Distant-Neighbor Exchange Constants in Dilute Magnetic Semiconductors

Abstract

Several exchange constants J i between Mn 21 ions which are not nearest neighbors were determined in Zn 12x Mn x X (X S, Se, Te) from magnetization steps at 20 mK. When the J i 's are listed in order of decreasing size, ratios between successive J i 's are material dependent, and differ from all predictions. The measured J i 's were identified by comparing the magnetization curves with simulations which assumed a random Mn distribution. Contrary to existing theories the second-largest exchange constant is not J 2 between next-nearest neighbors. The most likely alternative is J 4 , between fourth neighbors. [S0031-9007 (98)06413-8] PACS numbers: 75.30.Et, 75.50.Ee, 75.50.Pp, 75.60.EjThe distance dependence of the d-d exchange constants J i in dilute magnetic semiconductors (DMS's) has been discussed for more than a decade [1][2][3][4][5][6][7][8][9]. The focus has been on Mn-based II-VI DMS's with the zinc-blende structure. It has been established that the largest J i is the nearest-neighbor (NN) exchange constant J 1 . This J 1 is antiferromagnetic (AF), and is of order 210 K [5,6]. It is generally accepted that the second-neighbor (next-nearestneighbor) exchange constant J 2 , third-neighbor constant J 3 , etc., are all AF. What is at issue are the ratios J 1 : J 2 : J 3 : J 4 , etc.All existing theories, conjectures, and reported data as interpreted by their authors maintain that J 2 is the secondlargest exchange constant, after J 1 . The theory of Larson et al. [1] predicts that J 2 : J 1 , J 3 : J 2 , and J 4 : J 3 are all about 0.08. In the modified version by Rusin [9], J 2 : J 1 Х 0.08, and both J 3 and J 4 are less than 0.1J 2 . According to Bruno and Lascaray (BL), J 3 : J 2 J 4 : J 3 1͞2 (no prediction for J 2 : J 1 ) [4]. A power law dependence of J i on distance, J i~r

Effect of magnetic dilution in Zn1−xMnxGa2Se4 (0<x<0.5)

“…This global magnetic constant is about one order of magnitude higher than that reported for MnGa 2 Se 4 ͓͚͑z 0,i J i / k B ͒ =−3.9±2 K͔ 15 for which the magnetic superexchange pathways are of the type Mn-Se¯Se-Mn with antiferromagnetic constants of around J / k B = 0.1-0.5 K. However Mn-Se-Mn contacts must be also considered in the case of Zn 1−x Mn x Ga 2 Se 4 with 0 Ͻ x Ͻ 0.5. This is a magnetic pathway reported for zinc-blende structured Zn 1−x Mn x Se, which exhibits a related crystal structure, 22 with a corresponding superexchange constant of −12.2 K. 26 Therefore, the existence of higher magnetic interactions associated with shorter magnetic pathways is shown to play a significant role in the Zn 1−x Mn x Ga 2 Se 4 series when compared to MnGa 2 Se 4 .…”

“…Because only the order of magnitude of the superexchange constants for MnGa 2 Se 4 is known, 15 we have used the data published for zinc-blende Zn 1−x Mn x Se. 26 The exchange pathways are expected to be equivalent due to the similarity of the crystal structures. Those data for Zn 1−x Mn x Se provide J 1 / k B , the exchange constant corresponding to Mn-Se-Mn interactions ͑Mn-Mn distance= a / ͱ 2 Å, where a is the cubic unit-cell parameter͒, and three superexchange constants between next-nearest neighbors involving Mn-Se-Se-Mn-type contacts: One interesting aspect of the series under study is that the lattice sites available to the magnetic cations are not the same in MnGa 2 Se 4 ͑2d͒ as in Zn 1−x Mn x Ga 2 Se 4 with 0 Ͻ x Ͻ 0.5 ͑4d͒.…”

Nearest‐neighbour antiferromagnetic interaction as a limiting factor for critical temperature in a model DMS system

“…[6]). However, the same experimental results indicate that the AF interaction between Mn impurities in this DMS is not limited to NN ions.…”

Magnetization steps inZn1−xMnxO:Four largest exchange const

“…However, later theories, 11,12 and recent experiments, 5,20 indicate that there is no simple correspondence between size and distance.…”

“…2,5 Separate simulations are carried out for all competing possibilities for the DN classes. The six DN classes listed in Table I lead to 6ϫ5ϭ30 possibilities for the two DN classes associated with J (2) and J (3) .…”