Perturbed angular correlation study of the magnetic phase transitions in the rare-earth cobalt Laves phases RCo_ {2} (original) (raw)
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JOURNAL OF PHYSICS-CONDENSED MATTER 18, 253-264 (2006)
The magnetic phase transitions of pseudo-binary rare earth (R) Laves phases R(1-y)Y(x)Co(2) have been investigated by perturbed angular correlation (PAC) measurements of the magnetic hyperfine fields at the probe nucleus (111)Cd for R = Tb, Sm and Ho at various Y concentrations x and for R = Gd, Dy, Er, Nd and Pr at the concentration x = 0.3. First-order transitions were observed in Tb(1-x)Y(x)CO(2) and Sm(1-x)Y(x)Co(2) for x >= 0.3, in Ho(1-x)Y(x)Co(2) for x < 0.4 and in R(0.7)Y(0.3)CO(2) for R = Dy, Ho, Er, Nd and Pr. For Gd(0)(7)(.)Y(0.3)Co(2), the temperature dependence of the average magnetic hyperfine field is compatible with a second-order transition. The discontinuity of the magnetic hyperfine interaction at the first-order transitions of heavy R(1-x)Y(x)Co(2), which mainly reflects the jump of 3d magnetization of the Co subsystem at T(C), was found to increase monotonically with decreasing order temperature. The TC dependence of the normalized magnetic frequency v(M) (T(C))/v(M) (0) proportional to [1-(T(C)/T(0))(2)](1/2) with To = 203(5) K for the boundary temperature between first- and second-order transitions can be explained by the temperature dependence of the coefficient of the M(4) term of the free energy in the Wohlfarth-Rhodes-Shimizu theory of itinerant electron metamagnetism.
Journal of Alloys and Compounds, 2009
We present a study of the Banerjee criterion and its applicability to those cases in which the first-or second-order character of a magnetic phase transition is controversial. Detailed magnetization measurements on RCo 2 (R = Er, Pr, Nd, and Dy) are analyzed in the framework of Landau theory and the Banerjee criterion. We show that, in general, a simple observation of the curvature of H/M vs. M 2 is not sufficient to determine the order of a magnetic transition. Our detailed analysis, together with a complementary calorimetric study, has also been applied to the pseudobinary solid solution Er x Pr 1−x Co 2 (x = 0.9, 0.8, and 0.7), in order to check its validity in an unknown, non-trivial case. Magnetization measurements on Er x Pr 1−x Co 2 compounds with x = 0.6 and 0.4 show that this analysis is restricted to single phase and unfrustrated magnets. We present also the magnetic entropy change at the transition of the studied RCo 2 compounds obtained by applying the Maxwell relations.
Investigation of the magnetic hyperfine field of 111Cd in the rare-earth Laves phases RCo2 and RNi2
J. Phys.: Condens. Matter 12, 3423 (2000), 2000
The magnetic hyperfine field Bhf of the closed-shell probe nucleus 111 Cd on the cubic R site of the C15 Laves phase RCo2 (R = Gd, Tb, Dy, Ho, Er, Sm, Nd and Pr) and RNi2 (R = Gd, Sm) has been investigated at 9 K by perturbed angular correlation (PAC) spectroscopy. In RCo2 the hyperfine field increases with increasing R spin, both for the heavy and the light R. The experimental trend suggests that the total hyperfine field has two contributions of opposite sign: Bh f = B 3d - B 4 f , where B 3d is induced by the 3d Co moment and B 4 f is due to indirect 4f - 4f exchange. The 4f contribution can be estimated from the measurement of Bh f in RNi2 where the 3d moment vanishes. The 111 Cd hyperfine fields in GdNi2 and SmNi2 at 9 K are Bh f = 7.5(5) and 2.0(2) T, respectively. Assuming a linear dependence of B 4 f on the projection ( g - 1)J of the R spin on the total angular momentum J , as expected from the Ruderman-Kittel-Kasuya-Yosida (RKKY) theory of indirect 4f - 4f exchange, the variation of B 3d across the R series can be determined from the experimental values of Bhf . In heavy RCo2 (R = Gd, ... ,Er) B 3d follows very closely the variation of the Co moment µCo across the R series with B 3d /µCo = 28.7(2) T µ-1 B . With the same ratio for the light RCo2 (R = Pr, Nd, Sm) the experimental Bh f values indicate an increase of the 3d moment from µCo = 0.45 to 0.7 µB between PrCo2 and SmCo2 . "
Series of bulk magnetic-phase transitions in : A study
Physica B: Condensed Matter, 2006
Using muon spin rotation, well-defined bulk ∼ 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x = 0.85 with the highest transition temperature ever observed for x ≥ 0.75. This stresses the diversity of x ≥ 0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x = 0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 K and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.
Comment on “Nature and entropy content of the ordering transitions in RCo2”
Physical Review B, 2007
In their analysis of our perturbed angular correlation ͑PAC͒ study of the magnetic phase transitions of RCo 2 ͓Phys. Rev. B 68, 014409 ͑2003͔͒, Herrero-Albillos et al. ͓Phys. Rev. B 73, 134410 ͑2006͔͒ come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero-Albillos et al. on the order of the magnetic phase transitions of PrCo 2 and NdCo 2 . In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of PrCo 2 and NdCo 2 undergoes a discontinuous, first-order phase transition at T C . In a recent differential scanning calorimetry ͑DSC͒ study of the magnetic phase transitions of the rare earth ͑R͒ Laves phases RCo 2 Herrero-Albillos et al. 1,2 report that the magnetic phase transitions of the light R compounds PrCo 2 and NdCo 2 are second-order transitions ͑SOT͒. This conclusion is in conflict with the results of a perturbed angular correlation ͑PAC͒ investigation 3 of RCo 2 according to which the magnetization of PrCo 2 and NdCo 2 undergoes a first-order transition ͑FOT͒. As documented in Ref. 3, earlier studies of PrCo 2 and NdCo 2 have not been conclusive as to the order of the phase transition of these compounds.
PHYSICAL REVIEW B 72, 134402 (2005)
RCo2The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus Cd-111 on the rare earth (R) site of the pseudobinary Laves-phase compounds R1-xYxCo2 has been investigated by perturbed angular correlation (PAC) spectroscopy at 10 K for the rare earth R=Tb and Ho at various Y concentrations x <= 0.8 and for R=Gd, Dy, Er at the concentration x=0.3. Up to four components with different magnetic interaction frequencies nu(M)(i) could be resolved from the PAC spectra. The relative intensities of these components are in fair agreement with those of a binomial distribution of Y atoms on the four nearest neighbor (NN) R sites of the probe nucleus. For all R constituents, one finds a strictly linear relation between the number n(R) of NN R atoms and the magnetic hyperfine frequencies: nu(M)(i)=nu(M)(4Y)+Delta nu(M)xn(R).The frequency nu(M)(4Y)=35(2) MHz is independent of the R constituent and of the Y concentration up to x <= 0.6. These properties identify nu(M)(4Y) as the contribution of the Co 3d moments to the hyperfine interaction at the Cd-111 site. The frequency steps Delta nu(M)[<= 0.1 nu(M)(4Y)] reflect the spin polarization directly induced by the 4f spins at the probe nucleus. From Gd to Er, the spin polarization decreases much stronger than expected from the linear variation of the 4f spin in the heavy R series. An indirect 4f contribution caused by a dependence of the Co 3d moment on the number of R neighbors can be excluded. The relation nu(M)(i)=nu(M)(4Y)+Delta nu(M)xn(R) then implies that the contributions of the 3d and 4f spins to the magnetic hyperfine field in RCo2 have the same relative sign.
Journal of Alloys and Compounds, 1998
Electrical resistivity, magnetization and AC susceptibility measurements have been carried out to investigate the order of the magnetic phase transition in (Er Tb)Co compounds. In this system a changeover from first-to second-order transition at about x 50.60 was 12x x 2 c observed. This result is discussed in the framework of the generalized Inoue-Shimizu phenomenological model. Moreover, a microscope explanation about the order of the phase transition in terms of spin fluctuation and metamagnetism of the Co-3d subsystem is offered.
Transferred hyperfine interactions in ferrimagnetic RCo2 cubic Laves phase compounds
Physica B+C, 1977
The ferrimagnetic system (Gd. Y, ~)Co~ has been studied by the M6ssbauer effect of '~'Gd and by the NMR of '65Ho as a substitutional impurity. The data are interpreted in terms of a simple model which involves transferred hyperfine fields from the lanthanide and cobalt ions and which allows for crystal field quenching of the holmium moment.
Critical exponents of the ferromagnetic-paramagnetic phase transition of La1-xSrxCoO3 (0.20~x~0.30)
Physical Review B, 1999
The critical exponents of the second-order ferromagnetic-paramagnetic phase transition of La 1Ϫx Sr x CoO 3 (0.20рxр0.30) compounds are obtained. The results 0.43рр0.46, 1.39р␥р1.43, and 4.02р␦р4.38 are qualitatively different than those reported in the literature for parent manganites, and are taken as the indication of the absence of long-range interactions below the Curie point. Electronic phase segregation in hole-rich ferromagnetic and hole-poor LaCoO 3-like regions is supposed to explain the observations. The hole-poor regions would dilute the magnetic lattice and prevent the occurrence of long-range order. Also, a spin transition of the Co 3ϩ ions at T C or the hole-poor regions ͑acting equivalently to missed spin clusters in amorphous ferromagnets͒ could be the reason for the high  value in the framework of a Heisenberg model. ͓S0163-1829͑99͒02801-5͔