Use of Stevens coefficients for the prediction of magnetic transitions in pseudobinaryR1−xRx'Al

Abstract

Recent heat capacity and low field magnetic susceptibility measurements revealed unusual magnetic phenomena occurring in Er 1-x R x Al 2 systems (where R = Dy, Tb) in the vicinity of a "magic" concentration of x = 0.25. Empirically, the reasons for such behavior were attributed to different shapes of the 4f charge densities of the R 3+ ions, which are represented by the opposite signs of the second order Stevens' factors. Here we show that by using both the signs and magnitudes of the second order Stevens' factors, magnetic transitions can be predicted in a broader range of pseudo binary R 1-x R' x Al 2 alloys where R and R' are rare earth metals that have opposite signs of second orderStevens' factors. The predictions have been verified using Tm 1-x Tb x Al 2 system as model 2 using x-ray diffraction, magnetic susceptibility and heat capacity measurements. First principles calculations have also been performed in order to explore the behavior of the density of states near the Fermi level.PACS No's: 75.50.Cc, 75.10.Dg, 71.20.Lp, 71.20.Eh. 3 IntroductionThe fundamental magnetic properties of the RAl 2 compounds, where R is a rare earth element, have continuously attracted attention of researchers for more than forty years. 1,2,3,4,5 Specifically the isostructural RAl 2 compounds are well known for interesting properties including crystalline electric field effects 5 and low temperature ferromagnetism 4,5 that can be controlled by varying the rare earth component. Recent research results suggest that despite of broad experimental and theoretical investigations that have been already performed using these relatively simple MgCu 2 -type Laves phases, some fascinating and new phenomena in the RAl 2 compounds are yet to be discovered.Most of the earlier studies explored the properties of the pure binary RAl 2 compounds.Since the 4f charge densities of the R atoms are significantly different from one another, unusual behaviors may be observed when the R atoms in the RAl 2 alloys are partially replaced by a different R' atom to form pseudo binary R 1-x R' x Al 2 compounds. Such replacements have resulted in important discoveries such as zero net magnetic moment in annealing was not necessary. To check the phase purity and identify the crystal structure of the samples, room temperature x-ray powder diffraction (XRD) was performed on a PANAnalytical powder diffractometer employing monochromatic Cu K α1 radiation.Temperature dependent XRD measurements were performed on a Rigaku TTRAX rotating anode powder diffractometer employing Mo Kα radiation. 18 The diffractometer was equipped with a continuous flow 4 He cryostat controlling the temperature of a sample, which was prepared and mounted on a copper sample holder as described in The lattice parameter for pure TmAl 2 (7.7757 Å) was taken from the report of Harris et al. 21 As shown in the inset of Fig. 1, the lattice parameters depend linearly on the Tb concentration (x). Assuming that Vegard's law holds (which is reasonable considering that the structure is cubic and that ...

Tunable magnetism and structural transformations in mixed light- and heavy-lanthanide dialuminides

“…Mixing one lanthanide (R) with another (R') in pseudobinary compounds such as Er 1-x Dy x , Er 1-x Dy x Al 2 , Er 1-x Tb x Al 2 , and Tm 1-x Tb x Al 2 [8,9,10,11] results in unusual low temperature anomalies in heat capacity and multiple ordering phenomena. In some cases, additional first order transitions are observed below the main FM-PM transition at T C .…”

Structure-property relationships in rare earth intermetallic compounds RFe4Ge2 and R117M52+xX112+y

“…This approach has been successfully applied for rare earth-based magnetic systems, we refer readers to some of our recent publications. 47,48,49,50,51,52,53,54,55,56 Our low temperature XRD experimental results indicate that DyFe 4 Ge 2 undergoes a transformation from the high temperature paramagnetic (PM) tetragonal (P4 2 /mnm) to the low temperature antiferromagnetic (AFM) orthorhombic (Cmmm) structure at ~60 K.…”

“…The crystal structure, lattice parameters, phase concentrations were determined by performing Rietveld refinement using LHPM-RIETICA. 45 Single The starting atomic parameters derived via direct methods and the program SIR 97 48 were subsequently refined using the program SHELX-97 49 (full-matrix least-squares on F 2 ) with anisotropic atomic displacement parameters included for all atoms.…”

Magnetism of Ho1−xTbxAl2alloys: Critical dependence

“…2 Recent studies showed that when two lanthanides with 4f electron charge densities of different shape are mixed together in pseudo-binary R 1-x R' x Al 2 compounds (R and R' are lanthanide metals where R has a positive second order Stevens' factor and the same for R' is negative ), interesting magnetic phenomena are observed. 3,4,5,6,7 For certain critical concentrations centered at x = 0.25, sharp peaks, representing first order phase transitions, are observed in the heat capacity, C P , data of the R 1-x R' x Al 2 alloys. The peaks are suppressed upon the application of weak magnetic fields.…”