Magnetic properties and giant moment clusters in Be2Mn 1-xFex compounds R. Jesser To cite this version: R. Jesser. Magnetic properties and giant moment clusters in Be2Mn 1-xFex compounds. Journal de Physique, 1979, 40 (1), pp.23-38. 10.1051/jphys:0197900400102300. jpa-00208881 HAL Id: jpa-00208881 https://hal.archives-ouvertes.fr/jpa-00208881 Submitted on 1 Jan 1979 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. LE JOURNAL DE PHYSIQUE TOME 40, JANVIER 1979, 23 Classification Physics Abstracts 75 . 30C Magnetic properties and giant moment clusters in Be2Mn1-xFex compounds R. Jesser Laboratoire Pierre Weiss, E.R.A. n° 464 au C.N.R.S., Institut de Physique, 67084 Strasbourg, France (Reçu le 19 juin 1978, accepté le 9 octobre 1978) Resume. 2014 Les propriétés magnétiques des composés Be2Mn, _xFex ont été etudiees sur toute l’étendue de concentration 0 x ~ 1 et de champ 0 ~ H ~ 150 kOe à basse temperature (T ~ 100 K). Une representation des données d’aimantations en 03C32 = f(H/03C3) jusqu’a 150 kOe, a confirmé la nature inhomogène de la transition magnétique observée antérieurement dans ce systeme. Mais les mesures d’aimantations à bas champ (0 ~ H 1 kOe) ont montré que cette transition était graduelle : évolution graduelle du paramagnétisme (x ~ 0,06) au mictomagnétisme (0,11 ~ x ~ 0,30) jusqu’au ferromagnétisme (0,30 ~ x ~ 1,00). Le moment moyen 03BC des berylliures ferromagnétiques (0,30 ~ x ~ 1,00) décroît rapidement et de manière non linéaire quand on augmente la teneur en Mn. Mais les valeurs supérieures à l’unité que l’on a trouvées pour le moment normalise du fer (rapport 03BC/x03BC0) indiquent que deux types d’atomes (le Fe et le Mn) peuvent porter un moment dans ces bérylliures. Les données d’aimantations des composés Be2Mn1 _xFex à x allant de 0,06 à 0,25, ont été analysées en termes d’amas magnétiques à moments et concentrations dépendant de la temperature. Nous avons discuté les résultats insolites de cette analyse et nous avons tenté de les interpréter en invoquant les concepts (i) d’amas à moments géants ayant leur propre point de Curie 03B8c, augmentant avec la taille des amas, (ii) d’impuretés presque magnétiques devenant magnétiques en abaissant la température, vu les champs élevés utilisés ici (H ~ 80 kOe), et (iii) d’interactions antiferromagnétiques à courte portée, reliées au comportement mictomagné- tique de ces composés. Tout l’ensemble des résultats obtenus dans ce travail, peut être décrit en termes de magné- tisme d’environnement local, mais seulement dans un modèle adéquat, tenant compte de 2 types d’atomes por- teurs de moments (Fe et Mn). Il reste à élaborer un tel modèle au moyen d’autres techniques que les mesures d’aimantations (spectroscopie Mössbauer, diffraction de neutrons, etc.). Abstract. 2014 The magnetic properties of Be2Mn1 _xFex compounds have been investigated over the whole concen- tration (0 x ~ 1) and field (0 ~ H ~ 150 kOe) ranges at low temperatures (T ~ 100 K). The inhomogeneous nature of the magnetic transition previously observed in this system, has been confirmed by means of 03C32 versus H/03C3 plots up to 150 kOe. But low field (0 ~ H 1 kOe) magnetization measurements showed that this transition is gradual : gradual evolution from paramagnetism (x ~ 0.06) to mictomagnetism (0.11 ~ x ~ 0.30) then to ferromagnetism (0.30 ~ x ~ 1.00). The mean magnetic moment 03BC of ferromagnetic beryllides (0.30 ~ x ~ 1.00) shows a rapid non linear decrease with increasing Mn content but the corresponding normalized Fe moment (03BC/x03BC0 ratio) was found higher than unity, indicating that two kinds of atoms (Fe and Mn) are able to carry a magnetic moment in these beryllides. The magnetization data on Be2Mn1-xFex compounds with x ranging from 0.06 to 0.25, have been analysed in terms of magnetic clusters with temperature dependent moments and concen- trations. The unusual results of this analysis have been discussed and tentatively interpreted by invoking the concepts of (i) giant moment clusters with their own Curie temperatures 0, increasing with the cluster size, (ii) nearly magnetic impurities which become magnetic by lowering the temperature, in the high fields considered here (H ~ 80 kOe), and (iii) antiferromagnetic short range interactions related to the mictomagnetic behaviour of these compounds. All of the results obtained in this work may be described in terms of local environment magne- tism, but only with an appropriate model which takes into account two types of magnetic atoms (Fe and Mn). Development of such a model would be aided by the use of techniques other than magnetization measurements (Mössbauer spectroscopy, neutron diffraction studies, and so on...). 1. Introduction. - There are several alloy or - the inhomogeneous nature of the transition from compound systems in which magnetic properties are paramagnetism to ferromagnetism observed in such presently understood in terms of magnetic polarization systems, has been related to the existence of magnetic clouds (magnetic clusters) : clusters ; Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197900400102300 24 - giant moment clusters persist as superparama- - samples in the paramagnetic region (x = 0.11 gnetic entities well into the paramagnetic composition to 0.175) seem to obey a simple model of cluster range, even in perfectly random solid solutions ; superparamagnetism [14]. - the cluster concentration and magnitude increase A more recent work [ 15] reports specific heat measu- when the magnetic carrier concentration is increased rements on Be2Mno.865Feo.135 at low temperatures up to the critical composition ; (0.1 K to 23 K). The temperature dependence of the - the onset of ferromagnetism can be considered as excess heat capacity (anomalous term), somewhat resulting from the ferromagnetic coupling of giant like a Schottky function, could be reasonably inter- moment clusters -; preted in a magnetic cluster model. - these giant moment clusters have to be related to The present work was undertaken in order to obtain local environment effects [ 1 ], [2]. more information on the magnetic behaviour of over the whole concen- Let us mention as a justification of the above Be2Mn1 _xFex compounds tration 0 x 1, with to the propositions, some typical investigations on such range special regards in the critical for ferro- alloys or compounds in the papers referred to below. compounds composition range we have In all these papers, the magnetization data on the magnetism (0.06 x 0.25). Therefore, carried out a on these investigated alloys or compounds, were analysed systematic magnetization study over the whole field 0.00 to 150 kOe with the concept of magnetic clusters and the results beryllides range at low 100 After a brief des- obtained were described or discussed using various temperatures (T K). local environment models of magnetism. cription of the experimental procedures (section 2), we report in section 3 the new series of magnetization The most extensively studied system is that of Ni measurements on our Be2Mn1 _xFex samples binary alloys, especially Ni-Cu [3] ; Ni-Rh [4] ; Ni-Cu, (0.06 x 1.00) in fields up to 150 kOe at low Ni-V and Ni-Mo [5] ; Ni-Pd [6]. Giant moment clusters temperatures (T 100 K). The high field magneti- are shown to exist in other such as binary alloys, zation measurements offer a more accurate deter- Fe-V [7] or Cu-Mn [8]. mination of the saturation magnetization by allowing It in several studies on interme- appears MCX a true evaluation of the superposed paramagnetic tallic c ~ 1, M = Fe, Co or Ni compounds (with xH(T ) susceptibility. As will be seen in section 3, and X a non that being magnetic element) giant we simplify our Q(H, T) data analysis by attributing moment clusters can arise from excess M atoms the main part y(H, T ) of the magnetization to clusters X sites. A detailed on com- occupying study Co,Ga assumed to exist in all the investigated beryllides pounds [9] shows the existence of non magnetic, (0.042 x 1.00) in different magnetic states (super- and atoms nearly magnetic magnetic impurities (Co paramagnetic, -mictomagnetic or ferromagnetic, Ga The occupying sites). superparamagnetism depending on T and x). The new measurements in low observed in and was Fe,Al, CoAl Nival compounds fields (0 H 1 kOe) enabled us to observe micto- also attributed to Fe, Co or Ni atoms Al occupying magnetic behaviour on Be2Mn1 _xFex samples with sites [10]. 0.11 x 0.30 at low temperatures (section 4). The The magnetism of other binary or ternary alloy occurrence of mictomagnetism was ignored in our systems, which present a transition concentration z earlier investigations [13] on these compounds. from paramagnetism to ferromagnetism, can be Section 5 deals with the magnetization data analysis characterized in terms of magnetic clusters. This is on Fe rich beryllides (0.30 x 1.00). A detailed the case of the Be2Mn1 _xFex pseudo-binary system. analysis of the cluster superparamagnetism in beryl- We summarize briefly some aspects of previous lides with x around xCr (0.06 x 0.25) is presented investigations on these compounds. The ferroma- in section 6. The unusual results of this analysis gnetic compound Be2Fe and the paramagnetic com- (clusters with average moments and concentrations pound Be2Mn form an uninterrupted series of hexa- depending on T) are discussed and tentatively inter- gonal C14 solid solutions Be2Mn1 _xFex (0 x 1) preted in section 7.