Kang Ryong Choi

Magnetic Properties and Crystalline Transition for the NiCr 1.7 Fe 0.3 O

We have studied the temperature dependent magnetic properties and crystalline phase transitionn in small amount Fe doped nickel chromite. The Crystalline structure of NiCr 1.7 Fe 0.3 O₄ is spinel cubic (Fd-3m) structure with a lattice constant ^a0 = 8.317 A at room temperature. The magnetic Neel temperature (T N ) of the Fe doped nickel chromite sample is determined to be 250 K. The Mossbauer spectra exhibit that there are two magnetic phases with the two different sites for the Cr 3+ ions. The spectrum at 4.2 K is fitted to two magnetic components of the magnetic hyperfine fields Hhf = 496 and 485 kOe. From the spectrum at 295 K, the electric quadrupole splittings are observed with large values of 0.49 and 0.50 ㎜/s, respectively. The values of the isomer shifts at all temperature ranges show that the Fe ions are ferric states. We are suggested that the dynamic Jahn-Teller distortion and anisotropic magnetic relaxation effects due to the crystalline phase transition.

Stress Effects CoCr₂O₄ Film on MgO and MgAl₂O₄ Grown by RF-Sputter Process

Multiferroic CoCr2O4 film was deposited on MgO and MgAl₂O₄ substrates by the rf-sputtering process. The films were prepared at an RF-magnetron sputtering power of 50 W and a pressure of 10 mtorr (20 sccm in Ar), and at substrate temperatures of 550 ℃. The crystal structure was determined to be a spinel (Fd-3m) structure by means of X-ray diffraction (XRD) with Cu K￥a radiation. The thickness and morphology of the films were measured by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The magnetic properties were measured using a Superconducting Quantum Interference Device (SQIUD) magnetometer. While the ferrimagnetic transitions were observed at about 93 K, which was determined as the Neel temperature, the magnetic properties all show different behaviors. The differences between the magnetic properties can be explained by the stress effects between CoCr₂O₄ and the substrates of MgO and MgAl₂O₄.

Mössbauer studies of multiferroic spinel CoCr1.98Fe0.0257O4

In order to elucidate the role of Cr ions in CoCr2O4 exhibiting multiferroic property, we have substituted a small amount of Fe ions for Cr sites and investigated the magnetic behavior of Fe ions on atomic scale, using Mossbauer measurement. Polycrystalline CoCr1.98Fe0.0257O4 compound was prepared by wet-chemical process. The crystal structure was found to be a single-phase cubic spinel with space group of Fd-3m. The lattice constant (a0) and the internal structural parameter (x) of the oxygen were determined to be 8.340 and 0.264A, respectively. Mossbauer absorption spectra at 4.2K show that the well developed two sextets are superposed with small difference in hyperfine field. Isomer shift values (δ) of the two sextets are found to be 0.34 and 0.35mm∕s relative to the Fe metal, which are consistent with the high spin Fe3+ charge state. With increasing temperature, the sextets gradually split into two subspectra, and then around 28K the absorption line broadening of outer sextet appears rapidly. Above th...

Magnetic properties of Nd-Ga-Febal-Nb-B alloy

Here, we have synthesized Nd-Ga-Febal-Nb-B alloy by strip casting method. The crystalline and magnetic properties of sample were investigated with x-ray diffractometer (XRD), vibrating sample magnetometer (VSM), and Mossbauer spectrometer. The XRD pattern was analyzed with the Rietveld refinement method, indicating a tetragonal structure and the space group of P42/mnm. The temperature dependence of zero-field cooled (ZFC) magnetization curve was measured under applied field at temperature ranging from 4.2 to 740 K. From the ZFC curve, Curie temperature and spin reorientation temperature are determined to be 615 K and 130 K, respectively. Also, Mossbauer spectra were measured at various temperatures ranging from 4.2 to 620 K. Each spectrum was fitted with 6-sextets for Fe site (8j1, 8j2, 16k1, 16k2, 4c, and 4e), and magnetic hyperfine field, Isomer shift, electric quadrupole shift, and area ratio values were obtained from the fit. We observed the change in slope of magnetic hyperfine field and electric qua...

Bond frustration effect of Cr ions in magnetiochromite by Mössbauer spectroscopy

Polycrystalline MgCr1.98Fe0.0257O4 compound was synthesized by sol-gel process. The crystal structure was found to be single-phase cubic spinel with space group of Fd3(−)m. The lattice constant a0 and the fractional coordinate (x) of the oxygen were determined to be 8.336A and 0.260, respectively. The Cr–Cr linkages in ACr1.98Fe0.0257O4 (A=Mg,Zn) have bond lengths of 2.945 and 2.947A, respectively. We have observed larger value of Neel temperature (TN=12–12.5K) in Mg, Zn chromite spinels than those of Cd, Hg-chromite spinel (TN=6–8K). Mossbauer spectra of MgCr1.98Fe0.0257O4 were taken from 4.2to295K using a Co57 source in a rhodium matrix. MgCr1.98Fe0.0257O4 Mossbauer spectra below TN show the line broadening due to bond frustration. Above the Neel temperature, paramagnetic doublet is observed. The magnetic properties and Mossbauer results can be explained by the B–B exchange interaction and bond frustration in MgCr1.98Fe0.0257O4.

Characterization of

CoCr₂O₄(CCO) materials show multiferroic effect that ferroelectricity and ferromagnetism co-exist. CCO film was deposited on Pt/Ti/Si/SiO₂ substrates by pulsed laser deposition (PLD). The CCO film were prepared using KrF(248 nm) excimer lasers and with a pressure of 100 mTorr, substrate temperatures of 700^degC . The crystal structure was found to be oriented {111} planes by means of X-ray diffraction (XRD) with Cu radiation. The morphology of film has showed triangle shapes. The ferrimagnetic transition was observed at around 95 K, which was determined as Neel temperature and spiral magnetic transition temperature (<i>T</i> _S) was 21.5 K, while the <i>T</i> _S of bulk CCO was 28.0 K. We note that decreasing of <i>T</i> _S in CCO films is closely related to the preferred orientation of {111} direction.

\hbox{CoCr}_{2}\hbox{O}_{4}

Polycrystalline MnCr1.9857Fe0.02O4 compound was prepared by a wet-chemical process. The crystal structure was found to be a cubic spinel with a space group of Fd3m. The lattice constant a0 and the internal structural parameter x of the oxygen were determined to be 8.444A and 0.263, respectively. Mossbauer spectra of MnCr1.9857Fe0.02O4 were taken from 4.2to295K. The absorption spectra slow well-developed two sextets, superposed with small difference in hyperfine fields of 482 and 472kOe below the Neel temperature of 50K, and a paramagnetic doublet above the Neel temperature. We have observed a sudden change in sextet around 22K, which corresponds to the spin structure transition temperature. Even though the values of the magnetic hyperfine fields of MnCr1.9857Fe0.02O4 on B sites are close to those of CoCr1.9857Fe0.02O4, there is a noticeable difference in the Neel temperature between them, which is due to the A-B superexchange interaction with spin-orbit coupling attributable to ionic bond lengths.

on Pt(111) Grown by Using Pulsed Laser Deposition

In this paper, we elucidate the role of the manganese ion in the colossal magnetoresistance(CMR) effects by replacing the manganese ion in the CMR material with a similar transition metal ion Fe. The similar ionic radii of Fe/sup 3+/ and Mn/sup 3+/ mean that lattice distortion effects of the substitution may be ignored, and the electronic structure can be studied. Polycrystalline samples of La/sub 0.67/Ba/sub 0.33/Mn/sub 0.99//sup 57/Fe/sub 0.01/O/sub 3/ have been prepared with the aim of investigating the influence of the presence of a metal, which favors an antiferromagnetic coupling in the Mn-O layer.