarizations from 1.4 to 1.7 T. From hysteresis loops, hysteresis losses were separated and frequency-dependent anomaly factors were calculated. Frequency-dependent anomaly factors for all measured polarizations can be very well described by an empirical equation. This behavior can be explained by the fact that an increase in polarization at a fixed magnetizing frequency corresponds to an increase of magnetizing frequency at a fixed polarization. Both an increase in frequency and an increase in polarization activate a higher number of domain walls in the magnetization process. The power losses can be described only by the frequency dependence of the anomaly factor and by the additional knowledge of hysteresis loss.     

Magnetization processes in CoNiW films

The torque curves of hard magnetic CoNiW films with perpendicular magnetic anisotropy are analysed. Using the dependence of the rotational hysteresis energy losses on the field, both in the film plane and perpendicular to it, a conclusion is drawn about the magnetic structure of the films and the mechanism of magnetization reversal consisting of an inhomogeneous rotation of the magnetization vector.