Moments Of Rare-earth Ions Research Articles

Structural, magnetic, magnetocaloric behavior and magneto-transport, electrical polarization study in 3d based bulk and nano-crystalline multiferroic double perovskite Dy2MnCoO6

In this paper, we report a detailed investigation of the crystal structure, magnetic, magnetocaloric, magneto-transport and electrical polarization properties of a new multiferroic material in the polycrystalline and nanocrystalline form of the Dy2MnCoO6 double perovskite. Both compounds crystallized in the monoclinic structure with P21/n space group. The magnetic properties of both systems are mainly dominant ferromagnetic (FM) and weak antiferromagnetic (AFM). The FM/AFM coupling is related by the competing and combining functions of the radius and the magnetic moments of rare earth ions (i.e. 3d–4f exchange interactions). The reduction of the saturation magnetization in the isothermal magnetization curves can be explained by the existence of anti-phase boundaries and local anti-site defects in the system. Moreover, these materials hold reasonable values of magnetocaloric parameters and the absence of hysteresis makes the system a potential candidate for magnetic refrigeration. These compounds revealed two magnetic phase transitions, according to the appearance of two peaks in the temperature dependence of magnetic entropy change curves. The temperature dependent resistivity data for both the systems display semiconductor nature near room temperature and insulating like behavior at low temperature regime. The variable-range hopping conduction mechanism is used to best understand their transport mechanism. In addition, the electrical polarization loop at low temperature confirms the presence of ferroelectricity for both the studied systems. The decreases polarization under an external magnetic field evidence the weak magnetoelectric coupling. The coexistence of FM ordering with insulating behavior and ferroelectricity at low temperature promises new opportunities and improvements in next generation applications for information storage, spintronic, and sensors.

Temperature dependence of magnetic moment of rare earth ions in ErFeO3 and NdFeO3 single crystals.

The interaction between rare earth and iron spins in rare earth orthoferrites leads to remarkable phenomena, such as the spin-flip process. This is despite the rare earth spins not being magnetically ordered. Instead, they are polarized by the ordered iron spins. The interaction between the two spin families is not well understood. This study reports the temperature dependence of the net magnetic moment for rare earth spins, by measuring the overall magnetization for ErFeO3 and NdFeO3 single crystals. The obtained temperature dependence can be described well using a model based on the mean field theory, giving tanh(const./T) temperature dependence. This functional dependence is not disrupted by the spin-flip transition as the crystals are cooled.

Structural and magnetic properties of LaBa1-Sr CuFeO5+δ and YbBa1-Sr CuFeO5+δ (x = 0, 0.25 and 0.5) ceramic systems

We present a systematic study of structural and magnetic properties of LaBa1-xSrxCuFeO5+δ (La-x) and YbBa1-xSrxCuFeO5+δ (Yb-x) (x = 0.0, 0.25 and 0.5) ceramic compounds, synthesized with solid-state method. X-ray powder diffraction (XRD) technique indicates that La-x and Yb-x systems present single phases consistent with an orthorhombic symmetry (space group Immm (71)) and tetragonal symmetries (space group P4mm (99)), respectively, and a decreasing lattice parameter as the content of Sr increases. The results of infrared spectrums show the typical energy bands of a perovskite with a layered structure, and surface morphologic analysis evidenced the formation of polycrystalline material with diverse grain shape. The compositional characterization, obtained via the energy-dispersive X-ray spectroscopy technique, suggests that we achieve the desired material stoichiometry with the absence of impurities. Finally, vibrating sample magnetometry measurements evidenced an antiferromagnetic transition at a temperature around the 160 K only for x = 0, which is attributed to the dominant magnetic moments of rare earth ions.

Negative Magnetization and Exchange Bias Behavior in Nanocrystalline Ho2CoMn1−xNixO6 (x=0‐0.4) Double Perovskite

A gel combustion method is opted to prepare nanocrystalline samples of Ho2CoMn1−xNixO6 (x = 0–0.4) (HCMNO) double perovskite. Structural studies reveal a formation of nanocrystalline samples, which crystallize into monoclinic structure with space group P21/n. A drop in the lattice parameters including unit cell volume are noticed with Ni substitution. Magnetization versus temperature (M-T) analysis shows a single FM transition in the parent compound at 83 K (TC1) and it is ascribed to the super exchange interaction in Co2+ - O2- - Mn4+ networks. However, it is turned into two FM transitions (TC1, TC2) after Ni substitution, where TC2 arises due to possible super exchange interaction in Co2+ - O2- - Ni4+ networks. The transition temperature TC1, is decreasing from 83 to 76 K for x = 0.0–0.4 sample, whereas TC2 increases up to 88 K for x = 0.4 sample. A magnetization reversal (MR) has been observed in higher Ni concentration (x = 0.3 and 0.4) samples in the field cooled state with a compensation temperature (Tcomp) of 4.4 K and 10 K respectively. This MR is attributed to the competition between the ferromagnetic (FM) moment of the transition metal (TM) ions and the polarization of magnetic moments of rare-earth (RE) ions at low temperatures. A typical bipolar switching of magnetization has been demonstrated for x = 0.3 and 0.4 sample. At 5 K, the hysteresis (M-H) loops demonstrate a decrease in saturation magnetization (MS) and coercivity (Hc) value after Ni substitution. However, the observed MS of the order of 11.60–10.27 µB/f.u. (for x = 0.0–0.4) at 5 K is attributed to some visible contribution related to the polarization/ordering of Ho moment with the net magnetic moment of TM ions. A maximum exchange bias field value of 230 Oe has been observed at 5 K for x = 0.4 sample. It is due to the competition between the spin canted magnetic moments of RE ions and the FM moments of TM ions.

Nature of Griffiths phase and ferromagnetic 3d-4f interaction in double-perovskite Dy2CoMnO6

Double perovskites Dy2CoMnO6 (DCMO) was synthesized by a sol-gel method and the structure and magnetic properties were systematically investigated. Both X-ray diffraction and Raman spectrum indicate that double pervoskite DCMO has a monoclinic structure with a space group P21/n. A Griffiths phase is confirmed to be existing in DCMO by the magnetic behaviors, which is significantly different from the behaviors of the non-Griffiths phase of Gd2CoMnO6 (GCMO) recently reported in reference. It is suggested that the origins of the different short-range magnetic orders of them can be attributed to the different 3d-4f exchange interactions in DCMO and GCMO, besides the effect of the unavoidable anti-site disorder, interrupting the long-range ferromagnetic (FM) order of Co2+-O-Mn4+, resulting in the formation of short-range FM order. The effects of A-site rare-earth Re3+ ions on the magnetism of double perovskite Re2Co(Ni)MnO6 are dominated by the combining and competing functions of the radius and the moments of rare earth ions (or the 3d-4f exchange interactions). The FM/AFM coupling is not only related to the most outer 4f electrons of the rare earth ions, but also to the inner 5d/6s orbital electrons.

Magnetic Properties and Exchange Bias Behavior in Nanocrystalline (Ho1-xSmx)2CoMnO6 (x = 0 – 0.5) Double Perovskite

Single phase nanocrystalline samples of double perovskite (Ho1-xSmx)2CoMnO6 (x = 0 – 0.5) (HSCMO) were prepared by gel combustion method and their crystal structure and magnetic properties are studied. These samples crystallize into monoclinic structure with P21/n space group and their lattice parameters increase with Sm concentration. Temperature variation of magnetization measurements show that all the prepared samples exhibit ferromagnetic (FM) transition with transition temperature (TC) increasing from 83 K for x = 0.0 to 115 K for x = 0.5. A secondary rise in magnetization is observed for T < 50 K and is attributed to FM interaction between rare-earth (RE) and transition metal (TM) ions. The FM transition at higher temperature is attributed to strong super-exchange interaction in Co2+-O - Mn4+ networks. The M−H loops recorded at 5 K show a saturation magnetization (MS) in the range of 11.64 µB/f.u. for x = 0.0 to 7.86 µB/f.u. for x = 0.5 and it clearly indicates the contribution of RE ions to the net ferromagnetic moment. All the prepared samples show exchange bias behaviour with exchange bias field up to 237 Oe at 5 K and is attributed to competition between FM moment of TM ions and the spin canted magnetic moment of RE ions.

A comparative study of the magnetic induction heating properties of rare earth (RE = Y, La, Ce, Pr, Nd, Gd and Yb)-substituted magnesium–zinc ferrites

Rare earth (RE)-substituted magnesium–zinc ferrite (Mg0.5Zn0.5RExFe2–xO4) nanoparticles with different RE elements (Y, La, Ce, Pr, Nd, Gd and Yb) and different RE contents (x = 0–0.1) were synthesized via coprecipitation of metal hydroxides as the precursor, followed by calcination. Their crystal structures were characterized by X-ray diffraction (XRD) analysis, confirming that the RE-substituted Mg–Zn ferrites had a single-phase spinel structure at low x values. However, the Ce-substituted ferrites contained CeO2 as a byproduct. Scanning electron microscopy (SEM) showed that the particle diameters of the samples decreased from approximately 100 to 20 nm as x was increased regardless of the RE elements. The magnetic induction heating properties were evaluated using the intrinsic loss power (ILP) determined from the temperature rise profiles in an alternating magnetic field and the amplitude and frequency of the magnetic field. By using various RE elements, it was found that the increase in the magnetic moment of RE ions can increase the magnetization of ferrites, resulting in improvements in the ILP at low RE contents, except for Gd substitutions. The increase in RE content decreased the ILP due to reductions in crystallinity. The results suggest that the RE elements and contents can precisely control the magnetic induction heating properties, and RE-substituted Mg–Zn ferrite nanoparticles are promising candidates for magnetic hyperthermia applications.

Rare-earth doped Ni–Co ferrites synthesized by Pechini method: Cation distribution and high temperature magnetic studies

The present work was focused on the effect of rare earth doping on the structural and magnetic properties of Ni–Co ferrite materials with general chemical formula Ni0.5Co0.5Fe1.98R0.02O4 (R = La, Nd, Sm, Gd, Dy). All the ferrites, synthesized by the Pechini method, possess cubic symmetry with Fd3_m space group. A very small change in lattice parameter has been observed with rare earth (R3+) doping. The cation distribution between the tetrahedral (A-site) and octahedral sites (B-site), as estimated by X-ray diffraction analysis (XRD) revealed that the R3+, Co2+ and Ni2+ ions have migrated to the octahedral site. Room temperature magnetization results, obtained from Vibrating Sample Magnetometer (VSM) measurements, showed that saturation magnetization of rare-earth doped ferrites is strongly dependent on magnetic moment of rare earth ion. The coercivity increases with increase in the atomic number of rare-earth ion, which could be justified on the grounds of the stability of spin-orbital coupling that controls the magnetic anisotropy in ferrites. The results further suggest that the Curie temperature is dependent on lattice parameter and hence inter-atomic distance between magnetic ions.

Crystal-field approach to rare-earth higher borides: Dimerization, thermal, and magnetic properties ofRB50 (R=Tb,Dy,Ho,Er,Tm)

The microscopic justification for the dimerization in ladder-type rare-earth higher borides $R{\mathrm{B}}_{50}$ assumed by Mori [in Handbook on the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner, Jr., J.-C. G. B\unzli, and V. K. Pecharsky (North-Holland, Amsterdam, 2008), Vol. 38, p. 105] is presented. We consider a phenomenological model of crystal-field interactions and introduce Heisenberg antiferromagnetic exchange interactions between the rare-earth ions on the ladder rungs and the indirect exchange fields generated by magnetic interactions along the ladder legs. The crystal-field parameters and the exchange integrals for the rare-earth dimers are determined from modeling the temperature dependences of the heat capacity and static magnetic susceptibility. The field dependence of the low-temperature magnetization of polycrystalline samples of borides $R{\mathrm{B}}_{50}$ is simulated by making use of the proposed empirical expression for the renormalized magnetic moments of rare-earth ions. The characteristic anomalies of the thermal and magnetic properties of $\mathrm{Tb}{\mathrm{B}}_{50}, \mathrm{Dy}{\mathrm{B}}_{50}, \mathrm{Ho}{\mathrm{B}}_{50}$ and $\mathrm{Er}{\mathrm{B}}_{50}$ are successfully reproduced within the framework of this approach.

Calculations of the magnetic entropy change in amorphous through a microscopic anisotropic model: Applications to Dy70Zr30 and DyCo3.4 alloys

We report theoretical investigations on the magnetocaloric effect, described by the magnetic entropy change in rare earth—transition metal amorphous systems. The model includes the local anisotropy on the rare earth ions in Harris-Plischke-Zuckermann assumptions. The transition metals ions are treated in terms of itinerant electron ferromagnetism and the magnetic moment of rare earth ions is coupled to the polarized d-band by a local exchange interaction. The magnetocaloric effect was calculated in DyCo3.4 system, which presents amorphous sperimagnetic configuration. The calculations predict higher refrigerant capacity in the amorphous DyCo3.4 than in DyCo2 crystal, highlighting the importance of amorphous magnetocaloric materials. Our calculation of the magnetocaloric effect in Dy70Zr30, which presents amorphous asperomagnetic configuration, is in good agreement with the experimental result. Furthermore, magnetic entropy changes associated with crystal-amorphous configurations change are estimated.

マルチフェロイックMnWO&lt;sub&gt;4&lt;/sub&gt;と&lt;i&gt;R&lt;/i&gt;MnO&lt;sub&gt;3&lt;/sub&gt;（&lt;i&gt;R&lt;/i&gt;＝Dy or Tb）の構造物性研究

Multiferroic materials in which ferroelectricity is induced by magnetic ordering have been intensively investigated in the past decade. We performed magnetic and crystal structural studies on the typical multiferroic materials, MnWO4 and RMnO3（R＝Dy or Tb）and established that the spin supercurrent or the inverse effect of the Dzyaloshinskii-Moriya interaction is the origin of ferroelectricity in these materials by means of spin polarized neutron scattering and circularly-polarized X-ray diffraction. It is also shown that magnetic moments of rare-earth ions play an important role in incomprehensible electromagnetic effects in RMnO3.

THE SOL–GEL SYNTHESIS OF RARE-EARTH IONS SUBSTITUTED BARIUM HEXAFERRITES AND MAGNETIC PROPERTIES

In this paper, a series of rare-earth-doped barium hexaferrite powders ( Ba 0.95 Re 0.05- Fe 12 O 19 and Ba 0.95 Re 0.05 M 0.05 Fe 11.95 O 19: Re = La , Pr , Sm , Nd , Gd , Dy , Yb ; M = Zn 2+, Mn 2+, [Formula: see text]) were synthesized by the sol–gel self-combustion technology. The phase composition and the magnetic properties of the as-prepared barium hexaferrites were characterized and discussed with X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The results showed that the barium hexaferrites exhibited the magnetoplumbite phase structure with the average diameter of 45 nm. Magnetic properties study revealed that the variation of the saturation magnetization (Ms) was similar with the change of the rare-earth ions radius, but the change of Ms was low. This indicated that the magnetic moments of rare-earth ions could not affect Ms. The magnetocrystalline anisotropy field mainly influenced the anisotropism of hexaferrites, and the coercivity (Hc) of the rare-earth ions doped barium hexaferrites basically decreased with the increasing orbital quantum numbers (except Sm 3+ and Gd 3+). Further study showed the co-addition of Zn 2+ and Mn 2+ did not change the trend of Ms and Hc. Thus, it is concluded that the rare-earth ions played an important role for the anisotropy field of barium hexaferrites.

Controlled growth, intense upconversion emissions and concentration induced luminescence switching of bifunctional Tm3+ doped hexagonal NaYb0.55Gd0.45F4 nanorods

Bifunctional hexagonal Tm3+ doped NaYb0.55Gd0.45F4 nanorods with tunable size are prepared via in situ cation-exchange reaction using hydrothermal method. The measured field dependence of magnetization of the NaYb0.55Gd0.45F4 nanorods shows typical paramagnetic characteristics that can be ascribed to the non-interacting localized nature of the magnetic moment of rare-earth ions. When excited by a 980nm laser, these nanorods exhibit intense multi-color up-conversion (UC) emissions in infrared, red, blue and especially ultraviolet. In addition, luminescent switching between different UC emission wavelengths of 480nm and 450nm is observed by adjusting Tm3+ doping concentration. Based on power-dependent spectral analyses, it is found that with the increase of Tm3+ doping concentration, due to the suppressed saturation effect, the dominative UC process redistribute the populations at 1G4 and 1D2(Tm3+) states of Tm3+ ion resulting in the above luminescent switching. Our results indicate that bifunctional hexagonal NaYb1−xGdxF4 nanocrystals have potential applications in miniaturized solid-state light sources, optical processing sensors and fluorescent biolabels.

ChemInform Abstract: Magnetic and Magnetoelectric Properties of Rare Earth Molybdates

AbstractReview: 68 refs.

Magnetic and Magnetoelectric Properties of Rare Earth Molybdates

We present results on ferroelectric, magnetic, magneto-optical properties and magnetoelectric effect of rare earth molybdates (gadolinium molybdate, GMO, and terbium molybdate, TMO, and samarium molybdate, SMO), belonging to a new type of ferroelectrics predicted by Levanyuk and Sannikov. While cooling the tetragonal β-phase becomes unstable with respect to two degenerate modes of lattice vibrations. The β-β′ transition is induced by this instability. The spontaneous polarization appears as a by-product of the lattice transformation. The electric order in TMO is of antiferroelectric type. Ferroelectric and ferroelastic GMO and TMO at room temperature are paramagnets. At low temperatures GMO and TMO are antiferromagnetic with the Neel temperatures TN=0.3 K (GMO) and TN=0.45 K (TMO). TMO shows the spontaneous destruction at 40 kOe magnetic field. Temperature and field dependences of the magnetization in TMO are well described by the magnetism theory of singlets at 4.2 K ≤ T ≤ 30 K. The magnetoelectric effect in SMO, GMO and TMO, the anisotropy of magnetoelectric effect in TMO at T = (1.8–4.2) K, the Zeeman effect in TMO, the inversion of the electric polarization induced by the laser beam are discussed. The correlation between the magnetic moment of rare earth ion and the magnetoelectric effect value is predicted. The giant fluctuations of the acoustic resonance peak intensity near the Curie point are observed.

Critical Phenomena in Long-Range RKKY Ising Spin Glasses

We have investigated critical phenomena in spin glasses RxY1−xRu2Si2 (R = Dy, Tb, Gd). These compounds, where the magnetic moments of rare-earth ions interact by the long-range Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction via conduction electrons, has uniaxial magnetic anisotropy. The separation of the zero-field-cooled and field-cooled magnetization was found only along the c-axis in all compounds, and hence, they are classified into the long-range Ising spin glass. The magnetic anisotropic energies in these compounds are different from each other in two orders of magnitude, from 330 K to 1.8 K, however, the critical exponents are similar. It clearly indicates a presence of the universality of the long-range RKKY Ising spin glasses.

Magnetoresistance and magnetic ordering in praseodymium and neodymium hexaborides

The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB6 and NdB6) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB6 and NdB6 compounds. An analysis of the dependences Δρ(H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H2), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB6 and NdB6 compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB6 and NdB6, ferromagnetic nanoregions (ferrons) are formed in the 5d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χloc has been estimated. It has been demonstrated that, in the temperature range TN < T < 20 K, the behavior of the local magnetic susceptibility χloc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χloc ∝ (T − Θp)−1.

Magnetostriction in the rare-earth ferroborates RFe3(BO3)4, R=Pr and Tb

Recent experimental data for magnetostriction in the rare-earth (RE) ferroborates RFe 3 (BO 3 ) 4 with R=Pr and Tb are discussed from a theoretical point of view. Multipole moments of RE ions are calculated in the framework of a crystal-field model for the RE ion and the molecular-field approximation. Quadrupole approximation is shown to be sufficient for interpretation of data for longitudinal magnetostriction at the magnetic field along the trigonal axis. Parameters of PrFe 3 (BO 3 ) 4 are deduced when accounting for the experimental magnetization curves that manifest a spin–flop transition.

Magnetic properties of R 3Cu 4X 4 (R = Tb–Er; X = Si, Ge, Sn) compounds

dc magnetization and magnetic susceptibility measurements were performed on polycrystalline samples of R 3Cu 4X 4 (R = Tb–Er; T = Si, Ge, Sn) intermetallics which crystallize in the orthorhombic crystal structure of the Gd 3Cu 4Ge 4-type. All of the studied compounds are antiferromagnets showing complex behavior. For some of them additional transitions below Néel temperature are observed and few indicate metamagnetic transition at 2 K. Reciprocal magnetic susceptibilities of all the samples obey the Curie–Weiss law above the Néel temperatures with negative values of paramagnetic Curie temperatures and the effective magnetic moments of rare earth ions being close to the respective free R 3+ ion values.

Colossal magnetoresistance in perovskite manganite induced by localized moment of rare earth ion

The magnetic and transport properties of a single crystal of Pr 0.7Ca 0.3MnO 3 annealed by various magnetic fields were investigated by dc and ac magnetization measurements. The ordered Pr moment is almost absent in zero-field magnetization but is clearly observed when the sample was measured after annealing with a magnetic field. The Pr moment is enhanced when the annealing field is increased. Remarkably, the ordered Pr spins appear to induce colossal magnetoresistance (∼10 10%) as well as a metal–insulator (M–I) transition in Pr 0.7Ca 0.3MnO 3. This unique behavior is discussed in terms of the reorientation of Pr spins that are strongly coupled with the neighboring Mn spins.