摘要:

Iron‐impregnated medium pore zeolite ZSM‐5 (pore opening of 5.5 A˚) is an efficient catalyst for the conversion of synthesis gas (CO+H2) to high octane gasoline. Activity and selectivity of the ZSM‐5 (Fe) depends upon the manner in which Fe is impregnated on ZSM‐5. Magnetic and Mo¨ssbauer studies were conducted on catalysts prepared by two methods: (a) ferric nitrate impregnation and (b) carbonyl Fe3(CO)12impregnation on the zeolite. These studies were conducted for various stages of preparation and after use of the catalysts. The carbonyl impregnated sample in the as‐prepared form contained ultrafine &ggr;‐Fe2O3of d = 60–66 A˚, as evidenced by superparamagnetic behavior seen in magnetic and Mo¨ssbauer studies. In contrast, the as‐prepared, nitrate impregnated sample showed &agr;‐Fe2O3of approximate particle size of 100 A˚. Both samples were reduced to about 80% metallic Fe in flowing H2. On exposure to synthesis gas, the carbonyl impregnated sample yielded a substantial amount of Fe3O4in addition to &khgr; carbide, wheres the nitrate impregnated sample exhibited &egr;, &khgr;, and &Vthgr; carbides. The steady catalytic activity and selectivity of the carbonyl impregnated sample can be related to the ultrafine iron dispersions leading to the observed phases in the used catalyst.

ISSN:0021-8979    

DOI:10.1063/1.330966

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The magnetic structure of small CrO2particles doped with about 1.2 wt.%57Fe has been explored by57Fe Mo¨ssbauer spectroscopy as a function of the particle size. Mo¨ssbauer spectra, obtained with a longitudinal magnetic field applied, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect is the origin of this phenomenon. It follows that a core model proposed earlier for the magnetic structure of CrO2small particles is warranted.

ISSN:0021-8979    

DOI:10.1063/1.330893

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Landau theory predicts that the bcc, spin 1/2 Ising model with nearest‐neighbor ferromagnetic interaction K1and next‐nearest‐neighbor antiferromagnetic interaction K2has a continuous transition with a two component order‐parameter (n = 2) between the paramagnetic and the type two antiferromagnetic phases for K1/K2<3/2. For K1/K2≲3/2, we show that this system has a transition temperature, determined from matching high‐ and low‐temperature free‐energy series, which is well above the temperature at which the susceptibility diverges. This supports the conclusion, also drawn from renormalization group and Monte‐Carlo calculations, that the transition is driven first‐order by fluctuations.

ISSN:0021-8979    

DOI:10.1063/1.330978

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Globus model for magnetization processes in polycrystalline materials has led to significant improvements in the recognition of the contribution from different microstructural parameters. In this paper we present an analysis of the friction (pinning) term and the wall energy term that appear in the model. Experimental results show that the ratio of these terms is a constant, independent of temperature and the same for two different ferrites, YIG and Ni0.5Zn0.5Fe2O4. This result is analyzed by considering that the origin of the friction is the destruction of the wall, the shape of the grains, and the effect of demagnetizing fields.

ISSN:0021-8979    

DOI:10.1063/1.330979

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Energy loss in magnetic steels is generally taken to consist of two parts: static loss, associated with Barkhausen switching and assumed independent of drive frequency, and eddy current loss due to wall motion. Little effort has gone into analyzing the Barkhausen contribution. We consider a model of a magnetic sample in which the Barkhausen volumes are represented as a distribution of sites which reverse their moments at a uniform rate and only along a single axis. Using Green’s function the current distribution of symmetrically placed regions is determined subject to the boundary conditions of periodic bar domains in a thin sheet. In subsequent reports this is used to calculate losses.

ISSN:0021-8979    

DOI:10.1063/1.330980

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

We consider a model of a magnetic sample with distribution of Barkhausen switch sites which reverse their moments at a constant rate. Switching is considered in the case that reversal is induced by the incidence of a domain wall. The induced current distribution J of symmetrically placed regions has already been determined. The loss calculation involves summing J2over switching regions in the plane of the moving wall, and summing as the wall traverses a full cycle. Averaging techniques are employed and the expected loss over unit volume, per cycle is found, due solely to the Barkhausen regions. This is the classic static or ’’hysteresis’’ loss. In a later report the analysis is extended to determine the full losses.

ISSN:0021-8979    

DOI:10.1063/1.330981

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The total, temporally averaged, expected power density loss due to the combined actions of the Barkhausen (B) and Pry–Bean (PB) mechanisms is p¯ = (&rgr;/V)(JB+JPB)2 dV           (1)                = p¯B+p¯PB+2(&rgr;/V)JB⋅JPB dV  where p¯Bis given by Eq. (II.15),b)p¯PBis the expression derived by Pry and Bean, multiplied by (1−K)2, andVis the domain volume. The last term in Eq. (1) is a result of the simultaneus occurrence of both currents. In evaluating p¯Bit was noted in paper II that the B currents from each switching site/section only flow for the time &tgr; so that it was appropriate to consider energies instead of power. In terms of Eq. (II.15): p¯B=fE(w),wheref= frequency. This consideration must also enter into the cross term in Eq. (1). Space in this abstract limits the discussion of this term, but one may estimate its behavior by taking the currents to be rms values. Then             p¯ = ˙p¯B+p¯PB+2(pB pPB)1/2.              (2)   SincepPB∝f2andpB∝f, this cross term varies asf3/2; the energy density per cycle becomes             W= p¯ /f= ˙(A1+A2&sqrt;f¯+A3f)C,     (3)   where theA’s are functions of the model parametersKanda, andCis a common function of the material properties. The middle term in Eq. (3) is new. Comparison with experiments and analysis of data available in the literature is difficult and unreliable because of the uncertain sample parameters and magnetic state relative to the assumptions entering Eq. (3). However, several works on 3.5% Si‐Fe lead us to infer the values:a≊10 000/sec andK≊0.1. Similarly, direct comparison of Eq. (3) is inconclusive although some agreement is found in cases where the experimental magnetic parameters can be determined. The traditional view of magnetic reversal has large wall traps and smaller Barkhausen switches superimposed upon the continuous wall motion. These all may actually represent different regimes of a continuous distribution of behavioral properties. The present model separates and approximates the trapped regions and larger Barkhausen switches; the literature experiments on Barkhausen effect, from which the parameter valuesaandKwere derived, probably also reflect this separation. More work is necessary to develop the full distribution properties.    a)This work will be reported more fully in the near future.  b)B. E. Lorenz and R. L. Coren, J. Appl. Phys. 53 XXXX (1982) (preceding paper).

ISSN:0021-8979    

DOI:10.1063/1.330982

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Linear birefringence has been used to study the effects of random dilution on the critical behavior near TNin the anisotropic antiferromagnetic systems MnxZn1−xF2and FexZn1−xF2. In the critical region the temperature derivative of the birefringence is almost entirely magnetic in origin and is proportional to the magnetic specific heat. Measurements were made in magnetic fields 0⩽H0⩽20 kOe applied parallel to the easy axis. Such an applied field is expected to couple with the random magnetization generating effective random fields which cause large changes in the critical behavior. With increasing field, dramatic effects on the shape of the magnetic specific heat occur within a crossover region. Large shifts in TNwere observed as a function of the applied field for several concentrations. The observed effects are in qualitative agreement with the theoretical predictions.

ISSN:0021-8979    

DOI:10.1063/1.330937

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The role of random uniaxial anisotropy on critical behavior has been studied in the antiferromagnetic system FexMn1−xF2using linear magnetic birefringence. In the pure FeF2and MnF2crystals the deduced magnetic specific heat near TNshows no rounding for reduced temperature t as small as 10−4. Except for the effects of concentration gradients, no rounding is observed in the mixed crystals. In fact, for the Fe.75Mn.25F2sample, where such gradient effects are minimal, the transition is as sharp as in pure FeF2. Since MnF2and FeF2differ primarily in the size of the single ion anisotropy, it is clear that random anisotropy, by itself, does not broaden the transition. If anisotropy were not considered, MnF2and FeF2would be expected to have nearly identical TN. Although MnF2is itself asymptotically Ising‐like as T→TNbecause of its small but finite anisotropy, the larger TNin the more anisotropic FeF2and the drastic increase in TNwith x near x = 0 is attributable to the effects of crossover from Heisenberg to Ising critical behavior.

ISSN:0021-8979    

DOI:10.1063/1.330938

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The nuclear spin‐lattice (1/T1) and spin‐spin (1/T2) relaxation rates have been measured for19F nuclei with no magnetic near neighbors (F0) in diluted antiferromagnetic MnxZn1−xF2and FexZn1−xF2, for x between 0.1 and 0.8. ForT≫Tn, 1/T1in FexZn1−xF2varies nearly as x1/2. This result indicates that the relaxation process is dominated by the electron‐nuclear dipole interaction with electronic spins in exchange‐coupled clusters. In contrast, 1/T1in MnxZn1−xF2is independent of x, for x greater than 0.5. As x decreases, 1/T1increases until a maximum is observed at x≃0.1. The departure from x1/2behavior is found to be proportional to x(1−x)8, indicating that the exchange‐isolated Mn electronic spins dominate the nuclear relaxation for x less than 0.4. The absence of the exchange‐isolated electronic spin contribution 1/T1in FexZn1−xF2is attributed to the rapid electronic spin‐lattice relaxation rate (&tgr;sl)−1of the Fe2+ion at elevated temperatures. However, this contribution has been observed at 77 K where the dependence of 1/T1on x is virtually identical to that observed in the Mn‐Zn system. The temperature dependence of 1/T1in FexZn1−xF2exhibits a critical‐like divergence near the Ne´el temperature, for x above the percolation limit.

ISSN:0021-8979    

DOI:10.1063/1.330939

出版商:AIP    

年代:1982

数据来源: AIP