The onset of magnetic order in Y(Fe1-xMnx)2

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The onset of magnetic order in Y(Fe1-xMnx)2

G. Hilscher, H. Kirchmayr

To cite this version:

G. Hilscher, H. Kirchmayr. The onset of magnetic order in Y(Fe1-xMnx)2. Journal de Physique

Colloques, 1979, 40 (C5), pp.C5-196-C5-197. �10.1051/jphyscol:1979572�. �jpa-00218989�

The onset of magnetic order in YCFe^JMn^

G. Hilscher and H. Kirchmayr

Institut fur Experimentalphysik. Techn University, 1040 Wien. Austria

Abstract. — Magnetic measurements of the pseudobinary system YCFe^^MnJj, where a transition from ferro- magnetism to paramagnetism occurs at x

= 0.7, are presented. It is shown that the magnetic properties are strongly effected by annealing. From these results we suppose that magnetism in Y(Fe, Mn)

seems to appear in a heterogeneous way.

1. Introduction. — Alloys of the composition Y(Fe, Mn)

crystallize in the cubic MgCu

-structure, and exhibit as several other pseudobinary Laves phase systems with YFe

or ZrFe

magnetic transi- tions as function of alloy composition when Fe is substituted by Mn, Co, Ni or Al. Kirchmayr sug- gested [1] from Mossbauer and magnetic measure- ments of Y(Fe, Mn)

down to 77 K that the decrease of the magnetic moment and the Curie temperature T

is caused by local environment effects. The aim of this investigation was to extend the measurements to lower temperatures in order to study the onset of magnetic order.

2. Results and Discussion. — Samples were pre- pared by high frequency melting in a water cooled crucibel under argon atmosphere. For x < 0.8 only off stoichiometric samples could be obtained single phased without rather extensive annealing proce- dures. Thus all samples considered here were pre- pared according to the formula Y(Fz

_

Mn

)

. Magnetic measurements have been performed on the as cast samples and after two annealing procedures (750 °C 14 days water quenched, followed by 800 °C 14 ways water quenched) with a low frequency extraction magnetometer and a commercial vibrating sample magnetometer. Curie temperatures were deter- mined from A.C. susceptibility measurements and from M

- H/M plotts (Arrott plots).

The results Curie temperature T

and magnetization M(H, T = 4.2 K) are plotted against the Mn- concentration in figure 1. Both T

and M decrease almost linear with x. However for x > 0.55 the slope of T

versus x is changed. This coincides approxi- mately with a significant field dependence of the magnetization AM

While for x < 0.5 AM is small and equal ~ 1 emu/g,

AM increases as x -» x

and reaches the maximum value at x = x

. Additionally annealing procedures greatly influences the magnetization : the magne- tization increases with the annealing time over the whole range of concentration while T

is almost unchanged for x > 0.5 and only slightly reduced for x < 0.5. From the data in figure 1 the critical concentration for the breakdown of magnetic order is estimated to be x ~ 0.7.

Fig. 1. — Curie temperature T

. magnetization M(H. T) :

• M(0.5 T, 4.2 K), o M(7.2 T. 4.2 K). A M(7.2 T. 4.2 K) anneal- ed, A MCI.2 T, 4.2 K) 2 x annealed (see text) as a function of alloy composition x in YfFej.jJMn^

JOURNAL DE PHYSIQUE

Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-196

Résumé. — Nous avons mesuré le système pseudobinaire Y(Fe! - ^ M n ^ qui présente une transition ferromagné- tisme-paramagnétisme à x = 0,7. Un traitement à différentes températures change l'aimantation. On peut en conclure que le magnétisme dans ce système apparaît de manière inhomogène.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979572

THE ONSET OF MAGNETIC ORDER IN Y(Fe,-,Mn,),

Fig. 2. -

M Z _against

plots of Y(Fe,-,MnJ, for various x values and annealing times

as cast, o annealed,

an- nealed.

The magnetization curves (M-H) of various alloy compositions in the as cast condition and after two heat treatments are plotted as M' against HIM in figure 2. In the itinerant electron model the magnetic isotherms are over a large range of temperatures to a first approximation given by HIM

A + B M ~ , where the coefficients A, B can be correlated with the density of states and its derivatives [2]. Generally this equation, derived from the Landau theory holds for magnetic second order transitions in a limited concentration and/or temperature range with the additional assumption that the magnetization is homogeneous. It can be seen that the as cast samples do not fit such an expression

the Arrott plots are strongly curved. Deviations in a limited range from linearity in M2-HIM plots can be understood by Refer

KIRCHMAYR, H.

[2]

WOHLFARTH, E. P.,

[3]

SHTRIKMAN,

WOHLFARTH, E. P.,

[4]

HILSCHER, G., BUIS,

FRANSE,

M.,

[5]

HILSCHER,

Dissertation, T.

[6]

OESTERREICHER, H.,

including spatially dependent terms as ( V M ( ~ ) ) ~ in the 'free energy expansion [3]. By comparison of M2-HIM plots of Y ( F e o , 4 M n o , 6 ) 3 with Er6(Feo~,Mno~6)23 it turned out that even in the vicinity of T, the strong localized magnetic moment of Er causes a pronounced curvature, while Y6(Feo,4Mno~6)23 exhibits almost linear Arrott plots [4, 51. Therefore we suppose that the strongly curved Arrott plots of the as cast alloys are due to clusters which carry large magnetic moments. As shown in figure 2, annealing increases not only the magnetization but gives also less curved Arrott plots.

This leads to the suggestion that the distribution of magnetic moments becomes with annealing more homogeneous. In order to detect the presence of clusters by their influence on the M-T or M-H curves, hysteresis measurements have been performed with and without field cooling (7 T). Neither for the as cast nor for the annealed samples displaced hysteresis loops were detected, whereas in other Fe-Mn systems as Y6(Fe, Mn),, displaced hysteresis loops and peaks in M-T curves are observed [4, 61 which are charac- teristic for magnetic cluster freezing.

A preliminary analysis of the M-H curves in terms of the law to approach saturation gives up to 0.5 T and in the field range 0.5 - 2 T an a / H and b/H2 proportional magnetization respectively. The coeffi- cients a and b are only slightly effected by annealing.

However, the high jield susceptibility determined as AMIAH with AM= M(7.2 T)-M(3 T) at T=4.2 K is increased by annealing and exhibits a peak around the critical concentration. The maximum of the high field susceptibility at x z xc seems to be a generally observed feature within the range of the onset of magnetism and seems also to be correlated with a maximum in the electronic specific heat term

[7, 81. Furthermore it should be noted that the susceptibilities in the paramagnetic range (x > 0.7) do not obey the Curie-Weiss law. The 112-T plots exhibit a negative curvature with respect to the T axis. YMn, is, as reported by Buschow [9] a Pauli paramagnet and attained recently interest because of the pronounced change of the magnetic properties by hydrogen absorption [9, 101.

We may conclude from the strongly curved Arrott plots which are effected significantly by annealing and the almost linear Tc-x and M-x dependence which cannot be fitted easily with a Jaccarino Walker model that the onset of magnetism is due to magnetic cluster coupling and takes place in a heterogeneous way.

[7]

ODODO,

[8]

HILSCHER, G., GMELIN, E., Proc. Int. Conf. LT

Grenoble

J.

C

[9]

B u s c ~ o w , K. H.

SHERWOOD, R.

[lo]

WALLACE, W. E., MALIK,

K., TAKESHITA, T.,

Phys. 49 (1978) 1486.