摘要:

Grain‐oriented samples of highly magnetostrictive rare earth–iron compounds have been prepared. These samples possess lower inhomogeneous strains than found in the random polycrystalline RFe2compounds, resulting in much higher values of relative permeability (&mgr;r) and magnetomechanical coupling (k33). A partially oriented Th.20Dy.22Ho.58Fe1.95sample was prepared using a pyrolytic Bridgman type boron nitride crucible. At a bias field of 100 Oe k33=.73, which is considerably larger than found in the random polycrystal of the same composition. A relative permeability of 36 occurs in this same sample when a low ac drive of 1.6 Oe rms is used. A second fabrication method using a horizontal zone technique with a supporting ’’cold finger’’ was employed to grow the ternary Tb.27Dy.73Fe1.98. The low ac drive values of k33and &mgr;rwere .74 and 19 respectively. The permeability at low bias was found to possess a sharp ac drive dependence. Near zero bias, when the drive was changed from 1.6 Oe rms to 13 Oe rms, &mgr;rin the quaternary increased from 36 to 98. In the ternary the values of &mgr;rnear zero bias increased from 19 to 61. Magnetostriction measurements on both samples show a significant increase in d&lgr;/dH and &lgr;sover the random polycrystals.

ISSN:0021-8979    

DOI:10.1063/1.327234

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The longitudinal and transverse magnetostriction in EuS were measured at 1.8<T<146 K, in magnetic fields up to 130 kOe. In the paramagnetic phase (T≳Tc=16.49 K) the magnetostriction is predominantly due to the strain dependence of the exchange energy. The results are explained in terms of the theory of Callen and Callen supplemented by numerical calculations of the nearest‐neighbor and next‐nearest‐neighbor two‐spin correlation functions. The agreement with theory is good, but small deviations near Tcsuggest that EuS departs from the ideal Heisenberg model.

ISSN:0021-8979    

DOI:10.1063/1.327235

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The conditions under which stress alone can establish bulk magnetic moments in magnetostrictive ferromagnetic bodies are examined. It is shown that net moments may be realized by appropriately applied mechanical stress, in the absence of any coapplied field or existing net moment, if any or all of the factors influencing relative stress anisotropy (&lgr;,&sgr;, K) are unequal in the regions having antiparallel magnetizations. The regions may be compositionally homogeneous and contiguous with classical domain wall boundaries, or contiguous with structural or compositional steps or continuous gradients, or they may be spatially separated. Multiple pairs of such regions cannot be oriented at random in one ferromagnetic body if the net moment from each pair, due to a physically realizable stress distribution, is to add to a bulk moment. This configurational restriction constitutes an initial state of magnetization even though the pre‐stress flux closure paths are all internal. As the stress is applied and increased, the local magnetic field, accompanying the developing uncompensated moments, exerts a demagnetizing influence on this magnetization, in addition to that caused by the stress itself. When thresholds of irreversibility are passed, the initial magnetization is not regained during stress relaxation.

ISSN:0021-8979    

DOI:10.1063/1.327236

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

MnO is a low temperature antiferromagnet in which a spin orientation anisotropy within the plane of easy magnetization has been observed experimentally. The microscopic origin of this anisotropy has not yet been elucidated. As a consequence there is a certain uncertainty in the determination of the magnetic structure and in the interpretation of the magnon spectrum. We show here that magneto‐elastic coupling by dipolar interactions leads to well defined anisotropy axis which are determined to be the [1, 1¯, 0] directions modulo 2&pgr;/3. We have calculated the values of coupling coefficients which are derivatives of the components of the dipolar tensor with respect to the components of the strain tensor. Moreover using the mechanism proposed in this paper it should be possible to interpret correctly the magnetic properties of MnO in presence of external fields (magnetic field and mechanical stresses).

ISSN:0021-8979    

DOI:10.1063/1.327237

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Many experimental and theoretical investigations have separately characterized the magnetic properties of spin glasses and random ferromagnetic alloys. Magnetic systems which are on the borderline between the two ordering mechanisms show a double magnetic transition as a function of temperature. Two distinct phases have been found in AlFe, CrFe, AuFe, PdMn, etc. In this paper we will review the experimental properties of these systems and compare their behavior with the results of existing theories. The close analogy between the systems and the good agreement with the theoretical calculations suggest that a general magnetic phase diagram exists which can be applied to many random magnetic alloys with competing ferro‐ and antiferromagnetic interactions.

ISSN:0021-8979    

DOI:10.1063/1.327238

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

This talk reviews recent work on the theory of spin glasses. The Edwards‐Anderson approximation is explained and shown to be invalid even as a mean field theory because it predicts a certain positive definite correlation function to be negative. A method of restoring stability by breaking ’replica symmetry’ due to Bray and Moore is then discussed and shown to imply an infinite number of symmetry breakings in the ordered phase. Estimates of the ’lower critical dimension’ are reviewed. Several independent methods predict a value of four which, if correct, would imply that experiments have to be interpreted in terms of gradual freezing rather than a sharp phase transition. Finally we argue that the replica method gives correct results and previous difficulties arise from choosing an incorrect solution of the equations.

ISSN:0021-8979    

DOI:10.1063/1.327239

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We have proposed a cluster mean field theory of the spin glasses in which dynamical clusters of spins are the fundamental entity. We review experimental evidence for spin clusters and show that within our model it is possible to understand (i) why the specific heat and resistivity have a broad maximum at temperatures above TSGat which the magnetic susceptibility cusp appears and (ii) why the total neutron cross section d&sgr;/d&OHgr;khas a k‐dependent maximum at temperatures below TSG, where k is the momentum transfer to the neutron. These results suggest that there may, in fact, be a sharp transition to the spin glass phase, which turns out to be more apparent in susceptibility than in specific heat, resistivity, and neutron measurements.

ISSN:0021-8979    

DOI:10.1063/1.327240

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We present measurements and a theoretical analysis of the magnetic properties of dilute neutral (In) donors in CdS, which we demonstrate to be a well‐characterized example of an amorphous antiferromagnet with spins S=1/2. High‐quality data for relative values of sample magnetization have been obtained by the Faraday rotation method at temperatures ranging from 55 mK to 2.5 K and fields up to 15 kG. Data are given for crystals with neutral donor concentrations of 8⋅1016/cm3and 2.1⋅1017/cm3. The susceptibility, &khgr;, increases monotonically with decreasing T with no evidence of ordering, even though the median exchange coupling is ∼7 K. A model Hamiltonian is derived by scaling calculations of direct exchange for the H2molecule to the Bohr radius and effective Rydberg of the donor wave functions. A sample of such a Hamiltonian is generated in a computer simulation and solved approximately by dividing the system into clusters (and isolated spins) according to a coupling threshold and solving the cluster Hamiltonians exactly. Inter‐cluster interactions are included in a molecular field approximation. Excellent agreement with experiment is found, and &khgr; is shown to be dominated by clusters, both even and odd, at all temperatures.

ISSN:0021-8979    

DOI:10.1063/1.327241

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The melt‐spun amorphous FecNi80−cP14B6alloys show spin‐glass behavior for c?5, and mictomagnetic behavior for 5<c<∼10. We show that spin‐glass behavior is insensitive to annealing treatments below 300° C, which is well below temperatures that produce crystallization. The mictomagnetic alloys are slightly influenced by such treatments. This result suggests a long‐range nature for the magnetic interactions in the spin‐glass state. Annealing at 350° C results in the appearance of ferromagnetism, without obvious devitrification. The exact nature of this transformation is still not known.

ISSN:0021-8979    

DOI:10.1063/1.327242

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Ultrasonic measurements are presented providing the first direct measurement of the critical exponent &agr;′ for the spin glass CuMn. We find &agr;′=−1.9±0.2 for a sample containing 5 at. &percent; Mn.

ISSN:0021-8979    

DOI:10.1063/1.327243

出版商:AIP    

年代:1979

数据来源: AIP