Electron paramagnetic resonance and electron-nuclear double resonance study of Mn2+ ions in CdGeAs2 crystals

phys. stat. sol. (b), 243, No. 15, 4070–4079 (2006) / DOI 10.1002/pssb.200541162

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electron paramagnetic resonance and electron-nuclear

double resonance study of Mn2+ ions in CdGeAs2 crystals

S. M. Evans1, N. Y. Garces**, 1, R. C. DeMattei2, R. S. Feigelson2, K. T. Zawilski3,

N. C. Giles1, and L. E. Halliburton*, 1

1 Department of Physics, West Virginia University, Morgantown, WV 26506, USA

2 Center for Materials Research, Stanford University, Stanford CA 94305, USA

3 BAE Systems, Nashua, NH 03061, USA

Received 18 July 2005, revised 4 July 2006, accepted 2 August 2006

Published online 12 September 2006

PACS 61.72.Ji, 61.72.Vv, 76.30.Fc, 76.70.Dx

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used

to characterize Mn2+ (3d5) ions in CdGeAs2 crystals grown by the horizontal Bridgman method. These sam-

ples were p-type with room-temperature hole concentrations ranging from 2 × 1015 cm−3 to 5 × 1016 cm−3.

The Mn2+ ions substitute for the Cd2+ ions in this tetragonal chalcopyrite lattice, and form “deep” isoelec-

tronic centers. Their g values and crystal-field parameters are g║ = 2.0025, g⊥ = 2.0011, D = 118.8 MHz,

a + 2/3F = 18.4 MHz, and a = −0.3 MHz. The hyperfine and nuclear electric quadrupole parameters for the 55Mn nucleus are A║ = −154.94 MHz, A

⊥ = −155.69 MHz, and P = 0.622 MHz. Hyperfine parameters for

one set of nearby Cd nuclei and two sets of nearby As nuclei were also determined using ENDOR. No

evidence was found in these doped bulk crystals to suggest that Mn4+ ions substitute for Ge4+ ions.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Cadmium germanium arsenide (CdGeAs2) is a direct-band-gap semiconductor [1, 2] that is easily grown as large single crystals [3–5]. The unique optical properties of this material (i.e., a large nonlinear optical coefficient and suitable birefringence) make it a reasonable choice for mid-infrared frequency conversion devices. With a room-temperature transparency extending from near 2.3 to beyond 17 µm, the optical applications of CdGeAs2 include doubling CO2 lasers [6] and producing mid-infrared tunable laser output [7, 8] via difference-frequency generation or optical parametric oscillation. Recently, CdGeAs2 crystals heavily doped with Mn have been shown to exhibit ferromagnetic behavior at room temperature [9–14], thus making this narrow-gap semiconductor a viable candidate for future spintronic applications. The electrical and optical properties of CdGeAs2 are strongly affected by the relative concentrations of donors and acceptors unintentionally introduced during growth. Undoped crystals are usually p-type, but it is easy to produce n-type material by doping with donor impurities such as indium [15]. Photolumines-cence and optical absorption results show that most bulk CdGeAs2 crystals contain two distinct acceptors (both believed to be intrinsic rather than impurity in origin) and at least one donor [16–20]. Theoretical studies have provided guidance as to the identities of these defects [21, 22]. Electron paramagnetic reso-nance (EPR) has been used to characterize the dominant acceptor in CdGeAs2 [23]. A separate EPR study has shown that both Cr2+ and Cr4+ ions can be present in CdGeAs2, where they occupy the Cd2+ and Ge4+ sites, respectively [24].

* Corresponding author: e-mail: [email protected], Phone: 304-293-3422, Fax: 304-293-5732

** Present address: Naval Research Laboratory, Washington, DC 20375, USA

phys. stat. sol. (b) 243, No. 15 (2006) 4071

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In the present paper, we describe the results of an EPR and ENDOR study of Mn2+ (3d5) ions substitut-ing for Cd2+ ions in CdGeAs2 crystals. In semiconductor notation, this is the Mn0 center. The g-values and the crystal-field parameters are determined from EPR spectra, while the 55Mn hyperfine parameters and the 55Mn nuclear electric quadrupole parameter are determined from ENDOR spectra. Hyperfine parameters with one set of 111Cd and 113Cd neighbors and two sets of 75As neighbors are also determined from lower frequency ENDOR spectra. Although the manganese impurities could be expected to occupy both the Cd2+ and Ge4+ sites in CdGeAs2 (as Mn2+ and Mn4+, respectively), we find that these dopant ions prefer to be in the Mn2+ charge state and occupy only the Cd2+ sites. Previous studies of Mn2+ ions in chalcopyrites have focused on ZnGeP2, CdGeP2, CdSiP2, ZnSiP2, and ZnSiAs2 [25–28]. Recently, an ENDOR study of Mn2+ ions substituting for Zn2+ in ZnGeP2 crystals provided 31P hyperfine parameters for three sets of neighboring phosphorus ions [29].

2 Crystal structure

The crystal structure of CdGeAs2 is described by space group I42d and point group 42m. In general, the II–IV–V2 chalcopyrites are analogous to the widely studied III–V materials GaP and GaAs, except that the chalcopyrite structure is tetragonally distorted from the simpler zinc-blende structure of the III–V compounds. The unique axis in CdGeAs2 is labelled the [001] direction (or c-axis) and the two equiva-lent axes in the basal plane are labelled the [100] and [010] directions (or a-axes). Room temperature values for the lattice parameters are a = 5.941 Å and c = 11.210 Å [30]. The resulting c/a ratio is 1.887, which indicates that there is a significant compression of the lattice along the c axis. Ion positions can be generated using the following minimum set of (x, y, z) coordinates expressed in units of the a and c con-stants. Group II: (0, 0, 0), (0, 1/2, 1/4) Group IV: (0, 0, 1/2), (0, 1/2, 3/4) Group V: (u, 1/4, 1/8), (−u, 3/4, 1/8), (3/4, u, 7/8), (1/4, −u, 7/8) The parameter u has a room-temperature value of 0.2799 in CdGeAs2 [30]. It is a measure (when compared to 1/4) of the relative rotations of the arsenic tetrahedra about the cadmium and germanium sites. In the regular CdGeAs2 lattice, the “twist” angle that describes these small, and opposite, rotations of the two CdAs4 units about the [001] axis is 3.2°. This shifting in position of the arsenic ions occurs because the cadmium and germanium ions have different covalent radii and, thus, different bond lengths with the neighboring arsenic ions (i.e., the Cd–As separation is greater than the Ge–As separation). These clockwise and counterclockwise rotations of the arsenic ions about the cations result in two crys-tallographically equivalent, but magnetically inequivalent, sites for the Mn2+ ions that replace Cd2+ ions.

3 Experimental details

The CdGeAs2 crystals used in the present investigation were taken from a boule grown by the horizontal Bridgman method at Stanford University. Approximately 500 ppm (by weight) of elemental manganese was added to the starting material. The concentration of Mn2+ ions in the boule was estimated from the intensity of the EPR signal to vary from 2 × 1019 cm−3 in the first-to-freeze region to 9 × 1019 cm−3 in the last-to-freeze region. Small a-plate samples, oriented using back-reflection Laue X-ray diffraction, were cut from the boule. Absorption data were taken at room temperature using a Fourier-transform infrared (FTIR) spectrometer (ThermoNicolet Nexus 870). The EPR and ENDOR data were taken near 9.492 GHz using a Bruker ELEXSYS spectrometer. An Oxford Instruments ESR-900 helium-gas-flow system was used to control the sample temperature. The EPR and ENDOR measurements were made in a Bruker cylindrical TE011 ENDOR cavity (with an internal helix coil mounted on the cryogenic glass-ware). The ENDOR spectrometer normally provides rf output between 0.5 and 200 MHz. We extended the upper end of this range to 400 MHz by placing a frequency doubler (from Mini-Circuits) imme-diately after the rf synthesizer. Values of the static magnetic field were obtained with a proton gauss-meter.

4072 S. M. Evans et al.: EPR and ENDOR study of Mn2+ ions in CdGeAs2 crystals

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

Magnetic Field (Gauss)

3000 3200 3400 3600 3800

Fig. 1 EPR spectrum of Mn2+ in CdGeAs2. Data taken at 30 K with the magnetic field along the [001]

direction.

4 EPR and 55Mn ENDOR results

Figure 1 shows the EPR spectrum of the Mn2+ ions in CdGeAs2 when the magnetic field is parallel to the [001] direction in the crystal (this sample was cut from the first-to-freeze region of the boule). Figure 2 shows the Mn2+ spectrum from the same sample when the magnetic field is parallel to the [100] direction in the crystal. Both of these spectra were taken at 30 K. There are five sets of six lines in each spectrum (indicated by the “stick” diagrams above the data), as expected for an S = 5/2 electron spin interacting with the 100% abundant I = 5/2 nuclear spin of the 55Mn nucleus. This sample was p-type, as demon-strated by the presence of the intravalence-band absorption feature peaking near 5.5 µm at room tempera-ture [19]. A previous study [20] of CdGeAs2 has established a relationship between the intensity of this 5.5 µm absorption band and the hole concentration obtained from Hall measurements. We measured the intensity of the 5.5 µm absorption band in the sample used for Figs. 1 and 2 and determined, with the aid

Magnetic Field (Gauss)3000 3200 3400 3600 3800

Fig. 2 EPR spectrum of Mn2+ in CdGeAs2. Data taken at 30 K with the magnetic field along the [100]

direction.

phys. stat. sol. (b) 243, No. 15 (2006) 4073


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of this recently established relationship [20], that the room-temperature hole concentration in this par-ticular sample was approximately 2 × 1015 cm−3. For perspective, the room-temperature intrinsic carrier concentration of CdGeAs2 is approximately 2 × 1013 cm−3. Similar absorption measurements on other sam-ples cut from our CdGeAs2 boule showed that the hole concentration was not uniform. Of the nine smaller samples extracted from the boule, the lowest room-temperature hole concentration was 2 × 1015 cm−3 and the highest was 5 × 1016 cm−3. Variations of this order often occur in undoped CdGeAs2 boules, and thus we do not attribute this effect in our samples to the presence of Mn2+ ions. The following tetragonal spin-Hamiltonian describes the spectra from the Mn2+ ions in CdGeAs2 [31]:

0 4 0 0 0 0

4 4 4 2 2 4 4

Mn Mn Mn Mn N N Mn Mn

( 5 )

( ) .

H B O O B O B O

g

β

β

◊ ◊ +

◊ ◊ ◊ ◊ ◊

= + + +

+ + -

S g H

I A S I Q I H I (1)

Electron and nuclear Zeeman terms, second and fourth order crystal field terms, and hyperfine and quad-rupole interactions for the 55Mn nucleus are included. The spin operators are given below:

0 2

2= 3 ( +1) ,

zO S S S- (2)

0 4 2 2 2 2

435 30 ( 1) 25 6 ( 1) 3 ( 1) ,

z z zO S S S S S S S S S= - + + - + + + (3)

4 4 41

4 2( ) .O S S

+ -= + (4)

To analyze the EPR and ENDOR data, the spin-Hamiltonian was separately written in the coordinate systems that represent the local “cubic” axes of the two crystallographically equivalent Mn2+ sites (i.e., x, y, and z were parallel to the axes of the slightly twisted MnAs4 tetrahedra with z along the [001] axis of the crystal and x and y deviating from the [100] and [010] axes of the crystal by the small twist angle). Energy eigenvalues were then calculated by exact diagonalization of the 36 × 36 spin-Hamiltonian matrices. The g-values, crystal field parameters, and 55Mn hyperfine parameters were determined by a least-squares fitting procedure using data taken with the magnetic field along the [100], [110], and [001] directions. Final g-values and crystal field parameters are given in Table 1. The 55Mn hyperfine parame-ters obtained from the EPR data are not listed in the table because more precise values were subsequently acquired from the ENDOR data. We use a, D, and F instead of the q

kB parameters, where a = 1204

B , D = 3 0

2B , and F = 180 0

4B . Negative signs are assigned to the 55Mn hyperfine parameters [28, 31]. This

then fixes the signs of the crystal field parameters. We provide the value of the combined quantity a + 2

3 F in Table 1 (rather than F) because it can be more precisely determined than either a or F sepa-rately. The [100] and [110] EPR spectra appear very similar, and this requires the parameter a to be small in magnitude. We note that the positive sign we obtain for D in CdGeAs2 is consistent with earlier results for Mn2+ in the chalcopyrite structure. The sign of D for Mn2+ is positive in CdSiP2 and CdGeP2, but is negative in ZnSiP2, ZnGeP2, and ZnSiAs2 [25, 26]. The trend is a positive sign for D when the Mn2+ replaces a Cd2+ and negative sign when the Mn2+ replaces a Zn2+. This may be a result of the Mn2+ ion having a smaller radius than the Cd2+ ion, but a larger radius than the Zn2+ ion.

Table 1 Spin-Hamiltonian parameters for Mn2+ ions in CdGeAs

2.

parameter +from EPR −from 55Mn ENDOR

g║ +2.0025 (±0.0005) g⊥ +2.0011 (±0.0005)

D +118.8 (±0.5) MHz a + 2

3 F +18.4 (±0.5) MHz a −0.3 (±0.2) MHz A║ −154.94 (±0.04) MHz A

⊥ −155.69 (±0.04) MHz

P +0.622 (±0.004) MHz



We were unable to use EPR to determine a value for the twist angle that describes the small, and op-posite, rotations of the two MnAs4 units about the [001] axis. The value we obtained for the a parameter (see Table 1) is too small to produce a measurable splitting when the magnetic field is rotated from the [100] to the [110] direction [26, 32]. In the fitting process for the spin-Hamiltonian parameters that was described in the preceding paragraph, we used a value of 3.2° for this angle (which corresponds to the twist angle of the CdAs4 units in the regular CdGeAs2 lattice). The angle for the MnAs4 units is expected to be different because of lattice relaxation. Since the radius of the Mn2+ ion [33] is slightly smaller than that of the Cd2+ ion being replaced, the twist angle for the MnAs4 units should not exceed the regular lattice value. Figure 3 shows 55Mn ENDOR lines associated with the MS = +1/2 manifold. These data were taken at 25 K with the magnetic field along the [100] direction. We monitored two EPR transitions to observe all five of the ENDOR lines associated with this manifold. The magnetic field was set at 3302 G for trace (a) in Fig. 3 (this corresponds to the next-to-the-lowest line in the center set of EPR lines in Fig. 2) and was set at 3470 G for trace (b) in Fig. 3 (this corresponds to the next-to-the-highest line in the center set of EPR lines). ENDOR data were also taken from other MS manifolds. For the [001] orientation of mag-netic field, the MS = +5/2 and MS = −5/2 manifolds give ENDOR lines between 389 and 399 MHz and between 359 and 402 MHz, respectively, the MS = +3/2 and MS = −3/2 manifolds give lines between 242 and 246 MHz and between 206 and 234 MHz, respectively, and the MS = +1/2 and MS = −1/2 manifolds give lines between 88 and 95 MHz and between 55 and 71 MHz, respectively. We have observed all of these 55Mn ENDOR lines for the [001] direction of the magnetic field (except for the highest line from the MS = −5/2 manifold which was above the 400 MHz limit of our spectrometer). Although 55Mn EN-DOR lines were obtained from all the manifolds for the [001] and [100] directions, final values for the 55Mn parameters were determined using only ENDOR data from the MS = +1/2 and MS = −1/2 mani-folds. The 36 × 36 matrix form of the spin Hamiltonian was used in this fitting, and the results are given in Table 1. We use the parameter P to describe the nuclear quadrupole interaction (i.e., Q

xx = −P,

Qyy

= −P, and Qzz = 2P). The parameter P is defined as e2qQ/[4I(2I − 1)], where eq is the electric field

gradient and Q is the nuclear quadrupole moment. Experimentally, the sign of P is found to be opposite to the sign of A║ and A

⊥. This makes P positive since we have assigned negative signs to A║ and A

⊥.

The magnitudes of A║ and A⊥ given in Table 1 for the 55Mn nucleus are smaller than the values often

reported for Mn2+ ions in ionic crystals and in II–VI semiconductors. It was established early in the study of 55Mn hyperfine interactions that the origin of the large Fermi contact parameters was primarily core polarization of the 1s, 2s, 3s, and possibly the 4s shells by the half-filled 3d shell [34–36]. The combined contribution of the 1s, 2s, and 3s shells is predicted to be −304 MHz, and the contribution

Frequency (MHz)84 88 92 96 100

(a)

(b)

Fig. 3 55Mn ENDOR lines taken with the magnetic field

along the [100] direction. The splittings are a result of the

nuclear electric quadrupole interaction. Trace (a) is from

the MS = −1/2, m

I = −3/2 to M

S = +1/2, m

I = −3/2 EPR

transition. Trace (b) is from the MS = −1/2, m

I = +3/2 to

MS = +1/2, m

I = +3/2 EPR transition.

phys. stat. sol. (b) 243, No. 15 (2006) 4075


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from the 4s shell is predicted to be positive and smaller in magnitude [35]. Covalency leads to a partial filling of the 4s orbital, and causes a decrease in the observed magnitudes of A║ and A

⊥. Our hyperfine

values are very similar to the Mn2+ values found in GaAs and GaP [37, 38], thus verifying that the cova-lency in CdGeAs2 is comparable to the covalency in GaAs and GaP.

5 Cd and As neighbor ENDOR results

Figure 4 shows an ENDOR spectrum, from the MS = +3/2 manifold, that contains lines from nearby Cd and As neighbors. These data were taken at 25 K with a magnetic field of 3453.6 G applied parallel to the [001] direction (this field corresponds to the EPR transition from the MS = +1/2, mI = +5/2 level to the MS = +3/2, mI = +5/2 level). The ENDOR lines at 13.37 and 13.99 MHz in Fig. 4 are assigned to 111Cd and 113Cd nuclei, respectively. Both of these isotopes have I = 1/2 and their abundances are similar (12.8% for 111Cd and 12.2% for 113Cd). In this lattice, the nearest-neighbor shell of cations around one Mn2+ site contains four cadmium and eight germanium ions. No ENDOR signals attributable to 73Ge nuclei (I = 9/2, 7.8% abundant) were observed in our spectra. The four cadmium neighbors are crystal-lographically equivalent and are located 4.084 Å from the Mn2+ ion, assuming an undistorted CdGeAs2 lattice. Figure 5 shows the angular dependence of the 111Cd ENDOR lines for rotations in the three high-symmetry planes. Initially, data were taken for two rotations, [001] to [100] and [100] to [110]. This gave two sets of possible hyperfine parameters, so additional data were taken while rotating from [001] to [110] to determine the final correct set of parameters. Identifiable data points were not easily obtained in the middle of two of these planes, where a strong overlap of EPR lines resulted in the simultaneous presence of ENDOR lines from several MS manifolds. The 111Cd angular dependence data shown in Fig. 5 were analyzed by adding an I · A · S term to the spin Hamiltonian in Eq. (1) and then repeatedly diagonalizing the resulting 72 × 72 matrix to obtain a best fit. Eight magnetically equivalent Cd sites are considered, four for each Mn2+ site. Final values for the 111Cd hyperfine parameters are given in Table 2. Directions of the principal axes are specified by the polar angle θ measured relative to the [001] direction and the azimuthal angle φ measured relative to the [100] direction in the basal plane. Signs of the principal values were not determined experimentally; negative signs are arbitrarily assigned to the 111Cd principal values because the nuclear magnetic mo-ments of Cd are negative. Angular dependence data were also taken for the 113Cd neighboring nuclei. As expected, the principal directions are the same, while the principal values scale with the ratio of magnetic

Frequency (MHz)

10 11 12 13 14 15 16

75As

75As

111Cd

113Cd

Fig. 4 ENDOR spectrum of Cd and As ions near Mn2+ in CdGeAs2. These data were taken with

the magnetic field along the [001] direction, while setting on the MS = +1/2, m

I = +5/2 to M

S = +3/2,

mI = +5/2 EPR transition.



Angle (degrees)

EN

DO

RF

requ

ency

(MH

z)

13

14

15

[110]15 3030 60 [100][001] 30 60 [001]

Fig. 5 Angular dependence of the ENDOR lines from neighboring 111Cd nuclei. Discrete points are data

taken from the MS = +3/2 manifold of the Mn2+ ion. Solid lines were generated using the 111Cd parameters

in Table 2.

moments for the two isotopes. We note that the principal-axis directions given in Table 2 correspond to only one of the eight sites for the 111Cd nuclei (i.e., the principal-axis directions are different for each site and reflect the symmetry elements of the CdGeAs2 lattice). Expressions given in Ref. [39] can be used to generate the (θ, φ) pairs of angles needed to describe the directions of the principal axes at the other seven sites. The principal values of the 111Cd hyperfine matrix can be separated into isotropic and anisotropic parts using A = aI + B, where A1 = a – b + b′, A2 = a – b – b′, and A3 = a + 2b. This gives a = −11.10 MHz, b = −0.54 MHz, and b′ = 0.11 MHz. Contributions to this hyperfine come from the 4d and 5s orbitals, and possibly the 5p orbitals, on the Cd ion [40]. The 111Cd hyperfine matrix deviates measurably from axial symmetry, as indicated by the small but not zero value of b′. A unique principal axis, however, can still be identified, associated with the −12.19 MHz principal value. The nearest neighbor Cd ions are “connected” to the Mn2+ ion by an intervening As ion. Crystallographically, for one Cd site in the undis-torted lattice (out of the eight equivalent sites, four each for the two Mn2+ sites), the (θ, φ) pair describing the Cd-to-Mn direction are 133.3° and 0°, and the (θ, φ) pair describing the Cd-to-As direction are

Table 2 Hyperfine parameters for Cd and As neighbours of the Mn2+ ion in CdGeAs2. These principal-

axis directions for each interaction correspond to one arbitrary nucleus site among eight symmetry-related

sites. Estimated uncertainties in the principal values and angles are ±0.08 MHz and ±1.0°, respectively.

parameter −principal value θ φ

111Cd ENDOR A1 −10.45 MHz 142.1° 101.5° A2 −10.67 MHz 71.1° 37.5° A3 −12.19 MHz 121.5° 319.6° 75As ENDOR (set A) A1 4.45 MHz 125.5° 123.9° A2 8.38 MHz 59.2° 59.0° A3 12.94 MHz 129.2° 358.2° 75As ENDOR (set B) A1 4.43 MHz – – A2 4.43 MHz – – A3 15.88 MHz 125.6° 340.0°

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Angle (degrees)

EN

DO

RF

requ

ency

(MH

z)

3

6

9

12

15

18

[110]15 3030 60 [100][001] 30 60 [001]

Fig. 6 Angular dependence of the ENDOR lines from the 75As (Set A) nuclei. Discrete points are data

taken from the MS = +3/2 manifold of the Mn2+ ion. Solid lines were generated using the 75As (Set A)

parameters in Table 2.

122.1° and 311.8°. Experimentally, we find that the (θ, φ) pair for the unique principal axis of the 111Cd hyperfine matrix are 121.5° and 319.6°, a direction that lies close to the Cd-to-As direction with a slight deviation toward the Cd-to-Mn direction. In earlier studies of Mn2+ ions in GaP and GaAs, similar behav-iors were observed for the nearest shell of Ga neighbors [41, 42]. In those materials, the unique principal-axis direction of the Ga hyperfine pointed more toward the neighboring P or As than the Mn2+ ion. These earlier results and the present results verify the importance of the strong tetrahedral bonds in determining the principal-axis directions. Figure 4 shows ENDOR signals from 75As nuclei (I = 3/2, 100% abundant), at 12.08 and 12.83 MHz. These lines are assigned to two sets of 75As neighbors (henceforth referred to as Set A and Set B, respec-tively). Although 75As has a nuclear electric quadrupole moment, we were unable to identify any arsenic-related quadrupole splittings in our ENDOR spectra. These quadrupole-split lines may be broadened beyond recognition by random strain in the crystal, or the splittings may simply be small and unresolved. The angular dependence exhibited by 75As (Set A) is shown in Fig. 6 and the angular dependence exhib-ited by 75As (Set B) is shown in Fig. 7. When analyzing these data, we treated 75As as an I = 1/2 nucleus and expressed the spin Hamiltonian as a 72 × 72 matrix (i.e., possible quadrupole effects were ignored). When fitting each set of 75As angular dependence data, a 0° value for the MnAs4 twist angle was used.

Angle (degrees)

EN

DO

RF

requ

ency

(MH

z)

3

6

9

12

15

18

21

[110]15 3030 60 [100][001] 30 60 [001]

Fig. 7 Angular dependence of the ENDOR lines from the 75As (Set B) nuclei. Discrete points are data

taken from the MS = +3/2 manifold of the Mn2+ ion. Solid lines were generated using the 75As (Set B)

parameters in Table 2.



Final values for the two sets of 75As parameters are given in Table 2. Again, signs were not determined experimentally for the principal values. The 75As (Set A) hyperfine matrix is far from axial, while the 75As (Set B) hyperfine matrix is axial. Because the 75As (Set B) is axial, angles cannot be specified in Table 2 for the directions of its two non-unique principal axes. Separating the principal values of the 75As hyperfine matrices into isotropic and anisotropic parts (as was done for 111Cd) gives a = 8.59 MHz, b = 2.18 MHz, and b′ = −1.96 MHz for the Set A nuclei and a = 8.25 MHz, b = 3.82 MHz, and b′ = 0 for the Set B nuclei. The four nearest-neighbor arsenic ions surrounding a Mn2+ ion are equivalent, and are 2.633 Å away from the Mn2+ ion in the undistorted lattice. The next shell of arsenic neighbors divides into three sets, with each set containing four equivalent ions. These three sets are located approximately 4.646, 4.740, and 4.958 Å from the Mn2+ ion in the undistorted lattice. Unambiguously assigning the observed 75As (Set A) and 75As (Set B) results to particular sets of neighboring arsenic ions is difficult. Although it would be reasonable to assign one of the 75As hyperfine sets to the nearest-neighbor arsenic ions, even this appears to be questionable because the magnitudes of the two sets of principal values are very simi-lar. It is clear from the present 75As ENDOR data that the distribution of unpaired spin density on the ions surrounding the Mn2+ is complex in this tetragonally distorted chalcopyrite crystal structure. The observed interaction with the nearest cadmium ions and the lack of signals from germanium ions also support a complex distribution of unpaired spin density. Future theoretical investigations may assist in arriving at correct assignments of 75As hyperfine parameters to particular neighboring nuclei.

6 Summary

Isolated Mn2+ (3d5) ions have been investigated in single crystals of CdGeAs2. Using low-temperature EPR and ENDOR, we have determined the g-values, crystal-field parameters, and 55Mn hyperfine and nuclear quadrupole parameters. Hyperfine interactions with one set of Cd neighbors and two sets of As neighbors were also measured. Our results demonstrate that the Mn dopant ions enter the CdGeAs2 lat-tice as Mn2+ substituting for Cd2+, with a measurable spin density extending onto the surrounding Cd and As ions. No evidence was found that any of the Mn impurities become Mn4+ and substitute for Ge4+. Although the analysis of the hyperfine with the nearest neighbor Cd nuclei was relatively straight forward, the observation of two distinct As hyperfine interactions led to uncertainty as to which of the surrounding sets of As nuclei are responsible for each interaction. A theoretical study of the hyperfine interactions of the Mn2+ ion with its neighbors may help to clarify these As assignments in CdGeAs2. Crystalline CdGeAs2 is an excellent candidate for spintronic applications that require ferromagnetic behavior at room temperature. Large concentrations of Mn2+ ions can easily be incorporated on the Group II site in this material and crystals can be grown (independent of their level of Mn doping) as either p-type or n-type with large carrier concentrations at room temperature. These properties suggest that Mn-doped CdGeAs2 should exhibit carrier-mediated ferromagnetism (i.e., magnetic ordering resulting from the ex-change interaction between localized magnetic spins and delocalized carriers [43]). Future studies are needed to explore the magnetic, electrical, and optical properties of CdGeAs2 crystals that are co-doped with Mn and a donor (such as indium or selenium) or an acceptor (such as copper or potassium).

Acknowledgement This work was supported at West Virginia University and Stanford University by the Air

Force Office of Scientific Research (MURI Grant No. F49620-01-1-0428). One of the authors (S.M.E.) received

support from the West Virginia STEM Fellowship Program.

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Documents

Electron paramagnetic resonance and electron-nuclear double resonance study of Mn2+ ions in CdGeAs2 crystals