摘要:

In magnetically disordered materials, disorder is usually obtained by dilution with nonmagnetic ions. We present here a prospective work on single crystals of RbFeCl3−xBrx(x = 0/0.04/0.2/1/2/3), where magnetic disorder is expected to be induced via the ligands by eventually changing at random the superexchange between Fe2+ions. We have measured the optical, Raman, and magnetic circular dichroism (MCD) spectra of our crystals. We focus here on the dependence of absorption versus temperature and ligand, with emphasis on the magnetic properties of the chains. For RbFeCl3and RbFeBr3, the temperature dependence of the absorption follows the general trends predicted by Tanabeetal.; however spin‐orbit coupling is shown to restore significant absorption intensity at TNfor RbFeCl3. Mixed compounds exhibit a structural disorder as demonstrated by our Raman and optical experiments. Moreover, relying on the absorption data, we conclude that for x = 0.04, 0.2, and 1, the intrachain interaction is not essentially modified, whereas for x = 2 we suggest that magnetic disorder does exist. Finally, our MCD results are briefly mentioned, and their significance is discussed in terms of the magnetic properties of the pure compounds.

ISSN:0021-8979    

DOI:10.1063/1.330940

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

We report on magneto‐optical determination of the magnetic phase transition as well as the magnetic structure of crystalline CS3MnCl5. A new type of optical cryostat, with a closed cycle of He3has allowed us to study continuously the variation of the absorption bands down to 0.4 K. The temperature variation of hot and cold bands attributed to the 6A1→4E,4T2transitions proves that antiferromagnetic ordering arises at 0.6 K. Studying the MCD spectrum of the6A1→4T2zero phonon line in low magnetic field, we have observed that the antiferromagnetic phase reverts to paramagnetic state. From this we deduce a value of the weak exchange field (0.9 T) and the energy of the magnon (&ohgr;0= 0.28 cm−1). Using the latter value we show that the origin lines of the 4E and4T2states can be fairly well understood in terms of magnon sidebands. In contrast the &sgr; polarized absorption spectra under an applied magnetic field, perpendicular to the c axis, show no discontinuity. A log‐log plot of the absorbance versus magnetic field gives clear evidence for a power dependence with an exponent close to two, as expected when the spins are all orthogonal to the direction of the field.

ISSN:0021-8979    

DOI:10.1063/1.330941

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Single crystals of Y3−x−zCaxPr4+xPr3+z−xFe5O12have been successfully grown. Their magnetic moments have been measured and interpreted on the basis of Ne´el’s model. Cubic magnetocrystalline anisotropy and growth induced uniaxial anisotropy have been obtained on spherical samples using torque measurements.

ISSN:0021-8979    

DOI:10.1063/1.330942

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

In Ho3Fe5O12, canting of the rare‐earth magnetic moments is evidenced by neutron diffraction. Assuming the garnet to be cubic, analytic expressions of the superstructure lines are given as simple combinations of both components (parallel and perpendicular to [111]) of the Ho3+moments. New parameters of the ’’umbrella structure’’ are given.

ISSN:0021-8979    

DOI:10.1063/1.330943

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The effect of addition of Ti4+under oxidizing atmosphere on the structural and magnetic properties of Ni0.3Zn0.7Fe2O4is studied. The results of x‐ray diffraction, magnetization, and Arrott plot measurements are reported. The data are analyzed on the basis of a three sublattice Yafet‐Kittel model.

ISSN:0021-8979    

DOI:10.1063/1.330944

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

An applied magnetic field is known to produce novel effects in the phase behavior of magnetoelastic spin‐Peierls systems. Hence we report measurements of the differential susceptibility &khgr; and magnetizationMin fields up to 40 kOe (4T) on the spin‐Peierls compound TTF‐AuS4C4(CF3)4in the temperature region 1.1 K⩽T⩽4.2 K. The range of field and temperature encompasses an interesting phase region, including the zero‐field spin‐Peierls transition temperatureTC(0) = 2.03 K. The measurements of the differential ac susceptibility provide a more sensitive probe of the transition behavior than magnetization measurements. The first definitive evidence for significant deviations from mean‐field critical behavior appear in these measurements, and the appropriate criteria for determining the precise location of the transitions are thus provided by the thermodynamic theory of &lgr; transitions. Using the new criteria, qualitative and even quantitative agreement is obtained with current theories of the field dependence of spin‐Peierls transitions. A novel contour plot of &khgr;acin the H‐T plane is shown to be useful for the delineation of the global phase transition behavior. An investigation of the role of relaxation effects in &khgr;acrelative to the nature of the phase boundaries is conducted. A major feature is the observation of a striking degree of ’’universality’’ in the phase behavior of three spin‐Peierls systems, TTF‐AuS4C4(CF3)4, TTF‐CuS4C4(CF3)4, and MEM‐(TCNQ)2. These universal features are preserved through considerable differences in lattice structure and a variation inTC(0) of a factor of 10. A complete report on this work has been submitted to Physical Review B.    a)Supported in part at URI by NSF and at GE by AFOSR.  bOn sabbatical leave from URI, now returned there.

ISSN:0021-8979    

DOI:10.1063/1.330945

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The well‐known Barkhausen effect occurs when a domain wall moving through a crystal gets ’’hung up’’ at imperfections. A numerical model is described here that gives a possible mechanism for this effect. It is assumed that the imperfection is a spherical nonmagnetic inclusion in an otherwise perfect ferromagnetic material. The equilibrium wall shapes, as the applied field is slowly increased, are successively computed. When the wall ’’touches’’ the hole, the induced pole strength decreases and a sudden irreversible change is triggered. The field strength at which this takes place is the coercive force. A new numerical techique is introduced to overcome the ’’stiff’’ equations encountered. A discussion of these techniques will be included in this presentation.    a) Work supported by National Science Foundation grant.

ISSN:0021-8979    

DOI:10.1063/1.330947

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Light scattering is used to determine the average lengths and magnetic moments of magnetotactic bacteria in culture. The results are consistent with estimates made from electron micrographs.

ISSN:0021-8979    

DOI:10.1063/1.330948

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Magnetic desulfurization is an attractive physical method of coal cleaning which has been studied for a quarter century. High‐frequency selective heating of coal to convert nonmagnetic pyrite to moderately magnetic pyrrhotite has been suggested to enhance separation. Despite predicted feasibility, some successes have only recently been attained. We report on the use of Mo¨ssbauer spectroscopy and magnetization to follow indirectly the amounts of inorganic sulfur in coal. We observe significant conversion (≳10%) to pyrrhotite by microwave treatment of coarse coal granules at 915 MHz and 2.45 GHz at power levels of a few kW for about 1 min. Analysis of phase distribution allows independent calculation of the inorganic desulfurization resulting from microwave treatment and magnetic separation.

ISSN:0021-8979    

DOI:10.1063/1.330949

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A sample, after being saturated in a high magnetic field, is in an unstable remanent state in a low or reversed magnetic field. Application of a burst of ultrasound at this point will result in an irreversible decrease in the remanent magnetization. By subsequently changing the magnetic field, and applying a second burst of ultrasound, a reduced irreversible decrease is effected. This observed behavior is a measure of the statistical correlation between the two magnetic states of the sample. We have successfully described this behavior with a modified Preisach model. We have observed irreversible changes in the magnetization of Tb0.27Dy0.73Fe2(&lgr;111= 1.6×10−3) as large as 50% of the remanent magnetization, or 0.4% of the saturation magnetization, induced by ultrasound bursts of 107dyne/cm2at 20 °C. In MnBi (&lgr;s= −8×10−4) crystals in Bi, irreversible effects as large as 1.8% of the remanent magnetization, or 0.6% of the saturation magnetization, have been observed at 120 K.

ISSN:0021-8979    

DOI:10.1063/1.330950

出版商:AIP    

年代:1982

数据来源: AIP