Elsevier

Physica B: Condensed Matter

Volume 536, 1 May 2018, Pages 133-136
Physica B: Condensed Matter

Low temperature thermopower and magnetoresistance of Sc-rich CeSc1-xTixGe

https://doi.org/10.1016/j.physb.2017.10.075Get rights and content

Abstract

In CeSc1-xTixGe, Ti-alloying reduces the record-high antiferromagnetic (AFM) ordering temperature found in CeScGe at TN=46K and induces ferromagnetism for x0.5. In this work we focus on the AFM side, i.e. Sc-rich samples, and study their thermopower S(T) and magnetoresistance ρ(H,T). The measured S(T) is small in comparison with the thermopower of other Ce-systems and shows some features that are compatible with a weak hybridization between the 4f and band states. This is a further hint pointing to the local character of magnetism in this alloy. Magnetic fields up to 16T have a minor effect on the electrical resistivity of stoichiometric CeScGe. On the other hand, for x=0.65, we find that fields above 4T suppress the hump in ρ(T). Furthermore, the 4.2K magnetoresistance displays a strong decrease in the same field range, also in coincidence with magnetization results from the literature. Our results indicate that ρ(T,H) is a proper tool to assess the HT phase diagram of this system.

Introduction

CeScGe crystallizes in the La2Sb-type tetragonal structure [1], with nearly two-dimensional Ce double layers intercalated by Sc and Ge along the caxis. This compound stands out as having the highest antiferromagnetic (AFM) ordering temperature among Cerium-based magnetic systems, TN=46K [2], [3], [4]. As proposed in Ref. [5] and later verified by powder neutron diffraction experiments [6], at the Néel temperature the double layers order ferromagnetically (FM) and couple AFM along the caxis. Between TN and TL=36K [5], Ce magnetic moments lie in the basal plane [6]. When cooling below TL, there is a canting of the moments towards the caxis concomitant with a structural transition from tetragonal to triclinic of magnetostructural origin [6].

By introducing smaller Ti (3d2) in the Sc-site (3d1) the basal plane contracts, while the interlayer distance along the caxis is practically preserved: these structural and electronic changes should strongly affect the magnetism. The magnetic phase diagram of the CeSc1-xTixGe alloy was reported in Ref. [5]. Both T N(x) and TL(x) decrease at different rates, merging at a critical point xc0.35 at TN20K. At x0.45 the ordering transition changes to ferromagnetic, dropping continuously down to TC=7K at x=0.75, the limit of the La2Sb-type structure. From this research it was concluded that in CeScGe the Ce-4f orbital responsible for magnetism has a local character and that a number of factors, such as an optimized RKKY interaction and a low lying crystal field excited doublet at Δ1/kB35KTN, converge to produce the large ordering temperature.

Thermopower, S(T), is a convenient tool to study hybridization and crystal field effects in Cerium compounds, while resistivity and magnetoresistance measurements, ρ(T,H) can provide further information on the onset and stability of the magnetically ordered state and the nature of those phases. In this work, we present first results of a combined study of Sc-rich CeSc1-xTixGe using these techniques.

Section snippets

Experimental details

Well-annealed CeSc1-xTixGe polycrystalline samples were obtained by conventional synthesis and characterization techniques, as described in Ref. [5]. The samples for electrical resistivity and Seebeck coefficient measurements were cut using a low-speed diamond saw to typical sizes of 1×1×10mm3. Both zero-field and in-field ρ(T) measurements using a conventional four probe technique were performed with a LR700 ac resistance bridge. A zero-Lorentz force configuration jH was chosen for the

Results and discussion

In Fig. 1 we present the thermopower S(T) of four different samples with Ti-concentration between x=0 and 0.5. Our measurement on CeScGe confirms a previous result reported in Ref. [8] that found a sizable negative thermopower at high temperatures. Indeed, our data reaches S11μV/K around 300K, a value seldom observed in magnetic Ce-compounds in this temperature range. As Ti is introduced, the room temperature absolute thermopower is strongly reduced and changes sign for x0.2, reaching S(300K)

Summary

Our results on CeSc1-xTixGe show that in this system the thermopower S(T,x) has a reduced absolute value in comparison with other Ce-based systems. This implies a minor contribution from the 4f1 state of Ce to S(T), which seems to be a consequence of a weak coupling with conduction band states. A numerical estimation of the overall crystal-field splitting, Δ300K, serves as a means to assess the validity of this scenario. Additional valuable information could come from the measurement of some

Acknowledgement

We acknowledge S. Vanrell and F. Mangussi for their assistance at an early stage of this project. This work was partially supported by Conicet through project PIP 112-2013-0100576 and SecTyP-UN Cuyo through project 06/C513.

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