JOURNAL OF APPLIED PHYSICS
VOLUME 83, NUMBER 11
1 JUNE 1998
Magnetic measurements on the III-VI diluted magnetic semiconductor Ga12 x Mnx Se T. M. Pekareka) Department of Natural Sciences, University of North Florida, Jacksonville, Florida 32224
B. C. Crooker Department of Physics, Fordham University, Bronx, New York 10458
I. Miotkowski and A. K. Ramdas Department of Physics, Purdue University, West Lafayette, Indiana 47907
We have investigated the magnetic properties of Ga12x Mnx Se, which represent a new class of diluted magnetic semiconductors based on a III-VI semiconductor. These are layered materials; however the local environment is tetrahedral as in the II-VI materials. In contrast to the II-VI semiconductors, the Mn substitutional atoms have direct bonds to three Se atoms and to either a Ga or Mn atom. This leads to a complex temperature dependent magnetization. In fields of 100 G and below, a broad peak is observed in the magnetization centered at 160 K. In addition, a sharp change in magnetization is observed at 119 K. In a field of 100 G, the peak has a magnitude of 3 31025 emu/g above the background of 731025 emu/g. With increasing magnetic fields, these features are broadened which is suggestive of some type of short-range antiferromagnetic ordering. At 5 K we observe a magnetization which increases linearly with field up to 6 T similar to the Van Vleck paramagnetic behavior observed in the Fe substituted II-VI semiconductors. © 1998 American Institute of Physics. @S0021-8979~98!44011-8#
I. INTRODUCTION
exchange between magnetic ions. In addition, both Mn– Se–Mn superexchange interactions ~common in II-VI DMS!, and Mn–Ga–Se–Mn pairings are also possible @Figs. 1~a! and 1~b!#. In this article we present magnetization measurements taken on bulk Ga12x Mnx Se. The crystal structure of Ga12x Mnx Se is analyzed in terms of several distinct configurations of the magnetic ions within the III-VI lattice structure leading to a complex magnetic behavior.
The new class of III-VI diluted magnetic semiconductors VI ~III-VI DMS! include systems of the form AIII 12x Mx B , III VI where A B is a III-VI semiconductor and M is a transition metal ion. The III-VI semiconductors GaSe,1–4 InSe,1,5,6 GaTe,7 and GaS ~Ref. 8! have received considerable interest in the last few years because they have remarkable nonlinear optical properties and are promising materials for photoelectronic applications. Although Mn has been incorporated into GaSe up to ;1 at. % nominal level ~in samples grown from the melt!, this work is focused on photoluminescence,3,9,10 photoconductivity,5,10 Hall effect,3 electron spin 11,12 resonance, and deep-level transient spectroscopy measurements.3 No magnetization studies have been reported. Like the II-VI DMS,13 substitutional magnetic ions in the III-VI DMS are in a tetrahedral environment. However, in sharp contrast to the II-VI DMS, the III-VI semiconducting host is two dimensional ~Fig. 1!. Each Ga12x Mnx Se layer is comprised of a top two-dimensional layer of Se ions, two middle layers of Ga ions, and a bottom layer of Se ions as shown in Fig. 1~a!. Within each four atom thick layer, the bonds are covalent.14 The weak van der Waals bonding between the stacked four atom thick layers further enhances the two-dimensional nature of this crystal.14 Because each Ga ion has three neighboring Se ions as well as another Ga ion, substitutional magnetic ions would experience a host of different local environments. In III-VI DMS, directly bonded Mn–Mn pairs @shown as a dotted line in Fig. 1~a!# are expected which opens a channel for a direct
II. EXPERIMENTAL DETAILS
Single-crystalline Ga12x Mnx Se samples were taken from a boule with a nominal concentration of x50.05. Typically, samples grown by the vertical Bridgman method yield actual concentrations less then the nominal value. Atomic absorption spectroscopy ~AAS! was performed yielding a value for the concentration of x50.012. It is expected that the solubility limit for Mn in GaSe is higher then the amount of Mn incorporated into this sample and that higher concentrations will be achieved as the crystal growing technology for this system develops further. Magnetic measurements were made between 1.5 and 324 K in fields up to 6 T using a commercial superconducting quantum interference device ~SQUID! magnetometer. The diamagnetic susceptibility 22.031027 emu/g G for GaSe has been subtracted from the data.15 III. EXPERIMENTAL RESULTS AND DISCUSSION
Ga12x Mnx Se exhibits magnetic behavior that is significantly different from that observed in II-VI DMS. Magnetization versus temperature measurements taken in a 0.01, 0.05, 0.1, 1, and 6 T field are shown in Fig. 2. Focusing on
a!
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FIG. 1. Crystal structure for Ga12x Mnx Se. The big solid circles are the Ga lattice sites in GaSe or the substitutional Mn lattice sites in Ga12x Mnx Se. The small squares are the Se lattice sites. ~a! Cross section of a four atom thick layer. The tetrahedral environment of the Ga or Mn lattice sites is emphasized with three neighboring Se ions as well as another Ga or Mn ion. The direct Mn–Mn bond is shown by the bold dotted line. The bold solid line emphasizes the bonds in a Mn–Se–Mn pair. A Mn–Ga–Se–Mn pair can be seen where the bold dotted line is the bond between the Ga and the first Mn ions and the bold solid lines then completes the Ga–Se–Mn bonds. ~b! Top view of a single four atom thick layer. The top and bottom Mn ions are in a vertical line as are the Se ions. The bold solid lines are the bonds in the Mn–Se–Mn pair. The pair Mn;Se–Mn ~where ; denotes a van der Waals bond between layers! would have a van der Waals bond between the Mn ~in the four atom thick layer shown! and a Se ion directly above followed by a covalent bond to a Mn ion located at position A. The second layer would then be offset to the right by one half of the hexagon width.
the 0.01 T data, the key features are a broad peak between 120 and 195 K with an apex at 160 K and a sharp change in magnetization at 119 K. With increasing field, the peak appears to broaden and shift to lower temperatures, until it is barely visible above a rising background signal by 6 T. The sharp drop at 119 K is also rapidly broadened by the magnetic field. Given the observed broadening of these features in increasing magnetic fields, the expected antiferromagnetic interactions between the magnetic ions, and the twodimensional crystal structure, our data is suggestive of shortrange two-dimensional antiferromagnetic ordering.16 The sharp drop in magnetization at 119 K could then be interpreted as a transition to three-dimensional order as the interlayer coupling becomes important. Given the low concentration of magnetic ions (x50.012) as well as the increasing low temperature magnetization discussed below, it is likely that any order is short-range. We also cannot rule out the possibility that microcrystals of MnSe exist within our samples and are producing the observed magnetization signal. We note that the Ne´el temperature of thin films of MnSe was found to be 115 K by
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FIG. 2. Magnetization vs temperature for Ga12x Mnx Se in a 0.0100, 0.0500, 0.1, 1, or 6 T field. The diamagnetic contribution due to GaSe has been subtracted. The solid vertical bar at 210 K in each graph is 3 31025 emu/g tall to aid in a quick comparison of scales.
Samarth et al.17 which is very close to the observed sharp feature at 119 K. However, Ishchenko and co-workers11,12 found that the bulk of the manganese in their GaSe:Mn crystals did go in as substitutional Mn21 ions at the gallium lattice sites and have the e-GaSe layer structure.14 The e-GaSe layer structure has a two-layer basis. The top layer is offset to the right by one half of the hexagon width @i.e., one pair of Ga/Mn lattice sites is directly above position A in Fig. 1~b!#. Lee et al. also found the e-GaSe structure for their GaSe:Mn crystals.10 Shigetomi argued from conductivity measurements that a small fraction of their Mn was located in the interstices or between the layers of GaSe.3 Turning now to the low temperature behavior of these samples, we show in Fig. 3~a! the temperature dependence of the magnetization in Ga12x Mnx Se at 1 T between 5 and 325 K. As can be seen, there is a background magnetization that monotonically increases, in addition to the broad peak discussed above. The background rises above the peak at low temperatures and is rapidly increasing. To further investigate this background we show in Fig. 3~b! data on magnetization versus field taken at 5 K, which is well below the broad peak discussed above. As can be seen the increase in magnetization is nearly linear in magnetic field with M /H56.231027 emu/g G. This is much weaker than a S55/2 Brillouin function represented in the figure as a dotted and dashed lines for x5231024 and x50.012, respectively. Further, even at 6 T there is no evidence that the magnetization is saturating. The linear field dependence is similar to that seen in Fe based II-VI semiconductors. In that case it is attributed to Van Vleck paramagnetism.18
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saturation magnetization of the directly bonded Mn with S 55/2 spins would be roughly 0.025 emu/g. The fact that this number is a factor of 700 larger then the observed peak height, might be expected if the peak arises from antiferromagnetic ordering of the Mn–Mn pairs. The remaining 99% of the Mn atoms, which are not involved in direct bonds with a second Mn atom, would then contribute to the low temperature behavior. IV. CONCLUSIONS
Mn substituted III-VI semiconductors represent a new class of diluted magnetic semiconductors. We have studied Ga12x Mnx Se and found a magnetization peak at 160 K that is not observed in the II-VI DMS. We tentatively ascribe this peak to direct Mn–Mn bonds in this material. In addition, we find a linear magnetization as a function of field at low temperatures which is reminiscent of the Fe based II-VI materials. ACKNOWLEDGMENTS FIG. 3. The magnetization data for Ga12x Mnx Se are shown by solid circles. ~a! Magnetization vs temperature in a 1 T field. ~b! Magnetization versus field at 5 K. The dotted and dashed lines are a S55/2 Brillouin response for x5231024 and x50.012, respectively. The diamagnetic contribution due to GaSe has been subtracted from the data.
The authors wish to thank R. Sensemeier for his work on the AAS measurements. This work was supported by the National Science Foundation ~NSF! under Material Research Group Grant Nos. DMR-92-21390 and DMR-94-00415. 1
Some of the magnetic behavior of the II-VI DMS has been successfully understood assuming that the transition metal ions are randomly incorporated into the host lattice. The theory was then developed taking into consideration the fraction of magnetic ions incorporated as singlets, doublets, triplets, etc., assuming only the shortest-range interactions.13 Looking within a single layer in Fig. 1 we see that if Mn substitutionally replaces Ga in this material, then the probability p of a Mn being a singlet is p5(12x). 13 There is one Mn–Mn neighbor @ p5x(12x) 12# , six Mn–Se–Mn neighbors @ p56x(12x) 18# , and six Mn–Ga–Se–Mn neighbors @ p56x(12x) 18# ~note that there are two equivalent exchange channels between the two Mn ions!. If we also consider interlayer couplings, with one Mn ion in each layer there are three Mn;Se–Mn neighbors, three Mn–Ga;Se–Mn neighbors, and three Mn;Se–Ga–Mn neighbors ~note that ; denotes a van der Waals bond between layers!. For a concentration of x50.012 this leads to 1% of the Mn atoms having direct bonds with a second Mn atom. We expect this interaction to be much stronger than the superexchange interactions and would tentatively ascribe the peak at 160 K to these pairs and larger clusters. We note that the
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