摘要:

The excitation of first‐order spin‐wave instabilities in an antiferromagnet which may carry a nuclear moment is reexamined. Nuclear, electronic, and mixed magnon pairs can be excited by processes which are analogous to the ferromagnetic case. Various geometries of pumping fields are possible and their critical values are given. Contrary to the assumption of previous authors, we show that when photon pumping fields are applied to a flopped antiferromagnet, the uniform precessionis always excited, and its contribution to the threshold amplitude is significant andcan never be neglected. This implies that direct magnon excitation in which the linear susceptibility is negligibly small (parallel pumping) does not exist in an antiferromagnet when the weak dipolar effects are ignored. However, when phonon fields are applied, parallel pumping processes are allowed due to the change in symmetry of the effective magnetoelastic pumping field. In particular when a longitudinal wave is propagating parallel (or nearly so) to the dc sublattice magnetizations the effect of the negligibly small linear subsceptibility can be ignored and the process is analogous to parallel pumping in a ferromagnet.

ISSN:0021-8979    

DOI:10.1063/1.1659026

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

We observe 2‐ and 3‐pulse echoes in CsMnF3at 2°K, by pulsing the antiferromagnetic resonance. The 2‐pulse echoes have a short decay time <0.2 &mgr;sec while the 3‐pulse echo decay is quite long, >1 sec. The 3‐pulse echo is thought to result from indirect excitation of55Mn nuclear spins which are strongly coupled to the AFR mode. The AFR echo shows double resonance effects at all four NMR modes.

ISSN:0021-8979    

DOI:10.1063/1.1659028

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Excitation of two nuclear spin waves, as predicted by Hinderks and Richards, has been accomplished by parallel pumping in CsMnF3at 988 MHz. This is the first time to our knowledge nuclear spin waves of nonzero wavevector have been excited directly by electromagnetic fields without simultaneous excitation of electronic spin waves or other modes. Wavevectorskfrom near zero to 105cm−1are excited at 4.2° and 1.1°K. Relaxation times are sufficiently long that they can be measured directly from the transient decay, without having to rely on absolute measurement of criticalrffields. Order of magnitude or better agreement is obtained with the theory of Richards on the relaxation rate &eegr;kalthough thekdependence does not agree at 4.2°K. At 1.1°K, thekdependence is very close to that predicted by theory. The values of &eegr;kat 4.2°K are very close to those inferred by Seavey from more indirect measurements. A second threshold has also been detected at higher powers. This may be the result of nuclear spin and lattice heating which causes first an increase and then a decrease in the nuclear spin‐wave frequency.

ISSN:0021-8979    

DOI:10.1063/1.1659029

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

GdN is shown to be ferromagnetic but small amounts of oxygen (GdN1−xOxwithx≈0.05) cause paramagnetism and short‐range antiferromagnetic order to set in. We attribute this to nearest‐neighbor interactions having opposite sign.

ISSN:0021-8979    

DOI:10.1063/1.1659030

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

CsCoF3has hcp (h) and ccp (c) CsF3layers alternatinghhchhchhcat atmospheric pressure (9Lstructure of BaRuO3) but acchcchsequence (6Lstructure of RbNiF3) if quenched from 700°C at pressures greater than 20 kbar. The octahedral‐site Co2+ions are between the layers and give the 9Lstructure a pink color, the 6Lstructure a lavender hue. The 9Lphase obeys a Curie‐Weiss law above 70°K withCm=3.7 emu°K/mole and &THgr;p=−62°K. It hasTN=8°K with evidence for a spin‐flop transition atHc≈11 kOe and an anisotropy constantK≈5×104ergs/cm3at 4.2°K. The 6Lphase hasCm=3.3 emu°K/mole, &THgr;p=−65°K and is ferrimagnetic belowTC=50°K. The magnetization is not saturated at 17.2 kOe and 4.2°K, where &mgr;¯=0.8 &mgr;B/Co2+is smaller than the 1.23 &mgr;B/Co2+anticipated at saturation for RbNiF3‐type ordering. Optical data show a larger trigonal‐field splitting for the 6Lstructure. For both phases,Dq≈760 cm−1, spin‐orbit interaction parameter &zgr;≈500 cm−1, and long wavelength cutoff≈13 &mgr;m.

ISSN:0021-8979    

DOI:10.1063/1.1659031

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Effects of covalent spin densities on the neutron magnetic scattering are described using results of UHF‐MO‐SCF calculations for the (MnF6)4−cluster and for a model three‐atom system (Sc2+&sngbnd;F−&sngbnd;Sc2+) in a linear configuration. Full variational freedom was given to all the electrons and all multicenter integrals were included. From the computed spin densities magnetic form factors were calculated including nonspherical terms and core polarization effects. Both ferromagnetic and antiferromagnetic states of the three‐atom system were studied in order to investigate possible differences in covalent behavior and the contribution of covalency to measurements of zero‐point spin deviation.

ISSN:0021-8979    

DOI:10.1063/1.1659032

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The chemical and magnetic structure of MnAs0.92P0.08has been studied from 10° to 427°K using x‐ray and neutron diffraction. Below 440°K MnAs0.92P0.08is orthorhombic (Pnma;c>a>b), and a refinement of the x‐ray data at lower temperatures showed that there was a continuous change of the Mn and As(P) atomic coordinates. The nuclear scattering, calculated using the x‐ray parameters, was subtracted from the measured neutron intensities. The resultant magnetic scattering indicated there must be both ferromagnetic and antiferromagnetic spin components. The orthorhombic magnetic space group,Pn′m′a, is consistent with both types of spin components and the observation of the 001 antiferromagnetic peak. A least‐squares fit at 200°K gave a ferromagnetic component of 2.0 &mgr;Balong thecaxis and an antiferromagnetic component of 1.6 &mgr;Balong theaaxis. Below 150°K further weak magnetic lines appear, indicating the onset of long period modulations. The Mn moment is consistent with a high spin rather than a low spin, in disagreement with published bulk magnetization data.

ISSN:0021-8979    

DOI:10.1063/1.1659033

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Neutron diffraction and magnetic susceptibility measurements were performed on the MnPd3phase in both completely ordered and partially disordered states. The antiferromagnetic configuration of MnPd3was confirmed in the range of 23–30 at.% Mn. The angle &phgr;cof the magnetic moments to the tetragonal axis, which is 0° below 25% and 90° at higher Mn concentrations, was observed to become atomic order‐dependent at 25%, increasing from 0° towards 90° with decreasing order. A decrease in the Ne´el temperature with decreasing order was also observed. The 90° turn in the direction of the magnetic moments is thought to be due to the effect of the nearest‐neighbor Mn pairs which are formed either because of the partial disorder of the stoichiometric alloy or owing to the excess Mn atoms in the ordered nonstoichiometric alloys.

ISSN:0021-8979    

DOI:10.1063/1.1659034

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The orthorhombic compounds BaMF4(M=Mn, Fe, Co, or Ni) have recently been found to undergo antiferromagnetic transitions at low temperatures. Susceptibility measurements on BaNiF4reveal that a broad maximum occurs at about 150°K similar to that observed in a number of layer‐type structures. As the temperature is lowered, &khgr;∥ alongbdecreases rapidly, while &khgr;⊥ goes through a shallow minimum around 50°K. At 77°K, no magnetic order can be detected in powder neutron diffraction patterns of BaNiF4, but at 4.2°K there are magnetic peaks which can be indexed on a unit cell doubled along thebandcdirections (A21amorientation). The magnetic structure consists of puckered (010) sheets within which the moments are coupled antiparallel to neighboring moments about 4‐Å apart. However, the crystal structure is such that the net interaction between adjacent sheets is zero, and ordering in thebdirection depends upon interactions between second nearest sheets 14‐Å apart. This is very similar to the situation inK2NiF4and pronounced two‐dimensional behavior is therefore to be expected. The moments are directed along thebaxis and the ordered moment is about 2.0 &mgr;Bper Ni ion. The magnetic symmetry is actually monoclinic, and the simplest magnetic space group isP2a21.

ISSN:0021-8979    

DOI:10.1063/1.1659035

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Two main effects reduce the apparent moment associated with a cation in an ordered antiferromagnet. Covalency tends to spread the moment onto the adjacent anions resulting in a loss due to cancellation at the ligand,1and the nature of the antiferromagnetic ground state gives rise to zero point deviations from fully aligned spins.2The latter effect has proved very difficult to measure experimentally with accuracy. In principle the moment associated with the cation may be measured using neutron diffraction by extrapolation of the magnetic Bragg peak intensities to forward scattering. The present measurements were carried out on an ideal antiferromagnet, KNiF3, in order to test the current theories of covalency and spin deviations. The magnitude of both effects may be predicted in this compound1,2as the covalency parameters are known from NMR work.3From powder diffraction at 4.2°K we find a value of 1.495 (±0.020) for[g⟨Sz⟩0 f(&tgr;111)/F200]giving a value of 0.851 (±0.050) for the effective value of⟨Sz⟩0. The error in⟨Sz⟩0here includes both the random error of the experiment (0.011), and possible systematic errors (0.039) due to the uncertainties in the current values of the Ni2+gfactor, the form factor, and the nuclear scattering lengths. This value for⟨Sz⟩0may be compared with 0.803 calculated using the molecular orbital theory of covalency and spin wave theory, and 0.832 calculated using covalency parameters determined from a configuration interaction approach4and perturbation calculations of the spin deviation.5The experiments thus confirm the order‐of‐magnitude of the theoretical prediction, but the current systematic errors preclude an exact test of the theoretical approach at the present time. The measurements illustrate the difficulties in obtaining precise values of either covalency parameters or the spin deviation using this technique.

ISSN:0021-8979    

DOI:10.1063/1.1659036

出版商:AIP    

年代:1970

数据来源: AIP