Magnetic Behavior of Reduced Nickel Oxide Single CrystalsRamji Srivastava Citation: Journal of Applied Physics 39, 54 (1968); doi: 10.1063/1.1655778 View online: http://dx.doi.org/10.1063/1.1655778 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/39/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Anomalous magnetic behavior in nanocomposite materials of reduced graphene oxide-Ni/NiFe2O4 Appl. Phys. Lett. 105, 052412 (2014); 10.1063/1.4892476 Role of polaron hopping in leakage current behavior of a SrTiO3 single crystal J. Appl. Phys. 114, 224102 (2013); 10.1063/1.4842836 Magnetic behavior of reduced graphene oxide/metal nanocomposites J. Appl. Phys. 113, 17B525 (2013); 10.1063/1.4799150 Strong perpendicular magnetic anisotropy in Ni/Co(111) single crystal superlattices Appl. Phys. Lett. 94, 262504 (2009); 10.1063/1.3160541 Magnetic minor hysteresis loops of compressively deformed transition-metal single crystals J. Appl. Phys. 99, 08H908 (2006); 10.1063/1.2176592

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54 H. L. BURGE AND L. B. ROBINSON

its surroundings. Assuming the maximum loss with black-body radiation, the loss is only ",1.6X 10-3 W or ",o.34% loss of energy by black-body radiation. Since the source was not a black-body source the loss is less than this value. Also in this study the gaseous cpp product was obtained from the perfect gas and kinetic theory results. This results in less than 0.3% error at room temperatures. For the three gases of interest here, the product is essentially constant at ambient temperatures.

IU:SULTS

The determined experimental results are given in Table II for He, Ar, Ne, and mixtures of these gases along with the determined statistical variation in the results and the number of data points. A plot of the

He-Ar mixture data is given in Fig. 2 along with results taken from Ref. 12. .

CONCLUDING REMARKS

The results obtained with the line source indicate both accuracy and consistency when the rigorous line­source theory is used. The utility of the method does depend upon having available the cpp product data. For the light rare gases this product can be quite accurately calculated for ambient temperature and pressure conditions, and the method yields good results. The method should be equally applicable in liquids if the rigorous theory is used as indicated in Ref. 4.

12 J. O. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular Theory of Gases and Liquids (John Wiley & Sons, Inc., New York, 1954).

JOURNAL OF APPLIED PHYSICS VOLUME 39, NUMBER 1 JANUARY 1968

Magnetic Behavior of Reduced Nickel Oxide Single Crystals

RAMJI SRIVASTAVA

Physics Department, Allahabad University, Allahabad, India

(Received 22 November 1966; in final form 22 May 1967)

Magnetic behavior of antiferromagnetic nickel oxide single crystals, both in normal and reduced state, has been studied by magnetic susceptibility, anisotropy in susceptibility, and torque measurements. The observed magnetic behavior is interpreted in terms of T- and S-type domains, which are normally present in a nickel oxide single crystal.

INTRODUCTION

Antiferromagnetic nickel oxide (Neel temperature ,.....,523°K)1-3 has a fcc structure of NaCI type in the paramagnetic region. With the setting in of the anti­ferromagnetic order, the cubic unit cell is distorted .to the rhombohedral4 one, on account of contraction along any of the four [111J cube axes, which increases with increasing antiferromagnetic order.6.6 In the anti­ferromagnetic state, ferromagnetic (111) sheets of Ni2+ spins (the configuration being consistent with calculation of minimum dipolar energy)7·8 are anti­ferromagnetically coupled9 due to a strong antiferro­magnetic superexchange interaction between [l00J neighbors and a weak direct antiferromagnetic ex-

1 F. Trombe, J. Phys. Radium 12,170 (1951). 2 M. Foex and C. H. Blanchetais, Compt. Rend. 222, 1579

(1949). 8 H. Bizette, J. Phys. Radium 12, 161 (1951). 4 H. P. Rooksby, Acta. Cryst. 1, 226 (1948). 5 S. Greenwald and J. S. Smart, Nature 166, 523 (1950). .6 J. Kanamori, Progr. Theoret. Phys. Japan 17, 197 (1957). 7 J. I. Kaplan, J. Chern. Phys. 22, 1709 -(1954). 8 F. Keffer and W. O. Sullivan, Phys. Rev. lOS, 637 (1957). 8.W. L. Roth, Phys. Rev. 111, 772 (1958).

change interaction between geometrically nearest neighbors along [110J.6.6.10 The latter is also responsible for rhombohedral distortion.6os

Existence of two types of antiferromagnetic domains in a normal NiO single crystal is well established both experimentallyll-16 and theoretically.16.17 Those arising from degeneracy in the contraction axis (crystallo­graphic twinning), are known as T domains, and were observed by Slack,ll Roth,12 and Kondoh.13 The other type arising from ambiguity in the spin orientation within a T domain is known as S domain, and was observed by Kondoh et al.,14 and Yamada et al.16 In fact, multi-T-domain crystals represent the minimum energy state,I6.17 and actual domain structures are probably the result of accidental strains and other

10 P. W. Anderson, Phys. Rev. 79, 350 (1950). 11 G. A. Slack, J. Appl. Phys, 31,1571 (1960). 12 W. L. Roth, J. Appl. Phys. 31, 2000 (1960). 13 H. Kondoh, J. Phys. Soc. Japan 17, 1316 (1962). It H. Kondoh and T. Takeda, J. Phys. Soc. Japan 19, 2041

(1964) . 15 T. Yamada, S. Saito, and Y. Shimomora, J. Phys. Soc. Japan

21, 672 (1966). 18 T. Yamada, J. Phys. Soc. Japan 21, 650 (1966). 17 T. Yamada, J. Phys. Soc. Japan 21, 664 (1966).

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MAGNETIC BEHAVIOR OF REDUCED NiO SS

FIG. 1. Magnetic susceptibility (emu g-1) as a function of the direction of magnetic field for different magnetic fields for a nor­mal nickel oxide sample.

-6 80)(10

20

o

defects.12,18 It has been observed that T walls are highly mobile as seen under mechanical stress (",,106 dynj cm2)

and magnetic field ("-'2S kOe).11 Almost untwinned NiO single crystals have thus been prepared. It has also been claimed, both experimentally and theo­retically, that the easy spin-direction within the (111) plane is [II2].14,15,17

While Singer19 observed isotropic susceptibility similar to that of a. polycrystalline sample in a normal NiO single crystal, Kondon et al.20 do observe a small anisotropy. Removal of T domains by stressing pro­duced an anisotropy for its value parallel and per­pendicular to the (111) plane.19 It has also been ob­served that magnetic fields exceeding 2.4 kOe cause spin-rotation within the (111) plane in an untwinned NiO crystal.ll ,12,21

In this paper, we report magnetic behavior of reduced NiO single crystals, which may be classified

120 160 320 360

ANGLE IN D£GREES -

as defects of the short-range order. Roth22 has also made a study of short-range defects (of Frenkel and Schottky type) of FeH ion in antiferromagnetic FeO. For the present study, we have produced anion vacancies by creating a nonstoichiometry of oxygen ion in NiO single crystals.

EXPERIMENTAL RESULTS

Samples in the form of circular disks, have been cut perpendicular to the [111J axis from a boule of NiO obtained from Philips Research Laboratories, the Netherlands. The samples have been reduced susces­sively at 100°, 200°, 300°, 400°, and 500°C in vacuum for 6 h so as to ensure a steady state. Reduction has not been done in hydrogen atmosphere for fear of formation of the (OR) group inside the sample. Magnetic suscepti­bility along [111 J direction as a function of temperature, and anisotropy in susceptibility as a function of field

(400"C)

(100 'C)

NORMAL

FIG. 2. Anisotropy in suceptibility (emu g-l) as a function of the applied magnetic field COe) for normal and successively reduced nickel oxide sample~. Reduction temperature is indicated in parenthesis.

18 H. Kondoh and E. Uchida, J. Phys. Soc. Japan 17, 1318 (1962). 19 J. R. Singer, Phys. Rev. 104, 929 (1956). 20 H. Kondoh, E. Uchida, Y. Nakazurni, and T. Nagarniya, J. Phys. Soc. Japan 13, 579 (1958). 21 W. L. Roth and G. A. Slack, J. Ap,I>l. Phys. 31,352 S (1960). 22 W. L. Roth, Acta Cryst. 13, 140 (1960).

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56 RAM]I SRIVASTAVA

-6 3OltlO

2.0, , , , , ,

Reduced al 4tJOoCllnd ;oo·c

R£ducedat JO(lC ~~2:;::::::s! Reduwi aI 200 c'::;

fYoi-{,or FIG. 3. Magnetic suceptibility as a function of temperature for normal and reduced nickel oxide sample at a magnetic field around 3.5 kOe along [l11J direc­tion (in emu g-l).

o L. ___________ ~I': ::---------:-:!c:::--------:-~! :-----200 300 400

T (oK) ___

along with torque around 150 Oe (both at room tem­perature "-'27°C) in (111) plane have been measured using a sensitive magnetic balance23 and a torque magnetometer .23

For normal sample, anisotropy in susceptibility(..:lx = Xmax-Xmin,,-,5XlO-6 emug-l at 2 kOe) in the (111) plane is found to have sin20-type symmetry as shown in Fig. 1. Further ..:lX is found to decrease almost linearly with increasing magnetic field as shown in Fig. 2. It becomes vanishingly small above a critical field (He) of the order of 2.4 kOe. For successively reduced samples, although the intrinsic nature of ..:lx and its variation with field is unaffected, the value of He increases on successive reduction. It is also observed that ..:lx at a particular field and amplitude of sin 20 torque increase on successive reduction. The results of magnetic susceptibility measurement along [l11J on normal and reduced samples are given in Fig. 3. It is observed that susceptibility increases almost linearly with temperature and hence we can write

DISCUSSION

The sin20 nature of torque and hence anisotropy in susceptibility in the (111) plane for our normal sample is similar to that observed by Kondoh et al.l8 This, along with the fact that we do observe a slope similar to that observed by Singerl9 in the temperature varia­tion of magnetic susceptibility along [111J direction (perpendicular to the spin direction), indicates that distribution of T domains in our normal sample is not random: A redistribution of T domains is possible on account of stresses caused by hardening of the cement used to attach the sample to the suspension rodI8 and

23 Ramji Srivastava, thesis, University of Allahabad (1966).

those caused during cutting of the crystal. It cannot be said definitely why ..:lx decreases with increasing field even at fields lower than 2 kOe, but behavior above this field and existence of a critical field may be at­tributed to spin rotation similar to those observed previously.11·l2.2l The higher value of the magnetic susceptibility in our normal sample may be due to deficiency of oxygen in the NiO boule at the outset.

For successively reduced samples, we observe that the slope of X vs T curve is reduced, and the suscepti­bility has a tendency to become invariant with tem­perature. This is not likely to be related with removal of oxygen on reduction, since the oxygen defects, as a first approximation, are of short range order. The removal of T domains, responsible for observed ten­dency of X vs T curve towards invariance is probably due to the slow cooling of the samples carried out, after the reduction was over.

The observed increase of anisotropy in susceptibility, torque amplitude and critical field, for suscessively reduced samples, indicates that spins have to overcome successively higher magnetic potential barrier to align perpendicular to the direction of applied magnetic field. This is likely to be connected with the creation of short range defects as a consequence of reduction. These short range defects create inhomogeneities in the (111) plane which consequently increase the number of S domains and the magnetic potential barrier. There is also a possibility that short range defects resulting from reduction may increase barriers to wall motion and give rise to the observed magnetic behavior of the reduced NiO samples.

ACKNOWLEDGMENT

The author expresses his most sincere thanks to Dr. K. G. Srivastava for suggestions and discl.lssion in . course of these investigations.

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