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Magnetic transition in Nb1−yFe2+y
Journal of Magnetism
and
Magnetic Materials 140-144 (1995) 71-72
EISEVIER
Magnetictransitionsin wb, -yFe,,, M.R. Crook, R. Cywinski * JJ. Thomson Physical Laboratory, University of Reading, Recding RGS 2AF, UK
Abstract DC magnetisation has been used to map the magnetic phase diagram of Nb,-yFe*+.Y effects of coexisting and competing ferromagnetic and antiferromagnetic spin correlations
in the Cl4 hexagonal Laves phase compound, NbFe,, weakly localised Fe moments and mixed ferro (F)- and antiferromagnetic (AF) correlations [I] are superimposed upon a potential!y frustrated, Kagome lattice-like Fe-site topology. These features provide a system rich in interesting phenomena. For example, out resistivity measurements, in conjunction with heat capacity measurements (Hilscher, private communication) reveal a ground state characterised by strongly correlated electrons. Within the Kadowaki-Woods classification [2] NbFe,, with A/y= 5.25 X 10s4 pLn cm ml-’ mol, is not dissimilar to a light heavy fertnion (e.g. UIn,). Although there have been several previous studies of magnetic order in Nbl-,,Fez+,, [1,3,4] a detailed investigation of the magnetisation of this system close to stoichiometry is timely. We present here dc magnetisation measurements of Nbl-,Fez+Y over the concentration range - 0.04 C y < 0.04. Samples were prepared by argon arc melting the appropriate quantities of constituent metals (Nb = 99.9%~~ Fe = 99.98%). Mass losses during melting were typically 0.02%. The dc magnetisation measurements were made using an Oxford Instruments 12 T vibrating sample magnetometer. The magnetic isotherm of the stoichiomelric NbFe, compound (Fig. 1) at 4.2 K shows an S-like anomaly, consistent with previous reports [3]. Also shown in Fig. 1 is the dtfferentral, aM/>h’, - -whYch has a maximud at N 0.3 T. We have designated this f&l H,(T). It decreases with increasing temperature, approach& zero at 18 K (Fig. 2). At this temperature a peak, associated with the N&e1 point of NbFe,, is also observed in the ac susceptibility+ We therefore tentatively associate the temperature at which H,(T) approaches zero (i.e. T( H, = 0)) with the Nobel point. NbFe,, at stoichiometry, thus appears to be a weak antiferromagnet with H,.,.,(T) the critical field necessary to suppress the AF spin correlations.
between -0.04 are evident.
Immediately away from stoichiometry, with excess Nb, the magnetic isotherms develop a small ferromagnetic-lie component (Fig. 1). The magnitude of this component increases with increasing Nb content. However, in addition there is a persisting maximum in &l4/#f at H,!T). The temperature at which H,(T) vanishes, T(H, = 01, decreases with increasing Nb content (Fig. 21, tending to zero at approximatdy y = 0.02. This suggests that in this region ( - 0.02 < y < 0) F and AF correlations coexist, with the ferromagnetic correlations dominating as Nb content increases, At higher Nb concentrations the compounds show a transition to weak ferromagnetism. In this region
_I 000 internal
* Corresponding
author.
Fax:
f44
734 750203 email:
[email protected]. 03048853/95/$09.50
0 1995 Elsevier Science
SSDlO304-8853(94)00837-X
< y < 0.04. The
Field (lO’A/m)
Fig. 1, Magnetic isotherms and differentials, ah#/dH, of at 4.2 K for y = -0.0075, y = 0 and y * +0.0075, W-,Fez+, also shown is H,(T).
B.V. Att rights reserved
T (Kl Fig. 2. The critical field, H,(T), for JVb, -,,Fe2 f y as a funL?ion of temperature. The double values observed at low temperatures are the result of hysteretic behaviour.
( -0.04 < y < - 0.0125) the spontaneous magnetisation CM,), obtained from Arrott plots of the magnetic isotherms, fits the SCR IS) expression, M,’ =A(Tc4’3 - T413) for weak itinerant ferromagnetisrn (Fig. 3). The spontaneous moment per Fe atom at T = 0 is small, rising linearly with y from 0.035~, at y = -0.0125 to 0.078~, at y = - 0.04. The magnetic isotherms (Fig. 1) for compounds at low Fe-rich stoichiometries indicate that AF correlations persist to higher temperatures (Fig. 21 with increasing Fe content. T(H,,., = 0) reaches a maximum at y = 0.0075 (Fig. 4). The F-like character of the isotherms iTr this region develops more strongly than for the corresponding Nb excess
Nb excess
Fe excess
Fig. 4. Magnetic phase diagram of Nb,-,Fez+u obtained from dc maguctisation data. T(H,,, = 0) is the temperature above which a field-induced transition from the AP state is not cmbserved(see Fig. 2). q for FE excess stoichiometries is estimated from extrapolated Arrott plots. T, for Nb excess stoichiometries ir ‘(iained from the fit to SCR theory.
concentrations. In particular the magkztisation is much larger and hysteresis effects are mon pronounced. With further increases in Fe concentration (y > 0.015) the magnetic isotherms develop the character of a weak ferromagnet. The resulting magnetic phase d&ram of NbFe, is shown in Fig. 4. We see an extended AF phase close to stoichiometry, evolving via mixed AF + F phases to ferromagnetism for excursions to both h’b-rich and Fe-rich concentrations. The magnetic order in the NbFe, system is the result of a compiex interplay between electron concentration and local environment (ie antiuluctu:e Horn) effects [6]. It is also likely that the topological frustration associated with the Fe sites in these Cl4 compounds plays an important ro’?* To explore this point further we are currently using single crystal neutron diffraction and zero field ySR to probe the ground state of the IQ+-,Fe,,., system. References
T 4/3
(K4/3)
Fig. 3. The spontaneous magnetisation data with the SCR theory fit for Nb,-~Fc2+,, with y = -0.0125, y = -0.015, y = -0.02, y=-0.03and y=-0.04.
[l] Y. Yamada, H. Nakamura, Y. ffitaoka, K. hlyama, K. Koga, A. Sakata, T. Muiakami, J. Phys. Sot. Jpn. 59 (1990) 2976. {2] K. Kadowaki, S.B. Woods, Sol. Stat. Com.n. 58 (1986) 507. [3] M. Shiga, Y. Nakamura, J. Phys. Sot. Jpn. 56 (1987) 4040. [4] Y. Yamada, A. Sakata, J. Phys. Sot. Jpn. 57 (1998) 46. [s] T. Moriya, Spin Ructuations in Itinerant Electron Magnetism, Springer Series in Solid State Sciences, Vol. 55 (Springer, Berlin, 1985). [6] J. Inoue, M. Shimizu, 3. Magr. Magn. Mater. 79 (1989) 265.
and
Magnetic Materials 140-144 (1995) 71-72
EISEVIER
Magnetictransitionsin wb, -yFe,,, M.R. Crook, R. Cywinski * JJ. Thomson Physical Laboratory, University of Reading, Recding RGS 2AF, UK
Abstract DC magnetisation has been used to map the magnetic phase diagram of Nb,-yFe*+.Y effects of coexisting and competing ferromagnetic and antiferromagnetic spin correlations
in the Cl4 hexagonal Laves phase compound, NbFe,, weakly localised Fe moments and mixed ferro (F)- and antiferromagnetic (AF) correlations [I] are superimposed upon a potential!y frustrated, Kagome lattice-like Fe-site topology. These features provide a system rich in interesting phenomena. For example, out resistivity measurements, in conjunction with heat capacity measurements (Hilscher, private communication) reveal a ground state characterised by strongly correlated electrons. Within the Kadowaki-Woods classification [2] NbFe,, with A/y= 5.25 X 10s4 pLn cm ml-’ mol, is not dissimilar to a light heavy fertnion (e.g. UIn,). Although there have been several previous studies of magnetic order in Nbl-,,Fez+,, [1,3,4] a detailed investigation of the magnetisation of this system close to stoichiometry is timely. We present here dc magnetisation measurements of Nbl-,Fez+Y over the concentration range - 0.04 C y < 0.04. Samples were prepared by argon arc melting the appropriate quantities of constituent metals (Nb = 99.9%~~ Fe = 99.98%). Mass losses during melting were typically 0.02%. The dc magnetisation measurements were made using an Oxford Instruments 12 T vibrating sample magnetometer. The magnetic isotherm of the stoichiomelric NbFe, compound (Fig. 1) at 4.2 K shows an S-like anomaly, consistent with previous reports [3]. Also shown in Fig. 1 is the dtfferentral, aM/>h’, - -whYch has a maximud at N 0.3 T. We have designated this f&l H,(T). It decreases with increasing temperature, approach& zero at 18 K (Fig. 2). At this temperature a peak, associated with the N&e1 point of NbFe,, is also observed in the ac susceptibility+ We therefore tentatively associate the temperature at which H,(T) approaches zero (i.e. T( H, = 0)) with the Nobel point. NbFe,, at stoichiometry, thus appears to be a weak antiferromagnet with H,.,.,(T) the critical field necessary to suppress the AF spin correlations.
between -0.04 are evident.
Immediately away from stoichiometry, with excess Nb, the magnetic isotherms develop a small ferromagnetic-lie component (Fig. 1). The magnitude of this component increases with increasing Nb content. However, in addition there is a persisting maximum in &l4/#f at H,!T). The temperature at which H,(T) vanishes, T(H, = 01, decreases with increasing Nb content (Fig. 21, tending to zero at approximatdy y = 0.02. This suggests that in this region ( - 0.02 < y < 0) F and AF correlations coexist, with the ferromagnetic correlations dominating as Nb content increases, At higher Nb concentrations the compounds show a transition to weak ferromagnetism. In this region
_I 000 internal
* Corresponding
author.
Fax:
f44
734 750203 email:
[email protected]. 03048853/95/$09.50
0 1995 Elsevier Science
SSDlO304-8853(94)00837-X
< y < 0.04. The
Field (lO’A/m)
Fig. 1, Magnetic isotherms and differentials, ah#/dH, of at 4.2 K for y = -0.0075, y = 0 and y * +0.0075, W-,Fez+, also shown is H,(T).
B.V. Att rights reserved
T (Kl Fig. 2. The critical field, H,(T), for JVb, -,,Fe2 f y as a funL?ion of temperature. The double values observed at low temperatures are the result of hysteretic behaviour.
( -0.04 < y < - 0.0125) the spontaneous magnetisation CM,), obtained from Arrott plots of the magnetic isotherms, fits the SCR IS) expression, M,’ =A(Tc4’3 - T413) for weak itinerant ferromagnetisrn (Fig. 3). The spontaneous moment per Fe atom at T = 0 is small, rising linearly with y from 0.035~, at y = -0.0125 to 0.078~, at y = - 0.04. The magnetic isotherms (Fig. 1) for compounds at low Fe-rich stoichiometries indicate that AF correlations persist to higher temperatures (Fig. 21 with increasing Fe content. T(H,,., = 0) reaches a maximum at y = 0.0075 (Fig. 4). The F-like character of the isotherms iTr this region develops more strongly than for the corresponding Nb excess
Nb excess
Fe excess
Fig. 4. Magnetic phase diagram of Nb,-,Fez+u obtained from dc maguctisation data. T(H,,, = 0) is the temperature above which a field-induced transition from the AP state is not cmbserved(see Fig. 2). q for FE excess stoichiometries is estimated from extrapolated Arrott plots. T, for Nb excess stoichiometries ir ‘(iained from the fit to SCR theory.
concentrations. In particular the magkztisation is much larger and hysteresis effects are mon pronounced. With further increases in Fe concentration (y > 0.015) the magnetic isotherms develop the character of a weak ferromagnet. The resulting magnetic phase d&ram of NbFe, is shown in Fig. 4. We see an extended AF phase close to stoichiometry, evolving via mixed AF + F phases to ferromagnetism for excursions to both h’b-rich and Fe-rich concentrations. The magnetic order in the NbFe, system is the result of a compiex interplay between electron concentration and local environment (ie antiuluctu:e Horn) effects [6]. It is also likely that the topological frustration associated with the Fe sites in these Cl4 compounds plays an important ro’?* To explore this point further we are currently using single crystal neutron diffraction and zero field ySR to probe the ground state of the IQ+-,Fe,,., system. References
T 4/3
(K4/3)
Fig. 3. The spontaneous magnetisation data with the SCR theory fit for Nb,-~Fc2+,, with y = -0.0125, y = -0.015, y = -0.02, y=-0.03and y=-0.04.
[l] Y. Yamada, H. Nakamura, Y. ffitaoka, K. hlyama, K. Koga, A. Sakata, T. Muiakami, J. Phys. Sot. Jpn. 59 (1990) 2976. {2] K. Kadowaki, S.B. Woods, Sol. Stat. Com.n. 58 (1986) 507. [3] M. Shiga, Y. Nakamura, J. Phys. Sot. Jpn. 56 (1987) 4040. [4] Y. Yamada, A. Sakata, J. Phys. Sot. Jpn. 57 (1998) 46. [s] T. Moriya, Spin Ructuations in Itinerant Electron Magnetism, Springer Series in Solid State Sciences, Vol. 55 (Springer, Berlin, 1985). [6] J. Inoue, M. Shimizu, 3. Magr. Magn. Mater. 79 (1989) 265.