ISSN:0021-8979    

DOI:10.1063/1.1658983

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

In most diamagnetic substances the strength of dipolar interactions between nuclear spins is such that temperatures of the order of a microdegree or less must be obtained for cooperative effects to take place. Rather than cooling the whole sample we have cooled directly the nuclear spins of19F in CaF2, which are thermally isolated for the duration of the experiment by the weakness of the spin‐lattice relaxation mechanism. The cooling takes place in two steps. First, the spins are polarized in a high field by the ``solid effect'' to an equivalent temperature of the order of 5 mdeg. Second, they are demagnetized in the rotating frame reaching a temperature below one microdegree. For negative temperatures their transverse susceptibility as a function of initial polarization exhibits a plateau characteristic of antiferromagnetic behavior. Results are in qualitative agreement with a molecular field theory.

ISSN:0021-8979    

DOI:10.1063/1.1658984

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Low‐temperature specific heat and magnetization measurements with Ni&sngbnd;Cu solid solutions indicate the presence of magnetic clusters in these alloys between 47% and 62% Cu. Up to 57% Cu the interaction between clusters is sufficient for the alloys to become ferromagnetic at low temperatures.1The paramagnetic susceptibility of the alloys with higher Cu content is essentially independent of temperature above 200°K; in this temperature range the susceptibility depends on the Cu content. At low temperatures the susceptibility becomes temperature dependent and its value is strongly influenced by impurities, e.g., Fe and Co. The decrease of the ferromagnetic moment with increasing Cu content is not a result of the filling of thedband. Rather, it can be accounted for on the basis of the dependence of the moment associated with Ni atoms on their local atomic environment.2

ISSN:0021-8979    

DOI:10.1063/1.1658985

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

We have studied the magnetic properties of the rare‐earth alloys RAl, RNi, and R3Ni, with R = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, in high magnetic fields up to 70 kOe. For the three series, the magnetic properties differ according to the alloyed rare earth. The compounds RAl of DyAl type and RNi of FeB type are ferromagnetic or antiferromagnetic. The compounds R3Ni exhibit metamagnetism, except Tm3Ni which has a ferromagnetic behavior. We have studied the magnetic structure of NdAl, TbAl, HoAl, ErAl, TmAl, NdNi, ErNi, HoNi, and Er3Ni by neutron diffraction. They have all a noncolinear magnetic structure except NdNi, in which the magnetic moments are parallel. We have studied the stability of the magnetic structures by means of group theory; they are all stabilized by strongly anisotropic magnetic interactions between rare‐earth atoms.

ISSN:0021-8979    

DOI:10.1063/1.1658986

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The metal to insulator transition of V2O3proceeds from a low‐temperature antiferromagnetically ordered insulating state1to a high‐temperature metallic state. Pressures greater than 26 kbar completely suppress the metal‐insulator transition,2leaving V2O3metallic to the lowest temperatures. This allows study of the metallic‐phase magnetic properties at low temperature. Here we report high‐pressure51V nuclear magnetic resonance (NMR) measurements in V2O3which show that the metallic phase, unlike the insulating phase, does not order magnetically down to 4.2 K. The measurements were made with spin echo and free induction decay techniques at a frequency of 6.9 MHz in a cryogenic press with a nonmagnetic high‐pressure die, and are the highest pressure resonance measurements which have been made at cryogenic temperatures. The51V NMR was observed at pressures greater than 26 kbar with a linewidth of 300 G and a Knight shift of (− 1.0±0.2) % + (0.01±0.005)P% (P in kbar). At lower pressures the resonance intensity decreased rapidly, evidently as antiferromagnetic ordering produced larger frequency shifts and moved the resonance out of the observable range. The linewidth value implies that no static configuration of vanadium antiferromagnetism with moments larger than 10−3&mgr;Bexists in the metallic phase, while the Knightshift value shows that no space‐average spin magnetization greater than 0.5×10−3&mgr;Bper vanadium atom exists under the experimental conditions. It is concluded that there are no static localized moments in metallic V2O3. Any fluctuating moments would have to fluctuate at frequencies ≥1013sec−1to account for the 4.2 K relaxation rate of ≤200 sec−1. This is nearly a band frequency, indicating that metallic V2O3is best described by an exchange‐enhanced band picture. The pressure dependence of the Knight shift implies that thed‐spin component of the susceptibility is unusually strongly dependent on volume, withdln&khgr;d/dln V= 8±5. This strong volume dependence, together with a correspondingly anomalous volume dependence of the resistivity,2suggests that metallic V2O3is very nearly critical with respect to formation of an insulating paramagnetic state. Indeed, alloying of ≳1% of Cr with V2O3produces a first‐order transition atT>170 K to a paramagnetic insulating phase.3This transition upon Cr alloying involves neither a lattice symmetry change nor magnetic ordering and has all the features of a Mott metal‐insulator transition.

ISSN:0021-8979    

DOI:10.1063/1.1658987

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The magnetic properties of a series of Au80(V20−xFex) alloys, wherexvaried from 1 to 9, were investigated to determine the effect of adding a solute with a localized moment on the magnetic behavior of Au4V. Magnetization measurements were made over the temperature range from 4.5° to 300°K. All specimens were magnetically ordered at 4.5°K. Above the magnetic ordering temperature, the inverse susceptibility vs temperature curves can be represented by an equation which includes a Curie‐Weiss term and a Pauli paramagnetism term. The effect of adding Fe to the ordered Au4V alloy is to increase the effective moment and the Curie temperature of the alloys with up to 7 at.% Fe. However, the alloy with 9 at.% Fe did not chemically order under conditions employed for the other compositions, but did order magnetically at about 20°K.

ISSN:0021-8979    

DOI:10.1063/1.1658988

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The static magnetic susceptibility of Ni3Ga is strongly decreased by dilute Pd and Pt impurities. A linear relationship is found between that temperature and that impurity concentration which produce the same change of the susceptibility. The effective spin‐flip energy of the impurities is derived and found to agree with their spectroscopic spin‐orbit energy if differences of the effective local exchange enhancement between host and impurity are assumed to be negligible.

ISSN:0021-8979    

DOI:10.1063/1.1658989

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The pressure dependence of antiferromagnetic ordering in Cr&sngbnd;Ru, Cr&sngbnd;Mn, and Cr&sngbnd;Re alloys has been investigated. At low concentrations,TNdrops very rapidly and nonlinearly with pressure up to some point and then exhibits a much slower variation with a distinct break in slope. We attribute this break in slope to a transition from communsurate phase (C) at lower pressures to an incommensurate phase (I) at higher pressures. If we extrapolate the paramagnetic (P) to incommensurate boundary back to 1 atm we obtain values for the Ne´el temperature (TNI) forP‐Itransition, if theP‐Ctransition had not occurred first.TNIwhen plotted againste/ashows a sharp break at pure Cr, in accord with theoretical predictions based on the ``depairing'' effects of impurity scattering.

ISSN:0021-8979    

DOI:10.1063/1.1658990

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Neutron diffuse scattering measurements were made on Co alloys containing 5 at.% V, Cr, Mn, and Ni impurities in order to determine the distribution of magnetic moment around the impurity sites. The measurements were made at room temperature on polycrystalline samples with 1.09‐Å neutrons. The observed cross sections are of two distinct types. For the alloy containing Ni impurities, the cross section is independent of the scattering vector and indicates that Ni impurities produce no moment disturbances on surrounding Co atoms. With V, Cr, or Mn impurities the cross section decreases with increasing scattering vector in a manner which indicates that these impurities produce moment disturbances on their Co neighbors out to distances of approximately 5 Å. All three of these metals produce a net moment reduction when added to Co. For V and Cr this is mostly due to the negative disturbances produced on the neighboring Co atoms. For Mn impurities, an oppositely aligned Mn moment makes an equally significant contribution to the observed moment reduction.

ISSN:0021-8979    

DOI:10.1063/1.1658991

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

High‐field magnetic moment measurements have been completed in NixCu1−xalloys1near the critical concentration2(x≃0.44). Fields of 150 kG or higher and temperatures from 4.2 to 1.5 K were employed. The general features of the data are: (1) the differential susceptibility at high field, &khgr;HF, decreases with increasing field and with decreasing temperatures; (2) &khgr;HF increases with increasingxup tox=0.42 and is relatively constant fromx=0.44 to 0.48; (3) linear extrapolation toH=0 yields a moment &sgr;(0) which is linear withxforx≥0.44 corresponding to about 0.6 &mgr;B/added Ni atom; (4) forx<0.44, &sgr;(0) decreases less rapidly down tox=0.32 (the lowest value ofxinvestigated so far). These results are compared with recent low field measurements of Kouvel and Comly3and Robbinset al.4The general features of &khgr;HF and &sgr;(0) are compared with uniform and nonuniform (or clustering) models3,4for occurrence of ferromagnetism in Ni&sngbnd;Cu alloys.

ISSN:0021-8979    

DOI:10.1063/1.1658993

出版商:AIP    

年代:1970

数据来源: AIP