Anomalous exchange mechanism in Gd monopnictides

ELSEVIER

Physica B 230-232 (1997)472-474

Anomalous exchange mechanism in Gd monopnictides T. K a s u y a a'*, D . X . L i b aPhysics Department, Tohoku University, Aoba-Ku, Sendai 980-77, Japan b Oarai branch, Material Research Institute, Tohoku University, OaraL IbaraM 311-13, Japan

Abstract

Nearest and next nearest neighbor exchange constants 11 and I z were obtained for Gd monopnictides GdXp. Even though relative energy levels are different substantially, lattice constant dependences, as well as the absolute values, are nearly equal to those in Eu chalcogenides EuXo In particular, strong ferromagnetic 11 in GdN similar to EuO was explained by the process involving 4f8 configuration. On the other hand, contrary to electron doped EuXc, no ferromagnetic effect is observed in GdXp due to semimetallic carriers. This is also seen as a weak spin disorder scattering. This anomalous behavior was explained as cancellation of intra and intersite d-f scatterings characteristic to GdXp. Keywords: Gd monopnictides; Ferromagnetic interaction; Mixing mechanism; d-f exchange interaction

Both Eu monochalcogenides, EuXc, and Gd monopnictides, GdXp, crystallize NaC1 structure with essentially the same band structure, that is, the bottom of the conduction bands is at each X-point of Brillouin zone formed mainly by tzg type of 5d(R), R representing a rare-earth atom, and the top of the valence bands is at the/'-point, formed mainly by p(Xc or Xp) splitting i n t o / ' s quartet and /'6 doublet by the spin-orbit interaction [1]. However, in EuXc, where Eu is divalent with 4f 7 configuration, band gaps of several eV open between the conduction and valence bands making EuXc insulators of typical ionic character. The occupied 4f levels are in the gap and the unoccupied 4f levels are more than 12eV above the Fermi energy EF [2]. In GdXp, where Gd is trivalent with the 4f 7 configuration, the conduction and valence bands overlap slightly forming typical compensated lowcarrier-density semimetals with equal numbers of

* Corresponding author.

electron and hole carriers, n = ne = nh, of a few percent per Gd. This is a common feature in trivalent RXp [1]. The occupied 4f levels in GdXp are 8-10 eV below Fermi energy EF, and the unoccupied 4f levels are 5-6 eV above EF [3]. EuXc were the first rare-earth compounds received extensive theoretical and experimental studies on high-quality samples because of their interesting novel properties. Strong ferromagnetism in EuO, and its rapid weakening with increasing lattice constant a are typical examples. It was shown by Kasuya that all the anomalies are related with a common key concept, magnetic polaron [4], and the strong ferromagnetic nearest neighbor (NN) coupling constant 11 in EuO was explained as follows [5]. Through f~l mixing, an occupied 4f state is transferred to a 5d state on a N N Eu site, at which the 5d electron polarizes the occupied 4f spins ferromagnetically parallel to the 4f spins at the center site Eu through strong intraatomic d-f exchange interaction. Intraatomic d-f exchange constant/de defined by - 2IafSfSd is

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72 Kasuya, D.X. Li / Physica B 230 232 (1997) 472-474

obtained from the atomic optical spectra to be about 0.1 eV [5]. Strong lattice constant a dependence of 11 shown in Fig. 1 is explained well by strong a dependence of the d-f mixing. On the contrary, p-f mixing is insensible to a. Indeed, the next NN exchange constant 12 is considered to be due to conventional super exchange mechanism through the fourth-order of p-f mixing and thus 12 depends on a weakly. In GdXp, as shown in Fig. 1, their lattice constants are similar to those of EuXc and thus the d-f mixings of corresponding GdXp and EuXc should be similar. On the other hand, the excitation energy Ufp is different substantially, estimated to be 4 eV in EuXc and 10 eV in GdXp. Because 11 is proportional to Uf~2, I1 in GdXp is estimated to be at least a factor five smaller than that in EuXc. Recently, detailed studies have been performed by Li [6] on high-quality GdXp samples and the values for I1 and I2 are estimated as shown in Fig. 1 from the paramagnetic Curie constant 0p and the critical magnetic field Hc, at which the 4f moments align perfectly with the full moment of 7ktB. Note that G d N orders ferromagnetically at a Curie temperature of 58 K, while the value of 0p is 81 K. All other GdXp order antiferromagnetically at Nbel temperatures TN as shown also in Fig. 1. It is clearly seen that the antiferromagnetic values of

I

I

3O

0.5 × A

A 0

•

o

20~

~

o

X A

-0,5

lo N

0

L[ T~

P

ir 3

AsS

I~1

11 /1"~

Se Sb BI

6

alA)

Fig. 1. Nearest and next nearest neighbor exchange constants 11 and 12 are shown as functions of lattice constant a by • and o, respectively, for Eu monochalcogenides, while by • and A respectively, for Gd monopnictides. N6el temperature TN for Gd monopnictides are shown by × in the scale shown in the right-hand side.

473

12 are much larger in GdXp than in EuXc, corresponding to the fact that the excitation energy U p f t o create a 4f s configuration is much smaller in GdXp than in EuXc. On the contrary, the behavior in It is completely against our expectation described above. I1 in GdXp and EuXc are nearly on a common curve showing the same strong a dependence. It was already known that some GdN samples show large positive values of 0p, even though clear ferromagnetic ordering was not observed due to defects, and the ferromagnetic behavior in GdN was attributed to the RKKY interaction [7, 8]. Indeed, in EuXc, when a few percents per Eu of electron carriers are induced by substituting trivalent R for Eu, the value of 0p increases in common up to 30 K [7, 9] due to characteristic property of RKKY interaction in low-carrier-density systems. Because the similar amount of electron carriers exist in GdXp in common, we rather expect fairly strong ferromagnetism in common in GdXp. In the present detailed studies, however, such a common mechanism for ferromagnetism is shown clearly to be not applicable in GdXp. This is another puzzle and should be solved to be consistent with the strong I1 in GdN. In this situation, it is necessary to check other processes to give ferromagnetic coupling for I1 in which 4f 8 configurations are involved. Because the 5d bands are not occupied, these processes are of higher orders than those with 4f6 configurations. Therefore, I1 due to those processes was estimated to be one order of magnitude smaller for EuO [5]. These processes are, however, important in GdN due to following reasons. First of all, because the unoccupied 4f states are much more extended than the occupied ones in particular for the 4f7 configuration, both d-f and p-f mixings increase substantially. Secondly, the excitation energy for 4f8 is much lower in GdXp than in EuXc. Finally, the lowest-order process, p(Xp) -~ 4f ~ ~ 5d(NNGd) df exchange ~ p(Xp) , includes a low excitation energy Upd in GdXp due to the semiconducting character. Because complex mixing processes are involved, it is difficult to evaluate the value of I accurately. Therefore, other independent evidences to conform the p-f mixing mechanisms are required. The following evidence is clearly one of them.

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T. Kasuya, D.X. Li / Physica B 230-232 (1997) 472-474

The second puzzle is related clearly to anomaly near EF and thus related to anomalous magnetic scattering. Indeed, the spin-disorder magnetic resistivity in GdXp is observed to be anomalously weak, in particular in G d S b and GdBi [6]. Because of different symmetries at three X-points, the intrasubband scattering is the main contribution and then the intrasubband c - f scattering constant/eft is evaluated to be about 0.008 eV for GdSb, more than one order of magnitude smaller than/de mentioned before. This small value of coupling constant is explained well by cancellation of the ferromagnetic d - f exchange scattering and the antiferromagnetic p - f scattering as the second-order process of p - f mixing because they have opposite signs. This kind of cancellation does not occur in metals with large q scattering because of different q dependence in those two scatterings, but exists in low-carrierdensity systems with small q scatterings. In metals, those two scattering mechanisms give approximately independent additive resistivities. The semimetallic character is also essentially important because, as the p and d bands are nearly degenerated, even a small deviation of q vector from the symmetry point such as F and X points causes

a strong p - d mixing. The present study is the first case to show the cancellation clearly. The details will be published in a separate paper [10].

References [1] For example, see a recent review paper of, T. Kasuya, Physica B 215 (1995) 88. [2] For a reviewof EuXc, See T. Kasuya, CRC Critical review in Solid State Sciences 3 (1972) 131. [3] H. Yamada, T. Fukawa, T. Muro, Y. Tanaka, S. Imada, S. Suga, D.X. Li and T. Suzuki, J. Phys. Soc. Japan 65 (1996) 1000. [4] For a recent brief review, T. Kasuya, J. Appl. Phys. 77 (1995) 320O. [5] T. Kasuya, IBM J. Res. Devel. 14 (1970) 214. [6] D.X. Li, Dr, PhD Thesis, Tohoku University (1995). [7] A. Narita and T. Kasuya, J. Magn. Magn. Mater. 52 (1985) 373. For details, A. Narita and T. Kasuya, in: Extended Abstracts for US-Japan Seminar in Sendal (1977) p. 93. I-8] P. Wachter and E. Kaldis, Solid State Commun. 34 (1980) 241. [9] For example, F. Holtzberg, T.R. McGuire, S. Methfessel and J.C. Suits, Phys. Rev. Lett. 13 (1964) 18. [10] T. Kasuya and D.X. Li, J. Magn. Magn. Mater., to be published.