Component Of Magnetic Susceptibility Research Articles

Magnetic relaxation between ferromagnetic and helix spin configurations in holmium films

Multiple noncollinear spin structures and transitions between them initiated by field and temperature have attracted attention due to the extraordinary coexistence of complicated spin phases in Ho. We explore the magnetic relaxation dynamics within thin films undergoing transitions between ferromagnetic (FM) and helix states. When measuring susceptibility in a relatively high frequency range (∼100–1500 Hz) and magnetic viscosity at lower frequency (∼0.01 Hz), we focus on 400 nm thick Ho film. Notably, sharp variations in the real and imaginary components of magnetic susceptibility are discerned during the FM – Helix transition, occurring at temperatures between 15 and 30 K. Hysteresis effects in the magnetic susceptibility components are identified during cyclic variations in the external field, indicating a relatively rapid process with a timescale of approximately 10 ms accompanying the FM – Helix transition. The slow relaxation process is also found to exhibit sensitivity to this transition. Furthermore, the field dependence of magnetic viscosity displays a marked decline at the FM – Helix transition. The research methodologies proposed for the investigation of relaxation processes in materials with multiphase spin structures are deemed universally applicable and offer insights into transitions between spin states in materials manifesting non collinear spin structures.

Magnetic susceptibility and phase transitions in LiNiPO4

Detailed studies of specific heat, magnetization, and magnetic torque of a single crystal of ${\mathrm{LiNiPO}}_{4}$ olivine are presented. Olivines attract attention as promising for application as cathodes in Li-ion batteries and exhibiting a unique set of properties. ${\mathrm{LiNiPO}}_{4}$ is the unique olivine, in which antiferromagnetic order develops in two steps, i.e., at 21.8 K, the second-order transition to an incommensurate phase and then, at 20.9 K, the first-order transition to a commensurate phase appears. Specific heat studies, supplemented by the ``slope analysis procedure,'' revealed a splitting of the specific heat anomaly accompanying the first-order transition, which suggests that actually, these are two coupled transitions, one of which can be the ferroelectric one. The specific heat was measured as a function of temperature for a series of fixed external magnetic field $B$ values. As the result, analytical equations describing the phase transition lines in the $T\text{\ensuremath{-}}B$ plane were determined and evolution of a shape of the specific heat anomalies accompanying the phase transitions, observed in the powder sample under influence of $B$, was modeled. Angular dependence of magnetic torque and of magnetization for $B$ rotating within the $a\text{\ensuremath{-}}c$ and $b\text{\ensuremath{-}}c$ crystalline planes was measured for several fixed temperature and $B$ values. Based on these results, we found a new effect, that we called ``off-diagonal nonlinear magnetic susceptibility,'' i.e., we found that for each main crystallographic axis $(a,b$, and $c)$, an additional component of magnetic susceptibility, proportional to the square of the perpendicular to this axis component of $B$, exists. A phenomenological model of this effect, describing the experimental results correctly, was proposed.

Spin correlation function theory of spin-charge conversion effects

Theory of spin-charge conversion effects in spintronics are presented in terms of correlation functions of physical observables, spin and electric current. Direct and inverse spin Hall effects and spin pumping effect are studied considering metallic systems with random spin-orbit interaction and spatially nonuniform Rashba interaction. The theory is free from ambiguity associated with spin current, and provides clear physical picture of the spin-charge conversion effects. In the present approach, so-called the spin current transmission efficiency is essentially the nonuniform component of magnetic susceptibility.

Magnetic susceptibility of Griffiths like phase in poly- and nanocrystalline manganites La0.7Ca0.3MnO3

The Griffiths like phase is probed by the electron spin resonance and AC magnetic susceptibility in the poly- and nanocrystalline mixed valence manganites La0.7Ca0.3MnO3. The novel method to analyze the Griffiths phase allows to determine a quickly decaying temperature variation of magnetic susceptibility of Griffiths phase in a whole interval between the Curie and Griffiths temperatures. A location of Griffiths phase is additionally confirmed by a nonvanishing out-of-phase component of magnetic susceptibility.

Effect of Co2O3 Addition on Stability of Permeability to an Impulse Magnetic Field in NiCuZn Ferrites

The NiCuZn ferrites doped with Bi2O3 and Co2O3 were prepared by the solid-state reaction method. Micrographs were obtained by SEM. The complex permeability spectra were measured by Agilent E4991A and 16454A. Variation of the permeability under a magnetic field impulse was measured by Agilent 4284A. Effect of the Co2O3 content on the permeability spectra of the NiCuZn ferrite samples before and after exposing to an impulse magnetic field was discussed. The corresponding permeability spectra of the samples were decomposed into the domain wall motion component and the spin rotation component. The results show that the permeability at 1 MHz decreases and the resonance frequency increases with more Co2O3 content, and Co2+ have different effect on the two kinds of magnetic susceptibility components. Different variation was revealed for the domain wall motion component and the spin rotation component after exposing to an impulse magnetic field. The decline rate of susceptibility by domain wall motion is smaller than that of susceptibility by spin rotation.

Magnetic behavior of Bi5Nb3–3xFe3xO15–δ solid solutions

Most bismuth compounds with layered perovskite-like structure, analogs of Aurivillius phases, exhibit ferroelectric properties and are of interest from both practical and theoretical points of view. In this study, we carried out magnetochemical measurements of an electronic state and exchange interactions between iron atoms in the solid solutions Bi5Nb3O15, obtained by heterovalent substitution of niobium by atoms of iron (III). The iron-containing solid solutions were obtained in a narrow concentration range at x≤0.06. The iron-containing solid solutions at concentrations x> 0.005 are characterized by monoclinic distortion of the tetragonal cell (P2/m). The measurements of magnetic susceptibility of the solid solutions were carried out by the Faraday method in the temperature range of 77 - 400 K. The isotherms of a paramagnetic component of magnetic susceptibility of iron atoms are typical of antiferromagnets. The effective magnetic moment of single iron atoms increases with increasing temperature from 6.87 mB (90 K) to 6.92 mB (320 K). The study of the iron-containing Bi5Nb3O15 solid solutions has shown that, in the solid solutions iron (III) atoms aggregate to form dimers and tetramers with antiferromagnetic and ferromagnetic types of exchange. The values of antiferromagnetic and ferromagnetic exchange in dimers, Jdim = - 60 cm-1 and Jdim = 40 cm-1, and tetramers, Jtetr = - 40 cm-1 and Jtetr = 25 cm-1 correspondently.

Spin glass and ferromagnetic properties of Ce(Cu1-xNix)4Mn alloys: Multicritical points in the magnetic phase diagram

We report on the CeNi4Mn (ferromagnet FM) - CeCu4Mn (spin-glass SG) transformation leading to a complex magnetic phase diagram (MPD). It is verified that all the Ce(Cu1-xNix)4Mn alloys are isostructural and the transformation is governed only by the Cu-Ni substitution. MPD is built based on the magnetic dc/ac susceptibility measurements and reveals SG formation as well as the region of the coexistence of the FM and SG state in the middle range of the Ni concentration. The complex MPD is explained by clusters formation and a competition of interactions between various crystallographic sites of the hexagonal CaCu5-type structure, mainly the 3g-3g and 3g-2c interactions. The predominance of the SG state is confirmed by the analysis of the frequency dependence of the ac magnetic susceptibility components and the relaxation of the remanent magnetization. Additionally, the presence of two multicritical points is observed.

Quantitative β mapping for calibrated fMRI

The metabolic and hemodynamic dependencies of the blood oxygenation level-dependent (BOLD) signal form the basis for calibrated fMRI, where the focus is on oxidative energy demanded by neural activity. An important part of calibrated fMRI is the power–law relationship between the BOLD signal and the deoxyhemoglobin concentration, which in turn is related to the ratio between oxidative demand (CMRO2) and blood flow (CBF). The power–law dependence between BOLD signal and deoxyhemoglobin concentration is signified by a scaling exponent β. Until recently most studies assumed a β value of 1.5, which is based on numerical simulations of the extravascular BOLD component. Since the basal value of CMRO2 and CBF can vary from subject-to-subject and/or region-to-region, a method to independently measure β in vivo should improve the accuracy of calibrated fMRI results. We describe a new method for β mapping through characterizing R2′ – the most sensitive relaxation component of BOLD signal (i.e., the reversible magnetic susceptibility component that is predominantly of extravascular origin at high magnetic field) – as a function of intravascular magnetic susceptibility induced by an FDA-approved superparamagnetic contrast agent. In α-chloralose anesthetized rat brain, at 9.4T, we measured β values of ~0.8 uniformly across large neocortical swathes, with lower magnitude and more heterogeneity in subcortical areas. Comparison of β maps in rats anesthetized with medetomidine and α-chloralose revealed that β is independent of neural activity levels at these resting states. We anticipate that this method for β mapping can help facilitate calibrated fMRI for clinical studies.

Brain region and activity-dependent properties of M for calibrated fMRI

Calibrated fMRI extracts changes in oxidative energy demanded by neural activity based on hemodynamic and metabolic dependencies of the blood oxygenation level-dependent (BOLD) response. This procedure requires the parameter M, which is determined from the dynamic range of the BOLD signal between deoxyhemoglobin (paramagnetic) and oxyhemoglobin (diamagnetic). Since it is unclear if the range of M-values in human calibrated fMRI is due to regional/state differences, we conducted a 9.4T study to measure M-values across brain regions in deep (α-chloralose) and light (medetomidine) anesthetized rats, as verified by electrophysiology. Because BOLD signal is captured differentially by gradient-echo (R2*) and spin-echo (R2) relaxation rates, we measured M-values by the product of the fMRI echo time and R2’ (i.e., the reversible magnetic susceptibility component), which is given by the absolute difference between R2* and R2. While R2’ mapping was shown to be dependent on the k-space sampling method used, at nominal spatial resolutions achieved at high magnetic field of 9.4T the M-values were quite homogenous across cortical gray matter. However cortical M-values varied in relation to neural activity between brain states. The findings from this study could improve precision of future calibrated fMRI studies by focusing on the global uniformity of M-values in gray matter across different resting activity levels.

Understanding bifurcations in FC–ZFC magnetization of dilutely Fe3+ doped CdS nanoparticles

The bifurcation between field-cooled and zero-field-cooled curves in magnetization vs. temperature measurement has been investigated in Fe3+ doped CdS nanoparticles with different crystal structures (wurtzite and zinc-blende) and external magnetic fields. It has been found that the field-cooled and zero-field-cooled curves start bifurcating from ambient temperature in diluted (powder) samples irrespective of crystal structures. However, in the high density samples (prepared by pelletizing the powder samples at a pressure of ~200MPa) the bifurcation is significantly reduced within entire temperature regime. We have shown that the observed bifurcation in diluted (powder) samples is neither a consequence of blocking of super-moments in nanoparticle systems nor the spin-glass type transition, rather dominated by temperature independent component of magnetic susceptibility.

Stable Colloidal Suspension of Magnetic Nanoparticles for Applications in Life Sciences

Various utilizations of magnetitc nanoparticles (MNPs), highly biocompatible, in biomedicine and environmental sciences require not only fine granulated powder but also stable dispersion in fluid media. MNPs were synthesized by chemical co-precipitation route. MNP surface modification was carried out with oleate ions as coating shell able to ensure colloidal MNP stability and also allowing further adding of secondary organic shell for conferring hydrophylicity or possibility of grafting various biomolecules. Granularity of colloidal MNPs was analyzed using microscopy techniques, crystalline properties by X-ray diffraction while superparamagnetic properties by magnetometry were assessed. The influences of magnetic and non-magnetic stirring, used alternatively during the preparation steps - on the microstructural and magnetic features are discussed. The MNP suspensions response to electromagnetic fields was studied by measuring the components of magnetic susceptibility. The investigation results suggested the usefulness of the yielding technology based on non-magnetic mixing during both ferrophase preparation and surface modification.

Charge and magnetic states of rutile TiO2 doped with Cr ions

We observe that the electronic and magnetic properties of Cr-doped rutile single crystals are highly dependent on growth conditions. The ferromagnetic component of magnetic susceptibility is observed to be enhanced for samples grown under oxygen-rich conditions. To understand the charge state of Cr dopants and their role in response to an external magnetic field, we carry out density functional theory calculations for Cr-doped rutile . Using the results of formation energy calculations in the presence of oxygen vacancies and Cr atom substitution at the Ti sites, we demonstrate that the state is a source of Curie–Weiss-type magnetic response, whereas the defect states contribute to the ferromagnetic component. We also provide the electronic structures of various defect configurations and attempt to explain the optical and electronic properties of the Cr-doped system.

Low energy spin dynamics in trimer spin chain compound Ca3Cu2Ni(PO4)4: 31P NMR study

31P nuclear-magnetic-resonance (NMR) study in the trimer spin chain compound Ca3Cu2Ni(PO4)4 is performed in the presence of different external magnetic fields in the temperature range 4–300K. The results suggest that the three magnon mediated scattering process which was the dominant contribution to 1/T1 in the pure compound Ca3Cu3(PO4)4, has negligible contribution in Ca3Cu2Ni(PO4)4, with the two magnon mediated Raman process being dominant. The linear variation of nuclear spin lattice relaxation rate 1/T1T with the magnetic susceptibility χ indicates that the Q component of magnetic susceptibility χ(Q) is nearly proportional to χ, where Q is the antiferromagnetic wave vector. A change in the slope of 1/T1T with respect to χ near 15K suggests a change in the antiferromagnetic spin fluctuation spectrum. Below 15K, the spin diffusion mechanism governs the 1/T1 process. This could be a signature of the development of short range magnetic correlation or due to the contribution arising from the paramagnetic impurity spins.

Magnetic properties of Ce3+ ion in iron-containing oxypnictide CeFeAsO

Magnetic properties of a Ce3+ ion in two crystallographic phases of a nonsuperconducting oxipnictide CeFeAsO were studied. Ce3+-ion energy levels, g-factor values, temperature dependences of magnetic susceptibility components and the forth-order anisotropy constant were calculated within a modified crystal field approach. It was shown that in the tetragonal and orthorhombic phases, the easy direction for the magnetic moment of Ce3+ ions corresponds to [110] axis. Comparison of the calculated temperature dependences of susceptibility components to the respective experimental data showed that at temperatures below 100 K, it is necessary to take into account the internal field produced by the magnetic subsystem of iron ions.

Paramagnetic Behaviour in RE<sub>2</sub>W<sub>2</sub>O<sub>9</sub> Tungstates (RE = Pr, Nd, Sm – Gd)

Magnetic susceptibility measurements revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to the magnetic field and the temperature for Sm2W2O9 and Eu2W2O9 tungstates. The temperature independent component of magnetic susceptibility has a positive value for RE2W2O9 (RE = Pr, Nd and Gd) indicating a domination of van Vleck contribution. Only for Gd2W2O9 the magnetization is a universal function of µ0H/T, characteristic for the superparamagnetism.

51V NMR study of antiferromagnetic state and spin dynamics in quasi-one-dimensional BaCo2V2O8

We report on our ${}^{51}$V-NMR study of static and dynamical magnetic properties in the quasi-one-dimensional antiferromagnet BaCo${}_{2}$V${}_{2}$O${}_{8}$. Although the NMR spectrum shows well-defined antiferromagnetic (AF) order in the N\'eel ground state, the AF characteristic from the NMR spectrum is incomplete between 3.5 K and ${T}_{N}=5.4$ K, which could be affected by quantum spin fluctuations. The AF NMR spectrum indicates two V sites experiencing different magnetic field magnitudes, ${H}_{A1}=2.1$ kOe and ${H}_{A2}=3.8$ kOe. These internal fields could be explained by accounting for the classical and the pseudodipolar fields from Co${}^{2+}$ spins with a proposed magnetic structure based on the neutron diffraction measurements. In the paramagnetic state, the nuclear spin relaxation is dominated by AF spin fluctuations through the dipolar-type coupling between V and surrounding Co${}^{2+}$ ions. The linear relation between the nuclear spin-lattice relaxation rate $1/{T}_{1}T$ and the magnetic susceptibility $\ensuremath{\chi}$ indicates that the $Q$ component of magnetic susceptibility $\ensuremath{\chi}(Q)$ is roughly proportional to $\ensuremath{\chi}$, where $Q$ is the AF wave number. A change in slope of $1/{T}_{1}T$ with respect to $\ensuremath{\chi}$ around 150 K suggests a change in the AF spin fluctuation spectrum.

Single and multi-walled carbon nanotubes doped (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ superconductors

Carbon nanotubes (CNTs) added polycrystalline (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) superconductor samples have been synthesized at normal pressure. The X-ray diffraction (XRD) studies of these samples have shown an orthorhombic crystal structure following PMMM space group. The added CNTs have been found to diffuse at the inter-grain sites and act as cementing material. SEM images show that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the CuTl-1223 compounds improve the electrical connection between superconducting grains which in turn increase the Jc of the final compound. The room temperature resistivity of as-prepared samples decreases with the addition of CNT. The onset temperature of superconductivity [Tc(onset)] and zero resistivity critical temperature [Tc(R=0)] are found to decrease with the CNT additions. Except for the samples with CNT additions of 0.25%, the density [mass/volume] of final compound after CNT addition is decreased. Magnitude of diamagnetism is also suppressed with the addition of CNT which suggests that some of carbon atoms diffuse into the unit cells at the intra-grain sites. The Tc(R=0) becomes quite stable after post-annealing in oxygen atmosphere; the Tc(R=0) varies between 98 and 96K in polycrystalline (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) samples. The out of phase component of magnetic susceptibility observed as a broad peak in the as-prepared samples, is transformed to a sharp peaks after post-annealing of the samples in oxygen atmosphere. This shows that oxygen diffusion converts the inter-grain material to respective oxides and carbonates that increase the surface areas of the current flow around inter-grain loops that reduce the hysteresis losses. The electron-microscopy of these samples has shown that inter-grain voids population is suppressed with the addition of CNTs. FTIR absorption measurements of (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) samples have shown apical and planar oxygen modes at their usual positions.

A new highly sensitive broadband ferromagnetic resonance measurement system with lock-in detection

RF/microwave magnetic measurement systems can be categorized into broadband or narrowband systems. Narrowband systems utilize high Q cavities for magnetic material characterizations such as electron paramagnetic resonance or electron spin resonance. Example broadband characterization systems are permeameters or pulsed inductive magnetometers. Narrowband measurement systems have high sensitivity but limited bandwidth whereas broadband systems suffer from lower sensitivity. In particular, it has been challenging to precisely measure weak magnetization, high anisotropy and high ferromagnetic resonance linewidth across a broad bandwidth. In this work, we report a new highly sensitive broadband resonance measurement system with lock-in detection utilizing a broadband transmission line. The frequency of the RF source is swept while the magnetic field is fixed. The resulting curve of dP/dH versus f is similar to the dP/dH versus H curve from conventional narrowband magnetic measurement system. The imaginary component of magnetic susceptibility χ" is calculated by the equation of χ"(f)=C∫(dP/dH*dH/df)df, in which C is a constant and dH/df is determined from the Kittel equation for magnetic films. Broadband measurements of sputtered low-moment lossy NiCr (4πMs=1100 G) as well as NiFe (4πMs=11000 G) were taken and compared with conventional broadband measurement using a network analyzer.

Superparamagnetic-like behavior and spin–orbit coupling in (Co,Zn)RE 4W 3O 16 tungstates (RE=Nd, Sm, Eu, Gd, Dy and Ho)

Magnetic susceptibility measurements carried out on (Co,Zn)RE 4W 3O 16 compounds revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to magnetic field and temperature for ZnSm 4W 3O 16 and ZnEu 4W 3O 16 samples. The temperature independent component of magnetic susceptibility has a negative value for ZnGd 4W 3O 16 and a positive one for the rest of the tungstates, indicating a domination of van Vleck contribution. The magnetization isotherms of majority of the tungstates under study revealed both spontaneous magnetic moments and hysteresis characteristic for the superparamagnetic-like behavior with blocking temperature T B ∼30 K, except for ZnEu 4W 3O 16. Fitting procedure of the Landé factor revealed that the stronger the orbital contribution, the weaker the superparamagnetic effect, namely for ZnRE 4W 3O 16. In case of CoRE 4W 3O 16 a significant participation of the Co 2+ moment in the spontaneous magnetization was observed.

Magnetic properties of Ni3Al x Mn1 − x alloys

Paramagnetic susceptibility of alloys Ni3Al x Mn1 − x in the field of the transition between the ferromagnets described in the band theory (Ni3Al) and in the spin-localized (Ni3Mn) model of magnetic moments has been investigated. The concentration dependences of the paramagnetic Curie temperature, effective magnetic moment, and temperature-independent component of magnetic susceptibility have been determined.