2534 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009

Electric Signatures of Structural and Chemical Orderingof Heusler Alloy Films

J. Dubowik�, I. Goscianska�, K. Załeski�, Y. V. Kudryavtsev�, and Y. P. Lee�

Institute of Molecular Physics, Polish Academy of Sciences, Poznan 60-179, PolandDepartment of Physics, A. Mickiewicz University, Poznan 61-614, Poland

Institute of Metal Physics, National Academy of Sciences of Ukraine, Kiev 142, UkraineDepartment of Physics, Hanyang University, Seoul 133-791, Korea

In this paper, we present the results of in situ temperature measurements of resistivity for some amorphous or partially crystallineHeusler alloy (HA) films: Co�CrAl, Co�MnGa, and off-stoichiometric Ni�Mn���Sn� �, Ni�Mn���Ga� � that are known to exhibithalf-metallic properties and martensitic transformations, respectively. From resistivity versus characteristics we distinguish variousstages of chemical and structural ordering in the films. They appear to be quite distinct in both systems investigated due to differencesin their electronic structure. The resistivity results are compared with magnetic characteristics for Co�MnGa with a high Curie temper-ature. High-temperature resistivity measurement is a sensitive tool for tracing the chemical and structural ordering in HA films.

Index Terms—Heusler alloys (HAs), ordering, resistivity, thin films.

I. INTRODUCTION

T HIN films of Heusler alloys (HAs) are important forapplications in spintronics or in devices based on shape

memory effect (for review papers, see, for example, [1] and [2]).Epitaxial HA films prepared by molecular beam epitaxy havebeen shown to grow pseudomorphically on GaAs substrateswith L2 or B2 structures [3]. Frequently, however, HA filmshave been deposited by sputtering [4], [5], laser ablation [6], orflash-evaporation [6] at room temperature (RT) with subsequentannealing. HA films deposited at RT or lower temperatureshave, as a rule, a highly disordered structure (amorphous orpartially crystalline) [7] and chemical disorder significantlydifferent from that of typical HA with L2 or B2 structure.In bulk HA ordering is typically achieved by annealing attemperatures as high as 1000–1300 K for many days, but in HAfilms, such a procedure is obviously unacceptable. In thin HAfilms, annealing at moderate temperatures of 600–700 K for

60 min results in accomplishing the desired microstructure,magnetic, and electrical properties [5]. In this contribution, wepresent the results of in situ resistivity measurements of severaldisordered HA films. We show how the ordering in HA filmscan be monitored with the resistivity measurements. Underproper annealing conditions, properties typical of bulk orderedHA counterparts can be almost fully restored. The resultsof resistivity measurements are compared with the magneticcharacteristics of Co MnGa films.

II. EXPERIMENTAL DETAILS

Films of Co CrAl, Co MnGa, and off-stoichiometricNi Mn Sn were prepared by flash evap-oration [7] from HA powders of proper composition ontoliquid nitrogen cooled glass substrates. The off-stoichio-metric Ni Mn Ga ( 0.15, 0.4) films were prepared

Manuscript received October 15, 2008. Current version published May 20,2009. Corresponding author: Y. P. Lee (e-mail: yplee@hanyang.ac.kr).

Digital Object Identifier 10.1109/TMAG.2009.2018777

by rf-sputtering at room temperature. The Ni–Mn–Ga andNi–Mn–Sn films with off-stoichiometric composition werechosen since they are expected to reveal the martensitic trans-formation at higher temperatures [2]. Film thicknesses werebetween 100 and 200 nm except the sputtered Ni–Mn–Gafilm with thickness of 2 m and were measured with Dectakprofiler or by x-ray fluorescence (XRF). Film composition wasdetermined with XRF. Structure of the films was determinedwith x-ray diffraction (XRD) using Cu–K or Co–K radia-tion. The films were amorphous or partially crystalline exceptthe Ni–Mn–Ga sputtered films in which XRD shows a singlebroad (220) peak typical of microcrystalline A2 structure offully disordered HA structure. In as-deposited state, the filmswere nonmagnetic or very weakly magnetic Co MnGa andafter ordering they revealed crystalline B2 or L2 [7], [11] ortetragonal [2] structure and show ferromagnetic ordering. Theresistivity measurements were carried in situ in a high-vacuumchamber used for flash-evaporation at temperature varyingfrom 300 to 800 K with a scan rate of 4 K/min. For Ni–Mn–Gaand Ni–Mn–Sn films with shape memory effect, the resis-tivity measurements were also carried out from 200 to 400 K(and additionally from 4 to 350 K for Ni–Mn–Sn film) in alow-temperature setup. Magnetic measurements of Co MnGafilms with a high Curie temperature of 690 K [11] were per-formed using ferromagnetic resonance (FMR) spectrometerat 9.08 GHz and vibrating sample magnetometer (VSM) at atemperature range from about 200 K up to 600 K.

III. RESULTS AND DISCUSSION

Figs. 1–4 show changes in the resistivity of Ni–Mn–Ga(Fig. 1), Ni–Mn–Sn (Fig. 2), Co CrAl (Fig. 3), and Co MnGa(Fig. 4) films during temperature cycling from 300 to 700–800 Kand back to 300 K. After high-temperature cycling, we alsoperformed additional resistivity measurements in atemperature of 200–400 K. Usually, upon crystallization ofamorphous alloys, the resistivity decreases about two times[8] due to increase of structural and chemical ordering. Indisordered Ni–Mn–Ga(Sn) films, we observe a similar effect,however less noticeable: the resistivity decreases by 60%–70%.

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DUBOWIK et al.: ELECTRIC SIGNATURES OF STRUCTURAL AND CHEMICAL ORDERING OF HEUSLER ALLOY FILMS 2535

Fig. 1. Resistivity � changes of disordered Ni–Mn–Ga films on heatingto 800 K and cooling to 300 K. Low-temperature portion of � versus �

(200–400 K) was measured independently after full cycle of heating/cooling.Inset shows resistivity changes in a disordered film with different composition.Low-temperature portion of � versus � (�200–550 K) was measured indepen-dently and shows a hysteretic behavior of � (vertical arrows) characteristic of amartensitic transformation in ordered Ni–Mn–Ga ordered alloys.

Comparing Fig. 1 with Fig. 2, one can notice that disorderedNi–Mn–Ga films are more resistant to ordering than Ni–Mn–Snfilms, for which the resistivity decreases abruptly at about520 K due to crystallization. This is in accordance with resultsof EXAFS [9] measurements, which proved that the successiveappearance of ferromagnetic and shape memory propertiesin annealed Ni–Mn–Ga films are related to the change inchemical ordering and then to an increase in crystalline grainstructure, respectively. In the case of our Ni–Mn–Ga(Sn) films,the ferromagnetic ordering was achieved after full thermalcycle since in both films the characteristic “kinks” onare clearly seen at the Curie temperature 380 K and

320 K, respectively (see Figs. 1 and 2). Inset in Fig. 1shows for a Ni Mn Ga film. The main features of

dependence upon crystallization and further ordering aresimilar to that of Ni Mn Ga film but after orderingat 800 K, a hysteretic anomalous jump in is seen in atemperature range from 300 to 400 K. This is characteristic ofmartensitic transformation in some well-ordered Ni–Mn–Gafilms with shape memory effect [10]. In this case, however,the effect of the structural austenite/martensite transformationis significant (inset in Fig. 1). Usually, in thin HA films withshape memory effect, the anomaly in resistivity is blurred incomparison to that observed in bulk alloys [12]. In this context,a small anomaly in seen in the inset in Fig. 2 is typical of thefilms with shape memory effect in contrast to rather significantresistivity changes observed in bulk Ni–Mn–Sn alloys [13].

In Figs. 3 and 4, it is seen that for Co CrAl and Co MnGafilms characteristics are significantly different fromthat of Ni–Mn–Ga(Sn) films. The resistivity of amorphousCo CrAl film abruptly increases at 500 K and afterordering it attains higher values. Moreover, after ordering at600–700 K, the temperature coefficient of resistivity TCR

Fig. 2. Resistivity changes in a disordered Ni–Mn–Sn film on heating to700 K and cooling back to 300 K. Low-temperature portion of � versus �(�200–350 K) was measured after the full heating/cooling cycle. Inset shows� versus � for the ordered film. A small anomaly in � extended from 150 to260 K results from martensitic transformation.

Fig. 3. Resistivity changes in a disordered Co CrAl film on heating to 700 Kand cooling back to 300 K. Inset shows � versus � for the ordered film with ananomalous negative temperature coefficient of resistivity characteristic of theordered Co CrAl alloy [12].

in accordance with earlier observations [14] for L2 orderedbulk Co CrAl. Both substantial increase in resistivity uponcrystallization of Co–Cr–Al films at 500 K and nearly lineardecrease in resistivity with increasing temperature for orderedfilms suggest that the resistivity does not increase due to in-crease in scattering probability but as a result of a substantialchange in electronic structure. The electronic structure of theamorphous (disordered) Co–Cr–Al is not known, whereas theelectronic structure of L2 ordered Co CrAl is well known(see, for example, [15]). The characteristic features of theelectronic structure for ordered Co CrAl HA are: 1) a gap of0.18 eV in the minority band due to a strong hybridizationbetween Cr d and Co d states and 2) a large peak of the ma-jority density of states (DOS) at the Fermi level [15].

2536 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009

Fig. 4 Resistivity changes in a disordered Co MnGa film on heating to 700 Kand cooling back to 300 K.

Because is just located at a large peak of the DOS in themajority spin state situated in a gap of the minority spin state,the concentration of conducting electrons is lowered and thescattering processes to empty states above are enhanced.Disordered B2 structure causes decrease of the majority DOSat and disorder of Co–Cr type makes local DOS furtherblurred for Cr spin projected states. Hence, we can anticipatethat for amorphous (disordered) Co–Cr–Al films the electronicstructure is significantly different from that of L2 Co CrAl.

On crystallization and ordering of the amorphous Co MnGafilm the resistivity also increases, however in a different way.Fig. 4 shows clearly that at 430 K, the electrical resis-tivity experiences a substantial jump-like increase and thenat 500 K, it decreases in a similar way but attains valuesby about 30% higher than those in the amorphous state. Inordered L2 Co MnGa HA, the gap in the minority band isalmost open and the majority peak is slightly shifted belowbut there is still a considerable DOS at [16]. Similarly toCo CrAl, we may also expect that the disordered Co–Mn–GaDOS is significantly blurred resulting in different peculiaritiesin electronic structure at . Since resistivity in both Co CrAland Co MnGa experiences a notable increase upon ordering,we argue that scattering between conducting electronicstates to empty -states of majority band play a significantrole in this type of HA films. In Ni–Mn–Ga HA, the electronicstructure is completely different with moderate DOS at :specifically, with the minority spin DOS at varying withtetragonal distortion c/a and with the majority spin DOS atremaining almost unchanged [17]. These features of electronicstructure in Ni–Mn–Ga HA can be responsible for relativelysmall resistivity changes due to martensitic transformations.

With independent XRD and TEM measurements, wechecked that low-temperature annealing at 500 K leads toa very inhomogeneous mixed phase consisting of Co MnGaand Mn Ga (and/or Mn Co ) phases. Annealing of ourHA films can restore single-phase ordered HA structures,which are ferromagnets with Curie temperatures typical of

Fig. 5. (a) Changes in the effective magnetization ��� and (b) in the sat-uration magnetization � of an amorphous Co MnGa film in the course ofheating to �600 K and cooling back to RT.

bulk alloys. Due to a limited space, we would like to discussmore thoroughly characteristic changes in magnetic propertiesduring the crystallization process of the amorphous Co MnGafilms and compare them with the resistivity data shown inFig. 4. Since the Curie temperature of the ordered Co MnGais of 690 K, it is possible to monitor in situ the changes in themagnetic characteristics of the disordered film in the courseof heating and cooling back to RT. Fig. 5(a) and (b) showsthe temperature dependence of the effective magnetization

and saturation magnetization , respectively. Theamorphous Co MnGa is already very weakly magnetic: thereis seen a weak FMR signal though it has practically no mag-netic moment measured with VSM. It is seen that alsoexperiences substantial changes in the same temperature rangeas the resistivity (Fig. 4). As shown in Fig. 5(b), the saturationmagnetization does not change until about 500 K andthen begins to increase to a value of 250 G in a range of500–600 K. It is reasonable to conclude that crystallizationof the amorphous Co MnGa films, occurring below 520 K,leads to a strongly inhomogeneous polycrystalline structure inaccordance with the resistivity measurements. Further orderingleads to an enhancement of the magnetic characteristics of theCo MnGa films: the RT values of increase up to 6000 Gand to 500 G. Our resistivity and magnetic measurementsof the amorphous Co MnGa films at elevated temperaturesshow clearly nonmonotonic changes resulting from appearanceof mixed phases at 520 K followed by further homogenizationand ordering at 600 K.

IV. CONCLUSION

We investigated high-temperature resistivity in amorphousCo MnGa, Co CrAl, and partially crystalline off-stoichio-

DUBOWIK et al.: ELECTRIC SIGNATURES OF STRUCTURAL AND CHEMICAL ORDERING OF HEUSLER ALLOY FILMS 2537

metric Ni–Mn–Ga, Ni–Mn–Sn films. Structural disorderseverely affects the conduction electron mean free path as wellas the exchange interactions resulting in the high resistivity andthe lack of ferromagnetic ordering. Generally, at temperaturesof 500 K (Co MnGa, Co CrAl) or 600 K (Ni–Mn–Ga,Ni–Mn–Sn) the films crystallize. However, our results showthat chemical and structural ordering may have diverse effect onhigh-temperature characteristics of HA films. Resistivity maydecrease almost monotonically over a wide temperature rangefrom 600 to 700 K (Ni–Mn–Ga). It may drop (Ni–Mn–Sn)or rise (Co CrAl) in a narrow temperature range or it mayshow a characteristic maximum (Co MnGa) due to appearanceof mixed phases, which were confirmed with the magneticmeasurements. In conclusion, high-temperature resistivitymeasurement is a sensitive tool for tracing the chemical andstructural ordering in HA films.

ACKNOWLEDGMENT

This work was supported in part by the KOSEF throughQuantum Photonic Science Research Center at Hanyang Uni-versity, Seoul, Korea and in part by Scientific Network fromMinistry of Higher Education and Science, Poland.

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