55Mn nuclear magnetic resonance in Mn2Sb

I. Phys. Chem. Solids Vol. 41 pp. 7-10 Pergamon Press Ltd., 1980. PrInted in Great Britain

55Mn NUCLEAR K. V. S.

RAMA

RAO,

MAGNETIC S. L. PINJARE, T.

RESONANCE

RAJASEKHARAN

and C.

IN Mn,Sb RAMASASTRY

Department of Physics Indian Institute of Technology Madras-600036,

India

(Received 30 May 1978; accepted in revised form 21 March 1979)

Ahstraet-The observation of NMR signals from 55Mn nuclei in Mn,Sb has been reported. temperature variation study of the domain and domain wall signals in Mn,Sb confirms that magnetization is aligned parallel to the c-axis at temperatures higher than 240 K and that it is in basal plane below 240 K. The temperature dependence of the anisotropy in the hyperfine field at Mn(1) and Mn(I1) sites has been determined.

1. INTRODUCTION

The intermetallic compound MnzSb is ferrimagnetic with a Curie temperature of 550 K. It has a Cu,Sb type unit cell with Mn(1) atoms situated at the tetrahedral sites and Mn(I1) atoms situated at the octahedral sites. Neutron dtiaction studies show that there is a spin reorientation transition at 7’, = 240 K. The atomic spins are aligned parallel and perpendicular to the c-axis above and below 240 K respectively [l, 21. 55Mn nuclei in Mn,Sb have been reported to give zerofield NMR signals at 143.7 MHz and 126.26 MHz at 77 K [3]. Houghton and Weyhmann [4] have assigned these signals to Mn(1) and Mn(II) sites respectively from their magnetic field dependence. Above the spin-reorientation temperature, the magnetic moments in the domains in Mn,Sb are aligned parallel to the c-axis and the domains are connected by 180” domain walls, at the centre of which the magnetic moments are aligned perpendicular to the c-axis. The resonances from nuclei in domains and domain walls are expected to occur at different frequencies if the anisotropy in the hyperfine field is considerable [5,6]. In this paper, we report NMR measurements on the domain and domain wall resonances in Mn,Sb as a function of temperature. The anisotropy in the hyperfine field and the quadrupole coupling constants are also reported.

3. EXPERIMENTAL

A the the the

RRSULTS AND DISCUSSION

At room temperature (302 K), two sets of six signals each were observed: one set in the frequency range 128-132 MHz (Group A), and the other set in the frequency range 80-85 MHz (Group B); and the spectra are shown in Figs. 1 and 2 respectively. Group A signals consist of an intense signal at 127.90 MHz and five evenly spaced signals with a spacing of 0.76 MHz and centered at 130.62 MHz. Group B signals consist of an intense signal at 83.98 MHz and five equally spaced signals with a spacing of 1.10 MHz and centered at 82.41 MHz. The five equally spaced signals in both the groups are evidently the 55Mn (I = 5/2) resonances split by an electric quadrupole interaction. The observed spectra are similar to the “Mn spectra in the anisotropic ferromagnets MnP [5] and MnBi [6] and have the nature of resonances originating from 180” domain walls in the presence

2. EKPERIMJZNTAL PROCEDURE

Well-annealed Mn,Sb was prepared using Mn and Sb of 99.99% purity. The zero-field NMR measurements were carried out using a frequency modulated super-regenerative spectrometer. The resonance frequencies were measured using a Marconi R. F. signal generator in conjunction with a Grunding FZlOOO frequency counter.

FRE

Fig. 1. %fn

QUENCV

(MHz)-

NMR spectrum from Mn(l) site m Mn,Sb at 302 K.

K. V. S. RAMA RAO, S. L. PINJARE, T. RAJASEKHARAN

and C. RAMASASTFCY

of an electric quadrupole interaction and anisotropic hyperfine interaction. From the crystal structure of Mn,Sb, we expect the electric field gradient and the anisotropic hyperfine interaction at the Mn(1) and Mn(I1) sites to be axially symmetric about the c-axis. In this case, the resonance frequency corresponding to the m t, (m - 1) transition is, to !irst order in e’qQ, given by [6,7], V, = v,, cos20 + v,sin20

I

;-i;

FREQUENCY

Here vIIand Ye are the resonance frequencies when the electronic magnetic moment is in a direction parallel and perpendicular to the c-axis respectively, 0 the angle between the magnetic moment and the c-axis, q the principal value of the e.f.g. tensor and Q the nuclear quadrupole moment. At temperatures where the magnetic moments are aligned parallel to the c-axis in the domains, the direction of the magnetic moments in the 180” domain walls is distributed from the c-axis to a direction perpendicular to the c-axis. The resonance frequency and the quadrupole splitting vary with the position of the nuclei in the domain walls, and an unresolved single resonance signal is expected from the nuclei in the domain walls. From this consideration we conclude that the intense signals in groups A and B come from nuclei at the centre of the domain walls and the five equally spaced signals in the groups A and B arise from nuclei at the edge of the domain walls. Hereafter the resonances coming from the nuclei at the edge of the domain walls will be referred to as domain

(MHz)-

Fig. 2. 55Mn NMR spectrum from Mn(I1) site in Mn,Sb at 302 K.

70

60

I

I

I

I

I

I

I

I

I

100

140

160

220

250

300

340

380

420

50

TEMPERATURE(K)

Fig. 3. Temperature curves

dependence of Mn(I) and Mn(I1) resonance frequencies in Mn,Sb. The solid from 77 K to 260 K are based on ref. [4]. The plots showing the temperature dependence of domain and wall signals are marked D and W respectively.

9

ssMn nuclear magnetic resonance in Mn,Sb Table 1. Resonance frequencies, internal magnetic fields and quadrupole coupling constants of “Mn nuclei in Mn,Sb

Site

Temperature (K)

Resonance Frequencies (MHz)

Internal magnetic field (kOe)

127.90

121.16

Mn(I) Domain wall

Quadrupole coupling constants (MI-W

302 Mn(I1) Domain

129.16

129.80

Mn(I1) Domain wall

130.62

131.26

132.19

123.74

5.05

76.56

83.98 302

Mn(I1) Domain

Mn(I)

80.25

81.28

82.41

83.60

84.65

78.07

7.33

Domain wall

71

143.98

144.39

144.96

145.32

145.96

137.33

6.60

Mn(I1) Domain wall

77

125.53

126.03

126.48

126.93

127.41

119.82

6.26

appear as groups of five evenly spaced signals. At temperatures lower than the spin-reorientation temperature the magnetic moments being aligned perpendicular to the c-axis in both the domains and the domain walls, the domain and wall resonances would occur at the same frequency. Since the spins rotate in the base1 plane across the thickness of the domain walls, thus keeping the angle between the electronic magnetic moment and the e.f.g. axis constant, the signals from nuclei at the centre of the domain walls are expected to be quadrupolar split. The signals occurring in groups A and B at 77 K are quadrupolar split signals from nuclei in domain walls and domains, occurring at the same frequency. These signals in Group A occur, centered at 144.96 MHz and with a spacing of 0.50MHz, and those of group B occur centered at

resonances, since they have essentially the same frequency as signals expected from the bulk of the domains. The resonances from the nuclei at the centre of the domain walls will be referred to as wall resonances. The temperature dependence of the domain and wall resonances occuring in the groups A and B are shown in Fig. 3. The frequencies of the $* -$ transitions are plotted for the domain resonances. The domain and wall signals in Group A could be observed up to 440K, above which they lost their intensity to below the noise level. The group B signals could be observed up to 330 K. Neutron diffraction experiments have shown that on lowering the temperature the atomic spins become reoriented to the base1 plane at 240 K [l, 21. Below 240 K, the signals in Groups A and B would 1

I

I

I

I

I

4-

I

o-

Mn

.-

Mn(II)

(I)

2-

O-

3 CJ -r

-2 -

32 g

-6-

IO26()

I

I

I

I

I

I

I

260

300

320

340

360

380

400

4: 20

TEMPERATURE(K)

Fig. 4. The temperature

dependence

of the anisotropy in the hyperfine field (H,,-H,) and Mn(I1) sites.

at the Mn(1)

10

K.V.S.

RAMA

RAO, S. L.PINJARE,

126.48 MHz with a spacing of 0.47 MHz. The quadrupolar splitting at 302 K is approximately twice that at 77 K. The observed quadrupolar splitting supports the fact that the atomic spins are parallel and perpendicular to c-axis at 302 K and 77 K respectively (eqn (1)). The resonance frequencies of the “Mn signals at 77 K and 302 K, the calculated hyperfine fields and quadrupole coupling constants are given in Table 1. For the Mn(1) resonance the values of vjl and uI at room temperature (302K), are respectively 130.62 MHz and 127.90 MHz. The corresponding hyperfine fields I-IL1and H, at 302K are 123.74 kOe and 121.16 kOe respectively. Taking into account the negative sign of the hyperfme field [4], the anistropy in the hyperfine field EJ -I-IL is 2.58 kOe at 302 K. For the Mn(I1) resonances the value of vIl and vI at 302 K are 82.41 MHz and 83.98 MHz respectively. The anisotropy in the hyperfine field is +1.49 kOe. The temperature dependence of the anisotropy in the hyperfine field at Mn(1) and Mn(I1) sites is shown in Fig. 4. 4. CONCLUSIONS The nuclear magnetic resonance signals from “Mn nuclei in Mn,Sb have been observed and interpreted as arising from the nuclei at the centre and edge of the domain walls. A temperature varia-

T. RAJASEKHARAN

andc.

WASTRY

tion of the NMR frequencies has enabled us to confirm that the atomic spins are oriented in the c-direction in the domains in Mn,Sb above 240 K, and that on lowering the temperature below 240 K the spins become reoriented to the basal plane. The

temperature dependence of the anisotropy hyperfine field is determined.

in the

Acknowledgement-The authors are grateful to Professor Alarich Weiss, Darmstadt, for his interest in this work.

REFRRRNCES

1. Wilkinson M. K., Gingrich M. S. and Shull C. G., .I. Phys. Chem. SoZids 2, 289 (1957). 2. Alperin H. A., Brown P. J. and Nathans R., .I. Appl. Phys. 34, 1201 (1963). 3. Hirahara E., unpublished data (1963) from Portis A. M. and Lindquist R. H., In Magnetism (Edited by G. T. Rado and H. Suhl) Chap. 6. Vol. IIA. Academic Press, New York (1965). 4. Houghton R. W. and Weyhmann W., Phys. Rev. 20, 842 (1968). 5. Nagai H., Hihara T. and Hirahara E., J. Phys. Japan 29, 622 (1970). 6. Hihara T. and Koi Y., J. Phys. Sec. Japan 29, (1970). 7. Murray G. A. and Marshall. W., Proc. Phys. Sot. 315 (1965).

Left. Sot. 343, 86,