Robert D. Shull

Reduction of hysteresis losses in the magnetic refrigerant Gd5Ge2Si2 by the addition of iron

The magnetocaloric effect is the change in temperature of a material as a result of the alignment of its magnetic spins that occurs on exposure to an external magnetic field. The phenomenon forms the basis for magnetic refrigeration, a concept purported to be more efficient and environmentally friendly than conventional refrigeration systems. In 1997, a ‘giant’ magnetocaloric effect, between 270 K and 300 K, was reported in Gd5Ge2Si2, demonstrating its potential as a near-room-temperature magnetic refrigerant. However, large hysteretic losses (which make magnetic refrigeration less efficient) occur in the same temperature range. Here we report the reduction (by more than 90 per cent) of these hysteretic losses by alloying the compound with a small amount of iron. This has the additional benefit of shifting the magnetic entropy change peak (a measure of the refrigerators optimal operating temperature) from 275 K to 305 K, and broadening its width. Although the addition of iron does not significantly affect the refrigerant capacity of the material, a greater net capacity is obtained for the iron-containing alloy when the hysteresis losses are accounted for. The iron-containing alloy is thus a much-improved magnetic refrigerant for near-room-temperature applications.

Magnetocaloric effect in superparamagnets

The magnetocaloric effect is calculated for superparamagnetic materials as a function of temperature, field and cluster size. Assuming classical behavior, a universal curve is calculated from which an optimum cluster moment may be found for maximum entropy change upon application of a given field H at a given temperature T. Quantum effects are shown to be small for temperatures above 10 K and fields less than a few tesla. A comparison with results for a spin-72 paramagnet such as Gd3Ga5O12 (GGG) is made, which indicates that superparamagnetic materials such as magnetic nanocomposites offer the possibility of extending the upper useful temperature limit of paramagnetic materials for magnetic refrigeration.

Enhanced magnetocaloric effect in Gd3Ga5−xFexO12

The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field‐induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron‐substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.

Magnetic and optical properties of γ‐Fe2O3 nanocrystals

γ-Fe2O3 nanocrystals with a mean radius of 4.2 nm have been synthesized in a polymer matrix by an ion exchange and precipitation reaction. Magnetization and susceptibility data from experiment and computer simulations indicate that the system is superparamagnetic. The optical absorption edge is red shifted with respect to that of an epitaxially-grown single-crystal film of γ-Fe2O3. The red shift is attributed to lattice strain in the small particles.

Formation of superparamagnetic nanocomposites from vapor phase condensation in a flame

Recent work on the magnetic characteristics of nanometer scale materials has suggested that magnetically isolated nanometer magnetic particles would show magnetic behavior different than those found in the bulk. Such behavior could be explored if such materials could be synthesized in sufficient quantities where the magnetic particles could be isolated from each other via encapsulation within a non-magnetic host. We have investigated application of flame technology for the synthesis of this class of materials. A premixed methane/oxygen flame diluted with nitrogen has been used as the reacting environment in which iron pentacarbonyl and hexamethyldisiloxane were added as the magnetic and non-magnetic precursor materials. The results, based on x-ray diffraction, electron microscopy, Mossbauer effect, and magnetization data have shown that: (i) nanometer composite particles are formed containing 5–10 nm Fe2O3, encased in a silica particle whose diameter ranged from 30–100 nm, depending on loading and flame temperature, and (ii) the iron oxide clusters are magnetically isolated and in some cases show superparamagnetic behavior.

Iron magnetic moments in iron/silica gel nanocomposites

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low‐temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X‐ray diffraction, electron microscopy, Mossbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer‐sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie‐Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as‐cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After ...

Magnetocaloric effect of ferromagnetic particles

The entropy change accompanying the removal of an applied magnetic field (i.e. the magnetocaloric effect) is calculated for a system of magnetic spins independent of each other and also clustered together into independently acting magnetic particles. Mean-field-theory calculations are made for interacting, single magnetic spins and also for interacting, magnetic particles. In both cases, there are found temperature and field regimes where the entropy changes are larger when the spins are coupled together into nanometer-sized magnetic particles. Entropy change data is shown for a new Gd/sub 3/Ga/sub 5-x/Fe/sub x/O/sub 12/ (x >

Enhanced magnetocaloric effects in R3(Ga1−xFex)5O12 (R=Gd, Dy, Ho; 0<x<1) nanocomposites

A series of R 3 (Ga 1-x Fe x ) 5 O 12 (R=Gd, Dy, Ho; 0<x<1) compounds for potential magnetic refrigerants were synthesized by chemical routes and characterized by X-ray diffraction, and SQUID magnetometry. Dy and Ho were chosen since they respectively possess increasing orbital contributions to the total angular magnetic moment of the atom over the zero value for Gd. X-ray data showed that garnet structures were obtained and that improvements over the Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 compound, which was reported in 1992 as possessing enhanced magnetocaloric effects, may be achieved by equilibrating at 1473K for 15 h, rather than at 1173K for 15h as was done in the earlier studies. Magnetometry measurements showed that when Gd was substituted either by Dy or Ho, the material was superparamagnetic, possessing fine magnetic clusters resulting in enhanced magnetocaloric effects (ΔS m ) with respect to the basic paramagnetic garnet (i.e., x = 0). In addition, with variation in x, the optimal ΔS m was measured for the x = 0.5 compound, similar to that found for the Gd-containing garnet nanocomposites. The optimal ΔS m values for the Ho- and Dy-containing compounds, respectively, were found to be about the same or smaller than that for the optimal Gd-containing nanocomposite Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 , despite the increased total angular moment. We interpret these results as indicating a reduction in the interaction strength between the rare-earth elements and the Fe as the Gd is replaced by Dy or Ho, and that Dy reduces this interaction strength faster than does Ho.

Amorphous-FeCoCrZrB ferromagnets for use as high-temperature magnetic refrigerants

Magnetic metallic glasses, having large magnetic moments and high Curie temperatures (TC), have not been widely studied as magnetic refrigerants. In this study, we report on the magnetocaloric effects of five FeCoCrZrB amorphous alloys [(FexCoyCrz)91Zr7B2, with Fe:Co:Cr ratios of 100:0:0, 90:15:5, 85:5:10, and 75:15:10] chosen for efficiently surveying the high TC metallic glass composition surface. Magnetic isotherms were measured at 25K intervals from 25to525K. The entropy change (ΔS) due to the magnetocaloric effect was computed by integrating the magnetic isotherms. The refrigeration capacity (RC) was computed by Wood-Potter method. At 3979kA∕m (50kOe) the FeCoCrZrB alloys have RC values of 240–320J∕kg; at 796kA∕m (10kOe) the alloys have values of 32–54J∕kg. The RC values favorably compare with values reported for other refrigerant materials. The peak ΔS temperature depends strongly on composition, varying from 200to450K. The wide flexibility in choosing amorphous alloy compositions allows them to be ...

Vertically graded anisotropy in Co/Pd multilayers

Author(s): Kirby, Brian J.; Davies, J. E.; Liu, Kai; Watson, S. M.; Zimanyi, G. T.; Shull, R. D.; Kienzle, P. A.; Borchers, J. A. | Abstract: Depth grading of magnetic anisotropy in perpendicular magnetic media has been predicted to reduce thefield required to write data without sacrificing thermal stability. To study this prediction, we have producedCo/Pd multilayers with depth-dependent Co layer thickness. Polarized neutron reflectometry shows that thethickness grading results in a corresponding magnetic anisotropy gradient. Magnetometry reveals that theanisotropy gradient promotes domain nucleation upon magnetization reversal - a clear experimental demonstrationof the effectiveness of graded anisotropy for reducing write field.