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in a series of intermetallic compound rmn@xmath0 ( r = rare earth ) , an existence of magnetic moments on mn sites depends largely on an inter - atomic mn - mn distance@xcite . above a critical distance of @xmath11=2.7 @xmath12 m there exists a magnetic moment on the mn which is antiferromagnetically ordered in general . below this critical distance the ground state remains paramagnetic down to lowest temperature . this fact that the mn - mn inter - atomic distance plays a key - role in determining their magnetic properties indicates that these compounds may be magnetically sensitive to such a perturbation as a chemical substitution or an application of pressure . an interesting possibility is then that an application of pressure induces quantum critical phase transition governed by the zero point quantum spin fluctuation . in particular this is the case of ymn@xmath0 which exhibits antiferromagnetic ( af ) order with a nel temperature around 100  k at ambient pressure . a relatively low pressure of @xmath13=3 to 4  kbar is enough to suppress the af order@xcite . the paramagnetic to af magnetic transition upon cooling is of first order type accompanying by well pronounced hysteretic phenomena in various measurements . therefore , the af to paramagnetic phase transition across @xmath14 is anticipated to be first order type . so far it is reported that the paramagnetic state at ambient pressure is dominated by the af spin fluctuations @xcite which is well treated in terms of the self - consistent renormalized ( scr ) spin fluctuation theory developed by moriya et al . @xcite . as a matter of fact , the sc - substitution of y suppresses the af order and the spin fluctuations in the paramagnetic phase was reported to be well described in terms of the scr theory @xcite . on the other hand , the inelastic neutron scattering experiment has found that the singlet correlation among four mn spins forming the tetrahedron is developed to stabilize this paramagnetic phase , i.e. , a kind of spin liquid state is formed in the sc substituted system . thus , it is still controversial whether or not this novel character of spin fluctuations is actually related to the nature in the paramagnetic state above @xmath1 in ymn@xmath0 because the effect of disorder induces by the sc -substitution is not addressed yet . in other words , we address the question whether or not , near the critical point in ymn@xmath0 where the nel temperature is suppressed to zero , the af spin fluctuations are also critical with a damping energy tending to zero . an application of pressure to suppress the af order is promising to settle these issues , because this is the only way to stabilize the paramagnetic state in ymn@xmath0 at low temperatures without any disorderliness . the aim of this paper is to enligthten the evolution of low - energy magnetic excitations in the pressure - induced paramagnetic phase by approaching the critical point from the higher pressure side . a polycrystalline sample has been crushed in fine powder in order to ensure the penetration of the radio - frequency pulsed field . a conventional clamp - type pressure cell was employed for the nmr experiment under pressure with use of the fluorinert liquid as the pressure transmitting medium . the fluorinert volume was of the proportion as the sample volume and froze below 200  k. to calibrate the pressure at the sample position at low temperature , the pressure shift in @xmath15 of lead powder was monitored . the measurement of @xmath15 was performed by an inductive method using the in - situ nmr coil in a resonant circuit . the nqr spectrum has been obtained by plotting the spin - echo intensity as a function of frequency with an interval of 50  khz . the spin - lattice relaxation time , @xmath16 , was measured by the saturation recovery method in a temperature range of @xmath17  k. for the nqr transition of @xmath18 , the recovery function of nuclear magnetization , m , is given by @xmath19 and the spin - lattice relaxation time was uniquely determined by a fit of the data with the above formula . fig .  [ fig1 ] indicates the evolution of the @xmath20mn nqr spectrum for the @xmath21 transition at @xmath22  k under pressure . with increasing pressure , the spectrum experiences a slight shift towards a higher frequency side , reflecting an increase of the electric field gradient ( efg ) . the full - width at half - maximum in a series of nqr spectra does not change appreciably , which means that the pressure - induced distribution in efg can be ignored . the nuclear spin - lattice relaxation time @xmath16 was not measured above 120  k in order to avoid a melting of the transmitting liquid medium at high temperatures which increases the pressure effectively . the @xmath23 dependence of @xmath6 , which is plotted with the semi - logarithmic scale in temperature ( fig . [ fig2 ] ) , shows the possible existence of two regimes marked by a kink ( arrows ) . hereafter , we will refer to this cross - over temperature as a characteristic spin fluctuation temperature , @xmath24 , in the pressure - induced paramagnetic phase . @xmath5 increases from 30  k with increasing pressure . below @xmath5 , @xmath6 becomes weakly temperature dependent . the spin - lattice relaxation rate , @xmath25 , is in general related to the transverse component of the imaginary part in the dynamical magnetic susceptibility @xmath26 such as @xmath27 where @xmath28 is the gyromagnetic ratio of the nuclei studied , @xmath29 the number of atoms , @xmath30 the hyperfine coupling constant , and @xmath31 the nmr frequency ( @xmath32 represents the transverse component of the generalized susceptibility ) . in the case of nearly antiferromagnetic metals , the q dependence of the low - energy spin fluctuations are dominated by the components close of the af vector * * q**=*q * and the main contribution will thus be such as @xmath33 . therefore eq .  ( [ equ2 ] ) can be given by a simple form of @xmath34 the pressure and thermal dependences of @xmath6 thus allow us to probe corresponding dependences of af spin fluctuation . in the framework of the scr theory the dynamical susceptibility is given by @xmath35 where @xmath36 is the parameter to make the self - consistency equation valid , the non - interacting susceptibility @xmath37 and @xmath38 the coulomb intra - site interaction . the denominator of eq . ( [ equ4 ] ) is equivalent to the stoner factor which tends to zero at the criticality and @xmath39 . correspondingly the divergence of @xmath40 means the divergence in the static staggered susceptibility and as a result , it is expected that @xmath41 undergoes a divergence at @xmath39 or it is largely enhanced approaching @xmath42 . however , the experiment does not follow the above expectation . rather , the development of af spin fluctuations is saturated upon cooling and approaching the @xmath39 . namely , @xmath5 does not tend to zero , but seems to remain finite even at @xmath39 . this may due to the first order nature of the transition from the af state to paramagnetic at @xmath39 induced by pressure . above @xmath5 , the spin fluctuations are dominated by thermal fluctuations which are treated in terms of the scr theory , whereas 1/@xmath43 approaches a constant value below @xmath5 characteristic for the fermi liquid excitation . [ fig3 ] indicates the pressure dependence of @xmath44 deduced from the present @xmath16 measurement . from the electrical resistivity measurements under pressure , the spin fluctuation temperature was defined as the cross - over temperature below which the thermal variation of @xmath45 obeys the expected fermi liquid @xmath46 law@xcite remarkably , a fairly good agreement is obtained concerning the variation of both @xmath47 under pressure @xmath48 with these experimental implications , it is possible that the development in spin fluctuation remains in short - range upon cooling . from the thermal variation of the nuclear spin lattice relaxation rate , @xmath49 of @xmath20mn above the critical pressure of 4  kbar , the spin fluctuation feature has been found to change below the temperature @xmath5 . at temperatures higher than @xmath5 , the nuclear relaxation behavior is well described in terms of the self - consistent renormalized spin fluctuation theory for a nearly af metal , whereas below @xmath5 , a deviation from that is significant and @xmath50 exhibits a weak temperature variation . it is pointed out that @xmath47 coincides with the temperature below which a @xmath46 law in resistivity is valid as expected for the fermi liquid state . we proposed that these features are understood from the view that the development in af spin fluctuation remains in short - range associated with the singlet formation among mn spins in each tetrahedron as suggested by the inelastic neutron experiments on y@xmath8sc@xmath9mn@xmath0 .
we report the nqr study on the pressure - induced paramagnetic state in the antiferromagnetic ( af ) intermetallic compound ymn@xmath0 with the nel temperature @xmath1=100  k at ambient pressure . from the @xmath2 variation of the nuclear spin - lattice relaxation rate , @xmath3 of @xmath4mn above the critical pressure of 4  kbar 
 , the spin fluctuation feature is found to change below a temperature @xmath5 . at higher temperature than @xmath5 
 , the nuclear relaxation behavior is well described in terms of the self - consistent renomalized ( scr ) spin fluctuation theory for nearly af metals , whereas below @xmath5 a deviation is significant and @xmath6 exhibits a weak @xmath2 variation . 
 it is pointed out that @xmath5 coincides with the temperature below which a @xmath7 law in electrical resistivity is valid as expected for a fermi liquid ground state . 
 we proposed that these features are understood from the standpoint that the development of af spin fluctuation remains in short - range associated with the singlet formation among mn spins in each tetrahedron as suggested by the inelastic neutron experiments on y@xmath8sc@xmath9mn@xmath0 . 
 # 1@xmath10#1 pnumwidth2em