Showing papers on "Metamagnetism published in 1993"

Metamagnetic transition and susceptibility maximum in an itinerant-electron system.

Abstract: An itinerant-electron metamagnetism is discussed at finite temperature, by taking into account the effect of spin fluctuations on the Landau-Ginzburg theory. It is shown that the paramagnetic susceptibility always shows a maximum in its temperature dependence when the metamagnetic transition from the paramagnetic to the ferromagnetic state is induced by the external magnetic field at low temperature. This metamagnetic transition, associated with a hysteresis in the magnetization curve, is shown to disappear at high temperature. Moreover, the first-order transition in the temperature dependence of the spontaneous magnetization is shown to occur under a certain condition among the Landau coefficients. Three characteristic temperatures, at which the susceptibility reaches a maximum, the field-induced metamagnetic transition disappears, and the temperature-induced first-order transition of the magnetization occurs, are discussed. The present theory can explain qualitatively these anomalous magnetic properties observed in Co compounds Co(S,Se${)}_{2}$, ${\mathrm{YCo}}_{2}$, ${\mathrm{LuCo}}_{2}$, and others.

Unusual magnetic properties of the heavy-fermion compound CeCoGe3.

Abstract: Heat capacity, magnetization, and electrical resistivity measurements were carried out on ${\mathrm{CeCoGe}}_{3}$ and ${\mathrm{LaCoGe}}_{3}$. ${\mathrm{CeCoGe}}_{3}$ was found to be a heavy-fermion system with \ensuremath{\gamma}=111 mJ/mole Ce ${\mathrm{K}}^{2}$. Two magnetic transitions were found at \ensuremath{\sim}21 and \ensuremath{\sim}18 K in the absence of a magnetic field. The upper ordering temperature is believed to be a transition from the paramagnetic state to a c-axis ferrimagnetic (FERRI) state, which in turn transforms a colinear antiferromagnetic (AF) state at \ensuremath{\sim}18 K. The AF phase undergoes an irreversible phase transition to the FERRI state under an applied magnetic field. Consequently, magnetic glasslike behaviors such as thermal and magnetic hysteresis, frozen moment, and magnetic relaxation were observed. A nonhysteretic metamagnetic transition to a c-axis ferromagnet (FERRO) was observed at high magnetic fields. The critical spin-flip field, ${\mathit{H}}_{\mathit{m}}$, increases in the FERRI state, and decreases in the AF state as temperature decreases. The magnetic data also suggest that the Ce moments in the ab plane are antiferromagnetically aligned in all three magnetic phases. The existence of both ferromagnetism and antiferromagnetism implies that there may be close competition of ferromagnetic and antiferromagnetic exchange interactions between Ce moments leading to possible spin frustration.

Spin frustration and metamagnetic behavior in a molecular‐based quasi‐1D ferrimagnetic chain: (MnTPP)(TCNE)

Abstract: We report an experimental observation of spin frustration and metamagnetic behavior in a quasi‐1D ferrimagnetic chain, (MnTPP)+(TCNE)−. Metastability, hysteresis effects, and irreversibility of thermal and magnetic histories are observed in magnetization measurements. The ‘‘memory’’ phenomenon is seen in the temperature dependence of the thermoremanent and isothermal remanent magnetization. This system may be relevant to the magnetic behavior of the quantum/classical spin chains. The isothermal M(H) at T=2.25 K supports a first order transition which may be related to effects of local anisotropy.

dHvA Effect Study of Metamagnetic Transition in CeRu2Si2

Abstract: The de Haas-van Alphen (dHvA) effect measurements have been performed for both the field ranges above and below the metamagnetic transition field ( H m ) in the (010) plane in CeRu 2 Si 2 . The experimental frequencies and effective masses below H m agree well with the predictions of the first principle or the renormalized band structure calculations based on the itinerant 4f electron model. On the other hand, those above H m can be explained well by the localized 4f electron model. The one to one correspondence between the dHvA oscillations below and above H m can be made. The frequencies are found to change abruptly around H m . The effective masses decrease considerably around H m and continue to decrease with increasing field. These observations indicate that the 4f electron nature changes from itinerant to localized around H m .

Anisotropic effects in the antiferromagnetic Kondo compound CePtSn

Abstract: The magnetic, transport and thermal properties are reported on a single crystal of CePtSn which is isostructural with the Kondo semiconductor CeNiSn. The Ce ions in CePtSn are trivalent with 4f states governed by a strong crystal field causing an overall splitting of about 500 K. The Kondo effect is manifest by the double-peak structure of the magnetic part of the resistivity ϱm(T) possessing maxima at 10 and 140 K. Below the antiferromagnetic transition at 7.5 K, another transition appears in both the specific heat C(T) and ϱm(T) at 5.0 K. A metamagnetic transition occurs in magnetic fields around 9 and 11 T applied along the a and b axes, respectively. A magnetic phase diagram is presented based on the combined results of the field dependence of C(T) and the temperature dependence of the metamagnetic transition. These results are discussed in connection with the problem of the gap formation in CeNiSn.

Magnetism in RENiAl compounds

Abstract: Results of magnetic, resistivity and specific heat measurements of RENiAl compounds are presented. Except for CeNiAl, all the compounds with the unfilled f‐shell rare earths exhibit magnetic ordering at low temperatures. In the majority of cases, we observe one or two additional magnetic phase transitions below the ordering temperature. In RENiAl antiferromagnets, metamagnetic transitions are observed in moderate magnetic fields. Magnetization measurements performed on oriented powders reveal a strong magnetocrystalline anisotropy (except for GdNiAl and CeNiAl) which originates in crystal field interaction. The energy spectra of the RE 4f states were obtained from the analysis of the Schottky anomalies found in the specific heat data. This analysis also revealed indications of strong magnetic correlations in the paramagnetic state.

Itinerant electron metamagnetism and a large decrease in the electronic specific heat coefficient of Laves-phase compounds Lu(Co1-xGax)2

Abstract: High-field magnetization and low-temperature specific heat of the nearly ferromagnetic compounds Lu(Co1-xGax)2 have been investigated. A sharp metamagnetic transition has been observed in these compounds. The critical field Hc decreases with increasing x and becomes lower than 1 T at x=0.12. However, no spontaneous magnetization is confirmed even above x=0.15. With increasing x, the electronic specific heat coefficient gamma increases and reaches 36 mJ K-2 mol-1 for x=0.12 in 0 T, being 1.5 times as large as that of LuCo2. Such a mass enhancement is discussed in terms of the spin fluctuation. The gamma value of the ferromagnetic state for the specimen with x=0.12 in a magnetic field of 14.6 T is reduced about 50% compared with that of the paramagnetic state in zero field. This means that the metamagnetic transition results in a drastic reduction of the spin fluctuation in the compound.

Chapter 105 Valence-fluctuation and heavy-fermion 4f systems

Abstract: Summary This chapter deals with valence-fluctuation and heavy-fermion 4f systems, such as Ce, Sm, Eu, Tm and Yb alloys and compounds. Sections 1 and 2 summarize the most important physical properties of these systems, based on the main theoretical results achieved so far and on representative data. In the main part (section 3) we summarize neutron scattering data of VF and HF systems. By means of neutrons one can measure the magnetic structure of a system (elastic magnetic scattering) and magnetic and non-magnetic excitations such as relaxational modes (quasi-elastic excitations), magnons, crystal field, and spin-orbit transitions, and phonons. Some of these excitations have been observed in all VF or HF systems. Both VF and HF systems exhibit a quasi-elastic line due to spin fluctuations or the Kondo effect. Both expressions are used synonymously, but in VF systems the quasi-elastic line is quite broad, corresponding to a high characteristic energy, and is fairly temperature independent. Here one speaks in general about spin fluctuations. In HF systems the quasi-elastic line is narrower, temperature dependent, and has, at low temperatures, an halfwidth 1/2 Γ ⋍ k B T N K . A typical quasi-elastic line can be fitted by a Lorentzian and corresponds to a single relaxation time. In most HF compounds the line becomes Q -dependent at low temperatures, indicating spin correlations that at still lower temperatures often lead to magnetic order. These spin correlations, on the other hand, lead already to deviations from the Lorentz shape above the spin-ordering temperature. In VF compounds spin correlations do not play a role, but the high-temperature quasi-elastic line also changes shape and becomes narrower and inelastic at low temperatures. Ce-based HF systems have been investigated by many groups, whereas few data exist for the corresponding VF systems and for Yb-, Sm-, Eu-, and Tm-based VF and HF systems. As a consequence, our survey over Ce-based VF and Yb, Sm, Eu, and Tm systems is rather complete, whereas for Ce-based HF systems we could only select representative examples. Part of this strong research activity on Ce compounds is due to the fact that CeCu 2 Si 2 becomes superconducting, and it is an enormous challenge to find other superconducting HF systems. In addition, one has in these systems a variety of magnetic structures, such as ferromagnetic, antiferromagnetic and spiral order and metamagnetism. In many cases the detection of these structures is hampered by moments that are strongly reduced by the Kondo effect. In addition to the Kondo effect one has in HF systems strong crystal field effects, and the corresponding 4f-levels can be determined by neutron scattering experiments. The interplay between Kondo and CF effects varies from system to system, and the corresponding information is part of the content of this chapter. Since in HF systems the Kondo temperature is typically rather small, one observes at low temperature mainly the properties of the lowest CF level. Valence fluctuations couple to phonons, and our review gives a rather complete survey of the corresponding anomalies. In addition, one has phonon anomalies due to the CF-phonon interaction, since a lattice deformation around an R ion modifies the crystalline electric field. Finally, we mention the spin-orbit interaction, which is

Structure and magnetic properties of Nd6Fe13Cu

Abstract: A new phase δ in the ternary system Nd‐Fe‐Cu is identified as Nd6Fe13Cu. X‐ray powder diffraction patterns of a sample annealed at 540 °C for 1000 h are indexed on a tetragonal unit cell with a=8.09 A and c=22.26 A. The Curie temperature Tc is 190±5 °C and the small net magnetization at 4.2 K of 18 mT (2 J T−1 kg−1) after correcting for the presence of 1.5% Nd2Fe17 suggests a ferrimagnetic spin structure. There is a spin rearrangement in a field of 1.5 T at 4.2 K and a metamagnetic transition is also observed on hydrogenation. Mossbauer spectra at 15 K are fitted with four subspectra with hyperfine fields of 36.9, 33.2, 30.6, and 23.0 T. X‐ray measurements on oriented powder indicate that the easy direction of the net magnetization lies along the c axis, but Mossbauer spectra show that none of iron sublattices have their magnetization aligned along c.

Metamagnetic magnetization of heavy fermion single crystal UPd2Al3 in high magnetic fields

Abstract: High field magnetization and magnetoresistance of a single crystal UPd 2 Al 3 are investigated up to 50 Tesla (T) and a sharp metamagnetic magnetization is observed in the c -plane at 18 T. The metamagnetism is explained by assuming the magnetization-dependent heavy fermion energy.

The temperature dependence of the itinerant-electron metamagnetic transition in Laves-phase Lu(Co1-xGax)2 compounds

Abstract: The high-field magnetization and temperature dependence of the critical field have been investigated for Laves-phase Lu(Co1-xGax)2 compounds. A clear itinerant-electron metamagnetic transition is observed and its critical field increases with increasing temperature for x=0.09. The results are discussed on the basis of the Clausius-Clapeyron equation for the magnetic phase transition. This reveals that the magnetic entropy of these compounds decreases above the critical field, suggesting the suppression of the spin fluctuations due to the metamagnetic transition. This is consistent with the recent results on the field dependence of the electronic specific-heat coefficient of these compounds. The Arrott plots for the specimen with x=0.12, which has a very low critical field, have also been investigated. The squared hypothetical spontaneous magnetization Mh2 decreases linearly with increasing T2 and the reduced Mh versus T plot for Lu(Co0.88Ga0.12)2 is very similar to the reduced magnetization for an Invar-type ferromagnetic Lu(Cu0.83Al0.17)2 compound. This result suggests that Lu(Co0.88Ga0.12)2 in the ferromagnetic state is expected to exhibit marked magnetovolume effects.

Magnetic behaviour of γ-phase hydrides RCo3H∼4 in high magnetic fields

Abstract: High-field magnetization measurements have been carried out for single-crystal γ-phase hydrides RCo3H∼4 with R=Tb and Dy and for powder samples with R=Nd and Gd. The results indicate that these γ-phase hydrides with R=Y, Ho and Er become antiferromagnetic except for R=Gd at low temperatures due to the appearance of a negative Co-Co intersublattice interaction. A metamagnetic transition of the Co-sublattice magnetization occurs from the antiferromagnetic to the forced ferromagnetic state if the field is applied along the c-axis. GdCo3H≈4, being ferrimagnetic at low temperatures, becomes antiferromagnetic above 32K. The significant weakening of the R-Co interaction due to hydrogenation leads to a Neel temperature of about 200 K in all hydrides and an almost independent behaviour of the R and Co sublattices in the compounds with R=Nd, Ho and Er. The role of the R-Co interaction in the magnetic behaviour of the hydrides is discussed.

Metamagnetic transition and magnetic properties of UPt3.

Abstract: We report fully relativistic band structure and total energy studies of ${\mathrm{UPt}}_{3}$ in the different magnetic states. We find that the ground state of ${\mathrm{Upt}}_{3}$ is nonmagentic, even though stable ferromagnetic and antiferromagnetic solutions are also found. We find that the total energy calculations predict the direction of the ordered moment correctly and for the first time the ``metamagnetic transition'' is found from a first principles total energy calculation for heavy fermion compounds.

Anomalous Hall Coefficient in the f Electron System

Abstract: The 4f(5f) electrons in the rare earth(uranium) atom are pushed deep into the interior of the closed 5s(6s) and 5p(6p) shells because of the strong centrifugal potential 1(1+1)/r2 , where 1=3 holds for the f electrons This is a reason why the 4f (5f) electrons possess an atomic-like character even in the compound On the other hand, the tail of their wave function spreads to the outside of the closed 5s(6s) and 5p(6p) shells, which is highly influenced by the potential energy, the relativistic effect, the distance between the rare earth(uranium) atoms, and hybridization of the 4f(5f) electrons with the conduction electrons These cause the various phenomena such as valence and spin fluctuations, gap states, Kondo lattice, heavy electrons (fermion), metamagnetism and superconductivity for the f electron compounds1

New magnetically ordered materials with high Tc

Abstract: Magnets comprised of molecules, ions, and polymers is a focus of contemporary materials science research. The anticipated attributes of organic/molecular-based magnetic materials may enable their use in future generations of electronic, magnetic and/or photonic/photronic devices. Some organometallic solids comprising linear chains of alternating metallocenium donors, D, and cyanocarbon acceptors, A, i. e., -D-+A D+A, exhibit cooperative magnetic phenomena, i. e., ferro-, antiferro-, ferri-, and metamagnetism. For [FeIII(C5Me5)2]-+[TCNE] - (Me = methyl; TCNE = tetracyanoethylene) bulk ferromagnetic behavior is observed below the critical (Curie) temperature, Tc, of 4.8 K. Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean-field models developed for this class of materials. Extension to the reaction of a vanadium(0) complex with TCNE lead to the isolation of a magnet with a Tc ∼ 400 K which exceeds the thermal decomposition of the material. A new class ...

Chapter 6 Neutron scattering on heavy fermion and valence fluctuation 4f-systems

Abstract: Summary This chapter deals with valence fluctuation and heavy fermion 4f-systems such as Ce, Sm, Eu, Tm and Yb alloys and compounds. In sections 1 and 3 we summarize the most important physical properties of these systems, based on the main theoretical results achieved so far and on representative data. In the main part (section 2) we summarize neutron scattering data of HF and VF systems. By means of neutrons one can measure the magnetic structure of a system (elastic magnetic scattering) and magnetic and nonmagnetic excitations such as relaxational modes (quasielastic excitations), magnons, crystal field and spin-orbit transitions and phonons. Some of these excitations have been observed in all HF or VF systems. Both HF and VF systems exhibit a quasielastic line due to spin fluctuations or the Kondo effect. Both expressions are used synonymously, but in VF systems the quasielastic line is very broad, corresponding to a high characteristic energy, and is fairly temperature independent. Here one talks in general about spin fluctuations. In HF systems the quasielastic line is narrower, temperature dependent, and has at low temperatures the halfwidth Γ/2 ≈ K B T N K . A typical quasielastic line can be fitted by a Lorentzian and corresponds to a single relaxation time. In most HF compounds the line becomes Q -dependent at low temperatures indicating spin correlations that often lead at still lower temperatures to magnetic order. These spin correlations, on the other hand, lead already to deviations from the Lorentz shape above the spin ordering temperature. In V F compounds spin correlations do not play a role, but the high-temperature quasielastic line also changes its shape and becomes narrower and inelastic at low temperatures. Ce-based HF systems have been investigated by many groups, whereas few data exist for the corresponding VF systems and for Yb-, Sm-, Eu- and Tm-based VF and HF systems. As a consequence, our survey over Ce-based VF and Yb, Sm, Eu and Tm systems is rather complete, whereas for Ce-based HF systems we could only select representative examples. Part of this strong research activity on Ce compounds is due to the fact that CeCu 2 Si 2 becomes superconducting, and it is an enormous challenge to find other superconducting HF systems. In addition, one has in these systems a variety of magnetic structures such as ferromagnetic, antiferromagnetic and spiral order and metamagnetism. In many cases the detection of these structures is hampered by moments that are strongly reduced by the Kondo effect. In addition to the Kondo effect one has in HF systems strong crystal field effects, and the corresponding 4f-levels can be determined by neutron scattering experiments. The interplay between Kondo and CF effects varies from system to system, and the corresponding information is part of the content of this chapter. Since in HF systems the Kondo temperature is typically rather small, one observes at low temperatures mainly the properties of the lowest CF level. Valence fluctuations couple to phonons, and our review gives a rather complete survey of the corresponding anomalies. In addition, one has phonon anomalies due to the CF-phonon interaction, since a lattice deformation around an R ion modifies the crystalline electric field. Finally, we mention the spin-orbit interaction, which is modified in VF and HF systems due to the hybridization between f and conduction electrons. The few neutron scattering experiments on spin-orbit transitions in VF and HF systems show unexpected splittings, line shifts, and line broadenings if compared to the free ion case.

Critical Behaviour of an FeBr2 Model, a Monte Carlo Study

Abstract: A Monte Carlo simulation of the critical behaviour of a quite realistic FeBr2 model in a magnetic field has been performed. Two regions of different critical behaviour have been found in the temperature-field (T,H)-plane. At high temperature a region of continuous phase transitions is found with a crossover to a region of first-order (metamagnetic) phase transitions, in agreement with experimental results. Some of the main differences between FeBr2 and FeCl2 phase diagrams can also be reproduced.

Itinerant-electron metamagnetism of (Y, Lu)(Co, Al)2 at finite temperature

Abstract: The metamagnetic transition and paramagnetic susceptibility for the pseudo-binary compound (Y, Lu)(Co, Al), with the cubic Laves-phase structure are measured. The observed results are analysed on the basis of phenomenological spin-fluctuation theory for itinerant-electron metamagnetism. A satisfactory agreement between experiment and theory is obtained qualitatively.

Magnetization Process of a Dilute Ising-Like Chain

Abstract: Magnetization process at zero temperature is studied numerically for an Ising-like magnetic chain diluted with nonmagnetic impurities, which break up a magnetic chain into independent segments with various lengths. Calculations are made of the weighted contributions to the magnetization from each segment. The magnetization curve thus obtained shows successive metamagnetic transitions at lower fields in addition to the metamagnetic transition corresponding to the one in the pure case. These successive transitions are attributed to the instabilities of a single domain wall excitation including end-spin reversal process in various segments consisting of even numbers of spins. The characteristic features of the magnetization curve are consistent with recent experimental results on CsCo 1- x Mg x Cl 3 at very low temperatures.

METAMAGNETISM AND SPIN FLUCTUATIONS IN Co-BASED INTERMETALLIC COMPOUNDS

Abstract: High field magnetization and susceptibility of Co-based compounds Y(Co1−xA1x)2 are investigated in the paramagnetic region 0≤x≤0.11. In all the region, a sharp metamagnetic transition is observed, while the susceptibility shows a maximum at finite temperature Tmax. The transition field Hc exhibits a positive shift proportional to T2 with temperature. The Hc in the ground state is found to be proportional to Tmax. The experimental results are discussed with a new theory for itinerant electron metamagnetism based on the spin fluctuation model.

Strongly correlated fermions at low temperatures

Abstract: We summarize recent theoretical results concerning strongly correlated fermion system at nonzero temperature within a mean-field picture. In case of narrow-band systems (Part 1) we describe the Mott-Hubbard (localization) boundary, the presence of the T3 ln T term in the specific heat in almost localized systems, as well as the metamagnetism and spin-split heavy masses. We also mention the concept of a statistical spin liquid state and the stability of an extended s-wave superconducting state in two dimensions for this case. In Part 2 (hybridized systems) we introduce an interobital pairing mediated by a Kondo-type interaction and propose an universal scaling of superconducting properties for heavy fermion systems.