Field effect on itinerant electron magnetism of Y1$minus;xErxCo2 compounds (original) (raw)
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Physical Review B, 2009
The field-induced first-order phase transition in ͑Er 1−x Y x ͒Co 2 with the yttrium concentration x = 0.45 is observed to be accompanied by a butterflylike behavior and significant irreversibility of the specific heat. The coefficient ␥ of the T-linear specific heat decreases by ϳ48% under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of Co 3d electrons. The isothermal magnetic entropy change ⌬S m in Er 0.55 Y 0.45 Co 2 includes a large contribution associated with spin fluctuations induced by the f-d exchange interaction in the hybridized 3d-5d-electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the RCo 2 type compounds. The maximal ⌬S m value observed for ErCo 2 just above the Curie temperature is ascribed to the closeness of the T C value to the spin-fluctuation temperature T sf of itinerant Co 3d electrons. The nonmonotonous change in ⌬S m with the Curie temperature of R 1−x R x ЈCo 2 compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.
Physical Review B, 2000
The thermodynamic and transport properties of the Er 1Ϫx Y x Co 2 system were studied in the concentration range 0.0рxр1.0. In this system, the first-order magnetic phase transition observed in ErCo 2 at T C ϭ32 K is related to the itinerant electron metamagnetism of the d subsystem ͑Co sublattice͒ driven by the onset of magnetic ordering within the Er sublattice. By employing magnetic, specific heat, thermal expansion, and resistivity measurements we show that in a limited concentration range x cr Ј ϽxϽx cr and pressure P cr Ј Ͻ P Ͻ P cr the itinerant Co sublattice orders magnetically at T C Co , which is lower than T C R of the Er sublattice. This is referred either to a weakening of the effective molecular field acting on the Co sites owing to the yttrium subsititution or to a pressure-driven increase of the critical field necessary to induce a magnetic moment on the Co sites. On further increasing the yttrium concentration or the pressure only the Er sublattice exhibits longrange order. The theoretical calculations within the molecular field approximation are in agreement with the experimental magnetic x-T phase diagram of the Er 1Ϫx Y x Co 2 system and confirm the effect of a separate ordering of the magnetic sublattices with reasonable parameters used for the intrasublattice Er-Er and intersublattice Er-Co exchange interactions. A field-induced collapse of the Co moment, inverse itinerant electron metamagnetism, is well observable by magnetoresistance measurements at appropriate values of concentration and external pressure. The existence of itinerant electron metamagnetism in the Co sublattice is found to be limited in temperature by T 0 , a characteristic temperature which is sensitive to substitution and pressure.
Effect of magnetic field on the itinerant Co-subsystem in Ho 0.423 Y 0.577 Co 2
Applied Physics A: Materials Science & Processing, 2002
In order to study the band metamagnetism in the cubic Laves-phase compounds R 1−x Y x Co 2 [R = rare earth], neutron-diffraction measurements of the Ho 0.423 Y 0.577 Co 2 compound with yttrium content just below the critical value (x c = 0.58) have been performed in an applied magnetic field of up to 2 T and in the temperature range from 4.2 to 22 K. It has been found that the small external magnetic field produces an ordering effect in the R-subsystem which is accompanied by an irreversible growth of the magnetization of the Co-sublattice.
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.
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.
Magnetocaloric effect in the pseudobinaries (Ho1−cRc)Co2 (R = Er and Dy)
The European Physical Journal B, 2008
In this work, it is theoretically discussed the magnetocaloric effect in the Laves phase doped compounds (Ho1−cErc)Co2 and (Ho1−cDyc)Co2. To this end, it has been used a model Hamiltonian of localized states and itinerant 3d-electrons, including the crystalline electrical field and the magnetoelastic interaction. The theoretical calculations show that composite materials made up of several samples of the doped compounds (Ho1−cErc)Co2 and (Ho1−cDyc)Co2 exhibit large values of the magnetocaloric potentials in a wide range of temperatures.
Collapse of the itenerant Co moment in Er1−xLuxCo2 by the application of high magnetic fields
Physica B: Condensed Matter, 1997
The collapse of the Co moment in the ferrimagnetic Er~-xLuxC02 compounds has been studied by means of magnetization, magnetostriction and ~9C0 spin echo measurements. The collapse of the Co moment is observed at the critical concentration around x = 0.3 in zero magnetic field. The application of a high magnetic field also induces the moment collapse in the compounds below the critical concentration, which is observed as a metamagnetic transition. From the metamagnetic transition field of ErCo2 observed at 52 T, the exchange constant between Er and Co can be estimated to be JErCo =-1.3 x 10-22 J.
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.
Thermopower of ErCo2 in magnetic fields up to 15T
Physica B: Condensed Matter, 2002
Thermopower S and resistivity r of cubic Laves phase compound ErCo 2 were measured from 2 to 300 K in magnetic fields up to H ¼ 15 T. ErCo 2 shows a first-order magnetic transition from paramagnetic to ferrimagnetic state at Curie temperature T c E32 K, where the sharp jump in SðTÞ curve was observed with increasing temperature. T c increases with increasing H: The magnitude of the jump of SðTÞ at the first-order magnetic transition increases with increasing H in magnetic field up to 3 T. At higher fields, SðTÞ curve shows a double transition due to separation Er and Co ordering.