Excited state spin ordering

Journal of Magnetism and Magnetic Materials 31-34 (1983) 1123-1124 EXCITED

STATE

K. A M A Y A ,

1123

SPIN ORDERING

Y. K A R A K I

a n d T. H A S E D A

Faculty of Engineering Science, Osaka Umverslty, Tovonaka, Osaka 560, Japan

The spin ordering at excited state is estabhshed experimentally m a crystal NaN~Acac 3 benzene under the pulsed magneUc field w~th sweep rate 10 5 T / s and starting temperature below 1 K Thts spin ordering appears on a balance of competing rates between spin relaxatmns and external magneUc field sweep

between the excited f0) and ] + 11 level a r o u n d the level crossing field H~ ( = 2.11 T). If we define the spin t e m p e r a t u r e ~2 of these two excited levels, it should be very much reduced at H c. Considering the l n t e r a c n o n a m o n g spins on these excited levels, we could determine the spin ordering region corresponding to the ground state smglet case [2]. Using the pulse field with sweep rate 10 s T / s a n d starting from the initial temperature 7", below 1 K, we actually found the double peaks on d M / d t vs H curves a r o u n d H c and suggested the occurrence of spin ordering at excited state. However, there were some features left examined (1) double peaks of d M / d t appeared in the field increasing mode (2) The first peak of the doublet came up at field value a b o u t 0 1 T lower than H~ wxth decreasing T~, but the second always just at H e (3) The first peak was missed in the field decreasing mode, while the second one remained. (4) The shape of the doublet was a bit sample dependent, which referred the effect of impurity ions In NI 2+ salts, the c o n t a m i n a t i o n of Co 2+ runs usually h a p p e n s Co 2+ ions have a fast cross relaxation rate with Nl 2+ ions at some field value (4 95 kOe in this crystal) [3]. This m]ght cause the thermal mixing of populations a m o n g two levels preventing the establishm e n t of large r e d u c n o n of T~2 a r o u n d H c. So, we reduce the a m o u n t of Co 2+ ions in the specimen Fig l b is the results of the sample with Co 2+ ions of 0.016% N o w we observe the first peak growing up sharply with decreasing T, To confirm further, we tilt the angle 0 between the crystal axis a n d the field dtrecnon intending again

The posslbdlty of occurrence of spin ordering at excited state has been suggested in our prevmus p a p e r [1] for the crystal NaN1 trmcetylacetonate benzene (N, 2 + ' S = 1) m which one of the ground state doublet a n d the exc]ted slnglet level m a k e a crossing m an external magnetic field as shown in fig. l(a). W h e n the t e m p e r a t u r e of the spin system is sufficiently lower than separataon D / k ( = 3 15 K) between the excited and g r o u n d state level at zero external field, most of tons are m the ground state spin levels [ _+ 11. A p p l y m g a pulsed magnetic field with sweep rate faster than the possible spin relaxation rate a m o n g whole three levels, we expect one half of the total population goes up with one of the g r o u n d state 1+ 11 making a big population difference F_/k (K)

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K Aroma et al / E~-¢~ted state spin ordering

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FEg 3 (a) Expected d M / d t vs H cur,,e for the case of antlferromagneUc ordering (b) Observed d M / d t vs H curve (c) Expected T,2-H phase boundary and the ~sentropes (expected . . . . . and observed - ) m increasing and decreasing modes to prevent the ordering by enlarging the separation between 10) and l + 1> levels at H~. In fig 2, we find the first peak easily disappears with a slight tilting ol 0, making a strong contrast to the second one quite msensitive These results are definite evMence for the first peak to be the boundary of ordered phase at the excited state and for the second the p h e n o m e n o n at the crossing field

Now we discuss the problems (1)-(3) In fig 3, we give some schematic pictures of our interpretation fol the p h e n o m e n a (a) d M / d t vs H curve expected theoretically for a simple case with antlferromagneuc interacUons (b) The observed double peaks of d M / d t m increasing and decreasing field modes (c) 1 h e possible change of T,2 defined corresponding to the change ol d M / d t in (b) (solid lines) If the pulse field is suthclently fast so as to establish the reduchon of T,e, the spin ordering would occur msMe the phase boundary giving the double peaks of d M / d r , as shown m picture,, (a) and (c) (dotted hnes) above However, the phenomena are not the case hke as m (a) So lar we have assumed that the ume duration ~1 crossing the ordered phase region was expenmentall,; m the range satisfying the condition ~-: < ,r~ < ,r~, where -r: and ~'~ were defined as the s p i n - s p i n relaxauon rune a m o n g upper two levels and total three levels around H , respectively Just at H~. however. ~-~ should become very fast and comparable with -rd resulUng increase ol 7~2 This is just the case of pictures (b) and (c) One another characteristics of th~s new type ol spin ordering is that the system composed of 10) and I+ 1) levels is randomly diluted by tmxlng not of the nonmagneuc ions but of t - 1) level of the ground state

Reference~ [1] K Amaya 5c Karakl, N Yamada and T Haseda, J Ph,;~ Sac Japan 50 (1981) 3181 12] K AmayaandN Yamashlta, J Phys Sac Japan42(1977) 24 [3] T Haseda, K Ama~a, N Yamashua and S lkada, Ph~,~,t.a 69 (1973) 273