Tungsten carbide has been shown to be an effective catalyst for a number of reactions that are readily catalyzed by platinum, but not at all by tungsten, and it was speculated that this behavior is due to changes in the electron distribution when carbon is added to tungsten. A test of this hypothesis, made by measuring the valence band x-ray photoelectron spectrum of tungsten carbide and comparing it with the spectra of tungsten and platinum, shows that, near the Fermi level, the electronic density of states of tungsten carbide more nearly resembles that of platinum than that of tungsten.
Gadolinium nanoparticles have been produced at subambient temperature by alkalide reduction. The nanoparticles display maxima in the temperature dependence of their magnetization, cooled in the absence of an applied external field, at T(max) of 5.0 and 17.5 K for unheated samples and samples annealed at 1000 degrees C for 4 h, respectively. Field cooled behavior deviates at temperatures slightly above T(max), increasing at lower temperature. Curie-Weiss law fits of the high-temperature data yield magnetic moments in close agreement with those expected for noninteracting Gd(3+) ions, suggesting that the behavior seen is due to a magnetic transition rather than superparamagnetism. Magnetization is linearly dependent on field at temperatures higher than 7-8 times T(max) and shows remanence-free hysteresis at lower temperature, suggesting metamagnetism. Some annealed samples show evidence of additional ferromagnetic interactions below approximately 170 K. Magnetic entropy curves generated from magnetization data are consistent with that expected for a paramagnet.
Wasp-waist and pot-belly hysteresis loops have been observed in many materials. When only the major loop is reported, the results are insufficient to establish which processes are involved. We present two models for wasp-waist materials that produce virtually indistinguishable major loops, but show that first-order reversal curves can be used to separate the effects. In the simplest model, we take a soft magnetic material and a hard material and exchange couple them. When the exchange is positive, the loop is conventional. However, for negative (antiferromagnetic) exchange, the wasp-waist loop is obtained. Negative coupling of two materials with different switching field distributions leads to pot-bellied loops.
The Knight shift, % , measures th e magne ti c hype rfin e fi eld at th e nucleu s produ ced by th e conducti on e lec tro ns whi c h are po la ri ze d in a ma gne ti c fi e ld. Kni ght s hifts are ofte n dominated by th e Pauli term a nd , in it.s mo st s impl e form , can be writte n as % = (a) x p. H ere XP is the conduction electron Pauli s pin s usce ptibility whic h de pe nd s on th e de nsity of states at th e Fe rmi le vel, N(EF), and (a ) is an a ve rage magneti c hype rfin e couplin g' cons tant associated with th e wav e function characte r at the nu c le us, 1 1,.( 0) I", for co ndu c ti o n ele ctrons at th e F e rmi surface. The Kni ght s hift th~refore provides, through ( a ), insight into th e wave-function c haracte r assoc iated wit.h N(E,.·). Ca lc ul a ti ons of ( a ) involvin g a n a ve raging ove r k-s pace have bee n a tte mpted for a fe w simple me tals up to the prese nt tim e. For alloys and inte rm etaJJi c co mpound s, rath er diffe re nt ( a ) 's are expe rim entall y obse rved for diffe re nt local environm e nts, indi ca tin g that % sampl es th e variati on in local wav e-fun cti o n c ha racte r, or a va ri ati on in local de nsit y of sta tes . Th e re is no uniqu e wa y of se para tin g th e local va ri ati on of N(E,.) fro m 1 1,· 1 (0) 12.In thi s article the me thods deve loped for relatin g % to t.h e e lectroni c prope rti es for most of th e types of cases e nco untered in the literature are re vi e wed. We di sc uss "simple" me tal s in cluding probl e ms of orbital magne tis m a nd cha nge s in % cau sed by electroni c tran siti ons suc h as meltin g. Kni ght s hifts and the ir te mpe ra ture de pe nd e nce in metals and inte rme taJJic compo und s involvin g unfill ed d s hells , are di sc ussed . We give es timates of atomi c hype rfin e fi eld s due to sin gle electrons, appropriate to those cases where proble ms du e to electronic configurations do not mak e deduc ti ons from expe rime nt too ambiguou s. A density of states c urve calc ulated for Cu is given , showing th e relative importance of Sop , a nd d c ha racte r for th a t me tal. In a qualitative sense this C u c urve implies such information for other transition metals. We disc uss alloy solid solutions for the cases where a "rigid" band mod el might be us ed to explain the res ults, and for cases where local effects have to be taken into accounl. The charge oscillation and RKKY approac hes and their limitations are reviewed for cases of dilute nonm agne ti c a nd d-or !-t.ype impurities.Key word s: El ectroni c densit.y of states; hyperfin e fields; Knight shift; nuclear magnetic reso nan ce; s usce ptibility; wa ve fun cti ons.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.