Spin flopping in MnTiO3

1071

Journal of Magnensm and Magnetic Matermls 31-34 (1983) 1071-1072

S P I N F L O P P I N G IN M n T i O 3 H. Y A M A U C H I ,

H

HIROYOSHI,

M. Y A M A D A ,

H. W A T A N A B E

a n d H. T A K E I

The Research Inst:tute for Iron, Steel and Other Metals. Tohoku Untverstth Sendat 980, Japan

Susceptabdlty and magnetization have been measured on a single crystal of MnTtO3 Spin-flopping is observed m the magnet~zaUon curve wtth the field along the hexagonal ~-axJs The exchange parameter and the amsotropy constant are estimated from the experimental data, the latter being 2 3 rimes smaller than the calculated value for the dipole interactions

The antfferromagnetic llmenltes M T I O 3 (M = Mn, Fe, Co and N 1 ) have ordered c o r u n d u m structure (llmenite structure) in which M 2+ and T I 4 + i o n s OCcupy alternate hexagonal layers and M :+ layers are separated from each other by two oxygen and one T1 sheets In these llmenltes MnTtO 3 has a magnetic structure [1] shown in fig. 1, in whmh the Mn 2+ spins are directed antlparallel within each layer perpendicular to the hexagonal c-axis as contrasted with the other d m e m t e s (M = Fe, Co and NI) with the M 2+ spms parallel within each layer (c-plane) and the Mn 2+ spms direct along the c-axis. However, magnetic suscept~bmhty measured by Aklmltsu et al [2] on a single crystal as well as that on a powder specimen by Suckler et al. [3] show no anomaly at the N6el temperature determined from neutron diffraction measurement [2] and the magnetic resonance [4] and a broad peak at about 95 K showing a two-&menslonal character observed from neutron diffraction experiments by Aklmltsu et al [2]. In order to obtain more accurate information about

the magnetic anlsotropy and the exchange interaction, susceptibility and magnetization measurements on a single crystal of MnT103 have been made A single crystal of MnTIO 3 was grown in a lamp image furnace by the floating zone method This single crystal was about 8 m m in diameter and 30 m m in length Magnetic susceptlblhty measurements were carried out with a pendulum-type magnetometer in the temperature range from 4.2 to 300 K in applied field up to 20.5 kOe parallel and perpendicular to the hexagonal c-axis. Magnetization measurements were carried out with a motor-driven vibrating sample magnetometer with fields up to 108 kOe generated by a water-cooled Bitter-type solenoid Magnetic susceptibility vs. temperature curves of MnT103 are shown i n f i g . 2. XI I and X± are the susceptlblhtles measured with the applied field parallel and perpendicular to the c-axis, respectively. As shown in fig. 2, Nil i n c r e a s e s with mcreaslng temperature up to about 95 K and shows a change in slope of the susceptlbd~ty vs temperature curve at the N6el temperature of 63.6 K, while X± decreases slightly to a minimum value near 50 K and then increases to a maximum value near

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oxygen ~ons are omitted Prlmltwe rhombohedral umt cell, hexagonal lamce parameters and five principal exchange integrals J r , '12, J~ J4 and J5 are indicated 0304-8853/83/0000-0000/$03.00

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F~g 2 Magneuc susceptlbd~ty vs temperature curves of MnTiO 3 parallel (X0 and perpendmular (X ± ) to the hexagonal c-axis m an apphed field of 5 8 kOe The N+el temperature ~s 636 K

© 1983 N o r t h - H o l l a n d

1072

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Fig 3 Magnetlzatmn curves of MnTlO~ parallel (Oll) and perpen&cular (o x ) to the hexagonal c-axis at 42 K

95 K. N o t e that the a m s o t r o p y of X does n o t d i s a p p e a r at T N but persists up to 95 K T h e susceptibility in the p a r a m a g n e t i c region a b o v e 95 K is isotropic a n d follows the C u r i e - W e i s s law T h e paramagnet~c Curie t e m p e r a t u r e 0p, the value o f the m o l a r Curie c o n s t a n t Cmo I, a n d the effective Bohr magn e t o n n u m b e r per M n a t o m #~rr are - 2 2 0 K, 4 31 and 5 90fiB, r e s p e c t w e l y T h e s e values agree well with those o f Stickler et al. [3] m e a s u r e d on a p o w d e r s a m p l e T h e m a g n e t i z a t i o n curves at 4 2 K are s h o w n in fig 3 The m a g n e t x z a t m n m e a s u r e d in the a p p h e d fields p e r p e n d i c u l a r to the ~-axls, o ± , increases h n e a r l ) with field, while the m a g n e t l z a t m n m e a s u r e d in the a p p h e d fields parallel to the c-axis, Olp increases linearly with a very small slope up to 58 kOe w h e r e the s p m - f l o p p l n g p h e n o m e n o n is observed, and t h e r e a f t e r it c o m c M e s with o± In the m a g n e U z a t l o n p r o c e s s of the a n t f f e r r o m a g netlc s u b s t a n c e s there are two typical cases relating to the m a g n e t i c a m s o t r o p y field H A a n d the a n t l f e r r o m a g n e u c e x c h a n g e field H E. O n e case s h o w s a spin flop t r a n s m o n ( H A / H ~ < 1) a n d the o t h e r case a m e t a m a g n e t i c t r a n s m o n ( H A / H L > 1). M n T I O 3 belongs to the f o r m e r case, w h i c h could be c o n f i r m e d by e s u m a t m g the values of H A a n d H t, using the e x p e r i m e n t a l data M a k i n g use o f the umaxlal a m s o t r o p y c o n s t a n t

(2)

a n d is c o n n e c t e d to the a n t l f e r r o m a g n e u c e x c h a n g e p a r a m e t e r 2 Z J b e t w e e n the + a n d - s p i n sublattlces by the f o r m u l a A = 4 Z J / N ( g f f B ) 2 Here 2 Z J - 2 ( Z I J ; 4- Z2J2 + Z s J s + ) with Z t = 3, Z 2 - 1 and Z s = 3 (cf Fig 1) F r o m the e x p e r i m e n t a l values of H~, X L a n d Xll' a n d using eqs. (1) a n d (2), we o b t a i n K - 5 7 × 105 ( e r g / c m 3 ) , 2 Z J / I ~ = - 6 6 K and H A / t t I - - 1 5 × 10 ~, which satisfies the s p m flop c o n d l t m n T h e absolute value o f 2 Z J / k is larger than 54 K o b t a i n e d from the a n t f f e r r o m a g n e t i c r e s o n a n c e f r e q u e n c y m e a s u r e d bx Stickler et al [4] a n d the a b o v e value o f K F r o m the suscepttbllity of the h i g h - t e m p e r a t u r e region, we can e s t i m a t e the nearest n e i g h b o r e x c h a n g e integral Ji on the basis of the t w o - d i m e n s i o n a l model using the series e x p a n s i o n of the suscepttbfllt) of the h o n e y c o m b lattice, of w h i c h six terms have been obtained by R u s h b r o o k e a n d W o o d [5] We have tried to fit the series e x p a n s m n to the e x p e r i m e n t a l data a b o v e 150 K and f o u n d that the best value of J~/I, ~s - 11 K This value agrees with that of J / l , derived before lroln X± Hence, it is suggested that J~ is d o m i n a n t in MnTlO~ T h e m a g n e U c a n l s o t r o p y in MnT~O~ ts c o n s M e r e d to arise from the m a g n e t i c dipole m t e r a c u o n s b e t w e e n M n 2+ (3d 5) ions T h e calculated value for the magnetic d i p o l e l n t e r a c u o n s is K~,,l= t 2 8 × 10 s e r g / c m ~ (for /*M~ = 4 55fib d e t e r m i n e d from n e u t r o n diffraction study be Shlrane et al [1]) This calculated value is 2 3 tunes larger than the e x p e r i m e n t a l value This d t s c r e p a n c y m i g h t be ascribed to the mixing of o t h e r electromc c o n f i g u r a u o n s such as 3d a ( M n 3 + ) state which p r o w d e ~ the m e c h a m s m of e x c h a n g e interactions

Reference~ [1] G Shlrane, S I Plckart and Y lshlkaw'a, J Ph)s Soc Japan 14 (1959) 1352 [2] A Akimitsu, Y b,hikawa and Y Endoh, Solid State ( o m mun 8 (1970) 87 [3] J J Stickler, S Kern, A Wold and G S H e l l e r Phy,, Re,, 164 (1967) 765 [4] J J Stickler and G SHeller, J Appl Phys 33 (1962) 1302 [5] G S Rushbrooke and P J Wood, Molec Phys I (1958) 257