Y. Shapira

Spin-flip Raman scattering and magnetization measurements on (Cd, Mn)S

Abstract Spin-flip Raman scattering and magnetization measurements are reported for the semimagnetic semiconductor Cd1-xMnxS, x = 0.023. The data at 1.9 K and high fields (near saturation) give the exchange energy αNo = 217 ±11 meV. At low fields, the spin-flip energy for donor-bound electrons contains the predicted contribution from the excess local magnetization near the donors. Magnetoresistance and Hall data are also reported.

Magnetic phase diagram of MnAs: Effect of magnetic field on structural and magnetic transitions

Abstract The boundary, in the T − H 0 plane, separating the MnP-type and NiAs-type crystallographic phases of MnAs was determined from magnetization and volume measurements. Transitions on one part of the boundary are of first order, but are of second order on the remaining part. The two parts are separated by a tricitical point.

Exchange energy for conduction electrons in (ZnMn)Se derived from spin-flip Raman scattering and magnetization

Abstract Spin-flip Raman scattering, magnetization, and susceptibility data for Zn0.97Mn0.03Se are reported. The exchange energy Noα = 243 ± 10 meV for the conduction electrons is obtained from an analysis of the Raman and magnetization data. At large magnetic fields (H > 60 kOe), the spin-flip energy ΔE saturates at 14 meV. At low fields ΔE does not extrapolate to zero as H → 0, which is characteristic of scattering from donor-bound electrons. The low temperature magnetization curves are fit to a modified Brillouin function. The fit gives x /x = 0.67 as the fraction of active magnetic ions, and an effective temperature Teff = T + To with To = 1.1 K. The magnetic susceptibility follows a Curie-Weiss law between T = 150 and 280 K with a Curie-Weiss temperature θ = −33 K.

Observation of Antiferromagnetic Phase Transitions by Ultrasonic Techniques

The usefulness of ultrasonic techniques in determining the phase boundaries of antiferromagnets (in the H‐T plane) is discussed. The various magnetic phase transitions which occur in antiferromagnets are described with particular emphasis on transitions in uniaxial antiferromagnets of the easy‐axis type. The phase boundaries between the antiferromagnetic, spin‐flop, and paramagnetic phases are discussed in the molecular field approximation. Some results of more sophisticated treatments are also included. The main ultrasonic phenomena which occur near the various phase transitions of uniaxial antiferromagnets are described. They are (a) λ anomalies in the attenuation and velocity near the second‐order transitions, (b) an attenuation spike (velocity dip) at the spin‐flop transition, and (c) an abrupt attenuation increase (velocity decrease) at the spin‐flop transition. Examples of phase boundaries determined ultrasonically in MnF2, FeF2, CoF2, Cr2O3, α‐Fe2O3, Cr, and EuTe are summarized.

Effects of random fields on the phase diagram of Mn0.957 Zn0.043F2

The phase diagram of Mn0.957Zn0.043F2 was measured in magnetic fields H↘ up to 175 kOe, directed along the easy axis. The para‐antiferro transition, at T∥C(H), shows an increased roundling at finite H. At low H, the slope −dT∥Cd(H2) is considerably larger than predicted by mean field theory. The paraspinflop boundary exhibits a much larger ’’bulge’’ than in pure MnF2. All these features are attributed to random fields. Several discrepancies from early predictions for the effects of random fields are noted, and are attributed to the absence of a genuine order‐disorder transition on the line T∥C (except at H = 0).

Ultrasonic Propagation in EuO

The attenuation of 10 to 90‐MHz longitudinal and shear ultrasonic waves propagating along the [100] and [110] directions of EuO single crystals was measured between 63° and 78°K. At zero applied magnetic field, the attenuation was constant above the Curie temperature, TC=70°K, but increased as the temperature was lowered through TC. The increase in attenuation was much greater for the shear modes than for the longitudinal modes. Preliminary experiments at 4.2°≤T≤78°K with 30‐MHz longitudinal and 10‐MHz shear waves propagating along the [100] direction showed that the excess attenuation at T<TC passes through a maximum near 60°K and is small at 4 2°K. An external magnetic field H≳5 kG removed the excess attenuation at T<TC. No attenuation peak was found at TC. The excess attenuation at T<TC is tentatively interpreted as being due to ferromagnetic domains. The elastic constants at 78°K are (in 1011 cgs units) c11 = 19.2±0.6, c44 = 5.42±0.13, c12 = 4.25±0.85. These lead to an adiabatic bulk modulus B = (9.2±...

Magnetization and spin-flip energy in Cd0.9Mn0.1Se

Magnetization and spin-flip Raman measurements are reported for Cd1−xMnxSe, x = 0.106, at 1.9 < T < 4.2 K and magnetic fields H up to 80 kOe. The high-field results are combined to determine the exchange energy between donor electrons and Mn++ spins, αN0=261±13 meV. Empirical fits to the magnetization data are described.

The evidence for a Lifshitz point in MnP (invited)

MnP has a triple point where paramagnetic, ferromagnetic, and helicoidal (fan) phases meet. The following features of this point agree with those expected for a Lifshitz point: the order of the various transitions near this point (first or second); the qualitative shapes of the phase boundaries; the crossover exponent φ obtained from the phase boundaries; the variation of the wave vector q↘ in the fan phase; the exponent βk derived from this variation; and dispersion curves for spin waves.

Neutron scattering evidence on Lifshitz behavior in MnP

We have measured the variation of q↘ in the fan phase of MnP in order to test whether the para‐ferro‐fan triple point is a Lifshitz point. Along the para‐fan phase boundary, q↘ continuously decreases as the triple point is approached, extrapolating to zero at a temperature in good agreement with other measurements of the triple point. The temperature dependence of q↘ along this phase boundary is in approximate agreement with theoretical expectations for Lifshitz point behavior. These data support the conclusion that the triple point is a Lifshitz point.

Magnetoresistance near the metal-insulator transition of Cd0.99Mn0.01Se

Abstract At 0.5 ≤ T ≤ 4.2 K samples of Cd 0.99 Mn 0.01 Se which are near the metal-insulator transition exhibit a large positive magnetoresistance (MR) in fields H ∽ 10 k0e. The effect is accompanied by an increase in the Hall coefficient and a decrease in the Hall mobility. At the lowest temperatures and for fields well above 10 k0e a small negative MR is observed. The positive MR is tentatively attributed to an increase in the screening radius r s , which leads to enhanced Coulomb-potential fluctuations.