Volume 41, Issue 3, 01 March 1970
Index of content:
- PROCEEDINGS OF THE FIFTEENTH ANNUAL CONFERENCE ON MAGNETISM AND MAGNETIC MATERIALS
- NUCLEAR ANTIFERROMAGNETISM—Ni—Cu ALLOYS
41(1970); http://dx.doi.org/10.1063/1.1658984View Description Hide Description
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 of 19F 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.
41(1970); http://dx.doi.org/10.1063/1.1658985View Description Hide Description
Low‐temperature specific heat and magnetization measurements with Ni–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.1 The paramagneticsusceptibility 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 the d band. 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
- THE MOTT TRANSITION—ANISOTROPIC EXCHANGE
41(1970); http://dx.doi.org/10.1063/1.1658986View Description Hide Description
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.
41(1970); http://dx.doi.org/10.1063/1.1658987View Description Hide Description
The metal to insulator transition of V2O3 proceeds from a low‐temperature antiferromagnetically ordered insulating state1 to a high‐temperature metallic state. Pressures greater than 26 kbar completely suppress the metal‐insulator transition,2 leaving V2O3 metallic to the lowest temperatures. This allows study of the metallic‐phase magnetic properties at low temperature. Here we report high‐pressure 51V nuclear magnetic resonance(NMR) measurements in V2O3 which 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. The 51V 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 μ B exists in the metallic phase, while the Knightshift value shows that no space‐average spin magnetization greater than 0.5×10−3 μ B per 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 ≥1013 sec−1 to account for the 4.2 K relaxation rate of ≤200 sec−1. This is nearly a band frequency, indicating that metallic V2O3 is best described by an exchange‐enhanced band picture. The pressure dependence of the Knight shift implies that the d‐spin component of the susceptibility is unusually strongly dependent on volume, with dlnχd/dln V = 8±5. This strong volume dependence, together with a correspondingly anomalous volume dependence of the resistivity,2 suggests that metallic V2O3 is very nearly critical with respect to formation of an insulating paramagnetic state. Indeed, alloying of ≳1% of Cr with V2O3 produces a first‐order transition at T>170 K to a paramagnetic insulating phase.3 This transition upon Cr alloying involves neither a lattice symmetry change nor magnetic ordering and has all the features of a Mott metal‐insulator transition.
- TRANSITION METAL ALLOYS
41(1970); http://dx.doi.org/10.1063/1.1658988View Description Hide Description
The magnetic properties of a series of Au80 (V20−x Fe x ) alloys, where x varied 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 orderingtemperature, 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.
41(1970); http://dx.doi.org/10.1063/1.1658989View Description Hide Description
The static magnetic susceptibility of Ni 3Ga 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.
41(1970); http://dx.doi.org/10.1063/1.1658990View Description Hide Description
The pressure dependence of antiferromagnetic ordering in Cr–Ru, Cr–Mn, and Cr–Re alloys has been investigated. At low concentrations, TN drops 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 Néel temperature (TNI ) for P ‐ Itransition, if the P ‐ Ctransition had not occurred first. TNI when plotted against e/a shows a sharp break at pure Cr, in accord with theoretical predictions based on the ``depairing'' effects of impurity scattering.
41(1970); http://dx.doi.org/10.1063/1.1658991View Description Hide Description
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.
41(1970); http://dx.doi.org/10.1063/1.1658992View Description Hide Description
For Ni–Cu alloys on the paramagnetic side of the critical composition (∼44 at % Ni), the variation of the initial susceptibility with temperature is found to be resolvable into a Curie‐Weiss term C cw/(T−θ) and a temperature‐independent term χ′. In conjunction with the high‐field magnetizations measured at 4.2°K,1 the values for C cw reveal the existence of large superparamagnetic units whose number per atom falls rapidly from ∼0.3% (for the 44% Ni alloy) to ∼0.01% (for 32% Ni), but whose moment is essentially constant at ∼10 μ B over this composition range. Thus, the giant polarization clouds recently discovered in the weakly ferromagnetic alloys (50%–46% Ni) by neutron diffraction2 are shown to persist in the paramagnetic alloys. These results, including the smooth variation of χ′ through the critical composition, indicate that critical exchange enhancement in Ni–Cu is manifested in the spontaneous nucleation of magnetic polarization clouds at local, highly Ni‐rich regions, and not in the bulk ferromagnetism, which arises from the coupling between neighboring clouds.
41(1970); http://dx.doi.org/10.1063/1.1658993View Description Hide Description
High‐field magnetic moment measurements have been completed in Ni x Cu1−x alloys1 near 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, χHF, decreases with increasing field and with decreasing temperatures; (2) χHF increases with increasing x up to x=0.42 and is relatively constant from x=0.44 to 0.48; (3) linear extrapolation to H=0 yields a moment σ(0) which is linear with x for x≥0.44 corresponding to about 0.6 μ B /added Ni atom; (4) for x<0.44, σ(0) decreases less rapidly down to x=0.32 (the lowest value of x investigated so far). These results are compared with recent low field measurements of Kouvel and Comly3 and Robbins et al.4 The general features of χHF and σ(0) are compared with uniform and nonuniform (or clustering) models3,4 for occurrence of ferromagnetism in Ni–Cu alloys.
41(1970); http://dx.doi.org/10.1063/1.1658994View Description Hide Description
We have measured the resistivity of a series of Ni–Cu alloys as a function of temperature between 1.5° and 300°K, and as a function of field from 0 to 50 kG at 4.2°K. Experiments have been completed on nine paramagnetic alloys ranging in composition from 30 to 44 at.% Ni. A minimum occurs at low temperatures1 and the depth of the minimum increases with Ni content. At higher temperatures, the slope of the resistivity‐temperature curve decreases with increasing temperature, and in fact, becomes negative for high nickel content. The high‐field magnetoresistance is negative in all cases and is proportional to the concentration of giant polarization clouds2 as a function of Ni–Cu composition. These properties may be attributed to the scattering of conduction electrons from polarization clouds acting as large superparamagnetic moments. Results will also be presented for alloys beyond the critical composition for ferromagnetism.
- MAGNETIC SEMICONDUCTORS I
41(1970); http://dx.doi.org/10.1063/1.1658995View Description Hide Description
The existence of antiferromagnetism is investigated in the single‐band Hubbard Hamiltonian in the limit of bandwidth much less than intra‐atomic Coulomb interaction of electrons. We make use of the canonical transformation and ``spectral decomposition'' of the electron creation operators proposed by Harris and Lange, to write down a Green's function which describes electrons in the lower of the split bands of Hubbard's solution. The equation of motion is solved using the moment‐conserving decoupling approximation of Tahir‐Kheli and Jarrett [R. Tahir‐Kheli and H. S. Jarrett, Phys. Rev. 180, 544 (1968)]. We find within our approximation that it is impossible to have an antiferromagnetic state for other than one electron per site. To remedy this defect of the single‐band model we investigate a simplified two‐band model in the limit of intra‐atomic Coulomb and exchange interaction much greater than the bandwidth, and find that antiferromagnetism is possible for the two nearly half‐filled bands. We also discuss effects of the antiferromagnetic ordering on the conductivity in our simplified model, and discuss applicability of the theory to real transition metals and transition‐metal oxides.
41(1970); http://dx.doi.org/10.1063/1.1658996View Description Hide Description
We have considered the question of whether the Hubbard Hamiltonian can lead to properties characteristic of two types of semiconductors, depending on the value of the ratio Δ/I of bandwidth to intra‐atomic Coulomb integral. In one type there is a transformation, with increasing T, from a magnetic insulating state to a paramagnetic insulating state, and in the other the system goes from a magnetic insulating state to a paramagnetic metallic state. We have applied a new variational single‐determinantal approximation which, in contrast to the standard thermal Hartree‐Fock approximation, duplicates the exact behavior of the model both in the atomic limit, Δ/I=0, and the band limit Δ/I=∞. Limiting ourselves to well‐known types of one‐electron states, we have obtained stability boundaries (as determined by the free energy) between various phases. While the boundaries obtained to date have intrinsic interest, we find that further calculations are necessary to completely answer the question raised above.
41(1970); http://dx.doi.org/10.1063/1.1658997View Description Hide Description
Semiconductor‐to‐metal‐type transitions were studied by magnetic susceptibility measurements on pure single phases of Ti3O5, Ti4O7, Ti5O9, and Ti6O11. This work represents a more detailed investigation than those reported previously. From this and related studies the following model is proposed to explain the observed phenomena. Below the transition, groups of cations are distributed periodically throughout the lattice. Within these groups the d electrons are delocalized; however, ``constrained‐type'' antiferromagnetism sets in between specific neighboring d electrons through homopolar bonding of cations. Neighboring groups interact via thermal excitation of electrons. Above the transition this type of magnetic ordering is modified by changes in crystal structure; here 3d overlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice. The behavior is described by the free‐electron gas model. In addition, in all but Ti3O4, a Curie‐Weiss law phenomenon is seen to be superimposed on this transition, which may result from relatively small (<11%) interactions between the unpaired spins. In proposing this model, calculations of the effective electronic mass and Goodenough's ideas have been used.
41(1970); http://dx.doi.org/10.1063/1.1658998View Description Hide Description
The Verwey transition in magnetite is described on a band model, with the electrons self‐consistently breaking the symmetry by ordering in the self‐induced Coulomb potential. The order parameter is the difference in occupation numbers on alternate sites. There is a gap in the single‐particle excitation spectrum, and the gap and order parameter undergo a second‐order phase transition. The theory is BCS‐like, with the Verwey temperature being given in the weak‐coupling limit by an exponential in the reciprocal of the density of states at the Fermi level.
41(1970); http://dx.doi.org/10.1063/1.1658999View Description Hide Description
Vanadium ``monoxide'' VO x exists over a wide homogeneity range (VO0.79–VO1.30) in the cubic NaCl structure. With increasing oxygen content or V‐vacancy content, its properties progress from weak temperature‐independent paramagnetism to a stronger temperature‐dependent paramagnetism, and from metallic to semiconducting conductivity. In the present work we have observed the nuclear resonance of 51V in small single‐crystal pieces of pressure‐annealed VO0.86, VO1.02, and VO1.23, using cw and spin‐echo techniques in fields of 9–50 kOe at temperatures from 1.4°–300°K. In contrast with previously reported measurements, in no compound was there evidence of a sharp metal‐to‐insulator transition, and nuclear resonance was observable at all temperatures in all the compounds. A Knight shift of 0.4%, independent of composition and temperature, was observed, while the resonance linewidths increased with increasing susceptibility to a value of 5% of the applied field at 1.4°K in VO1.23. These results indicate that the bulk of the temperature‐dependent magnetization comes from local moments on a minority of sites whose nuclear resonances are unobservable. These moments, however, cause the broadening of the observed majority‐site resonance. A maximum in the transverse relaxation rates observed between 1.4° and 77°K in VO1.23 in a 47‐kOe field apparently results from the onset of spin‐spin correlations in this range.
41(1970); http://dx.doi.org/10.1063/1.1659000View Description Hide Description
Neutron diffraction measurements on powder and single‐crystal samples of V2O3 have confirmed the existence of long‐range magnetic order in the low temperature (monoclinic) phase. Paoletti and Pickart1 discovered an extra diffraction peak at low temperature which they attributed to nuclear scattering. A polarization analysis experiment on a powder sample showed that this peak was magnetic in origin. In addition, several much weaker magnetic peaks were identified. These peaks index in the monoclinic cell given by McWahn,2 with indices satisfying the relation h+k+l=2n+1. The magnetic peaks show little temperature dependence from 77°K up to the crystallographic transition at 170°K, where they vanish abruptly. On cooling, they suddenly reappear at about 154°K. The data agree with a model in which the V moments are ferromagnetically aligned in (010) m layers (perpendicular to hexagonal a axis) with a reversal between adjacent layers. The ordered moment of 1.2 μ B per V atom is oriented perpendicular to the hexagonal a axis at an angle of about 71° from the hexagonal c axis.
- MAGNON PAIR MODES
41(1970); http://dx.doi.org/10.1063/1.1659001View Description Hide Description
Far‐infrared transmission measurements have been made on pure and Fe2+‐doped MnF2 at 4.2°K using magnetic fields up to 105 kOe. By means of Fourier‐transform spectroscopy we have investigated the behavior of the spin‐wave modes near the spin‐flop transition. Both the AFMR and the two‐magnon absorption with the E field perpendicular to the c axis have been measured. In Fe2+‐doped MnF2 the spin flop occurs at higher fields depending on the Fe2+‐concentration. The localized mode which splits linearly in low fields no longer shows a linear dependence in the spin‐flop region.
41(1970); http://dx.doi.org/10.1063/1.1659002View Description Hide Description
The temperature dependence of magnon‐pair excitations has been obtained in the simple antiferromagnets RbMnF3 and FeF2, by the technique of inelastic light scattering. At low temperatures the pair‐mode frequency is nearly twice that of a Brillouin‐zone boundary magnon, decreased slightly by magnon‐magnon interaction effects. As the temperature is increased, the magnon‐pair modes broaden and decrease in frequency. The excitation evolves smoothly as T is raised above TN into the paramagnetic phase. In both materials, light scattering from the paramagnetic vestiges of the magnon‐pair modes could be observed up to 300°K. Together with earlier results on NiF2, these data indicate the trends in magnon‐pair temperature behavior for materials with different spins and different magnon‐magnon interaction effects. These trends may be useful in guiding theories for magnon‐pair excitations.