Search

Pressure dependence of exchange interactions among small Mn clusters scattered throughout the network of sp3 covalent bonds in a diluted magnetic semiconductor Cd1-xMnxSe has been studied by a cryobaric measurement of the exciton magnetophotoluminescence. The pressure is generated up to 2 GPa with a diamond anvil cell, being subjected at low temperatures to the static magnetic field up to 23 T generated with a hybrid magnet. The observed specific spin temperature T0 of the clusters gives the effective internal exchange constant J*/k≡J2/k +(10/3)J3/k +2J4/k to be –2.0±0.4 K at 1 atm, where Jn denotes nth-neighbor exchange constant. The nearest-neighbor interaction constant is obtained to be J1/k = -7.4±0.4 K at 1 atm from an analysis of the effect of the stepwise magnetization of Mn pairs. J*, as well as J1, increases rapidly with increasing pressure. The pressure coefficient dln|J*|/dP = 0.2-0.4 GPa-1 agrees with dln|J1|/dP = 0.25±0.05 GPa-1 within experimental errors. This result supports Larson's covalent spin interaction picture that the exchange interactions between the scattered, localized spins are determined by kinetic exchanges mediated by the extended p orbitals making the valence band of the host II-VI semiconductor.

Rare earth intermetallic compounds RX and RX_2 ( R=Gd, Tb, Dy, Ho and Er : X=Ag and Au) are antiferromagnetic compounds with the CsCl-type crystal structure and the MoSi_2-type one, respectively. Magnetization process is investigated for these compounds under static magnetic fields up to 270 kOe and pulsed ones up to 300 kOe. The observed field induced transitions are reviewed together with their magnetic phase diagrams.

We have made high-field magnetization measurements for some Mn alloys : ε-Mn_Ga with δ= -0.30, ε-(Mn_Fe_x)_3Sn with x=0.1 and 0.3, Fe_3Mn_4Ge_ and γ-(Mn_Fe_x)_Ga_y with x=0.44 and y=0.27. The ε-Mn_Ga has the hexagonal D0_ type structure and a distorted triangular antiferromagnetic spin structure. The hexagonal lattice is slightly distorted to an orthorhombic lattice at T_d of about 290 K with large magnetization. Pseudo-binary ε-(Mn_Fe_x)_3Sn has also the D0_ type structure, and the alloys with x has an hp13-type structure, and shows an antiferromagnetism below 470 K. The isothermal magnetization at 77 K shows a steep increase around 110 kOe, suggesting the metamagnetic transition. The γ-(Mn_Fe_x)_Ga_y is face centered cubic. The alloys with x=0.4～0.5 and y=0.27 show coexistence of an antiferromagnetic state and a ferromagnetic state below Neel temperature (T_N～300K), and a typical ferromagnetic behavior above T_N ( Curie temperature T_C>550 K). The magnetization of the coexisting state at 77 K in a field of 150 kOe was comparable to that of the ferromagnetic state at 300 K.

A study was made to investigate body function in high magnetic field of 8-tesla. In order to avoid magnetic-field effect on the measuring system, long distance was placed between a experimental animal and the measuring system. Changes in contractility of the heart were measured from frogs. Heart and respiratory rates were recorded from rabbits. Contractility of the heart was increased with increasing magnetic fields from 0 to 8 tesla in frogs. Heart and respiratory rates were decreased with increasing magnetic fields from 0 to 8 tesla.

General method of calculating magnetization curves in ternary system R-Fe-X is given, where R is a rare-earth element and X a non-magnetic element such as B. In this method the ferromagnetism of Fe sublattice is treated phenomenologically, while exchange and crystal field acting on R ions are exactly taken into account. Experimental data on R_2Fe_B and R_2Fe_N_x systems are summarized in comparison with the calculated curves. Detailed analyses have shown that in the light R compounds the inter-multiplet mixing via the R-Fe exchange interaction is important especially for the first-order magnetization process, which often occurs at low temperatures with external fields along the hard direction.

Magnetoresistance and Hall effect have been measured at 4.2 K using a hybrid magnet up to 26 T for several crystals of quasi-two-dimensional η-Mo_4O_ that has a charge-density-wave (CDW) induced nested multiple carrier band structure. The magnetoresistance is exceptionally large and the Hall resistivity shows a unique magnetic field dependence. We have confirmed the existence of characteristic hysteresis phenomena in both quantities when a field-sweep range exceeds a threshold value (?10 T). The hysteresis effects can be related with an effective and magnetic field dependent irreversible process for the CDW formation and destruction. We have also found that the hysteresis and quantum oscillations in the magnetotransport are strongly sample-dependent.

Absorption and luminescence spectra of Cd_Mn_xSe (x=0.10 and 0.38) and reflectance spectra of Cd_Co_xSe (x=0.012) were investigated in high magnetic fields up to 25 T by a hybrid magnet and a magnetospectrometer with an optical fiber system.

Recent developments in the use of high magnetic fields to investigate the magnetic properties of diluted magnetic semiconductors are reviewed here. The need for high fields to obtain reliable values of the exchange interaction are described. For a summary of previous work we refer to a presentation in the Proceedings of the Honda International Symposium on the Application of High Magnetic Fields to Materials Science. The most straight-forward approach for extracting the exchange interaction between pairs of magnetic ions is based on the observation of steps in the magnetization vs. field data. These steps result from the magnetic-field splitting of the angular momentum states and the level crossings that occur as the magnetic field is increased. It has been shown, however, that, even in the absence of observable steps, it is possible to estimate the exchange interactions from a fit to an expression containing terms representing both the magnetization of individual ions and that of pairs, if the magnetic field is high enough. A model, based on this approach, has been developed. In the present review this model will be summarized and the results applied to magnetization data taken on II-VI and IV-VI diluted magnetic semiconductors.

The Kondo-lattice compound CeNiSn behaves as a narrow-gap semiconductor at low temperatures below 7 K. The effect of magnetic field on the gapped state of this compound has been studied by the measurements of magnetization, magnetoresistance and specific heat on a single-crystalline sample. A weak metamagnetic transition and very large negative magnetoresiostance are observed only when a magnetic field higher than 13 T is applied along the α axis of the orthorhombic structure. These results show that the pseudogap is suppressed anisotropically by the magnetic field. The collapse of the gap is accompanied with the evolution of a polarized heavy-fermion state.