High-field Susceptibility Research Articles

Features of itinerant magnetism in Fe0.6Al0.4 film with B2-like short-range order

The complex relationship between spontaneous magnetization (M s) and structural parameters in Fe–Al systems should be attributed to the B2-like short-range order (SRO) and itinerancy-induced sensitivity of magnetism in B2 FeAl. We focused on the role of SRO in the magnetism of Fe0.6Al0.4 film with the B2-like SRO. The high-field susceptibility (χHF) was observed through magnetometry, X-ray magnetic circular dichroism, and magneto-optical Kerr effect measurement, through which different χHF values were obtained. The M s–T curves deviated from the Brillouin function, and it can be interpreted following the spin-wave theory with a low spin-wave stiffness of 0.75 meV nm2. Based on the observed χHF and M s trends, the magnetic moment in the B2-like SRO was found to be induced by the exchange field from the surrounding ferromagnetic region. The coexistence of weak and strong ferromagnetic regions can be a possible reason for the complex magnetism in the Fe–Al system.

An ultra-low field SQUID magnetometer for measuring antiferromagnetic and weakly remanent magnetic materials at low temperatures.

A novel setup for measuring magnetic fields of antiferromagnets (i.e., quadrupolar or higher-order magnetic fields) and generally weakly remanent magnetic materials is presented. The setup features a highly sensitive superconducting quantum interference device magnetometer with a magnetic field resolution of ∼ 10fT and non-electric temperature control of the sample space for a temperature range of 1.5-65K with a non-electric sample movement drive and optical position encoding. To minimize magnetic susceptibility effects, the setup components are degaussed and realized with plastic materials in sample proximity. Running the setup in magnetically shielded rooms allows for a well-defined ultra-low magnetic background field well below 150 nT in situ. The setup enables studies of inherently weak magnetic materials, which cannot be measured with high field susceptibility setups, optical methods, or neutron scattering techniques, giving new opportunities for the research on, e.g., spin-spiral multiferroics, skyrmion materials, and spin ices.

The fall of volcanic material from Mount Lawu, Central Java: a case study of magnetic susceptibility

Mount Lawu was recorded to have erupted last on November 28, 1885, releasing volcanic material including magnetic minerals. The material settles in various places carried by the wind and river flow. The movement of wind and river flow is very influential in the process of transporting volcanic material. Large material directly falls around the mountain, usually in the form of large rocks, and undergoes physical and chemical weathering. Small-sized materials such as volcanic ash and small-mass rocks will be carried by the wind and river flows in all directions. However, the displacement does not remove the magnetic minerals contained in it. Magnetic minerals can be evaluated by the method of rock magnetism using MS. Bartington MS2 device with a dependent MS2B sensor by mass. From this tool, the low field susceptibility values in the range of 1467.5 x 10−8 m3/Kg – 5262.1 x 10−8 m3/Kg and high field susceptibility in the range of 1467,1 x 10−8 m3/Kg. The volcanic material of Mount Lawu belongs to the Ilmenite (FeTiO3) group and belongs to the group of weak magnetism. Then further review the frequency-dependent susceptibility (%) to see the magnetic domains contained in the sample. From the magnetic susceptibility data, the percentage of frequency-dependent susceptibility (%) is less than 2%. From these data, the sample is not included in the superparamagnetic (SP) and in general the sample is classified as a multidomain item (MD).

Charge Storage and Reliability Characteristics of Nonvolatile Memory Capacitors with HfO2/Al2O3-Based Charge Trapping Layers.

Flash memories are the preferred choice for data storage in portable gadgets. The charge trapping nonvolatile flash memories are the main contender to replace standard floating gate technology. In this work, we investigate metal/blocking oxide/high-k charge trapping layer/tunnel oxide/Si (MOHOS) structures from the viewpoint of their application as memory cells in charge trapping flash memories. Two different stacks, HfO2/Al2O3 nanolaminates and Al-doped HfO2, are used as the charge trapping layer, and SiO2 (of different thickness) or Al2O3 is used as the tunneling oxide. The charge trapping and memory windows, and retention and endurance characteristics are studied to assess the charge storage ability of memory cells. The influence of post-deposition oxygen annealing on the memory characteristics is also studied. The results reveal that these characteristics are most strongly affected by post-deposition oxygen annealing and the type and thickness of tunneling oxide. The stacks before annealing and the 3.5 nm SiO2 tunneling oxide have favorable charge trapping and retention properties, but their endurance is compromised because of the high electric field vulnerability. Rapid thermal annealing (RTA) in O2 significantly increases the electron trapping (hence, the memory window) in the stacks; however, it deteriorates their retention properties, most likely due to the interfacial reaction between the tunneling oxide and the charge trapping layer. The O2 annealing also enhances the high electric field susceptibility of the stacks, which results in better endurance. The results strongly imply that the origin of electron and hole traps is different—the hole traps are most likely related to HfO2, while electron traps are related to Al2O3. These findings could serve as a useful guide for further optimization of MOHOS structures as memory cells in NVM.

Magnetic and magnetocaloric properties of Ho-=SUB=-1-x-=/SUB=-Y-=SUB=-x-=/SUB=-(Co-=SUB=-0.84-=/SUB=-Fe-=SUB=-0.16-=/SUB=-)-=SUB=-2-=/SUB=- compounds

Crystal structure, temperature and field dependences of magnetization, high-field susceptibility, and magnetocaloric effect (MCE) of Ho1-xYx(Co0.84Fe0.16)2 polycrystalline compounds (x=0-1) in magnetic fields up to 90 kOe in a temperature range of 5-400 K are investigated. An analysis of temperature dependences of magnetization (sigma) showed that, depending on the yttrium content (x), up to three "critical" temperatures can be simultaneously present in the studied samples: spin-reorientation (Tsr), magnetic compensation point (Tcomp), and Curie temperature (TC). The temperature dependence of the high-field susceptibility (chihf) of all samples with x<1 has an extremum in the vicinity of 100 K, where a significant MCE is observed, associated with a sharp change in the magnetization of the rare-earth sublattice. The temperature dependences of the magnetic entropy change (Delta Sm) and the adiabatic temperature change (Delta Tad) have a complex shape and reflect all the above-mentioned "critical" temperatures observed at different x. Keywords: magnetic properties, direct and inverse magnetocaloric effect, adiabatic temperature change, magnetic moment, Laves phases, Curie temperature, compensation point, spin-reorientation transition.

Magnetic and magneto-thermal properties of ferrimagnetic alloys (Er1−x Y x )(Co0.84Fe0.16)2 and their dependence on the orientations of resultant and sublattice magnetizations

In this paper structure, magnetic and magneto-thermal properties: magnetic entropy change (ΔS m) and refrigeration capacity (q) of Er1–x Y x (Co0.84Fe0.16)2 alloys (x = 0, 0.2, 0.4, 0.6, 0.8, 1) in magnetic fields up to 90 kOe in a temperature range of 5–360 K are investigated. An analysis of temperature dependences of magnetization (σ) and high-field susceptibility (χ hf) showed that in these compounds, three different ferri- and a one ferromagnetic structures are consequently realized. Concentration dependences of magnetic moment at 5 K (μ f.u.), Curie temperature (T C), residual magnetization (σ r) and coercivity (H c) have been shown to have an extreme at intermediate Y concentration. The character of temperature dependence of magnetic entropy change (ΔS m(T)) depends on the composition and originates from the type of magnetic structure of the compound and the mutual orientation of R- and 3d- element sublattices magnetization with respect to the resulting one. In compounds with x = 0.6 and x = 0.8 temperature regions with different signs of ΔS m are observed, reflecting the change of dominating R- or 3d- sublattice in the resulting magnetization.

Aggregation vs Surface Segregation: Antagonism over the Magnetic Behavior of NiCr Nanoparticles.

Annealing is a valuable method for fine-tuning the ultrasmall magnetic properties of alloy nanoparticles (NPs) by controlling their sizes, modifying their surfaces, and affecting their magnetic interactions. Herein, we study the effect of moderate annealing (450 °C) on strongly interacting NiCr nanoparticle assemblies (0 ≤ atom % Cr ≤ 15) immediately after deposition. Concurrent temperature-dependent electron microscopy and magnetization data demonstrate the interplay of two competing factors, namely, enhanced particle aggregation and element-specific surface segregation, on the magnetic properties, with the former boosting and the latter suppressing them. Strong interparticle interactions can lead to a magnetic response different from that of superparamagnetic particles, namely, from canonical spin-glass (0 atom % Cr) to correlated spin-glass (5–15 atom % Cr) behavior below higher spin-glass transition temperatures Tg (20–350 K). The observation of “high-field susceptibility” below cryogenic temperatures (≤20 K) is ascribed to the presence of inhomogeneity/defects caused by Cr segregation. This work emphasizes the necessity of taking into account the delicate balance of such competing factors to understand the magnetic properties of nanoparticulate samples.

Magnetic properties of R(Co0.88Fe0.12)2 quasi-binary compounds

In this paper the results of magnetic properties study for a series samples of R(Co0.88Fe0.12)2 type intermetallic compounds with a heavy rare-earth elements (R = Gd, Tb, Dy, Ho, Er) are presented. Such materials having a plateau-like temperature dependences of magnetic entropy change in the temperature range lower Curie point are considered as a promising functional materials for magnetic refrigerators. Their phase composition was controlled by X-ray diffraction analysis with a help of Bruker D8 Advance diffractometer. Magnetic field magnetization dependences – M(H) were measured using a SQUID magnetometer (MPMS-XL-7, Quantum Design) within the temperature range of 5 – 600 K in magnetic fields up to 5600 kA/m. The crystal lattice parameter – a, Curie temperature – TC, coercivity – Hc and residual magnetization – Mr values are analyzed as a functions of R-element atomic number. For compounds with Tb, Dy, Ho and Er the second picks on the high field susceptibility temperature dependences – χhf(T) were found in the temperature range lower their TC.

Formation of the magnetic subsystems in antiferromagnetic NiO nanoparticles using the data of magnetic measurements in fields up to 250 kOe

Abstract It is well-known that the fraction of surface atoms and the number of defects in an antiferromagnetic particle increase with a decrease in the particle size to tens of nanometers, which qualitatively changes the properties of the particle. Specifically, in antiferromagnetic nanoparticles, spins in the ferromagnetically ordered planes can partially decompensate; as a result, an antiferromagnetic particle acquires a magnetic moment. As a rule, uncompensated chemical bonds of the surface atoms significantly weaken the exchange coupling with the antiferromagnetic particle core, which can lead to the formation of an additional magnetic subsystem paramagnetic at high temperatures and spin-glass-like in the low-temperature region. The existence of several magnetic subsystems makes it difficult to interpret the magnetic properties of antiferromagnetic nanoparticles. It is shown by the example of NiO nanoparticles with an average size of 8 nm that the correct determination of the contributions of the magnetic subsystems forming in antiferromagnetic nanoparticles requires magnetic measurements in much stronger external magnetic fields than those commonly used in standard experiments (up to 60–90 kOe). An analysis of the magnetization curves obtained in pulsed magnetic fields up to 250 kOe allows one to establish the contributions of the uncompensated particle magnetic moment μun, paramagnetic subsystem, and antiferromagnetic particle core. The μun value obtained for the investigated NiO particles is consistent with the Neel model, in which μun ∼ N1/2 (N is the number of magnetically active atoms in a particle), and thereby points out the existence of defects on the surface and in the bulk of a particle. It is demonstrated that the anomalous behavior of the high-field susceptibility dM/dH of antiferromagnetic NiO nanoparticles, which was observed by many authors, is caused by the existence of a paramagnetic subsystem, rather than by the superantiferromagnetism effect.

Magnetic and Thermomagnetic Properties of Materials Based on R(Co1 – xFex )2 Laves Phases with Heavy Rare-Earth Metals

The crystal structure, the temperature and field dependences of the magnetization M and of the magnetic contribution into the entropy ΔSm, the temperature dependences of the high-field susceptibility χ and the heat capacity Cp of specimens of polycrystal R(Co1 – x Fex)2 compounds, where R is Gd, Dy, Ho or Er, are studied. The width of the ΔSm peak at half-height of its maximum (ΔTFWHM) is estimated. The dependences of ΔTFWHM and of the temperature behavior of ΔSm of the specimens on the magnetic field, on the iron content (x ) and on the atomic number of the element R are determined. The causes of the broadening of the ΔSm (T ) peak upon substitution of cobalt with iron in the R(Co1 – x Fex)2 compounds are considered.

Giant Pressure Dependence and Dimensionality Switching in a Metal-Organic Quantum Antiferromagnet.

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF_{2}(H_{2}O)_{2}]_{2}pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.

Intrinsic magnetic properties of hydrided and non-hydrided Nd5Fe17 single crystals

We report on the spontaneous magnetization Ms, the exchange stiffness constant A and the magnetocrystalline anisotropy constants K1, K2, K3 and K4 of Nd5Fe17 and Nd5Fe17H16 single crystals. Field dependencies of magnetization M(H) were measured along a, b' and c principal crystallographic directions within the temperature range of 10–600 K and magnetic fields up to 40 T. Large anisotropies of spontaneous magnetization and high-field susceptibility were revealed for both compounds. The exchange stiffness parameter A was determined using Bloch's T3/2 law. In order to provide high accuracy detection of K1(T), K2(T), K3(T) and K4(T), we used two different approaches: the modified Sucksmith- Thompson technique and the Néel's phase method.

Crystallization and magnetic characterizations of DyIG and HoIG nanopowders fabricated using citrate sol-gel

Abstract Dy and Ho iron garnets in form of nanoparticles were synthesized by citrate sol-gel method. Phase formation, lattice constant and average crystallite sizes of the samples were determined via XRD measurements. Morphology and particle size distribution were studied by TEM and chemical composition was checked by EDX. Magnetic measurements in temperature range 5–600 K and in the maximum applied field of 50 kOe were carried out by using SQUID and VSM. Their magnetic parameters, including Curie temperature, magnetization compensation temperature, spontaneous magnetization, high-field susceptibility, magnetic coercivity were discussed in the framework of three interacting magnetic sublattices, magnetocrystalline anisotropy, core-shell model and compared to those of the bulk materials. Based on these analyses further evaluation on the crystallinity and homogeneity of the samples has been made.

Lesion Heterogeneity on High-Field Susceptibility MRI Is Associated with Multiple Sclerosis Severity.

Susceptibility MR imaging contrast variations reflect alterations in brain iron and myelin content, making this imaging tool relevant to studies of multiple sclerosis lesion heterogeneity. In this study, we aimed to characterize the relationship of high-field, susceptibility contrasts in multiple sclerosis lesions to clinical outcomes. Twenty-four subjects with multiple sclerosis underwent 7T MR imaging of the brain, disability examinations, and a fatigue inventory. The inverse of T2* relaxation time (R2*), frequency, and relative susceptibility (from quantitative susceptibility mapping) were analyzed in 306 white matter lesions. Most lesions were hypointense on R2* (88% without a rim, 5% with). Lesions that were hyperintense on quantitative susceptibility mapping were more frequent in relapsing-remitting than in progressive multiple sclerosis (54% versus 35%, P = .018). Hyperintense lesion rims on quantitative susceptibility maps were more common in progressive multiple sclerosis and patients with higher levels of disability and fatigue. Mean lesion R2* was inversely related to disability and fatigue and significantly reduced in progressive multiple sclerosis. Relative susceptibility was lower in lesions in progressive multiple sclerosis (median, -0.018 ppm; range, -0.070 to 0.022) than in relapsing-remitting MS (median, -0.010 ppm; range, -0.062 to 0.052; P = .003). A progressive clinical phenotype and greater disability and fatigue were associated with lower R2* and relative susceptibility values (suggestive of low iron due to oligodendrocyte loss) and rimmed lesions (suggestive of chronic inflammation) in this multiple sclerosis cohort. Lesion heterogeneity on susceptibility MR imaging may help explain disability in multiple sclerosis and provide a window into the processes of demyelination, oligodendrocyte loss, and chronic lesion inflammation.

Features of magnetic and thermal properties of R(Co1−xFex)2 (х≤0.16) quasibinary compounds with R=Dy, Ho, Er

In this work the results of measurements of high field susceptibility, paraprocess susceptibility and thermal properties of R(Co1−хFeх)2 intermetallic compounds (R=Dy, Ho, Er and x=(0−0.16)) are presented (heat capacity and magnetocaloric effect (MCE)). A magnetic structure of the Ho(Co0.88Fe0.12)2 at 293K and 78K was studied by neutron powder diffraction. Some peculiarities of a high-field susceptibility were revealed at low temperatures and around the Curie point (TC). In temperature range lower than TC by (100–150)K, magnetic contributions to a zero-field heat capacity were found. Studying MCE in wide temperatures range, the large change of the entropy magnetic contribution (∆S) was observed which correlates with ∆T phenomenon. In particular, for the Er(Co0.84Fe0.16)2 compound the ∆S value at low temperatures is six times higher than that at Curie point. The possible reasons of such behavior were discussed.

The effect of magnetization anisotropy and paramagnetic susceptibility on the magnetization process

The Sucksmith-Thompson method is a widespread technique for the accurate evaluation of magnetocrystalline anisotropy constants K1 and K2 of a single crystal with easy magnetization axis or basal plane type magnetic anisotropy. In this work, a generalized form of the method is represented. It takes into account several magnetization process features: spontaneous magnetization anisotropy, high-field differential susceptibility (paramagnetic process), and stray fields in terms of the demagnetizing factor. Corrected anisotropy field expressions for both magnetic anisotropy types are also established herein. The modified approach was verified by magnetization measurements of single crystalline LaCo5 and YFe3.

Phase equilibria and magnetic properties of Heusler-type ordered phases in the Co–Mn–Ga ternary system

Phase equilibria at 800, 900 and 1000°C, as well as order–disorder and magnetic transitions of Heusler-type ordered phase were investigated in the Co–Mn–Ga ternary system. The β phase with bcc structure including the Heusler-type ordered phase exists in a wide composition range of the ternary system. It was confirmed by STEM–HAADF observation that the crystal structure of the Heusler-type ordered alloy Mn2CoGa is L21b-type Fm3¯m (i.e., (Mn,Co)2MnGa), but not inverse-Heusler (Hg2CuTi) type F4¯3m. The L21/B2 order–disorder transition temperature shows a maximum at the stoichiometric composition of Co2MnGa and the Curie temperature of the L21 phase decreases with increasing Ga content. Magnetic moments converted from magnetization curves at 4.2K for CoxMn(76−x)Ga24 alloys well follow the generalized Slater–Pauling rule in the composition of 25⩽x⩽50, and magnetization curves indicate very small high-field magnetic susceptibility.

Measuring material susceptibility using NMR

We report on a method of measuring the high-field susceptibilities of paramagnetic and diamagnetic materials using only a standard NMR system equipped with pulsed field gradients. We demonstrate the accuracy and sensitivity of the technique by measuring a series of 99.9% copper wires with diameters between 0.16mm and 0.79mm. We measured the volumetric susceptibility of the copper to be χ=-9.5±0.2·10-6, which agrees with the literature value of pure copper, -9.6·10-6. In addition to making quantitative measurements, this technique can also be used to evaluate the effectiveness of compensation schemes used to produce “zero-susceptibility” materials needed for construction of high-resolution NMR probes.

Advanced magnetic resonance spectroscopy and imaging techniques applied to brain development and animal models of perinatal injury

Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are widely used in the field of brain development and perinatal brain injury. Due to technical progress the magnetic field strength (B0) of MR systems has continuously increased, favoring 1H-MRS with quantification of up to 18 metabolites in the brain and short echo time (TE) MRI sequences including phase and susceptibility imaging. For longer TE techniques including diffusion imaging modalities, the benefits of higher B0 have not been clearly established. Nevertheless, progress has also been made in new advanced diffusion models that have been developed to enhance the accuracy and specificity of the derived diffusion parameters. In this review, we will describe the latest developments in MRS and MRI techniques, including high-field 1H-MRS, phase and susceptibility imaging, and diffusion imaging, and discuss their application in the study of cerebral development and perinatal brain injury.

Magnetic structure and interaction in (Sb, Co) co-doped ZnO thin films

The magnetic behaviour of (Co, Sb) co-doped ZnO thin films grown by pulsed laser deposition is investigated. The irreversibility (ZFC–FC bifurcation) in low field (H = 100 Oe) magnetization and small hysteresis below 300 K are similar in samples with or without Sb co-doping. Both the phenomena originate from the presence of blocked supermoments in the samples. Incorporation of Sb only increases the saturation magnetization and coercivity. The quantitative increase in moment due to Sb co-doping suggests a transfer of electrons from Co ions to Sb-related acceptor complexes. This is supported by a decrease in the number of electronic transitions from Co d electrons to the conduction band seen in optical transmission spectroscopy when Sb is added. The high field susceptibility data show the existence of supermoments with antiferromagnetic interaction between them. We find that the value of the effective antiferromagnetic molecular field constant decreases with increasing Co concentration, revealing that the supermoments are bound magnetic polarons around intrinsic donors, rather than coming from Co precipitates. True ferromagnetism (overlapping polarons) can emerge either with larger intrinsic donors, or with acceptors with shallower levels, than those created by Sb co-doping. Our results suggest that Sb-related acceptor states may be unstable towards accepting electrons from deep d levels of Co ions.