Low Temperature Hysteresis Loops Research Articles

Magnetic collective state formation upon tuning the interparticle interactions in ensembles of ultrafine ferrihydrite nanoparticles

The results of a study of the dynamic (alternating current magnetic susceptibility) and static magnetic properties, as well as 57Fe Mössbauer spectrometry and ferromagnetic resonance of two-line ferrihydrite nanoparticle systems with varying intensities of magnetic interparticle interactions are reported. The strength of the magnetic interparticle interactions has been tuned by coating (with various degrees of coating) the ferrihydrite particles (2–4 nm in size and an average size ∼2.7 nm) of the initial synthetic sample by arabinogalactan. Also, a biogenic ferrihydrite sample (an average particle size of 2-nm) with a natural organic coating was studied and it has the weakest magnetic interparticle interactions among of all the samples. Relaxation times of the particle’s magnetic moment were determined by the data of static and dynamic magnetic susceptibilities and from analysis of 57Fe Mössbauer spectrometry. Based on the temperature dependences of the relaxation times, it has been concluded that the predominantly collective processes of freezing of the particle magnetic moments occur under the action of the magnetic interparticle interactions. It is shown that an important role in these processes is played by a magnetic subsystem of the surface spins of the particles. The effect of the interplay between the surface spin and magnetic moment subsystems on the static magnetic properties (low-temperature magnetic hysteresis loops) and the parameters of the microwave absorption line under the magnetic resonance conditions is discussed.

Ce3+-doped nanocrystalline cobalt–zinc spinel ferrite: microstructural, magnetic, and optical characterizations

A series of Ce3+ ions-doped cobalt–zinc spinel cubic ferrite nanoparticles with the general formula Co0.5Zn0.5CexFe2–xO4 (x = 0.00, 0.03, 0.06 and 0.09) were synthesized using soft chemical co-precipitation route. The synthesized nanoferrites were in single-phase as ensured by the X-ray diffractograms. Average crystallite sizes were 16 nm, 12 nm, 10 nm, and 07 nm with increasing Ce content as determined using Scherrer’s formula. A significant enhancement in coercivity (HC) and decrement in saturation magnetization (MS) was observed in field-cooled (9 T) low-temperature (3 K) hysteresis loops with increasing Ce content. A correlation between the mean crystallite size of the synthesized nanoparticles and disordered surface spins has been established. The increment in Ce3+ ions substitution also favored the onset of superparamagnetic behavior as confirmed by the room temperature hysteresis loops. The blocking temperature was reduced with the increase of Ce3+ ions content which makes the nanoparticles a suitable candidate for various biomedical applications. The indirect optical bandgaps were observed to increase gradually with the increasing Ce content due to the nanosize effect.

Ferromagnetic excess moments and apparent exchange bias in FeF2 single crystals

The anisotropic antiferromagnet FeF2 has been extensively used as an antiferromagnetic layer to induce exchange bias effects in ferromagnetic/antiferromagnetic bilayers and heterostructures. In this work, an apparent exchange bias occurring in the low temperature hysteresis loops of FeF2 single crystals is investigated. A detailed investigation of the hysteresis and remnant magnetization indicates that the observation of an apparent exchange bias in FeF2 stems from an intrinsic excess moment associated with a distortion of the antiferromagnetic structure of piezomagnetic origin.

ZnFe2O4/CuO core–shell structured nanoparticles: synthesis, structural and magnetic properties

Synthesis of core/shell structured ZnFe2O4/CuO nanoparticles with varying core diameter and different weight percentage of CuO phase [(1−x) ZnFe2O4/x CuO: x = 0.15, 0.30, 0.45 and 0.60] using double stage standard chemical co-precipitation method has been reported in this article. For first four samples only the diameter of core was varied and for other four samples only the weight percentage of CuO phase was varied. The existence of both cubic spinel ZnFe2O4 phase along with monoclinic CuO phase was verified by X-ray patterns recorded at room temperature. The average core diameter was in between 8 and 13 nm as estimated using Scherrer’s formula and further verified by HRTEM images. A reduction in lattice parameter with increasing core thickness and also with increasing CuO phase was observed. A notable enhancement in coercivity and saturation magnetization were observed in field cooled (5 T) hysteresis loops obtained at 5 K for the first four varying core diameter core–shell structured nanoparticles in comparison to bare zinc ferrite nanoparticles of almost same size. A decreasing trend in coercivity and saturation magnetization was also noticed in low temperature hysteresis loops for higher CuO content samples. A considerable improvement in blocking temperature (TB) as well as Neel temperature (TN) for all the different core diameter samples were also found in M(T) protocols. Both blocking temperature and Neel temperature decreased for increasing CuO content in composite nanoparticles. The room temperature M(H) plots exhibited superparamagnetic nature with insignificant coercivity and negligible remanence.

Synthesis, structural, electrical and magnetic properties of Bi2Fe4-xGaxO9 (0 ≤ x ≤ 1.6)

We investigate the electrical and magnetic properties of the Bi2Fe4-xGaxO9 (0 ≤ x ≤ 1.6) polycrystalline samples synthesized via solid-state reaction technique. Magnetic susceptibility measurements reveal that lightly doped samples (x < 0.8) undergo successive transitions, from high-temperature paramagnetic to antiferromagnetic phase followed by a low-temperature spin-glass state while the samples with heavy doping (x > 0.8) demonstrate paramagnetic to spin-glass (SG) transition. The variation of irreversible temperature obtained from zero field cooling (ZFC) and field cooling (FC) susceptibilities versus measured magnetic field and low-temperature magnetic hysteresis (M-H) loops support the existence of spin-glass phase. The dielectric constant (Ɛr) of Bi2Fe4-xGaxO9 with Ga-dilution reveals a weak-temperature sensitivity in high-temperature range (300 K ≤ T ≤ 550 K), which is advantageous for high temperature capacitor applications and electronic devices.

Antiferromagnetic interlayer coupling of (111)-oriented La0.67Sr0.33MnO3/SrRuO3 superlattices**Project supported by the National Basic Research Program of China (Grant Nos. 2016YFA0300701, 2017YFA0206300, and 2017YFA0303601) and the National Natural Science Foundation of China (Grant Nos. 11520101002, 51590880, and 11674378).

We report a strong antiferromagnetic (AFM) interlayer coupling in ferromagnetic La0.67Sr0.33MnO3/SrRuO3 (LSMO/SRO) superlattices grown on (111)-oriented SrTiO3 substrate. Unlike the (001) superlattices for which the spin alignment between LSMO and SRO is antiparallel in the in-plane direction and parallel in the out-of-plane direction, the antiparallel alignment is observed along both the in-plane and out-of-plane directions in the present sample. The low temperature hysteresis loop demonstrates two-step magnetic processes, indicating the coexistence of magnetically soft and hard components. Moreover, an inverted hysteresis loop was observed. Exchange bias tuned by the temperature and cooling field was also investigated, and positive as well as negative exchange bias was observed at the same temperature with the variation of the cooling field. A very large exchange field (HEB) was observed and both magnitude and sign of the HEB depend on the cooling field, which can be attributed to an interplay of Zeeman energy and AFM coupling energy at the interfaces. The present work shows the great potential of tuning a spin texture through interfacial engineering for the complex oxides whose spin state is jointly determined by strongly competing mechanisms.

Thermally stimulated iron oxide transformations and magnetic behaviour of cerium dioxide/iron oxide reactive sorbents

The present paper is devoted to detailed study of the magnetically separable sorbents based on a cerium dioxide/iron oxide composite annealed at temperatures Ta=773K, 873K, and 973K. The X-ray diffraction and high resolution transmission electron microscopy are used to determine the phase composition and microstructure morphology. Mössbauer spectroscopy at room (300K) and low (5K) temperatures has contributed to more exact identification of iron oxides and their transformations Fe3O4→γ-Fe2O3 (ε-Fe2O3)→α-Fe2O3 in dependence on calcination temperature. Different iron oxide phase compositions and grain size distributions influence the magnetic characteristics determined from the room- and low-temperature hysteresis loop measurements. The results are supported by zero-field-cooled and field-cooled magnetization measurements allowing a quantitative estimation of the grain size distribution and its effect on the iron oxide transformations.

Ion Irradiation-Induced Magnetic Transition of MnGa Alloy Films Studied by X-Ray Magnetic Circular Dichroism and Low-Temperature Hysteresis Loops

The ion irradiation-induced magnetic transition of $L1_{0}$ -MnGa alloy films was studied in detail by measuring X-ray magnetic circular dichroism (XMCD) and the temperature dependence of their hysteresis loops. The XMCD of (001)-oriented MnGa films exhibited different spectral shapes depending on whether magnetization was parallel or perpendicular to the film plane; that is considered to be the origin of the large perpendicular magnetic anisotropy of the $L1_{0}$ -MnGa. The anisotropy of XMCD spectra was not influenced by the ion irradiation to the MnGa, while the intensity of the XMCD signal decreased with increasing ion dose. From the measurement of hysteresis loops at low temperature, the significant increase of coercivity ${H}_{c}$ in the ion-irradiated MnGa was found by lowering the temperature, while only a slight increase of ${H}_{c}$ was observed in the as-prepared MnGa. These results suggest that the ion-irradiated MnGa film has a composite structure of $L1_{0}$ -ordered MnGa nanocrystals separated by an $A1$ -disordered MnGa matrix, and that increasing the ion dose results in an increase in the volume ratio of $A1$ -MnGa to $L1_{0}$ -MnGa in the film.

Ferritin surplus in mouse spleen 14 months after intravenous injection of iron oxide nanoparticles at clinical dose

In this study, we followed the biodegradation of ultra-small superparamagnetic iron oxide nanoparticles injected intravenously at clinical doses in mice. An advanced fitting procedure for magnetic susceptibility curves and low-temperature hysteresis loops was used to fully characterize the magnetic size distribution as well as the magnetic anisotropy energy of the injected P904 nanoparticles (Guerbet Laboratory). Additional magnetometry measurements and transmission electronic microscopy observations were systematically performed to examine dehydrated samples from the spleen and liver of healthy C57B16 mice after nanoparticle injection, with sacrifice of the mice for up to 14 months. At 3 months after injection, the magnetic properties of the spleen and liver were dramatically different. While the liver showed no magnetic signals other than those also present in the reference species, the spleen showed an increased magnetic signal attributed to ferritin. This surplus of ferritin remained constant up to 14 months after injection.

Low-temperature ferromagnetic properties in Co-doped Ag2Se nanoparticles

β-Ag2Se is a topologically nontrivial insulator. The magnetic properties of Co-doped Ag2Se nanoparticles with Co concentrations up to 40% were investigated. The cusp of zero-field-cooling magnetization curves and the low-temperature hysteresis loops were observed. With increasing concentration of Co2+ ions mainly substituting AgI sites in the Ag2Se structure, the resistivity, Curie temperature Tc, and magnetization increased. At 10 T, a sharp drop of resistance near Tc was detected due to Co dopants. The ferromagnetic behavior in Co-doped Ag2Se might result from the intra-layer ferromagnetic coupling and surface spin. This magnetic semiconductor is a promising candidate in electronics and spintronics.

Electromagnetic and magnetic properties of magnetophotonic crystal based on opal matrix with Co and CoO nanoparticles

Magnetic properties of magnetophotonic crystals based on opal matrices have been studied as well as their electromagnetic properties in millimeter waveband. The particles of cobalt oxide are embedded into the inter-sphere voids of the matrix. After annealing in hydrogen the cobalt oxide particles transform to metallic cobalt. It has been shown that if antiferromagnetic cobalt oxide remains besides ferromagnetic cobalt, the low-temperature magnetic hysteresis loop is shifted along the field axis. Magnetic field influences essentially on the microwave transmission and reflection coefficients only after annealing in hydrogen that is if the ferromagnetic phase presents in the sample. The spectra of magnetic resonance and antiresonance are studied.

Synthesis, structure, and magnetic properties of SrFe12O19/La1−xCaxMnO3 hard/soft phase composites

SrFe12O19/La1−xCaxMnO3 (SFO/LCMO) composites with x = 0.1 and 0.5 were synthesized via a two stage combined citrate precursor sol-gel and hydrothermal method. The structure and composition of the samples were refined from the X-ray diffraction patterns. The morphologies of the composites were investigated using transmission and scanning electron microscopies, which revealed micron-sized hexagonal platelets of SFO with LCMO particles with average diameters of 110–130 nm formed at the surface. An investigation of the temperature and field dependence of the magnetization found that the SFO phase with a ferrimagnetic characteristic up to 720 K dominates the magnetic properties of the composite samples. The SFO/LCMO composite samples showed a reduction in saturation magnetization and coercivity when compared with pure SFO. However, low temperature hysteresis loops recorded after cooling in applied fields up to 5 T revealed the absence of exchange bias in the SFO/LCMO composite, ruling out the possibility of significant interfacial magnetic coupling between SFO and LCMO.

Particle size dependence of the superconductivity and ferromagnetism in YBCO nanoparticles

Magnetic properties of superconducting yttrium barium copper oxide (YBa2Cu3O7-δ) nanoparticles (31–43 nm) prepared by a chemical route have been studied. These nanoparticles have been found to clearly exhibit ferromagnetism at room temperature while superconducting transition is observed at lower temperatures. The low temperature hysteresis loops show evidence suggesting the presence of a large paramagnetic contribution in addition to the superconducting contributions from the particles. Bulk YBCO obtained by pelletizing and heating the same nanoparticles at a high temperature, displays the usual superconducting characteristics and gives no trace of ferromagnetism down to 10 K. The superconducting transition temperature of the nanoparticles is lower than for the bulk YBCO and there is a trend of decreasing T c with smaller size of the particles. In contrast the ferromagnetic moment increases with decreasing particle size. The development of ferromagnetism is attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. The simultaneous decrease of superconducting T c and the increase of ferromagnetism with decreasing size considered as being reflective of the increased role of finite size and surface defects that weaken the superconductivity and enhance the ferromagnetism. Possible coexistence of surface ferromagnetism and bulk superconductivity at lower temperatures is discussed.

Surface Spin-glass Freezing and Blocking in NiFe2O4 Nanoparticles

AbstractWe prepared single-phase nickel ferrite nanoparticles separated by silicon dioxide using sol-gel method with tetraethyl orthosilicate (TEOS) as a precursor for SiO2. The magnetic properties are investigated by using SQUID-magnetometry over a broad temperature range (4.2 – 350 K), magnetic field (2–70,000 Oe) and frequency (0.1 – 1000 Hz) range. The particle size is in the range 8 – 12 nm. Exchange bias and spin disorder appear at the core-shell interface due to broken bonds on the surface. Disorder and core-shell interaction induces spin-glass freezing which is manifested by a low temperature peak in the AC susceptibility well separated from magnetic blocking peak. This low temperature peak is assigned to spin-glass freezing. The proof of spin-glass freezing is managed by zero field cooled/field cooled (ZFC/FC), frequency and DC field dependence of AC susceptibility, low temperature hysteresis loop and time dependent thermoremanent magnetization at different temperatures. All the measurements stated above signify blocking/unblocking at higher temperatures and surface spin-glass freezing at low temperatures. The aim of our work is to contribute to a better understanding of “spin-frozen” magnetic ferrite nanoparticles at diameters 8 – 12 nm which could be important in future for stabilizing the magnetic state of “core-shell”-structured nanomagnets.

Exchange bias identifies lamellar magnetism as the origin of the natural remanent magnetization in titanohematite with ilmenite exsolution from Modum, Norway

Large and stable negative magnetic anomalies in southwestern Sweden, southern Norway, the Adirondacks, USA, and Quebec, Canada, are related to rock units with a magnetic fraction consisting primarily of ilmeno-hematite or hemo-ilmenite. It has been suggested that the unusual magnetic stability of these rocks results from lamellar magnetism. This is a type of magnetic remanence, carried by uncompensated magnetic layers at interfaces between nanoscale exsolution structures of antiferromagnetic (AFM) hematite and paramagnetic ilmenite. Here we present the first direct proof that this lamellar magnetism indeed is responsible for the natural remanent magnetization (NRM) of a rock from Modum, Norway. Our argument expands a previous observation, that, in mineral grains from this rock, the cooling of a positive-induced remanence from room temperature to 5 K – which is well below the ordering temperature of ilmenite (57 K) – leads to a large negative shift of the low-temperature (LT) hysteresis loop. This can only be explained by exchange bias due to exchange coupling across the hematite–ilmenite interfaces. In a different experiment, we now have cooled the original NRM of untreated grains to 5 K, and then measured the hysteresis loop. Again, in several separate grains we observed large shifts of the hysteresis curves. This shows that exchange bias develops also from the untreated NRM. This observation proves that the moments, which carry the NRM, also participate in the exchange coupling at the hematite–ilmenite interfaces. Therefore, the NRM is not carried by defect moments or stress-induced moments, which occur in normal bulk hematite. A closer look at the NRM-induced LT loops shows that exchange bias acts in both field directions, though one direction is clearly predominant. This observation can be interpreted as a frozen equilibrium of different proportions of oppositely directed lamellar moments, a key feature of the original lamellar magnetism hypothesis. We discuss lamellar aggregation, and the formation of exchange-coupled clusters to explain the observed high efficiency of lamellar NRM. We also conclude that remanence carried by lamellar moments should not be used for paleointensity estimates of terrestrial or extraterrestrial material.

Magnetic Properties of Heterostructured Co–Au Nanoparticles Direct-Synthesized From Gas Phase

Heterostructured Co-Au nanoparticles are direct synthesized from gas phase. Magnetic properties of Co-Au nanoparticles are characterized. Low-temperature and room-temperature hysteresis loops show that the magnetic properties of Co are kept in Co-Au nanoparticles. A blocking temperature of 290 K is measured for Co-Au particles with an average size of 12 nm. A field-cooled low-temperature hysteresis loop of Co-Au nanoparticles indicates the existence of oxide, even the particles have a core-shell-like structure

Enhanced alignment of Mn 12-acetate micro-crystals

A dilute Mn 12-acetate suspension composed of microscopic single crystals and single molecules in isopropanol was used for magnetic studies of Mn 12-acetate single molecule magnets. We observed magnetic properties of the frozen Mn 12-acetate suspension similar to large single crystals, specifically several sharp steps in the low-temperature hysteresis loop, indicating significantly enhanced alignment as compared to prior studies of micro-crystals. The greater the external magnetic field during alignment, the sharper the steps were in the low-temperature hysteresis loops, indicating that this method can be used for continuous control of alignment. A magnetic field of ∼0.5 T was sufficient to align the micro-crystals in the organic solvent to a degree previously observed only in much larger single crystals.

Enhancement of the exchange-bias onset temperature in a columnar nanocrystalline Ni80Fe20∕Co3O4 thin film

Typically, exchange coupling can only occur when ferromagnetic spins couple to antiferromagnetic spins below the Néel temperature. We present results on a Ni80Fe20∕Co3O4 thin film that is composed of ∼10nm diameter nanocrystalline columns. Field cooling the film reveals low-temperature exchange-bias hysteresis loop shifts that are the same magnitude as those measured in a similar Ni80Fe20∕CoO system, although the bulk magnetocrystalline anisotropy is a factor of 5 smaller. The coercivity and exchange-bias loop shift exhibit the same temperature dependence with individual “blocking” temperatures. Most surprising is that the onset temperature of the exchange-bias is nearly four times higher than the bulk Néel temperature.

Enhanced Magnetic Properties of Self-Assembled FePt Nanoparticles with MnO Shell

Self-assembled FePt/MnO nanoparticles with different morphology and size were synthesized with a polyol process. With the MnO coating, FePt nanoparticles exhibit a high blocking temperature and magnetic moment. The low-temperature hysteresis loop of FePt nanoparticles can be shifted through the AFM pinning of the MnO shell. The aggregation of FePt nanoparticles during the L10 phase transformation can be significantly decreased by coating with the MnO shell.

40Ar/ 39Ar dating and preliminary paleointensity determination on a single lava flow from Chifeng, Inner Mongolia

A precise 40Ar/ 39Ar age and paleointensity data for the Cretaceous lava flow from Chifeng, southern Inner Mongolia, northeastern China are presented in this study. Detailed rock magnetic investigations including the variation of magnetization with temperature, low temperature susceptibility and hysteresis loops show that pseudo single domain (PSD) grain size high-Ti titanomagnetite is the main magnetic mineral in the studied lava flow. Both the microwave and double heating Thellier techniques were used to determine the paleointensity, yielding mean flow paleointensities of 15.6 ± 3.2 μT and 23.9 ± 8.0 μT, respectively. However, the paleointensity results using the microwave technique are of higher quality (mean q = 12 for microwave compared to q = 2 for Thellier) and yield higher internal consistency for the flow mean (21% standard deviation about the mean for microwave compared to 34% for Thellier). The microwave paleointensity result, 15.6 ± 3.2 μT is therefore deemed the more reliable estimate for the paleointensity of the Niutoushan lava flow. 40Ar/ 39Ar age determination on the lava flow is 106.42 ± 0.48 Ma (2 σ, relative to GA-1550 biotite: 98.79 ± 0.96 Ma). Combining our 40Ar/ 39Ar dating and paleointensity results with other published paleointensity data suggests that the intensity of the Earth's magnetic field during the middle Cretaceous normal superchron (CNS) was weak, but variable throughout the whole CNS.