Showing papers on "Superparamagnetism published in 1975"
TL;DR: In this article, the magnetic susceptibility of non-crystalline FeF2 was measured in a field of H = 1 250 Oe and the temperature range 150 K and T > T and#x 3E; 25 K.
Abstract: 2014 In the temperature range 150 K > T > 25 K the magnetic susceptibility of noncrystalline FeF2 can be described by a Curie-Weiss law. A paramagnetic Curie temperature of 03B8 = (22 ± 1) K and an effective moment of peff = (5.2 ± 0.1) 03BCB are found. Temperature and field dependence of the magnetization between 15 K-20.5 K indicate superparamagnetic behaviour. At lower temperatures the magnetization shows hysteresis and a slightly time dependent remanence. The observation of moments which approach 4 03BCB/Fe2+ implies that the ordered state is ferromagnetic. Non-crystalline FeF2 can be prepared by condensation of a molecular beam of FeF2 on to a He-cooled substrate in a vacuum of 108 torr. We have shown previously by Mossbauer spectroscopy that noncrystalline FeF2 becomes magnetically ordered below N 21 K [1 ]. This is in contrast to the Neel temperature TN = 78 K found for crystalline antiferromagnetic FeF2 [2]. The non-crystalline samples of FeF2 are converted into the crystalline state only at temperatures above 800 K [3]. It is thus possible to warm such samples to 300 K, carefully brush the amorphous material from the substrate, and then transfer the powder under an inert atmosphere into a magnetometer (Foner type, vibrating sample method) without any structural changes taking place. The measured susceptibility in a field of H = 1 250 Oe and the temperature range 150 K > T > 25 K can be described by a Curie-Weiss law (Fig. 1). An extrapolated paramagnetic Curie temperature of 0 = (22 ± 1) K was found together with Peff = (5.2 :L 0.1) ,uB. The derived value g = 2.12 ± 0.05 of non-crystalline FeF2 is only slightly larger than g = 2.10 ± 0.05 for crystalline FeF2 [4]. The magnitude and temperature dependence of the quadrupole splitting and the magnitude of the magnetic hyperfine interaction as derived from the Mossbauer spectra indicate a less complete quenching of the orbital momentum in non-crystalline FeF2 compared to the crystalline phase [1, 3]. A simple (*) Work supported in part by Bundesministerium fur Forschung und Technologie. FIG. 1. Inverse susceptibility of non-crystalline FeF2 against temperature. perturbational treatment for the crystal electric field levels shows that an increase of the rhombic distortion at Fe2 + in the non-crystalline phase leading to a decrease of the splitting between the ground state and the first excited orbital state from 700 cm-’ 1 (crystalline phase [5]) to 500 cm-1 can explain the increase of orbital momentum. It would, however, only cause an increase of 0.03 in g-factor which agrees well with the data found. In figure 2 the magnetization curves between 1520.5 K are plotted as (M x Ms(T = 0))/M,(r) against H x Ms(T)/(T x Ms(T = 0)). The approximate ratios of the spontaneous magnetizations Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyslet:01975003607-8019700
30 citations
TL;DR: Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 1022 neutrons/cm2 at 425°C and 3.5 × 10 22 neutrons / cm2 at 500 and 600°C as discussed by the authors.
Abstract: Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 1022 neutrons/cm2 at 425°C and 3.5 × 1022 neutrons/ cm2
(E > 0.1 MeV) at 500 and 600°C. A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetization vs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiation and post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed 100 h at 760°C before irradiation and 1 h at 500°C after irradiation at 425°C.
18 citations
TL;DR: In this article, the superparamagnetic relaxation time for spherical particles with cubic magnetocrystalline anisotropy in zero applied field is calculated for larger particle size, namely larger energy barriers, than could be evaluated in the previous work.
Abstract: The superparamagnetic relaxation time is calculated for spherical particles with cubic magnetocrystalline anisotropy in zero applied field. Computation is extended to larger particle size, namely larger energy barriers, than could be evaluated in the previous work. Still, no asymptotic formula could be found to describe the behavior in this size region. It is found that the increase of the relaxation time with increasing particle size is much faster than can be obtained from a simple exponential, from the asymptotic formula for uniaxial anisotropy, or from a formula obtained by an analogous procedure to the uniaxial case. Unless these asymptotic formulas take over only for much larger barriers than studied here, it seems that the complications imposed by the cubic anisotropy potential cannot be ignored.
16 citations
TL;DR: In this article, the Curie points of the superparamagnetic (SP) and single domain (SD) particles in lunar soils and potential magnetic analogue materials were measured using a recently developed furnace.
Abstract: Using a recently developed furnace, ferromagnetic resonance (FMR) thermomagnetic studies up to 900 C were employed to measure the Curie points of the superparamagnetic (SP) and single domain (SD) particles in lunar soils and potential magnetic analogue materials. Based on measured Curie points of 775 C, the SP and SD particles in lunar soils 10084-853, 12070-29, 14161-46, and 67010-4 are essentially pure metallic Fe. Synthetic and terrestrial samples containing magnetite, titanomaghemites, and magnetite-like particles have measured Curie points below 600 C are thus not magnetic analogues of lunar soils.
15 citations
Patent•
22 Dec 1975
TL;DR: In ferromagnetic powders mainly composed of iron oxide, the superparamagnetic material provides a peak showing superparamagnetism by measurement of the Mossbauer Effect spectrum.
Abstract: In ferromagnetic powders mainly composed of iron oxide, the ferromagnetic iron oxide(s) comprise a superparamagnetic material which provides a peak showing superparamagnetism by measurement of the Mossbauer Effect spectrum. The ferromagnetic iron oxides are suitable for making highly sensitive magnetic recording media having excellent frequency response with lowered modulation noise.
15 citations
TL;DR: In this article, the Mossbauer spectroscopic study on some concentrated iron-chromium alloys FexCr1-x (with 0.2 < x < 0.3) gave evidence for the occurrence of a superparamagnetic state, which accounts for the typical transport and magnetic properties observed in these alloys.
Abstract: The Mossbauer spectroscopic study on some concentrated iron-chromium alloys FexCr1-x (with 0.2 < x < 0.3) gives evidence for the occurrence of a superparamagnetic state, which accounts for the typical transport and magnetic properties observed in these alloys.
9 citations
TL;DR: The magnetic susceptibility of a terrestrial, synthetic and lunar ilmenite specimen has been measured from 4 to 300 K as discussed by the authors, with a single Neel temperature transition which ranged from 56 to 57.7 K. The results of electron spin resonance measurements were also in general agreement with these findings.
7 citations
TL;DR: In this article, a transformation into a paramagnetic system is associated with the diffusion of nickel ions to anionic vacancies, and the imaginary part of the susceptability exhibits a typical superparamagnetic behaviour.
5 citations