Itinerant-electron Metamagnetic Transition Research Articles

Influence of pressure on itinerant electron metamagnetic transition in La(FexSi1−x)13 compounds

The influence of pressure on the itinerant electron metamagnetic (IEM) transition has been investigated for a La(FexSi1−x)13 compound. The critical temperature T0 defined as the disappearance point of the IEM transition decreases with increasing hydrostatic pressure. Since the pressure dependence of T0 is smaller than that of TC, the temperature range for appearance of the IEM transition becomes wider with increasing hydrostatic pressure. These results are consistent with the theoretical model based on the Landau expansion of free energy including the influence of spin fluctuations and the magnetovolume effect. On applying pressure, the critical field Bc for the IEM transition increases proportionally with temperature in low pressures, whereas it exhibits a quadratic temperature dependence with lowering the Curie temperature in high pressures. The change in the temperature dependence of Bc is explained by a change in the thermal growth rate of spin fluctuations.

Effects of carbon on magnetic properties and magnetic entropy change of the LaFe11.5Si1.5 compound

Effects of the interstitial carbon atoms on the magnetic properties, especially on the magnetic entropy change, of the LaFe11.5Si1.5 compound, have been studied. X-ray diffraction patterns reveal a monotonous increase of the lattice constant with the concentration of carbon, while the cubic NaZn13-type structure remains unchanged. The Curie temperatures TC of LaFe11.5Si1.5Cy are ∼195, 225, and 241 K for y=0, 0.2, and 0.5, respectively, increasing with the increase of carbon concentration. The maximal magnetic entropy changes |ΔS| of LaFe11.5Si1.5Cy at the respective TC under a magnetic field change of 0–5 T are ∼24.6, ∼22.8, and ∼12.7 J/kg K for y=0, 0.2, and 0.5, respectively, notably exceeding that of Gd (|ΔS| ∼9.8 J/kg K at TC=293 K). The |ΔS| of LaFe11.5Si1.5C0.2 is nearly as giant as that of the parent alloy LaFe11.5Si1.5 due to the first-order field-induced itinerant-electron metamagnetic transition that occurs in both compounds clearly observed for the LaFe11.5Si1.5C0.5 compound. With the increase of carbon concentration, the nature of magnetic transition has been changed from first order to second order, which results in the significant decrease of the magnetic entropy change. The large |ΔS|, convenient adjustment of TC and relatively low cost make the LaFe11.5Si1.5Cy interstitial compounds promising candidates for magnetic refrigerants in the corresponding temperature range.

Itinerant-electron metamagnetic transition and large magnetocaloric effects inLa(FexSi1−x)13compounds and their hydrides

The itinerant-electron metamagnetic (IEM) transition and magnetocaloric effects (MCE's) in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ and $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds have been investigated. The $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds exhibit large values of both the isothermal entropy change $\ensuremath{\Delta}{S}_{\mathrm{m}}$ and the adiabatic temperature change $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ around the Curie temperature ${T}_{\mathrm{C}}$ in relatively low magnetic fields. Such large MCE's are explained by a large magnetization change at ${T}_{\mathrm{C}}$ and a strong temperature dependence of the critical field ${B}_{\mathrm{C}}$ for the IEM transition. By hydrogen absorption into the compounds, ${T}_{\mathrm{C}}$ is increased up to about 330 K, keeping the metamagnetic transition properties. Accordingly, the extension of the working temperature range having the large MCE's in relatively low magnetic fields is demonstrated by controlling y in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds. The correlation between the increase of ${T}_{\mathrm{C}}$ and the large MCE's in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds is discussed by taking the magnetovolume effects into consideration.

Large magnetocaloric effects in NaZn13-type La(FexSi1−x)13 compounds and their hydrides composed of icosahedral clusters

The magnetocaloric effects (MCEs) in cubic NaZn13-type La(FexSi1−x)13 compounds and their hydrides composed of icosahedral clusters have been investigated because these compounds exhibit the itinerant-electron metamagnetic (IEM) transition just above the Curie temperature Tc: The value of Tc can be increased continuously up to about 336 K by hydrogen absorption into the La(FexSi1−x)13 compounds. Therefore, the values of the isothermal magnetic entropy change ∆Sm and the indirectly estimated adiabatic temperature change ∆Tad due to the IEM transition become -31 J/kg K and 15.4 K, respectively, in the magnetic field change from 0 to 5 T at Tc = 287 K for the La(Fe0.90Si0.10)13H1.1 compound. The MCEs in the compounds having the IEM transition are much larger than thosein the compounds exhibiting the second-order magnetic transition at Tc: The direct measurement of the adiabatic temperature change has confirmed such a large value. Consequently, the La(FexSi1−x)13 compounds and their hydrides having the IEM transition are promising as high performance magnetic refrigerants working in a wide temperature range covering room temperature in relatively low magnetic fields.

The effect of Mn substitution in LaFe11.7Si1.3 compound on the magnetic properties and magnetic entropy changes

The magnetic properties and magnetic entropy changes of La(Fe1−xMnx)11.7Si1.3 (x = 0–0.03) have been studied. The Curie temperatures TC decrease monotonously with increasing Mn concentration from 188 to 127 K, and the saturation magnetization μS decreases from 23.9 μB/fu to 22.2 μB/fu respectively, as x increases from 0 to 0.03. The maximal magnetic entropy changes |ΔS|, under a magnetic field change of 0–5 T, are 26.0 J kg−1K −1, 25.7 J kg−1K −1, 20.8 J kg−1K −1 and 17.1 J kg−1K −1 for x = 0, 0.01, 0.02 and 0.03, respectively. The appearance of negative slopes in the Arrott plots for all samples confirms the occurrence of a first-order field-induced itinerant-electron metamagnetic (IEM) transition. Furthermore, the full-width at half-maximum (FWHM) of |ΔS| peak, δTFWHM, increases obviously with increasing Mn content, which results in the decrease of the maximum magnetic entropy change.

Microscopic theory of magnetization processes in Y (Co1−x Alx)2

Employing ab initio electronic structure calculations we study thedevelopment of the magnetic properties in Y (Co1−x Alx)2for varying Al concentration. The effect of substitutional disorder is treated in thecoherent-potential approximation implemented within a tight-binding linearmuffin-tin orbital method. The experimentally observed reduction of the criticalfield of the itinerant electron metamagnetic phase transition with increasingcontent of non-magnetic Al is explained. It is shown, on the basis of aT = 0 KStoner type itinerant magnetism theory, that the alloying-induced changes in theshape of the calculated density of states, caused by the Al substitution, lead to (i) astabilization of the magnetic state, (ii) a smoothening of the first-order metamagnetictransition and (iii) a subsequent suppression of the metamagnetic transition aroundx = 0.15.Analysing the magnetization processes in Y (Co1−x Alx)2by varying the strength of the exchange interaction, we provide a microscopicalbackground to earlier phenomenological assumptions made in the literature.

Large magnetocaloric effect in La(FexSi1−x)13 itinerant-electron metamagnetic compounds

The magnetocaloric effect (MCE) originated from the itinerant-electron metamagnetic transition for La(FexSi1−x)13 compounds has been investigated. With increasing Fe concentration, the MCE is enhanced and both the isothermal magnetic entropy change ΔSm and the adiabatic temperature change ΔTad for the compound with x=0.90 are −28 J/kg K and 8.1 K, respectively, by changing the magnetic field from 0 to 2 T. Similar large MCE values are achieved around room temperature by controlling the Curie temperature by means of hydrogen absorption. Consequently, La(FexSi1−x)13 compounds are promising as magnetic refrigerant materials working in relatively low magnetic fields.

Large magnetic entropy change near room temperature in the LaFe11.5Si1.5H1.3 interstitial compound

The LaFe11.5Si1.5H1.3 interstitial compound has been prepared. Its Curie temperature TC (288 K) has been adjusted to around room temperature and the maximal magnetic entropy change (|ΔS|~17.0 J·kg-1·K-1 at TC) is larger than that of Gd (|ΔS|~9.8 J·kg-1·K-1 at TC=293 K) by ~73.5% under a magnetic change from 0 to 5 T. The origin of the large magnetic entropy change is attributed to the first-order field-induced itinerant-electron metamagnetic transition. Moreover, the magnetic hysteresis of LaFe11.5Si1.5H1.3 under the increase and decrease of the field is very small, which is favourable to magnetic refrigeration application. The present study suggests that the LaFe11.5Si1.5H1.3 compound is a promising candidate as a room-temperature magnetic refrigerant.

Magnetic entropy change in LaFe13−xSix intermetallic compounds

LaFe 13−x Si x compounds with x=2.4, 2.6, and 2.8 have been carefully prepared and characterized. X-ray powder diffraction patterns confirm that the samples crystallize in a single NaZn13-type phase. Magnetic measurements were carried out in a SQUID magnetometer. The Curie temperature Tc of LaFe13−xSix samples decreases nearly 50 K with x increasing from 2.4 to 2.8. Around Tc the samples exhibit a typical second-order phase transition; and no trace of first-order itinerant electron metamagnetic transition is found. So it is reliable to characterize the magnetic entropy change around Tc using magnetic measurement. The maximum values of the magnetic entropy change of LaFe13−xSix samples are found to be, at their Curie temperatures, 5.85, 5.9, and 3.7 J/kg.K for x=2.4, 2.6, and 2.8 under a field change of 5 T (0–5 T). For the quite large magnetic entropy change over a wide temperature range, LaFe13−xSix compounds appear, therefore, to be a potential material candidate for magnetic cooling.

Giant Magnetic Entropy Change in Hydrogenated La(Fe<SUB>0.88</SUB>Si<SUB>0.12</SUB>)<SUB>13</SUB>H<SUB>y</SUB> Compounds

A magnetic entropy change ΔS M due to the itinerant-electron metamagnetic (IEM) transition was estimated to be -23 J/kg K around the Curie temperature for La(Fe 0.88 Si 0.12 ) 13 in the magnetic field change from 0 to 5T. In order to control the Curie temperature while keeping such a large value of DS M , hydrogen absorption was carried out. La(Fe 0.88 Si 0.12 ) 13 H 1.0 with T C = 274K shows a large ΔS M due to the IEM transition around room temperature. The adiabatic temperature change ΔT ad from 0 to 2 T is about 7 K, comparable to that of Gd 5 (Ge 0.5 Si 0.5 ) 4 . By changing the hydrogen concentration, the Curie temperature can be controlled from 195 to 336K. It should be noted that the magnitude of ΔS M is almost the same after hydrogen absorption. Therefore, the hydrogenated La(Fe 0.88 Si 0.12 ) 13 H y compounds are promising magnetic refrigerants working in a wide range of temperature.

Itinerant-electron metamagnetic transition and large magnetovolume effects inLa(FexSi1−x)13compounds

Magnetic properties and magnetovolume effects have been investigated for itinerant-electron metamagnetic $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds. At the Curie temperature ${T}_{C1},$ a first-order magnetic phase transition takes place in the concentration range $0.86<~x<~0.88.$ With increasing Fe concentration, the Curie temperature decreases and the critical temperature of the itinerant-electron metamagnetic (IEM) transition increases, accompanied by a more sharp IEM transition. For the compound with $x=0.88,$ a large volume change of about 1% follows the thermal induced transition at ${T}_{C1}.$ The value of ${T}_{C1}$ is significantly decreased by applying hydrostatic pressure, whereas the pressure dependence of the spontaneous magnetization is relatively small. These results are explained by the negative mode-mode coupling among spin fluctuations.

Itinerant-electronmetamagnetism and magneto-volume effects in Lu(Co1-xAlx)2Laves phase compounds

The itinerant-electron metamagnetic transition (MT) and the effectsof hydrostatic pressure on the critical transition field BC ofthe MT, on the spontaneous magnetization MS and on the Curietemperature TC have been investigated for well homogenizedLu(Co1-xAlx)2 Laves phase compounds.The critical field BC decreases with increasing x, maintaining alinear relationship with the inverse susceptibility at thetemperature where the susceptibility exhibits a maximum value, χ-1(Tmax). On applying pressure, the magnetization M ofthe ferromagnetic compound with x = 0.100 is drastically decreasedat a critical pressure, resulting in a paramagnetic state. Inaddition, the metamagnetic transition from the paramagnetic to theferromagnetic state is induced by applying an external magnetic field.The effect of pressure on the Curie temperature TC is extremelylarge and negative in the vicinity of the criticalconcentration for the onset of ferromagnetism. The pressurecoefficient of the Curie temperature, ∂ln TC/∂P, is much larger than that of the spontaneous magnetization,∂ln MS/∂P, below x = 0.150. These results canbe explained by the theory for itinerant ferromagnets having anegative coefficient b of the fourth-order term in the Landauexpansion. The Landau expansion coefficients estimated from theexperimental results are in accord with the theories. From theseestimated values, it is concluded that the magneto-volume effectdecreases the critical transition field BC. It has beenconfirmed that the results for Lu(Co1-xAlx)2 are verymuch analogous to those for Lu(Co1-xGax)2.

Giant isotropic magnetostriction of itinerant-electron metamagnetic La(Fe0.88Si0.12)13Hy compounds

La(Fe x Si 1−x ) 13 compounds exhibit an itinerant-electron metamagnetic (IEM) transition above Curie temperature TC. The IEM transition in the compound with x=0.88 is accompanied by a giant volume change. From a practical viewpoint, TC was controlled by hydrogen absorption in order to obtain such a giant volume magnetostriction at room temperature. For the La(Fe0.88Si0.12)13H1.0 compound, the IEM transition occurs above TC=278 K, and a significant isotropic linear magnetostriction of about 0.3% at 7 T is induced in the vicinity of room temperature. This large magnetostriction is attributed to the giant volume magnetostriction of about 1% by the IEM transition.

Influence of negative lattice expansion and metamagnetic transition on magnetic entropy change in the compound LaFe11.4Si1.6

Magnetization of the compound LaFe11.4Si1.6 with the cubic NaZn13-type structure was measured as functions of temperature and magnetic field around its Curie temperature TC of ∼208 K. It is found that the magnetic phase transition at TC is completely reversible. Magnetic entropy change ΔS, allowing one to estimate the magnetocaloric effect, was determined based on the thermodynamic Maxwell relation. The achieved magnitude of |ΔS| reaches 19.4 J/kg K under a field of 5 T, which exceeds that of most other materials involving a reversible magnetic transition in the corresponding temperature range. The large entropy change is ascribed to the sharp change of magnetization, which is caused by a large negative lattice expansion at the TC. An asymmetrical broadening of |ΔS| peak with increasing field was observed, which is resulted from the field-induced itinerant-electron metamagnetic transition from the paramagnetic to ferromagnetic state above the TC.

The order of the magnetic phase transitions in RCo2(R = rare earth) intermetallic compounds

It has been found experimentally that the order of the magneticphase transitions in RCo2 compounds (R standing for rare-earthmetals) at Tc changes from second order for thelight-rare-earth series up to TbCo2 to first order for theheavier-rare-earth compounds DyCo2, HoCo2 and ErCo2. On the basis ofresults of fixed-spin-moment band-structure calculations for theisostructural compound YCo2 at different lattice constants, wepropose an explanation for this behaviour. In contrast to thewidely accepted Inoue-Shimizu theory for this class of compounds,our explanation also includes Pr, Nd which were thought to behavedifferently due to the influence of crystal-field effects. We showthat an itinerant-electron metamagnetic transition in thesecompounds can occur only over a certain range of lattice constantsand that the possibility of a first-order phase transition isconnected to features of the electronic structure rather than to themagnitude of the transition temperature as conjectured earlier. Theinfluence of the latter is only important if the transition takesplace at elevated temperatures, where effects of spin fluctuationscan suppress a first-order transition.

Itinerant electron metamagnetic transition in La(FexSi1−x)13 intermetallic compounds

A first-order transition above the Curie temperature for ferromagnetic La(FexSi1−x)13 (x=0.86 and 0.88) compounds has been confirmed by applying a magnetic field. The magnetic state changes from the paramagnetic to the ferromagnetic state and the transition field increases with temperature, indicating an itinerant electron metamagnetic (IEM) transition. The IEM transition is broad in x=0.86 and becomes clearer in x=0.88, which takes a negative slope of the Arrott plot. The volume change just above the Curie temperature for x=0.88 is huge, about 1.5%, which is caused by a large magnetic moment induced by the IEM transition.

Giant volume magnetostriction due to the itinerant electron metamagnetic transition in La(Fe-Si)/sub 13/ compounds

Ferromagnetic La(Fe/sub x/Si/sub 1-x/)/sub 13/ compounds with x/spl ges/0.87 show an itinerant metamagnetic transition above the Curie temperature T/sub c/. Since a large volume change of about 1.5% is caused by the transition, the present compounds have a possibility for practical applications as giant magnetostrictive materials. To adjust T/sub c/ around room temperature, the effect of the substitution by other magnetic elements on the Curie temperature was examined. The Curie temperature of La(Fe/sub 0.88/Co/sub y/Si/sub 0.12-y/)/sub 13/ compounds increases with increasing y and the itinerant electron metamagnetic transition appears above T/sub c/. For y=0.03 and 0.04, a huge volume change of 0.6/spl sim/0.9%, or a linear change of 0.2/spl sim/0.3%, due to the itinerant metamagnetic transition is observed around 250 K.

Itinerant electron metamagnetism in Y 1− xZr xCo 3

In Y 1−x Zr x Co 3 (0⩽x⩽0.1) the high field magnetization was observed in pulsed fields up to 40 T at 4.2 K, and the transition fields for the itinerant electron metamagnetism were determined in pulsed fields up to 100 T at 10 K. The results show that the itinerant electron metamagnetic transition is induced when the effective field acting on the Co, which is the sum of the applied and the exchange fields, attains the critical value.

Change in the itinerant-electron metamagnetic transition behavior by annealing in Lu(Co 1− xAl x) 2 Laves phase compounds

The itinerant-electron metamagnetic transition under the different annealing conditions has been investigated for Laves phase Lu(Co 1− x Al x ) 2 compounds with a C15-type structure. The metamagnetic transition feature of these compounds sensitively depends on the annealing condition. The compounds with x=0.080 and 0.085 annealed at 1273 K for 168 h exhibit a clear metamagnetic transition at relatively low magnetic fields, compared with the existing results. From both the metamagnetic transition behavior and EPMA, it is concluded that the anomalously broad metamagnetic transitions reported previously are caused by the concentration gradient of Al due to the insufficient annealing.

The lattice constant and itinerant electron metamagnetic transition in Laves-phase pseudo-binary Lu(Co1-xSix)2 compounds

The lattice constant, low-temperature specific heat and high-field magnetization of the Laves-phase itinerant electron metamagnets Lu(Co1-xSix)2 have been investigated. By partial substitution of Co by Si, the critical field Hc of the metamagnetic transition decreases and the temperature Tmax of the magnetization maximum shifts to lower temperatures with increasing x regardless of the decrease in the lattice constant. Both the electronic specific heat coefficient gamma and the magnetic susceptibility chi increase with increasing x up to x=0.09 and decrease above this concentration. The present results suggest that the decrease in the critical field is not due to the increase in the lattice constant but is due to the hybridization between 3d electrons of Co and 3p electrons of Si.